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NATL INST OF STANDARDS & TECH R.I.C.

gineering design : }77 C.1 NBS-PUB-C 19 NBS SPECIAL PUBLICATION 487

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Engineering Design

MFPG

25th Meeting

- r»iT--ti>ii-J'fittMnii;f iiff ii i>i<tiiii m
NATIONAL BUREAU OF STANDARDS

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MFPG

Engineering Design

SEP 2 2 1977

Proceedings of the 25th Meeting of the

Mechanical Failures Prevention Group,

held at the National Bureau of Standards

Gaithersburg, Maryland, November 3-5, 1976

Edited by

T.R. Shives and W.A. Willard

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

The 25th meeting of MFPG and these proceedings were sponsored by the Institute for Materials Research of the National Bureau of Standards, Washington, DC 20234; the Office of Naval

Research, Department of the Navy, Arlington, VA 22217; the Naval Air Development Center, Department of the Navy, Warminster, PA 18974; the Frankford Arsenal, Philadelphia, PA 19137;

the Federal Aviation Administration, Department of Transportation, Washington, DC 20591; and

the Energy Research and Development Administration, Washington, DC 20545.

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued August 1977
Library of Congress Cataloging in Publication Data

Mechanical Failures Prevention Group.

MFPG—engineering design. (National Bureau of Standards special publication ' 487) "Sponsored by the Institute for Materials Research of the National Bureau of Standards ... [et al.]." Supt. of Docs, no.: €13.10:487

1. Engineering design—Congresses. 2. Fracture mechanics—Congresses. I. Shives, T. R. II. Willard, William A. III. United States. National Bureau of Standards. Institute of Materials Research. IV.

Title. V. Title: Engineering design.. VI. Series: United States. National Bureau of Standards. Special publications ; 487. QC100.U57 no. 487 |TA174] 602'.ls [620'.004'2] 77-21810

National Bureau of Standards Special Publication 487

Nat. Bur. Stand. (U.S.), Spec. Publ. 487, 367 pages (Aug. 1977) CODEN: XNBSAV

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FOREWORD

The 25th meeting o£ the Mechanical Failures Prevention Group was held

November 3-5, 1976, at the National Bureau o£ Standards in Gaithersburg,

Maryland. The program was organized by the MFPG committee on Design

under the chairmanship of Jesse E. Stem o£ NASA/Goddard Space Flight

Center. The Design committee, the session chairmen, and especially the

speakers are to be commended for the excellent program.

With a few exceptions, the papers in these proceedings are presented as submitted by the authors on camera ready copy, except for some minor

editorial changes.

Appreciation is extended to the following members of the NBS Metallurgy

Division: T. Robert Shives and William A. Willard for their editing,

organization, and preparation of the proceedings, Paul M. Fleming for

handling financial matters, Todd Eudy for photographic work, Larry W.

Ketron for drafting work, and to Marian L. Slusser for typing. Appreciation is also extended to Sara R. Torrence of the NBS Office of

Information Activities for the meeting arrangements and to many members

of the staffs of the NBS Metallurgy Division and the NBS Institute for

Materials Research for their assistance in many ways.

HARRY C. BURNETT

Executive Secretary, MFPG

Metallurgy Division

National Bureau of Standards

III
TABLE OF CONTENTS

FOREWORD

CONFERENCE THEME: The Growing Web Around the Designer

Charles 0. Smith

SESSION I: TECHNIQUES OF DESIGN (I)

1. A Systems Approach to Materials in Design.

Bruce P. Bardes

2. Probabilistic Methods in Design. Edward B. Haugen

SESSION II: TECHNIQUES OF DESIGN (II)

1. The Finite Element Method in Structural Analysis.

Harry G. Schaeffer

2. NASTRAN Thermal Analyzer in Interdisciplinary Thermo

Structural Analyses. Hwa-Ping Lee

3. The Use of Cathode Ray Tube Graphics as a Computer

Aided Design Tool. Alyce Branum

SESSION III: MATERIALS OF DESIGN (I) - COMPOSITES

1. Review of the Processing and Properties of Pan

Graphite -Aluminum Composites. Roger T. Pepper

H. Gigerenzer and T. A. Zack

2. Composite Inlays for Jet Engines. W. Troha* and

K . Swain

3. Stress Concentrations in Composite Materials.

G . Kardos

SESSION IV: MATERIALS OF DESIGN (II) - LUBRICANTS

1. Elastohydrodynamic Lubrication. L. B. Sibley

2. Polyfluoroalkyl-Alkyl Polysiloxane Grease for

Instrument Lubrication. John B. Christian

IV
3. Antiwear and Lubricity Additives for Lubricants 172

S. M. Hsu

SESSION V: VALIDATION OF DESIGN (I) - TESTING

1. Piezoelectric Polymers and Their Applications. 189

Seymour Edelman

2. Exoelectron Emission- -Past Experience and Future 197

Expectations. W. J. Baxter

3. Detecting Structural Degradation by Acoustic Emission. 210

Michael P. Kelly* and Robert J. Schlamp

SESSION VI: GOALS, PURPOSES, AND REQUIREMENTS OF DESIGN - PRODUCT LIABILITY

1. Negligence Charges in Products Liability Law Can Be 241

Used in Engineering as Safety Design Criteria.

Raymond R. Hagglund

2. Prescription to Reduce Products Liability Losses.

A. The Defendant and Products Liability. 247

John P. Amess

B. The Plaintiff and Products Liability. 253

Milton Heller

Discussion 257

SESSION VII: VALIDATION OF DESIGN (II) - RELIABILITY

1. Safety Analysis: Qualitative, Quantitative and 265

Cost Effective. H. D. Wolf

2. A Failure Mode and Effect Analysis Program to Reduce 278

Mechanical Failures. Richard R. Landers

3. Fault Tree Analysis as a Part of Mechanical Systems 289

Design. J. B. Fussell* and D. P. Wagner

SESSION VIII: MATERIALS OF DESIGN (III) - COATING MATERIALS

AND TECHNIQUES

1. Ion Plating- -Concepts and Applications. D. M. Mattox 311

V
2. Sputtering. T. Spalvins 324

LIST OF REGISTRANTS 338

LIST OF MFPG PUBLICATIONS 343

APPENDIX

1. National Materials Policy in the 95th Congress- -Current 349

and Potential Government Impacts on Availability.

Franklin P. Huddle (Banquet Speaker)

2. Testing for Success. John C. New 356

(Presented at the 21st meeting of MFPG, Success

by Design: Progress Through Failure Analysis.

Manuscript received too late for inclusion in

the Proceedings of the 21st meeting.)

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

VI
ABSTRACT

These Proceedings consist o£ a group of twenty- three submitted

papers from the 25th meeting of the Mechanical Failures Prevention

Group which was held at the National Bureau of Standards in Gaithersburg, Maryland on November 3-5, 1976. The central theme of the

Proceedings is the application of design to failure prevention, with

enphasis on the techniques of design, materials of design, design

validation, and the goals, purposes, and requirements of design as related to product liability.

Key Words : Coating materials; composite materials; design; failure

avoidance; failure prevention; lubricants; materials of design; product

liability; reliability of design; techniques of design; validation of

design

UNITS AND SYMBOLS

Customary United States units and symbols appear in many of the

papers in these Proceedings. The participants in the 25th 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 references are given:

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."

E380-76 ASTM/IEEE Standard Metric Practice (Institute of Electrical and

Electronics Engineers, Inc. Standard 268-1976).

Disclaimer

:

Certain trade names and conpany products are identified in order to

adequately specify the experimental 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.

VII
ii
THE GROWING WEB AROUND THE DESIGNER

Charles O. Smith

Professor of Engineering

University of Nebraska at Omaha

P. O. Box 688

Omaha, Nebraska 68101

Abstract: The task of the designer has never been easy

since he has had to consider a number of factors, not all of

which are necessarily compatible. He has always had to make

iterative tradeoffs among various requirements, including

the very important one of cost. In the context of today's

technical and societal climate, the designer's task has become even more difficult, if not almost impossible.

This paper summarizes the interrelated factors the designer must consider. There is further complexity as not

all of the factors apply with equal weight in all designs,

and some requirements conflict with others, yet all must be

considered. None should be discarded without full justification, much less be ignored. The paper is essentially a checklist for the complex, confusing, and (sometimes) frustrating task of designing a product, not a "how to design"

paper.

Key words: Design; checklist for design; factors in design; requirements in design.

Introduct ion

The task of the designer has never been easy since he has

had to consider a number of factors, not all of which are

necessarily compatible. He has always had to make iterative

tradeoffs among various requirements, including the very

important one of cost. In the context of today's technical

and societal climate, the designer's task has become even

more difficult, if not almost impossible.

1
Today's designer is required to account for so many factors

and considerations that it is essentially impossible for

one individual to be thoroughly conversant with all of

these. At the same time, however, he can not afford to

overlook or slight even one of these factors. Thus he is,

in effect, forced to play a role something like that of an

orchestral conductor. He must have some special competence

of his own and a reasonable knowledge of the other "instruments". But above all, he must know how all these considerations fit together, what interactions are possible, and

what sort of tradeoffs can and/or must be made. The

various design considerations have been (somewhat arbitrarily) grouped into three categories as indicated in Table 1.

It is recognized that these categories intersect and overlap. This hard fact is the central core of the difficulty

in developing truly acceptable design in various products.

Functional Requirements

It is obvious that any design must necessarily be able to

meet performance specifications. Thus this item is at the

top of the list in Table 2. Performance specifications

must fully reflect a carefully and completely defined need.

One must be careful about specifications, however, since

these are often rather specific about what shall be in the

product rather than the functions required. The latter is

much more important and should be so emphasized. Design

can be improved as a result of clarifying the functional

requirements of a product. For example, if a surface must

only resist corrosion, coating or cladding may cost less —

and may require less material -- than using corrosion resistant material throughout (1) , and still adequately meet

performance requirements.

Awarding contracts to the lowest bidder encourages minimum

'material input (consistent with specifications) . As a result there is frequently a shift from first cost to maintenance cost (2), not necessarily good design from the viewpoint of total life-cycle performance and cost.

* Numbers in parentheses refer to items in the bibliography.

2
Any product must have some sort of design configuration,

i.e., it must take some physical, geometrical form. In all

cases, there will be some demand function which the product

must have the capacity to meet. Most efforts to balance

that equation, while providing some margin of safety, are

placed in a deterministic mode, using average values of demand and capacity. This, in effect, ignores the statistical nature of design parameters. To avoid unrealistic results, magnitude and frequency relationships, for both demand and capacity, should be considered. This suggests a probabilistic approach as portrayed by Haugen (3) and

others. In the context of mechanical design, coupling of

average stress from applied loading with the average available strength, while accounting for the variations in both,

permits designing to any desired degree of failure probability (4).

In making mechanical stress calculations, the designer must

give full attention to notches since notches render hardware highly vulnerable to failure (5). At the same time,

it must also be recognized that the developing field of

fracture mechanics provides evidence that some notches are

benign and acceptable to a degree in some design applications. The designer must very carefully assess this aspect .

Increasing emphasis is being placed on reliability in equip

ment. In this context, reliability is defined as the proba

bility of a device performing adequately for the period of

time intended under the operating conditions encountered.

There is no general definition of adequate performance, the

criteria for which must be carefully and exactly detailed

in advance of generating a design. In addition to reliability, there is also increasing emphasis on its probabilistic relatives such as maintainability, availability, and

repairability.

The designer must anticipate failure in his product. This

requires use of such tools as fault tree analysis, failure

modes and effects analysis, and hazards analysis.

In the process of generating a design configuration, one

should apply value analysis or value engineering, ie.,

appraising a design from the viewpoints of both function

5
and cost to assure maximum value, or achieving the required

function (without unnecessary functional capacity) at the

lowest cost. This is neither simple cost-cutting nor does

it lead to performance trade-offs.

Substantial improvement in function or cost can often be

achieved by redesign. Formal design review procedures have

proven very useful in enhancing product performance. In

this context, techniques such as failure-modes-and effectsanalyses are very valuable and, in many cases, essentially

imperat ive

.

Intensive and extensive standardization and simplification

of design can lead to substantial savings without loss of

performance. Retaining only the most efficient of the

variety of sizes, types, grades and models of a given product will lead to considerable economies. During World War

II, for example, the U. S. reduced the number of types of

steel, brass, and bronze valves from 4080 to 2500 with an

annual savings of 5000 tons of steel and 1900 tons of

copper (5)

.

Functional substitution offers great opportunity for improvement through redesign. The aim is not simply to replace but to find a new or different way to do a job.

Joining two pieces of metal can be done not only by using

stronger bolts and nuts but also by welding or by adhesives.

Jet engines replace piston engines and propellers. Telephones replace mail for transmitting some information (7)

.

Total Life Cycle

Materials, energy, and the environment form the physical

basis of the economy (8) . They also have a complex interrelationship (9) . The designer thus has the problem of using the optimum combination of materials, energy, input,

and environmental impact consonant with the functional requirements of a design. While many recognize that changes

in one area start changes in the others, probably no one

fully understands the interactions. The designer must recognize this and be prepared to cope as well as possible.
Designers often select a material for two incorrect reasons

(1) "It's always been used for this application," and (2)

"Its properties." With regard to the first incorrect

reason, the time when each application had its preferred

material and each material had its secure markets is gone.

In other words, tradition and good design do not necessarily mix. The second reason is incorrect because it implies

that properties are something mysteriously, almost metaphysically, connected with the material, something a material has without reference to the specific use. On the

contrary, a property should be regarded as the response of

the material to a given set of imposed conditions.

In selecting materials for a given application, the first

step is evaluation of service conditions. The characteristics (responses) of various materials are next evaluated

to select the most promising materials. One common question is that of adequate strength. While strength may well

be the primary selection criterion, it may also be ductility, toughness, corrosion resistance, specific gravity

strength/weight ratio, and/or one or more of several other

characteristics

.

Stability of material in service is intimately related to

temperature, fluctuations in temperature, and length of

time at temperature. All of these directly affect strength

and often lead to creep, but they can also produce changes

in the microstructure of the material, e.g., tempering of

martensitic steels and overaging in precipitation-hardening

alloys

.

Availability and fabricability are intimately related. A

given material may show excellent promise for a given appli

cation on the basis of laboratory tests. If it is not

available in the desired quantity and form at an acceptable

cost, or if it is not reasonably possible to develop processes to get the desired form, then it is obviously unreasonable to base a design on it. Fabrication processes

should be used which give minimum waste and/or maximum recovery of material.

In general, the designer is basically responsible for selec

ting materials. He should not, however, rely exclusively

5
or even heavily on handbook data but should consult with

materials specialists. At the same time, to best profit

from such consultation, the designer must give the specialist the complete story of intended use, possible sources of

supply; and fabricating facilities. To be most effective,

there should be continual collaboration throughout the design process.

The designer must be fully aware of the option of alternative materials. In some cases this will involve substitution for preferred, but unavailable material. In other

cases the alternative material will be selected because of

superiority in the specific situation. In one case a tool

manufacturer had been purchasing AISI 1078 and 1086 carbon

bar with overlapping analyses. The decision was made to

use only AISI 1078. This gives one less grade to inventoryo

Even more importantly, 1078 is rolled more frequently than

1086, thus effectively making it more available and easier

to schedule (10) . One aspect of alternative materials is

the possibility of using composites which might also be

considered as an aspect of value analysis and/or redesign.

The designer must be concerned with producibility . While

this is related to fabricability, it is much broader as it

necessitates consideration of manufacturing processes,

assembly processes, etc.

Durability is related to, but differs from, reliability.

Reliability is concerned with successful operation for a given period. Durability is concerned with how many cycles

of such operation are possible. In other words, durability

is a question of the length of life of a product.

In the context of total life cycle, the designer must think

of the material he selects from its starting point (e.g.,

ore) to its disposition. While he is properly concerned

with the durability of the product over the projected life,

he must also be concerned with possible recovery or reclamation. The ultimate goal should be to combine materials

selection, product design and manufacture, and recycle

processing in a systems approach to optimizing product development (11) . Emphasis must be placed on conservation of

nonrenewable resources, improved material effectiveness, and

much more intensive recyling of the discards of industry and

6
consumers (12). With more attention to recyling by the designer, it becomes easier to disassemble a discarded product

and feed components into appropriate material streams which

return them to the secondary industry for reworking (13).

With recycling, the same material may be reused many times

over a period of years. With each recycle, an equivalent

amount of virgin raw material is saved. Introduction of recycling, in effect, closes the loop on an otherwise open

system of extraction, consumption and disposal (14).

Recycling post-consumer waste materials is not a panacea

since it will not supply the majority of the Nation's raw

material demands. On the other hand, substitution possibilities, with regard to both total consumption and domestic

virgin material supply, are not insignificant (15)

.

In obtaining materials from basic sources (e.g., mining ore

and extracting metal from ore) , in fabricating into basic

forms (e.g., sheet, plate, rod, etc), in further processing

into semi-finished and finished products, etc, there is a continual requirement for energy. In fact, processing of

materials is highly energy- int ens ive . There are further

energy requirements in operating or using most products. If

the product is ultimately discarded, there is loss of nonrenewable material and energy plus some adverse impact on

the environment. If the product is reclaimed and/or recycled, there is still some energy requirement although this

is usually significantly less than in extracting virgin raw

material. The designer must consciously seek to minimize

the energy requirements of his product over the total life

cycle

.

The designer must consider the effect of his product on the

environment. The uses of energy and materials may have adverse environmental effects associated with generation (oil

drilling, water shortage, etc) , delivery (oil spills, exhaust fumes, etc), fabrication, application and disposal

(glare, noise, air pollution, water pollution, thermal pollution, debris, etc) (16). Consideration of the effects of

the product on the environment is in itself a complex task.

Nevertheless, it is a task which must be performed.

The designer must also consider the effects of the environment on his product. It may be argued that this is done

7
when considering stability, corrosion resistance, wear,

cavitation, etc. No matter when it is considered, it is

most important

.

One area often given too little attention is handling of the

product after completion. This includes such aspects as inspection, testing, packaging, shipping, and storage. Inspection and testing are important in ensuring that the

product is manufactured and functions as intended. Proper

attention to such considerations during design can make inspection and testing easier and more effective. Packaging,

shipping and storage can be a major cause of product malfunction or failure. For example, major home appliances

such as washing machines, dryers, etc, often are subjected

to stresses and environments during shipping and storage

which are significantly more harmful than the normal operating environment.

Further Major Factors in Design

The complex economic machinery of the Nation requires that

materials and products in commerce conform to prearranged

standards and specifications, which usually represent voluntary agreement among the elements of industry (17) . Although use of such standards (e.g., AISI, ASTM, etc) is

appropriate, the designer should also recognize that since

there is voluntary agreement, many standards are, in effect,

least common denominators or minimum standards. Good design

may well require more than this. At the same time, specification by standard sometimes requires a producer to meet

some requirement which is actually non-essential (18) . Emphasis in specifications should be on performance requirements.

A different area of standards relates to matters of safety

and health. Requirements of the Occupational Safety and

Health Administration (OSHA) and the Consumer Product Safety

Commission (CPSC) are of prime interest to the designer.

There are at least three aspects related to the CPSC which

often receive too little attention. The intended purpose of

a product is normally quite clear. Unintended uses of the

same product are not always as obvious. Obvious or not.

8
however, the designer has an obligation to foresee as many

unintended uses as possible and to develop his design in

such a way as to eliminate hazards to one who operates in an

unintended (from the designer's viewpoint), but foreseeable

manner. The designer must be aware that warnings are often

required on products. These warnings must be clear,

complete, unambiguous, and permanently attached. The designer must also be cognizant of all advertising material,

owner's manuals, operating instructions, etc, which relate

to the product since such information put out by the manufacturer has been held to constitute a warranty. There have

been situations in which such information essentially

promised the purchaser or user some characteristic which the

product neither had nor was intended to have.

Although OSHA and CPSC impose safety considerations, the designer must recognize that there may be additional safety

aspects which must be included in designing his product.

Consideration of human-engineering principles must proceed

concurrently with each step of the design process. Each

iteration implies a change in some aspect of configuration.

Each configuration, in turn, may imply a decision which is

crucial to operability and maintainability, two major aspects of the man-machine efficiency can be made more easily

by modifying the machine rather than the man.

While the overall appearance of the finished product may

have no influence on the capability of the device to perform

the required functions, appearance must be considered. It

is reasonably well established that if two machines are

"identical" in performance characteristics, the one which

has the greater aesthetic appeal will outsell the other.

This aspect is especially difficult since there is no uniform pattern of aesthetic appeal.

Last, but certainly not least, is consideration of cost. In

every case, the final configuration of a design depends on a number of compromises. One perpetual major determinant in

these compromises is the matter of economic considerations.

Industry has tended to optimize only that segment of the

total life that deals with the incoming material through to

the outgoing product (19, 20). There is growing recognition

that the designer must assess the total economic

9
consideration over the whole life cycle. This means assessing and including the cost of each factor, especially those

relating to materials, energy use, and environmental impact .

Summary

This paper has presented a brief discussion of the factors

which a designer must consider, explore, and use (or reject) in reaching a final design configuration for his

product to enter the marketplace. These factors do not all

have equal weight, nor do all of them apply in every design. All must be considered carefully, however, before

being omitted in a given design. More satisfaction of some

of these factors will mean less satisfaction of others.

The designer, as in the past, must make tradeoff decisions.

This task becomes increasingly complex. This paper tries

to point out all the problems' to be considered in developing a design. It makes no attempt to tell how to design.

Good designing

i

10
REFERENCES

1. Materials and Man's Needs, Summary Report of the

Committee on the Survey of Materials Science and

Engineering, National Academy of Sciences, 1973 (COSMAT)

p. 57.

2. Material Needs and the Environment Today and Tomorrow,

Final Report of the National Commission on Materials

Policy, June 1973, (NCMP) , pp. 4C-14.

3. Haugen, E. B., Probabilistic Approaches to Design, New

York, John Wiley and Sons, 196 8.

4. Smith, C. 0., "Design Relationships and Failure Theories

in Probabilistic Form," Nuclear Engineering Design,

(1974) , pp. 286-292.

5. Smith, C. 0., "Never Overlook Notches," Proceedings,

21st Meeting, MFPG, NBS, Gaithersburg , MD, 7-8 November

1974.

6. NCMP, pp. 4C-16.

7. COSMAT, p. 56.

8. NCMP, pp. 3-11.

9. Azaroff, L. V., "Materials, Environment, Energy - One

Vast System," Research/Development, March 19 76, pp. 32-

34, 36.

10. Metal Progress, September 19 74, p. 50.

11. COSMAT, p. 147.

12. NCMP, pp. 4C-4.

13. NCMP, pp. 4C-15.

14. Resource Recovery and Source Reduction (SW-122) , U. S.

Environmental Protection Agency, 19 74, p. 13.

15. SW-122, p. 15.

16. NCMP, pp. 5-3.

11
17. NCMP, pp. 10-13.

18. Metal Progress, September 1974, p. 45.

19. COSMAT, p. 78.

20. SW-122, pp. 109-112.

12
TABLE 1

General Categories of Factors in Design

Functional requirements

Total Life Cycle

Further Major Factors

TABLE 2

Functional Factors in Design

Performance specifications

definition of need

Design Configuration

probabilistic or deterministic approach

stress considerations

size/weight /volume

reliability, maintainability, availability,

and repairability

failure anticipation

value analysis

Redesign

design review

simplification and/or standardization

functional substitution

15
TABLE 3

Total Life Cycle in Design

Material selection

strength and/or ductilitystability

availability

fabricability

corrosion resistance

properties of unique interest

alternative materials

Producibility

Durability and/or recyclability

Energy requirements

for production

during use

for reclamation

Environmental compatibility

effect of product on environment

effect of environment on product

Handling

inspection and quality assurance

testing

packaging

shipping and storage

ih
TABLE 4

Further Major Factors in Design

State of the Art

prior knowledge

possible patent infringement

Conformance to Standards

Occupational Safety and Health Administrat

Consumer Products Safety Commission

unintended uses

warnings

advert ising

Environmental Protection Agency

American National Standards Institute

National Safety Council

American Society for Testing Materials

Industry standards

Safety

Human Factors

Ease of operating

Ease of maintenance

Aesthetics

COST

total economic consideration

15

I

I SESSION I

TECHNIQUES

OF

DESIGN

Chairman: R. E. Maringer

Battelle, Columbus Laboratories

A SYSTEMS APPROACH TO MATERIALS IN DESIGN

Bruce P. Bardes

Bimba Manufacturing Company

Monee, Illinois 60449

Abstract: A systems oriented approach to the problems of materials

selection and choice of manufacturing processes in engineering design is presented. The goal of this approach is to select the best

combination of shape, material and manufacturing process by optimization with respect to several previously established criteria, such

as performance, cost, availability of materials and manufacturing

facilities and personnel. A flow chart is used to describe the process

of evaluating the interactions between shape, material and process to

determine the best combination. The flow chart also illustrates the

use of prototype testing and reports of performance in service to provide feedback which may indicate changes in the design.

Key words: Engineering design; manufacturing processes; materials

selection; systems approach to design.

All too frequently an object is designed with careful attention to its

mechanical function or its esthetic appeal, but with little concern

for the material from which it will be made or the manufacturing

process by which it's made, A designer may have in mind the generic

material (e. g. steel) and a manufacturing process (machine to size

and heat treat), but specific choice of material and sequence of manufacturing processes may be neglected until a very late stage of the design process. The resulting object may perform satisfactorily, but

it probably isn't a very efficient design, either in terms of performance or cost. This approach to design and materials selection invites patchwork redesigns to cover up shortcomings, should the object not perform as intended. In this paper, an approach of optimizing the combination of shape, material and manufacturing processes

is described. The approach is not novel and it is used by good designers, consciously or otherwise. The reasons for describing this

approach are to reinforce the thought processes used by successful

designers and to emphasize the interrelation between shape, material and manufacturing process.

19
What the engineer finally designs to fill a particular function can have

many forms, depending largely upon whatever constraints are placed

upon his design. Identifying and describing the intended function of

the object is the basic constraint on the design. Governmental regulations, interaction with other objects and availability of potential

materials and manufacturing facilities are other examples of such

constraints. In general, a designer has very little control over these

constraints; they are derived from natural laws, public policy or management directive.

CHARACTERIZE REQUIREMENTS &

THEIR RELATIVE PRIORITY

DETERMINE PRIORITY OF

OPTIMIZATION PARAMETERS

HAZARD ANALYSIS

CHARACTERIZE MATERIALS & MANUFACTURING PROCESSES

SYNTHESIS & ANALYSIS

EVALUATION OF DESIGN

BUILD & TEST PROTOTYPE

BUILD & TEST PILOT MODEL

.TEST EVALUATION

FAILURE ANALYSIS

PRODUCTION & QUALITY CONTROL

CUSTOMER USE PUBLIC RELATIONS Figure 1. Flow chart depicting systems approach to design.

The engineer must order the priorities of the various factors which

define the performance of the designed object. The most obvious

performance criterion is the ability of the design to fulfill its basic

20
function. In some cases the criterion is absolute; either the design

works, or it doesn't. Particularly where service life is concerned,

there may be degrees of performance acceptability. Cost, availability of materials and manufacturing facilities and esthetic appeal

may be other criteria for evaluating the design; these criteria, in

order of their relative importance, represent the parameters against

which the design will be optimized. A flow chart, Figure 1, shows

how identification of design constraints and performance parameters

represent the first steps in engineering design.

Hazard analysis is the process in which a designer attempts to anticipate normal and abusive service conditions and the effect of these

service conditions upon the object iinder consideration. Is fatigue

likely to be a problem? Is impact loading likely? Will it be exposed

to extreme temperatures or corrosive environments? What additional

problems would be introduced by abusive service? The engineer must

translate service hazards, as well as design constraints and performance criteria, into factors which are amenable to manipulation during design. This is often a difficult process, demanding careful engineering judgments.

Characterization of materials is another problem facing the designer.

Unfortunately, readily measured and widely published data on tensile

strength, percent elongation at fracture and chemical composition

are not always useful in predicting whether or not a material can be

successfully used or processed as desired. Careful engineering judgment is required to relate available test data to actual applications.

Sometimes specialized testing which sim\xlates the intended use or manufacturing process will provide more useful information than the

usually published data.

Traditional usage of the word design is limited to the next step of the

overall design process, formxilating the shape of the object. Design

may be based upon prior experience, or it may be done by synthesis

or analysis. Relying upon the design of a similar object is convenient and often satisfactory, but there are cases where extrapolation

from existing designs produces disastrous results. Synthesis is devising an object to produce a desired response to a given stimulus;

analysis is predicting the response of a device to a given stimulus.

Synthesis and analysis are closely related and complement each other.

In practice design usually includes all of these techniques.

In principle, evaluation and optimization of a design could be accomplished by writing an equation which describes the performance as a

21
1 function of all possible design, material and process variables, setting its partial derivative with respect to each optimization parameter

j equal to zero, and solving for the optimum conditions. In reality,

that's impossible, so the designer must try to incorporate as many

good features (relative to the various criteria for design) as possible.

Evaluation of tentative designs begins during the previous design step;

obviously inadequate designs are discarded, perhaps before they are

even completed. Pencil and paper evaluation may suffice for simple

objects, or objects whose design is thoroughly understood. For more complex designs, computer simulation or physical modeling may be

useful. Changes can be made wherever necessary, and the evaluation

repeated. Several iterations may be required. When the design appears to be reasonably close to optimum, a prototype is built. A

frequent problem at this stage is that prototype materials and production methods are different from those planned for production.

Similarly, prototype testing may not represent typical service conditions. However, the data obtained from prototype testing does provide the engineer with the first record of the performance of his design. This is an obvious opportunity to further improve the design,

in retrospect, based on prototype performance. In the case of failure of the prototype, the engineer must identify the cause of the failure and take appropriate corrective action. The flow chart shows how

evaluation and corrective action fit into the design process.

The manufacturing process must then be scaled up to pilot lots, then

to full production. Early production units should be tested, both for

quality control reasons and to compare their performance with that

of the prototype. Any shortcomings in performance would suggest

design changes.

Whether the product is used in its manufactured form or incorporated

into some other product, the engineer probably loses contact with it. He must rely on service and sales personnel, who deal with the customer, to provide information concerning performance in service.

In the case of a service failure, he must obtain from the user information about the circumstances of failure. Then he can attempt to

isolate the cause of failure and determine what corrective action is

warranted by the particular incident.

A small bore pneumatic cylinder used to pvmch holes in plastic film

provides an example of how the systems approach to materials in design can function in practice. The perforated film is used in fabricating bags for fruits and vegetables. The piercing is accomplished

while the film is moving by quickly extending and retracting a series

of pneumatic cylinders.

22
When the piercing system was first constructed, stock cylinders

were used. V- slots were milled into the stainless steel piston rods

to act as cutters. Operation was acceptable, but there were several

deficiencies. Locations of the several cylinders used in the system

had to be changed frequently, depending upon the specified hole pattern, and the aluminum moxinting threads would be worn out rather

quickly. Because there are several cylinders operating simultaneously, the process was rather noisy. The piston rod was too soft to

hold a cutting edge for more than a few hundred thousand operations,

and the cutter sometimes became clogged with the circles pianched

out of the plastic film. The cutting edge could be reworked several

times, but the useful life of the cylinders was limited to about a milIon cycles.

NICKEL PLATED ROD GUIDE FELT WASHER

ELASTOMER BUMPERS

FRONT AND REAR

BUNA N "U" CUP SEALS

Figure 2. Cross- section showing internal construction of pneumatic

cylinder designed for punching holes in plastic film.

B/MBA

Figure 3. Pneumatic cylinder designed for punching holes in plastic film. Cutter is part of piston rod.

The customer's report of deficiencies came to the cylinder manufacturer through a routine sales call. This information became the basis for a second cycle through the design process. See flow chart.

Figure 1. The requirements imposed by the customer included sturdier mounting threads, some means of blowing the punched circles

out of the cutter, quieter operation and a goal of 10 million operations

without additional service. How these requirements were met is

shown in Figures 2 and 3. A hard nickel plated steel piston rod guide

provided corrosion resistance and a good bearing surface, but only

when the bearing was well lubricated. Thus a lubricant reservoir be23
came part of the design. To eject slugs from the cutter, the piston

rod was made from small inside diameter tubing. For cutter diameters from 5/16" to 5/8", mild steel tubing, centerless groxand and

buffed to the required diameter and surface finish, was chosen for the

piston rod . See Figure 2. The 1/4" diameter cutter, shown in Fig^-

ure 3, was part of the piston rod; a free -machining medium carbon

steel was gun drilled, then the end of the piston rod was hardened.

Because all piston rods have the same diameter, regardless of cutter diameter, the jig used in grinding the cutters can be used for all

cutter sizes. To reduce operating noise, elastomer bumpers were

attached to both ends of the piston. The return spring was designed

for the fastest possible cyclic operating rate, consistent with the largest possible space for the front bumper and long life without fatigue.

A special lubricant was developed to reduce the need for a mist-lubricated air supply.

The cost of the redesigned cylinder is about twice that of the stock

cylinder. But its service life has been more than five times that of

the stock cylinder; no maintenance or resharpening of cutters has been

necessary. The customer has been very pleased by the new cylinder.

The manufacturer has found a substantial market for this product. By

any standard, it has been successful. The success is due, at least in

part, to the systematic approach to the redesign of the stock cylinder.

2k
PROBABILISTIC METHODS IN DESIGN

Edward B. Haugen

The University of Arizona

Tucson, Arizona 85720

Abstract: It has sometimes been the case during the development of the

various branches of science and engineering that, on looking back,

initial solutions turned out to be special cases of more general

problems. Such has been the case with mechanical design and analysis.

Until recently, many of the implications arising from the probabilistic

nature of engineering phenomena were not fully realized, even though

with the announcement of "Heisenberg' s uncertainty principle," the

whole of science (including engineering) was recognized as ultimately

based philosophically on the concepts of experimental probability. A

partial explanation is that some of the necessary supporting science

and technology had not, until recently, sufficiently evolved to provide

a base for a probabilistic design theory and methodology.

Beginning with a probability based design theory and reducing the variability (in the design variables) to the vanishing point, the residue

amounts to a special case, i.e., a theory based on the deterministic

assumption. It is essentially this special case into which classical

deterministic design theory in its main features has evolved. Over the

past several centuries an enormous methodology and supporting literature has been developed around the special case of assumed determinism.

Although in many classes of engineering problems the variables have

been identified and the forms of their functional relationships cor- rectly described the behavior of many individual variables has until

recently remained imperfectly understood. In addition, probability

mathematics, for coping with functions of random variables-, has only

recently been developed in forms lending themselves to the support of

design synthesis and analysis.

In this paper, the focus of discussion is on a probabilistic design

theory and methodology. Several points of theory in which the assumptions of determinism has led to \intenable statements are examined.

Other points are identified, whose existence under deterministic

assumptions were not suspected.

The task undertaken here is to discuss points of theory and methodology

such that the practicing engineer can readily appreciate its power.

Design examples are given, to a reliability specification, in which

both over and under designing is avoided and variability information is

used not suppressed.

25
Key words: Probability; reliability; random variable; algebra of

normal functions; correlation.

In a general sense the satisfactory performance of an engineered

component or device in its use environment imposes the requirement

that for a defined mission:

strength > stress = S > s .

Except in the simplest cases S and s are each functions of more than a single variable. Thus, usually:

S = f (x^, x^, . . . , x^) (1)

and

s = g(y^, " "

Mathematical analysis and engineering science provide the vehicle for

developing equations (1) and (2)

These models, however, are mute concerning the nature of the individual

x's and y's. If the real conditions were such that each variable

^1' ^2' ^1' ^2' '**' single valued (as assumed in the

past) and the relationships of these variables to the variables S and s were described by the functions f and g, then the transformation from

x , x^, X and y^, y^, y^ to S and s would be single values.

If, however, tfie variables x^^, x , x^ and Y]^' • • • ' Y^ ^^^^

each multivalued, then the transformations to S and s would be distributions of probable values of S and s, the actual situation in engineering.

Since the solution of engineering problems will involve functions of

random variables, mathematically tractable models of random variables

and functions of random variables must be found. Certain familiar two

parameter distributions (normal, log normal, gamma, Weibull, etc.)

provide models that are mathematically simple and by means of which

random variables are uniquely defined by their mean values (y) and

standard deviations (a) . The mean values and standard deviations of

functions of such random variables and pairs of random variables (x and

y) are also easily calculated. Given the independent random variables

X and y, defined by statistics li and a and by y and O , the mean

X X V V

(y tz) and standard deviation O of the functions z = h(x,y) are calcued as follows:

26
Function Mean , y Standard Deviation, cr

a X

a + X

a y.

a + y

a a

X

(1)

(2)

X + y

2 ^ ^ 2

y + a X X

y + y X y

2 2 4

ayXXa + 2 aX

X o ^ + a V X 5

(3)

(4)

X y X X \xa+\io+oo 2 2 2 2 2 2 ,^,

(5) X z z X X y

x/y y /y X z

2 2 2 + y a

Z Z X

(6)

Functions of two independent random variables are often observed in

engineering. Such functions are illustrated in examples 1, 2 and 3:

Example 1 ; A particular spur gear tooth is siobject to a peak force, F,

inducing at each revolution a tensile stress (34,000; 2,600) psi.

Production processing has introduced a compressive residual stress

(11,600; 1,500) psi, at the_base of the tooth. What are the effective

applied stress statistics, s and -6 ? s Effective stress is the difference between the applied tensile and

residual compressive stresses. Thus,

3"= 34,000 - 11,600 = 22,400 psi

4 = /(2,600)^ + (1,500)^ ~ 3,000 psi

Example 2 : If a bending equation were to be applied to make a rough

estimation of the stress in a gear tooth, the moment at the weakest

section would be given by M = Fj^^- Given:

(F, , 4„ ) = (3,800; 460) lb. ^ ^b

i^, -4^) = (0.730, 0.016) in.

27
LINE OF ACTION

PITCH CIRCLE

BASE CIRCLE

WEAKEST SECTION

Figure 1. Beam Model of Gear Tooth

The statistics of M, assuming F, and £ statistically independent, are

D

M = 3,800 0.730 = 2774 in /lb.

2 2 2 2 2 21/2

•4.. = [(3,700) • (0.016) + (0.730) • (460) + (460) (0.016) ] ^ M

= 346 in /lb.

Example -3 : (s, 4 ) = (15,000; 1,600) psi are the statistics of stres

in the material o| a component having a modulus of elasticity (E, 4^) = (29*10 , 3.3*10 ) psi. If the model for elastic strain is

e = — , estimate the statistics of e, ^15^^ 10-^

^ 29-10^

-2, 2 -2, 2

s 4^ + E 4

E s

(1.5'10'^)^(3.3'10^)^ + (29'10^)^ (1.6*10"^)^ = 5.6'10 ^

(29'10^)'^

28
The mean value and standard deviation of arbitrary functions of

independent random variables can be estimated by a Taylor series

expansion. Given the function i|j = f(xw x„, x ), expand ^ about , y , y , . . . , y .

rr,! Then 12m

12 m

^ ^1 ^2 ^m

and

~ [ I ^ 2.1/2 (8) W . dx. X. J 1=1 1 1

Equation (8) provides a valuable tool with which to draw sensitivity

inferences, because the terms (9ip/8x.)^ • cjx.^ disclose the contri1 i. butions of each x. to the magnitude of a , . For instance, it is

desirable to minimize the variability in applied stress, s. An

examination of the expression for a will disclose which variables can most profitably be closely controlled. Further, the tolerances on

some variables (producing negligible effects) may be relaxed.

Example 4 ; Examine the sensitivity of the standard deviation on stress

to the contributions due to applied force and the dimensional random

variables. Consider the cantilever beam formula (rough approximation)

sometimes used to model stress (es) in a spur gear tooth:

s = r— lb/in.

Ft

(a)

= force = (W^; a„ ) = (5400; 360) lb.

I = dimension = (£; O^) = (1.30; .040) in.

F = face width = (F; ^) = (.625; .025) in.

F

t = root thickness = (t; a^) = (.75; .035) in.

The partial derivatives of s with respect to W^, I, F, and t evaluated

at the mean values are:

3s 6Z_ _

t Ft

g 6W

3f=— =92,160 (b)

Ft

29
utilizing equation (8)

:

^ ~ , ,ds .2^2 ,9s, 2 2 ,3s, 2 2 ,7)S,2 2,1/2

n

~

r 2 . -6Wj: -12W I

=

~ "^2> V - '=1' ' ^^^^

°F

^ Ft F Ft

• 0^]^/^ . (c)

Substituting values (b) into (c)

:

- 7 7 7 7 1/2

a ~ [6.39 X 10 + 1.36 X 10 + 2.30 x 10 + 12.5 x 10 ] s

= 15,020 psi. (d)

_ 6W •£

s = = 119,808 psi.

Ft

Equation (d) discloses that the variability in t is the largest

contributor to variability in s. Elimination of variability in t

would result in a reduction in to approximately 10,000 psi.

It is frequently the case that two engineering random variables may

be correlated, i.e., that the random value of one or more variables

in a function is related to the random value of one or more other

variables in the same function. The statistics of functions of

correlated random variables may be very different than the statistics

of identical but statistically independent random variables. Clearly,

in design the requirements of a component or system will be dependent

on the presence or absence of correlations among the relevant

engineering variables. The following formulas are those required to

estimate the statistics of correlated random variable pairs.

30
Function Mean u Standard Deviation O

z z

2 2 z = x + y^ u+y x y

v/a +a^ +2aa Pxy

2 2 z = x- y y-y v/cr + O - 2 o o x y uxypxy

z = xy uu + DO o,/uo+]iu+ao 222222

+2po o +po

222 o o xy xyyxy yx xz xz xy

2 2

y aa a y a a c^cr,,^

, Xr, , yx T r Y X, X r X y „ X y,l/2

z = x/y —[!+(— ] [— - P—] — • [ —r + - 2 p ^ /J

y 'y ' "-y "^y y '2 2 y y y X y y y y y

X y x y

Example 5 : Difference of Correlated Random Variables. A necessary

condition for equilibrium is that (assumed) deterministic forces acting

in any direction must sum to zero:

Zf = 0. (a)

The probabilistic analogue of equation (a) is:

Z(F, 0) = (0, 0). (b)

F

What are the necessary and sufficient conditions for equilibrium, that

satisfy equation (b) for a random pair of interacting normally

distributed force random variables?

The required conditions are:

1. Fi - ^2 = 0

2- S =^F

1 2

3. F^ and F^ correlated, with p = 1.

If F^ and F^ are dynamic loads, they must be in phase.

Example 6 ; Product of Correlated Random Variables. When a bar is

loaded in tension by a force, F, the strain, 6, is induced. Strain

energy, U, is:

f6 Work = total energy = U = — .

51
It is assxamed that F and 6 are linearly related, hence correlated,

p = + 1. Knowing the distribution of F, the distribution of 6 is

defined. The statistics of U are:

U = (1/2) (f6 + p O^) .

0^ = (1/2) [F'a/ + 6^ a/ + ' a/ + 2p¥6 c^a^ +

^fi'^^^''

Example 7 ; Quotient of Correlated Random Variables. The interaction

between loading and deformation is important in mechanical design^

Loading intensity, psi, called stress, s, is a random variable, (s,a ) Al^so, deformation, in. /in., called strain, e, is a random variable

(e,a^)

.

A unique feature of truly elastic behavior is that deformation in the

units above is a linear function of load intensity, since strain is a linear function of stress, they will be correlated, p = + 1.

E = s/e, the ratio of two correlated random variables, is a relationship called Hooke's Law, where:

_ — O O O

E : ^ [1 + (-^) .(-£)- p

_S)

] . e e e s

and

~ i" .""e^ ^s^ , Vs 1/2

e e s e»s

Example 8 : Reliability Calculation. Given the normal random variable

describing strength and stress in an eye-bar:

Strength = (S, 4 ) = 27,000; 3,200) psi.

Stress = (s", 4^) = 18,400; 1,500) psi.

Compute: R and P .

52
Figure 2

00 z

with S and s assumed to be normally distributed, reliability (R) is

given by the area under the curve in the figure above. The integration

is achieved by utilizing standard normal area tables

.

(1) S and s statistically independent, with p = 0.

Write the difference expression Z = S - s.

"z = 27,000 - 18,400 = 8,600 psi.

The transformation of the random variable Z to the standardized normal

R.V.t is:

z-z 0-z -z

t =

z z

Integration from 0 to °° is required, thus t = 0

-2.43

-8600

3534

= - 2.43.

f(t)

-00

Figure 3. Standard Normal pdf

Utilizing standard normal area tables:

R =

J dt = 0.9925, and = 0.0075.

-2.43

55
(2) S and s correlated, with p = + 1.

z = 8,600 psi;

a = /(3200)^ + (1500)^ - 2 (3200) (1500) = 1,610 psi.

With limits: t = - 8,600/1,610 = - 5.34 to «>, integrating as in (1):

R = 0-. 9999995, and = 0.0000005.

Recognition of the real nature of design variables has led to new

theory, revisions to older theory and has opened a Pandora's box of

incidental consequences in mechanical design. The end is not within

sight.

The remaining pages of this paper will be devoted to mention of several

non-trivial considerations arising from unique characteristics of

random variables.

Engineering random variables do not constitute orderable sets. Thus,

it is important in engineering to carefully scrutinize all inequality

relationships

.

The reason for the scrutiny is that with random variables it cannot be

said that one random variable is larger or smaller than another.

It can be seen that these remarks apply in particular to the concept

of principal stresses. These are usually designated as s^ and s^,

acting in the directions of axes 1 and 2 on the planes perpendicular

to which shear stresses are nominally zero. Referring to Figure 4

below the normal stress s^ has been referred to as the maximum

principal stress. The normal stress s^ has been referred to as the

minimum principal stress. Further, there is the suggestion that

s > s . The functional models for stresses s and s may be written:

1 2

s, = OA OC + CD

+ s fs ^^^~s I 7

X y . /, X y, 2 2

OC - CD

s X

+ s

2

s X

- s

2 Yx 2 2 —) + T

3h
tan 2ct)| = —

Figure 4. General Conditions of Two Dimensional Stress

The mean value estimators are s^ and s^, and the inequality, s > s^,

is a meaningful statement. It is permissible to make statements

concerning the maximxam {or minimiam) mean values of principal stress

.

The stress statistics, s and s , cannot, however, be considered

principal, because not only normal but also shearing stresses are acting on the planes perpendicular to the x and y axes.

max 2 V 2

The mean value and standard deviation of T are calculated, using

equations (7) and (8) .

In the 19th century it was generally accepted in engineering that:

(1) stresses were deterministic, and (2) principal stresses were

orderable. These were unstated beliefs when Rankline (1802-1972)

postulated the "Maximum Normal Stress" theory. However, the considerations above have shown such propositions to be untenable, in a strict

sense.

55
Classical maximiom normal stress theory postulated failure whenever

s^ > S or s^ > S , whichever was applicable. Loading was assumed

deterministic static, and s^ was the largest of the (deterministic)

principal stresses.

In the uni-axial stress case, ~ ^3 ~ ^1 simple (random)

variable. The maximiim normal stress theory can Be effectively

employed in probabilistic design of many component types, such as connecting rods, frame members, valves, leaf springs, etc., involving

ductile materials, subject to:

(1) A probabilistic requirement to avoid yield:

R(t) = p' [S^ > s^] .

(2) A probabilistic requirement to avoid tensile fracture:

R(t) = p* (S^ > s^) .

(3) A probabilistic requirement to avoid shear yielding:

R(t) = p(S = 0.577 S > s- ) . sy y 1

In the usual situation in design, s , s , and s^ must be treated as random variables. Thus, probabilistically the inequality ^ ^2 ^ ^3

is usually, in a strict sense, v/ithout meaning. The following

possible triaxial stress cases must, however, be considered:

1. s^, s^, and s^ are statistically independent.

2. s , s , and s are correlated.

If s , s^/ and s are independent, p = 0, then since the ranges are

0 < s. < °°, there will be a finite probability that s >_ s and even

Figure 5. Ordering of Correlated Principal Stresses

56
If the principal stresses are independent, the only possible ordering

is usually that of the mean values:

^1 > ^2 > ^3-

The scatter relationships not being ordered, component probability of

failure must be estimated for each axial direction, and summed.

Considering biaxial applied stress, the_strength of a material S^^ in

the direction of mean principal stress s is very likely not the same

as S 2_^^ direction of mean principal stress s^. Assume that

S , > S ^ and that S . and S . are correlated, p = + 1, in the follow-

.ul u2, ul u2 ing example.

Example 9 ;

The figure above suggests that for high intensity stress excursions

(larger than the mean values), the ordering s^ > s will be valid.

For very low stress excursions, there will be a calculable probability

that s^ < s^.

It is to be noted that the distortion energy theory of failure is not

dependent upon an assumption of ordered principal stresses.

The importance of recognizing the presence of correlation versus

statistical independence has previously been noted in this paper.

The following example will reinforce this point.

"2

5^

i

1

Figure 6. Simply Supported Link

Example 10:

If the reactions, and R^, at the supports of a link (modeled as a simple beam) are to be calculated, considerable insight might be gained

by considering correlation. In Figure 6, P, a, and £ are random variables. If c = a/Z, then. cP; R^ = (1 - c)P,

37
If^ c » -4 and ^ /c « 1, c may be treated as an approximate constant

(c,~0). S and can be considered correlated with P, with correlation

coefficient p ~ 1. Further R and will have distributions of

approximately the same general type as P, since the transformations

from the distribution of P to the distributions of R^ and R^ would be

nearly linear.

The unique considerations arising from the statistical and probabilistic nature of engineering phenomena, discussed in this paper, are far

from exhaustive. Clearly probabilistic treatment will bring important

changes to design practice. However, much important developmental work

remains to be done.

References

1. Haugen, E. B. , Probabilistic Approaches to Design , John Wiley & Sons, publ.. New York, 1968.

2. Haugen, E. B., "Probabilistic Mechanical Design," Aerospace and

Mechanical Engineering, The University of Arizona, 1976, to be

published.

3. Haugen, E. B., and P. H. Wirsching, "Probabilistic Design

Alternative to Miner ' s C-umulative Damage Rule , " Proceedings

of the Annual Reliability and Maintainability Symposium , Philadelphia, Pennsylvania, 1973.

4. Wirsching, P. H. , and E. B. Haugen, "Probabilistic Design of

Random Fatigue Loads , " Journal of the Engineering Mechanics

Division , ASEC , Vol. 99, No. EM6, December 1973.

5. Wirsching, P. H. , and E. B. Haugen, "A General Statistical Model

for Random Fatigue," Journal of Engineering Materials and

Technology, ASME, Paper No. 73-Mat/EE, December 1973.

58
SESSION II

TECHNIQUES

OF

DESIGN

(II)

Chairman: B. P. Bardes

Bimba Manufacturing Company

THE FINITE ELEMENT METHOD IN STRUCTURAL ANALYSIS

Harry G. Schaeffer

Associate Professor

University of Maryland

College Park, Maryland 2O740

Introduction

The finite element method is a technique that allows the analyst to

represent a complex structural system by means of an assemblage of

basic building blocks such as those shown by Figure 1. These basic

building blocks are termed finite elements and include:

• Truss -type elements that resist axial load and torsion

• Beam-type elements that resist bending, shear, axial load and

torsion

• Membrane elements that resist plane loads

• Plate bending elements that resist bending moments

• Three-dimensional elements

By utilizing these elements the analyst can represent such complicated

structures as an entire aircraft, a ship, or a nuclear containment

vessel as shown by Figures 2, 3 and 4, respectively.

The technology represented by computer codes for finite element analys was made possible by the almost simultaneous developments of:

• Large general purpose computers such as the IBM 704 and the

UNIVAC 1102

• The concept of the finite element in the late 1950 's

• Computer science advances including:

• higher level languages

• efficient computers

• data base management

kl
• Numerical analysis including:

• efficient numerical algorithms for solving large sets of

equations

• efficient eigenvalue extraction techniques

• efficient solution of ordinary differential equations

Today this technology is available in large computer codes which are called "general purpose finite element programs". These programs allow

the analyst to model structural systems having thousands of degrees of

freedom.

The purpose of this paper is to:

• Present an overview of the finite element method

• Discuss the technology incorporated in the current general

purpose finite element programs

• Discuss the impact of current developments in computer hardware,

computer science, numerical analysis and engineering mechanics

on future programs

• Discuss the impact of the finite element method on automated

design synthesis.

• Discuss the engineer's and the manager's responsibility in

design certification using a general purpose program.

Overview of the Finite Element Method

The bases of the finite element method are:

• The replacement of the continuous structure by a set of finite

elements which join each other only at a discrete number of

points called nodes as shown by Figure 5.

• The representation of the displacement of the element in terms

of the node point displacements

The element displacement is taken in the form

V = N A (1)

where

V is the displacement vector having component u, v, w in the

X, y and z -direct ions

i^•2
A is node point displacements

N is interpolation functions

Development of Element Stiffness Equations

Our goal is the development of a set of algebraic equations which relate

the element node point displacements to the node point forces in the

following form

^ (e) ^(e) = ^(e) (2)

where

. is the stiffness matrix for element (e) -(e)

A^^g-j is the set of all node point displacements for element (e)

P^g-j is the set of all nodal forces for element (e)

In order to derive the desired set of relations we start with the

principle of virtual work for the element which states that if the

virtual work is equal to zero for any possible virtual displacement in

harmony with the constraints the system is in equilibrium. We can thus

use the principle to obtain equilibrium equations provided we can

express the virtual work in terms of virtual displacements of the

element nodes.

The principle of virtual work for the element is expressed mathematically as follows

6U + 6V = 0 (3)

where, 6U is the virtual work of internal forces and 6V is the virtual

work of external forces.

The virtual work of the internal forces can be expressed in terms of

stresses a_ and strains e as follows

And, the virtual work of the external forces can be expressed in terms

of node point forces P^^-j and node point displacements, A^^-j as

6V. . = - P^ . 6A. . (5) (e) -(e) -(e) ^

h-5
Representation of Strains in Terms of Nodal Displacements

The strains are related to displacements through the well known straindisplacement relations of the form

XX

Bu

3x yy

'xy dy

9u ^ 3_y

- ^ zz 8w

8z

(6)

3w

9y ' zx 9w ^ du

9x 32

If we define the set of strain conponents £, as

^= ^"xx % 'zz V 'zx^ (7)

Then:

e = D V (8)

where,

D =

3x

3y

0

39z

0

38y

0

33x

_3_

3Z

33y

0

3y

33X

(9)

and where v is the displacement vector

Now we can relate the strains to the nodal displacements by using the

approximate displacement functions given by equation (1)

-(e) -(e) -(e)

kk
where we note that the shape functions, N^-^-^ are functions of position;

and that the node point displacements are displacements at specific

points in the structure.

The substitution of (1) into (8) then gives:

The virtual strains are then related to virtual displacements of the

node points by

where

Constitutive Relations

«£(e) = b^e) ^^(e) ^^^^

b. . = D N. ^ (12) -(e) (e) ^

^

The stresses, a, and the strains, £, are related by a generalized

Hooke's law as~follows

a = E £

Since there are six strain and six stress quantities, E is seen to be

a 6-by-6 matrix. The elasticity matrix, E, is symmetric, and thus has

at most 21 material constants. Most general purpose finite element

programs assume this general relationship between stress and strain, but

provide some convenient means for an analyst to define orthotropic or isotropic materials.

Element equations

We now have all the ingredients to define the element equilibrium

equations

.

