A111D3 0S77ME

NBS SPECIAL PUBLICATION 433

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

Natio'nai" Bufeau o! "Slandafds

library. Admla. BIdg.

OCT 1 1981

191033

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100

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NO. 43:

1976

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Success by Design:

Progress Through

Failure Analysis

MFPG

21st Meeting
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901.

The Bureau's overall goal is to strengthen and advance the Nation's science and technology

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NATIONAL BUREAU ^^

OF STANDARDS AiAJNDARDS

MFPG juCtTSm

Success by Design: qLtot

Progress Through Failure Analysis

-^^'^

Proceedings of the 21st Meeting of the

Mechanical Failures Prevention Group,

held at the National Bureau of Standards,

Gaithersburg, Maryland, November 7-8, 1974

Edited by

T. R. Shives and W. A. WiUard

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

The 2lsl ineoling of IVIFPG and these proceedings were

sponsored by ihe Inslitule for Materials Research of the

INalional Mureuii of Standards, Washington, D.C. 20234;

the Office <»f [Naval Research, Department of the INavy, Arlington. \ a. 22217; the I'Vankford Arsenal, U.S. Army,

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uikI the federal Aviation Administration, Department

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

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

U5 NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued July 1976
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Mechanical Failures Prevention Gro.up. Success by design.

(NBS special publication; 433) At head of title: MFPG.

"Sponsored by the Institute for Materials Research of the National Bureau of Standards ... [et al.]" Supt.ofDocs.no.: C13. 10:433

1. Engineering design-Congresses. 2. Metals--Fracture-Congresses.

3. Reliability (Engineering)--Congresses. I. Shives, T. R. II. Willard,

William A. III. United States. National Bureau of Standards. Institute

for Materials Research. IV. Title. V. Series: United States. National Bureau of Standards. Special publication ; 433. QC100.U57no.433 [TA174] 602'. Is [620'.004'sl 76-608132

National Bureau of Standards Special Publication 433

Nat. Bur. Stand. (U.S.), Spec. Publ. 433, 229 pages (July 1976)

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FOREWORD

The 21 SL meeting of the Mechanical Failures Prevention Group was held

November 7-8, 1974, at the National Bureau of Standards in Gai thersburg

,

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

the chairmanship of Mr. Robert E. Maringer of Battelle, Columbus Labs.

The Design committee, the session chairmen, and particularly the

speakers are to be commended for the program.

The papers in these Proceedings are presented as submitted by the

authors on camera ready copy, except for some minor editorial changes.

In addition to the papers, the Proceedings include the discussions of

the talks and a panel discussion. The discussions were recorded at the

meeting and have been edited to improve readability.

Appreciation is extended to Mr. T. Robert Shives and Mr. William A.

Willard of the NBS Metallurgy Division for their editing, organization,

and preparation of the Proceedings, to Mrs. Sara R. Torrence of the NBS

Office of Information Activities for the meeting arrangements, to

Mr. Harry C. Burnett of the Metallurgy Division for general coordination

and registration, to Mr. Ronald B. Johnson of the NBS Institute for

Materials Research and Mr. Paul M. Fleming of the Metallurgy Division

for handling financial matters, and to the entire staffs of the

Metallurgy Division and Institute for Materials Research for their

assistance in many ways. Special thanks are accorded to Mrs. Marian L.

Slusser and Mrs. Bronny Webb of the Metallurgy Division for their

diligent efforts in transcribing the recorded discussions.

EL 10 PASSAGE lA

Executive Secretary, MFPG

Chief, Metallurgy Division

National Bureau of Standards

III
TABLE OF CONTENTS

Pa^e

FOREWORD III

SESSION I: DESIGN PHILOSOPHY

1. Creative Student Engineering Design. Paul S. De Jong 3

(Presented by C. 0. Smith)

2. The Designer's Contribution in the Development of New 7

Products. James J. Lesko

SESSION II: CASE STUDIES

1. Successful Redesign and Launch Performance of the 19

ERTS/Nimbus Adapter after an Acceleration Test Failure.

Victor T. Sweet* and W. Brian Keegan

2. Agricultural Equipment for Underdeveloped Countries. 74

Charles W. Suggs

3. Safety Success through Design. David W. Logan 83

4. Reliability, Pollutants and Aluminum Raw Material. 89

Vaughn Dean Matney

5. Rotor Burst Protection Program. Guy J. Mangano 103

6. Airframe "Crashworthiness" Experiments. Nelson N. 122

Shapter

7. Redesign and Assembly of Anti-friction Bearing Housings 131

for Improved Life. G. D. Xistris and D. C. Watson*

8. Case History of Failures in a Hammer Mill. J. K. L. 144

Bajaj

IV
9. Solution of an Art Restoration Problem. Elio Passaglia 159

10. Elimination of Failures of U-bolts in Farm Tractor Dual 164

Wheels. Bruce P. Bardes

n. Structural In-flight Wing Failures. Michael L. Marx 166

12. Use of Cases in Engineering Education

a. Use of Cases in Engineering Education--Their 180

Special Value. Henry 0. Fuchs

b. What Can be Learned from Cases. Geza Kardos 183

c. Student Written Design Case Studies. C. 0. Smith 188

d. Teacher Feedback on the Use of Engineering Case 192

Studies. Byron J. Pelan

e. Panel Discussion 199

SESSION III: OVERSIGHTS AND OVERVIEW

1. Some Thermal Problems in the Design of Fluid Film 205

Bearings. Dudley D. Fuller (Presented by F. F. Ling)

2. Never Overlook Notches. C. 0. Smith 212

3. Innovation or Reliability. Henry 0. Fuchs 214

4. The "Model" Designer. R. E. Maringer 216

5. The Misuse of Tensile Strength as a Design Parameter. 218

Thomas J. Dolan

6. Inspection Consideration at the Design Stage. 222

Letter from John F. Erthal (Read by A. Wolff)

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

LIST OF REGISTRANTS FOR THE 21st MFPG MEETING 223

V
The following registrants gave presentations at the Symposium, but did

not submit a manuscript for publication:

Bryan R. Noton, "Design by Goals", Session I

John C. New, "Test Philosophy for Unmanned Spacecraft", Session I

Alan H. Schoem, "View of the Consumer Product Safety Commission",

Session III

VI
ABSTRACT

These Proceedings consist of a group of twenty two submitted

papers and discussions from the 21st meeting of the Mechanical Failures

Prevention Group which was held at the National Bureau of Standards in

Gai thersburg, Maryland on November 7-8, 1974. The central theme of

the Proceedings was improvement in design through failure analysis.

Emphasis was on design philosophy, the use of failure analysis case

studies as an educational tool, successful redesign through failure

analysis, and design oversights.

Key words : Design; engineering education; failure analysis; failure

analysis case histories; failure prevention; reliability; safety.

UNITS AND SYMBOLS

Customary United States units and symbols appear in many of the

papers in these Proceedings. The participants in the 21st 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."

E380-74 ASTM Metric Practice Guide (American National Standard Z210.1).

Disclaimer:

Certain trade names and company 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

DESIGN

PHILOSOPHY

Chairmen: R.E.Maringer

Battelle Memorial Institute

D.D.Fuller

Columbia University

CREATIVE STUDENT ENGINEERING DESIGN

Paul S. De Jong

Iowa State University

Ames, Iowa 5OOIO

The Creative Engineering Design Display is an activity of the Engineering Design Graphics Division of ASEE, The growth of this activity

in this particular division is the result of several causes, any one of

which might be debated by someone. Members of the division might argue

that it grew out of an attempt to re-emphasize the design aspects of

engineering in an otherwise science-heavy curriculum, A few might counter that it was an attempt to pump some life into what they consider

dull graphics courses.

Seriously, however, Alfred Whitehead advocated that universities

and their courses could not exist simply to disseminate information,

but had to explore the possibilities and implications of that knowledge.

Many individuals in the division felt that their drawing courses of the

mid-sixties had to be altered to meet this criterion. Since engineering

graphics is a language for conveying ideas, it was concluded that it

could be naturally combined with a course in basic conceptual engineering

design. The concept of open-ended problems had been established earlier,

and it seemed to be only a short step to consider problem-solving methods.

By doing this, several contributions might be realized. Students would

gain an appreciation for the role of engineering graphics in their

careers and for the courses included in various curricula. Students

would also have an opportunity to fulfill their desire to be creative,

which is probably why they selected engineering in the first place.

Finally, this early experience would motivate and tend to reduce attrition among promising students.

In 1967 1 division chairman Edward Jacunski appointed Percy Hill to

develop a summer school in creative design in conjunction with the 196?

ASEE conference at Michigan State University, Many people, too numerous

to mention, contributed to the success of that school, but Henry Fuchs

and Peter Bulkely should be recognized for their invaluable work. Many

schools were represented and incorporated these concepts into their

courses soon after that summer school. Professor Jacunski 's successor

as division chairman. Gene Pare, felt that the results of that summer

school ought to be displayed, and created the committee establishing

the first creative engineering design display for the I968 ASEE conference held at UCLA, The society accepted the display and its basic concepts immediately and enthusiastically, and it has been a major event at

every conference since. Judges from educational and industrial sectors

are invited to determine the winning entries. They are enthusiastic

about the display and are eager to return.

Initially, the display was limited to freshmen since the bulk of

the courses represented ' in the division were being taught at the freshman

5
level. However, many members within and outside the division who were

directing their efforts toward creative design at other academic levels

expressed a great deal of interest and today there are six categories

for entries: Freshman, Sophomore, Junior, Senior, Graduate and Co-op.

As a result of the efforts of Borah Kreimer and Ralph Blanchard of Northeastern University, financial assistance has been obtained to make several prizes available and guarantee the continuity of the committee's

operation, which requires a large annual budget for mailing, etc. All

students represented in the display receive a certificate on parchment;

entries winning first place in each category are awarded SlOO, and the

sponsoring school receives a plaque.

What the entries represent

The projects exhibit a broad range of subject material and sophistication, from bicycle wheel retainers to biomedical devices and operational computers. The students represented often show a surprising ability

to handle material far beyond their academic level when properly challenged. Almost all the entries represent the conclusions made in solving

an open-ended problem "situation", in which the instructor offers guidance and suggestions, but not rigid control. Entries are usually made

by teams of students, and include a formal written and graphic report;

frequently an extremely well-executed model or prototype. Study of the

entries discloses the influence of the summer school. Teams almost always

follow a series of steps, or design process, which converges upon a satisfactory solution. While no single set of steps is used, the processes

exhibit certain commonalities. In general, the teams must probably do

five things.

First, analyze the causes, consequences, and limits of occurrence in

order to define the true nature of the problem represented by the situation.

Second, investigate the market for existing solutions, and augment

their knowledge where obviously needed data is missing. The instructor

usually assists the typical freshman team at this point, suggesting

source material or experts in the field, and by asking questions that the

team cannot answer and suggesting a way of determining the answer.

Third, assemble and synthesize a wide variety of solutions to the

problem. Creativity is generally stressed; brainstorming sessions are

held and methods of idea-promotion are discussed. The rationale here

seems to be that the existence of the problem, when solutions are

available, indicates that none of the solutions are wholly satisfactory

and the problem is not completely understood or has not been adequately

investigated; a new and creative approach is needed.

Fourth, logically select the best solution from those they have generated, and defend that selection, frequently during an oral presentation.

Fifth, describe their solution well, demonstrating their understanding

and ability to create a high quality formal report using graphics and

report writing.

Lee Karrisburger says that every audience is motivated by personal

interests - the "What's in it for sue?" syndrome. What does this have to

do v^ith the Mechanical Failures Prevention Group?

k
students who have been exposed to this open-ended design experience

have a "mature" attitude toward design. They are aware that many solutions may exist for any problem, and that any new solution must compete

in the marketplace with established and future competition. They are

conscious of the many facets of a design that must be considered in

formulating it; reliability, production cost, safety, ease of operation,

failure modes of all kinds, human factors, and so on. They are unlikely

to accept blindly a pat definition of a problem without questioning its

validity or at least examining it. They are generally very motivated

about design; they have had at least one experience where they tried to

"put it all together" on a rational basis, succeeded to one degree or

another, and can benefit from their first mistakes.

Having received a rather classical and frequently tedious education

in mechanical engineering, I join the many associates with whom I have

discussed this topic in saying that I wish we had been exposed to this

material in our freshman year. Many of the things we are teaching freshmen now had to be found out the hard way after graduation and at no small

expense to the employer or the productive time of the employee.

I believe that the work exhibited in these displays indicates that

there is a new crop of young engineers becoming available who are very

conscious of problem-solving techniques and who will be able to perform

competent engineering design work much earlier because of this exposure.

5
DISCUSSION

B. P. Bardes, Bimba Manufacturing Conpany : One of the results from

offering a case study program to freshmen and sophomores is that these

people have not been told that they do not know how to solve the

problenio Therefore, they just go at it with the best they've got, and

very frequently they come up with not only workable, but elegant

solutions.

C. 0. Smith : Very true. Arizona State introduces a design project to

their freshman engineering students the day that they walk onto the

campus--not towards the end of their freshman year. This program has

been in existence for ten years. The results have been quite good,

particularly in lowering the attrition rate. After the design

projects have been completed, a panel of experts is assembled to judge

the students' efforts. I had the pleasure of serving on that panel at

one time. The primary consideration in the judging is method--not the

attractiveness or degree of sophistication of the specific project.

In my opinion, and in the opinion of many of my colleagues in many

institutions, method or approach is again considered to be the

critical aspect.

R. E. Marin ger. Battel le Memorial Institute : I think this kind of

approach to education puts a little fun into it for the students.

C. 0. Smith : And a different kind of frustration.

K. E. Ma ringer : I have heard that viewpoint expressed by other people

who complain bitterly about having all the rough courses in the first

semester so you can flunk out all the people who really are not

appropriate, instead of having those courses in the first semester

which will really grab the students' interest and get them

enthusiastic before you show them how tough life is.

6
THE DESIGNER'S CONTRIBUTION

IN THE DEVELOPMENT OF NEW PRODUCTS

James J. Lesko

Industrial Design/Mechanical Engineering

Purdue University

West Lafayette, Indiana 47907

Abs trac t

The designer's main responsibility is in the aesthetics of the

product, but he often can make a contribution in other areas as well.

A designer is trained to look at problems differently from engineers

concerned with the specifics of function and cost benefit analysis.

The designer is interested generally in the product and in its larger

context particularly as it relates to user needs.

The best way to illustrate that contention is to present a project

that I worked on while employed as an Industrial Designer at the

Westinghouse Electric Corporate Design Center. The project was the

Tampa Airport Shuttle System - often called Skybus. I arrived on the

project after specifications and contracts were signed. My job was

to work on a detail that was causing a problem and in desperation it

was given to design to see what we could "come up with". The problem

was a door located at the ends of the vehicle. After looking at the

concept for a few days it became quite obvious that the problem was not

with the door but with its location. I will discuss with slides my

method of analysis and conclusions, which resulted in a total revision of the specifications of the vehicle and of the architecture

of the building.

7
For the program Success by Design; Progress Through Failure

Analysis, I would like to begin my presentation, "The Designer's Contribution in the Development of New Products" by making four points

that cover some explanatory aspects of industrial design.

Traditionally, the Industrial Designer's primary responsibility

in product design is aesthetics. Aesthetics remains a prime concern

of the Industrial Designer but new approaches to problem solving are

changing this traditional approach.

1. By training, the Industrial Designer is conditioned to be sensitive to user needs and depending on the individual designer the

concern for user needs often takes precedence over the traditional

approach to design. Functional analysis with respect to user

need (or performance specification) has become the criteria for

making the aesthetic decisions in product design.

2. The Industrial Designer can make significant contributions to

product performace if he is a member of the product development

team. If the Industrial Designer is brought into the picture

after major concept decisions have been made and tooling has

begun, he can only make cosmetic contributions. This is true

of all professions, but the Industrial Designer (through his

own fault) has been considered more of a decorator of concepts than a contributor to the concept.

3. One method that the Industrial Designer uses in his approach

to design is a search for simplicity: simplicity in production

and simplicity for the user.

4. Pertinent to the subject of this conference the Industrial

Designer can, through analysis of user need, (a) help prevent

performance failure and (b) reduce the possibility of mechanical failure by the elimination of mechanisms in favor of the

development of simple and direct use of the product. This is

usually done with innovative use of existing or known

technology.

I would like to illustrate these points with a brief review of a project that I got entangled in as a young and recent addition to the

Design Department at the Westinghouse Electric Corp. in Pittsburgh,

PA. Although that was in 1965, the system has only recently been in

operation

.

The project was the Tampa Airport Passenger Shuttle System (often

called Skybus) which began operations in April, 1971. The Tampa System

has eight vehicles and a total track length of 7,600 ft, , with an

annual average mileage at 382,000 and about 13 million passengers. The

operations and maintenance cost per month is $23,000, with a total system

8
investment of eight and one quarter million dollars. Cost

per passenger is around $.07.

Back in 1965 shortly after July, the month I began

working at the design center, I was given the assignment to

investigate and design a door for the proposed vehicle at

Tampa. After discussing the project briefly with my boss he

left for a 3 week vacation. It soon became clear that the

specifications and contact for an end loaded vehicle would

cause many mechanical problems; but a more serious situation

was that it could not do the job that it was expected in the

time that it was expected.

Total turn around was to be 140 seconds per cycle. A

cycle was defined as; 50 people loading, 40 sec. travel, 100

people unload/load, 40 sec. travel, 50 people unload. ^ In

60 seconds 200 people were expected to pass through a door

that could not be greater than Ih feet. There were penalties

in the contract if the system could not meet the requirements.

The time requirements seemed totally unrealistic particularly

with the added problem of computer controlled docking and some

vague ideas on the population that would be using the airport.

At the end of the third week my boss returned expecting to

take the drawings or even a model of the door to a prearranged meeting with the hopeful engineers. Of course my

boss was less than pleased when instead of a model, I handed

him a paper with a critique of the end loaded proposal and my

alternate solution which was a double side, six door vehicle.

The engineers were equally disappointed and dismissed my proposal saying that the contract specifications were signed.

I would like to show some slides of the Tampa project

which may illustrate the design process, that was used to

change their minds. I will begin with the original end

loaded proposal.

Figure 1 is an artists concept of the original proposal

which was taken from an advertisement in the September 1965

Scientific American Magazine.^ It is obscure but the drawing

indicates some sort of folding door at the ends of the vehicle. I was given the job to make the end loading concept

work by designing a door that would magically solve the problems posed by the concept. As I began working on the project

a number of problems began bothering me. Most important was

the fact that the passenger would have to pass over a space

between the vehicle and the building. From our experience

with elevators we knew that people are uncomfortable with this

kind of situation. People prefer to step onto a platform that

feels more secure, like a platform parallel with the direction

of movement.

'Raised figures indicate literature references at the end of this

presentation.

9
i'lgure I

Another was the fact that the power lines were located

directly below, which is shown in figure 2. A number of

controls and mechanisms would be required to dock the vehicle

but that was mechanically possible if it was necessary to do

it that way. It was a "technologically limited" problem. A

problem that was "societally limited" and not solvable was

the fact that people simply could not move as fast as they

were expected in the time that was given through the door that

was proposed. Figures 3 and 4 is a diagram of the problem of

the problem of people unloading and loading in the end loaded

concept and the double side loader. The double side load concept separates unloading passengers from the loading passengers. Six doors were used to cut the time needed to load and

unload. Figure 5 shows one of the doors on the current installation with information for passengers of the system.

Figure 6 is a view of the vehicle showing doors on both sides

of the vehicle.

Returning to the end loaded concept for a moment, figure

7 shows the full scale mock-up of the end loaded concept which was built to attempt to show that people could pass

through the door in the required time. This study was conducted at the South Park site in Pittsburgh, PA. ^ The structure was then shipped to Tampa to try and demonstrate to the

Tampa people that the concept would work. Needless to say

the proponents of the end loaded system finally demonstrated

to themselves that their concept would not work. Besides the

10
Figure 5 Figure 6

time and gap problems the end loaded concept posed a number of

other serious design problems. (1) If the vehicle stopped

short of the dock or the vehicle was on fire the passengers

could not get out through the doors. They would have to

break the windows .( 2) If the vehicle made an emergency zero

deceleration period stop, which it was programmed to do, the

100 passengers would be forced against the door. With that

kind of force on it, the door in all probability would fail

and the passengers would continue out of the vehicle.

11
Figure 7

Figure 8 shows what a passenger would see in that case;

figure 9 is another view.

Figure 8 Figure 9

If any of you have used the Tampa Shuttle System, you

probably remember that it is one large box which can hold

100 people. I mentioned earlier that the vehicle could make

an abrupt stop which it did once during normal operations.

All of the passengers were forced against the end of the

vehicle, and some passengers were seriously injured. I had

recommended that the vehicle be separated into three compartments to minimize the effects on the passengers in the event

of a quick stop. An additional benefit was that one section

could be equipped for use by the elderly and infirmed

passengers. Additionally, internal bracing of the vehicle

would be possible which would increase the structural

strength, and it would also be lighter and cost less to build.

I mention that because it demonstrates point #1 that I made

in the beginning which I would now like to briefly develop.

12
First I would like to explain that after the concept

phase was completed the Tampa project was taken from the

design center and given to consulting firm. The decision to

ignore the report and to keep the vehicle open rather than to

have compartments was based on aesthetics and not on user

need. This decision was made with management level power

over engineering. There was a breakdown in the Product

Development team. The results of course were a number of

injured passengers and a number of law suits against the

manufacturer and owner. Further costs will be incurred in

the event of other accidents which will lead to the eventual

conversion to a compartmented vehicle. I believe that it is

necessary to maintain teamwork throughout the project, and

that all final decisions be based on user need.

I would now like to conclude with figure 10, a photo of

the final vehicle and installation and by quoting from a

letter by the project engineer which summarizes the important

points. "The Industrial Design Department contributed

significantly in the conceptual design phase of the project

. . . and was instrumental in providing information and a

design concept of a double side load vehicle which established an efficient station design . . . The double side

system eliminates passenger congestion and maximizes passenger movement, with a reduction in time for system turn

around. Additionally it made the control system design

significantly easier since it was not necessary to dead-end

the car to a mating platform at each end of travel."'*

Figure 10

15
REFERENCES

Westinghouse Electric Corporation, Pittsburgh, PA,

Research Memo 65-7d3-SYSTP-Ml, 1965.

Scientific American Magazine, September 1965.

Westinghouse Electric Corporation, Pittsburgh, PA,

Tampa Report Number 65-202-01, Tampa/South Park

Passenger Flow Study, December 10, 1965.

Letter from Mr. E. E. Hogwood, Tampa Project Engineer,

Westinghouse Electric Corporation, July 25, 1972.

BIBLIOGRAPHY

R. J. Benke, Westinghouse Electric Corporation,

Pittsburgh, PA, Technical Paper 330-201N, April 13, 1971.

E. E. Hogwood and R. B. Maguire, Passenger Transfer

System will Take Long Walk Out of Air Travel, Westinghouse Engineer, January 19 69, Volume 29, Number 1.

Tampa International Airport Passenger Shuttle System,

WTD 71-521A, Westinghouse Electric Corporation, Transportation Division, E. Pittsburgh, PA, 15112, December

1971.

ih
DISCUSSION

R. Lem'ch, Caterpillar Tractor Coinpany : In industrial design, do you

school the students in design features such as stress risers that

could lead to failures?

J. J. Lesko : At Purdue we have a joint program with the Mechanical

Engineering and other engineering schools. It is our philosophy to

develop dialogue and interaction among the various engineering

departments. For instance, last year our junior-senior project was a vehicle design, using hydrogen fuel because of the energy crisis. We

had engineering students acting as consultants to the industrial

design group and the design group acting as consultants to the

engineers much like it would happen in industry. We made major

decisions in our design process that were based on information from

students in other departments. There were a number of significant

design changes based on information that was given from engineering to

design, and also from design to engineering. We do not feel that

designers should be second rate engineers, nor do we think that

engineers should be second rate designers. We feel that it is more

important to develop the proper teamwork so that when a student

graduates and goes into a real -world situation in design, he will have

enough experience to communicate with an industrial engineer.

G. Wagner, Westinghouse Electric Corporation : A designer must

consider the trade-offs between safety and cost. If three people are

injured out of a total of 30 million handled by a system, is it worth

changing the design to eliminate these three injuries?

J. J. Lesko : I do not think the designer should be conditioned to

accept any injury. His primary role is as an idealist to say we want

zero injuries. He will be in constant battle because he has no

control over the funding of the project.

Go Wagner : That turns into a circular argument in terms of safety.

You can always hypothesize, but at some time you have to face

realistically the problems created in your field and in industrial

engineering.

J. J. Lesko : On every design development team, there should be a consumer's representative who is constantly saying, "I don't want to

get hurt regardless of the cost to you. I do not care what I have to

pay, but if I use your system I do not want to get hurt." He may be

unrealistic and idealistic, but I think in the long run he will save

the company quite a bit of money if he avoids law suits against the

company.

15

SESSION II

CASE

STUDIES

Chairmen: J. E. Stern

NASA.Goddard Space Flight Center

J.A.Houck

National Transportation Safety Board

SUCCESSFUL REDESIGN AND LAUNCFI PERFORMANCE

OF THE ERTS/NIMBUS ADAPTER

AFTER AN ACCELERATION TEST FAILURE

Victor T. Sweet

General Electric Co,

King of Prussia, Pennsylvania I958O

and

W. Brian Keegan

NASA/Goddard Space Flight Center

Greenbelt, Maryland 20771

Abstract

Both the Earth Resources Technology Satellite (ERTS) and the Nimbus

meteorological satellite play vital roles in the efforts of the National Aeronautics and Space Administration to focus the capabilities of space on the needs of

man.

Success of these two programs is, therefore, of the utmost importance.

This paper describes the redesign effort and subsequent successful launch performance of the ERTS/Nimbus spacecraft structure after a catastrophic failure

was encountered during the design qualification acceleration test on an engineering model spacecraft.

The derivation of the test loads, the configuration of the test article, and

the failure sequence are included. A description is given of the effort to reconstruct the spacecraft so as to glean clues as to the cause of the failure, and of

the static load test which was conducted on another launch vehicle adapter to

pinpoint the precise failure mechanism.

Whereas the prior mathematical model has shovm adequate margin for the

test condition, information gained from the static load test was used to correct

the mathematical model of the adapter so that the stress analysis accurately

predicted the failure which had occurred. This sequence is fully discussed in

the paper.

A description of the adapter redesign, the rationale for the redesign approach taken, and the successful performance of the redesigned structure during

a design qualification static load test and launch are discussed.

19
INTRODUCTION

Both the Earth Resources Technology Satellite (ERTS) and the Nimbus

meteorological satellite programs play vital roles in the efforts of the National

Aeronautics and Space Administration to focus the capabilities of space on the

needs of earth.

Since its launch in July 1972, ERTS-1 has facilitated studies of water quality

control, land use management, and related areas of oceanography and agronomy

previously impossible without the improved resolution of its current state-ofthe-art sensors. From its near-polar orbit, it scans the entire surface of the

earth once every 18 days providing spectral information acquired in 4 different

infrared bands. From such data, analysis of seasonal variations is made possible, thereby facilitating predictions of water resource availability and crop yield

as well as other vital environmental data.

At the same time, the Nimbus series of spacecraft continues to serve as

the principal program for remote-sensing research on which are developed the

sensors used in the operational weather satellite program of the National Oceanographic and Atmospheric Administration (NOAA). Over 50 nations currently

rely on the data transmitted from NOAA's satellites to provide timely, accurate

weather prediction. With sensors currently under development for Nimbus, prediction capability of two weeks or longer is anticipated within the next decade.

Timely and successful launches of spacecraft for both the ERTS and Nimbus

programs are therefore of the utmost importance, despite problems which might

be encountered during spacecraft development,

BACKGROUND

With the exception of the complement of scientific instruments, the ERTS

and Nimbus spacecraft are nearly identical. The two programs are managed by

the Goddard Space Flight Center. Both spacecraft are fabricated and integrated

by the General Electric Company at their Valley Forge Space Technology Center,

and both utilize the two-stage Delta launch vehicle to achieve orbit. The spacecraft contractor has the responsibility for the structure, power, control, command, and data handling subsystems as well as for integration of the scientific

instruments (developed separately by NASA and provided as government furnished equipment) and the subsequent system level performance tests and environmental tests. It is, however, the structure subsystem that is of primary

interest in this paper.

The ERTS/Nimbus spacecraft structure is depicted schematically in

Figure 1. The upper portion of the structure consists of the attitude control

box whose base plate is one-inch thick aluminum honeycomb. The base plate

is stiffened from above by a series of channel members attached in a cruciform shape. The control subsystem electronic components are mounted

between the arms of the cruciform, and the pneumatics tanlc and thrusters

are mounted to the top of the channel members.

20
Solar Array

Solar Array

Latch Line

Attitude Control

Subsystem

Paddle Shaft

Truss Tubes

Solar Array

Tle-Dovn Fitting

Sensory Ring

Vee-Band Clamp

Adapter

Launch Vehicle

Second Stage

Figure 1. ERTS/Nimbus Structural Schematic

21
Two large solar arrays (paddles) are supported from opposite sides of the

control box via hollow aluminum shafts. The solar array structure is one-halfinch aluminum honeycomb covered on one side only with solar cells. In orbit,

these paddles extend outward from the control box, but during launch they are

folded inward with their outboard edges joined along the latch line. During

launch, additional support for thrust axis loads is provided to the arrays by the

paddle tie-down fitting.

The control box is attached to the sensory ring by means of six tubular

aluminum legs arranged in a truss-like fashion. Two truss tubes attach to each

of three hard points on a triangular frame that is mounted to the underside of

the control box honeycomb base plate. Similarly, two truss tubes attach to each

of three hard points located 120 degrees apart on the upper side of the sensory

ring, Uniball joints are utilized at both ends of the truss tubes to eliminate

bending loads in the tubes themselves.

The sensory ring is a toroidal shell, rectangular in cross-section, that

houses the major portion of the spacecraft electronics in a series of 18 bays

into which the toroid is subdivided. The sensory ring structure consists basically of 18 machined aluminum separator castings joined by means of 4 aluminum rings, 2 on the outer edge of the shell and 2 on the inner edge, one of each

at both the upper and lower planes of the sensory ring. Extending across the

center of the toroid is an aluminum cross-beam structure which supports the

complement of scientific instruments. The instruments themselves extend downward inside the shell of the adapter.

Both the lower outboard ring of the sensory ring and the upper ring of the

adapter are machined surfaces joined by a tensioned vee-band clamp. After

powered flight, two bolt cutters release this clamp and, by means of push off

springs located in the adapter, the spacecraft is separated from the launch vehicle at this plane. The adapter itself is a frame and skin structure. The frame

consists of 4 aluminum rings, the upper and lower rings are machined and the

2 intermediate ones are bent~up channel sections, and 11 aluminum longerons,

8 main ones above each of the 8 bolt locations that attach the spacecraft to the

laxmch vehicle and 3 localized longerons. The entire frame is covered by a metallic skin which is part aluminum and part magnesium.

The above general description was provided to familiarize the reader with

the ERTS/Nimbus structure. The description of the adapter is necessarily

vague because it is that which failed during the acceleration test of a structural

development model spacecraft. Detailed descriptions of the original adapter

design, as well as of the failure analysis and the redesign effort are provided

in later portions of this paper. The design of the remaining portions of the

ERTS/Nimbus structure are as described above and were not revised as a

result of the failure of the adapter.

22
TEST PHILOSOPHY

In accordance with the test philosophy of the Goddard Space Flight Center,

the design of a spacecraft structure is considered qualified only after it has

successfully demonstrated the capability of withstanding loads equal to one and

one half times the maximum anticipated launch loads. These anticipated launch

loads are computed by conducting a flight dynamic loads analysis in which a mathematical model of the spacecraft coupled with one of the launch vehicle is

excited by a series of transients representative of the various load producing

events expected to occur during launch. Response predictions are made at each

spacecraft mass point to each of the various forcing functions. These responses

are then integrated to obtain loads at each major interface of the spacecraft

structure for each loading condition. Events typically included in this flight

dynamics loads analysis are first stage ignition and lift-off, wind gust loading,

first stage shutdown, first and second stage separation, and second stage ignition and shutdown.

For the Delta launch vehicle used by both ERTS and Nimbus, there is an

additional event termed POGO for which an analysis is conducted. On the Delta

vehicle, POGO is caused by propellant feed line oscillations that couple with the

launch vehicle structure and induce vibratory response of the entire launch vehicle at its first longitudinal or "accordian" mode. This large dynamic load is

coupled with high compressive static loads because it occurs immediately prior

to first stage burnout. Goddard test philosophy defines the flight acceptance

level at the mean plus two sigma value. For a normal distribution, this equates

to the 97.7 percent probability level. Observed data from previous Delta missions had established that the POGO vibration level at the spacecraft/launch

vehicle interface was 4.0 G at approximately 20 Hz. The simultaneous steadystate acceleration at the appropriate flight time was approximately 6. 0 G. The

lateral excitation of the launch vehicle at this flight time is negligible. The only

lateral response that must be considered therefore is that resulting from coupling through the spacecraft structure due to the longitudinal excitation. In the

case of ERTS and Nimbus, the coupling is appreciable due partly to the asymmetric spacecraft configuration and partly to the fact that several lateral resonant modes of the spacecraft lie in the POGO frequency range.

