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Mechanical Failure: Definition of the Problem

Proceedings of the 20th Meeting of the

Mechanical Failures Prevention Group,

held at the

National Bureau of Standards,

Washington, D.C. 20234

May 8-10, 1974,

Edited by

T. R. Shives and W. A. Willard

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

no I

Jointly Sponsored by

NBS Institute for Materials Research, Washington, D.C. 20234

Office of Naval Research, Arlington, VA 22217

Consumer Product Safety Commission, Washington, D.C. 20207

Frankford Arsenal, U.S. Army, Philadelphia, PA 19137

Naval Air Development Center, Washington, D.C. 20362

American Society for Mechanical Engineers, New York, NY 10017

Federal Aviation Administration, Washington, D.C. 20591

NASA, Goddard Space Hight Center, Greenbelt, MD 20771

"ates

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary Designate

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

U> NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director n \ Issued April 1 976
Library of Congress Catalog Card Number: 76-600003

National Bureau of Standards Special Publication 423

Nat. Bur. Stand. (U.S.), Spec. Publ. 423,, 228 pages (Apr. 1976) CODEN: XNBSAV

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WASHINGTON: 1976

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Abstract

These proceedings consist of 20 submitted papers and discussions from the 20th Meeting of the Mechanical Failures Prevention Group held at the National Bureau of Standards, May 8—10,

1974. The central theme of the proceedings is the definition of the problem of mechanical failure, with emphasis on modes of failure, consequences of mechanical failure, and implications of mechanical failure.

Key words: Brittle fracture; economics of failure prevention; fatigue failure; mechanical relia- bility; stress corrosion cracking.

iii
Units and Symbols

Customary United States units and symbols appear in many of the papers in these Proceedings.

The participants in the 20th Meeting of the Mechanical Failures Prevention Group have used the established units and symbols commonly employed in their professional fields. However, as an aid to the reader in increasing familiarity with and usage of the metric system of units (SI), the following

table of conversion factors and references are given:

Length

1 inch = 2.54 centimeters (cm)

1 foot = 0.3048 meters (m)

1 mile = 1.609 X 10^ meters (m)

1.609 kilometers (km)

Mass

1 pound (Ibm) = 0.4536 kilogram (kg)

1 ton (short, 2000 Ibm) = 907.2 kilograms (kg)

Stress

1 psi = 6.895 X 103 pascals (Pa)

1 ksi = 6.895 X 106 pascals (Pa)

Velocity

1 mile per hour (mph) = 0.4470 meter per second (m/s)

1.609 kilometers per hour (km/h)

Volume

1 gallon = 3.785 X 10"^ cubic meters (m^)

Volume/Time

1 cubic foot per minute (cfm) = 4.719 X 10~* cubic meters per second (m^s)

References

NBS Special Publication, SP330, 1974 Edition, "The International System of Units."

ISO International Standard 1000 (1973 Edition), "SI Units and Recommendations for Use of Their Multiples."

E380-72 ASTM Metric Practice Guide (American National Standard Z210.1).

Disclaimer:

Certain trade names and company products are identified in order to adequately specify the ex- perimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the products are necessarily the best available for

the purpose.

iv
Contents

page Units and Symbols iv Introduction: E. Passaglia vii Intent of the Symposium: D. C. Drucker xi

Modes of Failure

Session I:

1. What We Can Learn from the Examination of Service Failures: J. A. Bennett 3

2. Failure by Fatigue: A. J. McEvily 13

Session II :

1. Failure by Stress Corrosion Cracking—Current Approaches toward Failure

Prediction: J. Kruger 27

2. Elastohydrodynamics in Concentrated Contacts: H. S. Cheng 41

3. Failure in Gears and Related Machine Components: R. A. Wilde 55

4. Bearing Damage Analysis: R. L. Widner and W. E. Littmann* 67

Session III:

1. Anti-friction Bearing Design, Inspection and Recycling: K. M. Glaesner 87

2. Holographic NDI of P-3 Wing Plank Splices: A. J. Koury, M. J. Devine,

J. F. Erthal* and P. G. Bhuta 93

Consequences of Mechanical Failures Session III:

3. Economic Considerations in Failure Prevention: H. E. Morgan 107

4. Economic Impact of Tribology: H. P. Jost 117

Session IV:

1. Nuclear Power Plant Safety: P. A. Morris 143

2. Mechanical Failures and Public Expectations of Safe Transportation: H. H. Wakeland 153

3. Public Safety and Construction Surety: J. Feld 165

4. The People Considerations in Automotive Service: F. J. Uhlig 171

Implications of Mechanical Failures

Session V:

1. Mechanical Failure: Implication for Science: J. P. Hirth 181

2. Implications for Action for Engineering: H. W. Paxton 193

3. Mechanical Reliability—Implications for Engineering, Manufacturing and

Design: W. D. Compton 199

4. Implications for Action—Industry: J. E. Ryan 209

Session VI:

1. The Implications of Mechanical Failures for Consumer Product Safety—Vice

Versa: L. M. Kushner 215

2. Mechanical Failure—A Material Matter: R. W. Roberts ; 221

*Indicates speaker when a paper had more than one author.

List of Registrants at the 20th MFPG Meeting 227

The following registrants gave presentations at the symposium, but did not submit the manuscript

for publication: R. W. Staehle, "Mechanical Failure: An Overview," Session I H. Corten, "Failure by Brittle Fracture," Session 1

V

Introduction

Elio Passaglia

Executive Secretary, MFPG

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Mechanical failures are a pervasive fact of life in our society. Ranging from the failure of small items that all of us have experienced and that many of us take for granted, to the failure of a large

complex structure that often becomes front page news, they have undesirable consequences for our

society. The large ones many times cause loss of life or cause serious injury to many people. The minor

ones sometimes also cause loss of life or injury, and they always cause frustration and anger on the part of the one to whom they occur. Always they cause loss of valuable material, and have undesirable

social and economic consequences. Thus it is not surprising that a large segment of our technological community devotes its efforts

to the prevention of mechanical failures, and that numerous agencies of the Government are charged

with the responsibility of preventing mechanical failure. These efforts to prevent mechanical failures are wide ranging indeed. They go from the material scientist studying the fundamentals of fracture,

to the metallurgist assuring that a steel forging meets specification, to the design engineer designing a new

bridge, to the Government administrator considering new specifications to reduce the incidence of mechanical failures. Yet despite these efforts mechanical failures continue to occur, and continue to present

a problem whose solution, if possible, would have highly desirable consequences for our society.

The Symposium, the proceedings of which are presented in this volume, was designed to explore

with a broad brush the various aspects of mechanical failures. We say "with a broad brush" because no symposium of reasonable length could conceivably grope with all the various aspects of the problem. All that can be done in a reasonable time is to define the problem, not discuss in depth its many detailed aspects and ramifications. As the organizers of the Symposium saw it, the definition of the problem

of mechanical failures involved three aspects:

(1) The modes of failure (2) The consequences of failure (3) The implications (for action) of the occurrence of failures.

Thus the Symposium was divided into sections discussing each of these parts. The section on the modes of failure was constructed on the basis that all failures of machine and

structural components are eventually traced to material failure. Following this basis, the investigation

of large numbers of mechanical failures shows that the primary modes of failure are fatigue, stress corrosion, and brittle fracture. Thus these topics are treated in individual papers in the Symposium,

since they are of central importance to the problem of mechanical failure prevention. The reason this

is so, and that these modes of material failure are the most common, is that while adequate design pro- cedures are available for designing against simple overload, procedures to design against fatigue, stress corrosion, or brittle fracture, are either not available or are only now beginning to be used. Thus these modes of failure continue to be prominent because design methods are not yet available to deal ade- quately with them. These topics are dealt with in papers by McEvily, Kruger and Corten.^ When components are assembled into machines, particularly where moving parts are involved,

the situation becomes far more complicated. Here we have the situation where one part interacts with

another across their surfaces, and this leads to the whole problem of wear in its various forms, and

its prevention. Various techniques are used to prevent mechanical failures in these circumstances.

These include the use of fluid, solid or semi-solid films to separate the parts that are in relative motion,

and the "components" aspects of mechanical failures thus refers to the design, kinds of damage assoCorten's manuscript was not submined for piili] ir;\iion.

vii
ciated with various types of components, and inspection. These topics are discussed in papers by Widner

and Littmann, and by Wilde, and one of the underlying theories of the lubrication process, elasto- hydrodynamics, is described in some detail by Cheng. Not all aspects of the modes of failure could be discussed at the Symposium. The papers given were

all by renowned world authorities, and the topics they treated were obviously treated with great depth, but many topics had perforce to be left out. Among these might be mentioned crevice corrosion, fret- ting, erosion, corrosion fatigue, etc. In particular, one of the most important means of failure prevention, namely non-destructive evalution, was given only passing recognition in the paper by Erthal,

et al., where the topic of laser holographies, among others, is mentioned.

The consequences of mechanical failures are many and great, and this symposium attempted to discuss only a few of them: the public safety consequences, the economic consequences, and what might

be called the "societal" or emotional consequences. Public safety is the most important reason that the Federal Government is concerned with the prevention of mechanical failures. Indeed, many agencies of the Government owe their existence to the desire of the Government to protect the safety of the general public by the prevention of mechanical

failures. Only two were represented at this conference, the National Transportation Safety Board

(Henry Wakeland) and the Atomic Energy Commission - (Peter Morris). In his paper Mr. Wakeland

makes the point that some failures, because of their dramatic nature, assume an importance that tran- scends their eflfect on public safety, so that public emotional considerations are important in determining

public policy with respect to failure prevention. In the atomic energy field, similar considerations are important, and protection against malfunction or mechanical failure of reactor systems and components

in nuclear power plants is achieved by a very sophisticated combination of design and operational

procedures. To assure that the probability of serious accidents is acceptably low requires conservative

design, construction and operation, using redundant and diverse systems and components and a rigorous

program for quality assurance. The concern with public safety is, of course, not limited to the Federal Government. It is of concern

also to the whole private sector, but only one aspect is treated here. This is in the construction field (Feld). Most of the accidents and partial collapse in structures come during the construction period, often with serious effects on the life and well being of people involved in the work as well as the innocent public that is by chance in the area involved. However, accidents do occur even after completion. Mr. Feld discusses the historical aspects of their prevention, the design and legal considerations in the construction industry arising from failures, and discusses the sources of failures and the responsibilities for their prevention.

Another of the important consequences of mechanical failures are economic. No aggregate statistical data exist to measure the losses due to mechanical failures, or even the cost of mechanical failure prevention. Indeed, the measurement of these costs would be a formidable task. However, to the extent that many accidental deaths and injuries are due to mechanical failures, the economic losses undoubtedly

represent a substantial sum. However, the measurement of these losses poses a serious problem (Morgan). Economic analysis for quality control, which is one of the most important means of mechanical

failure prevention, relies on the techniques of marginal analysis and life-cycle costing. Typically there

is an optimal level of quality that is greatest relative to its cost. It is usually not economically feasible

to obtain zero defects, which implies that failures at some level of occurrence will always be with us. Although no aggregate statistical data exist as mentioned before, there exist, however, comprehensive data on a facet of failure in England. In a study underwritten by the Department of Education

and Science, the economic and technological significance of tribology was shown (Yost). Tribology is the science and technology of interacting surfaces in relative motion and of the practices related

thereto. The study detailed the annual saving which could be achieved by industry in the civilian sector in England through rigorous practice of tribology. This figure came to some $500 000 000. When

translated to the United States through the GNP, the relevant figure would indeed be staggering.

-The Atomic Energy Commission has since been reorganized anil its functions divided into two new groups: the U.S. Energy Research and Development Administration and the U.S. Nuclear Regulatory Commission. Peter Morris is with the U.S. Nuclear Regulatory Commission.

viii
Finally, we come to the emotional consequences of mechanical failures. These range from the

serious emotional consequences experienced by a large segment of the society when a dramatic failure

in a large structure occurs, to the annoyances we all feel when an item of equipment we own fails. A

particular case of wide importance of the latter type are automobile failures. Each of us has experienced the aggravation caused by such failures, and the anger with which we take our troubles to the

dealer. We have also experienced the anger and frustration caused when the repairs did not fix the

failure. These emotions are very real consequences of mechanical failures and are of very real con- cern to automotive manufacturers. How one of them dealt with the problem is described by Uhlig.

The occurrence of mechanical failures has implications for efforts of all concerned in their prevention. Those concerned range from basic scientists to Government policy makers, and the implications

for action on the part of the various groups involved are discussed in papers in the last section of the Symposium.

In the area of basic science, the main concern, as detailed by Hirth, is in the area of fracture, and

the understanding of the detailed mechanisms of fracture that is a prelude to its eventual control. In materials that are ideally elastic, the macroscopic theory of fracture is well worked out, but even in

this relatively simple case the atomistic details of the fracture process still require considerable attention before real understanding is attained. In engineering materials, where plasticity at the crack tip is important in determining the fracture process, the situation is far more complicated. Dislocation

motion at the crack tip must be taken into account in any atomistic treatment of the problem, and

mechanisms of void initiation, growth, and coalescence become important in the fracture process. While some continuum treatments of the plasticity problem, such as the / integral, are important steps lead- ing to a formalism that can be used by designers in fracture control design, the identification of the important material properties that control fracture mode await an atomistic treatment. In considering

the viewpoint of the design engineer in fracture control, Paxton makes the important point that there

is a great deal of impetus to reduce the number of engineering alloys since many specifications are redundant and obtaining data for fracture control design on all possible alloys poses an intolerable burden on the whole engineering community.

The existence of mechanical failures holds implications for action for industry as well. These are

detailed by Compton and Ryan, representing two very large industries. Both customer safety and

customer satisfaction considerations determine large programs aimed at failure prevention, and necessitate the development of highly detailed engineering data. In the case of fatigue, which is one of the major causes of mechanical failures, very sophisticated testing indeed is done on prototypes. Unfortunately, it was impossible in the time available to have discussions on what is done in failure prevention

in small industries that do not have the resources available to the two industries represented here. Consumers are also deeply concerned with the problem of failure prevention. This group has until recently not had a technical spokesman, but with the formation of the Consumer Product Safety Commission, such a spokesman has emerged, and the point of view of that organization was presented by

Kushner, one of the Commissioners. He described how it was the will of the Congress to protect the consumer from hazard, and pointed out that mechanical failure prevention was a deep and abiding concern of the Commission. Of the various technical aspects of failure prevention, Kushner singled out non-destructive testing as having the most promise for application to the prevention of failures in consumer items.

Finally, mechanical failures have had important implications for public policy, a topic discussed

by Roberts. The Federal Government has been a leader in mechanical failure prevention because of military activities, which are the sole responsibility of the Federal Government, and because of the space program. In both these areas technology is pushed to the limit. In the military case this is done

because military ability must be premier, and in the space area because the accomplishment of the mission required this. In addition to these two areas, the protection of the safety of the public required

the Government to be concerned with failure prevention. Roberts highlighted the emerging area of materials conservation as one in which the Federal Government could very possibly pass new laws,

and in which failure prevention would play an important part. Of the various technical issues in- volved, Roberts called for more attention to fracture control design and the greater and more intensive use of non-destructive evaluation.

ix
Clearly, this Symposium does not treat the whole problem of mechanical failures, even with the broad brush used. Each of the topics discussed could be the topic of one or several symposia. Indeed,

symposia could be built around the topics of many of the individual papers presented. Nevertheless,

as a definition of the problem, or the class of problems posed by mechanical failures, this symposium

should serve the purpose.

X
Intent of the Symposium

D. C. Drucker

Universitv of Illinois Urbana, Illinois 61801

The intent of this Symposium is to sharpen our understanding of the problem of mechanical fail- ure, its consequences, and its implications for research workers, practitioners, consumers, and the policy makers in government and industry. We hope that clear problem definitions will serve as a guide to engineers and scientists in industrial applications as well as in fundamental studies. Complete prevention of mechanical failure is, of course, an uneconomic, unattainable ideal. Working

toward it. however, will be essential as we move through the energy crisis into the materials crisis which lies ahead. The more effective our anti-failure engineering can become, the more viable will be our economy, the better our environment, and the safer and happier will be our people.

As time goes on. the public (quite understandably) expects us to do more and more, better and more

economically. We must respond. Therefore, the arrangers of the program, Fred Ling, Elio Passaglia,

and Robb Thomson, have provided superb speakers who will challenge our comfortable notions and

prod us to action. All of this is evident in the program before you. Although now I should speak only for myself,

perhaps there are others here in the audience, who like me. have fallen somewhat behind current knowledge in some aspect of mechanical failure prevention. All of us in this uncomfortable but common sit- uation express our gratitude to all those who organized this Symposium which will bring us right

up-to-date in so crucial an area in an efficient and incisive manner.

xi
1
MODES OF FAILURE

Session I

Chairman: Elio Passaglia

National Bureau of Standards
i iI
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechaniral Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

What We Can Learn from the Examination of Service Failures

John A. Bennett

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

The study of metal components that have fractured in service provides information that can

be of major importance in preventing future failures. The metallurgical and fractographic techniques

used in such studies are described so as to permit an evaluation of the reliability of the results. Examples are given of some case histoi ies in which the fracture studies were highly effective, and

of others in which the information was ignored and further failures occurred.

Key words: Crack propagation; failure analysis; failure prevention; fracture; fractography ; mechanical failure.

1. Introduction

Mechanical failures come in a wide variety of shapes

and sizes, and their importance varies from a minor

nuisance to a major disaster. When a failure causes

injury or serious property damage, there is usually an

investigation. However, these are often conducted with no clear understanding of what information can be

obtained from the investigation or how to use the in- formation that is obtained. I would like to consider

these questions in regard to one class of failures

namely fractures in metal components—and try to show where failure analysis fits into an overall failure avoidance program.

2. Techniques

First, what techniques are available for studying

service fractures? Obviously, one needs to know how

the information was obtained before he can evaluate

its validity. The most important requirement in such a study is to be sure that the fracture being analyzed

is the initial one—tiie one that really started the

trouble. This may be dilTicult to do if, for example, the fracture caused an aircraft to crash, but it can be done

in a surprisingly large number of cases. The characterislie of the initial fracture which is most useful in identifying it is usually the lack of

ductility. The cause of failure is apt to be a crack propagation mechanism, especially stress corrosion

cracking or fatigue, which has occurred at a stress level well below the yield strength of the metal, so there

will be little or no plastic deformation at the origin,

in contrast to fractures that have occurred due to overload. This characteristic is demonstrated by the two

specimens of identical material in figure 1. The upper

specimen was broken in a laboratory fatigue machine

by the application of many thousands of cycles of bending load, and the fracture appears completely

brittle. The lower sj)ecimeii was subjected to a larger

bending load which caused extensive plastic deformation, showing the iidierent ductility of the material.

FlGURi; 1. Identical steel specimens loaded in bending.

Top—broken by many ihousaiuls of stress cycles in a fatigue testing machine. Bottom —bent by a single ajiplication of load to show the ductility of ihc material.

In some components a failure due to stress corrosion

cracking or fatigue will occur at a different place than one due to a static overload. This is because the cracks

in the former cases will be initialed at the point where

the local stress is a maximum, while the overload frac- ture will occur at the location where gross plastic

deformation first takes place. A good demonstration of

this was given by the failure of some cylinder bold- down bolts from an aircraft engine. The two bolts on

the left in figure 2 were broken by fatigue cracks which started in the threads adjacent to the reduced

section in the shank. As each bolt failed the loads on

the remaining ones increased, until the stress became

greater than the yield strength in the shank, and over- load fracture occurred there, as in the bolt on the rigliL

Another good example was provided by the collapse

of the Point Pleasant bridge over the Ohio River, which was due to stress corrosion cracking adjacent to the pin hole of an eyebar [1].^ Eyebars are always de-

^ Figures in brackets indicate literature references at the end of this presentation.

3
Figure 2. Cylinder hold down bolts OAq in diameter) from

an aircraft engine. Two bolts at left (vere broken by fatigue cracking, one at the right by

Figure 3. Part of a tested eyebar showing normal overload

fracture in the shank.

(Smithsonian Museum of History and Technology.)

signed so that the nominal stress in the shank will be

greater than that in the head, to make up for stress concentration around the pin hole. This means that when an eyebar is overloaded in tension, fracture always occurs in the shank, as shown in figure 3. Consequently, when the eyebar shown in figure 4 was found

in the wreckage from the bridge, it was easy to guess

that it had been the initial one to break; this was

verified by a very careful investigation of the collapse.

If you are concerned with a large component, such

as a welded ship which has broken in two, there is no difficulty in identifying the first fracture, because

the component is all one piece. But you would like to know where the fracture started; this is usually important even on smaller components. Here the characteristics of the fracture surface often give the answer. When a tearing type fracture occurs in a reasonably

ductile metal, there are irregular ridges formed on

the fracture surface which are approximately perpendicular to the crack front. The ends of the propagating

crack are held back at the surfaces (fig. 5), so the

ridges tend to form a pattern which points back to- ward the origin. These patterns were first analyzed

carefully in plate materials, so they are often referred

to as "chevron" patterns. In components of a more

complicated shape the patterns may not look at all like chevrons, but they can often be used to trace the development of the fracture back to the origin, if it is kept in mind that the ridges are approximately perpendicular to the crack front. Figure 6 shows the area near

the origin of the fracture in a B-52 bulkhead wfiYch caused the loss of the plane and six crew members. Even

though the wreckage was spread over quite a bit of landscape, there was little difficulty in tracing the fracture back to this area, and it is fairly obvious that the fracture markings radiate from the dark area in the upper right. This failure was the subject of an

extensive investigation and a multimillion dollar lawsuit,

but no one ever questioned the statement that the fracture started near the point shown by the arrow. Another type of marking is generally found on

fatigue fracture surfaces, particularly in wrought

metals. These are the "clam shell" marks which rep- resent the location of the crack front at various

stages of its progress. Figure 7 shows the fracture of

Figure 4. Portion of an eyebar from the Point Pleasant, IF. Va.

bridge.

This fracture caused the collapse of the bridge.

Figure 5. Sketch showing the development of fracture markings

(solid lines) perpendicular to the moving crack front (dotted

lines).

4
an aircraft axle with a pronounced set of these markings; they result from variations in the magnitude of the alternating stress during crack propagation, so may not be present in some fractures.

It is not necessarily suflTicient to find a fracture caused by stress corrosion cracking or fatigue to be

sure that it is the initial cause of trouble. Failure in an- other component may induce an alternating load where

there was none in normal service, for example; so the conclusions about the initial failure must always

be checked to make sure that they are resonable in view of the service conditions. Once the initial fracture has been identified with

certainty, there are a number of techniques available

to the investigator which may give information regarding the factors contributing to the failure. Many of these involve the study of the fracture itself. Something

as simple as the location of the origin may be sig- nificant. The propeller fracture, figure 8, was found

to have started on the inside, figure 9. As one would

expect the bending load on the blade to produce a

Figure 6. Area near the origin of the fracture in a B-52

bulkhead. X4.

Figure 7. Fatigue fracture of a 2 in diameter aircraft axle showing pronounced "clam shell" markings.

higher stress amplitude on the outside, the location

of the origin showed that some extraneous factor had

reduced the fatigue strength on the inside. The

culprit was corrosion, apparently due to contamination

on the lead wool used for balancing, which was packed

in the hole in the blade. The orientation of the fracture surface may be

informative. The next two figures show failures of components that were stressed in torsion. Under this loading fatigue cracks may propagate on planes either

Figure 8. Fracture of a propeller caused by fatigue.

Figure 9. Surface of the fracture of figure 8 showing that the fatigue cracks started at the surface of the central hole.

5
parallel to the length of the rod or at 45° to it. Figure

10 is a torsion bar spring from a highway bus; the

initial crack was longitudinal and extended from A

to B. A 45° fatigue crack extended from B to C. and

the final overload fracture ran around from C to D. The fact that the initial crack was longitudinal indi- cated a weakness in this direction; metallographic

examination showed that there were unusually large

inclusions which had been elongated by the forging

operations and which undoubtedly contributed to the

failure. The helical spring in figure 11, on the other hand, cracked on a 45° plane. For a given discontinuity, the stress concentration is higher for the direct

stress on the 45° plane than for the shear stress on

the longitudinal planes, so this type of cracking suggests that a stress raiser had been a factor in initiating

the crack. As a matter of fact, there was a large pit

at the origin, probably the result of rolled-in scale. Finer details of the fracture surface can also provide

information regarding the stresses at the origin. In the absence of a stress gradient a fatigue or stress cor- rosion crack will grow fairly uniformly in all directions, so the crack front will be a circular arc. Figure

12 from reference [2] shows the fracture of a piston rod from a forging hammer where the fatigue crack

started at an internal defect and formed a nearly perfect circle. Under a bending load, portions of the crack

will be growing into a decreasing stress region, so the

front will be elliptical, with the minor axis perpendicular to the neutral axis of the component. Figure

13 shows this effect on a thin metal component from

an automobile suspension. The progressive positions

Figure 10. Torsion spring (lYs in diameter) from a bus in ivhich the initial fatigue crack developed on a longitudinal

plane.

iieiiiaimiUiiltiMlS'.

Figure 11. Portion of a helical spring (y^ diameter rod) in which the fatigue crack had initiated on a 45° plane at a a surface pit.

Figure 12. Fracture surface of a piston rod from a forging hammer.

The fatigue crack started at an internal flaw in the rod. (From reference [2].)

Figure 13. Fatigue fracture from a Yg in thick steel component of an automobile suspension.

Cracks started at several origins between the arrows.

6
of the crack front are often well defined by the clam

shell markings on fatigue fractures; they are much less apt to be discernible on stress corrosion fractures. The light microscope is of little use for examination

of fracture surfaces above about 30 magnification be- cause of its short working distance and limited depth

of focus. For a closer look at the fracture surface, the

investigator will go to the electron microscope. If the part cannot be cut up, or if the maximum detail is needed, the transmission microscope will be used by

making replicas of the surface. This is a time- consuming process, but the fine detail that can be

obtained is shown bv fipure 14 which is the surface of

Figure 14. Surface of a fatigue fracture in an aluminum alloy laboratory specimen.

The photograph is a transmission electron micrograph of a replica of the

surface.

Figure 15. Scanning electron micrograph of a fatigue fracture

ill an aircraft component.

The width of the picture represents 75 JLlm on the surface.

a laboratory fatigue fracture in an aluminum alloy.

The striations are 0.001 mm apart and represent the progress of the fatigue crack during each cycle of

stress. Such detail is seldom seen on service fractures,

in part because it is destroyed very quickly by even

mild corrosion.

If the fracture surface can be inserted into the vacuum chamber of a scanning electron microscope

the examination can be made much more quickly than

replicas can be prepared and the detail will usually be

sufficient to show the nature of the fracture. Figure 15

is a scanning electron micrograph of a fatigue fracture

in an aircraft flap hinge. The striations can be seen

clearly even though the resolution is not nearly as good

as the transmission micrograph. The SEM is also very

useful in giving an idea of the fracture toughness of a material by showing the amount of cleavage or inter- crystalline fracture in the overload part of the surface. In interpreting the results of electron micrography of fracture surfaces, it must be remembered that while

the presence of striations is a definite indication of a

fatigue fracture, their absence does not mean that it was not fatigue; they may have been obliterated by

corrosion before the part reached the laboratory.

After examining the fracture surface, the investi- gator next wants to learn all he can about the material;

for this he will use metallography, mechanical testing,

and chemical analysis. Of the three, metallography will probably provide the most extensive information in a majority of cases. The investigator is fortunate if there are other cracks in the component which appear to be

similar to the one which was involved in the fracture,

because metallographic examination of these will pro- vide information on the nature of the cracks much

more readily than examination close to the fracture,

where the material is often battered or otherwise

damaged. This is particularly true of stress corrosion

fractures where the surface of the fracture is often

Figure 16. Fracture of the collar on a %(3 in diameter aircraft fastener.

7
uninformativc, but metallographic examination of material containing cracks will show the wandering and

branching that is typical of this kind of cracking.

Figure 16 shows the fracture of a swaged-on aluminum

alloy collar on a %6 in diameter aircraft fastener. In addition to the fracture, there were numerous other cracks

in the collar, and figure 17 is a metallographic section through some of these. The nature of the cracks is typical of stress corrosion, caused in this case apparently by the residual stress resulting from the swaging

operation. Similarly, figure 18 shows the cracks in a

section of the fractured eyebar which I showed earlier; these are also believed due to stress corrosion cracking.

Metallographic examination is the best way to dis- cover many material defects that may have influenced

a service fracture. Figure 19 shows the two parts of a

failed aircraft wing spar fitted back together. The spar

Figure 17. Section of the collar of figure 16 showing cracks remote from the fracture. X 75.

Figure 18. Sectioji of the eyebar of figure 4 showing cracks

a short distance from the fracture. X 60.

was assembled by welding steel tubing, and the fatigue

crack had started at the toe of the weld at the left. It

is obvious that the weld caused a stress concentration, but in addition metallographic examination showed

another significant factor—decarburization (fig. 20).

The origin of the fatigue crack was in the low carbon

content material at the surface of the tube where the

fatigue strength was estimated to be only about half as great as it would have been in sound material.

Figure 19. Fracture in a light aircraft wing spar. The fatigue crack started at the toe of the weld. X 5.

Figure 20. Cross section of the tubing and weld of figure 19 showing decarburization (light-etching material near the sur- face). The wall thickness of the tubing is 2.5 mm.

8
Some of the other defects which have been found by

metallography are: excessively large inclusions, laps

and seams due to poor forging techniques, and im- proper heat treatment. Probably the most common technique used in trying

to learn about the material which has fractured is the hardness test. It has the advantages of low cost, rapidity,

and very little damage to the failed part, yet it can sometimes give enough information regarding the mechanical properties of the metal so that no further

testing is required. Figure 21 shows a cross section

of a stainless steel reaction plate from a dam gate;

these plates were found to be cracking after they were

fastened to the structure but before they had been

loaded. Near the surface where the cracking started the hardness was 45 Rockwell C, an unusually high

value for the 410 type steel from which the plates were made. The deformation of the plate after cracking

showed that there was a very high residual stress present, and apparently this combination was sufficient to initiate cracking under atmospheric conditions. The

problem was thought to be due to lack of tempering or improper tempering; these preliminary conclusions

based on only the visual examination and hardness

tests were verified by more extensive tests. There are, of course, many more sophisticated tech- niques for determining the mechanical properties of

metals, but these are of relatively little use in the in- vestigation of fractures; they are however of great importance in evaluating materials in order to prevent

future failures, and you will be hearing more about

these techniques from other speakers.

It is almost routine to ask for chemical analysis of

the failed component, but I cannot think of a single

instance in which the results were of real significance

in the investigation of a service fracture. This does not mean that the analysis should be skipped, because the component just might have been made of the wrong

alloy, but it doesn't happen often. The chemist may be

also called upon for the analysis of corrosion products

in an eflfort to determine the corrodent responsible for

stress corrosion cracking or corrosion fatigue. These

attempts have not been successful in many cases be- cause of the small quantities of material involved and

the complexity of corrosion processes. These techniques

will become more and more useful as our knowledge

of corrosion is increased.

Figure 21. Cracks in a 2.5 cm thick reaction plate which oc- curred before the plates were put in service.

3. Use of Information

I hope that this discussion has given an idea of the means which are available for investigating fractures,

and has shown that the techniques are capable of revealing a lot of important information concerning

the conditions leading to the fracture. The question now is, how can this information be used to best ad- vantage in avoiding further fractures?

If there are other components, similar to the frac- tured one, which are still in service, I believe that it is important to verify the results of the investigation

by duplicating the failure in the laboratory. If the fracture was in a large assembly, such as an airplane

wing, the duplication will have to be done by stressing

test samples which represent only a small part of the

total assembly. This introduces some uncertainty, but

there is no other way to get a base value from which

to measure the effect of corrective measures. And unless the failure can be produced in the laboratory under

conditions that might reasonably be expected in ser- vice, there is no assurance that the conclusions of the

investigation are correct. The duplication need not use

a full-sized component; for example figure 22 shows

the fatigue fracture of a truck axle in which the fatigue

crack propagated more than half way across before

final fracture occurred. This indicated that the operating stress on the axle was quite small, so the material must have had a very low resistance to fatique crack

initiation. This was attributed to severe decarburization combined with the grooves made by shearing off the forging flash. These factors could have been eval- uated on laboratory specimens much smaller than the

actual axle, and the observed improvement due to ehminating them could be applied to the full-sized parts with good confidence.

Figure. 22. Fatigue fracture in a truck axle. The vertical dimension of the axle is 7.5 cm.
Service failures due to stress corrosion cracking are more difficult to reproduce in the laboratory than those due to fatigue, but it is just as important to make

the effort. Because of the long exposure times required,

samples incorporating the possible corrective measures

should be tested at the same time as those representing the component that failed. For example, if the

corrective measure is an improved coating, samples

with the new coating should be tested simultaneously

with those being tested to duplicate the service fracture. Also the testing time can be shortened by examining specimens metallographically after various exposure

times to see if cracks have developed, rather than

waiting for complete fracture. A wide variety of corrective measures have been

used to reduce the likelihood of repetition of fractures,

and generally these will be indicated by the results of the service failure investigation. For the examples that

I have shown the indicated measures included, proper

torquing of bolts, elimination of corrosive contaminants, stricter surface quality requirements, cleaner

steel, grinding off of weld reinforcements, and eliminat- ing decarburization. Unfortunately, in our work on

service failures at NBS we were seldom informed as

to what corrective action was taken; we could only assume that if we did not hear about further failures, some successful modification was made. Figure 23

shows a case that we know a little more about; this

is a blower shaft which was broken by fatigue cracks

initiating in the severely fretted material under a

collar. The collar was held on by two set screws which

permitted considerable movement of the collar. When

the replacement shaft was installed, the method of

Figure 23. Severe fretting on a 10 cm diameter fan drive shaft where a collar contacted the shaft. The fatigue cracks started in the fretted area and resulted in fracture of the

shaft.

attaching the collar was changed to eliminate the

possibility of movement relative to the shaft, and no

further trouble has been reported. In closing, I would like to say that despite a large

amount of failure analysis work done in the United

States, failures continue to occur. This is illustrated by an excerpt from the Washington Post of June 18, 1973.

A letter to the [House Government Operations] committee from Rep. Edward Mezvinsky (d.-Iowa) referred to

tlie crash of a Beech Model 18 last April 19. The crash was

the result of a structural failure of a wing spar, according

to the Safety Board preliminary investigation. "Such a structural defect was first identified by a National Bureau of Standards metallurgical examination during the

investigation of a 1947 fatal crash. Since that time, seven

additional fatal crashes have been attributed to wing failure," Mezvinsky wrote

More encouraging is the experience subsequent to the failure of the Point Pleasant bridge. Another

bridge, identical in practically every detail, existed at

St. Mary's, West Virginia. After the investigation of the collapse of the Point Pleasant bridge (see figs. 4

and 18), it became clear that there was no way to make sure that a dangerous crack did not exist in the

St. Mary's bridge. Accordingly the authorities, in the face of violent local opposition, removed the bridge.

This was a courageous decision which may have saved

many lives. A great deal can be learned about failure prevention

from the analysis of service failures, and from the

related research in fatigue, fracture, stress-corrosion, wear, and non-destructive evaluation. One of the most

important tasks we face in the future is the more

effective utilization of information obtained from these

activities.

4. Reference

[1] Bennett, J. A. and Mindlin, Harold, Metallurgical aspects

of the failure of the Point Pleasant Bridge, Journal of Testing and Evaluation, JTEVA, Vol. 1, (1973), pp. 152-161.

[2] Bending and Tensile Failures in Shafts, Chapter 7 of How Components Fail, by Donald J. Wulpi, American

Society for Metals, Metals Park, Ohio, 1966.

5. Discussion

W. E. Littmann, The Timken Company: You spoke of metallographic examination often revealing excessively

large inclusions as a cause of failure. How do you tell the difference between a normal inclusion and one that

is excessively large and a cause of failure?

/. A. Bennett: I don't know any simple answer to

that, but comparison may be the best way. In the spring from the highway bus, the inclusions were

terrible. An inclusion at 100 magnifications couldn't be covered on a 4 x 5 print. I do have one theory,

and that is. in fatigue at least, the thing that is important is the size of the largest inclusion in the highly

stressed region. Most inclusion rating systems result

10
in some sort of an average inclusion size. I would like

to see a little more emphasis on the extreme values theory in order to trv to determine what is apt to be

the largest inclusion in the highly stressed region.

/. Kruger, National Bureau of Standards: You said you

had never seen a case where chemical analysis helped

you with a failure analysis. Surely this is not true. Analysis of corrosion products sometimes identifies a damaging species.

/. A. Bennett: You misquoted me a little. I said it hadn't been of major importance, and I was thinking

primarily of the analysis of the composition of the

failed component. I think the analysis of corrosion

products is becoming of increasing importance. The

difficulties lie in the complexity of the corrosion process and the small amount of material that is available. But

I do think that it is definitely one of the things that is coming up in importance as techniques for small sample analysis get better and as our knowledge of all the complicated reactions that go on in corrosion

improves.

A. J. Babecki, NASA, Goddard Space Flight Center: One aspect you didn't mention which we find in the space business to be rather significant is the fabrication of the part from the bulk material without due

consideration of the anisotropy. Mechanical properties

do vary with orientation. You didn't mention in any

of your analyses that this was the major problem.

/. A. Bennett: That certainly is an important consideration, although not in any of the examples that I've shown, \ears ago. Logan showed that in extruded

hinge fittings, the susceptibility to stress corrosion

cracking is much greater in the short transverse di- rection.

0. Jahari, IIT Research Institute: In regard to the

effect of inclusions on fracture, wc at the Institute have found an approach that I think has worked ex- tremely well. Instead of looking at the microstructure, we generate fractures and examine them with the scanning electron microscope. We have reported cases where two materials, with identical microstructures

have widely different fatigue properties because of the

location of the largest inclusion. In fact, many of the property differences which we have been attributing

to statistical analysis or statistical variations could be

explained by this fact. When there are existing cracks,

a good approach is to open the cracks and examine

the fracture surfaces. Recently I worked on a copper

casting in which there were existing cracks. When

the cracks were opened, inclusions were found that contained copper and phosphorus. Combining chemical

analysis with scanning electron microscopy offers a very

useful approach to failure analysis, particularly where

mechanical ]3roperties are concerned.

/. A. Bennett: I think that is a very good point. We

have had cases where we could see secondary cracks,

that is cracks not close to the fracture, which were

clearly initiated at inclusions.

11

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Failure by Fatigue

A. J. McEvily

University of Connecticut

Storrs, Connecticut 06105

The lifetime of manufactured items under intended service conditions is often limited by the processes of corrosion, wear and fatigue. Since such processes represent economic loss and perhaps

a safety hazard to the consumer, much effort has gone into the understanding of these phenomena

as well as into improved design procedures to guard against their occurrence. In the field of fatigue in particular a considerable advance in recent years has been made in the quantitative treatment of the fatigue process, especially with respect to the matter of fatigue crack growth.

Improved understanding of the fatigue crack growth process is timely as in certain circumstances,

as for example in the case of welded structures, it is not the initiation of cracks but rather the growth of cracks from preexisting defects which is the critical aspect in determining service

lifetime. Other advances have been made in improving the resistance of materials to fatigue either through the control of chemistry or by control of processing variables. Such procedures are

generally more important in affecting the crack initiation rather than the crack propagation stages. In this presentation a review of the current status of fatigue will be given from the mechanistic

as well as the design viewpoints. Areas in need of further understanding such as corrosion fatigue, creep-fatigue, and fatigue under variable amplitude loading will also be considered. Key words: Crack initiation; crack propagation; fatigue crack growth; fatigue failure; metal

fatigue.

1. Introduction

In this paper we are concerned with the problem of fatigue and the steps being taken to minimize its occurrence. Fatigue failure, of course, is due to the repeated application of stress. This type of failure can

occur in crystalline as well as noncrystalline materials,

with metals and polymers being of principal concern. Fatigue failure involves both the initiation and propagation of cracks, and irreversible plastic deformation

plays a key role in both, and it may be surprising,

therefore, that a completely elastic, i.e., brittle material, is not subject to fatigue. However, such materials

are not generally useful as engineering materials, a

notable exception being a composite material which

contains brittle fibers encased in a protective matrix.

Therefore, although ductility is a highly desired characteristic of a metal, under cyclic loading it leads to

failure. Perhaps Aesop could find a moral here. In considering the various components subject to fatigue, sometimes a division of these components into two categories is made. The first of these are components which comprise the primary load bearing

members of structures such as airplanes, cars, trucks,

bridges, pressure vessels, etc. The second category

includes the remainder of products subject to fatigue, a category which includes the bulk of mechanical

])roducts ranging from can openers to aircraft turbine

blades. This later category can also be broadened to include certain types of wear failures which in fact

result from contact fatigue. Of these two categories, more attention has been given in general to structural

failures due to fatigue. In addition to fatigue, other

forms of structural failure are: overload failure of a

ductile nature, overload failures of a brittle nature,

creep, stress corrosion and instability. Of these failure types fatigue is the most common form of failure simply because few structures are subjected to the

static loading assumed in design [1].^ In fact Freudenthal [2] states that 95 percent of all structural

failures are fatigue failures. Structural fatigue failures often attract considerable attention because of their sometimes catastrophic nature, which result in loss of

life as well as economic losses, as in the case of the Comet failures. Nonstructural failures are usually less dramatic, but nonetheless the integrated economic loss

is undoubtedly significant as is the drain on natural

resources as a result of such failures, although statistics on the frequency of occurrence as well as the economic

factors associated with such losses are not generally

available. In any event, such figures must be carefully

interpreted, for a low frequency of occurrence may not

necessarily indicate an absence of a fatigure problem

but may instead reflect a competent handling of the problem. Such is the situation in the transport aviation

field in recent years, where improvements in fatigue

analysis, fatigue testing, and in the capabilities of nondestructive inspection have served to control the

fatigue problem. In other areas of the consumer product

field, design against fatigue may not be as carefully

carried out and fatigue can be a more common occurrence. Some 700 000 product liability law suits are currently pending against manufacturers [3], an in- crease by an order of magnitude in a span of a few

^ Figures in brackets indicate literature references at the end of this presentation.

13
years [4]. Such an increase reflects a growing expectation for product reliability and safety on the part of the consumer. Undoubtedly many of these failures, rightly or wrongly, are attributed to fatigue resulting from

poor design.

Fatigue has now been the subject of research in- vestigations for over one hundred years, and despite

the progress made, failures continue to occur. This

situation is due in part to the complex nature of the

fatigue process and the stress-material-environmental

interactions involved therein. However, much of the fatigue jjroblem is simply due to poor communications.

Many designers do not know how to deal properly

with fatigue and do not treat it as a matter of concern

until product failures start to occur. This situation can

lead to a rapid but expensive development of an ap- preciation of the complex nature of fatigue and of the procedures available to aid in avoiding such failures. This is not to say that we can now design Holmes'

one-horse shay, or match the capabilities of Neville Shute's hero who accurately predicted that the plane on which he was a passenger was about to experience

fatigue failure, but we are getting closer. Today there are two approaches to design against fatigue [5].

One of these is to design for a safe life in terms of the S—N properties of the material. The other approach

is to design damage tolerant (fail-safe) structures so

that in the event that fatigue cracks do develop their presence will not be catastrophic. Nondestructive in- spection is an important aspect of this latter approach.

The purpose of the remainder of this paper will be to review the basic aspects of these two design approaches,

with emphasis placed upon the unit processes of crack

initiation and growth in simple specimens. Knowledge

of these processes is important in the design stage, with simulated service testing being relied upon to provide a final check of the fatigue analysis of many

complex structures.

2. Lifetime Determination

A representative S-N curve is shown in figure 1 [6]

.

To emphasize that scatter of test results is an important

characteristic of fatigue, survival probability estimates are also included. The safe-life design approach uses such curves, coupled with component testing, to establish fatigue reliability, particularly in the high cycle range. Fatigue is generally a surface sensitive phenomenon

with cracks in polished specimens initiating at sites such as slip bands in or at inclusions, figure 2, de- pending upon the nature of the metal. In total life determination of simple specimens, the distinction between crack initiation and propagation in the failure process is not made, at least at the design level. Cracks

of the order of a millimeter in size are usually visible only in the last stages of life, although they may be

present but of smaller size for a much greater portion

of the total lifetime, especially for lifetimes less than

1000 cycles. This total lifetime will be influenced by

the environment. For example, at elevated temperatures

oxidation can exert a deleterious effect, as shown in figure 3 [7]. Note that in this case the fatigue lifetime

is plotted as a function of plastic strainrange, rather than stress amplitude. At elevated temperatures the time dependent nature of deformation involving creep

or stress relaxation can also become important. Figure

4 shows a variety of hysteresis loop shapes as well as a thermal fatigue cycle [8]. Analysis of cyclic loading

under such conditions is obviously complex, but new

methods of analysis to deal with such loading histories are being developed, as for example, the strainrange

partitioning methods due to Manson and co-workers

[9]. The importance of surface finish on fatigue is indicated in figure 5 [10]. Allowance for scatter, sur- face finish, corrosion, and in-service surface damage

can be combined in certain instances to reduce the

0 55x10^

0 50

0 45

0.40

E< 0 35

m 0.30

0 25

%\ \

>

"^N^ Preibability of Survival, Per Cent

-T_

1 III 1 III 1 III

10= 10^ ' 10^ Number of Cycles to Failure

10*

Figure 1. Stress amplitude—fatigue life—probability (SNP) plot for an aluminum

alloy [6].

14
allowable design stress by as much as a factor of ten below the average smooth specimen stress for a given

lifetime. In the low cycle range where inelastic deformation

is dominant it is more usual to relate the lifetime to a

strain range rather than a stress range because rela- tively simple relationships, as indicated in figure 3, can exist between plastic strainrange and lifetime

stress amplitude for fully reversed strain. These rela- tionships are as follows, using the notation of Morrow

[11]:

Figure 2. Fatigue crack initiation sites in (A) a slip band, and

(B) at an inclusion. The STAGE I of crack growth is along primary slip planes; the STAGE II of crack, growth is perpendicular to the principal tensile stress axis.

Ac

2

" = e'f (2N,) _ o- f -{¥}"

(1)

(2)

(3)

where Asp is the plastic strainrange

e'f is the fatigue ductility coefficient 2N/ is the number of reversals to failure c is the fatigue ductility exponent

Ace is the elastic strainrange

(j'f is the fatigue strength coefficient b is the fatigue strength exponent

(Ta is the true stress amplitude K' is the cyclic strength coefficient n' is the cyclic strain hardening exponent

E is the elastic modulus

Equations (1) and (2) can be combined to yield

Aet

2 ^+^=^(2/V/)' + (4)

Maraging steel is an example of an alloy which

follows these average lifetime relationships as shown in figure 6 [12]. High strength aluminum alloys and ti- tanium alloys generally do not exhibit such good

agreement. In such cases a more complete testing pro- gram must be employed to obtain reliable fatigue life relationships.

Design situations often involve a mean stress whereas much fatigue test data, particularly those obtained

in the low-cycle range, are obtained under fully re- versed cycling. A Goodman diagram, figure 7 [13]

for example, can be used to determine the allowable

stress amplitude for a given lifetime, aa, at any mean

stress, CT,,,. If it is assumed that the stress amplitude for

15
Figure 4. A—F indicate a variety of hysteresis loop shapes ivhich can be developed

at elevated temperatures as a result of plastic deformation, creep and stress relaxation. G indicates a thermal fatigue cycle in Avhich both temperature and strain vary.

40 60 80 100 120 140 160 180

Tensile strength, ksi 200 220 240 260

Figure 5. Effect of surface on fatigue as a function of tensile strength of steel [10].

a given lifetime varies linearly with mean stress then we can write

era- (5)

where dn is the stress corresponding to a given lifetime

for fully reversed loading, and a„ is the ultimate tensile strength. For some materials it may be preferable

to use the true stress at failure, o-/, in place of ctu- Another design complication is that the in-service loading is rarely of the constant amplitude variety.

Random loading has to be considered, and a first problem is the selection of a statistical counting method

assuming that adequate load data is available. Current

statistical counting methods are modifications of the three basic types indicated in figure 8 [14]. These

three types are characterized by:

b.

The variable stress reaches a maximum or a minimum.

The variable stress changes from a minimum to a maximum, i.e. tive range.

it describes a positive or negac. The variable stress crosses a given level positive or negative direction.

in a

Recent methods such as the rain-flow and range-pair

cycle counting methods are variations of type b in which a second counting condition, the instantaneous mean value, is introduced. Figure 9 is a strain time

history in which a pairing of ranges to form a cycle

is made. For example, in figure 9, a range is counted

between peak 1 and peak 8, and each range that is counted is paired with the next straining of equal magnitude in the opposite direction to make up a complete

cycle. In figure 9 part of the range between peaks 8

and 9 is paired with the range counted between peaks

1 and 8. In figure 9 the counted ranges are marked

with solid lines and the pairea ^c^..^^ lines. Each peak is taken in order as the initial peak

of a range, except that a peak is skipped if the part of

id the ?d ranges with dashed

16
O-o, MEAN STRESS, ksi

Figure 7. A Goodman diagram for a steel of 180 ksi tensile strength [13].

the history immediately following it has already been

paired with a previously counted range [15].

Each strainrange can be converted to an equivalent

strainrange for fully reversed loading by using eq (5).

The fraction of life expended, —, for each cycle

i — i can then be computed. Miner's law, ^ '~Kr~ ~

is then used to predict the total number of cycles

which can be applied before failure. Dowling [15]

used this approach to predict the fatigue life of 2024

T4 aluminum alloy under a complicated stress-strain history and found predicted lives to be within a factor

of three of the actual lives in 83 tests. Thus far, we have considered lifetime prediction for simple specimens and modifications due to effects of surface finish, mean stress and random loading. Another important consideration is the effect of a notch on fatigue properties. In cases where eqs (1), (2), and

(3) hold they can be used in conjunction with Neuber's rule [16] (to be defined) to predict the fatigue

lifetime as a function of the applied R ratio (R being

the ratio of minimum to maximum stress in a simple

loading cycle). In considering the plastic behavior at the root of a notch it can be shown that the local R

ratio differs from the macroscopic value of R [17, 18]. For example, under R = 0 loading, the value of R at the notch root may be equal to —1. In order to esti- mate the local R value use is made of the Neuber rule: [16]

K\ = KJ<Cs (6)

which leads to = (7)

where Kr is the theoretical stress concentration factor

K(T = the local stress concentration factor Ke = the local strain concentration factor S = the applied elastic stress

(Ti = the local peak stress at the notch

e; = the local peak strain at the notch.

To take into account notch sensitivity and notch size effect (an effect related to the volume of material sub17
P»ak Counting

Range Counting

Level Crossing Counting

Figure 8. Example of three basic types of counting methods in the analysis of variable amplitude loading [14}.

ject to the peak stress at the tip of a notch), eq (7) can be written as — (T;F; (8)

where Kjy is the fatigue stress concentration factor de- termined at lifetimes such as 10' and defined as the

ratio of the unnotched to notched fatigue strengths

for fully reversed loading.

Figure 10 [19] is a simplified version of the stress- strain behavior at the root of a notch when yielding occurs on the tensile part of a cycle and is based upon

the experimental work of Wetzel [17] and Crews and

Hardrath [18].

Again, eq (5) can be used to convert a stress amplitude at a given stress to an equivalent stress or

strain amplitude fully reversed loading or vice versa. To obtain the R = —I baseline information, two ap- proaches can be used. One is to use eqs (1), (2), and

18
(3) together with the Neuber rule, eq (6), to obtain

+

^

{e'f)'" (2NfM (9)

where Sa is the appHed stress ampHtude for fully re- versed loading.

Alternatively, one can use the fatigue data for un- notched specimens tested under fully reversed loading

in the nominally elastic range. To obtain a value of Sa

for any lifetime, simply divide the unnotched stress value by Kf,. Then to obtain the value of Sa for that lifetime corresponding to the local R value, reduce 5^

N , cycle*

Figure 11. Comparison between experimental and predicted fatigue lives for two

stress raisers in an aluminum alloy.

19
by the ratio of a„/an given in eq (5). This latter pro- cedure was followed to prepare the curves shown in figure 11. For the low stress range the local yield stress

is not exceeded and the following relation is used: kA (10)

<^H + Ca» /

where Kp^ is the fatigue notch factor for /? = 0 loading, and (Tau is the fatigue strength of an unnotched

specimen at a specified number of cycles for /? = — 1 loading. It is noted that as the applied stress increases, the local R value changes rapidly and takes on a value

of -1.

These are the basic ingredients of the safe-life design

approach. In certain cases other factors may also have

to be considered. These may include fretting fatigue, contact fatigue, corrosion fatigue, and cyclic stability, for example. The principal subdivision of the safe-life approach is based upon consideration of whether the

fatigue process is of a low cycle or high cycle nature. A distinction between these two can be made by comparing the elastic and plastic strain amplitudes. The

low cycle fatigue region is that region in which the

plastic strain amplitude is larger than the elastic, and

the high cycle fatigue region exists where the elastic strain amplitude is larger than the plastic. At some

strain the elastic and plastic strain amplitudes are equal as seen in figure 6. Figure 12 [20] summarizes

the important characteristics associated with low and

high cycle fatigue as related to the transition lifetime.

In general the stress intensity will be of this form,

but modified to account for particular geometry, in which case we can write K = a\/a-Tra (13)

where ^ is a factor which takes into account the particular geometry. Consideration is now being .^ven

to the effect of structural modifications such as stringers on the rate of crack growth; however, the bulk of re- search in the past has involved specimens of constant

thickness in which material response rather than struc- tural response can be established. The characteristic dependence of the rate of fatigue

crack growth on the stress intensity factor is indicated

in figure 13. There are two asymptotic limits to the curve. The upper limit is set by the fracture toughness

of the material, K(. The lower limit is referred to as

the threshold for crack growth, Kth. This latter quantity is currently of considerable interest, for a new de- sign philosophy based upon the quantity is emerging.

Many parts before going into service already contain

crack-like defects as in the case of welded joints. Some

of these parts may have to be designed for infinite safelife. In the absence of defects this would entail designing at stresses based on the 10^ life of the S-N curve, but in the presence of defects the new approach is to insure that the stress intensity associated with defects is kept below the threshold level for crack growth. Wells

[21] has considered the implications of this approach

for both internal and surface flaws. The present capability for the detection of internal flaws varies from

3. Fatigue Crack Propagation

Consideration of fatigue crack propagation is important in the damage-tolerant approach to fatigue design. An analytical approach to crack propagation can

be obtained by the application of fracture mechanics

concepts to this subject. The most important aspect

of the use of fracture mechanics is the single-valued

correlation in the linear elastic range between the stress intensity factor and the rate of fatigue crack growth,

Aa/AN, where a is the crack length and is the number of cycles. The stress intensity factor, K, is related

to the stress concentration factor, Ka, through the following definition of the stress intensity factor.

K Urn Ka(T

p>0 (11)

with Ka and a based upon the gross cross-sectional area, and p is the tip radius. For the case of a central

slit in a sheet specimen subjected to tensile loading at right angles to the slit the expression for K (with the crack length much smaller than the specimen width)

becomes

K = (12)

Log AK.MN m"*'*

Figure 13. Schematic dependence of the rate of crack growth, da , • • t zr on the stress intensity factor K.

20
0.5 mm up to 5 mm depending upon particular conditions. If design were based upon the maximum size flaw in the most critical position, the allowable stresses would be unacceptably low. Rather than use such low

stresses, material processing is controlled so that the probability of such damaging defects is extremely low. The capability for detection of surface cracks is much

greater than for internal cracks, with surface crack

lengths of 0.05 mm being within the range of current

capabilities. For these smaller flaws the fracture mechanics approach could be used in the control of crack

growth even above the threshold level. However, for

these small surface cracks there may still be difficulties with the use of the fracture mechanics approach as in- dicated in figure 14 [22] where it is seen that surface

flaws grow more rapidly than do through-cracks at low

stress-intensity values. Such a lack of correlation with

the stress intensity factor suggests that the continuum

approach may break down for crack sizes comparable

to metallurgical features such as grain size, for example. Application of the fracture mechanics approach

above the threshold level will be therefore more

straightforward in dealing with structures which contain cracks of more significant size. For example, the

large scale of an aircraft wing structure favors the approach, where the small scale of an aircraft turbine

blade may restrict the approach.

LOADED

Figure 15. Schematic of the fatigue crack groivth process during a loading cycle in the absence of static modes of separation [23]

.

Kf*.

fff*

r >0154mi (aOt to) UMt

Itf*

10"

4

Mort Surfooi Cfocki (XOMinm - O.Smn (aOOOtS ht-OOZM Omp lb* S

10'

• t s e M w

AK,(lbfin»^)xlO'

Figure 14. Comparison of crack growth rate as a function of K for short surface cracks and through cracks.

After Pearson [22].

The rate of fatigue crack propagation can be ex- pressed as a function of the stress intensity factor, but

to put such an expression on a rational basis, consideration should be given to the modes of separation in- volved. At low crack growth rates, i.e., below 10"* in at

/? = 0, a ductile mode of crack advance associated with plastic blunting on loading, and tip sharpening on

unloading as illustrated in figure 15, is thought to be

dominant [23]. At higher rates of crack growth where

the peak stress intensity approaches the fracture toughness, static modes of separation such as ductile rupture

become operative and accelerate the rate of crack

growth. The following expression based on crack- opening displacement considerations has been developed

[19] to account for the contribution of each of these modes to the amount of crack growth per cycle:

Aa

AN <T,jE

(1 +

{AK^ - AK^j

AK

Kc — K„

:i4)

Since K„

AK

the effects of mean stress are

1 - R

incorporated in the equation. Figures 16 and 17 show

a comparison with predicted and experimental results for two aluminum alloys of different toughness levels. Note that mean stress effects are much more pronounced

in the case of the low toughness alloy, and they are vir- tually absent in the high toughness alloy. The threshold

level in both alloys is sensitive to mean stress, and

current research is aimed at understanding this de- pendency. Crack growth rates near the threshold are

21
AK.MNm"'-'* q 5 10 15 20 25

AK.dbf in''^*)x 10

Figure 16. Dependence of the Tate of crack growth on mean stress and the range

of the stress intensity factor for a low toughness aluminum alloy. Experimental results after Pearson [25] shown in solid lines; equation (14), dashed lines.

Figure 17. Dependence of the rate of crack growth on mean stress and the range

of the stress intensity factor for a high toughness aluminum alloy. Experimental results after Pearson [25] shown in solid lines; equation (14), dashed lines.

22
of the order of 10'^ in per cycle or less, representing

increments of growth on the order of the interatomic

spacing. Further, since oxides can form very rapidly in metals tested in air, the growth must be occurring

primarily in the surface oxide at these low rates rather than in the metal itself. The appearance of the fracture surface should, therefore, be influenced since oxide

rupture is a brittle process compared to the base metal

blunting process. The characteristic fatigue striations are not observed at such low growth rates since they

are resolvable only at crack growth rates above 10"^

in per cycle, but even in this range their ease of detec- tion will be material dependent.

The effects of variable amplitude loading on the rate

of fatigue crack growth are of interest. Barsom [24]

has recently reported on crack growth tests of an A514^B steel (yield strength 129 ksi) subjected to ran- dom loading. He found that the rate of crack growth

could be expressed as

Aa

AN

A' {AKn. ;i5i

where A' and n are material constants, and AKrms is the root-mean-square stress-intensity-factor fluctuation.

It is of interest to compare predictions based upon eq

(14) with this result. Since Barsom observed that crack

growth in this steel was independent of mean stress for crack growth less than 10"* in per cycle, the contribution due to static modes in eq (14), i.e., the

—r^ p term, can be neglected. An average rate

of crack growth under variable amplitude loading,

( AA^ ) A VG

taken to be equal to the average

contribution from each of the cycles over the increment

of crack growth considered, that is

Aa \

an) AVG

i =N + AN

(TyE > ;i6)

Note that the quantity

--NXaK'

AN

so that eq (16) can be written

^Aa \ A

is the square of A/C,-,

aA^ ; AVG (TyE

AK^u) (17)

Figure 18 shows a comparison between eq (15) and

eq. (17) and the data obtained by Barsom. (In eval- uating eq ill), A was taken to be 0.023 and AKth to be 8 ksi Vin-) Agreement of both equations is quite

good ; however, for other more extreme types of loading

the agreement may not be as good. For example, a

single overload can greatly retard the rate of crack

growth rate at a lower amplitude, and the above

equations would over-estimate the rate of crack growth.

Nevertheless, for a large number of random cycles, the amplitudes of which do not fluctuate too widely, the agreement shown above indicates that reasonable pre- dictions can be made.

10Q

>> 6

u

c

~ 4

Z*o

o

t- 2 <

X

»-o

-GR id*

AC 8 eo

UJ 6 3

O

tFA 4

• CONSTANT AMPLITUDE

o ASCENDING ORDER

o DESCENDING ORDER

-7 ASCENDING-DESCENDING H

ORDER

^ RANDOM-SEQUENCE

• I 0998 MN /m'^^ - 25 4 mm

60 80 100

Figure 18. Dependence of the rate of crack growth for va- riable amplitude loading on AKrms.

Equation 15 represented by the solid line; equation (17) represented by the dashed line. Data after Barsom [24].

4. Concluding Remarks

This brief review has primarily surveyed analytical

approaches to fatigue. To recognize that this review is not in depth, one has only to note that over 125 papers on various aspects of the subject were presented at a

recent conference. However, much of the nature of the current approach, if not all of the specific detail, has been indicated. In recent years, improvements in analysis rather than improvements in materials re- sistance to fatigue have been obtained. While it is to be hoped that material improvements will be forth- coming, thus far, this goal has been a difficult one to attain. Perhaps more modest goals such as reduction

in scatter, in notch sensitivity, and in environmental

sensitivity may be more realizable and useful than improvements in average properties. At any rate, a wider

dissemination of information already available about

fatigue design would constitute a positive step toward

the elimination of fatigue failures.

The support of this study under Air Force Grant

No. AFOSR-74^2703 is gratefully acknowledged.

23
5. References

[1] Week, R., Failure of steel structures: causes and remedies, Proc. Roy. A 285, No. 1400, (6 April 1965).

p. 3. [2] Freudenthal, A. M., Fatigue mechanisms, fatigue per- formance and structural integrity, Proc. of the Air Force Conference on Fatigue and Fracture of Aircraft Structures and Materials, Ed. by Wood et al, AFFDL

TR 70-144, (1970), p. 9. [3] Dolan, T. J., So, you are going to testify as an expert, ASTM Standardization News 1, (1973), p. 30. [4] Speigelman, A., An insurance approach to product lia- bility, ibid, p. 26. [5] Hardrath, H. F., A unified technology plan for fatigue and fracture design, presented at the Seventh ICAF

Symposium, London (July 1973).

[6] Hardrath, H. F., A guide for fatigue testing and sta- tistical analysis of fatigue data, ASTM STP 91-A

(1963).

[7] Coffin, 1,. F., The effect of vacuum on the high- temperature, low cycle fatigue behavior of struc- tural metals corrosion fatigue, NACE—2 Nat. Assoc.

of Corrosion Engineers, Houston, 1972, p. 590. [8] McEvily, A. J. and Crosby, S. R., Lifetime prediction methods for elevated temperature fatigue, presented

at the AGARD Specialists Meeting on Low-Cycle

High Temperature Fatigue, Washington, D.C., April

1974, to be published.

[9] Manson, S. S., The challenge to unify treatment of high

temperature fatigue—A partisan proposal based upon

strainrange partitioning, in Fatigue at Elevated Temperatures, ASTM STP 520, (1973), p. 744. [10] Lipson, C. and .luvinall, R. C., Handbook of Stress and

Strength (Macmillan Co., New York, N.Y.).

[11] Morrow, J., Cyclic plastic strain energy and the fatigue

of metals, ASTM STP 378 ( 1965 ) , p. 45. [12] Landgraf, R., The resistance of metals to cyclic defor- mation, ASTM STP 467 (1970), p. 3. [13] Landgraf, R., Fatigue Design Handbook, Society of Auto- motive Engineers, 1968. [14] Buxbaum, 0., Methods of stress-measurement analysis for fatigue life evaluation, in Fatigue Life Prediction for Aircraft Structures and Materials, AGARD-LS-62,

1973. [15] Bowling, N. E., Fatigue failure predictions for compli- cated stress-strain histories, J. Mater. ASTM 7, (1972),

p. 7L

[16] Neuber, H., Theory of stress concentration for shear

strained prismatical bodies with arbitrary nonlinear

stress strain law, J. Appl. Mechanics, ASME 28,

(1961), p. 544. [17] V/etzel, R. M., Smooth specimen simulation of fatigue behavior of notches, J. Mater. 3, (1968), p. 646. [18] Crews, J. H., and Hardrath, H. F., A Study of cyclic

plastic stresses at a notch root, Exp. Mech. 6, (1966),

p. 313. [19] McEvily, A. J., Phenomenological and microstructural

aspects of fatigue, presented at the Third International Conference on the Strength of Metals and Alloys, Cambridge, 1973, to be published.

[20] Coffin, C. H., Fatigue at high temperature—prediction and interpretation, Proc. Inst, of Mech. Eng. 188,

(1974), p. 109. [21] Wells, C. H., Design procedures for elevated temperature low-cycle fatigue. Presented at the AGARD

Specialists Meeting on Low-Cycle High Temperature

Fatigue, Washington, D.C., April 1974, to be published. [22] Pearson, S., Initiation of fatigue cracks in commercial aluminum alloys and the subsequent propagation of very short cracks, R.A.E. Tech. Report 72236, Sept.

1973. [23] McEvily, A. J., Boettner, R. C, and Johnston, R. C, On

the formation and growth of fatigue cracks in poly-

_ mers "in" fatigue—an interdisciplinary approach, Ed., Burke et al. (Syracuse University Press, 1964), p. 95. [24] Barsom, J. M., Fatigue crack growth under variable amplitude loading in ASTM A514-B Steel, ASTM STP

536, (1973), 147. [25] Pearson, S., The effect of mean stress on fatigue crack propagation in half-inch thick specimens of aluminum

alloys of high and low fracture toughness, Eng. Fract. Mech. 4, (1972), p. 9.

6. Discussion

/. M. Karhnak, Jr., U.S. Army, Ft. Belvoir: You

showed several different curves that looked like hysteresis-type curves in which you said that different time durations led to different reactions, etc. Who is doing further work in this area?

A. J. McEvily: The group that worked with Manson

at NASA, Lewis Research Laboratory. A rather comprehensive treatment of the subject is available in ASTM STP-520, Fatigue at Elevated Temperatures.

A. Beerbower, Esso Research & Engineering Company: We've been running into something we call sliding fatigue; fairly high stresses approaching a

plastic region, very environmentally responsive, and

tending to produce spallation. We have run into a lot of criticism for calling this fatigue because it only

takes between 2 and 30,000 cycles to generate complete surface roughness, sometimes called scuffing. Do

you think we could be seeing what you are calling low

cycle fatigue?

A. J. McEvily: Yes, I think so. There is an area called fretting fatigue which seems to be close to what you

are describing. I also think that a large number of what we would call wear failures, like in gear teeth, are actually contact fatigue failures. Each time the contact is made, a Hertzian type of stress system is developed which leads to cracking on a micro-level. If you want to broaden what the category of fatigue

might include, I would think what you described as sliding or anything that is repetitive of a contact nature could have an ingredient of fatigue associated with

it. In some recent scanning electron microscopy studies,

I was struck by the fact that a wear process looked

very much like a contact fatigue process involving sub- surface nucleation of a crack and a sheet of material

being removed from the surface.

24
MODES OF FAILURE

Session II

Chairman : Frederick F. Ling

Rensselaer Polytechnic Institute
iIII
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithershurg, Md. (Issued April 1976)

Failure by Stress Corrosion Cracking—Current Approaches

Toward Failure Prediction*

Jerome Kruger

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Stress corrosion cracking (SCO produces failures in a material when it is subjected to the combined effects of mechanical stress and reaction with an environment. All proposed mechanisms

of sec seek to explain how the combination of a given level of stress, a particular material, and

a given environment can lead to the initiation and propagation of cracks. The three major cate

gories of mechanisms that are generally proposed are (1) active path dissolution, (2) stress-sorption, and (3) embrittlement.

In recent years attempts to determine the mechanisms have been enhanced by new concepts and techniques that flow from the three major scientific disciplines that underlie the complex

problem of SCC—chemistry, metallurgy, and mechanics. From chemistry have come two main

currents: (a) increased awareness of the importance of characterizing the altered environment

inside a growing crack, and (b) a recognition of the importance of the regrowth rate of a protective film on a bare surface exposed when that film is broken by stress. The main emphasis in the metallurgy of SCC has been on the crucial role played by structure in general, and particularly

at the tip of a crack. Aiding this objective has been the use of the high voltage electron microscope

which can directly look at the interaction of the environment with structural defects, e.g., dislo- cations. Finally, the major thrust in the mechanics of SCC has been the application of the concepts and techniques of fracture mechanics. The impact of these new concepts and measurement techniques on predicting and preventing

SCC failure will be discussed.

Key words: Corrosion; failure prediction; film rupture-metal dissolution; hydrogen embrittlement;

stress corrosion cracking; stress-sorption

1. Introduction

Far too often the failure of a metal part involves an

alloy that was chosen because of its good corrosion

resistance to the environment in which it is designed

to operate. Moreover, it failed at stress levels well be- low its normal fracture stress. More than likely, this and similar such unexpected failures were due to stress corrosion cracking (SCC). The insidious nature of

this mode of failure results from the situation just de- scribed where, in the absence of the possibility for SCC, one would expect both good fracture and corrosion resistance. Figure 1 from a paper by Staehle [1]

illustrates how SCC markedly reduces the load carry- ing cross section of a part over that to be expected

from environmental attack (corrosion) alone or from

purely mechanical failure. Thus, it is from this synergistic interaction of corrosion and stress that SCC

arises, and the definition of the phenomenon reflects this fact. For example, the ASTM Committee on the Corrosion of Metals defines it as "a cracking process requiring the simultaneous action of a corrodent and

sustained tensile stress. This excludes corrosion-reduced

=i-This presentation was previously published as a part of NBSIR 74-583.

Figures in brackets indicate the literature references at the end of this presentation.

sections which fail by fast fracture. It also excludes

intercrystalline or transcrystalline corrosion which can

disintegrate an alloy without either applied'' or resid- ual stress." This definition embraces many of the other terms that focus on particular mechanisms such as "hydrogen embrittlement," "caustic embrittlement," "season cracking," etc.^ This diversity of terms has

tended to produce confusion. Many workers, for ex- ample Staehle [1] and Brown [2], have proposed that the term "stress corrosion cracking" be used as a generic term to avoid entangling and confusing the macro phenomenon with any special mechanism that may govern a particular situation. This proposal is being increasingly adopted in the modern literature. Because of the diversity of mechanisms affecting

SCC. and because it takes place in what are thought

to be innocuous environments, this mode of failure was not generally considered too important as late as 1940. In a discussion accompanying a 1940 paper by

Hodge and Miller [3] which clearly highlighted the problems of SCC. Brooks pointed out that ". . . the

industrial implications of this phenomenon do not loom

large. Failures are rare and will become more so as understanding spreads. . . . ' Not only were his pre-

- It does not include "corrosion fatigue" which involves a cyclic stress rather than a sustained static stress.

27
oxide or rust

(a)

section dimension capable —

of carrying load

good

1

metal

V

""1

H

V

Table 1. History of Stress Corrosion Problems

{Adapted from Brown['i]) When

stress

(b)

Normal fracture stress when tested in -an inert atmosptiere ( usually laboratory

air atmosptiere)

-Normal Engineering Tensile Curve

. Premature fracture at lower stress due

to active environment

strain

Figure 1. A comparison in the reductions in load-carrying

cross section as a result of (a) general corrosion and (b)

stress corrosion.

The efTect of stress corrosion on the tensile curve is shown in (c). From

Staehle [1].

dictions not realized, but the systems where SCC is recognized as the cause of a failure have increased

markedly throughout this century. It was first recog- nized at the end of the 19th century when brass car- tridge cases (containing residual stress) failed because

of exposure to the ammonia present in stables. Table 1 documents the marked increase in the recognition of SCC as a source of failure, going from the days of transportation by horse-drawn vehicles to the space

vehicles of today. The number of systems where pre- viously failures were not expected continues to grow,

especially as greater demands are made on materials. Because of the increased identification of SCC as a major cause of failures, there has resulted a significant increase in research directed toward understanding

the phenomenon and developing mechanisms to explain

it. Two recent proceedings of major meetings [5,6]

and a forthcoming one [7] give excellent accounts of

Late 19th Century

En5 19th Century

Early 20th Century

Early 20th Century

1930"s

1930's

1950's

1950-60's

What

Brass in ammonia

Boiler steels in caustics Mild steels in nitrates Aluminum alloys in moist atmospheres

Stainless steels in chlorides Magnesium alloys in moist atmospheres

Marlensitic steels in aerospace en- vironments Titanium—Hot salts, N2O4, methanol

the current state of thinking on the origins of SCC.

This paper will briefly outline the major mechanisms

that are currently believed to explain SCC and then show how these mechanisms have pointed the way to the development of new experimental approaches

which, besides enhancing our understanding of SCC,

have, more importantly, provided us with opportunities for the development of new capacities for failure prediction and prevention.

2. Mechanisms

In order to consider the important mechanisms that have been proposed to explain the origins of SCC, we

must first list the characteristics, most of which must

be present to produce this mode of failure. This is necessary because a valid mechanism must explain

the necessity for these characteristics to be present if SCC is to occur. Brown [4] has listed the following

features generally common to SCC:

(1) The existence of a tensile stress either applied

or present as a residual stress. (2) The alloy usually shows good resistance to cor- rosive attack by the environment causing cracking.

This is why alloys chosen because of their corrosion

resistance to an environment may be subject to SCC.

(3) SCC for a given alloy usually occurs more

readily in a rather specific environment. Moreover, the key damaging species in that environment need not be

present in large concentrations. Recent studies [9,10]

have indicated that the dependence of SCC on environmental specificity is not as pronounced as originally

thought in early SCC studies. (4) The microscopic appearance of stress corrosion cracks is that of a brittle fracture in spite of the fact that the metal itself may still be quite ductile in a mechanical test. The cracks frequently exhibit multiple

branching, but this does not always occur.

(5) In order for SCC to occur a threshold stress or

stress intensity must be exceeded. To Brown's list must be added another characteristic becoming increasingly recognized [9,11]. There exists a critical potential. Any metal in an environment where

its potential lies below this critical potential is thought

by some to be immune from SCC. This is a more

recently proposed characteristic that has been strongly

advocated by Uhlig [11]. At one time it was believed that SCC could not oc- cur for pure metals, but some recent work [13,14]

28
has found SCC for high purity copper. As with all the other characteristics, one can usually find exceptions. Using these characteristics of SCC as a basis, a number of mechanisms have been developed. At one time some believed that there existed a universal mechanism

that applied to all systems where SCC was found. It

is now widely held that SCC can be due to a number

of different mechanisms, and that there are alloyenvironment systems where one or more of the mechanisms outlined here may apply. Of the many pro- posed, three major mechanisms are currently the ones most actively used. Moreover, the others proposed can

be considered as variations or sub-sets of the three major ones. All of the mechanisms involve the complex

interactions of the three disciplines that control SCC:

chemistry (electrochemistry), metallurgy, and mechanics. None of the mechanisms will be discussed in great detail, and a critical evaluation of their pros and

cons is outside the scope of this paper. Instead these mechanisms will be briefly described, their strong and

weak points detailed, their failure prevention implications indicated, and, in the next section of this paper,

the techniques that they point to as possible tools for

failure prediction will be discussed. Stress-Sorption Mechanism—This mechanism, most

actively advocated by Uhlig [14,15], proposes that a

specific species adsorbs and interacts with strained bonds at the crack tip causing a reduction in bond

strength, or, thermodynamically stated, a reduction in the surface energy, y, which leads to a lowering in the

stress required to produce brittle fracture. The rela- tionship between the theoretical stress to produce brittle fracture, o-/, and y is given by the well-known ex- pression

" - (^)

'

where E is the elastic modulus and d is the atomic

spacing. The adsorption of a damaging species at a crack tip lowers Of and therefore the threshold stress for SCC is the minimum value to which y is lowered

by adsorption. Figure 2 outlines the mechanism sche- matically. Uhlig proposes that an important factor arguing for this mechanism is the existence of a crit- ical potential below which SCC does not occur. He

explains that the effect of potential can be attributed

to the strong dependence of adsorption on potential.

He bases this on many studies by electrochemists who

have found that the adsorption of an anion (usually

(a) Sub - CRITICAL (b) Adsorption Reduces

Stress Bond Strength

Figure 2. Stress-sorption mechanism for SCC.

(a) Chemisorption of ion at tip of crack.

(b) Weakening of interatomic bond at the tip of the crack so that applied

tensile stress leads to the breaking of this bond.

the damaging species for SCC) takes place at potentials above the point of zero charge for a given surface

in a given environment. (This strong dependence of

failure on potential explains why SCC can be prevented

by cathodic protection in some systems.) Environment

plays a role in the stress-sorption mechanism because

of the specificity of adsorption of a particular species on a surface, and because environment brings the potential to a value where adsorption can or cannot take

place. Moreover, if one can introduce into the environment an extraneous species that displaces the damaging

bond-weakening adsorbed species, cracking becomes more difficult. Uhlig [14] gives examples where such

extraneous species inhibit SCC. The stress-sorption

mechanism is more widely accepted in the case of

liquid metal embrittlement or stress cracking of plastics. In the former case, for example, Preece [15]

points out that the theory is successful in explaining

the observations of a brittle to ductile transition with

increase in temperature (desorption) , an "instantane- ous" failure mode, the increase in severity of embrittlement with increase in strain rate and flow

strength, and, possibly, specificity. For SCC in aqueous environments, however, a number of problems with the stress-sorption mechanism

have been pointed out [9,10]. Among them are the following:

(1) In the cracking of ductile alloys, the ductility

present is sufficient to produce extensive plastic defor- mation at a crack tip. The blunting that this produces

eliminates the infinite sharpness required for this mechanism to work.

(2) The large amount of metal dissolution observed

in many straining electrode experiments (to be dis,- cussed later) poses the question of what happens when

the metal dissolves out from under the adsorbed film. (3) Crack growth rates are considerably slower than one would expect from this model.

(4) Studies [17] have shown that thin protective

films exist on metals in SCC environments, and that these films reform rapidly, on the order of milliseconds

in some cases. The weakening of metal bonds requires

that rapid direct access to the base metal surface is necessary, but the existence of these films would pre- vent or strongly influence this adsorption process.

The measures suggested by the stress-sorption mechanism to promote SCC failure avoidance are as follows:

(1) Lower the potential of a metal to be protected

below the critical potential for SCC either by coupling

another more active metal to it or by cathodic protection using an applied current.

(2) Introduce inhibiting anions in the environ- ment.

To properly apply these measures requires tech- niques that enable both the determination of the critical potential for SCC and the potential of the metal in an environment. The latter requirement is not as easily

met since the environment that counts is that environment existing in flaws or cracks. In addition to a

29
realistic characterization of the environment, there

exists an equal need for realistic characterization of the stress system acting on a crack tip, since the model

requires adsorption at strained bonds or dislocations. Thus, techniques that yield both the geometry of stress application and expected dislocation configuration

would be valuable for failure prediction using the guidelines provided by this mechanism. Finally, ways

to measure the rate of adsorption of the relevant species as a function of stress geometry and potential

would be invaluable if this mechanism has merit. Film Rupture-Metal Dissolution Mechanism—This

mechanism was originally suggested by Logan [18]

and Champion [19]. It has also been refined and ex- tended by a number of workers [10,17,20-24]. The

mechanism proposes that the protective film normally

present on a metal surface (if it were not present the metal would fail by corrosion alone) is ruptured by

continued plastic deformation at a crack tip where the exposed metal becomes a very small and restricted anodic region where metal dissolution takes place. The

rest of the metal surface, especially the walls of the

crack, acts as a cathode. Susceptibility depends on the

rate at which the metal exposed by film rupture is repassivated vis a vis the rate of metal dissolution. Figure 3 gives a schematic representation of this mechanism. A clear picture of this process has been

provided by Vermilyea [24] in a recent paper where

he lists the following five steps in the process that leads

to crack extension:

(1) The film is ruptured at time zero. (2) Crack growth by dissolution occurs for a dis- tance of L during the time of repassivation t.[,. (3) The dissolution process removes the material

at the old crack tip and thereby creates a strain transient just ahead of the newly created crack tip as shown

in figure 4. (4) At a time a critical strain builds up to a value sufficient to cause rupture of the newly reformed

film at the base of the new crack.