# Principle of virtual work

-le) ^-^(e) "^(e) " ^(e) ^-(e) /

aT ^ 6e, ^ dv. . - pT„^ 5A. . = 0

with

£ = D V

V = N A
b 6A

a E e

E b 6A dv T

Pt ^ 6A. . -(e) -(e)

= 0

^^(e) ^^(e)

- pT O 6A, . -(e) ^ -(e)

0

k/- >, A, . -(e) -(e) (13)

where

b^ E b dv

(e) (14)

We see that the systematic application of principles of engineering

mechanics allows us to derive an expression for the stiffness matrix

for a general element. Looking back over this process we would note

that we have:

• Made a specific approximation for displacement field through

choice of N

• Assumed linear strain-displacement relations

• Assumed linear stress -strain relation

Element Libraries

General purpose finite element programs contain libraries of element

stiffness matrices which may be specified by the analyst to represent

a complex structure. A library of structural elements that is con- tained in a typical general purpose program includes those elements

shown by figure 1. These elements allow the analyst to model a structure as an assemblage of scalar (spring-type) elements, truss,

beam, membrane plate or solid elements.

Modelling Structures Using General Purpose Codes

The general purpose finite element programs incorporate technology that

allows the user to prepare a mathematical model in the following steps.

k6
• Define node point locations at which displacement variables are

to be determined

• Define the element connectivity; i.e. specify the type of element

and the node points that it connects

• Define the element properties

• Define the material

• Prescribe the loads which may be:

• Node point forces

• Enforced element deformations

• Enforced node point deformations

• Thermal gradients

• Define constraint relationships

By using the user defined input data the general purpose program then

performs the following operations which are transparent to the user.

I. Input Phase

• Reads input data and checks for syntax errors

• May resequence element numbers or node numbers to optimize

bandwidth

II. Generation Phase

• Generate the elemental stiffness matrices

• Transform stiffness from local to global coordinates

• Add elemental stiffiiess to form the system matrix

• Incorporate displacement constraints

• Calculate loads

III. Solution Phase

• Perform matrix decomposition

• Perform forward sweep -backward substitution for each load case
IV. Output Phase

• Calculate derived behavioral variables

• Print results

The specification of input data together with any control parameters

that are required for a specific program will cause the program to

execute and produce results -- and therein lies a real problem. There

is a tremendous amount of technology involved in the program requiring

a knowledge of the following areas

• The theory of Finite Elements

• The theory of Matrix Structural Analysis

• Numerical Analysis

• Solid Mechanics

The use of the general purpose program is not a turnkey operation that

may be intrusted to a junior engineer. Indeed, the validity of the

results obtained from the program depend on the expertise of the user

to prepare a valid model, and to interpret the results correctly. The

results of any program should be viewed with skepticism, and the

engineer must utilize all of the tools at his disposal to make sure that the results are reasonable . . . generally the "tools" are engineering judgement and a "feel" for the problem; tools that the

junior engineer may not have developed as yet.

Technological Content of General Purpose Finite Element Programs

There is probably no such thing as a typical general purpose finite

element program; therefore the technological content that we describe

here is a union of the capabilities of several well known analysis

systems. These capabilities may be characterized in general terms as follows:

• Matrix Abstraction Capability

• Numerical Analysis Capability

• Engineering Mechanics Capability

• Pre -and Post -Processors

Matrix Abstraction

There is an order -of-magnitude difference in the capability of programs

built around a general matrix abstraction capability and those which

U8
are not. The matrix abstraction codes have primitive matrix operators

such as

• Addition

• Multiplication

• Transposition

• Partitioning

• IVIatrix Decomposition

together with logical and executive functions and modelling modules.

These operations are then prescribed in a logical sequence to perform a specific analysis type. NASTRAnCI) is an example of a matrix abstraction program. NASTRAN contains these primitive matrix operations, as well as executive control modules and modelling modules. These modules

can then be linked together to specify a particular solution algorithm

by means of the NASTRAN higher level language called DMAP, an acronym

for "Direct Matrix Abstraction Programming". Specific sequences of

operations which define analysis options which are of interest to a large segment of users are pre -written and stored within the program.

These DMAP sequences are called rigid formats and may be specified

by the user. Because of the inclusion of these preprogrammed rigid

formats a matrix abstraction program such as NASTRAN may appear to be

a general purpose finite element program.

Numerical Analysis

In the sinplest terms a general purpose finite element program must

solve the set of equations

k u = P (19)

The program must, therefore, include an efficient algorithm for solving

sets of linear equations of very large order. By taking advantage of

the sparse matrix topology and either spill logic or virtual memory,

current programs can solve systems represented by thousand of degrees

of freedom at a reasonable cost.

In addition to static analyses there are other problems of interest to

the structural analyst which include

• Static Stability Analysis

• Dynamic Response

• Nonlinear Analyses

k9
In order to solve these classes of problems, a program must include the

following numerical analysis capabilities

• Unsymmetric Matrix Decomposition

• Eigenvalue Extraction

• Solution of Ordinary Differential Equations

• Efficient Solution of Nonlinear Equations

The typical general purpose code incorporates the latest advancements

in each of these areas in order to allow the analyst to solve a problem

in the most cost-effective manner. If the program incorporates more

than one method for a specific capability, the user may be required to

specify the correct algorithm. It then becomes the user's responsibility to specify the most cost-effective algorithm that is appropriate

for the analysis. A very general complex eigenvalue routine for finding

all the eigenvalues and eigenfunctions could be incorporated in the

program which would solve any eigenvalue problem defined by the user. On the other hand, if the problem is that of finding the lowest buckling

load and if the system matrix is symmetric, then the user would be

paying an enormous cost for the generality. Again, it comes down to my

previous point that the technology embodied in these programs is so sophisticated that one should not intrust their use to the junior

engineer. It is cost effective to retrain the mature engineer, if

necessary, in order to bring his experience and knowledge to bear on

the use of these high-technology tools to solve structural analysis

problems.

Engineering Mechanics

The engineering mechanics technology may be categorized by the following:

• Type of Strain-Displacement Relations

Linear

Nonlinear

• Type of Stress -Strain Relations

Elastic

Plastic

Time - dependent

50

I
Incompressible material

Temperature dependent

• Type of Analysis

Static

Dynamic

Stability

All these capabilities may not be available in any one program, but

these capabilities do characterize the types of engineering mechanics

representation that the analyst may request.

Pre- and Post -Processors

The amount of data to be prepared as input to the program, and the

amount of data that are generated by the program may pose a significant

problem in data definition and management. The engineer needs computerized assistance at both ends of the process. The design of these preand post -processors has been investigated by Hemess and Tocher C^) ^ who

identify the tasks of pre- and post-processors as follows:

Pre -Processors

• Generate node point coordinates

• Generate element connectivity

• Generate nodal loads

• Check data

• Reorder node/element sequences to reduce matrix solution time

• Graphic display of input data

• Prepare connectivity data for substructuring

• Format data for target analysis program

Post-Processors

• Plot behavioral variables

• Compute principal stresses and strains

51
• Plot deflected structure

• Generate time histories o£ behavioral variables

• Prepare plots and tables for reports

• Combine solution cases

• Convert output to format suitable for input to another program

Impact of Current Developments

The technology that directly supports computational analysis and design

has seen great recent advances. These include

• The development of mini- and micro -conputers

• The delivery of 3rd generation large general purpose computers

• Low cost computer graphics

• Conputer science developments including compiler design, new

languages and data base management techniques

• Numerical analysis developments including more efficient standard algorithms together with adaptive algorithms appropriate

to automated mesh refinement and nonlinear analysis

• Engineering Mechanics developments include higher order finite

elements and nonlinear analysis

These developments could well have an impact on both the types of

analysis problems that the analyst can solve, and the type of hardware/

software that is utilized.

The availability of relatively low cost mini-computer systems that have

the capability of an IBM 7094 or a UNIVAC 1108 is now a reality. These

mini-computer systems are not characterized by having sophisticated

software support; but due to their relatively low price we will see a trend for large volume users of structural analysis software to purchase

stand-alone mini -computer systems and to convert present software

systems to operate on these mini-computer systems.

While this development will be cost-effective, it is the recent development and the availability of low-cost micro processors and memory that

may significantly change the way that the user does business with the

computer. One can envision the use of a separate micro -processor to

represent each finite element, with all the micro -processors working in

52
parallel. Considering the low cost (approximately $10/chip) and the

small size of these devices, we may well see computing capability of

the order presently embodied in our large general purpose programs

in a desktop calculator -- at a fraction of the cost . . . and the day

isn't too far away. Certainly, we will see the start of these developments within the next five years.

We are also seeing the introduction of low cost graphics display

devices in the $2,000 - $4,000 range. At this price the hardware is

cost effective and we can thus expect to see a typical structural

designer sitting at his desk with a hand held NASTRAN analysis tool

communicating by means of a graphic display unit. Again, the current

technology will support such a fantasy; and its realization isn't too

many years downstream.

Automated Design Synthesis

The objective of structure design is to define a structure system which

is in equilibrium with the load and which satisfies all design con- straints and which extremizes some objective function. Posed in this

way, the problem of design may be considered to be a mathematical

programming problem as follows:

Minimize

<^ (u; a) = 0 (20)

Subject to:

k u = P (21)

i) (u, a) < 0 (22)

where

u is behavioral variables

a is design variables

Great strides have been made in the development of efficient algorithms

for obtaining the minimum weight structure of a specific topology that

will satisfy all behavioral and design constraints for all loading

cases. Generally these algorithms are costly and have not been included

in general purpose finite element programs. There does appear to be a trend toward the incorporation of fully stressed design procedures in

programs such as NASTRAN.

At the present time the reanalysis of the structure for all load cases

is expensive. Current research is being directed at the development of

55
efficient reanalysis algorithms that will significantly reduce the cost

of computer-aided design synthesis and thus bring analysis into the

design process rather than using analysis for design verification.

Manager's Responsibility

The technology represented by the general purpose finite element programs such as NASTRAN, ANSYSC^) , MARC (4

J

, and NONSAPC^) represent the

cutting edge of technology in both structural mechanics and numerical

analysis. While the technology is formidable, these programs respond

to a set of correctly formatted input data by producing a set of output data which is only as good as the input data and the codified

technology.

In my business of adult education, teaching graduate students and professionals to utilize large programs such as NASTRAN, I have become

concerned by the lack of appreciation of the technology incorporated in

the program. There is the danger that we will develop the undergraduate psychology of "don't try to teach me basics . . . just show me

what formula to plug". With the public welfare riding on the correct

interpretation of a computerized structural analysis of an aircraft

component it makes sense to require something more of conputer users than familiarity with the input data specification. Indeed the user

must be well versed in structural mechanics, in numerical analysis and

in computer systems. In short, he's a talented, high priced professional .

At the present time the engineering manager is the only certification

mechanism that is available to guarantee that qualified professionals

are utilizing computerized technology. It must be the manager's

responsibility, therefore, to be aware of the technological content of

programs utilized by his staff and to ascertain that the program users

are qualified to do so.

Considering the qualifications that have been identified for certified

users, it is clear that an organization cannot afford to hire a staff

of them. The answer would seem to lie in providing the qualified users with pre- and post -processors and with a suitable staff to support their

function . . . it's cost effective to do so.

Summary

The purpose of this paper has been to:

• Present an overview of the Finite Element Method

• Discuss the technology available in present codes
• Discuss present developments related to computer codes

• Discuss the use of the Finite Element Method in the design

process

• Discuss the responsibility of engineering management in design

certification

These objectives have been met by presenting a tutorial of the finite

element method and by identifying the technological content of typical

programs

.

The development of low cost hardware such as mini -computers and microprocessors and low-cost graphics is considered and it is speculated

that we will see a trend towards the development, first of stand-alone

mini systems which utilize present software, and then, the development

of low cost confuting systems based on low cost, large scale integrated

circuit technology which will provide the structural engineer with

present analysis capability on a desk-top calculator ... at a relatively low price.

Finally, management's role is seen to be that of protecting the public

by insuring that qualified professionals are charged with utilizing

computerized technology. It is concluded that it is cost effective for

management to support qualified users with pre- and post -processors as well as adequate staff and computer graphics support.

55
References

1. NASTRAN Users Manual, NASA SP-222, Available from Cosmic, Univ. of

Georgia, Athens, Georgia.

2. Hemess, E. D. , and Tocher, J. L., "Design of Pre- and PostProcessors", Structural Mechanics Computer Programs , ed. by Pilkey

et al; Univ. of Va. , 1974, pp. 887-898.

3. Swanson Analysis Systems, Inc.

4. MARC Analysis Corporation.

5. Dept. of Civil Engineering, Iftiiversity of California at Berkley.

6. Miller, R. E. and S. D. Hansen, "Large Scale Analysis of Current

Aircraft," On General Purpose Finite Element Computer Programs , P.V. Marcal (ed.), ASME Special Publication , New York, N.Y. , 1970.

7. Smith, C. S. and G. Mitchell, "Practical Considerations in the

Application of Finite Element Techniques to Ship Structures," Proc.

of Symposium on Finite Element Techniques, U. of Stuttgart,

Stuttgart, Germany, June, 1969.

8. Corum, J. M. and J. E. Smith, "Use of Small Models in Design and

Analysis of Prestressed-Concrete Reactor Vessels," Report ORNL-4346,

Oak Ridge Nat. Lab., Oak Ridge, Tenn. , May, 1970.

56
57
a. Boeing 747 Aircraft. (Cross-hatched area indicates

portion of the airframe analyzed by finite element method.)

b. Substructures for finite element analysis I of cross-hatched region.

j Figure 2 Finite Element Analysis o£ Boeing 747 Aircraft (from Ref . 6) ;

I

58

60
Original Domain

Descritization by Refined Elements

Figure 5 Finite Element Descritization

61
NASTRA.N THERMAL ANALYZER IN INTERDISCIPLINARY

THERMO-STRUCTURAL ANALYSES

Hwa-Ping Lee j NASA GODDARD Space Flight Center

I Greenbelt, Maryland 20771 I

Abstract: The NASTRAN Thermal Analyzer (NTA) is a unique, general purpose

heat transfer computer program that is an integral part o_f the NASTRAN (NASA

STRuctural ANalysis computer program) system. The NTA is based on the

finite-element method and was developed at GSFC but implemented by a conj tractor. This paper stresses the role played by NTA in interdisciplinary the rmostructural analyses and illustrates the versatility and capability of this computer program. NTA, together with its host program, NASTRAN, is best suited

j for analyzing large and complex thermo-structural problems such as those deal- | ing with stresses, deflections or deformations, vibration, buckling, etc., that are t induced by thermal effects. A comparison is made between (1) a unified approach '

to the thermo-structural problem, and (2) an approach that is a combination of

using (A) a thermal analysis computer program employing a lumped-nodal network

which is based on the finite-difference method, and (B) a structural analysis

computer program which is based on the finite-element method. Solution accuracy, procedural efficiency and savings of engineering efforts are discussed.

Key words: Finite-element method; general purpose heat transfer computer

program; interdisciplinary thermo-structural analyses.

INTRODUCTION

The evolution of the finite-element method has been one of the most notable

advances in engineering practice over the past two decades. This method was

originated by the aircraft industry in the structural mechanics field. It was a response to the need to provide precise and efficient solution methods that were

used to calculate the strength and stiffness of air frame structures. The early

analyses used an approach known as the force method, e.g. in [1]*, which was

soon augmented by the displacement method, e.g. in [2], and various hybrid

techniques, e.g. in [3]. The intrinsic nature of the finite-element method makes

it suitable for computer automation; and the ease of varying material properties,

*Numbers in brackets designate References at end of paper.

62
model configuration, the refinement in element representation and boundary conditions are principal advantages possessed by this method. As a result, finiteelement based structural computer programs proliferated, e.g. in [4-8], but

NASTRAN has emerged to be the largest and the most versatile general purpose

computer program for static and dynamic structural analyses using the displacement method [9, 10]. This NASTRAN was initially planned in 1964 with

a view toward establishing a computer program to be used as a standard at

NASA centers, and eventually to be accepted by the aerospace industry as well.

The NASTRAN Thermal Analyzer [11, 12], was originated and developed at

NASA Goddard Space Flight Center as one of the software products resulting

from a research and development program started in 1969. The software implementation was done by a contractor. The objective of this R&D program

was to provide analytical analysis capabilities in the multiple disciplinary areas

[13]. Special attention was paid to the interface problems impeding the reliable

predictions of the thermo-stresses or deflections that might degrade the optical

performance of a large space telescope system exposed to orbital thermal

inputs

.

The NTA is a general purpose heat transfer computer program being integrated

in the NASTRAN system. It is fully capable of rendering solutions of temperature and heat flow in structures subject to various boundary conditions that range

from prescribed temperatures and specified thermal loads to convective and

radiative exchanges at boundary surfaces, in both steady-state and transient cases. This heat transfer computer program was accomplished by the use of

applicable functional modules, which had existed in the NASTRAN, and the addition of new modules including new elements and new solution algorithms. The

feasibility of utilizing partial functional modules originally designedfor structural

analysis in thermal applications lies in the existence of a mathematical analogy

between these two distinct physical systems. The NTA is unique in that it is

compatible with the structural counterpart of NASTRAN with regard to both

problem size and the model representation. As a component of the unified

thermo-structural analysis capabilities in the NASTRAN system, the NTA is

especially suited to compute temperatures for large and complex structures

where thermo-structural problems under the influence of thermal effects are

of concern.

The public announcement of the NTA was made in September 1972 [11] after

this computer program had been delivered. Efforts were then focussed on the

verification of the program, debugging and maintaining, applications [14-16]

,

and new developments [17-19]. The NTA, having a full-fledged thermal analysis

capability, was integrated into the NASTRAN system in its Level 15.5 version,

which was made available for the general user through COSMIC in June 1973.

65
Experience with the NTA started after the delivery of the IBM-360 computer

program tape to GSFC in June 1972. Test problems were designed to verify

program capabilities and to unearth programming defects. At the end of that

year, the Colorado Experiment of the OSO-I was selected as the first flight i experiment on which to test the developed analytical tools [14, 15]. '

The NTA has since been employed to support many scientific instrument packages for various flight programs at GSFC, such as lUE (International Ultraviolet Explorer Satellite) [16], SMM (Solar Maximum Mission), MMS (Multiple \ Mission Satellite), etc. Figure 1 shows some thermal models of the lUE. The ! NTA has been employed by other users, who have contacted us for consultations, | engaged in the areas of nuclear reactors, weaponry, helicopters, automobiles,

j railroad cars, oil refineries, computer hardware, and electronics.

With proven capabilities and the inherent flexibility of the finite-element method,

the viable NTA should be utilized to the maximum extent to benefit engineering

designs in other applications. In view of the fact that thermal analysis using the

finite-element method is a rather new innovation to thermal engineers , a comparison with the finite-element and finite-difference methods will be made to distinguish their fundamental differences. An overview of the NTA organization

together with its functional features will be presented. The procedure of model

conversion in the interdisciplinary thermo-structural analyses will be given.

Solution accuracy, procedural efficiency and savings of engineering efforts will

be discussed.

DISTINCTIONS BETWEEN THE TWO NUMERICAL METHODS

The finite-element method and the finite-difference method are two distinct

numerical methods extremely valuable for solving complex engineering problems.

While the latter has long been a standard numerical process being employed in

thermal applications, e.g. [20, 21], the use of the finite-element method in

computing temperatures is a rather recent endeavor. Since the uniqueness of

the NTA lies wholly in its underlying theoretical basis of the finite-element

method, a comparison of these two methods is appropriate.

In the finite-element method, a continuum with infinite unknown variables can be approximated by an assemblage of finite number of discrete elements that

reduce the problem to finite number of unknowns. The elements are interconnected at the grid points consisting of the end nodes (in 1-D rod element case)

and vertices (in2-Dplanar or 3-D solid element case). The conditions of equilibrium and continuity are established at those grid points. The finite-element

6h
65
method deals directly with an approximate minimization of the functional and I applies an approximation to the terms of a variational formulation. In appljdng

j the Rayleigh-Ritz procedure or the Galerkin weighted-residual procedure [22], a variational principle valid over the entire region is postulated, and the desired ! solution is the one minimizing the functional, which is defined by a suitable in- j tegration of the unknown quantities over the entire domain. Detailed treatments

of using the finite-element method in thermal conduction and its extension to include the effect of radiative exchanges can be cited in [23, 24]. An essential

part of the finite-element formulation is to obtain matrices for individual ele- i ments. The types of matrices are shown in the resulting general heat equation I which is of the form: |

I

[C] {T} + [K] {T}= {Q^^ + {Q^} (1)

Where

{ T } a vector of temperatures at grid points

[ C ] a symmetric matrix of heat capacitance

[K] a symmetric matrix of heat conductance

{Q^} a vector of applied thermal loads

{Q^} a vector of nonlinear thermal loads that depend on

temperature

The general heat equation implies three classes of problems that require separate solution algorithms as will be discussed in the next section. Equation (1)

directly represents a transient heat equation. {Q^}is permitted to be a vector

of constant or time-dependent thermal loads and iQ^} is a nonlinear thermal

load vector arising from radiative exchanges or other temperature-dependent

variables such as the convective film coefficient. The steady-state equations

obtained readily when (t) =idT/dt} = 0 is imposed. However, two separate

solution algorithms are required for steady-state problems of the linear and

nonlinear cases. The linear case allows only an array of constant conductivity

coefficients and a vector of constant thermal loads. The nonlinear case allows a constant or a temperature-dependent conductance matrix and also a vector of

nonlinear thermal loads including radiative exchanges.

The theoretical basis of the finite-difference method, in contrast, is to apply a

direct approximation to the derivatives in a governing differential equation. The

distinctions between these two numerical methods from a user's viewpoint are augmented by the following illustrations:

A simple 1-D rod. Figure 2, is employed. In the finite-element method, the rod

is discretized by specifying nodal point locations along the length of the rod as

66
6?
shown in Figure (2-A). An element is defined as the subregion between two

adjacent nodal points. These nodes are numbered from 1 to m, and these elements are numbered from 1 to N. e is a typical element bounded by the two end

nodes i and j . A linear temperature variation is permitted within an element.

Transferring heat, whether via conductive, convective or radiative mode, is

automatically included in the derived element matrices. In the finite-difference

method, the rod is also discretized but on a totally distinct basis. The rod is

idealized into P elements, as shown in Figure (2-B), which are assumed to be

isothermal segments individually. The temperature of an assumed isothermal

element is represented by a lumped-node located at the center of that fictitious

segment. Conductive couplings are simulated by connecting thermal conductors

(the reciprocals of thermal resistance) between any two adjacent nodes, and

radiative couplings are likewise simulated by nonlinear thermal resistances

being connected between any two affected nodes.

Figures shows the difference between a 2-D conducting medium being discretized

by these two numerical methods. The continuum and the continuous independent

variables are again replaced by a discretized system in each case. Temperature

variables are represented at the vertices of each element in the case of the finiteelement method, and each isothermal area is assumed and represented by a lumped-node at its center in the case of the finite-difference method.

In dealing with irregular shaped regions , the nodes near the boundary, in the

finite-difference method, have to use separate equations other than those representing the interior points, but the finite-element method using triangular and

quadrilateral elements provide a much better approximation of the same region

than is provided by the other method. These finite-elements used at the boundary

are no different than any interior ones. Consequently, no additional programming

effort is required.

THE PROGRAM ORGANIZATION AND FUNCTIONS

An overview of the NTA program organization together with an introduction of

an NTA input deck would be useful for realizing the ease of using this computer

program and for appreciating its capability and versatility. Essential components in the NTA consist of two main parts: (1) the elements which provide

thermal capacitance and thermal conductance matrices and are able to accommodate thermal loading conditions, and (2) the methods of solution.

In modeling, heat conducting structure is represented by a finite number of

idealized heat conduction elements that are interconnected at grid points.

Various forms of heat conduction elements are provided to represent common

68
69
structural members including 1-D rods, 2-D triangular and quadrilateral plates

and axis5rmimetric rings of triangular and trapezoidal cross-sections, and 3-D

solids such as wedges, tetrahedra and hexahedra, as shown in Figure 4. Also

included are scalar heat conduction elements which can serve as linear thermal

conductors connecting pairs of grid points with specified conductances. A

special kind of boundary surface element is provided to serve as a medium for

exchanging heat from exterior environment to the overlaid conduction element

through the attached grid points

Thermal loads in both static and dynamic modes may be applied directly to grid

points, or indirectly via the boundary surface element. The primary t5rpes of

thermal loads included are concentrated loads applied to the points, internally

generated heat within an element volume, and uniform heat fluxes as well as

directional radiant sources applied to the surface of an element.

The NTA has been provided with three specialized solution algorithms that are

able to yield accurate, efficient and stable solutions for the following analyses:

(1) Linear steady-state analysis: This solution is, in essence, a matrix

inversion process.

(2) Non-linear steady-state analysis: The solution is an iterative process

essentially based on the Newton-Raphson methods [25]

(3) Transient thermal analysis including both linear and nonlinear boundary conditions : The integration algorithm is a special form of the

Newmark-/3 Method [26].

The NTA computer program inherent in the program structure of the NASTRAN

consists of a number of mathematical modules (subprograms) that are executed

according to a sequence of macro-instructions. Such a permanently stored prearranged sequence for solving a specific type of problem is called Rigid Format.

The described solution algorithms have formed three Rigid Forniats. The NTA

also permits the creation of a special program by adding and editing the program

stored matrix routines called DMAP (Direct Matrix Abstraction Program).

This is a user-oriented programming language of macro- instructions. A similar

modification, but to a lesser degree, to alter a Rigid Format is called ALTER.

A complete NTA input deck begins with the required job control cards. The

main body of the Data Deck consists of three sections:

(1) Executive Control Deck

(2) Case Control Deck

(3) Bulk Data Deck

TO

The Executive Control Deck identifies the job and the t5^e of solution to be

performed. It also declares the general conditions under which the job is to be

executed, such as maximum time allowed, type of system diagnostics desired,

restart conditions etc. The identification of the Rigid Format is declared along

with any alterations to the Rigid Format that may be desired.

The Case Control Deck makes selections from the Bulk Data Deck, and makes

output requests for printing, punching and plotting. It also defines the subcases,

if any, for the problem.

The Bulk Data Deck contains all of the details of the idealized thermal model

and initial and boundary conditions for the solution.

The functional flow of bulk data cards relative to the definition, constraints and

thermal loadings of the finite-element thermal model is shown in Figure 5. This Bulk Data Deck constitutes the main portion of a complete NTA input deck.

The grid point definition (GRID) forms the basic framework for the discretized

thermal model. All other parts of the model structure are referenced either

directly or indirectly to the grid points.

Heat conduction elements are defined on connection cards by referencing grid

points. The library of heat conduction elements are listed in Table 1. In most

cases the connection card refers to a property card, on which the cross-sectional

properties of the element are given. The property card in turn refers to a material card which gives the material properties. If some of the material properties are temperature dependent, a further reference is made to tables for this

information.

Table 1. Heat conduction elements.

Dimension Type Elements

1-D Linear BAR, R0D, C(/)NR(/)D, TUBE

2-D

Planar

TRMEM,TRIA1,TRIA2,

QDMEM, QUADl, QUAD2

SoUd of Revolution TRIARG, TRAPRG

3-D SoUd TETRA, WEDGE, HEXAl, HEXA2

Scalar ELASl, ELAS2, ELAS3, ELAS4

72
SEQGP

GRID POINT SEQUENCE

CONSTRAINTS

SINGLE-POINT MULTIPOINT

(OMITTED POINTS)

C0RDi COORDINATE

SYSTEM

DEFINITION

CONSTANT FACTORS DELAY DAREA

GRID

GRID POINT DEFINITION

GRID POINT PROPERTIES

STATIC THERMAL LOADS

INTERNAL HEAT GENERATION BOUNDARY HEAT FLUXES

DIRECTIONAL HEAT SOURCE

Cxxx CONDUCTION & BOUNDARY

ELEMENT

DEFINITION

Pxxx PROPERTY

DEFINITION

DYNAMIC THERMAL LOADS

TIME DEPENDENT THERMAL LOADS

MAT XX MATERIAL

DEFINITION

TABLEDi TABLE (TIME)

TABLEMI TABLE (TEMPERATURE)

Figure 5. Functional diagram of the NTA thermal model.

75
The heat boundary element is provided to accept external heat fluxes. It is also

used to form boundary surfaces of the heat conduction elements when thermal

convection and radiative exchanges are present. The radiative surface properties,

emissivity and absorptivity, can be entered on the associated property card.

This card is also used together with the thermal material card (MAT4) if the

convective film coefficient and the thermal capacity of the boundary film are

simulated.

Various kinds of constraints can be applied to the grid point. Single-point constraints are used in steady-state thermal analyses to specify prescribed temperatures. Multi -point constraints are used to specify a linear relationship

among selected grid point temperatures. Omitted points are used to reduce the

number of unknown temperatures in transient analysis.

Static thermal loads consisting of the internally generated heat, boundary heat

fluxes and heat input from directional radiant heat sources are provided to represent various t5rpes of heat inputs . The heat boundary element proportionates

the total energy received by the boundary element surface to the vertices of that

element according to its shape and size. The same loads may be modified by a time function to become time-dependent dynamic thermal loads.

The NTA has been specifically designed to treat large problems with many un- known temperature variables. The only limitations on problem size are those

imposed by practical considerations of running time and by the ultimate capacity

of auxiliary storage devices. There are no dimension statements in the program.

MODEL TRANSFORMATION

In unified thermo-structural analyses, the grid point temperature data, as required by NASTRAN for analyzing thermally induced structural responses, are

provided directly by the NTA. It is achieved by using a pair of compatible

models. Specifically, the input decks for these two physically distinct analyses

share the same basic model in a finite-element discretization. Two separate

input card decks, however, are still required. Either the NASTRAN structural

model or the NTA thermal model can be the first to become available in the backto-back analyses. Alterations of cards in the input deck for the second model

need be made only from the one first in existence to accommodate constraints,

loadings, physical properties, parameters, etc. in accordance with the problem

description. While remaining useable for those cards defining grid points and

connection cards (finite-elements), they generally constitute the main body of a bulky input data deck, which would be the most time-consuming effort to model

and prepare independently. When thermo-structural analyses are performed in

tandem, the structural model, satisfying mechanical requirements and design
criteria, is usually the first to be established. The transformation is, therefore, from a NASTRAN structural model to an NTA thermal model. This conversion process can be done very efficiently, if steps are taken in a systematic manner. The detailed procedure of model transformation and modeling

techniques are available, interested readers should consult with references

[12, 16, 27]. Some essential points are given below to highlight the model transformation process:

(1) Pay special attention to areas with high power dissipations and crucial

thermal contact couplings of the structure to be analyzed. It would

often be the case that the structure model would be of insufficient

representation in these areas, requiring additional modeling to be done.

(2) Select a proper Rigid Format to be used. It depends on considerations

of required temperature results as input to the subsequent analysis

and computer time, etc.

(3) Replace the Executive and Case Control Decks from a punched deck

copy of the NASTRAN structural model by the new ones specifically

designed for thermal analyses.

(4) Remove all Bulk Data Deck cards except those relating to the grid point

definition (GRID, SPOINT), coordinate, connection, property, cards.

In addition, applicable comment cards and common flag cards to the

program code (e.g. BEGIN BULK and ENDDATA cards) are retained for

thermal analysis.

(5) Remove any permanent constraints defined on the grid point definition

cards of the structural model. Use "1" only in thermal analysis when

degree-of-freedom information is requested by any input data cards.

(6) Examine any single-point-constraint, multi-point-constraint, or permanent constraints that have been removed from the structural model to affect the grid points properly coupled into the thermal model.

(7) Use the single-point-constraint to simulate specified temperatures at

grid points for either case of the two steady-state solution algorithms.

Specifically, SPCl is used to list all grid points which are to be held

at prescribed temperatures in the nonlinear steady-state case, and the

temperature values are entered on TEMP cards. The simulation for

specifying prescribed temperatures at grid points in transient analyses,

however, are much more involved. Documentation containing eight

detailed alternative modeling techniques for both constant-valued and

time-dependent temperatures are available [27].

75
(8) Add material cards to define thermal conductivities for each material

type referenced by the property cards in the model.

(9) Add the boundary surface elements (HBDY) to accommodate external

heat inputs to grid points , and to allow radiative and convective thermal

exchanges as well.

(10) Provide radiation matrix if gray-diffuse radiative couplings are present. RADLST and RADMTX cards are required to, respectively,

define the radiatively active CHBDY cards, and the area-times -view

factor coupling between the active CHBDY cards. A specially developed

VIEW computer program [28, 29], using the same radiatively active

CHBDY cards as input data cards, can automatically produce the required RADLST and RADMTX cards for NTA runs.

(11) Add PARAM cards to define the convergence criterion, the maximum

number of iterations , the datum temperature in the absolute temperature scale, and the Stefan-Boltzmannconstantfor the nonlinear steadystate case. Only the last two items are required for the transient

thermal analysis when thermal radiation phenomenon is present.

(12) Define a "guess temperature vector" for all grid points in the model

when the nonlinear steady-state solution algorithm is used. It is an

option to use a "guess temperature vector" in the nonlinear transient

thermal analysis. A special attention must be paid to those grid points

listed on an SPCl card, their temperature values be constrained consistently. In addition, to assure convergence, all guess temperatures

should be no less than 80% of the final steady-state absolute temperature of the respective grid points.

(13) Select appropriate input data cards to simulate various thermal loads

to be applied to any elements or grid points. In transient thermal

analysis, all thermal loads must be referenced via TLOADi (i = 1, 2)

cards, and TSTEP card must be added to specify the number of integration time steps, their length and the frequency of output.

(14) Supply an initial condition temperature set for a transient thermal

analysis

DISCUSSION AND CONCLUSIONS

For an accurate thermo-structural solution, the thermoelastically uncoupled

analysis requires reliable temperature inputs to the NASTRAN structural model.

76
Prior to the existence of the NTA, general purpose heat transfer computer

programs were all of the lumped-nodal network t5^e, which were based on the

finite-difference method. They were not only limited in capacity but seriously

handicapped by incompatibilities arising from the model representations inherent in the two distinct approaches. The intermodel transfer of temperature

data led to extensive interpolation and extrapolation. The extra work proved

not only a tedious and time-consuming process but also compromised accuracy.

The NTA eliminates the need to form two independent models with the concomitant requirement for intermodel interpolation of temperature data.

Table 2 compares the saving of modeling efforts with the two different approaches

when a thermal analysis and a structural analysis are performed in tandem.

With thermal radiation included in the analysis, a conventional approach requires

that three independent models be prepared: A model defines discretized isothermal surfaces for generating view factors, another model gives discrete

temperature nodes representing the isothermal surfaces for the lumped-nodal

network thermal analysis, and a third finite-element structural model is for the

NASTRAN structural analysis. In the unified finite-element approach, however,

the view factors can be generated by a specially developed VIEW program, which

uses the same boundary surface elements as required in the NTA model for

thermal analysis. The structural elements in the NASTRAN structural model

are virtually identical to those heat conduction elements used in the NTA. Only

one model with modifications is required to be prepared.

Table 2. Number of models required for thermo- structural

analysis including radiative exchanges.

Approach Conventional Method Finite-Element Method

View Factor Generation

Thernnal Analysis

Structural Analysis

(NASTRAN)

(Surfaces)

(Nodal-Network)

(Finite Elennents)

(Finite Elennents)

Total 3 Different Models 1 Unified Model

77
The economic advantage of using this computer program can be best expressed

by examining the computer time expenditures required to complete a run. From our experience with the OSO-I Colorado telescope, for example, this

model consisted of a total of 488 grid points. Each steady-state run required

10 min. CPU time and 12 min. I/O time, while a five-orbit transient solution

with 120 time steps required 24 min. CPU time and 28 min. I/O time. It must

be emphasized that structurally compatible temperature data cards were direct

outputs included in these indicated computer time expenditures. The engineeringsaving and cost-effectiveness is evident.

In conclusion, it is evident that the NTA, a finite-element based general purpose

heat transfer analysis capability in the NASTRAN system, is capable of providing valuable support to the engineering design processes. Principal advantages

in employing this for the interdisciplinary thermo-structural analyses are summarized as:

(1) virtually the same discretized finite-element model can be used

for thermal and structural analyses;

(2) temperature results determined by the program can be directly

applied, with no error-inducing and time-consuming interpolation

to the structural model. This permits the calculation of thermally

induced structural effects more accurately and efficiently.

To equally benefit general engineering applications in fields other than the

aerospace area, design engineers should become aware of the capability and

availability so they also can make good use of this proven and viable analytical

tool. The NTA is especially suited to handle interdisciplinary thermo- structural

analyses.

REFERENCES

1. De Veubeke, B. F.: Displacement and Equilibrium Models in the Finite

Element Method, Chapter 9 of Stass Analysis, O. C. Zienkiewicz and G. S.

Holister (eds.), John Wiley & Sons, London, 1965.

2. Turner, J. L., Clough, R. W., Martin, H. C, and Topp, L. J.: Stiffness

and Deflection Analysis of Complex Structures, Journal of Aerospace Science,

Vol, 23, No. 9, September 1956, pp. 805-825.

3. Pian, T. H. H.: Derivation of Element Stiffness Matrices by Assumed Stress

Distributions, AIAA Journal, Vol. 2, 1964, pp. 1333-1336.

78
4. Melosh, R. J., Diether, P. A., and Brennan, M.: Structural Analysis and

Matrix Interpretive Systems (SAMIS) - Program Report. TM33-307, Jet

Propulsion Lab., December 1966.

5. Greene, B. E. et al.: ASTRA-Boeing's Advanced Structural Analyses,

Proc. Int. Symp. on Structures Tech. for Large Radio and Radar Telescope

Systems, MIT, October 1967.

6. von Fuchs, G., and Schrem, E.: ASKA - A Computer System for Structural

Engineers, Proc. ISD/ISSC, Symp. on Finite Element Techniques, Institute

fur Statik und Dynamite der Luft-und Raumfahrtkonstruktionen, University

of Stuttgart, June 10-12, 1969.

7. Logcher, D., et al.: "ICES STRUDL H, the Structural Design Language

User's Manual", Vol. 1 = Frame Analysis, MIT, Research Report R68-91,

November 1968, Vol. 11: Additional Design and Analysis Facilities, MIT,

R68-92, June, 1969.

8. Mallett, R. H., and Jordan, S.: MAGIC - An Automated General Purpose

System for Structural Analysis: Engineer's Manual, Vol. I, AFFDL

TR-68-56. Air Force Flight Dynamics Laboratory, WPAF13, Ohio, January 1969.

9. MacNeal, R. H. (Editor): The NASTRAN Theoretical Manual, NASA SP-221,

with Level 15.5 updates, December 1972, Washington, D. C.

10. McCormick, C. W. (Editor): The NASTRAN User's Manual (Level 15), NASA

SP-222(01), with Level 15.5 updates, June 1972, Washington, D. C.

11. Lee, H. P., and Mason, J. B.: NASTRAN Thermal Analyzer - A General

Purpose Finite-Element Heat Transfer Computer Program, The 2nd

NASTRAN User's Colloquium, NASA TMX-2637, September 1972, pp. 443-

454.

12. Lee, H. P.: NASTRAN Thermal Analyzer - Theory and Application Including a Guide to Modeling Engineering Problems, Volume 1: The NASTRAN

Thermal Analyzer Manual, NASA Goddard Space Flight Center X-322-76-16,

December 1975.

13. Lee, H. P.: Structural-Thermal-Optical Program (STOP), NASA-GSFC

1970 Science and Technology Review. NASA-Goddard Space Flight Center,

January 13-14, 1971; also Significant Accomplishments in Technology:

Goddard Space Flight Center, 1970, NASA SP-295, 1972, Washington, D. C,

pp. 36-40.

19
14. Lee, H. P., and Jackson, C. E. Jr.: Thermal Analysis of the Telescope of

the OSO-I Colorado Experiment, NASA-GSFC Test and Evaluation Division,

Program STOP-Document, Greenbelt, Maryland, August 1973.

15. Lee, H. P., and Jackson, C. E. Jr.: Using the NASTRAN Thermal Analyzer

to Simulate a Flight Scientific Instrument Package, Proceedings of the 20th

Annual Meeting of the Institute of Environmental Sciences, Washington, D. C,

April 1974, pp. 152-158.

16. Jackson, C. E. Jr: Thermal Analyses of the International Ultraviolet

Explorer (lUE) Scientific Instrument Using the NASTRAN Thermal Analyzer

(NTA) - A General Purpose Summary, The Fifth NASTRAN User's Colloquium, NASA TMX-3428, Oct. 1976, pp. 99-118.

17. Lee, H. P., and Jackson, C. E. Jr.: Finite-Element Solution for a Combined

Radiative-Conductive Analysis with Mixed Diffuse-Specular Surface Characteristics, AIAA Paper No. 75-682, AIAA 10th Thermophysics Conference,

Denver, Colorado, May 27-29, 1975.

18. Lee, H. P.: NASTRAN Thermal Analyzer — Status, Experience and New

Developments, The 4th NASTRAN User's Colloquium, NASA TMX-3278,

Sept. 1975, pp. 523-528.

19. Lee, H. P., and Harder, R. L.: The GSFC NASTRAN Thermal Analyzer

New Capabilities. The Fifth NASTRAN Users' Colloquium, NASA TMX3428, Oct. 1976, pp. 119-126.

20. Dusinberre, G. M.: Heat Transfer Calculations by Finite Differences,

International Text Book Co., Scranton, Pa., 1961.

21. Thermophysics Branch: Nodal Network Thermal Balance Program, NASA

Goddard Space Flight Center, February 1968.

22. Hildebrand, F. B.: Methods of Applied Mathematics, Prentice-Hall, Inc. New York, 1954, pp. 120-226.

23. ZienkiewicZjO. C: The Finite Element Method in Engineering Science,

McGraw-Hill, London, 1971, pp. 116-120.

24. Lee,H.P.: Application of Finite-Element Method in the Computation of

Temperature with Emphasis on Radiative Exchanges, Progress in Astronautics and Aeronautics, Vol. 31 — Thermal Control and Radiation, MIT

Press, 1973, pp. 491-520.

80
II

25. Scarborough, J. B.: Numerical Mathematical Analysis, 4th Edition, The

Johns Hopkins Press, Baltimore, 1958, pp. 192-211.

26. Newmark, N. M.s A Method of Computation for Structural Dynamics, Proc.

ASCE, Journal of the Engineering Mechanics Div., EM-3, July 1959, pp.

67-94.

27. Lee, H. P.: On Modeling and the Use of the NASTRAN Thermal Analyzer,

NASTRAN: Users' Experiences, Fifth Colloquium held at Ances Research

Research Center, Moffett Field, California, Oct. 5-6, 1976, NASA TMX

3428, pp. 3-18.

28. Puccinelli,E. F.: View Factor Computer Program (Program VIEW) User's

Manual, NASA-GSFC, Test and Evaluation Div., X-324-73-272, July, 1973.

29. Jackson, C. E. Jr.: Programmer's Manual for VIEW — A Modification of

the RAVFAC View Factor Program for Use with the NASTRAN Thermal

Analyzer on IBM-360 Series Computers, NASA-GSFC Test and Evaluation

Div., X-322-73-120, March 1973.

ii

81
THE USE OF CATHODE RAY TUBE GRAPHICS

AS A COMPUTER AIDED DESIGN TOOL

Alyce Branum

Graphics Product Manager

Engineering Systems Group

Digital Equipment Corporation

200 Forest Street

Marlboro, Massachussetts 01752

Abstract: Considering how efficiently the human mind functions when communicating in pictorial terms, many graphics

experts expected the use of CRT's to sweep through the

engineering community much as the use of TV's had swept

through the home entertainment marketplace. In addition to

discussing this "boom that never happened" and why, the

paper will provide a general background in display technology and its progress over the past decade. Particular

emphasis will be placed on refresh calographic CRT's and

their use in the design process.

Key words: Cathode ray tubes, graphics, design, computer

aided design.

In going over some early papers to trace the history of

Cathode Ray Tubes as design tools and to speculate on some

of the reasons why they have not had as widespread use as

predicted; I came across a 1968 paper by Mr. Carl Machover,

a leader in the field and past president of the Society for

Information Displays. He ' questimated ' (and I would agree)

that there were about 1000 graphics consoles installed in

19 68. Mr. Machover went on to express the opinion then held

by most graphics experts that there would be a billion

dollars worth of graphics terminals purchased in 19 73.

Assuming an average terminal price of $50,000; twenty thousand of these graphics devices would have been sold in that

year! Interesting, especially considering that best

' questimates ' of the 19 75 market placed the volume at just

over $100 million. Today, these terminals are priced from

as little as $5000 to as much as $100,000. Assuming the

average to be $30,000 there were only about 3,300 terminals

placed last year. Somewhat under the estimate for 19 73!

Aside from a depressed economy, how can we account for the

over optimism - or under performance?

82
I propose the following:

1. Graphics and Design have been conceptually confused.

Many of these consoles were predicted for the

engineering, design and construction market - i.e.

for use in CAD. Quite honestly CAD and computer

graphics are not synonymous and have had parallel,

not convergent development paths.

2. Design is far more than a picture. Our prediction

authorities underestimated the level of computer

science and software expertise necessary to tie the

picture to a data base on one hand, and to a human

engineered application interface on the other.

3. When Graphics devices were applied to the design

process they were used to perform complex functions

at a stage in the process where the proof of payoff

was difficult at best; namely, the conceptual design

phase and the drafting phase.

To illustrate these points let us first examine the complexity of the total design process. (This may in fact be

a summary of all of the papers delivered here today on the

various uses of computers in design) . Secondly let us look

at the technology involved in Cathode Ray Tube Graphics and

how they are used in the picture creation and manipulation

process. Lastly, we should explore historic landmarks in

the use of Graphics in CAD - successful and unsuccessful.

Perhaps then, we can make some educated guesses about the

19 80 's.

I am indebted to Dr. John Adams of our Engineering Systems

Group for summarizing the very complex total design process

in a recent tutorial called "The Anatomy of a Project".

Actually, there are six distinct stages to any project.

Preliminary Investigation

Preliminary Specification

Preliminary Design

Final Design

Design Documentation

Commitment to Production

With each of these handled by a variety of people; and all

of them in some way drawing upon computer resources to

assist them in their particular project phase, we could

come up with a computational nightmare. For example, the

85
Corporate Manager uses the computer to do engineering

analysis and financial analysis and to ultimately prove

feasibility. His computer use payoff comes from the ability

to rapidly study alternatives and to provide quicker

decisions on project feasibility. The Project Manager is

the only person remaining with the project from start to

finish. He uses the computer for feasibility studies,

planning studies and preliminary scheduling. The Designer

and Specialties Engineers use the computer heavily for

computational analysis and occasionally for design investigation (PICTURES!) as well as for project control. Farther

down the line, Draftsmen use the computer to help produce

drawings. These will be final design drawings, working

drawings and most important of all, the revisions. While

drafting has been a traditional area for the use of

graphics devices, I venture to say that it is probably one

area in which the payoff is nominal, or at best difficult to measure. Specification Writers use computers for

text processing and total documentation. Finally, we have

the Fabricator using the computer to investigate alternatives and to keep track of scheduling, and of course

money

The conclusion I come to in looking at all this is that the

total process is highly complex and that in fact 'Pictures'

are not really that vital to the cost savings realized by

using computers.

At this point it is probably appropriate to diverge and

take a look at just what these picture generating devices

are and how they are applied to the total process. Besides

the CRT itself, there are several other devices of interest.

First, digitizers which come in many forms, types and

prices, but are all used to capture X,Y coordinate data in

a non-interactive mode. On the output side, plotters are

used to commit pictures to hard copy. Neither of these

devices is interactive and neither actually represents any

part of the problem other than the picture they capture or

reproduce. The CRT, or Scope, is different in that it can

be used in an interactive mode where the screen can be used

to represent and to communicate with the heart of the

problem. These Scopes come in many types and prices. They

can be Storage or Refresh, Random-draw or Raster scan and

in fact there are even many sub-varieties of these. The

most common display system is simply a tube (or bottle) with

a phosphorecent coating on the inside that can be activated

by an electron beam. If the phosphor retains the image it

is termed 'long persistance' and implies Storage graphics

Qk

I
that must be erased before new pictures may be drawn. When

the phosphor is low persistance, the image fades rapidly

requiring regeneration. The scope is then categorized as

Refresh. There is always some kind of instruction decoding

that sets up either a point mode, a character mode or a

vector mode. At Digital, because we are primarily an

interactive computer company, we prefer the refresh displays

because they allow for more dynamic changes and a higher

rate of interactivity. All refresh displays require some

type of memory from which the instruction decode unit picks

up its commands. This is often called a Display Processing

Unit. The Display Processing Unit operates like a CPU and

is attached to the I/O bus, or the UNIBUS, of a minicomputer.

In their simplest form the interpreters decode a command and

place it in the appropriate circuitry for picture creation.

More sophisticated systems add niceties such as Jump and

Sub-routining capability and other features such as hardware

windowing, scaling, and rotation. Some even have instruction look-ahead features. To complicate things even more,

dual scope capability is easily available for those brave

enough to tackle the software involved in providing two

workstations. Too, often purchasers have underestimated

the price of software development and the amount of extra

effort in making the picture actually do something more

than just be a picture. In the early days, few graphics

systems were programmed in any other mode than assembly or

macro code. Essentially 'Realtime' programming was needed

to allow the operator immediate response to his attention

getting interrupts. With faster Fortran, this is no longer

a problem, and most manufacturers of refresh random-draw

displays offer sets of Fortran Callable subroutines to

simplify the process. To date there is no one standard

interactive graphics language. Progress is being made and

the Association for Computing Machinery does now have an

active graphics standards committee ... and that is 16 years

worth of progress

.

Just to get the terminology straight:

Display Files are the array of commands designated

for building the picture. These are the commands

that the Display Processing Unit decodes.

A Primitive is any basic graphic element such as a

point, a line or some text.

A Sub-picture consists of a collection of Primitives

and calls to other Sub-pictures. Each Sub-picture

must be given a Name.

85
Finally, to interact and change any of these lines

or pictures a Pointer is used to refer to specific

Primitives.

Typical Fortran Subroutine groupings include those which

Control the picture program. That is, they initialize or

allow for its dimensioning. There are Drawing routines to

allow for creation of vectors, circles, or dots. There are

Sub-picture capabilities to allow for Manipulation , or the

changing, deleting or inserting of different sub-pictures

and Primitives into the Display File. In addition, almost

all refresh displays provide registers for setting parameters such as blinking and light pen sensitivity. As if

that were not enough, there must be routines for operator

interaction through such devices as joy sticks, track balls,

and writing tablets. The programming involved in attaching

to each of the sub-pictures or picture elements other

attributes such as component prices or nodal interconnections, or other geometries can be complicated.

There are actually two major parts to the problem of

inserting graphics into the design process. First, one

needs to be a fairly sophisticated computer scientist to be

able to understand how to develop data structures that

really relate to the problem; how to not only draw the

picture on the screen, but how to modify it and all its

attributes. Secondly, there is the need for the human interaction to be closely tied to the application so that the

person using the screen relates to his problem as he has

known it in the past. Rarely does the computer scientist

understand the human problems of the operator. Is his arm

going to get tired from holding the light pen? Is the part

he is designing presented so that he can understand it?

Quite typically there is a great need for communication

between the industry expert and the computer scientist.

Therefore we face:

The Design Process... A complex task

Computer Graphics ... Complex solutions

And now how do we put them together?

Actually, the use of Cathode Ray Tube terminals dates back

to the 1950 's when the SAGE system used terminals not

unlike those today in an Air Defense system. Incidentally,

Digital Equipment Corporation supplied Cathode Ray Tubes

86
with the PDP-1. In the early 1960 's General Motors

pioneered by developing a system still in use called DAC,

or Design Augmented by Computer. By 19 65 several large

companies had developed experimental computer aided design

systems using CRT's. Most of these were large and expensive. One of the most famous, the Lockheed CADAM system

is reputed to have cost in the vicinity of $12 million to

develope. I should add that this system is still in use

and is far more than an experiment. Actually close to ten

other aircraft companies are presently using the software.

In 19 69 the use of computer graphics in CAD became more

widespread when several small systems houses brought to

market turnkey solutions to the drafting problem. They

used storage tube CRT's digitizers and plotters under mini

computer control to provide simplified systems for

capturing the data, editing and producing final drawings or

"photo plots" for creating Integrated Circuits. I view

this as a major step in promoting the use of computer

graphics devices as design tools. Despite the fact that

the conceptual design phase remained manual, these systems

were successful because they could be shown to save money

in the manufacturing process and in the design verification,

or editing stage.

A few Turnkey "Total" Design systems have also been successful. These systems are used in the Garment Industry for

pattern layout, in Electronics for printed circuit layout,

and in Manufacturing Engineering for geometric construction

and for the specification of numerically controlled tool

path generation. One consistent item stands out in looking

at these systems. They all could be cost justified on $

savings realized in the production phase of the final

product. The routing problem solved by the REDAC PC design

system not only saved on people utilization, but improved

the product by minimizing the use of copper and ' real

estate' in the PC board.

The Garment Industry systems are quite popular because they

can be justified on fabric savings from optimal pattern

placement. They also produce a better product as the cut

pieces, being more accurate, can be used in automated sewing machines

.

87
One very successful package in the mechanical design area

is that originated by Patrick Hanratty of Manufacturing

and Consulting Services. He has developed a series of

programs called ADAM 3 - dimensional geometric construction

of parts to be machined using numerically controlled manufturing tools. One of the reasons for the widespread use of

his programs is that they combine a high degree of computer

science expertise with an indepth knowledge of the users

needs. They also go beyond the drafting and conceptual

design stage to relate that process to the associated data

such as parts lists, bills of materials and the NC tapes

for driving the tools. Once again payoff can be more easily

proven the closer to the manufacturing process we direct the

effort. On the computer science side, Mr. Hanratty

describes all his geometries in the form of parametric

equations so that a piece is defined as a surface generated

by revolving an arc about a vector contrasted to the series

of connected short vectors used by many automated drafting

systems. Obviously the representation of these surfaces is

not trivial, and requires considerable knowledge of the

geometries involved. From a human engineering standpoint,

the ADAM system allows a part piece to be constructed on

the screen by the designer complete with labels and dimensions and with the ability to be reconstructed altered if

an error is detected. In other words, by modifying the

point in error, all of the subsequent construction in the

process is regenerated to reflect the change.

Most of these complex systems have taken many years and

many iterations to reach the "successful" stage. Not many

buyers are ready to make the man/money commitment that it

takes to place computer graphics into their design process.

All have gained by the efforts of these few "winners", and

still their yield is low. I do not know of one system

vendor grossing $20 million per year.

Alternatively, I believe that the use of graphics in the

design process need not be complex if one capitalizes on

modern software tools and applies the products at the proper

place in the design process to justify purchase by proving

immediate cost savings.

Allow me to illustrate this with a problem recently solved

at Digital Equipment Corporation. We use some very expensive machine tools to punch out simple two dimensional

sheet metal pieces that are incorporated into our computer

88
cabinets and power supply boxes. The procedure for cutting

such sheets was quite lengthy and required the preparation

of a numerical control tape. To create this tape, we use

a simple, non-graphical parts programming language called

QUICKPOINT. Many such programs are commercially available.

The primary problem was how to check the accuracy of the

tape. Initially, this was done by running a first trial

run on the machine. A very expensive solution since

valuable machine time and precious metal were wasted.