Asa result of a preliminary loads analysis for both the ERTS-1 and Nimbus5 missions conducted by the spacecraft contractor, it was determined that the

POGO condition was a qualification load condition for both missions. After

adding the factor of 1.5 to the predicted flight loads, the values shown in Table 1

were defined as the qualification loads for both spacecraft at each of the four

major interfaces. While the final predicted set of launch loads computed by the

launch vehicle contractor were significantly lower than the ones shown in Table 1,

it should be noted that the loads used for the test were the best available estimates at the time. The reduction in the final loads was brought about primarily

because of reduced lateral axis coupled response of the spacecraft to the thrust

25
TABLE 1

ERTS-l/Nimbus-5 Design Qualification Loads

Interlace Type Load

Magnitude

Nimbus-5 ERTS-1

ACS/Truss Shear (lb) 2,478 1,695

Compression (lb) 7, 054 5,757

Bending (in. -lb) 31, 319 20. 553

Truss/ Shear (lb) 2,465 1,680

Sensory Ring Compression (lb) 7, 831 5,578

Bending (in, -lb) 161, 308 91, 762

Sensory Ring/ Shear (lb) 7, 923 4,934

Adapter Compression (lb) 32,531 30,636

Bending (in. -lb) 1luZ, '^9 ZoZ 989

Adapter/ Shear (lb) 7,914 4,917

Launch Vehicle Compression (lb) 33,613 31,710

Bending (in. -lb) 434, 712 239,682

axis POGO oscillation. The control box and the truss tubes had previously been

qualified to these loads, the control box during a controls subsystem acceleration test, and the truss tubes in a separate load test. It was decided that the

remainder of the structure would be qualified by conducting an acceleration test

of a structural development model spacecraft using the Goddard Space Flight

Center's Launch Phase Simulator facility.

Because of the structural similarity of the two spacecraft, a decision was

made to conduct one test to the worst-case loads, those defined for Nimbus-5,

and thereby qualify the structure for use on both missions.

The spacecraft model used for the test was actually configured with ERTS

peculiar secondary structure. The Nimbus peculiar hardware was qualified

either by similarity or else during separate component-level acceleration tests.

All structure was flight type hardware; the other subsystems were mass and

center-of-gravity simulations only. There was no functional electronic hardware on board the spacecraft.

Because the test setup utilized a pure static load to simulate a combined

dynamic and static load condition, it was impossible to precisely duplicate the

desired load at each structural interface simultaneously. The simulation adequacy was greatly enhanced, however, by redistributing some of the spacecraft

weight. One hundred pounds was removed from the sensory ring bays and

230 pounds of ballast was added to the control box baseplate. Thus the actual

2k
test article, which weighed approximately 2100 pounds, was 130 pounds heavier

than the ERTS-1 flight spacecraft and approximately 400 pounds heavier than the

Nimbus-5 flight spacecraft.

Table 2 defines the 100-percent test load at each of the three interfaces that

were to be qualified during the acceleration test and compares it to the desired

qualification load on a percentage basis. As can be seen, the bending and compression loads were reasonably well matched at all interfaces but, in order to

achieve this, the shear loads were substantially above those desired. Nevertheless, all parties involved agreed that the planned test was acceptable.

TABLE 2

Planned 100-Percent Loads versus Design Qualification Loads

Interface Type Load

Planned

100%

Test Load

Test Load/Nimbus-5 Qualification Load

Truss/ Shear (lb) 3, 007 1.22

Sensory Ring Compression (lb) 8,144 1.04

Bending (in. -lb) 153, 243 0.95

Sensory Ring/ Shear (lb) 10, 854 1.37

Adapter Compression (lb) 31,555 0.97

Bending (in, -lb) 244, 721 1.01

Adapter/ Shear (lb) 11,555 1.46

Launch Vehicle Compression (lb) 33,613 1.00

Bending (in. -lb) 512, 920 1.18

RESULTS OF PRE-TEST STRUCTURAL ANALYSIS

Prior to the acceleration test, the adapter was the subject of a rigorous

structural analysis for the expected test loads and was found to have sufficient

margin to survive loads 50 percent higher than the qualification level test loads.

The critical stresses for the applied test loads had been calculated using the

GE-proprietary finite element structural computer routine, "Mechanical Analysis of Space Structures" (MASS).

Basically, this routine requires the input of the geometric and stiffness

properties of the various structural elements, such as the geometric coordinates

of the structural nodes defining the boundaries of the structural elements, the

definition of the element as to its boundary nodes, its type (beam or plate

element, either straight or curved), and the element physical and cross-sectional properties. A special input allows the fixity or release in any of 12 degrees of freedom for beam elements and 24 degrees of freedom for 4-sided

25
plate elements. The loading conditions may be input as any of a system of concentrated loads or moments applied at the structural nodes, uniformly-distributed loads or moments applied to beam elements, or uniformly-distributed

pressures to plate elements, or any combination thereof. Restraints to the

structure may be applied by either inputing fixity or by the use of linear spring

rates applied in any of six degrees of freedom at preselected nodes. Output consists of nodal deformations, internal loads, stresses for each load component,

and an effective combined stress at the ends of each beam element or at the center of the edges of each plate element.

The original primary structure of the ERTS-l/Nimbus-5 adapter consisted

of a truncated, conical shell 60 inches in diameter at the lower launch vehicle

mounting ring, 24 inches high, and tapering inward at 4-1/2 degrees toward the

spacecraft separation ring. The shell was formed of 0.032-inch magnesium skin,

reinforced by nine full length longerons, two partial length longerons, and four

circumferential rings. The 11 longerons were 0,040-inch 7075-T6 aluminum

alloy rolled hat sections. The upper and lower rings were machined from 2014-

T652 aluminum alloy forgings. The intermediate rings were 0.032-inch 7075-

T6 aluminum alloy formed hat sections, and primarily used for skin panel stiffening and for local shear redistribution. Four structural access doors were

installed in the large center band skin panels. Launch vehicle mounting was

provided by eight bolt locations in the lower ring at the intersections of longerons

designated numbers 2, 3, 4, 5, 6, 7, 9, and 11. It should be noted that structural symmetry about the X-axis has only been broken by longeron number 9

which has been shortened by the installation of an interchangeable structural RF

plate to which antenna reradiators had been mounted. Structural continuity has

been provided by full length framing longerons numbers 8 and 10, and local skin

doubling adjacent to longeron 9.

The original analj^ical computer model defining this structure (illustrated

in Figure 2) consisted of 159 nodal points, 182 beam elements, and 45 plate

elements. The upper ring was modeled using 34 nodal joints, including 5 solely

to provide applied-loading points corresponding to the structure above, and

34 curved beam elements. The two intermediate stiffening rings and the lower

mounting ring were modeled using 29, 26, and 25 curved beam elements,

respectively. The nine full length longitudinal members were each modeled

using six straight beam elements. The partial length longitudinal members

designated as longerons numbers 1 and 9 were modeled using four and two

straight beam elements, respectively. Additionally, in the longitudinal direction, there were four separation spring brackets each modeled by five straight

beam elements. The skin areas defined by the rir^s and longitudinal members

were represented by 45 curved, trapezoidal plate elements.

The following potentially unconservative assumptions, the effects of which

on the structure will be discussed in greater detail later, had been utilized in

order to expedite the analysis:

26
Figure 2. ERTS/Nimbus MASS Computer Model of Adapter Structure

27
• Longeron loads were considered to act at the longeron centroid. That

is, the model did not permit any eccentricities, or the resulting

bending moments, to exist.

• Panels do not buclde. The MASS computer program is limited to a purely linear analysis and, therefore, cannot account for either panel

buckling or any secondary effects due to budding,

• To account for relatively low compressive buckling allowables on large

skin panels, center band adapter panels were made capable of carrying

shear, but not compression.

The adapter analytical model was loaded directly at the upper ring (at 18

nodal points designated by the numbers 21 to 38) by reactions calculated to have

occurred at the separation ring of the ERTS-1 spacecraft, above, when subjected to the qualification level test loads. The method of obtaining these spacecraft reactions, and their resulting applied adapter loads, will be described

below. Restraint to the adapter model was provided by the input of three linear

degrees of fixity at each of the eight mounting bolt locations.

The ERTS-l/Nimbus-5 spacecraft analj^lical computer simulations are of

interest to this paper only because they provide an insight into the manner in

which the applied spacecraft loads were redistributed to the adapter model.

Briefly, the spacecraft structure may be considered as having two major components: the six-member space framework known as the truss assembly and

the more complex sensory ring, which houses the electronic modules and earthviewing sensors. The truss assembly is composed of six pin-ended tubes interconnecting three points on the Attitude Control Subsystem (ACS) with three points

on the sensory ring. The sensory ring is basically a toroidal structure comprising 18 separator castings connected by four circumferential rings and a sheet metal web system. This structure is further stiffened by a cross-beam

system paralleling the Y-axis, which in turn, supports four tape recorders and

two major sensors.

The spacecraft computer simulations consist basically of two models, the

first being identical for both ERTS and Nimbus, and 'the second being partly

identical and partly unique to each spacecraft in the ERTS/Nimbus series.

Figure 3 defines, in detail, the truss assembly mathematical model, as

well as, giving an overall view of the assembly of the various components of

the spacecraft and adapter. The truss assembly supports the ACS, including

the major portion of the mass of the solar array. Because the truss assembly

was statically determinant, it was not included in the overall spacecraft computer model; however, based on a unit load type of computer analysis, the truss

assembly provides concentrated loads to be applied at three locations of the

sensory ring ananating from applied ACS and solar array inertia loads.

28
Attitude Control

Subsystem (ACS)

<^y^w AXIS)

Auxiliary Load

Panel Attachs.

Paddle

Damper

Attachs

.

Direc tion

of Orbit

Origin of

Coordinate

System

Sensory Ring Structure

(Ref. Figure ?) Adapter Structure

Figure 3. ERTS MASS Computer Model of Truss Assembly

29
The sensory ring MASS computer model consists of two parts: the torus,

identical for both ERTS and Nimbus, and the crossbeam assembly, which is

unique to each spacecraft. Figure 4 defines, in detail, the 137 node, torus

structural model, but, in the interest of clarity, only the basic outline of the

ERTS-1 100-node crossbeam structural model. An ERTS-1 configured structural development model spacecraft was the subject of both the structural analysis and the ensuing acceleration test.

In the performance of the analysis of the sensory ring, the following considerations and assumptions were made:

• The truss-induced loads described were applied eccentrically to three

rigid connector elements at nodes 84, 90, and 96. These rigid connectors represented the truss support fittings which were mounted to the

three separator castings designated, counting clockwise from +X-axis,

as numbers 4, 10, and 16. These separators transmitted the loads,

basically undiminished, to the adapter below.

• The sensory ring-induced loads resulted from accelerations acting on

the various masses within the sensory ring. These included major

mass items, such as the tape recorders, the externally-mounted (wideband video tape recorders (WBVTR)) electronics package, the orbit

adjust system (OAS), and the sensors, and distributed masses, such

as the structure, harnessing, thermal control system, insulation,

antennas, and electronic modules installed in the 18 torus bays. These

local and distributed acceleration loads were then allocated to various

torus and crossbeam nodal points, accounting for local eccentricities,

by a rigid-body type of analysis. These values then were the applied

loads utilized in the sensory ring MASS analysis.

• Reactions to the complex loading system were provided in the thrust

direction (Z-axis) by the input of 18 single degree-of-freedom spring

rates to the separation ring at the base of each separator casting.

These values each had been calculated based on local adapter

stiffness,

• Reactions in the shear direction (any combination of X-axis or Y-axis

components) have been provided by the input, around the periphery of

the torus separation ring, of a series of uniformly-distributed bearing

loads varying in accordance with the function 1/2 (1 + Cos 6). These

distributed loadings represent, in a sense, the bearing reactions provided by the relatively flexible adapter upper ring when clamped to the

torus. Theory indicated that this distribution would produce no in-plane

bending in the adapter ring, only hoop tension.

50
Direct ion

of

Orbit

R6

17,

21

07

[103) Side Plates

(18 places)

If 62/

140

.25

/6,

,84-

4-

/

V220

/ 76^ ^4-

J 7) 227

1)5

7f- 24

239.

(4.

Crossbeam

Structure

14 1^0

\

(44-<*/

245 j.SO^

34-

54

21Z

\ 2K)J

'3)

\

/ 1/ 1 n 1

/ / + Y 1 i/ /

/ ^?|TC M A%1S\ |/ / —Inboard '.Vebs

(18 places).

\ /

—

5"

234-

f/224-

'^7

^3

'2/ 173

Jl //Sl

32.

"MASS

Coord inate

System

So

7o

7.

21

no 2f

3/

Outboard 'Vebs

8 places)

6,i> A SO

30

KEY: Nodal Points - Shear Plates - flOl)

Figure 4. ERTS MASS Computer Model of Sensory Ring Structure

51
Based on the evaluation of the anticipated flight loads, an acceleration test

to be performed with the spacecraft in three orientations was selected to qualify

both the ERTS-1 and Nimbus-5 spacecraft. The test load conditions were

defined as:

• -16 g thrust (Z-Axis) and +5.5 g roll (+X-axis) • -16 g thrust (Z-Axis) and -5.5 g roll (-X-axis) • -16 g thrust (Z-Axis) and +3. 0 g pitch (+Y-axis)

In the sign convention presented, minus thrust denotes a downward load, and

plus roll and pitch denote loads applied to the forward (direction of orbit) and

right directions, respectively.

Inertia loads were calculated at the center-of-gravity of the various major

mass items such as the solar array, ACS, truss, orbit adjust, sensors, and

distributed masses of the sensory ring for each of the three conditions. These

loads were resolved into the following interface loads: Solar Array Tie-Down/

Sensory Ring, ACS/Truss, Truss/Sensory Ring, Sensory Ring/Adapter, and

Adapter/Launch Vehicle. From a comparison of these loads, two conditions

were selected as being the more critical (-16 g thrust/+5.5 g roll and -16 g

thrust/-5.5 g roll), and were subjected to further analysis.

The interface loads were distributed to the various nodal points of the

sensory ring MASS model in the manner outlined, and the sensory ring structural analysis was completed. From this analysis, reactions calculated at the

intersections of the 18 separators and the lower-outboard (separation) ring and

the reacting bearing distributions at the lower-outboard ring were extracted and

then used as the applied loads to the adapter MASS model.

It was apparent from an examination of the computer printouts, that the

areas of the adapter immediately beneath the sensory ring separators numbers

4 and 16 (truss assembly support castings) were critical for the +Roll/-Thrust

(+X/-Z) conditions, and the area immediately beneath separator number 10

(truss assembly and solar array tie-down support casting) was critical for the

-Roll/-Thrust (-X/-Z) condition. More specifically, these areas maybe identified as longerons numbers 4 and 5, 6 and 7, and 11 through 2, and their

immediately adjacent skin panels.

The most critical stresses and their resulting margins of safety from these

areas have been calculated and summarized. In Figure 5 the critical section of

a typical longeron is defined, and the crown and attachment flanges are immediately identifiable. Table 3 presents a summary of the longeron stresses and

margins. Because the computer analysis neglected thrust loads in adjacent skin

panels, these results have been adjusted to reflect an effective amount of skin

acting. The most critical margin for the actually-applied test condition (+R0II/

-Thrust) resulted at the attachment flange of longeron number 4, the point of

52
Section Properties!

A = .218 in.^

y = .^31 in.

I = .0^35 in.

XX

Crippling Allowables 1 at crown- F^^ = 58,500 psi

at flange- F = 4^,700 psi

Figure 5. Properties of Adapter Longerons

55
TABLE 3

ERTS-l/Nimbus-5 Critical Margins of Safety

From Pretest Analysis

Longeron

Number

Critical

Condition

Stress in

Crovm

(lb/in. 2)

Stress in

Flange

(lb/in .2)

Margin

of

Safety*

2

4t

5t

6

7

11

-X/-Z

+X/-Z

+X/-Z

+X/-Z

+X/-Z

-X/-Z

38, 820

22, 740

25,680

24,560

28,950

32,000

27,810

27,370

24,520

23,300

23, 960

26,300

+0.01

+0.09

+0.22

+0.28

+0.24

+0. 13

Skin Panel**

Location

Critical

Condition

Shear

Stress

(lb/in. 2)

Margin

of

Safety 1" ^

1 & 2 -X/-Z 4012 +0.24

2 & 3 -X/-Z 4453 +0.12

3 & 4 -X/-Z 3643 +0.37

4 & 5t +X/-Z 4920 +0.02

6 & 7 +X/-Z 4757 +0.05

7 & 8 +X/-Z 4602 +0.72

10 & 11 -X/-Z 2483 +1.01

11 & 1 -X/-Z 3376 +0.48

*Based on crippling allowables of 58, 500 psi and 44, 700 psi at crown

and flange, respectively. Ultimate factor of safety equals 1.50.

**Critical panels are in wide center band between longerons designated.

tArea of acceleration test failure.

•j-f Based on initial panel buckling allowable of approximately 5000 psi at

limit load, except panel numbers 7 and 8 (7910 psi) which have

flanged double r.

actual failure. It should be noted that the most critical margin results occurred

at longeron number 2 for the reverse axis condition (-Roll/-Thrust) . This is

also indicative of a potential failure at this point if this axis had been run first.

Also shown in Table 3 are the more critical center band skin panels and their

resulting margins of safety. In this case the most critical panel lies between

longerons 4 and 5 at the point of test failure.
ACCELERATION TEST DESCRIPTION

The acceleration test was performed using the Goddard Laimch Phase Simu

lator (LPS). This unique facility was designed for conducting combined environ

mental tests and has capabilities for simultaneously applying steady-state

acceleration, vibration, acoustic noise, and atmospheric venting profiles in an

automatically controlled real-time sequence. At the time of the qualification

test of the ERTS structural development model in mid- 1971, however, the LPS

vibration system was not operational when mounted on the centrifuge arm, thus

forcing the test to be a pure static acceleration test.

An overall view of the centrifuge is given in Figure 6. The nominal radius

to the base of the test item is approximately 65 feet, and capability exists for

accelerating a 5000-pound test item to 30 G at which time the arm is rotating at

approximately 40 rpm. The total rotating weight is 465,000 pounds. During

testing, the test item is enclosed in the test chamber , a 15-foot diameter,

20-foot -long shell which also houses the acoustic noise and atmospheric

venting systems. In preparation for test, the test item is mated to the LPS

end cap while in its normal orientation. A specially designed 6 degree-offreedom LPS loading vehicle then picks up the end cap, pitches it so that the

thrust axis of the test item is essentially horizontal, and places the end cap

against the test chamber where it is bolted into place for test. Figure 7 shows

the ERTS structural development model on the end cap about to be mated to the

test chamber by the loading vehicle.

The actual acceleration magnitudes of the test did not correspond directly

to any particular flight condition but rather they were calculated so as to produce the proper qualification loads. To induce these loads in the ERTS structural development model spacecraft, the required accelerations were 16. 0 G

along the thrust axis and 5.5 G along the lateral axisc To accomplish this

multi-axis acceleration, the spacecraft thrust axis was rotated to a position

15.6 degrees below the horizontal (see Figure 7) by mounting the spacecraft

on a variable angle tilt fixture specifically designed for LPS use. At this angle

when appl5^ng a centrifugal acceleration of 16. 9 G and including the effects of

gravity, the loads of Table 2 would result.

Monitoring instrumentation during the test was not extensive since it had

been viewed as a go/no go test. Although the applied acceleration is regulated

by controlling the centrifuge spin rate, there were four accelerometers on the

spacecraft. Two accelerometers were at the top of the solar paddle latch line

and two were at the spacecraft center-of-gravity on a bracket 4 inches above

the top of the sensory ring. At each location, one accelerometer measured the

applied thrust acceleration and the other measured the lateral acceleration.

More importantly, there were 19 strain gages on the structure. Of these, six

were located on the three separator castings of the sensory ring immediately

below the hard points at which the truss tubes are supported. The remaining

13 gages were mounted oh the adapter structure; three legged, rosette gages

35
Figure 6. Overall View of Launch Phase Simulator

56
were mounted on each of three selected skin panels, and single gages were

mounted on each of four longerons. These gages were located so as to measure

strains in members whose design had been modified as a direct result of the

loading condition to which the structure was being qualified in this acceleration

test. All 23 instrumentation channels were monitored in real time on an oscillograph and simultaneously recorded on magnetic tape for subsequent additional

data analysis.

During the test, the acceleration level was held constant at 25, 50, and

75 percent of full level in order that the measured strains could be compared

with those predicted for that loading condition. It was noticed from these intermediate readings that the loads in the skin panels were higher than predicted

and those in the longerons were lower than predicted. The real-time decision,

reached while the centrifuge was running, was that the differences between the

measured and predicted strains was not significant enough to warrant stopping

the test, and the authorization was given to proceed to the 100 percent test level.

At 94 percent of full load, the spacecraft collapsed, instantly falling to the bottom of and being forced against the end of the test chamber, leaving only the

lower ring of the adapter still bolted to the test fixture. The centrifuge was

shut down at once, and the failure analysis commenced immediately. A view

from within the test chamber of the untouched wreckage is shown in Figure 8.

FAILURE INVESTIGATION

The failure had occurred at 94 percent of the qualification level, although

the adapter stress analysis had indicated no expected failure of the adapter below

150 percent of qualification levels. The fact that the adapter had failed at less

than two-thirds of the predicted allowable loads led to the initiation of an extensive failure analysis that consisted of the wreckage analysis, a material analysis,

a review of the adapter history, revision and review of the stress analysis, and

a static test program, each of which are discussed in following sections of this

paper.

Wreckage Analysis

Initial inspection of the wreckage had indicated that the failure most likely

initiated in the +X, +Y quadrant of the adapter. Video tape pictures also

indicated that a hesitation occurred just after the start of collapse. These

pictures, however, were not adequate enough to determine the direction of

initial failure or whether this direction changed after the noted hesitation.

In an attempt to reconstruct the failure sequence, a detailed study of the

wreckage was performed. This consisted of physically arranging the pieces of

the wreckage into their original positions and studying the damage and markings

on the major spacecraft components and on the test fixture. Due to the large

degree of fragmentation of the compression side of the failed adapter, the attempt

57
Figure 8. ERTS Structural Development Model

in LPS Test Chamber after Failure
to reassemble it was not as successful as was hoped. However, some conclusions could be drawn from examination of the failed adapter, reassembled as

shown in Figure 9.

# The spacecraft collapsed in compression generally in the +X direction

inclined toward longeron 5.

• The -X side of the adapter experienced primarily a tension failure

of the skin at the lower adapter flange, indicating that this was a

secondary failure (this skin was theoretically still in compression

prior to failure).

A study of the damaged hardware was made to identify the probable sequence

of events that occurred subsequent to the initial failure and also to test the

credibility of some theories of initial failure events.

Through study of the damage to the adapter and the components mounted

within the adapter, and inspection of the markings on the test fixture mounting

ring, a sequence of failure has been developed which has appeared to be valid.

The locations of the significant components relative to the adapter and to the

fixture mounting ring are shown in Figures 9 and 10a. The derived failure

sequence is as follows:

1. An initial compression failure of the adapter occurs in the +X, +Y

quadrant (Figure 10b). The vehicle starts to rotate about an axis which

is approximately parallel to the Y-axis and which intersects the -X

axis near the periphery of the adapter. The wideband antenna (WBA)

No, 2 impacts the fixture moimting ring immediately; an analysis of

the geometry had shown that the nominal clearance with the ring is

essentially zero. As the vehicle starts to rotate, the lower end of the

Attitude Measurement Sensor (AMS) contacts the fixture ring leaving a

mark. The WBA is pushed inboard until the antenna bends and slips

below the fixture ring,

2. The vehicle continues to rotate until the Multispectral Scanner (MSS)

comes in contact with the fixture mounting ring, as shown in Figure

10c. This is confirmed by the damage to the +X, -Y corner of the

MSS as shown in Figure 9. The corner of the MSS acts as a new

fulcrum for the spacecraft and, coupled with the increased moment

due to rotation of the vehicle, causes a tension failure of the adapter

skin on the -X side. The WBA mount is torn loose at its base, but

remains trapped between the fixture and the MSS. The AMS tears

loose from its truss mount and falls to the bottom of the fixture cavity.

59
b. (T) Adapter Fails

(2) WBA bends inboard and snaps through fixture ring.

Support contacts ring.

(3) AMS contacts fixture ring

Figure 10. Failure Sequence

ko
c. (T) Spacecraft rotates about MSS contact

(D Adapter skin tears on tension side

(D WBA and AMS are torn loose

MSS rolls back into RBV

Sensory ring contacts fixture damaging castings and

crossbeam

Figure 10 (continued). Failure Sequence

hi
3. The increasing tilt of the spacecraft causes it to slide off the corner

of the MSS as shown in Figure lOd. This results in a second MSS

contact with the fixture ring. This time, the upper +X, +Y corner

hits the ring, breaking off a large piece of the wooden MSS simulation,

and exposing a steel mounting bar which also gashes the ring. The

MSS is torn loose from its mounting when the steel mounting bar hits

the fixture. It then rotates back into the Return Beam Vidicon (RBV)

simulation as verified by matching the RBV with the gouges on the

-X side of the MSS.

4. With the MSS out of the way, the sensory ring comes in contact with the

fixture ring (Figure lOd), damaging the crossbeam and separator

castings of bays 2 and 3. The spacecraft then rolls over the edge of

fixture and comes to rest against the cover and wall of the centrifuge

chamber.

Four theories were advanced to explain the initial failure of the adapter.

These are discussed in the following paragraphs as is the conclusion that was

drawn, based on the evidence noted, as to the validity of the theory presented.

Theory No. 1 ; The MSS mounting bolts failed and the MSS impacted adapter,

initiating failure.

Evidence

a. Video tape showed that the MSS stayed with spacecraft. If the MSS

had come loose initially, it would have fallen into the fixture cavity.

b. No zinc chromate marks on the MSS indicate no adapter contact.

c. Mounting bolts have shown a shear-type failure. Bearing damage to

holes in the sensory ring have indicated a CCW in-plane torque.

d. Damage on the lower -Y, +X corner of the MSS indicated that the

spacecraft rolled about this point.

e. Stress analysis had indicated a high margin of safety on the mounting.

Conclusion; The MSS did not initiate adapter failure.

1+2
Theory No. 2; The AMS broke loose, impacting the adapter and initiating

failure.

Evidence

a. No zinc chromate marks on the AMS indicate no adapter contact.

b. The buckled condition of the mounting legs and markings on the fixture

ring indicated the AMS was in place when it contacted the fixture.

c. Stress analysis had indicated a high margin of safety on the mounting.

Conclusion; The AMS did not initiate adapter failure.

Theory No. 3 ; Interference of the WBA with the fixture ring caused it to break

loose, impacting the adapter and initiating failure.

Evidence

a. No interference was noted by the test crew during spacecraft assembly

to the fixture.

b. The head of WBA bolt was coined into fixture ring. Coin tests showed

it required 5, 000 to 10, 000 lb to duplicate the indentation.

c. Gap checks at the attachment pads, with 2, 100 lb of spacecraft weight

on them, indicated less than 0. 003-inch gap at any point. This would

have been greater if interference was sufficient to cause coining loads.

d. The WBA mount would have acted as an additional adapter longeron,

supporting the spacecraft.

Conclusion; The WBA interference did not cause initial failure.

Theory No. 4 ; Insufficient adapter strength.

Evidence

a. The sequence of failure as previously described supports this theory.

b. Failure occurred on the most highly loaded side (+X).

^3
c. Longerons in failure area evidenced very low margins of safety in

pretest structural analysis.

d. There is no evidence of other failure before adapter collapse that can

be substantiated.

e. When included, the effects of the eccentricity of loading in the adapter

longeron analysis, as will be shown, indicated the existence of failing

stresses.

Conclusion; The adapter was understrength for the acceleration test loads

and initial failure was a compression collapse of the adapter

primary structure.

Adapter Materials Analysis

As part of the failure analysis, it was considered necessary to verify the

mechanical properties of the materials used in the failed adapter. To this end,

test samples were obtained from the skin and longerons of the failed adapter

and tested for tensile strength and elongation. The results have indicated that

the tested materials met or exceeded the material specification minimum

requirements. The surface pitting discovered in the magnesium skin was

probably present when the adapter was manufactured and may be typical of

the material. No indications of significant corrosion were found. From this

investigation, it was concluded that the adapter material properties were

satisfactory and did not contribute to the failure.

Adapter History

The adapter that failed in the acceleration test was Serial No. 001 and had

seen many uses and modifications since its initial fabrication. This led to the

possibility that the adapter failure could be attributed to a wealmess caused by

its prior experiences. To evaluate this possibility, a review of the adapter

history was performed. Usage on the adapter prior to the acceleration test

failure was as follows:

1. The adapter had experienced a total of about 2 hours of vibration

testing, or approximately 1 million cycles of loading at various levels.

An analysis of the theoretical fatigue damage was not performed because it would have been difficult and costly to review all the test data

and convert it accurately to the required stress/cycles data.

Additionally, inspection of the failed adapter disclosed no evidence of

fatigue. The conclusion was that fatigue was probably not the primary

cause of the failure.

kh
2, Open holes (0. 20 in, diameter) existed in the crowns of longerons 4,

5, and 6 due to the removal of Nimbus-B crossbeam simulation.

Longeron 4 had four holes near the bottom, longeron 5 had six holes

near the bottom, and longeron 6 had seven holes near the center.

Because the acceleration test data and the subsequent analysis review

showed the longeron crown to be a low stress region, the conclusion

was reached that the open holes had no significant influence on the

adapter failure.

3. X-rays and subsequent sectioning of rivets revealed that rivet holes

had been damaged when the reinforcing angles had been added to the

tops of longerons 1, 2, 4, 7, and 11 and to the bottom of longerons 2

and 11, The damage had been caused by the drilling out of the rivets

and the resulting voids are randomly oriented about the rivet circumference. Because the voids were randomly distributed and they did

not exist at the bottom of longerons 4, 5, 6, and 7, where the adapter

failure apparently started, it was concluded that this did not contribute

significantly to the adapter failure,

A check of the Product Assurance records for the adapter showed that all

changes, discrepancies, and material review actions had been satisfactorily

completed prior to the acceleration test. It was concluded that the adapter

used for the acceleration test was adequately representative of the flight

adapter and did not fail due to the effects of previous use.

Stress Analysis Review and Revision

Prior to the acceleration test, the adapter had been analyzed for the

expected test loads and was found to have sufficient margin to survive loads at

least 50 percent higher than the test loads. The adapter failure, and the

absence of any other obvious deficiency in the adapter, brought the validity of

the analysis into question. The impact of analytical model assumptions on the

adapter loads and the acceleration test failure was investigated.

The anal5d;ical model assumes all of the longeron centroids to be in the

plane of the skin. The geometric centroid is actually located 0.46 inches

inboard of the outer skin surface.

The MASS computer model was reconfigured to allow the actual eccentricities of longerons 4 and 5 (area of failure) to become effective as shown in

Figure 11, and the -Z, +X acceleration test condition was rerun. The following

longeron stresses were calculated for the critical longeron 5, midway between

the lower frame (hat section) and the lower moimting ring:

• Maximum skin stress = 31,590 psi, compression.
Figure 11. Comparison of Original and Revised Longeron Computer Models

U6
• Maximum longeron stress = 49, 650 psi, compression.

• Minimum longeron stress = 1, 300 psi, tension.

The minimum stress occurred in the crown of the hat-section and the

maximum stress occurred in the longeron at the skin attachment flange. It became apparent at this point that the low longeron crown strains measured

during the acceleration test were probably due to this longeron eccentricity.

This in turn indicated the presence of the high compression stresses on the

skin side of the longeron as shown above.

Previous analysis of the longeron had indicated a longeron crippling allowable of approximately 38, 200 psi for a bending load which induces compression

on the skin side of the longeron. This indicated that longeron load eccentricity

was a strong contributor to the adapter failure, and its absence in the analysis

was significantly unconservative.

In an effort to evaluate the potential effects on the adapter primary

structure of one or more panels reaching a buckled or otherwise ineffective

state, the following shotgun approach was taken:

• The panel or panels considered in a shear buckled state were removed

in their entirety from the analytical model. Note that in the model

these panels were already ineffective in compression.

• The critical -Z, +X acceleration test condition was rerun on MASS.

• The effects on the adjacent skin panels and framing structural

members were extracted from the MASS printouts and evaluated.

This is a rather gross approach, and represents an overly conservative

analysis, in that the skin shear-carrying capability prior to its buckling is

ignored.

The results of this analytical study indicated an insufficient increase in

shear loading in adjacent skin panels to precipitate a catastrophic failure such

as was witnessed. For example, an ultraconservative approach in which four

panels were considered completely ineffective (neglecting the skin shearcarrying capability prior to initial buckle) resulted in the following increase in

the critical panel shear stress:

• Initial panel shear = 3250 psi (all panels effective)

• Final panel shear ~ 3910 psi (four panels ineffective)

^7
The shear stress in the critical panel increased only 20 percent due to the

elimination of four adjacent panels. Considering the effects of initial shearcarrying capability of these eliminated panels would bring the panel shear stress

to a point between 3250 psi and 3910 psi. Because initial shear buckling for the

panel had been estimated to be 5000 psi, considering the stiffening angles

already installed, it was felt that panel buckling in itself was not the cause of

failure. Therefore, further analysis was not pursued since the static test

would verify or modify this conclusion.

All of the center panels (between hat section frames) were considered in

the MASS program model to be ineffective in compression, carrying only shear.

This assumption would be conservative for the longerons, because all of the

axial loads would be carried by them. However, ithis assumption is unconservative for the panels. Shear buckling allowables are greatly reduced in the

presence of compression, resulting in higher panel buckling loads being applied

to the longerons, reducing their load-carrying capability.

It was felt that compression buckling did occur in many of the adapter

panels during the acceleration test; however, it was not considered practical

to try to analytically predict this effect and it was evaluated in the static test.

From the failure analysis activities that were performed prior to the

static test of the adapter, several preliminary conclusions were derived.

The subsequent static test program would provide the necessary verification

or correction of these conclusions. The preliminary conclusions were:

• The adapter used in the acceleration test was adequately representative

of the design. Its strength had not been significantly reduced due to its

history, its configuration, or its material properties.

9 Failure initiated in the adapter; all other failures were secondary.

• Adapter failure was caused by the initial collapse of either longeron

4 or 5, the adjacent skin panels, or a combination of these.

• Initial longeron failure was probably, at least partially, the result of

the eccentricity between the load and the longeron centroid, causing

high combined bending and axial compressive stresses in the skin and

longeron legs.

• The panels adjacent to the longerons probably had buckled in combined

shear and compression. The buckling also significantly contributed

to bending in the longerons.

hQ
ADAPTER STATIC TEST

During the failure analysis activities previously described, it became

obvious that the true failure mode could only be determined and verified by

performing a static test of the adapter. The basic objectives of this adapter

static test were threefold:

• To identify the failure mode and the initial failure point as they

occurred during the acceleration test on the NASA centrifuge.