(5) The process repeats itself, the crack propagation rate, fj^ , given by Vermilyea as

Stress _ _ Stress _ _ Stress

(a) FILM (b) METAL (c) REPASSIVATION

RUPTURE DISSOLUTION

Figure 3. Film rupture-metal dissolution mechanism for SCC.

(a) Rupture of the protective film at tip of a cracic by tlie application of

tensile stress. (b) Dissolution of the metal exposed when film is ruptured. Tip of crack is an anode antl film covered walls of the crack are a cathode.

(c) Repassivation of the exposed metal at the tip of the crack.

A sub-set or variation of the active path dissolution mechanism just described is the brittle-film mechanism

[25,26]. It differs from the dissolution mechanism in that crack advance is mainly by the production of a penetrating brittle film which undergoes fracture when

tensile stress is applied. The crack produced enters

the ductile metal below this brittle film and a new

layer is formed on the bare metal so produced. The

process continues by a repetition of the film fracture- reformation cycle.

TIME

Figure 4. Time dependence of events occurring after film rupture. Repassivation time is 11. The time to produce the critical strain, £c, necessary to cause film rupture is (c. From Vermilyea [24].

One can conceive of at least three factors that affect the general mechanism.

(1) The rate of production of bare sites, which is dependent on the magnitude of the stresses applied,

the metallurgy of the material, and the geometry of the stressed system (the value of tc is important here).

(2) The rate at which the bare sites produced by

the applied stress are repassivated (i^, is important

here)

(3) The resistance of the protective film (its ductility) to rupture, see [24] (again tc is important).

The nature of the species in the environment has an

effect on tp, or probably more importantly, the rate of repassivation [17]. This is so because small impurities can affect the composition of the newly reforming

film and rates of growth are sensitive to the presence

of impurities [27]. Also, environment can affect tc through its influence on film ductility [28]. Potential

affects both film growth kinetics and metal dissolution

rate so that the balance of these two competing factors

is important, the latter affecting L, the distance of crack advance. The concept of a critical potential for SCC

is as valid for the film rupture-metal dissolution model

as it is for the stress-sorption mechanism.

30
There are a number of facts that create problems in accepting the film rupture-metal dissolution model for

all cases of SCC. Among the most important are the following:

(1) The quasi-cleavage character of some SCC trans- granular fracture surfaces indicates brittle failure rather than dissolution. (2) Crack propagation occurs at a surface which is itself rapidly dissolving at a high rate [29].

(3) There is a possible discrepancy between crack growth, rate and the currents observed in straining

electrode experiments which purport to measure the current involved in dissolution at crack tips [22].

The measures suggested by the active path dissolu- tion model to promote SCC failure avoidance will in- clude one suggested for the stress-sorption model, i.e., control of potential to a value below the critical potential for SCC. Another measure is the control of repassivation kinetics by environment or alloy modification. In order to apply the latter measure for SCC

avoidance, techniques for measuring repassivation ki- netics as a function of potential and environment are

required. However, in order to make such measure- ments one must, as suggested earlier for stress-sorption,

be able to characterize the environment and potential

existing in cracks. Finally, and most difficult, one must

have a technique for determining t,-, the time required

to build up a rupturing stress in a film as a function

of the metallurgy of the alloy whose failure we wish

to prevent. This requires the measurement of the

ductility of the protective film on an alloy and the geometry of the stress system acting on a crack. Hydrogen-Embrittlement Mechanism—This is con- sidered by some to be a different phenomenon from

SCC, but since cathodic reactions (which produce hydrogen under the proper conditions) are as much a part of corrosion as anodic ones (producing films and/or metal dissolution), it is difficult to exclude it in a discussion of SCC. In this mechanism fracture

results from the production of a brittle region at the crack tip because of the introduction of hydrogen into the alloy via cathodic reactions (see fig. 5). The pro- duction of this brittle region has been attributed to a number of causes among which decohesion [30], new

phase formation [31-33], and pinning of dislocations

Stress

OR Path

Figure 5. Hydrogen emhriulement mechanism for SCC.

Cathodic reduction of H"^ at the tip of a crack produces hydrogen which

enters the vicinity of the tip and embrittles the metal immediately ahead

of an advancing crack.

[34] can be mentioned. The environment can affect the process by influencing the rate of production of hydrogen ions and by enhancing or inhibiting the entry

of hydrogen into the metal by the interposition of ad- sorbed or reaction layer films. In aqueous systems the

potential at the crack tip determines whether the pro- duction of hydrogen at the surface is thermodynamically feasible. The composition of the environment

influences what potential is necessary to produce hydrogen, its rate of production, and by means of thq;^res- ence of inhibiting or enhancing species, whether it can

readily enter the metal lattice. In gaseous systems, hydrogen pressure plays the some role as potential in aqueous environments. Inhibiting species, e.g., oxygen, can also be added to gases [35].

It has been suggested by some workers, for example,

Troiano and co-workers [36], that almost all aqueous

SCC is hydrogen embrittlement. They found that austenitic stainless steels, which have been assumed to be

non-susceptible to hdrogen embrittlement, could be

made less ductile by hydrogen charging, and that one

could observe the permeation of hydrogen at positive currents (where one should only expect metal dissolu- tion). Other workers [31-33,37-39] have found the formation of a phase or phases capable of undergoing

hydrogen embrittlement.

All of these findings do point to greater universality

of application for the hydrogen embrittlement mechanism. However, as always, there are counter arguments

that rule against such universality. Recent work by

Wilde and Kim [40] showed that for an austenitic stainless steel while rate of hydrogen permeation was higher

at negative potentials, time to failure was also higher at these hydrogen producing potentials. This finding also agrees with results that show that cathodic protection

(which should promote the production of hydrogen)

does retard SCC for a number of alloys [16]. Indeed,

to confuse the picture further, it has been found by

Bernstein and Pickering [41] that anodic dissolution can occur in the cracks of cathodically polarized ferrous

alloys.

This difficulty in determining whether hydrogen embrittlement is an operative mechanism is most important

in developing measures for failure prevention. For ex- ample, unless one knows the operative mechanism, a change of the potential to what is believed to be a protective value may actually accelerate failure. Obviously

techniques that determine what are the real potential

and environment at the tip of the crack are crucial. Likewise, electron micrography of fracture surfaces which help in determining the mode of failure and can

thereby suggest the proper prevention measures are also of utmost importance.

3. Techniques Suggested by IVIechanisms

In the preceding section the discussion of the major

mechanisms of SCC pointed to the types of information

that are needed in order to develop our ability to predict SCC susceptibility and to institute suitable measures for prevention. In this section the techniques

developed in recent years, in many instances under the

31
influence and stimulation of the concepts of these mechanisms, will be discussed. The techniques brought into being in this manner have their roots in the three broad

disciplines that control the phenomenon of SCC: chemistry, metallurgy, and mechanics.

Techniques Dealing with the Chemistry of SCC—Two

areas have been attacked with increasing interest in re- cent years. As pointed out in the previous section they

are both important in all three major mechanisms al- though not always well recognized. They are the tech- niques directed towards determining the environment in cracks and the kinetics of repassivation.

(a) The Environment in Cracks—While it has been

recognized for a number of years that the environment

trapped in the interior of restricted regions on a metal

(cracks, pits, and crevices) has a difTerent composition

than the bulk environment surrounding the metal, it has

been only in recent years that such knowledge was con- sidered important, and serious attempts made to measure the composition of the occluded environment in a

crack. Brown and co-workers [42-44], using the in- sights provided by the large body of work of Pourbaix

(see the review in [45] ) that systematized the role of pH and potential in all corrosion processes, set about to measure these parameters in SCC cracks. Their original

approach was to immobilize the corrodent within SCC

cracks by freezing it in liquid nitrogen, breaking open

the crack mechanically, and then analyzing the thawed

out solution removed from the crack. A later development is illustrated in figure 6 where microelectrodes

determine the pH and potential in the crack's environ- ment. Other developments in this fast moving field have

been the use of a capillary to sample the crack solution [46] and the combination of a capillary technique with

thin layer chromatography [47].

Figure 6. Experimental arrangement for determining the acidity (pH) and potential at a crack tip. The glass and reference electrodes measure the pH electrocheinically by sampling the solution that ^vets the lens tissue after the solution travels the length of the crack. From Smith, Peterson, and Brown [44].

The value of these crack chemistry techniques be- comes evident when figure 7 is inspected. It shows for a

large number of steels of different compositions in en- vironments of either high or low pH that the potential

and pH inside the crack falls in a region of the pHpotential diagram (Pourbaix diagram) where one ex- pects both corrosion and, because the points on figure 7

are below the line marked H, the presence of hydrogen.

Thus, any failure prevention measures using bulk composition or potentials could be based on non-realistic conditions and lead to an improper choice. A good dis- cussion of the electrochemical ramifications of knowing

the proper environment in a crack is given by Pourbaix

[45.] In addition to knowing the nature of the solution

in a crack and the potential at the crack tip, it is of great

importance to know another aspect of the events occur- ring at the crack tip, e.g., how much hydrogen is enter- ing the metal lattice? As the discussion on the hydrogen

embrittlement mechanism stresses, this knowledge is of crucial importance. A tool for obtaining this information has been developed based on the work of Devanathan and Stachurski [48]. By using a thin membrane

of the metal studied it is possible to measure the amount

of hydrogen that is evolved on one side of the membrane when it passes through the metal by oxidizing it electrochemically in a chamber on the other side of the membrane. The anodic current density at steady state involved in the oxidation of the permeated hydrogen is a measure of the amount of hydrogen entering the lat- tice on the hvdrogen evolution side of the membrane. A

discussion of the technique, its difficulties, and validity

has been given recently by Bockris [49]. Figure 8 from

a paper by Kim and Wilde [40] illustrates the importance of this technigue. It shows that the rate of hydrogen penetration cannot always be related to SCC

susceptibility.

(b) Repassivation Kinetics—In all the mechanisms

proposed for SCC, access of the environment must be

provided to the metal surface. This surface, if it is suit- able for withstanding ordinary corrosion, usually has

a protective film. Therefore, as discussed earlier, the breaking of this film and the rate at which it regrows

(repassivation kinetics) are of great importance in de- termining the time during which the environment can

interact with the bare metal surface in any one of the

different ways the various mechanisms propose.

Scully [20, 50] was one of the earliest workers to emphasize the importance of repassivation kinetics, and

it has become an important element especially in the film rupture metal-dissolution mechanism because coupled

with the concept of slip step dissolution it explains why

restricted lateral dissolution occurs; the slower the re- passivation, the more of the metal slip step that has

emerged and ruptured the film can be dissolved before new film forms. If too little dissolves, no SCC occurs;

if too much dissolves, the dissolved areas are not re- stricted enough, and the crack is too blunt to propagate. Only when the environment and alloy provide a system where the repassivation kinetics produce a highly

restricted region for dissolution is SCC found. In addition to Scully. Staehle [10] has given a detailed dis- cussion of the significance of repassivation kinetics. These considerations have resulted in a number of techniques aimed at determining the repassivation ki32
2,0

1.8

I 6

I 4

I 210 08

06

z

uj 04

§ 02

> 0.0 -02

-04

-0 6 -0 8

-1.0 -02

-0.4 -0.6

MOST ACTIVE POTENTIAL DURING

STRESS-CORROSION CRACKING

- 17-4 PH

o = l3Cr-8Ni

= l8Ni MARflGE

« -- 9Ni- 4CO-0.2C

0 = 4340 AISI

1 = l2Ni-5Cr-3Mo

• = Fe 1- 0 45C

» = Fe +029C + ll.5Cr

I I I I I I I I I I

1.8

1.6 14

1.2 10

08

0.6

0.4 0 2 ui 0.0 O

-0.2 o

-0.4 -0.6 -0 8

-1.0 -12

-1.4 -1.6 -1.8 -10123456789

pH

10 II 12 13 14 15

Figure 7. Potential-pH diagram showing the potential-pH values in the crack tip environments for a number of high strength steels. From Brown in reference 16], page 197.

HYDROGEN ABSORBED (mC/5cm^)

20

10

• Type 304 Stainless Steel Temp 146 C

Sat'd Lithium Chloride

• " " "

\ Freely \ Corroding,

\ Condition \

+1 g/l NagAsOg

-3 -4 -5 -6

|-« CATHODIC -H

-6 -5 4 -3 k ANODIC H

LOGARITHM APPLIED CURRENT DENSITY (A/cm^)

Figure 8. The amount of hydrogen absorbed in 304 stainless

steel as a function of applied current.

NagAsOs (curve B) enables greater amounts of hydrogen to enter the metal.

In spite of the fact that more hydrogen enters the metal in the cathodic

region no SCC failures were observed for currents greater than a cathodic

current of around 10"'' A/cm-. From Wilde and Kim [40].

letics [51-56]. These techniques produce a bare sur- face either by straining an alloy wire or by scratching

or abrading a surface. In most of these techniques, the specimens are either strained or scratched while maintaining their potentials at a fixed value and measuring

the current transient that takes place during the dissolu- tion-repassivation event that occurs subsequent to ex- posure of bare metal. The rate of current decay is related to the rate of repassivation (figure 9) . In these mainly electrochemical techniques, one cannot separate

out the current involved in dissolution from that in- volved in passive film formation. A technique recently

developed by Ambrose and Kruger [56], however, does

attempt to measure separately the film regrowth transient by optical means (ellipsometry) and to compare it to the overall dissolution-repassivation current. With

this technique, called tribo-ellipsometry one removes the protective film on a metal in an environment by means

of a polishing wheel and then records both the film growth kinetics and the current as shown in figure 10. In figure 10 we can see that the rate of film growth on a titanium alloy is considerably slower in the susceptible chloride environment than it is in the non-susceptible ni- trate solution. With tribo-ellipsometry one can deter- mine the ratio of the total current to that involved in

33
Figure 9. A schematic representation shoiving the different current transients expected from

straining electrode experiments and relating these transients to the amount of material dissolved

after the rupture of a protective film by an emergent slip step.

I, rapid repassivation with little metal dissolution. II, intermediate repassivation rate producing restricted dissolution and

favoring SCC. Ill, slow repassivation producing extensive metal dissolution. From Staehle [10].

Cl

i

i

1

NO3

1

1 —

Figure 10. A comparison of oscilloscope repassivation transients for Ti—8A1—IMo—IV alloy in IN Nacl and NaNOa solutions at +706 mV (SHE) (ellipsometric film growth—jagged trace; current—solid trace). From Ambrose and Kruger [57].

repassivation only. This ratio Rp* when the exposed

surface area is know is given by the expression

Rr, Qr

(3)

where — charge involved in dissolution and Qr —

charge involved in repassivation.

Rji* is a measure of the effectiveness of the repassivation process. Work on the cracking of low carbon steel [17] has shown that SCC does not occur when the ef- fectiveness of repassivation is neither too high nor too

low. Thus, for low carbon steel at the time required to form a complete film is 2.8 in 1 N NaNOg (25 °C)

where it is nonsusceptible ; /?p* is 26 in 4 N NaNOg

(90 °C) where it does undergo SCC, and /?p* is >75 in

1 N NaCl (25 °C) where it undergoes widespread pit- ting attack but not SCC. Thus, a delicate balance be- tween repassivation and dissolution must be maintained

to lead to SCC. The determination of this balance is dependent on the environment, the alloy, and finally the metallurgy and mechanics of the system that control the

rate at which bare metal sites (generally slip steps) are produced as well as their density and size. This section, devoted to chemistry, has described tech- niques for evaluating the role of the environmental side

of the equation; the next two sections are concerned

with the other side of the equation.

Techniques Dealing with the Metallurgy of SCC—In

recent years two experimental aproaches have become

increasingly important in providing insights into the

role of metallurgy in SCC. They are constant strain

rate tests and a variety of applications of electron microscopy.

(a) Constant Strain Rate Tests—This test, which

produces experimental data as depicted in figure 11, can

be applied both to mechanistic studies as well as used

for a routine testing. The technique has been applied to studies of mild steels [59], Ti alloys [60] and Mg alloys

[23]. The mechanistic importance of the technique is

34
that it ties the metallurgically controlled rate of bare

metal exposure, resulting from film rupture by slip step emergence, to the chemical process of repassivation of the exposed metal. This can be seen by examining the

significance of the three regions shown in figure 11.

% Reduction in Cross Sectional

Area

Strain Rate

Figure 11. A schematic representation of the results obtained

in constant strain-rate tests. Region A—repassivation is rapid enough and strain rate slow enough to prevent damaging interaction with the environment. Region B—strain rate high enough so that repassivation does not prevent environmental access. Region C—strain rate is so rapid that ductible pulling apart takes place.

For Region A where strain rates are too low to lead to sec, the rate of repassivation is high enough to prevent

any significant and damaging interaction between the environment and the bare metal exposed during strain- ing. At the opposite extreme. Region C, the strain rate

is so rapid that the interaction of the exposed metal with

the environment is of no consequence because the rate that the metal fails by ductile fracture exceeds the rate by which the environment can affect fracture via any

of the possible SCC mechanisms (dissolution, hydrogen

embrittlement, or adsorption of damaging species). It

is only in the intermediate range of strain rates, Region

B, where the rate of production of bare metal sites has

a value sufficiently high so that the rate of repassivation

does not prevent environmental access. However, the

strain rate is sufficiently low so that environmental inter- action and not ductile pulling apart produces fracture. Under these conditions SCC takes place. If SCC is not

possible for the system studied, the dip shown in Region

B will not occur. Does this test, whose development and increasing use was stimulated for mechanistic reasons and was mainly

guided by the implications of film rupture and repassivation, provide a valid routine material-evironmental

evaluation test? Parkins [58] has pointed out that there are certain features of the constant strain rate technique

that make it a useful routine laboratory test. These fea- tures are as follows:

(1) It is a relatively severe test so that it will pro- mote laboratory SCC failures where other tests on smooth specimens will not unless inordinately long

testing times are used;

(2) It always produces fracture by SCC or some

other mechanism and therefore is a positive test; (3) The time of testing is relatively short.

These three somewhat interrelated attributes, severity,

positiveness, and rapidity, are especially valuable in comparative testing where one wishes to intercompare

materials and environments.

The promise that the constant strain rate approach

offers for laboratory testing is somewhat tempered by

the lack of good ways to quantify the results of the tests. For example, how does one relate the strain rate in Region B where SCC susceptibility is the greatest to crack

growth rate? Moreover, it is not known what relation,

if any, this strain rate bears to the strain rate that exists

at the tip of a growing crack. Parkins [23] has made

some valuable qualitative connections between this value and the threshold stress for SCC or the threshold value

of the stress intensity factor (described in the next sec- tion on mechanics) but the quantitative relationships

require new theoretical and experimental advances.

Despite these problems, the constant-strain rate tests have provided evidence of SCC in systems where other

tests have been difficult or not possible to apply [61-

63].

(6) Electron-Microscopy of SCC—With the advent

of such powerful tools as the scanning electron micro- scope (SEM), and high voltage electron microscopy,

have come new opportunities for the diagnosis of SCC

failures as well as for the prediction of the likelihood

of failure. The predictive ability provided by these tools

is based more on the mechanistic insights they provide

(by allowing the examination of the crack tip itself at nearly atomic resolution) than on any testing procedure

employing them. For example, using mechanisms that

rely on repassivation as a step in the SCC process, Scully

[50] has proposed that the width of the slip steps pro- duced when a metal is stressed plays a role in determining SCC susceptibility. He suggests that the wider the

step, the more time is allowed for the migration (under

a potential gradient) of a damaging ion (usually chlo- ride) to the point on the metal surface where the slip step is emerging. Thus, a greater concentration of this ion will build up at the emergence point and thereby

retard repassivation or enhance dissolution at this point. For fine slip steps, the time for the build-up of concentration of damaging ions via migration along the emerging step is less and repassivation is more effective. Therefore, Scully points out that the metallurgical substructure that determines the width of the step becomes

crucial. By means of transmission electron microscopy

one can determine the metallurgical substructure. For

example, it has been found that some alloys that suffer transgranular SCC have lattices of co-planar arrays of dislocations (figure 12a) which produce wide slip steps

(e.g., a brasses [64] ) . On the other hand, alloys where

cross slip is easy (figure 12b) are less susceptible to SCC because deformation results from fine slip and the process described by Scully would be less likely to lead

to cracking. The width of the slip steps produced is probably not critical for all combinations of environ- ments and alloys and, indeed, has been strongly ques- tioned [65]. Nevertheless, it is mentioned to ilhistrate

35
Figure 12. Transmission electron micrographs showing:

(a) Coplanar array of dislocations leading to wide slipsteps. (b) Tangled array of dislocations leading to fine slipsteps.

(Photographs courtesy of L. K. Ives.)

how a transmission electron microscopic study of metallurgical substructure can be used as a failure predictive

tool.Metallurgical substructure plays an important role in the formation of corrosion tunnels which have been

found by some workers using electron microscopy to be

associated with SCC [66,67]. Scamans and Swann

[68] describe a technique whereby they are able to look at U-bend specimens in a high voltage electron microscope and thereby observe the effects of stress and

environmental interactions. High voltage transmission

electron microscopy (500-1000 kV) allows the use of specimens approximately six times thicker than those used on conventional electron microscopes (100 kV)

and thus come closer to studying phenomena in bulk

material. With double tilting capabilities they could

analyze the three dimensional morphology of the crack

tip. By this means they could relate attack morphology

of austenitic stainless steels to slip traces and found that more susceptible alloy compositions showed a greater

tendency to promote the formation of corrosion tunnels. They found tunnel diameters increased with decreasing

susceptibility. For the purposes of this paper, a detailed discussion of the role of attack morphology and susceptibility is not necessary. What is important is to stress that by means of transmission microscopy one can deter- mine morphological or metallurgical features (tunnel

formation, dislocation array, etc.) which can serve as

potential failure predictive aids when their significance has been more firmly established. Unlike the just mentioned examples of applicatidhs of transmission microscopy, the use of SEM for the study

of fracture surfaces (fractography) is more useful for diagnostic, rather than predictive, purposes. As such, it

it extensively used for failure analysis which serves as a basis for suggesting measures to prevent future fail- ures. A comprehensive discussion of the use of frac- tography in the study of SCC is given by Scully [69]

Figure 13 shows a fracture surface that is indicative of a

failure that was brought by the hydrogen embrittlement

mechanism of SCC. The flat crystallographic damage

planes are typical of the brittle mode of fracture that would result from this kind of failure. In contrast, figure

14 shows the dimpled fracture that results from ductile

failure that could not be attributed to hydrogen embrittlement. The introduction of the SEM in recent years

into many laboratories has given an enormous impetus

to fractographic studies. This is so because, as pointed

out by Scully [69], the SEM has the following valuable advantages

(1) The microscope has a wide range of magnifications. This is important in SCC failure analysis because

it can quantitatively determine dimple/cleavage or dimple/intergranular ratios and thereby give information about the general pattern of crack propagation.

Also, by working at different magnifications it can look

at the different varieties of fracture features that can

result from different grain orientations. (2) The microscope has an extremely large depth of

field. Because in SCC where corrosion, corrison products, and rough surfaces are common, this feature is of

great, actually essential, utility. (3) No replication is necessary so that one need not be concerned about an alteration of the fracture surface (removal of corrosion products, for example) by the

replication process. (4) It can distinguish between the metallic and nonmetallic materials (corrosion products, inclusions) making up the fracture surface. This can be a source of misinterpretation when replication techniques are used.

Therefore, the advent of SEM fractography has pro- vided a powerful tool for the failure diagnosis which is an essential step in the prevention of SCC failure. Techniques Dealing with the Mechanics of SCC—Up

to this point most of the new techniques described have

had their origins in the different mechanisms of SCC

and are aimed at providing predictive abilities. However, while some offer hope that such opportunities to predict failure will materialize, the engineer's best pres- ent hope lies either in utilizing past experience or in uti- lizing laboratory tests that adequately model service conditions. Presently, the tests developed using the con36
Figure 13. A scanning electron micrograph of the fracture surface of a high strength steel showing transgranular quasicleavage typical of hydrogen embrittlement.

Arrows show dimpled rupture where some ductility is occurring at a few local

sites. (Photograph courtesy of C. G. Interrante and G. E. Hicho.)

titanium alloys, which appeared to be completely im- mune to sec because they did not pit readily and there- by provide initiation sites, were highly susceptible to sec once initiation sites were present. Therefore, since

initiation may be highly variable depending on whether

or not flaws exist in the specimen studied, the time to failure can be an unreliable and hence unreproducible measure of susceptibility.

The testing techniques for characterizing SCC susceptibility by the use of fracture mechanics seek to over- come these shortcomings of time-to-failure techniques

by providing a pre-existing flaw or crack and by clearly

defining the stress field around a crack through the use

of specimens whose geometry allows the characterization of a parameter A. ^ the stress intensity factor. This

parameter depends on the loading and configuration of the specimen, including crack size, and determines the magnitude of the local stresses. If a certain value of K is found to cause the growth of a stress corrosion crack,

any combination of structure and crack geometry which

duplicates this level of A. will produce the same crack

growth rate for the same material and environment.

Thus, using A. allows one to relate SCC tests carried out

for one crack and body geometry to another. Brown and

Srawley [73] have made calculations that allow the de- termination of K for a number of practical test speci- mens. The pre-existing flaw used to overcome the problem of crack initiation is for many specimens a

fatigue crack intentionally introduced for this purpose. The argument for this approach is that most structures have flaws in them so that crack initiation is not of great practical importance.

In Sec tests using specimens designed for the appli- cation of fracture mechanics concepts, results similar

to those shown schematically in figure 15 will be obtained. In these tests the rate of crack growth is measured as a function of K and, as is pictured, there can

be regions (I, II, III) where the functional dependence

of crack growth rate on K diff^ers markedly. Of more

importance for SCC testing is the existence of a thresh- old value of A., the ACrscc* shown in figure 15, below

Figure 14. A scanning electron fractograph showing typical ductile fracture of a high strength steel which indicates no hydrogen embrittlement.

(Photograph courtesy of C. G. Interrante and G. E. Hicho.)

cepts of linear elastic fracture mechanisms come closest

to answering this later need. A good review of the theory

and application of these techniques to SCC is given in a book edited by Brown [70]. Previously most laboratory

tests for evaluating the susceptibility of an alloy to SCC

in a given environment used smooth specimens subjected to a stress and measured the time required to observe a failure of the stressed specimen. One description of the kinds of specimens and experimental apparatus used can be found in the book by Logan [71] . The

two major criticisms of the test methods that do not use

the concepts of linear elastic fracture mechanics are that the geometry of the stress field around a crack is poorly

defined or unknown and that time to failure involves the time for crack initiation as well as crack propagation.

Brown and Beachem [72] showed, for example, that

LOGARITHM OF CRACK GROWTH

RATE

ARBIT^RYi

RATE

REGION III CRACK GROWTH

RATE STRONGLY DEPENDENT ON

STRESS INTENSITY REGION II CRACK GROWTH

RATE INDEPENDENT OF

STRESS INTENSITY REGION 1 CRACK GROWTH

RATE STRONGLY DEPENDENT ON

STRESS INTENSITY

DESIGNATED

STRESS INTENSITY '

Figure 15. Schematic diagram of the data obtained from frac- ture mechanics techniques applied to stress corrosion cracking.

From Smith and Piper [74].

^ For a crack, of length 2a in an infinite plate where the crack is normal

to the tensile stress field of value u. K = (T\/Ta.

* The I in the suhscript of Kiscc represents Region I and could be left out because SCC occurs only in Region I [4].

37

i
which the crack growth rate is zero or insignificant for

practical purposes. Determining A'rscc is an important

goal in sec testing because one can relate it to a critical flaw depth, a^,, and the yield-point stress that may

exist in a structure whose SCC behavior is being deter- mined. Brown [73] has shown that = o,2(^^y, (4)

where o-y is the yield strength. He proposes that Oc, be

used as a figure of merit which includes both A.iscc and

any yield strength stress whether applied or residual. Figure 16 shows how one can use a,., and ay to evaluate SCC characteristics of materials. Thus, a material whose

value of A^iscc in a given environment gives a value of X as shown in figure 16 would suffer SCC (crack propagation) if it had a flaw of 2.5 mm, but not if it had a flaw of 0.25 mm.

^Iscc OR

YIELD STRENGTH

Figure 16. A plot of Equation [4] using 0.1" and 0. 01"

values for acr assuming long surface flaws and yield strength

stresses operating. A material with a Kiscc value of X would not undergo SCC in the environ- ment where measurements were made if surface flaws were no deeper than

0.01". If. however, the flaws were as deep as 0.1", SCC would occur. From

Brown [73].

There have been a number of criticisms of the general applicability of the precracked specimens to SCC

testing. Parkins [58] has spelled out a number of these shortcomings. Among these are the following:

(1) In order to achieve conditions of plane strain (the conditions upon which the relationship between K

and specimen geometries are based) for ductile materials, test specimens of impractical size must be used ; ^

^ Brown in a private communication has pointed out that because all cracks are growing [irior to general yielding, the stress may be characterized by elastic mechanics. Therefore, the specimen size is determined by

the ratio of Kiscc/rr-,. Thus, one could have small specimens for relatively

soft materials.

(2) There may exist problems when precracked spec- imens are usually cracked transgranularly although the SCC process for the alloy is intergranular

(3) "The significance of the initiation stage may be to produce a proper environment for cracking rather than

just provide a notch to promote crack propagation;

(4) The frequent observation of crack branching

may modify the significance of the values of Kiscc de- termined in fracture mechanics testing.

These shortcomings are not serious for the many cases where they do not apply and where fracture mechanics

techniques have been successful, but it is obvious that one cannot rely exclusively on one testing approach.

Moreover, a recent paper [23] describes ways to inter- relate results obtained from precracked specimens using

fracture mechanics concepts and those obtained from

smooth specimens used in constant strain rate tests. Thus, a combination of the two approaches may be in the offing.

This brief survey of new approaches towards developing failure predictive tools to cope with SCC have

identified new opportunities provided by the three dis- ciplines that govern the phenomenon of SCC. From

chemistry comes the recognition of considering the changes in environment that occur in the crack and the important effect of environment on repassivation kinetics. From metallurgy comes the recognition of the importance of metallurgical substructure and the role that the rate of bare metal production via slip step emergence plays. From mechanics comes the recognition of properly characterizing the geometry of the stress field around a crack. When these tools have been refined fur- ther, both through experiment and theory, the eventual

goal of being able to predict susceptibility to SCC in go- ing from one environment to another (chemistry), from

one metallurgical structure to another (metallurgy),

and from one geometry to another (mechanics) will be

realized. This paper has described some of the beginning steps toward such a goal.

I am most grateful to the Office of Naval Research

which supported this work under contract NAONR 18-

89 NRO 36-082.

5. References

[1] Staehle, R. W., Proc. Conf. Fundamental Aspects of Stress Corrosion Cracking, R. W. Staehle, A. J. Forty, and D. Van Rooyan, Eds., Nat. Assoc. Corros. Eng., Houston (1969), p. 3. [2] Brown, B. F., Naval Research Laboratory Report 7130, June 1970.

[3] Hodge, J. C, and Miller, J. L., Trans. A.S.M. 28, 25

(1940).

[4] Brown, B. F., Stress-Corrosion Cracking in High Strength

Steels and in Titanium and Aluminum Alloys, B. F. Brown, Ed., Naval Research Laboratory. (1972), p. 2.

4. New Opportunities for Failure

Prediction and Diagnosis

38
Staehle, R. W., Forty, A. J., and Van Rooyan, D., Eds., Proc. Conf. Fundamental Aspects of Stress Corrosion Cracking, Nat. Assoc. Corros. Eng., Houston (1969).

Scully, J. C, Ed., The Theory of Stress Corrosion Cracking in Alloys, N.A.T.O., Brussels (1971).

Proc. Intl. Conf. on Stress Corr. Cracking and Hydrogen

Embrittlement of Iron Base Alloys, St. Etienne, 1973,

to be published by Nat. Assoc. Corros. Eng., Houston. Brown, B. F. in Stress Corrosion Cracking of Metals

A State of the Art, H. L. Craig, Jr., Ed., STP518,

A.S.T.M., Philadelphia, (1972).

Parkins, R. N., in ref. [6] p. 167. Staehle, R. W., in ref. [6] p. 223. Uhlig, H. H., and Cook, Jr., E. J. Electrochem. Soc. 116,

173 (1969). Pugh, E. N., Montague, W. G. and Westwood, A. R. C,

Corrosion Sci., 6, 345 (1966).

Escalante, E., and Kruger, J., J. Electrochem. Soc. 118

1062 (1971).

Uhlig, H. H., in ref. [5], p. 86. Uhlig, H. H., in ref. [7]. Preece, C. M., in ref. [7]. Ambrose, J. R. and Kruger, J., Proc. 5th Intl. Cong, on

Metallic Corrosion, Tokyo, 1972 in press. Logan, H. L., J. Res. Nat. Bur. Stand. (U.S.), 48, 99-105

(1952), RP2291. Champion, F. A., Intl. Symp. on Stress in Metals and

Alloys, Inst, of Metals, London, (1948), p. 468.

Scully, J. C, Corrosion Sci., 7, 197 (1967).

Parkins, R. N., in ref. [7]. Hoar, T. P., and Hines, J. G., J. Iron Steel Inst. 184, 166

(1956). Wearmouth, W. R., Dean, G. P., and Parkins, R. N., Corrosion, 29, 251 (1973). Vermilyea, D. A., in ref. [7]. McEvily, Jr., A. J., and Bond, A. P., J. Electrochem,

Soc. 112, 638 (1965). Pugh, E. N., in reL [6] p. 418. Lawless, K. R., Energetics in Metallurgical Phenomena,

1, M. W. Mueller, Ed. (Gordon and Breach, New

York, 1965), p. 345. Kruger, J., Ambrose, J. R., and E. Escalante, Nat. Bur.

of Stand. (U.S.), Report 10594 (May 1971).

Nielsen, N. A., Corrosion, 27, 173 (1971).

Oriani, R. A., Ber Bunsen Gesell. f. Phys. Chemie 76,

848 (1972). Troiano, A. R., Trans. A.S.M., 62, 54 (1960).

Birley, S. S., and Tromans, D., Corrosion, 27, 63 (1971). Whiteman, M. B., and Troiano, A. R., Corrosion, 21, 53

(1965).

Latanision, R. M., Staehle, R. W., Scripta Met. 2, 667

(1968). Hancock, G. G., and Johnson, H. H., Trans. Met. Soc,

A.I.M.E. 236, 513 (1966).

Shively, J. H., Hehemann, R. F., and Troiano, A. R., Corrosion 22, 253 (1966). Holzworth, M. L., and Louthan, Jr., M. R., Corrosion 24, 110 (1968). Vaughn, D. A., Phalen, D. I., Peterson, C. L., and Boyd, W. K., Corrosion 19, 315 (1963). Nechay Ye. P., and Popov, K. V., Fiz. Metal Metalloved. 19, 612 (1965). Wilde, B. E.. and Kim, C. D., Corrosion, 28, 350 (1972).

Bernstein, M., and Pickering, H. W., Carnegie-Mellon

University Report 036-099-2, May 1974. Brown, B. F., Fujii, C. T., and Dahlberg, E. P., J. Electrochem. Soc. 116,218 (1969). Sandoz, G., Fujii, C. T. and Brown, B. F., Corrosion Sci. 10, 839 (1970). Smith, J. A., Peterson, M. H., and Brown, B. F., Corrosion 26, 539 (1970). Pourbaix, M., in ref. [6] p. 17. Leidheiser, Jr., H., and Kissinger, R., Corrosion 28, 218

(1972). Bogar, F. D., and Fujii, C. T., Naval Research Laboratory Report 7690, March 1974. Devanathan, M. A. V., and Stachurski, Z., Proc. Roy.

Soc. 270A, 91 (1962).

Bockris, J. O'M., in ref. [7].

[50] Scully, J. C, Corrosion Sci., 8, 513 (1968).

[51] Hoar, T. P., and West, J. M., Proc. Roy. Soc. 268A, 304

(1962).

[52] Leidheiser, Jr. H. and Kellerman, E., Corrosion 26, 99

(1970) . [53] Murata, T., and Staehle, R. W., Proc. 5th Intl. Cong, on

Metallic Corrosion, Tokoyo, 1972, to be published by

Natl. Ass. Corr. Eng., Houston.

[54] Tomashov, N. D., and Vershinina, L. P., Electrochimica Acta 15, 501 (1970).

[55] Pessal, N., and Liu, C, Electrochimica Acta 16, 1987

(1971) . [56] Ambrose, J. R., and Kruger, J., Corrosion 28, 30 (1972).

157] Ambrose, J. R., and Kruger, J., J. Electrochem. Soc. 121, 599 (1974).

[58] Parkins, R. N., in reL [6] p. 449. [59] Humphries, M. J., and Parkins, R. N., Corrosion Sci. 7, 747 (1967).

[60] Powell, D. T., and Scully, J. C, Corrosion 24, 151 (1968).

[61] Parkins, R. N., Mazza, F., Royuela, J. J., and Scully,

J. C, Brit. Corr. J. 7, 154 (1972).

[62] Scully, J. C, and Powell, D. T., Corrosion Sci. 10, 371

(1970).

[63] Deegan, D. C, and Wilde, B. E., Corrosion, 29, 310

( 1973 )

.

[64] Tromans, D., and Nutting, J., Fracture, J. Gilmon, Ed.

( Interscience, New York, 1963), p. 637. [65] Nielsen, N. A. in reL [7]. [66] Nielsen, N. A., Corrosion, 27, 5 (1971).

[67] Swann, P. R. in ref. [6] p. 113. [68] Scamans, G. M., and Swann, P. R. in ref. [7]. [69] Scully, J. C, in reL (6) p. 127. [70] Brown, B. F., Ed., Stress-Corrosion Cracking in High

Strength Steels and in Titanium and Aluminum Alloys, Naval Research Laboratory. U.S. Government Printing

Office, Washington, 1972. [71] Logan, H. L., The Stress Corrosion of Metals (John

Wiley and Sons, New York, 1956), pp. 273-285.

[72] Brown, B. F., and Beachem, C. D., Corrosion Sci. 5, 745 (1965).

[73] Brown, Jr., W. F., and Srawley, J. E., Plane Strain Crack Toughness Testing of High Strength Metallic Materials, A.S.T.M., STP410 (1966).

[74] Smith, H. R., and Piper, D. E., Stress-Corrosion Cracking in High Strength Steels and in Titanium and Aluminum Alloys. B. F. Brown, Ed., Naval Research

Laboratory. U.S. Gov't Printing Office, Washington,

1972, p. 17.

6. Discussion

W. J. Baxter, General Motors Corporation: You indi- cated a distinction between fine slip and coarse slip, and that one was passivated more quickly than the

other. I don't understand the reason for this.

/. Kruger: The rate of passivation is the same for both

of them. The problem is that for fine slip, since there is less surface to deal with, the whole slip step can be re- passivated without forming a notch at the last portion

that's repassivated. For coarse slip, the whole thing

might repassivate, and a little notch forms at the end

because there is more surface.

W. J. Baxter: I don't understand why repassivation is like a creeping fungus. Why doesn't the whole surface

just uniformly repassivate?

/. Kruger: Because the whole surface is not exposed

at the same time. The first part that comes out is ex- posed first.

39
W. J. Baxter: You're saying that you get a little fine

slip and then it's repassivated and then the next one forms?

/. Kruger: That's right, it's a dynamic process. But

for coarse slip there is not enough repassivation time to repassivate the whole thing, so there is more corrosion

occurring.

D. C. Drucker, University of Illinois: Have you tried any calculations which would combine the mechanics

approach with the chemistry approach; that is, to see whether, for example, a particular species sitting at the

root of a crack interacts with the stress field in the mechanics sense, not quite the fracture mechanics sense, but the more detailed mechanics sense? Does it make

any sense to do that? Can you then show that you

should expect bond separation or bond breakage?

/. Kruger: We haven't tried that, but the kind of thing you are talking about is related to the stress- sorption picture. I believe for metals in aqueous en- vironments that the reactions that are taking place, for example the formation of a passive film or the dissolu- tion of metal, are so rapid compared to the rate of ad- sorption of a species that it is not important. For

non-metals, for example glasses, it is indeed important

and the kind of calculations you suggest would be worthwhile.

A. J. McEvilj, University of Connecticut: You indi- cated a repassivation trend for the precipitate free zone

of the Al-Zn-Mg alloy and for aluminum alloy 7075.

To which of the stages of the stress corrosion process does that trend apply?

/. Kruger: You mean the 3 regions in the V-K type curves?

A. J. McEvily: Yes.

/. Kruger: Region 1 is really the only one that most

people are concerned with in stress corrosion.

A. J. McEvily: So that's more like a crack initiation stage ?

/. Kruger: It certainly is involved in crack initiation, but I think it can also be involved in crack propagation.

Whether the crack will continue to grow, or whether

metal dissolution or active path cracking is involved

depends on the relative rate of repassivation and the

relative rate of slip step production versus the rate of corrosion.

A. J. Babecki, NASA, Goddard Space Flight Center: One solution for preventing stress corrosion cracking is to put the surface in compression, usually through shot peening or glass bead peening. Peening of some metals such as magnesium increases the corrosivity of the metal. If the corrosion reaction of the surface is in- creased by the peening procedure, does that also tend

to increase the stress corrosion cracking phenomenon?

/. Kruger: Not at all. Whereas the shot peening may

increase the rate of corrosion, it would decrease the

susceptibility to stress corrosion.

40
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974. held at NRS, Gaithershurg, Md. (Issued April 1976)

Elastohydrodynamics in Concentrated Contacts

Herbert S. Cheng

Northwestern University

Evanston, Illinois 60201

Typical geometrical features of elastohydrodynamic (EHD) contacts and major operating,

lubricant, and material parameters governing lubrication in these contacts are described. Operating

regimes for various machine components, such as gears, rolling element bearings, cams, and compliant bearings, are identified. A state-of-the-art survey is given to the predictability of major EHD

characteristics based on existing analytical and experimental EHD research. The significance of

film thickness, pressure fluctuations, friction, and temperature in affecting failures of concentrated

contacts by fatigue, scuffing, and wear is discussed. Finally, suggestions are made to incorporate

the EHD eff^ects in predicting fatigue life, scuffing load, and wear rate.

Key words: Contact lubrication; elastohydrodynamic contacts; fatigue; lubricant film thickness;

scuffing; wear.

1. Introduction

In concentrated contacts, load is generally supported

over a tiny contacting area known as the Hertzian contacting area, named after Heinrich Hertz who first de- veloped the elasticity theory to determine the size and

shape of these contacts. Even under a moderate load, the contacting pressure developed within a Hertzian

area can become enormous, and often can exceed the

tensile yield strength of the contacting solids. Because

of this severe condition, it was believed earlier that a continuous lubricant film cannot exist, and load is strictly supported by metallic contact through the asperities. Lubrication is provided mainly by a thin surface

film of one or several molecular layers recognized as boundary lubrication. The concept that concentrated contacts operate

strictly in the regime of boundary lubrication was un- challenged until 1949 when Grubin and Vinogradova

[1] ^ theorized that a fluid film of the order of many

times the surface roughness can be developed hydrodynamically. This thick film is mainly attributed to a rapid increase of viscosity with pressure under high

pressures for mineral oils, and to enlargement of the loading area caused by elastic deformation.

Grubin's work marked the beginning of an important

field known as elastohydrodynamic lubrication, devoted

to understanding of the lubrication behavior where

elastic surface deformation plays a dominant role in governing the process. During the past two decades,

rapid progress has been made both in the theoretical and experimental understanding of major variables in EHD lubrication, including detailed film, pressure, temperature, and traction distribution in lubricated Hertz-

^ Figures in brackets indicate the literature references at the end of this presentation.

ian contacts. Comprehensive reviews have been made by

Dowson [2] and more recently by McGrew, et al. [3].

The objectives of this presentation are to provide a general background of EHD lubrication in concentrated

contacts such as rolling element bearings, gears and

cams, to discuss how various failure processes are in- fluenced by EHD, and to suggest methods to incorporate some significant EHD eff^ects in failure predictions in concentrated contacts.

2. EHD Geometry

Considering a general case of a lubricated Hertzian

contact between two bodies as shown in figure 1, the geometry at the point of contact is characterized by the principal radii, Rri, Ryi for body 1 and Ry2 for body 2. In general, the principal planes containing R^i

and R,r2 may not coincide; however, for most EHD contacts, such as roller or ball bearings and gears, the principal radii R,ri and Rx2 do lie in the same plane. These

surfaces can be classified as convex, concave, or saddled

depending upon whether /?,,, and Ry are both positive,

both negative, or mixed. Thus, contacts between two

convex surfaces sucti as roller-inner race contacts or gear teeth contacts for external spur gears are counter- formal. Contacts between one convex and one concave

surface such as roller-outer race contacts or gear teeth contacts for internal spur gears are conformal. For contacts between one convex and one saddled surface such

as deep-groove ball-inner race contacts, the result is conformal in one direction and counterformal in the perpendicular direction. In these heavily loaded contacts, the normal load is distributed over an elliptical area approximately con- forming to the Hertzian ellipse of a dry contact as shown

41
at the top of figure 1. In most cases, the direction of roll- ing is along the minor axis of the ellipse with surface

velocities ui and uo for bodies 1 and 2, respectively. The

degree of sliding is measured by a quantity known as

the slide to roll ratio denoted by S = 2 ( ——,

— ] \«i + Uo

)

with S = 0 corresponding to pure rolling and S = 2 to simple sliding.

Figure 1. Geometry of an EHD

3. Governing Parameters

The variables which are well known in fluid film lu- brication also play important roles in EHD lubrication. These include speed, load, viscosity, and geometry. In addition to these, the elastic modulus of solids and the

pressure-viscosity dependence of lubricants also have a major influence in governing the EHD process. These

variables can be grouped together in four major dimensionless parameters listed below:

U — speed parameter

P — Phz/E load parameter

G = aE pressure-viscosity parameter

slide to roll ratio.

The above parameters can be readily calculated from

properties of the contact material, properties of the

lubricant, and operating conditions. Table 1 gives practical ranges of load, rolling speed, sliding speed, lubri- cant pressure-viscosity coefficient, and elastic modulus of the solid, commonly encountered in EHD contacts such

as gears, cams, rolling element bearings, and compliant

bearings. In this table, the ultra high-speed applications

such as those encountered in aircraft applications have

been excluded. Using the limiting values in table 1, the range of EHD load and speed parameters can be readily

computed for the above-mentioned EHD contacts. Using

load and speed parameters as the coordinates, one can

readily show in figure 2 in which regions these contacts operate. It is seen that gears and cams occupy the bottom left corner because of their relatively lower roll- ing speed and lower contacting stress. Rolling element

bearings settle in the middle region, and compliant

bearings occupy the upper right corner because of low

elastic modulus of solids.

Table 1. Range of operating and material variables in EHD contacts u/R Usju E Pnz a

Gears

10I250 c.p.10

i250 s-^

0

i

0.35 33 X 10" ps

2.5 X 10' psi

0.0001

i .0002 in=/lb

Cams

10

I250 c.p.

10i250 s-^

2.0 33 X 10" ps

1.5 X 10' psi

.0001 i

.0002 in=/lb

Rolling element

bearings

10

i100 c.p.

50

i2000 S-'

0

i

0.02 33 X 10" ps3.5 X 10' psi

.0001 i

.0002 inVlb

Compliant

bearings

1 I100 c.p.

10

i1000 S-'

2.0 300

i1000 psi

10i100 psi 0

42
IO""l I I I \ I I I I

I I I I I I I I I I'll

0015 .002 .003 .005 .010 .020 .03 .05 .10

Phz /E

Figure 2. Regions of operation for gears, cams, rolling element bearings, and

compliant bearings.

4. Major Characteristics

The behavior of EHD contacts is characterized by

the distribution of film thickness, pressure, temperature,

and frictional force within the pressurized region. These

variables will be discussed separately below.

4.1. Film Thickness

The effectiveness of EHD lubrication is largely measured by how well the asperities are separated by the lubricant film. A fully effective EHD lubrication is generally agreed to be where the center film thickness is at

least three times the composite r.m.s. surface roughness

of both contacting surfaces. A film thickness inferior to

this threshold condition is considered to be a partial

EHD lubrication. Figure 3 depicts the essential differ- ence between these two conditions.

Effort in the past has been mostly concentrated on

determining the nominal level and detailed distribution of film thickness in line contacts as well as in elliptical contacts. Among the experimental techniques [12, 13,

14] for the study of film thickness, the optical method

is most satisfactory in resolving the detailed gap distri- bution in EHD contacts. Figure 4 shows that the film shape in a heavily loaded elliptical contact consists of a

region of approximately uniform film thickness cover- ing the inlet and central parts of the contact. In the

exit region, the sudden drop in pressure causes the sur- face to protrude slightly and produces a constriction around the exit edge of the contact. The film shape is describable by local film thicknesses measured at three

strategic locations within the contact: the center film

or the average gap separating the contacting surfaces

in the inlet and central regions; hr, the trailing minimum film thickness along the center strip; and hs, the

side minimum film thickness at the two ends of the

elliptical contact.

FULL FILM EHD

PARTIAL FILM EHD

Figure 3. Full film EHD versus partial film EHD.

43
CONTOURS FOR f. CONSTANT FILM ^

Figure 4. Film shape in elliptical contacts.

The present ability in predicting the center film thick- ness he in EHD contacts of very small b/a ratios is shown in the left map of figure 5. The operating ranges

of load and speed for gears, cams, and rolling element

bearings are blocked out as dotted rectangles in this map. The predictability of he in each subregion in this map is indicated by a number with 10 for 100 percent accuracy and 0 for a total absence of predictabihty.

In the partial-film region, the mean central film thickness will be influenced by surface roughness effects, and very little analytical work has been developed so

far. Preliminary analysis accomplished by Chow and

Cheng [15] based on Christensen's stochastic model

[16] for transverse surface roughness showed that the mean value he can be altered by surface roughness.

Since there are no experimental data available for comparison, no great confidence can be attached to any ap- proximate means of estimating the mean he in the

partial-film region. Consequently, the entire region is covered with variable degrees of shade indicating marginal to poor predictability.

Figure 6 gives a survey of expected accuracies for the

Figure 5. Predictability of he in EHD line contacts.

Since the degree of separation is largely governed by

he, its prediction has been one of the most important

tasks in EHD research. This was first achieved by Grubin and Vinogradova [1] for line contacts and con- firmed by Dowson and Higginson [4] in their full EHD

analysis. For elliptical and circular contacts, similar analyses were also developed by Archard and Cowking

[5], Gohar [6], and Cheng [7]. For ultra-high-speed

contacts, thermal effects at the inlet region become sig- nificant, and center film thicknesses predicted from the isothermal theories can be corrected by thermal correction factors (introduced in [8, 9]). For lubricants with

low or moderate pressure-viscosity dependence, theories developed by Herrebrugh [10] and by Cheng [11] may

be used.

trailing film thickness A? in elliptical contacts and

the side minimum film thickness hs in circular contacts. The left map for Ay shows that the only open area, where

reasonable accuracy is expected from the Dowson and

Higginson theory [4], is the lower left quadrant. For

other conditions, one may use the thermal inlet theories for high speed cases, and the recent Jacobson's line contact theory [17] for high loads. However, some of these suggested procedures have not been confirmed by ex- periment, and, therefore are considered extremely uncertain. Turning to the right portion of figure 6 for the

side minimum film hs in circular contacts, there is very

little available analytically. Jacobson [18] recently de- veloped an impressive numerical method for the two

dimensional EHD film thickness based on a solidified

44
lubricant; however, the validity of his solidification model as well as the numerical accuracy is yet to be

confirmed. In view of these uncertainties, the entire re- gion is declared to be a dark area.

4.2. Pressure Distribution

The pressure distribution in the contact region of EHD contacts is not altered greatly from the Hertzian

distribution. For a line contact or an elliptical contact with very small hja ratio, the pressure distribution along the center strip of the contact is modified at the

inlet and exit edges. In the inlet region, the pressure

is built up gradually to the Hertzian distribution, as shown in figure 7. At the exit, EHD analysis for line contacts [4] predicts a sharp secondary pressure rise just prior to the termination of the film. This secondary

I. HIGH SPEED,

MODERATE LOAD

2. LOW SPEED,

X

Figure 7. Pressure profiles.

pressure is caused mainly by the strong pressureviscosity dependence of the lubricant. However, for very

heavily loaded contacts, this pressure spike occurs at the very end of the flattened region, and is not expected

to have a strong eff^ect on subsurface fatigue life. In spite of the extensiveness of EHD theories, the

predictability of the magnitude of secondary pressure peaks in EHD contacts is still not fully satisfactory. Present EHD theories can predict an abrupt pressure disturbance and its approximate location, but they can- not predict the degree of its severity. For this reason, only 30 percent confidence is rendered in the entire region in terms of predicting p* as shown in figure 8. Actually, in terms of relevance to failure, pressure dis- turbances caused by surface roughness in EHD contacts are far more important compared to the single pressure spike in full-film EHD lubrication. In terms of predicting the severity of these pressure fluctuations, under

partial EHD, very little is available. Tallian [19] made

an extremely interesting pioneering analysis in assessing the pressure ripples in EHD line contacts. It certainly

sheds some light in this otherwise totally dark chamber.

4.3. Temperature and Friction

Temperature and frictional forces in EHD contacts have a very strong mutual dependence, and, therefore, are discussed under a single heading. In general, sliding

speed is the most dominant variable governing temperature and friction. Except at very high rolling speeds,

temperature and friction in pure rolling contacts usually do not rise to a significant level to be of great concern. For sliding contacts such as gear teeth, both

quantities play major roles in affecting failure. Typical temperature profiles at the center of a lubri- cant film and on the contacting surfaces are depicted in

45
figure 9. In general, the temperature variation corre- sponds quite well with the pressure distribution, except

in the inlet region where the temperature rise occurs much earlier than the pressure curve. Most friction measurements in EHD contacts have exhibited characteristics like that shown in figure 10. For low sliding

speeds, the slope is approximately linear, and this slope can be predicted by a transient viscosity model proposed

by Harrison and Trachman [20]. With this transient

viscosity, the predicted friction and temperature based on an effective viscosity are fairly accurate, and a predictability of 70 percent is given in this low-slip region

in figure 11. As sliding speed increases, the relation between fric- tion and slip becomes non-linear. Analytical predictions

in this non-linear region are found to be not reliable,

T

Figure 9. Temperature profiles.

LOW-SLIP REGION

MODERATE-SLIP REGION

SLIDING SPEED

Figure 10. Sliding friction curve.

002 .003 .005 .010 .015 .002 .003 005 010 .015 P P

Figure 11. Predictability of temperature and friction.

46
and a 40 percent predictability is given. In the region

of high sliding speeds, the variation of friction with slip

becomes gradual, and reasonably accurate predictions

of friction can be made by using empirical friction models [21, 24, 25] and semiempirical models [22, 23,

26, 27]. The expected accuracy may vary somewhat

from one friction model to another, but the overall predictability in this region can be regarded as 70 percent. Analytical developments dealing with friction in partial EHD contacts is still in its very early stage. About the only theory available is the recent contribution by Dyson and Bell [28]. More research is defi- nitely needed here in order to obtain even a marginal

predictability of friction and temperature for partial

EHD contacts.

5. Effects of EHD on Failure

It has been recognized [29] that the life of concentrated contacts is generally terminated by one of the following modes of failure: subsurface or surface fa- tigue, scuffing, or mild wear. The advent of EHD has

triggered a sustaining effort to determine the extent of EHD influence on each major failure mode. For in- stance, fatigue experiments with discs under controlled conditions have yielded considerable evidence of the

beneficial effect of film thickness on surface fatigue.

Other relations between EHD and failure are currently

being uncovered. In the following sections, a brief dis- cussion is given to a commonly accepted view of failure mechanisms for each failure mode. In addition, a quantitative assessment is given to the relative influence of the major EHD variables on each failure mode.

In order to compare the relative influence of the EHD variables with other factors affecting various fail- ure models, an assessment is made in table 2 for each

failure model. The relative influence is measured by a number with ten being the most influential and zero be- ing least influential.

Figure 12. Mechanism of subsurface fatigue.

5.1. Subsurface Fatigue

Contact fatigue is generally referred to as the loss of contacting material under cyclic, high shear stresses. The material damage can appear in many forms from

very small pits to very large spalls. Regardless of the physical appearance of the final failed specimen, contact fatigue can usually be traced from cracks initiated around subsurface inclusions or surface flaws, or from

cracks caused by repeated asperity collisions on the

surface. This section will be mainly devoted to discus- sions of fatigue originating from subsurface inclusions. Fatigue originating at the surface will be covered in the next section. According to Littmann [30], subsurface fatigue originating around large inclusions can be depicted as a sequence of events as shown in figure 12. When a contacting surface is under a repeated normal jjressure, small cracks initiate around large inclusions located near layers of large alternating shear stress. These

cracks first tend to propagate normally upward until they reach the surface. Once this happens, further propagation can be aided by fluid pressure. Cracks then propagate along a plane slightly inclined to the surface. Spalling finally takes place when some of the branch

cracks intersect the surface.

Table 2. Relative influence of EHD and other variables on

various modes of failure Subsurface

fatigue Surfacefatigue

Scuffing Mild

wear hja 5 10 8 8

P*Iphz 5 8 5 5 T and t 4 6 10 5

Inclusion

density and

size 8

Slide to

roll ratio 8

Chemical and

physical properties

of surface film 10

For inclusion originated fatigue, the relative influenc- ing factors are assessed in table 2. The assessment is based on the fact that inclusion originated fatigue is mostly influenced by the subsurface stress distribution, inclusion density and size distribution. Film thickness does not alter subsurface stress distribution, and has

little or no influence. The normal pressure as well as

47
surface shear stress can have a significant influence on

the failure mode, if they produce a gross redistribution

of subsurface shear stress, particularly in the vicinity of maximum alternating shear stress. However, for a heavily loaded EHD contact, subsurface stress deviations resulting from an EHD pressure spike as well as from local sliding friction are not expected to be signifi- cant enough to cause a substantial change in fatigue life in inclusion originated fatigue. Thus, in predicting sub- surface fatigue life, the effects contributed by EHD are expected to be secondary compared with the influence from inclusion density and size.

5.2. Surface Fatigue

As the material improves in quality, the mode of fatigue is expected to shift from subsurface to that orig- inated on the surface. Littmann [30] called this mode

point source origin which is considered to be a major

factor in contact fatigue. Surface fatigue can be fur- ther divided into two types based on whether cracks are originated around surface defects, such as furrows and

nicks, or caused by asperity collisions under marginal

or poor EHD lubrication. The mechanism for surface fatigue from defects is shown in the left diagram of figure 13. The presence

of surface flaws tends to interrupt EHD lubrication, and

produce large pressure fluctuations and surface distress. This causes cracks to appear around the surface defects, which in turn set up propagation along an inclined plane beneath the surface leading to a fatigue spall. Figure 13 also depicts the scond type of surface fa- tigue which is caused mainly by microcracks initiated around asperities when they collide repeatedly with

each other under insufficient EHD lubrication. These

DUE TO SURFACE DEFECTS DUE TO ASPERITY COLLISION

Figure 13. Mechanism of surface fatigue.

cracks usually do not penetrate deep into the surface,

and propagate along a thin layer just below the surface. Before they have a chance to travel far, they usually

intersect with other cracks and break out into small

pits. They are also referred to by Littmann [30] as surface peeling, or by Tallian [28] as surface distress.

Since surface fatigue is mainly due to deterioration

of lubrication, EHD film thickness is expected to have

a strong influence on the failure mode. The relation be- tween EHD film and fatigue life was discovered inde- pendently by Dawson [31] with rollers and Tallian et al. [32], with rolling element bearings. After these two

pioneering efforts, many fatigue experiments with vari- ous contacting geometries have confirmed that fatigue

life increases with the specific film thickness A, follow- ing some power relationship. A typical empirical power

relation can be shown in figure 14 based on a series of

tests by Lund and Andreason [33]. Thus, EHD is ex- pected to have a strong effect on surface fatigue for contacts which are in the partial film regime and relatively free from inclusions. Qualitatively, the regions separating surface and subsurface fatigue can be shown in fig- ure 15, by plotting fatigue life against the inclusion density and the specific film thickness. It is seen that for a high quality contact material, surface fatigue dominates until A exceeds 3 where the inclusion originated

subsurface fatigue is likely to terminate the contact life. For material of high inclusion density, subsurface fa- tigue is expected to dominate the entire range of specific film thickness; EHD lubrication plays a minor

role in this regime.

200

100

' 50

20

y y

yy

y y^

y. - ^^10- '!0.A°=^

y^

0-2 0-5 12 5 10 20 50

A

Figure 14. Relation between A and surface fatigue.

Aside from the film thickness, table 2 also lists sev- eral other possible variables influencing surface fatigue.

The normal pressure fluctuation and tangential shear

stress during asperity collision as well as possible ther- mal strains may also have some effect on surface fatigue

however, these effects have not been fully explored. A

smaU amount of sliding superimposed on rolling has

been shown in past fatigue tests to have a significant

effect in reducing fatigue life. This may be explained by

the fact that sliding increases the number of collisions for a specific asperity, and hence increases the probabiUty of fatigue failure. There is yet no empirical

method to relate life to slide to roll ratios.

48
SUBSURFACE

FATIGUE

Figure 15. Effect of inclusion density and EHD specific film on fatigue life.

6. Scuffing

According to Blok [34], scuffing failure in rolling

and sliding contacts is characterized by material transfer from one surface to the other through asperity weld- ing which usually results in a rapid deterioration of the

surface. The exact mechanism which leads to scuffing is still not fully understood. Many hypotheses have been pro- posed, but none has been adopted as an undisputed

criterion for scuffing. However, most researchers seem

to favor the critical temperature concept which is shown

schematically in figure 16. In lubricated contacts, the surfaces are usually covered with an extremely thin surface film which is formed by physical adsorption,

chemisorption, or reaction between an additive and the surface material. Such a surface film is regarded as the

last line of defense against scuffing. When this surface

film is removed, any sliding of an asperity against the opposing surface will lead to a continuous cold welding

of the junction causing rapid surface deterioration known as scuffing.

The incipient conditions causing breakdown of these surface films have been under investigation by re- searchers in boundary lubrication for decades. The

problem is extremely difficult, and to this day has not received a fully satisfactory explanation. Nevertheless,

the majority of researchers seem to accept the so-called

critical temperature concept advanced by Blok for nonadditive mineral oils. He ])Ostulates the existence of a constant critical temperature for nonadditive mineral

oil (approximately 175 °C) beyond which the surface

film would be detached. Others have shown that this

critical temperature is not constant, and may be dependent on ambient viscosity.

T

Figure 16. Mechanism of scuffing.

Before one discusses the possible EHD effects on

scuffing failure, one must emphasize that scuffing is only

possible if there is asperity contact, i.e., one is dealing

with the regime of partial EHD. As such, the determination of the specific film is essential, because it not only

shows whether protection from a full EHD film is possible, but it also indicates the severity of asperity contact. For this reason, is interpreted as having a strong ef- fect on scuffing in table 2. There seems to be little dispute that the maximum

surface temperature is the key factor controlling scuff- ing. To determine this maximum surface temperature, one may use Blok's flash temperature equation [35], or one of the refined flash temperature theories described

in [34]. Whatever the method may be, it is essential to know the heat input through the frictional coefficient which still cannot be predicted with accuracy even for

full film EHD lubrication. Past predictions of scuffing failure have been based mostly on empirical friction formulas such as those given by Bennedict and Kelly

[36]. However, this is not regarded to be entirely sat- isfactory because there has been little confirmation or

correlation of the accuracy of these empirical friction formulas. Needless to say, a comprehensive, accurate, thermal friction analysis in the regime of partial EHD

would give a significant lift in this field. Pressure fluctations around asperities seem to have a more direct influence on fatigue rather than scuffing;

however, its indirect influence on local temperature

fluctuation could become an important consideration. While the attention here is focused upon effects of EHD, one should not overlook that prediction of incipi- ent failure based on partial film EHD behavior is only

possible if the critical temperature is known beforehand.

Keys for determining the accurate critical temperature

for a given condition, by and large, still lie in the hands

49
of chemists. The last factor in table 2 for scuffing failure serves as a reminder to us mechanists that EHD is only one facet of the entire problem on scuffing.

6.1. Mild Wear

Peterson [37] suggested that wear in lubricated contacts can be classified into two basic types: metallic contact wear and film wear. In the first mode, wear

particles result from the making and breaking of a junction between two sliding asperities, a purely mechanical process. Whereas, in the second mode, wear is a continuous removal of a solid film formed continu- ously through lubricant-surface interaction. Since EHD

is likely to have more influence on metallic wear, sub- sequent discussions will be confined to this type of wear

only. In figure 17, it is shown that if two asperities slide over each other under partial EHD, a junction is formed. If the level of surface temperature is sufficiently

with other asperities. Actually, this mechanism can also be interpreted as a surface fatigue failure. Since micropitting, in general, does not terminate life, it is usually referred to as mild wear rather than fatigue failure. From the description of wear mechanisms, it is not

difficult to see a strong relation between wear rate and

EHD film thickness, as suggested in table 2. Indeed,

experiments with a 4 ball tester [39] have discovered a marked reduction in wear rate as EHD film thickness

increases. However, these experiments also showed that the slope between the wear rate and specific film thick- ness appearing on a log-log plot still varies considerably

with lubricants and lubrication condition. It is not cer- tain what factors contribute to the variance of the slope for these cases. There has been speculation that besides

the influence of /i,./o', pressure fluctuations and tangential stresses as affected by different rheological properties of lubricants may also have some influence on the formation of wear particles if fatigue is the wear mechanism. These additional effects may have some in- fluence on the variance in slope observed in [39].

SCUFFING TEMP.

FiGURii 17. Mechanism of mild wear.

far below the critical temperature of the surface film, this junction usually does not grow in size, and is broken away as soon as the sliding action is completed.

In a large size model, Cocks [38] has shown that such

making and breaking of a junction usually produces a wedge of material adhering to one surface. Subsequent

sliding usually removes these wedges and produces wear

particles. This mechanism is sometimes referred to as adhesive wear. Besides the adhesive wear mechanism there is still another mechanism for producing wear particles known

as fatigue wear. In this mode, wear particles are caused

by micropitting around asperities after many collisions

7. EHD and Failure Prediction

As contributions in EHD are increasing at a rapid

rate, it is desirable to examine which topics in EHD are more useful in predicting incipient conditions for vari- ous modes of failure. In the following, equations for prediction of failure have been suggested. Included in these equations are factors which can account for the influence of EHD. Hopefully, these suggested relations may serve as a guide for establishing a priority for se- lecting future EHD research topics.

7.1. Subsurface Fatigue Life

In spite of the fact that early important EHD work

was partly motivated by its possible influence on sub- surface fatigue, all subsequent evidence seems to indi- cate that EHD has a much stronger influence on surface

fatigue than subsurface fatigue. Thus, it is believed

that for subsurface fatigue, improvement in life is likely

to come from better materials than from better EHD

performance.

Past standards for predicting life against subsurface

fatigue for rofling element bearings are based mainly

upon the classical Lundberg and Palmgren theory [40]

which relates the probability of survival to the maximum alternating shear stress, Tq, the depth at which

To occurs, the size of the contact, and the loading cycle.

The variation of inclusion size and density are not accounted for. Recently, two procedures have been

made available to include the effects due to changes

of inclusion density and size. Both methods propose

the use of a life adjustment factor for the standard

life predicted from the Lundberg-Palmgren Theory.

The first procedure, proj)osed by Zaretsky et al. [41],

is simple, easy to use; the adjustment factors are based on extensive fatigue tests either with full scale bearings

or with simulated 4 or 5 ball testers, for bearing materials with impurities controlled by chemical composition and by process variables. The second ])rocedure

by Tallian et al. [42 1, is more basic, but requires

50
much additional input for implementation because one must know beforehand the inclusion density, size dis- tribution, and stress concentration at the inclusion tip. There seems to be little room for argument that the use of adjustment factors in conjunction with the Lundberg and Palmgren Theory is an effective way

to predict subsurface fatigue life. Perhaps what is de- batable are the methods by which these factors are de- termined. In reference [41], these factors are entirely

empirical, and require extensive fatigue tests for each new material and process. On the other hand, the

factors in reference [42] were obtained purely analytically, and their reliability is somewhat uncertain. In view of these comments, it might be worthwhile to consider a compromised model, which is unrestrictive

to the material type or material process, and which

seeks an empirical relation between fatigue life and

inclusion size and density. This compromised model

can be written as

L'lO \ Phz)

In the above equation, the ratio of actual fatigue life (90 percent survival) Lio, to that based on the Lundberg-Palmgren Theory L\o, is assumed to vary

with p and F following a power relation, where p and r, are the inclusion density and the inclusion mean radius,

respectively, normalized by those found in materials

used in Lundberg-Palmgren's tests. In addition to these two factors, it is also suggested

that the secondary pressure peak caused by EHD ef- fects deserves a reexamination regarding its relative in- fluence on fatigue life. A power relation between

fatigue life and p*/p«^ ^^^^ suggested, and the ex- ponents n\, 7i2, and n.^ are to be determined by further

fatigue experiments with controlled inclusion density

and size.

7.2. Surface Fatigue Life

As described earlier, surface fatigue is strongly af- fected by specific film thickness hc/v if the contact operates under partial EHD. The film thickness effect has been incorporated in references [41] and [43] to predict fatigue life against surface fatigue. In reference

[41], an adjustment factor is used to account for the

effect of h/a, and this factor is largely based on a se- lected number of fatigue tests. Whereas in reference

[43], an analytical approach has been favored for de- termining the influence of h^/u. However, there have

been no efforts to correlate these two approaches.

So far, among the major EHD variables only one

identified the specific film thickness as a primary influ- ence on surface fatigue. Other factors such as pressure

fluctuations and slide to roll ratios may also have sig- nificant influence but are yet to be explored. It is speculated that for microcracks initiated around asperities, the local pressure variation, as influenced by rheological properties of different lubricants, and the frequency of asperity collision, as affected by the slide to roll ratio, must have some influence on its occurrence. If these two effects are included, one might

consider the following relation for future predictions

of surface fatigue life.

, , mi , , mo , V --a) (£) m

where L\q = surface fatigue life, based on 90 per- cent survival C = a bearing material constant mi = exponent for EHD film effect, ap- proximately equal to 0.6

m2,m.3 — empirical exponents for effects due

to pressure fluctuations and slide to

roll ratios.

7.3. Scuffing Load

Incipient scuffing for a concentrated contact has been

customarily measured by its ultimate load capacity

against scuffing when all other variables such as speed

and material properties are kept constant. This incipient load is known as the scuffing load, which is a familiar term in gear technology.

As described earlier this incipient scuffing load is regarded as the load at which the flash temperature,

as calculated from Blok's theory, or any one of its refined versions, reaches the critical temperature of a

lubricant. The functional relation between flash temperature and scuffing load can be expressed symbolically

as T; - Tb = F {P,f, Speed, Material, and Lubricant Parameters)

where Tf — the flash temperature or the maximum surface temperature — temperature of the surface before

entering the contact region

/ = EHD frictional coefficient, which is dependent upon the specific film

hc/(T and the shear modulus of the lubricant Coo P = contacting load. The symbol F stands for Blok's flash temperature function or one of the more refined but more complicated

versions. The inverse form of this function G permits one to compute the scuffing load P* provided that the critical temperature T*/ is given,

P* =G - Ts, /(^j:, C«), etc.|

The accuracy in predicting P* by means of the above

relation would depend on:

1. The validity of the flash temperature theory F in calculating Tf.

2. The predictability of the lubricant critical temperature T*f.

51
3. The predictability of the base temperature T^-

4. The predictabiUty of EHD frictional coefficient /.

It appears that the state-of-the-art of flash temperature

theory is overdeveloped compared with the predictabilities of T*f, TB, and /. There are uncertainties in

all available methods of predicting these three quantities.

While chemists concentrate their efforts in resolving

questions, such as whether there exists a single constant

critical temperature for a mineral oil or whether the

critical temperature is also speed dependent, mechanists

should accelerate their efTorts on a better heat transfer analysis of Tu and a more accurate analysis of EHD

friction. Thus, if these three terms can be determined

with reasonable accuracy, there is no reason to doubt

that the prediction of scuffing load based on existing

flash temperature theories will yield a satisfactory result.

7.4. Wear Rate

Even though there has been some scanty evidence

indicating that the wear rate in EHD contacts resulting

from the metallic contact mode appears to be amenable

to Archard's wear law, more correlations are still needed before it can be applied with full assurance. According to Archard's wear law, wear rate for roll- ing and sliding EHD contacts should obey the follow- ing relation [44]

where w -- wear rate K = a. wear constant = ratio of the real area of contact to the A

apparent area; ^ ^ (^)

— = specific film an — slope on log-log plot between w and

he/cy

Us = sliding velocity.

It is not certain to what extent pressure fluctuations or tangential shear stresses may influence K and n. One

can only hope that their influence will be relatively

mild so that the wear rate in EHD contacts can be pre- dicted with the simple relations suggested above.

8. Concluding Remarks

(1) It seems evident that among the major EHD

variables, the specific film thickness definitely

plays the most important role in influencing

failure by surface fatigue, scuffing, and mild

wear. Present methods in predicting the specific

film h,./(T for determining partial or full EHD

is considered to be quite reliable based on exist- ing EHD analyses. However, the accuracy when

h,./(T<, <3, i.e.. at the far end of the partial film regime, is still uncertain because of the lack of knowledge on the effect of surface roughness on EHD film analysis.

(2) For fatigue originating at subsurface inclusions,

EHD seems to play a secondary role compared

to material eff"ects. The EHD influence is from

the redistribution of the subsurface stress field due to secondary pressure spikes and from the

tangential shear stress. A quantitative determination of these effects is needed.

(3) For fatigue originating at surface defects and

surface asperities, EHD definitely plays a signifi- cant role. There are empirical relations now

available for predicting the surface fatigue life as influenced by the EHD film thickness; however, other EHD effects such as normal pressure and tangential shear are yet to be explored.

(4) Determination of scuffing failure seems to hinge

critically on the predictability of: EHD fric- tional force and surface temperature in the

partial film regime, base surface temperature

prior to entering the contact region, and the

critical temperature for a given lubricant. Considerably more effort is needed in the study of

friction under partial EHD to enhance the re- liability in predicting scuffing failure. (5) Prediction of wear rate, at least for rolling EHD

contacts, seems to be amenable to Archard's wear law which relates wear rate and EHD film linearly on log-log grids. However, the slope seems to be dependent on other EHD variables; further explorations are needed.

9. Nomenclature

a,b = semimajor and semiminor axis of Hertzian ellipse

A = apparent area of contact Ar = real area of contact C —a material constant in the suggested

formula for predicting surface fatigue

life E = reduced elastic modulus

/ = sliding frictional coefficient G — the pressure-viscosity parameter for EHD

contact: also used as a function for the

scuffing load

Goo = shear modulus of the lubricant hf — trailing minimum film thickness

hg = side minimum film thickness

he = center film thickness K = wear constant

Z-io = fatigue life based on 90 percent survival

L'lo = fatigue life based on Lundberg and

Palmgren theory for 90 percent survival mi.mo.m-j = exponents in the suggested formula for predicting surface fatigue life

n\,n>-,n;\ — exponents in the suggested formula for predicting subsurface fatigue life

52
Ph,

P*

r

s

Tf

T^f

u

«1,M2

UsW

aA

P

= exponent in the formula for predicting wear rate = contacting load; also used as the load parameter for EHD contact = sculTmg load = maximum Hertzian stress = pressure deviated from Hertzian profile = normalized inclusion tip radius = radius of curvature in the direction of

rolling for bodies 1 and 2 = radius of curvature perpendicular to the

direction of rolling for bodies 1 and 2 = slide to roll ratio = flash temperature or maximum surface temperature — critical temperature = base temperature = rolling velocity = velocity of bodies 1 and 2 = sliding velocity = wear rate = pressure-viscosity = specific film = ambient viscosity = normalized inclusion density = composite surface roughness

10. References

[1] Grubin, A. N., and Vinogradova, I. E., Central Scientific Research Institute for Technology and Mechanical Engineering, Book No. 30 (Moscow); D.S.I.R. No. 337.

[2] Dowson, D., Elastohydrodynamic lubrication: An in- troduction and review of theoretical studies. Lubrication a'nd Wear Group Symposium on EHL, Paper Rl.

[3] McGrew, J., Gu, A., Cheng, H. S., and Murray, F., Elasto- hydrodynamic Lubrication, Phase I, Technical Docu- mentary Report AFAPL-TR-70-27.

[4] Dowson, D. and Higginson, G. R., "The effect of material

properties on the lubrication of elastic rollers,' J. Mech.

Eng. Sci. 2 (3), 188. [5] Archard, J. P., and Cowking, E. W., A simplified treat- ment of elastohydrodynamic lubrication theory for a point contact. Lubrication and Wear Group Symposium on Elastohydrodynamic Lubrication Paper 3 (Inst. Mech. Eng., London).

[6] Cameron, A., and Gohar, R., Theoretical and exjierimental studies of the oil film in lubricated point contact. Pro. R. Soc, London, Series A, Vol. 291, pp. 520-536.

[7] Cheng, H. S., A numerical solution of the elastohydrodynamic film thickness in an elliptical contact, .Journal

of Lubrication Technology, Trans. ASME, Vol. 92,

Series F, No. 1. [8] Greenwood, J., and Kauzlarich, J., Inlet shear heating

in elastohydrodynamic lubrication. Journal of Lubrication Technology, Trans. ASME, Vol. 95, Series F, No. 4, pp. 417-426.

[9] Cheng, H. S., Calculation of Elastohydrodynamic Film

Thickness in High-Speed Rolling and Sliding Contacts, Mechanical Technology Technical Report MTI—

67TR24.

[10] Herrebrugh, K., Solving the incompressible and iso- thermal problem in elastohydrodynamic lubrication through an integral equation, Journal of Lubrication Technology, Trans. ASME, Vol. 90, Series F, No. 1, pp. 262-270.

[11] Cheng, H. S., Isothermal elastohydrodynamic theory for the full range of pressure-viscosity coefficient, Journal

of Lubrication Technology, Trans. ASME, Vol. 94,

Series F, No. 1, pp. 35.

[12] Dyson, A., Naylor, H., and Wilson, A. R., The measure- ment of oil film thickness in elastohydrodynamic

contacts. Lubrication and Wear Group Symposium on Elastohydrodynamic Lu!)rication, Paper 10 (Inst. Mech.

Eng., London).

[13] Sibley, L. B.. and Orcutt, F. K.. Elastohydrodynamic lu- brication of lolling contact surfaces, Am. Soc. Lubr. Eng. 4(2). 234. [11] Gohar, R., Oil film thickness and rolling friction in elastohydrodynamic point contact. Paper No. 70—

Lub-2, Presented in the ASME-ASLE Joint Confer- ence in Cincinnati, Ohio.