Several graphics techniques were considered including the

use of plotters and turnkey drafting systems. When all

commercially available systems were found lacking in some

way it was decided to develope a system of our own. The

solution was a simple, highly interactive technique based

on a Digital Equipment GT46 Display system consisting of a

Refresh Random-draw tube and a PDP 11/34 32K minicomputer

using RTll, a standard operating system and the DECgraphic11 Fortran extensions. The NC tape is loaded and the

screen displays the overall pattern. The operator, a

quality assurance inspector, checks this with the original

drawing by zooming in on any particular part piece and

requesting a dimension check. The inspector can accurately

retrieve dimensions from anywhere on the sheet to a

thousandth of an inch. This accuracy is not possible using

plotter output. He can specify a local origin on the sheet

and measure to or from it. He can place check marks on

inspected areas to record his process. He can check shear

lines for correct piece fallout. He can check the placement of multiple parts on the sheets for maximum metal

usage. He can pick optimum tool paths for maximum production throughput. What was the payoff? Approximately

25 hours a week of expensive machine time formerly used for

first run parts was eliminated, not to speak of the material

savings. How long did it take to develop and who did it?

The system was designed and developed in a little over

three months by a mechanical engineer (not a programmer)

,

Mr. Rich Simon, of Digital Equipment Corporation. Now,

having easily recaptured his original investment he has

justified the purchase of additional graphics and is

investigating other phases of the design process to

graphically automate. I would like to review an article I

recently read in the APEC Journal in which a graphics

questionaire sent to members was published. I was

pleased to note that about 40% of the fims responding had

been using graphics for design for about 5 years each.

Expenditures amounted to 10% of their computer budget.

However, it was their expectations and applications that

concerned me. Few of the applications have been proven

money savers. For example, the use of graphics for

drafting was a 3 to 1 favorite. I contend, it is difficult

89
to justify the purchase of $150,000 of capital equipment,

especially in union shops, on the basis of people replacement, or even improved productivity.

Reading on, at the bottom of desired benefit list was

"improved accuracy". I contend that more graphics systems

would be installed if improved accuracy and improved

products were used as the justification for the purchase

of graphics in the design process.

With todays improved hardware, software and understanding of

the products and problems, we may see a greatly increased

use of Graphics in the CAD process, lE^ we keep our

financial administrators in mind and direct the initial

use of graphics to the simplest, most cost effective phases

of the problem.

REFERENCE LIST

1. Carl Machover, "CRT Computer Graphics In The United

States," Presented at SICOB, France, October, 1968.

2. Patrick J. Hanratty, "Computer Aided Design, Drafting

and Fabrication On Low Cost Systems," Presented at SHARE

XLII, Houston, Texas, March, 19 74.

3. Leo M. Frawley, "APEC Graphics Questionaire Summary,"

Automated Procedures for Engineering Consultants, Inc.,

Summer of 1976.

90
SESSION III

I MATERIALS

OF

DESIGN

III

-COMPOSITESi

1

Chairman: C. 0. Smith

! University of Nebraska at Omaha

J
I'
REVIEW OF THE PROCESSING AND PROPERTIES OF

PAN GRAPHITE-ALUMINUM COMPOSITES

Roger T. Pepper

H. Gigerenzer

T. A. Zack

Fiber Materials, Inc.

Biddeford Industrial Park

Biddeford, Maine 04005

Abstract: The development of a liquid metal infiltration process and

successful fiber-matrix interface barrier coatings has led to an ex- citing new class of graphite fiber reinforced metals. Composite wire

preforms are continuously cast on a pilot production basis using the

liquid infiltration technique. Aluminum composites reinforced with PAN

graphite fibers have shown strengths in excess of 150,000 psi and

modulus of up to 20 million psi. Work is currently underway to develop

low-cost fabrication processes for hardware, using the wire preform material, by the modification of conventional mechanical working techniques

such as metal drawing. A review of the processes and mechanical behavior of graphite fiber reinforced aluminum is given in the paper below.

Key Words: Composite material; fabrication techniques; graphite fibersaluminum; mechanical behavior; pul trusion/drawing; wire preform.

Introduction

The simultaneous advents of potentially low-cost, high-strength and

stiffness multi fiber graphite materials and new process technology to

incorporate them in lightweight metals such as aluminum on a high volume

production basis gives the potential for dramatic improvement in structural reinforcement without significant increase in cost.

High performance polyacrylonitrile (PAN) precursor graphite fibers

of moderate cost have been available for the past several years and have

been widely used to reinforce organic resin matrices. Past attempts to

incorporate these PAN fibers into aluminum have been unsuccessful due to

a lack of chemical stability of the fibers causing severe degradation of

strength. Recently, significant progress has been made in the development of new interface barriers for PAN graphite fiber reinforced aluminum.

Graphite-Aluminum Composite Preparation

Aluminum does not readily wet graphite and attacks the carbon

93
surface forming a metal carbide (AI4C3). Such carbide formation degrades

carbon fibers severely, lowering their mechanical properties. Various

techniques have been developed to overcome the wetting and degradation

problems. Sara(l) shows that graphite-aluminum fiber composites can be

formed by first coating the fibers with a tantalum film by electrodeposition from a fused salt bath, outgassing the fibers by pumping them

down to a very low pressure and submerging the outgassed fibers into a pressurized molten aluminum bath to fill the interstices of the fibers.

A similar process is also described in another U.S. Patent issued to R.

V. Sara(2) in which the carbon fibers are first coated with silver or a silver-aluminum alloy by electrodeposition from a plating solution. The

fibers are then contacted with aluminum foil and the combined foil -fiber

is heated, while under pressure, to the solidus temperature of the foil.

In both of these systems, it is also suggested that the metal coating

can be applied by sputtering or by reduction of salts of the metal.

Whether using silver or tantalum, it is difficult to obtain uniform thin

coatings on the fibers and, in any event, the resulting composites are

not desirable since they contain substantial amounts of expensive, heavy

elements.

Electrodeposition and chemical deposition techniques have also been

used to deposit the matrix material directly on graphite fibers, the

aluminum coated fibers being subsequently hot pressed to form composites.

The major disadvantage of forming composites with such deposition techniques is that, for the most part, the matrix material is usually

limited to a pure metal which, for many purposes, has markedly inferior

properties compared to alloys. A fourth process which has undergone development and laboratory demonstration is a liquid metal infiltration

process (-^^ which uses nickel to promote the wetting of graphite fibers

by liquid aluminum. The nickel coatings required for good wetting and

protection of the fibers from attack by liquid aluminum are approximately 1000 angstrom units thick, giving rise to nickel contents of 5-10

weight percent in the aluminum alloy matrix at 30-50 volume percent

fibers. Such high nickel impurity levels result in a drastic lowering

of the ductility of the aluminum alloy matrix and the lowering of corrosion resistance.

A liquid metal infiltration process has been developed^'^'^

which involves the use of very small concentrations of titanium and

boron to promote the wetting of graphite fibers by commercial aluminum

alloys. The graphite fibers are coated with a layer of Ti/B between 100

and 200 angstrom units thick to promote wetting and to protect them from

attack by molten aluminum. This results in considerably less than 0.5

weight percent titanium and boron in the composite matrix. Similar

concentrations of titanium and boron have been successfully used as grain refining additions to commercial aluminum alloys for the past 20

years and have beneficial effects by increasing alloy strength, ductility, and corrosion resistance. The titanium and boron are deposited on the fibers by the reduction of titanium tetrachloride and boron

9h
trichloride using zinc vapor, the zinc being continually extracted from

the coating step of the process as zinc chloride gas. The coated fibers

are infiltrated by drawing through a molten aluminum alloy bath producing a unidirectional graphite-aluminum composite wire. Significant ad- vances have been made toward developing the Ti/B liquid infiltration

process for future large-scale manufacturing (Figure 1). For example,

CONTINUOUS PROCESS FOR ALUMINUM-GRAPHITE COMPOSITE WIRE

TAKE-UP REEL FOR COMPOSITE WIRE

LIQUID ALUMINUM

BATH

Figure 1 the introduction of textile techniques for feeding fiber into the Ti/B

coating chamber and the use of foundry style aluminum open melt techniques for infiltration both facilitate composite wire handling and

show the way for the future continuous casting of shapes directly from

the melt.

Figure 2 shows a scanning electron micrograph of a polished end

section of a graphite-aluminum composite wire containing approximately

11,000 filaments produced by the liquid infiltration unit. Good fiber

distribution and excellent metal wetting characteristics are apparent.

The graphite fiber in this composite is high modulus T50, a rayon pre- cursor graphite fiber. Prior to 1976, rayon based graphite fibers were

used almost exclusively in the development of metal matrix graphite

fiber composites. Rule-of-mixture tensile strengths and elastic modulus

have been attained utilizing a variety of aluminum alloys with these

fibers. Typical fiber properties are shown in Table 1, and composite

wire and fabricated bar properties are shown in Table 2.

95
THORNEL 50-ALUIVllI\IUM-5 WT. %

MAGNESIUM COMPOSITE WIRE X30

Figure 2

Table 1

TYPICAL FIBER PROPERTIES

Fiber Designation

T50 (rayon)

T50 (PAN)

T300 (PAN)

HMS, HM/PVA (PAN)

Strength

(xl03 psi

)

315 (2172 MPa)

300 (2068 MPa)

360 (2482 MPa)

340 min. (2344 MPa)

Elastic Modulus

(xlOP psi)

57 (393 GPa)

50 (345 GPa)

34 (234 GPa)

50-55 (3i(-5-379 GPa)

96
Table 2

STATE-OF-THE-ART T50 GRAPHITE/ALUMINUM COMPOSITE

MECHANICAL PROPERTIES

Property

Tensile

Strength

Tensile

Modulus

35 v/o T50/A201

Wire

120 Ksi

828 MPa

27 Mpsi

193 GPa

42 v/o T50/A201

Wire

155 Ksi

1069 MPa

30 Mpsi

207 GPa

30 v/o T50/A201

Test Bar

90 Ksi

621 MPa

20 Mpsi

138 GPa

Transverse

Strength

5 Ksi

34.5 MPa NOT TESTED

7.1 Ksi

49.0 MPa

Previous work using PAN based graphite fibers in metal matrices had

shown PAN based graphite to be more reactive with liquid aluminum than

rayon based graphite fibers. Therefore, poor translation of fiber

strength and elastic modulus resulted in the composite (Table 3).

Table 3

MECHANICAL PROPERTY COMPARISON PAN BASED GRAPHITE-ALUMINUM COMPOSITES

(PAST DATA vs. PRESENT STATE-OF-THE-ART)

Property

Tensile

Strength

Tensi le

Modulus

Fiber Content

As Received T300

A201 Alloy (Wire)

50.2 Ksi

346.4 MPa

45.9%

Barrier Coated T300

A201 Alloy (Wire)

180 Ksi

1242 MPa

20 Mpsi

138 GPa

40.0%

During the past year, a significant breakthrough has been made in re- search and development on PAN graphite fiber reinforced aluminum composites. Modified fiber barrier coatings have been developed and demonstrated which prevent attack and degradation of the fibers by aluminum

alloys during liquid metal infiltration and subsequent fabrication of

composite wire into shapes (Table 3). Both moderate and high modulus

PAN fibers have been processed with the new coatings. Typical mechanical properties for the newly developed PAN based graphite fiber rein97
forced aluminum matrices are shown in Table 4. Note the high translation of tensile strength and elastic modulus for fabricated test bars.

These bars were produced from PAN precursor composite wire (approximately .035 in diameter) using a hot pressing technique.

Table 4

PAN BASED GRAPHITE FIBER REINFORCED ALUMINUM COMPOSITE PROPERTIES

Property

40 v/o T300

A201 (Wire)

30 v/o T300/A201

Test Bar

40 v/o T300/A201

Test Bar

Tensile 180 Ksi 125 Ksi 150 - 160 Ksi

Strength 1242 MPa 863 MPa 1035 - 1104 MPa

Tensile 20 Mpsi 21 Mpsi

Modul us 138 GPa 145 GPa

Flexure 16 Ksi

Transverse 110 MPa

Tensile 5 Ksi

Transverse 34.5 MPa

Graphite-Aluminum Mechanical Behavior

The new fiber coating and bonding technology has resulted in the

development of a ductile Thornel 300 (PAN based) graphite fiber reinforced aluminum composite. This achievement of ductility is extremely

important since it implies that toughness or forgiveness necessary to

prevent sudden catastrophic failures in highly stressed hardware can be

developed in graphite-metal composites. To illustrate this composite

ductility effect observed in the newly developed PAN based graphitealuminum composites, a comparison of the mechanical behavior of rayon

based (T50) and polyacrylonitrile based (T300) graphite fiber reinforced

aluminum composites is made below. Figure 3 illustrates some typical

stress/strain data for rayon (T50) and PAN (T300) precursor graphite

fiber reinforced aluminum matrices in the as-cast condition^' K See

Table 1 and Table 5 for fiber and matrix properties, respectively. The

T50/A201 Al wire composite at 42 volume percent fiber shows a completely

elastic stress/strain relationship having a modulus of elasticity of 30

Msi (207 GPa) up to the ultimate tensile strength of 160 Ksi (1103 MPa).

The total strain-to-failure for this composite is 0.53%, which is representative of the strain range 0.45 to 0.55% usually observed for this

system. The stress/strain behavior is typically characteristic of a high modulus high strength composite material failing in a brittle manner. In contrast, the T300 (PAN based)/356 Al composites at 35 volume

percent fiber show a pronounced increase in total strain98
Table 5

TYPICAL MATRIX PROPERTIES

Matrix

Designation

A201

356

6061

NOTE:

Cu

0.2

0.25

Nominal Composition

(Weight Percent)

Ag Si Mg Cr Al

4.7 0.6 0.1 0.3

7.0 0.3

Bal

Bal

0.6 1.0 0.20 Bal

Strength

(xlO^ psi)

20(1

38(2

18(1

45^^

(138 MPa)

(345 MPa)

(172 MPa)

(262 MPa)

(124 MPa)

(310 MPa)

(1) As Cast

(2) Heat Treated

^ (3) Elastic Modulus of All Alloys Approximately 10° psi (69 GPa)

99
to-failure, (1.4%) and show an initial high primary elastic modulus

(typically 17-19 Ksi , 117-131 GPa) with a transition at approximately 20

Ksi (138 MPa) stress level and 0.1% strain to a lower secondary modulus

(typically 13-15 Msi , 90-103 GPa). The T300/356 Al composite exhibits a marked deviation from linear behavior prior to failure at the ultimate

tensile strength of 166 Ksi (1145 MPa). Cyclic loading the T300/A1 com- posites to their yield stress followed by unloading results in elimination of the primary modulus. The value of the resultant average elastic

modulus obtained on cycling is typically 17 Msi (117 GPa). Similar ductile-like behavior is observed, but to a lesser degree for high modulus

PAN based graphite fiber reinforced Al composites (Figure 4).

STRESS/STRAIN BEHAVIOR OF HIGH MODULUS PAN

GRAPHITE FIBER ALUMINUM COMPOSITE WIRE

stress. Ksi Stress. MPa

Figure 4

The linear stress/strain behavior of the T50 (rayon based )/A201 Al

composite in the as-cast condition suggests that the fiber and matrix

are essentially strained to failure elastically. There appears to be no plastic contribution by the matrix to the overall deformation behavior

of the composite. This suggests that, in the rayon based graphite-aluminum composite, the matrix is not bonded to the fiber well enough for

the matrix to contribute its plasticity to the composite when subjected

to a tensile strain. The observed strength at which the compos i te fai 1 s

,

160 Ksi (1103 MPa) and strain (0.53%), is probably due entirely to the

fiber. That this is the case is supported by examining the tensile

fracture surfaces of the as-cast T50/A201 Al composite (Figure 5). It

100
is evident that extensive fiber pull out has occurred, which is indicative of a weak fiber tn matrix bond.

T50 (RAYON BASED)/ A201 TEIVSILE FRACTURE ^/^

SHOWING FIBER PULLOUT

Figure 5

The stress/strain behavior of the PAN precursor graphite fiber aluminum composite (T300/356 Al ) , however, indicates that both fiber and

matrix are strained elastically only up to a stress level of approximately 20 Ksi. At this point, the elastic modulus of the composite

changes from the primary Ei (fiber and matrix elastic) to the secondary

E2 stage (fiber elastic, matrix plastic), and this change is attributed

to the onset of microplastic deformation in the matrix. In addition,

the T300/356 Al composite also shows a definite yield stress of 150 Ksi

(1035 MPa) at 1.1% strain, at which non-linear behavior (macro-plasticity) is evident. No plastic deformation occurs in T50 or T300 graphite

fibers; thus, the non-linearity prior to failure in T300/A1 is attributed to progressive failure of the fibers into discontinuous segments.

This has been observed on tensile fracture edges and on chemically ex- tracted fibers from tested tensile specimens. The total strain-tofailure of the composite is greater than the maximum strain-to-failure

(1.1%) of the T300 fiber because the matrix continues to deform after

fiber breakage but is still reinforced by the discontinuous broken fiber

segments. For this effect to occur, an improved fiber/matrix bond is a prerequisite and must be maintained during progressive fiber failure.

Examination of tensile fracture surfaces (Figure 6) of as-cast composite

wire specimens of T300/356 Al reveals little fiber pull out and

101
T300 (PAN BASED)/A201 Al TENSILE FRACTURE SHOWING

GOOD FIBER/IWATRIX BOND.

Figure 6

pronounced matrix yielding (cellular structure) around the fibers,.

These observations indicate that an improved bond has been achieved between the matrix and fiber which effectively transfers tensile load from

the matrix to the fiber by matrix shear.

Additional evidence of composite plastic behavior is shown by ex- amining progressive stress cycling data (Figure 7). Loading the com- posite to below 20 Ksi (138 MPa) and unloading (cycle 1) shows only com- posite elastic behavior. Loading/unloading above 20 Ksi (138 MPa) shows

E1/E2 modulus transition and retainment of permanent set (cycle 2 through 7) on unloading. The E1/E2 transition point increases rapidly

with small increments of strain indicating that a high rate of strain

hardening is occurring in the matrix (cycle 2 through 4). The abrupt

E1/E2 transition disappears at cycle 5 through 7. True composite plastic behavior is evidenced when one considers the loading and unloading

slopes of cycles 5 through 7. On unloading, the initial slope is parallel to the initial loading slope indicating that the first contraction

on unloading is elastic. The remainder of the slope on unloading to zero load is parallel to the secondary E2 slope on loading. This indicates that the remaining contraction on unloading is plastic. The

elastic contractive force exerted by the fiber on the matrix results in

plastic compression of the matrix. In conventional and completely

elastic materials, all the contraction would be elastic when the load is

102
Figure 7

released. Cyclic loading between 20 and 120 Ksi (138-827 MPa) shows a relatively small hysteresis loss indicating potential high fatigue re- sistance of the composite.

Little fiber degradation has been observed in processed graphitealuminum composites on heating in the solid state at times of up to

three hours. Figure 8. Some reaction does occur, however, when solidus

temperature (typically 500°C) is exceeded. Good compatibility exists

between graphite fibers and aluminum matrix up to 500°C and, thus, the

material is a prime candidate for applications requiring high strength

and stiffness to density at elevated temperatures.

Graphite-Metal Fabrication Development

The anticipated large volume utilization of metal -graphi te com- posites to satisfy the ever-increasing demands of present and future

specialized structural applications will critically depend upon the suc- cessful development of high rate fabrication processes to effectively

produce useful structural engineering shapes. As lower cost composite

preform materials become available (for instance, the utilization of PAN

based T300 fibers and the development of future pitch based fibers),

high rate fabrication processes must be developed which are not only

103
STRESS, KSI HEAT TREATMENT RESPONSE OF

T-300 Gr/A201 Al COMPOSITE

200 r — 1 HOUR — 2 HOURS — 3 HOURS

STRESS, MPa

1379

1310

1241

1172

1103

1034

965

896

827

758

690

621

552

483

414

345

300 400 500 600 700 800 900 1000 1100 1200

149 200 300 400 500 TEMPERATURE Figure 8

_^ , op

600 649 oc

capable of translating composite wire preform properties into bulk com- posite engineering shapes but, in addition, must also be capable to

achieve this end on a cost-effective basis. A development approach

being taken for the fabrication of premium quality graphite-metal structural engineering shapes is to modify conventional high rate metal

working processes for adoption to metal -graphite processing. One such

process having received much development and preliminary success is

controlled temperature pul trusion/drawing for the fabrication of uniaxial graphite fiber reinforced tubes and long cylinders. Other com- posite fabrication processes under development include hot isostatic

pressing (HIP) of complex structural shapes, mechanical hot pressing

of unidirectional and cross-ply structural panels, rolling of graphitealuminum tape preforms and strip, and casting of prewoven fiber shapes.

A large amount of matrix deformation occurs during the mechanical

working of composites causing buckling stresses at the fiber/matrix

interfaces which can lead to interface and fiber surface damage and, in

extreme cases, severe fiber fragmentation. The low strengths observed

in past attempts at hot working composites (extrusion) are thought to occur primarily because the fiber/matrix bond of the composite has been

damaged to the point where only limited load transfer from the matrix to

the fiber can be achieved or because the fibers are degraded by notching

of their surfaces. A pultrusion process is under development to

10k
fabricate high strength metal -graphite structural composites. This

process differs from other hot working techniques in that consolidation

of the fibers and matrix occurs with considerably less overall matrix

deformation. During pultrusion, the fibers are mainly loaded in tension^°'"', the direction of maximum fiber strength. Therefore, little

fiber breakage and interface debonding occurs during the consolidation

process since buckling stresses along the fiber/matrix interface are minimized.

Continuous T50 graphite fiber/A201 aluminum composite reinforced

tubes 1" OD X 20" long and cylinders 2" OD x 30" long have been successfully fabricated to date^-^*^^. Development of the process is being con- ducted on a modified 10 ton draw bench. Graphite-aluminum wire preform

is sealed in an evacuated stainless steel can and drawn through a silicon carbide die at a constant temperature below the solidus or in

the lower part of the liquid + solid range of the composite matrix alloy

(see Figure 9 for schematic illustration of process). Standard graphitebase extrusion lubricants are used to minimize friction between the

outer surface of the can and the die wall.

PULTRUSION OF CONTINUOUS METAL/GRAPHITE COMPOSITES

DIE FURNACE

DRAWING TUBE PACKED WITH DRAWING DIE CONTINUOUS ARRAY OF AL/G

COMPOSITE WIRE

Figure 9

Thin wall tube configurations are a necessity for structural efficiency. Thus, a major goal of the graphite-aluminum pultrusion development is development of fabrication technology for thin wall tubes and

cylinders. To achieve desired wall thicknesses in the range 1-60 mm.

105
hot working of composites is necessary. Current efforts are concentrating on the T300 class of graphite-aluminum composites which have shown

evidence of ductility at room temperature. Floating mandrel tube drawing

techniques are being used with the major deformation mode being 'draw'

(change in wall thickness) with minimum 'sink' (change in diameter).

Extensive studies of the effects of hot working graphite-aluminum com- posites by pul trusion/drawing are in progress with particular attention

being given to the effects of working on mechanical properties and fiber

or fiber-matrix interface damage which may occur during deformation.

Wrought aluminum alloys such as the 5000 and 6000 series, which are not

susceptible to hot shortness, are being used. To achieve low-cost metal

composites in fabricated forms such as tubes and other structural sec- tions, the adaptation of conventional metal working processes to com- posite fabrication will be necessary.

Historically, the most used fabrication technique for metal com- posites has been solid state hot pressing. Sample bars, panels, and

small parts have been prepared in this way by laying-up composite

preform wire or tape in a closed die or can and pressing at just below

the solidus of the matrix alloy (typically 500°C) for about 30 minutes.

Such a fabrication method is not foreseen to have potential in production due to the long set-up and bonding times required, but modified

versions of the process may develop for the closed die pressing/forging

of complex shapes such as jet engine fan blades. Hot isostatic pressing

is another technique that may have application to the fabrication of

complex shapes, where the complexity of the part justifies the use of a slow batch-type of process.

Summary

High strength aluminum and titanium alloys have been developed over

the past fifty years which are superior to steel on a strength to density basis and are now in widespread use. The development of premium

high strength and stiffness graphite fibers over the past ten years

surpasses the above developments of conventional metals in terms of

potential improvement in the strength and particularly the stiffness of

structural materials. For example, graphite fiber reinforced aluminum

is two and three times better than commercial high strength aluminum

alloys on a strength and stiffness to density basis, respectively. An

R&D data base has been obtained over the past three years with graphite fiber reinforced aluminum and excellent mechanical properties

have been attained. New technology is being developed for the production of high strength, high modulus graphite fibers from low-cost pitch,

and it is now anticipated that, within the next five years, performance

equal to that achieved with PAN graphite fibers will be realized. The

development of low-cost pitch precursor graphite fibers will have a tremendous impact on the industry since it will facilitate the production of lightweight graphite-aluminum composites of substantially increased strength and stiffness in comparison to conventional metals at

low increase in cost.

106
References

1. Sara, R. V., U.S. Patent No. 3,553,820.

2. Sara, R. V., U.S. Patent No. 3,571,901.

3. Perry, A. J., U.S. Patent No. 3,720,257.

4. Lachman, W. L. et al , Graphite Composite , U.S. Patent No.

3,860,443, 1975.

5. Lachman, W. L. et al , Graphite Composite , U.S. Patent No.

3,894,863, 1975.

6. Amateau, M. F., Progress in the Development of GraphiteAluminum Composites by Liquid Infiltration Technology , Aerospace

Corporation Report No. ATR-76 (8162)-3, 30 August 76.

7. Gigerenzer, H., Zack, T. A. and Pepper, R. T., Observations on the Mechanical Behavior of Rayon and Polyacryloni trile GraphiteAluminum Composites , Second International Conference on Mechanical

Behavior of Materials, Boston, 1976.

8. Spencer, R. A. P., Advances in Pultrusion of Carbon Fibre

Composites , Paper No. 21, Proceedings of the Second International

Conference on Carbon Fibers, Their Place in Modern Technology,

Plastics Institute, London, 1974.

9. Paton, W., Lockhart, A. H. and Montgomery, I. W., The Use of

Drawing Methods in C.F.R.P. Fabrication , Paper No. 22,

Proceedings of the Second International Conference on Carbon

Fibers, Their Place in Modern Technology, Plastics Institute,

London, 1974.

10. Pepper, R. T., Gigerenzer, H. and Zack, T. A., Development of

Graphite Fiber Reinforced Aluminum Tubes , Proceedings of the 8th

National SAMPE Conference, October 12-14, 1976, Seattle.

107
COMPOSITE INLAYS FOR JET ENGINES

W. Troha

USAF Aero Propulsion Laboratory

Wright-Patterson AFB, Ohio 45433

K. Swain

Detroit Diesel Allison Division

General Motors Corporation

Indianapolis, Indiana 46206

Abstract: A method of using composite material to improve tiorbine

engine blade vibration stability has been demonstrated on a TF41 fan

rig and engine. The procedure uses the high strength and stiffness

properties of B/SiC - titanium composite material, bonded into the

leading edge trip section of an all titanium fan blade. Comparisons

are made between the blade response characteristics with and without

the composite inlay. These comparisons are made for blade natural

frequency, untwist due to rotational speed, and shift in flutter boundary. Engine performance improvements due to blade shroud removal are

also presented.

Key words: Blade flutter control; composite materials; jet engines.

Introduction

:

The objective of this paper is to present the results of

a recent demonstration for improving turbine engine fan blade flutter

resistance by the placement of high modulus composite material in the

outer span portion of a flutter sensitive TF41-A100 first stage fan

blade. The TF41 is a two-spool low bypass turbofan engine in the

15,000-lb (6.7-kN) thrust class. The TF41 Fan-IPC (fan-intermediate

pressure compressor) consists of a three-stage fan with no inlet guide vanes followed by a two-stage IPC on the same spool. The flow entering the fan is split into a bypass stream and a primary stream by a flow splitter located at the inlet to the IPC compressor.

The AlOO compressor first stage rotor contains 25 monolithic, unshrouded titanium blades. Each airfoil is approximately 11 in. (28 cm) long

with a 5-in. (12-cm) tip chord. Tip speed is 1500 fps (457 m/sec) at

design. This blade experienced torsional flutter at a reduced frequency parameter (baj'^/Vj-g^) of 0.53 and at supersonic inlet relative

Mach number.

This paper was first submitted by the authors at the ASME Gas Turbine

Conference in March 1976, under publication No. 76-GT-29.

108
Design and Analysis: A parametric analysis was performed to define the • optimum geometry, location, and fiber orientation of the B/SIC-TI

(boron fibers coated with silicon carbide diffusion bonded on a volume

ratio basis of one to one In a 6AL-4V titanium matrix) composite Inlay.

The geometry of the Inlay area (Fig. 1) was selected so that flutter

stability could be significantly Improved, and to allow adherence to

customary blade vlbratlon-stress design criteria and practical machining practices. Fl^er orientations considered were radial, unidirectional at 45 deg, - 45-deg cross ply," and transverse. The lower boundary of the Inlay was restricted radially to an area of reasonably low

stress In recognition of the potentially high stress concentrations

occurring at the boundaries. The lower boundary was also restricted on

the basis of total Inlay length. A desirable objective Is the limitation of the inlay length to the point where the overall amount of inlay

twist and camber does not Induce excessive "oil canning" in the com- posite plies during the lay-up operation. The upper and leading-edge

boundaries, as well as the inlay cross-section, were established to

obtain maximum inlay volume while preserving a high confidence of fabricability. Fiber orientation was found to have an insignificant effect

on torsional stiffness of the inlay airfoil. Hence, a radial orientation was selected on the basis of its inherent fabrication advantages

by being in tape form. In addition, various inlay cross-sections were

analyzed, as shown in Fig. 2. As expected, the predicted dynamic re- sponses of the core and no-core configurations were essentially the

same. The constant thickness core cross-section was selected since the core provided ply locating advantages during fabrication. Selection of

the chordwise location of the inlay was based on consideration for its

effect on the position of the torsional mode twist axis. A more forward position (toward the leading edge of the airfoil) was considered

desirable. The aeroelastic analyses methods available at the time predicted that the forward location of the inlay would produce a forward

shift in the twist axis of approximately 2 percent C to a 46 percent

C position. Actual blade test, however, showed the twist axis was moved aft by about 2 percent C. Analyses using test results and supersonic unsteady theory have since indicated that this aft shaft had

little effect on stability of the inlay blade.

The calculated vibration characteristics of the inlay airfoil design

are compared to those of the baseline monolithic titanium LPl fan blade

in Fig. 3. Finite-element techniques similar to NASTRAN were used in

the analysis. No engine order coincidence problem was predicted.

The results of the finite-element stress analysis of the inlay airfoil

showed a maximum stress of 98 ksl (676 mPa) at the airfoil hub. This

compares with 104 ksl (717 mPa) for the all titanium airfoil. The

slight blade weight reduction associated with the lower-density com- posite material accounts for lower overall stress levels. A stress of

57 ksl (393 mPa) was calculated for the area near the lower inlay boundary on the suction side of the airfoil. This region was further analyzed to assess the stress concentration associated with inlay scarf

109
angle (Fig. 4). The results of this analysis are presented in Fig. 5 as a plot of maximum joint stress versus scarf angle. It was concluded

that joint tensile stress is not greatly influenced by scarf angle.

Fig. 6 is a Goodman diagram which shows that with a stress concentration of 1-^4 and a mean stress of 57 ksi (393 mPa) , an alternating

stress of - 28 ksi (193 mPa) can be tolerated. This is well above the

usual blade design requirement.

Fabrication

;

A contoured graphite electrode was used to electrical

discharge machine (EDM) the recesses in the blade airfoil. The electrode contained a series of coolant holes in the contour surface to

flush out spark-eroded titanium particles between the tool and the work

piece to obtain good surface finish and control recess geometry. These

holes leave small pedestals on the thin titanium core which must be

removed by a hand operation. A typical airfoil recess after EDM is

shown in Fig . 7

.

The first step in the blade lay-up was the cutting of composite monotape ply shapes. The computer-calculated outline was scribed on the

monotape panel which was then rough cut with a straight-edge shear to

the approximate contour, and hand ground on a rotating diamond disk to

the scribe line contour. A typical set of finish cut plies is shown in

Fig. 8. The monotape plies were etched to final thickness after rough

cutting. After final edge grinding, the plies were given a light dip

etch in the same solution, rinsed in water, dried, and stored for assembly. The blade recess and adjacent areas were scrubbed, acid

etched, rinsed, and dried prior to lay-up. The clean blade was then

positioned in a resin filled fixture, contoured to the blade geometry,

with welding ground wires clamped to the platform and blade tip for

capacitor discharge tack welding of the composite plies. Optimum tack

welding parameters were determined such that diffusion bond type rather

than nugget type welds were formed to minimize stress concentration

factor. The first composite ply was tack welded to the Ti-6A1-4V core,

with each succeeding ply tack welded to the ply below. A 0.020-in.

(0.51-mm) Ti-6A1-4V cover sheet was then welded to the blade airfoil

around the periphery of the recess, on each side of the blade, in an evacuated chamber. The final closed recess must be evacuated to obtain

successful diffusion bonds.

The diffusion bonding was done using a hot isostatic press (HIP) . A

typical blade after diffusion bonding is shown in Fig. 9. Figs. 10(a)

and 10(b) show the good filament array and excellent bond quality

characteristic of all examined sections. The final operation was to

hand finish the airfoil surfaces back to original dimension as shown

in Fig. 11.

Test and Evaluation: The first three natural frequencies of 27 blades were determined with the aid of an acoustical siren and a 30,000-lb

110
(13.3-kN) base load on the dovetail. Table 1 is a listing of the

average frequencies of 27 blades for three modes, and a comparison with

predicted values. The blade-blade scatter of the composite inlay

blades was approximately the same as that observed for a production fan

blade. Holographically determined mode shapes showed that the tor- . sional mode twist axis was shifted approximately 2 percent C, toward

the trailing edge in the tip region, to a 50 percent C position.

The fatigue or endurance strength of the inlay airfoils were determined

by a stress increment test. In this type of fatigue test, the airfoil

is vibrated in a particular mode for 5 x 10^ cycles at successively

higher alternating stress levels until failure occurs. The initial

blade fatigue test was conducted in the first torsional vibration mode.

First torsion was selected because the high speed flutter encountered

during rig testing of the standard all-titanium fan blades was a first

torsional mode response. The blade failed at an endurance strength of

- 42 ksi (290 mPa) in the parent titanium material about 1 in.

(2.54 cm) up from the platform on the trailing edge. The 1 42 ksi

(290-mPa) test value is comparable with that expected at the trailing

edge radius of an all-titanium blade. This would indicate that the

fabrication of the inlay blade does not degrade the fatigue properties

of the titanium portion of the airfoil and that the fatigue properties

in the inlay region are adequate if the blade were excited in first

torsion during operation. However, the stress survey indicated that

the relative stresses in the inlay area are very low when the blade is

excited in first torsion and that near-maximum alternating stress in

the airfoil does occur near the lower boundary of the inlay when the

blade is excited in the second bending mode. Subsequent blade fatigue

tests were^conducted in the secoijid bending mode. Alternating stress

levels of - 13 ksi (90 mPa) and - 23 ksi (159 mPa) in the parent airfoil material adjacent to the inlay interface were run for 5 x 10^

cycles each without failure. Failure was eventually induced at the

dovetail-stalk^radius after 3.6 x 10^ cycles into the third stress

level step of - 33 ksi (228 mPa) . Although the fatigue properties at

the interface were not defined, the test demonstrated adequate endurance strength for the planned rig and engine testing. In addition, it

indicated that the residual stresses and local stress concentrations

associated with diffusion bonding and tack welding were not of catastrophic magnitude. An inlay interface fatigue failure was produced

when a test specimen was vibrated in a free-free mode. A fatigue

strength of - 37 ksi (255 mPa) was demonstrated. Based on these tests

and analytical predictions, the inlay interface produces a notch factor of approximately 1.5. Subsequent specimen tests have shown a fatigue strength of 50 ksi at the composite/titanium interface. A limited

amount of static or bench type bird impact testing of one composite

inlay blade was done at the University of Dayton Test Facilities,

Dayton, Ohio. Preliminary results indicate that the inlay blade withstood an RTV 560 bird-strike (3/4 span) at a velocity of 1436 fps

(438 m/sec) . A 2.4-oz (0.068-kg) slice weight was obtained which (at

111
the given velocity) produced an impact approximately equivalent to a 3-lb bird-strike in an engine. The maximum blade tip deflection at

impact as determined from high speed photography was approximately 2 in.

(5.1 Cm). After testing, the blade was visually and X-ray inspected

and was found to be undamaged.

An uninstrumented spin proof test was run to demonstrate the structural

integrity of the B/SiC-Ti inlay blades. A first stage fan wheel was assembled with 21 "dummy" blades, two standard titanium blades, and two

composite material inlay blades. The "dummy" blades were sized to produce centrifugal loads comparable with those of actual blades. The

assembly was accelerated to 115 percent design speed and maintained at

that speed for 3 min. without failure. The planned rig/engine running was not to exceed 105 percent design speed. A second spin test was conducted for measuring airfoil tip untwist characteristics. High

speed photography was used to record tip rotation of composite inlay

and standard blades at various rotor speeds. Fig. 12 is a comparison

of the untwist characteristics for the two-blade types with predicted

values

.

An instrumented first stage fan stage with B/SiC-Ti inlay blades was installed in a TF41 Fan-IPG rig. Eight of the 25 inlay blades were instrumented with a total of 15 dynamic strain gages. Nine additional

dynamic strain gages were installed on the other stages of the rig. To

determine if the original flutter boundary of the all-titanium blades

had been eliminated, a compressor performance map was generated by

slowly accelerating from 90 to 105 percent corrected speed at succes- sively higher bypass pressure ratio loadings. The high speed flutter

phenomenon was encountered at a corrected speed of approximately 102

percent and a relative bypass pressure ratio of 1.17. These values are

in contrast to a 90 percent corrected speed and a relative bypass

pressure ratio of 0.92 at which high speed flutter was experienced

during rig testing of the all-titanium unclappered fan blades at com- parable inlet temperatures and pressures. The frequency at which

flutter occurred for both blade sets was characteristic of the first

torsional mode as identified in the bench test programs. This indicates little or no modal coupling existed, i.e., pure torsional flutter. This significant expansion of the flutter-free operating regime

of the compressor is best illustrated by the comparison in Fig. 13.

Note that the inlay blades moved the unstable boundary from within the

operating envelope to a region well above the estimated operating line

of the engine. Thus, on the basis of the rig test results, the B/SiCTi composite material insert showed sufficiently improved stability

over the TF41 unshrouded first stage fan rotor airfoils to be acceptable for operating in the engine. This improvement was later verified

by actual engine testing. A total of six hours of rig running time was accumulated. Analysis has isolated the stability improvement demonstrated by the B/SiC-Ti inlay blade over that of the baseline configuration as primarily the result of the 5 to 6 percent higher torsional

112
frequency of the inlay blade. Therefore, it is estimated that placing

the composite over the airfoil full-chord would improve high speed

flutter resistance 12 to 14 percent. The rig aerodynamic performance

characteristics show a 2.5 percent airflow increase and a 2.4 percent

higher efficiency at takeoff thrust conditions using inlay first fan

rotor airfoils as compared to shrouded airfoil blades. These improvements are attributed to elimination of the shroud aerodynamic losses

and improved matching of the fan stages at high rotor speeds as a con- sequence of increased first rotor airfoil untwist.

As stated previously, the results of the rig test indicated that the

B/SiC-Ti inlay improved the stability of TF41 unshrouded first fan

rotor airfoils to a level outside normal engine operating requirements

To demonstrate the engine operating characteristics using the inlay

blades for adverse operating conditions, both increased inlet temperature and pressure running was conducted on a TF41 engine. A test was

conducted at 78 F (298.7 K) , 28.25 in.-Hg (95.4 kPa) abs, and 103.9

percent of design corrected fan rotor speed. Additional tests were

conducted at ram pressures of 43 in. (145 kPa) to 49 in.-Hg (165 kPa)

abs with inlet temperatures of 84 F (302 K) , 146 F (336 K) , 200 F

(366.5 K) . No development of high speed flutter was indicated at any

time during a total accumulation of four engine test hours. The maximum speed pressure point run for the engine test is plotted in

Fig. 13. Demonstrated engine performance improvements due to replacement of the shrouded first fan rotor airfoils with inlay airfoils verified the rig test results. At rated thrust reductions in specific

fuel consumption and turbine inlet temperature of approximately 1.1

percent and 12 F (6.7 K) , respectively, were demonstrated.

Conclusions

The fabrication of viable B/SiC-Ti composite inlay fan

blades has been successfully demonstrated. This offers the designer a tool for controling flutter without degradation in aerodynamic performance, as compared to the more conventional approach which employs

part span shrouds and/or increased airfoil thickness.

Nomenclature: Abbreviations:

b = blade chord length/2 B/SiC-Ti = metal matrix composite

C = blade chord length material consisting of silicon

R = radius to engine center- ^^^^"^ 5°^°^ ^^^^^^

1 diffusxon bonded in a titanium ime

rpm = revolutions per minute

T/C = blade thickness to chord = isostatic press (diffu-

-j-g^^^Q sion bond process)

VreL = air velocity relative to = temperature vull^l^^Q^ canizing silicon rubber. A

v/o = volume fraction, i.e., the Product of General Electric Co

ratio, based on volume,

that the fiber portion of

a composite material is to

the whole

T = torsional frequency

113
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STRESS CONCENTRATIONS IN COMPOSITE MATERIALS

G. Kardos

Carleton University

Ottawa, Canada

KIS 5B6

Abstract: The use of high strength laminated composites has added a new dimension to the structural designers arsenal. The designer may

now tailor the properties of the material to match the load requirements of his structure. But the use of laminated composites is a mixed blessing; many design concepts have to be revised when applying

these composites. The most important of these is the significance of

stress and strain concentrations.

The analysis of simple laminated composite panels containing a hole

demonstrate that neither the stress nor the strain concentration has

the same significance as in isotropic homogeneous materials. Comparison with published test data shows that proceeding on the basis of

older concepts can lead to erroneous results.

Key Words: Anisotropy; composite laminates; failure; strain concentration; stress concentration.

Introduction

The use of high strength laminated composites holds great attraction

for the structural designer. The ability to tailor the material to fit

the load promises lighter, stronger structures. Analytical techniques

are available whereby the designer can predict, with some assurance,

the performance of the composite he selects. Unfortunately, this new freedom has made the situation more complex. The designer can no longer simply analyse his stresses and then simply choose a material

that matches his requirements. Now he must concern himself with the

internal structure of the composite before he can undertake his analysis.

In spite of all the advances made in analysis and theory, design must

start conceptually within the designer. The nature of this conceptual

design is dependent on his experience and his qualitative understanding

and prediction of the performance of his design. If his qualitative

understanding or experience falls short of reality, the best that can

be expected is that later detail analysis will reveal the shortfalls^

They can then either be corrected or the designer can move back to the

I beginning armed with the new information. But much more costly is the

121
case where misinformation results in the critical analysis never being

made, in this case the shortcomings may not show up until the design

is tested.

To take full advantage of laminated composites the designer should

select his composite so that the properties are anisotropic. When this

happens many of the responseswe take for granted no longer hold true.

Without the proper precautions simple tensile loads can cause bending,

failure may first take place in unexpected locations and directions;

and the maximum stress at a discontinuity may not be responsible for

failure.

One of the principal tenents of designing with isotropic homogeneous

materials is that where there are geometric discontinuities in a loaded structure the stress concentration will be the limiting factor

in the design. It follows that if the maximum stress (usually evaluated as the nominal stress times the stress concentration) is less

than the working stress of the material the structure is satisfactory;

and if the maximum stress is greater than the working stress failure

will occur at the point of maximum stress concentration.

With laminated composites the elastic constants are orthotropic, therefore the stress distribution about a discontinuity is different than

with isotropic materials, the magnitude and location of the maximum

stress are dependent on the elastic orthotropy. An additional complication is introduced because the strength properties are also orthotropic, which means the point at which failure takes place may not be

at the point of maximum stress. Finally with laminated composites

the strength of each lamina is different and failure may take place in

different lamina at different locations. Because of these properties

of laminated composites the designer must take great care in the

assumptions he makes when using composites having discontinuities.

To acquire some appreciation of these factors a computer study was undertaken at Carleton University to explore the characteristics of a simple laminated composite panel with a hole subjected to tension or

shear. The results are presented here to demonstrate what can be

expected due to stress concentrations in composites.

Composite Design

The details of analysing laminated composites will not be given here,

several excellent publications are available (Ref. 1 and 2) that

detail such analysis. We will limit ourselves to simply outlining

procedures that are necessary to design a satisfactory composite

structure.

122
The necessary design steps are illustrated in Figure 1. The process

is iterative, a composite configuration is chosen, analysed, and if

found not to be optimum a new composite configuration is chosen and

the process repeated.

A laminate composite design starts with the choice of the basic con- stituents, the reinforcing fibre and a matrix binder. In our analysis

we used boron fibres and an epoxy matrix. From the known properties of

the fibre and the matrix the properties of the individual lamina is

determined. The properties of the lamina will be dependent on the

volume fraction of fibre and can be computed using micromechanics

theory. The eTastic and strength properties of the lamina are orthotropic and are defined by a 3 x 3 symmetric matrix.

The lamina are assembled into a laminate. The elastic properties of

the laminate are determined considering the properties of the individual lamina, the number of layers, the orientations of the lamina

and the sequencing of the lamina. Lamination theory is used to

establish the consti tuati ve behavior of the finished laminate. The

result is based on the assumption that the strain will be continuous

throu^ the thickness of the laminate. The resultant orthotropic

laminate properties will be given by a 6 x 6 matrix. These relate the

applied stresses and couples to the deflections. The preferred

practice is to make the composite symmetric about the mid-plane which

decouples the bending and direct stress deflections, this is called a "specially orthotropic" laminate.

Once the constitutive behavior of the proposed laminate is established,

the structural analysis can proceed. The analysis must be in terms of

the orthotropy of the material. A number of closed form solutions for

structural elements are available (Ref. 3), and if closed solutions

are not available, finite element techniques can be used to give

specific solutions. Closed solutions are available for a panel with

a hole. The analysis is to give the stress and strain distribution

and in some cases the bending couples and deflections on any element

in the structure.

The local strain must then be resolved into the direction of the

principal directions of each lamina to find the local stresses and

strains in each lamina. The principal stresses and strains are then

checked against the orthotropic strength properties of the specific

lamina.

A number of theories of failure for composites have been proposed. In

the analysis discussed here the maximum strain theory has proved to be

as satisfactory as any other. For our purposes we assumed that the

structure would be considered to be overloaded if any single lamina

fails. In practice this would be considered a conservative design

125
procedure, the redundant nature of composite laminates permits con- siderable overloads to be carried even after a single lamina has failed.

If the failure analysis shows that the laminate selection is unsatisfactory, either because a lamina has failed or because all lamina are under-stressed, the designer must return to the beginning. On the

basis of his experience with the analysis he selects a new laminate

construction and proceeds through the analysis again.

Analysis Results

Greszczuk (Ref. 4) investigated the stress concentration in a single

ply composite, Fig. 2. He shows the tangential stress around a circular ' hole for different orientations of the princiji&l material direction | with respect to the loading direction. In this case the value and | location of the maximum stress shifts with orientation but the failure

j load and location remain very close to the same value. Stress concen- [ tration for a two lamina cross-ply composite, is given in Fig. 3, but j failure is not indicated. 1

In the Carleton studies (Ref. 6) four-nly symmetric laminates were considered with loading in the principal material direction. The

individual lamina directions a were symmetric with respect to the

principal material direction, i.e. [+a/-a/-a/+a] . ^

Figure 4 shows the tangential stress at selected points on the hole

periphery with changes in lay angle a. The figure also shows the

| absolute value of the maximum tangential stress with respect to lay ; angle. This indicates that in this case the stress concentration

j would be a minimum with a lay of a = 650. I

I Figure 5 shows the variation in tangential stress around the periphery

for three different constructions a = 0°, a = 65^, and a = 90°. The

point where the laminate will first fail is also indicated. It should

be noted that in no case does failure take place at the location of

the maximum tangential stress.

Figure 6 shows the tangential strain for selected points with changes

in lay angle a. The strain is always maximum at 9 = 90°. The smallest

maximum strain is at a lay angle of 15°.

Figure 7 shows the tangential strain distribution around the periphery

of the hole for selected lay angles. The points of failure do not

always correspond to the maximum strain locations.

Figure 8 compares the load capacity of a panel without a hole to the

load capacity of the same panel with a hole, for different lay angles.

The strength reduction ratio is shown. The optimum lay angle without
a hole is a = 10^. The optimum lay angle with a hole is a = 19° and the

minimum strength reduction ratio is at a = 8°.

Figure 9 is an expanded version of the load capacity vs. lay angle data

for a panel with a hole. It shows that the optimum panel design (a = 19°) would have a capacity of approximately 16,000 lbs/in. width.

Whereas a panel with a minimum stress concentration construction (a = 650) would only have a capacity of 2500 lbs/in. width.

Figure 10 shows the stress around the periphery of the hole for three

different composites when the specimen is subjected to pure shear.

With shear loading as with tensile loading the location of the first

failure is not at the point of maximum stress. Although in the ex- pected ideal lay (45°) it is nearly so.

Figure 11 shows the load capacity of a shear panel with a hole with

changing lay angles. It can be seen that the maximum capacity is not

at the expected ideal lay angle of a = 45° but at lay angle of a = 35°

and a = 550. Although in this case the load capacity is near constant

over the range a = 30° to a = 60°.

Significance

It can be seen from these analysis that neither stress concentrations

nor strain concentrations are a reliable criteria for load capacity of

a laminate composite structure. Nor can they be used to identify the

location of the probable point of failure. In fact it can be said in

general that stress and strain concentration as normally used or understood in isotropic design are not a meaningful index in the design of

anisotropic composites.

Furthermore since the objective of structural design is to optimize the

load capacity of the structure, the use of stress concentration as a design criteria would lead to a highly unsatisfactory sub-optimum

design (in this example a = 65° instead of a = 19°).

In addition, the introduction of a hole in a composite panel does not

simply reduce the load capacity, as it does in isotropic materials. It

requires re-evaluating the internal structure of the composite itself.

The introduction of the hole requires changing the direction of the

lamina lay to achieve optimum strength.

Comparison with Experiments

The results presented are based upon computer analysis. But how close do

these reflect what happens in a real composite? To determine if the

analysis could predict the performance of a real composite an analysis

was made of composite tests by Daniel and Rowlands (Ref. 5).

125
Tests had been carried out on boron-epoxy laminated composite panels 10

in. wide by 26 in. long, with a central hole. The laminate was a nine

ply specially orthotropic laminate with lay angles:

[90O/0O/+45O/-45°/0O/-45O/+45O/0O/90O].

Strain gages were implanted at 9 = 90° measuring the strain in the

tangential direction. The local strain was plotted against the

nominal applied stress.

An analysis was carried out on this laminate construction using the

elastic constants for the laminate quoted by Daniel and Rowlands, the

lamina allowable stresses and strains had to be taken from typical

values quoted elsewhere. The test results and the results of the

analysis are shown in Figure 12.

The analysis indicated the first failure should take place at 20,000

psi at 9 = 90° in the outer layers (a = 90°). Further increase in load

indicates the next failure should take place at 29,000 psi at 9 = 70°

in the third, fourth, sixth and seventh layers (a = + 40°). Further

loading should cause final failure at 34,500 psi at 9 = 90° in the

remaining lamina (a = 0°). In the analysis it was assumed that there

would be no change in elastic constants because of local failures.

The first failure predicted by analysis corresponds almost exactly with

test results, probably because the location of the failure corresponds

to the placement of the test gages. The second failure predicted by

the analysis does not have its counterpart in the tests. This is

probably because the failure is predicted to take place at 9 = 70°

where there is no test gages. As a check of this, an analysis was made

to determine when the a = 45° lamina would fail at 9 = 90°. This

should take place at a load of 40,000 psi. This value corresponds

closely to the final breaking strength on the tests. The predicted

failure of the a = 0° lamina at 34,500 psi corresponds quite closely

to the second discontinuity in the test results.

If we consider the test results to only reflect failure at 9 = 90°

location the sequence of failures as predicted by the analysis is as follows. First failure took place in the 90° lamina at 20,000 psi,

next the 0° lamina failed at 35,000 psi, finally the 45° lamina failed

at 40,000 psi. This corresponds quite closely with discontinuities in

the stress strain curve from the tests.

Thus we see that analytical methods used give reasonable predictions of

failure in laminated composites. Consequently the expectations predicted for the effect of holes in composites can be considered highly

reasonable

.

126
Conclusion

The designer when first faced with the use of composites must be

cautious about making any assumption based upon experience with other

materials. Geometric discontinuities do cause stress concentration

but their significance is not the same.

References

1. "Primer on Composite Materials: Analysis" J.E. Ashton, J.C. Halpin

& P.H. Pettit; Technomic Publications, Stanford, Conn., 1969.

2. "Mechanics of Composite Materials" R.M. Jones: McGraw-Hill; 1975.

3. "Theory of Elasticity of an Anisotropic Elastic Body", S.G.

Lekhnitskii; Holden-Day, San Francisco; 1963.

4. "Stress Concentration and Failure Criteria for Orthotropic and

Anisotropic Plates with Circular Openings", L.B. Greszczuk;

ASTM STP 497: American Society for Testing and Material; 1972.

5. "Experimental Stress Analysis of Composite Materials", I.M. Daniel

and R.E. Rowlands; ASME 72-DE-6; American Society of Mechanical

Engineers; 1972.

6. "Analysis and Design of Fibre Reinforced Composite Structural

Elements"; S.A. Allidina; M.E. Thesis, Carleton University; 1974.

127
Fi OR LAMINA

/

CHOOSE LAMIK^

IMITH MORE •SOXT'^BLE ^ pp.opEP.-ri ES

.^LTER. Sk. OR. n_ OP- BOTH

MiCROMECHANlCS r CoNSTn-UTiVE BrMAVioR. or V-amina

laminatiom "r^ETOR-v

NO-OP lavers ^ n. X-'. ^/

LAMiNATg (peoPeR-ties [Acjlj

OF LAMINATE.

1^

TRANSFORM STRAINS TO

LAMlKA PRINCIPAL DlftECTIONS

ANX) check. For. f/mi-URe:

' AHftLySIS. OF

COHPOS.17 E (^LAMiMATe}

(calcalahc/n trf midflaiic.

FIG. 1 DESIGN procedure: FOR LAMINATED COMPOSITES

128
129
130
28

10 20 30 40 50 60 70 80 9^

LAY ANaE a°

FIG. 6 TANGENTIAL STRAIN VS. LAY ANGLE BORON EPOXY

[+a/-a/-ci/+a]

131
LAY ANGLE a°

FIG. 9 LOAD CAPACITY OF COMPOSITE PANEL KITH HOLE, IN TENSION.