• To provide sufficient data to update and improve the analytical model

of the adapter.

• To determine the load paths and distributions within the adapter.

To accomplish these objectives, it was necessary to have a test article that

was a near duplicate of the one that failed in the acceleration test. This required the assembly of major spacecraft elements that would have the same

stiffness and geometric characteristics as the ballasted ERTS SDM. In

addition, the adapter used would have to be configured exactly the same as the

failed adapter. To achieve these goals, the following hardware (Figure 12) was

assembled as the test article:

• ACS - In this case, exact simulation was not imperative and a dummy

ACS was used that consisted of an aluminum loading plate to which

the upper ends of the truss members were attached, similar to flight

configuration.

• Truss - The truss members were flight quality and identical to those

used on the ERTS SDM.

• Sensory Ring - To obtain a proper stiffness representation, the Nimbus

Mechanical Test Model (MTM) sensory ring was used, including the

existing Nimbus leaded wood dummy bay modules.

• Separation Clamp - A flight band was used, installed in accordance

with normal procedures.

• Adapter - It was most important to simiilate this item exactly.

Therefore, a new adapter was obtained and was reworked, resulting

in a configuration which duplicated in all essential details the adapter

that failed in the acceleration test.

h9
"STRONG BACK

HYDRAULIC

JACKS

AND DIRiiCT

LOAD

R -'A DING

:-ieti-:r3

Figure 12. Schematic of Static Test Set-up

50
The test plan evolved in response to the specified objectives and

the approaches selected to achieve them. The following is a brief discussion of some of the decisions and rationale that went into the construction of this test plan„

Adapter Instrumentation

Figure 13 indicates the adapter instrumentation provided. Gages 1 through

40, 120, and 121 were rectangular strain rosettes, and gages 71 through 90

were axial bridges. All others were axial strain gages.

This extensive instrumentation of the adapter evolved as a result of

several requirements. The primary requirement was that to determine the

internal adapter loads it is necessary to know the externally applied loads and

reactions. That is, the load distributions at the sensory ring/adapter and

adapter/launch vehicle interfaces must be determined. No direct measurement

of these loads during the combined load test was considered feasible, so indirect methods were necessary.

As the means of establishing the sensory ring/adapter interface loads, all

of the upper skin panels were instrumented with strain rosettes. Similarly,

all of the lower skin panels were instrumented with strain rosettes as a means

for determining the shear reactions at the eight booster attachment points.

The axial (thrusts) loads at the eight booster attachment points were determined

by calibrating the longerons. This was done during a separate test, the

axial element test, in which an axial load was applied through the sensory ring

and the reactions were measured with load washers while the corresponding

longeron strains were being recorded. In later tests, the longeron strains

could be used to establish the thrust component of the pad reactions. This

established the requirement for instrumenting all of the longerons.

It was desired that correlation with acceleration test data be obtained

from the static test. To achieve this, similar instrumentation was provided

at the same locations as that provided on the ERTS SDM for the acceleration

test. This consisted of (Figure 13) rosettes 20, 21, and 28, and longeron

gages 61, 63, 66, and 70. It was suspected that the instrumented panels

buckled during the acceleration test, prior to the failure. Since panel gages

are only applied to one side of the skin, a buckle in the region of the gage

could produce unintelligible readings. To assess the effects of skin bending

on rosette readings, and to help in the interpretation of the rosette data,

gages 20 and 21 were backed up leg for leg on the inside of the skin with gages

120 and 121, respectively. These locations also coincide with gages from the

acceleration test. The interpretation of data obtained from strain rosettes

under complex loading is not precise or well understood. A shear element

test was included in the program. The intent of this test was to apply known

loads to an area of the adapter where the resulting panel shear loads could be

fairly well predicted and then to compare these predictions with the resulting

51
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52
strain rosette data. Results of this test would then be helpful in the interpretation of the strain rosette data to be obtained during subsequent testing.

Loads and Load Schedules

Since it was planned that the test adapter would be returned to the

fabricator for rebuilding for future use as a prototype adapter, no significant

damage was to be permitted during the static tests. This placed restrictions

on both the load levels and the sequence of loading.

The static test loads were to be representative of the loads which the

vehicle experienced during the acceleration test. A set of thrust and lateral

loads and a loading geometry were selected which would develop loads at the

spacecraft interfaces that were approximately equal to those at 100 percent

of the acceleration test load levels. However, because failure was not to be

risked, the test loads were to be restricted to approximately two-thirds of

these levels. Table 4 defines the static test interface loads and their relationship to the acceleration test loads.

The test loading schedules were selected to minimize the risk of an

unexpected failure by building up to maximum values in an orderly fashion.

This was accomplished by loading the sensory ring only in thrust, then loading

the sensory ring and ACS in thrust only, and finally combining lateral loads

and thrust loads up to the maximum test values. These final loads were

applied in the following sequence: sensory thrust, ACS thrust, sensory

lateral, and ACS lateral.

The -Z, +X loading condition in which the failure occurred was chosen

for the first static test in order to establish the failure mode as soon as

possible so that the adapter redesign coiild proceed. This was to be followed

by the -Z, -X condition, which was possibly the most critical, and then by

the -Z, +Y condition, considered to be less critical than the other two.

The test loads shown in Table 4 account for the weight of the test article.

Analytical Predictions

Prior to the actual performance of the adapter static test, a complete set

of analytical stress predictions was generated, with which to correlate the

measured strain data. At the time of preparation of this analysis, the loading

sequence and maximum levels had not yet been established. Therefore the

sequence of the pretest analytical effort was as follows:

1. Calculation of required maximum applied test loads to duplicate

the acceleration test.

2. Preparation of unit stress analyses for each of the load components

as required by 1, above.

55
TABLE 4

Static Test Load Combinations Analyzed

-I, +X

Interface Load

MCCc 1 • Test

Loads

Static Test Load s

1 UU/o I est Static/Accel

ACS/TRUSS Flat. c ,oyy 3,007 2,000 0.69

-7,830 -8,127 -6,156 0.79

Mlat. 14,757 19 1 nn \ c , \ UU o ,U 1 U 0.54

TRUSS/SENSORY hat. o ,UU/ 3,007 2,000 U .0/

-8,144 -8,144 -6,173 0.76

Mlat. 154,846 154,840 103,000 0.67

SENSORY/ADAPTER Flat. 10,854 10,854 7,500 0.69

-31 ,555 -J

\

,ou/ 00 AA'i 0.71

Mlat. 264,274 264,240 178,300 0.70

ADAPTER/BASE Mat. 11 ,555 10,854 7,500 0.65

-33,613 - J 1 , 300 -Lc. 00 ,4-3 AQl1 0.67

"= :

Mlat. 532,474 524,740 358,200 0.67

APPLIED LOADS ^acs -7,971 -6,000

(Add weight of Zs/r -22,093 -15,000

spacecraft for

total Z loads) LATacs 3,007 2,000

LATs/r 7,847 5,500

LAT. LOAD APPLICATION

(In. above sensory ring)

(in. above sep. plane)

^acs 51 .5

8.96

51 .5

8.96

TABLE 5

Applied Static Test Loads

Interface

Load

Comnonent

(ALL VALUES IN KILOPOUNDS)

Thrust Only Combined Thrust & Shear

1 2 3 1 2 3 4 5

Thrust - Sensory FIZ -7.5 -7.5 -15.0 -7.5 -7.5 -15.0 -15.0 -15.0

Thrust - ACS F2Z 0 -6.0 -6.0 -3.0 -3.0 -3.0 -6.0 -6.0

Lateral - Sensory FIX 0 0 0 1 .0 3.3 3.3 3.3 5.5

Lateral - ACS F2X 0 0 0 0 1 .2 1.2 1.2 2.0

3h
3. Combination of the above unit stress analyses to produce predictions

representing the actual test loading sequence and maximum levels.

Table 4 shows the resulting interface loads from the + roll acceleration test

(+5. 5g roll lateral and -16. Og thrust), which had been designed to attain 1. 5

times the flight loads. Also shown are the four applied static test loads, the

points of application, and the resulting interface loads which nearly duplicate

the acceleration test loads.

The adapter primary structure was analyzed in depth using MASS. Four

load cases were analyzed, each representing one of the (100 percent) test loads

of Table 4, The definition of these load cases is as follows (adjustment has

been allowed for weight of spacecraft components):

• Applied unit sensory ring thrust load

• Applied unit ACS thrust load

• Applied unit sensory ring lateral roll load (applied at a point 8. 96

inches above the separation plane)

• Applied unit ACS lateral roll load (applied at a point 4. 03 inches

above the truss interface, 51. 5 inches above the sensory ring)

Thrust loads were applied to the adapter separation ring at 18 points,

representing the bases of each of the sensory ring separator castings. Magnitudes of the 18 thrust loads were adjusted to represent the total thrust loads

and bending moments for the above four load cases, as redistributed by the

sensory ring stiffness characteristics.

Sensory ring shear loads were transmitted to the adapter separation ring

as a distributed bearing load varying in accordance with the function 1/2

(1 + COS e ). As noted previously, this distribution will produce no bending

in the adapter ring, only hoop tension, consistent with the actual condition of

a stiff sensory section separation ring bearing on a relatively flexible adapter

ring.

At the time the maximum test loads and testing sequence were finalized,

the imit stress analysis cases were combined in such a manner as to duplicate

these test loads. Predicted values were calculated for combined compression

and bending stresses in the longerons (total stress on the crown, centroid,

and skin elements of each longeron section) and for shear stresses in the skin.

These predictions were prepared for every instrumented longeron and skin

panel section as defined in Figure 13. Stresses were predicted for each of the

loading combinations and sequences outlined in Table 5.

55
The analytical model utilized in the prestatic test analysis was modified

from that used in the preacceleration test analysis as follows:

• Longerons 2, 4, 5, 6, 7, and 11 (the areas of maximum longeron

loads for any of the three loading axes) were reconfigured to simulate

the eccentricity of their centroids from the point of application of the

loads at the upper and lower rings. This change was expected to

eliminate the major portion of the unconservatism existing in the

previous analysis.

• The section properties of longerons 2 and 11, in full, and longerons

1, 4, and 7, locally, were modified to simulate the aluminum angles

added to these members prior to the acceleration test.

• The center bay (between hat section frames) skin panels were

unchanged; that is, they were capable of carrying shear, but not compression, in order to account for relatively low compression buckling

allowables at these locations. This assumption is somewhat conservative for the longerons, but unconservative for the skin panels.

Data Reduction

The reduction and analysis of data recorded during the earlier test phases

(specifically, the shear element test and the axial element test) led to the

development of two methods for evaluating the data output. A brief description

of the development of these methods is outlined in the following paragraphs.

First, it was necessary to establish a method of reducing rosette strain

data to values usable in comparison with standard analytical procedures, that

is, to stresses oriented with the edges of a rectangular plate. The strains

were recorded in three gages oriented with one gage vertical, one diagonal

at 45°, and one horizontal. This gage arrangement is described in the

literature as a rectangular rosette. The interpretation of strain rosettes in

a complex stress field (a combination of biaxial and shearing stresses) is not

well understood, in that the orientation of the principal stresses with respect

to the rosette or the edges of the plate is not precise. A study of the literature and a comparison of measured results with a known loading distribution,

the shear element test, led to the development of a logical and suitable method

of evaluation later used in reducing data from the thrust load test and the

combined thrust and lateral load test.

In contrast to the strain rosette data reduction, the reduction and

analysis of the longeron data is straightforward. The strains recorded by

three channels of data were converted to stresses. The channels represented

were at the crown, the centroid (axial bridge), and the skin intermediate to

56
the attachment flanges, respectively. From these data were extracted the

maximum stress in the longeron attachment flanges, the axial load, and the

longeron bending moment.

Data Analysis and Interpretation

Test data was recorded in the following forms:

• Strain gages and shear rosettes to measure internal stresses

• Load cells to measure input loads

• Load washers to measure base reaction forces

• Dial gages to measure selected deformations

Most significant of the strain data recorded were the strain-versus-load

curves plotted for longerons 4 and 5 (area of failure) during the thrust and

combined shear and thrust testing. In these plots, strain readings were

spotted during the ongoing tests, against preplotted analytically predicted

curves for five channels of strain information per longeron. Typical curves

have been reproduced for longeron 4 as Figure 14 for the combined sensory

and ACS thrust static test and as Figure 15 for the combined shear and thrust

static test. The most obvious fact, upon reviewing these curves, is that the

test values are, in all cases, significantly below the predicted values. This

leads to the obvious conclusion that the skin panels are carrying considerably

more axial load (pure thrust in Figure 14 and combined thrust and bendinginduced axial load in Figure 15) than analytically predicted. This conclusion

is verified by examination of the data reduction for skin panel axial stresses,

indicating significant stress levels in the panels adjacent to longerons 4 and 5.

In particular, for the combined test, data indicate very high compressive

stresses (probably greater than buckling) for rosettes 5, 22, 33, 120, and 121.

High tensile stresses in rosette 20 combined with the high compression evident

in rosette 120 (on the reverse side of the panel containing rosette 20) indicates

high local plate bending which is a very good indication of a buckled state.

Three other observations on the longeron strain-load curves are subject

to interpretation as follows:

1. The test-derived longeron axial stress curves, particularly for the

combined test, draw closer to the predicted curves at the higher

applied-load ranges.

Conclusion; Panel buckling reduces skin efficiency, allowing longeron

to carry a larger proportion of the load.

57
T'OTAL 3.5 5.5 7.5 9.5 11.5 13.5 15-5 IT. 5 19-5 21-5 23-5

TOTAL APPLIED TlffiUST LOAD (KIPS)

Figure 14. Longeron No. 4 Stresses during Combined

Sensory and ACS Thrust Static Test

58
OoO 1.0 2.0 3.0 h.O CONSTo SHEAE 5.0 6.0 7„0 ,0

APPLIED SHEAE LOAD (KIPS)

10.5 CONST. THRUST -| 12.0 1^.0 l6.0 l8.0 20.c(-21„0 CONST. THRUST

APPLIED THRUST LOAD (KIPS)

Figure 15. Longeron No, 4 Stresses during Combined

Shear and Thrust Static Test

59
2. The test-derived longeron skin stress curves "tail-off and, in some

cases, decrease, in the higher applied load ranges.

Conclusion: The stress in the longeron flange does not decrease,

rather, the skin between the two rows of rivets, reaching its buckling

limit, has a considerably reduced efficiency in compression,

3. The test-derived longeron crown stress curves are at very low values.

The spread between the crown stress and axial stress curves are

generally greater than predicted. Both of these phenomena are most

significant at the lower level of gages.

Conclusion; The crown gage location utilized during the acceleration

test was erroneously chosen, because this is the point of minimum

longeron stress. A better choice would have been the axial bridge

(the 70 and 80 series of gages as delineated by Figures 14 and 15).

As indicated above, the skin gage produces erratic results as its

efficiency varies with applied load. The optimum for maximum

stress would be on the attachment flange, but local disturbances due

to riveting makes this impractical.

Conclusion; The longeron bending moment is even greater than

predicted using an eccentric representation of the longeron in the

computer model. This is especially true near the base of the

longeron, additionally pointing out the necessity of a "bathtub-type"

fitting at this prime load-carrying point. A fitting of this type would

remove the eccentricity in the load path and make the longeron stress

nearly pure axial.

Not included are curves plotted for longerons 6 and 7, symmetrically

opposite to longerons 5 and 4, respectively, which theoretically should show

the same loading characteristics. It was apparent during testing, in performing tasks 5 and 6, that strain readings for longerons 6 and 7 were substantially

less than for 5 and 4, respectively. This phenomenon, which has become

known as the load bias, is probably attributable to the stiffness characteristics

of the adapter.

The interpretations presented here tend to support the theory of failure

as concluded in this report—forced crippling at either longeron 4 or 5. This

is evidenced by the high bending moment at the base of the longeron which,

when combined with axial stress, results in locally high compression stresses

in the longeron flange. High compression stresses in the lower skin panels,

when resulting in a compression buckle, could then have been the trigger that

completed the failure.

60
Results of Static Test Program

The static test program was stopped after completing only the first loading

condition (-Z, +X). The two remaining test conditions planned were deleted for

two reasons: (1) the test data obtained were considered to be sufficient to

achieve the basic objectives of the test program; and (2) the adapter design

being tested would never be flown and the redesigned adapter would receive

a complete qualification static and dynamic test. The static tests of the adapter redesign would also permit obtaining any additional data that might be necessary.

The static test results verified the failure mode that was previously proposed. That is, the adapter failure occurred when longeron 4 collapsed in

forced crippling due to the combined effects of eccentric loading and panel

buckling. Although no failure of the static test adapter was permitted, severe

and permanent buckling of the panels adjacent to the lower ends of longerons

4, 5, 6, and 7 was evident. These buckles carried the longeron flanges with

them as shown in Figure 16. As the applied loads increase, the buckle becomes deeper, until it forces its way through the radius of the flange (Point A,

Figure 16). When this happens, load redistribution occurs which causes the

total collapse of the longeron in compression and bending, buckling inward.

From the static test results, it appears that longeron 4 collapsed first,

rapidly followed by longeron 5, which would be near its critical loading when

4 collapsed and dumped its load into the adjacent longerons.

The static test results also indicated the existence of an unexpected bias

or anomaly in the load distribution in the adapter and the reactions at the

adapter base. This bias resulted in much higher reactions at longerons 4 and

5 than at 7 and 6, respectively, although theoretically, they should have been

close to equal. Additional tests were run to try to identify the source of this

anomaly; it appeared to be a property of the adapter. The identification of

this property was not pursued, but it is thought to be associated with the stiffness characteristics of the -X, -Y quadrant as related to the rest of the adapter.

ADAPTER REDESIGN EFFORT

The nature of the causes of the adapter failure set the requirements for

the redesign. After evaluations of the wreckage analysis and the revised

stress analysis, the following requirements for redesign were established:

• The first requirement was to reduce or eliminate the eccentric loading of the longerons. This was accomplished by adding "bathtub"

fittings between the longerons and the lower ring at the eight mounting points, as shown in Figure 17.

61
1. Lower panel adjacent to longeron buckles, carrying

longeron flange with it.

2. As load increases, buckle becomes deeper and progresses

through ra.dius at (a) .

3. This side of longeron dumps its load into rest of

member. Other leg cripples similarly.

^ , Longeron fails in compression and bending, buckling

inward

.

Figure 16. Longeron Failure Mode

62
OflO" 7075-T6 ALUM

ALLOY LONGERONiORia. .QUO")

ORIGINAL SKIN

(•032" MAGNESIUM)

Figure 17. Design Modifications to Lon

63
• The longeron thickness was doubled to 0. 080 in. in order to negate a

requirement for local beefups at the upper and lower ring attachments,

also having the effect of lowering the general level of stress.

• The third requirement was to reduce or eliminate the panel buckling

that apparently contributed to the failure of the longeron. This was

effected in the redesign by riveting 0. 032 aluminum doublers to areas

of the 0. 032 magnesium adapter skin, as indicated in Figure 18. All

of the changes were designed so that existing adapters could be readily reworked to the new configuration. As a result, although almost

all of the existing skin area required the doublers, it was not replaced

by a single aluminum skin.

Because practical considerations dictated that the adapter redesign be

accomplished at a time prior to the static test, the redesign details were established using only the results of the wreckage analysis and the revised structural analysis. This necessitated the use of an overlapping design, since the

quantitative effects of the various possible weaknesses indicated were not as yet

firmly established. The rationale of this overlapping design was as follows:

• Skin doublers were added to ensure every panel would be stable under

a maximum combination of axial compression and shear in order to

eliminate the trigger that initiated the failure, tentatively diagnosed

as forced crippling.

• The bathtub fittings were added at the mounting points in order to

eliminate the local eccentricity between the longeron centroid and the

flange of the mounting ring. This would virtually eliminate the induced bending which allowed the skin and longeron stresses to peak

at the skin attachment flange, having the effect of lowering compressive stresses at the skins immediately adjacent, thus reducing the

cause of the skin buckling.

• Rather than add local reinforcements to key longeron ring attachments,

as dictated by prior analysis and because longerons had to be removed

to allow skin doubling, it was believed that increasing the overall

thickness of the longerons would reduce the overall stress level,

again reducing the possibility of local crippling.

QUALIFICATION OF REDESIGNED ADAPTER

Rather than repeat the acceleration test to qualify the new adapter, it was

recommended that it be qualified by static test to both ERTS-1 and Nimbus-5

6k
-TRUSS TIE-DOWN

LOCATIONS (3 PLACES)

STRUCTURAL

ACCESS DOORS

4 PLACES)

NOTE I DOUBLED PANELS

SHOvVN SHADED

( .032" ALUf/I. )

®

ORIGINAL SKIN

( .032" MAG.

)

LONGERON NOS.

(11 PLACES)

Figure 18. Design Modifications to Adapter Skin

65
load levels (MECO-6) for three lateral load directions (+X, -X, -Y). Sufficient

instrumentation was provided to preclude unexpected failure, to support analytical modeling, and to identify the load distributions. The basic test objectives were:

• To qualify the prototype flight adapter, as modified, to 1. 5 times the

then anticipated Nimbus-5 and ERTS-1 launch vehicle flight loads.

• To provide complete data for verification and improvement, if required, of the adapter structural analytical model.

• To provide sufficient data to establish an experimentally-derived six

degree-of-freedom stiffness matrix for the adapter, for frequency

correlation between static and dynamic testing.

The qualification static test article, procedure, and instrumentation were

virtually identical with those for the failure analysis static test (see Figures 12

and 13). The qualification test article representation of the ACS, truss, sensory ring, and separation band were the same articles as those utilized in the

failure analysis test. The redesign adapter was the adapter utilized in the

previous test reworked to the new configuration.

The instrumentation was very similar to that previously shown in Figure 13,

but with additional axial strain gage locations on the key longerons. For instance, on longeron 4, instead of a total of 5 gages at 2 levels, there was a

total of 17 gages at 7 levels, the additional levels predominantly between the

circumferential skin stiffening rings. It was felt that these additional gages

would be an indication of the shear lag; that is, they would enable an estimation of the relative load distribution between skin and longeron. These additional gages exceeded the capacity of the recording equipment; therefore, the

gages were placed in three patch groups, one of which was recorded for each

axis of test determined by which group would be most active. Dial gages

to measure upper ring deformations were strategically located.

Qualification level loads were to be applied to the adapter representative

of both ERTS-1 and Nimbus-5 flight loads for both roll (X) and pitch (Y)

lateral axes. Tables 6 and 7 define the required test loads, their resulting

interface loads, and the relationship to the various flight interface loads for

roll and pitch axes, respectively. The columns labeled RATIO should indicate

1. 50 to provide the proper safety factor on qualification. Again loading

schedules were selected to build up to maximum values in an orderly manner

to minimize the risk of unexpected results. Basically, thrust loads were

first applied, then lateral loads were built up to provide maximum combined

test levels.

66
TABLE 6

Comparison of ERTS-l/Nimbus-S Roll/Yaw Qualification Loads

and Flight Loads for POGO Condition

NIMBUS5

INTERFACE LOAD FLIGHT TEST RATIO FLIGHT TEST RATIO

Appl ied Test Loads

Thrust-Sensory

Thrust-ACS

FIZ

F2Z

25,900

5,900

(1)

(2)

25,700 (1)

7,800 (2)

LaLcra 1 - ocilSOry

Lateral -ACS

FT Y r 1 A F2X

2,200

1 ,600

(3)

(4)

4,600 (3)

3,100 (4)

interTace Loaus

ACS/Truss Fx

FzMy

908

3,824

7,265

1 ,600

5,900

6,450

1 .76

1 . 54

0.89

1618

4684

22193

3100

7800

12500

1 .91

1 .66

0.56

Truss/Sensory FxFz

My

908

3,916

54,899

1 ,600

5,900

82,400

1.76

1 .50

1 .50

1618

5233

107330

3100

7800

159600

1 .91

1 .49

1 .49

Sensory/ Adapter Fx

FzMy

2,542

20,573

87,674

3,800

31 ,800

130,300

1 .49

1 .54

1.49

5140

21747

172548

7700

33500

256700

1 .50

1 .54

1 .49

Adapter/Base Fx

FzMy

2,545

21 ,190

148,674

3,800

31 ,800

221 ,500

1 .49

1 .50

1 .49

5147

22367

296298

7700

33500

441500

1 .49

1 .50

1 .49

NOTES: (1) Indicated load includes 1500 lb. weight of sensory ring

and loading fixtures.

(2) Indicated load includes 200 lb. weight of loading fixtures.

(3) Lateral sensory ring load applied by one loading strap at a plane 12.33 in. above separation plane.

(4) Lateral ACS load applied at a point 4.03 in. above Truss

Interface.

6?
TABLE 7

Comparison of ERTS-l/Nimbus-5 Pitch/Yaw Qualification

Loads and Flight Loads for POGO Condition

INTERFACE LOAD

ERTS- 1

FLIGHT! Test RATIO FLIGHT

NIMBUS-S

TEST RATIO

Appi ied Test Loads

Thrust-Sensory

Thrust-ACS

Lateral -Sensory

Lateral-ACS

FIZ

F2Z

FlY

F2Y

26,000 (1)

6,000 (2)

2,100 (3)

1 ,040 (4)

25,700 (1)

7,900 (2)

1,280 (3)

500 (4)

Interface Loads

ACS/Truss

Truss/Sensory

Sensory/ Adapter

Adapter/Base

-y

Mx

k

Mx

Mx

660

3,824

4,429

660

3,916

35,779

2,081

20,573

54,387

2,083

21 ,190

104,334

1 ,040

6,000

4,190

1 ,040

6,000

53,600

3,140

32,000

81 ,500

3,140

32,000

155,800

(2)

(3)

(4)

1 .57

1 .56

0.95

1 .57

1 .53

1 .50

1 .51

1 .55

1 .50

1 .50

1 .51

1 .50

298

4,684

3,051

298

5,233

17,206

1 ,197

21 ,747

27,417

1 ,196

22,367

56,121

500

7,900

2,020

500

7,900

25,800

1 ,780

33,600

41 ,000

1 ,780

33,600

83,700

1 .68

1 .68

0.66

1 .68

1 .51

1 .50

1 .49

1 .54

1 .49

1 .49

1 .50

1 .49

Indicated load includes 1500 lb. weight of Sensory Ring and

Loading Fixtures.

Indicated Load includes 200 lb. weight of Loading Fixtures.

Lateral Sensory Ring load applied by two loading straps with

centroid of applied load 6.85 in. above separation plane.

Lateral ACS load applied at a point 4.03 in. above Truss Interface.

68
Prior to the qualification static test of the modified adapter, a complete

set of stress predictions was generated in order to correlate with measured

strain data. The structural analysis was performed in a manner very similar

to that for the original static test: calculation of maximum required test loads,

preparation of four unit load stress analyses (one each for ACS-applied thrust,

sensory-applied thrust, ACS-applied shear, and sensory-applied shear), and

combination of the unit analyses in a manner to represent the applied loads

schedule. The following changes in the analyses in a manner to represent the

applied loads schedule. The following changes in the analytical computer

model were required:

• Longerons were reconfigured to eliminate the eccentricities at their

bases by assuming that the mounting bolts were located at the longeron

centroid, extended. This assumption now makes the mounting points

eccentric to the lower ring centroid, as in actuality.

• Longeron section properties were increased representing increased

thicknesses.

• Skin panel thicknesses were increased representing doubled areas.

• Axial load capability was added to all skin panel simulations.

As could be anticipated, with the overlapping design modifications, the

general stress levels were determined to be rather low. With the increased

crippling and buckling allowables, as a result of the thickened longerons and

doubled skin panels, there was no evidence of potential crippling stresses nor

initial panel buckling. All margins of safety were found to be substantial.

Static qualification testing was successfully completed. All of the test

objectives were met, post test inspections indicated no failures nor permanent

deformations, and reduction of data indicated no excessive measured deformations or stresses. As a result of the testing program the following conclusions were drawn,

• The adapter primary structure was qualified to both Nimbus-5 and

ERTS-1 qualification level loads as defined. Actual test loads and a

comparison with predicted flight loads are summarized in Tables 6

and 7.

• Strain data obtained, although only reduced to a limited extent, indicated that the static test tends to verify the analytically-predicted

load paths and stress levels and provided sufficient data to enable

69
prediction of loads and stresses for future loading conditions.

• Dial gage information obtained allowed construction of a 6-by-6

stiffness matrix of the adapter which verified stiffness characteristics

found from dynamic testing.

Because the qualification tests proceeded to a successful conclusion

without event, it was felt that only a limited reduction of strain data was required. One basic observation, however, was apparent. Although stress predictions were substantially more accurate than previously noted, the measured

stress levels in the longerons were still lower than anticipated. Obviously,

the MASS computer routine did not accurately predict the thrust, nor the

bending-induced thrust, load distribution between skin and longerons. In a

practical sense, the utilization of the MASS routine requires the breaking into

panel segments, only those areas bounded by structural members, such as:

longerons, rings, or other stiffening members. This technique has the effect

of softening the panel simulations; that is, the panel simulations were too

large relative to the stiffnesses of the framing members.

In addition, it was noted that a good correlation between computed skin

panel stresses and measured rosette strain gage data was not readily obtainable. These results, coupled with other observations obtained during the

failure analysis efforts, have led to the following conclusion: Axial and shearing stresses, in a skin panel adjacent to highly loaded framing members, vary

too rapidly across its span to be either accurately measured using a single

strain rosette, nor calculated using a single plate element.

CONCLUSIONS

Based on the failure analysis results, it was concluded that the acceleration

test failure was caused by inadequate adapter strength which was not predicted

by the pretest structural analysis. Adapter failure was caused by the initial

collapse of longeron 4, as a result of forced crippling, due to the combination

of panel buckling forces with the basic compression and bending loads. This

has been verified by both analysis and test.

Additionally, it was concluded from this failure analysis that the adapter

failure during the acceleration test was predictable by analysis if longeron

eccentricities and panel buckling effects could have been fully simulated in the

structural analytical model. In this case, it was not possible to completely

simulate both the local eccentricities and the skin buckling effects; therefore,

the computerized analysis had to be supplemented by additional analysis to

properly account for these effects. The structural analytical model has been

modified to include all local eccentricities as completely as possible and,

when supplemented by appropriate manual analysis, has produced results

idantif^'ing the mode of failure in agreement with the results of the subsequent

static test.

70
Finally, it became apparent during the failure analysis and subsequent

static testing that analytical techniques must be further sophisticated in order

to more readily predict a highly local failure in a structure loaded nearly to

capacity. Sophistication of local failure predictions would:

• Attempt to include in the analytical model the effects of all local

eccentricities, particularly those at major load-carrying attachments.

If not included in the analytical model, these effects must be considered

in a separate supplementary analysis.

• Include the relative effects of the compression-carrying capability of

all skin panels or other predominently shear-carrying members. The

relative axial stiffnesses of the longerons and the skin panels have

been assessed by the static test results. In this specific case, even

with full compression-carrying capability included, static testing

indicated that the skin still carries more axial loading than predicted

by the MASS analysis. Because, to a large extent, practical considerations dictate the modeling techniques used in panel simulation,

the effect is one of tending to soften the panel representation. Again,

a supplementary analysis appears to be a logical solution.

• Include a local analysis which would assess the effects of a combination

of local skin buckling adjacent to a highly-loaded compressioncarrying primary structural member. In lieu of this, ensure that all

panel members will not evidence initial buckling in combined compression and shear at ultimate rather than limit loads. Such a

shotgun approach was felt to be necessary in cases where overlapping

designs—thickened longerons and doubled skin panels—were utilized.

It has been found that relatively small eccentricities, while not radically

altering the overall structure load distribution, may have a significant effect

on a local area. It has been the authors' experience that, if due to practical

considerations on computer modeling, an eccentricity exists or panels have

been simulated too coarsely, an acceptable solution is to break out a small

section of the model to be more accurately simulated. Loadings may be provided by the original computer printout as internal loads at the boundaries of

the supplemental model- Such loads may then be more properly redistributed

fully evaluating the local effects. This technique, which provides the solution

to all three items above, has already been incorporated in the structural

analyses of several areas of ERTS and Nimbus series spacecraft components.

I 71
In summation, using the redesigned configuration of the adapter, as outlined in this report, ERTS-1 was successfully launched into orbit in July 1972,

and in December 1972 Nimbus-5 successfully attained orbit. After successfully exceeding their design lifetimes in orbit, both spacecraft are currently

operating in reduced modes, but still returning useful information to earth

stations.

Being readied for launch in early 1975 are two more in these series of

spacecraft— ERTS-B and Nimbus-F (to be designated ERTS-2 and Nimbus-6

after successful launch). Both spacecraft have been fully qualified and utilize

flight adapters structurally identical to that described.

72
DISCUSSION

J. J. Scialdone, NASA, Goddard Space Flight Center : Does the computer

program now include buckling instability?

V. T. Sweet : As we use it, it does not. I understand the developer

of our computer program has managed to mathematically simulate this

phenomenon, but this simulation has not been incorporated into our

program,, If we want to predict buckling, we have to do it in a series

of stages rather than have it taken care of mathematically.

J. J. Scialdone : Did you try the same program or a different

computation with the [\|ASTRAN program, or was it not available at that

time?

V, T. Sweet : At the time of failure we considered using NASTRAN, but

that approach was given up because it was too costly to completely

simulate the model. We had already made plans to do a static test and

we thought that would be sufficient to determine what the actual cause

was. In the future we may use a program similar to NASTRAN for such a problem.

G. Wagner, Westinghouse Electric Corporation : How did the nominal

values of strength that you used to get your factors of safety compare

with the actual measured values in the failure analysis?

V. T. Sweet : They were quite similar.

75
AGRICULTURAL EQUIPMENT FOR UNDERDEVELOPED COUNTRIES

Charles W. Suggs

Department of Biological and Agricultural Engineering

North Carolina State University Agricultural Experiment Station

Raleigh, North Carolina 27607

Equipment designed for underdeveloped countries should be simple to

adjust and operate. Where possible it should also be simple and easy

to construct using locally available craftsmen as well as materials.