[15] Chow, L. S. H., and Cheng, H. S., The effect of surface roughness on the nominal film thickness of elastohy- drodynamic contacts, to appear as a NASA Contractor Report in 1974. [16] Christensen, H., Stochastic models for hydrodynamic

lubrication of rough surfaces, Pro. Inst. Mech. Eng., Tribology Group. Vol. 184, Pt. 1, No. 55, p. 1013. [17] Jacobson, B., On the lubrication of heavily loaded cyl- indrical surfaces considering surfaces deformations and

solidification of the lubricant. Journal of Lubrication Technology, Trans. ASME, Vol. 95, Series F, No. 3. [18] Jacobson, B.. On the lubrication of heavily loaded spherical surfaces considering surface deformations and

solidification of the lubricant, ACTA Polytechnica

Scandinavia, Mech. Eng. Series 54, Stockholm.

[19] Tallian, T. E., Pressure and traction rippling in EHD

contact of rough surfaces, ASME Paper No. 73—Lub18. [20] Harrson, G., and Trachman, E. G., The role of compressional viscoelasticity in the lubrication of rolling contacts, J. of Lubrication Technology, Trans. ASME,

Vol. 95. Series F, No. 4, pp. 306-312.

[21] Johnson, K. L., and Cameron, R., Shear behavior of elastohydrodynamic oil film at high rolling contact pressure. Proceedings of Inst, of Mech. Eng., Vol. 182. p. 307. [22] Dyson, A., Frictional traction and lubricant rheology in elastohydrodynamic lubrication. Philosophical Trans.,

of the Royal Society of London. Vol. 266. No. 1170,

p. 1-33.

[23] Trachman, E. G., and Cheng, H. S., Thermal and non- Newtonian effects on traction in elastohydrodynamic

contacts. Submitted for presentation and publication

at the Second Internat'l Conference on Elastohydrodynamic Lubrication, Leeds, England.

[24] Smith, R.. et. al., Research on EHD Lubrication of High

Speed Rolling-Sliding Contacts, Technical Report

AFAPL-TR-56.

[25] Hammann, W. C. et. al, Synthetic Fluids for High Capacity Traction Drives, ASLE Paper 69-LC-lO.

[26] Allen, C. W., Townsend, D., and Zaretsky, E., Elasto- hydrodynamic lubrication of a spinning hall in a non- conforming groove, J. Lubr. Technol.. Trans. ASME,

Series F, Vol. 92, No. 1. pp. 89-96.

[27] McCool, J. I., et. al.. Influence of EHD Lubrication and

Operation of Turbine Engine Ball Bearings, SKF Technical Report AL73P014.

[28] Bell, J. C, and Dyson, A., The Effect of Some Operating

Factors on the Scuffing of Hardened Steel Discs, Second Symposium on EHL, Leeds. England, Paper Cll/72.

[29] Tallian, T., On competing failure modes in rolling contact, ASLE paper no. 67AMlc-3 presented at the 1967 ASLE Annual Meeting in Toronto, Canada.

[30] Littmann, W., The Mechanism of Contact Fatigue, In- terdisciplinary Approach to the Lubrication of Concentrated Contacts. NASA SP-237, pp. 309-377.

[31] Dawson, P. H., Further experiments on the effect of metallic contact on the pitting of lubricated rolling surfaces. Lubrication and Wear Group Symposium on EHL, Paper No. 7. [32] Tallian, T. E., Sibley, L. B., and Valori, R. R., Elastohydrodynamic Film Effects on The Load-Life Behavior of Rolling Contacts, Paper 65-Lubs-ll, ASME, New

York, N.Y.

[33] Andreason, S. and Lund, T., Ball-bearing endurance

testing considering EHL, Presented at the 1972 EHL

Symposium held at Leeds University, Leeds, England,

Paper no. C36/72.

53
[34] Blok, H., The postulate about the constancy of scoring temperature, NASA Special Publication-237. Inter- disciplinary Approach to the Lubrication of Concentrated Contacts, pp. 153-249.

[35] Blok, H., Surface temperatures under extreme pres- sure conditions, Congr. Mondiale Petrole Assoc., Franc,

des Techniciens du Petrole, Paris. [36] Bennedict, G. H. and Kelley, B. W., Instantaneous coefficients of gear tooth friction. Trans. Amer. Soc. Lubr. Eng. Vol. 4, 59. [37] Peterson, M., Mechanisms of wear, boundary lubrication—an appraisal of world literature, edited by

Ling, Klaus, and Fein, an ASME Publication, pp. 19-

34. [38] Cocks, M., Interaction of sliding metal surfaces, JAP 33. Vol. 7, p. 2152. [39] Tallinn, T. E., McCool, J. I., and Sibley, L. B., Partial elastohydrodynamic lubrication in rolling contact. Proceeding of Institution of Mechanical Engineers, Paper 14, p. 169, London.

[40] Lundberg, G. and Palmgren, A., Dynamic capacity of

rolling bearings, ACTA Polytechnica, Mechanical Engineering Series, Vol. 1, No. 3. [41] Zaretsky, E. V. et al.. Life Adjustment Factors for Ball and Roller Bearings—An Engineering Design Guide. ASME Publication. [42] Tallian, T. E., and McCool, J. I., An engineering model

of Spalling failure in rolling contact—the surface model. Wear 17, pp. 447-461.

[43] Tallian, T. E., An engineering model of spalling fatigue

failure in rolling contact—engineering discussion and

illustrative examples. Wear 17, pp. 463-480.

[44] Archard, J. F., A Review of Wear Studies, NASA SP181, Interdisciplinary Approach to Friction and Wear.

11. Discussion

/. E. Stern, NASA, Goddard Space Flight Center: I am

a little confused. Would you please explain the differ- ence in the mechanism between wear and scuffing?

H. S. Cheng: In scuffing, the surface temperature ex- ceeds the temperature at which the surface film breaks

down. There is no protection of the surface and there

is basically local welding. Material is transferred from

one surface to the other resulting in the junction

growth phenomenon. In the wear mechanism, surface temperatures are not high enough for the surface film

to break down. Asperities can slide over each other and, in so doing, produce wear debris. There is no

continuous propagation of plastic deformation.

A^. Classman, Naval Ship Research & Development

Center: There are possibilities in what you call lubri- cant parameters for chemical effects, particularly in the partial lubrication regime and even in the full film regime. We have data from rolling contact fatigue ex- periments with A factors larger than 3. With small amounts of water added to the lubricant, there are

early fatigue failures. In other work, possibly with A

less than 3, the use of different additives results in different load carrying capacities of gears. A large part

of the reduced gear load carrying capacity which might

be attributed to temperature effects might be due in- stead to the chemical reactivity of additives.

//. S. Cheng: I am aware of the effect of water content and other additives on fatigue life. There has been some work at Battelle which shows that additives have an influence on the film thickness. The actual film thickness is not the same as the film thickness calculated on the basis of the bulk viscosity.

A^. Classman: As I say it's possible. Both physical and

chemical effects may be operative.

H. S. Cheng: Yes.

D. C. Drucker, University of Illinois: I am puzzled

that you added the density of the inclusions to the list of fatigue variables. I would have thought that the size of the inclusions, rather than the density, was more

important. In a line contact problem, for example, the density enters in as related to location. Except for that,

I would have thought the size would be more important.

H. S. Cheng: I agree that the size of the inclusions also plays an important role in contact fatigue, and it should

have been included in the list of fatigue variables.

54
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Failure in Gears and Related Machine Components

R. A. Wilde

Eaton Corporation

26201 Northwestern Highway

Southfield, Michigan 48076

Tooth breakage and abrasive wear are gear failure mechanisms that are well understood. Adhesive wear (scuffing and scouring) and surface pitting are more complex failure mechanisms. When sliding is introduced into the rolling condition the rubbing between partially lubricated

surfaces introduce high surface tensile stresses which result in adhesive wear and surface pitting. Neither of these failures will occur if a continuous oil film separates the metal surfaces. Because these failures are the result of the interaction of many variables, a systems approach

is required to understand the nature of the failure. The following variables have a direct effect on

the thickness of the oil film and therefore on the amount of metal to metal contact: Surface roughness, direction of scratches relative to the sliding direction, oil viscosity, conjunction temperature,

rolling speed, relative sliding speed, oil composition and load (Hertz stress). These variables will have significantly different effects when sliding is introduced to the rolling condition as occurs in gear applications.

Key words: Abrasive wear; adhesive wear; gear failure; machine component failure; mechanical

failure; surface pitting.

1. Introduction

This paper will describe the types of failures that gears might experience. The general principles that are advanced will also apply to clutches, brakes and other components. Tooth breakage and abrasive wear are gear failure mechanisms that are rather well understood so they will be reviewed only briefly. Most of

the discussion will concentrate on failures that are caused by adhesive wear and surface pitting which are more complex failure mechanisms. Neither of these

failure mechanisms will occur if a continuous oil film separates the metal surfaces. The factor that distinguishes this paper from that of Dr. Walter Littmann's is that the two mating surfaces experience substantial relative sliding in addition to rolling.

2. Tooth Breakage

Gear teeth under load act like cantilever beams.

Bending loads on gear teeth usually cause the highest

stresses to occur at root fillets. Whole gear teeth or

substantial portions of the teeth can be broken off as the result of high overloads, often of an impact nature.

Also, fatigue failures can occur as a result of high

cyclic stresses. Gear tooth fractures may be averted by proper consideration of design, metallurgical and machining fac- tors [l,2].i

^ Figures in brackets indicate the literature references at the end of this presentation.

3. Abrasive Wear

Abrasive wear takes place whenever hard, foreign

particles, such as metal grit, metallic oxides and dirt from the environment are present between the rolling

and sliding gear surfaces. These hard particles first penetrate the metal, adhere

temporarily to one surface, and, in turn, tear metallic

particles from the second surface. Abrasive wear nor- mally does not occur in a gear drive unless the lubricat- ing oil becomes contaminated with foreign particles that are larger than 30 micrometres [3].

The best abrasive wear resistance is generally found

in an alloy steel with a structure of uniformly distrib- uted fine carbide in a martensite matrix containing some residual austenite. The bearing alloy 52100 properly heat treated has this type of structure. Residual

austenite is thought to add toughness to the matrix,

provide support for the carbide and be capable of un- dergoing transformation to martensite locally during wear [4].

The abrasive wear resistance of an alloy steel will increase with its carbide content but only if it is finely distributed. A carburized steel with 0.09/1.00

percent surface carbon heat treated properly to obtain

a good distribution of the excess carbides would be a reasonably good structure (figure 1).

4. Scuffing Failure

First, the failures caused by extreme conditions of adhesive wear, which will be called scuffing failures,

55
will be examined. These failures occur under rolling

and sliding conditions when a critical area of adhesive

contact takes place through the oil film which generates

enough heat so that the oil is rolled away from the surface ahead of the contact. This enables the contact area to spread in an uncontrolled manner causing

catastrophic scuffing. The way to prevent adhesive wear

is to prevent all metal to metal contact through the oil film. What are the factors that influence the oil film be- tween the surfaces? To simplify the discussion, we

will consider either gears or test rollers that are rolling

and sliding against one another at load pressures in the range of 50 000 psi to 300 000 psi mean Hertz

stress. In the case of gears at these loads, several things take place. First, both surfaces elastically deform. This

increases the load carrying contact area, and conse- quently the pressure distribution over the contact area

is changed. Second, the viscosity of the oil increases due to the rise of the pressure. Third, the shear rate has a permanent or semi-permanent effect on the vis- cosity of various oils. The higher the shear rate, the lower the apparent viscosity will be. This shear rate increases rapidly with the increase in relative sliding

velocities of the gear faces. Finally, the temperature of

the gear surface is raised by the elastic flexing of the material under load and by asperity contact through

the oil film in heavily loaded gears which causes a temperature rise in the oil and has the effect of lowering

the viscosity [5].

There is another way of looking at two metal sur- faces that are held apart by a film of oil. If any load

is applied the oil film will gradually be squeezed out from between the metal surfaces. The rate at which

the oil is squeezed out is dependent on a number of

factors. The higher the load the quicker the oil is squeezed out. The lower the oil viscosity the quicker

the oil is squeezed out. The higher the temperature of the oil the lower the viscosity and the quicker the oil

is squeezed out. The shorter the distance the oil has

to travel, i.e., the smaller the area of contact, the quicker the oil is squeezed out. The oil viscosity is increased somewhat by load and decreased by the shearing action in the oil as the relative sliding velocity

is increased. As two rollers roll against each other the

oil is wedged into the contact. The higher the speed the more oil is wedged between the rollers and the less time there is to squeeze the oil out. So far we can translate these statements into these relationships: Resistance to adhesive wear and scuffing

is increased by:

Higher oil viscosity

Higher roller or gear speeds

Lower operating temperatures

Lower unit load on surface Lower relative sliding velocity

Larger areas of contact

This brings us to the Critical Temperature Equation

of Blok. The Critical Temperature Theory proposes

that every oil (i.e., oil mixture) has a temperature

beyond which scuffing will occur. One of the basic problems of trying to develop mathematical formulas

56
for predicting scuffing is that scuffing initiates on a microscopic scale and evidence is being developed on

the basis of macroscopic or bulk measurements.

This critical temperature is the sum of two temperatures as follows:

T,r = Tr + AT,,. where Tc- = Temperature at surface for scuffing

Tr = Bulk roller (gear) temperature

AT = Momentary rise in temperature be- tween rollers (gears) from frictional heating (from Blok equation).

This rise in temperature is calculated as follows:

AT

where / We

V2

a

b

0.79 fWe (VFi - VF2)

a\/b

Coefficient of friction

Equivalent unit load, lb/in

Surface speed of upper roller, in/s

Surface speed of lower roller, in/s = A material constant, lb/in ^ s

-l/2„

Semiwidth of contact ellipse (minor

axis) , in a = Block thermal coefficient = (kpc)i/2 = lb/°F in si/2

k = thermal conductivity

p = density

c = specific heat

This equation shows that AT is increased and therefore

scuffing is adversely affected by an increase in sliding

velocity, load, and coefficient of friction. The coefficient of friction represents the energy losses when the two

surfaces are brought close together. The major loss would be adhesion and separation of asperity peaks

between the two surfaces. Minor losses would be from

resistance^ to oil shear and overcoming other attractive forces between the metal surfaces of a noncontact nature.The major deficiency of the critical temperature hypothesis is that it does not consider surface roughness,

which has a large effect on Tc- Some data developed at the Southwest Research Institute will give some idea

of the magnitude of this effect. The tests were run in a Caterpillar roller test rig using 3 in diam rollers with

a 14 in crown radius and phasing gears were used to develop the desired amount of relative sliding. The

rollers were ground in the circumferential direction. The surface roughness was measured in the circumferential and transverse (axial) directions and aver- aged together to obtain the composite roughness, in /xin CLA.

Figure 2 shows typical surfaces as ground and after

break-in. At this higher surface finish there was little difference in the surface roughness number. Figure 3 shows another roller with a rougher surface which was

substantially improved by break-in (composite 12.5 to 9.5-25 per cent improvement). During break-in the projecting asperities on the surface are worn away as the rolls are step loaded to a high preload level. It is obvious that the broken-in surface at 13.0/ 6.0 /xin is different than a newly ground surface of the equivalent

surface roughness. The pointed asperities have become

flats and the volume of the valleys into which the oil can be squeezed is considerably reduced.

Figure 2. Scanning electron micrographs of ground roller XX173L.

(a) As ground—X 540. Surface finish in tranverse and circumferential directions 5.0/4.5 RMS.

(b) As ground— X 1620.

(c) As ground plus break-in—X 540. Surface in transverse and circumferential directions 4.5/4.0 RMS.

(d) As ground plus break-in—X 1620.

The surface roughness in the range of 4 to 30 has a

significant effect on the critical temperature Tc,-. An

increase of 1 jxm S; reduces the critical temperature

by 5 °F. In the program S, varied by 12 uin so this

effect could change the Tcy by 60 °F. An increase of

1 jj.\r\ in 8/ reduces the critical temperature by 20 °F.

In the program, S/ varied by 8 /xin so this effect could

change the Tc- by 160 °F, which is substantial. The purpose of this example was to show that surface roughness of gears and rollers or other devices will have a significant effect on the load at which scuffing occurs. The direction of the grinding marks also has a big

effect on loads to failure. SWRI had some rollers ground

transverse to the direction of sliding and rolling and

57
r--

"

Figure 3. Scanning electron micrographs of ground roller XX95U.

(a) As ground—X 160. Surface finish in iranverse and circumferential di- rections 17.5/7.5 RMS.

(b) As ground— X 530.

(c) As ground plus break-in—X 160. Surface finish in transverse and cir- cumferential directions 13.0/6.0 RMS.

(d) As ground plus break-in—X 530.

10 RMS

Roller Surfaces

T Ground transverse to sliding

30

RMS

direction

Rolling and sliding direction

30 RMS

—^10 RMS ^ ^

f Ground circumferentially

Rolling and sliding direction

Figure 4. Wear scars in rollers ground in transverse and cir- cumferential directions.

loads to failure were compared with rollers ground cir- cumferentially (fig. 4). The rollers were run at a high

load with no break-in. The rollers ground circumferentially failed immediately with long circumferential

scuffing scars. The rollers ground in the transverse direction de- veloped wear scars, transverse to the rolling direction,

at substantial loads. The interrupted nature of the sur- face prevented the wear scar from spreading in a

catastrophic manner in the rolling and sliding direction. This shows that the composite surface roughness values.

which in this experiment would be the same, are

not enough to describe the critical surface condition. This also suggests that cutting gears transverse to the

rolling and sliding direction is an aid in resisting

scuffing but was not done for this reason, and that the

spline machine marks are in the wrong direction to minimize scuffing during axial movement.

In the case of heavy-duty gears such as truck ring

and pinion gears in the differential, the loads are in the range of 250 000/300 000 psi Hertz stress which

put the gears into boundary lubrication, i.e., inter- mittent metal to metal contact is occurring. To prevent

scuffing, additives such as sulphur and phosphorus are added to the oil. At the temperature range that the gears will see in service (250/300 °F), these additives

react chemically with the steel surface to form sulphides

and phosphides which prevent the two gear surfaces from adhering to each other. This allows the differential

to operate successfully in the boundary lubrication re- gime.

Another way of minimizing metal adhesion in heavyduty gearing operating in the boundary lubrication regime is to introduce some ammonia (5 cfm) in the

last stages of carburizing, especially in those cases where the gears are cooled to 1575/1600 °F before quenching. The steel surface will accept nitrogen readily

at these temperatures and the concentration of iron

nitrides in the surface will significantly reduce ad- hesion.

Scuffing type failures can be prevented by the follow- ing means:

(1) Reduce the conjunction temperature.

(2) Use as smooth surfaces as practical. If there are machining marks, have them perpendicular to the rolling and sliding direction. (3) Use lots of oil—direct it on the most heavily

loaded areas. (4) Cool the oil. (5) In those cases where the gears are operating

in the boundary lubrication regime, use E.P.

additives in the oil such as sulphur and phosphorus to form chemical films on the surface.

5. Surface Pitting

How does sliding affect the surface stresses and how

does this influence failure by surface pitting (fig. 5) ? What happens in the case of gears has been simulated

by means of two rollers, by numerous investigators, in which the rollers are rolling together in the direction

as shown by the arrows, but not at the same peripheral

speed. The relative motion of the rollers is such that the top roller (the faster one) appears to be sliding in the direction of rolling (positive sliding) and the lower

roller (the slower one) appears to be sliding in the opposite direction of rolling (negative sliding). In

the case where the rollers are forced hard together

radially so metal to metal contact can occur, tangential

frictional forces are developed at the surface which

are tension stresses in the areas marked t—t and compression stresses in the areas marked c—c. This will

58
occur under conditions of boundary lubrication where

there is intermittent metal to metal contact. When there

is no lubrication, adhesion would occur and catastrophic

failure would follow quickly.

Simulation of Gear Tootli Sliding by Two

Rotating Cylinders with Different Peripheral

Speeds

Friction of Sliding (Associated with Adhesion)

Introduces Surface Tensile Stresses Which

Lead to Pitting.

Figure 5. Stresses developed at the surfaces of rollers sliding and rolling together.

Figure 6 is an enlarged view of what is taking place on the surface. In this particular case, the oil is shown

being fed in advance of the rolling point. Any minute

cracks or imperfections on the surface on the lower

roller would be opened up and correspondingly closed up on the upper roller. The oil would, of course, then be forced into these cracks and raised to very high

pressures by its passage under the point of contact, with

resulting progressive fatigue failure which would finally

lift small pieces out of the surface. Observations show

that the pits invariably form on the roller where the

friction of the sliding would produce tensile stresses (negative sliding) and do not form on the roller where

the sliding would produce compressive stresses. This is so marked that even when the upper roller, the high

speed one which is in positive sliding, is made one half

the hardness of the lower roller, the upper roller de- velops no pits [6].

Enlarged View of Roller Surfaces

Rolling and Sliding Together

Motion

Figure 6.

Right away this would suggest that, if the surface could be put in compression to such an extent as to more than offset the tensile stresses which would be

caused by the sliding friction, then it might be possi- ble to preclude the formation of this type of pitting on

gear teeth.

What happens in the case of a gear set? On a driving- tooth face (fig. 7) the point of contact moves outwards,

the tooth rolling from the root towards the tip at an

increasing speed. On a driven-tooth face, the point of

Rolling and Sliding Directions

for Gear Teeth

Increasing Decreasing

Speed Speed Dedendum

— Direction of Rolling

Figure 7. Action and stresses on surface of gear teeth.

contact moves inwards from the tip to the root at a decreasing speed. The sliding velocity, i.e., the tan- gential velocity between the teeth, is zero at the pitch

circle; on a driving-tooth face the sliding direction is always away from the pitch line; on a driven-tooth face

the sliding direction is towards the pitch line. The con- sequence is that on the addendum, the rolling and

sliding motions have the same direction; on the dedendum, the directions of rolling and sliding motion are opposed to each other. This would suggest that the place

pitting failures would initiate on gear teeth is just on

the dedendum side of the pitch line. Figure 8 shows

a typical V-shaped surface pit on a gear tooth and a

cross-sectional view (fig. 9) shows that it progresses under the surface at an oblique angle in the direction

of rolling. In other words, the V-shaped surface pit

points in the direction of sliding and in the opposite

direction to the rolling direction. A typical surface pit on a test roller which failed in an industry sponsored

program at IIT Research Institute is shown in figure

10. Figures 11, 12, and 13 show how the pits develop on the surface of a roller sample in a program at IITRI. The next thing to consider are the factors that affect the development of the surface pits. The results from an

industry sponsored program conducted at IITRI entitled "Surface Fatigue of Gear Steels" will be dis- cussed. The purpose of the investigation was to determine the effect of metallurgical treatments and

processing of gears on the surface durability of car- burized and hardened gear steels under the high contact loads. The pitting fatigue tests were carried out on

cylindrical specimens in laboratory fatigue machines of the Caterpillar design. The geometry of the contacting

specimen surfaces and the testing parameters were carefully designed to simulate conditions existing between

meshing involute gear teeth, namely, loads of 275 ksi

to 420 ksi and a relative sliding velocity of 24 in/s.

59
Figure 8. V-shaped surface pit on gear tooth showing angle of crack propagation.

This figure reduced 58 percent in reproduction.

Figure 9. Micrograph of typical cracks originating at surface of gear teeth.

60
^'k^ '^'^-^'^'j^'^- ^,^;k^^ C'*v3C'^*'^

•1 , . .fr

(

if 4, y,*

' / S

1/25 in.-

^ Direction of Rotation and Sliding

Direction of Rolling

^

^ Direction of Rotation and Sliding

Direction of Rolling ^

Figure 11. Micrograph of surface of test specimen no. 209,

13.6 megacycles at 375 ksi, shoiving incipient development of surface macro-pit. Apex of pit, at bottom of photo, encircled by crack. X 40.

Figure 13. Micrograph of surface of test specimen no. 209,

13.6 megacycles at 375 ksi, showing enlargement of damage

area in the development of a macro-pit. X 40.

61
The results can be summarized as follows: No Significant Effect

(1) Grade of Steel No significant differences in pitting fatigue

durability have been found among seven dif- ferent grades of alloy steel commonly used for gearing. These results suggest that the chemical composition of the steel probably does not have a strong effect on pitting fatigue pro- vided that the surface microstructure of the

steel is free from soft non-martensitic trans- formation products.

(2) Heat Treatment

Contact fatigue tests conducted on a number

of different steels hardened after carburizing

by direct quenching and by reheat treatment

revealed little, if any, difference in pitting resistance for specimens heat treated by both

methods.

(3) Retained Austenite

Widely varying amounts of retained austenite

in the range of 5 percent to 60 percent have

shown no difference in pitting fatigue properties. One of the problems associated with the

testing of specimens having large amounts of retained austenite is the plastic deformation

that occurs on the surface due to the rela- tively soft microstructure.

(4) Excess Carbides

In several steels that were carburized to quite

high carbon levels there was a tendency for massive carbide formation at the carburizing

temperature or during furnace cooling from

1700 to the quenching temperature of 1550 °F. These colonies of carbides persisted

during direct quenching and reheat treatment, but there was no strong evidence in the test results to show that they influenced the pitting

fatigue resistance.

Significant Effect

(1) Carbon Content of the Carburized Case The pitting resistance of 8620 steel increases

significantly when the surface carbon content

is raised from 0.7 percent to about 0.95 percent, but no further gain in fatigue properties

is achieved with higher carbon contents. This

increase is thought to be due to two factors: (1) the hardenability effect of carbon in minimizing pearlite transformation in the surface, and (2) a strengthening of the

martensitic-austenitic matrix with increasing carbon.

(2) Hardness

The pitting resistance of carburized steels

falls off rapidly below 58 R,.

(3) Oxidized Grain Boundaries

The pitting resistance of the as-heat-treated surfaces containing grain boundary oxides was significantly higher than surfaces with the

surface layer ground off (fig. 14). This effect has been a baffling condition and seems to defy explanation. However, the following ex- planation is proposed. Surfaces with grain

boundary oxides tend to fail in a manner

shown in figures 11, 12 and 13. The shallow

surface cracks tend to be oriented transverse

to the rolling and sliding direction and there- fore act as little oil reserves which provide more lubricant to the surface than if the surface were smooth or ground in the circumferential direction.

• 1.27. C, Direct Quench

Depth Below Surface, Mils

Figure 14. Fatigue life as a function of the amount of material removed from the surface before testing.

Carburized SAE 8620 steel tested at 347 ksi.

As happens quite frequently in laboratory work, the variables that start out being the most suspected in influencing pitting resistance exert minor effects and

other variables that weren't the main object of the investigation have substantial influence on the results. Such was the case for the lubricant and surface roughness. In each case they were supposed to be held constant and therefore they should not influence the

results.

Lubricant

Although lubrication was not intended to be a variable in this study, it inadvertently became one through

the process of aging of the oil. The lubricant used for the tests was an SAE 80 containing sulfur and phosphorus E. P. additives. If a batch of this oil was used

for a long period of time, it was noted that the speci62
mens tested in the oil experienced a gradual but signifi- cant increase in pitting fatigue life. The reason for this increase in fatigue life with aging of the oil must be

associated with a change in the concentration of the

E. P. agents.

Other tests run on the same base stock with and with- out the E. P. additive showed significant interactions be- tween the type of oil and the surface of the test speci- men, and significantly influenced fatigue life. This

work, together with that of other investigators, has

demonstrated the importance of the role of lubricants

in pitting fatigue. There appears to be a stress corrosion eff^ect acting at the bottom of the crack.

6. Surface Roughness

During the progress of this investigation the exact technique for preparing the samples was inadvertently

changed because it was necessary to obtain the rollers from a different machining source. The changes in sur- face finish that occurred caused the results to be radically altered. This led to specific experiments to determine the effect of surface finish on surface pitting.

The surface finishes of the rollers were as follows:

Test Roller—1 in diameter

(1) As heat treated 10 ^uin RMS

(2) Ground (2 mils removed) 10/xin RMS (fig. 16)

(3) Ground and Polished—2 /xin RMS—Axially

(fig. 17)

(4) Ground and Honed—2 ixin RMS—Circumferentially

Load Roller—5 in diameter

A. Ground—10/15 /lin RMS

B. Polished—5/9 iJiin RMS

Lubricants

EP Oil—8 percent SP Additives

Proficorder traces of the surface of the best rollers are shown in figure 15. The data are summarized in figure

18.Curves (1) and (2) are for the polished load roller and the load to pitting is substantially higher than in the case of the ground load roller, curves (4) and (5).

. 1,/ " '"~

' > 1

Ii 'i /

'1

1 'i 1

'J,

Figure 15. Proficorder traces shoiving surface roughness of polished, honed, ground, and as-heat treated specimens.

63
c

Figure 16. SEM of ground specimen at (a) X 515, (b) X 1030, and (c) X 2575.

The surface condition of the test roller has a lesser

effect on load to pitting. In the case of the ground load

rollers, (4) and (5), there is practically no difference due to surface finish of the test roller. In the case of the polished load roller, (1) and (2), the surface finish of

the test roller has an effect—curve (2) for the ground

test roller is appreciably below that of the polished test roller, curve (1). When the roughness of the loading

roller (positive slip element) was reduced from about

15 /xin to about 8^in RMS and the test specimen

(negative slip element) was reduced from about 10 /xin to 2 /xin RMS, there was a gain of approximately 150

percent in load-carrying capacity.

From this figure it can be concluded that the roughness of the surfaces, and particularly the surface of the load roller which is in positive sliding, has a dominant

effect on pitting fatigue.

Specimens with as-heat-treated surfaces, curve (3),

have significantly better pitting resistance than ground

or polished test specimens when ground load rollers are used in the tests; however, they are not as good as specimens tested against polished rollers. The reason for these differences in service life are

quite apparent in figure 19. These show three combinations of surface finish on the test specimen and load

roller after 100 000 cycles at 363 ksi in non EP oil. The ground roller (fig. 19a) against the polished test specimen showed a very distinct contact track and evi- dence of heavy metallic contact between the two rollers. The polished roller (fig. 19b) against the ground test specimen showed a very faint but full contact track with

considerably less severe metallic contact. The polished

b

Figure 17. SEM of polished specimen at (a) X 450 and

(b) X 1350.

roller (fig. 19c) against the polished test specimen

showed only a random array of small contact track. In figure 20a at lOOX the contact track is quite

smooth as the result of the polishing action of the ground roller eliminating all transverse polishing marks

but there are many transverse surface cracks even after a short testing period of 100 000 cycles. This roller failed at 71/2 million cycles. (See fig. 20b.) At lOOX

very little polishing has occurred from the polished load

roller and only a few, very small, cracks are present

(fig. 20c). At lOOX the transverse polishing lines are

still present and the random spots of metallic contact seem to have occurred between high points on the load

roller and the tops of the polishing ridges in the test specimen. Total life for this roller was 32 million cycles

with no pitting.

This series of tests can be summarized as follows: Surface finish measurements show that the change in surface finish of the specimen, which is an indication of the amount of metallic contact, is a function of the roughness of the loading roller (the surface in positive

64
457

(T) Polished Specimens Vs. Polished Rollers © Ground © As Heat Treated " " Ground ©Ground " " "

" © Polished

No. Cycles

Figure 18. Composite of S—N curves for the various surface

finish combinations for tests conducted in the E. P. lubricant.

Figure 19. Illustration of severity of contact between various specimen-roller combinations after 10^ cycles of fatigue at 363 ksi in non-E. P. oil.

(a) Polished specimen against ground roller. (b) Ground specimen against polished roller. (c) Polished specimen against polished roller.

- ----

-M2S

Figure 20. Higher magnification illustrations of severity of damage in contact tracks for same combinations shown in figure 19.

(a) Polished specimen against ground roller, X 60. (b) Ground specimen against polished roller. X 60. (c) Polished specimen against polished roller. X 30. (d) Polished specimen against polished roller. X 60.

sliding) . When a rough loading roller is used, an initi- ally rough test specimen will tend to become smoother

during the test and an initially smooth test specimen

will become rougher, and both will approach about the same degree of surface roughness before failure occurs. When a smoother loading roller is used, there is only a

small change in the surface roughness of a ground test specimen and practically none in a polished specimen.

This program has been continued at I IT Research

Institute with a change in direction under the sponsorship of the ASME Research Committee on Lubrication.

While quantitative information is not available at this time, a general indication of the results can be given

(1) Fatigue life is lowered by an increase in the amount of slip between the rollers. (2) Additives in the oil will lower the fatigue life. This has been shown in other investigations and

tends to suggest that there is a stress corrosion

effect at the root of the surface cracks which

would increase the rate of crack growth.

(3) Polished surfaces have better fatigue lives than

ground surfaces. (4) High viscosity oils and higher roller speeds tend

to improve fatigue life. This is a reflection of

better lubrication between the rollers. As the speed of the rollers is increased there is less time to squeeze out the oil between the rollers and as the viscosity of the oil increases, its re- sistance to being squeezed out increases. (5) Surface temperature adversely affects fatigue

life—this reduces oil viscosity and enables it to be squeezed out faster.

65
7. References

[1] Wulpi, Donald J., How components fail, Am. Soc. Met.,

p. 48-51.

[2] Shipley, Eugene E., Failure modes in gears, p. 107, Gear

Manufacture and Performance, Materials/Metal Working Technology Series, ASM.

[3] Lorick, Robert R., Lubricating gears. Machine Design,

July 9, 1970, p. 108. [4] Hurricks, P. L., Some metallurgical factors controlling the adhesive and abrasive wear resistance of steels (A Review), Swansea Tribology Entr., G. B. Wear Volume

26, No. 3, (December, 1973), p. 285.

[5] Ku, P. M., Private Communication, Southwest Research

Institute, San Antonio, Texas.

[6] Buckingham, Earle, Prof. M. E., M.I.T., Surface fatigue

of ..plastic materials. Trans, of ASME, May, 1944, p. 297-310, Discussion by G. B. Warren, Design Eng., Turbine Engine., Div. G.E.

[7] Sheehan, J. P., and Homes, M. A. H., The effect of case carbon content and heat treatment on the pitting fatigue of 8620 Steel, SAE Preprint No. 720268.

8. Discussion

A. J. Babecki, NASA, Goddard Space Flight Center: I think you stated that in your roller tests where you had

differential speeds between 2 rollers you found that the one that moved the slower was the one that got the pits. And you said that hardness didn't seem to make any

difference if the roller was in compression. Yet, later on in your talk you said that hardness is one of the sig- nificant factors.

R. A. Wilde: Yes, but that would be only for the one of negative sliding.

R. Lenich, Caterpillar Tractor Company: Do you have

any comments on a solid additive like molybdenum di- sulfide on scuffing?

R. A. Wilde: No, I have never done any work in that

area. Have you had any kind of experience with it?

R. Lenich: We have done some work on oscillating

shafts, but little on gears.

A. Beerbower, Esso Research & Engineering Company:

In reference to the smoothness of the roller surfaces, you

said that too smooth gets rougher and too rough gets smoother. I had postulated this a couple of years ago. Is there any way of predicting what the equilibrium will be? It's well known that used oils have populations of particles that are 50 to 100 times the diameter, or at

least as big in diameter, as the gaps that you are talking

about, assuming that you are talking 1 to 1 film thick- ness to roughness. What happens when one of these

particles goes through the gap? In other words, have

you any indication as to the effect of oil cleanliness?

R. A. Wilde: In all of our experiments we very carefully filtered out all of the particles possible so that these would not become part of the problem. If there were

large metal particles going through the gap, it would be

the same as having high asperities on either side and it would lead to adhesive wear.

/. E. Stern, NASA, Goddard Space Flight Center: Has

any attempt been made to determine the effect of the concentration of the additive in the oil on the degree of

pitting of gears and rollers? In a r,ecent study of the

effects of additives in oils on bearings, it was found that there was an optimum additive concentration for a particular case.

R. A. Wilde: We have done some work in this area, but

all we have is general quantitative data. We don't have

good data in that area.

C. E. Vest, NASA, Goddard Space Flight Center: Were

all your components carburized?

R. A. Wilde: Yes, they were carburized by very careful practice because we recognized this to be very important.

C. E. Vest: What did the microstructure look like when

the ammonia was added to the carburized structure?

R. A. Wilde: Usually there isn't enough added to pro- duce a real strong iron nitride structure. The purpose of

this is to prevent deterioration of the surface as a result of grain boundary oxidation and the reduction in hardenability of the surface. There will be a very minor iron

nitride structure at the surface.

C. E. Vest: Is the structure changed when the 0.002

inch was machined off?

R. A. Wilde: No, the 0.002 inch was taken off simply

to get below the grain boundary oxides and the lower

products of transformation that might be at the surface.

V. C. Westcott, Trans-Sonics, Inc.: The particles from

gear wear that we are seeing on Ferrograms seem to confirm the mechanisms that you have described here. In particular the particles tend to be chunky and have

dimensions of thickness, width, and length, which are comparable. Have you photographed any micropits on

the surfaces of these gears at a magnification of the or- der of 1000 times?

R. A. Wilde: No.

/. V. Deller, Naval Ship Engineering Center: Mr. Beer- bower commented on the surface roughness of the test roller and the load roller. Can you elaborate on that?

R. A. Wilde: Apparently the load roller that's in positive

sliding acts much more heavily on the test roller itself because the load roller is in positive sliding and the test roller is in negative sliding. You tend to take more sur- face off the test roller by the action of the load roller. The load roller itself does not seem to be changed much.

Apparently this is the nature of the relationship between

positive sliding and negative sliding. The positive slid- ing surface tends to retain its surface and the one in negative sliding is either smoothed out a little bit if it's real rough, or is roughed up if it's real smooth to start with.

66
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Bearing Damage Analysis

R. L. Widner and W. E. Littmann

The Timken Company

Canton, Ohio 48076

Failure analysis of machine components is described with the major emphasis on tapered roller bearings and their interactions with other components of a system. Included in the discussion are the effects of loading, deflections, misalignment, lubrication, temperature, debris, water and electrical current on the type of damage and life of tapered roller bearings.

Key words: Bearing failure; bearing lubrication; bearing loading; bearing misalignment; bearing

temperature ; mechanical failure.

1. Introduction

Most mechanical systems have a complex combination of components that transmit forces across concentrated contacts. These contacts vary from hydrodynamic

film junctions to metal to metal junctions and from

essentially pure rolling to pure sliding motion. Such

components are journal bearings, gears, splines, rolling

element bearings, clutches, cams, and seals. As a result of repeated loads of varying magnitude

in service, high stresses in these concentrated contacts give rise to changes in the contact surfaces and in the region just beneath the surface. When these changes

are of sufficient magnitude as to adversely influence the subsequent performance of the contact surfaces, one

says they are damaged. When the damage is sufficient to threaten the essential function of the components, one

says they are failed. Continued operation would threaten

the performance of the entire mechanical system.

2. Control of Bearing Life

In a given system the ultimate life of a rolling ele- ment bearing is determined by factors controlled by

(1) the manufacturer of the bearing, by (2) the de- signer and builder of the machine in which the bearing

is applied, and (3) the user of the finished machine.

Knowledge of these factors can be coupled with an ef- fective method of bearing damage analysis to identify

and reduce some of the causes for mechanical system

failure.

3. Role of the Manufacturer

The manufacturer has the control of the internal design, materials, processing and inspection used in making the bearings. He also establishes load-life ratings

for each bearing and the revelant instructions and specifications for application, assembly, and adjustment of bearings.

3.1. Internal Design

The load capacity, torque, heat generation, and vibration characteristics of a given bearing are characteristics largely controlled by the producer in the design

and manufacture of bearings. The internal and external geometry of bearing components, the materials from

which they are made, and surface integrity of critical surfaces are the essential elements used by the producer

to control the performance capability of bearings.

3.2. Material

Material requirements for rolling element bearings

have been comprehensively described in relation to usual and unusual operating environments. They are

briefly summarized as follows:

( 1 ) Maximum hardness at the operating temperature

consistent with the fracture toughness required

for a given design and application environment.

(2) Corrosion resistance sufficient to maintain sur- face integrity under prevailing environmental

conditions.

(3) A low level of integral discontinuities such as nonmetallic inclusions, porosity, or microcracks.

These requirements can be met by a variety of alloy

steels of sufficient carbon content and hardenability to develop needed properties with appropriate processing.

Two major types of steels are used, low carbon bearing

steels which are case carburized and hardened or high

carbon bearing steels which are through hardened or

case hardened by surface heating methods. Bearings to be applied in high temperature environments require

materials that retain their hardness upon sustained ex- posure to expected temperatures. A summary of suit- able steels is given in reference [1].^

^ Figures in brackets indicate the literature references at the end of this presentation.

67
3.3. Processing

Bearing quality steels may be manufactured by sev- eral steel making processes as summarized in table 1. The frequency-severity distribution of oxide inclusions

resulting from the method of steel making used to pro- duce the bearing steel largely governs the load capacity

of bearing life as limited by the inclusion origin mode

of contact fatigue.

Table 1. Processing methods for manufacture of bearings

Melting and Casting

Electric Furnace Melting and Casting Consumable Electrode Vacuum Melting and Casting

Electroslag Remelting and Casting

Plastic Deformation Hot Rolling of Bars and Billets Hot Piercing, Rolling, or Extension of Seamless Tubing Hot Forging of Rings and Rolling Elements

Cold Forging of Rings and Rolling Elements Rough Machining

Conventional Chip Removal Machinery

Heat Treatment

Carburizing, Hardening, Tempering (High Carbon Steel) Hardening, Tempering (High Carbon Steel) Flame or Induction Surface Hardening (High Carbon Steel)

Finish Machining

Grinding

Honing

Polishing

Also, table 1 shows the various ways in which the

steel composition and processing can be varied to pro- duce the required material characteristics in the fin- ished bearing. This summary describes the methods

which are most widely used for processing of bearings.

A comprehensive summary of the influence of materials and processing is given in references [1] and [2].

3.4. Inspection

Inspection assures that parts meet the design, material, and processing specifications.

3.5 Establishment of Load-Life Rating

Using the relation of bearing internal geometry and

load to contact stresses, a load rating and methods for computing expected fatigue life are supplied by the bearing manufacturer. The factor representing material

fatigue strength is determined from bearing life tests under simulated service conditions. For one tapered roller bearing producer [3], an audit

sample of bearings from world-wide operations are life tested under a set of reference operating conditions to monitor the life rating of "standard" bearings. Additional testing has been used to determine the influence

of various environmental factors, such as temperature,

speed, misalignment [4], lubricant chemisty, lubricant

film thickness [5, 6, 7, and 8], etc., on bearing life. Adjustment factors derived from such tests can be used

to modify the expected life in applications that are known to place bearings in an environment significantly different from the reference operating conditions.

4. The Role of the Designer and Builder

A bearing is just one of many components that a machine designer and builder incorporates into a useful piece of equipment. Therefore, the manner in which a bearing is applied to a machine application and its in- teraction with other components become significant factors which influence the life of the bearing. Important considerations for the designer are the load, speed,

misalignment, and/or deflection under service loads,

bearing adjustment, heat generation [9] and heat flow within the system, the type of lubrication system [10], lubricant composition, and ambient temperature. In addition, he must consider stray electrical current, and

related requirements for shunting, debris, and the filter system to control debris recirculation, water entry, and

sealing problems.

If the designer can accurately predict the items above

with the help of the bearing application engineer, then

there can be greater confidence in the estimate of the bearing life in a given system.

The builder must consider correct bearing housing

alignment and the accurate machining and/or grinding

of the shafts and housings for proper shapes and fits. Before assembly, adequate cleaning is needed to assure removal of machining chips, abrasive debris from sand

castings and all foreign materials which could contaminate the lubricant. Finally, the careful assembly of bearings with the other components must be made to avoid damage to critical surfaces and assure proper bearing adjustment settings [11, 12]. It is important

that the designer and the builder jointly understand

how the cold setting of the bearing lateral (internal

clearance) is affected by heat generation in the system

during start-up and the run-in of gears and bearings.

The internal load distribution among the rollers in a ta- pered roller bearing is adversely affected by excessive

lateral. On the other hand, heat generation and contact

stresses at raceway and rib-roller end contacts increase rapidly if lateral goes to zero and into a preload condition. This is critical for new bearings and at start-up when heat flow has not established thermal equilibrium

in the system.

5. The Role of the User

If the manufacturer, designer, and builder have done

their work correctly, then the life of the individual components and the machine will be as-predicted unless something is altered by the user. The user can modify

the expected life of the bearings in a given system by:

(1) loads and speeds beyond the design ranges, (2)

improper maintenance, or (3) by using the machine in an environment in which it was not designed to operate.

Machine life and component life are statistical in nature. The contact fatigue life of individual bearings

has been found to be well represented by a Weibull dis- tribution. Early incidence of fatigue damage is usually

68
the result of a bearing from the portion of the population having the lowest intrinsic fatigue life being coin- cident with problems in machine design, building,

and/or use. In other words, the lowest life of a machine

or component is usually the result of a chance coinci- dence of factors which were caused inadvertently in the manufacture of the component, by the designer or builder of the machine, and by the user of the machine.

6. Modes of Damage

Whenever forces are transmitted across concentrated

contacts as between the elements of a tapered roller bearing there will be an eventual breakdown of the sur- faces by wear, fatigue, or some other process depending

on the load, speed, temperature, lubrication, and other environmental factors. Over the years, there have been

several papers [13, 14, 15, 16, 17, 18, 19, and 20] deal- ing with the classification of the various types of damage that have been observed. The classification schemes

have been based on (1) location of the damage (surface

or subsurface), (2) type of stress concentration (sur- face flaws or nonmetallic inclusions), (3) type of con- taminant (debris or water in the lubricant), (4) type

of propagation (hydraulic pressure or transverse crack- ing) , etc. The damage classification scheme in table 2 is pro- posed. The scheme varies from that previously used

[18] as a result of discussion at a Wear Mode Terminology Meeting in conjunction with an oil analysis

program, currently underway [21].

Table 2. Damage classification Wear

Adhesive Wear

"Normal," Mild, Smooth Wear

Scuffing, Scoring, Smearing, Galling, Seizure

Abrasive Wear

Corrosion Wear

Fretting Damage

Plastic Flow

Debris Denting

Brinelling

Burn-up

Electrical Damage

Fluting

Arcing

Fatigue

Contact Fatigue—Subsurface Origin

Inclusion Origin Geometric Stress Concentration Subcase Fatigure

Contact Fatigue—Surface Origin

Point Surface Origin

Microspalling (Peeling)

Section Fracture

A classification scheme with pictorial examples of the various types of damage is the most important tool

at the disposal of someone engaged in bearing damage

analysis. It is valuable because it provides a ready ref- erence to past experience and it enables the investigator

to communicate eff^ectively with those who will use the

results of his investigation.

The remainder of this paper will be concerned with

a description of the various modes of bearing damage

using examples of these modes from tapered roller bearings from service and laboratory applications. Although

the examples given are specific to tapered roller bearings, most of them are applicable to other types of roll- ing contact bearings, gears and cams.

7. Wear

Although contact fatigue, especially surface fatigue,

is considered by some to be a form of wear [22], the authors choose to separate the contact fatigue and wear modes of damage. There are two basic mechanisms of wear [22] in rolling element bearings: adhesive wear and abrasive wear. Corrosive wear also may be found

under some circumstances in chemically aggressive en- vironments. Cavitation wear has been hypothesized as a possible mechanism for some rolling contact surface damage. As with the competitive modes of contact fa- tigue, any one or all of them may be acting in a given

application. The cage (retainer), roller ends, and the large end rib (thrust rib) of a tapered roller bearing

are the sites of most wear in tapered roller bearings.

However, wear can and does occur on the rolling contact surfaces as well. The small amounts of wear nor- mally encountered do not interfere with bearing

operation. The amount of wear that can be tolerated in a given bearing is dependent upon the particular application. Excessive adhesive wear is an indication of in- adequate lubrication. Abrasive wear occurs when an

abrasive material contaminates the lubricant.

7.1. Adhesive Wear

The most serious adhesive wear damage in tapered

roller bearings has been called anything from scuffing,

scoring, seizing, to galling. The terms used for adhesive wear are somewhat indicative of the degree of damage.

Scuffing is least severe; scoring is a similar stage of damage often applied to gears; seizure and galling in- dicate more extensive plastic flow and metal transfer. Real surface contact is limited to asperity interactions under effective lubrication. When operating conditions

and surface characteristics permit metal to metal contact at asperity contacts, a welded junction is formed.

With relative motion between the contacting surfaces,

junction growth occurs by plastic deformation. Fracture

ultimately takes place within one of the two surfaces or

at the original interface. The resulting metal transfer and plastic deformations produce loose wear debris and

modify the original surface texture and microstructure

of the contact surfaces. At high loads and speeds, the

local temperature rise can be sufficient to cause tempering, rehardening and melting of the surface. Mechanical

69
activation, together with the high local temperatures, cause decomposition of the lubricant. Chemical reac- tions occur between the surfaces and the lubricant and

with the gaseous environment. The resulting modifications of surface texture, microstructure, and composition take place at astonishingly rapid rates and have

persistent effects on the subsequent performance of the contact surfaces. Because scuffing often results from transient conditions of inadequate lubrication, the loss of material by

adhesive wear is often of negligible importance com- pared to the changes in surface texture, composition,

and microstructure. These changes sometimes cause one

of the contact surfaces to become harder than the other. Since this change is often coupled with greater surface roughness due to scuffing, the wear rate in subsequent

operations is greatly increased, even if adequate lubrication prevents further scuffing damage.

The degree of damage caused by adhesive wear is dependent upon the material properties, surface tex- ture, EHD film thickness, surface films, lubricant additives, rolling and sliding velocities, surface and lubricant temperatures, speed, and load. In tapered roller bearings operating under so-called "normal" conditions essentially no adhesive wear occurs because the surfaces of the rolling contact and the roll- ing sliding contacts (rib-roller ends) are well lubricated and separated by an EHD and/or boundary lubricant

film.Under more severe operating conditions or when the lubricant supply is interrupted in a "normal" application the adhesive mode of wear can usually be observed on the rib and roller end surfaces and the cage pockets

as shown in figures 1 and 2. If effective lubrication is restored or the severe condition is intermittent, the sur- faces may have a chance to heal themselves. However,

1 0 mm (a) 1 mm (b)

Figure 1. Examples of adhesive type wear caused by inadequate lubrication.

(a) Metal pickup on a roUer O. D. from the sliding contact on the cage shown in (b). (c) The end of the same roller showing the scoring damage from the rolling-sliding contact of the L. E. rib or thrust rib shown in (d).

70
Figure 2. Scuffing (a mild form of adhesive wear) damage

on a cone L. E. rib or thrust rib in ( a).

In (b) and (c) the cage or retainer shows abrasive type wear where the hard

surface of the rollers cut the softer cage material.

if effective lubrication is not restored, the adhesive wear

can propagate from a mild form to a gross form (burnup). Progressive scuffing damage can generate wear

debris which will cause abrasive wear, similar to that in figure 3, when the lubricant is restored to the contact. Transfer of cage material, by adhesive wear, to the

roller body produces asperities which can lead to microspalling contact fatigue of the cone or cup raceway. An

example is shown in figure la. ..^

7.2. Abrasive Wear

Abrasive or cutting wear occurs when a "hard" sur- face abrades or cuts another "softer" surface. Another

type of abrasive wear results when a hard third-particle

between two surfaces abrades or cuts one or both sur- faces as shown in figure 3. Cage pocket wear, shown in figure 2b and 2c in tapered roller bearings, is mostly abrasive type wear. Wear at the rib-roller end under "normal" conditions can also be abrasive wear of a mild form. If conditions

are severe enough that adhesive wear (scuffing) occurs

at the rib-roller end contact, or on the raceway in high

speed applications, and conditions may improve somewhat, abrasive wear of either the roller ends or the rib occurs because one of the altered surfaces is now harder

or the other is softer. Examples of adhesive followed by

abrasive wear are shown in figures 4, 5, and 6. Third-particle wear occurs when debris from wear or

fatigue damage on other components, from external sources penetrating seals, or from improperly cleaned

components, circulates through the bearing in the lubri- cant. Cornish has described the abrasive wear damage in tapered roller bearings resulting from gear wear debris

and from other abrasive contaminants in automotive

• # • 9

1 1 mm

Figure 3. Abrasive wear caused by third particles.

Natural diamond dust (0-5 flm size) was deliberately introduced into the lubricant in a controlled test. Damage consisted of grooving of the raceway contacts and excessive wear of the rib and roller ends.

71
Figure 4. Adhesive wear caused by high speed skidding on a cone race in (a) and (c). The roller in (b) first experienced adhesive wear and subsequently abrasive wear when the very hard raised areas on the cone caused cutting wear on

the rollers as shown in figure 5. The arrows indicate the direction oi rotation.

rear axle assemblies [23, 24]. An extreme example of abrasive third-particle wear is shown in Figure 3 where

the bearing lubricant was deliberately contaminated

with diamond dust [25]. The obvious preventive measures against abrasive wear are prevention of gear wear,

cleaning of bearing housing and enclosures, effective sealing and adequate filtration of circulating lubricants.

7.3. Corrosion

Corrosion often occurs on bearing components as a

result of improper handling or storage of bearings after the protective film of lubricant is removed, as in Figure

7a. Fingerprint deposits are a common cause of corro- sion when bearings are handled after removal of oil films. Subsequent bearing operation usually results in removal of the brittle corrosion product by fragmentation. The fine abrasive debris can then cause abrasive wear, sometimes producing a smooth, lapped appearance. Perhaps the most common contaminant in a bearing

environment is water. It can enter through defective or inadequate seals or from condensation under certain conditions. The corrosion that occurs usually forms a black oxide commonly called "water etch." Corrosion

usually occurs when the parts are stationary and the

Figure 5. The roller from figure 4 showing the surface at high

magnification in (a) ivith the "normal" wear at left and the abrasive wear at the right.

(b) is a nickel plated chord section of the roller showing the depth of wear

(1.6 /^m) and the rehardening and tempering of the microstructure ac- companying the damage. The vertical magnification of the chord is about

10 times the horizontal magnification.

lubricant film has been lost, or the grease channeled

creating areas with little or no protective film. Under

such conditions, water can cause corrosion, especially

when "EP"' additives are present. In mild cases of "water etch" a trace of black oxide with minimum penetration into the metal occurs. In severe cases, the race- way surface is badly etched away and the bearing runs very rough and noisy. The pitted areas may become en- larged by fatigue spalling. Examples of "water etch"

are shown in figures 7 and 8. Contact fatigue life is substantially decreased by

"water etch" corrosion damage. Modification of surface geometry and texture by corrosion causes stress con- centrations which lead to early fatigue damage. Hydrogen embrittlement from the corrosion can also contribute to more rapid fatigue spalling at corrosion

pitted regions.

7.4. Fretting

Fretting is a form of wear which occurs when two

surfaces are subject to slight relative motion under high

contact pressures. The fretting process occurs in 3

stages which include adhesive metal transfer, oxidation,

and steady-state wear [26].

72
1 0 \m (a)

1 mm (b)

Figure 6. (a) is the spherical end of a roller from the same

bearing as in figures 4 and 5 showing the high speed skidding

damage (adhesive wear) that occurred between the roller and

the L. E. rib. ( In this instance the very hard raised areas caused abrasive wear to the L. E.

rib shown in (b) which was first damaged by the adhesive wear process.

" 1 mill (b) 100, .m (c)

Figure 7. Corrosion or "water etch" on bearing components

resulting from improper storage or inadequate seals in the

application.

(a) This type of corrosion damage is caused by the removal of the oil or

grease from the bearing and allowing the bearing to lie around unprotected,

(b, c) Corrosion damage typical of that found when seals did not keep

water out of the lubricating grease.

In tapered roller bearings and other rolling element

bearings, fretting occurs on the raceway of inadequately

lubricated bearings that are subjected to vibrations

without rotation or to oscillating motion. Fretting damage of this type [27] (also called false brinelling be- cause of the grooved appearance of the raceways)

results in noisy bearings that can accelerate fatigue in subsequent operation. Unlike true brinelling, the fret- ting damage on a raceway will remove material from

the surface and alter the surface texture. Examples of

fretting of raceway contacts are shown in figures 9 and

10. The fretting damage in figure 10 can be confused

with fluting damage caused by electrical current be- cause of a similar appearance. The damage in figure 10 was caused by vibration in the system when the bearing was operating under very thin EHD film conditions. Fretting damage also occurs on press fitted surfaces and, in fact, is more common there. Fretting occurs be- tween the cone bore and shaft, the cup O.D. and cup

seat, and between the bearing faces and spacers or

shoulders. Fretting can damage the shaft to the extent

that the fit and location of the bearing is destroyed. Or,

the fretting may be severe enough to initiate cracks in the shaft and/or the cone which propagate by fatigue

and break the shaft or the cone. Fretting of the bearing

faces can result in loss of the original bearing lateral adjustment. Examples of fretting of fitted surfaces are shown in figure 11.

8. Plastic Flow

There are several modes of damage that are directly related to the environment in which a bearing is operating and which impose extensive damage. Some are caused by foreign materials entering the bearing sys- tem, by overloads, or by failure of the lubrication system.

8.1. Denting

Denting of the contact surfaces is the result of particles much larger than the lubricant film thickness enter- ing the bearing as shown in figure 12. The sources of

these particles may be debris from spalling of other bearings or gears in the system, or from an external source through defective seals. The result might be

noisy operation or eventual fatigue because of the as- perities caused by the dents. Singular debris dents often lead to Point Surface Origin Contact Fatigue damage

when EHD film thickness is low.

8.2. Brinelling

Brinelling of bearing races by extremely high static or impact loads is usually seen as grooves located at the

roller spacing in tapered roller bearings as shown in

73
1 1" (a) <mmm,1 m« (b)

1 mm (c)

Figure 8. Corrosion or "ivater etch" caused by excessive water

in the lubricant that entered the bearing through defective

seals. Considerable spalling of the surface can result from damage of this type.

figure 13. True brinelling is characterized by retention

of the surface texture in the bottom of the brinell marks

because the plastic flow or yielding of the material oc- curs below the contact surface. A brinelled bearing is usually very noisy and fatigue may initiate from brinell damage after subsequent operation.

8.3. Burn-up

This mode of damage is probably the one most fa- miliar to the average bearing user because it is the

single most frequent cause of bearing damage. The

causes of the burn-up type damage are (1) an inade- quate supply of lubricant because of plugged lubrication lines, inadequate seals, too infrequent maintenance,

etc.; (2) overloading the system; (3) improperly ad- justed bearings which result in excessive preloading;

(4) excessive speed of operation for prevailing load and lubrication. Loss of effective lubrication causes heat

generation, and progressive scuffing causes severe metal

transfer from one bearing surface to another. The ab- sence of adequate lubrication causes the rate of heat energy input to the contact surfaces to exceed the rate of heat transfer from the region of scuffing damage. The

thermal expansion associated with the resulting temperature rise intensifies the contact stresses at the damaged

portion of the contact surfaces which causes an accel- erating increase in surface temperature. Finally, the material gets hot enough to flow plastically and the bearing geometry is destroyed as shown in figure 14. This mode of damage can be prevented by positive as- surance of effective lubrication for the prevalent operating conditions.

9. Electrical Damage

9.1. Fluting

Grounding of electrical current through rolling ele- ment bearings causes considerable damage. Fluting,

which results from continuous passage of electrical current, whether AC or DC [28], is a washboard effect on

the raceway which eventually destroys the raceway

geometry and results in generation of noise and fatigue

of the damaged surfaces. An example is shown in figure

15.

74
Figure 12. Denting damage to cone, cup and rollers from

debris entering the bearing from spalling in another component, through defective seals, or from improperly cleaned

housings.

1 mill

Figure 10. Fretting damage on a roller that closely resembles

fluting damage caused by electrical current. The mechanism suggested is excessive vibration under thin EHD film conditions.

Figure 11. Fretting damage of fitted sur- faces, (a) and (b) are fretted shafts from heavily loaded tests.

(a) had become worn from repeated use and the cones which are mounted on either side of the integral spacer had very little press*fit. (b) shows one seat

that was damaged and one that is still OK. (c) shows a cone bore with fretting damage and (d) shows a cup 0. D. with slight fretting damage.

(c)

75
Figure 13. Brinelling damage on cones caused by high static or impact loads in

(a) and ( b)

Brinelling at high magnification in (c) is compared to fretting damage (false brinelling) in (d).

Figure 14. An example of burn-up with plastic flow which is typical of a bearing that was inadequately lubricated, and/ or overloaded.

The damage starts out as scoring or adhesive wear on the rib and, if the heat cannot be dissipated as fast as it is being generated, then seizure takes place followed by hot plastic flow.

9.2. Arcing

Another type of damage resulting from electrical current is arcing (shown in fig. 16). This occurs by the same mechanism as fluting except that it occurs when

the bearing is stationary or rotating slowly and the cur- rent passes through only a very few asperity contacts. The individual arc-affected areas are damaged more

severely, but the bearing surfaces are not changed as in

fluting. Localized melting and vaporization occurs at the arc and the nearby material is rehardened and tempered.

One source of arcing damage is from grounding of arc welding through bearings when repairs are made

on railway or rapid transit cars. The other major

source of arcing damage is from drive motor current passing through the bearing to the ground.

10. Contact Fatigue

If a bearing is not damaged by one of the modes of damage described above, then the mode of damage ulti- mately seen will usually be contact fatigue. The load-life ratings of bearings are usually based on contact fatigue

as the expected mode of damage.

Contact fatigue involves the initiation and propagation of cracks from surface and/or subsurface origins.

The sites for crack initiation are usually at stress con- centrations such as non-metallic inclusions, grinding

76
lOOum

Figure 15. (a) Fluting damage caused by continuous passage

of electrical current through a bearing.

In (b) a roller was polished directly on the O. D. and etched in nital to show the many individual arc marks which are responsible for destroying

the geometry of the components.

furrows, edges of contact, and dents or nicks on the surface. Environmental factors such as lubrication, temperature, speed, load, and deflections have a strong

influence on the mode of damage and the propagation

rate of fatigue cracks. One or more modes of contact

fatigue damage initiate and propagate simultaneously

at diff'erent rates. The characteristics of each mode, the causes and factors which affect the propagation of each,

and examples of each are as follows.

11. Subsurface Origin Spalling

11.1. Inclusion Origin

The classical inclusion origin spall is the semiellipsoidal spall, shown in figure 17, having its origin

located at a depth corresponding to the region of maximum cyclic shear stress. Non-metallic inclusions of the

(c)

(d)

Figure 16. Electrical arcing damage to a cone and rollers caused by grounding of electrical current through the

bearing.

Here the current goes through only a few points and the individual points are more severely damaged but the geometry is not affected as much and the consequences are not as severe as in fluting, (c) and (d) are transverse sections of the cone and a roller, respectively. Nital etch.

aluminum or silicon oxide types (or hard alloy carbides

in some high temperature bearing steels) provide stress concentrations at which fatigue cracks initiate and propagate laterally, circumferentially and upward to the

surface. In tapered roller bearings operating under relatively thick EHD film conditions, the inclusion origin spall

will propagate slowly or not at all beyond the original

spall. If propagation does take place under these conditions it will be primarily in the lateral direction because

of the contact geometry [5, 16].

When conditions of low EHD film thickness are pres- ent the visual appearance of inclusion origin spalls varies from the typical semi-ellipsoidal spall to spalls

resembling the point surface origin mode, with extensive circumferential and lateral propagation [5]. Bearings operated under thin film conditions behave this way because the lower viscosity and/or thinner lubri- cant film alters the initial depth and propagation rate of fatigue cracks. It seems likely that presence of the thinner film increases the tractive force on surface as- perities which penetrate the film, causing the depth of the maximum shearing stress to approach the surface. There is also a likelihood, when the thinner film is due

to a low viscosity lubricant, that the lubricant penetrates

the crack and increases the crack propagation rate in a manner which is not fully understood.

77
(a) ilOOum

(b) lOum

Figure 17. The inclusion origin mode of contact fatigue.

(a) A typical semi-ellipsoidal inclusion origin spall on a tapered roller bearing cup or outer race, (b) An inclusion origin spall located at the edge of

the race where a geometric stress concentration and an oxide inclusion caused the spalling. The vertical line shows the plane of metallographic

sectioning tor finding the inclusion shown in (c) and (d). (d) was etched

in 4 percent nital.

lOOum (c)

01'

, _oi

(c) lOOi.in (d)

—ilOum (d)

Figure 19. Additional planes of polish [bottom lines of figure 18 (a) and (bj] showing the subsurface cracking associated with the inclusion mode of contact fatigue.

(a, b) Incipient spall that did not propagate: (c, d) incipient spall that propagated extensively. Note the shallow angle of the propagating crack in

(c) that starts at the arrow.

(e) (f)

Figure 18. Inclusion origin fatigue on a cone that ran in SAE

10 + 6.5 percent EP additive at 150 °F.

hg = 7.6 microinches (193 nm). (a) Incipient inclusion origin fatigue. Micros

in (c) and (e) were taken on the upper black line which is a transverse plane, (b) Inclusion origin similar to (a) but this one propagated extensively for over 100 degrees, (d) and (f) are on the transverse plane at the

top of (b). An additional plane is shown in figure 19.

Once a fatigue crack is formed under thin film conditions, the propagation is rapid and the Hfe of the bearing is shorter than under thick film conditions. Sometimes a semi-ellipsoidal spall is formed followed by

rapid circumferential propagation. Fatigue cracks can

also initiate subsurface and, when they reach the sur- face, the rapid circumferential propagation occurs as in figures 18 and 19 without formation of the semiellipsoidal spall. In some instances the fatigue will start

at or very near the surface, where an inclusion intersects the surface, without any of the usual crack pattern of a

typical inclusion origin spall.

78
11.2. Geometric Stress Concentration (GSC) 11.3. Subcase Fatigue (Case Crushing)

Tapered roller bearings, straight roller bearings and some other types of components have cylindrical shapes

of finite length which means there can be a corner to create a stress concentration [29]. In addition, any deflection or misalignment in the bearing will result in further concentration of the load at the end of contact [13]. The coincidence of a nonmetallic inclusion at the point of geometric stress concentration will also intensify the stress concentration on the material. The depth

of the fatigue crack origin is a function of the load, the geometry of the corner, and the lubricant film thickness. Geometric stress concentration spalls vary greatly in appearance, from very shallow microspalling (peeling)

with little lateral propagation, to deep subsurface origin

spalling with extensive lateral propagation as in figure

20, or to a single semi-ellipsoidal spall caused by an

inclusion coincident with the stress concentration as in figure 17b. Under thin EHD film thickness conditions, the geometric stress concentration can produce point

surface origin spalls that propagate extensively in the circumferential and lateral directions (see fig. 23).

Geometric stress concentration fatigue is generally not a problem at or below the rated load of a bearing

unless there is a substantial misalignment or large deflection that causes increased loading at one end of contact as shown in figure 20.

(e) —M»10 inm (f)

Figure 20. Geometric stress concentration fatigue.

(a, b) Cone (inner race) with GSC fatigue at the undercut, (c, d) Rollers

fr(im the cone in (a. b) with GSC where the roller contacted the edge of the cone race, (e) Cup or outer race with load zone typical of misaligned cup with microspalling at the edge of the raceway, (f) Another cup with

the same amount of misalignment (0.007 in/in) but with twice the load as* in (e).

Subcase fatigue is a mode of fatigue associated only

with case hardened components [30]. It is caused by

high overloads for a given case depth condition. It has

been observed most frequently in contact fatigue rig

tests where contact stresses are many times higher than

those encountered in most bearing applications. An ex- ample of subcase fatigue is shown in figure 21 from a 0.5" (12.7mm) diameter rig test specimen loaded "nut

cracker" style with crowned 8" (203mm) diameter

loading rings.

12. Surface Origin Sx^alling

In tapered roller bearings where the roughness of the contacting surfaces are only of microinch (nanometer)

magnitudes, surface origin cracks are usually associated with low viscosity lubricant, or other conditions such as high temperatures or low speeds which cause low EHD

film thickness. Extraordinary asperities of large magnitude may cause surface origin cracking with relatively thick EHD films. Combined rolling and sliding will also promote surface origin spalling because the maximum

shear stress range is nearer the surface than for pure

rolling contact.

12.1. Point Surface Origin (PSO)

The point surface origin spall has a characteristic "arrowhead" shaped origin pointing opposite the direc- tion of load approach as shown in figure 22. The crack

is initiated at the surface and propagates rapidly, cir- cumferentially and laterally. The depth of the crack in- creases as it propagates circumferentially.

The stress concentrations, that can lead to PSO spalling include prominent grinding or honing scratches that lead to microspalling under thin film conditions. Handling nicks and bruises from debris can cause large as- perities that act as stress concentrations leading to PSO

spalling. Examples of these are shown in figures 22 and

23.Another stress concentration, already mentioned, is the geometric stress concentration at the ends of roller contact which can result in PSO fatigue under thin EHD film conditions, as shown in figure 23. Under thin EHD film thickness conditions, there is an increase in asperity contacts in the contact area which causes the maximum shear stresses to occur

nearer the surfaces depending upon the magnitude of the tractive-force [31]. Leibensperger and Brittain [32] showed that there is also a micro-Hertzian stress distribution at surface asperities which is superimposed on the macro-Hertzian stress distribution. These factors

result in fatigue cracks initiated at the contact surface. A distinctive characteristic of PSO fatigue is the rapid propagation and the direction of propagation of the cracks from a variety of origins. Low lubricant vis- cosity at the bearing operating temperature promotes

the rapid propagation associated with PSO fatigue.

Since the lubricant has immediate access to the surface

cracks, physical and chemical eff^ects influence the prop- agation behavior earlier than when the origin is sub79
Figure 21. Subcase fatigue. The example shown here is from a heavily loaded contact rig specimen which is ivhere this mode is usually found.

The external appearance of this mode of fatigue is not usually indicative of what lies beneath the sur- face as is shown above in (a). The sjiecimen was

loaded to an equivalent bearing load of 750 percent which made the calculated shear stress depth at 0.012 in (0.305 mm). The total case depth was

0.0625 in (1.59 mm). The loading wheels were crowned which concentrated the load at the center

of the contact where the subcase cracking was

deepest as in (d).

' 1 mm ( a

)

Figure 22. (a, h) PSO spall on a cone

with the characteristic arrowhead shape.

(c, d) Handling nick on a roller responsible for the

stress concentration that caused the spalling in

(a, b). (e) Transverse section through the PSO

spall in (a. b) showing the shal low entry of the fatigue crack, Nital etch. Arrows show direction of load approach.

80
Figure 23. Point surface origin fatigue.

(a) PSO fatigue spaUing on a cone caused by an asperity adjacent to a debris bruise as shown in (b).

(c) Microspalling and PSO fatigue on a cup caused by a geometric stress concentration at the small end of the raceway. Arrows indicate the direction of load approach.

surface. Whether the origin is an inckision, geometric

stress concentration, a spot of micro-spalling, an asperity at a handling nick or propagating cracks from a subsurface inclusion, the arrowhead shape and rapid

propagation are also invariably present.

12.2. Microspalling

Microspalling is a very shallow fatigue spalling of the surface that occurs under low EHD film thickness. Sites of crack initiation are at peaks of the surface finish and

adjacent to prominent grooves from finishing operations, debris bruises, handling nicks, other fatigue

spalls, and edges of contact. This mode of fatigue was

formerly called peeling by the authors [14, 16, 18, 20]

and examples are shown in figure 24. The conditions that prevail for PSO spalling are identical for microspalling. The two modes often occur

together. On occasions microspalling develops into PSO

spalling. The cracking and microspalling seldom extend

below 0.001" (25.4^m) in depth. Tallian, et al. [33]

proposed a mathematical surface model and Leibensperger and Brittain [32] presented experimental evidence

of a micro-Hertzian stress distribution at surface asperities that explain the probable mechanism of the microspalling mode of fatigue.

Figure 24. Miscrospalling (peeling) on a 12 in (305 mm) O.D. double cup in (a) and a % in (19 mm) bore cone in (b).

81
When two surfaces of different roughness are run under thin EHD film conditions, the smoother of the two usually develops a general microspalling condition

while the rougher of the two will have only the peaks

of the roughness spalled away as in figure 25. Microspalling appears to relieve the stress concentration that caused it and may not propagate beyond this stage in some applications. Microspalling does generate a new

edge of contact if the spalling does not extend over the

entire length of contact. Such a discontinuity can be- come a site for GSC or PSO fatigue under certain conditions. If other damage modes do not accompany

microspalling, the useful life of the bearing or component will probably not be affected in most applications.

Figure 25. Microspalling (peeling) on a tapered roller bearing caused by a thin lubricant film compared to the composite

surface roughness.

(a) Cup showing fatigue on the peaks of surface texture, (b) Cone showing

fatigue on the peaks of surface texture, (c) Roller with a general spalled

area, (d) Transverse section through a slightly spalled area on a roller showing typical shallow cracking. Nital etch.

13. Section Fracture

Occasionally, the operating conditions in a tapered

roller bearing are severe enough to cause fracture of some components. Cage fracture often occurs under in- adequate lubrication conditions because of the increased

forces exerted on the cage by the rollers as shown in figure 26a. An improper system design may result in the cage rubbing against some component of the system

causing cage breakage as shown in figure 26b. Cone fracture ( formally called Transverse Cracking

by the authors) may result from excessive press fit from

fatigue propagated cracks generated by fretting between

the cone and shaft, from repeated thermal stresses in

10 mm (a)

^^10 mm (b)

Figure 26. Section fracture of cages. In (a) the cage has

fractured because of inadequate lubrication.

Plastic flow of the cage has also taken place. In (b) the bearing was mounted

improperly so that the cage ^vas rubbing against something which eventually

resulted in cage fracture.

burn-up damage, or from extensive fatigue spalling on

the cone raceway. Cup frature may result from foreign material en- trapped between the cup O.D. and the cup seat, from

extensive spalling on the cup raceway propagating

through the section when it is not effectively supported

or from cracks initiated by fretting of the cup O.D.

propagating through the section. In one example, for- eign material was entrapped between the cup O.D. and

the cup seat. The distortion of raceway geometry

caused early contact fatigue of the raceway and bending

fatigue fracture which propagated partially through the cup section. In another unusual situation, fretting damage on a cup O.D. caused a loss of support which subjected a cup to repeated bending stresses in a region

where there was extensive propagation of fatigue damage on the raceway. Cracking from the contact fatigue

finallv propagated through the section by bending fa- tigue.

When a bearing inner race made from through hard- ened steels is press fitted onto a shaft, the initial sub- surface or surface origin fatigue crack sometimes

propagates radially through the entire cross section.

82
Examples of this mode of damage are shown in figure

27. Propagation through the section may be so rapid

that no evidence of contact fatigue damage is seen on

the raceway surface, as in figure 27a. The low fracture toughness of the through hardened section, the residual

stress and press fit stress combine to cause this mode of fatigue fracture. In balls and rollers of all types, section fracture can sometimes occur under very high overload by fatigue

propagation from inclusion origin fatigue cracks.

i 1 mm (c)

Figure 27. Transverse cracking damage on tapered roller bear- ings made of through hardening steel.

(a) One crack has propagated through the section and there are at least 4

earlier stages of cracks showing, (h) A second cone with spalling at the transverse crack, (c) The fracture surfaces of the transverse crack shown

in (b).

14. Summary

Modes of damage to tapered roller bearings, many of which are seen in all types of rolling element bearings,

have been discussed. Classification of the various modes

of damage, according to a scheme which relates the

characteristics of each mode to its most probable cause, has obvious diagnostic value when service damage is encountered. In a broader sense the reliability of machines and components are dependent upon the cogni- zance by the manufacturer, designer-builder, and user

of the type of service damage normally encountered in a machine or component. Awareness of (1) the conse- quences of manufacturing variables affecting components, of (2) design and building variables in machines,

and of (3) the user's application of the machine can be

used to increase the reliability of mechanical systems.

When used in conjunction with an effective method of bearing damage analysis, such knowledge permits one

to diagnose and correct the cause of bearing damage

seen in prototypes and thus greatly improve the chances

for producing design life in a new machine.

15. References

[1] Zaretsky, C. V., and Anderson, W. J., Effect of materials—General background, NASA SP-237 Interdis- ciplinary Approach to the Lubrication of Concentrated

Contacts (July 1969), p. 379.

[2] Bamberger, E. N., Effect of materials—Metallurgy viewpoint, NASA SP—237 Interdisciplinary Approach to the Lubrication of Concentrated Contacts (July 1969),

p. 409. [3] Neifert, H. R., and Wren, F. J., The rating of tapered

roller bearings through continuing worldwide quality

audit. Preprint 690577, Soc. Automo. Eng. (September

18, 1969).

[4] Zantopulos, H., The effect of misalignment on the fatigue

life of tapered roller bearings, Journal of Lubrication Technology, Trans. ASME. Series F, Volume 94, No.

2 (April 1972), pp. 181-187.

[5] Littmann. W. E., Widner, R. L., Stover, J., and Wolfe,

J. 0., The role of lubrication in the propagation of contact fatigue. Journal of Lubrication Technology,

Trans. ASME, Series F, Vol. 90, No. 1 (January,

1968), pp. 89-100.

[6] Danner, C. H., Fatigue life of tapered roller bearings under minimal lubricant films, ASLE Trans. Vol. 13

(1970).

[7] Moyer, C. A., The use of elastohydrodynamic lubrication

in understanding bearing performance, SAE Preprint 710733 (September, 1971).

[8] Harris, T. A., Predicting bearing performance under

non-standard operating conditions, Mach. Des. (August 17, 1967), pp. 158-162.

[9] Witte, D. C, Operating torque of tapered roller bearings, ASLE Translations, Vol. 16, No. 1 (January,

1973), pp. 61-67.

[10] Leibensperger, R. L., An analysis of the flow of oil through a tapered roller bearing. Journal of Lubrication Technology, Trans. ASME, Series F, Vol. 94, No.

2 (April 1972), pp. 125-130.

[11] Cornish, R. F., Acro-Set: A new method for installation of tapered roller bearings with accurate end-play

setting, Soc. Automo. Eng. (April, 1966).

[12] Cameron, R. W., and Jenkins, J. A., Design parameters

for automated bearing settings, Preprint No. 700730,

Soc. Automo. Eng. (September, 1970).

[13] Kaufman, H. N., and Walp, H. 0., Interpreting service damage in rolling type bearings—A Manual on Ball and Roller Bearing Damage, ASLE (1953).

[14] Littmann, W. E., and Moyer, C. A., Competitive modes

of failure in rolling contact fatigue. Preprint 620A,

Soc. Automo. Eng. (January 1963).

[15] Davies, W. J., and Day, K. L., Surface fatigue in ball bearings, roller bearings and gears in aircraft engines,

fatigue in rolling contact, Inst, of Mech. Engineers,

London (1963), pp. 23-40.

[16] Littmann, W. E., and Widner, R. L., Propagation of contact fatigue from surface and subsurface origins, Journal of Basic Engineering, Trans. ASME, Series D,

Vol. 88, No. 3 (September 1966). pp. 624-636.

[17] Simpson, F. F., Failure of rolling contact bearings. Lubrication and Wear, Second Convention, Inst. Mech.

Eng., Vol. 179, Part 3D, (1965), p. 248. [18] Widner, R. L., and Wolfe, J. 0., Analysis of tapered

roller bearing damage. Technical Report No. C7.11.1, Am. Soc. Met. (October 1967).

[19] Tallian, T. E., On competing failure modes in rolling contact, ASLE Trans. Vol. 10 (1967), pp. 418-439.

83
[20] Littmann, W. E., The mechanism of contact fatigue, NASA SP-237, Interdisciplinary Approach to the Lubrication of Concentrated Contacts, July, 1969. [21] Belasco, S., Oil Analysis Program Quarterly Summary,

A/T A3400000/051B/4F53537401, WU #01, Naval Air Engineering Center (February 8, 1974).

[22] Burwell, J. T. Jr., Survey of possible wear mechanisms. Wear Volume 1, (1957/58), pp. 119-141.