BORON EPOXY [+a/ a/-a/+a]

132
20

16

2 12

t—

in^ 8

I—

10 20 30 40 50 60 70 80 90

LOCATION ANGLE 0"

FIG. 10 STRESS DISTRIBUTION AT PERIPHERY OF HOLE IR SHEAR PANEL

BORON EPOXY ^+a/-a/-a/^ a]

2.8

2.4

2.Q

1.6

y 1.2

0.8

0.4

10

2 —

n

1

1

20 30 40 50

LAY ANGLE a

60 70 80 90

FIG. 11 LOAD CAPACITY OF PAIiELS IN SHEAR^BORON EPOXY [W-c/-u/-ta]

133
13h
SESSION IV

MATERIALS

OF

DESIGN

(II)

-LUBRICANTSChairman: J. Scialdone

NASA,Goddard Space Flight Center
I
i

ELASTOHYDRODYNAMIC LUBRICATION

* L. B. Sibley

: SKF Technology Center

j King of Prussia, PA 19406

Abstract: The basic mechanism o£ elastohydrodynamic (EHD)

lubrication is described, including the historical development of the mathematical formulations and their experimental

verification. The results of recent studies of the thermal,

chemical and rheological conditions existing at the EHD

contacts in machine elements are reviewed, as well as new

! developments in EHD starvation theory and experiments.

i| Critical effects of EHD phenomena as they influence the per-

|| formance of rolling bearings are illustrated, and their

implications are given for the performance of other machine

elements having concentrated contacts, such as gears, cams,

seals, foil bearings and articular cartilage. The use of

, computers is described both for obtaining theoretical EHD

I solutions and for EHD calculations in rolling bearing design

! and application engineering. Typical computer computations

of EHD film thickness, traction, power loss and" starvation

effects are presented, showing how EHD analysis can be used

to design ball bearing lubricants and lubrication systems

for minimum friction torque with maximum lubrication

effectiveness and reliability.

Key words: Bearings; computer programs; elastohydrodynamics

;

I failure analysis; film thickness; gears; Hertz contacts;

lubrication; rheology; starvation; traction.

Elastohydrodynamic (EHD) lubrication concepts and analyses

play an important role in the application engineering and

I lubrication failure prediction of mechanical components.

Basically, EHD provides insight into the fundamental lubrication phenomena operating in a wide variety of mechanisms

containing concentrated contacts, some of which are listed

as follows

-the Hertz contacts between the balls or rollers and

. the ring raceways in rolling bearings,

1 -the contacts between gear teeth,

-cam and follower contact surfaces,

-foil bearing surfaces, such as in tape heads,

137
-lip-type elastomeric seal contacts,

-animal joints of articular cartilage surfaces.

In all these applications, the critical load-carrying contacts consist of "bearing" surfaces which deform elastically

under the action of the "lubricant" pressures generated

hydrodynamically by the relative motion of these deformed

surfaces. EHD lubrication prevails when the surface deformation becomes large compared to the lubricant film thickness in the contact. Theoretical solutions of the EHD

problem date back only 10 to 20 years (1)* and consist of the

simultaneous solution of both the hydrodynamic and elasticity equations with appropriate material property variations

A typical example of the hydrodynamic Reynold's equation for

two-dimensional steady-state lubrication with a Newtonian

fluid, including the compressibility effect, is

where jp = lubricant film pressure

"'>^i= surface velocities

U = film thickness

/io = film thickness where dp/dx = 0 = absolute viscosity of the lubricant

/«» = density of the lulDricant

= ambient value of lubricant density

X = distance along the surface in the direction of

motion

The boundary conditions are

= 0 at X = -oo

p = Q and /i = at X =

and the two-dimensional elasticity equation is

where R = the effective radius of curvature of the contact

ing surfaces in the direction of motion = 1/(1/Rl+1/R2)

^Numbers in parentheses refer to List of References at the

end of this paper.

158
the reduced Young's modulus o£ the contacting surfaces = 2/ [(1-1,^) /El + (1-1;^) /Ez] ^ = a dummy variable

"X, = a reference point where the elastic normal deformation of the surfaces is zero.

For applications where metal contacting surfaces are used,

such as in rolling bearings and gears, the EHD lubricant film

pressures in the contacts are so high that the following

lubricant property variations with pressure must be included

in the analysis

where o( = the pressure -viscosity coefficient

yj^ = absolute viscosity at atmospheric pressure

= the bulk modulus of compressibility.

Iterative solutions of the above equations using high-speed

digital computers are best represented by the diagrams

given in Figures 1 and 2 taken from the work by Dowson and

Higginson (1,2).

Figure 1 shows how with increasing contact load the contact

area increases rapidly, but the lubricant film thickness

decreases very slowly, which is a basic characteristic of

EHD lubrication quite different from the ordinary hydrodynamic lubrication of plain sliding bearings. This means

that once EHD lubrication is established at the load-carrying contacts in a mechanical device, very little effect on

the lubrication condition is caused by rapid large changes

in load such as from vibration or shock loading.

In Fig. 1 under high load the EHD pressure distribution in

the lubricant film is almost identical to the semi-elliptical Hertz elastic surface pressure distribution, so that

using Hertz formulas for computing lubricant contact film

pressures does not introduce significant errors under many

practical EHD lubrication conditions. Figure 2 shows the

variation in film shape and pressure over a wide range of

lubricant viscosities and surface velocities at a constant

load. Here the film thickness varies rapidly with the

viscosity- velocity product. The deformation and pressures

are very similar to the Hertzian except at the highest

levels of viscosity- velocity product where there is so

159
a,b,c,d = curves of increasing load on cylinders

X = distance from center of contact in rolling

direction

p = oil film pressure

h = oil film thickness

From Dowson and Higginson(2)

Fig. 1 Pressure distributions and film shapes with

pressure-dependent viscosity and elastic cylinders

for a wide range of loads.

little surface deformation that the lubrication mode there

is more hydrodynamic than elas tohydrodynamic

.

It is important that any new theoretical development be

supported by experimental evidence, and a variety of experimental EHD techniques have been developed (3) . Probably

the most striking experimental verification of the existence

IkO
-3

X = distance from center

of contact in

roll ing direction

Fig. 2.

= x/b

U - (1) 10-9, (2) xo-'o, (3) 10-Ji, (4) lO-«, (5) 10-«3, (6) 0 (diy eontaa). IT - 3 X 10-5, G = 5000. Pressure distributions and film shapes for a compressible lubricant as a function of the

product of viscosity times velocity.

From Dowson and Higginson (1

)

1
of EHD films at machine element contacts is the results of

radiotracer wear tests (4) shown in Figure 3. In this test

the wear rate of two steel balls in lubricated rolling/

spinning contact with each other in a rolling four-ball

tester was measured by making one of the balls radioactive

in a nuclear reactor and measuring the radioactive wear

debris in the lubricating oil using high- sensitivity radiotracer techniques. It can be seen that the wear rate was

indeed quite small, only a small fraction of a microgram

per 1000 ball revolutions, requiring the highly sensitive

radiotracer method for any practical test program. This

small wear rate fell even lower as the test speed was increased, all other conditions remaining constant, dropping

off at high speeds to what seems to be an asymptotically

negligible wear rate. This behavior is consistent with a hydrodynamic lubrication mechanism in which the film thickness increases with speed, and it is not consistent with a boundary lubrication mechanism in which the chemical reaction of the lubricant with the steel surfaces to form lubri

eating films would be expected to cause greater metal removal with increases in speed.

Studies using several new experimental EHD techniques presented at the recent American Chemical Society Symposium on

Lubricant Properties in Thin Lubricating Films show that

the shear conditions at EHD contacts are apparently severe

enough to cause structural changes in some lubricants.

Figure 4 from Lauer and Peterkin (5) in this symposium

shows how the infra red spectrum for a high stability polyphenylether lubricant taken directly on the EHD film in

the contact between a ball and a flat surface changes significantly with an increase in load on the contact. Sarrborn and Winer (6), however, could show no significant

molecular changes for lubricants having molecular weights

below 600. The effect of such lubricant structural changes

is mostly a modification of the contact traction characteristics which results in different heat generation rates and

friction losses of mechanical components. The EHD lubricant film thickness is not usually affected significantly,

since such a small quantity of lubricant is entrained in

the contact.

An important EHD phenomenon that does have significant

practical effects on both heat generation and oil film

thickness in mechanical components such as rolling bearings

and gears is the recently discovered EHD lubricant film

starvation (7) . Figure 5 shows plane views of two 3-dimensional EHD contacts which are identical except for the

lk2
SPINDLE SPEED (RPM)

Fig. 3. Progressive wear rate as a function o£ rolling speed

at a rolling/spinning ball-to-ball contact measured

by radiotracer technique.
t I I I I I I I t I I I I I 630 660 690 720 750 780 810 840

WAVENUMBER

a. 28.5 kg load, 480 rpm

b. 11.0 kg load, 480 rpm

From Lauer and Peterkin (5)

Fig. 4. Emission spectra of polyphenylether 5P4E in an

EHD contact.

degree of lubricant supply or starvation. The overall circular shape of these contacts results from the Hertzian

elastic deformation of the contact surfaces, in this case

between a ball and a flat. The lubricant films sticking to

the rolling ball and flat surfaces sweep into the contact

region from right to left on Figure 5, the direction of

the surface motion with respect to the contact. These

surface films merge together into a continuous oil film

between the converging surfaces at the meniscus line, which

separates the layer of air between the oil-wetted surfaces

on the right side of the line from the continuous oil film

on the left. As the amount of oil replenishment into the

track between successive ball contacts decreases, the me- niscus line moves closer to the Hertz contact region, as shown

in the progression from Sketch (A) to Sketch (B) in Figure

5. This partial EHD starvation results in a decrease in

the film thickness (from a plateau film thickness of 18.5

Ikk
(A)

Meniscus Line

Fig. 5. Contour lines of equal lubricant film thickness in

microinches at two identical ball/flat EHD contacts

except for location of meniscus lines where

separate oil films on converging inlet surfaces

merge into a continuous film of oil between the

surfaces

.

11^5
microinches to 14.5 microinches in the example shown in

Figure 5) and in a broadening of the plateau region of the

contact making it less hydrodynamic and more Hertzian.

The reduction of EHD film thickness at critical lubricated

contacts in mechanical components from whatever source,

either EHD starvation or simply too little oil viscosity or

velocity of the surfaces, is not necessarily harmful to

component performance. A useful parameter for judging the

lubrication condition at EHD contacts is the ratio of the

minimum film thickness to the composite roughness of the

two surfaces in contact, defined as

where ^ and 0^2 ^-re the rms roughnesses of the two contacting surfaces. Figure 6 shows the percentage of time,

t/to, that typical EHD contacts in a bearing or gear of

practical surface roughness are separated by a complete

lubricant film plotted against this ratio of minimum film

thickness to composite surface roughness. For typical high

speed bearings and gears, film thickness/roughness ratios

less than about 1.5 result in the generation of surface

distress in the tracks manifested by destruction of all

traces of the original surface finish and formation of a glazed appearance. Continued operation under surface distress conditions leads to the generation of surface microcracking and a multiplicity of micropits that eventually

result in surface initiated fatigue spalling at a much lower

fatigue life than would occur with the same surfaces operating under more favorable lubrication conditions. The

critical film thickness/roughness ratio needed to avoid

this lubrication-related surface distress is lower for

slower speed bearings and gears, down to about 0.4 for very

slow, medium size, heavily loaded roller bearings. As shown

in Figure 5, at very high film thickness/roughness ratios

there is evidence that L-^q fatigue life, the life at which

90% of a population of bearings or gears will survive without fatigue spalling, can be expected to increase by two

or three times over the accepted "rating life" at intermediate film conditions (8) . There is every reason to believe that the above EHD concepts

can be used to characterize the performance of concentrated

contact mechanisms other than bearings and gears, for which

their application so far has been most extensively developed

Even though the contact surface motion is essentially

sliding and usually not rolling in such components as cams,

elastomeric seal lips and foil bearings, there is still a

11^6
r- REGION OF

LUBRICATION

RELATED

RATIO OF MINIMUM FILM THICKNESS

TO COMPOSITE SURFACE ROUGHNESS

Fig. 6. EHD film condition and L-^q fatigue life as a function of the film thickness/roughness ratio.

substantial "sweeping" motion of the surfaces carrying

"lubricant" into the contacts to support EHD films. In the

case of seals and foil bearings the "bearing" surfaces

deform much more readily than rolling bearing or gear contact surfaces, so that EHD-type lubrication is realized

without the need to account for the changes in lubricant

viscosity and density under the high contact pressures

found in bearings and gears. The poroas, compliant, articular cartilage surfaces in animal joints, such as the

human hip joint, are another example cf "bearing" surfaces

that deform easily under the hydrodynamic pressures generIk-J
ated in the synovial fluid, which acts as a highly nonNewtonian "lubricant" film.

The effective use of EHD lubrication concepts in component

design entails the coordinated application of a wide variety

of complex calculations that demand high-speed digital

computer systems for their solution (9). A convenient

application of computers for this purpose is a time-sharing

conversational -mode system such as Program STARBRG illustrated in Figure 7. Program STARBRG obtains all required

input data for the analysis of a ball bearing application

in the answers to the six questions shown in the input data

section of Figure 7. The output data section of Figure 7 shows how, in this case of a very high speed angular-contact

ball bearing operating under a thrust load as in a typical

turbine application, the ball contact force on the outer

ring is much greater than on the inner ring, due to the

high centrifugal forces on the balls, causing the inner and

outer ring contact angles to be widely divergent. Under

these conditions the program computes the rolling velocities, maximum film pressures and dimensions of the contact

ellipses at the inner and outer ring contacts, as well as

the standard rating capacity for fatigue life calculation,

the spin/roll ratio as a measure of the amount of sliding

in the contacts and the cage speed for vibration prediction and comparison with measured values to determine ball

skid tendencies.

The EHD film thickness at each ring contact is computed by

Program STARBRG for the partially starved condition resulting from lubricant replenishment into the ball and outer

ring tracks only (due to the centrifugal forces on the oil

films in the bearing) , the replenishment being effected

only by surface tension induced capillary flow from the

track edges, as a conservative design base-line. The

corresponding meniscus distance and starved oil-film thickness reduction factor are also computed for each ring contact based on these replenishment assumptions. If it

should turn out that both the EHD film thickness and the

starvation reduction factor are lower than needed to obtain

the required performance in the application, the designer

knows to provide some means of obtaining direct lubricant

replenishment onto the bearing tracks, such as by the use

of an under race oil supply system.

More complex computer programs have been developed for

analysis of the effects of different amounts of lubricant

replenished directly onto bearing tracks as well as for

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150
calculation of EHD effects on bearing friction torque and

power loss. A partial output from such a program called

SHABERTH is shown in Figure 8. The computed EHD film

thickness/roughness ratio, labeled H/SIGMA in Figure 8, is

used to adjust the calculated L^q fatigue life of the five

different bearings included in the bear ing/ shaft system

analysis which comprises SHABERTH. The bearing frictional

heat generation rates are computed by including the tractions at all EHD contacts in the bearings both at the ball

or roller/raceways and all cage contacts. These tractions

are summed up for both EHD fluid film and surface asperity

frictions as well as for the viscous losses in the meniscus

films around each contact and the drag forces of the ball or

roller/cage assembly through the oil mist existing in the

open bearing spaces.

It is clear that EHD analysis provides insight into the

basic lubrication mechanisms at work in properly lubricated

mechanical components that can be used to guide their design

and to predict failure mode paths to be avoided when recognized early enough in the design and development cycle of

new machines. Computer simulation of these EHD lubrication

mechanisms has already proven to be a powerful tool for implementation of EHD analysis in advanced engineering design.

Acknowledgements

The author wishes to thank SKF Industries, Inc., for

permission to publish this paper. The support of various

government agencies in the development of some of the

technologies and computer programs described is sincerely

appreciated. The contributions of many members of the

SKF staff, especially Y.P. Chiu, W. J. Crecelius,

J. I. McCool and T. E. Tallian, to the technologies discussed in this paper are gratefully acknowledged.

151
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152
List of References

1. Dowson, D., and Higginson, G., " Elas tohydrodynamic

Lubrication, The Fundamentals of Roller and Gear

Lubr icat ion " , Pergammon Press, 1966

.

2. Dowson, D., and Higginson, G.R., "Numerical Solution to

the Elas tohydrodynamic Problem" Journal of Mechanical

Engrg. Sc. Vol. 1 No. 1 p. 6 (1959).

3. Seth, B. B., and Willis, T., "Techniques for Film Thickness Measurements in Elas tohydrodynamic Lubrication",

ASME Paper No. 76-DET-79 (1976).

4. Tallian, T.E., Chiu, T.P., Huttenlocher , D.V.,

Kamenshine, J. A., Sibley, L.B., Sindlinger, N.E.,

"Lubricant Films in Rolling Contact of Rough Surfaces,"

ASLE Trans. 7, 109-126 (1964).

5. Lauer, J.L., and Peterkin, M. E .

, "Analys is of Infrared

Emission Spectra of Elas tohydrodynamic Lubricant Films",

Proceedings of Symposium on Lubricant Properties in

Thin Lubricating Films, American Chemical Society,

New York, pp. 48-56 (1976).

6. Sanborn, D.M., and Winer, W,0., "Lubricant Properties

in Thin Lubricating Films," Proceedings of Symposium

on Lubricant Properties in Thin Lubricating Films,

American Chemical Society, New York, pp. 57-67 (1976).

7. Chiu, Y.P., "An Analysis and Prediction of Lubricant

Film Starvation in Rolling Contact Systems," ASLE

Trans. 1_7 pp. 22-35 (1974).

8. Liu, J.Y., Tallian, T.E., and McCool, J.I., "Dependence

of Bearing Fatigue Life on Film Thickness to Surface

Roughness Ratio," ASLE Trans. 18_ (2), pp. 144-152 (1975).

9. Sibley, L.B., and Pirvics,J., "Computer Analysis of

Rolling Bearings," Proceedings of Symposium on ComputerAided Design of Bearings and Seals, ASME, New York,

pp. 95-115 (1976).

153
POLYFLUOROALKYL-ALKYL POLYSILOXANE GREASE

FOR INSTRUMENT LUBRICATION

John B. Christian

Air Force Materials Laboratory

Wright-Patterson Air Force Base, Ohio 45433

Abstract: A wide temperature range grease has been developed for

instruments which operate under light to heavy loads and low to moderately high speeds. It is especially suited for use in applications

involving rolling, sliding or oscillating motions and where very little

wear can be tolerated. The grease is based on a fluorinated ethylene

propylene or polytetrafluoroethylene thickener in a polyfluoroalkylalkyl polysiloxane fluid. The grease's outstanding qualities are its

wide temperature range (-100° to 450 °F) capability, its extreme

pressure and antiwear characteristics, its non-migratory nature, and

its low foreign and/or opaque particle content.

The grease is suitable for use in aircraft actuators, gears,

gimbal rings, oscillation bearings, antifriction bearings, and plain

spherical bearings. It is especially suitable for use in applications

employing miniature bearings, and has yielded thousands of hours of

operation in some of these applications including blower motors, motor

generators, plastic clutches and gears, servo motors, and others.

Key words: Polyfluoroalkyl-Alkyl Polysiloxane; antimony dialkyldithiocar bamate; polytetrafluoroethylene; fluorinated ethylenepropylene;

extreme pressure; antiwear; grease; gear lubrication; bearing lubrication.

Advancing aerospace technology has placed ever increasing requirements on the lubricants industry. With the advent of high speed aircraft and aerospace vehicles and their support equipment, and the

increase in their scope of operations under extreme pressure conditions, severe demands have been placed on grease lubricants. To

meet these demands prior research and development efforts have

achieved substantial advancements in the synthesis of a variety of

grease lubricants which have many desirable lubricating characteristics and are capable of functioning under moderately high load conditions. Present demands and anticipated demands for grease lubrication in a variety of extreme environments have generated research

15^
and development efforts which have resulted in many advanced grease

lubricants. While a grease in its simplest form consists of a lubricating fluid thickened by a solid to impart a semisolid consistency, the

lubrication problems associated with specific applications have led to

the evolution not only of advanced fluid and solid materials, but also

third and fourth additive components. Various grease developments

have selectively ensured improved antiwear, antibleed, extreme pressure, high speed, antirust, and thermal characteristics. Among these

are greases based on the various types of esters and the chlorinated

silicones which showed improved lubricity over the straight methylphenyl silicones. Prior efforts have been made to develop extreme

pressure greases using these fluids individually and combined in

blends. Some success was achieved in developing a series of extreme

pressure greases which were usable over a relatively wide temperature range. Their extreme pressure characteristics were improved

to yield a Load Wear Index value of about 60 by the use of additives

such as pentachlorobenzene thiol and its thioether and heavy metal

salt derivatives, and by the use of pentachlorophenyl mercapto acetic

acid. Greases utilizing moderate concentrations of the latter extreme

pressure additive possessed Load Wear Index values approaching 100.

Anti-wear, however, was not one of the outstanding characteristics

of this series of greases. In fact, their antiwear properties were

very poor.

Today's requirements for grease lubricants to function at temperatures

as low as -65 to -100 °F and as high as 450 °F preclude the use of many

fluids as base materials. This wide temperature range requirement

together with requirements for antiwear and extreme pressure preclude all but the most advanced fluids for use in this type of grease.

Prominent among these base fluids are the polyalkyl-alkyl polysiloxanes.

This report discusses the development of greases based on this class

of fluid. This class of greases incorporates three very important

properties into single grease compositions, namely, wide temperature range, extreme pressure capability, and antiwear, and thus

represents an important break-through in grease lubrication technology.

Research efforts were conducted on the development of extreme pressure (EP), antiwear greases for lubricating heavily loaded surfaces

required to operate under rolling, sliding and oscillating motions at

slow to high speeds, and over a wide temperature range. The materials comprising the grease developed under this effort are described:

155
1. Investigations showed that the Polyfluoroalkyl Polys iloxane

class of fluids possessed most of the basic properties to render them

prime candidates as base materials for use in research on extreme

pressure, anti-wear greases. The polysiloxane fluid used in this

effort has the general formula:

CF3CH2CH2Si(CH3)^0 Si (CH3) o

CH CH CF,

2 2 3

Si(CH3)2CH2CH2CF3

n For the purposes of this research it was desired that the polysiloxane

have a molecular weight and value of n which would yield a viscosity

of about 50-100 centistokes, and preferably 65-85 centistokes at 100 °F,

The polysiloxane covered by the above general formula found to possess the desired properties has the formula:

(CH3)3SiO Si(CH3)0

CH^CH^CF3

n

Si(CH ) O.

3 2

.Si(CH3)3

n'

It was also desirable that the viscosity at various other temperatures

be in the following ranges:

200 °F

0 °F

-65 °F

15-25 cs 650-725 cs

10, 000-11, 000 cs

Other desirable properties included:

Pour Point

Flash Point

Fire Point

below -100 °F

above 500 °F

above 600 °F

It was found that the polysiloxane conforming to the second formula

above, a trifluoropropylmethyl -dimethyl polysiloxane, possessed the

desired properties. These properties are given in Table I.

2. A group of grease thickeners found to be highly effective in

producing grease based on the previously described polysiloxane

fluid are the fluorinated polymer powders polytetrafluoroethylene

156
(PTFE) and fluorinated ethylenepr opylene (FEP). The investigation

of these thickeners was under-taken first by defining parameters that

would allow the characterization of the thickeners. Tests were then

conducted by appropriate methods to characterize the thickeners as to

particle size, particle distribution, surface area, critical surface

tension, particle configuration, coefficient of friction, and bulk density. Results suggested trends of efficiency that can be observed in

grease formation with these thickeners.

PTFE powders are manufactured by a variety of techniques including

grinding, cryogenic pulverizing, molecular chain scission, and pyrolysis, and the behavior of thickeners in grease compositions produced

by the different above techniques varies. FEP thickeners present

entirely different characteristics. The low surface energy of the

PTFE and FEP thickeners means that little shear energy is required

to form a grease. The chemical resistance of these materials is

excellent and their critical surface tension is below the surface tension of most liquids. An unusual consequence of this is that most

greases in which PTFE or FEP is used as a thickener exclude water

from interface boundaries and are thus hydrolytically stable. The

surface characteristics of an FEP and a PTFE thickener found to be

especially compatible with the polysiloxanes to form greases in this

effort are described in Table II. The FEP thickener particles are

quite spherical. The PTFE, manufactured by molecular chain scission, is practically spherical, and is physically similar to the FEP.

These particle configurations were determined by scanning electron

microscopy (SEM).

Surface area is an important parameter when used in conjunction with

particle size distribution. The nitrogen adsorption method was used

in this investigation. A monolayer of nitrogen is absorbed on a known

amount of thickener solid. From the volume of nitrogen required and

the cros s -sectional area of the nitrogen molecule (15. 8 A^ ) the surface

area of the thickener particles may be calculated.

Critical surface tension is also related to thickening. It furnishes

information on the behavior of thickeners in oils or liquids. Liquids

whose surface tension is below the critical surface tension of the

thickener solid will wet the surface and show a contact angle of 0-

degree. The lower the surface tension of an oil with respect to the

critical surface tension of thickener solid, the greater the thickening

action of the solid.

Oil absorption number is a direct test for the relative absorption

of thickeners. This standard method involves the titration of linseed

157
oil onto a known amount of thickener until free oil appears.

3. Grease Formulations: A number of grease compositions were

prepared by using the polyfluoroalkyl-alkyl polysiloxane base fluids

and the previously described FEP and PTFE thickeners. The average proportions are given in Table III.

The greases were prepared by combining the base fluid and thickener

in a suitable vessel and mixing with a high speed stirrer. When mixing was complete a thick semi-grease consistency had formed. The

mixture was passed through a homogenizer set at 6, 000 to 7, 000 psi

pressure for a sufficient number of times to produce a grease having

consistencies representative of those given in Table III. Two to three

passes through the homogenizer may be necessary to result in the

desired consistencies. A three -roll mill equipped with floating or

fixed rollers may also be used with equal success in the preparation

of greases.

The use of antimony dialkyldithiocarbamate additive was used to

enhance the extreme pressure (EP) and antiwear properties of the

greases. The dithiocarbamate is characterized by the general formula

Sb S-C-N'^

Antiwear and EP properties of the carbamate additives are functions

of the alkyl group. Activity generally decreases as the alkyl group

molecular weight increases. Activity is also a function of the additive concentration in the grease formula.

Thermal stability of the carbamates increases as molecular weight

increases. Since EP activity decreases as molecular weight increases,

it was necessary to establish the molecular weight range that would

provide a good compromise between EP activity and thermal stability.

The greater emphasis was placed on EP activity.

For these investigations the antimony diamyldithiocarbamate was

selected. Typical grease formulations using this carbamate additive

with tr ifluoropropylmethyl-dimethyl polysiloxane base fluid and PTFE

or FEP thickeners are given in Table IV. The performance of the

PTFE and FEP thickened greases were very similar in preliminary

158
and subsequent performance investigations. Therefore, these greases

will be discussed as a class employing both thickeners.

The trifluoropropylmethyl-dimethyl polysiloxane greases were tested

by using Federal Test Method Standard 791 and ASTM procedures and

methods. Table V show the typical physical properties and some of

the performance characteristics of this class of greases.

1. Thermogravimetric Analysis (TGA): Thermogravimetric

analysis (TGA) was used as a screening test to determine the thermal

and oxidative stability of the base fluid, thickeners, additive, and

grease. Samples of ten milligrams were tested in an air atmosphere

at a programmed rate of 18 °F per minute. The heating rate was

controllable from 7 to 32 °F per minute over a temperature range of

32 to 1650 °F.

2. Coefficient of Friction: Coefficient of friction is a parameter

directly associated with lubricity. It is defined as the ratio of the

force required to move one surface over another to the total force

pressing the two surfaces together. The Four-Ball method was used

in determining the coefficient of friction involving the grease in this

investigation. A load of 1 5 kg was applied and the speed was adjusted

to 1,200 rpm. The test lasted for one hour. Friction coefficient and

temperature were measured at the beginning of the test and each 15

minutes during the one hour period. Temperature ranged from 77 °F

at the start of the test to 101 °F at the end of the one -hour test. Friction coefficient is given in Table V.

3. Wear Preventive Characteristics: The antiwear characteristics

of the grease were determined under sliding steel-on-steel conditions.

The standard Four -Ball Wear Test was used to conduct these tests.

Conditions were:

Speed 1, 200 rpm

Load 40 kg

Duration 2 hours

Temperatures 167, 400, and 450 °F

Specimens 52100, M-10, and 440C steel balls

159
The average wear scars in duplicate test runs were:

52100 steel at 167 °F 1.18 mm

M-10 steel at 450 °F 1.00 mm

440C steel at 400 °F 1.28 mm

4. Load Wear Index: The extreme pressure characteristics or the

ability of the grease to support heavy loads under sliding steel-onsteel conditions were measured by Load Wear Index and weld point

determinations using the standard Four -Ball Extreme Pressure Tester. The test is conducted by one load steel ball rotating at 1770 rpm

speed against three steel balls held stationary. Test temperature is

80 °F. The load wear index (an index of the ability of the grease to

prevent wear at applied loads) and weld point (the lowest applied load

in kilograms at which the rotating ball seizes and then welds to the

three stationary balls) are given in Table V.

5. Antifriction Bearing Performance: The grease was evaluated

for its ability to lubricate 20 mm anti-friction bearings. The tests

were conducted under the following conditions:

Bearing SAE No. 204, 20 mm

Speed 10, 000 rpm

Temperature 450 °F

Thrust Load 5 pounds

Radial Load 5 pounds

Cycle 2 0 hours /day until failure

The average of six tests is shown in Table V.

6. Oscillation Bearing Testing: The oscillation bearing test is

another tool to determine the ability of a grease to provide lubrication

under heavy loads. This test differs from the fretting corrosion tests

in which moving parts are subjected to oscillations of short amplitudes.

In this procedure the oscillations were conducted under large amplitudes. The grease lubricated plain spherical bearings were mounted

in the tester and a load of 3000 pounds was applied. The test was

conducted at 300 °F and 450 °F ± 10 °F and 250 cycles per minute with

a total angle of oscillation of ten degrees. The reported number of

cycles given in Table V are the average of four runs with a maximum

deviation between runs being not more than 20 percent from the average.

160
7. Starting and Running Torque: Grease torque characteristics

were determined in accordance with the ASTM-D 1478 procedure for

determining the low temperature torque data. Modifications were

made to the test equipment to allow for the use of miniature bearings.

The test bearing was an R-4, Grade ABEC 7, double shielded 440C

stainless steel. The bearing was packed with 80 microliters of grease,

a quantity which previously had proved to be the optimum for this size

bearing.

Torque characteristics at the two higher temperatures were obtained

on the Sunoco Bearing Grease Torque Tester, Model R-4. The same

bearing and quantity of grease were used in all torque tests. The

results of these determinations are shown in Table VI.

8. Dirt Content: The standard procedure outlined in Federal Test

Method Standard No. 791 was used to determine the concentration of

foreign or opaque particles in the grease, except determinations were

made for particles in the 10-35 micron range. The following results

were determined:

No. particles /cc

10-35 microns 23

Over 35 microns 11

The majority of the particles observed were opaque and are considered

to be agglomerations of the thickener rather than dirt or foreign particles. The grease is considered to be clean and essentially free of dirt

or foreign particles. One of the most significant characteristics of

this grease is its non-creep property. Creeping was not detected at

any stage during the extensive testing and evaluation phases of the

program.

Trifluoropropylmethyl -dimethyl polysiloxane grease is being implemented into numerous applications. One of its first uses involved the

actuators which control the variable exhaust nozzle (VEN) on the

engines of the F-5 aircraft. These actuators were previously lubricated by a proprietary dry film lubricant. The dry film lubrication

technique was unsatisfactory because it lasted only about 200 hours,

after which time the actuators were replaced. The implementation of

the grease extended the actuator lives to 2400 hours with a 400 hour

relubrication cycle. Relubr ication is accomplished by the addition of

lubricant, without removal of the actuator.

This new grease is currently being used in an increasing number of

161
instrument bearing applications. These applications were the result

of serious problems which had been assigned to the Rivet Gyro Project

Office (currently PRAM Project Office) for timely solution, (The

Rivet Gyro Project Office was established by a directive from the

Pentagon to develop specific corrective actions to improve the reliability of certain Air Force systems).

1. Analysis of various data had indicated that fully one third of the

failures occurring in LN-12 platforms were related to failure of the

servo-motors. The primary mode of failure was excessive breakaway

torque, which was related to lubricant failure in the bearings. A

number of the motors were disassembled, their bearings cleaned,

lubricated by the reported grease, and tested. It was demonstrated

that the motors repaired in this manner performed better than new

motors. It was also demonstrated that the new grease would extend

the life of the motors and improve their mean-time -before -failure

(MTBF).

2. Based on component failure data it was determined that a problem existed in the spin motor bearings and gimbal bearings of the 7900

D platform which was also related to lubricant failures. The new

grease was evaluated for use in the gimbal and spin bearings with the

goal of increasing life, reducing torques, and improving system MTBF.

The torque reduction would make it possible to eliminate the need for

a triple race bearing in the gimbal, thus eliminating the need for the

"agitator". This reduction in complexity would result in an increase

in MTBF. These evaluations proved to be successful.

3. Motor generators such as those in the Attitude Heading Reference Unit (AHRU) of the C-5 had shown a high failure rate. Examination of these units indicated no electrical failures, however, the

bearings were completely dry. Failure was due to failure of the lubricant. A number of these units were reclaimed, cleaned, and relubricated with the new lubricant reported herein. At the end of 1500

hours the tests were terminated and the bearings were examined. It was found that the new grease had performed without failure. Implementation of the grease into this application is allowing for the reclamation of "failed" parts and greatly extending the MTBF of these units.

4. The stabilator actuator rod-end bearing for the F-4 aircraft

had experienced excessive wear. The lubricant being used was showing indications of drying out and failing to carry the required load.

Implementation of the new grease into this application combined with

the resolution of a bearing outer race staking problem resulted in the

elimination of the entire problem involving the stabilator.

162
5. Implementation of the new grease into AAR-34 pumps is resulting in extended MTBF and improved efficiency of the pump. In an

efficiency test two pumps were lubricated, one with the prior lubricant,

and the other with the new grease. Both pumps were set at 1. 1 psi and

the 100 hour test was begun. During the test it was noted that the pump

lubricated with the new grease ran 20°F cooler. At the end of the 100

hour test, the pump lubricated with the prior lubricant was running at

0. 5 psi while the pump lubricated with the new grease was running at

2.2 psi. This indicated an increase in efficiency.

6. A clutch for a radar recorder in the F-111 is now lubricated

with the new grease. This clutch was considered to be one of the

leading causes of camera failure due to lubricant deterioration. This

clutch is now expected to last the life of the camera instead of a few

hundred hours.

The grease is being implemented into several other applications including servomechanisms , gears, gearboxes, velocity potentiometers,

elevation and azimuth drive motors, tachometer generators, micro

switch assemblies, reduction gears, vacuum pumps, NOD drive

assemblies, cryo-pumps, and camera drive mechanisms.

Conclusions: A tr ifluoropropylmethyl -dimethyl polysiloxane grease

lubricant has been developed and has been found to be capable of functioning over the temperature range of -100 to 450 °F. Its most outstanding characteristics are its non-creep properties, which preclude

the necessity of barrier films or coatings where anti-creep is an important lubrication factor; its extreme pressure capability or its ability

to support extremely heavy loads without allowing seizure or welding

of the moving parts to occur under dynamic conditions; and its ability

to prevent excessive wear of heavily loaded sliding or rolling surfaces.

These properties of the grease render it exceptionally effective as a lubricant in applications such as actuators, gear boxes, antifriction

and plain spherical bearings, and gears.

The grease has been extensively tested in many components where

failure of the component had occurred due to lubricant degradation.

These applications and components include the actuators of the variable exhaust nozzle on the engines of the F-5 aircraft, and miniature

bearings and gears in various blower motors, servomoters, gyro

motors, vacuum pumps, spur gears and discs, micro switches, velocity potentiometers, motor generators, radar recorders, cameras,

etc. , in the inertial navigation and attack radar systems of aircraft

such as the F-4, F-111 and C-5A. The grease is rapidly being

165
implemented into these applications and is resulting in greatly improv

performance, significantly increased mean-time -before -failure, and

highly improved reliability where high rates of lubricant degradation

and failures had previously occurred. Although it is impossible to

cite the magnitude of the cost savings attributable to the implementation of the grease into the various systems it is conservatively estimated to be several million dollars.

It is anticipated that this grease vv^ill be implemented into several

more applications and v^ill make further contributions to the improvement of Air Force operational capabilities.

I6k
TABLE I TRIFLUOROPROPYLMETHYL -DIMETHYL

POLYSILOXANE FLUID PROPERTIES

PROPERTY RESULTS

Viscosity at 200 °F, cs 22

Viscosity at 100 °F, cs 75

Viscosity at 0 °F, cs 692

Viscosity at -65 °F, cs 10,427

Pour Point, °F Below -100

Flash Point, °F 555

Fire Point, °F 680

Density, g/cc 1.136

Surface Tension, dynes /cm 25.2

Evaporation, 22 hrs at 400 °F, % 1. 1 Evaporation, 22 hrs at 450 °F, % 10.1

Four Ball Wear Scar at 1200 rpm,

40 kg load, 167 °F, 52100 steel

2 hr s , mm 1.3

165
TABLE II THICKENER PROPERTIES

PROPERTY FEP PTFE

Density, g/cc 2.1-2.2 2.1-2.3

Particle Size, microns

Scanning Electron

Microscope, microns 1-2 3-8

2

Surface Area, m /gram 10. 1 7. 6

Coefficient of Friction 0.07-0.10 0.06-0.08

Critical Surface Tension,

dynes/cm 21.9 20.9

Bulk Density (Uncompacted)

grams/liter 400 425

Oil Absorption No. , lbs

oil/lb thickener 47. 3 40.9

Molecular Weight 140, 000-190, 000 25, 000-40, 000

166
TABLE III TYPICAL POLYFLUOROALKYL-ALKYL

POLYSILOXANE GREASES

COMPONENTS AND PROPERTY RESULTS

Base Fluid, %

PTFE, %

FEP, %

65

35

65

35

Penetrations

0 Stroke

60 Strokes

10, 000 Strokes

Oil Separation

30 hours@400 °F, %

30 hours @ 450 °F, %

Evaporation

22 hours @ 400 °F

22 hours @ 450 °F

Coefficient of Friction

Load Wear Index

330

325

300

18

23

7. 1 10. 2

0. 144

70

315

335

340

20

26

5. 2

11. 1

0. 14

70

16?
TABLE IV

FORMULATED TRIFLUOROPROPYLMETHYL-DIMETHYL

POLYSILOXANE GREASES

COMPONENTS AND PROPERTY RESULTS

Base Fluid, % 62 62

PTFE, % 33

FEP, % 33

Antimony DiamyldithioCarbamate, % 5 5

Penetrations

6 Stroke 298 284

60 Strokes 311 313

10, 000 Strokes 310 315

Oil Separation

30 hours (a 400 °F 10.6 13.4

30 hours @ 450 °F 15.8 18.6

Evaporation

20 hours @400 °F 6. 4 3. 5

20 hours @450 °F 10.3 9.9

Coefficient of Friction 0.086 0. 091

Load Wear Index 104 115

168
TABLE V

PERFORMANCE CHARACTERISTICS OF

TRIFLUOROPROPYLMETHYL-DIMETHYL

POLYSILOXANE GREASE (TYPICAL)

Penetration

0 Strokes 290

60 Strokes 315

10, 000 Strokes 315

Density, g/cc 1.34

Evaporation, 22 hrs at 400 °F, % 5.50

Evaporation, 22 hrs at 450 °F, % 10.00

Water Resistance, % loss 3. 10

Coefficient of Friction 0. 089

Load Wear Index 107

Weld Point, kg 500+

Oscillation Bearing Test,

450 °F, Cycles 140,000

Oscillation Bearing Test,

300 °F, Cycles 300,000

High Temperature Bearing

Performance Test, 10,000

rpm, 5 lbs load, 450 "F, 20

mm bearings 2,200 hrs

169
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170
BIBLIOGRAPHY

1. Smith, R. K. "Development of High Temperature Heavy LoadCarrying Greases, " WADC TR 58-288, Part III (December I960).

2. Christian, J. B. , "Elasto -Hydrodynamic Lubrication with Fluor

Inated Polyor ganosiloxane Colloidal Lubricants, " NLGI

Spokesman, XXXV, 1, (1971).

3. Christian, J. B and Arkles, B, , "Polytetrafluoroethylene and

Fluorinated Ethylene -propylene Grease Lubricants," Lubrication Engineering, 30, 3, (1974).

4. Christian, J. B. , "Aerospace Lubricants -Lubrication Demands

and the State-of-the-Art, " AFML-TR-68-180, (September 1968).

5. House, W.E., "An Investigation of the Volatility Characteristics

of Greases by Thermogravimetr ic Analysis," AFML-TR-69 -78,

(1969) .

6. Zisman, W. A. , "Contact Angle, Wettability, and Adhesion, " ACS

Advances, Washington, D. C. , (1964).

7. Zettlemoyer, A. C. , "Spreading at the Teflon/Oil/Water/Air

Interface, " Journal of Colloidal and Interface Science, " 34, 4,

(1970) .

8. Poster, A. R. , "Handbook of Metal Powder s ," Reinhold Publishing, New York, (1966).

9. Schumaker and Garland, "Experiments in Physical Chemistry, " McGraw-Hill, New York, (1967).

10. 1973 Annual Book of ASTM Standards, Part 17, American

Society for Testing and Materials, Philadelphia, Pennsylvania,

(1973).

11. Federal Test Method Standard 791a, General Services Administration, Washington, D. C. , (December 1961).

12. Christian, J. B. (to the United States of America), U.S. Patent

3, 642, 626, (February 15, 1972).

171
ANTIWEAR AND LUBRICITY ADDITIVES FOR LUBRICANTS

S. M. Hsu

Amoco Chemicals Corporation

Naperville, II. 60540

Abstract: The physical and chemical characteristics of the common antiwear additives such as zinc dithiophosphate, trier esyl phosphate are discussed. Effective region of these additives is defined by the load,

speed, bulk and junction temperatures, and oxygen concentration. Interactions among additives in a lubricant is important. One additive can completely blank-out another additive. Some additive-metal interactions can also be critical. Proper consideration of these factors in

choosing the additive/ lubricant system during the design phase is

crucial in preventing costly premature failures.

Keywords: Antiwear additives; properties; interaction; lubricant;

metals; zinc dithiophosphate; tricresyl phosphate.

The trend of high performance, long service life in the design of

modern machinery has placed a greater emphasis on the quality of the

lubricant used. A variety of additives is used in different base

fluids to meet ever increasing performance criterion. Modern lubricants usually contain antiwear agent, corrosion and rust inhibitors,

oxidation inhibitors. Sometimes detergents, dispersants, viscosity

index improvers and other types of additives are also used in certain

applications

.

Antiwear additives are mainly used to reduce metal to metal contact

under highly loaded conditions. Under these conditions, a very thin

oil film exists between the surfaces, but intermittent asperity contact

occurs. It is generally accepted that the antiwear additive reacts with

the metal surface under localized high temperatures to form a surface

film to reduce metallic contact and wear.

Most antiwear additives are organic compounds containing one or more

elements or functions such as sulfur, chlorine and phosphorus. Carboxyl and carboxylate salts are sometimes used. Recently, the reduction

of wear by the use of compounds capable of forming a polymeric film

directly on rubbing surfaces have been reported (1-2). One example of

this is the use of one percent equimolar mixture of C38 dimer acid and

Cia glycol to form a polyester film. This additive has been shown to be

effective in reducing valve lifter wear in bench automotive engines.

Wide-spread use of this new additive type is not foreseen in the near future and will not be discussed here.

172
Common Antiwear Additives

Some of the common antiwear additives used in liquid lubricants are

listed below. Excellent review and listing of these additives can be

found in (3).

TABLE I

Common Antiwear Additives

Phosphorus

dialkyl phosphites

dialkyl phosphates

diaryl phosphates

tricresyl phosphate

tributyl phosphate

Sulfur

sulfurized oils

dibenzyl disulfide

alkyl polysulfides

zinc dialkyldithiocarbamates

Chlorine

chlorinated wax

chlorinated polypheny Is

chlorinated oils

Mixed

chlorbenzyl disulfides

zinc dialkyl dithiophosphates

zinc diaryl dithio

phosphates

Generally, additives containing sulfur and chlorine are more reactive.

They are usually used in highly loaded, high temperature applications

to avoid seizure and welding. Sometimes these additives are referred to

as extreme pressure (EP) additives. Phosphorus-containing additives are used in moderately loaded situations. The most important commercial

antiwear additives are probably tricresyl phosphate (TCP) and zinc

dialkyl (aryl) dithiophosphate (ZDDP) . These two additives will be ex- amined below in detail.

Zinc Dialkyl (aryl) Dithiophosphate (ZDDP)

Typical properties are listed in Table II.

173
TABLE II

Physical & Chemical Properties of ZDDP

Alkyl Aryl

Specific gravity

Flash point, °C

Viscosity, cs@210°F

1.1370 0.9847

Zn, Wt7o

P, Wt7o

S, Wt%

193

12.7

8-10

7-9

171

15

2-3

2-3

14-17 5-6

ZDDP is extensively used in automotive crankcase oils for gasoline and

diesel engines. Its antiwear properties are demonstrated in Figures 1 and 2. These results are obtained in a Shell 4-Ball wear tester with

the following conditions: 600 rpm, 75°C bulk oil temperature, 52100

steel on steel ball bearings. Fig. 1 shows that the minimum concentration of ZDDP in a highly refined base oil is about 0.01% by weight. The

normal dosage in most applications is from 0.5 to 3.07o(w). Fig. 2 shows

the effective load range for ZDDP is from 1 to 20 kg (17,000 kg/cm2).

Some extreme pressure protection is shown by ZDDP probably due to the

presence of sulfur in the molecules. Forbes (4) had demonstrated by

changing the metal of the dithiophosphates , reverse order of performance was observed in antiwear region and EP region. ZDDP showed the

best performance in antiwear, moderately loaded region while exhibited

the worst performance in the EP region when compared to Bismuth, Tin,

Antimony, Lead, Silver, Iron, Nickel and Cadmium dithiophosphates. ZDDP

is a very effective oxidation inhibitor. In automotive crankcase applications, it is used to control the oil thickening tendency. In the

bench oxidation type of test, a 0.5%(w) ZDDP can reduce the viscosity

increase by 2 to 3 times and cut the carbonyl formation by half.

It is generally accepted that ZDDP works by thermal decomposition (5,6).

The decomposed products then react with the metal to form a film that is

rich in phosphorus but low in sulfur. The oxidation inhibition of ZDDP

is probably by peroxide radical decomposition. The thermal decomposition of ZDDP normally is controlled by the alkyl group. There are indications that the lower the thermal stability, the better the antiwear,

load carrying capacity. Acids have been shown to catalyze the ZDDP decomposition. Alkaline agents on the other hand retards the decomposition. Its effect on frictional properties is insignificant. Residual

stress on the metal surface tends to reduce its antiwear effectiveness

Summing up, the nature of the alkyl group has a marked effect on the

antiwear activity of ZDDP. Considerable evidence also shows that the

more thermally unstable the ZDDP is, the better its antiwear properties.

No conclusive picture has emerged to describe the antiwear mechanism of

(7).

ZDDP.

17^
Tricresyl Phosphate (TCP)

H3C- © -a 0

0- Q) -CH3

TCP is a colorless liquid with a viscosity about 30-47 centistokes.

Its physical properties are as follows: S.G. = 1.167; Pour point = -25°C; Flash point = 250°C; Autoignition temperature = 590°C; Boiling

point at 10mm Hg = 260-275°C. It usually contains about 7.5 to 8.5%

phosphorus by weight and the average molecular weight is about 400,

Shell 4-ball wear data are shown in Figures 3 and 4. Fig. 3 shows the

minimum concentration required for wear inhibition under 10 kg load

(1,000 to 14,500 kg/cm^ contact pressure), 600 rpm and 75°C. It can be

seen that the minimum effective concentration is about 0.57o(w), con- siderably higher than ZDDP. The normal application range is about 1% to

57c(w) . Fig. 4 shows the effective load range of TCP as 1 kg to 40 kg

(up to 23,400 kg/cm2 contact pressure). The higher load data are obtained with a Shell 4-ball E.P. tester. It can be seen that the TCP has

no EP wear inhibition under high load, high temperatures. No oxidation

inhibition can be observed with TCP. It is also an excellent plasticizer and the compatibility with some vinyl based plastics and elasto- mers should be checked if the oil will be in contact with them. It has

no appreciable effect on frictional properties of the oil. Residual

tensile stress on the metal has been shown to increase the antiwear

effectiveness of TCP (7).

There have been a lot of studies on the mechanism of TCP (4, 9, 10).

Although confusion still exists on the exact mode of action, a general

picture has emerged. The polar components in TCP (acid phosphates) are preferentially adsorbed and reacted with the iron surface, to form both

organo/inorgano phosphate films which carry the load. Subsequent source

of the polar acid phosphate material may come from the hydrolysis/

thermal decomposition/oxidation of TCP.

General Mechanism for Antiwear Additives

Antiwear additives can be viewed as a functional chemical group attached to a hydrocarbon backbone which provides solubility and compatibility

with the base fluids. This also applies to other additives such as detergents (metal sulfonates, phenates, succinimides , etc.). The solution

state of these additives in base fluids, due to their polar functional

group, is probably in aggregate conglomerate. Watson (11) recently

suggested that all polar additives existed in micellar solutions. The

critical micelle concentration (CMC) defines the solution state of the

additive. In most additive solutions, the bulk of the additive exists

in micellar aggregates above the CMC, only 10"*^ to 10"''^ moles of the
additive exist as individually solvated molecules below the CMC in

equilibrium. These molecules then compete for the active adsorption

sites of the metal surface. There are also indications that multilayer

adsorption takes place. These adsorbed molecules then react with the

metal under high temperatures (260°-370°C) and pressures to form organometallic compounds. Sometimes the organic portion of the molecules may

be further decomposed/oxidized leaving only the inorgano-metal compounds

which constitute the protective film.

Base Oil

The base oil used in a specific application can affect the additive performance. Naturally occurring polar materials such as the oxygenated,

nitrogen-, sulfur-containing compounds can compete with the antiwear

additives for the metal surface. Aromatics generally have better antiwear properties as a base fluid. In some cases, different base fluidadditive interactions can occur depending on the chemical structures of

the base fluid. Fig, 5 shows that in a super-refined paraffinic

mineral oil where all the aromatics have been removed, further purification of the base oil gives better antiwear response to TCP, The data

are obtained from a Shell 4-ball wear tester at 75 °C, 40 kg, 600 rpm and

100 minutes duration. The purification step consists of percolating the

white oil through an activated alumina column to remove the polar impurities .

Additive Interactions

Interactions of various additives in fully formulated oils is a serious

problem. Recent trend in lubricant formulation centers around longer

service life and superior performance, sometimes even at the risk of

lower cost/effectiveness. This has prompted more additives at higher

concentrations to be used. Additive interaction can be both physical

and chemical. The physical interaction consists of competing for the adsorption sites from the standpoint of polarity, micellar aggregation of

different additives, solubilization or insolubilization of other additives. Chemical interactions are usually chemical reactions among the

additives. Very little is known on this phenomenon. In automotive

crankcase oil, the interactions between ZDDP and amine, dispersant and

detergent, dispersant and viscosity index improver are known. Additive

concentration can also affect the performance. Too high a concentration

of a single component can cause multiple interactions that is not observed at lower concentrations. Some antiwear additives are sensitive

to water, acidity e.g., ZDDP, When the base oil is changed to a fluid

where the major decomposition products are acidic, the effectiveness of

ZDDP in that fluid may be seriously impaired.

The composition for active adsorption sites on the metal surface can have marked effect on the additive performance (12). This is shown in

Table III,

176
TABLE III

Effect of Preferential Additive Adsorption

Shell 4-ball wear test with 40 kg load, 600 rpm, 75 °C

Base fluid: super-refined paraffinic mineral oil

Additive Wear Scar Diameter ,nmi Iron in Wear Debris

Organic Iron

Iron Iron Oxides

(A) 2.25% TCP

(B) 2.257o Cie Bromide

(A) + (B)

0.60 14 26 78

0.90 106 120 120

0.55 12 27 27

Both the degree of wear and the wear product analysis reveal no effect

of the octadecyl Bromide when TCP is present.

Other Factors

Variation in atmosphere and moisture content can have strong effects on antiwear additive performance (9). Since this relates to a particular

chemical interaction between the additive and oxygen, water, no generalized statement can be made. The presence of oxygen in a lot of cases

seems to help.

The metallurgy of the system has profound effect on the efficiency of

the load-carrying additives. Rounds (8) has demonstrated quite clearly

that different steels can have entirely different responses to the same antiwear additive. Although all of the steels contain 80'/o or higher

iron, the best additive for 52100 steel may not be the best for 440C

steel.

The temperature at the wear contact junction is another important parameter. All antiwear agents have their own effective temperature range.

The junction temperatures depend on the metallurgy, contact geometry,

load, speed and bulk temperatures. If this temperature is too high by

improper design, conventional lubricants containing TCP, ZDDP may not

work and premature failure occurs. Modified Blok-Archard flash tempera

ture calculations can be used (13, 14) to estimate the junction tempera

ture.

Recommendat ion

Antiwear additive performance is affected by a wide range of parameters

such as load, speed, atmosphere, base oil type, metallurgy, other

additives. This article illustrates the complexity and the "art" state

of formulating a lubricant. Since very little information is available

the recommended action for a designer is prototype testing or bench

177
simulations where the important parameters mentioned before are care- fully matched with the expected service conditions. If special lubricant/additives are required, specification of such should be made, A

design in which the lubricant is an integral part of the system can prevent costly premature failures.

178
REFERENCES

(1) Furey, M. J., "The Formation of Polymeric Films Directly on Rubbing

Surfaces to Reduce Wear," Wear , 26, 369-392 (1973).

(2) Zaslavsky, Y. S. et al, "Laboratory Investigations and Industrial

Use of Friction Polymer Forming Additive Treated Industrial Oils,"

JSLE/ASLE International Lubrication Conference, Tokyo, Japan, June

1975.

(3) Smalheer, C. V., Smith, R. K. , "Lubricant Additives," The LeziusHiles Co., Cleveland, Ohio, 1967.

(4) Forbes, E. S., "Antiwear and Extreme Pressure Additives for Lubricants," JTribology^, August (1970).

(5) Rounds, F. G. , "Some Factors Affecting the Decomposition of Three

Commercial Zinc Organodithiophosphates , " ASLE Trans , 18 , 79-89

(1975).

(6) Coy, R. C, Quinn, T. F. J., "The use of Physical Methods of

Analysis to Identify Surface Layers Formed by Organosulfur Compounds in Wear Tests," ASLE Trans , 18, 163-74 (1975).

(7) Sakurai, T. et al, "Effect of Internal Stress on the Wear Behavior

of Steel During Boundary Lubrication," ASLE Trans , 17 , 213-23,

(1974).

(8) Rounds, F, G. , "Influence of Steel Composition on Additive Performance," ASLE Trans , 15, 54-66 (1972).

(9) Goldblatt, I. L. and Appeldoorn, J. K. , "The Antiwear Behavior of

TCP in Different Atmospheres and Different Base Stocks," Presented

at the ASLE/ASME Conference, Houston, Texas, Oct. 14-16, 1969.

(10) Klaus, E. E. and Bieber, H. E., "Effect of P^^ impurities on the

Behavior of Tricresyl Phosphate-32 as an Antiwear Additive," ASLE

Trans , 8, 12-20 (1965).