Malfunctions, although undesirable, can be tolerated provided they can

be rectified locally. What cannot be tolerated are long delays while

a part is shipped in from some distant point. Also, machines requiring

special service tools or skills are difficult to maintain.

Because the safety of operators, passengers or community is not

usually at stake in the malfunction of simple agricultural devices, mechanical failure does not have the same meaning as on airplanes, automobiles, space crafts and nuclear power stations. Malfunction, especially of critical components, in these machines often results in serious

and immediate jeopardy of life. Malfunction in simple agricultural

machines does not usually constitute such a hazard. If the machine cannot be quickly repaired the jeopardy can be expected later in the form

of increased labor to manually perform the machine's task or loss of the

goods or services which the machine produced. In extreme cases, especially in tribal societies, this could mean famine.

In addition to simplicity the over-riding design philosophy is that

the machine should be built as much as possible by local craftmen using

local materials. If they can build a new one they should be able to

repair an old one.

Shafts and bearings are items that sometimes must be supplied from

"outside". However, it has been shown that slow speed shafts may be

made of wood and fitted into wood bearings. Shafts of several hundred

rpm are usually made of steel but may be fitted into hardwood bearings

soaked in oil. The desirability of wood as a building or construction

material is obvious both from the standpoint of availability as well as

the ability of local craftmen to work and shape it. Village blacksmiths

in many remote areas are able to work metal quite effectively and produce the metal brackets, wear surfaces, hinges, etc., needed.

7^
Because wood cannot be welded like steel or other metals, care must

be exercised in fastening parts together. Nails, bolts and screws work

well provided sufficient shear surface is provided. Otherwise the fastener tends to cut through or pull out of the wood. Mortise and tenon

joints of various types in which part of one member is inserted in an

opening in another have been used for centuries and overcome many of

the disadvantages of nails, bolts and screws. They should be used as

extensively as possible. Modern technology has produced excellent glues

but these are often not available in underdeveloped or remote areas.

Use of available glues in load bearing applications should be limited to

mortise and tenons where they can keep these joints from coming open

under load reversals (tensile loads). Reinforcing high stress areas

with metal straps, corner to brackets, etc., is a useful practice.

Over the past several years the writer has designed several agricultural machines for use in underdeveloped countries. This work has

been done in response to requests from VITA (Volunteers for International Technical Assistance) which serves as a clearing house between technical cooperators and the person requesting assistance. A few of the

devices designed are described below.

Animal Powered Pump

Figure 1 shows a mechanism for applying animal power to the handle

of an existing man-operated pump. It consists of a beam, mounted on a

vertical shaft, to which a draft animal can be hitched and constrained

to move in a circle. The shaft drives a beveled gear and pinion set

to which a crank arm is attached. A connecting rod between the crank

arm and the pump handle provides the required reciprocating motion.

This design is more difficult to repair than it should be as the

gear set would not be available in many remote areas. Some ideas have

been advanced which would provide the necessary motion without the

gears. It has appeal because it applied animal power to deep wells

where the Persian Wheel is not applicable.

Coffee Bean Sheller

A machine to remove the pulp and outer covering from coffee beans

is shown in Figure 2. It consists of upper and lower wooden counterrotating disks fitted with interdigitating spikes. A feed hopper supplies the beans to the shelling area through the hollow shaft of the

upper disk. Bearings may be of wood. The support frame (not shown)

would also be of wood for ease of construction and repair.

75
Tobacco Twister

A tobacco processing machine, designed to twist leaves of chewing

tobacco into a rope, is shown in Figure 3. It is man powered by means

of a treadle and looks like a heavy duty spinning wheel. Again, the

construction is of wood with a minimum of complex detail. Leaves with

midribs removed, are arranged on the table, twisted by the spindle and

then wound on the drum. The machine should operate trouble free if

properly constructed, but if malfunctions do occur they can be easily

repaired. As this machine converts a bulky crop into a product which

can be stored, transported and marketed it could be a valuable asset to

a tribe or other small production group.

Twine Spinning Machine

Various types of naturally growing fiber can be twisted into twine

or rope by the machine shown in Figure 4 which was originally designed

to spin a particular variety of palm leaves. It is very similar to

the tobacco twisting machine except that it operates at a higher rpm

in recognition of the greater number of twists required. Also the

twisted material can be wound onto the spool without stopping the machine's rotation. When the tube and spool rotate together no windup

occurs. When the spool is stopped, by hand pressure, for example, the

twisted thread is drawn through the tube and wound on the spool. A

slight drag introduced between the spool and frame makes for automatic

windup

.

Pedal Powered Thresher

In many areas of the world human beings are used as a source of

mechanical power because motors or engines are not readily available and

animal applications may not be easy to make. Figure 5 shows a pedal

operated thresher which is significantly more efficient than man powered flails or animals stomping out the grain with their hooves. Pedal

actuation was selected because the legs are stronger than the arms. In

order to conserve power and energy, only the heads of grain are allowed

to enter the threshing mechanism.

The mschine is built on a wooden frame. Figure 6, which is mortised

together. The threshing mechanism consists of a wooden barrel-like container inside which a series of bars, mounted on disks at each end, is

rotated. These bars pass in close proximity to fixed bars in the bottom

of the barrel-like container and strip or thresh the grain from the

heads. The straw is then withdrawn by the operator and the grain falls

through slots into a tray.

Everything except the shafts, the threshing bar surfaces and the

drive belt can be made from wood and the belt can be made of local rope

76
or leather. Shafts turn In oiled wooden bearings but a metal sleeve

or antifriction bearing could be added if available. Also, it would

be possible to mechanical power the machine wherever motors or engines

are available. An additional pedal set could be added to divide the

load between two people.

A cleaning attachment was added, Figure 7, to remove the chaff

from the threshed grain. This consisted of a belt driven wooden propeller fan arranged to blow air across the stream of grain as it falls

into the collecting tray.

In all of these applications the emphasis has been on simplicity

of design, ease of construction using local materials and workmen, and

the capacity to be repaired without requiring service parts from "outside." Although the tools described cannot increase productivity enough

to make the user competetive with mechanized agriculture, they may support the production of small amounts of goods for trade purposes which

is a first and necessary step toward further mechanization. Also, these

simple devices could well be the first step in the development of a ladder of technological skills which would pave the way for the introduction of more complex and more productive devices. Much depends on

the ease with which machine failure can be prevented.

77
K-Akl l"m to"

Figure 1. Animal powered reciprocating pump.

Figure 2. Coffee bean depulper.

78
Fig>'.r. 3. Machine for twisting cured tobacco leaves into a rope.

Figure 4. Twine making machine.

79
plans for a soybean thresher by a VITA volunteer

Figure 5. Perspective view of grain or soybean thresher.

80
, giM^, .l.iM

sui£_aii)tFigure 6. Drive and threshing mechanism showing slots for threshed

grain to fall through.

^^<m.: -a> |Htji:ALL EXN IIHIJ- UCJCUJ will IMCStASE.

CUT i^^w/IDt JJ'otcP C4J4NHH-

^^^^

1]

XTi—-am W 1 I //'

rAH PLTAIL. tbljgWIHC

0W& «J aeaw^ l^t+c- com*! tf* -GOV j -autu

-^-i^-Ta 1 — *^ —

Figure 7. Design of cleaning fan for thresher,

81
DISCUSSION

R. M. Thomson, National Bureau of Standards : Agricultural design is

probably several hundred thousand years old. I wonder if design can

still be improved on. Do you find yourself going back to old designs

and putting things together?

Co W. Suggs : Sometimes. Designs are available for a great deal of

the equipment that is needed in some of the underdeveloped areas.

Usually adaptation is necessary because the conditions are different.

J. Qroshnik, U. S. Navy : I am curious to know how the coffee grower

ultimately separated the coffee bean husk from the bean itself.

C. W. Suggs : I do not know. I wish that it was possible to have more

feedback from the people in the field so we could see how they make

use of a design. This system was not complete; it only carried the

bean part way. From there, a floatation technique would be needed to

separate the hull from the bean. We had to assume that the operator

had available some way of doing this.

82
SAFETY SUCCESS THROUGH DESIGN

David W. Logan

U.S. Army Agency for Aviation Safety

Fort Rucker, Alabama 5656O

The U.S. Army Agency for Aviation Safety has the mission of determining ways and means to enhance the combat capabilities of the U.S.

Army through the conservation of aviation resources. The aviation safety effort to be discussed here involves the use of computer stored mishap data, the research and analysis of that data, and the application of

the results of that research.

We are convinced that if the hazards that produce accidents can be

absolutely identified, then design, training and supervision can reduce

the potential of these hazards becoming cause factors. With this conviction we want to get plugged into design and training cycles as early

as possible. The past solution to safety problems has been "fly-fixfly." This has been replaced through the use of the system safety process which uses not only problems from similar aircraft, but problems

from similar subsystems and analysis of the new design.

The order of precedence in the resolution of hazards includes the realization that not all hazardous conditions can be "designed out." If

the presence of a hazard must be accepted then safety devices, warning

devices or special procedures have to be developed.

The first example of the application of this process involves the

flight helmet used by aviation crew members. Injury data for the calendar year 1967 to 1969 shows that nearly one in four fatalities was due

to head injury (Fig 1). The vast majority of these people were wearing

helmets. Why then was the head so vulnerable? The helmet became dislodged during the accident.

Army Aviation Accident Fatalities

Calendar Years 67-69

Fatal Head Injury

Total Non Thermal Fatalities

Percentage as Head Injury

175

766

22.8%

Fatal Thermal Injuries

Total Fatal Injuries

Percentage as Thermal

328

1094

30%

Figure 1

83
Qk
<1 ^

C/2 "-^

85
Substantial redesign of the helmet was accomplished. The general

appearance wasn't changed except for an "earmuff" look (Fig 2). The

major changes are inside. The new helmet uses a suspension system not

unlike a football helmet (Fig 5). The suspension system attaches to

the head and the helmet is attached to the suspension system. Injury

data from fiscal years 1972 to 1974 shows only one in 10 fatalities due

to head injury (Fige U & 5)« This is a substantial improvement achieved

through well founded design and design criteria.

Army Aviation Accident Fatalities

Fiscal Years 72-74

Fatal Head Injury 38

Total Non Thermal Fatalities 364

Percentage as Head Injury 10.4%

Figure 4

Army Aviation Accident Injury Data

Old Helmet New Helmet

No. Percent No. Percent

Personnel Wearing Helmet 160 739

Helmet Dislodged from Blow 5 3. 12% 19 2. 57%

Helmet Dislodged from Deceleration 11 6. 89% 7 0. 95%

Helmet Contributed to Injury 9 3. 75% 6 1. 22%

Figure 5

Another example of safety achievement through design relates to the

fact that 30% of the accident fatalities are thermal. Aircraft carry

fuel. Fuel burns. After the violence of a crash impact, fuel can be

ignited by a variety of ignition sources and engulf the entire aircraft

in a fraction of a second. The weakness of the fuel cells, fittings and

lines resulted in conceptualization and design of crashworthy fuel

systems. This includes fuel cells that are relatively rupture proof,

fuel fittings that are self sealing at breakaway, and fuel lines that

pass loads on to the breakaway fittings. Again the design that is

oriented towards the environment - the crash environment - has produced

some impressive results. The occurrence of thermal fatalities in impact-force survivable accidents has gone to zero (Fig 6). In the

System Safety Logic Process this represents a "safety device." There

are cost and weight penalties associated with these crashworthy systems,

but the savings in resources have been overwhelming.

86
Crash Injury Data

Survivable Accidents

100 T

Figure 6

There is more work not yet proven. Crashworthy structures are to be

a part of the new Utility Tactical Transport Aircraft System (UTTAS) and

the Advanced Attack Helicopter (AAH) . Hydraulic systems and power train

systems are being designed with an eye towards the prevention of the

mission abort failures as well as the catastrophic failures.

There are many areas in which safety success can be achieved by

component design. We in the Army have engineering capabilities. We use

these capabilities to sort out the problems. But we rely very heavily

on industry to apply the principles of design engineering to achieve the

needed safety success. Identification of problems in terms of criteria

that can be "design criteria" is challenging, but it produces results.

It's the only way to go.

(Opinions stated above are not to be construed as an official Department of the Army position unless so designated by other authorized

documents .

)

87
DISCUSSION

R. E. Maringer, battel 1e Memorial Institute : Has the crashworthy fuel

system technology been applied to the automotive industry?

D. W. Logan : To the best of my knowledge, no. However, some of the

automotive manufacturers have positioned fuel tanks in less vulnerable

positions rather than building them of flexible material as the Army

is requiring.

R. E. Maringer : To civilian aircraft?

L). W. Logan : I believe that it has not been applied to other than

crop duster aircraft.

R. Lenich, Caterpillar Tractor Company : Would you elaborate on the

Tele-torg method vs the torque wrench?

i). W. Logan : The Tele-torg bolt that Boeing selected has a hollowed

out shaft. A pin goes all the way through the head to the threaded

end of the bolt. Then the bolt is filled with fluid and a glass cap

is put on Lop. As the bolt is stretched by tightening a nut, the pin

that is against the glass window is pulled away from the window and

fluid covers the head of the pin. So, it indicates the preload in

tension and not torque. From what I understand, the Tele-torg bolt is

reusable.

88
RELIABILITY, POLLUTANTS AND ALUMINUM RAW MATERIAL

Vaughn Dean Matney

General Dynamics Corporation

Convair Aerospace Division

Fort Worth, Texas 761OI

SUMMARY

This paper describes how subtle pollutants from the storage

atmosphere led to a sizeable corrosion condition in aluminum tubing raw

stock that was suspected of adversely affecting the reliability of the

end products. This paper is divided into the following major topic

segments. A description of the factors contributing to the corrosion

condition is first given. This is followed by a discussion of the

effects of the corrosion condition on the reliability of the products.

Next is a review of the planned actions to resolve the condition.

Specifics of the test program are then discussed which include the

different types of testing performed and the test results. At this

point enough data is available to draw some basic conclusions to

conclude the paper.

The investigation covered the following three basic types of

aluminum tubing in various heat treated conditions: (1) 2024, (2) 6061,

and (3) 5052. The 2024 which has a high copper content for aluminum

tubing and is used for more structural applications was the most

severely damaged by the corrosion and as a result required the most

corrective measures.

DESCRIPTION OF CONDITIONS

The fact that subtle atmospheric pollutants infiltrated our

aluminum raw stock area to the degree that corrective measures became

necessary establishes the basis for this paper. Seldom do we have great

concern over such a simple thing as the apparent normal environment of

the storage area for raw materials. Warehouses across the country being

somewhat open to the elements and shipments being made in open trucks

result in a tendency to accept raw material storage with less concern

than we reserve for completed end products. We were concerned enough

with our raw material storage, however, that new dry and ventilated

warehouse facilities were provided at the beginning of the production

line. Yet under these conditions pollutants in the form of corroded

tubing were detected in the production cycle. Tracking back through

the production steps the corrosion was traced to the raw material stores

89
area. Investigation in the raw material stores area revealed that dust

particles settled on the aluminum tubing while stacked vertically in

racks. The particles were on both the inside and the outside of the

tubing. The large diameters had more inside than did the small diameter tubing. The dust particles absorbed humidity from the air along

with just enough fume traces from material cleaning and processing dip

tanks located in the next building bay to be corrosive. The shape of

each corrosion pit followed the shape of a dust particle resembling a tiny worm track. For the most part they were minute shallow surface

pits no larger than a dust particle. The density of dust particles

settling on the surface of the aluminum determined the lateral extent

of corrosion. A review of air ventilation movement disclosed that air

coming in from the roof ventilation could induce fume traces. For, on

occasion, the wind could be from the proper direction to move vented

processing fume traces outside the building across the ventilation

openings above the aluminum storage area and down into and onto the raw

stock. Such movement could allow the fume traces to be trapped by the

dust particles on the aluminum. The corrosion appeared minute and

negligible in most cases, even though some aluminum pieces did give the

appearance of being somewhat heavily worm tracked. This in itself would

have little to no effect on the reliability of the end products which

include such things as air conditioning ducts, hydraulic lines (high and

low pressure), drain lines, hot air flow ducts, push pull tubes for

control systems, structural braces and other things. Samples of the

tubing were subjected to laboratory analysis to determine the extent

and effect. The analysis revealed shallow surface pitting which was no

real concern for alarm provided the corrosion was arrested prior to

finish application. However, in addition to the surface pitting some

intergranular corrosion was detected which was considered to be severe

in nature for critical applications. Intergranular corrosion, if not

arrested, will go deep into the metal. It progresses along between the

grains and eventually destroys its structural properties resulting in

part failure. Critical applications in our case were such things as

high pressure hydraulic and pneumatic lines and control system linkage

members (push pull rods) . The intergranular corrosion was detected in

all three types of tubing analyzed but was the most prevalent in the

2024 due to its copper content. In those cases where the intergranular

corrosion was found, it most usually started in the bottom of a corrosion pit and extended down into the aluminum. The paths of corrosion

were very evident in specimens that were cross sectioned and polished

for examination. The findings from these analyses resulted in ttie

establishment of a plan of action to prevent an impact on the reliability of the end products.

EFFECTS ON RELIABILITY —-

'

A major concern created by the presence of the corrosioh was the

potential effect on the reliability of the end products. This effect

would be a reduction in reliability due to part failures. The presence

of slight surface pits would not affect the functional use of the

90
aluminum tubing parts. However, the threat of the presence of intergranular corrosion caused a classification of all affected aluminum

parts into the two categories; critical and non- critical . It was

decided that a test program to thoroughly measure the extent and effects

of corrosion on the three aluminum types involved was necessary to

evaluate corroded parts in tne critical category for final disposition.

It was felt that intergranular corrosion could damage the structural characteristics of the aluminum causing it to fail under load. It

was concluded that such failures would occur after the parts had been in

service for a period of time but before they should reach their life

expectancy. High pressures and/or physical loads would be seen by the

critical parts. The non-critical parts were such things as low pressure

air flow ducts, return lines and light load bearing members. As such

they would not be stressed to the point that they would be likely to

fail.

Another concern was to determine when the corrosion started and

how far it reached into production. A special study was made in this

direction to determine the most likely production lots affected. This

portion of the investigation keyed from a likely date that the corrosion

could have started. Since the cause was associated with the new storage

area, the date it was put into use service became a key in determining

part lots that could be affected. The condition was primarily isolated

to parts in work and in stock. To eliminate this apparent threat to

product reliability, a plan of action was decided upon.

PLANNED ACTIONS

It became evident that an overall plan of action was necessary to

cover all major elements of the condition. It would be necessary to

purge the raw materials receiving and storage area, review parts in work

on the production line and evaluate the effect on completed parts for

possible rework or replacement action. A detail plan evolved which

encompassed the following major actions.

Stock Purge

The entire aluminum tubing raw materials stores area was purged.

New receipts were inspected during this time and some corrosion was

found. However, it was not to the extent of that in stock. This

amounted to the immediate rejection of approximately 620,000 feet of

aluminum tubing which had exhibited corrosion under the visual inspection of a lOX hand magnifying glass. Test samples sent to the materials

processing lab were cross sectioned and examined under magnifications

up to lOOOX to confirm intergranular corrosion. This material was held

until there was enough valid test data to make a reasonable disposition.

Approximately one- third of this tubing was ultimately reclaimed.

91
All critical tubes were pulled from stock, stripped of paint, reinspected and corroded parts were rejected. Half of all non-critical

painted tubes and all non-painted non-critical tubes underwent similar

treatment to evaluate the extent of the corrosion and to evaluate if it

was necessary to inspect all of the non-critical painted parts. The

other half of the painted non-critical tubes were held and ultimately

not reinspected due to subsequent faborable test results which indicated

that chemical cleaning prior to fabrication and the part finish (anodize

and paint) had arrested any corrosion. The effects of corrosion on

these parts was not of sufficient magnitude to cause part failure. All

tubes in process of fabrication were reinspected and the corroded parts

rejected. All rejected raw material and parts were reviewed for the

proper disposition.

Tubing Control Board

A special tubing control board was established to recommend disposition to Material Review Board (MRB) for rejected tubes and raw

material

.

The board members consisted of:

1, Design Groups

2. Tooling

3. Product Assurance

4, Process Control - assisted by:

a. Inspection

b. Manufacturing, and

c. Manufacturing Control

The board reviewed all rejected tubes from manufacturing and made

a documented recommendation to MRB for disposition. These dispositions

were based on results from the test program taking into account the

material type and the application of the part by the board members.

When necessary, the control board would request special testing (i.e.,

sample sectioning and inspection under 500X - lOOOX magnification") on

lots of parts to aid in making their recommended dispositions. This

board was established in the factory area and ultimately reviewed

approximately 60,000 parts of which approximately 17 percent were

scrapped by MRB and 837o were returned to production as acceptable for

use. The scrappage occurred primarily on those corroded tubes that were

to be used in what was considered to be a critical application. Critical applications were considered to be such usage as high pressure lines

and load bearing members of structural or linkage mechanisms.

Over half of the raw material was ultimately scrapped due to the

extent of the corrosion, for only that material that was considered to

be usable for both critical and non-critical applications was retained.

It was chemically cleaned in dip tanks to arrest the active corrosion

and placed in polyurethane bags for restocking.

92
Industry Conference

Experts from the leading aluminum companies were called upon to

review the condition for recommendations and opinions. It was the

opinion of ttie aluminum companies that chemical cleaning in dip tanks

and subsequent anodine/paint finishing would permanently arrest the

corrosion. However, they recommended a testing program to determine:

(1> if the corrosion was in fact arrested; (2) the effects of the corrosion on the structural characteristics of the aluminum tubing; and (3)

the effects of weather environment on a large sample of different

aluminum finishes on the different aluminum types. These recommendations

were subsequently implemented with the development of a test program on

the aluminum tubing.

TEST PROGRAM

The test program associated with the corroded aluminum tubing

condition consisted of the following types of tests performed on several

heat treated conditions of the three types of aluminum involved; (1)

2U24, (2) 5052, and (3) 6061. Each type of test and its results are

discussed separately. The results of these tests shaped the actions

taken to prevent detrimental effects on the reliability of the products.

Intergranular Attack Examination (Visual-Microscopic ")

Raw stock, partially fabricated, and completed production parts

were examined to determine:

1, If corrosion was present

2, Type of corrosion

3, Depth of corrosion

A total of 735 parts was selected for this examination. The

samples were from both raw materials and production parts.

Each specimen was sectioned at areas of maximum appearing corrosion,

with up to four sections par part. These sections were mounted,

polished, and visually studied. Each section had from zero to four

corrosive areas, thus giving up to 12-16 pits per part to evaluate, with

the maximum depth being the recorded figure.

All studies were made at magnifications up to 500X, and the intergranular suspects verified at lOOOX magnification. The results are

shown in the following table:

95
Total Intergranular Surface No

Material Specimens Corrosion Corrosion Corrosion

2024 52 33 15 4

5052 501 50 416 35

6061 182 _64 _99 19

TOTAL 735 147 530 58

The maximum depth of corrosion per specimen is shown in the

following table.

IG = Intergranular

Surf = Surface

Depth in inches.

Corrosion

Depth

2024 5052 6061

IG Surf IG Surf IG Surf

Surface 0 7 1 181 23 71

- .0011 6 5 12 119 13 14

.0011 - .0020 24 3 32 110 19 14

.0021 - .0030 3 0 3 5 7 0

.0031 - .0040 0 0 2 1 2 0

33 15 50 416 64 99

TOTALS 48 466 163

TOTAL 677

Depth summary. The above table measurements show that of the 677

specimens evidencing corrosion, 654 had maximum corrosion depths of

0.0020 inch or less, and no depths greater than 0.0040 inch.

A percentage breakdown of corrosion depth of the 677 specimens is

as follows:

Corr o sion Depth No. Specimens % of Total

Surface 283 42

- .0010 169 25

.0011 - .0020 202 30

.0021 - .0030 18 2.6

.0031 - .0040 5 0.7

Conclusions. Of the 2024 aluminum specimens exhibiting surface

corrosion 68% of it was intergranular in nature while 39% of the 6061

was intergranular and only 12% of the 5052 was intergranular. As for

depth, 97% of all corrosion was less than .002" deep.

9^
Salt Spray

The purpose of this test was to determine if active corrosion can

be arrested by the chemical film process (MIL-C-5541) and Type I anodize

process (MIL- A-8625A) and if tensile and elongation properties were

affected. It was very important to know this to support decisions of

what to do with completed parts.

Specimens. Pitted and corroded tubes were selected from raw stock

and cut into 8 inch lengths.

No . Type

Specimens Aluminum Surface Condition

3 2024-0 These nine specimens were

3 5052-0 tested for control - no

3 6061-T6 salt spray exposure

5 2024-0 Anodized PS 74.02-3

5 5052- 0 Anodized

5 6061- T6 Anodized

5 2024- 0 Chemical film PS 74.

5 5052- 0 Chemical film

5 6061- T6 Chemical film

2 2024- 0 No surface processes

2 5052- 0 No surface processes

2 6061- T6 No surface processes

Procedure. Five specimens from each alloy and process were placed

vertically in the salt spray (per OQ-M-151") along with two untreated

specimens of each alloy. All specimens were removed at the same time

after 240 hours exposure. The physical properties - ultimate tensile

strength and percent elongation - were determined on each specimen along

with controls which were not subjected to the salt spray test. The

results of the test are shown in the following summary table.

95
Salt Spray Te s t S ummary

240 Hours OQ-M- 151

Material Finish

Physical Properties

Tensile Elong

.

7o Loss 7c. Loss

2024-0 Bare

Alodined

Anodized

-10. 7

- 2.1

Nil

-25.0

-11.0

- 8.7

5052-0 Bare

Alodined

Anodized

Nil

Nil

Nil

Nil

Nil

Nil

6061-T6 Bare

Alodined

Anodized

Nil

Nil

Nil

Nil

Nil

Nil

Conclusions. The chemical filmed and anodized 5052-0 and 6061-T6

alloys showed little or no change in physical properties after 240 hours

of exposure to the salt spray test. Minor additional corrosion

occurred on the non-treated (bare) 5052-0 and 6061-T6 alloys, but with

no measurable effect on their physical properties.

The 2024-0 alloy showed a significant loss in physical properties.

Recommendations

:

1, 5052-0 and 6061-T6 tubing showing evidence of corrosion was

satisfactory for non-critical and critical use.

2. All 2024- 0/2024- T3 tubing showing evidence of corrosion in raw

stock, in process of fabrication, or finished unpainted parts

was recommended for disposition by accept /re j ect criteria to

be established to prevent a degradation of reliability in

certain applications.

NOTE: 2024-T3 (final condition of fabricated parts) was

slightly more susceptible to corrosion than 2024-0.

Verification. Technical experts from two aluminum tubing producing

companies reviewed in-house processing, cleaning, alodining, and anodizing procedures and verified that such processing arrested the

corrosion.

Tens i le - Compression Tests

Specimens. Specimens of 2024-T3, 5052-0 and 6061-T6 corroded and

non- corroded tubing were taken from production stores and tested as

representative of structural and hydraulic return line (low pressure)

96
tubing.

Procedure. Tensile and compression tests were conducted in a Baldwin Lima Hamilton Universal test machine of 120,000 pounds capacity,

equipped with an autographic load-deformation recorder. Tensile yield

(0.2 percent offset) was measured with a T- IM extensometer clamped to

the tubing. Elongation was measured over a 2 inch gage length. Full

section tensile specimens were tested in accordance with Method 211 of

Federal Test Method Std. 151, July 17, 1956.

Compression specimens were tested per ASTM Method E9-67. A

spherically seated upper compression head was used. The yield strength

was measured with a PC- IM compressometer using the 0.2 percent offset

me thod

.

Results. The tensile and compressive properties of the tested

tubing, both new and corroded, are shown in the following summary table:

Material Fty,KSI Ftu.KSI 7o Elong Fcy,KSI

5052-0 New 12,,5 28 .7 12.6

5052-0 Corroded 11,.7 26 .8 11.6

Corroded vs New, % - 6,,4 - 6 .6 - 7.9

2024-T3 New 50,,2 70 .9 20 53.2

2024-T3 Corroded 50,,6 71 .3 16 49. 7

Corroded vs New, % + 0..8 + 0 .6 -20 - 6.6

6061-T6 New 41,.5 48 .0 20.3 40.3

6061-T6 Corroded 39,.4 47 .6 20. 7 42. 9

Corroded vs New, % - 5,, 1 - 0 .8 + 2.0 + 6.5

The tensile properties met the strength requirements of the applicable AMS specifications, 4071G, 4082H and 4086H, except for one low

elongation value on the 2024-T3. The tensile strength requirements of

WW-T-700/3C for 2024-T3 and /4C for 5052-0 and /6D for 6061-T6 were

also met.

Relative tensile and compressive yield strength values between the

two types of tubing (good and corroded) were:

1. 5052-0 aluminum, corroded, 6.4 percent lower tensile yield

and 7.9 percent lower compressive yield strength.

2. 2024- T3 aluminum, corroded, .8 percent higher tensile yield

and 6.6 percent lower compressive yield strength.

3. 6061-T6 aluminum, corroded, 5.1 percent lower tensile yield

and 6,5 percent higher compressive yield strength.

97
Since different manufacturing lots of material were evaluated, this

spread could represent normal variation between production runs.

Conclusions. Corroded specimens averaged about 5 percent lower

yield and ultimate values than non-corroded specimens. This difference

was too small to be considered significant especially in view of the

fact that the corroded and non-corroded specimens came from different

manufacturing lots.

Recommendations, No restrictions were recommended to be imposed on

any of the three alloys from the standpoint of yield-ultimate physical

property degradation. The 20 percent reduction in the 2024-T3 elongation was considered to be significant, and it was recommended that it

be considered in the overall evaluation of the usability of 2024-T3 to

prevent a degradation in reliability for certain parts due to their

application.

Fati gue Tests - Rotating Beam

Specimens. Total specimens tested:

16 3/8 OD X .022 Corroded 6061- T6

12 3/8 OD X ,022 Good 6061- T6

4 3/4 OD X .028 Corroded 5052- 0

6 3/4 OD X .028 Good 5052- 0

5 3/4 OD X .049 Corroded 6061- T6

9 3/4 OD X .049 Good 6061- T6

25 Total Corroded

27 Total Good

Procedure. All 52 specimens were tested using the cantilever beam

method with free end rotary motion (MIL- F- 18280B , para. 4.8.7.1).

Test results. The rotating beam test applies uniform stress around

the circumference of the tube. Despite the fact that maximum stress was

applied to areas with known defects, all specimens (27 good and 25

corroded) failed in the tube flare. There was no significant difference

in the number of cycles to failure of the good and corroded specimens.

Conclusions. Existing corrosion was not sufficient to cause a tubing failure outside the flare area, and the corrosion damage in the

flare area is insignificant and not sufficient to cause early fatigue

failure

.

Recommendations. No restrictions were imposed on 5052-0 and/or

6061-T6 tubing from the standpoint of fatigue as determined by the

rotating beam test. No significant effect was expected on reliability

98
of the parts from this aspect.

Fatigue Tests - Tension- Tension

Specimens. Total specimens tested;

D (jJJ X

n/i Q uOOCl

1i (JU X . U'H-y /UZ'+- i J bOOu (^rirb;

3 3/4 OD X .058 2024-T3 Corroded

3 3/4 OD X .058 2024-T3 Corroded

1 3/4 OD X .058 2024-T3 Corroded (MPG)

2 3/4 OD X .049 6061-T6 Good

2 3/4 OD X .049 6061-T6 Corroded

2 3/4 OD X .049 6061-T6 Corroded

MPG: With molded plastic on ends of specimen.

Procedure. Specimens were tested in a Baldwin SF-IU fatigue

machine with a 5:1 multiplying fixture. Cyclic rate was 30 cps. The

specimens were fitted witn a rounded-tip, steel mandrel inside the

tubing and a smooth collet outside on each grip end. This placed the

tube walls in compression between the mandrel and the collet, affording a firm grip on each end of the tube. Each specimen was loaded to

24,000 - 4,800 - 24,000 psi stress (each wall thickness).

Test results. About 2/3 of the corroded 2024-T3 specimens tested

failed at the junction of the collet grips and the test section. In

contrast only one of the non-corroded 2024-T3 tubes failed at the

collet. Although failures at the collet are undesirable and represent

more severe stress conditions than found in normal fatigue tests, the

results of the corroded vs. non-corroded are directly comparable since

they were run identically.

Average Fatigue Life

2024-T3 Tubing

24,000 psi maximum

R = 0.2 Fatigue Stress

Corroded

Non- Corroded

Non-Corroded (1)

288,000 cycles

1,578,000

553,000

(1) With 0.003 deep surface imperfections from drawing during

manufacturing

.

Surface crack failures were typical fatigue failures, and in spite

of failure originating at the collet interface, many fatigue cracks

were present beyond the collet clamping area. Fatigue cracks did

99
start from corrosion on the tube. The corrosion was of both types,

intergranular and pitting corrosion.

Limited fatigue testing on 6061-T6 tubing was also conducted.

Conclusions. 2024-T3 fatigue life loss was:

82 percent corroded vs. non-corroded (good surface"*

48 percent corroded vs. non-corroded (.003 surface

imperfections)

No significant difference in corroded and non-corroded 6061-T6

tubing was observed. Alodining seemed to benefit the fatigue strength

of the 6061-T6 corroded tubing.

Recommendations. Due to the significant percentage loss of the

fatigue strength of the corroded 2024-T3 tubing, action was recommended

to establish accept/reject criteria for the corroded 2024-T3 tubing to

prevent a degradation in reliability where fatigue strength is a necessity.

Sea Shore Environment

Purpose. The purpose of this test was to represent an accelerated

life test to determine if being exposed to extreme environments would

result in corrosion which could affect the life expectancy of the

part, thus reducing its reliability.