[23] Cornish, R. F., A critical look at tapered roller bearing wear in hypoid axles. Preprint 650440, Am. Soc. Automo. Eng. (May, 1965).

[24] Cornish, R. F., Wear of Tapered Roller Bearings, American Society for Metals 1969 Wear Conference.

[25] Fitzsimmons, B., and Clevenger, H. D., The effect of contaminated lubricants upon tapered roller bear- ing wear. To be presented at the 8th Annual Fluid Power Research Conference at Oklahoma State University, October, 1974. [26] Hurricks, P. L., The mechanisms of fretting—A review, Wear, Vol. 15 (1970), pp. 389-409.

[27] Culp, D. v., and Lieser, J. E., Fretting corrision in au- tomobile wheel bearings. Lubrication Engineering

(October, 1971).

[28] Simpson, F. F., and Crump, W. J. J., Effect of electrical currents on the life of rolling-contact Bearings, Lubrication and Wear Convention, 1963, Inst. Mech. Engr.

(1963), pp. 299-304.

[29] Moyer, C. A., and Neifert, H. R., A first order solution

for the stress concentration present at the end of roller contact, ASLE Trans. Volume 6 (1963), p. 324. [30] Pederson, R., and Rice, S. L., Case crushing of case carburized and hardened gears, SAE Trans. Vol. 68

(1960), p. 187. [31] Smith, J. 0., and Liu, C. K., Stresses due to tangential and normal loads on an elastic solid with application

to some contact stress problems, J. Appl. Mech. ASME

(June, 1953), pp. 157-165.

[32] Leibensperger, R. L., and Brittain, T. M., Shear stresses below asperities in Hertzian contact as measured by

photoelasticity. Journal of Lubrication Technology,

ASME (July, 1973), pp. 277-286.

[33] Tallian, T. E., and McCool, J., I. An engineering model

of spalling fatigue failure in rolling contact, II. The

surface model. Wear, Vol 17 (1971), pp. 447-461.

16. Discussion

W. Gaylord, United States Postal Service: Do you have

any information which would show how to correlate

loading in the case of a steady load as compared to high

frequency, intermittent loading?

W. E. Littmann: You're talking about how to do a life prediction when the loading is not going to be constant, but going through some spectrum. In other words you

have a periodically varying load.

JV. Gaylord: It's constant but it's periodically applied.

It goes from zero to the maximum in the case of a letter machine where the shaft has to flex to compensate for various thicknesses of letters.

W. E. Littmann: Many real mechanical systems operate over a range of loads and one uses a weighted average summing up of damage together with the load-life re- lationship, that is the life is inversely proportional to the load, to empirically determine power. This accumulation of damage can be used by simply integrating over the significant range of loading and ignoring the lower load levels to come up with a weighted average

equivalent load and then compute the expected life from that load.

C. E. Vest, NASA, Goddard Space Flight Center: In a starvation situation, did you have a good lubricant at

all times?

W. E. Littmann: Yes, except for the adhesive wear and

burn-up types of fatigue.

C. E. Vest: Were most of the lubricants oils or greases?

W. E. Littmann: The majority were circulating oil sys- tems, although, for example, the AP freight car bearing

that had suffered corrosion damage when water got in

it was a grease lubricated bearing. This is a case where

channeling of the grease will tend to develop a dry condition on the raceway and make it susceptible to cor- rosion. So with grease lubrication, if there is a tendency

for channeling I would say that it's important that the

seals be kept in condition to avoid ingress of water.

84
MODES OF FAILURE

Session III

Chairman: J. R. Strang

Air Force Materials Laboratory

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Anti-Friction Bearing Design, Inspection and Recycling

Kay M. Glaesner

St. John's Evangelical Lutheran Church

Wittenberg Avenue at Columbia Street Springfield, Ohio 45502

The paper will describe the ramification of failures of antifriction bearings related to aircraft applications, including engine bearings. The paper will endeavor to describe the problems of anti- friction bearings in four categories:

I. Design of application. Here, the paper will describe the inherent design failures as noted in applications. The paper will also include parameters of bearing design which will best provide

operational success and life expectancy.

II. Failure modes in bearing applications will be described and an outline of the most popular

types of failure as detected in bearings, will be discussed.

III. A recovery process for the reclamation of bearings will be outlined and again limits for the

acceptability of defects on bearings will be presented.

IV. New inspection techniques now under study will be outlined and the method of inspection

will detail failure prevention once the bearing is mounted in the application.

Key words: Aircraft engine bearings; antifriction bearings; bearing failure; bearing failure modes;

bearing inspection; bearing reclamation; bearing recycling.

1. Introduction

The performance of an aircraft engine is directly

proportional to the precision and quality of the anti- friction bearings mounted in that engine. There is no

single part in engine design that plays such a vital role and determines performance more than that of a bearing. For many years in the military service, we have

had an unusual opportunity to study thousands of bearings, not only as they are originally designed into the

aircraft application, but also through their service life and reuse, after engine overhauls. Through the years, we have noted a longer life ex- pectancy in our heavy loaded bearings and this has

been accomplished because of a number of factors, some of which will be outlined in this paper. In a recent

life expectancy study on ten bearings picked at random

from a number of positions in a number of engines, we

discovered that most of the bearings had accumulated

between 5000 to 7000 serviceable hours and that all of

the bearings still had usable hours so that they could be

remounted into the engine. This is a long way from ten years ago when the service life of these bearings was

running from 500 to 2000 h. The bearing in an aircraft engine is critical for sev- eral reasons: (1) It is subject to all the rotational

stresses of the engine; (2) it carries all the radial thrust and torque loads imposed upon the engine assembly;

and (3) because of its sensitivity of design it is most

vulnerable to any variations in normal engine operation, such as: heat transfers, misalignment, improper

load distribution, shaft deflections, shock loads, or im- proper assembly operations. As soon as one or more of

these discrepancies occurs the antifriction bearing is the

first to pick up the signal and sooner or later, will relate

to itself such discrepancies in the form of some bearing

distortion, fatigue or failure. Many of the improve- ments in bearing aircraft design have resulted from our

analysis of bearing failures. As indicated, failures of bearings are not only attributed to the original design

for the particular application, but also to components

associated with the system. By noting these factors, we

have been able to improve the life expectancy of air- craft engine bearings over the last 25 years.

2. Design of Application

The following design factors for bearing application

are of great importance to insure precision operation

and maximum life. (a) —Fit-up between Bore and Shaft. The fit-up be- tween the bore of the bearing and the shaft is extremely

important. If an out-of-roundness or taper exists in a particular bearing bore, then you can be assured of a reduction in the life of the bearing. In bearing design

there is a great effort made to control the tolerances of the bearings, but often little attention is given to the matching dimension of the shaft. An improper fit-up or

a taper will place undue stress upon the rings as well

as upon the rolling elements as they pass through the loading zone. Oftentimes, as a result of an improper

bore mounting, a fracture will occur or a fatigue failure

will develop in the raceway proper. Sufficient equipment

is available today to determine proper bore diameters

and fits. (b)—Parallelism of Sides. One of the most outstand- ing causes for misalignment of the bearing itself is at87
tributed to non-parallel sides of the bearing. Here again

the bearing will assume a cocked position in the instal- lation and cause undue stress on the rings and rolling element.

(c) —Eccentricity. Here is one design factor which

is often overlooked, especially in high speed applications. Inherent in the bearings is this built-in problem

which during operation is prone to develop vibratory

loads. A good way to measure eccentricity is simply to mount the bearing on an arbor, having a very slight

taper (0.0001 to 0.0002 inch in diameter per inch of length), and apply an indicator on the center of the

stationary outer ring. The eccentricity is the difference between the minimum and maximum readings when

rotating the arbor one revolution. With this inherent

problem, considerable out-of-balance may be experi- enced. (d) —Side Runout. Another important factor is the

side runout of the bearing which creates undue thrust loads on the bearing itself, and if such loads exceed the

thrust load capacity, it is not long before fatigue will

set in. For measuring this, use the same arbor technique

as described above. Apply the indicator against the side

of the inner ring. The side runout is the difference be- tween the maximum and minimum reading when rotat- ing the arbor one revolution.

(e) —Groove Parallelism with Side. One traditional way to create undue stress on the rings and rolling ele- ments is to have some discrepancy in the parallelism

between the groove of the raceway and the side of the

ring. During operation, a counter axis is developed and

the stress centers between the balls and the raceways,

resulting in a shifting of race curvature.

(f)—Outside Diameter. As with the bore, so often we overlook the necessity of a proper fit between the outside diameter of the bearing and the housing. De- pending on design, this fit should be prescribed. If the outer ring is to be fixed, especially for cross loading, then the fit-up should follow. If the outer ring is to float to provide for thermal expansion, then the fit-up for

this type of application should follow. Although it is very difficult to require exacting tolerances in housings,

such as are required in antifriction bearings, the solu- tion for a proper mounting may be accomplished by

the technique of "selective fits." Other design factors related to engine bearing problems are also associated with such problems as the constant flow of lubricant to reduce the effect of heat, shaft

deflections which may occur after an engine is assembled which' causes misalignment of the bearings, and

damage to the bearing parts due to contamination and

improper handling.

Another important factor in designing a bearing for an application is the "internal fit-up within the bearing." Except for special bearings, the internal fit-up in antifriction bearings does not have a uniform standard

dimension established by the manufacturers. Generally

speaking, they are classified into four groups:

(a) —Standard Fit-up. In this class, internal un- mounted clearances are to be of such magnitude that, as a result of the bearing's internal design, when

mounted with standard ABEC fits for revolving shafts,

the average radial clearances will be best suited for

single row annular bearing service. (bj —Loose Fit-up. Here the bearings can accommodate heavier press fits with a resultant small amount of

radial clearance.

(c) —Ti^ht Fit-up. This type of design will actually create a preload condition within the bearing.

(d)—Selective Fit-up. This is the type of internal fit- up that best describes the high precision, heavy load or high rpm bearing. This type of internal clearance is a pre-calculated one in which consideration is given to the thermal flow, the lubrication, torque loading, and the

resultant ability of the bearing to make compensation

for variations in operation.

The above factors have been the most important de- sign needs in bearing manufacturing for aircraft engines. Of course, the internal design of the bearing such

as race curvature, angle of contact, retainer drag, and

type of retainer, are also important in choosing a bearing, but these factors are usually taken up by the bear- ing manufacturer, rather than the application designer.

3. Failure Modes in Aircraft Engine

Bearings

Visual inspection for bearing defects has been, up until recent years, the main technique for determining

the serviceability of bearings, especially where such

bearings have been in a previous application. Visual

inspection is now passing out of the picture and more

sophicated techniques are being employed to determine

the serviceability of the bearings. Later in this paper

an outline of some of these new techniques will be pre- sented. The purpose now is to provide descriptions and

service limitations of defects most commonly found in aircraft engines.

(a) —Indenting. This indenting may occur any place on the bearing rings, raceways, or rolling elements. Usually indentations will appear as cavities with smooth

bottoms and sides. The cavities are depressions in the operating surface where the metal is displaced but not removed. This condition is caused by the passing of foreign or loose material between the races and the roll- ing elements. Usually any dent up to 0.060 in long

which cannot be felt with a 0.030 in radius scribe, is considered acceptable unless there is a cluster of four or more dents within a quarter in diam area. (b) —Fatigue Pitting. This kind of pit may be recog- nized by the occurrence of irregular, relatively deep

cavities on the rolling surfaces. These cavities are usually the result of subsurface, molecular breakdown or voids and can be further distinguished by a characteristic fractured appearance on the side. Fatigue pitting on

a bearing renders it unserviceable. Geometric stress concentrations may often generate this defect. (c) —Corrosion Pits. Abrasive wear results from

small droplets of corrosive material attacking the ex- posed surface of the bearing. They are distinguished by

irregular shaped, ragged edged pits which have a red- dish color. Usually such corrosion is widely distributed and if such pits cannot be felt with a 0.030 in scribe.

88
the bearing can be serviceable if the cluster is less than

four within a quarter-in diam area. (d)—Nicks. Nicks on bearing surfaces result from

bearing components striking together either in shipment

or during application. Normally, this type of defect is acceptable and not cause for rejection unless it exceeds

0.060 in in length.

(e) —Scratches. These are linear abrasions of the sur- face which again occur from improper handling and are not usually a cause for bearing rejection unless they can

be felt with a 0.030 in scribe. (f) —Adhesive Wear. Circumferential bands and

grooves on rolling elements result from foreign material

which becomes trapped or embedded between rollers and retainer pockets. It can also be produced by break- down of lubrication, scuffing, smearing or galling. Axial

scoring occurs often during assembly or disassembly of bearings. When the bearing has two or more such defects, the bearing is considered unserviceable. (g)—Axial Galling. This results from an ineffective lubrication at the sliding contact surfaces. Galling may

be recognized by the presence of metal from one part

remaining attached to another. Often it is also caused

by metal removed from the retainer by the rolling ele- ments and deposited on the races. No form of galling is acceptable.

(h)—True Brinelling. This results from extremely

high shock loads which leave permanent impressions at each rolling element position on the loaded side of the bearing. Often this is the case when a shock load occurs while the bearing is in a stationary position and even

while it is in transit. Visual inspection does not often

detect true brinelling, but it can be seen by reflected

light measurements. No form of true brinelling is ac- ceptable.

(i) —False Brinelling. This is a specialized form of

fretting occurring only at the rolling contact surfaces

of the bearing raceways. It may be recognized by a

series of shallow indentations in the race at each rolling

element position and is also accompanied by a red iron oxide substance. This brinelling does not occur when a bearing rotates.

(j) —Bearing Discoloration. Discoloration may come

from heat or stain due to oil or grease and is not con- sidered harmful unless it has advanced to an etching or oxidation process on the race. Of course, the obvious failures of bearings such as

fractures, broken retainers and dimensional changes

are often seen in engine bearing applications. The above

outline of failure modes has provided bearing engineering with the background for the development of more

advanced bearing inspection equipment and techniques. In 25 years of research, we would say that the subsurface, as well as the surface, of the bearing plays as vital a role as does the geometry of the bearing itself. Of

course, unusual temperatures, shock and operating conditions vitally affect the various bearing defects and

accelerate bearing failures.

4. Recovery Process

One of the techniques developed by the Air Force and currently used to analyze bearings arises from the

Air Force program to recover used antifriction bearings. About 75 percent of engine bearings are now being

recycled in a program of bearing reclamation. The new

technology now being developed will increase the number of bearings which can be reused in aircraft installa- tions. The Air Force is at the present time considering an enlarged facility to reclaim antifriction bearings. An

inspection at one of our bearing shops would include

the following:

Initial inspection—to remove obviously damaged

bearings

Cleaning—to remove foreign elements

Polishing—to restore surface finish Rework—to accomplish retainer repair

Measurement—to determine internal clearance, race curvature and dimensions

Calibration—matching angular contact and preload

Inspection—to determine metallurgical and operational condition

Preservation—to protect against corrosion

Packaging—to provide for adequate protection dur- ing transportation.

The recovery process is timely because of the energy

crisis, as well as the scarcity of man hours and raw materials. The dollar savings in this program is estimated

at 50 percent over the cost of new bearings.

5. New Inspection Techniques

A considerable amount of study on causes and effects of bearing failures has indicated that many of these fail- ures occur beyond the visual concept and are oftentimes

related to subsurface structure and to the bearing race

finish per se. The classical methods for nondestructive

evaluation have proved to be too limited in meeting the new requirements of the space age. The limits of sensitivity have now been defined to a point where conventional analysis for bearing serviceability is inadequate.

New modern techniques for bearing inspection are well underway and will not only prove to be very efficient in detecting bearing failures but also will provide assur- ance for bearing life predictability.

The purpose here is to call attention to some of these advances, and while some of the techniques are not new, we do believe that only recently have they been applied

to bearings. Some of these new techniques are as fol- lows: (a)—Magnetic Field Perturbation. We have known

for many years that the performance of high speed and

heavily loaded bearings is directly related to the condition of the subsurface structure of the bearing metal. Subsurface voids and contaminants provide flaws which

will in turn affect the smooth operation of the bearing

and eventually become the actual cause of bearing fail- ure as the subsurface deteriorates. Many experiments

have proved that most all outward signs of fatigue and

failure are related initially to subsurface flaws. Magnetic perturbation is a system to measure these subsurface imperfections. Using the Hall effect, this system is capable, through sensitizing a probe signature,

of measuring the subsurface defects and voids when a uniform magnetic field is induced into the bearing

89
raceways. The signature yield from the subsurface in- clusion or void becomes a measurable value. The density of such inclusions or voids, their location, and the depth from the surface can all be detected in a second,

measured mechanically, and thus used to determine

actual serviceability as well as life prediction.

(h)—Stress Measurements by Means of the Barkhausen Effect. This is a system which has objectives as follows

(1) To prepare Barkhausen stress measurements on

the reference bearing at the beginning and end of its normal life's service. (2) To analyze the records obtained and from this analysis evaluate the effectiveness of the Barkhausen

noise stress measurement method for determining the uniformity of the residual surface stresses in new bear- ings and measuring the change of the stress caused by

service life and environment.

(3) Laser optical inspection. This is a system which

aims to develop a laser optical technique for the auto- matic inspection of bearing parts which eventually will eliminate the need for visual inspection. The design of

this system is based on the principle that the directional

characteristics of light reflected by a surface contain

information pertaining to the profile of the surface. The

photodetectors are used to measure certain aspects of

the directional distribution of the reflected light and the analyzer, which might be a simple electrical analog

computer, processes the photodetector response data. The work being accomplished by this system will help

us to ascertain surface geometry as well as being a method to determine defects not noticeable by the naked

eye. This system will provide automatic characterization

of bearing surfaces in terms of defect parameters.

(4) Automated bearing life cycles. Considerable

work has been done to establish a data storage system

through the use of automation. In order to provide a

profile and statistical analysis on critical bearings, such

a collection of data would include:

Number of hours of life Population statistics Magnetic perturbation signature

Barkhausen signature

Optical-metallographic examination

Physical dimensions and tolerances.

Work in this area is enabling us to predict life expectancy, performance ability, and premature failure. In

the space age, flights require a precision and determinability beyond anything of the past. There is no reason why bearings cannot be included in the present science which allows for automated data. Automation now

makes it possible for us to measure operational conditions of a bearing under a given load so that simultaneously we can empirically measure such factors as film thickness, retainer drag, ball contact, temperature variations, and torques at any given instant and at any position while the bearing is under mission operation.

The author wishes to acknowledge the following for certain concepts included in this presentation: J. R.

Barton, Southwest Research Institute, Pat Hampton,

Bobbins Air Force Base, Warner Bobbins, Georgia,

Charles Eddy, Hdqrs. AFLC, WPAFB, Richard L. Jentgen, Battelle Columbus Laboratories, Columbus,

Ohio.

6. Discussion

C. E. Vest, NASA, Goddard Space Flight Center: Do

you use grease or oils in most of your applications?

K. M. Glaesner: Oil.

H. Ferris, Hughes Helicopters: I am interested in your

predictions for the future for replacing aircraft engines,

transmissions, and bearings, which as you say, is costly.

Will it be possible to predict on an "on condition"

basis rather than a TBO basis by using diagnostic means like acoustics so you don't have to overhaul

something at some prescribed period?

K. M. Glaesner: Yes, that is what we are working for.

If we can analyze a bearing or require of the bearing

manufacturer certain parameters which we know are important in ascertaining the life expectancy of a bearing, then we can be reasonably sure that that bearing

is going to operate for the maximum life that we will need to perform our particular mission. That will, of

course, reduce a lot of the rework and inspection. Some

of the factors involved in our program are the scarcity

of bearings, the delay oftentimes experienced in obtain- ing them, and having the technical knowledge to repair

a bearing in the event that we would have a national emergency. The biggest factor has been that of endeavoring to establish a quality control on all of our bearings. This is one of the things that we are hoping will be accomplished when we develop this technique of examining the subsurface of the bearing, the proper

condition of the bearing, the surface condition of the bearing, and inner stress of the bearing. In instrument

bearings right now I think it's safe to say we are using

only 30 out of every 100 bearings we receive from the manufacturer. We don't blame the manufacturer for

this. It's just that our requirements have risen so greatly

that by the time the manufacturer gets down to ful- filling our requirements the bearing gets so costly that

it's almost prohibitive for us to buy. By inspecting the bearings ourselves, we find that we get better deliveries and better production requirements.

H. Ferris: Why overhaul it at all if it is running good?

K. M. Glaesner: We don't. If the bearing is satisfactory

when we take it out, we don't do anything with it except

clean it.

H. Ferris: Why take it out at all, why not leave it on

the aircraft and let it run?

K. M. Glaesner: Usually a bearing is taken off an air- craft when the aircraft is being overhauled for other purposes. When I first started working in bearings at Wright Field it was mandatory to take the bearings

off at regular time intervals such as 300 hours, 500

90
hours. Now we'll operate an aircraft engine for as many

hours as that engine will operate showing no defects or

defaults and we won't do anything with the bearing.

H. Ferris: You don't have TBO's in the aircraft engines

in the Air Force?

K. M. Glaesner: Not in all of them.

/. E. Stern, NASA Goddard Flight Center: You mentioned detergents as being deleterious to the surface of a bearing. Can you name names and tell us what the

effects are?

A.. M. Glaesner: Most any of the standard detergents

used for cleaning raceways of bearings will leave some

sediment on the bearing. Some of the new systems of alcoholic ether spray seem to be less deleterious than

the normal detergents which you can buy from various

manufacturers. For thirty years I have worked with

bearings, and I never dreamed that the problem of wetability could be one of the big factors in the break- down of a bearing. For instance, in a gyro, I am sure

that is one of our greater problems today.

/. E. Stern: What is the frequency of types of failures that you have encountered in the bearings that have

failed in the Air Force program? Do most failures in- volve retainers, races, or materials? Are most failures by fatigue?

K. M. Glaesner: Forty percent fatigue of the raceways

in rolling elements, forty percent due to mishandling of the bearings, and twenty percent having to do with

corrosion.

/. E. Stern: How do you evaluate the remaining life of recycled bearings?

K. M. Glaesner: Only through experience. We keep a log on each bearing, recording the hours of operation

each time it is removed. Examination of these bearings

over a 30 year period has given us the knowledge of how many defects, how many pits, what size of indentation, etc. we can stand in order to provide enough

operating hours in the bearing. It is by experience only,

and to my knowledge we have never lost an aircraft engine because of one of these bearings. I hope some

day that we can predict the service life with more

accuracy. I think if we could measure the subsurface

structure of the bearing and if we could calculate the amount of load, heat, RPM's and some of the environmental conditions of the bearing, we could design a bearing and predict a life expectancy of X number of hours and I think when that X number of hours came,

the bearing would all collapse.

/. E. Stern: Do you use acoustic signature analysis?

K. M. Glaesner: We do on small bearings in generators

and in similar auxiliary equipment.

/. V. Deller, Naval Ship Engineering Center: You indi- cated that 75% of the bearings are recycled. Is that the first recycle? How is the number of hours before

the first recycle related to the Lm life of the bearing?

K. M. Glaesner: As I indicated before, we keep a log on

bearing life so we can tell how many hours a bearing

has been in service. With the exception of a few engine

bearings, in no place do we approach the actual life design expectancy of a bearing. When the Barkhausen

analysis and measurements of subsurface changes due

to operational conditions are obtained, it may be neces- sary to decrease the life expectancy of the bearing.

/. V. Deller: Would you try to revise your design cri- teria to a higher Ljo life?

K. M. Glaesner: No, most aircraft engines are well over our design requirements.

H. P. Jost, K. S. Paul Products, Ltd: To what extent do

you use vibration analysis to determine the life that is

still in the bearing? In some applications of larger

bearings we now know through vibration analysis

whether the bearing has 5 hours, 50 hours or 500 hours

life still in it, so we can use it right to the very end

of its commercial life.

K. M. Glaesner: Ten years ago we designed what we

call an andarometer, and since it turned out to be a

lousy design, I might as well tell you I designed it. Its purpose was to measure the vibratory loads and the frequencies of the vibrations in a bearing. We found

it valuable for research and development, but not for

actual bearing monitoring in the shops. We didn't know

how to pick up the vibrations 10 years ago as we know

today. But in the Air Force we do not use any mechanism to measure vibratory forces on bearings.

H. P. Jost: I don't know of any application in Britain

in aircraft, but on rolling mills it's being used to lengthen the life of bearings. It cuts down on mechanical

failures to a tremendous extent and it is used actually on the rolling mills themselves.

K. M. Glaesner: Battelle Memorial Institute has done some marvelous work in automating what happens

within a bearing during its normal cycle of operation

at a given speed. Because of automation, they can tell, for instance, the kind of vibration in the retainer, the thickness of the lubricant film, the drag of the retainer on the ball, and many other things simultaneously. I don't think that a bearing has really been examined

until it has been subjected to a dynamic analysis.

T. L. Daugherty, Naval Ship Research & Development

Center: We find increasing importance in the use of the andarometer in low noise bearings in submarine

applications. We use it not only in failure analysis but

also in inspection testing.

K. M. Glaesner: I can go to heaven in peace now.

91

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Holographic NDI of P-3 Wing Plank Splices*

A. J. Koury

Naval Air Systems Command

Washington, D.C. 20361

M. J. Devine and J. F. Erthal

Naval Air Development Center Warminster, Pennsylvania 18974

P. D. Bhuta

TRW Systems Group

Redondo Beach, California 90277

Continuous wave and pulsed laser holographic techniques were applied to inspect stress cor- rosion cracking in P—3 wing plank splices. Holographic results were compared with those of cur- rently used ultrasonic inspection techniques. Holographic techniques successfully detected all the

stress corrosion cracking areas that were identified by the ultrasonic inspection technique and, in addition, several other crack areas were located whose presence was later verified by dye penetrant

inspection after careful disassembly of the panel joints. It was also shown that the holographic

technique can locate areas of stress corrosion cracking without removal of paint and sealant materials. The currently used ultrasonic technique requires a paint and sealant stripping operation prior

to inspection. Considerable savings in cost will be realized when inspection using holographic tech- niques is implemented. The holographic technique was demonstrated to Navy personnel at the Naval

Air Rework Facility, Alameda, California, in an air rework environment. The concept of "structural signature" to verify structural integrity of complex aircraft structures is briefly described. Key words: Aircraft component failure; continuous wave holograms; crack detection; holographic

techniques; pulsed laser holograms; stress corrosion cracking; ultrasonic inspection.

1. Introduction

Naval aircraft are subject to stress corrosion crack- ing because of constant exposure to the ocean environ- ment. A particular problem under investigation dealt

-with the Navy's P-3 patrol aircraft. The commercial

version of the P-3 aircraft has also shown the presence

of stress corrosion cracking. Approximately 900 wing

plank splices have shown stress corrosion cracking as reported in references [1] and [2].^ Stress corrosion

cracking results in the necessity to repair wing fuel

leaks. The current method of inspection requires ultra- sonic inspection by a point probe. Prior to performing

the ultrasonic inspection, it is necessary to strip the area completely of paint and render the surface clean. This technique provides satisfactory results; however,

it is time consuming and expensive because it is neces- sary to inspect large areas of the wing, and also the stripping process necessitates repainting.

*Work performed under contract N62269-72-C-040O for the Naval Air Development Center in support of Navy's Analytical Rework Program.

^ Figures in brackets indicate the literature references at the end of this presentation.

In the interest of cost savings, it is desired to perform

the inspection without requiring the removal of the paint and the sealant material which is time consuming.

To satisfy this requirement, holographic interferometry

techniques were considered for this application. Holographic techniques have the capability of recording very small displacements. These techniques take advantage of the fact that a structural member with a crack

or flaw will respond differently to a load than will a member which is not defective. These differences in response are evidenced by marked changes in the inter- ferometric fringe patterns as they traverse a defective

area. Based on the nature of the materials and the

details of the structure, it was considered that holo- graphic techniques offered a potential to inspect P-3

wing plank splices in a cost effective manner. Using continuous wave holographic techniques in the laboratory, its ability to detect stress corrosion cracking was verified by comparing the holographic results with

those of ultrasonic inspection. The presence of additional flaws or cracks indicated by the holographic

techniques was verified by comparison with dye penetrant results after disassembly of the structural joint.

93
Subsequent to the continuous wave holographic in- spection of small P—3 wing plank panels, a larger wing

plank panel was inspected using the pulsed ruby laser

in a demonstration at the Naval Air Rework Facility

at Alameda on December 5 and 6, 1972. The purpose

of the demonstration at Alameda was to verify that inspection can be performed using pulsed laser tech- niques in a Naval Air Rework Facility environment

and to brief Navy personnel on the potential use of holographic techniques. Details of these results and

their comparison with ultrasonic and dye penetrant

results, after disassembly of the structure, are described. Use of the holographic techniques to perform in- spection without requiring removal of the paint or

sealant was later verified by performing the inspection on a panel on which four different thicknesses of paint were applied and a bare strip was left at the top of the panel. Cracks discovered in the earlier inspection

showed up again even after the coats were applied.

A brief description of the pulsed laser holographic

system capable of performing in situ stress corrosion

cracking inspection on an aircraft in a Naval Rework

Facility is given.

2. Description of the Problem

Figure 1 illustrates the geometry of the wing plank

splice specimens taken from a P-3 Naval aircraft. Particular attention is drawn to the fillet and to the crack originating there despite the installation of seal- ant. The method currently used to locate these cracks is a point-by-point search with an ultrasonic probe.

Figure 1. Wing plank splice configuration.

Figure 2 shows the original locations of the three wing panels. Because of apparent mislabeling these panels will be identified in this paper by their physical

dimensions to eliminate ambiguity.

At the outset, it was postulated that a properly

strained panel would exhibit localized structural weaknesses in the vicinity of cracked areas. Anomalies in deformations would be recorded as irregularities in the holographic fringe patterns.

WS 145.00 NAC 221.00

Figure 2. Station sketch showing location of samples.

3. Procedure for Continuous Wave

Holographic Inspection

One of the smaller panels, 12" x 13", was clamped

in cantilever fashion so as to be able to strain it by

applying a simple bending load. It was expected that the application of a small preload before the first ex- posure would enhance the detection process. The apparatus used for testing the small panels in the laboratory was as follows: one 15 milliwatt con- tinuous wave Helium-Neon laser, one beam splitter, four mirrors, two spatial filters, one neutral density

filter, one film plate holder, two right-angle blocks, as- sorted clamps, rised blocks, optics holders and a stable shock isolated mechanical platform. Figure 3 is a photograph of the actual setup. The various elements are arranged so as to provide a scene illuminating beam

and a reference beam. The various elements and the

optical paths followed by the reference and scene beams

Figure 3. CIF holographic setup for testing P-3 wing plank

splice.

94
He-Ne

Lase r

Sample

Reference

Beam Mirror

Spatial Filter

Figure 4. Diagram of CW holographic setup.

are shown in figure 4. The specimen was clamped to an

angle block and was loaded in bending by means of a knurled machine screw at the right-hand edge of the specimen.i Initial testing in the laboratory was per- formed using a Helium-Neon laser because it permits

use of stored beam interferometry techniques which

provide real-time inspection. A continuous wave

Helium-Neon laser is not suitable for use in a Naval

Air Rework Facility environment because it requires a greater degree of mechanical stability and isolation from the environment.

A holographic interferogram of the 12" x 13" panel was made by performing the following steps.

1. Arrange optical setup so as to match path lengths

of the illuminating and reference beams.

2. Apply preload to specimen and make first ex- posure.

3. Apply small additional load, minute rotation of knurled screw, and make second exposure.

4. Develop, fix and wash film plate. Allow to dry.

A hologram made in this manner can be "reconstructed" by illuminating it with the reference beam

only, impinging upon the hologram at the same angle

employed in the original exposures. When the eye (or a camera) looks through the hologram, as if it were

a window, it sees a virtual image of the panel sitting there in space, even though the panel is removed from

the scene. Presence of a flaw is detected by examining

the fringe pattern obtained in the holographic inter- ferogram. Any anomaly in the fringe pattern, such as discontinuities in the fringes themselves and/or their

slopes, indicates the presence of stress corrosion cracks.

CRACK INDICATED BY FRINGE DISCONTINUITY

Figure 5. Small 12" % 13" panel subjected to continuous-wave

holographic inspection.

Figure 5 is a picture of an interferogram of the 12" X 13" panel. It was strained and holographed and

the consequent fringe pattern contains a marked dis- continuity, shown in the encircled area. The discontinuity in fringe pattern is identified by reconstructing

the hologram and viewing the fringes in three dimensions. The second of the two smaller panels measured 121/4" X 1714". It was inserted into the same setup as the first

95
panel and inspected in the same manner. Figure 6 depicts the holographic results. The fringe orders have

been assigned numbers to aid the reader in tracing them

across the joint. Four cracks were indicated by discontinuities in the fringe pattern and are shown in rec- tangular blocks marked 1, 2, 3, and 5.

Figure 6. Crack locations in 12.25" x 17.50" panel as indicated by holographic inspection.

Cracks are indicated by fringe discontinuities shown in rectangular blocks marked 1 through 3 and 5. Numbers assigned next to fringes indicate fringe orders and serve as a means for improved data interpretation.

4. Pulsed Laser Holographic Inspection of

Larger P—3 Wing Plank Panel

In this section pulsed laser holographic inspection of the larger P-3 wing plank panel is described. To per- mit inspection in a Naval Air Rework Facility en- vironment, pulsed laser holographic techniques were

utilized. The section provides a description of doublepulse ruby laser holography, test setup, timing sequence and making of the holographic interferogram.

The double-pulse approach consists of exposing the

film plate while the panel assumes an arbitrary "background" pose and then exposing it a second time after having impacted the panel at a low level by a ballistic pendulum. Both sets of wave fronts reflected off the panel are thereby captured on the same emulsion and

interfere constructively and destructively, depending

upon their relative phases at various stations on the hologram. This is, in turn, a function of the relative positions of the panel at the times of the first and sec- ond exposures. Upon reconstructing the hologram, in- terference fringes will be formed wherever the displacements equal odd numbers of quarter wavelengths.

The double-pulse system permits firing of two pulses

in rapid succession, several microseconds apart. This

time interval is so short that the panel cannot experience any appreciable displacements other than the ones induced by the impact. The pulse duration is approximately 50 nanoseconds, which permits "freez- ing" of the motion due to the induced stress waves.

The pulsed ruby laser system, which was used for

this program, is shown in figure 7. This laser has the

capability of producing up to four, long-coherence,

optical pulses (wavelength, A = 6943A) within one

millisecond. The separation between pulses can be

varied from one to 250 microseconds.

Figure 7. Multipulse ruby laser system.

The basic components of the ruby laser are two

mirrors, a ruby rod, a flashlamp, and a Pockels cell Qswitch. These are shown schematically in figure 8. The

two mirrors form opposite ends of the optical cavity.

The ruby rod provides the optical energy (light) in a coherent, monochromatic form. The flashlamp

"pumps" the ruby rod, making it ready to lase. The

Pockels cells is situated between the mirrors and acts as a shutter for the cavity. When the proper voltage

is applied to the Pockels cell, light is allowed to pass through it forming a complete optical cavity allowing

the laser to lase. When there is no voltage applied, the Pockels cell isolates one mirror from the other and the

optical cavity is no longer complete.

The actual operation of the pulsed ruby laser consists of a series of steps which must be performed in the proper order. The flashlamp requires considerably more power than can be supplied on a continuous basis

so it is necessary to store energy for use during the pump cycle. This is accomplished by charging a large

capacitor. The amount of stored energy is adjusted by

controlling the voltage to which the capacitor is charged. Once the capacitor has been energized, the

laser is ready to operate. In order for the laser to lase, the ruby rod must be

"pumped" by a Xenon flash tube. The pumping period

approximates one millisecond, after which the ruby

rod is capable of lasing. It is necessary, then, to initiate the pumping cycle before the ball impacts the specimen.

The signal for this is provided by breaking the CW
Partially

Reflecting

Mi rror

Light

Out

Figure 8. Schematic representation of a pulsed ruby laser.

laser timing beam with the ball as it nears the speci- men. This signal is fed into a delay circuit where it is held in abeyance as the ball continues its swing. At the proper moment before impact, this signal is fed into the flashlamp circuit and the pumping cycle begins.

As the ball contacts the panel ^t acts as a mechanical

switch, closing the path through the conducting sup- port wire, the ball, and the panel. This circuit activates the Pockels cell control which in turn causes the laser

to fire the first pulse. The stress wave is launched upon

ball impact and the first pulse may be fired simul- taneously or may be delayed to give the stress wave

time to progress. At an arbitrary time later, approximately 10 to 300 ^s, the second pulse may be fired capturing the interferogram on the film plate.

The large wing panel measured 2514" x 48" and

contained four splices. It was clamped, cantilever fashion, to the left side of a support structure which was secured to the table on which the test was con- ducted. See figure 9 for a photograph of the setup. A

small preload was applied to the panel by means of

Figure 9. Inspection setup.

the small knurled screw shown at the lower right side

of the panel.

The ballistic pendulum was a 1" diameter brass

ball suspended by a wire which was 16'^4" ^ong. It was caged and dropped by a special release mechanism

seen at the top center of the picture in figure 9. A

camera shutter cable can be seen dangling from this device, and is used to trip the release lever. The drop

height of the ball was 61/^". On top of the taller aluminum pedestal near the left side of the panel was a small rectangular mirror which was used to direct a one milliwatt CW laser beam

across the face of the panel and into a photocell se- cured to the right end of the support structure. This

served as a timing beam, the function of which is described previously in this section. The path of the timing beam and the positioning of the other optics are shown in the diagram of figure 10. The optical elements for the illumination beam are,

in order, one beam splitter and two negative lenses. The reference beam path incorporates the same beam

splitter, a neutral density filter, a small mirror, a negative lens, an iris diaphragm and a final mirror. The

holographic setup is completed by the pulse laser, the

film plate holder, and the CW timing beam elements. The hologram is made in the following manner. The

pendulum ball is placed in the retaining nest, auto- matically cocking the release lever. The room lights are turned off (during an on-site inspection, the area to be inspected can be covered by light proof curtains)

and a fresh film plate is placed in its holder. Following an oral warning, the capacitor bank of the pulse laser

is charged. This takes only a few seconds. With a countdown of "3, 2, 1, fire," the operator then presses the release cable and the ball commences its swing.

The event proceeds from this point according to the programmed timing sequence. After the firing of the laser, the film plate is removed

and placed in a carry box (before turning on the room

lights). The film plate is then taken to the darkroom

for developing, fixing, washing, and drying. The fin- ished product is referred to as a hologram.

97
PHOTOCELL

WING

PANEL

Figure 10. Diagram of pulsed holographic inspection setup.

5. Holographic Results of the Pulsed

Laser Inspection

Several interferograms of the larger panel were pro- duced which revealed a total of 10 marked discontinuities in the fringe patterns. One particular hologram was chosen for illustration in this paper in that it shows

all 10 of these suspicious areas. Refer to figure 11 for a picture of this interferogram. Again, anomalies in the fringe pattern are enclosed in rectangular boxes

Figure 11. Fringe pattern on 48.50" x 25.50" panel obtained

using the pulsed holographic inspection technique.

(The areas of stress corrosion cracking are indicated.)

and are numbered for ease in identification. Inspection was confined to the two center splices. The flaw loca- tion chart of figure 12 indicates the positions of the cracks along these splices as detected by the pulsed

laser holographic system.

Figure 12. Flaw location chart.

6. Comparison of Holographic and

Ultrasonic Results

Figure 5 shows the holographic results of inspection

of the 12" X 13" panel where the discontinuity in the fringe pattern indicates presence of one crack. The

98
same panel was tested by the Naval Air Rework Fa- cility/Alameda personnel using the current ultrasonic technique. The results of ultrasonic testing are shown

in figure 13. A comparison of figures 5 and 13 shows

that both holographic and ultrasonic techniques lo- cated the same crack.

Figure 13. Ultrasonic crack location in 12.00" x 13.00" sample.

Results of holographic inspection of the 12.25" x

17.5" panel are shown in figure 6 where 4 crack loca- tions are indicated by numbers 1, 2, 3, and 5 identi- fying rectangular blocks. The same panel was tested by the ultrasonic technique and the results provided

by Naval Air Rework Facility/Alameda are shown in figure 14. The ultrasonic technique detected two cracks

which were also detected by the holographic technique.

However, the holographic technique indicated the pres- ence of four cracks. To further investigate these suspected crack indications, the panel was carefully dis- assembled and subjected to dye penetrant inspection.

The results of this comparison are provided in a later section. Holographic results of inspection of the 48.5" x

25.5" panel are shown in figure 11. For ease in identi- fication and location, these flaws are shown in figure

12. Figure 15 shows the locations of 4 cracks detected by the ultrasonic-system used by the Naval Air Rework

Facility/Alameda. The flaws located by the ultrasonic system correspond to crack Nos. 1 through 4 shown

in figure 12. Holographic results indicated presence of additional cracks and/or anomalous joint behavior. To

further investigate this aspect the panel joints were

disassembled and subjected to dye penetrant inspection.

Figure 15. Locations of the 4 cracks detected using ultrasonic

inspection.

UlTRASCNIC CRACK ICCATICNS IN 12.25" X 17.50" SAMPLE

CRACK LOCATIONS

Figure 14. Ultrasonic crack locations in 12.25" x 17.50" sample.

7. Comparison of Holographic and Dye

Penetrant Inspection Results

Figure 16 shows the results of dye penetrant in- spection after disassembly of the 12.25" x 17.5" panel. In the figure the crack locations are identified by

circled areas 1 through 5. Further magnified (5 times)

views of each of the crack locations are shown in the same figure. Figure 16 shows that the cracks do not

lie in the fillet but rather on the flat portion of the

tank, a short distance away from the fillet. During the demonstration at the Naval Air Rework Facility/Alameda, a comment was made that the ultrasonic system

currently used does not provide crack locations accurately. The current repair practice of removing the sus- pected area as indicated by the ultrasonic technique

might miss the actual defective site and hence the defective area would not be repaired. A comparison of figure

16 with figure 6 shows that the holographic technique

99
detected only 4 of the 5 cracks. Crack No. 4 in figure

16 was not detected by the holographic technique. By

developing an improved straining technique, it should

be possible to obtain a more uniform and finer grid

spacing of the interferometric fringes on the joint.

Finer and more uniform grid spacing on the joint

should provide improved sensitivity to detect the crack

whose presence was not visible in the results obtained

to date. Figure 17 shows the photographs of the two center

splice joints showing locations of stress corrosion

cracks, upon dye penetrant inspection, after disassembly. For a comparison of the holographic results with

dye penetrant results, it is more convenient to work

with one splice at a time. The left photograph in figure

17 shows crack locations by circled areas 1, 2, and 6. Holographic results in the flaw location chart (fig. 12)

indicate the presence of the same cracks and, in addition, an anomaly in the holographic fringe pattern at location 7. Since the dye penetrant results do not con- firm the relationship of the fringe pattern irregularity

at location 7 to the presence of a crack, it was sus- pected that the anomaly in the fringe pattern may be

attributed to a loose rivet which is indicated by the irregular deformation pattern. This was later verified. The right photograph in figure 17 shows the location of cracks determined by dye penetrant inspection and

are indicated by Nos. 11, 9, 3, 4, 4a, and 10. The

holographic results are shown in figure 12. Cracks 3 and 4 are located in both holographic and dye penetrant results. Cracks 8 and 9 detected to be separate cracks by the holographic results are both confirmed

UliitL- 1 lov 1 ire

.iC > i.iri'. i;- in rc ser t i on

Figure 17. Photographs of the two center splice joints showing

stress corrosion cracks located by a dye penetrant inspection.

to be present by the dye penetrant results as a single

crack shown in location 9 in figure 17. Crack No. 10

is located in both holographic and dye penetrant results. An unlabeled crack between locations 3 and 4 in figure 17 giving the dye penetrant results was not

detected by holographic results. At the area of location

100
5 in figure 12 giving the holographic results, no crack was found by dye penetrant inspection. However, the presence of an anomaly in the fringe pattern, it is felt, results from the fact that there is an abrupt change in the cross-sectional thickness of the panel. This abrupt

change in the cross-sectional thickness is indicated by

a white glow line in figure 17. Since we would indeed

have in our possession the drawings of structures we

wish to inspect, we would have a prior knowledge if such behavior is to be encountered from sudden

changes in thickness. The dye penetrant result at location 11 in figure 17 does not indicate a crack but rather a "scab" of en- crusted material. In summary, the pulsed laser inspection technique

of the 48.5" x 25.5" panel located 3 additional stress corrosion cracks which went undetected by the ultra- sonic system and also indicated anomalous behavior of the joint at locations 7 and 5. The reasons for the anomalous behavior were postulated and verified by

subsequent testing.

8. Effects of Surface Paint

The question arose as to whether a painted wing

panel would respond as well to holographic inspection

as a bare panel. To resolve the question, the one panel

remaining intact, the 12" x 13" panel (the other two had been disassembled), was taken to Naval Air Re- work Facility/North Island to be painted. Four differ- ent thicknesses of paint were applied and a bare strip was left at the top of the panel. Starting at the top of

the picture of figure 18, the respective thicknesses, in horizontal strips, are: bare, 6, 12, 18, and 24 mils.

Figure 18. Demonstration that surface paint does not degrade

the quality of the interferometric fringe pattern.

Original flaw still evidenced by discontinuity in fringe pattern in boxed area.

The panel was set up as before and strained in bending after an initial preload. The resulting fringes were

sharp and of good contrast, showing that in no way

does the paint degrade or hamper the holographic

process. As pointed out earlier, the holographic inter- ferometry inspection technique eliminates the necessity

of stripping the wings as a part of the process of searching for stress corrosion cracking.

It will be noted also that the crack discovered in the

earlier test shows up again in this picture. It is out- lined by the upper rectangular box. Another short discontinuity appeared immediately below this one

(lower box) which may be a separate crack or an

extension of the effect of the original crack.

9. Description of a Portable Holographic

System

A conceptual design of a portable holographic sys- tem for in-situ inspection of Naval aircraft wings is shown in figure 19. The design requires mounting the

optical elements and laser head on an optical mounting

plate as shown in figure 19. The power supply and the cooling system would be mounted on a transporter or on the floor in a Naval Air Rework Facility area.

Figure 19. Inspection setup in an air rework facility.

The concept of "structural signature" to verify

structural integrity of complex aircraft structures is described. In brief the concept is based upon comparing the structure against itself periodically as the

service life of the structure progresses. As the structure experiences progressive duty cycles, changes in the

"structural signature" of a highly redundant structure should result from fatigue or reduction in a cross- sectional area of a structural component. It is proposed

101
to use the "structural signature" to verify integrity of a complex structure without its disassembly or any

special surface preparation.

10. Conclusions

The following was accomplished:

(1) All the cracks previously detected by the cur- rently used ultrasonic technique by Naval Air Rework

Facility/Alameda in P-3 wing plank panel splices were

detected by the holographic technique.

(2) Additional cracks were also detected whose

presence was later verified by dye penetrant inspection after careful disassembly of panel joints.

(3) Stress corrosion cracks in P—3 wing plank

splices were detected without requiring stripping of

the paint prior to inspection. The confirmation was made by performing the inspection holographically

with and without the panel painted with up to four

coats of polyurethane paii\t. This feature of the in- spection technique offers substantial savings because

stripping and resealing operations, which are currently necessary when the ultrasonic technique is used, will be eliminated. Reduction in inspection time by a factor

of 3 for location of stress corrosion cracks in P-3 air- craft wing plank splices should result when holographic

techniques are used.^ If automated film processing and

data reduction techniques are developed and incor- porated into the holographic inspection system, a re- duction in inspection time by a factor of 5 may be

achievable.

The authors wish to thank J. E. Wright, J. L. Jacoby,

W. S. Tierney and E. K. Burchman of the Advanced

Technology Staff, Space Vehicles Division, TRW Sys- tems Group, for their providing the experimental

results reported in the paper. The work described in the paper was performed under the sponsorship of the Analytical Rework Program, Naval Air Systems Command (AIR-4117) to verify structural integrity of Naval aircraft. The authors also wish to thank Naval

Air Rework Facilities personnel at Alameda and North

Island, California, for their cooperation.

11. References

[1] Case histories—stress corrosion and corrosion enclosure

(4) to letter NARF-313-RAW, from A. W. Tucker, Naval Air Rework Facility/Alameda, to P. G. Bhuta,

TRW Systems Group (12 June 1972).

[2] P-3A BUNO 151387, wing panel splices; excessive crack- ing of, Naval Air Rework Facility/Alameda from S. G. Kurz, Enclosure (5) to above letter NARF-313-RAW.

12. Discussion

A. J. McEvily, University of Connecticut: I was inter- ested in the fact that you identified the cracks as stress corrosion cracks in the Electra and wondered how you

^ Verbal communication between Naval Air Rework Facility/Alameda and TRW personnel. Sixteen h to prepare splice gaps, 40 h to ultrasonic inspect and 75 h to reseal gaps.

made that determination in the sense that here is a structure that is subjected to cyclic loading? How do

you know whether you have corrosion fatigue or stress corrosion cracking as your primary problem?

/. Erthal: This work was done by the engineering and

materials group and a metallurgist. Experience in ex- amining planes over the last 15 years has led to the belief that it's stress corrosion rather than fatigue. Of

course it could be a combination; pitting corrosion can develop that leads to nuclei for the stress raisers that allow fatigue to enter the picture. But, they are

usually identified as stress corrosion cracks.

A. J. McEvily: What led them to believe that they were stress corrosion cracks?

/. Erthal: The decision is based on a failure analysis

by the metallurgical department.

D. C. Drucker, University of Illinois: I would think,

to look at the effect holographically, that the loading

would be equivalent to that which the aircraft had in

service. I wonder why impact type loading was used

for the hologram instead of the much simpler, and

perhaps more direct, method of putting a vibrator at the end of the wing. I believe this would open the cracks in the proper direction while the impulse might

or might not do that.

/. Erthal: It is planned to determine the best way of loading the wing.

C. E. Vest, NASA, Goddard Space Flight Center: What

do you do after the crack is found?

/. Erthal: That depends on where it is and what kind

of data we have for crack propagation for the particular aircraft. It is up to the stress analyst, the de- signer, and the materials people to decide from the fracture toughness data, or fracture mechanics data, to say fix it, take it out, repair it, and so forth.

C. E. Vest: Are the cracks large or very small?

/. Erthal: The smallest one was roughly a little less than a quarter of an inch. We found them up to one and one-half inches long.

C. E. Vest: How big does a crack have to be in order,

to ground an aircraft?

/. Erthal: That depends on the aircraft, where it is, and who says ground it, or what kind or operation it's used in. No doubt, I have seen aircraft fly with 3 or 4

inch cracks.

A. J. Babecki, NASA, Goddard Space Flight Center: Inasmuch as you are using lasers in the inspection

system, can you discuss the safety aspects?

/. Erthal: The laser beam is similar to a welding arc. The light should be kept from the eyes. We follow

102
OSHA requirements as well as requirements of our own safety people.

/. K. Miska, Naval Ship Engineering Center: How

often do you perform a holographic inspection? What

is your confidence that if the crack does exist, you

really find it? In other words, what is the reliability of

that kind of inspection?

/. Erthal: The inspection is so far about 95 percent

reliable, but as I said, this is a feasibility study and

limited in extent. It does look feasible.

/. K. Miska: And how often do you perform these? Is

it routine?

/. Erthal: If it proves satisfactory, it would be used

as a routine tool when an aircraft comes in for rework.

103

CONSEQUENCES OF MECHANICAL FAILURE

Session III Ccont)

Chairman: J. R. Strang

Air Force Materials Laboratory

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Economic Considerations in Failure Prevention

Howard E. Morgan

Office of the Director

National Bureau of Standards

Washington, D.C. 20234

Economic considerations normally determine if product failure prevention is justified by a producing firm. Some failures are not worth preventing, even if prevention is technically feasible. Since the risk of failure is inherent in all products, the expected benefits of failure prevention should be compared with expected costs, both discounted to present value in a life-cycle context and compared with alternatives. Producers and users of a product may evaluate the risk of failure and its economic cost

differently. This may lead to inefficient and inequitable production decisions affecting the public

interest. Consequently, externalities (third-party effects) may require that regulatory or other agencies of Government intervene to protect the public and to insure socially efficient and equitable

allocation of resources to failure prevention. Failures represent a gap between expectations and performance. Both involve measurement and

standards technology which the NBS can provide. However, economic analysis should be used to determine the conditions under which failure prevention is desirable. Examples of economically

desirable conditions for failure prevention are provided.

Key words: Economic cost of failure prevention; frequency of failure; government action; life-cycle costing; marginal analysis; quality control; safety standards.

1. Failure Prevention Management

Requires Economic Considerations

Some mechanical failures are not worth preventing.

This statement may be heresy to an audience composed

of professional people dedicated to failure prevention.

Yet, upon reflection, most probably would agree with

it. This paper provides arguments in defense of that statement. The central question addressed here is: "When is it economically desirable to prevent mechanical failures?" If this question can be answered, we have a basis for setting priorities and allocating resources to failure prevention. We can then sort the

trivial from the important problems and perform

needed research and engineering work more efficiently. Since zero defects (nonfailure) usually is an economically unrealistic objective, economic considerations

should prevail in the management of a failure prevention program.

Mechanical failure prevention is a problem of importance to manufacturers of durable consumer goods

and industrial goods which increasingly are becoming more complex. Servicing of these products involves

the correction of defects often not recognized at the time of manufacture. These failures cost a firm money,

both in warranty costs and the loss of repeat business. In addition, mechanical failures are receiving increas- ing public attention with numerous Government pro- grams designed to improve public safety endangered

by these failures. Automobiles and other consumer

products are receiving particular attention. Economic

losses in property and personal injuries—often involv- ing passive third parties—have made failure prevention important to both private businesses and

Government.

This discussion proceeds by indicating some of the

possible economic consequences of failures. It is then

argued that failure prevention can be managed by

quality control techniques. This leads to a discussion

of the definition of quality, the frequency distribution

of failures, and the economic implications. Techniques

for economic analysis in quality control are then

presented and illustrated. The paper concludes with a

brief discussion of the appropriate role of Govern- ment in failure prevention.

Specifically excluded from the paper are techniques

of statistical quality control, theoretical issues of in- terest to economists, and a discussion of risk-benefit analysis to decide "how safe is safe enough." Rather,

the focus is on those analytical techniques of value to business management in deciding which failures are worth preventing.

2. Economic Cost of Failures

Comprehensive statistical data on the economic cost of mechanical failures do not exist. Sellers do not wish

to publicize their mistakes; this may account for the

fact that we have only anecdotes rather than aggregate

data on failures. Consequently, one is forced to use

107
proxies to indicate the possible magnitude of the economic cost of mechanical failures. Some of these data appear in tables 1 through 6. Some fraction of all accidental deaths undoubtedly

are caused by mechanical failures. Table 1 indicates the number of deaths and death rates from accidents occurring in 1967. Of a total of 113 169 deaths, nearly

half were attributed to automobile accidents. Another

one-fifth was caused by accidental falls. If only one

percent of these nearly 75 000 deaths were attributable

to mechanical failures, 750 deaths would have been

caused by mechanical failures in 1967. Table 2 in- dicates the total cost of accidental injuries and deaths

in 1963. This amounted to nearly $10 billion dollars of which a large share is the discounted value of lost earnings of those killed or injured in accidents. If only 1 percent of these injuries were caused by mechanical failures, the economic cost would have been

around $100 million dollars in personal injuries and

deaths, exclusive of property damage.

Table 1. Deaths and death rates from accidents, 1967.

Death rate Number of per 100,000 Cause deaths population

all accidents: 113,169 57.2 Motor vehicle 52,924 26.7 Railway 997 0.5 Aircraft 1,799 .9 Accidental falls 20,120 10.2 Miscellaneous Machinery 2,055 1.0 Fire and explosion 7,423 3.8 Firearms 2,896 1.5 Drowning 5,724 2.9 Poisoning 4,080 2.1

All causes

° 1,851,323 935.7

" Mostly due to diseases. Source: Statistical Abstract of the U.S., 1970, p. 57

Table 2. Total economic cost off accidental personal injuries, 1963.

Cost of illness Amount (mil. dollars)

Direct expenditures 1,703 Indirect costs

1,811

Mortality cost

6,427

Total cost 9,941

" Lifetime earnings discounted to present value at 6 percent Source: HEW, Estimating the Cost of Illness, Health Economics Series, No. 6, 1966, Table 32

Where do accidents occur? Most injuries occur to persons at home or at places other than at work. Of

nearly 50 million persons injured in accidents in 1968,

20 million were injured at home and another 20 million

in motor vehicles, or at other places. This suggests that mechanical failures in equipment used in the home or

at locations other than work may be worthy targets for investigation (table 3).

Table 3. Accidental injuries by circumstances, 1968.

Persons injured (mil.)

While at work 9.3 While at home 20.5 Other (includes motor vehicles) 20.8

49.0

Source: Statistical Abstract of the U.S., 1970, p. 82

The incidence of injuries in industry is indicated in table 4. The metal producing and forming industries,

of particular interest to this audience, are not especially high in injury frequency or severity rate. On the other hand, the extractive industries, especially coal mining, is hazardous. The construction and transportation industries also have a high injury frequency rate, some portion of which may be due to mechanical

failures. Injuries occurring at home are now receiving the

attention of the new Consumer Product Safety Commission. Among the more hazardous sources of in- juries, thus far identified, glass bottles and glass doors

rank conspicuously (table 5). Undoubtedly, some of these injuries are attributable to mechanical failures

in glass. Perhaps this audience should give attention to this problem, along with mechanical failures in other

materials. A somewhat different way of viewing mechanical

failures appears in table 6. Between 1960 and 1970,

the cost of automobile repairs as percent of depreciation declined from about two-thirds to one-half. To the extent these data reliably indicate the magnitude of the economic cost of mechanical failures in auto- mobiles, the auto industry appears to have made

progress in reducing the economic burden of these

failures on consumers, if not in the inconvenience of required repairs [1].^ Similar analyses might be per- formed for other types of consumer durables and in- dustrial equipment where mechanical failures may

have considerable economic cost. (Life-cycle costing

discussed below provides a technique for comparing

the lifetime cost of alternatives.)

To identify industries using metals and other materials subject to mechanical failure, the Input-Output

table for the U.S. economy is useful [2]. This permits one to measure the dollar value of purchases of materials by using industries; if the failure rate of that material were constant in all applications, one would

have an indication of the magnitude of failure-relative economic losses in all industries using that material. For example, if the impact of copper corrosion were in proportion to the copper purchases by the industries using primary copper in 1963 (industrial chemicals

$11.1 million, blast furnace products $12.5 million, iron and steel foundries $6.9 million, primary copper manufacturing $659.2 million, copper rolling and

^ Figures in bracltets indicate the literature references at the end of this presentation.

108
drawing $684.1 million, nonferrous wire $257.8 million, pipe valves and fittings $45.7 million) this would

indicate where copper corrosion is apt to be found a problem. Of course, this offers only a first approximation to the incidence and magnitude of copper corrosion

because a corrosive environment (e.g. chemicals) may

cause higher corrosion losses in some industries than

others.

Table 4. Injury frequency rates and severity measures: selected industries, 1968.

Industry Frequency rate injuries per mil. man-hours

Severity rate days per mil. man-hours

Manufacturing: Food products 26.8 1,023 Lumber and wood 36.1 2,973 Furniture 22.3 945

^fr»n*> 1 Q \r (tIqcc 21.6 1 919 Primary metals 10.

z

Fabricated metals 21.1 924

• JVlining

Coal mining 4(1 R

IVtetal mining and milling 21.1 "i 997 Nonmetal mining 22;5 3,010 Sand and gravel 20.2 2,440 Stone quarrying 16.6 2,505

Construction 26.9 1,992

Transportation:

Passenger 23.6 1,220 Motor freight 31.7 1,821

Services: Auto repair 14.5 568

Misc. repair 20.1 710

State hospitals 20.0 535

Source: Statistical Abstract of the U.S., 1970, p. 237

Table 5. Personal injuries by class of consumer product, 1969.

(Hospital emergency room reporting)

Glass bottles and containers

Floors and flooring materials Outside walls and fences

Bicycles

Stairs Gas furnaces

Structural glass and glass doors

Frequency/severity index

127.7

119.8

110.7

96.0

83.2

70.1 40.9

Estimated extra cost

to reduce hazard

2/10^' per bottle N.A.

N.A.

N.A.

N.A.

N.A.

110 per door

Source: National Commission on Product Safety: Final Report, 1970, pp. 40 and 68

N.A.: Not available.

Table 6. Estimated 10-year cost of automobile depreciation and

repair: 1960, 1968, 1970.

1960 1968 1970

Depreciation 12,542 $2,806 13,185 Repairs and main- 1,722 1,788 1,521

ten-ahce.

Repairs as percent of 68 64 48

depreciation.

Source: Statistical Abstract of the U.S., 1970, p. 547

109
3. Failure Prevention = Quality Control

The title of the group sponsoring this conference

reflects a pessimistic viewpoint. Failure prevention

implies a pessimism which can be translated into the more optimistic term, "quality control." Technical

know-how and effective quality control management

can prevent those failures which are worth preventing.

Quality control represents the optimist's point of view

[3].

Statistical quality control relies heavily on the use

of probability sampling to estimate the characteristics

of a group from which the sample is drawn. We shall not discuss this technique here; it is important only to note that some variability is inherent in the characteristics of any product. Quality control for failure pre- vention should measure and recognize the implications

of that variability.

There are two concepts of quality which must be

distinguished in quality control for failure prevention;

table 7 identifies and illustrates these concepts. Design

quality is a specification a product must meet in a given use; it may be expressed either as a set of product attributes or as a set of performance require- ments for a particular use. Design quality is focussed on user needs. In contrast, conformance quality indi- cates the extent to which a product meets the design

requirements; i.e., to what extent do the physical at- tributes of the product comply with design specifica- tions? The term "standard" may refer to either the

quality of design, considering use, or the extent to which a product conforms to design.

Shewhart, in his classic Economic Control of Quality

of Manufactured Product, neatly illustrates this dis- tinction graphically as illustrated in modified form in figure 1 [4]. In figure la., design quality is represented

as a three-dimensional box having specified limits or dimensions. A product whose attributes lie within the box meets the specified design quality or standard. In

figure lb., conformance quality is illustrated by a particular point lying entirely within the box. A mechanical failure could occur because of either a design

failure or the failure of a product to conform to design.

Table 7. Quality of design versus quality of conformance.'^

Term Defined Example Source of information

for quality control

Design quality

A specification for a given

functional use. "Grades" of steel differ withuse. Value to user may de- pend on grade of steel.

Users of product and other

sources external to producer.

Conformance quality The fidelity with which a product conforms to the de- sign specification.

Steel of a given grade which

meets the design specifica- tion for that grade of steel.

Internal to producing firm

laboratory or production de- partment. "A "standard" may refer to design quality (described by physical characteristics or by performance in use) or to conformance

quality.

X3

3)

Figure 1. Quality of design versus quality of conformance. (Source: Shewhart, page 40.)

(a) Quality of design is represented by a multi-dimensional space with specified limits. (b) Quality conforms if a point falls within design space. Q = Xi , X2 , X3 P =Xn, X21, X31

110
4. Distribution of Failures

Experience indicates that there are a few important

failures and many trivial failures [5]. A frequency

distribution of economic losses or severity of failures typically is skewed and described by one or more of

the familiar mathematical functions. Figure 2 illus- trates one such distribution described by the Lorenz

curve which indicates the cumulative percent of failure

losses plotted against the cumulative percent of quality

defects. For example, 80 percent of warranty costs may

be due to 20 percent of all of the defects in an auto- mobile.

100 TIME TO FAILURE

100

100 Cumulative percent of quality characteristics

Figure 2. Pareto- (Lorenz-) type curve NOTE: The Lorenz

curve has incorrectly been called a "Pareto" curve by quality

control specialists.

(Source: Joseph M. Jurin and Frank M. Gryna, Jr., Quality Planning and

Analysis, copyright 1970 by McGraw-Hill Company. Used with permission

of McGraw-Hill Book Company.)

The typical frequency of failures over time is illus- trated in figure 3 with the exponential distribution. A

high proportion of defects will appear early in the life of a product; the frequency of defects then declines as the product ages [6]. This suggests that efforts to correct defects early in the life of a product may be

economically more promising. However, without in- formation about the value of prevention and the cost of correcting defects, one cannot assert that this is true. These and similar statistical distributions suggest,

however, that not all failures are equal in magnitude

and therefore, the prevention of some may be economically more worthwhile than others.

FlcUBE 3. Distribution of time of failure.

(Source: Joseph M. Jurin and Frank M. Gryna, Jr., Quality Planning and

Analysis, copyright 1970 by McGraw-Hill Company. Used with permission

of McGraw-Hill Dook Company.)

5. Economic Analysis for Quality Control

In deciding how to allocate resources for failure prevention and quality control, familiar techniques of economic analysis are applicable. Specifically, marginal

analysis and life-cycle costing are useful techniques

illustrated in figures 4, 5. and 6, and tables 9 and 10.

First, however, table 8 indicates the elements of value and cost which should be considered in quality

control. It is necessary to obtain information on some

of these items before the techniques of marginal analysis and life-cycle costing can be used to make choices.

Table 8. Components of the value and cost of quality.

Value of quality considers

user's —Knowledge of product and

market —Service needs over time —Aesthetic preferences —Safety and health —Life cycle cost

Cost of quality considers producer's

Quality design costs: —Market research costs —R&D costs for product —Advertising quality features

Conformance costs: —Planning and maintenance

of quality control processes and machines —Quality control inspection and action to correct defects —Warranty and other costs due to defects or failures

Note: Assume quantity sold is given, unit price may vary

with quality. Source: Juran, pp. 42-46.

Table 8 suggests that the value of quality (design

quality) must consider the users needs and wishes. This includes both economic and noneconomic considerations facing the user of a product. The cost of

111
quality includes those costs associated with design and

the cost of insuring conformance with the specified

design. To the extent that these value and cost elements can be identified and appropriate data obtained, the

analytical techniques which follow can be used.

An economically optimum level of design quality is illustrated in figure 4. As the quality of design is improved as one moves on the horizontal axis from 1 to 3, the value of quality first increases more rapidly than

the cost of quality, then declines. Area B is greater

than Area A, indicating that the marginal value of quality improvement exceeds the marginal cost of quality control as one moves from design quality 1 to

2. At 2 a maximum difference (i.e., profit) exists be- tween the value and cost of quality. Between design

quality 2 and 3, the marginal value of quality improvements represented by Area D is smaller than the marginal cost of quality control in Area C.

0 1 2 3

Quality of design

Figure 4. Economics of quality of design.

(Source: Joseph M. Jurin and Frank M. Gryna, Jr., Quality Planning and

Analysis, copyright 1970 by McGraw-Hill Company. Used with permission

of McGraw-Hill Book Company.)

Marginal analysis similarly can be used to decide

upon the best level of quality conformance as illus- trated in figure 5. As the percentage of defects declines along the horizontal axis, the cost of quality control

rises first slowly, then rapidly to infinity as one tries to obtain zero defects. At the same time, however, the

cost of defects is high at a level of 50 percent or more

defects, but declines rapidly to zero with zero defects. Where the cost of quality control and the losses due to defects intersect, the total cost of quality conformance

is at a minimum. This point lies somewhat to the left of zero defects.

100 percent No

defective defectives QUALITY OF CONFORMANCE

Figure 5. Economics of quality performance.

(Source: Joseph M. Jurin and Frank M. Gryna, Jr., Quality Planning and

Analysis, copyright 1970 by McGraw-Hill Company. Used with permission

of McGraw-Hill Book Company.)

Frequently, a manufacturer is faced with defects in each of several products which may be components of a finished product. Table 9 illustrates a benefit-cost analysis for quality control of defects in five products

(A through E) . The second column indicates that products A and B have the major share of all defects, but

the present annual cost of these defects (column 3) is not as great as for products C and E, which account

for only 14 and 9 percent of all defects, respectively.

The probability of success in quality control is in- dicated in the fourth column for each of the five products. Multiplying this likelihood of success by the present annual cost of defects, one obtains the data in the fifth column. Products C and E still offer the

greatest potential savings in quality control. But, the

five products differ in their expected life as indicated

in the sixth column. Recognizing this fact by calculat- ing the expected annual value of quality control over

112
lO ^ CO f""

0 rH \o r-^ in

0\ l6 00 tr~^ ^ CO CO CM CN) CM

1 1—1 1 ^ r-

<sfi>

-0 2

(D 0 0

•t; 0 ij "id 1^ S ^ c c 20 J^C S0 cj 00000 0

S cu 0 fci CM ^ CO ON

<n y V Q (-1 0

""^ W rv ^

r—

1

s

"53

Discoun

resent

v

of

contr

percent

i-H vo ci in CO r-^ in

vd m ^2 CO CO

(-H CM ^ f—

l-H s S*^

o

"o

ontr t3 s >^ a p 0 CO CO in X

"ea

s

for V

oj u

(/J > *j 0 C

(D -~ 0

1/5 CO 0 LO 0

Ti'* CO 10 00' (n' COCM CO 0^ On e

i-H 1—

1

Q

OO

en —

1

M v> 0

03 <D ^ 2 0 c 0 LO LO 0 0 as 3 0

r-; CO \q 10 CO u -Q Ul 0 0

kJ 0 n

< Cl, o.g

annu defec 10 LO 0 to 0 in

CO NO c'o

°

CO CN NO

°

LO Tt* LO ON gI-H

0

0

-<i ro ( ^ 1 rvi l-M ^ I—J W

13

CL, 0

the expected product life of the five products, one obtains the discounted present values shown in the seventh column. Products C and E still account for the major potential value of quality control, but products

B and A have increased in relative importance because

of their longer expected life. The eighth column indi- cates what it will cost in present dollars to control the quality of each of the five products. Net benefits (column 9) indicate that quality control of E may

yield large dollar benefits, while quality control of A

may incur losses. The last column indicates the

priority which should be assigned to quality control of each of the five products. Note that product E, which

has the lowest percent of all defects, may yield the

largest dollar benefit from quality control. One of the important user considerations noted in table 8 is the life-cycle cost of operating alternative pieces of equipment. Table 10 illustrates the application of life-cycle costing to two machines which have

the same design quality. Machines A and B each meet

the same performance standard or design quality in use.

Table 10. Life cycle cost to user of two machines having the same design quality but differing in conformance quality.

Initial cost Annual operating cost (include^ down time)

Service life Salvage value Discount rate

Machine

A B

.15 mil. $0.3 mil.

40 years

$0.1 mil. 10 percent

$6 mil. $0.2 mil.

50 years

$0.5 mil. 10 percent

Life cycle cost

° $7.87 mil. $7.74 mil.

oc,

(1 + r) (1 + r)'

/ "Based on formula: LCo = FC ! + 2

t-l

Where: LCo = Present value FC = First or initial cost OC = Annual operating cost

I — Service life of machine

5; = Salvage value at end of life

r = Discount rate

t = Time in Years of Operation

However, Machine B has higher initial cost, but lower

annual operating cost than Machine A. The life-cycle cost of Machine B is less than that of Machine A, con- sidering the differences in initial cost, operating cost, service life, and salvage value as calculated with the formula shown in table 10. Buyers of industrial equipment and consumers buying durables frequently consider only the initial cost of the product. A failure to recognize the other elements of cost (including the

cost of defects) may yield an incomplete picture of the

total cost of alternative products [7]. Figure 6 illus- trates graphically the tradeoff existing between the acquisition or initial cost of equipment and the net operating cost. The intersection of these cost curves

indicates the optimum tradeoff and minimum life-cycle

cost. In realistic applications of life-cycle costing in a system involving many elements, the tradeoff analysis

becomes much more complex. However, the principles

of marginal analysis and life-cycle costing remain

valid.

113
SCALE OF QUALITY

Figure 6. Life cycle cost of quality.

(Source: Joseph M. Jurin and Frank M. Cryna, Jr., Quality Planning and

Analysis, copyright 1970 by McGraw-Hill Company. Used with permission

of McGraw-Hill Book Company.)

6. Role of Government in Failure

Prevention

One may ask why a paper dealing with economic

considerations in failure prevention should discuss the

role of Government in preventing failures. The answer

is that business firms operating in the marketplace

frequently cannot or do not allocate resources efficiently in failure prevention. Apart from humanitarian considerations, failures which threaten public safety

represent an economic loss [8] . It then becomes neces- sary and desirable for Government to intervene in some manner. In short, a more efficient allocation of resources and greater public satisfaction may result from Government action to correct imperfections in the marketplace.

Some rules (criteria) for deciding when Govern- ment action is appropriate appear in table 11 [9].

There are three basic conditions or criteria for Govern- ment involvement, either in the form of Government

programs which produce services related to failure prevention or Government policy governing private

business.

Table 11. Criteria for Government action in failure prevention.

1. When private firms cannot capture in the market for their product all the henefits (disbenefits) which accrue to third

parties.

2. When production by decreasing cost industries leads to monopoly and output less than socially optimal.

3. When imperfect knowledge about production or use dis- courages socially desirable high risk investments by private

firms.

First, Government action is appropriate when private firms cannot capture in the marketplace all of the economic benefits which may accrue to third parties other than the initial users of their product. For ex- ample, efforts by the Ford Motor Company to sell safety several years ago did not appear successful because the public was not aroused to the need for safety and be- cause buyers of safety equipped automobiles may not have been willing to pay a premium for safety benefits which may accrue to other persons as well as the users of the vehicle. Similarly, firms frequently do

not charge buyers of their product for adverse eco- nomic harm which accrues to third parties. For ex- ample, a railroad may not charge its customers for the

cost of injuries inflicted on third parties occasioned by

a train wreck brought about by mechanical failure in wheels or track. If third parties suffer economic losses without being compensated by users of the railroad. Government intervention may be necessary to insure

equity.

Secondly, Government action is appropriate when an

industry has decreasing unit costs at an output less than- that which is socially optimal. The best example

of this is the public utiUty selling electric or gas

service. These firms obtain economies of scale and

lower unit cost as their systems expand and they are granted a monopoly position to service a given area.

If left to their own decisions, their prices probably

would be higher and their level of service lower than

the public might wish. In such cases, Government

regulatory action is appropriate to insure reasonable

pricing and service levels leading to more efficient resource use. For example, the Federal Power Commission regulates gas pipeline pricing in the public

interest. Recently, pipelines have become a matter of public concern because of mechanical failures which

have resulted in damage to persons and property.

Government regulation is then appropriate (under the

first but not second rule) to insure public safety.

Thirdly, Government action is appropriate when

private firms are discouraged from making high risk investments in socially desirable projects because they

lack the resources or knowledge about the costs of production and payoff from such investments. This is illustrated by NASA research to reduce defects in space

vehicles when the space program was considered to be

important to the U.S. Similarly, the Department of Housing and Urban Development and National Bureau

of Standards research in buildings is intended to reduce

failures which may affect public safety or waste build- ing materials. Private firms in the construction in- dustries, left to themselves, may be discouraged from

making the high risk investments necessary to provide

technology needed to reduce failures. In such cases, Government acts as a risk-taker in providing technical

service to protect public safety and improve the use of materials. Many of the technical activities at the National Bureau of Standards assist industry and Government

in quality control to prevent mechanical failures. For

example. Standard Reference Materials which provide

114
a basis for accurately measuring the metal content of engine oil may help prevent mechanical failures by

monitoring engine wear; should wear become excessive,

engines may be overhauled sooner than normally and

failures avoided. Similarly, Standard Reference Materials are available for determining the composition of

steel and other products. These assist in insuring con- formance quality; i.e., the extent to which the product

complies with a given standard. The Standard Refer- ence Data system provides evaluated data on the characteristics of materials which helps design engineers

determine design quality. Some of these reference materials and data are sold to users at a fee when their benefits are capturable by users in the marketplace.

When some users or beneficiaries cannot be excluded

from the benefits in the marketplace, it is appropriate

for the Government to supply these at the expense of the general taxpayer, provided the economic benefits exceed the economic cost relative to alternative invest- ments in public funds. In reality, many of these technical activities are carried out cooperatively by industry

and Government.

7. Summary

Mechanical failures may impose substantial economic

losses on business firms and the public. No aggregate

statistical data exist to measure these losses; however,

to the extent that accidental injuries and deaths are due

to mechanical failures, the economic losses may repre- sent a substantial sum. This is an economic loss which

may justify much of the technical effort to prevent mechanical failures. But, not all failures are worth preventing. Typically,

there are a few large and many trivial failures. This

suggests that the prevention of some failures may be

worth more than others. The techniques of quality control are useful in managing technical work in failure prevention. These tech- niques are well established and rely upon the disciplines

of statistics and economics. Quality control relies heavily on the concept of probability sampling to estimate the

characteristics of a group from which the sample is drawn. It also requires that a distinction be made be- tween design quality and conformance quality. Design

quality refers to the characteristics of a product in use and may be stated in terms of standards. Conformance

quality refers to the extent to which a product complies

or conforms with the given standard or design quality.

Economic analysis for quality control relies on the techniques of marginal analysis and life-cycle costing.

Typically, there is an optimal level of quality at which

the value of design quality is greatest relative to its cost. Similarly, the cost of conformance quality usually reaches a minimum at some point less than zero defects; i.e., it is usually not economically feasible to obtain zero defects. Life-cycle costing permits a comparison of two or more alternatives available to users, considering all costs over the life of the product. This

method is preferred over a consideration of initial cost only. These techniques permit one to decide which

failures are worth preventing so that technical and other

efforts to reduce mechanical failures can be more ef- fective in reducing the economic losses incurred.

Government action to help reduce failures is appropriate when public safety is endangered by the in- ability of firms to capture in the marketplace the

benefits or disbenefits accruing to persons other than

users of their product or service.

8. References and Notes

[1] One estimate of the economic cost of failure of five common automoljile parts is four billion dollars per

year or $400 over the average vehicle life. This source

also notes the inadequacy of data on vehicle parts

failures. See: Booz Allen Applied Research, Maintainability and Repairability of Vehicles in Use: Volume I Summary (National Technical Information Service, U.S. Department of Commerce, June 1971, Report # PB202 531), pp. III-17 and IV-2.

[2] Bureau of Economic Analysis, U.S. Department of Commerce, The Input-Output Structure of the U.S. Econ- omy: 1967, Survey of Current Business (February

1974), p. 24. [3] In 1748, Voltaire defined an optimist as a madman who

maintains that everything is right when it is wrong.

[4] Shewhart, W. A., Economic Control of Quality of Manufactured Product, (New York, D. Van Nostrand Co., 1931), p. 40. [5] Juran, Joseph M. and Gryna, Jr., Frank M., Quality Plan- ning and Analysis (New York, McGraw Hill Co., 1970),

p. 63. [6] Nelson, Wayne, Charts for confidence limits and teests for failure rates. Journal of Quality Technology (October 1972), p. 190. [7] For a discussion of product durability and service as af- fected by the competitive structure of producing industries see: Richard W. Parks, The demand and supply

of durable goods and durability, American Economic

Review (March 1974), p. 37. [8] There is considerable economic literature on the economic

cost of safety and legal aspects of liability. See for ex- ample: Simon Rottenberg, Liability in law and eco- nomics, American Economic Review, March 1965, p. 107 and note by T. E. Borchering on that article in the December 1970 issue of that Journal, p. 946. See also, R. A. Holmes, On the economic welfare of victims

of automobile accidents, American Economic Review

(March 1970), p. 143. [9] See: Richard A. Musgrave, The Theory of Public Finance (New York: McGraw Hill Co., 1959).

9. Discussion

IV. D. Compton, Ford Motor Company: You made the comment, and I think it's quite correct, that there are examples where the private sector has not had proper market incentives to move as a total industry in a particular direction. I was curious, however, that you did

not suggest how government controls might be used to develop market incentives. How, in fact, does the gov- ernment develop the incentives that will allow the free market to operate?