(11) Watson, R. W. , "Petroleiam Additives-The Role of Akyl Groups,"

JSLE/ASLE International Lubrication Conference, Tokyo, Japan,

June, 1975.

(12) Hsu, S. M. , "The Effect of Chemical Reactions in Boundary Lubrication," Ph. D. Thesis, Chemical Engineering Dept., Pa, State

University, University Park, Pa. 16802, 1976.

179
(13) Bos, A., "The Temperature at the Wear Scars of the Four-Ball

Apparatus," Wear, 31, 17-27 (1975).

(14) Francis, H. A., "Interfacial Temperature Distribution Within A

Sliding Hertzian Contact," ASLE Trans , 14, 41-54 (1970).

180
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183
181^
Fig 5

Effect of base oil purity on the wear

inhibtion of TCP

0.1 0.3 0.5 1 3 5 7 10

Wt% TCP

185

SESSION V

VALIDATION

OF

DESIGN

(I)

-TESTINGChairman: W. W. Gunkel

General Motors Corporation

PIEZOELECTRIC POLYMERS AND THEIR APPLICATIONS

Seymour Edelman

National Bureau of Standards

Washington, D.C. 20234

Abstract: Polymers with significant piezoelectric activity have been

developed. This activity has been used to make measuring instruments

of thin polymer sheets. Sensors have been made to measure dynamic

stress, strain, acceleration, sound, vibration, and the effects of

mechanical impact or shock. Polymer sensors differ from conventional

instruments in being light, flexible, thin, highly damped, easily attached to curved, twisted, or compliant surfaces, uniform in response

over a wide range of frequencies, and cheap. Neither the instrument or

its leads is subject to mechanical fatigue. Polymer instruments have

obvious applications in failure prevention. For example, the condition

of bearings can be monitored by cementing polymer gages to any available portion of a bearing, the signal analyzed to provide the noise

spectrum of the bearing, and the spectrum checked at periodic intervals

so that deterioration in the condition of the bearing is noticed in

time to prevent catastrophic failure. Other components of machinery

can be monitored in similar ways. Typical noise spectra will be shown.

The process of poling the material will be outlined. The construction,

characteristics, and uses of typical sensors will be described.

Key words: Acoustic emission; damage detection; failure prevention;

noise spectrum; piezoelectric polymer; vibration sensor.

The most surprising thing about piezoelectric polymers is that they

exist. Polymers differ from conventional piezoelectric materials in so many of their properties that it seems strange that they should be com- parable in their piezoelectric response.

Conventional piezoelectric materials are typically crystalline or polycrystalline, hard, stiff, brittle, and dense. It is difficult to machine them into thin layers or to obtain them in sheets of large area and they are expensive. Polymers are compliant, flexible, tough, and

light. They are available commercially in layers as thin as six micrometers (one-fourth of one thousandth of an inch); they are available

in 1000 meter rolls over one meter wide; and they are relatively cheap.

189
Measuring instriiments using conventional piezoelectric materials usually have metal bases and housings and require threaded holes or specially ground flat surfaces of appreciable area for mounting. Polymer

instruments [1,2] typically consist only of the active material and a lead of the same polymer. The active material can be cut to any shape

that is suitable for a particular use and can be attached to a surface

whose only preparation is cleaning. Rubber cement, cyanoacrylate

,

epoxy, or other cements can be used. A polymer gage can be attached to

curved, twisted, or compliant surfaces.

Conventional instruments tend to have many high-Q resonances both because of their internal spring-mass systems and because of the effect

of the mass of the instrument on the elastic compliance of the mounting

surface. Polymer instruments are flat with frequency over wide ranges,

typically into the megahertz range, their resonances are low-Q, and

their mass is so low that they do not resonate with the mounting surface. The piezoelectric modulus, g, governs the usefulness of a material as a sensor. Its value for the usual piezoelectric polymer,

poly (viny lidene fluoride) is about six times as large as for a typical

lead zirconate titanate ceramic. On the other hand, the value of the

d modulus , which governs the usefulness of the material as a generator

of motion or as a sound source, is about 21 times larger for the ceramic.

There are some cases of considerable importance where polymer instruments are uniquely suited. Usually the advantages of polymer instruments show up in dynamic measurements. A typical case is measuring the

level of vibration at a point on a thin metal sheet. Any point on the

metal sheet to which an accelerometer is attached will have significantly greater surface density than the rest of the sheet. The vibration pattern of the sheet will rearrange itself so that the motion at

that point is minimized and the measurement is unrepresentative. On

the other hand, cementing a small piece of polymer film to the metal

sheet need not change the surface density significantly and a meaningful measurement can be made. The amplitude of vibration and the variation of vibration level with frequency measured by the polymer are more nearly representative of what the level at the point would be with

no instrument attached. Conversely, both the vibration amplitude and

the spectrum of resonances measured by an accelerometer would be affected by the presence of the instrument. Thus, it is practicable to distribute a number of polymer vibration gages over the surface of a panel

and to infer from their reading the mode of vibration of the panel un- der various conditions. This kind of study cannot be performed effectively with conventional vibration measuring instruments.

Our Program Chairman, Jesse Stern, suggested another application for

which polymer gages are well suited. This is monitoring the noise

spectra of bearings, gear trains, and other transmission systems. Such

monitoring has been done with various kinds of vibration measuring instruments. The use of conventional instruments involves a number of

190
difficulties. In some cases, the size of the instrument has prevented

it from being mounted directly on the source of the sound and when It was mounted on a nearby structure, it responded to sound from a number

of sources. Sometimes its mass affected the motion being measured.

Sometimes its own resonances obscured the spectrum of the noise it was measuring. Occasionally, the relatively high cost of the gage made it

uneconomic to mount a gage permanently at each point where a spectrum

was needed so that a single gage had to be moved from point to point

and variations in mounting conditions caused changes in the spectra

taken at different times.

Polymer gages eliminate or minimize these difficulties, A polymer gage

can be cut to fit whatever surface is available for a given measurement.

For example, a crescent-shaped gage can be mounted to the stationary

race or the dust shield of a bearing. There is little doubt that it

responds to the noise generated directly beneath it. Neither its mass nor its internal resonances affect the signal. The low cost of the

gage makes it practicable to mount one permanently at each point of

interest so that, if periodic readings of the signal indicate a change

in the spectrum, the change can be ascribed to changes in the bearing

and not to differences in mounting conditions.

A typical spectrum of the noise of a ball bearing is shown in Figure 1: Two ways of mounting the gage to the bearing are also shown.

Polymer gages are handy for acoustic emission studies. They can be

bonded to irregularly shaped surfaces and provide good coupling. Their

high internal damping minimizes the effects of ringing. They have a wide useful frequency range extending well into the megahertz region.

Their sensitivity is adequate for most work and can be Increased by

stacking and by the use of bias, if necessary. It is cheap and convenient to use gages in sufficient numbers so that noise sources can be

located by triangulatlon.

The detailed mechanism of piezoelectric activity in a polymer is com- plicated and controversial. The composition of the polymer, its division into amorphous and crystalline parts, the crystal structure, the

presence of surface and space charges and of ionic impurities have all

played parts in different explanations. We get useful guidance in preparing material for use in measuring instruments if we consider the

polymer to consist of long chains of identical units called monomers.

In the case of poly (vinylldene fluoride) (PVF2), the commonly used

piezoelectric polymer, the chains consist of linked carbon atoms and

each monomer consists of two carbons, one joined to two hydrogen atoms

and the other joined to two fluorine atoms. Each monomer has a strong

dlpole moment. As the material is received the dipoles are oriented

randomly as shown in Fig. la. We heat the material, apply a strong

191
electric field, and return the material to room temperature with the

field applied as shown in Fig. lb. We can consider that this process

results in the alignment of a significant number of the dipoles normal

to the plane of the sheet. The dipoles can be considered to be stiff er than the bonds between dipoles in adjacent chains so that any stimulus

that changes the thickness of the sheet will change the surface density of charge. A compensatory flow of charges through the circuit

connecting the electrodes forms a signal as shown in Fig. Ic. More

detailed analyses have been published [3].

A typical polymer sensor is made from two sheets of the polymer. The

active area is a circle, one centimeter in diameter. The active area has evaporated aluminum electrodes on each side of each sheet. The

sheets are cemented or fused together so that charges of the same polarity appear on the inner faces and charges of the opposite polarity

appear on the outer faces. Each inner face has a thin evaporated aluminum lead running from the active area for about ten centimeters to

the center conductor of a coaxial connector. Each outer face is completely covered with evaporated aluminum which is connected to the shield

of the coaxial connector. This kind of coaxial construction allows the

signal to be completely shielded and all exposed surfaces are at ground

potential. The evaporated metal lead on the same sheet of polymer as the active material allows the comparitively massive connector to be

far enough from the point at which the measurement is taken to avoid

any interference with the measurement. The thin polymer leads can be

bent, twisted, or stretched without damage and they are not subject

to fatigue. They can be cemented to a supporting surface so that they are not damaged by a blast of air or a reasonable amount of friction

and they do not generate noise by their motion.

To avoid leaving you with the impression that polymer gages are perfect

in every way, I will describe the effect of temperature [4]. Returning to the picture of a poled polymer as containing a large number of

electrical dipoles aligned normal to the plane of the sheet, we can see

that lowering the temperature will make the sheet thinner, just as presSTjre will. In either case, the effect is to change the surface density

of charge on the surfaces, giving a signal. Since a signal due to pressure comes from the same phenomenon as a signal due to temperature,

there is no way to separate the two effects if they occur at the same

frequency. Fortunately, many of our measurements are made at constant

temperature except possibly for the effect of adiabatic heating which

can be taken into account in the calibration process. In many other

cases, the temperature changes slowly compared to the rate of pressure

change so that the effect of temperature can be filtered out. Nevertheless, it must be realized that the polymer gages have a very large

pyroelectric effect and also that the sensitivity of the pressure gage

varies with temperature.

192
In those cases where the temperature changes at the same frequency as the pressure or where the ambient temperature changes drastically, we

have evaporated a bismuth- antimony thermocouple right onto the polymer

surface. This polymer thermocouple is an interesting instrument in

itself since the metal is only a few hundred angstroms thick giving a very fast response. It is not subject to fatigue and has many of the

other advantages of polymer instrumentation. In this application, the

thermocouple signal is proportional to temperature but not to pressure.

The pressure gage signal is a function of both temperature and pressure

A little arithmetic gives the effect of pressure alone.

The last figure shows a kind of non- invasive , non- destructive measurement that might have some application to personal failure prevention.

References

1. S. Edelman, S.C. Roth, and J.F, Mayo-Wells; U.S. Patent 3,970,862,

July 20, 1976.

2. S. Edelman, L.R. Grisham, S.C. Roth, and J. Cohen, J. Acoust, Soc.

Am. US, Part 1, 1040 (1970).

3. F. I. Mopsik, and M. G. Broadhurst, J. Appl. Phys . , 46, 4204 (1975)

4. J. Cohen, S. Edelman, and C. F. Vezzetti, Nature Physical Science,

233 , No. 36, 12, (1971).

193

(a)

(b) I u n i i i

+ + + + +•# + + =vT

\

I M n w ( (A)

Figure 2a Unaligned dipoles.

Figure 2b Dipoles aligned by field,

Figure 2c Signal due to pressure.

195
196
EXOELECTRON EMISSION - PAST EXPERIENCE AND FUTURE EXPECTATIONS

W. J. Baxter

Research Laboratories

General Motors Technical Center

Warren, Michigan 48090

Abstract: During fatigue testing of metallic components there is a gradual accumulation of "damage", leading ultimately to total failure.

An early manifestation of this damage is the development of surface

deformation which eventually provides sites for fatigue cracks to nucleate. However prior to the development of a crack in the metal itself,

the surface deformation produces microscopic cracks in the natural layer

of surface oxide. Under ultraviolet illumination photoelectrons from

the underlying metal can escape more easily through these microcracks,

such emission being referred to as exoelectron emission.

Thus exoelectron emission provides a measure of the accumulation of

fatigue deformation. By scanning the surface with a spot of ultraviolet

radiation the distribution of emission (i.e. deformation) can be displayed, and the severity of the damage assessed from the localized

intensity of the emission. After only 2% of life the ultimate fatigue

life may be reliably predicted. This technique should be applicable to

all metallic materials, but the range of variables such as stress level,

yield strength, oxide thickness, etc., has yet to be completely

explored

.

Key words: Assessment of fatigue damage; exoelectron emission;

failure prediction; laser scanner; oxide rupture; surface deformation.

INTRODUCTION

Design engineers ultimately resort to some type of test procedure to

validate a structural or engineering component and ensure its reliability in service. This is an essential part of the design process, and

can be very costly in terms of the time expended. This is particularly

true in applications where the ultimate failure is due to metal fatigue.

Such failures occur with little or no warning after apparently satisfactory operation during millions of cyclic loadings at low stress levels. Thus the determination of the fatigue performance requires at

least days, but more often weeks or months. At the end of that time the

designer obtains a vital (but solitary) piece of information, namely the

number of cyclic loadings to cause failure. In some applications it may

be necessary to design for an infinite fatigue life, i.e. ensure that

the service stresses are below the so-called fatigue limit for a given

197
material. A test in this case is usually eventually terminated after

-10^ load cycles, the engineer justifiably concluding that the design is

certainly adequate. On the other hand the structure may be overdesigned

and require further refinement and testing to be cost effective. This

situation represents the extreme in terms of the testing time required.

Thus the desirability of an abbreviated fatigue testing technique has

long been recognized, and a wide variety of approaches have been

explored but with very limited success. The problem is far from simple because of the complexity of the fatigue phenomena. The latter may

be conveniently regarded as consisting of two processes i) the initiation of a fatigue crack, and ii) propagation of the crack to ultimate

failure. Neither of these processes are completely understood, but they

are both known to involve plastic deformation within very localized

regions of the metal. Due to the lack of any quantitative measure, or even a useful definition, this deformation is generally referred to as fatigue damage. The objective then is to detect and measure this damage early in the fatigue life, and thereby be able to terminate the test

and predict the number of cycles to failure (Nf)

.

Most techniques which have been tried have been aimed at detecting the

initial fatigue crack, which usually develops after about 10% Nf. The

exoelectron method on the other hand detects and measures the accumulation of surface deformation after -1% Nf, i.e. during the crack initiation stage. 2 The origin of this so-called exoelectron emission is now

understood, and a scanning technique has been developed which provides

quantitative measurements of the all important localized fatigue deformation. This is illustrated by results for metals with quite different

yield strengths. Experience with the technique is now sufficient to

permit the main requirements to be specified, but there are still some aspects requiring further investigation.

ORIGIN OF EXOELECTRON EMISSION

An early manifestation of fatigue damage is the development of surface

deformation which eventually provides sites for fatigue cracks to nucleate. However prior to the development of a crack in the metal itself,

the surface deformation produces microscopic cracks in the natural layer

of surface oxide, revealing microscopic regions of bare metal. Under

ultraviolet illumination, photoelectrons from the underlying metal are able to escape more easily from the freshly revealed metal surface in

the microcracks than from the surrounding oxide coated surface. In this

way the accumulation of surface deformation increases the photoelectron

current, this increase being known as exoelectron emission.

The validity of this model was irrefutably established by direct observation of the exoelectron emission from a specimen deformed in the vacuum of a photoelectron microscope . 3 j 4 This is illustrated by the

photoelectron micrographs in Fig. 1. The exoelectron emission produced

by unidirectional tensile deformation of SAE 1018 steel appears as white

198
lines in Fig. la. These sources of emission are slip steps, where the

very thin (~4 nm) natural oxide has ruptured to reveal the fresh metal

surface of the step itself. If the step is allowed to reoxidize by

simple exposure to ambient air for a few hours, the exoelectron emission is completely suppressed (Fig. lb)

.

The surface deformation produced by fatigue cycling is more complex

than that for the unidirectional deformation illustrated in Fig. 1. The

deformation is much less homogeneous, and very localized regions of

severe deformation develop. The course of events for 1018 steel is sum- marized by the scanning electron micrographs in Fig. 2, which show the

small regions of most severe deformation at various stages of fatigue

life. At -0.5% of the fatigue life, the only visible deformation is

the appearance of isolated arrays of slip traces as in Fig. 2a. These

traces are in fact very small extrusions of metal, which again will rupture the surface oxide. The surface area of the extruded metal is small

at this stage, but as fatigue cycling continues the slip traces develop

more structure and broaden (Fig. 2b), and gradually merge together to

form a very mutilated surface (Fig. 2c). Eventually i.e. ~6% of life, a localized "honeycomb" surface structure develops with a very large sur- face area as shown in Fig. 2d. (It is only at this stage that the first

fatigue crack develops in the metal itself.) It is this large increase

of surface area of extruded metal in these localized regions which

results in the large increase of exoelectron emission during fatigue.^

ASSESSMENT OF FATIGUE DAMAGE

The details of the localized surface deformation outlined above are of

course effectively lost in the initial topography of a real surface.

Nevertheless their presence can still be detected by means of the asso- ciated exoelectron emission. The procedure consists of first fatigue

cycling the sample inside a vacuum chamber, then scanning the surface

with a small spot of ultraviolet radiation, and measuring the intensity

of the photoelectron emission. As the spot passes over a fatigued

region the electron emission is more intense (i.e. exoelectrons) , and

it is this localized exoelectron emission which provides a quantitative

measure of the accumulation of damage.

This technique was initially developed^ using a simple optical system

with a scanning ultraviolet spot of -70 ym diameter and a scan field

of only -0.3 cm x 0.2 cm. The test samples were correspondingly small

and were fatigued in the reverse bending mode. As the spot scanned

along the gauge length of the sample the intensity of the electron emission was displayed as a function of position on an X-Y recorder. An

example is shown in Fig. 3 of the distribution of electron emission from

a sample of 1100 aluminum. The onset and growth of exoelectron peaks

identify regions of fatigue damage. The initial background emission in

the undeformed areas remains unchanged, and corresponds to the grey

areas of intact oxide in Fig. la.

199
From the microscopy studies (e.g. Figs. 1 and 2) it is reasonable to

expect that the largest localized emission peak will identify the location of the most severe accumulation of fatigue damage. Furthermore,

the intensity of a peak should be some measure of the extent of the

local damage. Strong support is provided by the results for SAE 1018

steel shown in Fig. 4. In this graph only the largest peak is plotted

for each of twelve samples fatigued to failure. The intensity (1) is

normalized with respect to the initial background emission (Iq) • Since

the samples were fatigued at different stress levels to cover the indicated range of fatigue lives, the abscissa is also normalized and

expressed as fraction of life. All the data points fall within a scatter band which is quite reasonable for fatigue experiments.

Results of this type provide a calibration curve for the assessment of

the extent of accumulation of fatigue damage, from which the remaining

number of cycles to failure can be predicted. For example, in this

case, if an emission peak develops with a normalized intensity value of

10, then the sample has been damaged to such an extent that between 0.8

and 3% of its ultimate fatigue life has been consumed. Such a measure- ment constitutes a very abbreviated fatigue test, and predicts the

fatigue life as accurately as normally experienced by the time consuming procedure of cycling to failure.

LASER SCANNING SYSTEM

The application of the above technique to the evaluation of the fatigue

performance of structural or engineering components requires a small

spot of ultraviolet radiation to be scanned over a large area. A frequency doubled argon-ion laser (wavelength=257 . 3 nm) is well suited to

this task, since it has sufficient energy (4.8 eV) for the emission of

electrons from metals, and it may be readily focused to a small spot

and scanned over a large area. In these initial experiments a spot

diameter of 14 ym and a scan path of 9 cm were selected, as appropriate

for measurements on cylindrical samples of martensitic steel subjected

to torsional fatigue.

The laser scanning system and the procedure for recording the electron

emission is shown schematically in Fig. 5, and has been described in

detail elsewhere.^ The electrons photoemitted from the specimen by the

incident laser beam are collected by an electron multiplier. The

entrance cone of the multiplier protrudes through a grid; the latter

improves the collection efficiency of the multiplier particularly for

electrons emitted from the ends of the specimen. The intensity of the

electron emission can be displayed on a pen recorder, or used to control

the beam intensity of an oscilloscope. Since the beam of the oscilloscope is synchronized with the scanning of the laser beam, a photoelectron emission image of the specimen is formed on the oscilloscope screen.

Immediately after passing through the focusing lens (not shown in Fig. 5),

the now gradually converging laser beam is deflected by the mirrors of

200
an X-Y scanner, and enters the vacuum chamber through a quartz window.

There are two modes of scanning. In mode 1 the laser beam is scanned

on an X-Y raster while the specimen is stationary, so that a side view

is displayed on the oscilloscope. This mode of scanning is suitable

for flat surfaces; the example in Fig. 6 shows the emission map of a small specimen of 1018 steel at 8% of its fatigue life. The emission

is much stronger from the central portion of the gauge section where

the fatigue deformation has accumulated. The intensity of this emission was in good agreement with the earlier results shown in Fig. 4.

In mode 1 it is difficult to collect electrons emitted from the lower

portion of the cylindrical specimen indicated in Fig. 5. However in

mode 2 the electron emission can be measured and displayed from the

entire gauge surface. By observing the beam position on the oscilloscope, the Y position is adjusted and held fixed such that the X scan passes along the centerline of the rod. While scanning the laser beam

continuously back and forth along this centerline, the rod is slowly

rotated about its axis. The angular position of the rod is monitored

by an angular transducer which now controls the Y axis of the oscilloscope. In this way an electron emission map of the entire surface is

unfolded onto the oscilloscope screen. The example in Fig.' 7 shows the

distribution of electron emission from the central portion of a martensitic steel rod after 100,000 cycles of torsional loading. This sample

was at 12.8% of its fatigue life and the highly localized nature of the

fatigue damage is revealed by the isolated spots of bright emission.

This example also illustrates the purpose of the oscilloscope display:

it identifies the major regions of intense emission (exoelectrons)

where the fatigue damage is most severe. Subsequent quantitative readout on the pen-recorder can now be restricted to these regions of primary interest.

The results from four rods of martensitic steel, fatigued in torsion,

are summarized in Fig. 8. For each specimen, only the largest emission

peak is plotted regardless of its location. As before (i.e. Fig. 4),

the intensities of the peaks are normalized with respect to the initial

background emission and plotted as a function of the fraction of

fatigue life. The scatter band is quite small, particularly during the

first 10% of fatigue life. For comparison the dashed lines represent

the scatter band for the earlier results on 1018 steel shown in Fig. 4.

The agreement between the two sets of experiments is quite good despite

a threefold increase of material strength and a different mode of

fatigue loading. In each case the development of exoelectron emission

not only reveals the presence of fatigue deformation in less than 1% of

the fatigue life, but more importantly provides a quantitative measure

of the severity of the damage which can then be expressed in terms of

the fraction of life expended, or the useful remaining life.

201
SUMMARY AND PROSPECTS

The above results illustrate that the laser scanning system can be suc- cessfully applied to the early detection and assessment of fatigue damage in small engineering components as well as very small laboratory

samples. In this initial demonstration the limitation on specimen size was imposed by the robustness of the readily available vacuijm equipment,

but this is not a major obstacle to testing of larger structures. The

laser beam may be scanned over much larger distances e.g. 50 cm, by

scanning with a larger spot of 40 ym diameter.^ The other important

factor is uniform electron collection efficiency, and larger components

may require several electron detectors. However the one essential

requirement is that the component under test must be mounted in a vacuum chamber and then the cyclic load applied. Otherwise the fresh

metal surfaces extruded by the cyclic deformation will reoxidize and no longer be sources of exoelectrons.

The localized intensity of exoelectron emission is clearly quite a good

measure of fatigue damage. This result emphasizes the importance of

local damage in determining the fatigue life. Furthermore, despite the

complexity of the fatigue process, the accumulation of surface deformation is very systematic when viewed on the appropriate microscale. The

fundamental material requirements are that the metal under test must

i) have a brittle surface oxide and ii) develop regions of

microplasticity prior to crack formation in the metal itself. Thus it

is not known whether the technique is appropriate for a very brittle

metal, but all metals appropriate to fatigue applications should satisfy

these requirements. Certainly the fact that the growth of the normalized emission from the martensitic steel torsion specimens is in close

agreement with that from bending tests on 1018 steel, is a good indication of the universality of the technique.

Experience with the exoelectron method has so far been confined primarily to the low cycle regime of fatigue (i.e. Nf<106 cycles). The

applicability to high cycle fatigue should be investigated, because it

is in this regime that the greatest benefit is to be derived from an abbreviated test. Of particular interest would be studies close to the

fatigue endurance limit. Eventually the quantitative capability of the

technique could also be profitably applied to studies of spectrum loading. In all cases the predicted life is of course that in the strictly

inert environment of a vacuum. In practice, where the ubiquitous cor- rosion accelerates crack propagation, the consequent reduction of

fatigue life would have to be taken into account.

One aspect requiring immediate investigation is the effect of oxide

thickness. The very thin (~4 nm) natural oxide has been shown to rupture precisely along slip traces and thereby permit the details of the

surface deformation to be monitored by the exoelectron emission. However it is not known to what extent thicker oxides will rupture in this manner and thereby serve the same function. Clearly a thick oxide

202
scale with weak adhesion to the underlying metal is unlikely to be satisfactory, since it will undoubtedly hide the intricate details of the

surface deformation. Also very thick oxides may fracture independently

of the detailed distribution of deformation in the metal, in the manner

of a stresscoat lacquer. It is not known how this mode of oxide fracture will influence the results.

Finally it is worth noting that the above unique capabilities of this

scanning exoelectron technique can be attributed primarily to two fundamental factors:

1) Fatigue damage is usually initiated at or very close to the surface,

and electron emission is very sensitive to surface changes.

2) The scanning system can examine all regions of interest on a com- ponent or structure and pinpoint the very small region containing

the all-important localized severe damage at the site of eventual

crack formation.

In this regard, since the most intense source of exoelectron emission

corresponds to the most severe accumulation of damage and provides an overall assessment of the "state of fatigue" of the specimen, it is

entirely reasonable to regard the distribution of less intense emission

as a display of the complex distribution of fatigue damage elsewhere in

the sample or component under test. Thus the technique can not only

assess the severity of the primary fatigue site, but can also locate

and assess regions of secondary weakness. This capability would be particularly useful in evaluations of structural or engineering components,

where there are often several locations of potential failure.

ACKNOWLEDGMENTS

The author is very grateful to his colleague, S. R. Rouze, for the photoelectron micrographs shown in Fig. 1.

REFERENCES

1. Barton, J.R. and Kusenberger, F. N. , ASTM STP 495 (1971 p. 123.

2. Baxter, W. J . , Met. Trans. 6A (1975) p. 749.

3. Baxter, W. J . and Rouze, S. R. , J. Appl. Phys. 44 (1973) p. 4400.

4. Baxter, W. J . and Rouze, S. R. , J. Appl. Phys. 46 (1975) p. 2429.

5. Baxter, W. J . , J. Appl. Phys. 45 (1974) p. 4692.

6. Baxter, W. J . , Submitted to J

.

Testing and Evaluation.

7. Grosskreutz

,

J. C. and McNeil, M. B. , J. Appl . Phys. 40 (1969) p. 355

203
(a)

100 m

(b)

FIG. 1. Photoelectron micrographs of sample of 1018 steel

a) after 4% tensile strain in vacuum,

b) after exposure to room air for 60 hours.
0.5% LIFE 1% LIFE

2.5% LIFE I I 6% LIFE

10fim

FIG. 2. Scanning electron micrographs of localized surface fatigue

deformation during crack initiation in 1018 steel.

205
LiJ ^

Q.

= 2

"1 ^ r

xio^

4000 CYCLES

1000 CYCLES

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.6

POSITION OF SCANNING LIGHT SPOT (KM)

2.0 2.2 2.4 2.6

FIG. 3. Exoelectron emission from 1100 aluminum after 1000 and 4000

fatigue cycles. Fatigue life = 140,000 cycles.

C 1^

10 o 172.000

11 1%,000

12 O 942,000

0.1 1.0 10 FRACTION OF FATIGUE LIFE 17.1

100

FIG. 4, Growth of the largest exoelectron peaks from twelve samples of

SAE 1018 steel with a range of fatigue lives.

206
i

FIG. 5. Schematic diagram of the laser scanning and electron emission

recording system.

207
1 mm

FIG. 6. Photoelectron emission image of gauge section of fatigue sample

of 1018 steel at 8% of fatigue life.

FIG. 7. Unfolded photoelectron emission map of the surface of the cen- tral portion of a rod of martensitic steel after 100,000

fatigue cycles (12.8% fatigue life).

208
' 0.1 1.0 10 100

FRACTION OF LIFE (%)

FIG. 8. Normalized intensity of largest exoelectron emission peak from

four rods of martensitic steel as a function of the fraction of

fatigue life. Dashed lines represent data for 1018 steel shown

in Fig. 4.

209
DETECTING STRUCTURAL DEGRADATION BY ACOUSTIC EMISSION

Michael P. Kelly

Dunegan/Endevco

San Juan Capistrano, California 92675

Robert J. Schlamp

Dunegan/Endevco

San Juan Capistrano, California 92675

Abstract: The application of acoustic emission techniques to the detection and location of structural defects in a variety of materials and

geometries is presented. Results obtained during the preservice inspection of the Baltimore Gas & Electric Company Calvert Cliffs Unit 2 Nuclear Power Plant will be discussed. Additional results obtained

during proof testing of two petrochemical hydrotreaters will also be

presented.

A multichannel computerized source location system was utilized to monitor petroleum drill pipe during proof loading. Several sections of new

and used, corroded and sandblasted drill pipe were monitored. Initial

results indicate an ability to detect, locate and identify fatigue

cracks, coating cracks and corrosion during proof loading. Test re- sults from high reliability petrochemical vessels demonstrate that incremental crack growth during pressurization can be accurately located

and assessed.

Key words: Acoustic emission, crack growth, drill pipe, failure. Kaiser

Effect, pressure vessels, sources.

INTRODUCTION

Acoustic emission (AE) is the term applied to the spontaneously generated elastic waves produced within a material under stress. Plastic

deformation and the nucleation and growth of cracks are the primary

sources of acoustic emission in metals. AE is unlike most other non- destructive test (NDT) methods in that the energy comes from the material itself. As a result, AE is more sensitive to growing defects and

potentially less reliant on operator interpretation than conventional

NDT techniques. Acoustic emission offers a further advantage in the

inspection of complex structures which include inaccessible areas. Normally just a few stationary transducers can detect, locate and qualify defects over a very large area, including regions inaccessible by

alternate methods.

210
The most common type of structural degradation in pressure vessels is

in the form of subcritical crack growth caused by fatigue, stress cor- rosion or hydrogen embri ttlement. The detection of subcritical flaw

growth by acoustic emission can be accomplished in two ways. The first

and most obvious is the continuous on-line surveillance of a structure

throughout its operating life. This approach is expensive and the

transducers and instrumentation are often subjected to hostile environments and/or excessive background noise.

Structural degradation due to subcritical crack growth can also be detected using acoustic emission in conjunction with periodic proof testing. This approach takes advantage of the irreversibility of acoustic

emission, which is known as the Kaiser Effect (1). If a pressure vessel, for example, is pressurized to a proof pressure and then operated

for a period of time at a lower working pressure, no acoustic emission

will occur during subsequent pressurization to the same proof pressure

unless structural degradation (crack extension) has occurred in the

meantime. If crack propagation occurs in a vessel during service,

acoustic emission will be observed above the working pressure during

the next overpressurization.

Since proof tests are usually performed during periods of low background noise, such as when a vessel is temporarily out of service, and

in a fairly benign environment, the difficulties encountered in con- tinuous on-line surveillance are normally nonexistent.

This report presents the acoustic emission results obtained from a variety of structures, materials, and geometries monitored during proof

testing.

INSTRUMENTATION

The portable, computerized multichannel source location system utilized

in this work is shown in Figure 1. A block diagram of the system is

presented in Figure 2. The system accepts inputs from up to 32 transducers arranged in 4-transducer arrays for planar location and 2-transducer arrays for linear location. The system display oscilloscope (CRT)

provides a real-time display of the location of each source in relation

to the position of the transducers (array) involved. In addition, a histogram or bargraph showing the overall activity of each active array

is constantly updated and displayed on the CRT along with the currently

displayed array. Figure 3 represents a hardcopy reproduction of a typical CRT display including the array transducers, active sources, and

the histogram of events per array. In addition to the real-time display

the important information associated with each eventlocated by the system is printed out by a high speed teleprinter. A typical printout is

shown in Figure 4.

211
EXPERIMENTAL PROCEDURE AND RESULTS

Drill Pipe

The detection of drill pipe defects is of considerable importance to

the oil production industry since downhole failures often cause substantial losses. In an effort to improve drill pipe inspection techniques Continental Oil Company sponsored a program to assess the applicability of AE techniques to the detection and location of anomalies in

this important class of structures. The method employed in carrying out

the program is generally described in Continental Oil Company's U.S.

Patent No. 3,911,734. Thirty sections of 114 mm (4.5 in) O.D. and

140 mm (5.5 in) O.D. drill pipe were monitored for acoustic emission

during proof loading in two separate studies. In each case, the drill

pipe was capped at each end and loaded in tension to the proportional

limit of the pipe. Several grades of coated and uncoated pipe were

evaluated to obtain a representative sample of various pipe conditions.

The tool joint- test machine interface generated considerable extraneous

frictional noise during the loading, so special lockout transducers were mounted in each tool joint header to spatially filter out these

emissions. Each pipe was instrumented with a total of six 450 kHz

resonant frequency transducers; one in each tool joint header to guard

against extraneous emission and four more in two 2-channel arrays for

linear location along the length of the pipe. Each pipe monitored for

acoustic emission was later inspected by an alternate NDT technique for

correlation purposes.

Figure 5 shows the transducer locations for a typical drill pipe. The

acoustic emission activity expressed in cumulative events as a function

of location was plotted for each drill pipe tested. Figure 6 represents

actual test data received from sections of four pipes monitored during

the initial drill pipe study.

Pipe #11 (Figure 6a) contained a fatigue crack at 175 cm (70 in) in

addition to moderate pitting throughout the pipe. The acoustic emission

record shows a peak at 179 cm (70.5 in) and moderate activity in several areas demonstrating good correlation between the two inspection techniques. The lack of emission in Figure 6b is typical of a quality pipe

without defects and agrees with the post-test inspection which reported

no anomalies. Pipe #23 was found to contain several cracks including

eight in the 220 cm (87 in) to 241 cm (95 in) region. The AE results

in Figure 6c show good correlation in the 200 cm (79 in) to 250 cm

(98 in) region and another peak at 102 cm (40). The peak at 102 cm can probably be attributed to another crack or cluster of cracks. Pipe #21 was reported to contain internal pitting and coating cracks at 287 cm

(113 in) and 330 cm (130 in). The acoustic emission results in Figure

6d confirm the inspection report.

The results of the first study indicated that while AE was a viable

technique capable of detecting drill pipe defects it was difficult to

212
differentiate between internal coating cracks and fatigue cracks. Coat

ing cracks were often interpreted as the more important fatigue cracks

and vice versa. In an effort to determine differences in the AE "signa

ture" obtained from these anomalies, a laboratory investigation of

CONOCO supplied specimens of coated and fatigue pre-cracked uncoated

drill pipe material was initiated. This study revealed that coating

crack propagation actually generated higher amplitude signals than fati

gue crack extension. A second group of drill pipes was monitored with

acoustic emission during proof loading to substantiate the results obtained during the laboratory investigation. Figure 7 represents the

acoustic emission activity expressed in events and counts (event size)

as a function of location for a 114 mm (4.5 in) O.D. drill pipe monitored during the second study. The post-test inspection reported a fatigue crack at 419 cm (165 in) and coating scratches or cracks in the

355 cm (140 in) to 380 cm (150 in) region. While the coating cracks

generated fewer events than the fatigue crack (Figure 7b) there were

more counts associated with the coating anomalies (Figure 7a) This

confirms the results of the laboratory investigation and indicates that

the fatigue and coating crack phenomena can be identified on the basis

of magnitude (counts).

High Reliability Petrochemical Pressure Vessels

Acoustic emission source location techniques were applied to the detection of defects in high reliability petrochemical pressure vessels.

The purpose of the program was twofold:

1) to prevent the unintentional failure of units during proof

testing

2) to enhance the confidence in the structural integrity of units

surviving the proof test.

Under American Petroleum Institute (API) regulations, each of these

vessels is subjected to at least one proof test to twice the working

pressure. Several vessels were instrumented with two arrays of four

140 kHz resonant frequency transducers and monitored during proof testing. Figure 8 is a hardcopy CRT reproduction of the actual sources received from a defect-free vessel.

An artificial defect in the form of a sharp notch was instrumented in

the high stress region of one structure to assess the locational accu- racy of the acoustic emission instrumentation. Figure 9 presents the

data obtained in the hydrostatic proof test of that vessel. The nearly

horizontal pattern above the center transducer of the array represents

the emission obtained from the artificial defect. The last 50 events

received during the proof test are shown in Figure 10. Nearly all of

these events originated in the region of the artificial defect.

A third vessel monitored during proof test contained a natural 7.6 cm

(3 in) long crack located midway through the 10 cm (4 in) thickness.

213
Twenty-seven events located in an area 4 cm (1.6 in) X 5.3 cm (2 in) were received from the crack during the test. Figures 11 and 12 present

the source activity recorded during the proof test of this vessel. It

is interesting to note that although this structure survived a proof

cycle to twice the working pressure, it contained a growing subcritical

crack. Crack extension actually started well below the vessel working

pressure.

Petroleum Hydrotreaters

Two Gulf Oil of Canada petroleum hydrotreaters, manufactured by Japan

Steel Works, were .moni tored with acoustic emission during initial hydrostatic proof testing. The vessels were fabricated from ASTM A-336-F22

steel, normalized and tempered; and included a 6.35 mm (.25 in) stainless steel weld overlay on the I.D. surface. The two cylindrical hydrotreaters, designated R-201 and R-401 , measured 37.9 m (124 ft) and

29.4 m (96.5 ft) in length, respectively and were 22 cm (8.7 in) thick.

The cylindrical sections were 33.5 m (110 ft) and 24.9 m (82 ft) in

length. Each hydrotreater was instrumented with eight arrays of four

transducers each, a total of 32 channels. The 140 kHz transducers were attached to the structures with magnetic hold-downs. A thin layer of

silicone grease was placed on the surface of each transducer to help

couple the stress waves into the transducers.

In accordance with the American Society of Mechanical Engineers (ASME),

"Proposed Standard for Acoustic Emission Examination During Application

of Pressure" (2), the source location system was calibrated as a com- plete system prior to and immediately after the inspection of each

hydrotreater. Total system operation was verified through the use of

pulsing transducers (1 per array). In addition, all acoustically active sources recorded during the tests were graded, using analytical techniques such as event rate, count rate, and source activity as a function

of location and/or pressure. The classification of sources by grade

requires the following actions:

Grade A - If it occurs during pressure build-up, pressuri zation

should be halted. Confirm results as relevant by other

nondestructive examination methods.

Grade B - Recordable and reportable for future comparison.

Grade C - Further evaluation or correlation not required.

Figures 13 and 14 represent graphic displays of all active sources re- corded during the AE examination of R-201 and R-401. A tabulation of

these sources including grade classification is presented in Table 1.

During pressuri zation of R-201, several events were received from a relatively small area within the monitoring area of Array 4 (see Figure

15). The events all occurred in the region of a steel measuring tape

which was draped ci rcumferential ly around the structure. The tape was

21k
removed midway through the test and emission from the suspect region

ceased. This indicated that the emission was frictional in nature and

of no concern. A post-test ultrasonic inspection revealed no defects

in the active area, confirming the tape measure suspicion.

Gulf Oil of Canada will periodically requalify these hydrotreaters during their useful life through similar proof tests. The AE results obtained from this initial preservice inspection represent a valuable

baseline for future monitoring of these structures. Any crack initiation or propagation which might occur during the operating life of

these vessels, will be very evident during subsequent proof tests.

Calvert Cliffs Unit 2 Nuclear Power Plant

Sixty-four channels of source location instrumentation were utilized

during an AE examination of the Baltimore Gas & Electric Company Calvert

Cliffs Unit 2 nuclear reactor and primary coolant system (3). The primary coolant system consists of four reactor coolant pumps, two steam

generators, one pressurizer and associated piping. The AE examination

was performed in conjunction with the preservice cold hydrotest of the

system, and in accordance with the ASME "Proposed Standard for Acoustic

Emission Examination During Application of Pressure".

The 140 kHz transducers were attached to the carbon steel components

using magnetic hoi d-downs and a nuclear approved couplant. An approved

adhesive was used for attachment to the stainless steel members. Quad

arrays (see Figure 2) were utilized in monitoring the reactor top head,

the reactor vessel below the nozzles, the two steam generators, and the

system pressurizer. The four reactor coolant pumps, the two steam generator hot legs, the four cold legs, and the pressurizer surge line were

covered by multi sensor linear arrays. The region of the reactor vessel

between the nozzles and the upper flange was also monitored by four

transducers in a single contiguous linear pattern.

The reactor vessel and primary coolant system were monitored during the

entire pressure excursion to 220 kg/cur (3125 PSI) except for hold periods. The movement of customer personnel visually inspecting for leaks

prevented the acquisition of data during hold periods.

The AE examination of Calvert Cliffs Unit 2 revealed 35 insignificant

sources. No significant sources were found to be present under the

stress conditions imposed by the hydrotest. Table 2 presents the location instrumentation. Figures 16 and 17 represent graphic displays of

all active sources received from the reactor vessel and top head.

A post test examination of the Calvert Cliffs Unit 2 primary system is

complete in the areas which experienced Grade B and Grade C to B indications during the acoustic emission inspection. These examinations re- vealed no indications which exceeded ASME code standards. However,

minor acceptable indications were found in the regions defined by

215
acoustic emission, which attests to the sensitivity and locational

accuracy of the acoustic emission examination method.

CONCLUSIONS

The four tests presented were for widely varying conditions and materials, yet the results showed that standard acoustic emission source location techniques provide a powerful tool for the verification of

integrity of pressure vessels and other structures. The results re- ported here are typical of the results being obtained from bridges,

nuclear and petrochemical pressure vessels, pipelines, rocket motor

cases, composite materials and aircraft (4-14).

It is our belief that many additional applications will arise in the

future when the technique becomes better recognized by the technical

communi ty

.

216
REFERENCES

1. J. Kaiser, Untersuchungen uber das Auftreten Gerauschen Beim

Zuguersuch; Ph.D. Thesis, Technishe Hochschule, Munich, 1950.

2. "Proposed Standard for Acoustic Emission Examination during Application of Pressure", available from the American Society of Mechanical

Engineers, United Engineering Center, 345 East 47th Street, New York

N. Y. 10017.

3. M. P. Kelly and R. J. Schlamp, "Acoustic Emission Preservice Inspection of Calvert Cliffs Unit 2 Nuclear Power Plant", Dunegan/Endevco

,

San Juan Capistrano, California.

4. D. 0. Harris and H. L. Dunegan, "Verification of Structural Integrity of Pressure Vessels by Acoustic Emission and Periodic Proof

Testing", STP-515, Testing for Prediction of Material Performance

in Structures and Components , American Society for Testing and

Materials, 1972, pp. 158-170.

5. A. T. Green and H. L. Dunegan, "Acoustic Emission Analysis of Crack

Propagation and Fracture in Pressure Vessels and Pressure Vessel

Materials", presented at the First International Conference on Structural Mechanics in Reactor Technology, Berlin, Germany,

September 1971

,

6. N. 0. Cross, L. L. Loushin, and J. L. Thompson, "Acoustic Emission

Testing of Pressure Vessels for Petroleum Refineries and Chemical

Plants", Acoustic Emission, STP-505, American Society for Testing

and Materials, 1972, pp. 270-296.

7. A. A. Pollock and B. Smith, "Stress-wave Emission Monitoring of a Military Bridge", Non-Destructive Testing , December 1972, pp. 348-

353.

8. M. P. Kelly and R. L. Bell, "Detection and Location of Flaw Growth

in the EBOR Nuclear Reactor Vessel", Dunegan/Endevco, San Juan

Capistrano, California, DE-73-4.

9. R. Gopal , "Westinghouse Acoustic Emission Studies at EBOR", Final

EEI-TVA Rp 79 Meeting, San Antonio, Texas, December 11, 1972.

10. D. L. Parry, "Acoustic Emission Analysis in Oil and Chemical

Industry", Exxon Nuclear Company, Richland, Washington, April 1972.

11. M. P. Kelly, D. 0. Harris, and A. A. Pollock, "Detection and Location of Flaw Growth in Metallic and Composite Structures", STP-571

,

Monitoring Structural Integrity by Acoustic Emission , American

Society for Testing and Materials, 1975, pp. 221-240.

217
12. T. F. Drouillard, R. G. Liptai, and C. A. Tatro, "Industrial Use of

Acoustic Emission for Nondestructive Testing", STP-571, Monitoring

Structural Integrity by Acoustic Emission , American Society for

Testing and Materials, 1975, pp. 122-149.

13. R. R. Corle, "The Use of Acoustic Emission to Improve Rocket Motor

Case Structural Reliability", presented at AIAA/SAE 9th Propulsion

Conference, Las Vegas, Nevada, November 1973, AIAA Paper 73/1258.

14. C. D. Bailey and W. M. Pless, "Acoustic Emission Used to Nondestructively Determine Crack Locations in Aircraft Structure Fatigue

Specimens", 9th Symposium on Nondestructive Testing , San Antonio,

Texas, April 1973.

218
I

TABLE 1 Hydrotreater R-201 Sources

Acoustic

Source Event Density Counts Discussion Grade

SI Low High Minor Nozzle Weld Discontinuity C S2 Medium Medium Minor Nozzle Weld Discontinuity C S3 Low Medium Minor Discontinuity C Hiah n 1 y M MpHi 1 1^ U 1 umUNI Fxtranpoii^ - Fri r t i nna 1 B S5 Medium Low Minor Discontinuity C S6 Low Medium Minor Discontinuity C

S7 Low Medium Minor Discontinuity C S8 Medium Low Minor Discontinuity C S9 High Medium Support Drag C to B SIO Medium Low Support Drag C Sll Medium Low Minor Discontinuity C S12 Medium Medium Minor Weld Discontinuity c

Hydrotreater R-401 Sources

Acoustic

Source Event Density Counts Discussion Grade

SI Low Medium Minor Nozzle Weld Discontinuity C S2 Medium Low Minor Nozzle Weld Discontinuity C

S3 Low Medium Minor Nozzle Weld Discontinuity C S4 Low High Minor Discontinuity or Support

Drag C S5 Low Medium Support Drag C S6 Medium Low Minor Discontinuity or Support

Drag C

S7 Medium Low Minor Discontinuity c S8 Medium Low Minor Discontinuity c S9 Medium Low Minor Discontinuity c SIO Med i um Medium Support Drag c

-219
1

TABLE 2 Calvert C I iffs Uni t_ 2 Acoustical ly Active Sources Acoustic Event Source Density Counts 1L nrUV,0A fL

11 nnU 1

1

Di scussion Grade

SI Low Low Reactor Minor Nozzle Discontinuity C S2 Low Low Reactor Minor Nozzle Discontinuity C S3 Low Low Reactor Minor Nozzle Discontinuity C S4 Low Medi urn Reactor Minor Nozzle Discontinuity C 35 Low Low Reactor Minor Nozzle Discontinuity c S6 Low Medium Reactor Minor Nozzle Discontinuity C to B 37 Low Medium Reactor Minor Nozzle Discontinuity C to B 38 Low Low Reactor Minor Nozzle Discontinuity C S9 Low High Reactor Nozzle Discontinuity B SIO Low Low Reactor Minor Nozzle Discontinuity C Sll Low Low Reactor Insulation Drag or Minor Weld Discontinuity

C

S12 Low Medi urn Reactor Minor Weld Discontinuity C to B S13 Low Low Reactor Minor Weld Discontinuity C 314 Low Medi urn Reactor Top Head Insulation Drag or Minor Discontinuity

C to B

S15 Low Medi urn Reactor Top Head Insulation Drag or Minor Discontinuity

C to B

S16 Low Low P res su r i ze r Minor Discontinuity C 317 Low Low Pressurizer Minor Discontinuity c 318 Low Low Pressurizer Minor Discontinuity c 319 Low Low Pressurizer Minor Discontinuity c 320 Low Low Pressurizer Minor Discontinuity c

CO! DC 1 Low Low Pi pi ng Minor Weld Discontinuity c S22 Low Low Piping Minor Weld Discontinuity c S23 Low Low Pump 21A Minor Discontinuity c

324 Low Low Piping Minor Weld Discontinuity c 325 Medi urn Medium Piping Minor Weld Discontinuity C to B 326 Low Low Pi pi ng Minor Weld Discontinuity Q S27 Low Low Steam Generator 21 Minor Discontinuity in

V 1 L 1 111 \,j U 1 He 1 U c

S28 Medium Low Steam Generator 21 Minor Discontinuity in

V IK, 1 1 1 1 K,y \j 1 nc I VI c

329 High Medium Steam Generator 21 Discontinuity in Vicinity of Weld

B

330 High Medi um Steam Generator 21 Discontinuity in Vicinity of Weld

6

331 Low Low Steam Generator 22 Minor Discontinuity in Vicinity of Weld

C

332 Medi urn Medi um Steam Generator 22 Minor Discontinuity in Vicinity of Weld

C to B

333 Medi urn Low Steam Generator 22 Minor Discontinuity in Vici ni ty of Weld

C

534 Medi urn Medi um Steam Generator 22 Discontinuity in Vicinity of Weld

6

S35 High Medi um Steam Generator 22 Discontinuity in Vicinity of Weld

B

220
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238
SESSION VI

GOALS.PURPOSES

AND

REQUIREMENTS

OF DESIGN

-PRODUCT LIABILITY

Chairman: J. E. Stern

NASA,Goddard Space Flight Center
I

i

I

)

i

I
NEGLIGENCE CHARGES IN PRODUCTS LIABILITY LAW

CAN BE USED IN ENGINEERING AS SAFETY DESIGN CRITERIA

Dr. Raymond R. Hagglund

Mechanical Engineering Dept.

WORCESTER POLYTECHNIC INSTITUTE

Worcester, Massachusetts 01609

Abstract: Society expects manufacturing companies to make products that

are safe but the meaning of the word safe is not very clear. In products liability law, the legal requirement is that a product be "reasonably safe". The charges of negligence, as used in products liability

suits, can be used as safety design criteria in design and manufacturing

to insure that reasonable care will be exercised in producing products

that are reasonably safe.

Key words: Product Liability; Safety

Designing a "Reasonably Safe" Product

An interdisciplinary engineering team composed of design, material,

manufacturing and quality control engineers have the responsibility for

creating products which are useful, reliable, reasonably durable,

profitable and "reasonably safe". These last words are in quotes and

underlined because they are the focus of this paper.

It is fairly easy to establish tests for measuring product usefulness,

reliability, durability, and profitability, but it is not so easy to

define a test for measuring product safety. Based on my experience as

an engineer in products liability litigation, I have found that many

engineers have not seriously considered safety. I have also found that

some management do not place much emphasis on product safety because

safety generally increases the cost of the product. I predict that the

increasing number of product liability suits will force both engineers

and management to seriously study the meaning of safe and especially the

terms "reasonably safe". In analyzing these words, there are two problems. The first is the difference in meaning between safe and"reasonably safe"; the other is the need for developing a set of engineering

design criteria, based on these words, which can be introduced into the

design and manufacturing process.

Some Background on the Products Liability Problem

It is necessary to have some understanding of the product liability

problem in order to fully understand why this paper is placing such

emphasis on the terms "reasonably safe".

2kl
Today, if someone is injured or killed while using a product, the injured person or his estate, will almost automatically enter a suit

against the manufacturer (s) , the distributor, and even the retailer,

claiming that a defect in the product directly caused the accident.

During the last six years there has been a rapid rise in the number of

such suits and as the general public becomes more aware, because of

newspaper articles, there will be even more suits entered and it is

predictable that the number of suits will rise even more rapidly.

When an accident occurs, a suit is filed against a manufacturer but the

insurance company carrying the companies products liability insurance

actually handles the suit. At the very beginning of a suit, the insur- ance company investigates the accident and studies the charges of

negligence. If the case is black and white and in favor of the injured

person, the insurance company will settle; otherwise they will fight.

Some cases go to court where a jury makes a decision as to whether the

product was defective or not and if the defect, assuming one existed,

caused the accident.

I have found that insurance companies are settling many claims because

the engineers, who designed and manufactured the product, did not

exercise "reasonable care" and, as a result, these engineers would have

a hard time convincing a jury that the product was "reasonably safe".

The actual costs of unsafe products is quite clear today. The social

costs are the loss of life or the social cost of a person living with

an injury that may affect him for life. The financial costs are associated with settlements and jury awards which are causing insurance

premiums for manufacturing companies to rapidly escalate. Some com- panies cannot even get insurance.

In some ways, the products liability problem is getting out of hand.

For example, a company which designed and manufactured a machine thirty

years ago can be sued in some states even if the machine, which caused

an accident, had been resold several times and installed improperly.

Insurance companies still have to spend money to investigate and defend

these suits. In some states laws have been passed to control this type

of suit.

To get back to the main point of this paper, engineers need to understand the meaning of reasonably safe. To do this engineers need to

know a little about products liability litigation. Specifically they

need to understand the role of the technical expert, the legal strategies used by the attorneys, and the charges of negligence that are used.

With this understanding, I feel that an engineer can develop legaltechnical criteria which can be used in engineering design.

2U2
Understanding the Role of the Technical Expert in a Suit

If a person is injured or killed, an attorney, called the plaintiff's

attorney, is generally retained by the injured person or his estate.

This attorney calls in a technical expert, an engineer in many cases,

to conduct an investigation.

The technical expert attempts to reconstruct the accident and develops

technical opinions which answer specific technical-legal questions.

Examples of these general questions are:

1. Was the product defective?

2. Did the defect cause or contribute to the accident?

3. Was the product being properly used?

4. Was the product reasonably safe for its intended use?

If the technical expert develops opinions which show that the product

was defective, then a suit is entered against the manufacturer (s) . The

sued company then contacts its insurance company and the insurance company will perform its own accident investigation and develop some opinions on the charges of negligence which were entered. The insurance

company then makes a decision to settle or to fight the suit.

If a decision is made to fight the suit, the insurance company generally

brings in a defense attorney skilled at trial law. The defense attorney

retains his own technical expert who performs an accident reconstruction

analysis and who develops his own opinions about the accident and the

product.

While the suit is being developed, the engineering expert may help to

educate their respective attorneys in the technical details of the

product, draft technical questions (interrogatories), review depositions

and give depositions. The experts may even help in developing ideas for

presenting the technical-legal case to a jury.

In presenting the case to the jury, each lawyer will use a certain legal

strategy. While using their respective strategies, the jury will hear

testimony from witnesses, technical experts and medical experts. When

all of the testimony is heard, the jury will make a decision in the case.

The following sections give a brief description of the legal strategies

used by lawyers at least as understood by an engineer.

2^3
General Legal Strategies

An understanding of the strategies that the attorneys will use in the

courtroom will develop, in the engineer, more awareness to product

safety in general. Also, the knowledge could be very useful if the

engineer needs to appear as an expert in a products litigation.

The plaintiff's attorney will attempt to show that the product was defective and that the company was negligent because they did not exercise

reasonable care in one or more of the areas of design, materials, manufacturing, testing, quality control, safety devices, user and maintenance

instructions, and even in anticipating alternative uses for the product.