Procedure. A sixteen (16) month sea shore environment test was

conducted by a leading aluminum company along the Atlantic Coast to

determine the effect of a sea shore environment on aluminum tubing

specimens with different types of finishes. Two groups of specimens

were tested. One group was located 80 feet from the shore line and

the other group 800 feet from the shore line. These specimens were

left exposed to the environment for a 16 month period. There were

multiple specimens of the three types of aluminum: (1) 2024, (2) 5052,

and (3) 6061 at both the 80 and 800 feet levels. Each aluminum type

at each level had three specimens with the following type of finish

condition:

Bare

Alodine 1200

1200 + Epoxy

1200 + Epoxy + Acrylic

Anodized

Anodize + Epoxy

Anodize + Epoxy + Acrylic

Test results. With regard to the 800 feet level test group of

specimens, only the bare aluminum and Alodine 1200 coating variations

100
developed corrosion as a result of the 16 month exposure - both considered superficial. All other specimens were unattacked.

With regards to the 80 foot level test group of specimens pittingtype general corrosion developed on the bare control and Alodine 1200

coated tubes. The pitting on the 80 foot samples is more severe and

general than on the 800 foot tubes previously reported. No other

coating variation showed pitting.

Conclusions. These tests conclude that corrosion was arrested

by the surface finish normally applied to the end product and such

finishes should sustain the part during its normal life. Alodine 1200

is not normally used as a finish coat. After weeding out the parts

recommended from the test program, those remaining were considered to

be adequate to meet the reliability requirements and their finish

coats should guard them from corrosion during their life expectancy.

CONCLU SIONS

In reviewing the factors leading to the corrosion condition and

the subsequent investigation and corrective measures, a number of

basic conclusions have been drawn. Following is a listing of these

conclusions

:

1. Storage location and environment are very important to prevent

corrosion in aluminum raw material and should be kept pollutant free and under close surveillance,

2. Aluminum tubing should be adequately inspected upon receipt

to assure that corroded material is not introduced into

inventory, especially since it may be housed in warehouses

for an extended period of time prior to delivery.

3. Precautions should be taken to protect aluminum raw stock

while in inventory such as covering with a protective coating

of oil and/or storing in polyethelene bags.

4. Soma types of aluminum tubing such as 2024 may require

special handling and storage due to the significant changes

that corrosion has on its strength characteristics to prevent

adverse effects on desired reliability.

5. Issues from stock should be inspected closely for corrosion

prior to release to production, especially where the application is critical in nature.

101
DISCUSSION

H. 0. Fuchs, Stanford University : Who found the corrosion first?

V. D. Matney : It was found by an inspector in the production area who

was looking a little closer than inspectors normally look.

102
ROTOR BURST PROTECTION PROGRAM

G.J. Mangano

Naval Air Propulsion Test Center

Trenton, New Jersey O8628

Introduction

A seemingly irreducible number of uncontained gas turbine engine rotor

bursts occur each year in U.S. commercial aviation. The potential for

catastrophy that can be associated with these events has prompted NASA

to sponsor the Rotor Burst Protection Program. This program was developed and is being conducted by the Naval Air Propulsion Test Center in

conjunction with the Massachusetts Institute of Technology. The basic

goal of the program is to develop criteria and data for the design of

optimum light-weight devices that can be used on aircraft to protect

passengers and vital parts of the aircraft structure from the lethal and

devastating fragments that are generated when a gas turbine engine

rotor bursts.

The intent of this paper is to acquaint you with the RBPP by:

- Explaining what motivates us to pursue this effort.

- Describing the test facilities that were developed and are being

used to implement the program.

- Presenting some of the experimental results that have been produced.

Motivation for the RBPP

Impetus and motivation for the RBPP stems from the statistics of the

rotor failure situation or problem in U.S. commercial aviation. The

data that will be presented on this problem comes from analysis of

Flight Standards Service Difficulty Reports (SDR) that are published

daily by the Department of Transportation, Federal Aviation Administration (F.A.A.).

It has been stated that an irreducible number of uncontained rotor

bursts occur each year. In fact, the data shown in figure 1 indicates

that on the average 32 uncontained rotor bursts occur each year. A

more detailed presentation of the statistics on rotor burst for the year

1972 are shown in figure 2. Here, the data is presented in terms of

what part of the engine was affected, and how many rotor failures and

bursts occurred. A rotor burst being defined as a failure that produced

fragments. These data indicate that 196 rotor failures were experienced

105
in 1972. These failures accounted for approximately 7% of the 2854 shut

downs that were experienced by the gas turbine powered U.S. commercial

aircraft fleet. The data shown in figure 3 further characterizes the

rotor burst problem by identifying: what types of fragments are being

generated; where in the engine bursts occur; and what percentages of the

bursts are uncontained. This type of data serves to establish where in

the engine the burst problems exist and what type of fragment has the

most potential for doing damage and therefore must be protected against.

This in a sense directs our efforts so that the most critical aspects

and locations of the rotor burst problem are addressed by the program.

The conclusion that we've drawn from these and other more detailed data

is that rotor burst in commercial aviation is a relatively sizeable problem with potentially serious consequences - at stake is the welfare and

safety of literally thousands of airline passengers.

The question that we've addressed ourselves to is this: What can be

done to minimize or eliminate the hazards and risks that are attendant

to rotor burst? As we see it there are two basic alternatives: The

level of safety that is needed can be achieved either through improved

reliability or protection. We've chosen to pursue that goal which involves developing methods of providing light-weight protection. This

position has been adopted, because the statistics of rotor burst show

that some limit to reliability has been reached and we can expect to experience some minimum number of rotor failures each year. And, in

order to ensure safety some measure of protection must be provided.

Test Facilities for the RBPP

From the outset, it was recognized that extensive experimentation and

testing would be needed to meet the goals of the RBPP. Meeting these

goals would involve:

- Characterizing the burst and fragment control processes.

- Conducting parametric studies to establish functional relationships

between significant process variables.

- Evaluating the effectiveness of various rotor burst protection

devices and configurations.

To accomplish these tasks a Rotor Spin Facility was designed and constructed at the NAPTC. This facility is shown in figure 4. It consists

of a control and data acquisition area which houses the controls and

instrumentation used for test; and a test area which contains the spin

chambers and auxiliary equipment such as the vacuum and lubrication

pumps.

- The facility has two spin chambers. The smaller chamber, which is

shown in figure 5 can accomodate rotors up to 40 inches in diameter and

has a working height of 32 inches. The large chamber, shown in figure

6, is comprised of a heavy walled (1 inch thick) vacuum vessel that is

protected by a 5 inch thick laminated steel inner liner. The working

space in this chamber is 10 feet in diameter with a height of 6 feet.

It was designed to accomodate rotors from the largest aircraft engines

that are made. This chamber has ports on its walls for instrumentation

lOh
feed-thru and optical access. In a normal test situation the rotor to be

burst is suspended vertically from an air turbine drive motor which

mounts on the chamber lid as is shown. A family of air turbine motors

are available to produce rotor speeds up to 150000 rpm. To minimize

the power required to accelerate the rotors to failure, the chambers

are evacuated to produce a vacuum of approximately 5 mm Hg. The main

data acquisition systems used for test, aside from those used to monitor

facility operational variables such as rotor speed, chamber pressure and

the like, are the impact strain measuring and high-speed photo- instrumentation systems. The strain measuring system which consists of balanced bridge millivolt signal conditioning equipment and two dual beam

oscilloscopes, is used to measure and record the strains that are induced in the containment and deflection devices as a result of rotor

fragment impact. The high-speed photo system is comprised of a continuous framing camera and photo lighting unit. The camera is capable

of producing 225 pictures as a framing rate of 35000 frames per second.

The lighting unit has an output of 12 million beam candlepower.

These are just some of the salient features of the Spin Facility.

The Development of Design Criteria

The experimental development of rotor fragment protection design

criteria, which is what this program is all about, has progressed

through two distinct phases. The first phase involved the conduct of

exploratory tests that were performed to determine what mechanisms were

involved in the rotor fragment containment and deflection processes; and

also to establish what variables significantly influenced these processes. The second and current phase involves the conduct of systematic

experimentation to generate data for the design of fragment containment

rings. The results of some of the earlier exploratory tests are shown

in figures 7 through 12. These results are in the form of high-speed

photographs that depict the ring and fragment behavior and interactions

that result from the impact phenomena and processes associated with

rotor fragment containment and deflection.

The systematic rotor burst containment experimentation evolved from the

results of the exploratory testing, which provided insight and an understanding of how certain variables significantly influenced the rotor

fragment containment process. From this information a program was

developed that aims at establishing a functional relationship between a measure of containment ring capability and the significant variables

that are involved in the containment process. These relationships, once

established, constitute the criteria needed for design. The significant

variables for fragment containment, as determined through experimental

observation are:

Rotor: Di ameter

Length

Tip-to-hub-ratio

Weight/inertia

Speed

Material

Number and type of

fragments

105
Ring: Diameter

Radial thickness

Axial length

Materi al

For purposes of developing the design criteria and the test scheme needed to generate this criteria, a dependent variable was formulated which

measures the containment capability of the ring. This variable was

called the specific contained fragment energy (SCFE) and is derived by

dividing the rotor energy at burst by the weight of the ring required to

contain this fragment energy. This variable is a combination of several

of the significant variables, namely: The rotor mass inertia and speed;

and a gross description of the ring in terms of its weight. The remaining significant variables become independent or experimental

variables; that is, factors that are varied from test to test to determine what influence these variations have on the containment capability of a ring.

Conceptually the relationship and therefore the design criteria developed through experiment would take the form shown in figure 13. Here the

SCFE is plotted against the number of pie sector fragments generated at

burst and the ring/rotor diameter. The rotor to ring axial length ratio

is the parameter. The relationships between these variables are being

derived experimentally by varying the: Rotor diameter; Number of

fragments; Ring axial length; and Ring radial thickness. These relationships, when fully established, will provide all of the information that is needed to design an optimum weight ring for the containment of rotor burst fragments.

106
107
108
COMPONENT AND FRAGMENT TYPE DISTRIBUTIONS

FOR CONTAINED AND UNCONTAINED ROTOR BURSTS^'i 1972

ENGINE

ROTOR

COMPONENT

TYPE OF FRAGMENT GENERATED

TOTALS

DISK RIM BLADE

IF UCF TF UCF TF UCF TF UCF

FAN 1 1 0 0 17 10 18 11

\^\J mr K C ^ w w

n

2 2 0 0 33 6 35 8

TURBINE 2 2 1 1 71 8 74 11

TOTALS 5 5 1 1 121 24 127 30

(1) FAILURES THAT PRODUCED FRAGMENTS

TF - TOTAL FAILURES

UCF - UNCONTAINED FAILURES

100 -

2 75 - Z

<

z

O 50

u

Z

25

100% 100%

TYPE FRAGMENT

19.8% 2 3.6%

TOTAL

FIGURE 3

109
110
Ill
>

U3

LlJ ZD

CD

112
FIGURE 7

115
Ilk
FIGURE 9

115
X "

UJ

FIGURE 10

116
FIGURE n

117
FIGURE 12

118
SCFE Specific Contained Fragment

Energy

RD - Ring (Rotor) Diameter

NF - Number of Fragments

LRG Ring Axial Length

LRT Rotor Axial Length

SCFE

ROTOR BURST CONTAINMENT, FUNCTIONAL RELATIONSHIP, OR DESIGN CURVES (CONCEPTUAL)

FIGURE 13

119
DISCUSSION

J. J. Scialdone, NASA, Goddarci Space Flight Center : Uo you simulate

the temperature?

6. J. Mangano : No, we are not considering temperature. Later on in

the program we will examine the effect of temperature. Right now, we

are just trying to develop information that will give us a gross idea

of what is involved in providing containment protection for these

aircraft engines.

J. J. Scialdone : Would it change the energy at first?

G. J. Mangano : It won't change the energy at first, but it will

probably change the behavior of the rings or devices that we use.

H. S. Link, U. S. Steel Research Laboratory : Are there any systems

for containment?

G. J. Mangano : Yes, for blade containment. Containment is required

for blade failures in most commercial engines and in some military

engines. But close to 20% of those types of failures are uncontained

despite FAA regulations. That is why we are alarmed and conducting a program such as this.

Rc Lenich, Caterpillar Tractor Company : Was any previous work done by

turbine manufacturers in regard to containment or was this the first

effort?

G. J. Mangano : This is probably the most comprehensive effort, but it

is not the first. The nature of the aircraft engine manufacturing

business is such that when a problem develops, only that specific

problem is treated. Therefore, no generalized data are available for

design

.

R. Lenich : Are you suggesting that action be taken to prevent the

wheel burst in the first place?

G. J. Mangano : In some cases, that is done. In other more

complicated cases where the problem cannot be solved through design,

containment protection must be provided.

G. Wagner, Westinghouse Electric Corporation : Were there any

surprises in the evaluation concept from MIT?

G. J. Mangano : I just presented the experimental aspects of this

program. In fact, MIT has a very ambitious structural dynamics

120
program, and hopefully at the end of our efforts, their theoretical

programs vn'll become the design criteria used by the engine

manufacturers.

W. R. McWhirter, Jr., Naval Ship Research & Development Center : Di

d

you implant the faults?

G. J. Mangano : Yes, in these experiments we are examining the effect

of the number and shape of the fragments. So we do implant faults so

that the rotors fail in the desired manner.

W. R. McWhirter, Jr. : Do you document what kind of fault you implant?

G. J. Mangano : Yes

.

W. R. McWhirter, Jr. : Have you done any work in trend analysis for

failure prediction?

G. J. Mangano : We do collect data on the causes of failures and when

in the flight envelope they occur.

P. Hal lick. Federal Aviation Administration : Insofar as the

monitoring is concerned, when the disk failure occurs, it develops so

rapidly that we do not have the real capability for detecting

incipient failure. Failure is almost instantaneous. Our approach has

been in design and testing to limit the service life of the disk to a number of cycles, hours, etc. At this time, the disk is removed from

service and sent back to the manufacturer for further testing to

determine the remaining life.

121
AIRFRAME "CRASHWORTHINESS" EXPERIMENTS

Nelson N. Shapter

Department of Transportation

Federal Aviation Administration

Washington, DC 20591

When technical accuracy, reliability, and human performance can be

adequately monitored to detect degradation before final failure, then

there is a basis to prevent accidents, and in airplane design or operation, the airframe is called safe. We believe there is a concomitant

need, namely a need to initiate remedial action. That is: The degradations, the failures need to be assessed collectively so as to change

the design standards, where necessary.

For the most part, aviation safety efforts are concentrated on

assuring successful flight through the design and construction of airplanes, airports, and navigational aids, and the day-to-day operation

and maintenance of them. The airworthiness and safe operation of the

airplane are related primarily to it physically. Going beyond hardware

integrity and safe operation, however, is the consideration of assuring

the protection of those onboard the airplane, the passengers and crew,

once degradation becomes a real possibility.

Aviation safety, therefore, is aimed at a twofold objective: First,

and primary, safety of the airplane relative to the operating environment, and second, safety of the occupants relative to the airplane.

The integrity of the airplane, its reliability, maintainability,

performance, and factors of cost and operation are considered in the

design development from the outset. In recent years, more attention has

been given to the protection of persons onboard the airplane from the

minor or so-called "survivable" crash landing environment. This has

been a direct result in large measure to the overall assessment of

accidents.

Changes in Federal Regulations have stemmed from the results of the

investigations of accidents, research by both industry and government,

and recommendations from many interested groups. Amendments FAR 25-32

of May 1972 reflect crashworthiness standards that were the result, in

large part, of a major government /industry research program.

The present wide-body transport airplanes--B-747 , DC- 10, and L-1011--

were designed to these upgraded standards. These airplanes reflect many

design improvements, particularly, in the cabin interiors. The remodeling or "new look" of the older narrow-body transports, such as the B-707,

DC-8, B-727, and DC-9, reflect these improved interiors. The success of

these new designs is the result of many factors, such as improved materials. There is one area of improvement which is being verified by tests

at the NASA Johnson Space Center,, specifically, full-scale aircraft cabin

flammability tests of improved fire-resistant materials. These tests are

outlined in NASA Technical Memorandum X58l^l, June 197^.

122
As background it should be pointed out that for many years, tests for

f lammability standards for cabin materials remained relatively unchanged,

calling for a simple horizontal bunsen burner test, with a maximum burn

rate of 4 inches per minute. This was intended primarily to get the

extremely fast burning materials out of the cabin and to prevent the

start of a major fire from common minor cabin sources, such as dropping

a lighted cigarette.

The work started in the mid-1960' s, however, aimed far beyond this

simple objective. It began to view cabin materials as a source of

potential fuel in a major cabin fire, and sought to assess the fire

hazards of this potential from the standpoints of not only f lammabi lity

,

but of smoke and toxic gas emission as well, all of which affect

passenger survival. The work was concentrated initially on the primary

hazard of flame propagation, complemented by work on the secondary

hazards of smoke and gas emission. Flame was considered to be the main

threat, since it in itself is lethal and consumes life supporting oxygen

within the cabin. This priority on flame not only sought directly to

reduce a primary hazard, but to reduce the secondary hazards as well by

reducing a major gas generating heat source which might be present in a

cabin fire, the burning cabin materials themselves. Full-scale cabin

fire tests conducted by NASA have confirmed that this was a prudent

action, and that the newer fire retardent materials, by virtue of their

overall lower potential for fire spread, do give the cabin interior a

lower potential for smoke and gas emission during the first few minutes

of a cabin fire.

Several graphs in the NASA report show how the pre- 1968 materials

generated heat, smoke and carbon monoxide, and correspondingly depleted

oxygen more rapidly and significantly than the newer materials which are

available. They point out that the use of the improved materials

provides additional safety during aircraft cabin fires and that substantial ignition sources are required to ignite such improved materials.

These tests are continuing.

Another program of interest concerns the mechanical failure concept

from an airplane structural point of view. It is a general joint crashworthiness program. One problem has been the use of static design

criteria for crash conditions. In actuality, crash conditions are

dynamic, and survivability depends on the dynamic response of the

structure in relation to the occupants. Another problem is that

mathematical methods used in dynamic analyses generally are in the

elastic range but only marginally valid beyond the elastic range.

In order to develop the techniques for treatment of dynamic structures

in both elastic and plastic ranges, a joint FAA, NASA, and GAMA

(General Aviation Manufacturers Association) program has been established

which encompasses the following phases:

1. Man-seat Model - Dynamic Criteria

2. Airplane - Math Model

3. Analytical Technique - Computer Models

125
The program seeks to achieve a means of realistically predicting

failure modes in the early design stages of an aircraft so that design

variations may be studied to optimize the structure for crashworthiness

,

The mathematical modeling will result in programs that will take into

account the plastic deformations of structural elements and the

redistributions of internal loadings which result from these deformations. The programs will lead to choices of materials and structural

shapes that will most efficiently dissipate the kinetic energy of motion,

thus protecting the occupants as far as possible from severe decelerations . Development of this program requires knowledge of force/time inputs

to the structure from the impact. Thus the effects of resistency of the

impacting surface becomes part of the problem. It also requires the

determination of peak shock waves through the structure from the points

of impact of the occupants. To obtain and verify this information,

full-scale crash tests are being conducted at NASA.

12k

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126
6.81x10^ 6 r5.68

4.54

CVJ

3.41 £ 3

2.27

1.14

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- Pre-1968 materials

New materials

New materials and

smokeless fuel

60 120 180

Time, sec

240 300

Figiire 5c Heat Flux at Center of Test Section,

Center Aisle.

127
128
SESSION II

ft

CASE

STUDIES

(continued)

Chairmen: G.Hurt

Detroit Diesel, Allison Division

B.R.Noton

Battelle Memorial Institute

REDESIGN & ASSEMBLY OF ANTT-FRICTION BEARING

HOUSirxIGS FOR IMPROVED LIFE

G.D. Xistris, Assoc. Professor

Concordia University, Mtl., Oue.

Also, Engineering Consultant

Naval Eng'ng Test Estbl., Canada

D.C. Watson

Project Officer

Naval Eng'ng Test Establishment

La Salle, Que. Canada

ABSTRACT

A survey of anti-friction bearing failures in marine equipment, has

shown that 95% of the failures occurred before the predicted mean service

life had been reached. The principal causes of premature failure v;ere

bearing duress created by deficiencies in the design and assembly stages

as well as, damage caused during the initial break-in periods. The investigation has also established that rotor rundown time (RDT) is a reliable indicator of the quality of assembly workmanship and the effectiveness of the break- in runs. This paper describes the revisions performed on a 25 HP motor driven pump and discusses the implementation of

RDT as a quality control tool. Redesigning in coniunction with controlled assembly procedures, have resulted in significant improvements in

equipment service life.

INTRODUCTION

The Naval Engineering Test Establishment (NETE) has conducted an

investigation of Anti-Friction bearing failures on behalf of the Canadian

Forces (CF) . The purpose of this study v/as to ascertain the principal

causes of premature failure of anti-friction bearinfr assemblies incorporated in the auxiliary eauirment installed in the destroyer fleet of the

Sea Element of the CF

.

In al]. . about 1.500 in-service bearin'.'^ failures fron 10 tvpcs of

mechanical and electrical enuipment v/crc analyzed in detail over a period

of four years. This analvsis shov.'ed tb.at 957. of the observed defects

v/ere the direct result of innroper assembly practices and inadecmate

knovjle.df^e of anti-friction bearing technolon-y. It should be nointcd out

that these findings suiistant la te to a larpe extent the conclusions of

surveys conducted independently by a number of b. -aring manufacturers oi- other types of industrial machinery (1,2)''". Therefore, it would appear

tiiat in many applications, installed beariri'^ ]ife is only a fraction of

the attainable service life predicted by the bearing designers. From an

economic standpoint , tlie inabil Itv of te.chni cal personnel to obtain consistently the rated service life fron beariu" asser biles is a particularly costly and v;asteful practice.

A study of the failure patterns obtninorl ir, our invest ir^at ion , has

stiown that in the najority of cases, a considerable i!,!provenopt in equipment relialiility could be acliieved if the bearinf i\ousjn?-',s were redesigned and assembled under rifrldly controlled conditions. The general

aim of these chaof^es is to assure tiiat the bear J,n<"/^-re,ase/housin(^ com-

''"Figures in parentheses indicate literature references at the end of

this presentatl oPo

151
bination would be subjected to a force system that is within its design

limits. Such modifications have been successfully field tested for

seven of the ten units included in this study and have resulted in substantial increases in equipment service life.

During the evaluation stai?e of the proposed revisions and various

alternatives which were considered, rotor rundown time (RDT) x/as found to

be a reliable indicator of assembly v/orkmanship and the effectiveness of

the initial runninp in period. It was also established that ppod correlation existed betv/een attained service life and initial rotor rundoxim

time. In addition, this parameter was used successfully to predict the

remainine useful life of the system throueh periodic monitoring after the

equipment was returned to normal operation. In viev; of these findings,

it v.'ould appear that measurements of PJ)T, reinforced with measurements of

temperature and vibration levels. \;ould constitute a prom.isine method of

predicting incipient defects and preventing catastrophic failures.

This paper discusses the changes in design and assem.bly procedures

which were adopted in the case of a 25 FiP motor driven- seav;ater pump

with sealed, grease packed, lubricated for life anti-friction ball bearings of extra v/ide design. Ancillary changes involved redesigning the

motor casing components to ensure continued submerc'ed operation and modifications to the pump bushing in order to minimize corrosion effects.

The implementation of assembly and maintenance procedures for antifriction bearing assemblies using rotor KDT measurem.ent s is also illustrated .

REDESIGMTMO FO? EIPROVED BEARIHH LIFE

Description of the En Ipment

The principal causes of anti-friction bearing failures that this investigation encompassed, w^ere the result of fabrication and assembly

faults v;hich in some cases were compounded by subiecting the unit to applications outside its design ranfe. It was found that in all the units

analyzed- substantial improvement to bearine reliabilitv was achieved by

introducing design nx)dif Ications x;hich ensured that the anti-friction

bearines were subjected only to the force environment thev were intended

to accommodate.

In order to attain the desired clearances durin" the operating cycle

of the equipment and to avoid damage to the hearings unon installation

and durine the initial break in runs, it became necessary to redefine and

revise the existing fitting and assembly procedures. One of the first

pieces of enuipment to be modified in this manner was a 25 IIP motor driven sea\,ater pump.

A general view of the pump unit as orifcinally manufactured, is shov/n

on Fie. 1. The motor free and drive end anti-friction bearings have been

designated as .'o. 1 and ivO. 2 respectivelv , The steady bearing between

motor and pump- x.'hlch v.^as eventuallv eliminated, is the ;io. 3 bearing.

The pumn bearings are identified as No. h and ^'o . 5 vvitb t'^e bottom bushing beinr t!ie :Io. 5. The main function of t:":is pump is to sunnly seav/ator at IDO psi at a rate of AO tons per b.our to the ship's fire mains.

It i.s a].so used to drav? from various waterti^'it connartments and dischargo overboard. As can l-^o seen, the induction type 25 HP AAOV three

phase motor is enclosed in r-n air bell to nernit operation xv/hile submerg152
ed to a maximum depth of 13 feet of water alcove the purin soaf. in,».

History of Failures

Fxarnnatlon of 23 f-'a5]ed motor bearinrs received over a two year

period, indicated tliat failure was primarily due to outer rir\-:' rotation

and excessive radial thrust loads. The hifdi thrust loads x-;ere caused by

improper installatioi-i, as \jell as nvalfunctiou of the flexil'le coupling,

between motor and pumu. Some bearin^^s failed duo to overheating but it

is not possible to ascertain whether overlieatxn^ was caused by imiproper

lubrication, severe misalignment, or because the aml:iient temperature

exceeded the desi^^n value.

The pump end has a double row anti-friction bearing at position No.

4 and a steel backed DU bushinc at the lov/er support. Analysis of the

28 failed num.n bearings received, sh.owed that t'cie majority of failures

was due to overspeed . inner rinp, rotation, misal i<^nment and excessive

thrust. Inner ring rotation and misalignment are defects v;liich stem

directly from installation and assembly errors and partly from, inadequate

sunnort of the bearing inside the housin;^. The latter emits a certain

amount of skewing and excessive thrust loading to take place, particularly v.^hen tlie hold-dov.Ti bolt pressure is uneven. Also, excessive

thrust may be generated by incorrect fitment of the coupling.

Another major cause of pump failures was attributed to malfunctioning of the cooling v.^ater system brouf^ht about bv clo?'C'ing of the

strainer. This affects tlie supnly of coolant to the pump seal and the

bottom bush. It should be noted that the bottom bush in the original

design is dead-ended viith no flov^ through provision.

Available information indicated tliat the average operating life of

this unit v;as 500 hours. Breakdovms v^ere equally distributed betv/een

motor and pump ends. A review of tlie performance of the subject equipment together v/ith an analysis of the failure patterns exhibited by the

bearings received , revealed that design improvements were required in

the following areas

:

(a) Motor bearing housings in order to eliminate outer ring rotation and bearing distortion,

(b) Flexible counling and rotor support arrangement so that a

greater degree of misali frnment could bo accommodated vjithout subjecting bearings to excessive loads,

(c) Pump bearing housings and coolinc water circu.lation svstem so

that adequate flow of coolant could be maintained , and

(d) Air bell enclosure and seals to retain operating canability

under submerf^ed conditions without overhcatin? of th.e motor bearings.

The design chances in each of these areas will be described in the

follov/ing sections.

Modifications to the Air Bell

The existing air bell enclosure fulfils the subm.ergence requirements

but at the same time it seyere].y limits accessibility to the motor components, "ormal servicing of motor parts is cumbersom.e and relatively

costly even for minor repairs. Furthermore, temperature measurements

135
taken inside the motor casin?: sho^;ed tii.-it under fnlL load conditions rhe

enclosure v^as the cause of excessive local overheat in!' vTiiich in turn

caused bearinj^ and electrical x^rindinr^ breakdovms.

'Lhe suhmerf^ed operation canabilitv was retained by reriacliinin-^ tlie

motor end covers true to the casinf centerline to + 0,001", sealin? all

bolt recesses v:ith a joint comnound and fahricatin?' a snecial electrical

cable entry box. A relatively inexpensive lip seal vas introduccul at

the shaft opening' in t];e drive end cover to nake tiie motor assembly

v.'atertiftht

.

These revisions rendered the air bell enclosure obsolete and thus

the overall unit irelvht v/as reduced bv approximately lb. v/hile sti.ll

retaininrr the canabilitv of operatinp; under subnerr^ed conditions. A.c~

cessibility to motor components was improved an.d tiie careful refacin^' of

the end covers and motor casin.r made it pos3ii)le to obtain better alignment betx-reen motor and pumn.

r'.odif ications to the Counlinp; Arrangement

The severe m.isal ipnmient wliich was evident in the bearinf.s examined,

was attributed to malfunction of the coupling and to the three point

support of the rotor vrhich made alip.ninf: the motor shaft to the recjuired

tolerances very difficult to acliieve.

It was decided to eliminate the steady bearin" altopet'ier and dcsip^n

a r\e\<f motor sl;irt. This change also entailed manufacturii'f^ a nevj p-otor

sii^ift which v;as about 12" shorter than th.e original item.. Jn addition,

a nev.' coupling', v/ith greater misalignmcTit tolerance and simpler coaslruc--

tion V7as specified. The end effect of these chan'^es was that alignm.ent

betvzeen motor and pum.p could be easily controlled to wm'.t]iin + 0.002".

I'.odif ication to the Fump End

Failures of the pump end were caused by excessive misalignment,

overspeed and inner ring rotation of the !o. 4 ijearin'"' and by inadeauate

coolant flox>7 to the pur.p seal and tl'.e /io. 5 bushing.

The cooling water piping was altered by routine the flo\' throu<"h

the bottom end of the pumn casing and discb.arginr into tlie inlet cliambcr.,

providing separate supply and return, lines to and from the Durametallic

seal; introducing a larger strainer and approximately doubling the water

flow rate. These chiinges improved coolant supply to the pump seal and to

the bottom bushing.

At position No. 4.. a double rov/ angular contact ball bearing had

been specified by the supplier. Sucli an arrangement is not conducive to

the h.iph degree of misal ifrnment t'aat appears to arise durinp operation

of this unit. The bearing v/as replaced by a single row. shielded, deep

proove. cartridge type ball bearing: v/hich can accommodate masalirnment

to a greater degree and also withstand higher operatinp sneeds. A number of diiaensional changes were made to the bearinp housinc^ and associated components to ensure that the final assem.bly satisfied the tolerances recommended by the bearing manufacturers for this application.

At position 'lo. .5. the steel backed DU bushin.p had exhibited a substantial rate of corrosion mainly due to tlie use of m,aterials with different electrolvtic potentials. The DU bushinp v/as replaced Vv'ith a

I3I+
Ferrobestos bushing and the 316 S/S shaft sleeve was ceramic coated.

The revised arran.^rement effectively isolates all dissiinilar materials

resulting in a much lower rate of corrosion of the non sacrificial components.

A further modification, not related to bearinp; performance, was

implemented at this time. This involved ceramic coatinf> the whirl

chamber and imneller. Accurate data on erosion rates were not readily

available, primarily because these units very seldom operated for

periods lon<T enough to yield such information. tiowever some pumps did

exhibit significant erosion at the cut-off point and of the impeller.

Consequently , the whirl chamber casting and imneller v/ere ceramic coated

and tested on one trial unit. Evidence to date suc^sests that this modification sh.ould be extended to all pumns of this tyne.

ftodif ications to the llotor

Service failures of the motor were attributed to a combination of

high ambient temperatures and a number of assembly and operational deficiencies wliich caused the bearings to nerform under duress. The high

ambient temperature problem vras eliminated by removal of the air bell

and improved coolant flow resulting from chan<?es to the cooling system

piping.

Bearing operating conditions were improved by remachininf the Iiousings and shaft. These c?\anges assured that:

(a) xMl machined surfaces such as casing faces., cover spigots and

bearing housings were parallel and true to tlie motor center line.

Perpendicularity of spigots to the housing centerline was !ieid to

4- 0.001".

(b) The drive end bearing cap spigot was fitted to acquire n.(;05"

interference. Since bearing width tolerances pe.rnlt a variation of

0.005". this allowed the maintainer to acliieve a clamLPinr interference OP the bearin^'^ of 0 to 0,002" without distorting' the outer

ring

.

(c) Tlie bearing housing radial tolerances v/ere increased to an

ISO 06 fit at the free end to nllow ur^^ibited axial travel accommodating vrhatever temnerature growth occurred under full load condit iop s

.

(d) A load snrin^'' waslier \;as inserted at the free end bearing, to

arrest outer ring rotation i/nich ^70uld he a'ggravated by tlie increase in the diametral clearances of tiie nev;' housin". The load

spring v.-asher is expected to reduce ball si- id at tb.is bearing. An

axial preload of 100 lb was used.

A general view of tlie nodlfied unit incornornt in^^ all the revision

desci'iiieci in the preceding para'^ranhs, is sb^otTi in FL'-' . 2.

Test "Jesuits

The modif ication.s outlined in this pa^er vjere irplemon to^' into one

notor/pump unit and field tested. The motor operated for n7i hours,

an iniprovenent of 3'^ times- over the nr^^vious m.ean time to bearing

failure. The reason for removing tlie motor from service was to permit

a detailed examination of tb.e bearin.'-'s and grease, esnecially. iri viex'

155
of the fact that the service acquired was almost three times the originally estimated life. Inspection of the motor bearings indicated that

both were in excellent condition. Some minor fretting corrosion on the

outer ring was noted at the top bearing and it is suspected that this

was caused by axial movement generated by thermal growth of the rotor in

an unlubricated housing.

During this time , the pump end was replaced three times giving a

mean life of 5000 hours. The cause of failure \<!as severe erosion or

corrosion of the impeller and casing brought about by the presence of

materials with widely differing electrolytic potentials. A pump with a

ceramic coated impeller and whirl chamber coupled with an aluminum

bronze shaft has been subsequently tested, and after 6000 hours showed

no visual evidence of significant deterioration. Preliminary results

appear to indicate that these modifications will make it possible for

the pump end to reach a service life comparable to that of the prime

mover.