H. E. Morgan: We are looking at that question with our Experimental Technology Incentives Program (ETIP)

here at the National Bureau of Standards. Experiments

are conducted with private firms to identify the barriers

to technological innovation, some of which may relate

to mechanical failures, and then suggest to the Congress

changes in policies which will provide incentives to firms. Of course, many of us think about tax incentives

115
or a reduction of anti-trust restrictions. These are the

classic arguments often used by business for relaxation

of controls so that they will have more incentive. Our

program is looking particularly at procurement practices of government. What sort of innovations, for ex- ample, specifying performance in use and letting manufacturers come in with various designs to meet these performance requirements, are necessary to achieve

objectives which are publically desirable. We're looking

at regulations that inhibit technological innovation in order to suggest changes which will still protect the public and yet which will provide incentives to firms. Although the effort is underway, it is too early to say how it will pay off. Many other countries have gone way beyond this in that government acts as a partner

with business in attempting to solve these problems.

116
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Economic Impact of Tribology

H. Peter Jost*

Angel Lodge Laboratories K. S. Paul Products, Ltd. Nobel Road

London N18 3DB, England

The emergence of tribology as a subject in its own right arose out of industrial need. In the

early 1960's there was a large increase in reported failures of plant and machinery due to wear

and associated causes. At the same time, technology was increasing the capital intensity of plant and the use of more continuous processes; thus breakdowns of such plant and machinery were becoming more costly. In 1966 a U.K. Government investigation came to the conclusion that by application of

tribological principles, very large savings were obtainable, in most cases without appreciable

capital investment. Tribology was defined as "the science and technology of interacting surfaces in relative motion and of subjects and practices related thereto." In other words, it deals with all aspects of rubbing,

sliding and rolling surfaces and included the subjects of wear, friction and lubrication. Application

of tribological principles would lead not only to greater operational reliability, efficiency and productivity, but also to conservation of materials and energy. To gain the advantages outlined in the above Report, estimated at the time to be in the region of £515 million per annum (p. a.) ($1285 million approximately), the British Government

set up the Committee on Tribology, to implement the recommendations in the education, research and industrial fields. Actual savings already resulting to British industry are conservatively estimated to be in excess of £100 million p. a. ($230 million), and potential savings (allowing for inflation since the 1966 estimate) to over £1.000 million p.a. ($2,300 million). In less than seven years, the new concept of tribology has been accepted by the majority

of the industrial countries as part of their scientific and technological background in the field of obtaining greater plant efficiency, better performance, fewer breakdowns and significant savings in other directions. Key words: Economic savings of failure prevention; friction; lubrication; mechanical failure; tribology; wear.

1. Foreword—The Consequences of

Preventing Mechanical Failure

Friction and wear-caused mechanical failures and

maintenance normally have their root in phenomena

based on tribology, the science and technology of inter- acting surfaces in relative motion.

Like other new industrial technologies, the problems

of tribology are seldom solved by the application of the knowledge or techniques of a single subject area, such

as physics or chemistry, but often require teams of people, bringing their expertise and techniques together

for successful operation. New concepts and techniques

develop from the linking of ideas from two or more dif- ferent disciplines in new ways, but there are often diffi- culties in persuading specialists in established fields of knowledge to co-operate in such multi-discipline activities. Developments in tribology since the publication of

*Chairman, Committee on Tribology, Department of Industry, U.K.,

President International Tribology Council. Tbe views expressed by the Author in this paper are his own and not necessarily those of Her Britannic

Majesty's Government nor those of H.M. Committee on Tribology or of the International Tribology Council, of which tbe author is Chairman and

President respectively.

the "Jost" Report have utilised existing and new knowledge from physics, chemistry, mathematics, statistics, engineering, etc. and advances in diverse fields from

metalworking to medicine or space technology have

been produced as a result. The economic benefits that may accrue to industry

from the application of such new technologies can be

substantial; in the case of tribology in the United

States, they could amount to as much as $16 billion per annum. Some of the means which were utilized in the United

Kingdom in order to reap the benefits attainable by application of the principles and practices of the multi- disciplinary subject of tribology and some of the results obtained are described in the following paper.

2. Introduction

It is only eight years ago that the word "tribology" was used for the first time. It saw the light of day in a Government Report [1],^ published by the United

^ Figures in brackets refer to reference and bibliography listings at the end of this presentation.

117
Kingdom Department of Education & Science, as a re- sult of an investigation of a Working Group under my

Chairmanship.

Whilst none of the constituent members of this new

science and technology were new, some had indeed been

known for many centuries [2], tribology brought to- gether—for the first time—all the constituents under

the overall description of "interacting surfaces in rela- tive motion." This approach was new, inasmuch as no- body seemed to have regarded such subjects as "wear,"

"friction," and "lubrication" as wholly interdisciplinary—embracing Physics, Metallurgy, Chemistry, Mechanical Engineering, Mathematics, etc., as well as being parts of the same concept, namely that of forces being transferred from one surface onto another, at a time when the surfaces were in relative motion. Tribol- ogy is therefore a realignment and a polarisation of thoughts in many spheres, all basically dealing with

"force transference between surfaces moving relative

to each other," and as such, it was new. Now it is not unknown that a new concept, particularly one that consists of a new orientation of some very

old ones, may not always be willingly accepted and may

indeed meet with fierce resistance, particularly from the experts in a particular constituent of the new concept.

This is exactly what happened in the case of tri- bology. Indeed it is true to state that some of the greatest reluctance to accept the new concept came from a few experts in lubrication—some of great eminence and

distinction—and from those countries, including the United States of America, in which lubrication research was particularly advanced. Yet in spite of this, there are now tribology societies and tribology groups—as

distinct from lubrication societies and groups—in the United Kingdom, Western Germany, Holland, France,

Norway, Sweden, Poland, India, and in formation, in

Italy. Tribology, or tribotechnology, has also become a recognized subject in the German Democratic Republic

and others. Why did this happen in such a short span of time and

in spite of the lack of support from some academic

quarters? The reason is very simple: the new approach

which tribology provided was needed by industry.

The findings of the original Working Group ex- pressed the view that with increasing mechanisation

and automation, with greater use of capital intensive equipment and higher production rates, breakdowns

and mechanical failures were becoming increasingly ex- pensive, both directly and consequentially. Amongst the principal reasons for the normal and general occurrence

of maintenance, breakdowns and mechanical failures in industry, breakage, corrosion and wear feature prominently, the latter being the predominant group of causes

of maintenance and not insignificantly those of failures. Efforts to prevent such wear-caused maintenance,

breakdowns and mechanical failures have led to the fundamentals of tribology, i.e., the fundamentals of "interacting surfaces in relative motion," becoming rec- ognised at a principal consideration in engineering

design, and therefore of considerable importance to in- dustry.

There is no doubt that, at least in the United Kingdom, the tribology movement was carried on the shoulders of industry. When, in five years' time, our present

instructional campaign is completed, it is hoped that there will be at least 150 000 people in the United Kingdom, most of them at the technician level, but also quite a number of professional and qualified engineers and

other scientists, capable of utilising knowledge of the

practical facets of tribology for design purposes, in much the same way as they now use "strength of materials" and other "materials science" subjects.

A great deal of this training will have to be per- formed by industry and in industry; plans to achieve

this are now well advanced. There is little doubt or argu- ment that if industry is to profit from tribology, in- dustry itself will have to learn how to apply tribology

in all its aspects.

3. The Macro-Economics of Tribology

In 1965, the Department of Education and Science

Working Group estimated that tribology could reason- ably save British industry about /515 million per an- num ($1236 million). Details of the investigation and

the basis of the estimated savings were presented in 1966 in a U.K. Government Report which has since been referred to and may be known to you as the Jost Report [1].

Figure 1 gives a breakdown, taken from this report, which shows that the vast majority of the savings,

namely those falling under headings d-g, have a direct relevance to the theme of this conference. One can rea-

£ Million $ Million

a). Reduction in Energy Consumption

through lower friction 20 48

b). Reduction in Manpower 10 24

c). Savings in lubricant costs 10 24

d). Savings in maintenoce and replace- ment costs 230 552

e). Savings of losses consequential upon breakdowns 115 276

0- Savings in Investment due to higher

utilization and greater mechanical

efficiency of machinery 22 52

g). Savings in investment through

increased life of machinery 100 290

1965 Totals

(as published in 1966) (') 515 1,236

Conversion rate of S2.40 per £1.00 Is used throughout

this paper.

Figure 1. Estimated figures of savings obtainable by British

industry.

118
sonably assume that after taking into account the infla- tion of the last 9 years, this figure is likely to be in the region of £850 million per annum ($2040 million). It should however be noted that these and the following

calculations do not take account of the increased oil prices due to the recent Middle East crisis. There is some evidence that, in the United Kingdom,

through application of tribological principles and practices, industry is effecting considerable savings which

may exceed £200 million per annum (480 million) [3].

The direct cost of government expenditure, including

the salaries of the Civil Servants involved, to achieve

this end was in the region of £1.25 million ($3 million)

.

However, included in this figure are about £750 000

($1.8 million) for the financing and setting up of three Centres of Tribology. These are principally consulting

centres, all of which sell their services to industry, all of which are now self-supporting and which together

have earned foreign currency to the value of nearly $1.5

million. What would the equivalent figures of potential sav- ings be for the United States? " Only very rough approximations can be made, based on limited comparative parameters, and even these may

not be wholly accurate. Taking the employment in manufacturing in 1971 to be: U.K. 8.1 million. United

States 20.9 million; crude steel production in U.K. 24

million metric tons, U.S.A. 109 million metric tons; gross domestic products (purchasers' values in U.S. million dollars); U.K. 134 838; U.S.A. 1 045 753, and

estimated energy consumption in million metric tons

coal equivalent: U.K. 307, U.S.A. 2327, [4, 5] the es- timated range of potential savings obtainable at present

in American industries, through tribology, could be be- tween $12 and 16.3 billion (fig. 2).

Whilst these amounts are of course only a small portion of the gross domestic products of either country,

they are both substantial and significant and represent sums which the industries of both our countries would

be very pleased to save, particularly as such savings can be obtained without the deployment of large capital

investment. For the macro-economics of tribology are of

a nature which enable large returns to be obtained from

minimum capital outlay.

It is not often that an intermediate technology can

claim to procure a measurable economic effect within

a period of a few years. However, at least as far as the United Kingdom is concerned, tribology—in the eco- nomic sense—has become one of the few applied sci- ences and technologies which is believed to have

succeeded in making an impact, albeit a small one, on

both national productivity and the economy.

4. Micro-Economics of Tribology

The spectrum of micro-economics of tribology ex- tends from the summation of a multitude of small savings, obtained by the application of correct tribological

principles, designs and practices on the one hand, to single or group savings of large individual amounts, the

significance of which has been outlined in many papers

in Europe and which is of particular interest to this conference. For the purpose of this paper, these two

types of savings will be referred to as indirect and direct savings, respectively.

Direct savings are those which the direct application

of tribological knowledge can procure, e.g., by direct expert advice leading to a design change.

Indirect savings are those savings which are applied

only when the tribological knowledge, say that acquired

1971 1971 1971 1971 1974

Employment in manufacturing

Industries

Crude Steel ProductionGross Domestic

Product

(purchasers volue)

Estimated Energy Consumption

Estimated Savings

through Tribology

(A) (8) (C) (D) (E)

Millions

Millions Metric TonsMillion

Dollars

Million Metric Tons Coal Equivalent Million Dollars

UK 8.1 24 134,838 307 2040

USA 20.9 109 1,045,753 2,327 12,000 to 16,000

SOURCES: (A) ILO Year book of Labour Statistics (1972) Table 2 Adjusted for U. K.

(B) U.N. Statistical Yearbook (1972) Table 124

(C) U.N. Statistical Yearbook (1972) Table 188

(D) U.N. Statistical Yearbook (1972) Table 140

(E) Author's calculated estimates, based on D, E. 5. Report and price Indices Numbers, U.N. Statistical Yearbook

(1972), and above tables.

Figure 2. Potential savings comparison, UK—USA.

Although the figures for both U.S.A. and U.K. are based on the present systems of National Accounts of our countries, comparisons of this nature must of necessity be somewhat inaccurate. However, even if, for a number of reasons, such inaccuracies were ± 25 per- cent (or even ± 50%), the order of magnitude of sav- ings is so great that they should not be lightly ignored.

at school or university, has penetrated to its ultimate

points of application, e.g., design offices. The procurement of savings by the indirect route must be part of an integrated policy of developing and

transferring tribological knowledge into industry. This

includes knowledge transfer of the application of multitudes of minor elements of tribology into the every119
day life of the industrial bloodstream. This normally

necessitates a policy involving education, training, and

appropriate promotion on a nationwide scale. If this is done, a permanent improvement of industrial processes,

of significant magnitude, and of industrial designs of all machinery and equipment can be obtained. Indirect savings may, however, be difficult to quantify accurately; to do so requires the establishment of a bqsis

connected to more definite and to more quantitatively

measurable savings, after which by extrapolation and

other means, the more difficult measurable indirect sav- ings through tribology can be estimated. In any case, as in any other subject, the industrial

benefits of education and training are difficult to quantify with any degree of accuracy; however, there is evidence to show that—as far as tribology is concerned —in terms of productivity and profitability, their value

is very considerable. The other end of the spectrum of the micro-economics

of tribology are the direct savings, i.e., those of indi- vidual or group savings of significance; these are more

easily measurable. Within this group fall the large tri- bological savings obtainable by those industries which

are based on capital intensive plants with costly production rates, particularly the process industries. An interesting example of such industry, in which

the application of tribological principles and practices

has led to considerable and measurable savings in both

the United Kingdom and in the United States, is the iron and steel industry [3].

When, in 1965, the original British Working Party

estimated savings obtainable through better application

of tribology and better education and research in tri- bology, out of an estimated saving of £515 million per annum ($1236 million), as being applicable to the whole British Industry [1], a saving of about £20 million per annum ($48 million) was calculated in respect

of the Iron & Steel Industry. It was found that such

saving could be obtained by the application of modern

tribology to iron and steelmaking practices and machinery.

About years ago, the British Steel Corporation set up a Tribology Section. This was an entirely new venture, being distinct and different from the Lubrication

Engineering activities which the British Steel Corporation engages in in its many steelworks. To run this section, which is headed by Dr. Fred

Westlake, a former student of Imperial College, London

and of MIT, costs the British Steel Corporation approximately £50 000 ($120 000) per annum. (It would probably cost twice as much in the United States.) One of its main purposes is to look at tribological problems, which

cause significant losses, and to solve them.

The criteria employed in the selection of its work

were outlined by Dr. Westlake in May, 1973 [3]. They

are as follows:

Firstly: The solution of a problem must be capable

of leading to savings of at least £1 million ($2.4 million) per annum.

Secondly: A problem must be capable of being

solved within a period of 12 months.

Thirdly: There must be a 90 percent plus chance

that the problem can be successfully

solved.

It was estimated that through tribology, by the end

of 1973, the British Steel Corporation would have al- ready saved £6 million per annum ($14.4 million), and

a figure of savings of £20 million per annum ($48 million) was confidently expected by the end of this year. Thus the original 1965 estimate of a total of £20 million per annum ($48 million) capable of being saved by

the British Steel industry, through the application of tri- bological principles and practices, was an underestimate; at least that is the view of the British Steel Corporation. It may therefore be equally true that this degree of under-estimation may also apply to the potential savings of £515 million per annum ($1236 million p.a.) in respect of British industry as a whole. As a result of similar actions in the field of tribology, the equivalent savings that would accrue to the U.S.

Steel industry, calculated on the basis of the difference

in crude steel production between our two countries, could exceed the $200 million per annum mark. You

may feel that even if through greater application of technology in U.S. steelworks this figure were only

seventy-five percent achieveable, it is still one of con- siderable substance. Savings at the above level are, however, not confined

to the Iron and Steel industry. Evidence from another

great process industry, viz: the chemical industry,

showed that by the correct application of tribological

principles, considerable savings were made:

(a) by the reduction of outage time of plant

(b) to the production efficiency of the new plant,

largely through increased reliability, and

(c) by direct savings in purchasing costs.

On the latter point it is interesting to note that correct tribological design need not be more expensive, but can in fact save money.

Two outstanding examples relate firstly to the start of the low pressure ammonia plant, on which a rolling

bearing experienced three catastrophic failures on the vetrocoke pumps. Because of the inherent hazards in- volved, the process operators refused to run the plant

not unreasonably—until reliable bearing arrangements

could be installed. The Engineering Department's tra- ditional proposals were to convert the pumps to plain

bearings using an oil circulation system; this would

have taken 3 months. Tribological investigation suc- ceeded in identifying the cause of failure; and this was so well explained that the Trade Union representatives accepted the modified rolling bearing arrangement

which reduced the outage time from 3 months to less than 3 weeks, representing an ammonia saving of 126 000 tons or (at 1966 prices) £1.26 million ($3

million).

Another example from the same company dealt with

reciprocating compressors, which handled a variety of

different gases. Before the tribological exercise was con- ducted, a conservative estimate of cost (both mainte- nance and production losses) over the 2-year period and

arising mainly from tribological causes connected with

120
piston rings, cylinder liners and glands, was a'bout £500 000 ($1.5 million). Since the modifications result- ing from the tribological exercise were carried out, the production losses in this area were negligible and the maintenance costs small. Summing up, we have on one end the micro-economics

of indirect application, largely introduced by a change

of practices brought about by education, training and

awareness promotion, and on the other end of the spec- trum, significant direct savings arising out of individual

applications or groups of applications.

To meet the requirements of indirect application,

facilities for education, training, and appropriate pro- motion are required; to meet the requirements of direct application, expertise must be provided.

Since in industry the majority of tribological applications occur between these two extremes, both education,

training and promotion on the one hand, and direct ex- pertise on the other hand, are necessary in order to enable industry to gain the benefits of tribology.

4.1. Some Indirect Means of

Tribological Savings

In the United Kingdom, in order to effect industrial savings through the application of tribological principles and practices, the Government Committee on Tri- bology set up a programme to encourage tribology to penetrate the mainstream of industrial life, in particular

in education and training.

Universities, Technical Colleges, Industrial Training

Boards and others were encouraged to cover the whole

spectrum of tribology, and industry was encouraged to make use of the facilities thus made available. In addition, advice on research was offered and some research

priorities established. Advisory services to government

Departments and public bodies and others were pro- vided by the Committee on Tribology.

The work of the Committee on Tribology was fully

reported in "The Introduction of a New Technology." [6] Let it suffice to report that there are now in the United

Kingdom three Chairs in Tribology, over 70 Masters' Degrees in T-ribology have been awarded, a basic tri- bology module—representing the minimum coverage of tribology which should be present in the education of all mechanical engineers—was established; and teaching

material in tribology is now available from school to post-graduate levels.

4.2. The Tribology Handbook

The original Working Group found that there was a need for a reference book for use by engineering

draughtsmen, designers and works engineers, its contents to be expressed in simple terms, primarily non- mathematical, fully illustrated, comprehensive and free from bias. The work—called "The Tribology Handbook" [8] —

was produced by an independent publisher, under the auspices of the Committee on Tribology, and is regarded

by many as a breakthrough in the presentation of a

new technology to the designer, draughtsman, techni- cian and works engineer for the purpose of enabling

him—without the use of much mathematics—to select tribologically correct designs, to use the tribologically suitable materials, and in general to ensure that tribo- logical considerations are not omitted in design. In addition the effects of many tribological failures are shown,

the causes explained and remedies listed. This 500 page book—believed to be the first of its kind in any technology—is already helping U.K. in- dustry to prevent numerous breakdowns, save costs and

improve the quality of its products. A list of subject

headings as well as examples of some of the information

contained in it, are given in appendix 1. It is intended to use the Tribology Handbook as a

training means for technician and crafts courses both

at technical colleges and within industry and to train at

least 45,000 engineers and technicians in its use.

4.3. Industrial Centres of Tribology

The Tribology Handbook already veered in the di- rection of direct savings through tribology by providing

the user with some "expertise" in condensed and readily useable form. In the main, however, to deal with the

direct application of tribology, some of the largest companies in the United Kingdom now employ their own

trained tribologists or have tribology sections. To provide expert advice for the majority of companies, however, three Centres of Tribology were estab- lished with the help of governmental funding [6, 9].

They are the Industrial Centre at the University of Leeds, the National Centre of Tribology at Risley, and

the Tribology Centre at the University College of Swansea. (In addition the National Engineering Laboratory

at East Kilbride, Scotland, also provides tribological services of high quality.)

These Centres, although started off with government

deficiency payments [6], are now viable concerns, sell- ing their services to industry, for which they have pro- cured savings of considerable magnitude, and—as will be seen—generally at comparatively very low cost.

It is important to point out that the work of these Centres does not conflict with the work of the many excellent universities offering their services in the field of tribology. The principal difference is due to the Centres having to earn enough money to keep going and

growing without the need of further government aid,

e.g., the amount of research work that they can afford

to engage in on their own is very limited; indeed such

work would normally have a direct selling relevance,

as in the case of the National Centre of Tribology, which

spent some money on developing a window hinge, which was then patented and is now licensed to commercial

manufacturers.

Thus, in general, in the LInited Kingdom the universities on the one hand, and the Industrial Centres on the

other, do different jobs and have different functions. Since the Industrial Centres are obliged to be eco- nomically viable, companies seeking their advice, or considering seeking advice, may ask themselves "How

much will tribology cost me?" (This is, of course, the wrong question. The right question should be "How

much is bad tribology costing me at present?")

121
That a company need not spend much money to get

profitable advice from an Industrial Centre is shown in figure 3. The cost of the contracts placed with the Leeds

Industrial Centre, varied in value from £10 ($24) to over £20,000 ($48,000) ; the graph shows the distribu- tion of contracts by value, and demonstrates that the majority of problems were solved for less than £500

($1200), and a high percentage for less than £100

($240).

The National Centre of Tribology at Risley, which is a larger unit than either the Leeds Industrial Unit of Tribology or the Swansea Tribology Centre, shows a

distbution curve not dissimilar, but the average value

of individual jobs is somewhat higher (fig. 4). This

LU< 50 - >

t25 126 1100 1250 1500 11000 12000 15000 110,000 120,000

or TO TO TO TO TO TO TO TO AND

under 199 1249 1499 1999 11999 14999 19999 119,999 OVER VALUE OF CONTRACT (NON-LINEAR SCALE)

Figure 3. Distribution of contracts according to value.

Industrial Unit of Tribology—Leeds University.

122
table excludes the $1 million contract which was

awarded to the National Tribology Centre by E.S.R.O.

for the setting up of European Space Tribology Laboratory.

A detailed government survey (in 1971) of the benefits of the Swansea Centre's work to its clients showed a

net benefit considerably more than £125 000 ($300-

000) to 50 clients on projects costing them only £16 000

($38 000) . This attractive financial picture is supported

by a significant proportion of contracts being repeat

orders, which would seem to show the satisfaction of

clients with the work of the Centre. Some work was carried out by the Swansea Tribology

Centre, in conjunction with the Department of Eco- nomics of the College, to find potential savings through

tribology in a very ordinary and not very modern en- gineering works. The survey identified 4 areas as the most obvious in need of improvement. The estimated

annual savings were £8040 (nearly $20 000) for an

investment of £5245 (nearly $13,000), a financial re- turn of the order of 150 percent per annum. Even allow- ing for some inaccuracy in the calculations, this still leaves a very big margin over the usual financial return on industrial investment.

A cost benefit analysis conducted in respect of the University based Centres revealed the following interest- ing features [7]

(1) More than 70 percent of the Centres' customers had approached them for assistance in solving particular problems. The remainder were about equally divided

between those making a general enquiry and those who

wanted organisational assistance. (2) Seventy percent of the Centres' customers who

co-operated stated that as a result of their use of the

Centres' services, they had either saved money or ex- pected to do so. (3) Out of these, 30 percent had saved, or expected

to save, a reduction in maintenance and down-time, 20

percent had saved the time of an information search, 16

percent had benefitted from the building up of general

background knowledge, and the remaining 34 percent

reported savings of a miscellaneous nature.

(4) Even the most conservative extrapolations of the assessed benefits suggested that the net savings, dis- counted to the net present values, were well in excess of

the total grants given to the Centres by government.

(5) At one Centre, the cost/benefit ratio was as high

as 1:6.

4.4. Direct Tribological Savings and Costs

The work of the three centres has been of a wide variety. A few examples to illustrate the range of subjects

in the sphere of tribology with which the Industrial Centres of Tribology try to meet industry's needs of ser- vices in tribology are given in appendix 2. They generally show the savings obtained and the cost of the work

by the Industrial Tribology Centres in the United Kingdom. They include some of the applications which the widening of the spectrum of the new concept of tribol- ogy entailed, and show how these Centres are slowly

becoming an integral part of the industrial design

process. The seven cases referred to in a])pendix 2 cover: (1)

industrial problems of reduction of friction in hostile environments (wire in acid bath) ; (2) the effect of difficult materials on seals (molasses) ; (3) co-operation

in the development of probably the most important

breakthrough in car cafety since the last war (safety

tyre) ; (4) the more traditional redesign of pump bearings; (5) the detection of the tribological cause of computer amnesia; (6) the saving of energy (float glass

process); and (7) medical applications.

It must be added at this juncture, that since complete

commercial security is maintained in the work at these Centres, only those examples can be quoted which have

specifically been cleared by the customers of the Centres, and only in such form as has been permitted. Confidentiality between the industrial centre and the customer is of vital importance and the heart of the customer relationship of the centres. It is for these rea- sons that some of the interesting technical details of the cases quoted in appendix 2 are omitted. The significance of these examples is therefore not

their relative or actual values nor the tribological technicalities involved, but the exposition of a trend towards an integration process of tribological design services in industry which they reveal [9].

However, the Industrial Centres do not merely deal with tribological problems of a mechanical engineering

nature. Since tribology is based on natural laws, and

since the laws of nature apply equally in other spheres,

the Tribology Centres helped in the interface between

tribology and medical engineering, in which area there are many unsolved problems encompassing the concept

of interacting surfaces in relative motion. Some ex- amples are given in Case Group 7, Medical Applications.

It is a source of great satisfaction that, as a byproduct of our work for industry, the application of tri- bological knowledge has in some areas contributed

considerably to the alleviation of human suffering and

to the enjoyment of life.

5. Conclusion

It has been shown that the acceptance and application

of the concept of tribology can be of benefit to industry,

in the spheres of macro-economics as well as microeconomics. It may well be that the savings, reasonably

obtainable in the United States, could exceed a figure of $16 billion. Whether this figure is wholly accurate or whether it is 25 percent or even 50 percent inaccurate

is immaterial. A very high value is involved. The question now is whether such savings can be

effected in the United States. There are many examples

in the sphere of science and technology in which the United States has led the world. There are numerous

areas in the application of modern technologies in which

the United States may reasonably be regarded as being without equal. However—and this may come as a surprise to you—in the acceptance and application of

the admittedly diffuse concept of tribology, and in the

realization of the economic impact of this new technol- ogy, amongst all major industrial nations, the United

123
States could rightly be called—even by its best friends

perhaps not backward, but certainly underdeveloped.

It is therefore a question of some considerable interest whether, in spite of the large savings obtainable without

correspondingly appreciable capital investment, the American system permits full advantage to be taken of the benefits of a diffuse and intermediate technology,

such as tribology, and other similar intermediate tech- nologies, particularly as such benefits can be gained

only by an almost complete integration of the subject

into the educational and industrial fabric of the nation. To achieve such integration is not an easy task in a

free and democratic society which particularly values,

and rightly safeguards, its academic, educational and

commercial freedom. However, in this matter, there is little difference between our two countries and, as has

been stated, we have advanced some way towards suc- ceeding in our task. In other words, it can be achieved

by the free and voluntary cooperation and collaboration

of all concerned.

Therefore, irrespective of whether in the case of the United States the estimate of potential savings of $12—

16 billion per annum is accurate or npt, I suggest to you that it is worth every effort that needs or should be

taken to promote the science and technology of tribol- ogyIn so doing, you will not only help your own industries and provide support for the research workers in the field, but you will also help to increase the wealth of

the world without having to find corresponding invest- ment capital. If only for the last reason and the need

for the industrial nations to create the additional wealth

necessary to assist in meeting the requirements of the

rest of the world, you may feel as I do, that introduction

of the concept of tribology is a task well worth undertaking.

The author wishes to thank the Directors of the University of Leeds Industrial Unit of Tribology and the Swansea Tribology Centre, and the Manager of the National Centre of Tribology, Risley, for their help in providing the Case Studies contained in this paper.

6. References and Bibliography

[1] Lubrication (Tribology) —A report on the present position and industry's needs. Department of Education and Science, (1966), H.M. Stationery Office, London.

[2] Dawson, D., Tribology, (Inaugural Lecture 1969), (Leeds University Press).

[3] Owen, Kenneth, Tribology speeds the wheels of British industry. The Times, London (1 June 1973).

Darnell, H., Applying tribology to British steel, iron and

steel internation (1973).

Carvel, John, Cutting down on wear and tear. The

Guardian (1 June 1973). Tribology savings £200 million a year, The Birmingham

Post (1 June, 1973).

[4] I.L.O. Year Book of Labour Statistics (1972).

[5] U.N. Statistical Year Book (1972).

[6] The Introduction of a New Technology. Department of Trade and Industry, 1973. H.M. Stationery Office, London.

[7] Tribology Notes on Science and Technology in Britain. Prepared for the British Information Services by the Central Office of Information in London. Ref. NST 56, Classification 6 (October. 1973).

[8] The Tribology Handbook, Edited by M.J. Neale, (Butter- worths, 1973, London).

[9] Jost, H. Peter, Centres to help industry, The Financial Times (Nov. 25, 1969).

'Eight years after Jost,' 'Tribology in action'—Leeds industrial unit of tri- bology. (University of Leeds, London, England, 1974).

Jost, H. Peter, Tribology in engineering design. New

Scientist (June 8, 1968).

6.1. Books

Summer-Smith, D. Introduction to Tribology in Industry (Machinery Publishing Company, Brighton, England. 1969).

QuiTin, J. J. F., Application of Modern Physical Techniques to Tribology (Butterworth & Co. Publishers Limited, London,

England, 1971). Hondros, E. D., Tribology (Mills & Boon Limited, London,

England. 1971).

6.2. Friction

Bowden, F. P. (deceased) and Tabor, D., An Introduction to Tribology, Anchor Books (Anchor Press, Doubleday Garden

City, New York. 1973). The Principles of Tribology, Edited by J. Hailing. (MacMillan's London, to be published 1974). War on Wear, Tribology in Iron and Steel Works. Proceedings

of the Iron and Steel Institute, London, S.W.I. M69 Price

£12.50. Tribology Projects for Schools, Department of Trade & Industry, London (1972).

7. Appendix 1. Subject Matters of

Tribology Handbook

A. Component Selection, Design and

Performance

Bearings

Al Selection of bearing type

A2 Selection of journal bearings

A3 Selection of thrust bearings

A4 Dry rubbing bearings

A5 Steady load, pressure-fed journal bearings

A6 Ring and disc-fed journal bearings

A7 Grease, wick and drip-fed journal bearings

A8 Pourous metal bearings

A9 Hydrostatic bearings

AlO Gas bearings

All Crankshaft bearings

A12 Oscillatory journal bearings

A13 Spherical bearings and universal couplings

A14 Instrument pivots

A15 Palin thrust bearings

A16 Profiled pad thrust bearings

A17 Tilting pads thrust bearings

A18 Selection of rolling bearings

A19 Rolling bearing installation A20 Plain bearing form and installation A21 Radial stiffness of bearings

A22 Bearing vibration

124
Cams, gears and roller chains

A23 Cams and tappets, performance and materials

A24 Gears, selection of type and materials

A25 Metal gears, hardness, finish and lubricant A26 Roller chain drives, performance and materials

Reciprocating components

A27 Wire ropes and control cables A28 Slides, selection and design

A29 Yalves, selection and materials

A30 Piston design

A31 Piston rings

A32 Cylinders and liners materials and finish

Seals

A33 Selection of seals A34 Mechanical seals A35 Lip seals A36 Packed glands

A37 Soft piston rings

A38 Mechanical piston rod packings

A39 Labyrinths and throttling bushes

A40 Oil flinger and drain grooves

Wear-resistant components

A41 Wear resistant parts, material selection A42 Hard surface coatings, selection and application

Metal forming and cutting tools

A43 Sheet forming and forging tools A44 Wiredrawing dies A45 Metal cutting tools

High friction components

A46 Selection of belt drives A47 Belt drives, design, materials performance

A48 Selection of friction clutches A49 Friction clutches, design and materials

A50 Brakes: performance and selection A51 Brakes: design data A52 Damping devices

A53 Wheels, rails, tyres, performance and life A54 Capstans and drums performance and design

A55 Selection of industrial flooring materials

6. Lubrication

Lubricants

Bl Selection of lubricant type

B2 Mineral oils B3 Synthetic oils B4 Greases B5 Solid lubricants and coatings

B6 Other liquids

Lubrication components

B7 Plain bearing lubrication B8 Rolling bearing lubrication B9 Gear and roller chain lubrication BIO Slide lubrication Bll Coupling lubrication B12 Wire rope lubrication B13 Lubrication in metal-working and cutting

Lubrication systems

B14 Selection of lubrication systems

B15 Total loss grease systems

B16 Total loss oil systems

B17 Dip, splash systems

B18 Mist systems

B19 Circulation systems

B20 Design of storage tanks B21 Selection of pumps

B22 Selection of filters and centrifuges

B23 Selection of warning and protection devices

B24 Selection of heaters and coolers B25 A guide to piping design

B26 Lubricant change periods and tests B27 Biological deterioration of lubricants B28 Lubricant hazards; fire, explosion and health B29 Commissioning lubrication systems

B30 Running-in procedures

C. Properties of Materials for

Tribological Components and Surfaces

Materials

CI Plain bearing materials

C2 Bearing surface coatings and treatments C3 Wear-resistant materials and surfaces C4 Rolling bearing materials

C5 Gear materials

C6 Flexure and knife edge materials

C7 Friction materials

C8 Frictional properties of materials

D. Environmental Factors

Environmental data for design

Dl World ambient climatic data

D2 Industrial plant environment data

D3 Human limits of noise and vibration

Machine design data for particular environments

D4 High pressure and vacuum D5 High and low temperatures

D6 Dirt and dust D7 Chemical effects D8 Vibration and shock

D9 Storage

125
E. Failures and Repairs

Failure of common components

El Failure patterns

E2 Plain bearing failures E3 Rolling bearing failures E4 Gear failures E5 Failures of friction surfaces E6 Seal failures E7 Fretting problems

Operating and failure limits of components

and machines

E8 Failure detection methods

E9 Failure limits of noise and vibration

ElO Failure limits of operating temperatures

Ell Allowable wear limits

Repair and maintenance methods

E12 Repair of plain bearings

E13 Repair of worn surfaces

E14 Repair of friction surfaces E15 Lubrication maintenance planning

F. Basic Information

Basic tribology

Fl Nature of surfaces and contact F2 Surface topography

F3 Friction mechanisms, effect lubricants F4 Viscosity and rheology

F5 Methods of fluid film formation

F6 Mechanisms of wear

General design information

F7 Heat dissipation from bearing assemblies

F8 Shaft deflections and slopes

F9 Shape tolerances of typical components

FIO Relevant standards ISO and BSS

Fll SI units and conversion factors A selection of examples are shown in figs 5-11.

126
10000 000

1000 '0 000 ^—

1000000

-1000000

10000

n, rev/s Rabhiag pUia bMriag* in which the surfaces rub logrthrr.

The braring is usually non-mriallic.

Plaia bcaiiags of porous mrial imprrgnaird with a lubricant.

RolUaf bmiiags. Thr matrrials arr hard, and rolling rlrmrnts

separate the two moving components.

Flaid film plaia bcariags. A hydrodynamic pressure is generated by the relative movement dragging a viscous fluid into a

taper him.

Figure 5. Example from Tribology Handbook.

Selection by load capacity of bearings with continuous rotation.

127
Figure 6. Example from Tnbology Handbook.

General guide to limiting speed for ring and disk lubricated bearings.

128
A33 Selection of seals

CHECK-LIST FOR SEAL SELECTION

Temperature (see Fig. 12): seals containing rubber, natural

fibres or plastic (which includes many face seals) may have

severe temperature limitations, depending on the material;

for example:

Natural rubber - 50 to + 80°C

Nitrile rubber - 40 to + 1 30 Viton rubber - 40 to +200

PTFE plastic - 100 to + 280

Speed (see Fig. 13). Pressure (see Fig. 13). Size (see Fig. 14).

Leakage (see Fig. 15).

Fluid compatibility: check all materials which may be exposed to the fluid, especially rubbers.

Abrasion resistance: harder sliding contact materials are

usuallv better but it is preferable to keep abrasives away

from the seal if at all possible, for example by flushing with

a clean fluid. Vibration: should be minimised, but rubber seals are

likely to function better than hard seals. Having narrowed down the choice of seals the reader

should refer to the detailed treatment of the seals in question but is also recommended to discuss his require- ments with a seal manufacturer or an independent information centre such as the British Hydromechanics Research

Association.

Figure 7. Example from Tribology Handbook.

Selection of seals.

129
RELATIVE VELOCITY AT SEAL CONTACT, fi/s

RELATIVE VELOCITY AT SEAL CONTACT, m/;

Figure 8. Example from Tribology Handbook. Limits of pressure and rubbing speed for various

types of seals

130
Plain bearing failures E2

Figure 9. Example from Tribology Handbook.

Plain bearing failures.

131
Plain bearing failures

Wire wool' damage

Characteristics

Causes

Figure 10. Example from

Plain bearing failure—'*i

Wire wool' damage

Characteristics

Causes

Tribology Handbook.

'ire wool" damage.

132
E6 Sea) failures

fig 2 Tungstan carbtdm mmchsnicml smml fmct showing tymmatrics/ tur1mc9 polishing chmrmctmristic of mild hydrmulic or thmrmst distortion

fig 3 Tungsfn cmrbidm machsnical saal face showing

localisad polishing dua to lack of flatnass this saal laakad

badly , - , • 'n fiodiu'fi iig' ; . , • ' .

• ; , . 11 micro inch • . • ' » . ,••

•

Figure 11. Example from Tribology Handbook.

Seal failures.

133
8. Appendix 2. Case Studies from the

Work of the Tribology Centres Including

Savings and Costs

8.1. Case 1 (National Centre of Tribology)

Tribology in Hostile Conditions (fig. 12)

Wire, in an acid bath, is processed round two sets of immersed pulleys in order to achieve the requisite

residence time. The life of the set of 2 shafts was 13 weeks and that of the associate pulleys, only 3 weeks. The replacement costs for these components amounted

to over £1,500 ($3,600) per annum. In addition, pro- duction losses and maintenance costs were extremely

heavy and believed to be multiples of that figure. This wear problem was analysed by tribological consideration in metallurgy, in engineering design and in materials science. Thereafter the assembly was redesigned

according to proper tribological principles.

Figure 12. Adding plating bath pulleys—National Centre of Tribology.

The National Centre of Tribology manufactured a new assembly. This has now operated for more than 12 months without significant wear. The charges which covered the redesign and manufacture of the first set of shafts and pulleys and subse- quent supply of two further sets was under £1,000

($2,400). Savings must have been extremely high be- cause the company is now operating two machines on a

2-shift system with a much increased rate of production

and virtually no down-time.

8.2. Case 2 (National Centre of Tribology)

Molasses on Seals (fig. 13)

One of the leading British food cake manufacturers

uses positive displacement pumps to meter quantities of molasses into animal feedstuffs. Leakage from the shaft

seals had been continually causing trouble. Molasses is a difficult substance to handle, since besides being very

viscous, it corrodes cast iron and mild steel. Moreover,

it is abrasive, containing crystals of sugar at low temperatures, whilst at temperatures about 50 °C it turns

irreversibly into a kind of caramel toffee. Existing seals needed replacing every 3 months with tightening at much more frequent intervals; wear of the shaft aggra- vated the situation. Ultimately the stainless steel shafts themselves required replacing.

The requirement from the National Centre of Tribol- ogy was to look into this matter tribologically and design some specific seals that (a) could be incorporated into existing pumps and, (b) would be leaktight for at least 6 months without requiring any maintenance.

After a study of the problem on the basis of tribological principles, a complete test rig was set up so that the

typical pump conditions were simulated and an ingeni- ous arrangement of seals packed with special lubricants was tested. The pump was operated under start/stop

Figure 13. Mollasses pump—National Centre of Tribology.

and continuous running conditions and no significant leak had occurred during 600 h of running and 60,000

starts and stops. The pumps were therefore modified and have now

operated for periods well in excess of the target period

without the slightest leakage. Similar pumps used in the works are in the process of modification. Several other

factories of the same Group will be adopting the same

design.

The cost to the customer, including laboratory prov- ing tests, was less than £1000 ($2400). Total savings are now well over £5000 ($12 000) per annum.

134
8.3. Case 3 (Lrceds Industrial Unit of Tribology)

The Ehinlop Safety Tyre (figs. 14^17)

Perhaps one of the most interesting and unusual in- volvements in an industrial centre with industry took

place during the development of the Dunlop Safety

Tyre. This is a tyre which, when punctured during use,

is capable of running on at speeds of 50 m.p.h. for up

to 100 miles, and with little or no effect on the handling

of the car. The special features of the wheel are that it contains no well.^ The rim is such that the tyre stays firmly

against it even under conditions of tight cornering.

The special design of the side walls enables them to squash down onto the inside of the tread section and

continue to support the weight of the car. When the tyre deflates a special type of lubricant, de- signed by the Leeds Unit of Tribology, is spread be- tween the side walls and the tread, performing as follows

Firstly, it lubricates the surfaces between side wall and tread, thus preventing the degradation of the rub- ber by frictional heat. Full or elasto-hydrodynamic

lubrication conditions apply in this condition which is referred to as "internal aquaplaning".

Secondly, it seals any small holes. Thirdly, it partly vapourises and reinflates the tyre to a small pressure. Fourthly, it has a high specific heat and acts as a

transfer medium, removing heat from the rubbing sur- faces and flexing parts. Figure 14 shows the difference between a standard

tyre and the new type of safety tyre, which contains

pressurised lubricant containers. When the tyre is punctured, it is deflated, figure 15, until the canister releases the lubricant which coats the internal surfaces and seals them, figure 16. A partial reinflation takes place which

allows the vehicle to continue running (figure 17).

It has been said that the development of this tyre is the most fundamental and most important breakthrough

in road safety since the war. It is a source of great sat- isfaction that one of our Industrial Tribology Centres should have been involved in its development.

The actual cost/benefit of this particular exercise is impossible to establish; the benefits are so vast that they can be measured in £ millions; on the other hand,

the work described in tribology was a team effort be- tween the designers of the company and the Industrial Centre of Tribology at the University of Leeds.

135
Figure 17. Partial reinfiation.

8.4. Case 4 (Swansea Tribology Centre)

Redesign of Pump Bearings (fig. 18)

This involved the redesign of water cooling plant

bearings for a large international chemical company

where pump bearings required replacing every 750 h. Modification by the pump manufacturers produced very

little improvement and the Swansea Tribology Centre was called in to solve the problems.

The Swansea design, including construction, installa- tion, strain gauging, bearing load data and testing on

the Swansea bearing test rig, was completed in a total "down-time" of the pump of 72 h and a total charge of i340 ($816). Since modification the pump has run for over 2000 h without bearing failure. This represents a saving to the operators of at least £2400 ($5760) per annum for their once only investment of £340 + £25 own costs.

8.5. Case 5 (National Centre of Tribology)

Computer Amnesia (figure 19)

This case was known as "computer amnesia" or "loss of memory". It concerned a large memory store for

digital computers capable of holding up to 1200 milHon

'bits' at one time (fig. 19) . The store consists of a number of magnetic discs which rotate about a horizontal axis at a speed of 1200 rpm. Small magnets, imbedded

in pads which float above the disc surface on a film of

air, can record or read ofif information on the disc.

Original Design {Pump Capacity Swansea Design providing a vortex lOOOOgallons waterper minute at discharging oil into the bearing end

150 ft head) carrying it through by the bearingi

naturalpumping action. Since modification the pump has run over 2000hours without bearing failure. This represents a saving to the operators ofat least £2400per annum for their once

only investment of £340.

Figure 18. Pump bearing redesigned by Swansea Tribology

Centre.

This was a classic example requiring both redesign

and manufacture of the modified components by the Centre, and in respect of which the improvement in performance and the resultant savings could be esti- mated with some accuracy. The key to the Swansea tribology analysis was the tri- bological principle that bearings of this type exhibit a natural pumping tendency in the reverse direction to the thrust, i.e., the direction (a) figure 18. The problem

in the original design was a pumping ring which was

attempting to force oil through narrow passageways

path (b) in the opposite direction to the natural pumping action.

Figure 19. Computer flying heads—National Centre of Tribology.

The pads, or flying heads, have to swivel about a pivot in order to fly at the correct angle and to adjust

the inevitable but small amount of wobble of the disc as

it rotates (the one metre diameter discs have a per- mitted wobble of =t 0.07 mm at their outside edge). It was found that under certain conditions, a head would

stiffen unduly on its pivot and result in "touchdown"

onto the disc. Then, in an avalanche process caused by

dust, many heads would "crash" and the whole disc, (or even several discs) would then be spoiled, with all the recorded data being lost. Tribological examination found that the cause of this stiffening was due to a "fretting" action between the pivot spindle and the bore of the pad. A build-up of

fretting debris was identified by the very sensitive tech- nique of electron micro analysis and the Centre recom- mended a change in the manufacture and setting up

procedures to reduce the risk of seizure, including a

136
number of chemical treatments designed to eliminate

fretting.

The diagnosis of the problem as being a tribological one of fretting damage has led to not only the savings of several thousand pounds per annum, but to the prevention of loss of possibly invaluable information. The

charge made by the Centre was £500 ($1200)

.

8.6.Ca8e 6 (National Centre of Tribology)

Energy Preservation (fig. 20)

This was the first case in which the prime principal

objective was in the sphere of energy saving. Since it is acknowledged that a third of all energy generated is used in friction, the relationship between the subjects

of tribology and energy is not difficult to establish. The case concerned dealt with the "float" process of

glass making. Higher output at reduced power was the requirement (fig. 20).

Figure 20. Float glass process—National Centre of Tribology.

In the manufacture of "float" glass, a ribbon of glass

up to 11 feet wide leaves the float tank and enters the annealing lehr at temperatures in the region of 600 °C. In the design of a new lehr employing nearly 200 rollers

( 18 ft long and weighing 1300 lbs) it was necessary to cater for an increase of the speed of the glass by a fac- tor of about 3. The plant is designed to run for two

years without stopping.

By careful analysis of the whole design of all inter- acting surfaces in relative motion, including materials,

lubrication, etc., it was possible to give the increased

output without an increase in energy.

8.7. Case Group 7

Medical Applications (fig. 21)

Tribology has helped in the interface between engineering and medicine, in which area there are many unsolved problems encompassing the concept of interacting surfaces in relative motion.

The following are but a few examples of this work:

(a) The studies of synovial joints have led to a better understanding of the performance of these remarkable

"bearings" and contributions have been made towards

the alleviation of osteo-arthrosis. The introduction of synthetic lubricants into joints showing early signs of osteo-arthrosis and the development of total endoprostheses are instances. Experiments have been undertaken to develop lubri- cating fluids which are compatible with the body and

which possess rheological characteristics similar to those

of synovial fluid. Clinical trials designed to assess the

potential of this approach to some forms of failure of

the human tribological system have begun.

(b) Design and development of total endo-prostlieses

replacement "bearings" are now available in a variety

of forms for the hip, knee, shoulder, elbow and finger

joints. Work on 'total replacement' artificial knee joints

is progressing.

(c) But perhaps one of the most unusual applications

of tribology was that in respect of the design of artificial heart valves (fig. 21). Heart valves are required to

Figure 21. Heart valve—National Centre of Tribology.

operate continuously for 40 miflion cycles per annum

for many years. Plastic replacements for diseased heart

valves, therefore, need to be extremely reliable and free from significant wear. At the request of the manufacturer of the newly de- signed heart valve, the National Centre of Tribology

137
subjected it to tests for placidity, load and impact resis- tance, and sterility under various environmental conditions and temperature.

The substance pumped, which also forms the lubri- cant, is like salt water with very badly suspended semicolloidal particles in it. From a colloidal dispersion

point of view it would not pass the tests of any reputable

manufacturer. Wear took place on the hinges. In some

cases this would become severe after 18 months to 3 years only. Tribological study, a knowledge of the op- eration of the valve, the effect of the environment on its materials, and the employment of tribological principles, led to the redesign of the valve. As a result the hinges will not now become troublesome for many

years.

9. Appendix 3. Particulars of Industrial

Centres of Tribology in the

United Kingdom

National Centre for Tribology,

UK Atomic Energy Authority,

Risley, Lancashire.

Warrington 31244

Manager:

Dr. W. Roberts.

Industrial Unit of Tribology,

University of Leeds,

Leeds, Yorkshire, LS2 9JT

Leeds 31751.

Director: Dr. R. Wakelin.

Swansea Tribology Centre,

University College of Swansea,

Singleton Park,

Swansea, Glamorgan, SA2 8PX

Swansea 25678.

Director: Dr. A. R. Lansdowne.

National Engineering Laboratory,

East Kilbride, Glasgow.

East Kilbride 20222

Director:

Mr. R. Weir, CB.

10. Discussion

H. Corten, University of Illinois: It wasn't quite clear what you were suggesting to be introduced into the cur- riculum of a university mechanical engineering depart- ment.

H. P. fast: I didn't suggest you should do anything. I just told you what we have done. You cannot be a mechanical engineer designer without a minimum of tribological knowledge. Therefore we have laid down, and

when I say we, I mean not the Government but the Tri- bology Group of the Institute of Mechanical Engineers

together with the universities and colleges, a module

which is a minimum amount of tribology which every mechanical engineer ought to have in his curriculum, even if it means leaving something else out. Because

without it, he cannot design properly and half of the

failures which you are talking about will still be in, whereas by proper design, they could be left out. The

polarization of the subject has been of great help to our universities, but basically the minimum knowledge

in the curriculum of a mechanical engineer is the vital point.

A. J. Babecki, NASA Goddard Space Flight Center: We

do, in the United States, teach our mechanical engineering students subjects which are all encompassing in the word tribology. We teach them metallurgy, chemistry,

mathematics, etc., so the fact that we don't have a course named tribology does not mean that mechanical engineering students do not get the basic information. How

did you arrive at the numbers that you were quoting

as the estimated savings the United States could effect by using tribological applications? Did you assume that

there's no technology being used currently in solving

these problems? Certainly in the United States we have

literally hundreds of organizations that are not necessarily called tribological centers that have research

facilities that are in the lubrication area, the friction and wear area, and they are solving these problems.

Are you discounting all of that work when you quote

those numbers?

H. P. Jost: No sir. In the first place I didn't suggest any

degree courses in tribology; in fact, I am very much

against a first degree in tribology. Tribology is a part

and parcel of mechanical engineering. I am very much

for a post graduate course to become specialized in the

subject. In the second place, my figures are based on a very thorough investigation of United Kingdom figures

transposed using the United Nations statistics. Now, in the United Kingdom, we too, have all these research

laboratories and schools. But we've found that the trans- fer of knowledge, in general, throughout industry to the designer is insufficient. Our savings in Great Britain are based taking into consideration all of the things you

mentioned that can be obtained by a better transfer of

that kind of intermediate type of technology in a way

which is completely multidisciplinary.

A. }. Babecki: We have a lot of technology centers in

this country to which industry can resort to solve its problems. I think it would be a duplication of effort to

set up additional organizations in the United States as you have in England which would be called tribology

centers, because we have those facilities already avail- able.

H. P. lost: I very much question that from personal

knowledge. You have the same type, and perhaps better, organizations available as we have; you have excellent

universities, you have excellent research institutes. But

138
I very much doubt whether you have any means of transferring this knowledge, whether from research or knowledge that exists, and most of this knowledge is in existence, into the everyday curriculum at work and the

industrial bloodstream of your industry. Mind you, the argument you put forward is extremely good. It was put

forward by our own people when they tried to oppose

the original setting up of the Committee on Tribology.

/. E. Ryan, General Electric Company: I think philosophically this deserves comment because here we've had numerous component technologies including lubri- cation, the metallurgy of wear, etc., in existence often

at the graduate level. It takes people at Battelle or in similar laboratories to "put it all together" themselves,

but they do it only in limited areas. I'm not waving the

flag for your tribology, but I do think that it gives us some pause for the value of putting things together in what we might call intermediate packages so we can

see everything at once. I am a mechanical engineer by

training and I've practiced mechanical engineering. I had to learn to "put it together" myself, and many a seal manufacturer I wrestled with over what you now call tribology. We didn't have the name but we had the problem. I think that there is some value in putting to- gether for consideration new alignments of basic knowledge so that the practicing man who has to, I use the term again, "put it all together", may see not only the kinematics and the strength of materials or material

science as such, but may see it integrated with lubrication and all the other relevant component sciences.

H. P. Jost: Thank you very much. This bears out the point which I mentioned: The whole tribology move- ment in England was borne on the shoulders of industry

who felt they needed it, and against the reluctance of

some of the finest and best experts in the field of re- search who just couldn't see that something wider was

involved. Tribology in Britain serves industry, it's been

borne on the shoulders of industry, and carried forward

by industry.

G. M. Ugiansky, National Bureau of Standards: Since a

lot of the failures in industry are caused by corrosion,

a similar argument could be used for introducing studies of corrosion into the universities for mechanical engineers.

H. P. Jost: I quite agree. You're absolutely right. We

have made a similar investigation into corrosion. Last year a committee on corrosion was set up by the Department of Trade and Industry with very similar terms of reference to a very similar work. There are one or two

other intermediate technology subjects being considered

for similar treatment. If you want to go to the moon, the government is prepared to appropriate millions of dol- lars so you can do it, but they are unwilling to fund

areas of study in which basically all of the knowledge is already known, knowledge you need for everyday work.

Corrosion and tribology are parallel sister subjects that are being dealt with in the same way. Talking as an in- dustrial dealing with industrial problems, if there were no more fundamental research in these two subjects for the next 10 years I believe that all the answers are al- ready there. It's just for us to use them.

/. R. Strang, Air Force Materials Laboratory : I don't think the study of tribology should be limited to mechanical engineers. There are tribo-chemical and tribo- physical aspects to consider and I think tribological

education should be provided for chemical engineers, surface chemists, and surface physicists also.

139

CONSEQUENCES OF MECHANICAL FAILURES

Session IV

Chairman: H. P. Jost

K. S. Paul Products, Ltd.

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Nuclear Power Plant Safety

Peter A. Morris

Office of Operations Evaluation U. S. Nuclear Regulatory Commission

Washington, D.C. 20545

Protection against the potential radiological consequences of malfunction or mechanical failure of reactor systems and components in nuclear power plants is achieved by a combination of pro- cedural and technical constraints. For routine operation, plants are to be designed and operated

so that radioactive effluents are maintained as low as practicable. To assure that the probability

of serious accidents is acceptably low requires conservative design, construction and operation using redundant and diverse systems and components and a rigorous program for quality assurance at all times.

Key words: Failure prevention; mechanical failure; mechanical malfunction; quality assurance;

reactor systems and components ; safety standards.

1. Introduction

The potential consequences of mechanical failures in nuclear power plants conceivably could be very severe. The character of the consequences, where radiation

and radioactivity are involved, are different from the consequences of mechanical failures in nonnuclear activities, and generally are not well understood by the layman. This paper discusses the general subjects of nuclear power plant design, nuclear safety philosophy

and policy. Atomic Energy Commission nuclear plant

regulatory procedures and nuclear power plant operating experience. Through this discussion, it is hoped

that the importance of prevention of mechanical fail- ures in nuclear power plants is put in proper perspective.

2. Discussion

2.1. Nuclear Power Plant Description

Nuclear steam supply systems for electric power gen- erating plants are made up of extensive, complicated,

advanced technology equipment and systems. Because

of the nature of the nuclear chain reaction taking place

in the reactor cores, there are very high radiation fields and very high levels of radioactivity involved in the operation of such plants. Because of the goal of achiev- ing economic production of electricity, the steam- generating aspects of the plants require high power,

high temperature and high pressure of water and

steam, which leads to the storage of an immense quantity of energy in the reactor system. Further, because of

the delayed release of heat from the decay of radio- active fission products in the reactor core, large amounts

of heat are released even after the reactor itself has

been shut down for a long time. The combination of a very large amount of stored energy in conjunction with

a very large amount of radioactivity presents a signifi- cant potential hazard to the public and to the environment in the vicinity of a nuclear power plant.

Many different kinds of nuclear reactors for use in

electric power plants have been designed and operated.

All of them depend upon the fissioning of uranium,

thorium or plutonium for operation. The major differ- ences in kind are in the design of the fuel elements, the way in which heat is removed from the fuel, in the choice of materials used to cool the fuel elements and

in the method used to control the average energy of the neutrons produced from fission before further inter- action. Most fuel element designs have used natural uranium, enriched uranium dioxide or uranium-metal

alloys. Moderators, the materials used to absorb the en- ergy of fission neutrons, principally have been graphite,

ordinary water (called light water) or heavy water (which has an extra neutron in the nucleus of the hydrogen atoms). The primary coolants have been light

water, heavy water, liquid sodium, carbon dioxide, and

helium gas. Within the United States, the great majority

of the power reactors in operation today and to be built

in the foreseeable future are of two basic designs and

the rest of this discussion will be confined to those two

designs. Both designs use uranium dioxide pellets, slightly enriched in the isotope uranium-235 for fuel, Zircaloy tubing for the fuel cladding and light water as both moderator and primary coolant. In the boiling

143
water reactor (BWR) type, the pressure in the reactor primary coolant system is designed to be approximately

1000 pounds per square inch (psi). The primary cool- ant water is heated sufficiently while flowing through

the reactor core to form steam in the reactor vessel itself and this steam is piped directly to the turbine. In the pressurized water reactor (PWR) type, the pressure in the reactor primary coolant system is designed to be

approximately 2200 psi. In this design, the primary

coolant water is piped to heat exchangers, i.e., steam

generators, where the heat is transferred to a secondary

coolant water system at lower pressure, in which steam

is generated to drive the turbine. The BWR design is called a direct cycle system and the PWR design is called an indirect cycle system.

The light water reactor systems are made up basically of the fuel elements in the reactor core, a reactor pressure vessel and associated pipes, pumps and valves, control and auxiliary systems, engineered safety fea- tures and a containment building. Of course, both de- signs have a large turbine and generator. While this sounds relatively straightforward, consider for a mo- ment or two what really is involved in the principal

systems of a nuclear power plant. (The following listing

is taken from a draft of a user's manual being de- veloped by a nuclear subcommittee of the American

National Standards Institute, N18-20, Nuclear Plant

Reliability Data) :

Standard Sysiem Coding of Nuclear Plants

System Description

Reactor

Reactor vessel internals Reactivity control systems Reactor core

Reactor Coolant System and Connected Systems

Reactor vessels and appurtenances

Coolant recirculation system and controls Main steam systems and controls Main steam isolation systems and controls Reactor core isolation cooling systems and controls Residual heat removal systems and controls Reactor coolant removal systems and controls Feedwater systems and controls Reactor coolant pressure boundary leakage detection systems

Other coolant subsystems and their controls

Engineered Safety Features

Reactor containment systems

Containment heat removal systems and controls Containment air purification and cleanup systems

and controls Containment isolation systems and controls Containment combustible gas control systems and

controls Emergency core cooling systems and controls Control room habitability systems and controls

Other engineered safety feature systems and their controls

Instrumentation and Controls

Reactor trip systems

Engineered safety feature instrument systems

Systems required for safe shutdown

Safety related display instrumentation

Other instrument systems required for safety

Other instrument system not required for safety

Electric power systems

Offsite Power Systems and Controls AC Onsite Power Systems and Controls DC Onsite Power Systems and Controls

Onsite Power Systems and Controls (Composite AC

and DC)

Emergency Generator Systems and Controls Emergency Lighting Systems and Controls Other Electric Power Systems and Controls

Fuel Storage and Handling Systems

New fuel storage facilities Spent fuel storage facilities Spent fuel pool cooling and cleanup systems and controls Fuel handling systems

Auxiliary Water Systems

Station service water systems and controls Cooling systems for reactor auxiliaries and controls Demineralized water make-up systems and controls Portable and sanitary water systems and controls Ultimate heat sink facilities Condensate storage facilities Other auxiliary water systems and their controls

Auxiliary Process Systems

Compressed air systems and controls Process sampling systems

Chemical, volume control and liquid poison systems

and controls Failed fuel detection systems

Other auxiliary process systems and their controls

Other Auxiliary Systems

Air conditioning, heating, cooling and ventiliation systems and controls Fire protection systems and controls Communication systems

Other auxiliary systems and their controls

Steam and Power Conversion Systems

Turbine-generators and controls Main steam supply system and controls (other than

CCX)

144
Main condenser systems and controls Turbine gland sealing systems and controls Turbine bypass systems and controls Circulating water systems and controls Condensate clean-up systems and controls Condensate and feedwater systems and controls (other than CHX)

Steam generator blowdown systems and controls Other features of steam and power conversion sys- tems (not included elsewhere)

Radioactive Waste Management Systems

Liquid radioactive waste management systems

Gaseous radioactive waste management systems

Process and effluent radiological monitoring systems

Solid radioactive waste management systems

Radiation Protection Systems

Area monitoring systems

Airborne radioactivity monitoring systems

Over 60 systems are identified in this table, almost every one of which has direct relevance to nuclear

safety. Although the primary characteristics of these systems may be thought of as thermal-hydraulic, electrical, mechanical, or what-have-you, every one of

these systems has important mechanical features whose

failure would jeopardize the availability, reliability or

safety of the plant. Needless to say, the possible failure modes cover just about the entire spectrum known to man.

Aside from the problems of high pressure and high

temperature steam, which are common to any steamturbine system whether fueled with uranium, gas, oil or coal, the different potential hazard from a nuclear

plant arises from the possible release of radioactivity

either as a routine effluent or as a result of an operational transient or accident. The fuel elements themselves, both fuel and cladding, become intensely

radioactive. The uranium dioxide pellets retain some

of the products of fission and transmutation, but during

normal operation some of the products, particularly the gaseous isotopes, escape from the fuel material into the spaces between the fuel pellets and between the pellets

and the tube-like containers in which they are located. Some of these radioactive materials diffuse through the

fuel cladding, i.e., the walls of the tubes, and some leak out through pin-holes or other defects in the cladding,

into the primary coolant water. In addition to this source of radioactivity during normal, routine operation, radioactive materials are also formed by the irra- diation of the coolant water itself and any contained

impurities, such as corrosion products and abrasion

particles from pump seals, that might be in the water. As just described, it is, therefore, expected and nor- mal that the primary coolant water, after the reactor has been operated for any appreciable time, will contain a certain amount of radioactivity. It is also ex- pected and normal that a certain amount of water, gas

or steam containing radioactivity will escape from the

reactor primary coolant system. Some radioactivity will

escape during normal operation as a result of the irre- ducible low leakage of water and steam that occurs from high pressure systems with many valves, pumps,

flanges and seals. Some radioactivity will escape when

the reactor vessel is opened up—by removing the vessel head to permit replacement of fuel elements (about once a year) and some radioactivity will escape during

the time that newly discharged fuel elements are stored underwater in a storage pool before shipment off-site to a fuel reprocessing plant. There are other minor

sources of radioactivity release, such as from water

chemistry sampling procedures, that are not of concern

in this discussion. Because radioactive fluids and gases are constantly

being generated and escaping from the primary coolant system during operation of the reactor, there need to be extensive systems and equipment provided for pro- cessing these effluents. Such equipment needs to operate

reliably for long periods of time, just as do the reactor

itself and the electricity generating equipment.

2.2. Consequences of Reactor Accidents

As mentioned earlier, it is very easy to conceive of accidents in which large amounts of radioactivity might

be released. The character of the consequences and the magnitude of the conceivable consequences of a very

large release of radioactivity are clearly more severe than for the consequences of accidents in non-nuclear

activities that normally are considered. Severe nonnuclear accidents that one thinks of are explosions, fires, collisions, ship sinkings, aircraft crashes and the like, as a result of which a hundred or a few hundred people

might be killed and from which damage might cost in the tens or hundreds of millions of dollars to repair.

From natural catastrophes, such as floods, tornadoes

and earthquakes one might think of consequences re- sulting in thousands of people killed and property

damage costs exceeding hundreds of millions of dollars. While such events would cause severe social con- sequences, they would not generally be expected to produce severe generic consequences or to render uninhabitable large areas of land, as might be the case following a nuclear accident. Nevertheless, they are

serious accidents of a kind that have occurred and will occur well within the span of human history.

The consequences of several hypothetical large re- leases of radioactivity from a nuclear power plant were

studied and reported by the Brookhaven National Laboratory in 1957 in a report known as WASH-740

[1].^ The results of this study are frequently misrepresented to the public by those who are critical of the nuclear community. The largest release—for which no mechanism was postulated—assumed that most of the

radioactivity contained in the fuel that could be released from the fuel if all of it were in a molten state was in

fact not only released from the fuel, but also completely

escaped the primary system and completely escaped

from the containment building and was transported

by unfavorable weather conditions to regions of high

population. These conservative assumptions; assumed

^ Figures in brackets indicate the literature references at the end of this presentation.

145
to apply simultaneously, constitute an almost impossible situation. No estimate of probability of such a re- lease was made in the Brookhaven report, except to characterize it as very small. While this study did

quantify the upper limit of consequences that could be

conceived for such a hypothetical accidental release of radioactivity,^ it did not and was not intended to, characterize the risk of nuclear power plant operation.

The upper limit of theoretical consequences calculated

in the Brookhaven report was used as a basis for selec- tion of the maximum level of third party liability in- surance to be provided by the government under the Price-Anderson Act, but was never intended to imply

that there was any meaningful probability that such a release would in fact occur. Any discussion of risk needs

to discuss not only conceivable consequences, but also probability or frequency of occurrence of such con- sequences. A more recent AEC discussion of the probabilities

and consequences of postulated accidents is provided in the report, WASH-1250 [2], "The Safety of Nuclear

Power Reactors (Light Water-Cooled) and Related

Facilities", that was made available in July 1973. Among others, two very irr/portant results were pre- sented in this report. First, it was shown that when

realistic models for postulated accidents are used, rather than extremely conservative models, the cal- culated off-site consequences for a given event are

several orders of magnitude less. Second, quantitative estimates were reported of the probability of accidents leading to release of radioactivity. Assuming only in- dependent mechanisms for failure, that is, not common

mode mechanisms, it was estimated that the chance of an accident leading to the release of the equivalent of about five million curies of radioiodine is approximately one in 100 trillion each year (i.e., approximately 10"^* per reactor year) . This, of course, would be a really very serious accident, and it is comforting that the esti- mate of the probability is as small as it is, even though

the chance might be increased by a factor of 10 or so

if common mode failure possibilities were taken into account. For a relatively innocuous release of from one

to ten curies, the probability is still only approximately one in a thousand per year (i.e., approximately 10"^ per reactor year)

For about the last year and a half, a special study of reactor safety has been in progress for the AEC under

the direction of Professor Norman C. Rasmussen of the Massachusetts Institute of Technology. This study was undertaken to provide a realistic assessment of the

risk of nuclear power plant operation using the reliability analysis technology developed principally by the aerospace and missile industry for the Defense Department and NASA to calculate the probabilities of accidents where actual experience data were not avail- able. Seven major tasks were defined for carrying out

this study, as follows:

(1) Identification of Accident Sequences—A systematic study was made of possible accident initiators, such as equipment failures or natural

2 A maximum of 43,000 deaths and the equivalent of seven billion dollars damage.

catastrophes, that could lead to melting of fuel and subsequent release of radioactivity to the environment.

(2) Assignment of Probabilities—Existing experi- ence data for components and systems similar

to those in a nuclear plant were adapted to synthesize overall probabilities for the various ac- cident sequences, using fault tree methodologies.

(3) Definition of Source Terms—For each of the accident sequences, estimates were made of the amounts of radioactive isotopes that would be

released to the environment, taking into account

filtering, plate-out or other mechanisms for re- tention within the containment structure. (4) Dispersion of Radioactivity—Models were de- veloped for calculating the dispersion of the various isotopes as a function of meteorology.

(5) Health Effects and Property Damage—Conse- quences of accidents were calculated in terms of

fatalities, injuries, long term health effects such

as latent cancer and genetic effects, and property

damage.

(6) Other Risks—To provide a basis for placing nuclear risk in perspective, some risks in nonnuclear activities were investigated. These

included high probability risks resulting from

events that happen often enough so that their frequency can be measured and low probability

risks resulting from events that have not happened, but whose frequency can be estimated.

(7) Overall Risk Assessment—This last task was an

effort to convey an accurate impression of the

risk of low probability events to a non-technical

audience.

Although the report of the Rasmussen study is not

yet available, some general conclusions have been re- leased by Dr. Ray, Chairman of the AEC. The chance

of a reactor core meltdown is caluclated to be in the range of one in a million to one in 10 million per re- actor year (10"*"— 10~^ per reactor year). The

probability of such an event leading to severe conse- quences affecting a large number of people is calculated

to be in the range of one in a billion to one in 10 billion

( 10~''-10~^^) per reactor year. The consequences of

this latter event are characterized as being comparable

to the consequences of a large dam failure or to a large

aircraft crash in a large, fully occupied stadium. In other words, even for the very serious nuclear incident,

whose probability is extremely low, the consequences would be the order of several hundred people killed immediately. Depending on specific weather conditions,

population distribution and emergency measures, sev- eral hundred additional persons might be killed or injured and land areas might be inaccessible for a number of years but not several tens of thousands of people

killed as one hears from those who quote or extrapolate

from the early Brookhaven report.

2.3. Prevention of Failures

The AEC regulatory program for assuring that operation of nuclear reactors does not present an undue risk

146
to the health and safety of the public has been evolving

for some 20 years, i.e., since the passage of the Atomic

Energy Act of 1954. Originally, this program reflected

to a large extent the safety concepts that had grown up

since 1942 during the design, construction and operation of the Commission's production, test and research

reactors, which generally were located at remote sites. Power reactors are preferably located near large population centers or industrial complexes, however, so that

isolation alone cannot be relied upon for protection of the public from potential accidents. Current designs of nuclear power plants incorporate engineered safety features and containment structures in addition to the basic plant safety features to compensate for smaller

isolation distances. In addition to conservative siting practices, the three general concepts used in assuring that nuclear safety

objectives are achieved are:

(1) Conservative design, construction and operation.

(2) A defense-in-depth philosophy.

(3) Quality assurance.

Clearly, the best way to avoid unforeseen and unacceptable consequences resulting from nuclear plant op- eration is to be sure that unforeseen performance and

accidents don't happen in the first place. This truism

leads to the requirement that designs be conservative, even though extra cost is incurred. Conservative designs

are achieved by careful selection of materials and de- sign principles, by use of redundant systems in parallel,

and by use of systems incorporating diverse principles of operation and providing backup systems and physical

separation of all systems.

Similarly, the use of conservative practices during

construction helps to provide reliable systems and the use of conservative practices during operation reduces

the chance for mistakes in operation, or deterioration of operating conditions. Even though conservative design, construction and

operation are mandatory and do produce a relatively small chance for unacceptable performance, the degree

of assurance needed that public health and safety are protected has led to the requirement of a policy of defense-in-depth. What this means is that even though

accidents should not happen, equipment and systems are required to be built into the plant first, to detect reliably

any abnormal or impending abnormal condition and

second, to provide reliable and positive control measures. The most important example of this policy is the requirement for first, an elaborate control system,

second, an automatic, fast acting shutdown system and

third, an independent and separate slow acting shut- down system to be provided in each nuclear plant to shut down the nuclear chain reaction, should this be- come desirable. In addition to control and safety sys- tems provided to manage the nuclear chain reaction of

the reactor core, in such a way as to prevent an acci- dent, engineered safety features are nevertheless always

provided to mitigate the consequences of an accident

should it occur. Such features might include, for ex- ample, filters or sprays to remove radioactivity from the

reactor building atmosphere.

Another way of thinking about defense-in-depth is to consider the barriers that impede release of radioactivity from the fuel to the environment. First, even when molten, not all of the radioactivity escapes from

the fuel itself, and the fuel cladding, when intact, retains most of the activity. That activity that does escape to the primary coolant is retained in the primary system,

unless there is a significant breach in that system. That

activity which escapes the primary system will be re- tained, to a large extent, by the containment buiJiling

(which is required in all cases). Finally, all plants are required to have an exclusion area, of some 600 yards

radius or more, and are located in a low population zone of radius generally in the range of 2 to 5 miles,

both of which serve to isolate the source of radioactivity

from the public.

The third general concept, quality assurance, as used

in the nuclear industry, is extremely important and is perhaps unique. It is defined to comprise "all those planned and systematic actions necessary to provide

adequate confidence that a structure, system, or component will perform satisfactorily in service". As such,

it is meant to require a rigorous discipline throughout

design, construction, test, operation and maintenance

of a nuclear plant on the part of all concerned to pro- duce a very high degree of assurance that objectives are indeed met. Frequently, quality assurance programs

provide a check or an audit by a separate organization

of what is done by line organizations, to see that the job really is done and is done properly. In addition to the three general concepts just dis- cussed, the bases for reactor technical safety reviews in- clude the following

(1) General Design Criteria

There are 55 general design criteria listed in Appendix A to 10 CFR Part 50, which is the AEC's rule on facility licensing. These criteria are grouped in relation to overall requirements,

protection by multiple fission product barriers,

protection and reactivity control systems, fluid systems, reactor containment and fuel and radioactivity control.

(2) Radiation Protection Standards

These are provided in 10 CFR Parts 20 and 50

of the Commission's rules and are quite specific on allowable doses to people and releases of radioactivity to the environment.

(3) Site Criteria

Site criteria for reactors are provided in 10 CFR

Part 100. No fixed requirements are listed, but

rather procedures are described by which the

suitability of a site is determined.

(4) Regulatory Guides

In many cases, requirements of the Commission's rules are general. To give additional guid147
ance on implementation of the rules, regulatory

guides have been developed (a) to describe and

make available to the public descriptions of methods acceptable to the AEC regulatory staff for implementing specific parts of the Commission's regulations and in some cases to delin- eate techniques used by the staff in evaluating

specific problems or postulated accidents; and

(b) to provide guidance to applicants concern- ing certain of the information needed by the regulatory staff in its review of applications for permits and licenses. The guides are not in- tended as substitutes for regulations, and therefore, compliance is not mandatory. There are

ten divisions of the guides dealing with differ- ent facets of the licensing process; Division I deals with power reactors.

(5) Industry Codes and Standards

The Commission strongly believes in the in- creased use of codes and standards to improve

the quality of design and construction and to accelerate the safety review process. Where

such codes and standards meet Commission re- quirements, this is explicitly stated in Commission rules.

(6) Letters of the Advisory Committee on Reactor Safeguards

These letters comment not only on the acceptability of specific designs, but also on generic

matters, including such things as research and

development.

(7) Decisions by the Atomic Safety and Licensing

Boards, the Appeals Board, and the Commission

These decisions may very well deal with policy matters and therefore set precedents for future

licensing actions.

Aside from the bases and requirements contained in the materials just described, licensing decisions are based on the technical analysis of the proposed plant

design and proposed operation, to evaluate the ability of the plant to perform as designed for both routine

and transient conditions. To evaluate the consequences

of non-routine conditions, several "design-basis acci- dents" are postulated. The consequences of such acci- dents are calculated conservatively and the resulting calculated doses at the site boundary are compared with

the guideline dose values that are provided in the Commission's site criteria and which are used in evaluating

the suitability of a site. Thus, for a low probability, but

a very severe postulated accident, such as the instanta- neous break of the largest pipe attached to the reactor pressure vessel, the site and the reactor plant design

must be such that an individual at the site boundary

would receive a calculated whole body dose less than

25 rem and a calculated dose to the thyroid from iodine

less than 300 rem, in the two hours following the accident. While these guideline dose values may appear

large compared to background dose rates and to those acceptable normally from medical X-rays, for example,

they are still in the range where biological effects can

only begin to be observed. How does the Commission assure itself that plant

construction and operation really are as they are sup- posed to be?

First, the Commission, in an operating license in- cludes license conditions, in the form of technical specifications, that are required to be met by the licensee. These specifications, for both nuclear and environmental control, include items in the following categories:

(1) Safety limits and limiting safety system settings.

(2) Limiting conditions for operation.

(3) Surveillance requirements.

(4) Design features. (5) Administrative controls.

Second, the Commission has a program for periodic,

and mostly unannounced, inspection of licensed facilities.Throughout the construction period and throughout

the lifetime of a nuclear plant the AEC makes inspections, on-site and at vendor and manufacturers' facilities, to ascertain compliance with Commission rules and

regulations and to make on-the-spot assessments of the quality and safety of construction and operation. Such

inspections include visits to the shops of the reactor pressure vessel manufacturers, observation of concrete and steel placement, review of welding procedures and

inspections, witnessing of preoperational and startup

tests and routine operation and maintenance, review of operating procedures and emergency plans and assess- ment of overall management organization and effective- ness.

2.4. Operating Experience

At the end of February of this year there were 42 li- censed nuclear power plants in operation in the United

States, representing more than 5 percent of the nation's

electric generating capacity. The oldest plant started operation in 1959. The cumulative operating experi- ence now totals approximately 175 reactor years. The

safety record has been perfect, to the extent that no

incident at a licensed nuclear power plant has caused

any detectable exposure of persons outside the plant

site. The operating record for nuclear plants has not been perfect, but nevertheless, has been quite comparable to that for other generating stations of the same

size and age. For example, for units producing 600

megawatts electrical and up, the forced outage rates and

plant availabilities have been nearly the same for fossil- fueled and nuclear-fueled power plants of the same age. An indication of the kinds of equipment failures and

malfunctions that have caused forced outages of nuclear power plants is provided in table 1.^ It should not be inferred that these forced outages were necessarily safety related. In fact, only approxiTaken from AEC report OOE—ES-OOl, Evaluation of Nuclear Power

Plant Availability, January 1974.

148
Table 1. Equipment

Number

of Portion of facility outages Hours

A. Primary coolant system

1. Valve & pump seats 20 1523

2. Steam generator tube leaks 4 1402

3. Jet pump failure 1 1322

4. Main steam flow restrictor 1 312

5. Main steam isolation valve 4 148

6. Recirculation pump flow control 4 68

7. Installation of pipe

restraints 1 36

8. Recirculation system valve operation 1 35

9. Miscellaneous 4 24

Sub-total 40 4870

B. Secondary system

1. Condenser tube leaks .... 8 2003

2. Turbine control & stop

valves 19 737

3. Reheaters, deaerators & FW lines 11 233

4. Thrust bearing wear

detector 3 201

5. Feedwater pumps 9 181

6. Feedwater control systems 13 163

7. Turbine-Generator oil system 6 162

8. Airlocks into condenser or turbine 4 107

9. Superheaters 2 29

10. Steam jet air ejectors 1 28

11. Turbine seal steam 3 7

Sub-total 79 3852

C. Control rod system

1. Control rod failed to insert 3 1113

2. Primary system leakage

through seals 2 918

3. Control rod slipped into core 5 57

4. Control circuit 2 9

Sub-total 12 2097

D. Electrical generation system

1. Generator stator windings . 1 1323

2. Turbine Generator control

circuits 1 161

3. Generator bearing seal . . 2 106

failures and malfunctions initiating forced outages. Percent of forced outage time Portion of facility

Number Percent of

01 forced outages Hours outage time

4 96 0.64 3 24 .16

11 1710 11.44

10.19 9.38 8.84

2.09

0.99

.45

.24

.23

.16

32.58

13.40

4.93

1.56

1.34

1.21

1.09

1.09

0.72

.19

.19

.05

25.77

7.45

6.14

0.38

.06

14.03

8.85

1.08 0.71

E. Containment system

1. Torus baffles 1

2. Inspect drywell 1 572

11

Sub-total 2 583

F. Electrical distribution system

1. Rainwater in outside

1

2. 1

3. Main transformer 3

4. Auxiliary transformers &

control circuit 3

5. Loss of Offsite power . .

.