In addition, the plaintiff's attorney will probably claim breach of

warranty stating that the product cannot do what it is supposed to do.

He will also try to convince the jury that the product left the manufacturer in a defective condition, having previously defined one or more

specific defects, and that the plaintiff's injuries occurred because the

product contained these defects.

On the defense side, the attorney will attempt to show that the company

did exercise reasonable care in the areas of design, materials, manufacturing, product testing, quality control, safety features, user and

maintenance instructions, and that the company anticipated all reasonable alternative uses of the product. He will also attempt to show that

the product can do what it was designed to do.

As a key part of the defense strategy, the attorney will attempt to prove

that the person, who was injured or killed, was misusing the product

and/or that the user assumed the risk when he used it in an unusual way

which he knew to be dangerous.

In the trial, both attorneys will use prepared questions to develop their case so that it will appear to be logical to the jury. They will use

information already gathered in interrogatories, depositions and experts'

reports to help in framing these questions.

The. plaintiff attorney's questions will probe deeply into the areas of

negligence which have been charged with the goal of proving that reason- able care had not been exercised, thus resulting with an unsafe product.

The defense attorney will, of course, use questions in an attempt to

show that the charges of negligence are not true.

The engineer needs to know what the charges of negligence are in order

to develop technical-legal design criteria. These are given next.

2kh
Some Charges of Negligence

Some of the specific claims of negligence that might be entered are

listed below; you can probably think of others.

Ten possible charges of negligence are:

1. The product was defectively designed.

2. The materials used In the product were defective.

3. The product was defectively manufactured,

4. The product was not tested.

5. The product lacked sufficient quality control testing.

6. The designer was negligent because he did not foresee

all possible uses of the product.

7. The product lacked safety devices.

8. The product lacked proper warning signs.

9. Instructions for the proper use and maintenance of the

product were not given.

10. The manufacturer did not notify all owners of their products

that new safety devices were available, or that a design

modification to Improve the safety of the product had been

made and was now available.

In the trial, the plaintiff's attorney will ask questions and subquestions In any one or more of the areas where he feels the company

was negligent. For example. In the first charge listed, defective

design, the plaintiff's attorney will attempt to show that the manufacturer did not use state of the art Information; did not use codes

such as those developed by OSHA, ANSI, ASTM, etc.; did not prepare

proper stress calculations or use special computer programs which were

available. These are only a few examples of how the attorney might

prove negligence In design.

The plaintiff's attorney will also probe Into the record keeping system

and he will attempt to show that records, such as calculations, tests,

etc. do not exist or that they are not correct. Of course, this type

of evidence, If It Is found, will suggest to a jury that reasonable care

was not exercised by the manufacturer.

Obviously the defense attorney will need correct facts and evidence to

show that reasonable care was exercised In all of the areas listed.

2k3
Using the Charges of Negligence as Design Tools In Engineering

The charges of negligence, previously listed, can be used in design and

manufacturing as a method for defining a "reasonably safe" product.

But to be effective, engineering will have to develop defense answers

for each of the charges listed and they will have to have records to

back up these answers.

Most likely an interdisciplinary team of engineers will have to develop

detailed defense type answers for charges of negligence listed and for

others that they can think of. Such a team would ensure that the state

of the art from other disciplines has not been neglected. If all of

these charges can be defended on paper, in a thorough way, then the

product can be defined as being "reasonably safe".

These charges of negligence can also be used as a management tool. It

is proposed that management request engineering to develop a Safety

Design Report for each consumer or industrial good manufactured. An

interdisciplinary team of engineers should develop the report and they

should devote a chapter to each of the charges of negligence listed.

In each chapter, the engineering team should summarize the state of the

art, what was done, what was feasible but not done. For the latter,

they should explain why it was not done if it was feasible. The report

should be reviewed by company management, and by technical and legal

experts outside the company.

If this action is taken, products will be manufactured that are safe - at least "reasonably safe".

Summary

The law of products liability, especially the charges of negligence

listed in this paper, can be used in engineering and manufacturing as criteria for determining if a product is "reasonably safe". But in

order for these legal-technical criteria for safety to be effective,

company management will have to formally request that engineering develop a Product Safety Design Report where this report contains a chapter for each of the charges of negligence listed in this paper.

References

1. Legal ; E. N. Swartz, "Hazardous Products Litigation", 1973,

Chapter II, Theories of Liability, published by

Lawyers Cooperative.

2. Codes and Standards : "Accident Prevention Manual for Industrial

Operations", 1974, published by National

Safety Council.

2h6
PRESCRIPTION TO REDUCE PRODUCTS LIABILITY LOSSES

A. THE DEFENDANT AND PRODUCTS LIABILITY

John P. Amess

Hogan 5 Hartson

815 Connecticut Avenue, N.W.

Washington, DC 20006

We have taken a brief look at the program for this symposium and find

that it is literally filled with highly technical and useful information.

I am quite sure that you will not get that kind of information from our

talks because Milton Heller and I are just a couple of lawyers who are here to add a little bit of confusion to an area that is already as confused as it can possibly be. I am going to try to approach the

situation from the standpoint of a defense counsel representing a target defendant in either a manufacturing or an engineering setting

and give you a game plan with which to approach the very perplexing

problem of products liability and the ever increasing costs that are incident to it. After I am finished Milton Heller who has been doing

a good job for a long, long time scaring the "heller" out of me, is

going to do the same to you.

As Mr. Stem told you one of the sidelines that I have is to be a participant in the Defense Research Institute. This is a nationwide

organization of some 6,000 individual members and 400 corporate members.

Among them are representatives of manufacturing companies -- maybe many

of them represented at this conference -- and most of the major insur- ance companies. It is a most unique organization because it is the only

one that I know of that is a real "think tank" organization. It started

in 1960 and it is devoted almost exclusively to the production of data

and proposals to help target defendants defend themselves against claims

and lawsuits that might be filed against them. DRI is particularly

interested in the unfounded claim of the type that Professor Hagglund

has mentioned to you this afternoon.

Now, you must understand when you think in terms of products liability,

and when you think of anything in terms of law, that you are dealing

with fluid and imprecise concepts. I know, being scientists such as you are, it is going to be a little bit difficult. But law is a dynamic

thing, ever changing. It has no rigid precepts. It has nothing that

you can count on from day to day. Because the law tries to accommodate

to the ever changing needs of society and in doing that focuses upon

every case that comes along. If a particular case has certain factual

connotations and cries out for a remedy, the judge is there to try to

set things right. If you will analyze the situation I think you would

conclude that you would not have it any other way even though the imprecision may offend your scientific mind.

2l^7
The products liability situation is relatively new. Even as short a time as 25 years ago when I began my practice of law a products

liability case was a rare thing. But fads come along and right now we

are in that period where it seems fashionable to condemn a manufacturer

or an engineer or designer for something that has caused injury. An

after the fact Monday morning quarterback finds it easy to place blame

particularly upon a deep pocket --a company or an individual. Notwithstanding what I said about not having rules upon which you can depend

in the law, a safe product can be defended. The law, although it does

try to accommodate changes in circumstances, is a very reasonable thing.

People come into courtrooms before a judge and a jury and once in a while you will have an aberration, but, by and large, that jury and that

court is there to see that the fair thing is done. So we are going to

try to talk for a few minutes this afternoon about what kind of program

you can have to put yourself in the posture when you come before a court to appear reasonable with respect to your product and your conduct.

There are basically three ways in which liability can be affixed upon a manufacturer or design engineer. One is on the theory of negligence,

and as Professor Hagglund told you, negligence is a very simple concept.

It is defined by saying that one is not supposed to do that which a reasonable person would not do. Or the corollary of that is that you

are supposed to do what a reasonable person would do under the same or similar circumstances. The second theory upon which liability can be

imposed is breach of warranty. That was originally a contract theory.

It merely means that when you sell something, whether to an ultimate

consumer or to anyone else, you warrant that that product is reasonably

fit for the purposes for which it was intended and if it isn't you have

breached your contract. You haven't given the purchaser what you said

you would give him. The third theory, and that is the one that is

giving people the most trouble these days, is a concept that has become

known as "strict liability". Milton Heller is going to tell you a lot

more about that a little later but I will just touch on it. Strict

liability, as it originated, was not supposed to be what it has come to

be. There has been an awful lot of hysterical talk about it. When you

say "strict liability" you kindle emotions. Does it mean that I am

liable no matter what I do? No -- that is not what strict liability is.

Strict liability merely means that when you put a product into the

stream of commerce and it is defective in that it contains some feature

that is unreasonably dangerous, you will not be heard to say that you

were careful in its manufacture and therefore you shouldn't be liable

for injuries caused by it. You put it in commerce -- it is unreasonably

dangerous -- and you are going to pay for it. That is what strict

liability means --it does not mean you are liable unless it is unrea- sonably dangerous.

The first thing that a design engineer has to think about in dealing

with liability situations is the design of the product as it relates to

man; as it relates to the machine; and as it relates to the environmental

2k8
interface or the circumstances under which that machine or that product

is going to be used. I would appreciate it if you would dwell on that

concept for a minute, because in my experience in defending products

liability cases and in a few cases prosecuting them as well, liability

has come about in every instance that I can recall because somebody did

not think about how that product was going to interface with society.

So that is the first thing you have to do. You have to think about the

product as an entire concept, not just the product in its technical

sense, but how is that product going to fit in with man and with the

function that it is going to perform.

There are many vulnerable areas. Professor Hagglund has touched upon

some of them, as a matter of fact he has hit the ten most important ones. But let me review them for you. There are design errors; material

selection errors; component selection errors; assembly; testing;

instructional materials and techniques; sales literature; labels; advertising; warnings of latent danger; warnings of hazards which cannot be

eliminated; inadequate safety measures or features and failure to foresee misuse or abuse of the product. There are some kinds of abuse that

you know from your experience people are going to subject the product

to. You are required to foresee that which a reasonable person would

expect. You must anticipate some alteration to the product or some aborting of safety features. I have seen products, and you have too,

where safety features are designed and you have to do this or that to

make it work and you know a lazy workman is going to take a shortcut and

he is not going to operate that safety feature. You have to make it

easy for them to put it on, you can't make it difficult. Liability

itself is premised sometimes on the failure to have that safety feature

in a proper position or susceptible to easy use. Failure to take protective action after discovery of a defect or failure to properly

consider the life of the product and make accurate representations with

respect to it are additional vulnerable areas. You have a constant

fight between the design engineer and the manufacturer on one hand, and

the sales force on the other. You make a product and you know it is

going to be good, you know it is going to perform for a certain period

of time, but when your salesman attempts to sell it, he is not going to

say after 4 or 5 years you should replace this item -- because if he

does he is going to meet sales resistance. So after 7 or 8 years and

the user is using the product far beyond the life for which it was

designed, you have an accident. There is liability because there has

been an inaccurate representation with respect to the life of the

product and its effective and safe use. Packaging is another area of

concern. The product must be packaged properly because sometimes

accidents happen when it is not.

Other areas in which liability can crop up involve warranties and the

way you go about making repairs and replacement parts available when

they are necessary. I am not going to talk about each one of these

areas of vulnerability because in this limited time we have this after2k9
noon I would rather talk about the approach to doing away with the

liability that can come about from these vulnerabilities. First, I would strongly recommend that you establish in your companies, by whatever name you wish to call it, a design review committee -- design

review committee which has interdisciplinary members, not just engineers

as Professor Hagglund mentioned. Your design review committee should

have people from the sales force on it, personnel from management on it;

and it should have lawyers on it. Now, what should the design review

committee do? First, it should be given the "DOwer to make decisions

and to enforce those decisions. A discussion over coffee and doughnuts, once a month, is no good to anyone. The design review committee should

be charged with the duty of evaluating all new products and with the

duty of re -evaluating, from time to time, old products to see that they

conform to current safety standards. The design review committee should

be familiar with all current applicable state and federal regulations,

as well as industry standards whether they be mandatory or merely

recommended. The committee should be charged with the responsibility of

keeping abreast of trends; it should be aware of what other manufacturers in the same industry do. It should be particularly alert to

safety features to be incorporated into the products. The committee

should document all of its deliberations during the design phase and it

should explain why a particular modification or safety device, having

been considered, was rejected. The same stringent controls should be

exercised by the review committee during all phases of production and

distribution, even though the design phase may be the most important.

In addition, your companies should have a testing program. The program

should be effective so as to thoroughly test the design of any product

before the design is finalized. The testing program should also provide

effective quality control during the production phases. It should be

charged with the duty not only of checking the product that is being

manufactured by your company, but should check all components supplied

by other manufacturers. In addition, all materials should be checked

for suitability.

Next, we recommend that your company have an effective and detailed

record retention program. One of the most difficult tasks for a trial

lawyer trying to defend a client after an accident has happened is to

go iDack, many times, two, three or four years, to try to find out what

the facts are. Any many times he is frustrated to find out that there

were no records to show what the facts were. Records are the most

important single tool of defense and they are the most important thing

you can do to save your company's money in products liability defense

situations. What kind of records? Records showing how the design came

about. Records identifying the materials and showing how the materials

were selected. Assembly drawings, including all the guards and safety

features. Records of the suppliers of the component parts. Records

showing testing, quality control and inspection. Check lists in

250
shipping to show that warning labels were included, to show that proper

instruction material was sent along with the product, and to show that

operating manuals were included. Records to show what the state of the

art was at the period of time when you are going to be charged with

having breached it and reliance upon recollection is hazardous when you

are dealing with such a fluid situation as development of the art. I think that type of program will be very, very beneficial.

There are a few additional points I would like to make. By all means,

gentlemen, don't let your companies keep the fact that there is a defect

in the product a secret. If you discover a defect and then try to

cover it up while taking remedial action to see that it does not occur

again you are being naive and reckless. It will be discovered. And

when it is, gentlemen, a showing of attempted cover-up will lead to an award of punitive damages. You know it is one thing to have somebody

injured and to be soaked by a jury for fifty to seventy-five thousand

dollars. That is hard enough to bear, but when that jury is told that

you found the defect and that you knew about it and you hid it, that is

when the millions of dollars get tossed on the top. That is the pill

that is really throwing product liability litigation all out of economic

reality. It is the millions of dollars in punitive damages that you

cannot adequately predict that is driving manufacturers up the wall.

When you find a defect, publish it, remedy it, get that product back,

send out the warning, take care of it and stop that accident that is

going to happen tomorrow. Never keep a defect a secret.

My next subject is personnel. You have to find some way to prevent

untrained workers and unqualified personnel from participating in these

very important matters. Qualified personnel have to make design

decisions. They have to have the technical ability; they have to have

the background and the scientific knowledge to make the decisions that

are going to be tested later on in court. You just cannot expect

unqualified people to make that kind of decision. Yet, history has

shown that many manufacturers try to set up design review committees

complemented with borderline qualified personnel in order to save money.

Next, pay as much attention as you can to warning situations. I heard

a few questions after Professor Hagglund's remarks. We all know situations where liability has been imposed under circumstances which would

lead one to ask "How in the name of justice could someone be held to

pay damages for such an occurrence? An occurrence like that is almost

necessarily going to happen." Another way of thinking about this

subject is that some products are impossible to manufacture absolutely

safe. Some machinery and some equipment that is designed to do particular jobs are just inevitably going to expose human beings to injury

and damage. You cannot make everything safe. You cannot put a guard

over a butcher knife blade. So, what can you do? You can adequately

warn. You can make certain that there are no latent dangers. You can

251
make certain that you have the right kind of labels on your product so if somebody is going to throw a switch he knows what is going to happen.

If you manufacture a product which has a hole with a piston inside of

it, you can put a big red sign over the hole which says "Don't poke your

head in this hole." Adequate warning is the easiest, safest technique

that you can enploy to improve the safety of a device or a product that

is going to inevitably cause danger if it is not used properly. Put

it on your product and save yourself from liability. I cannot believe

that a court would hold somebody responsible for an injury that came

about from something where there was a great big red sign that said

don't do it and yet the person went ahead and did it anyway.

Now, let me briefly mention the need for proper packaging. Some products

are dangerous unless they are handled properly, unless they are packaged

properly, unless the right kind of warning is put on the package. Be

very careful about such details and pay just as much attention to the

package as you do to the design.

The last thing I would caution you to heed is the need for prompt

investigation. Liability frequently flows from an energetic plaintiffs'

attorney who gets to the product before you do. He gets all of his

homework done and then files a claim and you sit around and try to pick

up the pieces afterwards. Get in there and make the investigation

promptly. Save the evidence, identify the witnesses, take their statements and then you will be in a position to set the record straight once a claim is made. In conclusion, I would like to wTap up this part

of our presentation by suggesting that, in order to carry out these

things that we have recommended, reduce your intercompany procedures to

writing so that policies will not be misunderstood, then enforce them

rigorously. Establish effective communication to facilitate your loss

prevention program. Keep those records and make sure that you have

established a mechanism for conducting a prompt investigation.

252
PRESCRIPTION TO REDUCE PRODUCTS LIABILITY LOSSES

B. THE PLAINTIFF AND PRODUCTS LIABILITY

Milton Heller

Investment Building

1511 K Street, N.W.

Washington, DC 20005

Now you know why John is so successful. He is a gentleman, knowledgeable, and articulate. If any of you become expert witnesses I think

those are the three qualities that you should possess. The jury trial

often functions as a popularity contest coupled with some good hard

facts and theories and a cause of action. Although everything that

John says is true, most of the engineers in this country don't work for

manufacturers who are willing to adopt the programs John suggested. A

lot of you might work for people who cannot afford to adopt all of the

safeguards. I doubt very much if even the largest corporations do all

of the things that John says they should do. A lawsuit is therefore

inevitable when a plaintiff's attorney is retained in a case where a defective product causes injury. If the defect existed at the time the

product left the manufacturer's or distributor's hands, the attorney

will inevitably find that one or more of the suggestions John made were

not adopted by the defendant. If the failure to comply with one or more of the points was a competent contributing cause of the manufacture

and distribution of this defective product, and that defective product

in turn, without misuse or abuse, caused injury, the plaintiff is

supposed to collect.

Regardless of what you read or hear, pursuing a products liability case

is a very hard job. It is especially hard to get good experts if you

represent the plaintiff, because if you are suing a particular member

of an industry like the auto industry, most of the good experts are aligned with the auto industry in one form or another. Those who

aren't, and set up their own consulting firms, I have found, as a general rule, are not as knowledgeable with the state of the art and

industry. They don't have the facilities and funds to run tests and

keep abreast of advances that are so essential to an expert being able

to express an opinion on the efficacy and safety of a particular

product

.

Before I go any further, many terms have been used, like "reasonably

safe" and the converse, "unreasonably dangerous." The term "unreasonably dangerous" is the one used in the Restatement of Torts, Section

402A, which is the basis for strict liability. Strict liability is not

absolute liability, as John told you, and it requires proof of the

defect and injury. An official comment to 402A states that an "unreasonably dangerous" product is a defective product that injures because

253
of a condition o£ the product not contemplated by the ultimate consumer. Some of the typical exanples of defects not contemplated are a steering

wheel that comes off while you are driving or wheels that roll off.

Those are obvious exan^les; ones that are not as obvious appear when

you get into the failure to warn, or failure to instruct area. Defining

terms is most important.

I served on a committee with a group of doctors who was trying to define

when a person is legally dead. Is it when the heart stops or brain

waves are flat? We kicked it around and finally decided that a man is

truly dead when he cannot arise and litigate. The other day I was listening to the Today Show and they were talking about someone who ran

for election on a fun ticket in Washington or Oregon, and promised that

if he is elected, heads would roll on Capitol Hill. When they asked him

what he meant by that, he said he would take portable Johns, put wheels

on them, and roll them down the hill. Defining terms is so important.

What is a defect? A product can be defective if it doesn't meet design

specifications, if it doesn't meet federal regs., other regs., if it

doesn't meet industry standards evidenced by association standards. If

the product falls below those, and the plaintiff buys the product, and

it causes injury, he has a case. Cases like these are few and far

between. They are usually settled. Cases you read about in the news- papers are the ones where the product is destroyed, or there is no ostensible defect in the product after some accident or collision that

half destroys it. Here we are in a gray area where expert witnesses

come in and quite logically and precisely opine that a defect caused

the accident. They can reconstruct, based upon the way it happened, and

come up with a very good explanation. Those cases are rarely settled

because for every one of those experts, the industry can find ten to

refute him and call him a prostitute or whatever.

The tenth factor that Professor Hugglund mentioned, a continuing duty

on the part of the manufacturer to modify or warn, is the hot area in

the field of products liability at this time. But you must limit that

concept to a product that has a long life expectancy. It can be a punch press, an airplane, truck, or industrial equipment. When it is

manufactured, distributed and sold it must not be designed or produced

in a negligent way. By that I mean the manufacturer has exercised

reasonable care. Putting it another way, the manufacturer has done

everything that a reasonably prudent manufacturer would have done. In

addition, it was not defective according to the state of the industry

or the state of the art. But then, you have a Catch 22 situation. If

the manufacturer remains in the business, to be competitive, he has to

continue to improve his product, research, and keep abreast of improvements in the field by his competitors. When he does that he eventually

becomes aware that the product he sold ten years ago can be vastly

improved from a safety standpoint by a modification that costs very

23h
little. Now, the other situation is when the manufacturer, after ten

years or so, is informed that his product is causing a number of

injuries and deaths, but knows of no reasonable method of improving

the product. In the latter situation the manufacturer has a duty to

inform his purchasers that there is a danger which he does not think can

be obviated, so that the user or purchaser can act appropriately. In

the initial instance, where the manufacturer learns that the product can

be improved, he has a duty to offer a modification that can improve the

safety of that product, or in the alternative, at least inform the

purchaser that he can purchase a new product with the new safety

features. There aren't many appellate cases around the country on it,

but there are a few, one in Illinois, one in Massachusetts that I am

aware of, one in Maryland. I don't know of any recent opinion that

holds that a manufacturer, once he sells a product and title has passed,

has no obligation or duty to that purchaser to keep him informed when

the manufacturer knows that the product can be improved from a safety

standpoint. Now, some examples. Take a manufacturer of an anesthesia

machine used in an operating room that hooks up to an outlet in the

wall. Another manufacturer has sold and installed a main central system

that feeds oxygen and other gases through the walls to let's say nine

operating rooms in that hospital. The mechanisms hook up in the

operating room. They work by pressure, although there are electrical

ones too. Now, for years an anesthesiologist was a computer. He had

to watch the vital signs periodically, and at the same time, periodically glance over at the flow meter to see that the nitrous oxide and

the oxygen were flowing. Anesthesiologists are well skilled. However,

not all operating rooms have anesthesiologists on duty at all times.

Some hospitals have nurse anesthetists or dental residents or other

quasi -medical personnel who are inducing patients and watching them.

They are not as skilled. Occasionally a hose kinks, something goes

wrong, the connection is poor, and the nitrous oxide flows but the

oxygen doesn't. The person administering the oxygen doesn't glance

over at the flow meter, the patient has brain damage and eventually

dies

.

The manufacturer knows the tube kinks, and knows that many of the

hospitals around the country are teaching hospitals with trainees who

are not as qualified as the M.D.'s. The manufacturer knows that since

the gas flows by pressure, a little whistle that costs about $2- $3

could be put on the machine so that when the gas pressure falls below

a certain level, the whistle will go off and warn them that the gas is

not flowing.

Now, to me it is negligent for the manufacturer not to inform the

hospital that they have this little device on their newer models,

because the burden of telling the hospital personnel that they have the

whistle is very slight. The manufacturer regularly does business with

hospitals, they have detail men going into every hospital at least

255
once a month. The cost of the whistle is slight, that's the burden,

compared to the benefit of saving a life.

Another illustration appears in the motorcycle industry. As you know,

a driver has available to him a hand throttle, manually operated by his

right hand. If he is propelled backward out of his seat, the motorcycle racer is prevented from being propelled too far back by a "sissy"

back. In older models, a return spring was not installed which would

have automatically reverted the hand throttle to neutral. In newer

models, the return spring is installed. For some reason certain manu- facturers failed to include the return spring, and in others the return

spring is defectively designed. Should manufacturers inform purchasers

of older models that return springs are readily available at a reason- able cost for installation on older models?

In the failure to warn field the most "far out" case of its kind was handed down by the Court of Appeals of Maryland a year or so ago. A

well known conpany makes perfumes and colognes that are flammable.

They also incorporate their fragrances into candles. The candles are

a popular item for the pre-adolescent kids who enjoy the fragrance at

birthday parties. Some 12 or 13 year old kids were having such a party, but the candles were unscented. The children were accustomed to

scented candles. They took mommy's cologne or perfume and poured it on the lit candle. A child was horribly burned. The Court of Appeals

held that the manufacturer had a duty to place a warning on the perfume

or cologne cautioning that it be kept away from fire. Although the case might sound extreme, it contained abundent evidence that such a warning was on the cologne aerosol bottles, and since the kids were

conditioned to the fragrant candles, it was foreseeable that children

would engage in the devious, mischievous and ingenuous play that caused

the injury.

256
DISCUSSION

Question : Many catalogs of manufactured equipment or instruction

manuals contain a warranty statement something to the effect that whatever is warranted against applies to defects, workmanship and material

for a period of such a length of time or, in the case of an automobile,

so many miles. All other warranties express or implied are null and

void. Another statement that often appears is something to the effect

that in keeping with its policy of constant product improvement the

manufacturer reserves the right to change specifications from those

noted in the catalogs and manuals. Are either of these statements

legally meaningful in today's legal climate?

Mr. Heller : With regard to the first statement, in most jurisdictions,

that is usually an attempt on the part of the manufacturer to limit

their liability for breach of warranty to replacement of the parts. The

typical auto warranty says we guarantee this to be free from defects in

material and workmanship and we will replace parts. That is a com- mercial warranty, and although they go further usually and say "We are not liable for anything else", the Courts have held this disclaimer

unenforceable and most statutes among the States specifically exclude

cases for personal injury or consequential damages. That attempt to

limit their liability has no effect at all in personal injury cases.

It does in commercial cases.

Mr. Amess : Yes, I would agree with that except for one thing. I spoke in my remarks earlier about the life of the product. This is a concept that hasn't been brought to the fore. However, I think it is a very viable concept once you get your sales force under control. Many

products just shouldn't be used beyond a certain period. I think a warranty that is weasel worded is just an attempt to limit legitimate

liability and might not be enforced. But if a manufacturer clearly

said this product should be used for four years and that anyone who

uses it beyond four years is taking his life in his own hands, I think

that type of limitation would be heeded and would provide a measure of

protection in a court of law. On your other question pertaining to the

other type of disclaimer, you just cannot rely on any kind of a disclaimer automatically. You are still going to be held to a standard of

reasonableness. You can put disclaimers on all you want to, but if

you have violated one of these reasonable standards you are going to be

held liable for the injury consequences of that violation.

Question : The question I had about the disclaimer is more directed

toward the follow-up, toward getting information back to the ultimate

user. The manufacturer is trying to tell the user that he can expect

that the manufacturer is probably going to change the product next year,

next week, five years from now, because the manufacturer may find in the

future that what he is selling today is not going to be up to tomorrow'

s

standards

.

257
Mr. Amess : You are trying to put the burden on the user to follow up

whereas Milton Heller told you the burden is on the manufacturer. You

have to get to the user and you cannot put something on there that says

". . . Look we may change this next month, or next year, and you get it

when we do." The manufacturer is going to have the burden, the

reasonable burden. I do not subscribe to those who say that a manu- facturer who puts out millions of products is going to have to find

where everyone of those millions of products are and get word to the

person who has it every time there is a modification. You are going to

have sales and resales and things of that nature and tracing the ownership of these products is an impossible burden. But, in the case of

capital goods, normally that transaction is easy to trace. In the case of large industrial equipment, that is going to be relatively easy. In

the case of vehicles, that is going to be easy. In the case of smaller

products, the manufacturer, in order to comply with the burden, is

probably going to have to resort to mass media advertising or something

of that nature. But they are going to have to do something.

Question : I notice something was carefully avoided in this presentation

about the changes to the product to cover safety features which were

missing from the original product. A manufacturer found that a handy,

dandy whistle, costing $5, would make his product a lot safer. Is he

responsible for paying for the $5 or can he go to the users and say

"Look we have now found a safer way to do it and we can sell you a handy, dandy whistle for $50?" Does that cover him automatically?

Mr. Heller : Oh yes, I didn't mean to suggest that you had to furnish

the whistle free. In fact it is good business, when you modify and

improve an obsolete product or an outmoded product, to sell that

improvement to the old users. I think the wise thing to do is to keep

a record of the fact that you called on that person and warned him of

it. You can't force him to purchase it.

Mr. Amess : I think that is absolutely right. But I want to tell you

something. I represent one manufacturer that really follows these

things to a fetish. He has developed a very interesting technique. He

sends out with his product those postcards which have the return card

attached. Then he has a follow-up system, if they do not return the

card, he follows up with them. Then if there are subsequent improvements he sends out another card or letter. There are all kinds of ways

to do it.

Question : I believe it was Attorney Amess who in passing referred to

products liability as the latest legal fad. Does this imply that this

is something which you think is going to run its cycle and is going to

be de- emphasized in the future?

Mr. Arness : I certainly do and I will tell you why. There is such a hue and cry already abroad in this land that I am certain something is

258
going to be done to keep product liability litigation within proper

bounds. As Professor Hagglund told you, the insurance premiums are going out of sight. I am told that certain manufacturers' premiums

have tripled in this last year alone. Whenever that kind of a syndrome

occurs and things get out of joint, this nation has the imagination to

deal with it and there will be changes made that will stop it. I do

not know what the changes are going to be, but legislators will make it

unprofitable for people to bring products liability cases. But we have

had trends in the law ever since the law has been used as a vehicle for

solving human disputes. Plaintiff's lawyers 40 years ago, had one kind

of toy, now they have products liability and five years from now they

will have something else to play with. In my role as defense attorney

I probably will be standing before some group 10 years from now and

talking about something else because there will be a rash of those

different cases to defend.

Mr. Heller : Let me comment on that if I may. What John says might be

true, but I don't think so, because the law of torts, especially in the

products liability field, has been a force for good. It has been found

that, but for some of these big verdicts and liberal laws, the manu- facturer would not voluntarily make a safe product. Mind you, I am not

a great fan of Ralph Nader. But if he had not pushed that National

Highway Safety Act or whatever, we wouldn't have these recalls that we

see and what I am saying is reflected in the laws that are coming out

from the legislatures around the country. Every act in this field

that passes through Congress, specifically states that this does not

mean that a person does not have a common law right to sue for products

liability recognized prior to the enactment of this law. They specifically reserve to the public their right to sue when injured by a defective product.

Mr. Amess : I think that is absolutely accurate, but that is not

contrary to what I said. What I said is, manufacturers are now being

exposed to a much greater number of product liability litigations at a rapidly increasing rate - much more so than they were five years ago.

The answer certainly is that rigid controls are going to be imposed and

. manufacturers are not going to put out unsafe products with impunity.

Once the science develops better, safer designs and manufacturing

techniques, and improved quality controls, then products liability

volume will be cut down. It is going to be too hard to make out a products liability case. Then something else will be easier and come

into vogue so as to replace the current products liability fad.

Question : What effect would a change in model number have on product

liability? In other words, would a yearly change in model limit the

liability for discontinued models?

Mr. Heller : Generally on your duty to design from a negligence standpoint, you have a duty to design the products that employ all of the

259
knowledge that the industry knows at the time. Now, the model T goes

out, but then if you find that the model T is unreasonably dangerous

and you know how to improve it, I would say there is a duty to warn the

purchasers. If you find that it cannot be modified and yet you still

know it's unreasonably dangerous you have a duty to warn. Something

that is obsolete and was manufactured in accordance with a standard of

care different from present standards, is, in itself, no indication

of negligence. This is not the kind of question that can be answered

yes or no. You have to talk about a specific part of the model T. If

you are talking about the brakes or something else then I could be more

specific.

Question : I am just curious, I will put this to both of you. In the

future is this a possibility - say someone is driving a car and they

get hurt. They feel that their injury was due to a defect in the car,

hire an attorney and file suit. The jury finds in favor of the defense

Can the manufacturer countersue the plaintiff's lawyer and the person

injured or his estate for defamation of character or the equivalent, in

any v/ay. Is that a method that will be there maybe five years from nov; to limit the suits?

Mr. Arness : No, and the reason for that is very simple. I think that one of the greatest - and now we are waxing mellow, I grant you - but I think one of the greatest privileges that we have in this country is

open resort to the Courts for solving disputes. I do not believe that our society will ever tolerate a penalty for that access. Now if you

ever had that kind of penalty, people with resources could keep people

without resources from an attempt to vindicate their rights in a civilized manner. It will never come about.

Mr. Heller : There has always been a civil abuse of process lawsuit

available to anyone who is sued without any basis. Proving such a case

is very difficult. Now a doctor in Chicago proved it and got an $8,000

verdict on a counterclaim. I do not know what has happened to it after

that.

Mr. Amess : That verdict has not been tested yet, it is still in Court

It could never come about when a person was truly injured and tried to

get a recovery. The only way it could come about would be if somebody

wasn't really injured, but came into Court and fraudulently represented

that they were injured and fraudulently manufactured a claim then maybe

you would have civil abuse of process, but it would have to be almost

willful. A recovery for civil abuse of process would never be imposed

on someone making a bona fide attempt to get a recovery even though

their theory was way out.

Question : What is going to happen to cut down this spiral of lawsuits

and insurance premiums? Will the government be the insurer of last

resort?

260
Mr. Amess : There is a presidential commission meeting this week. As

a matter of fact, they met on Monday and Tuesday. This commission,

composed of representatives from all types of manufacturing concerns,

as well as lawyers from both sides of the street, was meeting in an

attempt to grapple with this product liability situation. I am certain

they will come out with some kind of recommendation. What it will be

would take a crystal ball to predict.

Mr. Heller : Let me say one thing, one of the arrangements that has been

criticized a great deal is the contingent fee that lawyers take these

cases on. Of course, you read about $3,000,000 recovery and the lawyer

gets $1,000,000. First of all, the contingent fee, as far as I am

concerned, is probably the most equitable fee arrangement that an attorney can have with his clients. If he loses the case, the attorney

gets nothing, but more inportant, if you have a good injury case, it

matters not to the attorney whether you are the president of a bank or

that president's janitor. Sometimes you would prefer to represent the

latter. All that matters is the merit of your case, and that person can

walk into any law office and hire the attorney he wants without putting

up a nickel. To me that is very fair.

Question : Would either of you like to comment on the proposals to

change the process for trying products liability cases. That is to

impose a technical evaluation, a technical pretrial before the litigation goes to the jury.

Mr. Amess : That is one of literally dozens if not scores of proposals

for handling the current products liability crisis. As a matter of fact

that is one of the more viable alternatives to the situation that we

have now. Particularly with respect to strict liability. Strict

liability is supposed to be a doctrine that makes you pay when the

product is found to be defective. A defective product is one which,

when you put it out in the stream of commerce and when used as it is

supposed to be used, has the ability to hurt someone. You cannot really

test that premise in an arena where you have someone lying in a pool of

blood after they have had an accident. It is not really fair to judge

on a Monday morning after someone got hurt the night before. If you

are going to test the product to find out if it is in fact defective and

if you are going to ascertain whether the product, in the environment in

which it v/as supposed to be used, is dangerous, you do it objectively.

You do it scientifically. The fact that somebody happened to get hurt

might be a factor, but it certainly would not be the compelling factor

that it is now in the lawsuit trial. The people who recognize and

appreciate that fact of life say, "Let's have a bifurcated trial - let's

try the product first in front of a jury - let the plaintiffs' lawyer

come in and say, "This product is a dangerous product - it's got this

wrong with it - it was designed in such a way that someone is liable to

get hurt"; and let the jury look at the case and evaluate the product."

261
Their focus will be on the product and not upon the unfortunate plight

of the person who happened to get hurt. That approach, objectively

speaking, is a fair way to approach the situation. However, the

American Trial Lawyers Association and plaintiffs' groups all over the

land are going to fight such proposals like wounded eagles and it

probably won't get through.

Mr. Heller : I say that we should have a bifurcated trial. Bear in mind

it really doesn't affect anything. If you represent someone with both

legs off he will be in the courtroom because he has a right to be

present and the jury will see him anyhow. Trying the case outside of

the Court, and I think that is what was being suggested, is not

necessarily a satisfactory practice since you can't deprive a person of

his right to come to the Court. Besides, I am not so sure it is a good idea, because, like in the medical malpractice field, it is very

difficult to get objective experts, or specialists to sit as a jury.

The argument that juries are sympathetic to people who are injured, is

not true. Let me cite an example. I tried a case, for three and a half months against an automobile manufacturer on the design of one of

their cars. I represented a quadraplegic girl, 17 years old, couldn't

be prettier or sweeter and had everything going for her; couldn't move

anything except her head. We had a lot of good testimony. We went to

the jury. We lost. We didn't get a nickel. Evidently the automobile

manufacturer had better expert testimony. What I am saying is that

there are some jurisdictions that have emotional jurors, but there are many that do not.

Question : Referring to the cologne case, I was wondering if there were

any other perfume manufacturers who indicated on the container that the

product was flammable. If they did not, wouldn't the sued manufacturer

have some recourse?

Mr. Heller : That is a good point, and some of the evidence showed that

the sued manufacturer itself put a warning on its spray cologne - "Don't

spray in the presence of flame". But there was no evidence that any of

the manufacturers put "Flammable" on any of their perfume or cologne.

Again, just because no one in the industry does it doesn't mean that

the industry is safe. If it is unreasonably dangerous and the whole

industry adheres to that standard, then any one of them is liable. In

other words, what you are saying is that they were not negligent, true,

but in addition to negligence there are other theories. I think

Professor Hagglund might not have made it as clear as John did, that

there are three theories of recovery; there is negligence, if you adhere

to the standard of care of your competitors you are not negligent; but

in addition there is breach of warranty, regardless of negligence; and

there is strict liability, regardless. So strict liability means that

if it is unreasonably dangerous even though everybody puts it out that

way, you are liable.

262
SESSION VII

VALIDATION

OF

DESIGN

(II)

-RELIABILITYChairman: G. Hurt

Detroit Diesel, Allison Division
I
SAFETY ANALYSIS: QUALITATIVE, QUANTITATIVE AMD COST EFFECTIVE

H. D. Wolf

Grumman Aerospace Corporation

Bethpage, New York 11714

Abstract: No system involving man can be 100^ safe since he is not

error-free. The objective of any safety analysis effort is to identify what to "worry" about in the system and how to take steps to

eliminate that "worry". With the proper attention to (l) past

history of similar systems; (2) development of safety controls in

all program phases; (3) reviews of program efforts; {k) aggressive

accident/incident analysis, corrective action and closeout; and

(5) an educational program for all project personnel from corporate

managers on down, a system can approach "failure-free" conditions.

The various approaches to safety analysis are described in this

paper, including preliminary^ system and operational hazard analyses.

The advantages and limitations of the several methods are discussed.

Examples are used to emphasize specific points and to enhance the

reader's understanding. Some ideas on safety feature decisionmaking are included.

Key Words

Accident prevention; hazard analysis; risks; safety.

Introduction: Prevention of accidents and incidents is the primaiy

function of safety analysis- This means preventing personnel injuries

and damage to equipment from the point where the idea is germinated

until the ultimate user relegates it to the scrap heap. The techniques

are simple, rigorous, time consuming; they can be applied to a railroad wheel, to a space vehicle or to a nuclear reactor. They require

a different point of view from other system analyses. To provide the essence of safety analysis and describe what it can accomplish, this

paper has been organized into two major sections: Safety Analysis

Techniques and Risks/Benefits.

Safety Analysis: The primary tools of an early or preliminary safety

analysis are the worksheet, a hazard log book, a checklist on hazards,

a system description and a "challenging" nature. Using the work done

by W. Hammer (Reference A) in categorizing potential hazards and historical information on how similar systems survived, each element of

the system to be studies is challenged by each hazard category to

determine which hazard category applies to which system element.

The tentative results are listed on the worksheet. Figure 1, indicating

where potential safety problems may be. Simplifying the first cut is

265
done by constructing Essential Hazardous Situations (EHS); these are the fundamental safety issues to address. A logbook is established

with files on each of these EHS; tracking of the hazard to closeout

becomes essential once the design is frozen.

Identification of a hazard is only the first step. In a Preliminary

Hazard Analysis^ Figure 2., an iterative process is initiated where

design approaches^ safety devices, warning devices or crew procedures are used to eliminate, control or resolve the hazard situation.

The method(s) for handling the hazard are compared with the user's

requirements and resolved to the user's satisfaction. This can be

done formally or informally. If a specification is used, the spec

is changed to reflect the new design. At a logical point in the

system evolution, a design review is performed. The purpose of this

review, from safety's viewpoint, is to assess how and to what extent

the system designers have satisfied the safety requirements . It is

not meant to be a contest between safety and engineering as to how

a design requirement was satisfied, but rather to answer the question:

Does the selected design satisfy the "no personnel injury/no equipment

damage" criteria? New hazards may be identified, which may require

additional design effort.

At this point the safety program has accomplished several things

:

1. identified general hazards to worry about; i.e. design boundary

conditions

2. established additional design/operational requirements to include

in the design.

3. identified where additional fertile areas for "improvement" can be found.

^. provided detailed procurement specification data/sections.

5. created a data bank that can subvert (and potentially support)

any legal action taken in association with this design.

The logbook on hazards data is expanded and embellished as the design

is refined and as more detailed information is developed. At specific

points, the analyst can say, "I've carried the the hazard closeout as far as possible for this portion of the design." That means, for

example, the specification includes the proper requirements or the

detailed drawings include the proper material specifications. Ultimate closeout may be when the finished product successfully completes

an acceptance test or the user successfully opens his beer can without

cutting a finger.

266
For complicated systems, each segment of the design (system, subsystem,

assembly, component, part) can be broken down into its basic elements ' and be subjected to a comparable hazard analysis. Figure 3 presents

the next hazard analysis level, a System Hazard Analysis. Normally,

this is conducted during the period prior to freezing a final design

and starting final manufacturing and assembly operations. The technique used is similar to a Preliminary Hazard Analysis except the

level of detail is greater because the analyst is digging more deeply

into the basic design. To maintain traceability, hazards identified

in one category are maintained in that category throughout the program.

Resolution of hazards now may require more than just simple discussions with the cognizant engineer. Program management may be brought

in as the cost of implementing a safety feature or redesigning may

require a trade study to determine the cost/benefits. Just prior to

the design freeze, another design assessment is conducted to set the

stage for the final push to completion. The purpose at this point is

to identify all known safety problems and establish a schedule, budget

and approach to their resolution. One approach may be to "accept the

risk." The ramifications of such a decision are financial, contractual, legal, and at times, political; stranding two American astronauts

on the moon in full view of the world is not a tenable position for a company. If your customer can agree that such a risk is acceptable,

you have at least shared the responsibility.

Data that have been collected on system safety problems serve the

program in ways other than just design. The manufacturing and test

personnel use the data bank as a source of information on things to

be careful about during their efforts. Failed open valves that

startle a user by their "noise" can also cause the test operator to

jump if a system is pressurized and a valve fails. If the user can flick switches in an incorrect sequence applying power to the wrong

circuit, so can the test man. It also works the other way. Problems

that occur during test can be analyzed by asking the questions : What

if this happened during normal use? How would the user react? What

is the impact? Also, accident and incident analysis can identify

generic problems whose resolution results in design improvements.

This type of information is fed into the design change loop and also

into the next type of hazard analysis: Operating Hazard Analysis.

People make mistakes; even the best trained, most highly motivated

personnel can err. Operating Hazard Analysis (Figure k) serves the

purpose of identifying those situations where personnel "Failure" is

likely. Such situations can stem from a design compromise that dictated "its too late to redesign now" to an operator limitation: man

cannot exert x lbs. of force on an escape system handle under a cen- trifugal force of 2 g . Though the technique is similar to the other

aforementioned analyses, the results are quite different. Design

changes are highly unlikely at this

267
point in the development^ so crew procedures are used. The hazards

identified are compensated by the crew. To do this they must know

how to identify the situation (cues to hazard onset )^ the backout

steps to take to prevent the situation if timely action is taken or^

at least, to minimize the impact of the situation if it cannot be

prevented. The output, then, of this analysis are these types of

data:

- procedural steps

procedure sequence constraints

caution notes

warning notes

reference data bank: cues, clues and data

support equipment, if required

Fed into operator manuals, training courses, crew procedures documents

test program plans and procedures and technical management planning

documentation, human failure situations can at least be controlled

even if they cannot be completely eliminated.

At this point in an idealized program, the design has satisfied safety

requirements where feasible, the product has been manufactured and

tested verifying the quality and the safety margins of the design,

and the unit is ready to "go on the air." Complex systems are subjected to one more review: a readiness review. A blue ribbon panel

is formed of customer and corporate "experts" to review the design,

the test results, the closeout of open issues and the safety risks

that have been identified and to render a judgement. Is the product

really ready to go?

Special studies may be directed before go-ahead is given. A Warning

Time Analysis may be one of them. Though normally done in parallel

with the Systems and Operating Hazard Analyses, it can be done at

any time. Time critical situations identified in prior analyses are compared with system and human response times. The worksheet is used

to assure no hazard categories are forgotten and to help understand

the causes and effects of these time critical situations. Judgments

are made as to whether the design and procedures are adequate or not.

In some cases the result is traumatic. For example, human reactions

are too slow to initiate escape mechanisms if the fireball of explosion travels at speeds of detonating hydrogen.' Automating such a system and assuring "No False Initiations" is a career unto itself.

The results of the Warning Time Analysis are elimination of time

critical sequences, automation of inadequate responses and identification of potential residual risks (see Figure 5).

268
Residual risks require constant re -evaluation . Prior to each

opportunity for personnel injury or equipment damage^ it is essential

that attention is focused on the risks. User and program management

must re-decide based on current data if the risk is still worth taking. It is obvious that these risk discussions can become thorns in

management's side. Diplomacy is the only way to avoid rapid ejection

from a program under such circumstances.

The various types of hazard analyses have been presented above.

Related analyses, FMECA and Fault Tree, will be covered in the following papers. Several basic questions always asked about these hazard

analyses are: What do we gain and why do it? Figure 6 summarizes the

pros and cons of performing such analyses.

Risks/Benefits

The second part of this paper deals with some thoughts on safety risks and benefits. If we define Expected Loss

Index (ELI) as the product of the probability of event occurrence

and the dollar amount at risk, we have a potential decision-making

tool for accepting/rejecting safety features. Let's assume for the

moment that an acceptable safety risk is one that has a probability

of occurrence of one in a million (10"°). Figure 7 is a log plot

of dollars at risk versus the probability of the event occurring.

If we decide that an ELI of 1.0 is allowable, then Figure 7 is

divided into Area A - event probability less than 10-°; Area B - ELI less than 1.0; and Area C - ELI greater than 1.0. If, for a given design, the ELI is 1000, your goal is ELI=1 and the event probability is 10"'^; then your approach is either to reduce the probability of the event by eliminating or limiting the exposure to the

hazard and/or reducing the dollars at risk, for example, by moving

a high cost element into a low risk area.

There are several problems with this approach

1. Price tags on men's lives are emotional issues and highly variable.

2. Dollars at risk include elements difficult to pin down: user

costs, training costs, test costs, etc.

3. Event probabilities are only estimates which range from

"substantiated" by accident data to "best guess."

To resolve this question, when is it beneficial to add a safety feature

or make a change for safety reasons, the emotional element of the cost

of men's lives must be eliminated and a realistic accident data base

must be constructed to improve event probability guesses.

269
Conclusions

:

Safety hazard analyses are another tool set in the kit

of the assurance disciplines. They are capable of identifying bound

ary design conditions, controlling the safety risks, alerting manage

ment and supporting any corporate actions to resolve legal suits.

Methods and techniques for deciding when it is beneficial to add a safety feature are still in the early development stages.

References

:

(a) Hammer, Willie, "Progressive Qualitative Hazard Analyses" AIAA

Fourth Annual Meeting and Technical Display No. 67-935,

October I967.

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277
A FAILURE MODE AND EFFECT ANALYSIS PROGRAM

TO REDUCE MECHANICAL FAILURES

Richard R. Landers

Reliability and Maintainability Department

Gould Ocean Systems Division

Cleveland, Ohio 44117

Abstract: Described is an interdisciplinary technique to reduce the

number of mechanical failures in an ongoing torpedo program through

the use of failure mode and effect analysis techniques. The paper

describes the procedure and forms for completing the analysis and

provides representative examples. FMEA's are performed at the component level early in the design phase to find possible ways that equip

ment can fail and to determine the effect of such failures on the system

The FMEA is used to assure that: all component failure modes and

their effect have been considered and either eliminated or controlled;

information for design reviews , maintainability analysis and quantitative reliability analysis is generated; data for maintenance and operational manuals are provided; and that inputs to safety and hazard

analyses are available in a Critical Item List (CIL)

.

Key Words: Failure mode and effect analysis; reliability; failure reduction; critical item list; system safety.

INTRODUCTION

The old maxim, "an ounce of prevention is worth a pound of cure" is

quite applicable to today's products. Mechanical failures can be prevented through the application of fairly recently developed techniques

One such technique is the failure mode and effects analysis , which is

usually abbreviated into the acronym FMEA. This analysis method

provides for early evaluation of a design and, as applicable, generation of recommendations for design improvement

.

This paper is organized in the following manner . The following major

section is written as a standard practice as if it were to be put in a organizational manual . The second major section provides an example

of an FMEA to illustrate the method . The paper concludes with a summary of observations regarding the practice and application of this

analytic technique.

2T8
STANDARD PRACTICES FOR FAILURE MODE AND EFFECT ANALYSIS

PURPOSE

The failure mode and effect analysis (FMEA) is a design technique to

provide a degree of assurance that an equipment and system are safe

and reliable. The design office specifies which equipments/ systems

are to be subjected to an FMEA.

DEFINITION OF FMEA

A failure mode and effects analysis is a reliability design evaluation

procedure initiated early in the design phase and periodically updated

which:

a. Documents possible failure modes in a system design which

fall within specified ground rules.

b . Determines by analysis the effect of each identified failure

mode on system operation

.

c. Identifies items critical to mission success and safety.

d . Assists in the evaluation of design features , such as redundancy

,

failure detection, override, and fail-safe characteristics.

e . Generates recommendations for design improvements or contingency options

.

f . Provides an input to reliability analysis and design review

data packages.

g. Allows priorities to be met for testing, procurement, inspection , etc

.

h . Provides information for fault isolation and maintainability

analysis

.

i. Classifies each potential failure effect with a criticality category and assigns a probability of occurrences for the potential

failure

.

j . Provides a data base for subsequent comparison in a closedloop failure reporting and corrective action program

.

k. Provides information for system safety analyses.

1. Provides information for operation and maintenance manuals

,

handbooks and training programs

.

279
Thus , the FMEA is an engineering technique which allows evaluation

of critical failure sources by predicting the observable reaction of an

item to a failure within it and the effect which the failure has on performance and operational requirements . A failure is defined as the

inability of an item to perform its required functions within specified

limits . (Defects and discrepancies that do not adversely affect function are not considered as failures.) A failure mode is the physical description of the manner in which a failure occurs (e.g. , open, leak,

structural failure) . Figure 1 shows a typical completed FMEA form

.

RESPONSIBILITIES

The design office is responsible for the preparation of FMEA lists

.

These lists identify by work breakdown structure the items requiring

FMEA. Design engineering is responsible for reviewing and approving

the FMEA's. Reliability Engineering reviews the FMEA's and prepares

a critical item list (CIL) for subsequent hazard analyses . As the system becomes operational, field results are compared with analyses and

adjustments made as required . Figure 2 shows an FMEA procedures

flow chart.

PROCEDURES

1. Establish Ground Rules

FMEA's generally are prepared by a number of persons. Therefore , it is important that a comprehensive set of rules be provided to assure consistency and completeness . Among these

rules are treatments for:

a. Operational and environmental requirements.

b . Lowest level of hardware tier to which analysis is to be

performed (generally down to the piece part level)

c. Work breakdown structure and other coding systems for

FMEA item identification

.

2. System Description

A functional statement is prepared for each system , a subsystem and assembly being analyzed giving a narrative description

of the operation of each item for each operational mode. Schematics are included . A system block diagram is prepared

showing series and redundant relationships along with inputs

and outputs. Separate block diagrams are prepared for each

operational phase or mode as applicable

.

3. Potential Failure Mode Analysis

Potential failure modes for each hardware item are analyzed

and their effects on the system determined . The FMEA con280
siders multiple failures only where functional redundancy

and/or component redundancy exists . Interfaces within redundancy and isolation techniques for redundant elements are analyzed to ensure that the desired redundancy is not negated

due to a failure of any of these interfaces or isolation techniques

.

In addition to hardware failure modes analyses , the FMEA includes an analysis of the effects due to loss of inputs . The

analysis provides for each failure mode:

a. The effect of the failure on the operation of the subsystem and complete system

.

b. The indication (s) of failure presented to the operator (s) or maintenance personnel during use or checkout.

c . The failure isolation and repair procedure required in

use and checkout and corrective action available to

operator (s) and maintenance personnel

.

d . The controls or features in effect to eliminate or reduce

failure mode frequency and/or criticality of occurrence.

Analyses for various products would likely require varying

degrees and kinds of the above items . For example , in

FMEA's prepared for aircraft it is desirable to include information detailing: means of indication of failure to crews;

means to isolate faults; other failures that produce similar

indications; variation due to flight phase; etc. It may also

be desirable to have a preliminary analyses early in the

design stage and then modify the analysis as more detailed

information becomes available.

Criticality Categorization

Each failure effect is categorized into one of the following:

a . Critical - Loss of life or a disabling accident or extensive damage to equipment resulting in the inability to

perform the mission.

b . Major - Not critical , but loss of reserve capability or early (but safe) termination of mission

.

c . Minor - Not major , but causes a non-serious delay in

start or completion of mission or an unscheduled adjustment or other action

.

Specific characteristics of a program probably require modification of the above categories to better describe and tailor

criticalities to the product and its use

.

281
5 . Loss Frequency

A frequency (or probability) of loss is assigned for each failure

mode . In most cases this frequency or probability is simply

the hourly failure rate of the component part in its operational

mode and environment. In some cases it may be desirable (or

sufficient) to merely categorize as "high," "medium" and "low."

However , this simplified categorization should have defining

rules for determining into which category a failure falls, such

as greater than 1 chance in 10, 1 chance falling between 10

and 1000, less than 1 chance in 1000.

6. Critical Item List

Once the FMEA is complete, a critical item list (CIL) is prepared.

The CIL identifies those failures whose effects exceed a predetermined threshold of seriousness . Generally , this threshold

is the product of the criticality and loss frequency. For example,

if there were three criticality and three frequency classifications

,

there would be nine categories of seriousness. The CIL threshold could then, say, be set at the "major criticality and medium

frequency" level and include all failure events at and above this

level (i.e. , all "critical" plus "medium and high major") as shown by the shaded portion of Figure 3

.

The CIL becomes an input to the safety analysis effort . That

is , each item on the CIL is examined in detail to determine

the nature and extent of the safety hazards associated with

the item . This analysis aims at the elimination or reduction

and control of hazards

.

EXAMPLE OF A FMEA

A somewhat abbreviated example of a FMEA is provided below . The

example follows the six steps listed in the procedure section

.