ASSEIIBLY OF ANTI-FRICTION BF^\RING HOUSINGS

Anti-Friction Bearing Run-in Procedure

Running in procedures for grease lubricated anti-friction bearings

have not been standardized throughout industry. As a rule, such procedures vary from nothing at all to up to four hours of continuous running under no load at full voltage. Experimental data for a number of

motor designs., show that with across-the-line starting under no load,

the rotor attains full operating;', speed mthin 250 milliseconds. Such

high rates of acceleration cause the inner ring to skid on the rolling

elements. Skidding occurs because the unchannelled grease creates

enough friction to inhibit the rolling elements from rotating normally

and consequently, flat spots are formed. This has been repeatedly

substantiated through detailed visual inspection of bearings subjected

to across-the-line under no load start up procedures. Microscopic examination of the rollinp, elements revealed the presence of flat spots and

temper rings. Once the rrease has been pronerly channelled, localized

friction is significantly reduced and hence the rolling elements are no

longer prevented from rotating on the inner ring. However, the damage

caused by the initial ap-n location of high start up torques is irreparable and invariably leads to premature failure.

In order to guarantee that start up torques do not set up unacceptable localized friction forces, we adopted a two stage., low voltage

start combined with conseqnentive run-in periods of 1 minute, 5 minutes,

10 minutes, 20 minutes, 30 minutes and finally 1 hour. The purpose of

the gradual increase in the running tine is to ensure that the build up

of frictional heat which is generated during the initial grease channelling process., will not be permitted to reach a level that might cause

damage to the bearing. The two-step controller assures that acceleration rates are lov7 and hence, initial ball skid will be minimized.

Rotor PJ)T As A Ouality Control Tool

Measurements of rotor T^DI during the running in tests liave provided

an excellent indication of the effectiveness of the breaking in pro156
cedures and of the quality of the bearing assembly xjorkmanship . Rundo^vTi

time curves obtained during the breal: in trials of the 25 IIP motor discussed in this paper are sho^^n in Y±<z. 3.

The KOtor shaft PJ)T increases relatively steeply at the bepinninp.

as a function of the total running time (TRT) . The slope of the increase becomes gradually less and one v/ould expect the PJ)T to asymptotically approach a constant value which would depend on the bearing design

and the lubricating cliaracteristics of the employed grease. However,

preliminary measurements indicate that the rotor PDT is a continuously

increasing function of the TRT, provided that the bearing is not subjected to any duress. This behavior may be typical of the medium tem.perature type cnSB 3--GP-691 grease (equivalent to US !riL-G-2A139 and

British XG-27A) and bearing combination that vras used exclusively in our

tests. Further experimental work is reauired to determine v/hether this

phenomenon apnlies to other bearing designs and grease combinations.

The gradual increase noted in the rotor RDT can be attributed to

the reduction of frictional losses expended within the moving parts of

the system.. Frictional losses diminish as operating clearances are attained and hence RDT increases. Test units which exhibit a rotor RDT to

TRT relationship similar to that depicted on Figure No. 3 are considered

to have been installed correctly and the grease/seal/rolling element

combination broken in without any damage to the system components.

Subsequent decreases in the PDT indicate that the bearing is not

operating as freely as it had been. Such decreases are a direct measure

of the formation of defects which bring about an increase in the system

friction. Fig. 4 shov/s typical curves of PDT vs TRT obtained from equip

ment in service. In the cases where bearing defects had developed, a significant decrease in RDT was obseirved. Measurements of RDT therefore

could also be used to monitor the condition of bearings in operation and

to provide an accurate method of establishing the amount of remaining

service life in a particular piece of equipment. Admittedly, such

measurements are somewhat cumbersome in that they reauire tem.porary removal of the equipment from service and partial disassembly of the coupl

ing. However, our experience has shown that bearing defects influence

RDT much before there is a significant change in ultrasonic or vibration

levels. It is expected therefore, that in cases x^^here PDT measurements

could be implemented , such a monitoring tool v/ould be a m.uch more reliable indicator of bearing performance and relative condition than is

potentially possible with monitoring programs based on measurements of

other parameters.

SUM^IARY

This study of anti-friction bearing failures, has shoTm that 95% of

the bearings investigated had failed x-;ell before the potential bearing

life had been attained. The primary causes of these failures v/ere bearing duress created by deficiencies in assembly procedures; operation of

bearings in a force environment that they were not intended to accommodate and lastly. damag,e caused during the initial run-in period.

The first two factors can be eliminated by selecting the appropriate bearing for each application, remachining housings and revising

157
installation procedures to ensure that the operational environment conforms to the bearinf^ manufacturers' specifications. Removal of the

third source of failures requires the use of controlled running, in procedures. Break-in runs must be conducted under conditions which V7ill

avoid local overheating and p;uarantee the [gradual channelling of grease.

'. leasurenents of rotor PDT time have been successfully used in assessing the effectiveness of the running in tests and the quality of assembly workmanship. The RDT also provides an accurate measure of the

remaining useful life of the bearing assembly.

REFERENCES

(1) Ransome and Maries Bearing Co. Ltd., "Ball & Roller Bearings",

Monograph 1965

(2) Direction of the Chief of the Bureau of Naval Weapons, "!'aintenance

of Aeronautical Anti-Friction Bearings" NAVWEPS ni-lA-503, Aug. 1965

158
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DISCUSSION

R. Lenich, Caterpillar Tractor Company : How did the coupling perform?

Do C. Watson : Excellently. Some have been in service over 30,000

hours and we have not replaced a component yet.

J. E. Stern, NASA, Goddard Space Flight Center : What prompted you to

investigate bearing failures? Were there case histories of systems

failing or did you have a regular maintenance schedule?

D. C. Watson : There were two reasons. Firstly, somebody at

headquarters thought the Armed Forces, the Navy in particular, were

going through too many bearings. Secondly, I did not like the

instruction books that called for opening a unit after 9 months of

service. I knew perfectly well that when the units were opened to see

why they were running well, the bearings were being changed. Or if

the bearings were not changed, they were being damaged. Bearings are

in very short supply and our whole concept was to try to cut the

wastage of bearings.

R. L. Lundeen, Boeing Commercial Airplane Company : Can the run-down

test technique be used on oil-lubricated bearings?

U. C. Watson : I have used the run-down time concept on gas turbine

generator sets. People are also trying this concept in traction

motors in the locomotive field, and I believe they have cut bearing

failures by 75%. It can be used to check for misalignment in a gas

turbine unit; it can also be used on roller bearings as well as on

grease-lubricated ball bearings.
CASE HISTORY OF FAILURES IN A HAMMER MILL

J.K.L. BAJAJ

Technical Director, Taylor Stiles

Day Mixing

Cincinnati, Ohio 45212

When I learned that the Conference "Success by Design: Progress through

Failure Analysis" was to be organized by The Mechanical Failures Prevention

Group, I thought a brief presentation about the failure of a few components used

in High Speed Mills might be of general interest to this elite group. In this

presentation, I will talk about the failure of hammers and screens and some of

the steps taken to reduce the frequency of failure.

Hammer Mills are generally defined as Impact Mills, using either Fixed or Swing hammers, with the tip of the hammers operating in close proximity to a

screen, that either partially or completely surrounds the hammers. Basic mill

design permits continuous size reduction and mixing of many products ranging

from viscous liquids and slurries to dry solids.

In a Hammer Mill, the factors that influence the Power, Capacity, etc., are:

(a) RPM of the machine: The material breaks down by impact and attrition.

When the speed is too low, we don't get the desired end results and the machine

may get clogged, whereas high speeds produce fine material and waste energy.

The tip speed is a function of the diameter and the RPM whereas the centrifugal

force is a function of the diameter and RPM^.

(b) Screen and its area: Screen is used for classification. Certain minimum

screen area is required to efficiently handle the material.

These Mills are available in two configurations — Horizontal and Vertical,

The Mills referred to here are the Horizontal Mills. The basic principle is illustrated in Figo 1 which shows a horizontal shaft with fixed hammers and a

screen for classification. Hammers apply a large tensile force to the infeed

material and hurl the material against the breaker plate, followed by strong

attrition forces. Normally, the hammers are so placed on the rotor so as to wipe

the whole area of the screen. A variety of blade, rotor, screen, and throat

options make possible an extremely wide range of processing applications.

The various types of blades and feed throats used on the above-mentioned

Mills are illustrated in Fig. 2 and Fig. 3.

Ihh
Before I go into detail relative to the main subject matter, a few relevant

specifications of the Mills would not be out of place.

Mill MaXo No. of Blades Screen Area Calculated Tip

Model No. RPM (Old Design) Sq.ln. Speed-Max. F.P.M.

T-6 9600 14 105 28,000

TS-12 7200 30 210 21,000

MS-12 6250 38 305 27,500

KS-20 5400 48 or 72 650 29,300

KS-32 3600 72 1040 19,500

These Mills have found a wide application in processing tomato products.

They are used in "Catsup" processing for the most part to improve consistency

in solids particle size, but in some lines, the Mill pre-grinds the feed material

to a fine particle size to increase the yield. On "Sauces", they are used to

improve or increase viscosity. Another application of the Mills is to reclaim

pulp from the tomato finishers. Stating generally, the Mill as used in the tomato

industry has provided a homogenizing effect and also grinding of the solids.

For application in Tomatoes Industry, the need has been for the screen with a

hole size of 0.023". The size of the screen has to be carefully selected for

proper retention time and the size of the product passing through it. These

screens have normally been made of two layers — the top layer 0.015" thick with

a hole size of 0.023" and an open area of 25% and a back-up plate 0.109" thick

with one inch perforated holes and 60% open area. These two screens were spot

welded at various places to form a composite. The top screen has mostly been

Inconel, but Stainless, Monel and Nickel screens have also been used. None of

them had given satisfactory performance.

The "failure of the screen" terminology used here means rupture or breakage

of the screening material to allow larger sized particles to pass through.

Some of the problems encountered with the screens have been that the thin

screen material at the top was pushed through the backing plate as if a very hard

product was being run and driven through it. It appeared that the internal

pressure was too great and hence, the failure. In some cases, because of uneven

feed and surge load, there was a large fluctuation of pressure and hence, the

failure. In such cases, failure appeared always on the longitudinal side of the

lk3
screen. It was analyzed that this type of failure might be due to the fatigue

cracking where the screen had been welded to the backing plate. These cracks

were often patched with silver solder. It was thought that Monel screens during

processing, might have failed due to the ductility of the screening material.

These screens had averaged 2 weeks and in some cases, lasted only one day.

To overcome some of the failure problems as mentioned above, one piece screen

1/8" thick was designed. The top half of the thickness was with 0,023" hole

diameters and in the lower half the hole size was enlarged to 1/16" to ease the

material out. This screen was about 7-8 times more expensive and had to be

imported from Europe for one test trial. No one in UoS.Ao could manufacture it

and the cost of importing it was prohibitive -hence, this approach was shelved.

Other alternative design of using small hole openings of 1/2" - 3/8" hole size,

instead of 1 " hole diameter in the backing plate were tried. These have proved

to be fairly successful in preventing frequent failures and bulging of the top thin

screen. Another approach that was tried to reduce or eliminate the screen problem

was to put longitudinal liners on top of the thin screen over each rib of the backing

plate. This meant that when spot welding, the thin screening material would be

sand wiched between the two thicker screens. It also meant that the spot welding

would be indirect. This seemed to help in some instances.

The second and major part of this talk is related to the failure of hammers and

knife bars. The blades or hammers were mounted on the rotor via a knife bar.

Knife bar and hammers are shown in Figure 4. The earlier designs called for

mounting of the hammers in the knife bar as illustrated and then forcing the bar

into the head.

The hammer or the knives were first pressed into the knife bar and then these

bars were pressed into the dovetail slot into the head. The dimensions and the

tolerances of these mating parts is very critical. Any gap would cause possible

contamination. Another concern (valid for high speed operation) was that when

the fit was not perfect it might affect the balance of the machine. Gauging of

the bar and the slot is a time-consuming job. The specifications also called for

10,000 lbs. to 30,000 lbs, force for pressing operation. The bars were built up

by chrome plating if the force required for pressing was less than 10,000 lbs, and

the material was removed when the press fit was too tight and required more than

30,000 lbs, of force for assembly.

Before going into any analysis about the repeated failure of the different types

of hammers, the following modes of failure were most predominately noticed.

Figures 5 and 6 illustrate ifie different modes of failure,

1 . Complete wearing off on the top part of the hammer resulting in rounding at

1U6
the top edge. (Illustrated in Figures 7 and 8).

2. Pitting — Erosion at the top part of the hammer (illustrated in Figs. 9 and lO),

3. Cracking at the bottom part of the hammer where there is a change of section

or even breaking of the complete hammer, (illustrated in Figs. 11 and 12)o

4o Initiation of cracks at the base where the hold down screws are mounted.

(Illustrated in Figs. 13 and 14).

In the early stages of design, the criteria for the selection of hammer and bar

material was to have good wear and strength characteristics. The two principal

metallurgical methods used to make stronger stainless steels are by "Precipitation

Hardening" and, by the addition of strengthening elements like Nitrogen.

17-4 PH Precipitation hardening steel was extensively recommended and used in

most applications for hammers. The important advantage of this steel is that it can

be formed into required shapes in their annealed, soft condition and then heattreated to higher strength after forming. The dimensional changes from hardening

ore very slight. In general, corrosion resistance of these steels is generally

inferior to the basic stainless steel grade 304 and the cost is also higher. The

hammers heat-treated to 44 Rc have about 190,000 PSI Ultimate strength and 60 ft.

Ibso of Izod Impact strength.

The selection of 17-4 PH steel was not a bad choice if wear and strength were

the only criteria of selection of material. This was not true, as noticed later.

During the operation of these mills, repeated failure due to plain wear, pitting

and arosion and also breaking of the complete hammers were experienced. If one

hammer broke during processing, it would hit all other hammers causing damage to

the rotor, screen and other hammers. Figure 15 illustrates the damage done to the

knife bar and other hammers because of breakage. The initiation of cracks and,

hence the breaking at the bottom part of the blade or hammer, where there is a

change of section from the dovetail to flat, could be attributed to the stress

concentration. Provision of a general radius might have solved the problem. The

breaking or cracking of the rest of the hammers could be attributed to low impactstrength of the hammers at such high value of hardness. When one broken hammer

or any extraneous material came in contact with other blades rotating at 5000 RPM,

it would either crack the blades or completely break these because of low impact

strength thus damaging the complete rotor.

The pitting - erosion resistance of 17-4 PH steel is also poor and failure due to

this is quite often noticed in liquid slurry applications. The wearing or rounding

of the hammer was only at the top where the centrifugal force is maximum. When

the hammer wears off, it would lose its effectiveness of size reduction. In one

instance, when the 17-4 PH blade failed, die penetration examination was also

1^7
performed to find if the knives were defective, but were found to be metal lurgically sound and well made. 17-4 PH steel hammer should have had a life of at

least 200 - 300 hours, but many of them failed much earlier.

Due to repeated failures, because of one reason or another, the following knife

materials were also tested o Hammers made of stainless steel hardened to

Rockwell 56-58 were tested without any success. (These were supposedly perfect

metallurgically „) Another type of knife that was tried had carbide insert and these

were (a) 17-4 PH with a type K-6 flat insert, (b) 17-4 PH with type 703-45° rear

insert, and (c) 304 SS with type 703 flat insert. The failure of the carbide knives

is also well illustrated in the Figure 16.

One of the design modifications done was the addition of jack screws below

the hammers. Later on, it was found that in some hammers the cracks had initiated

at the places where the screws were being forced against the hammer. Also, the

"area" of the dovetail where the hammers and the knife bar or the knife bar and

the rotor was corroded o This could be attributed to the poor corrosion resistance

of the material

.

As mentioned above, the hammers would break or wear and would need replacement, but the problem encountered in the design was that due to frequent failures

of the blades, and hence in resembling all the components the fit of the mating

parts could not be maintained. With frequent blade changes, these bars had badly

scarred in pressing them out of the rotor. It was also found that the manufacturing

and assembly of new or replacement hammers and bars was quite expensive. As it

was virtually impossible to eliminate failures which had been caused because of

one reason or the other, another approach was tried to at least reduce the frequency of failure , the initial cost and the cost of replacement.

After analyzing years of data and considering the availability, strength, wear

characteristics, weldability price, corrosion- e rosion characteristics, a new

approach of using stamped hammers made of 316 stainless steel and hard-faced with

stellite 6 was tried. (Figure 17) These hammers were so designed that they could

be used on the existing machines in the field. Hammers were installed on the

rotor just by sliding them in the dovetail slot (originally meant for the knife bar)o

They were grouped together along with spacers (also stamped) to provide the

desired configuration and held in position by end locks. The cost of hammers,

hard-facing and assembly on the rotor was significantly lower as compared to the

original design of heat-treated machined hammers and bars. This approach met

with tremendous success in the field. Some of the stamped hammers also cracked

in the field. (The initiation of crack is illustrated in Fig. 18o) But the breakage

of these did not cause catastrophic failures of the whole machine because of the

ductility of the material from which the other hammers were made. When one

1U8
hammer broke, it was replaced within minutes and the mill was back in operation.

Hard-facing the hammers improved the wear and pitting characteristics. The

stamped hammers could again be hard-faced if the material wore off.

By using stamped hammers, the knife bars have been eliminated. As the

stamped hammers slide in ifie dovetail slot, therefore there are no close tolerances

to be maintained.

Another solution undertaken to overcome the problem of breakage of the

hammers has been to make an integral one-piece knife and ifie bar from the same

material. This is the most expensive approach taken as the different knife-bar

blocks mounted on the circumference of the mill are all different and each of them

have to be individually machined and hardened. This has worked quite well in one

application, but costs of manufacturing are prohibitive.

In conclusion, I would just like to add that partial success was achieved after

repeated failures, but one thing to bear in mind is that the manufacturer and the

user were willing to cooperate at all stages and the project was not abandoned. If a similar attitude is adopted, all parties concerned would gain tremendously in

better understanding of the process, and hence help in designing better machines

for the future.

Ik9
Figure 1: Schematic diagram showing the principle of a high speed

impact mill o
feetf—conveys materiais from hopper to rotor by horuontei sSl J8I1 pan feed— materials fed to rotor from horizontal pan. ^OH feed—ciefivers sheet materia! to blade at measured fa-

'

Liquid-dorizontat Seed—accepts liquid stream by gravity

Figure 3: The various types of feed throats used with the Miilso

Figure 4: The Knife bar and the different types of hammers used.

151

Figures 7 & 8: Complete wearing off on the top part of the hammer.

155
Figures 9&10 : Pitting- Erosion at the top part of the hammer.

15^

Hammers broken at the base.

156
Figure 15 : Damage done to the knife bar and other hammers

because of breakage.

Figure 16 : Failure of the hammers with carbide insert.

157
3^

1

Figure 17 : Stamped Hammers with and without the Stellite hard

facing. The bent hammer shows the ductility of

stainless 316

.

Figure 18 : Crack initiation prior to complete breakage in

stamped hammers.

158
SOLUTION OF AN ART RESTORATION PROBLEM

Elio Passaglia

Chief, Metallurgy Division

National Bureau of Standards

Washington, D. C. 20234

In Washington, D. C, behind the Lincoln Memorial, there are four

large equestrian statues „ Two of them, representing the "Arts of War",

front on the Arlington Memorial Bridge, and are named "Valor" and

"Sacrifice". Two others, representing the "Arts of Peace", front the

entrance to Rock Creek Drive, and are named "Aspiration and Literature"

and "Music and Harvest". The groups were originally sculpted in plaster

by the American sculptors James Earle Eraser and Leo Friedlander. The

statues as they now stand were gifts of the Italian Government to the

people of the United States, presented in thanks for the Freedom Train

sent to Italy by the U„ S. after the second World War. They were

installed in their present locations in 1951, and were dedicated by

President Truman.

The statues were cast in bronze by four Italian foundries,

located in Milan (Valor), Florence (Sacrifice), Rome (Aspiration and

Literature) and Naples (Music and Harvest). There are two interesting

facts about their fabrication. First, they were cast in bronze by

use of the lost wax process, a process well-known and widely used in

Italy since Benvenuto Cellini wrote his treatise on the casting of his

"Perseus" by this process o Second, the statues were gilded using the

"fire-gilding" process, in which an amalgam of gold and mercury is

put on to the statues and the mercury subsequently boiled off by heating

with torches.

By 1970 the statues had begun to show definite signs of

deterioration, particularly "Music and Harvest" and "Aspiration and

Literature". Instead of their original gold color, the statues were

turning dark-grey-green, evidence that the gold gilding had

deteriorated and was not carrying out its function of preventing

corrosion of the underlying bronze. "Valor" and "Sacrifice" were by

comparison much better, but they also were showing signs of

deterioration.

In 1970, the National Capital Parks, which has the responsibility

of maintaining the statues in the Nation's Capital, asked the National

Bureau of Standards what needed to be done to restore the statues.

After a preliminary investigation of the problem, the following options

were laid out.

159
1. Do nothing, in which case the statues would continue

to deteriorate, but eventually reach a final stage of

patina. What the condition would be like in this final

stage, and how long the process would take, could not

be predicted because of the presence of the gold coating.

2. Sandblast off the gold coating and let the statues assume

a natural bronze patina.

If the gold color had to be preserved, the following options were

available.

1. Clean the statues and paint with gold gilt paint. Such

paint would have to be replaced every 5 years.

2. Clean the surfaces and apply gold leaf.

3. Clean the surfaces thoroughly and plate with gold by

the "brush-plating" technique, a technique normally

used for industrial purposes. This would require that

all the surfaces to be repaired be free of pits and

pores that would be the source of future corrosion.

In any of the cases above, structural repairs would have to be

carried out as necessary.

The National Capital Parks chose the last option, and restoration

work was begun in the fall of 1971. The work fell under two broad

categories: structural repair, and surface restoration.

Structural Repair

The various parts of the bronze castings had been bolted together

by steel bolts, and steel angle irons had been used in the interior of

the statues to provide structural rigidity. Unfortunately, the statues

were set on hollow plinths that were open to subterranean chambers.

This provided excellent opportunity for condensation of moisture on the

inside surfaces of the statues, causing classic galvanic corrosion

between the steel and the bronze. This had proceeded to the point where

an incipiently dangerous situation had developed with respect to the

bolts holding the various parts of the statues together, many of which

had corroded almost in two. The steel angle irons had corroded to the

point that they had very little structural strength left.

The correction of these conditions was straightforward, but

required considerable work. All the steel bolts and angle irons were

removed and replaced with bronze counterparts, and the statues are now

structurally sound.

i6o
There were, however, other structural defects. There were

numerous cracks in the castings, some very large indeed, being over two

feet long. Moreover, the abutment between the pieces of the statues

had in some cases not been completely sealed. These and the cracks

provided openings from the inside to the outside, and gypsum left from

the casting molds on the inside worked out through these openings,

adding to the corrosion problems. To correct these conditions, the

insides of the statues were thoroughly cleaned, and the cracks filled

with 97% Sn-3% Ag solder. Welding proved to be unsatisfactory. The

largest cracks were reinforced by strapping.

Surface Restoration

The surface restoration proved to be the most difficult and the

most challenging. First the surface was sandblasted, using care to

preserve the surface detail left by the artists. After the surface

was cleaned and thus available for close inspection, it was found that

the castings were of exceedingly poor quality. This was particularly

true for "Music and Harvest" and "Aspiration and Literature". They

were very porous and in many places hundreds of pits ranging in size

from less than one millimeter to several millimeters covered the

surface. Some of these were deep enough to penetrate the wall. These

were drilled out and filled with bronze rod, or the tin-silver solder.

However, filling in all the pits was impractical, and protection of

this surface by a gold plating was impossible. What was done, therefore, was to plate the statues using the brush technique with a 500

microinch coating of nickel followed by a 160 microinch coating of

gold. Then the exterior was covered with a clear protective lacquer

especially formulated for the protection of bronzes.

Conclusion

The restoration was esthetically successful. The statues

recovered their heroic character, and were a pleasing warm gold color.

The technical success of the restoration remains still to be determined.

It is expected that the lacquer coating will last at least five years,

and then may require another coat. A few additional pits that penetrated the wall of the statues have been found and repaired. Some

discoloration apparently due to insufficient washing away of the

plating solution has been found and corrected. Only minor and easily

correctable evidence of corrosion has been seen, and in that sense, at

least, the restoration has been a technical success.

l6l
DISCUSSION

R. L. Lundeen, Boeing Coiiiinercial Airplane Company : Did you plug the

hole in the bottom of the statues?

E. Passaqlia : There was a lot of discussion about whether to plug the

hole in the bottom. Once we got all the gypsum cleaned out and

replaced all the steel bolts with bronze, all we would have is a bronze surface. Then there would be no need to plug the hole. Some

people considered putting heaters inside the statues to keep them dry.

What we did, in fact, was to fill up with epoxy compounds places where

water would collect so that the water now runs down the inside of the

plinth. The inside of one of the statues was coated with a polysulfide. In my view, that was not really necessary because bronze

is supposed to be able to take weather. If it patinas on the inside,

you do not really care.

R. Lenich, Caterpillar Tractor Company : How long did the restoration

process take?

E. Passaglia : The whole process took about 60 days. It was all done

with unskilled workmen. There was really nothing tremendously

difficult about it. Sandblasting, of course, required a certain

amount of skil l--buff ing you have to learn. Brush plating can be

taught to people very quickly. The most skilled people there were

probably the solderers and the welders. The work was all done by

National Park Service personnel.

One of the problems that occurred was that no one realized that these

plating solutions would turn dark under sunlight. The way the Navy

uses the brush plating technique is to plate objects and then put them

out into the air where rain water washes off any remaining plating

solution. On the statues, a coating of lacquer was applied after

plating. This coating prohibited the removal of the excess solution

and allowed the statues to turn dark. I am quite confident that now

that this has been removed, the statues will remain bright. In the

places where there was no plating solution, there was no evident

deterioration after 3 years.

J. K. L. Bajaj, Envirotech Corporation : What was the cost of

restoration?

E. Passaglia : That I do not know.

J. J. Scialdone, NASA, Goddard Space Flight Center : What was the

effect of hydrocarbons on corrosion?

162
E. Passaglia : This is a complicated problem. Patinization of bronzes

is one of those areas where art meets science. There are people who

know what conditions and what kinds of alloys are needed to obtain

certain colors. I do not know of anything in the scientific

literature about this. I would not expect hydrocarbons to cause any

significant problem. However, sulfides could be a very serious

problem in that they would result in the formation of a black patina

rather than the desirable green patina. No one feels that traffic

fumes had anything to do with the condition of these statues. The

deterioration of these statues was simply caused by galvanic corrosion

between the gold covering and the copper substrates because the gold

finish did not completely cover the bronze.

165
ELIMINATION OF FAILURES OF U-BOLTS

IN FARM TRACTOR DUAL WHEELS

B. P. Bardes

Bimba Manufacturing Company

Monee, Illinois 60449

When farm tractors are to be used on sticky or wet soils, many farmers

will equip their tractors with dual driving wheels. One common design

for these dual wheels includes a semicircular hub and two or more

U-bolts which are used to fasten the wheel to the axle. Shortly after

introducing a new line of dual wheels, one manufacturer experienced

a number of failures of U-bolts after only a few hours of service.

Cold finished 1 1/8" diameter AISI 1045 steel was used for these Ubolts. The bars were cut to length, the threads were cut, the blanks

were bent to shape, the U-bolts were oil hardened and tempered to

Rq30 and then the U-bolts were rebent if necessary.

Without exception, the failures occurred through the threads of the

U-bolts, immediately adjacent to one of the nuts used to tighten the

U-bolt in place. The fracture surfaces exhibited positive evidence of

fatigue. An analysis of the service loading indicated that there were

three components of the loading. There was a constant preload, at a level determined by the farmer who installed the dual wheel. There was

a periodic variation as the hub and U-bolts alternately bore the weight

of the tractor. There was also a load caused by the widely varying

torque applied by the power train to the drive wheel. It was concluded

that the composite loading could logically cause fatigue failures.

Metallurgical examination of several failed U-bolts indicated that only

a shallow case of each bolt contained the desired tempered martensite

microstructure. The interior of the bolt was a mixture of ferrite and

pearl ite. For immediate relief of the failure problem, the specified

material was changed to AISI 4140, which has sufficient hardenabil ity

to obtain the desired tempered martensite microstructure throughout

the U-bolt. The material change prevented service failures, although

the cost of the alloy steel was greater than that of the plain carbon

steel it replaced.

The U-bolt failures also initiated a general review of the manufacture

of this item. As a result of this review the entire operation was

changed. The material was changed to warm drawn AISI 1527, 1.070"

diameter. The bar stock was cut to length, threads were rolled and

the blanks were then stored. Blanks were bent to size only to meet

161+
orders. No heat treatment was required. No U-bolt made in this manner

has failed in the two years since the production methods were changed.

Besides totally eliminating service failures, the new production

methods have substantially reduced manufacturing costs. The weight

per foot of the steel is approximately 10 per cent less. Roll threading is faster than cutting threads (and produces a thread more resistant

to fatigue). By cutting all blanks to the same length, that of the

longest U-bolt, only one item was kept in inventory, instead of 12

different U-bolts. Storing blanks rather than U-bolts greatly reduced

the required storage space. And the costs of heat treatment, transportation and rebending were eliminated.

The original U-bolt design evolved from smaller U-bolts used on

smaller tractors, where the hardenabi 1 i ty of AISI 1045 was adequate.

Only because the part designed by evolution failed was the general review started. In the review the engineers examined the interrelations

between the function and design of the U-bolt, its material and the

methods by which it was made. From this systems approach to design, a far superior product at a far lower cost was achieved. The success of

the redesigned product may be judged by considering that every competitor in this market has since adopted the same material, manufacturing methods and design.

165
STRUCTUEAL IN-FLIGET WING FAILURES

Michael Lo Marx

Bureau of Aviation Safety

National Transportation Safety Board

Washington, DC 20591

ABSTEIACT

An overview of the types and number of structural in-flight wing

failures occurring within the 10-year period of 196^ through 1973 is

presented. Particular attention is given to failures caused by fatigue

fractures of wing components. Some case histories involving those

fatigue failures of the wing which relate to design are presented.

INTROnJCHON

The Congress of the United States recognized the importance of

aircraft accident investigation as a means of preventing accidents when

it passed the Civil Aeronautics Act of 1938 » This Act gave the Civil

Aeronautics Board the authority and the responsibility for investigating

and determining the cause of all United States civil aircraft accidents.

In 1966 the Transportation Act created the National Transportation

Safety Board, to whom the Civil Aeronautics Board's accident investigation functions and other safety activities were transferred. Since this

paper deals with the period of 1^6k through 1973; information on accidents was, in part, taken from records and files accumulated by the

Civil Aeronautics Board but now maintained by the National Transportation Safety Board.

The Safety Board conducts its own investigations of all air carrier

and air taxi accidents, most fatal light plane and helicopter accidents,

as well as other accidents or incidents that may be of special interest

because of their safety implications » The Safety Board has delegated

the authority to investigate most nonfatal light plane (under 12,500

lbs, gross weight) accidents to the Federal Aviation Administration, but

retains the responsibility for determining the probable cause of these

accidents. To assist in the determination of probable cause in accidents or incidents involving a material failure, the Safety Board

maintains a metallurgical laboratory in Washington, D. C.

This paper will be confined to United States -registered fixed-wing

aircraft having in-flight structural failures of the wing (within the

10-year period of 196^ through 1973) • Descriptions of failures precipitated by fatigue fractures of wing components and some case histories

where design was a contributing factor will be presented.

166
FAILURES IN GENERAL

Data complied by the National Transportation Safety Board for 196^

through 1973 disclosed I70 accidents involving United States -registered

civil aircraft that were caused hy structural failure of the wingo

Table 1 shows a breakdown of these accidents by failure type.

TABLE 1

IN-FLIGHT WING STRUCTURAL FAILURES (l96h THRU 1973)

FAILURE TYPE NUMBER OF j

OVERLOAD 117

UNKNOWN 29

FATIGUE 15

FABRIC

FIBERGLASS 7

OR WOOD

CORROSION 2

DETHKIORATTON

TOTAL 170

The failiores included in Table 1 are partial or total wing separations, folding without separation, wing support structure failures, and

deformations due to aerodynamic loads (such as wrinkling of the wing

skin or partial breaking of structure.)

Failures listed as "UNKNOWN" include those where the wreckage was

destroyed by fire or lost at sea, and those not reported as being caused

by overload, fatigue, stress corrosion or corrosion. Listed as fabric,

fiberglass or wood were failures known to have been caused by separation

or breakage of these nonmetallic components. All the fatigue fractures

shown were substantiated by examination in the NTSB metall-urgical

laboratory.

FATIGUE FAILURES OF THE WING

Table 2 lists chronologically the I5 accidents attributed to

fatigue fractures of wing components. Also shown are the probable

cause(s) and/or contributing factor(s) for each of the failures.

167
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168
Inherent design Is shown as a cause or contributing factor to the

failure when the design itself does not appear adequate to prevent

fatigue cracking under the normal flight profile loading conditions of

the aircraft. It should be pointed out that all the aircraft shown in

Table 2 were not required by the existing regulations to be designed

and substantiated in fatigue at the time of their certification.

Ten of the fifteen accidents involving fatigue fractures occurred

on aircraft types having more than one failure; Eeechcraft l8 series

(5 failures). Piper PA-25 (3 failures), and DeHavilland lO^J- type

(2 failures).

Some examples of fatigue failures which show design as a cause or

contributing factor are illustrated in the following examples.

BEECHCRAPT E-l8S ACCIDENTS

A good example of failures attributed to inherent design where a

change in design of the original part is impractical is the Beechcraft

18 series accidents of April 28, I96J, June 22, 1972, and April I9,

19T3o All three accidents occurred as a result of fatigue fractures

almost Identical in nature.

In the accident of June 22, 1972, the failure of the wing was

precipitated by a fatigue fracture through the lower spar cap that

initiated at the toe of a weld used to secure a gusset plate to the top

side of the lower spar cap (see arrows "0" in Figures 1 and 2). The

spar cap is an elliptical shaped ^130 steel tube procured to MIL-T-6736A

and heat treated to l80-200 KSI tensile strength range. MIL-T-6736A

allows up to 0.15 inch total decarburization on the outside diameter

of the tube for the wall thickness specified (O.IO inch). Decarburization such as this lowers the tensile strength of the surface of the spar

cap to about 80 KSI; more than half of the internal material strength

of the spar cap. This low strength at the surface and the geometric

stress riser at the toe of the weld contributed to fracture initiation

in fatigue. The spars, as they were designed and manufactured, were

inherently susceptible to fatigue cracking » Changing the original

design in these cases was not a practical solution, and inspections

were performed periodically to detect cracking before ultimate failure

occurred.