. 1 6. Substation Switchgear . .

.

. 1

10

259

18259

24

18

2 544

G. Reactor protection instrumentation

1. Tubing in the steamline

venturi 2 318

2. Miscellaneous 11 100

Sub-total 13 418

H. Engineered safety system

1. High pressure coolant

injection 3

2. Low pressure coolant

injection 1

3. Isolation condenser 1 4. Breaker for ESS (DB-50) 2

68

42

14

7

131

1. Instrument transformer

2. Loss of power to instrument bus ....

Sub-total 7

I. Process control instrumentation

1 85

1 14

Sub-total 2

J. Auxiliary system

1. DC to motor generator set 1

Sub-total 1 Grand Total 178 14,382

99

79

79

3.83

0.07

3.90

1.73

1.22 0.39

.16

.12

.01

3.64

2.13

0.67

2.80

0.47

.28

.09

.05

.89

0.57

.09

.66

0.52

.52

96.2

149
mately 45 percent had any potential safety significance

and none of the forced outages in any way endangered

the health and safety of the public.

The causes of the failures (not in any order of importance) included the following

( 1 ) Stress corrosion cracking of stainless steel pipes.

(2) Inadequate design for dynamic loading of pipes

and fixtures. (3) Fatigue failures from vibration.

(4) Short circuits from rainwater leakage into electrical systems.

(5) Packing and seal leaks in cooling water, steam and lubricating oil systems.

(6) Hydrogen explosions in waste gas treatment sys- tems and fires in cable trays. (7) Personnel errors, including failure to follow pro- cedures, poor workmanship, inadequate design,

installation errors and lack of adequate perfor- mance verification testing.

3. Conclusions

From a discussion of the design and operation of nuclear power plants it is clear that there are a great

many systems and components whose reliable operation

is essential to achieve a low level of risk to the health and safety of the public, either during routine operation or as a result of incident. From a discussion of the consequences of reactor accidents, it is seen that the character of a nuclear incident and the conceivable consequences are such that a very low probability of the severe incident is necessary to achieve an accept- able level of risk. Thus, very low mechanical failure rates of systems and components are required.

The previous discussion of prevention of failures de- scribed the philosophy and regulatory review proce- dures for assuring reliable performance of components

and systems in nuclear power reactors. While this ap- proach is thought to result in suitably safe reactor designs, it does not permit a quantitative estimate of the

risk being assumed by the public as a result of the oper- ation of nuclear power plants. Also, the present ap- proach does not provide a systematic procedure for identifying the relative safety importance of systems

and components. The current criteria for acceptance of mechanical designs of systems and components, the

relative emphasis on certain systems and components

and the incentives for considering alternatives have

been the product of what I call "community judgments"

of all those directly involved in the reactor safety assess- ment process. Primarily, those involved have been those

qualified to understand the highly technical issues in- volved. These have been the designers, constructors, architect-engineers and regulators. More recently, as a

result of the rather innovative procedures of the AEC,

which provide for public hearings of an adjudicatory

nature on each nuclear power plant, because of a positive policy of the AEC to make public its policies, pro- cedures and decisions, because of the increased access by the public to government documents and proceedings under the Freedom of Information Act, and be- cause of the National Environmental Policy Act which

requires extensive consideration of costs and benefits of a particular licensing action in addition to assess- ment of environmental impact, many public interest groups and individuals have become a part of the overall decisionmaking process. Speaking as an individual—and in no way reflecting the AEC position one way or another—I first conclude

that reactors can be built and operated safely, and I further think that the risk to the public presented so

far, by operation of a limited number of reactors for a

relatively few years, has been minimal, and has been

far offset by the advantages I believe have been or are

certain to be derived. Two such advantages that come

to mind immediately are the conservation of fossil fuels and the reduction of atmospheric pollutants that result from operation of nuclear rather than fossil-fired plants.

I also believe, however, that to an ever increasing

extent, with hundreds of reactors to be built and each

to be operated for twenty to thirty years or more, that it is important, if not essential, to know quantitatively

(even if only approximately) what the actual risk is from operation of nuclear plants so that comparisons

can be made with other risks, that in turn would permit

either public (or "community judgement") decisions

to be made on possible alternatives, or, overt acceptance

of the nuclear risk, whatever it is. To make possible a more quantitative assessment, in my opinion, will re- quire a coordinated effort to use more extensively in the nuclear power industry the reliability technology that has been developed recently in the aerospace and mili- tary arenas. In addition to using probabilistic and

statistical methods for analysis, this will require greater

effort to understand failure modes and mechanisms and

to predict and measure the in-service performance of components and systems.

Finally, in the nuclear business, I believe there is a new dimension in which success is to be measured. With

respect to the control of radioactive effluents, for ex- ample, the AEC regulatory policy for review of system

designs for retention or processing of liquid and gaseous

wastes has changed in the last few years from a policy

of deciding whether systems met particular numerical

criteria (i.e., "speed limits") to a policy of deciding

whether systems were designed to do all that was reason- able and practicable (with the current state-of-the-art technology) to retain radioactivity within the plant.

Analogously, it seems appropriate to me to argue that systems and components should be designed, constructed and operated to be as reliable, i.e., failure-free, as is reasonable and practicable with today's technology.

I recognize that there are arguments against this position. For example, on the surface such a policy seems to

fly in the face of current emphasis on standardization.

Also, such a policy, because of the imprecision of the

definition of goals, presents uncertain and moving tar- gets to the designer and owner-operator. Finally, there

is the difficult problem of balancing the cost of achiev- ing higher reliability or availability or safety against

the benefits to be derived. Nevertheless, so long as any

radiation exposure is considered potentially harmful

(which is a basic, long-term assumption of all national and international radiation protection standards), it will be hard to defend not requiring the application of

150
any available technology to achieve levels of risk to the public from operation of nuclear power plants that are as low as practical. It follows that reducing the fre- quency of, or preventing, mechanical failures will be an

important part of this effort.

4. References

[1] WASH-740, Tlieoietical Possibilities and Consequences

of Major Accidents in Large Nuclear Power Plants (March 1957).

[2] WASH-1250, Draft, The Safety of Nuclear Power Reactors (Light Water-cooled) and Related Facilities (July 1973).

5. Discussion

A. J. Babecki, NASA, Goddard Space Flight Center: Does the AEC have any research facility or laboratory

that it is supporting which is looking at material problems or material failures in nuclear reactors, or is this work contracted out to private industry?

P. A. Morris: All of the AEC work is done by contract, usually in the national laboratories. The program that comes to mind first, is the one on heavy section steel plates at Oak Ridge. The most uncertain event in the safety of light water nuclear power plants is failure of the reactor pressure vessel. One worries about this be- cause if you destroy the geometry of the vessel, you can no longer cool the fuel, and because of the decay heat, the chances of rupturing the containment vessel eventually are very large. So it is extremely important to un- derstand the technology of the heavy section steel in reactor vessels. There are other programs, primarily

carried out by manufacturers on Zircaloy because of the problems with Zircaloy cladding of the fuel.

151

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Mechanical Failures and Public Expectations

of Safe Transportation

Henry H. Wakeland ^

National Transportation Safety Board

Department of Transportation

Washington, D.C. 20591

Although, in the interaction between society and technology, it is desired to minimize the incidence of mechanical failure under all conditions, the practical ability to circumscribe and

control failures in transportation (for example) varies widely. Depending upon identifiable variables in circumstances, society may effectively require a range of performance between complete freedom from failure and general acceptance of repeated failures of a life-threatening nature. An attempt is made to classify the variables of circumstance which seem to govern public expectation and to create a structure in which the degree of freedom from mechanical failure which

will be tolerated by the public can be reviewed. The possibility of using this structure for esti- mating public reaction to failures is discussed.

Key words: Effects of mechanical failure on environment; failure prevention; government action; mechanical failure; safety standards; transportation safety.

Most scientists, engineers, or managers in failure pre- vention work are sooner or later required to decide how

much failure to prevent, who must bear the risks or

losses of mechanical failures, and who is to pay for the prevention work and receive the benefits therefrom.

Such decisions, once hardly recognized or thought to be

invisible, are now more often made in the open. Every- one recognizes that these decisions govern the gross

losses felt by society from accidents and costly failures. But that is not all. The correctness of a great many of these decisions also governs the economic success or

failure of the systems which we are developing or operating. Where scientists or engineers are making the decisions, the Tightness of our actions will be judged by

results and will affect the way that society evaluates our professions. Need I say also that correct decisions may somewhat affect our individual progress?

The public need for correctness and a broad base in these decisions is intense, whether the decisions are made by technical experts or public bodies. We now see

that mechanical failures can govern our nation's access

to natural resources and useful technology. Public con- cern for the effects of mechanical failures on the natural environment has played a role in delay to the Alaska

pipeline, delays to ofTshore drilling for oil, and delays

in the licensing of badly needed nuclear power plants.

No one has calculated the effects of these delays on our energy position, but the effect is certainly not negliThe views expressed are individual views of the author and are not those of the National Transportation Safety Board.

gible. In each of these fields, public bodies have insisted on employing their own decisionmaking approaches,

reviewing the arguments and taking the decisions out

of the hands of technical decisionmakers, who formerly were seldom questioned. In the case of nuclear power

plants, the questions were raised without the example

of a single accident having major consequences. The

public concern was for what might happen in the fu- ture.Major consequences may also flow from wrong judg- ments of public expectations or unanticipated adverse

public reaction. Adverse economic estimates, even those developed by careful analysis, may reverse safety gains or deny technically feasible safety. Congress is now

interested in the detailed arrangements for promoting

or requiring the use of safety belts and automobile

crash injury prevention equipment, for example, and

may reverse various safety regulations already in effect. Before the interest by Congress, the courts had also been asked to reverse various safety regulations. Overall, there is an impression that Congress or the courts

will take over wherever the values to be served are not agreeable or do not, for some reason, meet public ex- pectations.

Congress itself, of course, serves a variety of values

in loss prevention. One committee will advocate strict discipline of all program decisions according to benefits and costs, while another committee seems not to hear

that argument and insists that innocent school-children

must be protected. Still another committee says that not

153
one single nuclear power accident involving the public

is tolerable. If Congress disagrees, how can scientists and engineers be expected to make predictive decisions

that will meet public expectations? Yet it appears that we must make such decisions. The different public consequences that can flow from

countless possible mechanical failures are simply too numerous to be sent to Congress or the courts for ad- judication. The need for decisions may also be more

immediate than can be handled by the courts; very seldom do the courts permit themselves to be asked in advance whether a certain engineering decision is ac- ceptable. Such answers are often not discoverable

through legal processes until a failure has occcurred. Most legal processes operate in judgment only after the

loss has been sustained. The judgment may apply to all later design. Thus scientists and engineers must learn how to cope with public expectations on a predictive

basis as a matter of protecting investment.

Scientists and engineers are in the best tactical posi- tion to make the publicly acceptable failure prevention

decisions. Our skills are used first chronologically. We

are building better technical understanding of failures and failure prevention. We know better how to organize a new system technically to halt the effects of both

mechanical and human failures. Given enough time and

money, we would be able to quantify many of today^'s

qualitatively defined failures and assemble them into

failure prediction and loss estimate for system design.

By studying individual accidents and through the use

of statistical accident data, we can find the loss-inducing

failures in old or existing systems. We can determine

what changes will have the greatest effect and what

the costs of failure correction will be. We could even determine what groups in society are bearing the risk or the loss, and even whether the consequences of fail- ure can be countered by those involved. What we lack for predicting public expectations,

however, is something that scientists and engineers consider a basic tool for prediction. In the problem of public expectations there is no logical pattern to be

probed. Instead, there are conflicting aims. The fatal weakness for a predictive effort is the lack of general

theory. From our scientific perspective, absence of ac- cepted theory precludes teaching and understanding,

and tends to require each new decisionmaker to develop

his own theories and approaches. Yet, in each field of knowledge there was a time of conflict before theories appeared.

Perhaps it is incorrect to say that there is no generally accepted theory of what is valuable in safety or

failure prevention. Cost/benefit analysis is rather widely

accepted. A feeling that loss reduction efforts can be

justified only by showing that total benefits of a change

will exceed total costs is clearly visible. Sometimes this view is accompanied by a corollary; that any decision

in which costs exceed benefits is irrational, or mere

"emotionalism." From this economic point of view,

non-cost-beneficial decisions are acceptable only as tem- porary waste, accepted because the less enlightened who

hold the power happen to be making a compromise. Yet many decisions in safety continue to be made ir- respective of cost/benefit urgings and without cost/

effective criteria. As will be seen, these essentially eco- nomic reasons for loss reduction effort are only part

of a fairly large family of values respected in American

society. They have a useful, but circumscribed, role in safety decisionmaking, and may be opposed to other values of society having far better recognition.

The oral delivery of this paper consisted of an illus- trated discussion of a number of accidents, all involving

mechanical failures, with an explanation of the public

values which the author diagnosed as having been present. These were concerns or values implied in the back- ground, circumstances of action, or followup actions. The illustrative accidents were among those investigated

by the National Transportation Safety Board since 1967; however, the social concerns noted were not find- ings of the reports, but observations. Other situations in transportation safety in which the public values of safety problems were analyzable were also presented.^

A "Schedule of Concerns" developed in an attempt

to classify such observations, was then presented, and is a part of this paper. This schedule seeks to list major

classes of socioeconomic concerns and public values which have been the apparent basis for actions (or in- actions) in mechanical failure safety decisions made by

technical personnel or others exerting controlling re- sponsibility.

The sources for these concerns and values were de- veloped from the field of transportation safety and, in candor, are not limited in derivation to safety problems

involving only mechanical failure. The field of trans- portation safety involves human failure as well as physical failures. Nevertheless, it is considered that all the concerns in the schedule apply to some circumstances

or situations that could be found in mechanical failure

loss. The concerns are not yet a full list for transportation safety. Some concerns or values are listed in section IV which are specialized to the interested viewpoint of the mechanical system designer. This was done simply

to provide specific coverage to a field of interest to those

in the Mechanical Failures Prevention Group. Some of the concerns are of very broad application as to the organizational situation; others are narrowly stated for application to rather specific circumstances.

The purpose of the schedule is to provide a rudi- mentary organization to assist the decisionmaker or the analyst of decisions in reviewing a safety situation and

evaluating a variety of concerns or values. Word de- scriptions have been written which seek to grade each

of the concerns. The user looks at the situation and the possible approach to failure prevention from the stand- point of each concern individually, and he estimates

its grade according to the word descriptions. To record

his analysis or estimate, the user marks the circle in line with his selection. The circles are placed so that marks

to the right indicate generally increasing need or desir- ability of a failure prevention action, as suggested by

the author's observations. These are believed to be generally agreeable.

The schedule is designed to aid decisionmaking, but

it is also useful for questioning decisions.

" Schedule of Concerns for Safety by Mechanical Failure Prevention or Control of Consequences, MFPG 1974, H. H. Wakeland.

154
The first gain for the decisionmaker is. an assurance

that he has at least considered the major values and

concerns in which the decision may be evaluated later by others. This tends to handle one of the problems of poor prediction, which is simply that of overlooking some objections or value system pitfalls. Other ways to use the chart will be noted later. Four groups of socioeconomic concerns and public

values have been classified:

I. Economic Efficiency in Control of

Loss to Society

This group of values reflects the desire to reduce fail- ure associated losses in American society as a whole

according to economic factors or economic criteria. The six concerns in this group come into play according

to the circumstances and the amount of information

available for the decision. These concerns are relatively new in the historical

sense, and with the exception of I.C and I.E, they rely on failure consequence data or cost data that were

hardly visible a century ago. At the present period, most analysts try to select safety changes (as distin- guished from safe design of new systems) on the basis

of economic cost benefit status. This is concern I.D. Benefits in losses avoidable by failure prevention are compared with the cost. At a minimum, a certain change is cost/beneficial if the benefits in savings

(reduced to money values) are greater than the esti- mated costs over a long time period. Where all benefits and costs can be determined, the benefit/cost ratio can

serve as a basis for priority judgment among competing

changes. To the degree that a population at risk is ac- curately reflected in the loss data gathered, it can be

argued that discussions based first on benefit/cost status of a proposal come close to an elusive ideal called "The

greatest good of the greatest number." Benefit/cost

would produce the greatest benefit with limited funds

when workable.

The weaknesses of an exclusive benefit/cost criterion,

however, are many. Data are difficult to gather and

because data change, decisions can seem more logical for incremental improvement than for planning. The

necessary summation of benefits must total lives saved,

injuries avoided, and property damage prevented. In- evitably, money values are placed on human life and

suffering, with illogical results on philosophical

grounds. Methods of evaluating life tend to emphasize

the individual's value as contributor to an economic

system, and absurdities may result as soon as the analysis begins to try to include system failures that affect non-wage earners. Yet, when a high benefit/cost ratio

is determinable, there may be wide agreement and few

countervailing concerns seem logical.

Priority in cost effectiveness, item LB, seeks the same result, but avoids the embarrassing effect of totaliz- ing non-comparable losses. Effectiveness is judged by

fatalities prevented, or injuries prevented, or property

damage prevented, stated separately. The result is some- what more open and easier to deal with on a policy

level. Decisionmakers can still decide to prevent large

property losses or numbers of costly minor injuries

instead of a few fatalities. The difficulties of data and

of application to new systems or long range plans are

still present. Cost effectiveness decisions can be more

easily adjusted to "human" values. When loss data and cost data cannot be logically

combined for cost effectiveness judgment, the next lower step on the economic judgment scale is the size-of- the-target. This concern is item I.A, size of total losses in continuing accidents. Among the economic concerns,

society at large easily grasps the idea that a field of large loss ought to be attacked. Since the loss is great, any effort has a better chance of being effective, it might be thought. Size of the target decisionmaking

sometimes produces effective results, but heavy waste

of resources can also eventuate. Where a field is completely new, however, size of the target is necessarily

the first approximation to economic effectiveness. Size

of the target is listed first because, in the public view,

it is expected that something be done when failures are producing large losses. The demand to "do something"

will be present, even when the absence of economic

analysis suggests that results are uncertain. The need to take some action has traditionally been the first concern

because the earliest knowledge of accident failure losses arises when it first becomes possible to measure the

loss. This consideration does not, of course, say that the amount of effort must be proportional to the loss. That

question has to be weighed on several bases. The size of the total loss in continuing accidents is graded as a proportion of the losses which are within

public recognition of the field of loss. The perception

by the public or Congress of field of loss is quite important and has been closely related to the visibility of accidents, discussed in IIIA. The absolute money cost of prevention measures has

historically served to permit or limit safety efforts re- gardless of considerations of the efficient use of funds. Failure prevention methods can often produce an absolute cost-saving, especially where operating efficiency

and safety can be improved by the same action. But in many cases, where cost must be incurred, funds are simply unavailable. Alternatively, costs incurred could even be financially disastrous to those involved. The

grading of this item reflects apparent public expectations; namely, that there will be less public pressure

of necessity where the cost is high. The losses may also be high ; high enough to overcome the cost. Yet the cost must be considered. Note that this form of absolute

cost assessment refers to every level of the economy.

The absolute cost will be important to producers, middlemen, consumers, ticket purchasers, maintenance operations, public agencies.

The importance of an activity to the national econ- omy is a de facto concern in loss or accident prevention

work, but one which requires study more than specific

argument. At the public level there are occasional ten- dencies to use safety or loss prevention arguments to bring about changes which are more economic than

safety-oriented. There may be genuine situations in which the economic health of a portion of the economy

is associated with failure prevention. For example, eco- nomic conditions in a major portion of the transport

155
economy may cause deterioration of the equipment or a moratorium on safety improvements may be sought.

Though there are few published examples of this con- cern as a real influence, known examples have had

strong effect. The role of importance-to-the-national-economy as a concern appears to change drastically after a system

investment has been made. When there is a possibility of a major scope system change, the fact than an activity is important to the national economy can serve to magnify the scope of effort. Safety was introduced into the Urban Mass Transportation Administration under

conditions before the major-investment. However, once an important investment has been made, costs of change

for safety rise, and the costs may tend to be viewed as threatening the economic operation. When that circumstance is present, there may be less resistance to cor- recting failures if the system is not economically

important. Item I.E applies to a local economy as well as to the national economy. Again, it may be wiser for

the decisionmaker to be aware of the effect than to argue it. In Item I.F the fact that a certain system has a low or high status of loss rates in relation to its competitors is noted. The fact that there are high loss rates may not generate a strong expectation of correction among the public unless that loss rate is known. The importance of

loss rates is probably strongly influenced by concomitant concerns in equity and social justice and in public awareness concerns in Class III. That is, there may not be a strong public component of concern where those who are suffering the loss seem to be participating in economic benefits. The Class I concerns for economic efficiency in loss control seek to treat society as a whole. The thrust of these concerns is the most efficient use of public funds

to reduce the totality of loss. Where such judgments are

not possible, the economic efficiency concerns still seek

efficiency in the use of funds, but through other, less precise criteria. The economic efficiency concerns could

SCHEDULE OF CONCERNS

FOR SAFETY BY MECHANICAL FAILURE PREVENTION OR CONTROL OF CONSEQUENCES

SOCIO-ECONOMIC CONCERNS AND PUBLIC VALUES WHICH MAY BE BASES FOR NECESSITY OF ACCIDENT LOSS PREVENTION ACTIONS BY DESIGNERS OR OTHERS EXERTING CONTROLLING RESPONSIBILITY.

I. ECONOMIC EFFICIENCY IN CONTROL OF LOSS TO SOCIETY

I. A. Size of total losses in continuing accidents

o Small percentage of total losses in field

o Significant proportion of losses in field

o Losses large as proportion of field or in public

recognition.

I. B. Priority of failure prevention in cost effectiveness

o Preventive measure has low cost effectiveness in controlling risk relative to alternatives

o Preventive measure has medium priority among alternatives on cost effectiveness basis

o Preventive measure shows highest effectiveness

for cost among alternatives

I, C. Absolute cost of preventive measure

o Preventive measure has cost saving

o Cost of prevention insignificant

o Moderate cost

o Cost of prevention disastrous to those who must bear

cost

INCREASING NECESSITY OF FAILURE PREVENTION

OR CONTROL OF ACCIDENT CONSEQUENCES

O

o

o

o

o

o

o

o

o

o

156
also be credited with a strong thrust toward fairness. That is, it may be fairest to society as a whole to maximize the total benefits of failure prevention without

respect to whether one element of society may be con- tributing more than another. Such an approach would seem logical for those social systems in which the good

of the state is seen as paramount. All of the concerns

for economic efficiency treat the losses as being undesirable because of their numbers. Numbers and costs confer priority. As noted earlier, the concern for benefits versus costs leads directly to evaluation of the human

according to his contribution to the economy through

earning power. This is most significant when seen be- side the rule that only a benefit/cost ratio exceeding one can justify a safety change. These two features of judgment seem to say that the good of the economic

system is the measure of safety effort, and that the individual is not to be valued beyond his economic

contribution. Safety effort must show a profit for the economic system.

A good many safety proposals have actually been

rejected in recent years without any stated reason other than a showing that their cost was greater than the present value of future earnings of those who might be

saved. Such statements do not prove that our value system is narrow. The intent of such judgment is to save funds for some other safety technique that might

save more lives. No analyst is known to have evaluated

safety for retirees on the basis of present value of future earnings, nor has safety effort for newborns been limited to what might be justified by the investment made.

Society is not really in danger of suspending safety

effort should zero population growth become a goal.

Our de facto social concerns extend beyond en masse

evaluation. Let us try to formalize some of these values which arise from our status as individuals and groups.

SCHEDULE OF CONCERNS.

I.D. Economic benefit/cost status of prevention measure

o Payoffs of prevention are other than economic

o Economic benefits and costs about equal

o Economic benefit somewhat greater than costs

o High payoff of expenditure in reduced loss or risk

I.E. Importance of activity to national or local economy

o Activity widely regarded as harmful to society or economy

o Activity supports economy, but could be abandoned

without damaging effects

o Activity important to national economy

o Key economic or defense activity which must continue to operate

NOTE: The order of these grades is for design

effort prior to system construction. Reverse the order for a system already

installed in the national economy.

I«F, Status of activity's loss rates amon^ competitors

o Activity has lowest loss rates of possible modes

in activity

o Loss rate of activity near median of competitive

ac tivities

o Competing activities have much lower loss rates

Increasing necessity

o

o

o

o

o

o

o

o

o

o

o

157
II. Concerns for Equity and Social Justice

Mechanical failures involve risk and losses to people,

and people are individuals. Safety regulations or other

safety decisions based on economic efficiency often do

not act equally on individuals and groups. The circumstances of failures place groups at risk in different degrees. Society has developed ways of avoiding inequities or unfairness, or of providing special protection in cer- tain social problems. For example, laws provide that individuals can recover money for wrongful loss in an

accident.'' Laws provide welfare payments for the dis- advantaged, or special tax relief for the elderly or blind

persons. Men who make a career of flying military aircraft receive risk payments, effectively as compensation from

the rest of society for presumed higher risk that benefits everyone. The cry of "Women and children first," effectively followed at the lifeboats of the Titanic,

reflects one of the general principles of social priority decisions where risks are present. Ideas of equality,

equity, and the avoidance of unfairness have been followed in Western society much longer than ideas of economic efficiency. In America it was declared to be

self-evident almost 200 years ago that "all men are

created equal." Above the entrance of the U.S. Supreme

Court building are inscribed the words "Equal Justice under Law."

Yet there is still very little explicit recognition of these values when engineering designs are evaluated and decided. Standards and regulations do cover some

hazards because they may be unfair or wrongful to some group, but little is said to make the reasons explicit. Many explanations can be given for this. There

may be a lack of advocacy because groups are not or- ganized or do not perceive the inequity in risk bearing.

Historically, the raising of questions of group equity

and fairness has temporarily threatened economic effi- ciency and produced controversy. (Consider, for ex- ample, child labor, mine safety, and the raising of crops with the aid of human slavery.) From a practical

viewpoint, scientists and engineers are not trained to see instantly inequities caused by product design. Eff'ort

is required to discern these values in system design,

SCHEDULE OF CONCERNS .

II. CONCERNS FOR EQUITY AND SOCIAL JUSTICE

II, A. Share of persons at risk in general benefits of the Activity

o Persons at risk fully compensated

o Persons at risk receive benefit of employment

o Persons at risk gain benefits about same as general population

o Persons at risk bear risk almost entirely for benefit of others

o Persons at risk bear risk without compensation for benefit of same organization which controls their degree of risk

II. B. Voluntary-involuntary status of riskbearing by persons at risk

o Persons at risk bear risk voluntarily

o Risk could be avoided at cost or inconvenience

o Persons at risk cannot avoid risk-laden situation

II, C, Ability of persons at risk to know and avoid hazard

o Persons at risk fully appreciate hazard and can avoid

o Some appreciation of hazard, but degree of risk unknown

o Persons at risk cannot know the risk

o System tends Co minimize awareness of risk or encourage unrealistic confidence

Increasing necessity

o

o

o

o

o

o

o

o

o

o

o

o

158
effort of a differenL kind than required to analyze

effects in mass economics. When equity or equality are

studied, however, it will be found that system design

which efficiently reduces risk for the multitudes may

concentrate the risk on the few. Scientists and engineers

thus face a design decision not yet in the manuals. In

actual experience, failures to recognize these values can

result in reversals of technical decisions by policymakers who are more directly trained in these matters.

It is far preferable to detect these values at the design

stage and perhaps make a different decision. Let us review some examples in the six Equity and

Social Justice categories of the Schedule of Concerns. We will draw upon problems treated in publications of

the National Transportation Safety Board (NTSB).

Concern II. A provides a scale for varieties of situa- tions in which groups of persons bear risk for themselves, for others, or for society in general. Unfairness

is avoided by making failure prevention more necessary

as the degree of riskbearing for others increases. Persons who live in the hazard zone along high pressure

transmission pipelines bear the risk of a wide variety of mechanical failures. They bear this risk for the benefit of the remainder of society, and with no compensation.

In a specific situation, it had been proposed that regulations for natural gas pipeline safety be justified by a showing that benefits in reduced losses would exceed the

costs of preventing the failures. The NTSB pointed out

that many of the fatalities and injuries occurred to per- sons living along the pipeline who were bearing the

risks and losses, uncompensated, to provide the benefit

of pipeline transportation to the rest of society. The

Board said that benefit/cost analysis was not a basis for decision and that safety regulations should seek to equalize the risk between those who live near pipelines

and the rest of the population. (The effect of balancing

benefits and costs to adjudicate the risks of persons along the pipeline was to judge the safety of those per- sons as though they were witbin the pipeline economy.)

Category II. B describes the observation that society

is willing to require greater protection against failures

to persons who cannot avoid the risk. This observation

SCHEDULE OF CONCERNS.

II, D. Protective attitudes of society toward deserving

classes of persons

o Persons at risk regarded as taking risk recklessly or for profit

o Those at risk neither reckless nor especially

deserving of protection

o Risk to persons In protection-deserving

categories. Pregnant women, children,

handicapped, elderly, otherwise disadvantaged

o Persons protected for symbolic or national

interest reasons

II. E. Equal protection of groups by amount of effort

o Prevention efforts applied with approximately

equal efforts to groups suffering loss, or at risk

o Prevention efforts not equal

o Prevention efforts appear very unequal

II. F. Apparent locus of operational responsibility

o Individuals considered primarily responsible

o Both Individual and organizational responsibility

o Private organization regarded responsible

o Public organization regarded responsible

Increasing necessity

o

o

o

o

o

o

o

o

o

o

o

159
has been detailed for several classes of persons by

Chauncey Starr.^ The NTSB, in several documents re- lating to the safety of children riding schoolbuses, has

cited their presence in the buses, not entirely of their own volition, as justifying higher priority efforts. Category II.C covers the variety of situations as to knowledge of the hazard when there is ability to avoid

it, if the hazard were known. Society's practice here is largely fixed by legal action after an accident has oc- curred, in which the failure to warn of hazards or possible failures can have a strong effect on liability.

Recognition of this principle in the design of equipment

not only protects against liability, but will also remove

the unfair hazard. Examples at the high value end. include booby-traps and practices which seek to fulfill liability criteria while submerging awareness of the hazard by vagueness or euphemisms.

Category II. D describes society's low estimate of the need for safety of motorcycle thrill riders compared to the actual heavy protection provided to persons in de- serving categories. The social practice of providing this protection goes back at least to the prophet Jeremiah

and is found to some degree in every civilized society. In a very recent example, the NTSB urged that the need to protect innocent children riding in schoolbuses outweighed the probable adverse benefit/cost ratio of a

certain schoolbus safety improvement. After a period

of time, the NTSB also recommended a way to implement a regulation without disturbing other priorities for use of government funds. Legislation proposed in Congress was opposed by the cognizant administrative

agency on benefit/cost grounds, but Congress nevertheless enacted this form of special protection for children. Category II. E parallels the idea of equal protection

under law, translated to equal protection under regulation, and the similar idea of equal protection by tech- nical effort. Notice that equal protection by amount of

effort is an approach that will frequently require ac- tions contrary to concerns for economic efficiency in loss reduction. This situation occurred in a study of Department of Transportation efforts directed to the protection

of pedestrians, in which it was found that funded efforts for the direct benefit of pedestrians were of the order

of Yioih the funded efforts which benefited motor ve- hicle occupants. After showing that the pedestrians were not the same group as the vehicle occupants, the NTSB recommended increased funding for pedestrian

safety efforts, a recommendation which was carried out by the administrative agency. Category II. F describes the variety of values based on private-public responsibility varying between low

necessity of action where individuals are responsible

and high necessity when the public is considered re- sponsible. It is observable that Congress and the press respond much more strongly to failures involving public

responsibility than private responsibility. An implicit

example is seen in the difference in fatality rates be- tween commercial and general aviation. Commercial

aviation over a 5 year period has a low rate typical of

^ Starr, Chauncey, An Overview of the Problems of Public Safety, Paper

presented to the Symposium on Public Safely —A Crowing Factor in Modern

Design, National Academy of Engineering, May 1, 1969.

all regulated carriers, in the region below 0.2 fatalities per hundred million passenger miles. General aviation

(private flying) has a fatality rate between 16 and 20

fatalities per hundred million passenger miles, about 2

orders of magnitude greater.

III. Concerns in Estimating Public

Acceptance of Change or Need for Change

Where failure prevention measures apply to the public or need the assistance of government, further con- cerns must be taken into account. Extremely strong

demand for change or sympathetic attention to a subject grows and wanes under certain practical conditions. The concerns graded in Section III were arrived at by

long term observation. They are practical publicopinion-generating forces, which affect the public necessity of change, and must therefore be concerns of the system designer or decisionmaker. These concerns tend

to aid or to interfere with economically logical loss re- duction decisions. They may be deplored by some

scientists or engineers, and actions which take place are

frequently called "emotional" or "irrational." There is, therefore, a strong tendency by scientists or engineers

to reject these concerns rather than to make use of the motivations or changing tactics. In Concern III.A it is observable that accidents of the type which attract heavy public notice, such as large

explosions or commercial aircraft disasters, do attract public attention and interest which makes corrections more in the public eye and therefore more necessary. At the other end of the spectrum, public information

must constantly be generated for highway problems

which produce smaller losses visible only in statistics. Concern III.B separates the amount of loss in a

single accident from the public visibility as a differentiation to aid analysis. Some accidents will be highly

visible, but produce little loss. Concern III.C describes the apprehension of the public as a factor. This factor can be so strong as to completely determine actions, regardless of other factors. "Freedom from Fear" is of sufficient importance that it was named as one of the Four Freedoms in the Atlantic Charter. The scope of apprehension ranges from vir- tually zero, as in driving to work, to the extreme fear of exposure to radioactivity which delayed new nuclear power generation plants for a period of years during an energy crisis. The fact that highway accidents cause 150

fatalities per day, while there has never been a public

fatality from a nuclear power plant demonstrates the

role of public opinion as compared to statistical importance. The difference in effects is essentially one of

differences in apprehension generated in the public by

the activities. Concern III.D, the impingement of preventive measures on personal freedom, reflects apparently deep- rooted but not necessarily universal American ideas of the proper role of government in protecting individuals. This is a cultural value and it must be considered.

The full range of these concerns is not tested, but is classified here as a matter of structural order. Strong

leadership can raise or lower the effects of these con160
SCHEDULE OF CONCERNS.

III. CONCERMS IN ESTIMATING PUBLIC ACCEPTANCE OF CHANGE

OR NEED FOR CHANGE

III. A. Public visibility of loss

o Individual accidents attract short term attention

o General awareness by public that losses are occurring

o Heavy public notice of accident occurrences

III.B. Possible loss in single accident

o Accidents never involve many fatalities

o Typical single accident regarded as catastrophic

o Large-scale loss to population in one accident

III.C. Public apprehension of activity to which failure is related

o Activity generates little apprehension

o Sufficient apprehenson to affect individuals' economic decisions

o High degree of public apprehension

III.D. Impingement of preventive measure on personal freedom

o Preventive measure violates supposed constitutional

freedoms

o Measure requires timely application and is res trie tive

o Preventive measure obtrusive, requires operation,

but is nonres trie tive

Increasing necessity

o

o

o

o

o

o

o

o

o

o

o

o

cerns. At one end of the spectrum, motorcycle helmets were attacked as unconstitutional, but they are nevertheless a required form of protection in many States. In another case, the personal inconvenience of auto- mobile safety belt interlocks is producing a rather strong effort against them in Congress.

A general observation of the concerns in Class III

is that they are somewhat subject to leadership, but that leadership effects may be unpredictable. There is a ten- dency for the imperfect knowledge of hazards possessed

by the public to produce concerns that run counter to logical loss reduction, but there are situations in which

illogical concerns aid loss reduction.

IV. Concerns for Post-Accident Judgments

of Design or Failure Control Decisions

This group of concerns is felt most strongly by per- sons who are responsible for design or regulation to control failures or failure consequences. The concerns

are important because they do affect, or should affect, decisions as to whether mechanical failures must be

prevented or guarded against. These concerns are un- derstandable as written and do not require justifica- tion.

Decision Problems with a Schedule of Concerns

The purpose of a Schedule of Concerns is to aid

decisions of failure prevention or control of conse- quences, not to make the decision. The task of the decisionmaker has always been to know the concerns

present and to weigh them to meet a variety of public

expectations. Thus the schedule adds a certain kind of order, but the task is still highly judgmental. Decisions

still require balancing of values and concerns with less than a comfortable amount of knowledge. The schedule,

for example, shows linear spacing of the circle marks,

where nonlinearity is frequent.

The user first studies the situation in which the deci- sion is to be made. He may begin by listing the various

publics whose expectations may be touched, for that is important in estimating each concern. These may or

161
may not include a consumer product public, a certain public which uses the transportation, a public of inter- acting administrative agencies, local or national public

opinion, a professional peer group, legislative bodies,

regulation enforcement agencies. The choice of the publics to be served can dramatically change the estimated

concerns. Making specific choices of whose expectations are to be served is part of the value judgment.

The decisionmaker may then go through the schedule, studying each concern, perhaps initiating information gathering to clarify estimates. In so doing, he will almost be forced to consider many more factors than in an unstructured decision. He will be required to explicitly determine how various groups will be affected. The decisionmaker may very well find ways to decrease

the necessity of controlling failure, as by opening a hazard to full knowledge rather than hiding it. He may

find that responsibility should be in a different place.

The decisionmaker will see directly some differences between decisions motivated by broad public concerns (Groups I, II, and III) and those which are needed to forestall his own possible embarrassment (Group IV).

These concerns tend to be additive: it is not a loss to

anyone if more safety is provided while protecting the designer's reputation.

When the schedule is marked with estimates of concerns, the conflicts between necessities inevitably stand

out. The numbers of high and low estimates can be

totaled, but that is not sufficient. The high-valued con- cerns can be assembled in a list. Take special note of the lowest valued concerns and list them. (Note, however, that some low-valued concerns are the result of ignorance while others are genuine contraindications of

action.) When the lists are compared, the concerns in the broad judgment will be capsulized.

It will also be useful to write a separate description

of the conflicts that appear. For example, Winston

Churchill, in his decision at Dunkirk, knew explicitly that he was promoting overall economic efficiency by

rescue at Dunkirk and permitting unfairness and sacrifice as smaller garrisons not to be rescued. It is important to know what concerns are served and what

concerns must be sacrificed. Thus are reached those ultimate conflicts, often suppressed, which require the exercise of knowledgeable

judgment. There will also be happy discoveries where it

SCHEDULE OF CONCERNS.

o Preventive measure unobtrusive, requires no attention or operation

IV. CONCERNS FOR POST-ACCIDENT JUDGMENTS OF DESIGN

OR FAILURE CONTROL DECISIONS

(Basis is not legal responsibility, but factors which influence society's judgments.)

IV-A. Predictability of loss by those responsible

o Possibility of failure leading to accident is obscure or unpublished

o Failure involvement in accident analyzable by known

techniques

o Failure involvement in accidents documented

o Role of failure in accidents shown by statistical

data

IV. B. Time since accident consequences of failure identified

o Possible failure consequences pointed out recently

o Possible failure consequences known for some time

o Possible failure consequences pointed out repeatedly

over period of time

IV. C. Involvement of deciding organization in consequences of accident failure

o Deciding organization will not suffer money loss from accident

o Deciding organization status re loss uncertain

o Deciding organization must itself absorb money

or personnel loss

Increasing necessity ^ o

o

o

o

o

o

o

o

o

o

o

162
is found that economic efficiency, equity, and public

acceptance aid each other. A schedule can be a wooden thing. The word implies,

sometimes, a mindless or routine repetition. But this schedule is an early approach, the writer believes. Some

additional steps are already visible. They include classi- fication of recurrent application situations, detailed case

studies of concerns as they apply to situations, search

for patterns and combinations of concerns which have

defined major failure prevention decisions of the past. Are there other concerns not generally identified? In whatever \vay they study the problem, scientists and en- gineers need to devote more attention to finding social concerns and values in their work, for public expectations are not based on optimizing one index, but on

balancing many values.

5. Discussion

H. P. lost, K. S. Paul Products, Ltd: It appears to me

listening to Dr. Morris and Mr. Wakeland that, as a

society we are more and more pressed and more and more willing to accept large numbers of fatalities. I wonder whether 10 years ago we would have been pre- pared to accept the possibility, if not the probability, of 350 people being killed in one air accident but with the use of jumbo jets we seem to be accepting it. Perhaps

one day we will have to accept the possibility of 10,000

people being killed in one accident.

W. D. Compton, Ford Motor Company: The determination of the risk associated with a failure and the post

evaluation of a failure is one thing; the translation of

this information into standards or regulations to be

used in subsequent designs is quite something else. Do

you foresee this sort of thing happening? Do you think

that a set of criteria or regulations will evolve, based on

information from failure investigations, that can be applied to a broad range of situations in which the cost effectiveness may be quite diflferent?

H. H. Wakeland: Yes, certainly it's going to happen. It

is the nature of humanity to evaluate things in the way

they are put before them, not in the way that a cost benefit analyst, even though he may be working under

OMB's strictures or under the orders of the Joint Eco- nomic Committee of Congress, would evaluate them. It

is very unlikely that the Silver Bridge investigation was a cost effective action. However, it fulfilled another

of society's basic social concerns and needs. You have

to remember that the first thing stated in the Declaration of Independence is not "cost benefit and maximum

efficiency for all", but "all men are created equal". The

sign above the Supreme Court does not say "maximum

cost effectiveness for minimum fatalities in society as a whole". It says "equal justice under law". Equal justice

under law means the sort of thing that you manufacturers reach in product litigation after an accident. That's entirely an equality and equity action. It may or may not come out to be cost effective. But society is going to go on asking for those things simply because

that's the way the people think and that's the way Congress is organized.

W. D. Compton: Isn't it also appropriate to ask that there be something developed for a review of these cri- teria as technology changes so that one isn't faced with

a long list of possibly obsolete restrictions?

H. H. Wakeland: It's very appropriate to ask for that. What should be asked for from organizations like Congress is a clear look at the priority system being used. Are priorities decided on a cost benefit basis or on a safety basis? These values conflict with each other and

someone must decide which point of view will prevail. In the meantime, conclusions reached by the public on the basis of the sheer horror of a situation are not wrong conclusions. They may be uneconomical; they

may steer us in the wrong direction; it's up to us to decide. I'll give an example. We were before the Transportation and Aeronautic Subcommittee of the House

Interstate and Foreign Commerce Committee recently.

We wanted to talk about soft cushions for the front of

railroad locomotives for which the Federal Railroad

Administration has a feasibility study that shows a ten

to one benefit to cost ratio in lifesaving in a field which

involves one-half of all the fatalities now assignable to railroads. They didn't disagree with us, but they wanted

to talk about requiring money to be spent for inspectors

to look at broken rails, even though broken rails are re- sponsible for only about six fatalities per year. When

we have our values organized and when they have their values organized, we can discuss the priorities intelli- gently.

163

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Public Safety and Construction Surety*

Jacob Feld

Consulting Engineer

114 East 32nd Street New York, New York 11016

Structures are conceived, designed and constructed for use by man and located in proximity to other structures and open spaces occupied by man. How safe are these structures when first occupied, and what degree of safety can be expected of them with age?

Who is responsible for the original condition of safety of the structures, and for the degradation of safety with age and use? Who is responsible for the maintenance of sufficient safety to permit continued use and

occupancy ? In addition to the broad aspects of the problem, the paper will describe avoidable use of incompatible materials that caused local distress or failure of the structure. Public safety and construction surety depend on proper control of each factor involved in the design, construction, maintenance, and use of a structure. Any deviation on the part of any

one factor makes the structure vulnerable to damage or even collapse. Absolute safety is not

obtainable. The historical record of structure behaviour indicates the high reliability of present techniques and procedures.

Responsibility for defects and for the small percent of structures that do not perform properly

is a deterrent to proper control, but considerable pressure is needed to further reduce the number

of failures to insignificant numbers. It is the duty of the professions involved to provide that pressure, since legal and political action has not been successful, and to control the proper per- formance of the work in every factor and so provide public safety and surety in the construction

industry.

Key words: Construction safety; construction surety; degradation of safety with age and use; maintenance of safety; public safety; safety responsibility.

1. Introduction

Structures are conceived, designed and constructed

for use by man and located in proximity to other struc- tures and open spaces occupied by man. How safe are

the structures when first occupied and what degree of

safety can be expected with age? Who is responsible

for the production of safety in the structures, and who

for the degradation of safety with age and use? Who

is responsible for the maintenance of sufficient safety to permit continued use and occupancy?

Historical Notes: The problem of safety in construction must be separately treated for incidents, accidents

and even collapse occurring during the production of the work, from the condition of the completed structure and its reaction to use and to natural causes of deterio- ration and reduction in its resistance against failure. Most of the accidents and partial collapse in structures

*This presentation is based on a papet presented to Section of Engineering, New Yorlc Academy of Sciences, December 10, 1969.

come during the construction period, often with serious

effect on the life and well-being of people involved in the work as well as the innocent public by chance

within the area involved. This discussion is, however,

limited to the accidents coming after completion when

public use is begun and the people involved in the construction are away from the area. The problem of safety has a long history and there are many references even in ancient regulations and

discussion setting forth the limit of responsibility and

of expectation. If the code of Hammurabi (about

2000 BC) is typical of ancient regulations, few engineer

builders had the opportunity to learn from their own

mistakes.

"If a builder build a house for a man and do not make its construction firm and the house which he has

built collapse and cause the death of the owner of the house—that builder shall be put to death."

Certainly such strict punishment for failures must have

been a deterrent to shoddy construction practice.

165
Some warnings in the Hebraic literature against

building on shifting sand may stem from experiences

encountered when the Hebrews were the construction

battalions in Egypt. That entire ancient city complexes

collapsed from normal aging as well as from enemy

action and tectonic disturbance, is found in the archeological exploration of separated layers of rubble. There

is no record to indicate if any mortal was judged re- sponsible for the collapses and the loss of lives. In Greco-Roman times the construction industry was

in the hands of trained slave artisans and successful work was rewarded by gifts of substance as well as of freedom. With no competitive financial or time schedule

to worry the builder, work was well done and with ap- parent great success, as is seen in the many examples

still existing. Similarly, in the Medieval period time was no object and the great successful religious and

governmental structures have lasted for centuries. One

doubts that our modern works will exist to comparable

age.In the common law developed in England, as is found in the 15th century court records from the reign

of Henry IV, the rule was "if a carpenter undertake to build a house and does it ill, an action will lie against

him." Of course, by "ill", is meant "not well", and if all work were done well, there would be no record of

failure. So historically the burden rests on the construction industry to see that work is done "not ill" and each

partner in the industry, from the architect who conceives the project to the foreman who directs the per- formance of a part thereof, must lend every effort to avoid and eliminate every possible cause of "ill" structures. The Napoleonic code has become the basis of the common law wherever the original settlers were French.

In many ways there is a greater responsibility placed on the designer and the professional in charge of the work as agent of the owner, to safeguard the investment

and guarantee proper and adequate performance, than

is expected under English legal procedure.

In the period from 1870 to 1900, construction be- came a large industry in the United States, chiefly

influenced by the expansion of the transportation sys- tem with its necessary bridges, storage facilities, and

industrial plants. Comparatively little is recorded of

failures in buildings, most of which were heavy masonry

wall bearing structures with sound timber floors. Bridge

spans soon exceeded the capability of timber trusses and a great competition in the sale of iron spans re- sulted in many failures. These were spectacular and

were almost daily newspaper headlines. Even foreign

technical journals commented on the great number of unsuccessful designs of bridges in the United States. Engineering magazines from 1875 to 1895 were as full of reports of railroad accidents and bridge failures as today's daily newspaper reports of automobile traffic accidents. Even as late as 1905, the weekly news sum- mary in Engineering Record described the most serious railroad wreck of the week, usually tied in with a bridge failure. The Railway Gazette in 1895 published

a discouraging summary of iron bridge failures result- ing from railway traffic, listing 502 cases in the period

from 1878 to 1895 inclusive, collected by C. F. Stowell,

and noting that the first 251 occurred in 10 years while

the second 251 occurred in only eight years. In the years 1888-1891 inclusive there were 162 such acci- dents. These reports received wide publicity and atten- tion and must have had considerable influence on the designing engineers as well as the bridge salesmen of those days. In this discussion, the term "safety" is defined in the Webster Dictionary as:

(1) freedom from danger or risk (2) freedom from injury

(3) harmlessness or being without hurt, loss or li- ability

and "construction" is the man-directed assembly of in- animate components to make up:

(1) shelter for man in his various activities (2) tools to extend his power and mobility

(3) instruments to extend his senses.

2. Absolute Safety

Absolute and unqualified safety at all times can only

exist if the designer has prepared a plan entirely free of any error in judgment or computation, the contractor has not built in hidden errors which void the intent of the design, the owner maintains the structure so that there is no degradation of strength in any component

and the public does not exceed the expected loadings,

both static and dynamic, nor modifies the strength of the structure either by removal of support or by imposition of new conditions from adjacent construction or

demolition. One cannot economically design for zero probability

of failure, nor for infinite life of a structure. There is always a risk in being alive or even in being; the only

certainty against death of a living being is not to be

alive.. The concept of a factor of safety in structural design came about a century ago. If the ratio of available

resistance to the effect of the loading always remains

greater than unity during the existence of a structure, safety is assured. But the factor of safety is merely a

ratio of two assumptions and is therefore set by either regulation or agreement at values large enough to allow

for considerable error in assumption. For normal design

of buildings 2.5 was considered sufficient protection

and lately has been reduced to about 1.6, for bridges

3.0, but for the stability of earth mass embankment or dams, 1.5 is normal, since factors of safety of 2.5 or 3 would bring the cost so high that these earth structures would never be constructed. Under extraordinary water

pressure loading, a computed factor of safety of 1.1 is often considered sufficient. One can readily see why the

relative incidence of failure is greater in earth struc- tures than in structural frames or mansonry buildings.

The proper choice of a factor of safety is often de- pendent not only on the physical design procedure and

uncertainty concerning material strengths and construction control, but also on sociological and psychological

effects of visible cracking which may not be struc166
turally significant, on political grounds to avoid public

criticism if failure occurs, and on economic grounds

when the cost of extra strength is evaluated. Where failure, should it occur, would not affect life, many designers and owners are willing to take a cal- culated risk in degrading the normal value of the factor of safety. In foundation design and performance, Professor George F. Sowers of the Georgia Institute of Technology has warned that more care is necessary

since designs are becoming increasingly more daring,

modern structures are now less tolerant to movements

from foundation settlement, sites for new construction

are becoming poorer in bearing value and populated

areas, with their load imposition on the land, are tak- ing up a larger proportion of the total available areas. Separate designs of foundation and superstructure,

each with ample factors of safety, in combination can prove insufficient if the two are not compatible in the

elastic and plastic strains which will result. Absolute

safety requires complete evaluation of strains which come from loading and provision for control of the

strains to within the safe limits of the various materials

used in the structure. Similarly, bridge design over water courses cannot

economically provide resistance against a 1000 year

flood for a bridge expected to exist 100 years. During

the 1964^5 floods, scour of piers and foundations caused some 76 million dollars of damage. If all the structures were designed for a 50 year flood expectation, the risk taken could be economically justified. Yet a 1000 year

flood has a 2.5 percent chance of occurrence in a 25

year period and a 10 percent chance in a 100 year

period. Even if we could predict the magnitude of a 1000 year flood, this would not be a certainty of safety

in the 100 year life of a bridge.

Guaranteed safety of unusual bridge designs in the U.S.S.R. is proof tested with the design engineer and

the construction supervisor standing under the bridge

during the test. This may sound as fiction, but a Soviet

engineer of railway bridges at an International Prestressed Concrete Congress held in Rome in 1961

showed a photograph of such a test with two lines of locomotives on his design with the constructor and himself standing under the bridge during the test. Structural integrity under all possible forces re- quires continuity in the structural design, complete compliance in construction performance and soundness of the finished structure. Failure of structures during

earthquake loading is often blamed on lack of continuity which would provide alternate support after local failure. For every construction failure there must be

many near misses with some small excess strength per- mitting the structure to weather the storm. Can these

structures be classed as having absolute safety against

the next storm? The criterion of public safety was analysed by David

E. Allen of the National Research Council of Canada

from data collected in both the U.S.A. and in Canada

of fatal accidents as a ratio of exposure. For one hour of exposure, either occupancy or use, the death rate times

10'^ is listed as:

Failure in completed structure less than one

Construction worker 218

Failure of temporary structure ele- ment 35

Bus or rail travel 80

Walking along a road 420

Automobile travel 1040

Air travel 2400

Swimming 3500

Motorcycle 4420

Alpine rock climbing 40,000

The incidence of fatality in a completed structure indi- cates almost absolute safety, yet failures do occur. Allen

estimates that to reduce the probability of failure from

10"^ to lO"**, the cost of completed structures would in- crease by 6 to 7 percent, but the cost of structural elements and assembly methods would increase by 20

percent.

3. Reliability or Insured Safety

Reliability stems from four separate controls in the construction program. The designer assures that his analysis and choice of materials will produce a safe

structure. The contractor assures that his assembly con- forms to the design and that the materials furnished

conform to the desired strengths. The owner assures

that the building will not be used under floor loadings

exceeding the basis of the design and that the maintenance program will keep the materials and assembly

in substantially the original state. The public repre- sented by the governmental agencies assures that adjacent conditions will not be altered so as to degrade

safety of the structure. A perfectly produced design and carefully controlled construction does not guarantee perpetual life of a structure. Maintenance must be provided to prevent change

in the material characteristics. Some of the natural agents causing deterioration and loss in safety margin

are oxidation, solution, dehydration, solar radiation,

thermal reversals, fatigue from load reversals, plastic

flow, internal stress corrosion, bacterial action and many

others. They require special built-in protections, such

as timber preservatives, anodic corrosion protection, control of thermal eflfects, sealers and paints. But even

the protections require renewal with age. As an example of public responsibility, some simple

cases of failure caused by extraneous conditions are

cited. With the advent of steel bridges, many failures of bottom truss members were attributed to dogs. The

steel members under the Riverside Drive Viaduct near

125th Street were eroded almost to the point of failure by the contact with acidic nesting accumulation from

the local pigeons. After major repairs, the underside was sheathed in aluminum with no ledges on which the pigeons can roost and nest. Air contamination, especially the sulfur content, modifies and weakens cement

mortar. Dewatering of adjacent areas causes a loss of foundation support, usually non-uniformly, with tip- ping and tearing apart of structures.

167
In 1846, Robert Stephenson, then president of the

British Institution of Civil Engineers, advised the members to meticulously study failures of structures to de- termine limits of design reliability. Freudenthal, in his 1950 book on "Inelastic Behaviour of Engineering Materials and Structures", warns that a factor of safety in the design is only a measure of uncertainty, since it compares design stress, an intellectual concept, with

measured material strengths. The designer should analyse the variability of all influences, during the ex- pected life of the structure, on available resistances and

on the service loads and external conditions. Designs are based on assumed failure patterns. Carl

J. Turkstra in discussing choice of failure probability

provides as a good approximation for the best economic

design the criterion that no further strength is needed

when the increment to the basic design cost divided by

double the economic loss resulting from failure is effec- tively zero. Just how close to zero is a decision based on experience and judgement.

At the Washington International Conference on

Structural Safety and Reliability, held in April 1969,

certain basic warnings were sounded. There is a limit to

statistical application of structural reliability prediction. Structural failure is an emergency, whereas electronic failure is a contingency, hence more susceptible to statistical analysis. There is great need for study of competence in predictions of reliability of materials. In the case of fighter aircraft, an empirical accumulation of

data, statistically analysed, shows areas for more intense inspection and control. No such procedure can be of great value for structures. There is at present no test- ing procedure for design and life estimation of fatigue

sensitive structures. Witness the collapse of the Silver Bridge over the Ohio River in 1967. Engineering de- signers concentrate on technical matters that they can

control. But the investigation of failures shows many

parameters outside of direct control, not physical in nature, which should be considered.

The reliability of materials as delivered to the site for assembly is one of the questionable facets of pro- ducing safety. There are three classes of available materials: those proven by years of use, recent improve- ments of older materials, and completely new materials. In the second class are such acceptable items as new

steels, better cements, laminated timber, Plexiglass and

aluminum alloys. Newly developed materials are rapidly

marketed with little historical proof of suitability. A

large amount of research is needed to prove any new

material as capable of providing required long time

resistances. Considering soil as an engineering material, it must

be kept in mind that the design depends on results of laboratory tests on small samples that one hopes are

truly representative and also that the interpretation of the tests is structurally applicable.

The failure mechanism chosen for determining safety

may not necessarily be the one with largest probability

of failure. In both soil and structure problems, distor- tion can be tolerated without collapse, but the effects of fatigue, wear and corrosion will then set a shorter life to the structure. As an example of the basic problem, reliability of

welding can be cited. It is recognized that flaws in the weld, just as flaws in any example of glue adhesion,

have a detrimental effect on strength. Various test pro- cedures have been developed for nondestructive testing of welds, such as radiography and ultrasonic scanning.

How and who interpret the test data? Radiography

shows a two dimensional picture of a three dimensional

problem. Ultrasonic waves reflect from surfaces at a dis- continuity, which may not be in the weld volume. The

old welding specifications required welds to be "sound"; we are still looking for a foolproof test of soundness. D. A. Olsson of the Bethlehem Steel Company tabulates

failures from welding in order of frequency as:

ship hulls—fatigue, gross failure, brittle failure, cargo handling—brittle failure, gross failure, fatigue,

bridges—brittle failure, fatigue,

buildings—gross failure due to loading above yield

point.

The marked reduction in hull failures after the National

Research Council investigation and imposition of cri- teria for ship assembly following the World War II experiences, shows that research and control can pro- vide greater reliability.

Reliance upon compliance with established regulation as proof of reliability was subjected to some question in 1969, when the revised New York City Building

Code increased the minimum wind load to be resisted by multi-story frames. What about the reliability for safe use of the thousands of structures designed, ap- proved and constructed under the requirements of the previous code, all of which bear legal certificates of occupancy. Are they now of questionable sufficiency?

Similarly the 1969 Code of the American Association

of State Highway Officials, the recognized criteria for design of all highway bridges, modified all earlier edi- tions in a considerable reduction of safe stress for members or portions of members subjected to tensile force

if any welding, even of such minor items as clips, brackets or studs, is performed. This reduction is listed under the heading of fatigue from change in stress level under live loads, not necessarily requiring reversal of

stress. The eff"ect is to degrade allowable traffic loading on bridges designed under the provision of earlier codes, or in other words, a sudden reduction of reliability of the expected strength of these structures. If reliability is referenced to consistency with code provisions, the date of the code becomes significant.

4. Responsibility for Safety

The total responsibility for safety of a structure is therefore the sum of the warranty or surety which can be given by the four separate entities involved. The de- signer can only assure of a safe plan for assembly of

safe components; the constructor, of proper materials

safely assembled; the owner, of control of loading to the maximum design assumption and of continuous

maintenance to prevent degradation of material

strength; and the public, of control of extraneous conditions which would void the proper assumptions of the design. Each factor must assume responsibility for its

168
duties. Recent legislation in New York State crystallized

this responsibility and forbids by agreement or dictate, the transfer of responsibility to another. Various decisions by courts of the several states control the legal responsibility of the designer. In the case

of Gage vs. Buttram, a California decision is based on: "The general rule is applicable that those who sell their services for the guidance of others in their economic

and financial and personal affairs are not liable in the absence of negligence or intentional misconduct. . . . Those who hire experts are not justified in expecting

infallibility but can expect only reasonable care and

competence. They purchased service, not insurance."

And in another case, the decision reads: "For defects

in the original plans and the approval of detailed plans

arising from negligence on the part of the architect,

liability resulted". So the Architect-Engineer is liable for negligence and for breach of an implied-warranty.

The period for legal liability has been limited by some

states to an average of eight years (from 4 to 12 years

in different states) usually from the date of completion

of the agreed services to be performed. Several states, including New York, have no time limitation for such

liability.

The Architect-Engineer has three areas of legal ex- posure in the practice of his profession

:

(1) Liability to the owner for defects in plans and

specifications or for negligent supervision.

(2) Liability to third persons, not involved in his contract for services, for personal injury result- ing from negligence.

(3) Liability to the contractor for interference with

his rights in performing his work, and other miscellaneous types of indirect liability.

The general definition of negligence liability often quoted is by Professor George M. Bell in "Professional

Negligence of Architects and Engineers" (Vanderbilt

University Press, 1960). "An Architect-Engineer does

not warrant the perfection of his plans nor the safety or durability of the structure any more than a physician

or surgeon warrents a cure or a lawyer guarantees the winning of a case. All that is expected is the exercise of ordinary skill and care in the light of the current knowledge in these professions. When an Architect or Engi- neer possesses the requisite skill and knowledge common

to his profession and exercises that skill and knowledge

in a reasonable manner, he has done all that the law

requires. He is held to that degree of care and skill and

that judgment which is common to the profession."

The question often arises as to the safety of an exist- ing structure and how can it be evaluated. An answer

has been suggested by Committee 437 of the American

Concrete Institute whose mission is the study of Safe

Loads for Existing Concrete Buildings, as either a strength evaluation by analysis or a load test for any

structure that has undergone general or local damage,

excessive loading, impact, blast, fire, earthquake, war damage or deterioration caused by the environment. To

set the criteria for such evaluation, full scale loading

tests to failure are planned on existing structures sched- uled for demolition, in abandoned exhibitions and in

redevelopment areas. Some valuable empirical data has

been collected on degree of compliance with the plans,

and on the extent of deterioration. As to recent new construction, similar indictment of the financial interests recently came from such diverse

parties as Engineer A. A. Colder in Canada and A. C. Jenkins, then President of the American Institute of

Architects, for allowing the erection of sub-standard

buildings with too much emphasis on aesthetics for rentability and too little for utility, as affecting public

health, welfare and safety.

Under the subject of "Social Benefit and Technological Risk" Chauncey Starr in "Science" of 19 September

1969 describes the cost of accidental deaths from tech- nological developments for public use, especially since engineering developments are seldom tried out in small, localized areas test out the risks. There is now a very rapid acceptance of new designs and voluntary

risks are accepted readily by the public, usually induced

to do so by advertising and general awareness only of the advantages.

The problem is not limited to the United States. In 1966, after a rapid succession of structural failures,

mostly of novel designs, an editorial in the British "Construction News" asked "When is it going to stop?"

All of these happenings prompted people to ask

whether or not there was not some underlying common

reason or a vital link missing in the safeguards. The

editor suggests two possible reasons:

(1) The system and method used for checking both

design and construction.

(2) The allocation of responsibility.

England does not license professional engineers. Within

the London County Council there are 28 district sur- veyors with absolute authority, who check structural calculations and deal with the details of construction

stage by stage. Outside of London, local enabling legis- lation and Building Regulations control all new construction but with little personal contact with the work.

A common national system is being studied with the background that almost all the failures were outside of

the London area. Sweden places responsibility for quality control and

adherence to design and specifications on the contractor's engineer and this function is taken seriously.

France, Holland and Belgium place all responsibility for checking design, material strengths, construction

methods and performance on groups of experts hired

by insurance carriers. The owner pays the premium and

is insured against collapse, defect or abnormal future maintenance costs, both during construction and after completion. Insurance is not available to every project,

only where the designer and constructor are considered

qualified. In Rio de Janeiro, after the collapse of an elevenstory apartment, some 30 minutes after the city engi- neers had inspected the building and declared it safe, the "0 Cruzeiro", a leading paper in Brazil published a

list of 20 buildings that collapsed in Rio between 1947

and 1957, and also identified a number of major struc- tures on the main streets, Rio Branco and Presidente

169
Vargas Avenues, which were eight to twelve inches out

of plumb. Many tenants including government agencies

moved out. However, the City did nothing to institute better control. A local newspaper in Italy, after the collapse of a new apartment in Barletta in 1960, taking 58 lives, said: "Here in Italy we prefer to weep for our dead

rather than perform an autopsy and find out why they

died." Five stories had been put on top of an existing one story garage building, avoiding all construction

laws and by some passing of graft, the chief health of- ficer issued a permit for use and occupancy before the work was even finished. The trial of a dozen engineers,

contractors, health and construction officials resulted in a total of 70 years jail sentences. The worst recent failure, also in Italy, was the Vaiont

Dam disaster, claiming 2300 lives. In spite of continu- ous day and night inspection of conditions, some 700,-

000 cubic meters of rock slid as a mass into the full reservoir causing a tidal wave over the dam and into the villages below. Investigations and litigation went

on for several years to pin the responsibility for not heeding the warnings of incipient slide failure.

5. Summary

Public Safety and Construction Surety depend on

proper control of each factor involved in the design,

construction, maintenance and use of a structure. Any

deviation on the part of any one factor makes the

structure vulnerable to damage or even collapse. Absolute safety is not obtainable. The historical record of structure behavior shows a high reliability from present

techniques and procedures but with too many failures. Responsibility for defects and for the very small percent

of structures that do not perform properly is a deterrent

to poor control, but considerable pressure is needed to further reduce the number of failures to insignificant

numbers. It is the duty of the professions involved to provide that pressure and control the proper perfor- mance of the work to provide public safety and surety

in the construction industry.

170
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg. Md. (Issued April 1976)

The People Considerations of Automotive Service

F. J. Uhlig

GM Technical Center General Motors Corporation

Warren, Michigan 48090

The presentation will cover some of the human factor considerations in servicing automobiles

at GM dealerships. Emphasis will be placed on the communication channel between customer and mechanic and possible attendant breakdown that can limit the effectiveness of repairs. Specifically, the repair challenge will be described relative to:

( 1 ) Vehicle relation and consumer expectations

(2) The customer item-service writer interface (3) Technical service information for internal service shop communications.

In addition, the presentation will cover some recent developments of GM service research which

address themselves to the above problems for example

:

(1) Service writer check sheet—a vehicle simulator oriented approach to customer order write

(2) "STAR"—A computerized reading comprehension method for use in improving the

readability of service manuals used by mechanics.

(3) "USDA"—Universal Symbol Diagnosis Approach—that uses graphic symbols in a logic

tree format to aid in vehicle system problem diagnosis. (4) Shop operation management system.

Key words: Automotive repair; automotive repair effectiveness; automotive service; customer- mechanic relationship; human factor considerations; universal symbol diagram approach.

In keeping with the theme of this symposium on fail- ures, I would like to address some consequences of failure associated with a complex mechanical system

called an automobile. Specifically, I wish to focus on some people considerations of automotive service after highlighting the statistical impossibility of eliminating

mechanical malfunction.

According to the U.S. Office of Consumer Affairs, motor vehicle service is the primary area of consumer

dissatisfaction in the nation. Studies have also been

made by the subcouncil on performance and service

of the National Business Council for Consumer Affairs. In this report, data were presented from the Harris-Life

Survey, conducted in February of 1972, that indicate

the extent of consumer concern over keeping his auto- mobile running; 40 percent of the respondents expected

difficulty in obtaining repairs for their automobiles and

31 percent believed auto repairs were made improperly,

up from 16 percent in 1967.

It is generally agreed that the failure rates of most

current consumer durables is lower now than in years

past. However, the sheer volume of consumer items in

use results in a large volume of consumer complaints

being registered with the manufacturers, dealers, ser- vice agencies, and public and private consumer protection organizations.

The National Business Council report went on to say

that "no matter where complaints are first directed,

manufacturers are usually blamed for product sales or

service deficiencies." The manufacturers' challenge then

is to reduce the probability of malfunction.

By specification, motor vehicles are required to per- form well under a great variety of operational and en- vironmental conditions. Due to these conditions and the inherent variability of our production process, there

are times when the product does not perform its in- tended function, that is, it fails in some manner.

Specifically, we have a situation which can be illus- trated as shown in figure 1. On the left of the figure we

plot a curve which can be used to estimate the probability that a user will impose a given load or requirement on a part or system. On the right, we show a curve which can be used to estimate the probability that a

part or system can handle a particular load or require171
THE REAL WORLD

PRODUCT CAPABILITY

DISTRIBUTION

POTENTIAL FAILURE ZONE IMPOSED LOAD

Figure 1.

ment. As shown here, some few users with higher-than- average requirements may encounter some few parts

with lower-than-average capabilities. When this happens

there is a possibiHty that failure will occur. This is represented by the potential failure zone. One might

suppose that all we have to do to control or eliminate

failure is to control or eliminate variation. Just how

feasible is this?

First, let's consider customer demand. There is no

product I know of that is subject to a wider range of uncertain use conditions than a motor vehicle. These

include variations in road surface, climate, loads, operation time, operator skill and maintenance. The combinations are infinitely varied and are continually

changing. The ability to have a thorough understanding of customer use variation is a challenge to the motor vehicle manufacturer all by itself. Now what about the product? We make about six million vehicles a year in about 300 different models.

There may be up to 15,000 parts in each vehicle, as many as a hundred variables for each part, thousands

of manufacturing and assembly steps in putting all the parts together, and millions of people involved in its manufacture and maintenance. Someone has estimated

there are one and a half million places for variation in a typical vehicle. Variation is simply the name of the game.

Obviously, we can't eliminate variation, but we per- haps can control it. A fundamental way to reduce the

failure zone is to reduce the curve overlap. The spread

of the customer variation curve could be reduced or

shifted to a lower range by government regulations

which affect drivers, road conditions or other use factors. But there is little the manufacturer can do about customer use variation. On the other hand, the product variation curve can be altered by the manufacturer in two general ways. One method, as illustrated in figure 2, is to reduce the

THE REAL WORLD

PRODUCT CAPABILITY

DISTRIBUTION

potential failure zone imposed load

Figure 2.

172
THE REAL WORLD

Figure 3.

spread by holding very close tolerances, limiting product variety or providing many tiers of inspection and

screening.

Another method as shown in figure 3, is to shift the

distribution to a higher capability range by possibly

using heavier structures or more exotic materials. Now that I have touched on some of the technical

realities involved with automobile usage. I would like

to focus on some human factors or people considerations

that we judge are deserving of examination-human fac- tors and reactions that become consequences of a vehicle

failure incident. In our studies of the automotive service system, we

have been able to identify some of the vulnerable points

that can cause the system to breakdown from a human

factors standpoint. Psychologists tell us that one of the most important and pervasive ingredients in maintaining effective interpersonal relationships is communication; without effective communication, negative human

factors consequences will result.

I would like now to treat some specific communication elements or links within the dealer service system

that we judge are in need of help. In addition, I would

like to explain what GM has done, or is doing on a trial basis, to increase the signal to noise ratios in these communication links, and thereby help to mitigate further

possible adverse consequences.

In looking at the communication networks within a

service shop, and there are many, three main channels

are deserving of special attention. In order for a mechanic to perform effective repairs,

it is required that he has an accurate description of the owner complaint. Since this description is initially pre- pared by the service advisor in consultation with the owner, it is imperative then that the service advisor

capture this vital information on the repair order in mechanic style language.

One effort we have made to improve communications

in this vital area is the development of a rather simple

device called a Service Advisor Check List. This printed

form covers both sides of an 8I/2 by 11 in sheet of paper, and is a rather complete list of common automotive service problems along with their probable cause. Major classifications of service problems such as "noise

in engine compartment" or "air conditioning" are grouped together on the form. Within each major clas- sification a list of typical symptoms or problem descriptions are shown. The object of this list is to help the customer and service advisor arrive at the most accurate

description of the service problem.

By going through a question and answer series, a general complaint of hard starting might be narrowed

down to a specific problem like "hard starting in wet weather;" a particular situation that may not be evident

in a general check-out in the service shop. Thus, the mechanic can be guided to the specific problem area

with a high probability of correction the first time. In practice, the service shops using the check list also have enlargements of the list displayed near each

mechanic's work area. This allows the repairman to see the list of questions that led to the service instruction on the repair order. The information shown on the check list was developed by General Motors Institute for GM Service Research by interviewing scores of ser- vice managers, service advisors and technicians. The

appearance of the chart has been modified several times,

and further refinement is anticipated after more field experience. A field trial of this communication device

is underway in select dealerships to determine the practicality and merit of daily use. The second major communications channel that we

judge could be improved involves technical service in- formation generated specifically for use by the mechanics. This type of information most often is in printed form, and the most general example is the shop

service manual. Ostensibly, the primary purpose of the shop manual is to provide the mechanic with a compilation of the necessary procedures and specifications to

173
enable him to diagnose and repair all major systems of a car. As a communication tool then, it is expected that the mechanics are able to comprehend what they are read- ing in the manuals and apply the information in con- junction with tools to the repair of automobiles. We took

a look at the writing contained in many of our manuals,

and found that much of the written content was not ex- pressed clearly in simple terms. We reasoned that if a mechanic cannot understand what he is reading in a manual, then a high probability exists that repairs could b^ done incorrectly leading to customer dissatis- faction. Ineffectual communications to the mechanic,

therefore, could produce subsidiary adverse conse- quences. Readability of service manuals became a target for improvement.

Research into ways to measure reading ease showed

that several measuring methods had already been de- veloped. Of these methods, the one developed by Rudolf

Flesch best fitted the needs of GM Service Research.

Dr. Flesch based his Reading Ease Formula on sentence

length and syllables per word. He found that the higher

the syllable count and the longer the sentence, the harder it is to understand the sentence meaning. Dr.

Flesch's formula gave the research group a means to measure the reading ease of written material. Tests on

sample text confirmed that shop manuals are hard

to understand. It was found, in fact, that the equivalent

reading grade levels required ranged from 12th to 16th. We questioned whether mechanics could really read on

that level. At present, the only available reading skill measure- ments are on future mechanics—those students now

enrolled in vocational school programs. The reading

comprehension of 239 students in the twelfth grade

studying automotive repair and diesel engines was

tested. The students were given a comprehensive read- ing test with the following results: only three percent were comprehending at the twelfth-grade level, the majority at ninth grade and twenty-five percent of the

class sixth grade or less. We must remember, however,

that these students are in the vocational arts and usually

represent the lower half of the twelfth grade in reading

achievement. A textbook written to a twelfth grade

level would be understood by about thirteen percent of

the class. To reach at least half of the group, the text must be written to a ninth grade level. If this is any

indication of the reading ability of our mechanics,

books for their use must be rewritten to just below ninth

grade level. In our initial analysis of shop manual readability,

each measurement had to be done manually by having someone count words, syllables and sentences, and apply

these numbers to the Reading Ease Formula. A quicker

way to test was needed, that is how the Flesch Reading

Ease Formula and the computer came together. The

result is a program designed for a common computer

that can be used by anyone with a little experience. The

computer program provides a fast, useful measurement

of reading ease. We have called this program "STAR",

an acronym for A Simple Test Approach For Readability. In working out this theory of reading ease, Dr.

Flesch was convinced that two factors in wjitten material tend to reduce understanding. They are long sen- tences and long words. Flesch saw that modern English

writing has come a long way in reducing sentence

length ; but that it has lessened this gain by using longer

words. This tends to cram too much meaning in too small a unit. This "density" of language is now the main problem in efforts to write clearly. In using STAR to sample the readability level of written material, the text is entered into the computer

through a keyboard-type terminal tied into a time- sharing system. Unlike other programs, plain text is used without any additional program markers. There

are only a few special instructions used when typing

the text into the time-sharing computer. As an example,

a portion of our automative air conditioning manual

was found to have an equivalent reading grade level of

16. To reduce the reading level, the sentences were

shortened by breaking up clauses and substituting "low

calorie" words for heavily loaded words; cooling for

air conditioning . . . car for automobile . . . and inside

for interior. Rewriting the portion reduced the grade

level by 4 down to a 12th grade level. Splitting up sentences and finding replacement words

creates a real challenge to the writer trying to communicate. It cannot be an automatic process just to change

words ... or as some have suggested, a computer pro- gram to do just this substitution ... it requires a writer who knows his subject and has a good working vocabulary. We cannot replace a writer with a machine, nor is

it recommended.

General Motors is now in the process of applying

STAR on an increasing basis to all of our service manuals. It is our goal to make the language in shop manuals as effective as we can in communicating the service information to the mechanic. General Motors is making

this program available to all interested parties through

a booklet called "S.T.A.R." It is available through

GM's Public Relations Department, Detroit, Michigan.

The booklet contains background information and the complete computer program instructions written in basic computer language. This will help a computer program- mer to place the STAR Program into action. You may

be interested in knowing that this presentation was

analyzed by STAR, and the required reading comprehension grade level is thirteen. As we continued to study the communications effec- tiveness of our service manuals, it became apparent that more was necessary than just improving the readability. The construction and format were judged hard to follow for a mechanic, and had complicated approaches

for presenting diagnostic instructions. Our interest was

directed to work that numerous highway departments

have done in standardizing road signs using graphic

symbols to communicate important information. Ex- amples of these symbols are familiar to all of us. Once

a person is made aware of the meaning of each symbol, one no longer needs to read the words; the meaning

is conveyed at a glance. Last year we decided to try this approach on a diagnostic procedure for a new system that was being intro- duced on 1974 models and would be common to all General Motors' car lines; namely the seat belt starter

174
STARTER CRANKS WITH SEAT BELT UNFASTENED

TEST CLCCK FUSE AND DIRECTION

SIGNAL FUSE. REPLACE IF NECESSARY. CHECK FOR GOOD CONNECTION AT THREE WIRE CONNECTOR

.

STARTER XCES NOT CRANK

I SYSTEM

NOW OK

STARTER CRANKS

REMOVE BY- PASS RELAY

STARTER

DOES NOT CRANK

DEFECTIVE BY- PASS RELAY

STARTER

CRANKS

GROUND OK. GREEN WIRE AT " THREE WIRE CONNECTOR

STARTER

DOES NOT

CRAr.K

STARTER CRANKS

PROCEED TO SEAT SYSTEM DIAGNOSIS CHECK CONTINUITY OF DK . GREEN WIRE FROM INTERLOCK RELAY TO THREE WIRE CONNECTOR. CHECK FOR 12 VOLTS AT INTERLOCK RELAY PINK WIPE. REPAIR

WIRES AS NECESSAR^ . IF WIRES ARE

0 K, REPLACE INTERLOCK RELAY.

Figure 4.

interlock system. Figure 4 shows a portion of the diagnostic procedure for this system that, if put in the usual format, would have looked like this in service manuals.

The failure mode is that the starter motor cranks with

the buckle unfastened. In looking at the procedure, we

determined that the graphic symbols shown in figure 5 might be applicable to the starter interlock procedure

and be readily understood by a mechanic.

Our first attempt was made utilizing both symbols

and pictures to guide the mechanic into the proper re- pair as shown in figure 6. You v/ill note that the pictures are as the wiring and connectors would appear to the mechanic working on a car and not just a picture of a component out of its environment. You will also note

that the system failure complaint is shown in symbolic

form at the top of the page. It was determined at the beginning that words should be used along with the symbols until enough familiarity could be developed to eliminate them.

Various attempts were made to develop a format that seemed less confusing and easier to follow than the one you just saw. Again, selected words were added next

to the symbols to make it more understandable. The fi- nal approach at formating is shown in figure 7. This is what has proven to be the most usable, the most flexible and the most understandable. It is the format as it ap- pears in our 1974 Service Manuals.

STARTER

Figure 5.

This symbolic system was tested on mechanics pos- sessing varying degrees of experience and skill. We

found that highly skilled mechanics had no objection to using it and found it effective. Also, mechanics with

moderate skill, who had difficulty in following the old word-tree format, were able to use this one satisfactorily.

And most importantly, mechanics of low skill level, who

175
Figure 6.

found it impossible to use the old word-tree system, were able to follow the new procedure and, in fact, fix the car with little difficulty. As an interesting sidelight, one mechanic with little experience, was still having difficulty following even

the new procedure, because as we found out, he didn't read well. He was still stumbling over the words. When

this became apparent, the words were scratched out and the man was then able to follow the procedure with

virtually no problem.