1. Ground Rules

a. The purpose of the torpedo is to seek out and destroy

assigned targets. The operational environment is 10

to 1000 feet depth, -55°C to 105°C, and a launch shock

of 10 g. The torpedo is designed to meet its mission

requirements without redundant elements and is not to

pose a hazard to its operators or to other equipment

.

b. This FMEA is performed at the subassembly level and,

as such, considers the effect of each piece part failure

mode on neighboring items, the mission, personnel,

and associated equipment . Possible modes of failure

282
are considered in the activation, operational, handling

and storage phases . The failure frequency and criticality criteria are presented in IS 2345.

c . A block diagram of the torpedo as it relates to this portion of the FMEA is shown in Figure 4

.

2. Torpedo Steering Subsystem Functional Description

Four three -position servovalves are mounted on the manifold

forward face . The servovalve controls the hydraulic fluid

flow to and from twin hydraulic control piston cylinders

.

The servovalve receives a fin drive signal from the gyro control unit (GCU) which positions the valve as commanded. A

combination of hydraulic fluid flow is mechanically converted

into control surface movement, thus altering the course of the

torpedo. Each piston pair is connected to a rocker arm which

pivots on a rocker arm shaft which extends through a hole

in an aft tailcone stabilizing fin . A control surface (fin) is

spline connected to the rocker arm shaft . As rocker arm position varies, the linear transformer provides a fin feedback

position voltage to the GCU . As the control surface is moved

either port or starboard , the gear movement varies the output

voltage of the linear transformer which is fed back to the

GCU fin control servo loop

.

CONCLUSION

Methods and details for preparing and applying FMEA's may vary from

company and , indeed , from program to program within a company . The

important consideration here is that the FMEA procedure be tailored to

specific needs. To be avoided is the preparation of FMEA's as a

ritualistic exercise where form is considered more important than content. Also to be avoided is a cookbook approach whereby previously

prepared FMEA's are lifted in total with little consideration for currentness or applicability to altered requirements

.

Experience indicates that FMEA's are best prepared as a cooperative effort between the design and reliability engineers . The design engineer

generally is best qualified to identify failure modes and establish the effects. The reliability engineer, as an objective participant, usually

can best evaluate the results and make beneficial recommendations

.

Also, the reliability engineer usually is in the best position to perform

the expected frequency and criticality of occurrence portion of the

analysis whereby the seriousness of the failure can be established and

categorized

.

283
When properly prepared , the FMEA provides an orderly , early documentation of product elements important to performance, reliability and safety.

This provides a strong data base for program planning and a basis for

design improvements or contingency options to eliminate or control the

effect of failure

.

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DESIGN GROUP

ESTABLISH LIMIT OF FMEA

REQUIREMENTS, SCHEDULE

AND CRITERIA FOR COMPLETION

COGNIZANT DESIGN SECTION

1

1

1

PREPARE FMEA'S

MODIFY/UPDATE

1 ACTION ITEMS ORIGINAL OR REVISIONS EXPERIENCE

• MODIFY • MODIFY

• CLARIFY RELIABILITY ENGINEERING • UPDATE

REVIEW FOR TECHNICAL

ACCURACY AND COMPLETENESS

AND SIGN WHEN ACCEPTABLE

DESIGN REVIEW

• FMEA PRESENTED AND

EXAMINED AS PART OF THE

DESIGN REVIEW PACKAGE

• ACTION ITEMS ASSIGNED

AS REQUIRED

L 4 >

REPRODUCTION/DISTRIBUTION

SIGNED ORIGINAL TO

DOCUMENT CONTROL

COPIES TO:

• DESIGN

• RELIABILITY

• MAINTAINABILITY

• SYSTEMS SAFETY

FIGURE 2. FMEA PROCEDURES FLOW CHART

286
CRITICALITY

CRITICAL MAJOR MINOR

LOW

FIGURE 3. CRITICAL ITEM LIST CRITERIA

28t
288
FAULT TREE ANALYSIS AS A PART OF

MECHANICAL SYSTEMS DESIGN

J. B. Fussell and D. P. Wagner

The University of Tennessee

Knoxville, Tennessee 37916

Abstract: Fault tree analysis is a method of system reliability and

safety analysis that is rapidly gaining favor as a design tool. Fault

tree analysis has the potential of offering an objective basis for

analyzing system design, performing trade-off studies, analyzing common

cause failures, demonstrating compliance with safety requirements, and

justifying system changes and additions.

In this paper emphasis is placed on the use of fault tree analysis in

mechanical system design with particular concern for:

(1) Introductory concepts

(2) Schemes for integration into the design process

(3) Limitations

(4) Related application

In addition, a review of the latest production computer programs is

given.

Key words: Availability; fault tree analysis; probabilistic analysis;

reliability; systems analysis.

1.0 INTRODUCTION

Fault tree analysis is a technique of reliability and probabilistic

safety analysis and is generally applicable to complex dynamic systems.

Fault tree analysis provides a versatile, mathematical tool for

analyzing complex systems. Its application can include a complete plant

as well as any of the systems and subsystems. Fault tree analysis

provides a basis for analyzing system design, performing trade-off

studies, analyzing common cause failures, demonstrating compliance with

safety requirements, and justifying system changes or additions.

The logic of the approach makes it a visibility tool for both

engineering and management. The fault tree method is concerned with

assuring that all critical aspects of a system are identified and

289
controlled. The fault tree itself 1s a graphical representation of

Boolean logic associated with the development of a particular system

failure, called the TOP event, to basic failures, called basic events.

For example, the TOP event could be the failure of an automotive braking

system to stop the vehicle during an emergency with the basic events

being failures of the individual braking system components.

In 1961 the concept of fault tree analysis was originated by Bell

Telephone Laboratories as a technique with which to perform a safety

evaluation of the Minuteman Launch Control System [1]. At the 1965

Safety Symposium, sponsored by the University of Washington and the

Boeing Company, several papers were presented that expounded the virtues

of fault tree analysis [2]. The presentation of these papers marked the

beginning of a widespread interest in the possibility of using fault

tree analysis as a reliability tool. In the early 1970's progress was made in the solution of fault trees to obtain reliability information

about relatively complex systems [3-7]. The collection and evaluation

of failure data is still of the utmost importance [8-11].

Fault tree analysis is of major value in:

(1) Providing the analyst with insight into system behavior

(2) Pointing out the aspects of the system important in respect

to the failure of interest

(3) Providing options for qualitative or quantitative system

reliability analysis

(4) Providing a graphical aid giving visibility to those in

system management who are removed from the system design

changes

(5) Allowing the analyst to concentrate on one particular system

failure at a time.

The cost of development in first-time application to a system is

substantial. As in the development of engineering drawings, the cost is

somewhat offset by future application of the models in accident

prevention, maintenance scheduling, and system modifications. The

expense is justified by detail of the qualitative or quantitative

analysis resulting from fault tree analysis. Another aspect of fault

tree analysis that limits its application at this time is the relatively

few persons skilled in its techniques. Even skilled personnel might

develop a fault tree for a given system in different ways.

Although certain single failures that can result in several component

failures simultaneously, called common cause failures, can be pointed

out by a detailed fault tree analysis, the analyst must properly account

for common cause failures, specifically at some point in a detailed

analysis. The analyst should be aware that fault tree analysis does not

290
inherently ferret out all common cause failures. Additional limitations

are discussed in Section 4.0.

2.0 CONCEPTS OF FAULT TREE ANALYSIS

2.1 Fault Tree Terminology

Fault tree construction is the logical development of the TOP event. As

the construction proceeds, each fault event is also developed until

basic events are reached. A fault event is a failure situation

resulting from the logical interaction of basic events. The development

of any fault event results in a branch of the fault tree. The event

being developed is called the base event of the branch. The branch is

complete only when all events in the branch are developed to the level

of basic events.

Fault events are classified as either primary faults, secondary faults,

primary failures, or secondary failures. A primary failure is defined

as the change of a component from the working to the non-working state

for which the component is held accountable, and repair action on the

component is required to return the component to the working state. A

primary fault is defined as the change of a component from the working

to the non-working state for which the component is held accountable,

and repair action on the component is not required to return the

component to the working state, i.e., the component is "self-healing".

Secondary faults and failures are the same as primary faults and

failures except the component is not held accountable for its change of

state.

The graphical symbols used in a fault tree fall, basically, into two

categories: logic symbols and event symbols [1,12].

The logic symbols, or logic gates, are used to interconnect the events

that contribute to the specified main event, or TOP event. The logic

gates that are most frequently used in fault trees are the basic AND and

OR Boolean expressions. The AND gate provides an output event only if

all input events occur simultaneously. The OR gate, specifically the

inclusive OR gate, provides an output event if one or more of the input

events are present. More than one variation of AND and OR gates exist.

A priority AND gate requires that the input events must occur in a specified order before an output is passed. An exclusive OR gate passes

an output when any single input occurs, however, if more than one input

event occurs, the exclusive OR gate does not pass an output. The third

type of OR gate, the mutually exclusive OR gate, also passes an output

when any single input occurs, however, the occurrence of more than one

291
input must be physically impossible if the mutually exclusive OR gate is

to be applicable.

The more frequently used event symbols are the rectangle, circle, and

diamond. The rectangle represents a fault event resulting from the

combination of more basic faults acting through logic gates. The circle

designates a basic system component failure or fault input that is for

the purposes of quantitative analysis usually assumed to be independent

of all other events designated by circles and diamonds. The diamond

symbol describes fault inputs that are considered basic in a given fault

tree. The fault tree is not developed further, either because the event

is of insufficient consequence or the necessary information is

unavailable. In order to obtain a quantitative solution for a fault

tree, both circles and diamonds must represent events for which

reliability information is input. Events that appear as circles or diamonds are treated as basic events.

The triangles shown in Figure 1 are not strictly event symbols although

they have traditionally been classified as such. The triangle indicates

a transfer from one part of the fault tree to another. A line from the

side of the triangle (transfer out triangle) denotes an event transfer

out from the associated logic gate. A line from the apex of the

triangle denotes an event transfer into the associated logic gate from

the transfer out triangle with the same identification number.

The TOP event of a fault tree occurs when the system passes from the

unfailed to the failed state. A given basic event can appear in many

places in a fault tree. A fault tree representation is shown in Figure

2. An explanation of the symbols used in fault trees is given in Figure

1.

2.2 Problem Definition

System definition is often the most difficult task associated with fault

tree analysis. Of primary importance is a functional layout diagram of

the system of interest showing all functional interconnections and

identifying each system component [a]. An example might be a detailed

[a] For some systems that are not hardware oriented such a diagram may

not exist and, indeed, the fault tree itself can be the only feasible diagrammatic system representation. For the discussion presented the emphasis is directed toward hardware-oriented systems.

However, the implications and terminology extend to all fault tree

analyses.

292
electrical schematic. Physical system bounds are then established

focusing the attention of the analyst on the precise area of interest.

A common error is failure to establish realistic system bounds and

thereby a diverging analysis is initiated.

Sufficient information must be available for each of the system

components to allow the analyst to determine the necessary modes of

failure of the components. This information can be available from

experience of the analyst, from the technical specifications of the

components, or from a library specifically for this purpose.

A further step in the system description is to establish the system

boundary conditions. These analytical boundary conditions should not be

confused with the physical bounds of the system. System boundary

conditions define the situation for which the fault tree is to be drawn.

A most important system boundary condition is the TOP event. For any

given system, a multitude of possibilities for TOP events exists. The

selection of the "correct" TOP event is sometimes a difficult task. The

system initial configuration is described by additional system boundary

conditions. This configuration must represent the system in the

unfailed state. Consequently these system boundary conditions depend on the TOP event. Initial conditions are then system boundary conditions

that define the component configurations for which the TOP event is

applicable. All components that have more than one operating state

generate an initial condition. System boundary conditions also include

any fault event declared to exist or to be not-allowed for the duration

of the fault tree construction. An existing system boundary condition

is treated as certain to occur and a not-allowed system boundary

condition is treated as an event with no possibility of occurring.

Neither existing nor not-allowed system boundary conditions appear as events in the final system fault tree.

2.3 Fault Tree Construction

An example is given here to demonstrate some of the fundamental aspects

of fault tree construction. The example system is a dual hydraulic

braking system. The system physical bounds include the master cylinder

assembly, front and rear brake lines, and the wheel cylinder and brake

shoe assemblies. The system boundary conditions are:

TOP Event = Loss of all braking capability

Initial Conditions = Brakes in released position

Not-Allowed Event = Failures due to effects external to the system

Existing Events e Parking brake inoperable

Tree Top = Shown in Figure 3.

293
Figure 3 reflects the inductive reasoning that all braking capability is

lost if the brakes on all the individual wheels fail to operate.

From a knowledge of the components, the fault tree shown in Figure 4 is

constructed. The event "right front brake fails to operate" occurs if

the right front brake shoes fails to operate or the right front brake

shoes are not applied. The event "right front brake shoes are not

applied" occurs if the right front wheel cylinder falls to operate or

the wheel cylinder is not activated. This logical development continues

until the fault tree is completed to the level of basic events, as shown

in Figure 4.

2.4 Qualitative Analysis

Qualitative analysis includes determining (a) the system modes of

failure and (b) the components of the system that share an alliance such

that they are candidates for a common cause failure. A system mode of

failure, called a minimal cut set, is a group of basic component i failures, called basic events, that are collectively sufficient to cause

the TOP event to occur. The occurrence of each basic event in the

minimal cut set is necessary if the occurrence of the TOP event is a j result of the failure of the minimal cut set in question. The minimal

| cut sets are the minimum amount of information required to construct the I complete truth table for the logic model. Computer programs for [ determining minimal cut sets from existing fault trees are available [3, I 13,14,15]. I

I

A common cause failure [16] is a secondary cause of failure that is \ applicable to the development of more than one component malfunction. ! For the purpose of discussing qualitative analysis of common cause

' failures, definition of several terms is necessary. I

A significant common cause event is a secondary cause that is common to ! all the basic events in one or more minimal cut sets. If, in addition,

all the components represented by the basic events in that minimal cut i set share a "common location", that minimal cut set is a common cause ^ candidate. Components share a common location if no barriers are present that are capable of insulating the components from the secondary

cause of failure. Components may share a common location irrespective

| of the physical distance separating them. I

i

On occasion, a significant common cause event may not be specified for a common cause candidate, but rather the common cause candidate is

identified based solely on a "significant common cause condition". A

29h
significant common cause condition is a condition that closely links all

the basic events in the minimal cut set. For example, all components

indicated by the basic events in a minimal cut set being produced by the same manufacturer is a significant common cause condition. The common

cause candidate is then identified without concern about a common

location. Other significant common cause conditions arise from

components being tightly linked by a common location. Components in the same electrical circuit, chemical flow loop, or even tightly clustered

in a cabinet can give rise to common cause candidates based on significant common cause conditions rather than as the result of

specifying the secondary cause susceptibility and location of each

component.

2.5 Quantitative Analysis

Since the introduction of fault tree analysis, the area receiving the

most research and development effort has been the evaluation of fault

trees [5,7,17]. The evaluation of a fault tree is obtaining reliability

information about the TOP event and perhaps the minimal cut sets from

the data supplied for the failure of the basic components.

Quantitative reliability analysis is of value for analysis of system

design, demonstration of compliance with safety requirements, and

justification of system changes or additions. From minimal cut sets and

the component reliability characteristics, the reliability

characteristics for a particular system failure can be calculated.

Component reliability characteristics are completely described by their

time-dependent failure rate (hazard rate) and their time-dependent

repair rate. In practice, both failure rates and repair rates are often

assumed to be constant.

From these component failure rates, component repair rates, and minimal

cut sets, the time-dependent characteristics for a particular system

failure that can be calculated include:

(1) Availability and unavailability

(2) Reliability and unreliability

(3) Rate of failure

(4) Expected number of failures

(5) Failure rate (hazard rate)

(6) Repair rate.

Availability and Unavailability . The availability is the

probability that the system failure does not exist at some specified

time in the future. The unavailability is the probability that the

system failure does exist and numerically is equal to unity minus the

295
availability. The general assumption used in the availability

calculation is that no system component failures exist at time t = 0.

Availability is usually associated with repairable systems. In which

case, the availability at time t contains no information about system

failures before time t. However, the availability is a useful

characteristic if the occurrence of the specified system failure is

tolerable at least some expected fraction of the time. The average

unavailability during a time interval is the expected fraction of the

time the system failure exists. Therefore, the availability is a useful

characteristic when system downtime has economic implications.

As an example of a system for which unavailability is a factor of merit,

a building fire extinguishing system that will extinguish any building

fire, given it functions, is considered. The unavailability is the

probability that the system will not be available to extinguish a fire

because a sufficient number of system components are failed. Although

for the extinguishing system to fail to operate during a fire would

probably be considered intolerable, the unavailability still has merit

because the situation requiring the system to be available is expected

to exist only a small fraction of the time. The unavailability at time

t can be interpreted here as the probability that the extinguishing

system is failed on demand.

In summary, availability and unavailability are:

(1) Generally reported only for repairable systems

(2) Useful characteristics if the occurrence of the system

failure is tolerable at least some expected fraction

of the time.

Techniques for calculating availability and unavailability are well

documented and appear in Reference [13].

Reliability and Unreliability . The reliability is the probability

that the system has experienced no failures to a given time t. The

unreliability is the probability that the system has experienced one or more failures to time t and numerically is unity minus the reliability.

Reliability is generally a pertinent characteristic of both repairable

and nonrepairable systems. For nonrepai rable systems, the unreliability

is numerically equivalent to the unavailability since if the system

fails, it is thereafter unavailable. For nonrepairable systems,

reliability is conventionally the reported quantity rather than

availability.

296
Repairable system reliability calculations account for the possibility

of a redundant component failing and being repaired without system

failure necessarily occurring. Repairable system time-dependent

unreliability, called the distribution of time to first failure by

mathematicians, cannot be calculated exactly for the general case. Conservative, bounding approximations for repairable systems do exist,

however, for the distribution of time to first failure [18,19],

System reliability is in general an interesting system characteristic

and is the pertinent factor of merit when the system failure is

considered intolerable. For catastrophic system failures, such as explosions, system reliability is reported; system avail alDility has

limited engineering meaning for such failures.

Rate of Failure and Expected Number of Failures . The rate of

failure as a function of time is the expected number of failures per

unit time at time t. The rate of failure should not be confused with

the failure rate (a misnomer) to be discussed later. The changes in the

rate of failure as a function of time are of interest from a qualitative

point of view as well as from a quantitative point of view. Also, the

rate of failure is used to calculate the expected number of failures.

The expected number of failures during a specified time interval is

given by the integral of the rate of failure over that time interval.

If the cost of each system failure is known, the expected number of

failures multiplied by the cost per failure gives the expected cost

(risk) during the time interval due to the specified system failure.

The expected number of failures is in a sense a more global system

characteristic than either availability or reliability because it

contains information about the system with respect to an entire time

interval. Unlike probabilistic quantities, the expected number of

failures for repairable systems can be greater than unity.

Failure Rate and Repair Rate . The failure rate as a function of

time t is the probability of failure during t to t + dt, given no failures before t. The term failure rate is a misnomer; hazard rate is more fitting and is sometimes used. The term failure rate is so intrenched in the literature, however, that little chance exists that it

will be discarded.

The time-dependent system failure rate completely describes the density

of time to first system failure and, consequently, the distribution of

297
time to first system failure (unreliability). If the repairable system

failure rate is constant, the mean time between system failures is

given by the inverse of the system failure rate.

The repair rate as a function of time is the probability of repair

during t to t + dt, given no repair before t. The repair rate is a misnomer also for the same reason as is the failure rate. The timedependent system repair rate completely describes the probabilistic

system repair characteristics. If the repair rate is constant, the mean time to repair is the inverse of the system repair rate.

From the system failure rate and repair rate, the time-dependent system

reliability, availability, and expected number of failures can immediately be determined. Also, the failure rate and repair rate

supply the quantities needed to treat a system as a component of

another system in a subsequent more global analysis.

3.0 INTEGRATION INTO THE DESIGN PROCESS

Qualitative and quantitative fault tree analysis results should be

integrated into the design process and can provide valuable assistance

in upgrading system design and performance. An organized approach to

this integration of methods is shown by the flow chart in Figure 5.

As shown in the figure, the first reliability and safety models are formed after preliminary design and capability evaluation. These

models are then analyzed qualitatively and quantitatively. Qualitative

analysis provides information about the system modes of failure and

potential common cause failure candidates. Single failures and

potential common cause failures which cause an undesirable situation

can be identified and corrected.

4.0 LIMITATIONS OF THE METHODOLOGY

The limitations of the system reliability methodology fall into two

categories: limitations occurring during implementation and

limitations in the theory.

In spite of the large effort that has been directed towar^i the

theoretical aspects of system reliability methodology, many individuals

feel that, in general, implementation of system reliability methodology

298
is "still in the academic stages" [20]. Computer programs used to

determine the minimal cut sets cannot always handle th,e large fault

trees encountered in practice because of astronomical computer storage

requirements or excessive computer execution time. Similarly, even if

only several hundred minimal cut sets are considered to adequately

represent the TOP event in question, exact quantitative solutions are not often possible (nor desired) because of computer restrictions.

Theoretical system reliability methodology is limited because of the

binary assumption - either an event is failed or it is working.

Degraded performances, other than totally failed, cannot be considered.

Priority (sequential) AND gates and cold standby cannot be treated

quantitatively by analytical solutions in the general case; but rather,

Monte Carlo solutions must be used.

A methodology for quantitative treatment of secondary failures has not

been published. A secondary failure is failure of a system component

that is due to that component being forced to operate outside its

design envelope, that is, failure due to excessive environmental or operational stresses. Fault trees that contain secondary failures are generally not coherent [21] because repair of all the basic events that

have failed does not guarantee repair of the TOP event. In short,

fault trees that display development of secondary failures are not

coherent because removing the secondary cause of failure does not

repair the component that has failed.

No exact solution for repairable system reliability exists for the

general case. For TOP events that represent hazardous conditions, the

reliability is often the quantity of interest rather than the

availability. Common practice has been to consider all basic events

nonrepairable and to use the resulting TOP event unavailability as an upper bound on unreliability. The unreliability is thereby largely

overpredicted in most cases because the possibility of redundant basic

event failure and then being repaired without the TOP event occurring

is not taken into account.

Although considerable effort has been made to catalogue data that

reflect the probabilistic failure characteristics of components [22-24]

inapplicable or insufficient data continue to be a problem for the

system reliability analyst. Fortunately, a great deal of information

is available from a system reliability analysis without the benefit of

good data. Qualitative analysis does not rely directly on data and is

often the source of most of the useful information obtained from an analysis. Ranking components in order of importance is possible even

if data are only relatively correct. However, good data are required

299
to calculate the expected number of system failures, availability, and

reliability. System reliability methodology suffered a major setback

in the nuclear industry in the late 1960s because of poor or inapplicable data being applied to complex systems and an effort made

to convince system management the qioantltative analysis results were

valid [25].

5.0 RELATED APPLICATIONS

System reliability methodology is applicable to a wide range of

problems. Formal analysis methods are relatively new. The fault tree

technique originated at Bell Laboratories in 1961 and was later

improved by the aerospace industry with individuals from the Boeing

Company making many published contributions [4,17,26]. Until about

1970, publicized applications of the technique remained almost entirely

hardware oriented. Then in 1970, Crosetti and Bruce [27] announced

application to administrative and management systems as well. Examples

given included evaluations of marketing alternatives and optimization

of freeway traffic control systems.

With system reliability methodology being applied to intangible

systems, industry began to realize the full potential of the technique.

By organizing pitfalls in a logical manner in a graphical

representation, system management is better able to evaluate risk and

communicate managerial reasoning to peers, supervisors, and

subordinates. Decision making is consequently carried out more objectively, and a powerful aid to selling ideas results.

In 1973, a tome entitled MORT - The Management Oversight and Risk

Tree [28] was made available. The volume is concerned with application

of the fault tree technique to tangible and intangible systems as an aid to evaluating risk and to making management decisions. While the

MORT methodology will undoubtedly be extended, the volume is a worthwhile addition to any management library. Recently, a large scale

study was undertaken to evaluate the risk associated with nuclear power

plants. This study, the Rasmussen study [29], made extensive use of

system reliability methodology to evaluate the probabilities necessary

to determine risk. The overall approach used in the Rasmussen study is

subject of cause-consequence methods [30-32].

6.0 CONCLUSIONS

Fault tree analysis is a versatile reliability tool that has rapidly

won favor with those involved in reliability and safety calculations.

500
Fault tree analysis is a relatively new subject, continuously

developing and expanding. Concepts and techniques of fault tree

analysis have been developed over the past decade and now results from

this type analysis are important considerations in the design of many

systems such as aircraft, ships and their electronic systems, missiles,

and nuclear reactor systems.

Fault tree analysis is applicable to problems concerning tangible and

intangible systems. Application of the technique to complex hardware

systems encountered in practice is emerging from the academic stages.

Considerable research is needed to complete the theory associated with

fault tree analysis. Methods for more efficient analysis of fault

trees must be formulated before system reliability engineering becomes

a fully mature engineering science. In spite of the limitation of

fault tree analysis, it remains the best approach available for a large

number of the systems analysis problems encountered in practice.

Fault tree analysis can be the most simple or sophisticated analytical

reliability tool depending on the needs of the analyst. Although the

concepts of fault tree analysis are outstandingly simple, constructing

suitable fault trees for complex systems requires thorough

understanding of the principle of fault tree construction and

evaluation as well as an understanding of the system being analyzed.

7.0 REFERENCES

1. Haasl, D. F. , "Advanced Concepts in Fault Tree Analysis", System

Safety Symposium , June 8-9, 1965, Seattle: The Boeing Company [g],

2. "System Safety Symposium", Proceedings of symposium sponsored by

the University of Washington and the Boeing Company, Seattle,

Washington, June 8-9, 1965 [a].

3. Semanderes, S. N,, "ELRAFT A Computer Program for the Efficient

Logic Reduction Analysis of Fault Trees", IEEE Transaction on Nuclear Science , Seattle, Washington, November 1970, p. 79.

4. Nagel, P., "Importance Sampling in System Simulation", IEEE

Transactions on Nuclear Science , Seattle, Washington, November

1970, p. 101.

5. Vesely, W. E. , Analysis of Fault Trees by Kinetic Tree Theory , IN-1330, (October 1969).

301
6. Vesely, W. E., "A Time-Dependent Methodology for Fault Tree

Evaluation", Nuclear Engineering and Design , 13(2), August 1970.

7. Crosetti , P. A., Computer Program for Fault Tree Analysis , DUN-5508, (April 1969).

8. Crosetti, P. A., "Fault Tree Analysis with Probability Evaluation",

IEEE Transactions on Nuclear Science , Seattle, Washington,

November 1970, p. 132.

9. Greger, G. E., D. A. Snyder, P. D. Gross, "Description and Uses of

a Critical Systems Data File for Nuclear Plants", The American

Society of Mechanical Engineers, United Engineering Center, 345

East 47th Street, New York, New York 10017, January 9-12, 1972.

10. Yarosh, M. M. , "Accident Analysis", Nuclear Safety , 3(4), 1962.

11. Leonardini, L., "The Third Reliability Meeting at Riso", Nuclear

Safety , 11(4), July-August 1970.

12. Mearns, A. B., "Fault Tree Analysis: The Study of Unlikely Events

in Complex Systems", System Safety Symposium , June 8-9, 1965,

Seattle: The Boeing Company [a].

13. Vesely, W. E. and R. E. Narum, PREP and KITT: Computer Codes for

The Automatic Evaluation of a Fault Tree , IN-1349, August 1970

TbT

14. Worrell, R. B., Set Equation Transformation System (SETS) , SLA-72-0028A July 1973. Available from Sandia Laboratories,

Albuquerque, New Mexico 87115, USA.

15. Fussell, J. B., E. B. Henry, and N. A. Marshall, MOCUS - A

Computer Program to Obtain Minimal Sets from Fault Trees , ANCR-1156 August 1974, [bj.

16. Fussell, J. B., et. al., A Collection of Methods for Reliability

and Safety Engineering , ANCR-1273, April 1976 [b].

17. Michels, J. M. , "Computer Evaluation of the Safety Fault Tree

Model", System Safety Symposium , June 8-9, 1965, Seattle: The

Boeing Company [a].

18. Esary, J. D. and F. Proschan, "A Reliability Bound for Systems

of Maintained, Interdependent Components", Journal of the American

Statistical Association , 65, No. 329 March 1970, pp. 329-338.

19. Ross, S. M., Reliability and Fault Tree Analysis , SIAM 1975,

pp. 331-350.

502
20. Vesely, W. E., "General Modeling Techniques Applied in the

Rasmussen Safety Study", Reliability and Fault Tree Analysis , SIAM 1975, pp. 775-804.

21. Esary, J. D. and F. Proschan, "Coherent Structures of Nonidentical

Components", Technometrics , Vol. 5 1963, 191-209.

22. "Reliability Stress and Failure Rate Data for Electronic

Equipment", Military Standardization Handbook, MIL-HDBK-21 7B.

Available from the Chief Bureau of Naval Weapons, Department of

the Navy, Washington, D.C. 20360 , USA.

23. Failure Rate Data Handbook (FARADA), U.S. Naval Fleet Missle

Systems Analysis and Evaluation Document #SP 63-470, April 1970.

Available from Naval Weapons Station, Seal Beach, Corona,

California 91720, USA.

24. Ablitt, J. F. , "An Introduction to the 'SYREL' Reliability Data

Bank", UKAEA REPORT SRS/GR/14 1973.

25. Griffin, C. W., "The Fault Tree as a Safety Optimization Design

Tool", ANS TOPICAL MEETING ON WATER-REACTOR SAFETY, CONF-730304

March 1973 [b].

26. Nagel , P. M., "A Monte Carlo Method to Compute Fault Tree

Probabilities", System Safety Symposium , June 8-9, 1965,

Seattle: The Boeing Company [a].

27. Crosetti , P. A., and R. A. Bruce, "Commercial Application of Fault

Tree Analysis", Proceedings of Reliability and Maintainability

Conference , Vol 9, 1970, pp. 230-244.

28. Johnson, W. G. MORT - The Management Oversight and Risk Tree , U.S. Atomic Energy Commission, SAN 821-2, UC-41 (February 12, 1973

[c].

29. "Reactor Safety Study", WASH-1400 (NUREG-75/014) , October 1975 [b].

30. Nielson, D. S., "Cause-Consequence Diagrams", NARS PUBLICATION 2,

December 1972 [d].

31. Nielsen, D. S., "The Cause/Consequence Diagram as a Basis for

Quantitative Accident Analysis", RISO-R-5-71 May 1971 [d].

32. Nielsen, D. S., "Use of Cause/Consequence Charts in Practical

System Analysis", Reliability and Fault Tree Analysis , SIAM

1975, pp. 849-880.

[a] Available from the University of Washington Library, Seattle,

Washington.

305
[b] Available from the NTIS, 5285 Port Royal Road, Springfield,

Virginia,

[c] Available from the Superintendent of Documents, U,S, Government

Printing Office, Washington, D.C. 20402, USA,

[d] Available from the Library of the Danish Atomic Energy Commission

RIS0, Dk-9000, Roskilde, Denmark.

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Figure 2: Sample Fault Tree Representation

R.F. BRAKE

FAILS TO OPERATE

BRAKES FAIL

TO OPERATE

X

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FAILS TO OPERATE

R.R. BRAKE

FAILS TO OPERATE

L.R. BRAKE

FAILS TO OPERATE

Figure 3: Tree Top Boundary Condition For Braking System Example

306
507
PRELIMINARY DESIGN AND CAPABILITY EVALUATION

FAILURE DATA ACQUISITION

QUANTITATIVE —EVALUATION

nz

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PROPOSE DESIGN

MODIFICATIONS

EVALUATE SYSTEM

CAPABILITY

DEVELOP RELIABILITY AND SAFETY MODELS

QUALITATIVE

EVALUATION

IS CAPABILITY

ADEQUATE

Figure 5: Flow Chart For Integration Of

Fault Tree Analysis In The Design Process

308
SESSION VIII

MATERIALS

OF

DESIGN

-COATING MATERIALS

AND TECHNIQUES

Chairman: A. K. Wolff

T. R. Mallory and Company

ION PLATING - CONCEPTS AND APPLICATIONS

D. M. Mattox

Sandia Laboratories, Albuquerque, N. M. 87II5

Abstract: Ion plating is a generic term applied to deposition processes in which the substrate is subjected to a flux of high-energy

ions prior to and possibly during film deposition. This flux of ions

cleans the substrate surface to allow intimate substrate-film interactions. Ion bombardment during deposition may be used to control the

properties of the deposit and to increase the "throwing power" of the

deposition technique. Recent advances in the understanding of ion

plating processes and applications will be reviewed. These include

sputter cleaning, reactive plasma cleaning, gasless (or vacuum) ion

plating, and several new source configurations. Related deposition

processes, such as activated reactive evaporation, will be discussed

and specific large scale and small scale industrial applications of

ion plating will be detailed.

Key words: Adhesion; coatings; films; gas discharges; sputtering.

Introduction: Failure of materials often begins with surface defects.

Surface processes such as corrosion, erosion, wear, microcracking, etc., can often be controlled by the proper application of coatings which protect by their resistance to failure or by sacrificial means. Coatings

may be applied by a variety of techniques, each of which has its strong

and weak points. Some desirable coating properties are good adhesion,

high durability, low cost, and the ability to provide the functional

properties desired. The coating process itself must not degrade the

properties of the base material. Examples of materials degradation

caused by the coating process are the hydrogen incorporation in high

strength steels during electrodeposition giving hydrogen embrittlement

and the microstructural changes in the base material res\alting from the

high temperature required for some chemical vapor deposition processes.

TOO

The ion plating deposition process was developed ' to obtain good

results in vacuum processing where adhesion was. normally a problem

because of poor film nucleation, barrier layers (contaminants or natural) or film/ substrate material insolubility. ^5 5 ion plating is a generic term applied to atomistic film deposition processes in which

the substrate surface is subjected to a fl'ux of high energy ions sufficient to cause appreciable sputtering before and during interface formation. The ion bombardment may, or may not, be continued during film

growth. Ion plating is usually performed with the substrates immersed

in a gas discharge while a dc or rf potential is applied to the surface

511
to be coated. Sources for the film atoms include ' --^ ; (1) resistivelyheated filaments, (2) electron beam heated crucibles (e-beam ion plating), (3) rf-heated crucibles, (k) sputtering cathodes (high-bias

sputter deposition), (5) metal-bearing gases (chemical ion plating),

and (6) sources in which vaporized atoms pass through an electron cloud

or plasma to enhance ionization of the metal atoms.

From the standpoint of adhesion, the principal benefits of the ion

plating process are: (1) cleaning is an integral part of the deposition process and the surface is maintained "clean"^ \intil the interfacial region is formed, (2) a high energy flux to the substrate sur- face during deposition enhances diffusion and chemical reaction in the

surface region without necessitating bulk heating, and (3) alteration

of the surface and interfacial region in a manner conducive to good adhesion by creating high defect concentrations''' developing a compressive

stress^ and creating a "pseudodiffusion"- type of interface. 5 in addition, it has been shown that ion bombardment during film growth can improve surface coverage, alter the morphology of the deposited film,

and affect the physical properties of the coatings.

9

Figure 1 shows a typical ion plating system using a thermal vaporization source and a dc diode gas discharge to provide ion bombardment of

the substrate. In a typical operation, the precleaned substrate is

placed into the vacuian system, the system evacuated to an ultimate of

< 10"^ Torr, the evaporant premelted and cooled with the shutter closed.

The system is then backfilled with argon to a pressure of 0.1 - 1

("bleed-back pressure") m Torr, The vacuum pump is then throttled to

give a discharge pressure of 1 - 20 m Torr, and a dc gas discharge is

established at 2 - 5 kV dc and />' 0.5 m A/cm^ substrate current density.

The surface is then sputtered for a period of time which depends on the

material surface condition, and level of background contamination.-^^j-'-'-

At the end of the sputter cleaning portion of the process, the evaporator filament temperature is raised iintil the film material begins to

evaporate. A shutter between the source and substrate is then opened,

allowing the material to deposit on the substrate. The important step

is the formation of the surface -coating interface without interruption

of the ion bombardment. Of course, for a film to form, the deposition

rate must exceed the sputtering rate.

Figure 2 schematically depicts the processes that occur in a dc gas discharge used in ion plating an electrically conducting substrate. The

primary source for ionization in the discharge gas is electron-atom

collision e" + -» G"*" + 2e". The gas ions that are produced may then

become part of the plasma and ultimately lost to the chamber walls, or they may be accelerated across the cathode dark space to the cathode.

Very few of these ions reach the cathode with the full cathode fall

potential. In transit they undergo charge exchange collisions with gas

atoms + G+ + G^, producing a spectrum of high energy neutrals

and high energy ions, which bombard the cathode surface and surrounding

fixtiiring.

512
TABLE 1

Film-Substrate Couples

Electrical Contact

Ag-Si

Ag-Ge

Cu-Si

Cu-Ge

Ai-Si

V-Si/Ge

In-CdS

Cu-Steel

Corrosion Coatings

A-e-U

AX-Steel

C-Steel

Au- Steel

Lubrication Coatings

Au-Steel

Pb-Steel

MoSg-Carbon

Ag-Steel

Joining

Cu-Mo

Cu-Ta

Cu-Nb

AZ-Be

Ag-Fe

Cu-Ti

Cr-Mo

Ag-Be

TABLE 2

Reactive Ion Plating

CdS HfW

Si HfC

TiC Ai^Oo

In 0 ZnTe

TiN ^

315
The high energy atoms/ions that impinge on the cathode may (l) produce

secondary electrons, (2) sputter cathode atoms (S), (3) sputter surface

contaminants (sputter cleaning), {k) become neutralized and reflect

from the cathode s\irface as high energy neutrals or as metastable atoms

(G^), and/or become incorporated into the cathode surface modifying the

surface properties. In ion plating, the impinging atoms may sputter

film atoms (M) .

The secondary electrons that are produced are accelerated back across

the cathode dark space, causing ionization which sustains the gas discharge .

The sputtered surface atoms (film, substrate, or contaminant) can be

(1) scattered back to the cathode, (2) ionized by electron-atom collision or met astable -atom collision + G"^ -» S"*" + G^ + e" (Penning ionization and accelerated back to the cathode or lost from the cathode

region, and (3) transported as high energy neutrals to some surface

(sputter deposited). Penning ionization is probably the most important

ionization process for sputtered cathode atoms. The energy distribution

of sputtered atoms is higher (1 - 100 eV) than that of thermally vaporized atoms (0.2 - 1 eV) . The return of sputtered atoms to the cathode

surface is a function of many discharge parameters'^ as well as the

system throughput. This return is important in sputter cleaning, generation of surface features (such as cones and whiskers), surface coverage and also in the sputter deposition of alloy material. It has been

estimated that at 0.1 Torr gas pressure, 90% of the sputtered atoms are scattered back to the cathode. It should be noted that only neutral or negatively-charged particles can escape from the cathode surface.

The gas atoms that are incorporated into the cathode s^urface may affect

the surface properties, such as sputtering yield (number of cathode

atoms sputtered per incident ion). The gases trapped in the surface

during sputter cleaning may be released after film formation and cause poor film adhesion. Gases trapped in the surface or deposited film may

be released by heating. 12

If the sputtering gas (or impurities in it) can react with the cathode

STirface, the ion bombardment can be used to form a compound or alloy

surface. This makes sputter cleaning the surface of a reactive metal

difficult if a reactive gas is present. Contaminant gases in the discharge are often more reactive than they are in the simple gaseous state

since a portion of them are disassociated and/or ionized. If reactive

gases are present during film deposition, compound films may be formed

(reactive ion plating).

In ion plating, film atoms are injected into the discharge from a source such as a resistively-heated filament electron beam heated crucible,

sputtering target, or rf-heated crucible. Such a source is shown in

Fig. 2. Here the film atoms M may be (l) scattered back to the filamait,

(2) deposited as neutrals on the substrate (cathode), (3) ionized by

3lh
electron-atom or metastable-atom collision and accelerated to the substrate, or (k) nucleated in the vapor phase to form small particles

(gas evaporation) which become negatively charged and may be collected

on a positively charged surface or lost to the system walls. In any

case, the film atoms act as foreign particles in the gas discharge and

can cause changes in the discharge parameters.

It should be noted that in a sputtering discharge only a small percentage of the gas atoms are ionized, so the cathode and. other surfaces in

the system are being continuously bombarded by thermal neutrals. In

addition, the discharge acts as a rather intense source of ultraviolet

radiation hv-, which can affect organic surfaces and possibly insulator

surfaces. The cathode surface will become heated during sputtering,

since most of the energy of the bombarding ions is given up as heat and

will radiate energy in the infrared, hv^* Soft x-rays may be emitted

from the cathode surface and affect semiconductor materials. The cathode current is not totally due to ion current, but is a combination of

ion current and secondary electron current. It is, however, valuable

to know the total cathode current density in order to establish reproducible ion bombardment.

Cleaning: Even though cleaning is an integral part of the ion plating

process, it is important that gross contamination be removed prior to

insertion into the ion plating system. In our experience, carbon is one of the worst contaminant materials since it reacts to form carbide

layers which prevent diffusion.^ In ion plating, the in situ cleaning

can be performed by (1) sputter cleaningl*-* by ion bombardment using

inert gas or (2) reactive plasma cleaning!! using a reactive species in

the discharge gas which will chemically react with the surface materials

to give volatile species.

Sputter cleaning is a well established method for obtaining atomically

clean surfaces in a clean environment. Sputter cleaning is usually ac- companied by heating in order to desorb sputtering gas incorporated

into the substrate surface and to restore the surface crystallography.

Sputter cleaning in a less clean environment can pose problems due to

recontamination and the reactive nature of the sputter-cleaned surfaces.

It has been found using in situ surface analysis by soft x-ray appearance potential spectroscopy (SXAPS) that the best way to sputter clean

metal surfaces is to use rf sputtering at a low discharge pressure.!^

Insulator siirfaces can also be cleaned using rf sputtering.

Reactive plasma cleaning-'-'- uses a reactive species in the gas discharge

and may or may not use a potential applied to the surface to be cleaned.

Gases having halide species have been found to be effective in cleaning

metal surfaces. Figure 3 shows the cleaning of a titanium surface by

argon in sputter cleaning and reactive plasma cleaning using EC Jl or CCX^ vapor as measured by SXAPS carbon peak height. Using low power

inputs, the reactive plasma cleaning is more effective than sputter

515
cleaning and ECl is better than CCZ]^ apparently due to residual carbon

left by the Cdi^. Reactive plasmas may be formed using gaseous species

of chemicals which would normally be used to etch the material and which

form volatile species. Such reactive species might be the halides (F,

C£) or hydrogen.

For carbon removal, a reactive plasma containing oxygen is very effective, but may result in excessive oxidation of easily oxidizable materials. The oxygen plasma produces ultraviolet radiation which causes bond scission in carbon compounds and active oxygen species react with

the resulting fractions to form volatile gases. -^3 ion bombardment also

increases bond scission and reaction.-*-^ The oxygen plasma may be used to

clean insulator and gold surfaces, and may also be used to remove hydrocarbon and carbon contamination from metals which form passive oxides.

An oxygen plasma is also useful in cleaning and desorbing gases from the

walls of stainless steel vacuum systems .-'-Sj 15

Energy Deposition: High energy particles bombarding a surface will lose

their energy by (1) sputtering surface atoms, (2) generating heat, and

(3) forming defects in the near-surface region. Since sputtering yields

(ratio of sputtered atoms to incident ions) are rather low and the ener- gies of the sputtered atoms are far less than that of the incident particle, only a small percentage of the incident energy is carried away by

the sputtered atoms.

Little can really be said about the distribution of lattice vibrations

(heat) in the near-surface region. One could speculate that atoms in

the first few layers are subjected to vibrations equivalent to extremely

high temperatures.

It has been shown that ion bombardment of surface creates a large con- centration of point defects.l° These defects can be of such great mmber as to destroy the surface crystallography and to trap diffusing gas

atoms. Thus the surface defect concentration and entrapped gas is dependent on the incident particle energy and dose. This surface condition probably influences the sputtering yield.

Good adhesion is promoted by5 (l) strong bonding within the interfacial

region, (2) low local stress levels, (3) absence of easy deformation or fracture modes, and (k) the lack of long term degradation modes. These

properties depend on the nature of the interfacial region. The most desirable type of interface is one which exhibits a gradual change (grade)

of material properties across the interfacial region and which contain

no defects which act as stress concentrators or crack initiators. This

type of interface can result from interdiffusion or the formation of a "pseudodiffusion" type of interface. A pseudodiffusion interface is one formed by energetic processes such as melting/quenching, ion plating,

ion implantation, codeposition, or processes involving nonequilibrium

defect concentrations. Even a very thin (10 - 1000 A) interfacial

region may be effective in distributing interfacial stress.

516
In ion plating, the surface is bombarded by ions prior to film deposition. This bombardment cleans the surface of contaminants and surface

layers which might inhibit diffusion or chemical reaction. The bombardment also generates high defect concentrations which may accelerate

diffusion (radiation enhanced diffusion) and introduces stresses in the

surface region which may promote diffusion. The surface heating from

ion bombardment will enhance diffusion and chemical reaction and aid in

the solution or disaruption of contaminants which remain on the surface.

Bombardment by ions during interface formation will give an enhanced

nucleation density. ^7 Bombardment by high energy ions/neutrals of the

film material will give "ion implantation" into near-surface region.

Ion bombardment will cause film atoms to be injected into the substrate

surface. 18 Heating will also enhance diffusion and chemical reaction.

The return of backsputtered substrate and film material to the surface

will tend to generate a graded interface by codeposition and implantation.19

Film Properties: Deposition in a gas atmosphere and under ion bombardment has been shown to improve surface coverage over that obtained by

vacum deposition. This is due to several effects: namely, (1) gas

scattering, 20 (2) backsputtering and redeposition,-'-9 (3) forward

sputtering, 9 and {k) ionization and direction along field lines. Gas

scattering alone normally gives poor adhesion and a low density film.

Ion bombardment during deposition may be used to affect a number of film

properties. Figure k shows the intrinsic film stress and density of

sputter-deposited chromium as a function of substrate bias during deposition. 9 The density of both dc and rf sputter- deposited chromium are

shown. In general, ion bombardment during deposition will give a more

dense film both for metals and ceramics. 9521 under ion bombardment,

thick deposits do not develop the coliminar morphology often associated

with thick vacuum or sputter deposits. 9 j 22 Film porosity is also decreased.

Gas may be incorporated into the growing film and may be deliberately

used to achieve high gas concentrations in deposited films. -'-2 This gas

may usually be desorbed at rather low temperatures during deposition.

Ion bombardment during deposition may give compositional changes in

glass, 2-^ alloy, or compound materials.

An important parameter in the modification of films by ion bombardment

is the ratio of impinging high energy particles to depositing film atoma

If this ratio is small, obviously the effects will be small

.

Applications: Identifying applications of the ion plating process is

often rather difficialt since it is called by other names such as "ion

vapor deposition," "glow discharge physical vapor deposition," "ionization electrostatic plating," "bias deposition," and others. The use of

electron beam evaporation sources (e-beam ion plating) and rf-heated

crucible sources23 has greatly expanded the field of applications.

517
Japan has led in the development of the ion plating process for commercial applications.2^ The Japanese have "been interested in the ion plat

ing process as one of the "dry plating" processes which may he used to

replace electrodeposition and its associated pollution problems. The

process is being used for depositing both functional and decorative

coatings.

In the United States most applications have been rather specialized

where other deposition techniques have not succeeded. Ion plating of

alminum on enriched uranium reactor parts has been successful in preventing corrosion. 25 Aluminum coating of high strength steel bolts has

been developed into a large scale commercial application. ^6 Metal lubricant has been ion plated on titanium, zirconium, and other such

metals to aid in subsequent joining. Ion plated metals have also been

used as a "strike" to allow subsequent electroplating. 28 in many appli

cations, ion plating is of interest for its ability to uniformly cover

steps and complex geometries. An example is the aliaminijm metallization

of silicon semiconductor devices. 29

Table 1 siammarizes some of the applications of the film- substrate systems listed. Table 2 gives some materials that have been deposited by

reactive ion plating processes.

References

1. D. M. Mattox, Electrochem. Technol. 2 295 (I96U).

2. D. M. Mattox, J. Vac. Sci. Technol. 10 (1973).

3. D. M. Mattox, (Proc. 6th International Vacuum Congress 197^) > Jpn,

J. Appl. Phys. Suppl. 2, Pt. 1 p. hk3 (197^).

h, D. M. Mattox, "Interface Formation and the Adhesion of Deposited

Thin Film" Sandia Laboratories Report SC-R-65-82 (Jan. I965),

Available from NTIS*

5. D. M. Mattox, "Thin Film Adhesion and Adhesive Failure" to be presented to ASTM Symposium on Adhesion Measurement of Thin Films,

Thick Films, and Bulk Coatings, Philadelphia, Pa. , (II/2- Vl976)

.

To be published in the symposiimi proceedings.

6. D. M. Mattox, "Surface Cleaning in Thin Film Technology," Sandia

Report SAOT)7^-03^^ (Jan. 1975). Available from NTIS*

7. R. R. Hart, H. L. Dunlap, and 0. J. Marsh, J. Appl. Phys. k6 19k7

(1975).

8. G. Dearnaley, Appl. Phys. Lett. 28 2kk (I976).

9. R. D. Bland, G. J. Kominiak, and D. M. Mattox, J. Vac. Sci. Technol

11 671 (197^).

10. G. J. Kominiak and J. E. Uhl, J. Vac. Sci. Technol. 13 1970 (1976).

518
11. G. J. Kominiak and D. M. Mattox, "Reactive Plasma Cleaning of Metals

Metals," to be published in Thin Solid Films (Sept. 1976)

12. D. M. Mattox and G. J. Kominiak, J. Vac. Sci. Technol. 8 19k (1971).

13. R. R. Sowell, R. E. Cuthrell, R. D. Bland, and D. M. Mattox, J. Vac.

Sci. Technol. 11 k7h (197^).

Ik. L. Holland and S. M. Ojha, Vacuum 26 53 (1976).

15. R. M. Lambert and C. M. Comrie, J. Vac. Sci. Technol. 11 530 (197^).

16. E. V. Kornelsen, Radiation Effects 13 227 (1972).

17. M. L. Tarng and G. K. Wehner, J. Appl. Phys. k3 2268 (1972)

18. L. E. Collins, J. G. Perkins, and P. T. Stroud, Thin Solid Films k

kl (1969).

19. G. J. Kominiak, "Metallization Adhesion Layer Formed by Backscattering during dc Sputter Etching," to be published J. Vac. Sci. Technol

(Sept. 1976).

20. Kurt D. Kennedy, Glen R. Schevemann, and Hugh R. Smith, Jr.,

Research/Development Magazine, p. ^0 (Nov. 1971).

21. D. M. Mattox and G. J. Kominiak:, J. Electrochem. Soc. 120, 1535

(1973).

22. T. A. Thornton, J. Vac. Sci. Technol. 11 666 (197^).

23. T. Spalvins and W. A. Brainard, "Ion Plating with Induction Heated

Sources," MSA Report TM-X-3330 (Jan. 1976)^

2k. Chikara Hayashi, Jpn. J. Appl. Phys. Suppl. 2 Pt. 1, p. 2^1 (197^),

(Proc. 6th International Vac. Congress 197^).

25. R. D. Bland, Electrochem. Technol. 6 272 (I968).

26. K. E. Steube (McDonnell-Douglass), "Glow Discharge-Tumbling Vapor

Deposition Apparatus," U.S. Patent 3,926,1^7, U.S. Pat. Off. Gaz.

9^1 , 1128 (12/16/75).

27. N. Ohmae, T. Nakai, and T. Tsukizoe, Wear 30 299 (197^).

28. Bernard C. Strupp, Plating 6I I090 (197^).

29. J. L. Vossen, "Method of Metallizing Semiconductor Devices,"

U.S. Patent 3,6^10,811 (2/8/72) RCA.

^National Technical Information Service

U. S. Department of Commerce

Springfield, VA 22l6l

519
VARIABLE LEAK INSULATOR

MOVEABLE SHUTTER GROUND

SHIELD

SUBSTRATE

CATHODE DARK SPACE EVAPORATOR FILAMENT GLASS CHAMBER

HIGH CURRENT FEEOTHROUGHS

APPARATUS FOR ION PLATING

TYPICAL ION PLATING APPARATUS USING A DC

POTENTIAL ON THE SUBSTRATE AND A RESISTIVELYHEATED VAPORIZATION SOURCE

FIGURE 1

320
SCHEMATIC REPRESENTATION OF THE PROCESSES

WHICH OCCUR IN A DC GAS DISCHARGE WITH A

THERMAL VAPORIZATION SOURCE.

FIGURE 2

521
TITANIUM SPunERED AT 300 vrt

1 I I L.

0 1.0 2.0

SPLHTERING TIME (Hrl CLEANING OF TITANIUM BY KF SPUTTER CLEANING

AND REACTIVE PLASMA CLEANING (Ref . 11)

FIGURE 3

522
CHROMIUM

150 f

DC BIAS VOLTAGE

AN EXAMPLE OF HOW FILM PROPERTIES MAY

DEPEND ON DEPOSITION CONDITIONS. DATA

TAKEN FROM REF. 9.

FIGURE k

323
SPUTTERING

T. Spalvins

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

Abstract: This paper primarily reviews the potential of using the sputtering process as a deposition technique; however, the manufacturing and

sputter etching aspects are also discussed. Since sputtering is not regulated by classical thermodynamics, new multicomponent materials can be

developed in any possible chemical composition. The basic mechanism for

dc and rf sputtering is described. Sputter-deposition is described in

terms of the unique advantageous features it offers such as versatility,

momentum transfer, stoichiometry , sputter-etching, target geometry (coating complex surfaces), precise controls, flexibility, ecology, and sputtering rates. Sputtered film characteristics, such as strong adherence

and coherence and film morphology, are briefly evaluated in terms of

varying the sputtering parameters. Also described are some of the specific industrial areas which are turning to sputter-deposition techniques.

Key words: Sputtering; Thin films; Protective coatings

INTRODUCTION

Sputtering is a process where a surface, when immersed in a plasma such

as an ionized inert gas, is bombarded with energetic particles that cause ejection of surface atoms. The heart of the sputtering process is essen- tially the disintegration of the target material under ion bombardment.

These ejected sputtered atoms can be collected onto a substrate to form

a film.

Using the sputtering process as a film deposition method is receiving

an enormous amount of interest and acceptance. New ways of applying the

sputtering process are constantly being found for those areas where the more conventional deposition methods can not solve the problems encountered. In addition, as a film-deposition method, the sputtering process

is used in sputter fabricating intricate mechanical components which are difficult or almost impossible to manufacture by machining, casting, or powder metallurgy techniques. The sputtering process is also used as a universal, nonchemical etching technique - for example, sputter etching,

ion milling, or micromachining

.

Especially in the film deposition area, the unknovm potential for sputtering originates from the knowledge that it is possible to prepare

new multicomponent materials (such as new alloys and compounds in any

32k
chemical proportions and relations) and also, if needed, to form a con- tinuous compositional change or concentration gradient. Since the

sputtering process is not regulated by classical thermodynamics and

Gibbs phase rule relationships, one is not forced to remain within the

framework of the rigid regulations. All of these unexplored features

show that sputtering is really in its infancy; therefore, at present the

potential future impact of sputtering on industry is inestimable.