X-ray radiographic inspections were performed on the aircraft

involved in the accidents on June 22, 1972, and April I9, 1973^ with no

cracks being reported. Post accident examination of the X-ray films

disclosed that substantial cracks were in the spars during some of

these Inspections and should have been detected during the inspection.

The probable cause of these accidents were, therefore, attributed to

Inadequate inspection, .

169
As a result of the April I9, 1973, accident, the FAA issued an

amendment zo the existing inspection Airworthiness Directivel requiring

modification of the spar within 6OO service hours. The modification

consisted of attaching steel straps to increase the structural integrity of the area.

HELIO AIRCRAFT AUGUST 21, I97I

The original part which failed was redesigned after the Helio

aircraft H-295 accident on August 21, 1971. That was the steel main

spar carry-through fitting shown by arrows "F" in Figures 3 and ^4-.

Multiple fatigue cracks had originated in the inside surface of

the fitting along the line where the inner wall changes from a cylindrical to a conical surface near the closed end of the tubular section.

This area is at the bottom of a longitudinal 1.25-inch diameter drilled

holeo The fatigue cracks had propagated through the wall thickness and

across approximately kO percent of the cross sectional area of the

tubular section before the fitting failed completely (shown in Figure

The points where the fatigue fracture originated were in a rough

drilled surface where the edge of the drill had left a sharp corner at

the change in section thickness near the bottom of the hole. The manufacturing drawing for the fitting showed a requirement for a Oo062-inch

radius and 125-microinch RMS surface finish in this area. The metallurgical examination on the failed fitting, however, indicated that no

attempt had been made to form a true radius or improve the surface

finish after the drilling operation. These discrepancies probably

contributed to the cause of the fatigue fracture. A subsequent fatigue

analysis of the primary aircraft structure indicated that the steel

carry-through fittings were subjected to high stress levels and the

development of fatigue cracks and, therefore, had a limited fatigue

life,

2"

As a result of this investigation, the National Transportation

Safety Board issued a safety recommendation3 to the FAA to require

inspections of fittings of the same design for cracks, adequate internal fillets, and surface roiighness in the internal fillet area at the

earliest practical time by an FAA-approved method. Also recommended

was the replacement of fittings with an improved design if possible,

at the time of this inspection. A suggested design change to greatly

increase the structural integrity of the fitting was to decrease the

depth of the 1,25-inch hole to move the 0.62-inch radius to an area of

less stress. Such a design change, however, would require the manufacturer to change materials because the original material chosen

(^130 steel, 180-200 KSI heat treatment) could not be heated to the

strength level required.

Raised figures indicate literature references at the end of this

presentation.

170
The manufacturer did, however, change the design Incorporating the

original material and heat treatment. This was done by changing the

conical shaped bottom of the hole to a hemispherical shape, thereby

decreasing the stress intensity in the area. The hole depth was also

decrea,sed somewhat to increase the cross-sectional area near the bottom

of the hole. The newly designed fitting was incorporated in the new

assembly which contained factory installed carry-through reinforcing

straps

.

The FAA, as a result of this accident, issued an Airworthiness

Directive^ (AD) requiring a gamma ray inspection of the fittings within

10 hours time in service, and thereafter at 100-hour intervals, on all

Hello Model H-250, H-295, H-39I, H-391B, H-395, H-395A land airplanes

with 3^000 or more hours of service, and all float equipped planes of

these models with 1,500 or more hours in service.

The AD further required that, within 350 hours in service after the

initial inspection, carry-through reinforcing straps were to be installed

in the field. With the installation of these straps, the repetitive

inspection interval was Increased from 100 hours to 5OO hours. The AD

also provided that if a new assembly with the newly designed fitting was

installed, then the inspections would no longer be required.

DEHAVILLAJ^D DH-IO^-TAKC, MAY 6, I97I

Apache Airlines DeHavllland Dove lO^-TAXC crashed into a cotton

field on May 6, 1971? after separation of the right wing. 5 The failure

was the result of a fatigue fracture in the wing attachment fitting

(see Figures 5 a-nd 6). Origination of the fatigue fracture was in an

area of fretting and corrosion in the hole for the main wing-to-fuselage

attachment bolt.

The aircraft involved was originally a standard DeHavilland Dove

which had been substantially modified by Von Carstedt Corporation, C-Alr,

Long Beach, California. The modification consisted primarily of increasing the fuselage length to accommodate more passengers and installing

more powerfull engines for Increased speed. To accomplish this, the fuel

tanks in the wing were relocated and the main wing attachment fittings

were redesigned. The design was approved by the Federal Aviation Administration and Supplemental Type Certificate SA-17^7WE for the aircraft

was issued July 23, I968.

The redesigned wing attachment fitting consisted of increasing the

tensile strength and cross -sectional area of the fitting over that of

the original Dove fitting to accommodate the increased load expected

due to the modifications. The new fitting was made of ^130 steel heat

treated to l80-200 KSI tensile strength (originally fitting had 175

KSI tensile strength).

171
Since the new fitting was to be structurally similar to the original fitting, having similar internal stress, the design was approved

without a requirement for substantiating fatigue tests. The fatigue

life of the Von Carstedt fitting was predicated upon the life of the

original DeHavilland fitting, provided that the new fitting maintained

the same precise tolerances and joint-sealing procedures employed

in substantiating the life of the original DeHavilland fitting.

The aircraft that crashed, 51^9227, had accumulated a total service

time of 5^593 hours and was the first production aircraft modified by

Von Carstedt under STC SA-lT^i-TWE.

Hardness measurements in the failure area gave results indicating

the fitting had an average tensile strength of 157 KSI, well below the

engineering drawing requirements (l80-200 KSl). The alloy chosen for

the fitting was ^130 steel known to have low hardenability properties.

According to Military Handbook 5-^, used in the design of the modification, a part fabricated from ^130 alloy steel with the size and geometry of the fitting could not be consistently hardened throughout the

section thickness to attain the specified tensile strength required in

the engineering drawing; tables in the handbook indicated that ^3^0

steel would have been the preferred alloy. The hardnesses obtained on

the failed fitting were found consistent with hardenability data

supplied for the material certification for the barstock used in fabricating the fitting. Although this data was available to the designers

and checkers at the time of the design change, the drawing was approvedo

The stress analysis submitted to and approved by the Federal

Aviation Administration noted that the service life of the new fitting

was predicated upon maintenance of the original DeHavilland tolerances.

The engineering drawing checked and approved by a designated engineering representative of the FAA, however, specified a tolerance allowing ^ an 0.0022 inch greater diametrical clearance than that specified in the

fatigue analysis.

The conformity inspections of the aircraft during manufacture of

N^922V were performed by an FAA manufacturing inspector for the local

district office. The inspector testified in a public hearing on the

accident that he had rejected the fitting on the basis of its strength.

The part was rejected because a hardness test on another part from the

same heat treat lot was not within its hardness specifications and,

therefore, the entire lot was rejected. The inspector did not, however,

follow up to assure compliance with his request for a subsequent

inspection to determine if the part was properly heat treated. Although

the procediores used for the ultimate acceptance of the wing attach

fitting were never determined, the fitting was subsequently installed

and the aircraft was certificated.

172
Shortly after the accident, the Federal Aviation Administration,

in an emergency order, suspended the airworthiness certificates on the

remaining four aircraft modified by Von Carstedt. These aircraft

remained grounded until October 22, 1971^ when a Supplemental Type

Certificate SA-23^8WE was issued. The STC modified the aircraft to

incorporate a lower wing-to-wlng strap that could carry the total load

should the lower wing joint fail (fail safe). This was performed, of

course, only after inspecting each fitting to assure that they were

crack free.

REFERENCES

1. Federal Aviation Administration Airworthiness Directive 72-20-5,

Amendment 39-1526 as amended by Amendment 39-1632. Amendment 39-1632

became effective May 7, 1973

o

2o Federal Aviation Administration letter to owners of Hello Model

H-250, H-295, H-391, H-39IB, H-395 or H-395A airplanes dated October 5,

1971.

3. National Transportation Safety Board Recommendation A-7 -h6 and ^7

to the Federal Aviation Administration adopted September 23, 1971, and

issued October k, 1971-

k. Federal Aviation Administration Airworthiness Directive 71-21-11

Amendment 39-1320 effective October I6, 1971.

5. Accident Report NTSB-AAR-72-19, Apache Airlines at Coolidge,

Arizona, 5/6/7I (available from NTIS, Springfield, Virginia, 22161).

175
Figure 1. Overall view of portion of center section truss

removed from accident of Beechcraft E-l8S, June 22, 1972.

Arrow "0" denotes fatigue fracture origin in lower spar

cap.

Figure 2. View of fracture origin showing relation to weld

for gusset plate. Accident Beechcraft El8S, June 22, 1972.
Figure 3. Fatigue fractured fitting, arrow "F", in relation

to mating structure. Accident Helio aircraft H=295 on August 21,

1971.

175
Figure k. Overall and closeup views of fractured fitting. The fatigue

fracture originated from numerous origins on the inside diameter of the

hole shown, by bracket "0" in top photograph which is the same as that

shown between arrows "0" in the bottom photograph. Accident Helio aircraft H-295 on August 21, 1971.

176
Figure 5* Lower wing root attachment area as laid out at

the accident site. Carry through spar is shown on left and

fractured wing attachment assembly is shown on right. Accident Delfevilland DH-10^-7AXC, June 6, 1971.

Figure 6. Closeup view of fracture siirface of lower wing attach

fitting. Arrow "0" denotes origin of fatigue fracture. Accident

DeHavilland DH-10i4-TAXC, June 6, 1971-

177
DISCUSSION

R. Lenich, Caterpillar Tractor Company : Was the overload failure due

to an over gross weight condition or due to aerodynamic overloading?

M. L. Marx : Aerodynamic overloading.

J. Srern, NASA, Goddard Space Flight Center : You mentioned that a probable cause of failure is inherent design. Would you explain what

you mean by inherent design?

M, L. Narx : It is a term that I use to describe parts that, as

designed and manufactured, are inherently susceptible to fatigue

fail ure.

J. A. Bennett, National Bureau of Standards : The Beech-18 keeps

turning up at these meetings. At the time that we looked at the

Beech-18 accident in 1964, we referred back to the 1947 accident. The

whole approach was to try to save these aircraft by radiographic

inspection. This approach obviously did not work. At that time there

was complete refusal to even consider modifications such as grinding

off the weld reinforcements or shot peening the area to improve the

fatigue strength. Is that still the case?

M. L. Marx : Modifications were made eventually. In the past,

modifications were resisted because it was felt that inspection

requirements would be adequate. In the investigation of the last

accident, we found that the cracks were growing very slowly. These

cracks could be seen on radiographs taken as early as 6 years prior to

the accident. As it turned out, the inspections that were performed

were sometimes adequate to detect the cracks, but the people that were

reading the radiographs just didn't see them.

H. Ferris, Hughes Helicopters : It looks to me that design is blamed

for failures whereas the failures really appear to be related to

quality assurance. What was done about the decarburization which

caused the strength to be half of what it should have been?

Mo L, Marx : The design permitted up to 0.15 in decarburization.

H. Ferris : I thought that 4130 steel heat treated to 180 to 200,000

psi was specified.

M. L. Marx : Right.

H. Ferris: But that was not obtained.

178
M. L. Marx : The material was heat treated to the proper strength.

These aircraft were designed in about 1925, and at that time, the

applicable design specifications permitted a certain amount of

decarburization.

179
USE of CASES in ENGINEERING EDUCATION

THEIR SPECIAL VALUE

Henry 0. Fuchs

Design Division, Dept. of Mechanical Engineering

Stanford University

Stanford, California 9U305

Case studies such as we have heard in these sessions can

be of iimnense value in educating engineers , particularly when

they become case histories suitable for use in classrooms.

Their contribution as a supplement to the other media which

we use is unique.

Let us compare the work of learning engineering to the

work of learning a language: We find that in engineering

education the greatest fraction of time is spent in learning

the scientific background. In learning languages this would

correspond to grammar, the general rules of the language. A

smaller fraction of time is spent in design projects or

other project work. In language learning this corresponds

to composition in the foreign language. A very small amount

of time, frequently none at all, is spent in observation and

discussion of the work done by engineers. In language studies this corresponds to reading and discussion of the literature written in the foreign language.

Engineering education is unique

,

I believe, in omitting

from most of its curricula the study of the works of practitioners. Language students study literature, music students listen to recorded music, art students go to museums.

The place of case histories in engineering corresponds

to the place of literature in the study of language or to

that of records in the study of music.

An engineering case history is an account of an engineering job as it was actua lly encountered , told with the

circumstances of time constraints, resource limits, and personalities which influenced the outcome of the job. A technical report which tells only the problem and its solution

is not an engineering case: It omits the circumstances and

all the efforts which might have been avoided if foresight

had been as good as hindsight. Cases taken from their own

experience were often told by professors who were also practicing engineers. Today very few engineering teachers have

had responsible engineering experience.

Our collection of written case histories provides

material which can supplement lectures, problem sessions,

laboratories, and projects by providing snapshots of outside

180
reality for study in schools. These written case histories

have some advantages over oral accounts by experienced engineers: They can be studied more leisurely, they can be discussed more critically, and they can easily be used as

sources of problems and role-playing situations.

The ASEE-Stanford Case Library is a central clearing

house for engineering cases. It was started ten years ago

with an NSF grant to the late Professor John Arnold and is

now supported by a few industrial sponsors. It has about 200

case histories available, in pamphlet form for distribution to

students. These pamphlets have been used in classes by well

over 100 schools. Almost as many schools keep bound volumes

of the case collection available as research material.

The sources of case histories are varied: For instance,

both Professor Smith and Professor Kardos have contributed to

the collection. Some of their cases are taken from their own

experience, others are skillfully assembled from historical

accounts or from newspaper articles. One of these deals with

the collapse of false work under a bridge which led to the

deaths of many people. It is a fine example of the many uses

to which a single case can be put: I have used it in a class

on failure prevention for problems in the analysis of buckling. Professor Kardos has used it to discuss professional

ethics. Others have used it to bring out concepts in liability and in engineering organization. Each of these different

uses is enhanced by the fact that the problem which we analyze is embedded in the circumstances of the case.

Professors are not the only contributors of cases. Graduate students have written excellent cases under the supervision of their professors and practicing engineers have contributed case histories from their experience. We have also

made available some previously published case histories, for

instance, some of the incidents reported by the Flight Safety

Foundation, and Mr. Graham's article on the development of

his variable speed transmission which he entitled, "Twelve

Years to Discover the Obvious"

.

To me it seems obvious that the study of specific instances, reported with the circumstances surrounding them,

is essential to an engineering education. This does NOT mean

that cases should replace lectures, textbooks, laboratories

and projects. Cases should supplement them.

When they first begin to study case histories, students

often are reluctant to believe that good engineers can be so

human as the cases show them, so fallible, so lacking in complete knowledge. To me, this reluctance indicates the need

for case histories; it suggests their power to show engineering, as it really is, to engineers and to others, and it shows

why we must restrict ourselves to true histories . We might

perhaps invent better stories, but we could not overcome

un-belief with them.

I8l
To sum it up: We need good true case histories. They

can be used in many different ways. We have an engineering

case library as a central clearing house for cases. We would

like to receive good case histories from you and your ideas

on the use of cases in engineering education.

182
WHAT CAN BE LEARNED FROM CASES

G. Kardos

Carleton University

Ottawa, Canada

Introduction

For the last three years we have been running a core course in

engineering design with the entire lecture period devoted to Case discussions. Because the use of Engineering Cases is still comparatively

new, we have had the students make a course evaluation at the end of

the term. The purpose of this evaluation is to provide a feedback as

to what students felt they had learned from the Cases.

The Course

At Carleton University the four year engineering program consists of a three year common core and a final year of professional specialization

in Civil, Electrical or Mechanical engineering. Because of this

structure, the core classes must emphasize fundamentals . Engineering

82 .323-Engineering Design Studies, was introduced in the second term of

the junior year to give the students information about the design

process and engineering practice.

To meet this objective it was decided that the course would not have a specific information content. The format of the course was two hours a week of Case discussion and one three hour laboratory period for design

projects. The design projects were of the "make and break" type in

which the students worked in teams on a competitive basis. The Cases

used for the discussions were from the Stanford Case Library. Approximately eleven Cases are used during the term. A final written critical

essay was required on one of the Cases.

The Case discussions were carried out in groups of approximately 25-30

students. Over the three years six different instructors were involved.

Only two instructors participated all three years.

The specific objective of the course was laid out at the beginning as

follows

:

To give the student a realistic view of and various experiences

in engineering design as actually carried out in industry.

To develop the students skills in using and appreciating the

analytic tools he has and to develop his engineering judgement.

To demonstrate to the student how he may use his learning skills

to tackle engineering situations beyond his immediately available

analytical tools.

185
To give the student a real design experience with hardware

feedback

.

The Questions

The questions asked of the students in the evaluation are shown in

Figure 1. This questionnaire was originally developed by Karl H. Vesper

at Stanford University. In generating the original questionnaire Vespa:

had classified the questions into four categories: Skills (1-6), Habits

(7-15), Knowledge (16-21) and Attitudes (22-29). Although these

categories were retained they were not identified as such on the

evaluation sheets.

In Vesper's original questionnaire students were asked to rate the

Case method with respect to other teaching methods on a scale from 1 to

5- Although we found the questions the kind we wanted to ask, we

found the rating method somewhat difficult to interpret. For our

purposes we changed the rating to have the students indicate their

relative agreement with the statements. The degree of agreement to be

indicated as:

1. Disagree

2. Can't tell

3. Probably

k. Agree

5. Strongly agree.

Although both Cases and projects were rated, in this presentation I

will confine my remarks lo what the students felt they had learned from

the Engineering Cases.

The Results

The results of the responses of 300 students over three years are

shown in Figure 1.

For our purposes, we interpret a response of "Probably (3)" or better

as showing that some learning of the type indicated had taken place.

A response of "Disagree (1)'' was interpreted as showing that learning

of the type indicated had not taken place.

With the above interpretation, what did students feel they had learned

from Cases?

Better than Q0% of the students felt that Cases had probably:

increased their appreciation of what engineers do. (17)

181^
increased their appreciation of criteria for judging engineering

procedures and for evaluating design. (l8)(19)

increased their ability to define practical problems and to spot

key facts amid less relevant data. (9) (10).

increased their tendency to discriminate between fact and

opinion and to search for more alternative solutions (2) (3).

In addition 70^ of the students felt that Cases had probably:

increased their appreciation of Historical episodes as lessons. (I6)

increased their appreciation of technical facts in engineering

other than theory. (21)

developed their ability to view problems with perspective, (l^)

developed their ability to sell their own ideas as well as

increase their tolerance for other's ideas and errors. (15) (29)

More than GO°lo of the students, in addition to the above, felt that

Cases had probably;

increased their ability to foresee consequences of alternative

action, to think more carefully before speaking, to prescribe actions

more specifically and developed their ability to communicate. (ll)(6)

(M(8)

increased their knowledge of their own abilities and limitations.

Certain kinds of learning did not take place with our use of cases.

Forty percent of the students felt they had not learned anything about

formulating idealized mathematical models of real problems or about

how to manipulate such models for solutions from the Cases. (12)(13)

In addition, 30 percent of the students felt that Cases had not contributed to:

their ability in using unfamiliar tools or methods (?);

their self confidence (22);

their aesthetic sensitivity (26);

their enthusiasm or motivation toward engineers (25);

Learning From Cases

As a result of this evaluation we can say something about what can be

learned from Cases as we have used them. Students can learn about what

engineers do and about the criteria necessary to carry out a real

engineering project. Students learn the necessity of properly identifying and defining the real problem and to look for alternate solutions.

With Cases students learn to interact with their peers in a cooperative

and tolerant manner toward the solution of a common engineering objective.

185
We can also say something about what we have not been able to do with

Engineering Cases.

The students have not learned or appreciated the use of the mathematical and theoretical tools available to them. This may not be the fault

of cases but in the manner in which we have applied them. In the

future we plan to make changes which may correct this.

Cases have not been successful as a medium for motivation. This result

is most unexpected. Instinct would suggest that cases would be highly

motivational; for us this has not proved to be so. We have speculated

about why, but we have been unable to come up with a reasonable

explanation.

186
Figure 1

CARLETON UNIVERSITY

Faculty of Engineering

Engineering Design Studies

Eng ineering Case Evaluation

300 Responses

323 has increased my tendency to:

1 Reason quantitatively (use numbers) whenever possible

2 Discriminate between fact and opinion

_

2 Search for more alternative solutions

k Prescribe action more specifically

5 Pay meticulous attention to- detail

6 Think more carefully before speaking

323 has helped develop my ability of:

Using unf amiliar exercise tools or exercise methods

Communicating (writing or speaking or drawing)

Identifying and defining practical problems

% Positive Response

80 100

7

8

10 Spotting key facts amid less relevant data

11 Foreseeing consequences of alternative actions

12 Formulating idealized math models of real problems

13 Manipulating and solving idealized math models

Viewing problems and complications with perspective

1 5 Selling ideas or arguing more persuasively

323 has increased my appreciation of:

16 Historical episodes as lessons

lY What engineers do

2_Q Criteria for judging engineering proceoures

19 Criteria for evaluating designs

20 Mathematical, scientific, or engineering theory

21 Technical facts used in engineering (besides theory)

323 has increased my:

22 Self confidence

23 Perseverance in spite of setbacks

2h Concern with questions unanswered for me yet

2 ^ Enthusiasm, motivation for course or engineering

26 Aesthetic sensitivity

27 Interpersonal sensitivity (feel for people)

28 Self knowledge (own abilities and limitations, etc.)

29 Tolerance for others' ideas and errors

1

187
STUDENT WRITTEN DESIGN CASE STUDIES

C. O. Smith

University of Detroit

Detroit, Mich. 48221

Case studies are of immense value in the education of

engineers, both in and out of the classroom. When they are

written for classroom use, they become a unique supplement to

the more traditional classroom tools of lecture, problem

solving, etc. In using case studies or case histories, there

is an opportunity for both writers and users to profit from

them. It is recognized, however, that the potential profit

for the writer is substantially greater than for the user.

As a consequence, I have offered a course at the University of Detroit called "Design by Case Study" at the gradu-^

ate level. The objectives of the course are: (1) become

acquainted with the case method; (2) gain greater insight into design and engineering practices; (3) assess the decision

making process as it functioned in a series of cases studied;

and (4) write a case study.

During the first five weeks (about 1/3) of the course,

the class studied, prepared written discussions, and then

verbally discussed a series of case studies. Some of these

were from the Engineering Case Library at Stanford University. Others were student-written case studies generated at

the University of California at Berkeley under the direction

of Dr. R. F. Steidel.

During the next five-to-six week period, the class

worked in small teams (usually three students per team) in

the "real world". Each team worked with an engineer who was

in responsible charge of a project in a local engineering

organization. Both the engineer and the project were rather

carefully selected in advance in concert with the cooperating

organization, since the project engineer served as a "clinical professor". The instructor accompanied the team on its

initial meeting with the project engineer at his place of

work but not on subsequent visits. During this period, the

class did not meet together but the instructor was available

188
for consultation. Thus, the team had to function by itself

under non-academic circumstances.

The first time this course was offered, the team visited

the project engineer, listened to his exposition and asked

questions (recording the discussion on tape) . Between interviews, the team went through the results of the interviews

and any file documents and drawings loaned to the team. The

team members were expected to review the design from concept

to completion (including field performance if possible) with

particular attention to chronology, key decisions (and factors involved in reaching them) , wrong directions taken as

well as successful directions, and critical review of technical aspects.

The second time the course was offered, there was some

shift in emphasis. This time, the team again met with the

project engineer and discussed the background of the problem

and arrived at a problem definition. The team then was expected to try to develop a conceptual solution. On each successive visit with the project engineer, the work of the team

was reviewed by the project engineer and discussed. Additional information was presented to the team and they proceeded further with their conceptual solution (or substantially altered it, as appropriate). At the end of the series

of visits, the project engineer told the team how the problem had actually been solved in practice.

In both modes of emphasis, the team had to prepare a

written case study. A due date was established for a draft

with copies being provided for the other students as well as

the instructor. Each team was given time to prepare a critical review of the work of each of the other teams. The

class then met with each team presenting its study and the

other students asking pointed questions. Following this,

each team had an opportunity to confer again with the project

engineer, fill in gaps in the study, and develop a final

written draft. At the end of the course, each team made a

public oral presentation of its study. Faculty, project engineers, and other students were welcome at the final presentations .

This is not a "cake" course (for students or instructor)

189
with each team producing a completed document of perhaps 50

to 80 pages, especially relating to a design project in an

area in which they may have classroom background but little

practical experience. The reaction of students, project

engineers and instructor is that the students have had a

valuable experience. It has been agreed that they gain a

greater insight into real product development with its interplay of analysis, testing and decision making, than would be

possible through academic study alone. In at least one case,

the student team proposed a conceptual solution which the

project engineer admitted had not occurred to the industrial

team but which appeared superior to the one that had been

developed

.

190
Append ix

Student-Written Case Studies

and Cooperating Companies

FAST - A New Friction and Wear Test for Brake Linings

Ford Motor Company, Dearborn, Michigan

Development of an All-Paper Asphalt Container

Owens-Illinois, Toledo, Ohio

Development of Military Industrial Standard Engines

with Specific Consideration of the Governor

Teledyne Continental Motors, Warren, Michigan

Design and Development of a Brush Burning Plant

City of Detroit, Detroit, Michigan

First Harmonic Analyzer

Uniroyal, Detroit, Michigan

Automatic Cam Brake Adjuster

Eaton, Yale and Towne, Southfield, Michigan

Transformer Load Management Research Program

Detroit Edison, Detroit, Michigan

Design of the Pocket Select Gate for the Burroughs

B9134-1 Reader Sorter

Burroughs Corporation, Detroit, Michigan

Evaluation of Passenger Car Eyellipse for Visibility

Requ irement s

Ford Motor Company, Dearborn, Michigan

A New Air Flight Control Power System

Sperry Vickers, Troy, Michigan

These case studies have been reproduced and are available

cost

.

191
TEACHER FEEDBACK ON THE USE

OF ENGINEERING CASE STUDIES

Byron J. Pelan

Associate Professor of Mechanical Engineering

Department of Mechanical,

Industrial, and Aerospace Engineering

Rutgers University

New Brunswick, N. J. 08903

How do I as a teacher of mechanical engineering design and a former

practicing engineer regard the engineering case method? Let me lead into

my response by first briefly commenting on several considerations.

In recent years there has been a widespread movement in engineering

education to expose the student to realistic engineering situations,

sometimes contrived but often from real life. In part, our reason has

been for stronger motivation. We have also believed that somehow we

could then better prepare him as an engineer. I would be less than candid if I did not include our concern for an antidote to the attrition and

disillusionment of some engineering students. There are other reasons,

too, of course. Part of this widespread movement has been the institution of programs in which the student becomes an intelligent, active

participant. The use of the engineering case method, the authentic involvement of students in industrial problems, and the selection of design projects that culminate in such useful hardware as prosthetic devices and unique instrumentation are instances of this movement. The

particular approach adopted may depend upon several parameters, including

time limitations, teacher preference, and industrial support as in the

instance of authentic student involvement cited. However, the adoption

of one approach does not preclude the adoption of another. Indeed, the

use of more than one can have a reinforcing effect. A properly-selected

case study can provide background for a design project and also motivate

the student by adding the realistic ingredient of an actual engineering

situation. My comments here are addressed to the case method, in particular to case problems and case histories that are available, not the

writing of them. As you are aware, in a case problem the student is in

the position of an engineer facing a problem to be solved whereas in a

case history the student has the opportunity to see the problem and the

outcome

.

Primarily the teaching method to be used depends upon the teaching

objectives. For example, to more nearly individualize instruction, selfpaced instructional programs complete with special course material,

including text, audio-visual tapes, and self-administered progress checks.

192
are currently used by a number of colleges. Traditionally, although

possibly not always with complete justification, many engineering courses

have been taught using the lecture-recitation method. It is frequently

the most viable method to employ, the economics alone being a major consideration. In such courses, some faculty, wishing to implement motivation of their students, have included case problems to illustrate and to

add realism. For instance, case problems have been used in undergraduate

courses as a supplement to conventional problems to show how practicing

engineers attack real problems. Professor Hirsch of the U.S. Naval Academy has introduced a real case concerned with the dynamics of a recoiloperated, automatic weapon at a timely point in his first course in

undergraduate engineering mechanics [I]-'-.

The most likely teaching objectives of the case method in engineering

were investigated by Professors Vesper and Adams [2,3] . Three groups were

polled: professors who had attended summer institutes where they had

written cases and practiced on them, professors who taught case studies

on the first-year graduate level, and, lastly, graduate students in that

course. All groups placed high emphasis on these teaching objectives:

1. Knowledge of what engineers do and how they work

2. Enthusiasm, motivation for course or for engineering

3. Search for more alternate solutions

4. Identification and definition of practical problems

5. Discrimination between fact and opinion

6. Detection of key facts in the midst of less relevant data

7. Need to be more specific in prescribing action

8. Need to foresee the consequences of alternative actions

It is interesting to note that another survey in their investigation revealed that a second group of graduate students participating in

a four-week summer institute composed entirely of cases agreed that two

major effects of case instruction were exercise in thinking and increased

realism [2]

.

Case studies may be used at different levels. It is not unusual to

use the same case at several instructional levels: freshman, senior,

and graduate. At the first level it may be used to expose the student

to the real engineering situation and have him prepare engineering

sketches of student-generated alternatives. The senior may find the

case a justification for the time spent on fatigue failures in his

stress-strain-strength course and may even have that delight of detecting

the reason for the failure and of being able to competently recommend an

alternative correction. For the graduate student, on the other hand,

here is an opportunity to diagnose the case from an administrative viewpoint .

Cases have been assigned in a number of ways, as you have come to

suspect: homework reading assignments, problem formulation, background,

illustrations, research material, and so on. Normally, I prefer at some

figures in brackets indicate the bibliography references at the end

of this presentation. ig^
point to have group discussion. Such a discussion may require more than

one class period; for instance, it may be in sequential parts with the

students not receiving the entire story until later. My students have

had an opportunity to review each part prior to discussion. A group discussion gives the student an opportunity to do ~ and ideally should demand that he do ~ independent, constructive thinking and make positive

contributions. Additionally, it provides the real environment of an

engineering case as background. Initially I find that the attention of

the class is divided between the case and myself, but the case and the

developing discussion must become the prime focus of their attention.

The case presents an opportunity for detached objectivity, free from the

emotional involvement of the original participants. At times the student

may be called upon to defend his position in open discussion. This is

part of his opportunity to grow. This total experience, we hope, eases

his transition from college to industry. Exposure to a number of cases

is often desired. Ideally in such instances the point is approached

where he does make independent, good judgments. But not all students

can or do achieve this and not all faculty would be successful with case

studies. Good cases in poor hands can be disastrous; poor cases in good

hands still might be used effectively.

The role of the instructor is unique. In a good case activity, he

is present, prepared to intervene as subtly as needed but nearly forgotten by the students. As Professor Kenneth Andrews of the Harvard Graduate Business School has stated in his paper entitled "The Role of the

Instructor in the Case Method": "However his judgment prompts him to

behave in the classroom, the instructor plays a multiple role. He is

student, listener, and analyst. He is questioner, paraphraser, and

minuteman lecturer. He plays these parts without costume changes, and

he never steals the show from the rest of the cast" [4]

.

In conclusion, I would be delinquent if I did not mention that the

teacher, too, benefits ~ and in many ways. First, he himself has had an

opportunity to grow, for as he left the structured teacher-student relationship of the traditional lecture-recitation format, he also left the

environment of almost predictable student response patterns. Now, he

must play that role which will demand all of his skills as a teacher.

Second, he has an opportunity himself to be exposed to different, often

exciting engineering situations and thereby to add to his own professional experience.

BIBLIOGRAPHY

[1] Hirsch, R.J., "The Use of Engineering Case Studies for Teaching Undergraduate Mechanics" included in the paper portfolio "Five Ways To

Use Cases in Teaching", Event No. 251, Annual Conference, American

Society for Engineering Education, June 24, 1974, U.S. Naval Academy,

19k
Annapolis , Maryland

[2] Vesper, K.H. and Adams, J.L. , "Evaluating Learning from the Case

Method", Engineering Education (ASEE) , October, 1969, Part 1, pp.

104-106

[3] Vesper, K.H. and Adams, J.L., "Teaching Objectives, Style, and Effect

with the Case Method", Engineering Education (ASEE) , April 1971, pp.

831-833

[4] Andrews, K.R., "The Role of the Instructor in the Case Method": The

Case Method at the Harvard Business School , edited by Malcolm P.

McNair, McGraw-Hill, 1954

[5] Vesper, K.H., "Task Corporation-Failure of a Ball Bearing" (Stanford

University Case Library No. ECL-14) : Case Studies in Mechanical Engineering Design , edited by R.F. Steidel, Jr., College of Engineering,

University of California, Berkeley, 1968

[6] Gragg, C.I., "Because Wisdom Can't Be Told": The Case Method at the

Harvard Business School , Ibid.

[7] Fuchs, H.O., "Outside Reality Inside the Classroom", Engineering

Education (ASEE) , March, 1970, pp. 745-747

[8] Kardos, G. , "Conducting Engineering Case Class Discussion", paper

presented at Engineering Case Study Workshop, Annual Conference,

American Society for Engineering Education, June, 1974, Rensselaer

Polytechnic Institute, Troy, New York

[9] Vesper, K.H., "On the Use of Case Studies for Teaching Engineering",

Engineering Education (ASEE) , October, 1964 , pp. 56-57

[10] Smith, CO., "Case Studies as Laboratory Exercises", Annual Conference, American Society for Engineering Education, June, 1974,

Rensselaer Polytechnic Institute, Troy, New York

[11] Brewerton, F.J., "The Engineering Case Study - An Interdisciplinary

Comparison", SAE Paper 700003, Automotive Engineering Congress,

Detroit, Michigan, January 12-16, 1970

195
CASE STUDY ILLUSTRATION

(Illustration referred to at the closure of "Teacher Feedback on the Use

of Engineering Case Studies")

I have a double purpose in taking a further moment of your time to

describe one case: first, to show one way a case has been composed and,

second, to hope this may suggest to you a problem that you have encountered that might be used for a case study.