So ultimately, we hope that diagnostic and repair

procedures all might look something like this. A mini- mum of words, an uncluttered sequence arud one that is easy to modify since adding or rearranging steps is obviously quite easy. All the reports from the field have

been very positive. It is well accepted and seems to be

working.

The ultimate critique, we believe, was received from

one mechanic who did not like the new system. He said,

"This makes it possible for anybody to fix it.^' The third and last communications area I would like to talk about is a very broad one, the intrashop communications network. After we have effectively com- municated with the owner through the service advisor

Figure 7.

176
and have determined why the owner is at the shop, the

service challenge becomes one of expeditiously repairing

the car. The vital elements of the ideal repair situation converge on the ability to fix the car the first time, on

time and with minimum inconvenience to the owner. A service shop may be fortunate enough to have the most empathetic and understanding service advisors,

competent mechanics, the latest in service tools and

equipment, the most modern facilities; but if its shop

scheduling system is inadequate, the potential exists for additional breakdowns in the customer-dealer hu- man factors area. Customer surveys we have conducted, specific to auto- mobile service, indicate that the service shop's ability

to return a repaired car to its owner at the promised

time is of great importance. For a service shop to be

able to give an owner a reliable promise time, it is required that the service dispatcher have an accurate means of determining the incremental time elements

associated with the vehicle to be repaired and the avail- able shop resource that can be assigned to perform the

repairs.

Specifically, the dispatcher should be able to deter- mine the total vehicle work content as derived from

discrete time estimates of routine maintenance services,

possible diagnostic time required and the estimate of

total repair time. He will then compare his available shop resources such as mechanic specialty, equipment

availability, floor space and parts availability to the work required and arrive at a promise time. To accomplish this task effectively requires a communication sys- tem with a high degree of information feedback.

Most dealerships employ a manual entry scheduling

system that captures all in-process repair work on one

large sheet of paper. The ability to keep track of all cars in the repair flow and provide current in-process

status information hinges on the skill and expertise of each dispatcher. Smaller service shops, processing less than 50 repair orders per day, do not generally experi- ence severe scheduling problems. Customer promise

times are usually met.

In shops handling over 100 repair orders per day,

scheduling problems resulting in missed promise times

do happen more frequently. Keeping track of all those

cars in the repair system is really quite a task for a human being. When these scheduling breakdowns do

occur and promise times are not met, customers understandably become annoyed and angry. When this hap- pens to the customer, another layer of adverse human

factor consequences has been added to his initial ex- perience with his vehicle failure. Recognizing the importance of vehicle scheduling, our organization. Service Research, is now developing

a system for improving dealership service shop operations. Utilizing the power and speed of the computer,

the proposed system will provide assistance to the fol- lowing dealership service operations personnel: service advisor, dispatcher, parts, cashier, and dealer management. I will touch on only the service advisor and dis- patcher portions.

With the aid of special equipment connected to the computer, the service advisor will be more capable of analyzing vehicle problems and communicating this information to technicians. He will also be assisted in: preparing time estimates, that are realistic to meet;

diagnostic repair problems, aided by the service advisor check sheet through computer logic trees and pass- ing the information on to the technician via a technician

write-up sheet; preparing repair cost estimates; providing vehicle status information, instantly and accurately;

and in reducing paperwork. For the dispatcher, the system will provide assistance through; scheduling of work loads in an efficient and

orderly manner according to technician skill availability

and promise times; reducing paperwork; job status information instantly without searching long lists; re- duction of phone calls by providing service advisors and

cashiers with accurate and instant job status. The computer will schedule repair orders into the

service shop work load flow according to the technician

skills required and the promise time of vehicle completion. As technicians become available for service work,

the system will assign properly sequenced jobs to the technicians. The dispatcher has the ability to override

any computer decision. What we have presented here is a service shop scheduling system representing state-of-the-art technology. A

system of this magnitude would quite understandably

not be required in all dealerships. Therefore, in recognition of this fact, we are also developing a manual and/

or semi-manual system for dealer service shops which

will offer many of the improvements of the more sophisticated computer system. In summary, I have attempted to outline some special

types of human factor consequences that can result from primary vehicle failures. I have also touched on some partial solutions that may decrease the owner ag- gravation index. The challenges facing those of us con- cerned with minimizing or preventing human factor breakdowns in the service system are great. The type

of failure consequences we have dwelt on are people

orientated and, as such, require people oriented solutions. We must continue to re-evaluate our traditional ap- proaches to solving these types of problems. In addition

to the technical aspects of automotive service that in- clude tools, equipment and the vehicle, we must focus on the service environment as it relates to human be- havior. This we must do since environment shapes

behavior and attendant human factor consequences.

Constructive modification of the automotive service en- vironment from a human factors frame of reference is a worthy goal.

Discussion

/. M. Karhnak, Jr., U.S. Army, Fort Belvoir: Those of us who are shade tree mechanics generally recognize

the difference in quality level of things such as tools. We generally go into the life cycle cost which was mentioned in some of the other talks. We compare the price we pay for different things with the useful life. I have

not noticed the same kind of practice in the automotive

industry. You mentioned the stainless steel car which we probably couldn't afford, but there might be two or

177
three different price levels of a given automobile that we might be able to buy with different life expectancies.

F. J. Uhlig: It's an interesting thought.

H. H. Wakeland, National Transportation Safety Board:

I spent 5 years in the automobile industry myself and

often like to ask people the embarrassing questions that

I couldn't ask when I was an employee. Early in your

talk you said that you had trouble defining the early

performance envelope in which the vehicle would run and that you had difficulty telling the customer how to use it. When we look at the speedometer of a modern

automobile which shows the figure of 120 mph on it, does that mean that it's safe to operate that vehicle at 120 mph idenfinitely, or is this an example of a piece

of implied information that's been given the customer

irrespective of your view that not much could be said about it?

F. J. Uhlig: I think you're right.

H. H. Wakeland: Is the area of a rated maximum speed

for automobiles established by the manufacturer as part

of his performance envelope in which he makes his design predictions desirable pr practical?

F. J. Uhlig: We could design automobiles for a maximum speed of 55 mph. But passing maneuvers require

a little more speed. So ultimately, the top speed of a

vehicle has to be a little higher because of the passing

ability.

A^. Glassman, Naval Ship Research & Development Center: I got the impression that all the diagnosis that was

occurring in this interaction at the automobile service

department was the opinion of the driver and the opinion of the advisor. Isn't there a place here for some

diagnostic techniques?

F. J. Uhlig: Yes, there is a place and I'll have to say

quite candidly, that there isn't much really good diagnosis being employed. In the smaller independent ser- vice shops, where the mechanic can actually interface with the owner on a one-to-one basis such that the owner

can describe his problem and the mechanic can go

through a line of interrogation to determine the symptoms, the owner will probably get his car fixed. But as greater and greater the distance in the communication

link gets between the owner and the mechanic a lot of misinformation is introduced. The service advisor check

list that we mentioned was a means to sharpen up the dialogue between the customer and the service advisor. Many times a service advisor isn't even a mechanic, it could be anybody. Many of our dealerships employ

ladies that don't really have much of a working knowledge of automobiles.

G. McPherson, University of Missouri, Rolla: You mentioned using a computer to schedule the flow of repair

work. Is that going to be a mini-computer located on

the premises or it is a time-sharing system? How large a work load is required to justify this?

F. J. Uhlig: We're now studying the communication

system itself to answer that question.

H. P. Jost, K. S- Paul Products, Ltd: It might interest the audience to know that, not in the United Kingdom,

but in some continental European countries, diagnostic means of checking cars are becoming more and more

prominent now and, in my view, will become the thing

to do in a few years time.

178
IMPLICATIONS OF MECHANICAL FAILURES

Session V

Chairman: R. M. Thomson

National Bureau of Standards

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Mechanical Failures: Implications for Science

John P. Hirth

Ohio State University

Department of Metallurgical Engineering

Columbus, Ohio 43210

The early scientific advances by Griffith for continuum cracks and by Orowan, Taylor, Hall and

Petch, and Hirsch and co-workers for effects of microstructure have contributed to our current understanding of mechanical failure. However, this understanding remains incomplete. Here, a

brief historical account and survey of the present status of various fundamental aspects of the

failure problem are presented. Specific unsolved problems and areas for needed research are

suggested.

Key words: Continuum cracks; continuum mechanics; failure mechanisms; mechanical failure; microstructural effects on mechanical failure.

1. Introduction

There are several approaches that one can take in considering contributions of science to failure prevention. One could argue that the most important problem

is that of flaw detection which would imply that ad- vances in instrumentation and experimental devices are

points of greatest potential for science. Alternatively, one could make a case for statistics since flaw distributions, the likelihood of failure at a given flaw, and the

local material properties themselves are all statistically variable. In the present article, however, we take the position that the greatest potential for science in failure prevention is in providing a better understanding of mechanisms of failure. For convenience in discussion, the scientific advances

which have contributed to the understanding of mechanical failures can be categorized roughly as arising in the

disciplines of continuum mechanics, materials science and solid state physics. The first category comprises

elasticity and plasticity, the second the relation of properties to microstructure at the size scale of about 1000 A

and larger and the third structure at the atomic scale. There has been considerable progress, particularly for metals, in the first two areas and some in the third. However, a number of important problems remain unsolved so that not much can be done at present in pre- dicting the mechanical failure of materials from first principles. Moreover, there is some lack of communication among disciplines with regard to those advances

which have been made.

In this article, we present in sequence, (a) a brief

historical perspective, making no attempt to be comprehensive, (b) a progress report on our current state of understanding of the problem, and (c) a listing of topics of needed research. A general discussion of all types of mechanical failures would require too much

space. Hence, for brevity, we primarily consider the room-temperature failure of structural parts. Also, the

discussion deals mainly with metals and materials ex- hibiting nominal ductility, and for the quasi-static or

low-cycle-stress cracking failure mode. Brief discussions

of brittle materials and of high-cycle-fatigue cracking

are presented at the end of the paper. Considerations of polymeric materials and composites

would require a far lengthier treatment. One should

regard the following development as a typical set of problems for one portion of the general class of mechanical failures. There is some direct carry over of the following ideas, but other considerations would also be

required in other cases. For example a whole new set of problems associated with viscoelastic-plastic behavior

arises for the polymer case.

2. Historical Perspective

The concept of a stress concentration at a flaw in a material was illustrated by the early work of Inglis

[1] ^ on the stress distribution about a cylindrical hole with elliptical cross section in a body under uniform re- mote loading. This idea was extended to the reversible extension of a planar brittle crack by Griffith [2] : the balance of elastic energy released with the free-energy

^ Figures in brackets indicate the literature references at the end of this presentation.

181
dissipated in forming the crack surfaces for reversible crack propagation was involved in his work. Subsequent

developments on energy release rates in cracking and

on stress intensity factors for flaws of various shapes were made by Irwin [3] and others; compilations of

stress intensity factors are given by Sih and Paris [4]

In view of these developments it is highly questionable

to treat flaw problems in terms of, say, net section stress instead of stress intensities or concentrated stresses. For

purposes of discussion, we mainly restrict ourselves to the mode I crack under uniform remote loading, figure

1, producing forces acting normal to the crack surface;

Mode m

Figure 1. Relative crack surface displacements for three modes

of cracking.

work was the result that the / integral applies not only

to propagation of a brittle crack in an otherwise elastic material, but also to an elastic-plastic crack in the limit of small scale yielding as discussed in a following section.Of course, numerous advances in the theory of plasticity have been useful in the development of elastic- plastic solutions for cracks. Particularly relevant to the present discussion, as revealed in a subsequent section,

is the theory of characteristic slip fields in plastic zones,

i.e., lines along which the plastic strain is purely shear. The influence of discrete microstructural features on

the applicability of continuum plasticity models has

been discussed by Drucker and co-workers [7].

Equivalent advances in materials science might be

regarded as beginning with the finding by Rosenhain

and Ewing [8] that plastic deformation of metals oc- curred in discrete slip steps. Subsequently, important

work included studies of crystal plasticity by Taylor [9]

and others, the discovery of dislocations by Orowan

[10] and Taylor [11], and various developments of dislocation theory. Particularly pertinent to the crack problem was the work of Orowan [12] showing that flow stress was inversely related to the mean free-path

between microstructural particles which strongly impede dislocation motion and of Hall [13] and Petch

[14] in showing that flow stresses of polycrystalline

materials were universely related to the inverse square

root of grain diameter. Finally, we note the development of transmission electron microscopy by Hirsch,

Horne, Howie, Whelan [15] and Bollmann[16] which

has led to the enormous number of observations of de- formed structures at scales down to tens of Angstroms.

In the solid-state area, work relevant to the cracking

problem mainly involves the construction of atomic potentials. Rather than discussing this topic in detail we

cite the numerous reviews in the recent book edited by

Gehlen, et al [17].

this is the usual practical failure mode of engineering

materials. In this case the stress intensity factor is given

by

K = C <Jr (1)

with (Tr the uniform remote stress, a the crack length,

and C a geometric factor, generally near unity. In turn

the near crack tip local stress field a,; has the form = fij (e) K/y/7 (2)

with r and 0 cylindrical coordinates fixed on the crack

Certain unique integral expressions involving energy

conservation concepts for virtual reversible processes have been derived by Eshelby [5] and by Rice [6].

The former applied the integral to the force on an elastic singularity in an external stress field and in its most

general form to his so-called energy-momentum tensor. Rice related his path-independent integral, the so-called / integral, to the energy-release rate in crack propagagation. A most important development in the latter

3. Current Status

3.1. Contained Ductile Fracture with

Planar Crack

Consider the case of cracks with planar cracked sur- faces and straight fronts in nominally plane strain or plane stress fields. A recent review of the mathematical

methods of analysis of such cracks is given by Rice

[18], while a summary of such results together with a discussion of cracking mechanisms and of the role of microstructure is presented by McClintock [19]. The

mechanism of fracture of many high-strength materials,

e.g., heat-treated AISI 4340 steel, is one that we have

termed contained ductile fracture and is well established by numerous metallographic results. We call this frac- ture mode contained because the plastically deformed

region adjacent to the cracked surface is largely confined to a thickness equal to the spacing of microstructural features of the material. In this mechanism

holes nucleate at inclusions or second-phase particles in the triaxial tensile stress region ahead of a crack, plastic flow occurs in the crack tip region with the region of

182
large plastic deformation roughly limited to a size equal

to the hole spacing, and crack advance proceeds by

failure of the ligaments left between the locally blunted

crack tip and the growing holes. This failure can occur by completely ductile necking down to a point, by a

repetition of hole nucleation and growth on a finer scale and associated with finer scale particles in the microstructure, or by plastic instability as discussed in the next section. On a macroscopic scale, the quantitative analysis of

the process is fairly firm [20]. Figures 2 and 3 show

plastic zones around blunt and sharp cracks in Fe-3

percent Si alloys [21]. The zones are revealed by etch- pitting which occurs below 5 percent plastic strain [21]. In both cases the shape of the zone agrees quite

Figure 2. Plastic zone at a sharp crack tip in an iron—3 percent silicon alloy revealed by etch-pitting of sample

loaded to (Ki/Y) =0.38 Vin. X 200.

Courtesy of G. T. Hahn and A. R. Rosenfield.

infinitesimal region near the crack tip, so that the /

integral approach applies. Even with an appreciable

plastic zone, the small-scale yielding approach can be

used wherein the linear-elastic sharp crack field is used as a boundary condition at the elastic-plastic

boundary and the plastic field, including blunting, is calculated [22]. The result agrees fairly well with

complete finite element plasticity results. Provided that the proper constitutive equation is known, whether nonlinear elastic, work-hardening plastic or other, the /

integral approach still applies. The plastic or elastic

Figure 3. Plastic zone at a blunted crack tip in an iron—3 percent silicon alloy revealed by etch-pitting of sample

loaded to (Ki/Y) =0.21 Vin.X 54.

Courtesy of G. T. Hahn and A. R. Rosenfield.

well with expectation from elastic-plastic stress analysis

(see Rice [20]). The details of plastic flow at the

dislocation or grain size scale do not markedly affect the gross form of the plastic zone. Moreover the / in- tegral approach provides a basis for calculating the energy release in crack propagation. Consider the case

of a plastic zone small relative to other characteristic dimensions of the problem. In the limit the field ob- viously becomes essentially an elastic one except in an

"blunting" essentially leads to the result that the long range field is that of a sharp crack equal to the true crack length plus the plastic or nonlinear elastic zone

radius. The basic requirement for the solution to apply, in addition to the usual ones for a virtual reversi- ble process, is that the form of the field fixed on the crack tip remains unchanged as the tip propagates;

otherwise path independence is not retained. This is the essential reason for the breakdown of the model

183
when the plastic zone size approaches the crack length or either approach the specimen size." The same basic concept of a virtual reversible displacement would

work in such a case, but the work would specifically

have to be calculated for the particular cut surface which forms the new crack surface. Although there have been the above successes in treating cracks a number of problems remain:

(1) Hole nucleation depends on either cracking of

the inclusions or decohesion of their interfaces with

the matrix and little can be predicted at this time about

when such events occur. There are a few [23], but

growing, number of observations in smooth tensile specimens of cracking and decohesion events in a

locally uniaxial stress field which do indicate that typically the events occur at plastic strains roughly

between 5 and 50 percent. One expects similar events

in the multiaxial stress field in the plastic zone at the

tip of a crack. If plastic strain is critical in the latter case, as in the former, these events will occur at some

point within the plastic zone as the particle passes through it. Particularly if these events occur at the smaller of the above strains, the whole set of problems

of deformation at the dislocation scale becomes important to the solution of the problem. These include

anisotropic elastic effects both for matrix dislocations and for particle fields because of the different elastic constants of the latter [24], nonlinear elastic effects at interfaces and in dislocation pileups [25], aniso- tropic elastic and microplastic compatibility effects at grain boundaries and particle interfaces [26], inter- face structure including interface dislocations [27],

interface cohesive strength, and local dislocation inter- actions including cross-slip [28]. There have been some

advances in understanding of these individual defor- mation phenomena as discussed in the cited references,

but by no means sufficient to treat the crack problem.

(2) The results discussed above suffice to rationalize correlations of crack velocity with stress intensity but

do not suffice to predict a constitutive equation from

first principles for crack growth in a material. The

major problem again resides in an expression for hole

nucleation. In some special cases, some progress has been made. For example. Rice and Johnson [22]

supposed that particles decohere at a fixed strain ahead of the crack tip and that consequently both the zone of heavy plastic deformation and the crack- opening displacement should equal the particle spacing X. Then the critical stress intensity factor K,c should vary as the square root of the particle spacing. These

relations hold fairly well for manganese sulfide spacings

in alloy steel [22]. For general particle distributions

the same idea would lead to Kic « (ZF)'''^ where Y

is the yield strength. When the parameters X and Y

are interrelated, the expression takes the form Kic a Y". Such relations hold over limited ranges of Y but not at

all over large ranges, as shown in figure 4. The probable reason is again the unknown variability in hole

nucleation with stress, plastic strain and particle size. One factor which is established qualitatively is that particle cracking and decohesion occur at lower stresses and strains the larger the particle size [29]. This factor could account for part of the variation in figure 3.

600

550

500

- 450

400

350

240 250

- While the path inilependent integral does not apply in such a case, a more general J integral relating the area under a load versus load-point-' dis[)lacemcnt to crack size is applicable and provides useful correlations with experiment [see various results of such / integral analysis by J. R.

Rice, P. C. Paris and J. G. Mcrkle, STP 536, Am. Soc. Test. Mat. Phila- delphia, 1973, p. 231].

Figure 4. Data for an 18Ni (250) maraging steel aged for 3

hr. at the indicated temperature and air-cooled.

Curve C, 0.2 percent offset yield strength Y versus aging temperature. Curve A, Kic versus Y, underaged. Curve B, Kic versus Y, overaged.

(3) The above solutions are for planar cracks only.

Branching has been treated only for special two- dimensional crack geometries [30]. Elastic-plastic so- lutions have not been developed for roughly branched

nonplanar cracks, for planar cracks with wiggly crack

tips, for cracks which meander three dimensionally or

for planar cracks in the presence of random statistical distributions of growing three-dimensional holes. (4) The above analysis leads to a prediction of a

critical stress intensity and hence to a critical flaw size for a given loading, eq (1). However, the details of analysis of the nucleation of a crack at a flaw in a nominally continuous material are largely unknown

for the same reasons as discussed for hole nucleation.

All particles or inclusions may or may not crack, and

the resultant cracks may or may not be equal in size

to the particle size. In certain circumstances entire grains, whether flawed or not, can act as virtual flaws. Thus, one can only roughly associate inclusions and

other particles or structural features with potential

flaws. Hence, at present, one can only design the alloy structure to avoid microstructural features of certain

sizes, assuming them to be potential flaws of that size. (5) At present, the above theory does not include

dynamic effects in detail. Particularly for rapidly growing cracks, problems arise in that the crack tip region

is unlikely to remain isothermal because of dissipation

of work in the plastic region, and damping effects may

become important in the elastic region. Also, the details of crack propagation may involve localization of

flow along characteristic slip lines in the plastic zone, as discussed in the next section. This localization is a

rate process which perforce involves dynamic effects.

184
3.2. Localized Ductile Failure at Shear

Instabilities

Another failure mode that may or may not be

associated in part with hole growth as in the preceding

case is that of cracking arising within a thin lamellar region exhibiting plastic instability in pure shear. This

mode has been discussed by Spretnak, et al. [31,32],

and McClintock, et al. [33], who also provide experimental confirmation for the model. The shear instability along characteristic shear lines is distinct from

plastic instability associated with necking. A clarifica- tion of this distinction is that in a constant-rate-of- extension tensile test of a round smooth bar, necking

initiates at a load instability, and while plastic flow

is macroscopically inhomogeneous at the size scale of the specimen diameter, it is microscopically homogeneous. The shear instability begins at a local maximum

in true flow stress and is microscopically inhomogeneous, occurring in a region typically a /xm or less in thickness along a characteristic slip direction. The local

stress instability condition is

In the case of figure 5, the microscopic propagation

of the crack occurred by hole growth with holes initi- ated at carbide particles. In other cases, including some

instances of single crystals of pure metals, the crack

surface gives no indication of microscopic hole growth.

Phenomenologically, it has been established that such

failures along lines of instability require that, (a) the

rate of work-hardening be small (consistent with eq

(3)), (b) that a free surface be present since initia- tion occurs there, and, (c) that a large plastic strain gradient exist.^ These conditions were present for the case of figure 5. Pertinent to the previous section,

they would be applicable in the ligament region be- tween growing holes so that ligament failure could oc- cur by shear instability.

Unsolved problems in this case include:

(1) What is the microscopic failure mechanism in cases where hole growth does not occur?

(2) No elastic-plastic solution is available for the ragged crack case such as that in figure 5. (3) When hole growth accompanies failure along an

instability line, does formation of the instability pre- cede hole nucleation or vice-versa?

(4) What is the mechanism of initiation at a free surface?

and is evidently rate dependent. Here e is strain, e is strain rate and T is absolute temperature.

An example of such failure in an AISI 4340 steel is presented in figure 5. The specimen was in plane strain

Figure 5. Mid-thickness crack configuration in V-notched AISI 4340 specimen loaded close to maximum load [31].

Steel tempered to 178 ksi ultimate tensile strength. X 90.

SO that the characteristic slip lines are orthogonal as indicated. Obviously, the path of the crack follows the

characteristic slip lines, in one case actually switching

from one set of characteristics to the other and back.

3.3. Stress Corrosion Cracking

In this section, we discuss corrosion cracking in al- loys such as austenitic stainless steel and alpha-brass

which are in either an annealed or cold worked condition but which are at low strength levels relative to the elastic modulus compared to the high strength alloys

such as high strength steel and precipitation-hardened

alloys. The features of such stress-corrosion cracking

(SCC) in comparison to the cases of the previous sec- tions are given in figure 6 [34]. In higher strength

ranges, SCC occurs at lower stress intensities than does

ordinary cracking. Also, in SCC a regime of slow crack

growth is exhibited at low stress intensities, stage I in figure 6b. Crack initiation occurs at surfaces and the

useful life prior to failure is dominated by crack growth

so that nucleation is not as important as in the previous

cases. In some cases stage I smoothly merges into the stage

III rapid crack growth which follows the same v-K

curve as ordinary cracking, figure 6b. In many cases a plateau stage II is also found. As expected, stage III usually corresponds to ductile cracking by hole growth

as discussed above and with the crack proceeding trans- granularly. Stage I corresponds to either intergranular

or transgranular cracking with a much finer scale of features associated with plastic deformation at the crack

tip, as would be expected at the lower stress intensity,

and with a smoother fracture surface containing fine features resembling slip steps. Stage III corresponds to crack branching (meandering) or to rumpling of the crack surface (quasi-cleavage) . Both effects would tend

to reduce the local stress intensity and hence would

Indeed it appears that crack nucleation is dependent on the attainment

of a critical strain in this case in contrast to brittle cleavage crack nucleation within a plastic zone which follows a critical normal stress criterion

[e.g., see R. O. Ritchie, J. F. Knott and 5. R. Rice, J. Mech. Phys.

Solids 21, 395 (1973].

185
sec

o

Figure 6. (a) Typical plots of Kic and Kiscc versus U.T.S.

for high strength alloys, (b) Typical plot of crack growth rate v vervus Ki in the SCC

case.

and that dissipated in plastic flow, but since the latter

is difficult to calculate, a constitutive relation cannot be

constructed. Moreover, in most cases there appears to be a purely corrosion part of the SCC cycle producing

a locally changing stress field at the crack tip, involving

another rather poorly defined variable in the cracking

process.

(3) Superposed on the previous problem is that

of establishing surface boundary conditions. Local crack

chemistry, diffusion in solution, rates of adsorption,

degree of polarization or film formation, and electrostatic effects all affect the surface boundary conditions. These effects appear in the mechanical portion of the

cycle as an effective surface energy appearing both in the work dissipation and as an influence on the emergence of dislocations at the surface and through local image stresses acting on near-surface dislocations. In

the chemical portion of the cycle, they directly control the local dissolution rate. (4) In the cases where SCC is intergranular, the question remains unsolved of why it is so. Possibilities for the effect include: grain boundary grooving which

changes the local solution chemistry and influences

the chemical step, solute segregation to the grain

boundaries which influences either the chemical step or

the mechanical step via local solid solution hardening,

elastic and plastic compatibility effects which generate

differential mechanical behavior at the boundaries.

3.4. Failure Following the Temper

Embrittlement of Steels

logically reduce the slope of the crack velocity versus average stress intensity plot. While still somewhat controversial, the balance of the prevailing view is that SCC proceeds by a cyclic process of local plastic flow producing slip steps with

surfaces freshly exposed to the solution, dissolution at these surfaces causing crack advance and increase of

local stress intensities, causing the cycle to repeat. Other models mostly also involve the concept of a mechanical-plastic flow portion and a chemical disso- lution portion of the cracking cycle.

The / integral, energy-release methods again apply

to this case, but some of the unsolved problems are even more difficult than in the previous case: (1) No elastic-plastic solution exists for the meandering crack which occurs to a marked degree for systems with stage II cracking.

(2) The problem in constructing from first principles an equation to describe crack growth rates for

stage I cracking is more severe than for ductile cracking, even with known surface boundary conditions. The plastic zone size is smaller and hole growth has

not been shown to be a factor in cracking (though it could occur on a near atomic scale). The plastic zone

size is of the order of the spacing of dislocation sources, of dislocation cells, or of grains. Hence a de- scription of the plastic flow involves all of the structural features discussed previously: anisotropic elasticity,

compatibility, cross-slip, etc. One can balance the en- ergy release with energy to create new crack surfaces

A case with some features of each of the preceding

cases is that of temper embrittlement of heat treated

alloy steels. Cracking occurs at approximately a critical stress intensity, crack velocities are relatively high,

and the nucleation stage is largely undefined as for the contained ductile fracture case of section 3.1. However,

cracking in the embrittled condition occurs at prior

austenite grain boundaries or at ferrite boundaries

[35] and evinces no dimples or other features characteristic of a large plastic zone, in this sense resembling

the SCC case. The unsolved problems are mainly those mentioned

above, but some new features merit emphasis:

(1) There is definite evidence that solute segregation to boundaries is an important factor with evidence

for both transient [35] and quasi-equilibrium [36]

adsorption effects. Mechanistically, the solutes could

either cause a highly localized, contained elastic-plastic

cracking, wherein the critical step is local plastic de- cohesion as in the previous cases, or could cause the

critical step to become atomic bond breaking at the crack tip, with or without some minimal plastic defor- mation ahead of the tip. (2) If atomic bond breaking is important, a number of possible unresolved mechanisms exist at the atom-dislocation scale. Does the breaking occur nearly

reversibly with groups of bonds gradually parting or does it occur irreversibly bond by bond? If the latter occurs, then the mechanism is likely to resemble the nucleation and growth of double-kinks over the Peierls

186
barrier in low temperature dislocation glide. The phenomenology of such an approach has received some

attention [37]. However, the elastic field of a kinked

crack has not been determined and the form of the potential barrier is not known.

(3) Atomic interaction potentials are required for

solute-solvent groupings in highly nonlinear elastic configurations to even begin to predict potential barriers from first principles. This problems is severe even

for a crack in a single crystal of a pure metal. The

progress that has been made in the physics of solutions has been for somewhat idealized, homogeneously

strained solutions. The development of a potential for highly strained configurations is an outstanding chal- lenge in solid state physics.

(4) For the case of cracking at boundaries, even

with known atomic interactions it is important to know

the boundary structure at an atomic scale, including the

dislocation structure at the boundaries.

(5) With any irreversible portion in the cracking

process, the problem in development of a first-principles relation to describe cracking is exacerbated. The energy

released for a given crack advance is still known, but

of the corresponding dissipation terms the irreversible portions usually are not. For example, if a crack ad- vances by partly irreversible bond breaking with no

plastic deformation, the thermodynamically reversible part of the energy released would appear as surface en- ergy of the cracked surfaces while the irreversible part

would be dissipated as elastic pulses eventually at- tenuated and dissipated by heat flow. Similarly with an elastic-plastic crack, part of the energy could be

dissipated as elastic pulses in addition to that used

to form the crack surfaces and the plastic zone. Then,

independent of one's knowledge of the details of the

plastic region, one still could not develop the needed

equation without information on the irreversible re- lease mechanisms. Acoustic emissions which have been

observed in a number of cracking cases provide direct evidence for such irreversible terms [38].

3.5. Hydrogen Embrittlement

Hydrogen embrittlement includes both cases where

alloys are exposed to hydrogen gas and cases of cor- rosion cracking where hydrogen is introduced as a consequence of hydrogen evolution being the cathodic

reaction in the local electrochemical cell. Hydrogen

embrittlement of alloys is manifested in two ways. At

high hydrogen fugacities, the hydrogen forms molecules in voids and generates internal pressure which

assists in the plastic growth of voids and subsequent

cracking by the contained ductile fracture mode [39].

Alternatively, for high-strength alloys such as high

strength steels, hydrogen at low fugacities (corresponding to pressures well below one atmosphere) leads to failure at stress intensities less than the critical value

for a hydrogen-free alloy [40]. A three-stage v-K curve

as in figure 6b is often exhibited. Scanning electron microscopy indicates that, as for SCC, stage I is inter- granular and stage HI is contained ductile rupture

[41]. Hydrogen entry definitely has a role in the crack- ing process since oxygen inhibits hydrogen cracking

[40], presumably by adsorption, blocking sites from

hydrogen adsorption, while catalysts for hydrogen adsorption, such as hydrogen sulfide, enhance cracking

Perhaps it will evolve that vapor embrittlement will be more appropriate terms for this type of failure. The

work of Kerns and Staehle [42] has shown that slow

crack growth can occur in a variety of gases containing

hydrogen as a constituent and one, chlorine, that does

not. Their work indicates that for AISI 4335 tempered

to an ultimate tensile strength of 284 ksi, exposure to gases at 100 to 300 torr led to X^/scr values decreasing

from 80 to 40 ksi \/in, respectively, for CH4, CoHo,

Ho. NH3, HCl, HBr and Clo. Again a number of problems are unanswered:

(1) For the intergranular mode, the problems of grain boundary solute adsorption, grain boundary

structure, and compatibility effects exist. (2) The mechanism is not known as in the previous

case. Again the critical step in crack advance could be

either the onset of decohesion in a contained plastic

region ahead of the crack tip or bond breaking at the

tip. Because interstitial hydrogen is highly mobile at room temperature there is an additional uncertainty in where hydrogen influences the cracking process. Plausible models have been advanced for hydrogen diff^using

ahead of the crack into the contained plastic zone and

aiding in decohesion [40] ; for hydrogen adsorbing

as a tightly bound surface layer hindering plastic re- laxation [43], and thus decreasing the K value required

for crack propagation, for electrostatic interactions of protons and electrons in fields arising from elastic- electrostatic field crossterms [44], for hydrogen aiding

in bond-breaking (verified in an approximate atomistic

computer simulation model [45] ) , for hydrogen in- fluencing decohesion in the near vicinity of the crack

tip [46]. and for hydrogen influencing dislocation motion by forming atmospheres in the elastic fields of the

dislocations [47]. Which model applies remains unsettled, although observations [42] of slow crack

growth for the same steel in dry hydrogen or in dry

chlorine tend to support the surface effect models, since chlorine solubility and diflusivity are much less than

the equivalent values for hydrogen.

We do note that there are some inconsistencies aris- ing from lack of communication between disciplines in the above models. However, some aspects of dislocation theory have not been considered in models involving

solute atmospheres [48] ; and, as pointed out by Rice

[20], the region of maximum normal stress for the plane strain case has incorrectly been taken as equal to the tensile yield stress Y and to occur at the boundary

of the plastic zone rather than the continuum mechanics result of a value ZY at a distance roughly onetenth of the extent of the plastic zone (in plane stress the maximum normal stress would equal F).

(3) The need for interatomic potentials discussed in the previous section is also critical in the present case.

3.6. Brittle Fracture

Fracture with no plastic deformation, even on a microscopic scale is defined as brittle fracture. There is

187
some question whether such fracture can ever occur. However, in principle, the problem of brittle fracture

exists at three levels. First, there is the Griffith's crack

in an otherwise perfect crystal. As discussed in earlier sections for cases where decohesion at the crack tip is controlling, problems exist in degree of reversibility of cracking (equilibrium surface energy as the dissipa- tion step or that plus elastic wave release), planar

versus double-kink type advance with lack of attendant

three dimensional elastic field solution, and atomic potentials. A further problem is whether an atomically

sharp crack can be retained in crack propagation with- out local microplastic relaxation by dislocation for- mation. A model three-dimensional calculation for deciding upon the relative likelihoods of these processes has been presented by Rice and Thomson [49] . Further development requires consideration of the local three-dimensional double-kink problem for both the crack and the generated dislocation. A second case is that of cleavage fracture initiated after some plastic flow at a preexisting stress concentration [50,51]. There is evidence that in steels the

requisite is for a critical stress to be achieved at a

characteristic microstructural distance of about two

grain diameters ahead of a preexisting crack. This criti- cal stress is in turn related to a Griffith criterion with

the flaw size equal to carbide particle diameters or

inclusion diameters. In other instances initiation has

been associated with dislocation intersections [52] or

dislocation-twin boundary intersections [53]. The problems in both initiation and propagation parallel those

of section 3.1, e.g., particle decohesion and cracking or non-planar cracks, and hence are not repeated here. In connection with cleavage, the work of Hoagland,

et. al. [54], is also of interest. They have initiated cleav- age cracks, propagated them in a decreasing stress in- tensity field, and arrested the cracks at a critical dynamic stress intensity. The understanding of the arrest problem in a microstructural mechanistic sense is beset with the same problems as above together with

various dynamic problems. The latter include the

elastic-field of a moving nonplanar crack, dissipative

heating in the crack tip region, and local strain rate dependence of the flow stress in the tip region.

At another level there is the problem of apparently

brittle fracture in materials such as silicon nitride and silicon carbide now being considered for some

structural applications. (Not all ionic or covalent crystals, or course, fail brittlely. Sodium chloride is ductile

in compression and zirconium hydride fails in tension by localized plastic shear flow, for two examples.)

These silicon compounds have quite complicated microstructures, which give rise to a set of problems in understanding:

(1) The fracture problem is primarily one of nucleation since growth is essentially instantaneous. Thus

even with an expectation phenomenologically of crack- ing at a critical stress intensity—critical flaw size there

is a most formidable problem in the statistics of ex- treme values in statistical distributions [55].

(2) There is a current need for characterization

of microstructures to make possible even the assess- ment of typical flaw sizes for these materials.

(3) The calculation of local stress distribution is a

difficult problem requiring consideration of anisotropic

elasticity and of compatibility eff"ects.

3.7. Fatigue

Space does not permit treatment of other fields of mechanical failure such as creep, failure under shock

and impact loading, hot corrosion cracking and fatigue,

all of which would require treatment equal to the above

to reproduce even the above type of cursory survey. However, there are some direct analogies [56] in the

particular case of high cycle fatigue which merit brief

discussion. Similar to SCC, fatigue cracking is almost invariably

surface nucleated and proceeds by a period of slow crack growth. Surface stresses, surface finish, and sur- face chemistry thus influence crack nucleation. When

failure does occur, however, most of the time to failure (roughly ninety percent or more) consists of the crack growth stage. During cyclic loading, work-hardening

occurs in a few thousand cycles after which a nominal

steady state is achieved in which cyclic plastic defor- mation occurs in persistent slip bands. Extrusions form

at surface intersections by the slip bands with corre- sponding formation of internal porosity in the bands. Cracks nucleate at the interface between the slip bands

and the matrix and grow. The internal dislocation structure in the bands comprises cell walls of multipole arrays which are relatively unstable under stress compared 40 the relaxed interwoven arrays characteristic of monotonically loaded specimens. Environment af- fects crack growth rates in analogy with the SCC

case. The local crack propagation event in some sense resembles the contained ductile rupture case [57].

This parallelism suffices to rationalize the phenomenological correlation between crack growth rates and

stress intensity factors [57] . The analysis of this process in terms of continuum elasticity-plasticity is complicated by the occurrence of deformation in a highly localized slip band but, on the other hand, over

distances (of the order of millimeters) large compared

to the band width. Contrariwise, the scale of the defor- mation tends to rule out mechanisms based on dislo- cation events at the atomistic scale [58]. With these

provisos, many of the problems (excepting those related

to brittle fracture) discussed in connection with a monotonically loaded crack apply to the fatigue case.

4. Needed Research

The various problems in the context of the fore- going discussion suggest areas of needed research,

which are enumerated here with little elaboration. Some similar listings have been presented recently for

the cases of corrosion cracking and fatigue [19,20,48].

We note particularly that the following list includes

those areas of potential interest which do not now

seem to be receiving sufficient support or attention and

does not include important research topics now being

supported. The latter, which of course deserve con188
tinued support, include studies of solution and elec- trode kinetic effects in corrosion cracking, correlations

of microstructure with crack velocity-stress intensity factor studies, dynamic effects for elastic cracks [59],

and so forth. The listing follows:

1. Theory

a. Continuum Mechanics

(1) Elastic and elastic-plastic solution for a plane

crack with a straight front in the presence of a ran- dom distribution of spherical holes. (2) Elastic and elastic-plastic solution for a crack

meandering in three dimensions.

(3) Elastic and elastic-plastic solution for a plane

crack with a wiggly crack front or for a branched

crack.

(4) Inclusion of dynamic effects for intermittently

irreversible cracks and for rapidly accelerating

cracks, including both dynamic K-v and energy

dissipation-v relations.

b. Elasticity-Plasticity

(1) Dislocation scale models of decohesion of a matrix-particle interface. (2) Elastic-microplastic compatibility effects at inclusions and second-phase particles including

anisotropic elasticity.

(3) Nonlinear anisotropic elastic treatments of

dislocation pileups and interactions. (4) Elastic field of a plane crack with a double- kink on an atomic scale in an otherwise straight

crack tip. (5) Elastic-microplastic treatment of a crack tip near a grain boundary with compatibility effects and a dislocation model of plastic relaxation. (6) Tensor treatment of dislocation motion in multiaxial stress states characteristic of crack tip elastic fields; interrelation with stress-invariant

plasticity models.

(7) Influence of surface films and electrochemistry on dislocation—crack interaction. (8) Development of a criterion for initiation of

failure by shear instability and of a mechanism

for cracking when hole growth does not occur.

c. Atomic Structure

(1) Models of grain boundary (interface) structures. (2) Models of grain boundary (interface) dislocations. (3) Estimations of interface cohesive strengths.

(4) Estimations of the Peierls energy of an ad- vancing crack.

d. Interatomic Potentials

(1) Potentials for pure metal atoms in highly nonlinear elastic strain fields characteristic of crack

tips including electrostatic crossterms arising from

electron redistribution and including free surface

relaxations. (2) The same for solute-solvent interactions also including screening effects at solute sites.

e. Statistics (1) Theories of statistics of extreme values for flaw distribution and flaw formation.

f. Computer Simulation

Some comment is added here since some suggest

that simulation is not worth while until better atomic

potentials are available. Granted that simulations at present are not likely to exactly replicate real materials, the work is most valuable in studies such as those listed below where even order-of-magnitude re- sults would provide better guidelines than presently

available. In other words, actual mechanisms are

tested, albeit in artificial materials.

(1) Model of kinked crack as in item D-l-b-4.

(2) Model of dislocation relaxation at sharp crack

tip and test of theory [48].

(3) Study of the role of solute adsorption, or ad- sorption at a sharp crack tip. (4) Study of the degree of irreversibility in propagation of a sharp crack.

(5) Development of lattice Green's functions for

a cracked body to permit the fixing of boundary

conditions on an atomic region.

2. Experiment

a. Deformation Studies.

(1) As suggested by Rice [20], studies of hydro- gen embrittlement for cracks with rounded tips of various radii to give regions of maximum normal

stress at various distances ahead of the crack tip

and hence to resolve the controversy about where

the hydrogen exerts its influence. (2) Studies of interface decohesion of particles in the vicinity of similarly rounded notches to assess the process of decohesion in the presence of multiaxial stress states. b. Atomistic Kinetics

(1) Study of adsorption and absorption in the presence of hghly nonlinear elastic fields by per- forming such measurements on whiskers and platelets which can sustain roughly five percent elastic strain. (2) Study of hydrogen diffusion in the presence

of multiaxial stresses to determine the tensor nature of the stress dependence of the diffusion acti- vation energy pertinent to crack-tip elastic fields.

c. Microscopy

(1) Characterization of solute adsorptionabsorption at interfaces, grain boundaries, dislocations, and crack tips (Auger microscopy, atom

probe)

(2) Study of grain boundary structure (atom

probe, electron microscopy of molecular crystals).

(3) In situ studies by high-voltage-transmissionelectron microscopy of SCC, hydrogen embrittle- ment cracks. (Studies [60] of ductile rupture of

films from 0.1 to 100 fim show that some degree

of triaxiality-'hole growth is achieved in the center

of the films so that the studies provide some semblance of plane strain conditions.)

(4) Characterize the structure of complex materials (mostly ceramics) subject to brittle fracture.

189
The author is happy to acknowledge helpful discus- sions of the present topic over the past five years with

colleagues in the ARPA Materials Research Council,

in particular F. A. McClintock and J. R. Rice, whose

work is drawn upon in the present article. The author

is grateful also to R. G. Hoagland, J. E. Sinclair, J. W.

Spretnak and R. W. Staehle for helpful comments, to G. T. Hahn, A. R. Rosenfield and J. W. Spretnak for supplying micrographs, and to the U.S. Office of Naval

Research for support of this work.

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Paris, P. C, in Fatigue—an Interdisciplinary Approach,

Edited by J. J. Burke, N. L. Reed, and V. Weiss

(Syracuse Univ. Press, New York, 1964), p. 107. Avery, D. H., and Backofen, W. A., in Fracture of Solids, Edited by D. C. Drucker and J. J. Oilman (Interscience, New York, 1963), p. 339. Freund, L. B., Int. J. Engineering Sci. 12, 179 (1974) ; see

also ref. 18. Bauer, R. W., Lyles, Jr., R. L., and Wilsdorf, H. 0. F.,

Z., Metallk, 63, 525 (1972) ; R. W. Bauer and H. 0. F. Wilsdorf, Proc. Int. Conf. on Dynamic Crack Propagation, Noordhoff, in press (1974).

190
6. Discussion

0. Jahari, IIT Research Institute: You showed a curve

of volume fraction of inclusions versus the strain de- pendence. In such calculations, don't you have to take

into account the size of the inclusions also?

/. P. Hirth: Yes, that was a correlation where the

particles were all nominally the same size. It is true, failure is more likely to occur with larger particles.

However, that correlation assumed that all samples

failed independent of particle size. That was only a rough correlation with experiment. There's not enough

detail to give a correlation with spacing and the size of the particles.

0. Jahari: But in general, except for certain types of alloys such as TD nickel, the inclusion size would vary

a great deal.

/. P. Hirth: Yes.

0. Jahari: What happens in materials that do not have inclusions, like high purity zone refined materials?

/. P. Hirth: It's really not clear what goes on. For

single crystals, of course, there are some cases where

necking right down to a knife edge takes place; that is, no internal voids ever open up. However, recent elec- tron microscopy studies by Wilsdorf of silver single

crystals and large grained stainless steel indicate that internal voids open during crack propagation. Wilsdorf's specimens were not "thin" films in the sense of being thousands of angstroms in thickness, but they are

not thick either. So there's a question of whether plane

strain or plane stress plays a role here, but at least there is one case where a high purity metal fails by

internal hole growth.

0. Jahari: We looked at high purity zone refined iron and found dimples, but at the bottom of the dimples.

instead of finding particles we found what looked like holes. Unfortunately the resolution of the scanning

electron microscope is not good enough to get further

details.

K. G. Kreider, National Bureau of Standards: Would

you comment on George Ansell's treatment of the role of inclusions in the fracture of ductile metals?

/. P. Hirth: There is some controversy here. Ansell's work is really most applicable to dispersion hardening

systems and more related to work hardening rates. His idea is that Orowan type pile-ups around particles cause them to crack. Other people say that that cannot

take place and instead there is dislocation generation

and motion around the particles that cause a hole to nucleate near the particle. Experimentally, when people

look around particles, they tend to see only one or two

dislocation loops which would tend to discount the Ansell idea. On the other hand, the resolution of the in- strument is such that there could be a number of loops

right next to the particle that are not resolved. So

it's really still an open question.

K. G. Kreider: Would you express an opinion?

/. P. Hirth: My own bias is that the dislocation relaxation takes place. But the evidence is still out.

B. W. Christ, National Bureau of Standards: In the Hahn-Rosenfield work on silicon iron, how do you

get a kidney bean shaped plastic zone at the crack tip

without lots of etch pits in the region where the plastic zone must have been when the tip got to the point shown

in the slide?

/. P. Hirth: They got the sharp crack by propagating

a fatigue crack to that point, which would have

characteristically a much smaller plastic zone, and

then they pulled it in tension.

191

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Implications for Action for Engineering

H. W. Paxton

Camegie-Mellon University

Oflice of the Director for Research

Pittsburgh, Pennsylvania 15213

The demands on materials for fail-safe operation have increased enormously since the days of "Galloping Gertie" and Liberty Ships. The necessity to use higher strengths has increased demands

on selection, design, acceptance testing, nondestructive evaluation of structures, and manufacturing

methods. Extremes of temperatures, thicker sections, and environmental effects have all accentuated

the tendency for mechanical failure. This paper will discuss the current status of selected problems, and by assessing our fundamental understanding of certain important engineering parameters, viill examine prospects for improved materials and techniques.

Key words: Environmental extremes; fail-safe operation; high strength materials; mechanical

failure; potential service conditions; proliferation of specifications.

Many engineering structures are not "materialsintensive" and selection of material is made on eco- nomic grounds, all too frequently on a first-cost basis with little though to subsequent maintenance costs. Mechanical properties are important to a point, but

since over-design is common, it is extremely rare to find

failures from gross plastic deformation following overloads. Occasionally, massive flaws are present, for ex- ample, large internal cavities in castings, but this problem is operational rather than conceptual and

will not be pursued further. As requirements become more stringent, and the strength-to-weight ratio increases, there are two other

causes of failure resulting from thinner sections which

I will not discuss. One is buckling, usually plastic, and

the other is general corrosion resulting in significant

cross-sectional loss. In the last two decades, the demand for materials to perform at higher strengths, greater thicknesses, and

in extremes of environment such as high and low temperatures, various kinds of radiation, aggressive chemistries, etc. has grown enormously. It is in these areas where I would like to place most of my emphasis.

Fortunately, only a small fraction of failures occur

in service, but in many cases these tend to be very

expensive in terms of immediate damage (the "Silver

Bridge"), lost or delayed production (e.g., several nuclear plant failures), expensive design precautions (e.g., much space hardware) and personal hazards or loss of

life.Let me now outline what I will cover. The points I would like to make really can be summarized as fol- lows:

(1) How well do we know the service conditions?

(2) How well do we know the total properties of

the material?

(3) What can be done in specifying materials and

processes to make the final structure more predictable, and, in particular, less sensitive to unexpected conditions in service?

I will give some examples to illustrate these points.

I will also apologize just this once for belaboring the obvious, but somehow the obvious is not always so. There are many engineering structures which are

sufficiently standardized that historical precedent can be utilized effectively and even an acceptable statistical chance of failure may have been established. It is the new and extrapolative designs which are of most in- terest here. Let me illustrate what I mean by an

interesting example with (so far) a happy ending where

prediction of service conditions was very difficult but

also crucial. It involved the general concept of protection of a system at a critical point from the results of failure at another critical point. The Peach Bottom

reactor of the Philadelphia Electric Company is a pro- totype of a very interesting competitor to the light water

reactors. It uses carefully purified helium to extract heat from a graphite moderated core. This helium then

is used as the heat source in a relatively conventional steam generator. The hot helium and the cooler return are in concentric pipes, and of course separating insulation is necessary. Provided the helium can be kept

"pure," the problem is not particularly interesting

because several layers of thin metallic sheet can be

used to provide stagnant gas spaces in series. However,

193
if we have any leakage from the steam generator

thereby transferring water vapor to the heHum, we have

a potential problem. This water vapor reacts with the hot graphite in the core to form a mixture of CH4 and

CO which can be strongly carburizing to the FE-Ni-Cr

alloys which typically would be candidates for the in- sulation. Carburization can produce an extremely brittle material which could have all kinds of unwanted

effects. The probability of this sequence is small, but

how small? As you might expect, we cannot solve this completely by analytical means, but it was possible

to combine analysis with a few critical empirical data and produce a solution which has worked.

Another example of imperfectly known service conditions is from the bridge world. The "Silver Bridge"

across the Ohio River failed catastrophically because

load and the local industrial atmosphere caused slow crack propagation to a critical size, and this likelihood was just not forseen. A second general principle vio- lated here was the lack of redundancy in the structure, a problem of increasing significance with materials

demonstrating only fair resistance to crack propagation. There are a number of other service conditions

which sound trivial but are often tragic. The best de- sign in the world is of no use if the concept in the brain

of the designer is not in fact the one which actually goes into service. The problem is compounded if the designer is under the impression that it really did. Two examples illustrate this point. Metallic implants

to help repair skeletal damage are commonplace and

essential. I have a number of thoughts on inadequate

design and material selection which are not appropriate

here. But I was shocked to find that a 316 L bone plate was being installed with vitallium screws. Ah, you

say! But surgeons don't know any metallurgy and

have perhaps never heard of electrolytic effects be- tween dissimilar metals. I am sure this is truer than we would like, but in this case the surgeon was not

to blame. The vitallium screws were in a package

clearly labeled as stainless steel. Another example concerns the failure of a large pressure vessel used in the manufacture of ammonia.

The design was perfectly adequate but because of var- ious time constraints an inspection for cracks in or near the internal surface of the welds was not made

after stress relief. No cracks were found prior to this process and since reinspection was awkward and ex- pensive, the judgment was made not to do it, even

visually. Unfortunately, a crack did develop and under

the influence of operating stress and a substantial

partial pressure of hydrogen this propagated completely through the weld. The resulting shutdown and

loss of production was of course very expensive. The

designer thought he had designed a safe and proper

system and indeed, in the absence of a crack and high

pressure hydrogen simultaneously, in all probability he

had.The more general question which this audience may

wish to debate is the cost-effectiveness of inspection.

How much is enough? We have seen in recent years

a new attitude on such matters as assigning economic

values to environmental controls or safety of nuclear

reactors. As scientists and engineers, we know that data on which such decisions are made lack the precision we would wish, but nevertheless action is taken and has legal backing. I think we have at the moment

an inadequate basis for designers to assess the probable consequences of the differences between their brainchild and the structure actually in service. This is com- pounded as we work with untried designs, higher

strength materials, and interactive effects (e.g., cracks and environment) . An important factor in ameliorating

this problem is to stimulate recognition of the importance of understanding the totality of a material to be specified—its "pedigree" if you will, since this word connotes not only ancestors from melting stock (sometimes of a standard questionable by the equivalent of the Kennel Club) but also off-spring as it in turn becomes re-melted and reused in the recycling which

seems unavoidable in the near future. This is my

next topic. In their more pejorative moments, metallurgists are prone to refer to designers as "handbook engineers."

There is some truth in this. Designers have a primary

responsibility to design and this involves specifying

a material. Certain properties are necessary and are in handbooks. In a great majority of cases the design

works and everyone is satisfied. It is the relatively rare situation where selection of one property in ig- norance of others causes problems and ultimately ex- pensive failures.

I am reminded of a classic case where a very important material characteristic for a particular design was the thermal expansion coefficient. Consultation of physical property tables and price lists suggested that Type 430 (a 17% Cr stainless steel) was ideal. It would have been except that in service around 500 °C,

as was planned, this material suffers a drastic loss of

ductility and would rather soon have lost mechanical

integrity. This fact is not in handbooks of physical

properties, although it is reasonably well known in metallurgical circles. Many such difficulties can be

avoided if everyone concerned asks the proper questions of the right people—the standard communications

problem. Information transfer happens a lot of the time but when it doesn't, failures are more likely. And

here designers get their own bark at the materials

specialist! They might have a life cycle involving

combinations of creep, fatigue and thermal cycling. So, they ask, what do I use? What stress or strain limits do I have? Are the welds likely to be better or worse than the base material? And so on. The truly

knowledgeable metallurgist will be faster on his feet than Mohammed Ali in avoiding direct answers to such questions.

I have painted perhaps too extreme a picture, but,

I maintain, one with substantial elements of truth. Let me discuss now some factors which may eventually

contribute to a higher probability of using materials

successfully.

A casual glance at a list of specifications, be they

SAE, ASTM or whatever, really boggles the imagination. To an informed metallurgist, who has an idea of the function of the various elements listed in the chemical composition, there is a certain pattern. But we must

194
forgive an uninitiated person for wondering if we are

really serious about the need for all the compositions

listed. We have perhaps heard of the old shop supervisor to whom there were only three kinds of steel, hot-rolled, cold-rolled and stainless. This is slightly

apocryphal but I feel fairly certain that if these three were expanded to ten or twelve compositions, the great

majority of our needs could be covered by more emphasis on careful processing.

A study of how these specifications arose is perhaps

helpful. The chain of logic—composition structure —> proj)erties—is something we teach to all our stu- dents. However, the degree of sophistication and un- derstanding along this chain varies enormously. There

are many situations where even on the research frontiers, a semi-quantitative appreciation is lacking.

Looking back, we see that variations in chemical

composition have been a principal consideration in developing new alloys. This is understandable and in many cases logical. If we wish to have a stronger

alloy, we know various ways to produce this—solid solution hardening, precipitation, cold work, and so on. Certain alloying elements can be used and it is a matter of relatively simple experimentation to find out how much. We thus get into the habit of creating a "structure" by compositional variations with the implicit assumption that the processing variables—melting, casting, working, quenching, ageing, etc.—will not be very different from those we have used in the past. We will end up with a stronger alloy. Now we ask

about the other properties—the more subtle ones like fracture toughness, stress corrosion crack rate, ability

to resist cyclic loading, and so on. Here our predictive

ability is much poorer and all too often we find empirically (and expensively) that our stronger alloy

has marginal or unacceptable other properties. The

next step is to try some other variation in chemical

composition to produce our strength, and again measure

our "subtle" properties. Eventually, we find the least harmful combination of increased strength and minimally impaired other properties.

I chose strength as the primary variable of interest in an alloy. Of course, others could be of maximum

interest, but they do not change my argument. Many

of the compositional specifications were designed to optimize one property. We then scramble as well as we can to avoid possible shortcomings in other properties.We are obviously a long way from having the knowledge to design an alloy with a particular combination of properties. However, experience over the

last decade or so in controlling processing variables more closely for a specific composition has proved to be a major step forward. We may note here particularly

the pioneering work of Irvine and Pickering [1] ^ in simultaneously improving the yield strength and

fracture toughness of low-carbon low-alloy steels, a development rapidly picked up by all the major steel producing countries. Over-simplifying slightly, this was

done by recognizing the vital positive contribution of

ferrite grain size and the negative contribution of

^ Figures in brackets indicate the literature references at the end of this presentation.

pearlite to toughness. Chemistry still has effects but

these are quantifiable separately.

A second example of processing control developed

in the last few years is the ability to influence inclusion shape in steels, especially sulphides. Traditionally, the

difference in toughness in rolled products, for example,

between "longitudinal" and "transverse" directions is quite significant, and is convincingly explained by the presence of elongated MnS inclusions. Fatigue properties are also markedly different. There are other sul- phides which have a higher melting point than MnS

and thereby are stronger at rolling temperatures, re- maining substantially spherical in the finished product.

However, while the idea of using elements other than

manganese to combine with sulfur is not new, earlier

results were erratic, in part because oxygen and sulfur compete for these scavenging elements. When the proper techniques of addition were perfected [2], rare- earth silicides proved most effective and produce substantially equivalent longitudinal and transverse impact properties throughout a whole ingot. In higher

strength steels, zirconium, (now reasonably economically available as Zircalloy scrap) seems to hold promise

of achieving the same result. In steels at the highest strengths conveniently used

today, say in the range of 180 000 to 250 000 psi yield

strength, one is struck by the large variations in toughness, crack growth rate and K/scc at a given yield

strength, even for the same nominal composition What

this means of course is that our understanding of the

fine details of structure which control these properties

is inadequate.

Some progress is being made in this area but it is, perhaps understandably, slow. Fast fracture of such

steels occurs typically by the nucleation, growth and

linking of voids nucleated at second phase particles.

Hirfh [3], in the preceding paper, has outlined some

of the present theoretical approaches. Experimentally,

detailed studies to provide quantitative understanding

are fairly sparse. We note here an interesting unpublished study by Cox and Low [4] to compare behavior

in a carbide-strengthened steel (quenched and tem- pered AISI 4340 at 200 ksi) with that of an 18 Ni

maraging steel at the same strength level. In both

cases, the first voids form at relatively massive inclu- sions (at the MnS—matrix interface in 4340, and

by fracture of the titanium carbonitride inclusions in the maraging steel). These voids grow in the maraging

steel until impingement occurs leading to coalescence and final fracture. However, an instability sets in earlier in 4340 in that void sheets nucleate at the cementite

particles responsible for strengthening and lead to premature failure. Apparently the strengthening particles in the maraging steel do not nucleate these sec- ondary voids, and thus we obtain the typical extra toughness of maraging steels. Since our knowledge

of nucleation of voids in solids is still rudimentary, it is not surprising perhaps that we can still get large variations in toughness in apparently equivalent steels.

It will require the most careful experimentation to doc- ument the subtle contributing factors. In stress corrosion cracking the microstructural fea195
tures are also very difficult to relate to such parameters

as Kiscc or crack growth rate. Some plausible reasons

for this difficulty have been outlined by Hirth [3]. A

variety of experimental studies in steels where a serious

effort was made to vary one microstructural feature at a time was unable to uncover major effects [5,6].

Since these studies were on a single heat, another study was made to examine possible effects of variations in trace elements from heat to heat [7]. Again the effects were slight.

As we consider fatigue, one can find a similar story. Quoting from a recent NMAB report [8]

"Contributing to the complexities of using a fracture- mechanics model for fatigue-life prediction are various facts including:

( 1 ) Life estimates are subject to error since observed

crack-propagation rates (da/dN) at specific values

of the stress-range intensity factor (AK) show as much scatter as fatigue lives at specific values of the stress amplitude in conventional fatigue tests. (2) On the same metal, a scatter range of at least 1:3 in observed crack-propagation rates must be anticipated, while crack-propagation rates in the same class of metal may vary still more. "A similar uncertainty applies to the determination of the critical crack size. The rate of slow crack propagation is not a simple power function of the (constant) stress-range in- tensity factor {AK), as is assumed usually, but deviates sig; nificantly from such a function at both ends of the AK range."

The examples cited above, while representative rather than comprehensive, illustrate the point that even on

"well-known" materials the properties of interest in

failures are incompletely understood. The strength is fairly well understood and is reproducible within sat- isfactory practical limits, the role of undesirable but

unavoidable second phases is understood in a qualitative fashion, e.g., the effects of different melting and

solidification technologies on toughness, but there are other important properties, {da/dN,Kiscc, etc.) which

cannot yet be clearly identified with microstructural

and compositional features. With some trepidation, I would propose two items for discussion:

(1) We try to arrive at a very limited number of materials (two or three at most) in each class with suf- ficient strength to be susceptible to occasional cata- stropic failure. A concerted effort should be made to characterize these materials so that we understand much

better than now:

(a) what factors are responsible for the variance

in fracture properties

(b) what processing techniques are necessary to manufacture a truly consistent product

(c) what joining technique (s) will be effective (d) what characteristic defects can be found by

present (and future?) inspection techniques.

The designer then will ultimately be able to prescribe

with greater confidence and rigor a material to do his job. The first cost of a carefully processed material may

well be somewhat increased, but I am confident that there will be many examples where these costs can be

saved in maintenance, inspection and general "peace-ofmind."

(2) This item may not go over well but I hope it will be discussed vigorously and even bitterly rather than

ignored. Very simply, I propose that we have far too many specifications which cause confusion and unfor- tunate choices. Can we devise a situation where any

specification has a prescribed finite life and will auto- matically be delisted unless a sponsor or sponsors can be found for it? This sponsor would have certain rights

and responsibilities which need to be worked out carefully, but more importantly, the lack of a sponsor would

allow a marginal specification to die.

Let me summarize the points I have tried to make.

(1) In untried and sophisticated designs, an imaginative assessment of potential service conditions, includ- ing interactions, must be made by the designer at an

early stage in conjunction with someone who understands the various possibilities.

(2) There are various pitfalls, some technical and some institutional, which can cause good conceptual

designs to result in disaster-prone products.

(3) A serious effort must be made to reduce the variety of materials available which overlap each other

in many properties. I recommend a concerted approach

to provide a few well-characterized materials which are reproducible in all of their significant properties.

(4) We should rely on careful processing rather than

changes in chemistry much more than we are accus- tomed to doing.

Finally, as a sting in the tail, let me add something

which for reasons of time (and perhaps cowardice), I did not discuss in detail in the text. More and more, we

will be held accountable for our actions and designs. We

are, or at least insist we are, professionals. Therefore, we must know our levels of confidence, we must understand where our expertise fits in with that of others and we must not be embarrassed to admit ignorance

when appropriate and to ask for advice. Meetings like this are fine, if we keep up the cooperative spirit when we go home.

References

[1] Pickering, F. B., The effect of composition and microstructure on ductility and toughness, in Toward Improved Ductility and Toughness (Climax Molybdenum

Development Company of Japan) 1971, p. 9. [2] Wilson, W. G., and Wells, R. G., Metal Progress, (Dec. 1973), p. 75. [3] Hirth, J. P. This volume, pp. 179. [4] Cox, T. B., Thesis, Carnegie-Mellon University, 1973 (to be published).

[5] Stavros, A. J., and Paxton, H. W., Metallurgical Transactions 1 (1970) p. 3049.

[6] Proctor, R. P. M., and Paxton, H. W., Trans. ASM 62

(1969) p. 989. [7] Proctor, R. P. M., and Paxton, H. W., Corrosion Science 11 (1971) p. 723. [8] Application of Fracture Prevention Principles to Aircraft. NMAB-302 (Feb. 1973), p. 41.

196
Discussion H. W. Paxton: I couldn't agree with you more.

R. S. Fein, Texaco, Inc.: The last two talks were con- cerned primarily with the implications for work that needs to be done in developing new information. Yesterday we heard Dr. Jost talk about the transfer of technology that's already on the shelf and the tremen- dous financial implication that the use of this information has. It was estimated that about one percent of the gross national product in terms of energy or raw materials could be saved. I think the real challenge for engineers, particularly, is how to put this information

in such a form that the practicing engineer can use it everyday and use it effectively. This is something that needs considerable effort on the part of knowledgeable

people such as research engineers and research metallurgists.

R. Thomson, Federal Energy Office: It seems to me

that this may be a very laudible approach to try to de- crease the total number of specifications and the total number of classes of materials. How do you think we'll get there? It seems to me that the natural tendency,

with the pressure on to develop new materials, is to go

just exactly in the opposite direction. Also, there is the tendency for every materials producer to get his own

special material out. What kind of practical approach

do you think we should take here?

//. W. Paxton: May I remind you, the title of this symposium is the "Definition of the Problem". I agree with

you it's going to be tough, but evangelists must get out and sell the bill of goods.

197

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Mechanical Reliability—Implications

for Engineering, Manufacturing and Design

W. Dale Compton

Director, Ford Scientific Research Laboratory

Ford Motor Company

Dearborn, Michigan 48021

The automobile is a complex assembly of some 15 000 different components in a network

of functional subsystems and systems. The definition of the problem of mechanical performance

is complicated by the absence of a fixed vehicle performance envelope. Control over use of the

vehicle is in the hands of the individual driver. This is substantially different from military and

aerospace hardware for which there are specified performance envelopes, and from many con- sumer products in which the performance follows a fixed pattern, almost independent of con- sumer preference. This situation imposes special implications for engineering and design, and

for testing. Improved methodologies for predicting component durability and lifetime under real-life conditions, when combined with information on material and structure properties derived under

realistic test conditions, can be of great value in developing designs with improved mechanical

properties. Likewise, the high volume production rates associated with automotive components

involve consideration of material processing and manufacturing factors if the proper levels of mechanical performance are to be achieved. Key words: Design engineering; material deterioration; mechanical failure; mechanical integrity; mechanical reliability; performance envelope; predicting product reliability.

Although life was much simpler then, few of us would

be willing to return to the era where communication was limited to the newspaper and the wireless, where

the principal labor was devoted to the production of

food, and where the upper level of education seldom

exceeded the twelfth grade. The demand of the late- twentieth-century consumer has been for ever-increasing

comforts and new products. And this demand has been

met, but at the expense of increasing complexity. But

since complexity brings with it the increased probability for failure, it is little wonder that the consumer

and the supplier are continually caught up in the conflict between the demand for inexpensive products and

the demand for products with high reliability. Zero

initial defects, infinite life, and low initial cost are

objectives that are very difficult to achieve in the same

product. But, the buying public is really rather realistic about what it expects. While it doesn't demand

infinite life at zero cost, it does want a product with

increased mechanical integrity. Surveys of what bothers automobile buyers clearly indicate that they want a

vehicle with greater reliability, durability and increased

handling ease—all within the constraints of low cost and high operating efficiency. Althugh they tend to consider squeaks and rattles as a greater measure of mechanical integrity than is probably justified, they

are clearly concerned by mechanical failures that impair the operation of the machine. It is essential that we continue to develop improved ways of providing an improved product.

It seems clear that the manufacturer that can reduce

troublesome failures even in the face of the demand for increased complexity will have an advantage in the marketplace. How, then, do we proceed to try to satisfy

this need, remembering always that the final decisions

as to what is cost effective for the product will be set- tled in the marketplace?

In the automotive business there are five key ele- ments to the process of assimilating those 15 000 or so discrete parts into the mechanical, electrical, chemical

and physical system that we call an automobile. These

elements are:

Design

Manufacture

Assembly

Customer Usage

Service

Since Mr. Uhlig has already discussed the matter of

service in some detail, I will not dwell upon it here

except to reiterate the point that service must be con- sidered on a par with design, manufacture, and assembly in the total context of producing a failure-free

199
system. It is important, though, to understand that attempting to anticipate how the customer will use (or

abuse) the vehicle and how it will be serviced and maintained in the field has a significant impact upon the design, manufacture, and assembly of the total system.

My purpose here is to try to give you some feeling

for the complexity of the problem of reliability in the automotive business, some insight into how we try to attack these issues, and some view of how we look at the

future. First, for the complexity.

The design objectives in this area are reasonably

straightforward: achieve the optimal cost design that

will meet the objectives for the part while providing

the required reliability. Of course the performance

objectives of the part must be generated from the per- formance objectives of the subsystems and these, in turn, from the performance envelope for the total vehicle. Thus the tendency would be to start with a detailed performance envelope for the vehicle—an envelope that can be taken as representative of customer

usage. But one immediately recognizes that the loads

that are applied which lead to the forcible deterioration of a vehicle will be of varying amplitudes, will be

dependent upon the type of usage that each customer

imposes, and will vary with the environmental conditions. Thus, the performance envelope that the vehicle must satisfy is very broad. As will be seen shortly, this has a profound impact upon how the design process is carried out. Clearly, design "prove-out" is a major task that in- volves extensive laboratory and field testing of components, subsystems, and vehicles under a variety of conditions. This is costly and time-consuming and often indicates that a design change is needed, with another

"prove-out" required. Thus, if we can provide the de- signer with better techniques, we can look forward to more reliable products at lower design costs. But no

matter how complete the design process becomes, it will always be necessary to verify experimentally that the functional performance and the durability levels are acceptable by utilizing tests that produce results that correspond as closely as possible to those arising

from customer usage. While it is not appropriate here to catalog all of the

efforts that are under way to develop better information

and techniques for the dsigner, I will mention two that are being developed and used. The first is a response to the question, what is the accumulated effect of usage in the field? In order to obtain a better measure of accu- mulated loads, a method is being developed that appears

to have broad applicability in determining the struc- tural loads that selected components will experience in service [1].^ A number of S/N Fatigue Life Gages with

various mechanical multipliers are bonded to a component. A permanent change in gage resistance is ob- served which depends upon the magnitude and the number of cycles of load that the part experiences. The

relationships are schematically shown in figure 1. Under

a proper set of conditions, these resistance changes can be used to reconstruct a good approximation to the

STRAIN RELATIONSHIP BETWEEN COMPONENT

AND MULTIPLIERS

(Cmax.jjo = 20.0

(€max.),. = 15.0

LINEAR SCALE

(^max.;

(€max.)|

GAGE 1 STRAIN (20:1 MULTIPLIER)

STRAIN (15:1 MULTIPLIER)

STRAIN (10:1 MULTIPLIER)

ACTUAL COMPONENT

1 10 10^ 10^ lO" CUMULATIVE CYCLES, LOG SCALE

Figure 1. Relationships between the strain experienced by a component and responses of strain gages having mechanical

multipliers of 1:1. 10:1, 15:1 and 20:1.

(Taken from reference [1], figure 7.)

STRAIN-TIME DATA

^Figures in brackets indicate literature references at the end of this presentation.

Figure 2. A typical strain-time response for a component.

load-range history that the part has experienced. For

example, the strain-time sequence shown in figure 2 can

be reduced to the form shown in figure 3, which is a "cumulative distribution" of the strain versus the "cu- mulative number of cycles" that produced that strain. Figure 3 also shows the laboratory verification of the method through a comparison of the actual load his- tories with the load histories estimated by this rather simple technique. The good agreement is obvious. The

correlation between the estimated and recorded load

histories then was made for a suspension component af- ter 50 and 100 circuits on the test track. The results are shown in figure 4. The estimated load history derived

from the S/N gage readings compares quite favorably

with the actual strain history that was recorded during one circuit on the track. While this technique appears

to hold considerable promise for developing a realistic measure of the loads that structural components have

experienced in service, there will be a very long period

of data taking before the actual performance envelope

for many components can be reasonably established. The second method is a response to the question, what

are the localized strains that are present in various com200
COMPARISON OF ACTUAL AND ESTIMATED LOAD HISTORIES LABORATORY TEST

t LOAD

LBS

10' 10' CUMULATIVE cyclesFigure 3. Comparison of actual and estimated load histories for a laboratory test sample.

Cumulutive load is plotted as a function of the cumulative cycles thai produced the strain. (Taken from reference [1], figure 16.)

COMPARISON OF SUSPENSION COMPONENT LOAD HISTORIES - PROVING GROUND TEST

(S/N GAGE ESTIMATION VS. ACTUAL PROJECTED STATISTICALLY)

300 r

250

210 200

STRAIN

^111 IN 100

50

0

STATISTICAL PROJECTION OF ACTUAL

100 CIRCUIT ^^»T^50 CIRCUIT ~

. r CIRCUIT^> ^^CTUAL

S N GAGE ESTIMATION

'^v^,^ 50 CIRCUIT

li - 44 1

in

'"^^^^^^^^^^^CmCLHT^^^^

10 10' CUMULATIVE CYCLES

Figure 4. Comparison of suspension component load histories as estimated by S/N gage and as projected statistically. Data accumulated from proving ground test. (Taken from reference [I], figure 19.)

ponents that are subjected to "typical" usage? This

problem is approached, as shown in figure 5, by attach- ing strain gages to the component of interest and record- ing the output of the gages when the vehicle that incorporates this part is operated over a "typical" test cycle. The surface conditions of the test track have been

determined through experience to subject the vehicle to a reasonably good approximation of the varied stresses that a vehicle will experience in normal service. The

recorded strain dependence, as is shown in figure 6 for three different components, is then translated into a control algorithm that allows a computer to simulate the track conditions in the laboratory and subject a sample

to a strain history that will duplicate that seen by the part in service [2]

Figure 5. Component shown with attached strain gage.

l-SUSj'ENSION LOAD

"ifi'"rinT-iw!iiirii|-

L.. Uil J. Kill -J

r+-r

Figure 6. Strain dependence as a function of time for three components as measured during a vehicle test.

An expanded portion of the strain history of the

central trace, as taken from the component on the test track, is reproduced in figure 7. This is the strain sequence that is duplicated in the laboratory. The rela- tionship between the stress and strain is far from linear, as can be seen by a comparison of the strain sequence

with the stresses needed to impose the observed strains on the test sample shown at the bottom of figure 7. The

determination of such complex stress-strain responses provides an essential input into subsequent cumulative

201
FATIGUE ANALYSIS PROCEDURE

S-60

Figure 7. Determination of stress response of specimen shown

at left during complex strain sequence.

(Taken from reference [2], figure 2.)

MATERIAL

PROPERTIES

MECHANICAL

ENVIRON.

MECHANICS

ANALYSIS

CUMULATIVE DAMAGE

ANALYSIS

LIFE PREDICTION

Figure 8. Essential aspects of fatigue analysis procedure.

(Taken from reference [2], figure 4.)

damage analyses and can be accomplished either ex- perimentally, as shown, or analytically if the appropriate cyclic stress-strain properties of the material are known. Clearly, one is interested in the most critical elements of a component in such an analysis, so this technique is most often applied to such areas as those near holes, notches or welds. Not only is the strain spectrum as measured on the track an approximation

of the true effects that will be seen in the field, it is further assumed that a succession of runs on this track segment will subject the part to an accumulated effect that is representative of long in-service effects and,

therefore, can be used to accumulate damage typical of several years of vehicle usage while in the hands of the consumer. There is justification for this assumption,

since experience has shown a good correlation with

performance under average customer usage. It is clear, however, that the width of the performance envelope

that is generated by the wide variety of usages is quite

large. Thus, the accumulated load measurement tech- nique mentioned earlier will give a better measure of the average loads experienced under customer usage

rather than the distribution in the loads that arise from

the wide variety of usages. Even so, these data will then make it possible to more effectively utilize the measured

local strains in evaluating component design. But the development of an improved performance

envelope is only the first step, for the designer must then

translate a desired performance into a design. At this point, all of the material properties that are of importance to the performance of the part must be available

to him, or he must be able to estimate the important

properties of the material from the limited data that may be available. The design procedure must now proceed along the paths shown in figure 8 where a fatigue analysis, lead- ing to a life prediction, is developed from the material

properties, the mechanical environment, and the geometry of the part. The performance envelopes mentioned

above, albeit very imprecise, must be introduced as a part of the mechanical environment to which the part

will be subjected.

Let us look at the problem from another direction for a moment and consider some of the multitude of ways

that a component can fail mechanically. These include

fatigue, after a few cycles and after millions of cvcles, fracture, both brittle and ductile, creep, gross yielding,

and buckling. The loading mode, type of stress, and

operating temperature strongly influence the type of material that can be used in a given application. Further, it must be expected that the material will deteriorate in use. It wears and galls; it oxidizes and corrodes; and it changes its metallurgical structure through diffu- sion and grain growth. With all the information avail- able on material properties, it seems incongruous to assert that the absence of suitable material properties

presents one of the serious deficiencies in developing an

adequate life prediction for the part. But, unfortunately,

this is true. The most easily determined properties are

often not the most useful. This situation can be illus- trated by considering fatigue analysis in a bit more

detail. Those properties of most direct bearing to a fatigue

analysis are the cyclic stress-strain response of the material. These are much more significant for life prediction than are the more commonly available monotonic

relations. Also needed for the damage analysis are material fatigue-life curves. Examples of each of these types of data curves are shown in figure 9 for a low

carbon steel that is used prominently in the vehicle. For

the design engineer, however, these curves must be sup- plemented with the sort of detailed specifications listed in figure 10. We have collected all of this type of data

that we can find into a Materials Handbook for use by

the design engineer. By way of emphasizing the limited extent of the information available, the present handbook contains data on only 41 steels, 7 cast irons, and 8

aluminum alloys.

As the design engineer considers the requirements

for efficient energy management in a structure, require- ments that arise from the need to protect the vehicle and occupant against damage and injury during a col- lision, it is necessary to know whether the various properties of materials depend upon the strain rate. The

dependence of the yield and the ultimate tensile strength as a function of strain rate is shown in figure

202
Condi tion: oar (I200»F) - 225 BHN

HATERIAL CHARACTERIZATION SHEET —

Material: SAE 1045 Sptcliren lltf. Mo.: IX

CofKtltlMi: oar (IZOO'F) - 225 BHN Test Caid. : R.T.

Honotonic Properties: Cyclic Properties:

Nod. of Elast. . E 29.5 x lo' kst Yield Strenqth. 0.2t Sj.' 60 ksi

Yteld Strength. 0.21 S 92 ksi strain Hard. Exp., n' 0.18

Ultlmte Strenqth. S 105 ksi Strenqth Coeff.. K' 195 ksi

Red. in Area. % RA 65 Fatique Strenqth Coeff.. c^' 178 ksi

True Frac. Strength, 178 ksi Fatique Ouctility Coeff. . t,' 10

True Frac. Ductility, 1.04 Fatique Strenqth Exp. . b -0.095

Strain Hard. Ew.. n 0. 1 3 Fatique Ductility Exp., c -0.66

Strenqth Coeff. . K 166 ksi Transition Fat. Life. 2llj 10,000 rev

True Tou*ne5s. U 165,600 in-lb/in'

Convents

CHEMISTRY

C Mn P S Si Cu M Cr Mo

.47 .77 .007 .035 .23 .08 .02 .02 .01

HEAT TREATMENT

AUSTENITIZED : \500°f/ 30 MIN., BRINE QUENCH, TEMPER : I200»F/ 1 HR.

source: W. Landgrof (7) om: I/T'O

Microstructure: TEMPERED MARTENSITE

Figure 10. Properties of SAE 1045 steel. (Taken from reference [3].)