The purpose of this paper is to acquaint the mechanical engineer with

the potentials of the sputtering process, particularly with the physical

rf-sputtering technique. The following sections on sputtering modes,

unique features of sputtering, characteristics of sputtered films, and some practical applications of sputtered films are written in an introductory nature in order to bring out and emphasize the key features of

the method and the resulting films. For more detailed information one

is referred to Refs. 1 to 6.

SPUTTERING MODES

There are two basic types of sputtering, depending on whether the glow

discharge plasma is generated by direct current (dc) or radio frequency

(rf) fields. To induce sputtering, a negative surface charge has to be

built up on the target. Besides the two basic dc and rf diode-type

techniques, the sputtering modes and configurations are also as varied

as the number of sputterers in the field. All of these sputtering modes

and configurations arise essentially from (1) the way in which plasma

is created (dc, rf, auxiliary electrodes), (2) the target and substrate

positioning and their geometrical configurations, (3) the number of

sputtering targets in the system, (4) the type of gases used (inert or reactive), and (5) the utilization of magnetic fields. A few modes are

dc diode and triode, rf and dc/bias, dc-rf combinations, reactive sputtering, and magnetron sources.

dc Sputtering

The simplest sputtering configuration is the dc diode type, which consists of two electrodes. The coating material is the target or cathode,

and the specimen to be coated is the anode. When these electrodes are

immersed in an inert gas (argon) atmosphere of 10 to 100 ym and a dc

potential of 500 to 5000 V, is applied across the two electrodes, ionization of the gas occurs, and generates a glow discharge plasma. The

positive argon ions are accelerated toward the target with a kinetic en- ergy high enough to knock off or sputter atoms from the target surface.

The oldest, simplest, and least expensive method is dc sputtering. The

limitations to this method are that only conductors and certain semiconductors can be sputtered. Nonconductors (ceramics, glass, etc.) cannot

be sputtered by the dc method. The reason is that a positive charge ac- cumulates on the target's surface and acts as a barrier to further ion

bombardment and thus the sputtering process. This problem, however, can

525
be overcome by using an rf potential,

rf Sputtering

In rf sputtering a high frequency potential in the low megacycle range

is directly applied to the metal electrode behind the target. The target (an insulator) is bonded to the metal cooled electrode, thus forming

a capacitive coupling. Since the rf voltage is applied to the target

assembly through rf fields generating plasma, the sputtering current is

controlled by the rf voltage. Efficient plasma generation requires

frequencies above 10 megacycles, and the resulting rf diode sputtering

system operates usually at the allowed band of 13.56 megacycles. The

insulator surface in contact with the plasma is alternately bombarded by

electrons and positive ions during each rf cycle. When the surface is

positive it attracts electrons, when it is negative it attracts ions.

Since the electrons in the plasma have a higher mobility than the ions,

the electron current to the target surface is initially much greater

than the ion current. The cathode acts as a diode and charges the coupling capacitor to the peak value of the rf input voltage and then attains a negative bias.

If a voltage greater than 500 volts is applied, the surface bias will be

sufficient to accelerate ions from the plasma to an energy high enough

to cause sputtering of the insulator. A simplified schematic of the

sputtering process is shown in Fig. 1.

All rf diode systems use the previous mechanism which makes the technique universally applicable. In addition to sputtering insulators,

rf-sputtering can operate at lower argon pressures (1 to 5 ym) and attain higher sputtering rates than dc sputtering.

rf Sputtering with dc bias

Of the many possible sputtering modes and arrangements which are used

today, rf sputtering with a dc bias will be described very briefly.

Fig. 2 shows schematically and photographically a sputtering system

which consists of two independently operated sputtering processes: rf

sputtering (deposition) and dc sputtering (cleaning) . The dc sputtering

process is used strictly for cleaning or sputter etching metallic substrates before rf sputter deposition. The specimen is thus capable of

sequential substrate cleaning or etching followed by sputter coating or simultaneously etching (biasing) while sputter depositing.

DISCUSSION

Unique Features of Sputtering

The unique, advantageous features of rf sputtering make it the most versatile of all the deposition methods used today. From an industrial

point of view, for many applications the following features are of great

526
significance: versatility, momentum transfer, stoichiometry , sputteretching, target geometries - coating complex surfaces, precise controls,

flexibility, ecology, and sputtering rates.

Versatility . - Virtually any solid material, regardless of its chemical

complexity, can be sputtered in the same stoichiometry on practically

any type of specimen. Alloys, intermetallics , inorganic compounds,

glasses, ceramics, selected organics such as teflon (PTFE) , polyimides, . and cattle bone have been successfully sputter deposited.

Momentum transfer . - The sputtered atoms from the target are transferred

by a momentum transfer process. The sputtered atoms are dislodged by

impact evaporation as opposed to thermal evaporation. Sputtering will

occur only when the actual energy by the bombarding argon ions transferred to the target surface exceeds the usual lattice binding energy of

3 to 10 eV. Sputtering does not depend on the vapor pressures of the

constituent elements, and since there is no direct heating involved, it

is sometimes referred to as a "cold process."

Stoichiometry . - When the sputtering parameters are carefully controlled,

multicomponent solids can be deposited with the same chemical composition. The target must be water cooled at all times to avoid thermal

evaporation and bulk and surface diffusion since during sputtering less

than 5 percent of the kinetic energy of the bombarding ions goes into

the kinetic energy of the sputtered atoms, the other 95 percent goes as heat into the target.

If the target is kept at a sufficiently low temperature to avoid evaporation and diffusion, the composition of the sputtered material will be

identical to the composition of the target, even though the chemical

components have different relative sputtering yields. Stoichiometry in

the sputtered coating is retained because the component having the highest sputtering yield cannot diffuse from the bulk to the surface as fast

as it is removed by sputtering. Following this, the lowest yield component will be sputtered along. Very soon a steady-state condition is

reached in which the material is transferred from the target to the substrate in the same composition.

An important property in sputtering is the sputtering yield which is defined as the number of atoms ejected from the target per incident ion.

Carbon has the lowest sputtering yield, less than 0.1 atom per ion,

while silver has the highest, 2.7 atoms per ion. Sputtering yield,

which is synonymous with sputtering rate, will be discussed separately

in the last section.

Sputter-etching . - Instead of applying the potential to the target, the

potential is applied to the substrate to induce sputtering, which is es- sentially reverse sputtering. Before the substrate is sputter deposited

it is sputter etched. The purpose is to clean the surface of contaminants, oxides, and skin effects of cold working that may be produced by

327
mechanical polishing. Most sputtering systems are capable of sequential

substrate cleaning or etching or of simultaneous etching while sputter

depositing.

Target geometries - coating complex surfaces . - Targets can have any size

and shape such as planar, cylindrical, hemispherical, or some other

suitable configuration. It can be made from one material or several

sections of different material composition. Planar targets (12.7 to

20.32 cm diam.) are the most widely used. When such targets are sputtered, atoms leave the target at all possible angles as shown in Fig. 3.

As a result, most of the specimen surface is in direct line of sight with

some portion of the target. Also, because sputtering in a plasma occurs

at relatively high pressures (5 to 25 ym) , the mean free path of the

sputtered atoms is relatively short (<1 cm). Thus, the sputtered atoms

will be scattered in random directions by collisions with particles in

the plasma; such scattering enables sputtered atoms to reach surfaces

that are not in a direct line of sight with the target. As a consequence, irregular, nonsyimiietrical surfaces can be coated in cavities and

around corners without any rotation in just one operation.

Precise controls . - Sputtering offers an extraordinary control in terms

of deposition rate, film thickness, uniformity, density, and film morphology. Tolerance requirements can be controlled to a millionth of a centimeter

.

Flexibility . - Sputtering offers many options of various parameter com- binations. As a result, the chemical composition of the coating can be

controlled in any desired ratio from stoichiometric to nonstoichiometric

compounds. In addition, graded compositions, laminated structures (composites), dispersion strengthened structures, and insoluble additions

can be formed. These various compositional changes can be accomplished

in a number of ways; for example, multitargets can be used where con- trolled mixing of materials is performed by sputtering from different

targets or by reactive sputtering where the flow rate of the gases carrying one of the reaction product constitutents is carefully controlled.

Ecology . - Sputtering does not create any disposal problems. ,

Sputtering rates . - Sputtering has the disadvantage that deposition rates are relatively low. The average rate is 0.005 to 0.3 ym/min. However,

equipment improvements are gradually yielding higher deposition rates.

In specially designed high rate sputtering systems rates up to 250 ym/hr

have been achieved. In order to increase the rate, efficient ways must

be found to cool the target sufficiently with increased power input.

The sputtering rate (yield) to a first approximation can be related to

the type of the material being sputtered and the sputtering parameters

(bombarding ion flux density, angle of incidence, gas pressure, target

and substrate temperature, etc.). The general trend as to how the sputtering parameters affect the rate are shown in Fig. 4. The rate in528
creases with rf-power input to a maximum value and then falls off.

Above the ion bombarding energy of ^^7 keV, there is no gain in sputtering yield. An increase in spacing between the target and the specimen

decreases the deposition rate. Increases in the target and substrate

temperatures also decrease the deposition rate. Although an increase in

the gas pressure initially increases the sputtering rate, it eventually

causes a decrease in the deposition rate because of a decrease in the

ion mean free path. Applied magnetic fields increase the plasma density

and thus increase the deposition rate.

The low deposition rates also have certain advantages; for example, they

do afford a high degree of film control. The slow rates have a tendency

to form denser films as compared to higher rates.

CHARACTERISTICS OF SPUTTERED FILMS

Functionability of a coating, regardless of its intended use, depends

primarily on the degree of adherence, coherence, and coating morphology.

Sputtered coatings grow in a complex plasma environment. The coating

adherence, coherence, and morphology are directly affected by (1) sputter etched or biased surface, (2) kinetic energy of the sputtered

species, (3) plasma conditions, and (4) substrate temperature and topography .

The strong adherence normally obtained with sputtered coatings can be

attributed basically to the surface cleanliness and the relatively high

arrival energies of the sputtered material. These energetic submicroscopic sputtered particles have certain activation energies which not

only favorably affect the surface adherence but also increase the cohesion between the sputtered particles. The strong particle to particle

cohesion is responsible for the formation of high density films. The

submicroscopic particle size is important in both compactability and

final density of the coating as well as strength. Strength is generally

related to the final grain size - the smaller the grain, the stronger

the compact. Due to these characteristics, relatively thin film.s in the

0.2 to 10 ym range can be used where previously thicker films were required. It can be implied, therefore, that the coating attachment to

the surface is more important than the volume of coating present.

It is well known that stress induced peeling, which is caused by internal stresses in the film, increases with film thickness. Therefore, in

sputtered films, which are usually very thin (<1 ym) , the stress induced

peeling effect is minimized.

A diverse range of coating morphologies and properties can be reliably

produced by controlling the various sputtering parameters and the substrate conditions. The nucleation and growth of the sputtered film can

be varied and this would in turn affect the film properties. Morphological changes (preferred texturing: columnar, equiaxed and even

epitaxial growth) in the coating can be initiated. The grain size can

529
be decreased substantially, and this reduction will increase the density

of the coating by simply bias sputtering. One of the most obvious bias

related properties of many coatings is the optical reflectance at ground

potential and dull and matte appearance at bias potential. Since a detailed discussion of this subject is outside the scope of this paper,

one is referred to Refs. 1 to 9

.

SOME PRACTICAL USES OF SPUTTERING

The sputtering process is primarily used in sputter deposition technology. However, high rate sputtering techniques have been developed which

can achieve sputtering rates up to 250 ym/hr. Thicknesses up to, but

not limited to, 0.63 cm have been achieved. As a result, commercial interest has developed in thick deposits. Fabrication by sputtering of

free standing shapes such as sheet, tubing, and inner and outer cylindrical structures with coolant passages for thrust chambers has been developed as illustrated in Fig. 5.

Sputter etching provides an alternate technique to chemical etching.

The outstanding features are that it is universally applicable to all

materials, it eliminates undercutting, and it is widely used for pattern

delineation.

Of all the specific industrial areas the greatest sputtering activity is

in microelectronics and microminituarization such as integrated thin film

circuit technology. In the last 10 years sputtering has not only spread

rapidly in the mechanical area but also in practically all areas which

require films that are difficult or impossible to handle by other means. Just a few areas where sputtered coatings have an increasing impact are corrosion and high temperature oxidation protection, reduction of friction and wear, solid film lubrication, decorative purposes, replicating

techniques, biomaterials for surgical implants, and solar cell development .

Several specific examples will illustrate how sputtered films function

in these areas. VThen solid film lubricants such as M0S2 are sputtered

on sliding surfaces, only 0.2 ym thick films are required for effective

lubrication in vacuum or dry air [10, 11]. An endurance life evaluation

is shown in Fig. 6 where sputtered M0S2 films are compared to two other

application techniques. The endurance lives of bearings sputter coated

with a duplex coating (0.1 ym thick underlayer of Cr3Si2 and subsequently

with 0.6 ym of M0S2) were greatly improved over the lives of those

bearings which had M0S2 films directly applied as shown in Fig. 7.

Teflon PTFE (polytetraf luoroethylene) has been successfully sputtered

with excellent adherence on metal, glass, paper, and wood surfaces.

Fig. 8 shows 0.1 ym thick PTFE films sputtered on hypodermic needles.

Special formulations of modified silicon carbide and other refractory

carbides have been sputtered on airborne components such as first-stage

compressor blades. These components operate in severe environments and

have to be protected from corrosion, oxidation, erosion, and abrasion
[12]. An interesting approach is to sputter cattle bone on metallic

prosthetic devices used as surgical implants for hip bone replacements.

The sputtered bone film promotes bone growth and attachment to living

bone [13].

SUMMARY OF RESULTS

The sputtering process is primarily used in thin-film deposition technology. Because higher sputtering rates have been attained, sputtering

is now used for fabricating intricate mechanical components (tubing,

cylinders with passages, etc.). Finally, the sputtering process is being used as a universal nonchemical etching technique.

In coatings technology sputtering has yet unexplored potentials due to

the fact that almost any material can be sputtered in proportions and

relations that are not regulated by classic thermodynamics and phase interrelationships. Many sputtering modes and configurations can be used

to achieve chemical compositions which range from stoichiometric to non- stoichiometric . Coatings with gradual changes in composition and laminated and dispersion strengthened structures can also be obtained using

these configurations.

The unique sputtering features of versatility, momentum transfer, stoichiometry, sputter etching, target geometry, precise controls, flexibility, and sputtering rates contribute to the outcome of the desired coating.

The sputtered films generally exhibit strong adherence and coherence,

and the coating morphology can be controlled by the sputtering parameters. As a result, thin sputtered films having 0.2 ym thicknesses are sufficient for many applications. Sputtered M0S2 lubricant films 0.2 ym

thick exhibit superior performance during sliding friction, and sputtered

Teflon (PTFE) and sputtered cattle bone have found new useful applications .

REFERENCES

1. Wehner, G. K. , and Anderson, G. S., Handbook of Thin Film Technology,

Chap. 3, L. I. Maissel and R. Glang, eds., McGraw Hill, 1970, pp. 3-1 to

3-38.

2. Miassel, L., Handbook of Thin Film Technology, Chap. A, L. I. Maissel

and R. Glang, eds., McGraw Hill, 1970, pp. 4-1 to 4-44.

3. Carter, G., and Colligow, N. S., Ion Bombardment of Solids, Chap. 7,

Heinemann Educational, London, 1968, pp. 310-353.

4. Mattox, D. M. , Sputter Deposition and Ion Plating. American Vacuum

Society, 1973.

531
5. Brown, S. C, Introduction to Electrical Discharges in Gases, John

Wiley and Sons Inc., 1966.

6. Vossen, J. L. and O'Neill, J. J., RCA Review, Vol. 29, pp. 149-173,

1968.

7. Chopra, K. L., Thin Film Phenomena, McGraw-Hill, 1969.

8. Thornton, J. A., Journal of Vacuum Science and Technology, Vol. 11,

No. 4, pp. 666-670, 1974.

9. Notis, M. , High Frequency Heating Review, Vol. II, No. 8, Lepel High

Frequency Laboratories, Inc., pp. 4-6.

10. Spalvins, T., "Sputtering - A Vacuumed Deposition Method for Coating

Material," Paper No. 72-DE-37, presented at the Design Engineering Conference and Show of the ASME, May 8-11, 1972.

11. Spalvins, T., ASLE Proceedings - International Conference on Solid

Lubrication, American Society of Lubrication Engineers, 1971, pp. 360-

370.

12. Centner, K. , "Potential of Sputtered Films as Protective Coatings,"

NADC-75038-30, Naval Air Development Center, 1975 (N76-17251).

13. Shaw, B. J. and Miller, R. P., "Sputtering of Bone on Prosthesis,"

Patent 3,918,100, United States, Nov. 1975.

552
RF POTEm"IAL

Figure I. - Schematic of sputtering process.

335
High -voltage dc power supply (5kV)

Matching network

E

rf power

supply,

1 kW

Specimen

(cathode /

or ground)-^ Metal ring

(anode)

Top plate

—Target

"~~ —Rotatable shutter

^^Pyrex cylinder

-Base plate CS-60020

(a) Schematic diagram.

(b) View of apparatus during sputter coating. Figure 2. - Radiofrequency diode sputtering apparatus with direct-current bias.

33h
TARGET

-DIRECTIONS OF SPUTTERED ATOMS

—-BEARING CAGE

Figure 3. - Schematic of sputter coating complex surfaces.

RF INPUT POWER

CARRIER GAS PRESSURE

OPERATING PARAMETER

Figure 4. - Deposition rate as a function of various operating parameters.

335
Figure 5. - Fabrication of cylinc^ers by sputtering.

NO EVIDENCE OF

FAILURE AFTER 5.8xl05cYCl£S

o>-o

BURNISHED

FIIM RESIN BONDED

COMAAERCIAL

FILM

(130 000 A

THICK)

FILM APPLIED BY DC

SPUTTERING

(2000 A

THICK)

Figure 6. - Endurance lives of M0S2 films applied by various techniques.
Figure 7. - Endurance lives of 440C

stainless-steel ball bearings with sputtered M0S2 films on races and cage -

with and without a Cr3Si2 underlayer.

!

CS- 74111

Figure 8. - Hypodermic needles and protective housings with sputtered teflon.

557
MECHANICAL FAILURES PREVENTION GROUP

25th MEETING

Engineering Design

List of Registrants

A. Anderjaska

Federal Aviation Admin.

800 Independence Avenue, SW

Washington, DC 20591

J. Amess

Hogan and Hartson

Washington, DC 20006

B. P. Bardes

Bimba Manufacturing Co.

101 E. Main Street

Monee, IL 60449

W. J. Baxter

General Motors Research Lab

12 Mile and Mound Roads

Warren, MI 48090

D . K . Benson

Midwest Research Inst.

425 Volker Boulevard

Kansas City, MO 64110

E. P. Blyskal

Eastern Airlines

International Airport

Miami, FL 33148

A. Branum

DECdesign Marketing

200 Forest Street

Marlboro, MA 01752

L. G. Buhr

Hughes Aircraft Company

Bldg. 117, MS 1 P. 0. Box 90515

Los Angeles, CA 90009

H. C. Burnett

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

J. B. Cardoso

Aerospace Corporation

2350 E. El Segundo

El Segundo, CA 90245

0. Casarella

Washington Navy Yard

Naval Weapons Engineering Support

Act., Bldg. 219.2

Washington, DC 20374

J. B. Christian

Wright -Patterson Air Force Base

Lubricants and Tribology Branch

Dayton, OH 45433

A. A. Conte, Jr.

U.S. Naval Air Development Center

Code 30212

Warminster, PA 18974

R. A. Coulombe

Naval Ship Engineering Center

Code 6146, NC-4, Rm. 268

Washington, DC 20382

R. M. Davison

Naval Weapons Engineering Support

Act.

9205 W. Stayman Drive

Ellicott City, MD 21043

558
p. R. deBruyn

Industrial Liaison

National Bureau of Standards

Washington, DC 20234

0. Decker

Mechanical Technology, Inc.

968 Albany- Shaker Road

Latham, NY 12110

E . J . Deemer

Naval Air Systems Command Rep.

Atlantic

NAS Norfolk, Bldg. R-52 (ASCR 332)

Norfolk, VA 23511

M. G. Dhawan

Naval Ship Engineering Center

National Center 4, Code 6145F

Room 378

Washington, DC 20382

S. Edelman

National Bureau of Standards

B-324, Polymers Building

Washington, DC 20234

G. Ellis

Potomac Research, Inc.

7655 Old Springhouse Road

McLean, VA 22101

R. M. Evan-Iwanowski

Syracuse University

141 Link Hall

Syracuse, NY 13210

S . Flanagan

Federal Aviation Admin.

800 Independence Avenue, SW

Washington, DC 20591

P. M. Fleming

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

J. B. Fussell

University of Tennessee

Knoxville, TN 37916

W. W. Gunkel

General Motors Corporation

EAS-PA, GM Tech. Center

Warren, MI 48090

R. R. Hagglund

Worcester Polytechnic Inst.

Worcester, MA 01609

F. Hall

Washington Navy Yard

Naval Weapons Engr. Support Act.

Bldg. 219.2

Washington, DC 20374

P. Hal lick

Federal Aviation Admin.

800 Independence Avenue, SW

Washington, DC 20591

H. R. Hegner

CARD, Inc.

7449 North Natchez Avenue

Niles, IL 60648

M. Heller

519 Investment Building

Washington, DC 20005

P. Henry

Peoples Gas Light ^ Coke Co.

122 S. Michigan Avenue, Rin. 1725

Chicago, IL 60603

J. D. Hoffman

Director, Inst, for Materials Res.

National Bureau of Standards

Washington, DC 20234

S. M. Hsu

Amoco Chemical Corporation

Naperville, IL 60540

339
G. Hurt

Detroit Diesel

Allison Division

Indianapolis, IN 46201

R. B. Johnson

National Bureau of Standards

Institute for Materials Res.

Washington, DC 20234

G. Kardos

Carleton University

Ottawa, Canada KIS 5B6

W. D. Keeley

Vibration Specialty Corp.

100 Geiger Road

Philadelphia, PA 19115

M. P. Kelly

Dunegan/Endevco

Rancho Viejo Road

San Juan Capistrano, CA 92675

H. K. Keswani

Gulf Research ^ Development Co.

P. 0. Drawer 2038

Pittsburgh, PA 15230

E. E. Klaus

Pennsylvania State University

108 Fenske Laboratory

University Park, PA 16802

R. A. Kuehl

John Deere Plow § Planter Works

501 Third Avenue

Moline, IL 61265

R. R. Landers

Gould Ocean Systems Division

Cleveland, OH 44101

H. P. Lee

NASA

Goddard Space Flight Center

Greenbelt, MD 20771

P. Lewis

Mechanical Technology, Inc.

968 Albany- Shaker Road

Latham, NY 12110

R. E. Maringer

Battelle Columbus Labs

505 King Avenue

Columbus, OH 43201

J. H. A. Mason

British Embassy

Defence Research § Dev. Staff

3100 Massachusetts Avenue, NW

Washington, DC 20008

D. M. Mattox

Sandia Laboratories

Albuquerque, NM 87115

R. S. Miller

Office of Naval Research

Arlington, VA 22217

R. Misialek

U. S. Navy

6762 NAVSEC Phila. Div.

Philadelphia, PA 19112

E. Passaglia

National Bureau of Standards

Institute for Materials Research

Washington, DC 20234

R. Pepper

Fiber Materials, Inc.

Biddeford Industrial Park

Biddeford, ME 04005

J. Perso

Whirlpool Corporation

Monte Road

Benton Harbor, MI 49022

M. Peterson

Wear Sciences

32 Sutherland Drive

Scotia, NY 12302
A. G. K. Pong

Consumer Product Safety Comm.

5401 Westbard Avenue, Rm. 935

Bethesda, MD 20207

C. Portillo

John Deere Plow ^ Planter Works

501 Third Avenue

Moline, IL 61265

R. Predmore

Goddard Space Flight Center

Code 313

Greenbelt, MD 20771

M. Rayl

RCA Corporation

RCA Labs

Princeton, NJ 08540

H. Schaeffer

University of Maryland

College Park, m 20740

A. Schmudde

Caterpillar Tractor Co.

Industrial Division-Engr.

Peoria, IL 61629

J. Scialdone

Goddard Space Flight Center

Greenbelt, MD 20771

G. W. Scott

Oak Ridge National Laboratory

P.O. Box X, Bldg. 4500-S, Rm. Y-15

Oak Ridge, TN 37830

T. R. Shives

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

W. H. Skewis

Telcom Systems, Inc.

2300 South 9th Street

Arlington, VA 22204

L. Sibley

SKF Industries

King of Prussia, PA 19406

C. 0. Smith

University of Nebraska at Omaha

P. 0. Box 688

Omaha, NB 68101

J. R. Sodorquist

Federal Aviation Admin.

800 Independence Avenue, SW

Washington, DC 20591

T . Spalvins

NASA

Lewis Research Center

Cleveland, OH 44101

J. E. Stem

NASA, Goddard Space Flight Center

Code 721

Greenbelt, MD 20771

J. N. Strohlein

National Bureau of Standards

Building 304, Room 136

Washington, DC 20234

J. W. Taylor

David W. Taylor Naval Ship Research

and Development Center

Annapolis, MD 21402

W. A. Troha

U.S.A.F. (AFAPL/TBP)

Wright -Patterson AFB, OH 45433

C. E. Vest

NASA, Goddard Space Flight Center

Code 313

Greenbelt, MD 20771

W. A. Willard

National Bureau of Standards

Metallurgy Division

Washington, DC 20234

3hl
W. 0. Winer

School of Mechanical Engineering

Georgia Institute for Technology

Atlanta, GA 30332

H. Wolf

Grumman Aerospace Corporation

Bethpage, NY 11714

A. Wolff

T. R. Mailory and Company

N.W. Industrial Park

Burlington, MA 01803
MFPG PUBLICATIONS

Both printed and microfiche copies of the following MFPG publications

(1-11) whose catalog numbers start with "AD" or "COM" may be obtained

from the NTIS.

National Technical Information Center

5285 Port Royal Road

Springfield, VA 22161

1. Glossary of Terms AD 721 354

2. Proceedings of Meetings 1-9 (set of 5) AD 721 359

Meeting Nos. 1-5 Papers and discussion on failure

analysis and control.

"Detection, Diagnosis and Prognosis",

December 6, 1968.

"Failure Mechanisms as Identified

with Helicopter Transmissions",

March 27, 1969.

"Critical Failure Problem Areas in

the Aircraft Gas Turbine Engine",

June 25-26, 1969.

"Potential for Reduction of Mechanical

Failure Through Design Methodology",

November 5-6, 1969.

3. Proceedings of Meeting No. 10 AD 721 912

"Vibration Analysis Systems"

January 21-22, 1970.

4. Proceedings of Meeting No. 11 AD 724 475

"Failure Mechanisms: Fatigue"

April 7-8, 1970.

5. Proceedings of Meeting No. 12 AD 721 913

"Identification and Prevention of Mechanical

Failures in Internal Combustion Engines"

July 8-9, 1970.

Meeting No. 6

Meeting No. 7

Meeting No. 8

Meeting No. 9

3h3
6. Proceedings of Meeting No. 13 AD 724 637

"Standards as a Design Tool in Surface

Specification for Mechanical Components

and Structures"

October 19-20, 1970.

7. Proceedings of Meeting No. 14 AD 721 355

"Advances in Decision-Making Processes

in Detection, Diagnosis and Prognosis"

January 25-26, 1971.

8. Proceedings of Meeting No. 15 AD 725 200

"Failure Mechanisms: Corrosion"

April 14-15, 1971.

9. Proceedings of Meeting No. 16 AD 738 855

"Mechanical Failure Prevention Through

Lubricating Oil Analysis"

November 2-4, 1971.

10. Proceedings of Meeting No. 17 AD 750 411

"Effects of Environment Upon Mechanical

Failures, Mechanisms and Detection:

April 25-27, 1972.

11. Proceedings of Meeting No. 18 AD 772 082

"Detection, Diagnosis and Prognosis"

November 8-10, 1972.

12. Proceedings of Meeting No. 19 COM-74-50523

"The Role of Cavitation in Mechanical Failures"

October 31 - November 2, 1973.

Printed copies of the following MFPG publications (stock numbers

starting with SN) may be obtained from the Government Printing Office.

Superintendent of Documents

U.S. Government Printing Office

Washington, DC 20402

Microfiche copies of these publications (catalog numbers starting with

NBS-SP) may be obtained from NTIS.

13. Proceedings of Meeting No. 20 SN003-003-01451-6

"Mechanical Failure - Definition of the Problem"

May 8-10, 1974.

Cost: Printed copy $6.10

3hk
14. Proceedings of Meeting No. 21

"Success By Design: Progress Through

Failure Analysis"

November 7-8, 1974.

Cost: Printed copy $3.50 SN003-003-01639-0

Microfiche copy $2.25 NBS-SP-433

15. Proceedings of Meeting No. 22

"Detection, Diagnosis and Prognosis"

April 23-25, 1975.

Cost: Printed copy $4.25 SN003-003-01 556-3

Microfiche copy $2.25 NBS-SP-436

16. Proceedings of Meeting No. 23

"The Role of Coatings in the Prevention

of Mechanical Failures"

October 29-31, 1975.

Cost: Printed copy $2.65 SN003-003-01 664-1

17. Proceedings of Meeting No. 24

"Prevention of Failures in Coal

Conversion Systems"

April 21-23, 1976.

Cost: Printed copy $3.00 SN003-003-01 760-4

5U5
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APPENDIX

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I
NATIONAL MATERIALS POLICE IN THE 95th CONGRESS

CURRENT AND POTENTIAL GOVERNMENT IMPACTS ON AVAILABILITY

Franklin P. Huddle

Congressional Research Service

Library of Congress

Washington, D. C. 20003

An interesting and constructive circumstance about national materials

policy as well as the broader subject of national science and technology policy, is that it is very largely bipartisan. We are fortunate

that in both Houses of Congress and in the relationship between the

legislative and executive branches of our national Government, there is a healthy consensus on these matters. Thus, in the elections that took

place this week, the question relevant for this meeting was not over

issues of materials or science policy, but whether the winners in the

various campaigns will be motivated to preserve this same bipartisanship, and will be equipped to deal knowledgeably with the subjects in

question.

As far back as 1970, the National Materials Policy Act setting up the

National Commission on Materials Policy, was jointly supported by seven Democrats and five Republicans, including the Chairman and the Ranking

Minority Member of the Senate Subcommittee that brought the legislation

into being. There was no opposition to the bill in either House of

Congress.

Similar bipartisanship has characterized other materials and science

policy legislation. The National Commission on Supplies and Shortages

was created in response to the joint initiative of the Senate Majority

and Minority Leaders, Senators Mansfield and Scott. The four legislative members of the Commission are a Democrat and a Republican from

each House of Congress.

Also, the National Science and Technology Policy, Organization, and

Priorities Act of 1976, approved by the President last May 11, was

originally introduced by Chairman Teague and Ranking Minority Member

Mbsher, of the House Committee on Science and Technology. Once

differences in legislative approach had been ironed out between the

House and Senate versions, there was no opposition to the measure and

the White House accepted it with ceremony.

Shortly thereafter, on last June 17, a bill to establish a national

materials policy was offered in the House of Representatives as H.R.

14439, under the sponsorship of James Symington and Charles Mosher,

Chairman and Ranking Minority Member, respectively, of the House Subcommittee on Science, Research, and Technology.

3h9
For one last illustration o£ this healthy bipartisan consensus, I offer the example of the Office of Technology Assessment, whose Board

consists of three Democrats and three Republicans from each House of

Congress, and whose studies are responsive to the interests of congressional Members of both political parties.

I suggest that it is beneficial to the scientific, the technological,

and the materials communities of the United States to cherish and

preserve this state of affairs. We should also recognize- -as did the

National Commission on Materials Policy- -the need to deal with policy

for materials, energy, and environment as one interlocked and interconnected subject. Similarly, we should deal with the global flow of

materials and scientific knowledge as a subject of shared interest of

all nations, a matter of inescapable interdependence. And thirdly, as the authors of the Constitution of the United States proclaimed in the

Preamble, it is necessary to strike a balance between the demands of

the present and the needs of the future- -to serve both "ourselves and

our posterity." These should not be partisan matters, and there is

evidence to show that they have not been.

Then, what ought to be the role of the industrial community, the

materials community, and the science-and-technology community with

respect to national policy for materials, energy, and environment? I suggest two broad areas of activity in which you, as members of one or more of these three communities, can best serve.

First, you can help to guide the Congress and the President in the

effective inclementation of important legislation passed by the 94th

Congress, whose term of office ends at the close of 1976.

Second, you can help to guide the Congress to design future legislation

and action programs in the remaining unfinished business of the 94th

Congress which is likely to become the priority business of the 95th

Congress, beginning next January.

Inplementing Policies Promulgated by the 94th Congress

Among the actions of Congress up to the close of the 94th Congress are several of great importance to the materials community.

First, there is the Act that created the National Commission on Supplies

and Shortages, whose final report is expected by the close of 1976.

The Act calls for advice to the Congress on means to anticipate and

deal with future important shortages of industrial materials and agricultural products. What actions this Commission will propose, and how

the Congress will respond to the recommendations will not be known until

1977, but should warrant the careful scrutiny of the materials community.

350
The scope of the Commission's report is expected to encompass the

management and analysis of supply and demand information, stockpiling

decisions, and materials conservation programs.

Second, in Title III of the National Science and Technology Policy,

Organization, and Priorities Act of 1976, approved May 11, a "President's Committee on Science and Technology" is created and instructed

to report in two years on 13 policy questions. These include:

government organization for science and technology, the management of

scientific information, technology assessment, the relations of the

Federal Government with the States and private industry, elimination

of red tape obstructing innovation, support for basic research and

education, manpower training, the strengthening of community uses of

science, and long range plans for the management and use of science and

technology. It will be of interest to this audience to learn how the

President's Committee proposes to resolve these important policy

questions, and how the Congress responds to its recommendations.

Three other subjects decided by the 94th Congress will be of interest

next year and thereafter. One is the inplementation of the Toxic

Substances Control Act, and another is the administration of the new

Resource Conservation and Recovery Act of 1976, approved October 22.

Both of these two pieces of legislation are so conplex and broad in

scope that all I can do here is commend them to your careful study.

How they are carried out will have much to do with the successful

response of industry to the policies these Acts prescribe.

Finally, one legislative decision was reached by non-action. That was

the response of the Congress to the earlier proposal of the President

to liquidate most of the remaining reserves in the National Stockpile

of Strategic and Critical Materials. For several years and more, the

Congress had deferred action on this matter. Finally, last October 1,

the Federal Preparedness Agency, which now has jurisdiction over stockpile planning and management, announced the results of an extended

study of stockpile objectives. According to a press release from FPA,

the new objectives policy was recommended by the National Security

Council and approved by the President. There were three changes:

1. Planning will be based on the first three years of emergency of

indefinite duration, conpared /with/ one year under the previous

guidelines.

2. The civilian portion of the economy will be provided for after some reasonable allowances for "belt tightening." Previous policy did

not separately consider civilian needs.

3. For each year used in planning, stockpile needs will be estimated

separately for defense and civilian requirements.

351
In addition, Director Bray of the Agency promised that goals and

policies would be reviewed at least every four years, or sooner if

required

.

The issue remaining to be examined in 1977 is whether this policy

modification in the Strategic and Critical Materials Stockpile is

sufficient to meet future national industrial needs, or whether a national economic stockpile or international buffer stocks should also

be established. The first alternative has been reported on by the

Office of Technology Assessment, and will also be discussed in the

final report of the National Commission on Supplies and Shortages. The

second alternative was suggested by the Secretary of State, Dr.

Kissinger, in his major address to the United Nations General Assembly

in its special session in September 1975.

Unfinished Congressional Business in Materials for 1977

I have already mentioned the prospect that the next Congress --the 95th--

will be called on to implement proposals of the National Commission on Supplies and Shortages, and the science proposals of the President's

Science Advisory Committee. The Congress will also be concerned with

monitoring the execution of other materials and science legislation,

including the several important measures adopted in 1976.

What may be the most important item of unfinished business of interest

to the materials community is the bill proposed in the House as H.R.

14439 and introduced separately in the Senate by Frank E. Moss of Utah

as S.3637. Title I of this bill sets forth a proposed national policy

for materials. Title II would create a principal presidential adviser

on materials and a policy board. To coordinate implementation of

national policy there would also be created a commission representing

principal Federal agencies with existing functions in materials management or materials research. Paralleling the commission in scope would

be two congressional committees on national materials policy. The

purpose in introducing this legislation was explained by Congressman

James Symington of Missouri, last June 17. It was, he said, "designed

to promote some serious thinking about where we go from here with the

remainder of Earth's resources. What are we going to do with the stuff

which still remains accessible to us? How can we best use it?" The

bill was intended, he went on to say, "primarily for the purpose of

stimulating discussion on what is probably the most widespread and

serious problem facing modem civilization- -lack of any cohesive

policy to govern, or at least to guide, the use of materials." Such

a policy was central to any modem technological society, a prime

source of opportunity for energy conservation, and a basis for achieving

environment quality.

352
At the request o£ the House Committee on Science and Technology I have

been inviting and collecting views of informed persons in the materials

community on this proposed legislation, and would welcome further

contributions of opinions from this source. I should add that last

week I had occasion to discuss the subject with a senior executive of

one of the larger companies in the field of plastics, who suggested

that perhaps he had been too harsh in his comments, and that for that

reason they might not be welcomed. I assured him that the purpose in

sending the bill out for criticism was to invite it, and that compliments were of no particular value. What the Committee is looking for

is a consensus in which all can share.

Incidentally, I received another letter from an industrial society

declining to comment on the bill, and explaining that such comment was

considered premature. My reaction to that statement is that the best

time to comment on legislative proposals is in the early stages of their

formulation, instead of waiting until they are, so to speak, frozen in

concrete. Comments are most useful in the earliest stages. If you

wait until the end of the line, you may find yourself in the position

of "take it or leave it" without any real opportunity to help shape the

final product.

A number of materials policy issues have been under study for the past

two years or so in the Office of Technology Assessment, and are the

subject of reports either issued or about to be. These are (1)

Economic Stockpiling, (2) the Management of Materials Information, and

(3) Access to Minerals in the Ground. All three subjects are likely to

become of interest to the 95th Congress.

In addition, the O.T.A. has been planning or starting up other studies

to meet congressional committee requests in such additional subjects as the following:

Resource Recovery, Recycling, and Re -Use

Conservation through Reduced Wastage

Mineral Exploration Technologies

Alternative Responses to Materials Shortages

Substitution Alternatives for Critical

Imported Materials

Existing Federal Coal Development

Conservation of Energy through Materials

Management

These, too, are likely to become of interest to the 95th Congress, in

the years 1977-78.

555
The Legislative Situation in 1977

As I suggested at the outset of this statement, there are some difficulties in forecasting what the interest of the Congress is likely to

be in 1977. This week, you and I went to the polls to vote for a President and a Vice President, one of the 435 Members of the House of

Representatives, and perhaps also one Member of the roughly one- third

of the Senate selected by the voters in 1976.

The 95th Congress will be very different from the 94th, particularly in

its leadership. Many Members of both Houses who have taken a leading

role in developing national materials policy did not run for reelection.

These included Senators Mansfield and Scott, who were responsible for

the initiative that led to the National Commission on Supplies and

Shortages; House Speaker Carl Albert, who joined with Mansfield and

Scott in urging the President to work with the Congress on national

materials policy; Congressmen James Symington and Charles Mosher, the

sponsors of the national materials policy bill; and a number of other

very inportant Members.

A number of other members of the 94th Congress were defeated at the

polls, including Senators Tunney and Brock, both members of the National

Commission on Supplies and Shortages, and Senator Frank Moss, who

sponsored S. 3637, the proposed national materials policy act, in the

Senate

.

Thus, when the new Congress is convened in January, there will be a number of new faces. The opportunity- -indeed, the necessity- -will be

at hand for the materials community to help build a new national

consensus on materials policy.

In the 94th Congress upwards of 500 separate legislative proposals were

introduced that involved national materials policy. Surely this was

some kind of a record! But will the 95th Congress have the same recognition of the significance of materials for economic health and progress, energy conservation, and environmental quality? I suggest that

the time will soon be at hand for the materials community in the United

States to help the Congress to assess the importance of national

materials policy. What are the elements of this policy that the

materials community would wish the legislature to promulgate and

implement? How is the national wellbeing, with respect to materials

policy, to be determined and acted upon?

It is the constitutional function of the Congress to shape the national

policy on important questions like these. But it is the constitutional

function of you, the electorate and the informed technical community,

to advise the Congress wisely on these matters.

35^
What mechanisms and means are available to you to exercise this con- stitutional function? I have observed quite a number of these. As a matter of fact, a very useful and proper relationship can be built up

between the Congress of the United States and the scientific/technical

community. Among the appropriate mechanisms that characterize this

relationship are -

- The appointment of scientific or technological interns to serve as voluntary staff members in congressional offices.

- The organization of legislative liaison committees to entertain

requests for technical information and advice, and to channel such

requests to qualified respondents within the technical society.

- The organization of ad hoc committees to assess and evaluate

particular legislative proposals.

- The acquisition and dissemination of congressional literature of

particular interest to the members of the technical society.

- Establish liaison with the four policy advisory services to the

Congress (the Congressional Research Service, the General Accounting

Office, the Office of Technology Assessment, and the Congressional

Budget Office) and provide these services with resumes of technical

people best qualified to provide advice on special categories of

technological subjects, problems, and issues.

It is important to note that the legislative function generates an insatiable demand for scientific and technological information and

advice. The advisory offices to the Congress are constantly called on

to satisfy this demand, manifestly, we cannot do this without recruiting

a great staff of experts in all imaginable fields. So the alternative-

which we follow- -is to establish ourselves as a bridge between the

Congress and the various technical communities. That is why I am here

tonight, and it is the essence of my message to you. In sum, the

Congress needs your help, and I ask you to supply it.

555
TESTING FOR SUCCESS

John C. New

National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Maryland

In 1959 the Goddard Space Flight Center was created as

a major new space flight center of NASA. It was franchised

to oversee the design, development, testing and flight of

unmanned spacecraft. It was my privilege to be a participant in the creation, construction, and operation of a new

test facility for providing assurance that these spacecraft

would be successful in space. It is the purpose of this

discussion to review the fifteen year record of the test and

evaluation program developed at the Center.

On the earth, we deal with equipment and materials

which cost a few dollars per pound. The costs of space

hardware are upwards of $15,000 - $50,000 per pound,

delivered into space. The complexity of this space material

can be likened to insertion of a scanning electron microscope into space orbit, operating it by remote control and

then transmitting its data back to ground. Generating

confidence, then, that these types of complex systems will

function reliably after the launch and in the space

environment, is the purpose of the test facility and test

program at Goddard Space Flight Center.

In the early sixties, the objectives of NASA were to

explore space; gain scientific data on the space environment, planetary and astral bodies; develop applications and

instruments for communications and weather observations;

survey and observe earth resources; place man in space and

return him safely. This period will be remembered as one

when the technical supremacy of the United States was

being challenged. The technological emphasis was on

achievement and the avoidance of failure. The eyes of man

world-wide were focused alternately upon the US and USSR

space programs as they raced for a moon landing. This

paper will deal only with the unmanned space program; the

manned program deals with problems of a different order of

magnitude

.

Some of the different types of spacecraft over which

Goddard Space Flight Center has cognizance cover the range

556
from the initial U.S. spacecraft Vanguard, a relatively

simple system, to the highly sophisticated scientific and

applications satellites such as the Orbiting Solar

Observatories and the LANDSAT (Earth Resources Technology

Satellites) . Also included are such spacecraft as Echo,

OGO, OAO, TIROS, NIMBUS, SYNCOM, and several foreign

satellites. The information covered in this paper was

derived from 57 missions. Some of these missions investigated energetic particles, cosmic rays, sun corona, solar

energy, radiation belts, weather phenomena and communication

techniques. Most of the spacecraft carried a large number

of scientific or application types of experiments.

Figure 1 is a typical spacecraft. It is actually one

of the early Explorer type spacecraft. The figure shows

distribution and the subsystems complement of the spacecraft. In the early Vanguard days launching an eight

pound spacecraft was considered a problem. Today we are

still concerned with the weight of spacecraft, but we are

now dealing with thousands of pounds, orders of magnitude

greater than the early days. A typical unmanned spacecraft

may weigh from a few hundred pounds to a few thousand

pounds. The largest, heaviest unmanned spacecraft weighed

4600 pounds. This is the Orbiting Astronomical Observatory,

with a one meter telescope. It is still in space collecting

data that is unavailable from earth because of the ultraviolet light absorption by the earth's atmosphere.

Figure 2 shows the various steps in the spacecraft

development cycle from the spacecraft concept through the

dissemination of the scientific information derived from

the spacecraft experiments. The length of time in each

step varies with different spacecraft but the overall time

is typically about two years. Many of the steps are

performed concurrently, in parallel, in order to meet

demanding and critical schedules. In some cases, the

scientific information being sought is tied to a planetary event to which the launch schedule is timed. Missing

the schedule means delaying the launch for the next

planetary cycle, a delay of many months or even years.

Insofar as the test programs are concerned, there is

generally ample time to plan and replan, but little time

to execute them. This is due to the fact that more time is

generally used for the concept, design and development

than is allotted for these stages. Consequently the test

program period is squeezed between the fixed launch date

and the increasing time demands of the development stage.

Product assurance requires an organization which has

the test equipment and trained personnel to generate

357
techniques and develop a facility that will execute the

evaluation program and do it correctly the first time. The

invested cost of a spacecraft, at the time it is being

tested, is large several millions of dollars. For the

Orbiting Astronomical Observatory, it was about 60 million

dollars. This did not include the costs of the scientists'

time and their years of research up to this point, it was

just the labor and hardware costs to produce the spacecraft

system. One thinks twice about rehandling and retesting a massive device that has this kind of invested cost. Also

it becomes more obvious why the tests should be done

correctly the first time when one considers that delays in

schedules at this point just prior to launch are

very costly (several thousands of dollars per hour) since

project manpower loading costs are at their peak.

The various operating conditions and environments,

natural and induced, to which a spacecraft is exposed are

shown in Figure 3. The unmanned spacecraft test program

is concerned with the environmental conditions of prelaunch,

powered launch, and orbital spaceflight. The test

philosophy developed by and insisted upon by the Test and

Evaluation Division at Goddard Space Flight Center was to

test the flight spacecraft, the one-of-a-kind unit, at test

levels that were expected to occur during launch and in

orbit; and to test the complete system in it's flight

assembled condition. This test approach was unique in the

formative days of the space program and it generated

controversy and debate before it was accepted. This

philosophy and practice was pursued at Goddard Space Flight

Center from it's inception and has resulted in a success

rate of 97% for 57 spacecraft for which GSFC was responsible. For the 33 spacecraft which passed through GSFC '

s

Test and Evaluation Division facilities and personnel over

the 15 year review period, no failures were experienced.

A 100% mission success rate was achieved. This is not to

say that failures did not occur. They did both in ground

tests and in space flight. On the ground they were fixed;

in space flight, redundancy and software work-arounds

permitted mission success to be achieved.

This is a record of which the Test and Evaluation

Division is extremely proud. It was accomplished by use of

excellent facilities, a positive and highly supportive

attitude by Center and NASA management and the dedication

of the Test and Evaluation technical staff.

Our system test program has several objectives. These

include proving the hardware design and applications,

eliminating weak links, locating and identifying subsystem

358
interactions, training launch personnel and developing

design guidance.

It has been the experience at GSFC, as it pre' ably

is and has been elsewhere in the engineering worlaV that

engineering design is a process of successive approximations, a series of iterations converging toward a practical

optimum. It is exceedingly rare to find a complex

mechanism which works perfectly the first time. New devices

are developed through use and through the correction of

known malfunctions. In space projects there is no way to

correct mistakes once the spacecraft is launched; there is

no way to measure what may malfunction or fail. If one

knew where to locate the failure detector, the design

could have been corrected so it would not have failed in the

first place. Very little direct failure information has

been brought back from space. Generally, failures in space

have been deduced from the telemetered data indirectly

related to the failure itself.

The reliability of spacecraft c.nd assurance of flight

success was found in environmental testing, where the

spacecraft was exposed to conditions simulating those that

were expected during launch and in space. General

Environmental Test Specifications were developed and became

the "design criteria" for the spacecraft project. The

weak links in the system were discovered and eliminated.

It was found that there was considerable interaction between

various subsystems including electronics, mechanical,

control, telemetry and power subsystems. The interactions

were identified; their effects were evaluated; and the

systems were modified to remove or correct those interactions which interferred with the satisfactory functioning

of the spacecraft system. Teams of people were trained with

the actual flight hardware, so that they were familiar with

the spacecraft functions and operations. These teams

could then satisfactorily recognize and respond to launch

or flight emergencies through use of redundant systems

or software reprogramming. Out of this space flight

experience design-guide-concepts were developed and fed back

into the later programs most often through a revised

General Environmental Test Specification.

Based on the launch environment, which is measurable

and predictable, a philosophy for flight units was

adopted in which the test severity was set at a level of

two sigma above the mean. At that level the probability

of missing a failure is about five percent. The overall

record is somewhat better than that. For prototype units.

559
the test severity was increased to 150 percent of the flight

test levels, which, statistically, is about the three sigma

level. This choice of prototype test level was more

fortuitous than scientific, a kind of engineering factor of

ignorance based on instinct tempered with past experience.

Figure 4 graphically describes the test level selections.

While the time spent in the launch environment lasts

for only a few minutes, the time in the orbital

environment can extend over a period of months or years.

Based on spacecraft system failures that can be expected

in orbit, a test philosophy was developed whereby infant

mortality in space was virtually eliminated. This was

achieved by exercising the spacecraft on the ground for a sufficient length of time so that the early failures

were discovered on the ground. The failure was analyzed;

corrections and modifications were incorporated in the

design and the failure mechanisms were eliminated. Figure 5

shows the effects of the test program and space environments

on the occurrence of failures in spacecraft systems.

The environmental test program centered on simulating

the space environment on the ground, exercising the spacecraft systems long enough under space environment conditions

until the infant mortality was virtually eliminated and

failure rates were reduced to an acceptable level. Studies

are being pursued regarding the length of time a spacecraft

system should be operated in a thermal-vacuum chamber (space

environment simulator) . The present thinking is that thirty

days would be an ideal exposure. However, the economics of

testing must be considered. A balance must be made

between the costs of testing and the costs of launching a

spacecraft with high confidence that a minimum failure rate

has been achieved.

One of the problem areas in spacecraft systems is the

heat transfer in the space environment. The normal gas

pressure environment internal to the spacecraft is less than

10-4 torr. This precludes convective heat transfer and

we must depend upon conductive and radiative processes.

Figure 6 shows a typical electronic part failure caused by

poor heat transfer.

Although there was radiative heat transfer between the

electronic part and its environment, the adequacy of the

thermal design depended upon heat conduction to the base

plate or heat sink. Poor thermal contact between the

part and heat sink caused the temperature of the part to

exceed safe limits and a failure occurred. It is difficult

to insure good contact between mating surfaces. This is a

360
function of the contact area, the physical and chemical

character of the contact surface, the bearing or contact

stresses, the nature of the base material, the torque

applied to the fastenings. Add to this the people

problems in quality control and you find that it is

necessary to environmentally test the design on the ground

and determine whether a space failure is likely.

Let us now look at a unique type of problem that

occurs in spacecraft components, A loose metal particle,

generated during part manufacture or during environmental

test, migrated and shorted out two adjacent leads. Now

this is not a big bother in a television set on the ground.

One can use the standard terrestrial solution, kick the set,

and the particle will generally move out of the way. The

gravity field does our work. However, in a zero gravity

field, the particle can float around, under small

spacecraft disturbances and maneuvers, creating opportunities to cause frequent malfunctions.

Various types of spacecraft subsystem problems that

were uncovered by environmental testing are shown in Figure

7. A problem malfunction or failure is defined as

a performance which does not meet or surpass functional or

operational specifications. The P and F stand for the

Prototype or Flight test level severity. It is quite

evident that the test program had isolated many problems

that needed correction and that a marked reduction in

problems has occurred when the modified and improved flight

units were retested. It also shows that the largest source

of problems are due to the exposure of the subsystems to

thermal-vacuum environment,

A flight subsystem which has had more than its fair

share of problems is the tape recorder. The typical flight

tape recorder is a device made up of all kinds of materials metals, polymers, organic and inorganic components

of wide variety, and is a composite of electrical,

electronic and mechanical elements. It is expected to have

a useful life, for as long as five years, during which time

it must faultlessly record and play back data acquired from

flight experiments. A typical recorder records at one

speed and plays back at a speed about 100 times the

recording speed. With such stringent demands upon the tape

recorder, we find that design improvements must be made to

overcome the failures observed in space and in testing on

the ground.

The tape is the source of failure due to wear and

load; bearings are a source of trouble due to lubrication

561
problems; lubricants can generate contamination problems

the recording play back head suffers wear degradation.

Environmental and life testing has helped solve some of

these tape recorder problems. Continued design changes and

life testing are the bases for improving this necessary

device for spacecraft flight systems.

Figure 8 shows how well we have done in space. We

find that about 50% of the first month's problems show up on

the first day. It is conjectured that the causes for this

high percentage of first day problems is due to the first

exposure to a weightless condition, not to the launch

environment. The weightless condition is conducive to the

movement of loose particles. These particles have the

opportunity to upset electrical and electronic circuitry as

previously indicated, and to interfere with the functioning

of mechanical elements by lodging in bearings, joints, etc.

Upon reaching orbit a space observatory consisting of

several hundred thousands of parts is first turned on after

experiencing weightlessness, energetic particle bombardment,

outgassing pressure changes, and radiative heat transfer

conditions. Is it surprising that 50% of the malfunctions

occur on this first day? The other 50% of the problems

are spread over the remaining days of the month.

The success of our test program is indicated in Figure

9. The comparison between the malfunctions uncovered

during system test and those subsequently occurring in

flight indicate the effectiveness of our test efforts. A

success rate of 97% has been achieved. We do not get all

the malfunctions the test screen is not 100% effective.

However, it is interesting to note that between the system

test malfunctions and the flight malfunctions there is a

reduction of 10:1 on all devices, 12:1 on electrical

devices; 13:1 on electromechanical .devices

.

Based on our successful test program experience, it is

felt that the concept of full systems test at expected

levels has been proven, especially for the space program

where the quantities we deal with are not large number

production units but single systems, one of a kind type,

of a research and development nature

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tS-114A (REV. 7-73)

U.S. DEPT. OF COMM. 1 . PUB L IC ATION OR R E PORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No. SHEET NBS SP 487

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4. TITLE AND SUBTITLE

Engineering Design- -Proceedings o£ the 25th. ^feeting o£ the

Mechanical Failures Prevention Group

5. Publication Date August 1977

6. Performing Organization Code

EDITORS

T. Robert Shives and William A. Willard

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Library of Congress Catalog Card Number : 77-21810

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) These Proceedings consist of a group of twenty-three submitted papers from the 25th

meeting of the Mechanical Failures Prevention Group which was held at the National

Bureau of Standards in Gaithersburg , Maryland on November 3-5, 1976. The central

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Coating materials; composite materials; design; failure avoidance; failure prevention;

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