11 for two high-strength low-alloy steels. A strain rate of 5 X 10^ in/min is equivalent to a 50 mph strain rate. By contrast to the behavior shown for the HSLA steels, the aluminum alloys tested to date show essentially no

strain rate effects. Data are available for only a very few

materials in the high strain rate regime.

When we look beyond the usual metal alloy systems

to those materials that are finding increased usage (for

_ HIGH STRENGTH LOW ALLOY

I I I I I I 1 1 .01 0.1 1.0 10 100 1000 10,000 100,000

Strain Rote, min''

Figure 11. Yield and ultimate tensile strength as a function

of strain rate for two high strength, low alloy steels—YST

50 and YST 80.(Taken from reference [4]), figure 5.)

example, plastics and elastomers) , we find even less data on materials of direct interest, and less fundamental understanding of properties that can be used to make reasonable extrapolation of material properties.

Phenomenologically, many plastics behave similarly to metals [5]. For example, the general response of ductile polymers to cyclic strain is a cyclic softening. There are, of course, distinct effects of the microstructure on the

details of the softening mechanisms; for example, single

phase structures, such as polycarbonate, tend to exhibit an incubation period before softening, whereas multi- phase microstructures, such as nylon, show immediate

203
softening with no incubation period. But for the highly

heterogeneous systems that we use so frequently, for ex- ample the rubber-modified plastics and the glass- reinforced plastics, an overall cyclic softening occurs which continues throughout the life of the material. Figures 12 and 13 present the comparison of the monotonic and cyclic stress-strain relations for glass- reinforced nylon and ABS, respectively. Although these curves have the same general form as are observed for most metals, the magnitudes of the effects differ by an

order of magnitude or more. The maximum stresses in

plastics are substantially less than for metals, whereas

the corresponding strains are much larger than occur

for metals. The temperature effects are quite important

for both plastics and metals. What about fatigue failure of the plastics? Figure 14

gives a comparison of the fatigue life curves for pure

nylon and for nylon loaded with 13 percent and 33 per- cent glass fiber. The form of the upper curve is characteristic of the fatigue of those polymers that do not

craze. The shape of the lower two curves is characteristic of polymers that craze. In the heterogeneous polymer

0 2 4 6 8 10 12 STRAIN (7o) Figure 14. Fatigue life data showing the effects of reinforcing

glass fibers (chopped) on the fatigue life of nylon at 298 °K. Figure 12. Cyclic and monotonic stress-strain curves for ABS

at 13 percent glass fiber reinforced nylon at 298 °K.

(Taken from reference [5], figure 22.)

6

5

^ 4

^^^^ Tensile

^ ^Cyclic

ABS

1

1

298 °K

1

1 1 0 2 4 6 8 10 12 STRAIN (%)

Figure 13. Cyclic and monotonic stress-strain curves for ABS

at 298 "K.

(Taken from reference [5,] figure 23.)

systems, the added material, such as the glass fibers, produces an effect on the mechanical behavior that is equivalent to crazing in a homopolymer. It appears

that any perturbation that disrupts the bulk flow of

the polymer and promotes a localized accommodation

to deformation can be expected to introduce an abrupt

transition in the fatigue response at short lives. There is no question but that the fundamental understanding of the response of heteropolymer systems to mechanical

stresses is inadequate and deserves much more attention.The strain rate dependence of the glass-reinforced

plastics is much greater than that of any of the metals

studied thus far. The dependence of the ultimate tensile strength upon strain rate is shown in figure 15 for two

types of commercially available glass-reinforced plastics. An 80 percent increase in the ultimate tensile strength occurs over the seven orders of magnitude

change in strain rate. When we turn to elastomers, the material problems

become even more complex. Figure 16 is a plot of the number of cycles to failure against a parameter that is directly related to the strain energy. These data suggest

that fatigue failure is essentially a cut growth process

that takes place at small flaws or stress risers that are present in the material. The initial size of these flaws is estimated to be about 2.5 x 10"^ cm. Such cracks are

not readily visible to the eye and will not become so

204
10 100 1000 10,000 100,000

strain Rate, min"'

Figure 15. Ultimate tensile strength as a function of strain

rate for two commercially available glass-reinforced plastics. (Taken from reference [4], Figure 8.)

Life N

cycles

lO

I

t

• •

1 r 1 r

I •

I 2 3 4 5 6 7

Maximum Extension Ratio ^

Figure 16. Cycles to failure as a function of strain energy for die-stamped dumbbell test samples of vulcanized natural rubber.

(Taken from reference [6], figure 7.)

until about 90 percent of the useful life has elapsed,

in agreement with the general observation that failure may be imminent when the deterioration of rubber com- pounds becomes visible. The combined effects of mechanical stresses and the atmosphere must be carefully considered in specifying

rubber components. This is illustrated for unprotected

natural rubber in figure 17 where the crack growth is seen to be dependent upon both the cycling rate and the

presence of oxygen. The addition of an antioxidant to the material removed much of this frequency dependence.

crack growth

per

cycle cm

10* 10' 10' 10' tearing energy erg cm'^

Figure 17. Effects of frequency on mechanics-oxidative crack growth in an unprotected vulcanized natural rubber measured in laboratory atmosphere : (X)IOO cycles/ min.; (0)1

cycle /min.; (*)0.0l cycle/ min.

The solid curve was measured in vacuum at (o)lOO cycles/min. Broken line

is the same as the solid but displaced by a factor of 2.6. (Taken from

reference [7], figure 16.)

I am compelled to observe at this point that it is an

unusual design engineer who understands all of the key variables in such diverse materials as metals, plastics and rubbers well enough to be able to start from a design concept and arrive at an optimal design without

a long series of "prove-out" testing. Just to emphasize

again the point made by Harry Paxton in this Symposium, this is particularly true if there are no historic data available on a similar component.

There are many areas in which data are needed but

are not adequate. A better characterization of the engineering properties of the more complex materials must

be developed. An understanding of failure phenomena

in these complex material systems is of vital importance.

And this is particularly critical where we are engineering new materials. It seems to me that this represents a genuine opportunity for the science and engineering

communities to cooperate in satisfying these needs. I submit that the engineering community needs more sup- port from the materials engineers and the materials

science community if it is to successfully meet the demands of the consumer for increased reliability with

increasingly complex systems. But as you well know, the choice of material, the de- velopment of a design, and the testing of the prototype

components are only the first aspect of the design process. The component has to be manufactured, with per- haps a number of different companies involved, and

205
then assembled into the system. Each of these steps presents opportunities for improper manipulation with a

resulting reduction in overall reliability. The designer is confronted with a particularly difficult task when it comes to considering process specifications. It is usually

preferable to specify performance rather than process,

for in this way the processor will be able to exercise much more initiative and will produce a better product

if he is given the maximum amount of freedom in de^ veloping a process that will yield a part with the pre- scribed properties. But still the designer has to be able

to control the subtle differences in material properties

that can result from even minor changes in the processing. This is precisely the reason why the process validation procedure is so stringent and can lead to what some people view as a reluctance on the part of the automotive industry to readily accept new processes. A

new process can be introduced only when there is sufficient confidence in it to be certain that it will not adversely affect the reliability of the product. Establish- ing this may be a laborious process. Process variability,

quality control, the statistics of sampling, the relation- ship of testing to field failures and the characteristics of the failures must all be established. When this is being done on a new desigft for which no field history

exists, the problem is greatly compounded.

Although performance specifications are the most

desirable from the processor's point of view, the sensitivity of some material properties to the process steps is so great that detailed processing specifications must be

followed in some cases. As an example, I need only mention the importance of the close control that is needed to eliminate the possibility of hydrogen embrittlement on

electroplated components. The ASTM has issued a detailed recommended practice for the preparation of high

carbon steel for electroplating with a minimum of hydrogen embrittlement. This ASTM standard proceeds to note that "the test for the effectiveness of the procedures

used to control embrittlement lies in the subsequent ser- vice use of the material." With no known rapid test for hydrogen in the part and no really adequate means of characterizing hydrogen embrittlement failures, the de- signer has no recourse but to specify the process steps.

I will not dwell on the effect of the assembly operations upon reliability. A general build-up takes place

from the assembly of individual components into sub- assemblies. The final assembly operation then combines

these into the complete vehicle. Testing and quality control are carried on throughout this operation in order

to minimize assembly errors. The designer, however,

has the task of developing his component and sub- assembly designs such that it minimizes the opportunity

of inadvertent damage to the part during handling and

reduces the possibility of having an improper connection, alignment or fixturing of the part. Obviously, the

reliability is affected by the complexity of the final assembly.

What I have tried to indicate here is the breadth and

complexity of the problem that confronts the designer

as he attempts to consider the modes by which the component can fail. In an effort to help him prioritize the tasks that face him in his attempts to reduce the fail- ures, we have formalized this process into what we call

Failure Mode Effects Analysis (FMEA). This is a sys- tematic approach to assessing the probability of occur- rence of a failure and the effect that such a failure will have. Field experience is a great help in assessing fail- ure modes, for such data are derived from systems that are in the hands of the consumer. But FMEA is particularly important for a new design where a particular

component or part has never been evaluated by the customer and for which field experience is not avail- able. In its simplest terms FMEA consists of describing

the anticipated failure mode, the effect of the failure in terms of customer reaction, and the cause of the failure. The designer then proceeds by giving a numerical esti- mate to each of the following: the probability of occur- rence of the failure, the consequences of the failure (which means the severity of the failure), and the probability that failure will be detected before it reaches

the customer or will give the customer a warning that an incipient problem might lead to failure. These nu- merical estimates, when combined, offer a priority rank- ing among all the various conceived failure modes.

Although this is, admittedly, a subjective process, it does require a careful examination of the trade-offs that are possible in designing, manufacturing, assembling,

using, and servicing the part or system.

Throughout this discussion, I have made mention of the great benefits derived from field experience. While

this is absolutely true, I must point out that the infor- mation channel between the customer and the manufacturer is oftentimes very noisy, appears to have numerous delay lines in it, and frequently has such an impedance mismatch at the ends that it is very nonlinear. The information content is oftentimes highly

subjective, sometimes self-serving to the customer and

may not reflect engineering reality. So we often develop

multiple information paths. For example, we get infor- mation on warranty problems from our dealers; we

support various surveys that ask present, past and, hopefully, future owners what they like or dislike about the product and what difficulties they have experienced with

it; we survey repair data obtained on commercial fleets of vehicles; we collect information on our own fleets; we

try to understand what product strengths and weaknesses affect the used car market; etc. All of this continues for several years for each model. Thus, a lot of data is developed and, imperfect as the individual seg- ments may be, in the aggregate the information has

been used effectively to improve the product. But as an

illustration of the type of short-fall in the data, the time

constant for developing high mileage field histories is, by its very nature, long, and the process of accurately

verifying and correcting assumptions relative to cus- tomer usage is extremely difficult. This brings me directly to the issue of this Symposium—Mechanical Failure—Definition of the Problem.

I believe that the problem for the automotive industry can be rather succinctly stated in the following fashion:

it is impossible now and for the foreseeable future to predict from first principles the absolute rate of failure for a mechanical system as complex as the automobile

when it is subjected to the wide diversity of uses and to irregular maintenance that is characteristic of customer

usage. Further, since the customer has the freedom to

206
utilize his vehicle in almost any fashion, it is impossible

to test against all contingent circumstances. Thus, we

cannot expect to eliminate all mechanical failures. We

can expect to minimize them, however. But in order to accomplish this, we will need a more accurate description of the performance envelope that the vehicle must

satisfy, a much greater understanding of material properties, how these properties relate to mechanical

behavior, and how the behavior is affected by process parameters and environmental effects. We need the con- tinuing help of the universities, the national laboratories, and the not-for-profit laboratories in developing

the proper models of mechanical behavior for materials

that are -not of the ultimate purity, ideal crystallinity,

and of the simplest chemistry. Those aren't the materials we use now nor will we be able to use them in the future

to any extent. And all of these material problems are going to be compounded as we search for alternate materials that can replace those in short supply, as we

search for ways to reduce vehicle weight in order to improve fuel economy, as we search for materials that

will survive the higher temperatures brought on by

many of the emission control systems, and as we try to build vehicles that will withstand crashes at higher and

higher speeds.

And finally, let us not forget that the cost that the consumer must pay for providing any solution must

be commensurate with the effect that it produces.

References

[1] Sfieth, N. J., Bussa, S. L. and Merces, N. M., Determination of accumulated structural loads from S/N gage

resistance measurements, SAE Paper 730139 (Jan. 1973 )

.

[2] Landgraf, R. W., and LaPointe, N. R., Cyclic stress-strain concepts applied to component fatigue life prediction, SAE Paper 740280, (Feb. 1974).

[3] Landgraf, R. W., Mitchell, R., and LaPointe, N. R., Monotonia and cyclic properties of engineering materials. Scientific Research Staff, Ford Motor Company

(June 1972).

[4] Davies, R. G. and Magee, C. L., The effect of strain- rate upon the deformation of materials I—tensile behavior, (to be published in Journal of Engineering

Materials and Technology).

[5] Beardmore, P., and Rabinowitz, S., Dynamic Factors af- fecting the structural uses of plastics, presented at the SAM Materials Symposium on Relationship Between

Structure and Mechanical Behavior of Polymeric Solids, Chicago, October, 1973. Proceedings of the symposium

to be published as an ASM seminar volume.

[6] Gent, A. N., Lindley, P. B., and Thomas, A. G., Cut growth and fatigue of rubbers. I. The relationship between cut growth and fatigue, to applied polymer

science. Vol 8, (1964), p. 455.

[7] Lake, G. J., Corrosive aspects of fatigue. Rubber Age,

p. 30. (Oct. 1972).

[8] ASTM Standard B242-54 (Reapprove.d 1971). See sec.

8.2.

Discussion

E. Passaglia, National Bureau of Standards: You've

concentrated your discussion on desifin considerations

in trying to prevent failures. Yet thei? are two things

right now on all automobiles which, while they don't prevent failures, do indicate the approach of incipient

failures. These are the ammeter and the oil pressure

gage. Are there any thoughts of using predictive devices

of this kind for other critical components in automobiles?

W. D. Compton: Yes, there is a considerable amount

of research effort directed at developing better information transducers for use onboard vehicles. You have

probably found the change from ammeters and oil pressure gages to lights that blink frustrating. That

change was made because many people won't read a

dial. But there are a lot of other indicators under fcbn- sideration such as tire pressure indicators. If electronic transmission controls are employed, undoubtedly there

will be some kind of torque indicator. This would be a

direct measure of the condition of the transmission. As

these kinds of sensors are added, the wiring harness be- comes more complex, and the probability of an electrical failure is increased.

R. E. Maringer, Battelle Memorml Institute: You stated that a service history is of great value to the designer in correcting defects. Are there any serious attempts in the industry to analyze the cause of failure in a catastrophic

accident?

W. D. Compton: We expend a lot of effort in research

activity doing just that. Obviously, it has to be done in

all cases of litigation and it is done in a large number of

others. The service organization does a great deal of

this also. One of the problems is that sometimes a part

or component is so badly damaged that little information can be obtained without an unbelievable amount

of work. The analysis of a failed transmission, which

had not been abused after failure, revealed a problem

with one of our material suppliers. If the owner of the car had abused the transmission by continuing to drive

the car after the transmission failure, this problem probably would not have been discovered.

H. Corten, University of Illinois: You were talking

about your performance envelope. Once you have this,

I am sure it's going to show that there are a few vehicles

that are abused rather severely, a few more that are abused less, and the remainder which are in the normal

usage range. What philosophy are you going to use to decide how much abuse to design for?

JF. D. Compton: The design is based on evaluation of

cost effectiveness. That will get us right into the argu- ment as to whether the governmental regulation should

insist that you design for all possible uses or for average

use. The clean air amendments dealing with the control

of vehicle emissions require the emissions of all auto- mobiles to be below a specified value. It costs the con- sumer a substantial amount to prevent any vehicle from exceeding that value. We believe that this kind of regulation must be approached with the realization that there are going to be statistical distributions and that there should be allowance made for some excess if it^s balanced by the proper distribution below the average. That's a somewhat easier thing to discuss in terms of vehicle emissions than it is in terms of safety, because

207
the vehicle emissions contribute to an overall ambience

whereas safety devices protect the life and limb of one

individual.

A. J. McEvily, University of Connecticut : I was inter- ested in the emphasis you placed on strain rather than

stress determination in connection with fatigue analysis. Does that carry over to the idea that low cycle

fatigue due to abuse at the high level of hitting pot

holes, etc., is controlling and causing fatigue faihires,

when they do occur, rather than high cycle fatigue in which case you might use stress or strain interchangeably?

JV. D. Compton: There is an effort to try to eliminate

the assumption as to what the relationship is between

stress and strain and to use directly in the laboratory

that which is measured on the component in the field. If the stress-strain relationship is known there is no

reason to use one in preference to the other.

/. M. Karhnak, U. S. Army, Fort Belvoir: You mentioned that transducers were built into some components

to give information outputs at periodic intervals. Would

you comment on that? W. D. Compton: These are accumulated load gages. It's simply a use of three or four of these gages in conjunction with mechanical multipliers, so that there are sev- eral levels of sensitivity as you go up the line. The

outputs of these devices can be used to produce a histo- gram of the total amount of accumulated damage.

There's a SAE paper on this subject that is referenced

in my paper.

208
NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS. Gaithersburg, Md. (Issued April 1976)

Implications for Action—Industry

John E. Ryan

General Electric Company

570 Lexington Avenue

New York, New York 10022

This paper will review product trends for industrial and consumer durables; the foreseeable environment of production and use of these products and systems; and the implications for preventive action in this industrial area based upon the growing body of knowledge of mechanical

failure mechanisms and the increasing concern for economic and social consequences of mechanical failures. Key words: Consumer products; marketing; mechanical failure; product performance; product

reliability; product testing.

My position on the program, as a representative of industry, implies that we are the ones who will "put it all together." The public—which is but a collective term for our customers—expects this; the title of my

paper "Implications for Action—Industry" underscores

it; and industry gladly accepts responsibility for achiev- ing integrated, optimized results in the products it supplies. Let's pause and consider what the word "failure"

really means before we charge off on action plans. Running through all the shades of meaning listed in the dictionary is the common thread of being unsuccessful,

inadequate, falling short of the goal. All of these imply

that there is a goal, a bench mark performance, for goodness. Indeed there is, but it is often a largely subjective one in the mind of the user of a product and

likely to be considerably different from one individual

(or group of people) to another. Add to this variability the fact that the bench mark is often a moving target

and you will appreciate that an industrial company

has to be a pretty good marksman to avoid failures of the product or the business. Failure of products, we all know, is of two distinct types: (1) the slow deterioration of performance as in bearing wear; (2) the sudden, often catastrophic break- down of a part or assembly such as occurs in fatigue

fracture. The first type can often be mitigated by proper maintenance but with consumer products—and many

capital equipment products too—users have come to expect a relatively maintenance-free product. So we

marshall all available technology and try to give them

what they want even though we may have difficulty in

communicating to each other an acceptable definition of

reliability. Failures of the second type—the sudden

breakdowns—can, of course, be minimized by careful design but the very word "design" implies a certain kind of usage, a certain intensity of loading. Here's where a manufacturer and a user may have quite dif- ferent ideas—each in all sincerity.

This is all part of what has been called the Space

Syndrome. The consumer says, in effect, "If you (meaning industry) can put a man on the moon, why can't you make a really reliable product for me?" Not only

the individual consumers say this; the big industries and utilities sing the same tune, sometimes with concern

for substantial loss if failure should occur. The reasons behind this attitude are quite understandable. On the one hand, we are becoming increas- ingly dependent upon our machines, both in the home

and in industry. On the other hand, we cannot afford the extra cost of standby equipment, at least not to the extent that we once thought was prudent and justifiable. In a more fundamental sense, such attitudes seem to stem from a growing concern for the rights of the in- dividual. Arthur E. White, in last October's issue of the "Public Relations Journal" calls this phenomenon

"the galloping psychology of entitlement"—the process by which a person's wants and desires become converted

into presumed rights. He points out that this is "indeed

a very old trend, long recognized as part of a world- wide revolution of rising expectations. But in recent years it has accelerated and assumed new political and

institutional forms."

The earlier papers in this Symposium considered

quite thoroughly what implications the facts of mechan209
ical failure have for science, engineering and manufacturing. Trying to figure out what I should cover, I wrote a little equation.

Industry — Science — Engineering — Manufacturing = Marketing + Finance +e

I threw in the epsilon to cover ingredients I might

have overlooked but the more I thought about it, the more I realized that epsilon contained a mighty important entity—general management.

The Marketing function can, and certainly should,

describe in functional terms for the business, what vari- ous segments of their market want in a product and

what value (plus or minus) these potential customers

attach to each product characteristic—at least stochastically. The scientists, engineers and manufacturing men respond, preferably in a give-and-take iterative fashion, while the Finance fellows organize the cost and price consequences of the various alternatives into a meaningful array. Then comes the moment of truth, when the general manager and his functional associates, as a team, choose the course that will be followed. The inevitable trade-offs are made, including those in- volving level of performance and reliability. At these

critical junctures, the voice of the engineering manager

must be clearly heard, not only as a technical witness but as an active advocate. However intuitive such final product decisions may

be, once all measurable facts are on the table, the guiding principle is to maximize the ratio of total value

as the intended customer regards it—to cost, the total cost to produce, sell and use it. Since price can usually

be set in relation to value, at least as the intended cus- tomer perceives value, that all-important difference be- tween price and producer's cost, viz., PROFIT, is the crucial consequences of these general management

trade-offs that I have been talking about. "What's

new," you say, "that's elementary economics." Yes,

you're right; at least in theory, product decisions have

always been based on what the management of the busi- ness believed was a maximum ratio of perceived value

to total cost. Note that I said "perceived"; I mean "as

the customer perceives it." A reputable, ethical manufacturer knows that perception extends far beyond the day of purchase. He intends to stay in business so he

needs repeat business. That means that actual perfor- mance must coincide pretty well with represented—or implied—performance; otherwise the disillusioned cus- tomer will go elsewhere. "Elsewhere" implies that there is indeed a better source. We fervently hope that the time will never come when further improvement is not possible, when no one

ventures to improve lest he fail. However, under the increasing pressure for "more" that exists today, there

is a grave but understandable temptation for manufacturers to weight less heavily such characteristics,

like reliability, that are not so easily or so immediately

perceived and rewarded by the customer. To be sure, such cutting of the corners, either deliberately or through oversight, eventually catches up with those who

play that way but the time lag between purchase and

disillusionment introduces instability in the marketplace and a frustrating distrust and hostility among all parties concerned. We believe the only way to arrest such deterioration is for industrial companies to subscribe to, and live by, professional principles from the top to the bottom of their organizations. In the dic- tionary sense "professional" means "pertaining to a vocation that requires specialized knowledge for its practice." I mean that—and more. As one writer put it: "Professionalism is characterized by obedience to the unenforceable." As manufacturers we must expect our customers to "ask for the moon," so to speak. For the most part, they

have only a general idea about what product characteristics are difficult to obtain simultaneously. Consider

the following opposed pairs, both elements of which a customer might expect in a product offering.

Versatility versus Operational simplicity

Accessibility versus Safety

Compactness versus Serviceability

Special features versus Reliability

Speed or response versus Stability.

These rising expectations in the marketplace, along

with rising costs, mean that manufacturers will have to rely increasingly on the best technology available to deliver durable, reliable products. Those comfortable

"factors of ignorance", euphemistically called "factors

of safety", are going to have to be reduced because of both economic forces and the social pressures for con- servation of natural resources through the more efficient use of engineering materials and energy. We shall have

to use not only more complete and exact theory; we

shall have to do better planned, more truly indicative

testing—and do it more efficiently, in less time.

1

'] 1 _

1 1 1 1 ^ CONlSeqUENlCE OF

TECHnIOLOG-ICAL ACCELtl^ATION

Figure 1.

This is a big order, but you'll see from figure 1 why

it is inevitable. We all acknowledge that technological

developments are occurring at an accelerated pace. Let's quantify that; let's assume that advances in a given

field occur in quantum steps of uniform magnitude but,

210
as I said, at an accelerating rate. These I have repre- sented as dots on a uniform time scale, the time incre- ments decreasing according to the equation:

A„ = Ke-s\

where n is the increment number and g is a positive constant. If a given dot is plotted at the point in time

when it emerges as an established research finding, consider the consequences of a fixed length of time between

that moment and the time when it is embodied in a proven product, ready for sale. Some while ago such a

gestation period would have spanned, let us say, two

technological milestones; in other words, only two as- yet-unapplied units of knowledge would be in the "mill"

of reduction to practice. Contrast this with a later period

in which, perhaps, three such units would be advancing

through the development, design and test sequence. Note the advantages that a fast-moving but sure-footed manufacturer has in such a situation. With proper tech- nical assurance, he can elect to run with a later, more

advanced and presumably more productive technology

and arrive at the market place one quantum jump ahead

of his competitors.

Recognizing that the winner always sets the standard

for the next try, it is obvibus that means must—and

will—be found to accelerate development, design and

test activities. I have chosen to comment mainly on test- ing because it is often the most time consuming, not only for the final product, but for components all dur- ing development and design.

Testing products or components for degradation or

failure in service is so time consuming that efforts have

always been made to accelerate the degradation process,

relative to real life,

(1) by making the cycle rate faster and

(2) by increasing the intensity of the various stresses (temperature, force, velocity, etc.).

Simple as this sounds, it can lead to erroneous results, as every engineer who has lived a while knows.

The main traps I believe, are these:

(1) Increasing the cycling rate may produce second- ary effects, such as higher temperature, res- onances not present in real use, etc. (2) Mutual effects of several applied (or environmental) conditions in real life may be unexplored if the common "one variable at a time"

approach is taken for the sake of procedural

simplicity.

(3) Intensifying the independent variables, singly or in combination.

(4) Extrapolating limited data without knowing, or worse yet, knowing but not applying the avail- able theoretical knowledge of the failure mechanisms at work.

Because linear relationships are eaiser to handle

mathematically than nonlinear ones, we are all too prone to linearize phenomena even though we know that most cause-effect relations are, in fact, nonlinear. But

bcause that nasty reality makes superposition of ex- perimentally determined effects questionable, if not downright wrong, we tend to ignore nonlinearities. Another testing foible, found in academic as well as industrial circles, is to do very thorough and precise

testing on variables for which the testing is easy or where our laboratory's superdeviometagraph can be

employed. Other equally important variables may be

considered only sketchily. We may even be following

meticulously the official test procedure of the ASKY ( !

)

but applying it to a situation that could not have been

anticipated when that hallowed document was written!

I cannot leave this matter of accelerated testing with- out commenting on extrapolating limited data without

benefit of theory. Among the practical engineers of industry there is still a tendency to lean on intuitive ex- perience more heavily than on theory when assessing limited data. Fortunately, theory is gaining in credibility every year, not only with the younger engineers,

fresh from school, but with the older ones as well. The

famous English consulting engineer Urwick, speaking

after years of experience, stated the point very well when he said:

"We cannot do witliout theory. It will always defeat practice in the end for quite a simple reason. Practice is static. It does, and does well, what it knows. It has, however, no principle for dealing with what it doesn't know

Practice is not well adapted for rapid adjustment to a changing environment. Theory is lightfooted. It can adapt

itself to changed circumstances, think out fresh combinations and possibilities, peer into the future."

Fortunately, we have available today some techniques

based on sound theory, that can help us to hew closer to the line in the use of materials while predicting with

quite satisfactory accuracy the risks of failure. The

list includes:

Failure mode and effects analysis

MTBF and MTTR analysis

Decision analysis

Design optimization methods, extended through

computerized procedures

Probability applications of many types.

So far I have discussed only testing in the manufacturer's laboratory, how to accelerate the process without

impairing validity, etc. How about testing in actual service? When a product goes into service, a long and

very real experiment begins, whether we call it by

that name or not. Unfortunately, we all too often do

not take even cursory data on it. True, a given product

unit may be thousands of miles away and we can't ex- pect our customer to take its pulse regularly for our

benefit. But when malfunctions do occur, especially

within the warranty period, we do investigate wifh our own field service people. Resourcefully, these men fix the trouble, write a brief report (no time to communicate more than the highlights!) and go on to the next problem.

Unless such failure reports and diagnoses get or- ganized and classified somehow as they come into headquarters from every direction, much of their value is lost. This need not happen. For more than a decade

211
computers with large memory capacity have been avail- able and, in fact, have been successfully used by many

companies to store, then sift the field service reports so that corrective action, both in theory and practice, can

benefit future products. These companies have dis- covered that, though the significant facts had always

been present in those file drawers of reports, the design

engineers saw only glimpses of field performance until the computer brought its searching and organizing

power to bear. This improved procedure is far from uni- versal today. We in industry who have experienced the

benefits must urge them upon our more reticent fellow manufacturers.

In concluding, I want to say a few words about the

role of government in all of this. Certainly it is proper

for government—federal, state or local—to enact and

enforce laws that protect the citizen's person and property from (1) risks that he, in general, cannot be

expected to detect and understand, and (2) those that he cannot control even if he does know of them and

understands them. But inherent in all regulatory authority is the tendency to prescribe in even greater detail how a certain degree of reliability or safety must be

achieved rather than what is to be achieved. This ten- dency toward the "how" in codes and regulations occurs because it is often difficult to define a result or effect in precise quantitative terms. Yet it must be done or we

in industry who are subject to the regulations will be

caught on one of two horns of the dilemma: (1) We

will have our opportunity to innovate stifled by detailed prescriptions based on past technology, or (2) we will be subject to the whims of bureaucrats as they

interpret such weasel words in the law as '^substantial hazard" and "evidence that reasonably supports viola- tions". Ethical industrialists have nothing to lose and

much to gain by assisting our lawmakers and administrative agencies in making regulatory legislation fair, properly aimed and precise in language. Unless we are

willing to get into the game, we cannot, in good grace,

criticize from the sidelines. As Thomas Jefferson once

said, "For evil to prevail, the only requirement is that good men do nothing."

212
IMPLICATIONS OF MECHANICAL FAILURE

Session VI

Chairman: E. E. Klaus

The Pennsylvania State University

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Provention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

The Implications of Mechanical Failures for

Consumer Product Safety—Vice Versa

Lawrence M. Kushner

Commissioner

Consumer Product Safety Commission

Washington, D.C. 20016

In years past, manufacturing a safe product (rather than one that is simply marketable) for

the consumer was regarded as a nice, but not necessarily imperative, thing for a manufacturer

to do. Recent legislation, however, mandates consumer product safety, thereby putting additional

responsibilities on product designers and engineers and the research scientists who support them. Experience in operating a regulatory product safety program discloses that materials failures, many

of which are mechanical in nature, are an important factor in consumer product safety. A discus- sion will be presented of the implications of the Consumer Product Safety Act for materials

engineers. Some specific examples of mechanical failures in consumer products that have created

serious safety problems will be given. Key words: Consumer product safety; failure prevention; government action; government re- sponsibility; mechanical failure; safety standards

I believe that what I have to say will be both interest- ing and encouraging to you in your efforts to attract the level of attention to mechanical failures—their causes and prevention—that the subject merits. I'm personally sympathetic with any group of scientists and engineers who is trying to motivate its research

in terms of detailed knowledge of real-life problems. For that group of scientists and engineers to be con- cerned also about transforming the increased knowledge

resulting from its research into specific developments

which benefit the public is an additional plus. It is my

understanding that both of these are consistent with the

objectives of the Mechanical Failures Prevention Group.

As far as mechanical failures are concerned, I have a personal scientific interest in the subject which goes back to my days with the Metallurgy Division here at the Bureau. But more recently, my interest in mechanical failures has been heightened by realization of both

the implications of consumer product safety for the study and prevention of mechanical failures and, on the other hand, the implications of success in dealing with

mechanical failures for the safety of consumer products.

Indeed, this interest is more than a personal one. The

agency which I represent today—the Consumer Product

Safety Commission—has acknowledged its interest in the purposes and programs of the Mechanical Failures Prevention Group by its cosponsorship of this meeting

along with NBS and six other organizations.

The principal way in which the national concern for consumer product safety is going to impact on the objectives of MFPG is through the provisions of the Con- sumer Product Safety Act of 1972. The impact is going to be a beneficial one. For those of you whose companies make consumer

products or supply the companies who make consumer

products, this law is going to heighten your management's concern about mechanical failures, and indeed

about any other aspect of product design and perfor- mance which bear importantly on the safety of con- sumers when using their products. For those of you who are not in consumer product

industries, some of the programs that will be set up

under the Consumer Product Safety Act—and the way

in which they will be run—will be of interest and offer the opportunity for useful participation in technical matters of public concern. Let me turn now to a brief discussion of the Act and

detail further its implications for this audience.

In 1972, Congress and the President made a forceful commitment to the notion that consumer products

should be safe. That commitment is embodied in the Consumer Product Safety Act. That Act offers opportunities—and responsibilities—for engineers and scientists. The law heralds a new commandment—"Consumer

Products Shall Be Safe." Its purpose is to introduce a

215
safety ethic into consumer product industries—to put

safety on an equal footing with the traditional concerns

of top management—cost, style and fuction. As a matter of fact, safety may well become a little more equal than the others. It isn't against the law for a product to be too expensive. Nor is it against the law

for a product to be unattractive or not to work. But under the new law, putting an unsafe product on the market can lead to civil penalities as high as $500 000

or criminal penalties up to $50 000 and a year in jail. The Consumer Product Safety Act established a new, independent regulatory agency, the Consumer Product

Safety Commission, which is charged with reducing the hazard posed to consumers by unsafe consumer products. The definition of a consumer product under the law

is broad. The law says that a consumer product is any

article that is manufactured for sale to, or for use by, a consumer in or around the home, at school, in recreation or otherwise. There are some exclusions—products

that are already regulated by other Government agencies. These include foods, drugs, cosmetics, automobiles,

boats, airplanes and firearms. But what remains is impressive, generally estimated at about 10 000 products

including toys, gas and electrical appliances, power

tools, recreational equipment, furniture, clothing, doors,

windows and on and on. The range of regulatory options available to the Commission is also broad. Which one is used in a particular case depends on the severity of the hazard. For "imminently hazardous" products, immediate ban and

seizure are in order. For those requiring less immediate

action but which are serious nevertheless, there are mandatory standards, nonconformance to which is a

violation of law. For lesser hazards one can consider

voluntary standards. And certain kinds of hazards may

best be dealt with not through standards or other regulatory actions but through information and education

programs directed to the consumer. One of the features of the law requires that if a manufacturer, distributor or retailer obtains information

that a consumer product either does not meet an appli- cable standard or has a defect in it and therefore could

present a "substantial hazard" to its user, then that party has an obligation to inform the Commission immediately. The Commission can then require the manufacturer to recall, repair, replace, or repurchase the defective product. The Commission can require that the public be notified through a widely distributed press

release. This is the kind of publicity any responsible

company would like to avoid. This spectrum of regulatory options plus the penalties

that can be assessed should provide effective motivation

for the development of a safety ethic in the consumer

product industries. We should see the development of an environment in industry in which scientists and engi- neers of all kinds can express their latent interest in safety. In this context, the prevention of mechanical failures should surely get high priority attention.

I have been told that before the Consumer Product

Safety Act became law, the top management of many

companies manufacturing consumer products believed

that building safety into their products would make

them too expensive to be successful in the highly competitive consumer product market. My perception now

is that it may be too expensive for a company not to. The Consumer Product Safety Commission has been

called potentially the most powerful Federal regulatory

agency ever created. One of the most imaginative and

amusing descriptions of the Commission is that offered by the syndicated columnist, James Kilpatrick, last year.

It must have been quite a moment for Dr. Frankenstein when his monster got off the table and walked . . . The

Congress might well have put itself in the same fix with

the Consumer Product Safety Commission.

Is the law overkill? After all, we're only dealing with

stoves, toasters, lamps, power lawnmowers, matches,

and other apparently innocuous products. Yet the acci- dent statistics are appalling and do, in fact, describe a

serious problem which merits national attention.

It is generally estimated that there are over 20 million consumer product-related injuries a year which require

medical treatment. These 20 million accidents include over half a million hospitalizations, more than 100 000

permanent disabilities and 30 000 deaths. These are the dimensions of a problem to which industry must now

direct attention. For those of you in industry, the challenge—and the opportunity—are clear. And hopefully the challenge

will be transmitted to the academic community as well. The opportunities for imaginative, exciting, and useful research and development are plentiful. Particularly

when one considers the cost constraints that apply. After

all, this R&D is being motivated not by "cost plus" contracts with the Government, but by the demands of a competitive market—an extra constraint, but an additional challenge.

The opportunity for scientists and engineers goes beyond laboratory R&D. It extends to the development

of effective, technically valid consumer product safety

standards—both voluntary standards under private

sponsorship and mandatory ones being developed for the Commission. The motivation for industry people to be heavily involved in developing these standards is clear. But there is a long-standing complaint that the interests of consumers have not been adequately represented

in the development of even those few voluntary stan- dards that apply to consumer products. We hope that more voluntary safety standards will be developed. But

in order for such standards to be credible to consumers,

there must be technically competent consumer repre- sentatives on the committees which develop them. And

this is equally true for mandatory standards. The basic problem is a financial one. When engineers

from industry or the Government serve on standards

committees, their expenses are paid by their respective

companies or Government agencies. But there has been no one to pick up the tab for technical people to serve on these committees on behalf of consumers. The Commission is going to do two things to help im- prove this situation:

216
We're going to develop a list of qualified, technical people who are interested in serving on standardization committees as consumer representatives. The list will be compiled following a public solicitation ad- dressed to the technical community and will be

available to any standards-developing groups seek- ing qualified technical experts as consumer representatives. More importantly, the Commission will pay the expenses of such representatives in the de- velopment of standards in which the Commission has

a particular interest. This applies to both mandatory

and voluntary standards.

I think you will agree that the Consumer Product

Safety Act provides many avenues through which the technical community can address a whole set of safety- related problems including mechanical failures. Are mechanical failures an important source of consumer product hazards? I believe the experience of the Commission amply demonstrates that they are very important indeed.

I mentioned earlier in the talk the requirement of a manufacturer, distributor, or retailer to report to the Commission any time he has reason to believe that a product in commerce could, because of a manufacturing or other defect, present a substantial hazard to a consumer. In the year that the Commission has been

in existence, we have received well over a hundred

such defect notifications including, but not limited to, bicycles, TV sets, gas ranges, power mowers, cigarette

lighters, air conditioning system controllers, garden

tractors, and even reclining chairs with heaters built into them. In approximately 40 percent of these cases, the hazard resulted from actual or potential mechanical

failures. In some cases it was poor mechanical design. In others, it was improper materials selection. In others,

it was a combination of both. In still others, the haz- ardous products appeared on the market because of a quality control breakdown.

Let me cite a few cases:

CASE A—The product is a heavy duty rotary lawn- mower. The problem is the improper selection of materials for the unit which attaches the blade to the drive

shaft. In this particular case the attachment assembly

became embrittled after welding and subsequent car- burization and heat treatment. The critical part suf- fered brittle fracture while the mower was in use, the blade separating from the shaft. The manufacturer had

to replace the defective parts on approximately 450

production units that had already left the factory

some of which were in the hands of consumers.

CASE B—A ride-on lawnmower. Because of improper design of some spiral pins which fastened gears

to shafts in the steering mechanism, under repeated

high load stress reversals, the pins worked their way

out and steering control was lost. Over 4,500 mowers

were affected. All of the known purchasers of these

units were contacted by phone or letter and provided

repair kits at no cost.

CASE C—Defective design in a bicycle fork stem

involving over 200,000 bicycles. Corrective action is being undertaken by the manufacturer.

CASE D—Another ride-on lawnmower—this one

presenting the possibility of failure of a brake cable due

to improper design and materials selection. Over 6,000

production units are involved. The company is identi- fying all of the customers and sending service bulletins and repair kits.

As a final item let me quote from the Product Safety

Letter, a newsletter which covers the consumer product

safety scene in the regulatory agencies in Washington.

This is from the May 6, 1974 issue.

Schwinn recalled 70 000 bicycles to replace imported

pedals after spindles fractured due to faulty heat

treating.

Mechanical failures are a very important problem in consumer product safety. The costs to manufacturers

of taking the corrective action required by the Commission are high. Not only is one concerned about the immediate, out-of-pocket dollar cost of the new parts

and labor, in the long run the unfavorable publicity

that inevitably accompanies a manufacturer's recall may be far more costly.

One of the important components of any program

to reduce mechanical failures is the development of good nondestructive evaluation techniques. Let me give

you a specific example of their importance to the Commission.

The Commission has been involved in rather extensive debate about the incorporation of sampling plans

in its mandatory standards. The debate is not yet over. The sampling plans that are the subject of this debate would set the sampling procedures which a manufacturer must follow in testing his product for conformance to the standard. One side of the argu- ment holds that sampling plans are needed in those

cases in which the test embodied in the standard is de- structive or, for one reason or another, 100 percent

testing of all production items is unfeasible. A sampling

plan in a mandatory standard would acknowledge the

practical fact that short of 100 percent testing, no pro- duction process can be completely foolproof and that in any production batch there is some expected fraction of defective products. In other words, putting a sampling plan into a standard makes the standard applicable to a production batch or a production lot rather than to an individual product.

Now this position is great as far as manufacturers

are concerned. And the use of sampling plans makes'

sense for those consumers such as DOD, GSA, or in- dustrial purchasers who buy large quantities of a particular product and are willing to accept a statistical statement of quality. But on the other hand, how about the individual

private consumer who buys only one item and not an

entire lot? If he gets one of the defective products,

217
the likelihood of which the standard acknowledges if

it has a sampling plan built into it, what protection

has that consumer been given by the mandatory safety

standard?

The answer to the dilemma is the development of nondestructive evaluation techniques which can be in- corporated in the production process and could give

the manufacturer confidence of 100 percent compliance, even though he may not have subjected 100 percent

of his production to the proof tests in the standard it- self.

It shouldn't be necessary to lower the level of protection afforded by a standard because of inadequacies

in the technology of nondestructive testing. You people

are in a position to do something about this. In closing, let me point out that the real purpose

of the Consumer Product Safety Commission is not to issue regulations. It is not to levy fines, nor put people

in jail. Our real purpose is to get manufacturers—and

indeed all of those who are involved in the sequence

of events from product design through production and

distribution—to think product safety. In my view, this is an area in which technology that has been developed for sophisticated space programs

and military programs can be converted to be useful

in the consumer product industries. The challenge is to do it at a price that manufacturers and consumers

can afford. The threat of regulatory action in the consumer product industries is now so real that an environment

conducive to achieving the objectives of the Mechanical Failures Prevention Group should exist. I hope you

will take advantage of it. The safety of consumers would be greatly enhanced

by your success.

Discussion

/. E. Ryan, General Electric Company: I would submit

that sampling techniques have long been established as being the best assurance within a cost benefit frame

that we can get. The fact that the DOD has accepted

sampling, after much study, as the best cost effective way of handling the quality of weaponry signifies that

it is technically sound. To use the case of the DOD,

consider the soldier who goes into the jungles of Viet

Nam with an M16 carbine and because of the sampling

plan there is a remote chance that his gun won't fire, so he gets killed. Are we going to try to shield our consumers beyond the point to which we would shield our soldiers?

L. M. Kushner: The fundamental point here is the

difference between the DOD as a consumer and the individual consumer as a consumer. The DOD as a consumer is buying millions of rifles and, as I mentioned in the talk, it's reasonable for them to accept some statistical statement of quality control. I don't think that that is necessarily a reasonable thing for the individual consumer. Now I recognize that in instances

in which the only test available is a destructive test, and 100 percent testing is 100 percent destruction and

complete safety, 100 percent testing is not reasonable.

I don't think there is a simple answer to the question.

I think the way the Commission is ultimately going to come out here is eminently intelligent. We will use sampling plans where they have to be used and try

to avoid using them where they don't need to be used. One of the ways that you can avoid having to use them is to have the necessary equivalent nondestructive testing that can be incorporated right into the production line so that every single item is ,s^b- jected to the nondestructive test which, if passed, could

give the manufacturer essentially complete assurance

that the item would pass a destructive test if one needed

to be performed.

/. E. Ryan: I will defer to the experts on non-destructive

versus destructive testing. But I would rather take a

destructive test sample plan than to temperize with a supposedly equivalent non-destructive test which in- evitably, if it is to be more than a token, will greatly increase the cost on the line.

L. M. Kushner: As I mentioned, the challenge is to do

it at a cost that people can afford.

H. Corten, University of Illinois: In looking at products we see a spectrum of strengths and a spectrum of loads and you have been talking primarily about the removal of defects in the spectrum of strengths. How

will you decide what is a reasonable spectrum of loads and when something has been "used" as opposed to "abused" in judging failures?

L. M. Kushner: We will try to write into the test methods incorporated in the standard a range of loads

that is reasonable to expect in terms of normal use

as the product ought to be used and ways in which we can reasonably expect that the product is going to be misused or abused. I would put the same responsibility on the designer and the manufacturer when

designing a product. They must take into account that the product will probably in most cases end up being

abused or misused as many times as it is used properly.

One of the messages that we are trying to get across

to product designers is that it isn't going to be sufficient to make a product which is safe only if the instructions are followed very carefully, because it is known ahead

of time that instructions are not going to be followed very carefully.

/. Rabinow, National Bureau of Standards: Returning

to the subject of testing, isn't it a fact that if you do

100 percent testing, there is still no assurance that, in practice, the product will not fail? For example, in testing bottles, each bottle may pass the test, but some may still rupture later. In fact, the testing may in- crease the possibility of rupture. There is no guarantee

that 100 percent testing will guarantee 100 percent

reliability.

L. M. Kushner: I'll have to accept that, particularly

if the test could weaken the product.

218
C. G. Interrante, National Bureau of Standards : Many

of the primary metal producers have equipment operation poHcies that involve extensive safety programs. The principal benefit from these programs is lower

accident rates. These programs have been effective to the point that it is claimed to be much safer for the husband to work in a steel mill than it is for the wife

to be at home. Are there plans to make these kinds

of programs available to consumers?

L. M. Kushner: Absolutely. Information and education

programs are an important part of the total range of options that the Commission has to deal with particular

subjects. Of the 20 million accidents that occur each

year, only 20 to 25 percent of them could be eliminated

by a change in the product. We're sufficiently sophisti- cated to understand that. In order to make dramatic

inroads, that is approaching a 50 percent cut in acci- dent rates, there is a tremendous educational and in- formational responsibility requirement.

C. G. Interrante: Do you have any idea why the media hasn't undertaken such educational programs?

L. M. Kushner: The news media is simply not really an educational device as such; its primary function is presenting news. Banning a product is much more news- worthy than telling people how to use it effectively and

carefully.

W. R. Bozman, National Bureau of Standards: How

do you feel about the safety of products which are used in a way that the product was not originally in- tended to be used? For example, the small shelf on a stepladder that is designed to hold a bucket of paint

is frequently used by people to stand on. There may

be a sign on the ladder warning people not to step on

the shelf, but they will do it anyway.

L. M. Kushner: Product misuse such as that is foolish,

but on the other hand it's to be expected and that is the situation that the Commission is trying to cope

with. We think that manufacturers have a responsibility

to deal with this problem too. Misuse is often predict- able and in some cases a product can be designed to anticipate that kind of misuse or abuse. It's also true

that the intelligent consumer is going to have to pay

a little bit more because of a concern for the proclivities of the unintelligent one.

W. D. Compton, Ford Motor Company: What is your

thinking about self-certification by industry in terms

of compliance? Do you foresee establishing fixed nu- merical standards or fixed performance criteria that the manufacturers will have to certify having met be- fore a product can be sold much as the clean air amendment now requires in the emissions area?

L. M. Kushner: The law does require that products

subject to a mandatory standard of the Commission

must be certified by the manufacturer as meeting those standards. I have no objection to self-certification in a mandatory program because there are substantial sanctions for violating the standard. If we find a violation, we can take people to court and we can impose penalties, so there are real incentives for everything being on the up and up. On the other hand I am a little bit more leery of the use of self-certification in voluntary

programs because there are no sanctions to give people

the assurance that there are sufficient motivations for

the companies to be totally honest in the way the

certification programs are used.

W. D. Compton: What would be the result in a certifi- cation program where the product was properly certi- fied and approved by the agency but then was found

to be faulty in the field? Is that still the manufacturer's

responsibility ?

L. M. Kushner: The Commission's action would depend

on a whole set of circumstances other than just the

specifics that you identified. Conformance with a stan- dard, as far as private liability suits are concerned, does

not grant any immunity in the courts.

R. F. Scott, Jr., Travelers Insurance Company: Are

there any mandatory standards in effect at this time?

L. M. Kushner: Yes. There are no standards that have

been issued under the Consumer Product Safety Act

itself, but the Commission has been given responsibility for several older laws. The three most important of these are the Flammable Fabrics Act of 1953 as sub- sequently amended, the Hazardous Substances Act, and the Poison Prevention Packaging Act.

R. F. Scott: But you have not propagated any new

standards of a mandatory nature?

L. M. Kushner: That's right, because, under the law, there are requirements in terms of the kinds of pro- cedures under which mandatory standards can be de- veloped for the Commission. We have taken a long time

in writing regulations and procedures because we're

trying to encourage many new kinds of groups to get involved in the sponsorship of standards development.

Regulations have recently been published in the Federal Register and we are now in a position to undertake

the development of mandatory standards. We will be

doing that for architectural glass, power lawn mowers,

matches, electrical extension cords, and one or two

other products.

G. McPherson, Jr., University of Missouri, Rolla: Are

you empowered to do research, to develop standards

and perhaps develop non-destructive testing techniques,

and is the Commission going to do the research or is

it going to sponsor research in companies and universities?

L. M. Kushner: We have authority to conduct research and development programs related to the mission of the Commission. We have our own technical staff that by and large spends its time fighting fires. We haven't

yet formulated a long term R&D program, but when we

do, we will be seeking proposals from the entire tech- nical community.

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NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 423, Definition of the Problem, Proceedings of the 20th Meeting of the Mechanical Failures Prevention Group, May 8-10, 1974, held at NBS, Gaithersburg, Md. (Issued April 1976)

Mechanical Failure—A Material Matter

Richard W. Roberts

Director

National Bureau of Standards

Washington, D.C. 20234

The Federal Government has long been concerned with minimizing mechanical failures. This concern has come about for two reasons. First, the Federal Government purchases and operates

a great deal of equipment, much of it, as in the space program and in the DOD, equipment of extreme technological sophistication. The Government, in purchasing, using, and maintaining this equipment, has often led the way in devising new approaches to mechanical reliability. The second

reason involves areas of broad public concern, such as safety. Here, Congress has granted Govern- ment agencies the authority to set mandatory standards that are designed to prevent mechanical

failures in the private sector. It is clear that in the future both these activities will be continued.

In addition, several current national trends call for enhanced concern with mechanical re- liability. These trends are related to the conservation of natural resources, the continuing need

for new methods of improving product quality and maintainability in the face of rising labor

costs, and the growth of new technologies, such as nuclear energy, requiring a new plateau of operating reliability. These trends are analyzed in terms of future government policy and future modes of in- teraction between the government and the private sector. Key words: Conservation of material resources; failure prevention; government action; mechanical

failure; mechanical reliability; safety standards.

1. Introduction

Man is a great user of materials. With them he has

created homes, offices, factories, cars, television, in fact, civilization as we know it. Creating, building, using are

constants of the human condition; differences are merely of degree. While one culture constructs its dwellings of adobe and another uses steel and concrete, the intent is the same. Take away materials and our

grandest plans and projects become meaningless. Take

away materials and most of mankind would disappear.

We use enormous amounts of material, and the con- sumption rate is accelerating. In 1950 we used 1.2 million metric tons of aluminum; in 1970, over 4.5

million metric tons. The 1950 use of nonmetallic build- ing construction materials was 500 million metric tons, contrasted to 1600 million metric tons in 1970. To put

it on a more personal basis, about 18 000 kilograms

(40,000 pounds) of new materials are used each year

for every American. This figure includes 4000 kilo- grams of gravel and sand, 350 kilograms of cement, 550

kilograms of iron and steel, 7 kilograms of lead, 3500

kilograms of oil, and 2300 kilograms each of natural gas and coal. In terms of products, we used up and

threw away last year 100 million tires, 7 million cars and trucks, and 7.6 million TV sets. Not only is gross materials use rising, but there is a constant demand for new and better materials. For

example, the search for energy alternatives is creating the need for high temperature, high strength ceramics

for use in higher efficiency turbine generators. Engineering data on such new materials are needed in a hurry, as are reliable ways to fabricate them and eval- uate their expected performance in service.

2. Mechanical Failures Do Occur

With the production of millions of products and

structures, I don't find it surprising that some are subject to mechanical failure. Almost no system is foolproof. Rather I find it surprising that so many

things work so well for so long. This is certainly a credit

to American technology. But mechanical failures do

occur, and our goal should be minimization of their frequency and consequences. Mechanical failures are nothing new. The old nursery rhyme "London Bridge is Falling Down" is really a piece of history, celebrating the slow degeneration

of that famous span. The cause of the London Bridge

troubles is lost in antiquity, but when the Ashtabula

Railway Bridge collapsed in 1877, killing 90 persons,

the fault was traced to insufficient knowledge of the wrought iron used in its construction. The fall of the

Silver Bridge into the Ohio River in 1967, with a death

toll of 47, was another case of mechanical failure, initiated by stress corrosion cracking.

221
There have been other infamous cases of mechanical

failure, of course. The breakup of Liberty Ships during

World War II, the failure of Comet Aircraft in the

1950's, and the recent rash of gas pipeline ruptures,

remind us that we live in a mechanically imperfect

world. And as we build larger structures, the potential for disaster is magnified. For example, a new class of barge has been designed to carry 42 thousand cubic metres (11 million gallons) of petroleum products. Last winter one of them broke apart and sank in New

York harbor despite having met approved construction

specifications. Luckily the tanks were loaded with water at the time, or we might still be cleaning up the waterfront.

3. Consequences of Failure

There are, of course, varied consequences of mechanical failure. Death or disability can be one tragic

outcome, causing anguish to the victims or their sur- vivors and often leading to lawsuits for the recovery of damages. The assessment of responsibility and liability

is a complex problem, one that I shall expand on later in my remarks.

Economic loss is also associated with mechanical

failure, and sometimes this can be a double-action

effect. For example, when a machine on the assembly

line breaks down, there is not only the cost associated with repairing or replacing it, but also the cost of lost productivity.

Another consequence of mechanical failure is that of materials waste. When an item breaks down and we

replace it, a finite amount of irreplaceable raw materials is consumed. This aspect of the problem will be

reduced as we improve our recycling ability, but we've

got a long way to go in this area. There are other con- sequences associated with the "fail-discard-replace"

cycle, such as the energy used and the pollution created

in producing an item, and the problem of disposing of

it if the materials are not recycled.

A final consequence of mechanical failure is that of frustration. Almost everybody has experienced the

flash of anger when the family chariot burns out a bearing on the Interstate, or the local repair shop tells you the central air conditioner will be out of action

all summer because compressors are back-ordered at

least 3 months. Not only is this frustrating, but it also leads to disillusionment with the system. When things

break down, when repairs are slow, difficult, costly, or impossible, some people turn off from technology.

And I can't always blame them.

4. Government Concern

I think we all agree that failures do occur, with consequences ranging from major disaster to minor

inconvenience. Why is the Federal Government con- cerned? There are a number of reasons. First, the Government has certain missions that, because of their nature or sheer size, are clearly its special responsibility. For example, providing for the common defense,

as spelled out in the Constitution, is a unique function

of the Federal Government. Today this means designing,

building, and maintaining the most modern and so- phisticated machinery that technology is capable of producing. It also requires some assurance that the machinery will perform efficiently and without failure. The Government also is the prime mover in the fields of atomic energy and space exploration.

The requirements in defense, space, and atomic

energy have influenced the science of failure avoidance

in the areas of fracture control and nondestructive

evaluation. The very high strength materials that have

been developed for aerospace construction, and the thick section steels developed for the pressure vessels of the AEC, have in common their susceptibility to the presence of cracks. The need for designing against

failure despite inherent flaws gave a great impetus to the science of fracture mechanics, and much was

learned about the behavior of brittle materials that perhaps might not have been learned, or certainly

would have been learned more slowly. With the added

impetus of the troubles with the F 111 and C5A, the Air Force now writes fracture control specifications into the requirements for aircraft. In a parallel effort, pres- sure vessels for use in nuclear power plants are sub- jected to a rigorous scrutiny for cracks. The zero

defects program of NASA owes much to fracture control methods. These three developments added great

impetus to an emerging new science, that of fracture

control design.

5. Failure Prevention and NDE

V Failure prevention need not be based on the statis- tical probability of failure alone, but on the concept

that the life of a given piece of equipment can be ac- curately predicted if the use of it can be specified, and

if the flaws in it can be determined. Both of these are achievable aims, and I view this development as having

far reaching consequences for the science of design not only in equipment for the military, for space, and for nuclear energy, but for the private sector as well. Fracture control design and fracture proof manufacture is an emerging discipline that will assume ever more

importance.

A necessary partner to the concept of fracture control design is the location of flaws in a structure through

the techniques of non-destructive evaluation (NDE).

Beginning with early work on radiography at Water- town Arsenal in the early 1920's, and reaching recently

to the elaborate requirements for the location of cracks

written into the contractor specifications on the Bl

bomber program, the Federal Government has had an

intense interest in NDE. This interest was intensified in the post World War II period when high-strength, flaw- sensitive materials came into extensive use. Very small

flaws indeed must be detected reliably and accurately,

if such materials are to be used with the kind of effec- tiveness required in the military and space programs of the nation. Until very recently, NDE has been mainly a qualitative tool, very useful for the detection of major flaws in materials. However, with the advent of a drive toward

222
fracture safe design NDE is moving to a higher level of precision. The reason is simply that fracture control design places quantitative requirements on the detection of flaws, and these new requirements are causing

rapid developments in the quantification of NDE.

Despite the advances that have been made, NDE is not yet a precision technique. Consider ultraspnic test- ing, one of the most popular NDE approaches. No

standard is available against which to make meaningful calibrations; phase and frequency data that could

greatly increase the information output are ignored;

and automation to increase efficiency and reduce operator variability is not used widely. Similar problems are common to other NDE techniques, and a great deal of fundamental work lies ahead if NDE is to become a

truly useful, quantitative tool.

6. Health and Welfare

The Government is also concerned with the health and welfare of the people. Congress has created special

agencies whose primary concern is public safety, and

mechanical failure prevention is a major concern of these agencies. One of the newest is the Consumer Product Safety Commission. Its interests have been ably and

thoroughly covered by the previous speaker, my good

friend Dr. Kushner, so I will not say anything more

about that one. The Federal Highway Administration, responsible

for the correction of highway conditions that contribute

to accidents, has had a major program on the detection of cracks in highway bridges. Some of the techniques

being investigated for this purpose are derived from

those developed for the defense, space, and atomic energy efforts I previously discussed. The Coast Guard, in carrying out its responsibilities for merchant marine safety, has recently required frac- ture mechanics-based design for tankers carrying liqui- fied natural gas. The Federal Aviation Administration has the responsibility of enforcing rules, regulations, and minimum

standards relating to the manufacture, operation and

maintenance of aircraft. The interest of this agency in failure prevention is well known to all of you, an in- terest attested to by the fact that they are cosponsors of

this Symposium.

This concern for public safety is such that, when the private sector cannot do the job alone, it is appropriate

for the Federal Government to join in. The Government is also concerned with the economic

health of the nation, and has legitimate interest in mechanical failures that may cause undue economic dis- location. And, finally, the Government is the world's

largest consumer, and like us as individuals, wants its purchases to be safe, economical, and long lasting. In 1972 Uncle Sam purchased over $1.6 billion worth of supplies and equipment for civilian agencies, and $58

billion for the military. Through this huge purchasing

activity the Government can do much to influence the reduction of mechanical failures, an area under investi- gation by the NBS Experimental Technology Incentives

Program.

7. Public Policy

In all of its concerns, be they military, health and

welfare, or space. Government policy is concerned with

mechanical failures. First a few definitions. To define public policy to everyone's satisfaction reminds me of the old story of the blind men and the elephant, in that the answer is different depending on the way in which

you approach the subject. Policies provide the basic

framework of principles and rules that are used as guides in decision making. According to some experts,

policy has two distinct aspects. One aspect considers

what is being done, and the other considers how it is achieved. There are many other definitions, but let me

talk about how public policy comes into being, and how

it is implemented.

The President, of course, helps formulate public

policy. President Nixon has done so in his Buyer's Bill of Rights, in which he states that the consumer has a

right to safety in the products he buys. This credo ob- viously has implications for mechanical failure prevention, as there is often a direct tie between failure and

injury.

Congress formulates policy through legislative enact- ments, and many Acts of Congress have impact on

mechanical failure. For example, the Natural Gas Pipeline Safety Act is directly concerned with protecting the public from this class of mechanical failure. Neither the President nor the Congress can carry out

policy on a day-to-day basis, and a number of Federal

agencies have been established to do so. I mentioned some of them earlier. These agencies, in interpreting

and executing national policy, often establish rules, re- quirements, and procedures that are in effect minipolicies of their own. For example, the FAA has

established stringent rules concerning air safety, and

every time I fly I'm glad they have. Now none of this policy setting is done in a vacuum,

nor is it a one way street. The President and Congress

reflect the will of the people, and respond to the needs

of the day. The creation of the Consumer Product

Safety Commission was a response to a new wave of consumerism that has grown over the past 10 years. The standards setting efforts of the private sector also have policy impact, for such standards are often used

in the requirements of various Government agencies.

8. Product Liability

The courts, too, play a major role in policy setting.

They not only interpret formal legislation, but they also provide a forum for the challenge of specific rules set by Federal agencies. And the courts play a major role

in their interpretations of common law—by common

law I mean that arising out of precedents and not out

of legislation—and here, too, they are having broad

impact. First a few statistics. The public is turning more and more to the courts in cases of product liability,

and in terms of this conference that often means mechanical failure. In 1960 there were 50,000 such suits in this country; in 1970 there were 500,000.

Not only is the absolute number of product liability suits growing astronomically, but there has been a dras223
tic change in court climate regarding liability. A hundred years ago courts held that the manufacturer had no

liability except where a contractual relationship existed. Over the years there was a shift to the situation in which

the plantifl had to prove negligence on the part of the manufacturer to the situation today in which the plaintiff need only prove that the article was unsafe in some way which caused him injury or damage to win his

ease.

This shift has potential implications for public policy.

Perhaps the notion of no-fault insurance for manufacturers should be considered. The no-fault concept is being applied in the area of automobile insurance, and

extension in some form to the area of product liability

is worth debating. Certainly it would go a long way

towards reducing court backlogs, as a million liability cases per year are foreseen within the next decade. At a time when sophisticated new engineering appli- cations and shortages of traditional materials put a premium on innovations in materials and on imagination in design concepts, this legal climate may lead to an overly cautious approach by industry. For example,

the avoidance of new materials, overdesign, and delays

in marketing. This is an appropriate area for new policy —for strong Governmental support of materials characterization research and of research and development

of NDE test methods. Advances in these technical areas would benefit the public, the manufacturer and would

lessen the load on the courts. There is a pretty good

track record in such areas as aerospace, and the time is right for an active extension of NDE to consumer

products. This is a big job, an expensive job, and one

in which the Government could well be a willing part- ner. This policy area should also be explored.

9. Other National Concerns

There are other national concerns that have both materials failure and policy implications. Let me cite just a few. In energy generation, the desire for higher efficiency

has led to development of new high temperature turbine

blade materials. These are new alloys with special shock

sensitivity properties, or new high temperature ce- ramics, both subject to fracture from mechanical and

thermal shock. Reliable assessment techniques are needed for such materials. In building, steel-fiber reinforced concrete can be

used without further reinforcement, but new tests are needed to assess this material's long term strength and

durability. Asphalt roofing is subject to mechanical

failure as a result of thermal cycling, and reliable tests are needed for proposed replacements.

The use of recycled materials will bring about new

testing requirements. For example, the stress-rupture re- sistance of high strength nickel base superalloys is seriously impaired by parts per million traces of se- lenium, a common element in scrapped electronics. New,

fast, reliable analytical techniques will be needed in

this area. There are mechanical failure problems in the de- velopment of synthetic body implants, in the use of

plastics as load bearing materials, and in the use of polymer-impregnated concrete. Mechanical failure has

a double impact on our foreign trade. First, our productivity must be high if we are to compete successfully

in foreign markets. Productivity is in turn linked to new products, new materials, and a minimum of break- downs in the manufacturing process. Second, the products we sell abroad must also have a reputation for

reliability, and that means few mechanical failures.

10. Policy Needs

I said earlier that policy is a many faceted thing. It embodies what we as a nation hope to achieve as well as the courses of action by which we get the job done.

If you look hard enough, you'll find that there is national policy on materials and mechanical failure, but

it is a diffuse, uncoordinated, stop-gap policy that lacks

a real focus. We as a nation need a coherent materials

policy. Public safety, new materials, productivity, materials shortages, pollution are all part of a problem that

affects all Americans. There are obvious mechanical

failure components in each of these categories, and fail- ure prevention should be an integral part of policy

formulation.

I'll be specific about the needs. The Government

should foster the development and use of reliable NDE

techniques so that safe, long-lasting products can be

made available at reasonable cost. There will be other

benefits. With longer lasting products there will be

reduced needs for raw materials, less energy used in the production and transportation of goods, and less pollution at both the production and disposal ends of

the cycle.

We need both policy and technology that leads to in- creased recycling and reuse. We need a better understanding of the relationship between flaws and failure. We need research into the characterization of materials,

so that they can be better matched to service require- ments. And, finally, there should be a clear definition of liability when the rare but unavoidable failures do

occur. Of course, such policies must be developed in terms

of the latest and best information. You and I, both as concerned citizens and technical specialists, can and

should contribute to the creation of such policies. This

is a large assignment, but a necessary one if we are to make the best use of our national resources—materials, energy, technology and people.

11. Discussion

H. Piehler, Carnegie-Mellon University : I would like to react somewhat to your posture toward product liability

and its role in this entire mechanical failure milieu. There seems to be a general feeling here that a govern- mental agency is all that is required in order to alleviate these ills and that the liability suits provide no other function than to goad manufacturers and create backlogs in the courts Having been involved in a study of

the whole litigation process at Carnegie-Mellon with

people from the Duquesne Law School for the last 2

years, let me just comment that this vehicle tries to

224
incorporate all of the societal judgements into situations where, in fact, consumers have been injured and have

no other form of recourse. If I were given the option of doing away with this system and replacing it with a no-fault system, I would definitely opt for the present

system but with refinements from the technical community. One goal of this group ought to be injecting

realistic criteria into the present legal system so that

strict liability can work the way it really should.

R. W. Roberts: There is merit in what you say. By

bringing up _a concern such as no-fault you not only

examine what is good and bad about that but you also examine what is good and bad about the current system.

The major thrust that I was trying to make with respect

to what was happening in the courts is that perhaps we

can avoid many of these suits if we take the tack that Commissioner Kushner was pushing of trying to build

better reliability into products. If we take some of the

techniques that have been developed in the aerospace,

nuclear, and defense industries and apply these to the consumer industry, we probably will be able to do that. You know there haven't been any reactors that have

had major failures. There are very few aircraft that

fail. But what's the product failure rate in some of the consumer product lines? I would submit that they are

a good deal higher than you would find in the nuclear

industry. With respect to cost, and that's really the key, we're going to have to work fairly hard to build NDE

techniques into any manufacturing line, but I think

unless we try, we're never going to succeed. It's my impression that some of the larger manufacturing corporations in this country want to do that, too, because the

profit margin isn't very large on certain items. The first service call that occurs under warranty can wipe out the margin on many, many products, not just the one that's being serviced. There is a real incentive to the corporation to be concerned with product reliability.

225

LIST of REGISTRANTS

Mechanical Failures Prevention Group

20th Meeting

Mechanical Failure—Definition of the Problem

National Bureau of Standards, Gaithersburg, Maryland

May 8-10, 1974

Frank Almater

Consumer Product Safety Commission

Bethesda, Maryland 20016

E. Ambler

Deputy Director

National Bureau of Standards

Washington, DC 20234

Clifford Astill National Science Foundation

Washington, DC 20550

Alfred J. Babecki

NASA—Goddard

Code 755

Greenbelt, Maryland 20771

Bruce C. Baird

The Boeing Company

P. 0. Box 58747

Houston, Texas 77058

William F. Bangs

NASA-Goddard Code 321

Greenbelt, Maryland 20771

Bruce P. Bardes

Bimba Mfg. Co. 101 E. Main Street Monee, Illinois 60449

William J. Baxter

General Motors Research

12 Mile & Round Roads

Warren, Michigan 48013

Alan Beerbower

Esso Research & Engineering Co.

P. 0. Box 51

Linden, New Jersey 07036

Dennis C. Bely

U.S. Army Ballistic Research Labs

131 Law Street Aberdeen, Maryland 21001

Russell E. Benner

Lehigh University

Dept. of M.E. & Mech.

Bethlehem, PA 18015

J. A. Bennett

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

Isadore Binder

U.S. Army TACOM

Warren, Michigan 48090

Raymond L. Bisplinghoff

National Science Foundation

1800 G. Street, NW.

Washington, DC 20550

T. Blair Travelers Insurance Company

1401 Elm Street

Dallas, Texas 75202

Alfred P. Brackney

Monsanto Polymers & Petrochemicals Co. 730 Worcester Street Indian Orchard, MA 01051

David C. Broom

Alcan International, Limited

P. 0. Box 8400

Kingston, Ontario

Canada K7L 4Z4

H. C. Burnett

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

Herbert S. Cheng

Northwestern University

Technological Institute Evanston, Illinois 60201

John F. Clayton

Gulf Oil of Canada

Sheraton Park

Ontario, Canada

Maurice G. Cockcroft U.K. National Gas Turbine Establishment

British Embassy

3100 Massachusetts Ave., NW.

Washington, DC 20008

L. A. Colucciello National Transportation

Safety Board

Washington, DC 20591

227
W. D. Compton

Director, Ford Scientific Res. Lab.

Ford Motor Company

Dearborn, Michigan 48021

Herbert Corten

University of Illinois 321 Talbot Laboratory

Urbana, Illinois 61801

Raymond A. Coulombe

Naval Ship Engineering Center

Prince Georges Center

Hyattsville, Maryland 20782

James J. Coyle

Villanova University

Villanova, Pennsylvania 19085

Arnold R. Daddario

Pratt & Whitney Aire. Div. UAC

400 Main Street East Hartford, Connecticut 06108

Paulo Sergio daSilva

Instituto de Pesqisas Techno Pogicas

Sao Paulo Cx Postal F141, Brazil

Thomas L. Daugherty

Naval Ship R&D Center

Annapolis, Maryland 21402

Ann Davis

Naval Ship Res. and Dev. Center AnnapoHs, Maryland 21401

Evan J. Deemer, Code ASCR 3370

Naval Air Systems Command

NAS Norfolk

Norfolk, Virginia 23155

John V. Deller Naval Ship Engineering Center

(Sec. 6148)

3700 East West Highway

Hyattsville, Maryland 20782

Edward J. Devine

NASA Goddard Space Flight Center

Greenbelt, Maryland 20771

Madan Dhawan

Naval Ship Engineering Center

Hyattsville, Maryland 20782

V. Dobroselskiy

USSR Embassy

1125 16th Steet, NW.

Washington, DC 20036

Thomas J. Dolan

Engineering Consultant

510 S. Highland Avenue

Champaign. Illinois 61820

Stanley W. Doroff

Office of Naval Research

Arlington, Virginia 22217

John J. Dougherty

Boeing Vertol Compay

P. 0. Box 16858

Philadelphia, Pennsylvania 19142

James F. Dray

Naval Ship Engineering Center (Code 6148)

3700 East West Highway

Hyattsville, Maryland 20782

D. C. Drucker

College of Engineering

University of Illinois Urbana, Illinois 61801

Robert J. Dudash

American Telephone & Telegraph Co.

P. 0. Box 665

Herndon, Virginia 22070

George Earle Boeing Vertol Company

P. O. Box 16858

Philadelphia, PA 19142

M. Keith Ellingsworth

Office of Naval Research

800 N. Quincy Street Arlington, Virginia 22217

Dean F. Elliott NASA, GSFC

6703 Lamont Drive

Lanham, Maryland 20801

John Erthal Naval Air Development Center Warminster, Pennsylvania 18974

James Fei Naval Ship Engineering Center

Prince George's Center, Code 6107B1

Hyattsville, Maryland 20782

Richard S. Fein

Texaco Inc. P. 0. Box 509

Beacon, New York 12508

228
Harry Feingold

U.S. Naval Ship R&D Center 11801 Prestwick Road

Potomac, Maryland 20854

Jacob Feld

Consulting Engineer

114 East 32nd Street New York, New York 11016

Hal Ferris Hughes Helicopters

Centinella & Teale

Culver City, California 90230

Paul Fleming

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

William R. Fontaine

Naval Ship Res. and Dev. Center

Bethesda, Maryland 20034

J. L. Frarey

Shaker Research Corp.

Northway 10

Executive Park

Ballston Lake, New York 12019

A. C. Gangadharan

Foster Wheeler Corp.

110 S. Orange Avenue

Livingston, New Jersey 07039

C. Gerald Gardner

Southwest Research Institute P. 0. Drawer 28510

San Antonio, Texas 78284

Frank S. Gardner

Office of Naval Research

495 Summer Street Boston, Massachusetts 02210

Luciano R. Gatto Metcut Research Association, Inc. 3980 Rosslyn Drive

Cincinnati, Ohio 45209

Wilmer Gaylord

U.S. Postal Service

Research and Eng. Dept.

11711 Parklawn Drive

Rockville, Maryland 20852

B. K. Genetti

General Electric 14-3

2901 East Lake Road

Erie, Pennsylvania 16501

Peter S. Given

SKF Industries, Inc. 1100 First Avenue

King of Prussia, PA 19006

Kay M. Glaesner

St. John's Evangelical Lutheran Church

Wittenberg Avenue at Columbia Street Springfield, Ohio 45502

Nathan Glassman

Naval Ship R&D Center

Annapolis, Maryland 21402

Irving W. Glater

Pratt and Whitney Aircraft. Div. UAC

Box 611

Middletown, Connecticut 06457

William R. Godwin

Westinghouse Electric Corp.

Box 153 MS 7380

Baltimore, Maryland 21203

W. G. Grenier, Code 755

NASA-Goddard Space Flight Center

Greenbelt, Maryland 20771

Edmund J. Gutowski

Naval Ship Engineering Center 3700 East West Highway

Hyattsville, Maryland 20782

Barry Hammond

Naval Ship R&D Center

Annapolis, Maryland 21401

R. Emerson Harris

NTSB

6513 Byrnes Drive

McLean, Virginia 22101

Keith L. Hawthorne

Association of American Railroads

1920 "L" Street, N.W.

Washington, DC 20036

Donald E. Hayes

Research Center ARMCO Steel Corporation

Middletown, Ohio 45043

Henry Hegner

IIT Research Institute 10 West 35 Street Chicago, Illinois 60616

Rolf M. Hellerung

Burns and Roe, Inc. P. 0. Box 663

Paramus, New Jersey 07652

229
John P. Hirth

Ohio State University

Dept. of Metallurgical Eng.

Columbus, Ohio 43210

Stanley Hlady

Commonwealth Edison Company

Chicago, Illinois 60690

Charles P. Hoffman

NTSB-FOB-lOA

800 Independence Avenue, S.W.

Washington, DC 20234

J. D. Hoffman

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Rudy Hohenberg

AVCO Lycoming Division

550 South Main Street Stratford, Connecticut 06497

E. Horowitz

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Paul L. Howard, Jr. SKF Industries, Inc. 1100 First Avenue

King of Prussia, Pennsylvania 19406

Jack Hubacher

Value Engineering Laboratory

2550 Huntington Avenue

Alexandria, Virginia 22303

Patrick W. Humphrey

NASA/GSFC

Code 754

Greenbelt, Maryland 20771

Redin G. Jaime (INEN)

Institute Ecuatoriano de Normalizacion Ponton 136

Quito, Ecuador

Om Johari IIT Research Institute Chicago, Illinois 60616

Carl R. Johnson

NASA-Goddard Space Flight Center

Greenbelt, Maryland 20771

Charles W. Johnson

U.S. Government Printing Office Washington, DC 20401

R. B. Johnson

National Bureau of Standards

Institute for Materials Research

Washington, DC 20234

Frank C. Jones K West

9371 Kramer Avenue

Westminster, California 92683

H. Peter Jost Angel Lodge Laboratories K. S. Paul Products, Ltd. Nobel Road

London N18 3DB, England

John M. Karhnak, Jr. U.S. Army MERDC

Attn: STSFB-HM

Fort Belvoir, Virginia 22060

James Kerley

GSFC—NASA

Glenn Dale Road

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Elmer E. Klaus

The Pennsylvania State University

133 Chemical Engineering Building

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Joseph D. Kochevar

Chrysler Corporation Defense Engr.

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General Electric Knolls Atomic Power Lab

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Naval Research Laboratory

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National Bureau of Standards

Metallurgy Division

Washington, DC 20234

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Commissioner

Consumer Product Safety Commission

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U.S. Coast Guard

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Caterpillar Tractor Co. 100 N.E. Adams Street Peoria, Illinois 61602

Robert Leon

Vibration Specialty Corporation

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Lawrence Leonard

Franklin Institute Res. Labs

20th & Race

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Samuel J. Leonardi

Mobil Research & Development

Paulsboro, New Jersey 08066

Frederick F. Ling

Mechanics Division

Rensselaer Polytechnic Institute Troy, New York 12181

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Manager, Metallurgy Research

The TIMKEN Company

Canton, Ohio 44706

Seng Hin Low

Singapore Inst, of Standards

and Industrial Research

18 Hai Sing Road

Singapore 19

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12 Carriage Lane

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Jesse B. Lurasden

Ohio State University

Dept. of Metallurgical Eng.

116 W. 19th Avenue

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R. Maringer

Battelle Columbus Labs

505 King Avenue

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R. F. Martin

National Bureau of Standards

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Ontario Hydro Research Division

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Colorado School of Mines

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Metallurgy Department, U-136

University of Connecticut

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Atomic Energy Commission

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Federal Aviation Administration

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U.S. Atomic Energy Commission

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National Bureau of Standards

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H. W. Paxton

Carnegie-Mellon University

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232
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National Bureau of Standards

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Superior Tube Company

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Eaton Corp.

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M. Wildmann

Ampex Corporation

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National Bureau of Standards

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Ward 0. Winer

Georgia Institute of Technology

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Victoria C. Wittenbreder

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Massachusetts Materials Research

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•d U.S. GOVERNMENT PRINTING OFFICE; 197S O—588-797

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NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP-423

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Mechanical Failure;

Definition of the Problem

MFPG

20th Meeting

5. Publication Date March 1976

6. Performing Organization Code

7. AUTHOR(S)

T. R. Shives and W, A„ Willard, Editors

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsorine Organization Name and Complete Address (Street, City, State, ZIP)

)ffice of Naval Research, Arlington, VA 22217; Consumer Product

Safety Commission, Wash» , DC 20207; Frankford Arsenal, U«S« Array,

'hil, PA 19137; Naval Air Dev. Center, Washo , DC 20362; Am Soco

lecho Eng., NY, NY 10017; FAA, Wash., DC 20591; NASA, Gr^nbg^^^^^

13. Type of Report & Period Covered

Final

14« Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 76-600003

16. ABSTRACT (A 200-word or /ess factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

These Proceedings consist of 20 submitted papers and discussionas from the

20th Meeting of the Mechanical Failures Prevention Group held at the National Bureau of Standards, May 8-10, 1974o The central theme of the proceedings is the definition of the problem of mechanical failure, with em- phasis on modes of failure, consequences of mechanical failure, and implications of mechanical failure.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Brittle fracture; economics of failure prevention; fatigue failure; mechanical

failure; mechanical reliability; stress corrosion cracking.

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