Technically, the case* I have selected concerns the failure of the

front-end ball bearing of an electric motor driving an aircraft pump.

Figure 1 shows the condition of this bearing after 1800 hours of operation. This bearing and the one at the other end of the motor shaft were

cooled by immersion in the circulating hydraulic fluid. Figure 2 shows

a cross section of the pump and motor assembly. Considerable documentation has been provided for the study. For instance. Figure 3, which is

my composite shows some of the typical data that has been included: there

we see an excerpt of the specifications of the bearing from the manufacturer's catalog and also an excerpt of the specifications of the hydraulic

fluid. Other documentation and commentaries re-establish in part a sense

of the original atmosphere of concerns and urgency. The motor is but one

of 300 on order. Each is specified by contract to have a failure-free

life of 2500 hours at least. Yet, the first failure occurred during the

life qualification test after the motor and pump had been connected. But,

by then, tooling had been finished, production started, and some motors

already had been made. The pump manufacturer had also started production.

There were several conferences. The bearing manufacturer was consulted.

Figure 4 shows a portion of the manufacturer's letter of analyses and

recommendations. As a consequence, a different bearing was substituted

on all units. Then again a failure ~ it was the substitute bearing. The

case progresses rapidly. Outside consultants were engaged; there were

differing opinions. Then another failure of the bearing occurs. Again

questions are raised. Could it be that the pump produced an axial loading

even though no reference was made to one in the original specifications?

Could there have been a misalignment or a binding? Thus, the case develops and, as it does so, opportunities are presented to explore more than

just the technical aspects if desired.

*Vesper, Karl H. , "Task Corporation ~ Failure of a Ball Bearing" Stanford

University Case Library No. ECL-14.

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198
PANEL DISCUSSION : Use of Cases in Engineering Education

Panel Moderator : George Hurt, Detroit Diesel, Allison Division

Panel Members : Henry 0. Fuchs, Stanford University

Geza Kardos, Carleton University

Byron Pel an, Rutgers University

C. 0. Smith, University of Detroit

G. Hurt : Have you tracked any of these former students into industry

and seen whether they could, or have, answered any of these same

questions after they have had some experience?

H. 0. Fuchs : Without deliberately tracking them, I have gotten some

volunteer responses from students. One of them told me that the case

course that he took had been more valuable to him than most of the

other courses that he had taken. We got a similar response from

another student who came back for a master's degree after working in

industry.

C. 0. Smith : In the graduate case course that I was talking about, vfe

have some part-time students with 5 to 10 years experience. Without

exception, these students said that the course was beneficial.

G. Kardos : My experience is the same. We have had this course for 3

years and it is a little too soon to get any direct feedback. But,

anyone who has any working experience immediately recognizes this

course as being useful . The working engineer seems to be much more

responsive to the course than the new, green engineer.

R. E. Maringer, Battelle Memorial Institute : My feeling is that this

is an ideal way of teaching. It takes a student who does not know

anything about industrial life and gives him a little more of a feel

for what he is going to be doing the rest of his life. It seems to me

one way of judging whether the case studies approach is effective is

how broadly it is used. Is it increasing in use; are there demands

for these case studies?

H. 0. Fuchs : Last year there were 7,000 copies of case histories

requested from us at cost. The case studies approach is increasing in

use. My guess is that it is used in half the schools, and in about 3%

of the classes taught in those schools.

G. Kardos : Stanford has a very liberal policy on distributing case

studies. If you want to buy it at cost, they will let you have it at

cost. If you do not want to pay cost, they will be glad to save you

the money by letting you reproduce it yourself. I am sure there are a

199
lot of copies of case studies being used that we don't know

about— there is no way of getting the feedback.

C. 0. Smith : Bob Maringer used the term "ideal". My response is that

it is not ideal —it is realistic. Teaching a course with case studies

is not as easy, or as comfortable, for the instructor as a lecture

course. Once you have lived with it for a while, it becomes more

interesting. If you get good interaction among the students, you may

learn something.

A. Uolff, Massachusetts Materials Research^ Inc. ^ Do you use real

industrial cases from your area so that the students can meet with the

industrial people involved?

C. 0. Smith : Yes, they have worked with people from the city of

Detroit, Ford, Bendix and others.

A. Wolff : Do the students deal with the quality control managers and

the technical managers?

C. 0. Smith : They deal with whoever is responsible for the project.

G. Wagner, Westinghouse Electric Corporation : Do you precede these

case history studies with some theory on the logic of design?

H. 0. Fuchs : We at Stanford go easy on the logic of design. We do

mention it in other courses, but we do not treat it as a separate

subject.

G. Kardos : We do not precede the case studies with the logic of

design. At the end of the case course, a student must turn in a critical evaluation of the design process from one of the more

extensive cases against a written text book design process.

C. 0. Smith : To try to teach the logic of design, per se, would be

very dul 1

.

Go Wagner : The natural extension of understanding the logic of design

is recognizing that engineering is more certain in terms of

uncertainties than otherwise. Again, the logical argument is that the

student does not understand or appreciate that the nominal design is

not necessarily the design that he gets at the end of the production

1 ine.

G. Kardos : In some of the essays that I mentioned previously,

students often point out that the textbook design process was not

followed. But the important thing is that the design worked.

200
G. Wagner : One other point, when an engineer is exposed to an actual

industrial situation, he is rarely alone. Team development is a worthy consideration.

G. Kardos : We agree that team effort is important. We have team

projects. When a project is completed, it is marked for performance

against specifications by other members of the class.

C. 0. Smith : I might add that we also require a design project at

freshman and senior levels. The freshman projects consist of two or

three-member teams. The senior project is an interdepartmental

operation and has teams of three to six members.

201

OVERSIGHTS

AND

OVERVIEW

Chairmen: A.K.Wolff

Massachusetts Materials

Researchjnc.

B.P.Bardes

Bimba Manufacturing Company

SOME THERMAL PROBLEMS IN THE DESIGN

OF FLUID FILM BEARINGS

Dudley D. Fuller

Dept. of Mechanical Engineering

ColiJinbia University

New York, New York 10027

The analysis of fluid film behavior in journal and thrust bearings

is rather well developed and much useful material is presently available

to permit the prediction of performance in such bearings. However, a common error is to concentrate on the fluid film itself and to more or

less neglect the total bearing. The total bearing is the fluid film plus

the bearing structure that contains it and which allows it to behave as

fluid-film theory predicts that it should.

The mechanical design of the bearing does influence its ability to

operate properly. Proper mechanical design is often overlooked, especially the effects of temperature and temperature gradients. This discussion will be concerned with these thermal aspects of bearing design.

The fluid film bearing is similar to all other common forms of bearings in that when operating it does have some friction. This friction

produces an energy loss that appears as heat. Eventually this heat must

be dissipated in some way through radiation, convection, conduction or

evaporation until the rate of heat dissipation is just equal to the rate

of heat generation. At that point thermal equilibrium will have been

achieved and the steady state operating temperature of the bearing will

have been established.

A complete bearing analysis must include, in general, a heat balance so that the leveling off temperature of the lubricant and the bearing materials can be predicted and also so that the thermal growth of

the bearing package and possible distortion of the housing may be considered.

Details for doing this for both self-contained bearings and for

externally-cooled bearings have been set forth in a number of texts on

the subject, as for example. References (1) and (2). Because of the

importance for industrial applications of the self-contained bearing I would like to make a few comments with special reference to this type.

For self-contained or self-cooled bearings, the housing and its

supports should have sufficient bulk to produce an acceptable heatdissipating surface. The housing surface of such bearings acts as a radiator to dissipate from the bearing by convection and radiation the

heat generated by bearing, fluid-film friction. From a maintenance point

205
of view accumulation of oily dirt on the housing is very undesirable.

The extra insulating layer may upset the natural heat balance of the

bearing and cause overheating. Many layers of paint may accomplish the

same result. The heat-dissipating action of a bearing and its housing

should never be modified without serious consideration.

In spite of all the obvious variables that enter the picture, the

limits of size and speed of industrial bearings that have been successfully operated as self-contained bearings without auxiliary cooling are

fairly clear and Fig. 1 represents these limits. Since the curve is

based on typical designs it does have value as a rough design measure.

w 3000 -

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Fis. I 4 a /X

SHAFT D/AMETE/Z, ^/A/C»E^)

20

A rather interesting example concerns a 1 radioactive liquids. The unit had a shaft 6 i at 1800 RPM. Its design incorporated standard

thrust bearings. Reliability requirments for

was servicing, demanded that the unit continue

four hours in the event of failure of the circ

ings. This means effectively that the bearing

ing characteristics of self-contained bearings

bearings where the heat was carried away by ci

originally designed.

arge pump to handle waste

nches in diameter turning

Kingsburg journal and

the plant, that the pump

to function for twentyulating oil to the bears needed to have the coolrather than those of

rculating oil as they were

The first attempt to do this resulted in failure. When the circulating oil was shut off, the bearing temperatures continued to rise until

seizure took place. There was no hint of a leveling-off temperature

being reached. Fig. 2. It seemed as though there could be no hope of

ever reaching the goal of having a self-cooled bearing with this design

concept.

Before going through a complete analysis and making a heat balance

for these bearings we can spot the diameter of 6 inches and speed of

1800 RPM on Fig. 1 and observe that it falls right on the line. This is

206
a quick but approximate indicator that this design concept could be selfcontained.

Cut opp T/Me

Cfn.CUUkT/A/C

OfL.

Further investigation showed that the bearings were very lightly

loaded and could be reduced in size without sacrifice of safety. This • would reduce the shear area and heating. Accordingly the journal bearings were shortened to about 2/3 of their original length and every

other shoe of the thrust bearings was removed. In addition, a new cover

with extended ribs and fins was put on the bearing housing. With these

modifications the bearing was converted to a truly self-contained, selfcooled bearing. The heat generation was reduced and at the same time

the heat dissipation capacity of the bearing housing was increased by

extending the surface. No further modification was needed, but if it

were, an axial blower could have been attached tothe shaft, blowing air

over the housing and thus increasing the rate of heat transfer from its

surface.

Bearings operating at higher speeds than those indicated in Fig. 1 do need some form of external cooling. This is often accomplished by

forced feeding of the lubricant through the bearing. However the hot

lubricant on returning to the reservoir or sump must release its heat,

in some manner, to the outside world. Thus the reservoir itself must be

evaluated in terms of its heat dissipating capacity.

This is a continuing problem with automobile engines for example,

especially when the car is driven at high speed. Cooling can only take

place by flow of air over the surface of the crankcase as a result of

the motion of the vehicle. If the crankcase is small, with a typical 4

quart capacity, the surface dissipation area is also small, especially

if it is not ribbed, and the ability of the crankcase to dump heat into

the atmosphere is limited. When super-powered cars are converted for use

in stock car racing, under NASCAR regulations (National Association for

Stock Car Auto Racing) it is necessary to about double the crankcase

207
capacity and in addition, in many instances, add an oil cooler. Ref (3).

Sports cars often have ribbed crankcases to increase the surface cooling

area. To the best of the author's knowledge, no stock passenger car

manufactured in this country has a crankcase oil cooler. The MercedesBenz Diesel engine passenger car has such a cooler and so does an MG

sport sedan. Manufacturers of industrial equipment like tractors and

trucks recognize this and use extra large crankcases. Transcontinental

trucks frequently have separate oil cooling radiators and crankcase

temperature control since bearing temperatures in truck engines, with no

external cooling, have been measured as high as 310°F, even in the winter.

On rainy days with water spray from the tires and especially on

snowy days where slush is being splashed on the crankcase, temperatures

as low as 70°F have been measured in automobile crankcases even when

driving at 40 to 50 MPH. On the other extreme, high-speed turnpike

driving in the summer will show crankcase temperatures approaching 300°F,

especially if the body sheet metal work tends to shield the crankcase.

There have been instances where this has happened. Such elevated temperatures will tend to soften bearing materials and make them more prone to

failure. The service life of oil lubricants is also sharply reduced, due

to chemical deterioration.

The comments made above for conventional lubricants and bearing

materials apply with some modification to more unusual kinds of fluid

film lubrication where the lubricant may be water, acid, liquid refrigerants, molten metals, mercury, gas, etc.

Conditions of pressure and temperature must be established within

the bearing package, the bearing film and where the lubricant leaves the

bearing to be sure that vaporization or boiling will not occur. If the

vaporization takes place within the film, cavitation may result; real

cavitation, which can cause serious physical damage to the bearing materials, as the vapor bubbles are swept into areas of high pressure in the

film and collapse. This has been a serious problem with mercury lubricated bearings in some of the space-power systems.

Material softening is usually not a problem with these unusual

lubricants since conventional, low-melting point bearing materials are

generally not employed for most of these systems. Instead, such things

as Monel , stainless steel, Stellite, Alundum (al . oxide), tungsten

carbide, al . bronze, etc., may be used.

Of course, vaporization can occur at low temperatures with volatile

liquids such as alcohol, gasoline, or liquid refrigerants. We were

running a journal bearing on Freon F-11 at room temperature conditions

and fairly light loads. The Freon was being supplied to the bearing at

70°F. Vaporization occurred in the film so that the bearing was cooled . As a result the film temperature dropped to 32°F and frost accumulated on

the outside of the bearing shell.

208
Gas, of course, does not vaporize at elevated temperatures and this

is one of its advantages as used in gas-lubricated bearings. At the other

end of the temperature spectrum, namely approaching the cryogenic level,

gas may liquify. Thus with a change of state, either vaporization or if

a gas vapor is used, condensation, there is a marked change in viscosity

which means a large change in the performance characteristics of the

bearing. This must be considered in design.

Temperature changes also mean variations in bearing clearances. Most

bearing design calculations are based on idealized bearing clearances at

room temperature . These are usually not the clearances that are found at

operating remperatures. This may make a serious difference.

A very common problem is to find that the clearance has reduced as

the unit reaches equilibrium temperature. If the clearance is reduced,

the friction losses increase. At the same time, with a smaller clearance,

the flow of lubricant is cut down, tending to raise the temperature.

This in turn reduces the clearance still further, so that at times a condition of thermal instability is reached which can lead to failure.

The source of heat is in the film itself. The bearing housing has a greater capacity to get rid of heat than the journal. The amount of heat

conducted through the journal is actually very small. Thus while the

journal will increase in diameter as the temperature rises, and the

bearing diameter will also increase, the increase in the bearing diameter

will typically not be as great as that of the journal, so that there will

be a loss of clearance. The prediction of just how much this will be

requires a very careful thermoelastic study of the structure. However,

in general, a slightly more generous value of cold clearance will usually

take care of the problem.

For example, a large gas line compressor, with a shaft 5 inches in

diameter, running at 5000 RPM, failed 3 sets of bearings. The failure

pattern indicated loss of clearance. Design specifications, based on

tolerances of manufacture for the shaft and bearings, called for a diametral clearance between 0.004 inches and 0.007 inches. Thus it was

perfectly possible to start out with a cold clearance of 0.004 inches on

a 5 inch diameter shaft operating at 5000 RPM. A loss of a mil or two in

diametral clearance, would spell the difference between thermal stability

and instability.

-6

With a coefficient of thermal expansion of 6.5 X 10 per °F, a temperature difference between the bearing and shaft of 30°F would yield

a loss of diametral clearance of 30 X 6.5 X 10"^ X 5, or about 1 mil.

Thus one could be trying to operate this high speed bearing with an

actual diametral clearance of 3 mils. Calculations would show that this

is too small

.

The recommended cold clearance was changed to 0.0007-0.010 inches

209
and no further failures occurred. For the same hypothetical equivalent

temperature differential, resulting in a loss of diametral clearance of

1 mil, the net minimum operating clearance would still be 0.006 inches or

twice that achieved before.

Another similar example concerned a large centrifugal compressor

pumping chlorine gas. Speed 12,000 RPM, shaft diameter 3.3 inches. Cold

diametral clearance 0.004 inches. Bearing failures resulted from overheating. Cold clearances were opened up to 0.007-0.008 inches and

eliminated these failures.

Another interesting example is the Haystack Antenna recently builti

by North American Aviation in Columbus for the Air Force in accordance

with plans developed by the Lincoln Laboratory at M.I.T. Ref. (4) shows

a combination thrust and journal bearing, 14 feet in diameter. Fig. 3.

This is a hydrostatic, externally-pressured bearing. In order to maintain a proper balance between flow and pressure in the journal bearing

pads, a radial clearance of 5 mils was desired. This imposed severe

manufacturing problems, but consider the thermal condition! Suppose the

temperature difference is 9°F between the inner journal and the cap. The

change in diameter would be 14 X 12 X 6.5 X 10-6 x 9, or 10 mils. All of

the clearances would be gone!

X——

\

PAX>S

Pai>s

One suggestion was to place a ring of strip heaters around the

circumference of the cap, and then if necessary, the cap could be

heated to expand it and maintain the required hydrostatic bearing

clearance. However, the secret of the solution to the problem was in the

elastic distortion of the geometric elements. When under hydrostatic

bearing pressure, the cap would expand and the inner journal would

contract, thus tending to compensate in this way for loss of clearance

due to thermal conditions. In a very oversimplified way, if we think of

the cap as a thin ring, with a radial wall thickness of 3 inches, but

with a diameter of 14 feet, and internal pressure of 1000 psi it will

show an increase in diameter of about 0.150 inches. The real problem

210
then was to manufacture the cap so that it could be assembled. After

that, the hydrostatic pressure itself would maintain a clearance because

of the elasticity of the structural elements themselves.

Sometimes it must be recognized that in a complex structural package

subjected to various temperature gradients, the clearance of a bearing may

increase for certain designs as the unit comes up to thermal equilibrium.

As is well known, half-frequency whirl in journal bearings is sensitive to

clearance. If the clearance increases by 10 or 20% certain bearing designs can become unstable and cause serious whirl problems. This has also

been encountered in the space-power development program and has at times

been a serious factor as it affects rotor dynamics and system stability.

For this kind of inverse thermal problem the solution may well be to have

a cold clearance which is smaller than the optimum value so as to allow

for its increase under steady-state operating conditions.

It is clear then that for reliable design of fluid-film bearings,

analysis of the film itself is necessary but not sufficient. In

addition the contribution of the bearing structure or bearing package

must be carefully evaluated. One of the most important aspects of this

structural behavior may be concerned with thermal problems caused by both

high temperature levels and temperature gradients. These should not be

overlooked.

References

1. Fuller, D.D., "Theory and Practice of Lubrication for Engineers,"

John Wiley & Son, New York, N.Y. 1956.

2. Rippel, H.C., "Cast Bronze Bearing Design Manual," Cast Bronze Bearing

Institute, Inc., 1604 Chicago Ave., Evanston, Illinois, published 1960.

3. Owens, C, "How to Build a Grand National Stock Car-Racer," Hot Rod , Feb. 1963, pp. 36-41, 114-118.

4. Carroll, G.R., "A Hydrostatic Bearing for Haystack," A.S.M.E. Paper,

62-WA-299 presented at the Annual Meeting Nov. 29-30, 1962, at New

York, N.Y.

211
NEVER OVERLOOK NOTCHES

C. O. Smith

University of Detroit

Detroit, Mich. 48221

For several years, I have been consulting on analysis

of failure of various items of hardware and machinery,

mostly relating to products liability litigation. It has

been my experience that at least 80% of these failures involve fatigue. I have yet to see a fatigue failure which

did not start at a stress concentration of some description,

i. e., a notch. In addition, I have observed some failures

which did not involve fatigue but did initiate at a notch.

In other words, notches render hardware highly vulnerable to

failure

.

Granted a biased viewpoint on my part, I find myself

dismayed by what appears to be neglect or oversight of

notches in design and/or manufacture. Certainly, knowledge

of the effect of notches on stress concentration has been

known for years. For example. Den Hartog (1) in discussing

a stepped shaft with the larger diameter twice the smaller

with a fillet radius between the two sections has said "if

the fillet radius is made equal or better than 1/4 of the

small shaft radius the stress-concentration factor is about

1.35 or better, which is low. Therefore more generous fillets than 1/4 are not practically justifiable. However, if

the fillet corner is sharp, the stress-concentration factor

becomes enormous." Elsewhere (2) he has said "stress concentration depends greatly on the local fillet radius of the

reentrant corner: for a mathematically sharp corner (zero

fillet radius) the stress becomes mathematically infinitely

large, which in practice means very large, equal to the

yield point of the material."

It so happens that I have in my personal library two

textbooks (3, 4) published just prior to World War II which

point out difficulties resulting from notches. One of these

also refers to a paper by G. B. Jeffery in the Philosophical

Transactions of the Royal Society in London, published in

212
1921. No doubt, a very little research effort would disclose even earlier publications. Comments on the effect of

notches appear in a large number of textbooks and handbooks.

Notches come from a great variety of sources. In some

cases, this appears to be a design oversight with much too

small a fillet being indicated. In other cases, it appears

to be a machining problem with too small a fillet resulting

during fabrication. In other cases, notches result from surface defects in the material, such as inclusions in the surface or just below the surface. Machining marks, weld defects, and similar fabrication difficulties can be sources of

notches and subsequent failure. Even quality control can be

a source of difficulty as there are some cases on record in

which the inspector's stamp effectively caused a notch which

initiated failure.

It is extremely important to eliminate notches from all

possible sources. This requires very careful design and

manufacturing techniques with painstaking quality control.

Production of finished hardware under rigid scrutiny from

design to shipping will reduce (but probably never completely

eliminate) the problems.

BIBLIOGRAPHY

(1) J. P. Den Hartog, "Advanced Strength of Materials",

McGraw-Hill Book Co., 1952 (pg. 48)

(2) Ibid, pg. 17

(3) V. M. Faires, "Design of Machine Elements", MacMillan

Publishing Co., 1941

(4) A. Vallance, "Design of Machine Members", McGraw-Hill

Book Co. , 1938

21?
INNOVATION OR RELIABILITY?

Henry 0. Fuchs

Design Division, Dept. of Mechanical Engineering

Stanford University

Stanford, California 9^+305

Some of my friends wait until a new type of airplane has

been in service for several months before they fly in it, and

they do not buy a car until its engine design has been proven

in service. As a rule, new designs are less reliable than

old designs. Novelty and reliability compete for the available resources of time and money in engineering design. Are

we achieving a balance between the two which is close to the

most desirable balance? I doubt it.

Reliability means painstaking attention to detail. The

pay-off is delayed; we must wait for service experience. If

we succeed and all is well there is no triumph, just an absence of complaints. Innovation means creativity, the immediate thrill of having an idea and the continuing satisfaction of seeing the idea become reality. If the new idea

succeeds the pay-off can be fairly quick and spectacular.

The pursuit of innovation is far more attractive than

the pursuit of reliability. If we want to balance the two

we should compensate the greater intrinsic rewards of innovation by greater social rewards of reliability. Is the inventor of the Rollamite worth more to us than a detail designer who produces consistently reliable designs? Do we

have a surplus of good detail designers and a shortage of

inventors? We act as if we did.

Universities can obtain support for innovations far more

easily than support for the development of good concepts. We

do have many courses in creative conceptual design but few in

the design of reliable hardware. We do have courses in

reliability engineering, but these are concerned with mathematical techniques for measuring reliability, or things like

"The Use in Probabilistic Design of Probability Curves

Generated by Maximizing the Shannon Entropy Function". They

are not concerned with the achievement of reliability by

careful design. Teachers are rewarded for new contributions

to knowledge, not for teaching the importance of details.

21k
Case histories of failures show that they are caused

more often by overlooked details than by ignorance of advanced theories. It seems to me that in schools we should

study such case histories and that in engineering practice

we should give good detail designers more recognition and

more pay, so that attention to detail becomes more attractive.

215
THE "MODEL" DESIGNER

by

R. E. Maringer

Battelle, Columbus Laboratories

Columbus, Ohio 43201

My own pet problems which show up consistently in design have to do

with what I believe to be a fairly common misuse of "models". The

misuse is not only by the designer himself, but also by those who

transform the design criteria into the reality of the final product.

These problems show up most prominently in precision equipment such as

bearings, gyros, accelerometers , mirrors, etc.

A good example is the use of Hooke's Law which, in its simplest form,

states that the strain is directly proportional to the stress over

some stress range. As near as we can determine in the laboratory,

there is no material which obeys Hooke's Law rigorously. This is to

be expected, really, for Hooke's Law is simply a mathematical model.

The usefulness of any mathematical model depends on how close it

approximates reality. It is never more than a useful approximation,

just as a doll is never more than an approximation of a human being.

The design problem enters when needs require extreme accuracies under

changing or difficult environmental conditions.

Let us suppose that a designer must make a structure that retains its

sjnitmetry under changing load conditions ( a gyro is a good example).

If he looks in the handbook for the elastic modulus of steel, he will

find a number of about 30 million psi. In fact, the modulus of a given steel can vary 10 to 15 percent around this number, depending on

how it is processed. There can be a significant difference in the

modulus depending upon whether it is measured parallel to or transverse to the rolling direction of sheet. A structure made of strips

cut at different orientations from a rolled sheet would, therefore,

warp or tilt under stress. The modulus, obviously, is not the "constant" implied by the tables.

Let us suppose, however, that the designer succeeds in building the

structure of components with adequate uniformity of modulus. It is

not unreasonable that such a structure would be required to respond

to imposed stresses in a manner that Hooke's Law would predict, with

deviations from Hooke's Law being of the order of parts in 106. The

design tables will often report tensile strengths, but these are useless for the purpose at hand. The tables will usually also report

yield strengths, but these normally refer to the 0.2 percent offset

yield. These mean 2000 x 10"^ strain at the yield stress value. The

only value they have to the designer is that they indicate a stress

which is much too high. On occasion, the tables will list the proportional limit or the elastic limit, but these values are extremely

2l6
dependent on the sensitivity of the tools with which they are

measured. Without a quantitative indicator of this sensitivity, these

terms are meaningless. I believe, in fact, that the true elastic

limit of all materials is precisely zero. The use of tabulated data

requires both care and judgment.

Similar arguments can be made for virtually all of the "constants"

with which the designer has to work. Thermal expansion coefficients,

thermoelastic coefficients, electrical resistivity, density, and a host of other physical and mechanical parameters are amenable to

mathematical modelling, and thus the designer can predict the effect

of the environment on the response of the property of interest. But,

the designer must remember that there is a real and philosophical difference between a model and reality. When you stretch such models to

their limits, the agreement between the model and reality can fail

completely.

While many of the mathematical models of reality are of undoubted

value, some are a source of difficulty all by themselves, since they

describe the wrong thing very accurately. Surface roughness measurement is a very illustrative example. This is a part of our manufacturing technology which has only just recently been attracting the attention it deserves.

Manufactured surfaces are normally described by a mathematical parameter which averages out all the hills and valleys and expresses them

in a simple number. The problem is that when a surface is prepared,

especially by conventional machinery, the surface layers of the

materials are altered, often to a depth of many thousandths of an inch.

These layers are usually highly deformed and contain severe residual

stress gradients. It is readily demonstrable that these surface

layers can have a more profound effect on the function of the part

(in fatigue, for example) than the nature of the surface undulations.

We achieve a sense of security by specifying a surface quantitatively,

hoping thereby to be exerting quality controls on our product. But,

in some cases, we might just as well specify that the machinist eat

Wheaties for breakfast for all the good it does for the product.

The point I'm trying to make is simple. Mathematical models are just

rather sophisticated "rules of thumb". They are easily (and often)

stretched beyond their applicability, especially by computers (electronic and human) which do not take into account nature's delight in

frustrating man's attempts to describe her simplistically

.

217
THE MISUSE OF TENSILE STRENGTH AS A DESIGN PARAMETER

Thomas J. Do! an

Engineering Consultant

Champaign, Illinois 61820

One of my pet peeves is that engineers tend to treat tensile strength

as the major design parameter of materials. So many specifications

and codes base design stresses on tensile strength and yield strength.

For example, most of the aircraft industry uses the limit load factor

based on static loading to determine the factor of safety, which is

the ratio of yield strength to load limit.

What we need to look at is, "What will the mode of failure be if this

component or structure fails?" There might be more than one possible

failure mode and we need to know what significant parameters to

measure in order to determine the material's resistance to these

particular modes of failureo The mode of failure might be low cycle

fatigue, hydrogen embri ttlement, stress corrosion cracking, etc. In

many instances, of course, you do have to make some trade-offs between

the properties of the material and various modes of failure. My point

is that tensile strength is the least likely of the mechanical

property parameters that will indicate resistance to failure.

Even in the case of wire, tensile strength may not be the major

parameter, as evidenced by a failure some years ago of bridge wires.

The tensile strength was too high which led to other problems. In

general, increased tensile strength may lead to trouble because it is

detrimental where the potential mode of failure might be brittle

fracture, low cycle fatigue, stress corrosion cracking, etc.

Realistically, a tensile stress-strain curve is no longer valid once a material has been subjected to fatigue loading. In some instances,

the cyclic stress-strain curve will give more useful information than

the static stress-strain curve.

Most young engineers come out of undergraduate engineering courses

with an idealistic attitude toward tensile strength. They consider

tensile strength to be a far more important parameter than it really

iSo It is unrealistic to approach design considering tensile strength

and yield strength to be the only important parameters. Most standard

specifications do require that, when purchasing a steel, the tensile

strength, yield strength, and percentage elongation be specified.

These are not really indicative of the inherent mechanical properties

that are needed for service conditions. Material with a higher

tensile strength may perform very poorly under certain service

conditions, whereas a lower tensile strength material might perform

218
satisfactorily. People in the aircraft industry think in terms of a strength-to-weight ratio, meaning usually good tensile strength or

yield strength-to-weight ratio. This again is an inappropriate

parameter.

Remember also that metal components may be subjected to a number of

processing variables. They may be cold stretched, bent, stamped,

machined, ground, welded, brazed, etc. On the way to becoming a finished product, they may undergo mechanical treatments, thermal

treatments, and chemical treatments. We need to know the significant

mechanical property(ies) in the localized zones of a component where

the critical stresses occur. Most designers do not think much about

the critical stress zones.

High strength weldments produce some special problems for the

designer. The first thing that comes to mind is to use a high

strength steel. The fatigue strength of a welded structure with a tensile strength of 130,000 psi may be no better than that of a structural grade steel with a tensile strength of 60,000 psi. Most

engineers would not believe this-- they have to try it and suffer a few

failures before they understand what is going one Remember that when

operations such as welding are performed, the properties of the

material may be vastly modified. These processes may result in

residual stresses that are nearly equal to the yield strength of the

material. Microvoids and other flaws are introduced into the

material --the tensile strength in the region of the weld is lowered.

The residual stresses and the flaws may lead to failure under cyclic

loadingo So there is a need to know the service environment, the

range of anticipated loading, and the probable failure mode should

failure occur. Forget tensile strength as a criterion for material

performance.

In many instances, a component may deteriorate with time due to

chemical corrosion, radiation, wear, etc. The original tensile

strength cannot predict deterioration, but, for a given material,

deterioration will be more rapid for higher tensile strengths than for

lower tensile strengths.

219
DISCUSSION

A. VJolff, Massachusetts Materials Research, Inc. : Today, there is a great need for impact data, particularly on materials having

transition temperatures, especially when they are being used in

cryogenic applications. Published impact data are very difficult to

relate to the real world. The direction of the notch, the effect of

very minor residual impurities, processing, etc., make it very

difficult to use this information. How can we better utilize impact

data, or is there some alternative to using impact data to predict

brittle transition failures?

T. J. Dolan : The fracture mechanics people have devised some rather

standard tests for measuring toughness characteristics, primarily of

the higher strength steels, which give us a good indication of how

large a flaw can be sustained before there is danger of a sudden,

running, rapid catastrophic fracture. They have also done quite a bit

to understand how flaws grow from submicroscopic size either under

sustained stressing or under cyclic loading. These are the things

that are very valuable for the designer to know. Design must be

approached assuming that all parts contain some flaws. We must know

the smallest flaw that can be detected by inspection because design

must take into consideration the fracture toughness characteristics of

the material so that the designed part will perform satisfactorily

either with no flaw growth, or with a gradual, calculable flaw growth.

A. Wolff : Then you feel that fracture toughness, even for the

so-called garden variety of carbon steels, is going to have to become

a part of specifications?

T. J. Dolan: It may well be so. A few years back, I examined a fracture in a gas transmission pipe. The fracture had propagated for

850 ft in what appeared to be a shear mode. It is well known that

long, running cracks occur in a brittle mode. It turned out that this

particular steel had very low Charpy impact values. It had a lot of

manganese sulfide layers in it. These fracture characteristics could

have been predicted if fracture toughness tests had been conducted

before the pipeline had been installed.

R. L. Lundeen, Boeing Commercial Airplane Company : You said that you

wanted to know what the smallest flaw was that could be detected.

What you really want to know is what is the biggest flaw that can be

overlooked.

T. J. D o lan : Definitely, right.

220
G. Kardos, Carleton University : The designer's principal problem is

to make predictions based on the information available to him.

Unfortunately, this information is often not a true reflection of the

characteristics of the material.

T, J. Dolan : Data published in the producers bulletins are usually

those attainable in the rolling direction. For low alloy, high

strength steels you will find in the handbook tensile strengths, yield

points, elongations, etc., in the direction of rolling. They do not

tell you what the properties are transverse to the direction of

rolling because these properties are often not as good as those

attainable in the rolling direction. In the thickness direction, the

properties may be terrible. That is why there are a lot of lamellar

tearing failures in welds on structural components. These are things

young designers are not aware of.

221
INSPECTION CONSIDERATION AT THE DESIGN STAGE*

John F. Erthal

Naval Air Development Center

Warminster, Pennsylvania 18974

The designs of systems that require the assembly of parts and

components which are expected to function for a length of time

assisted by preventive maintenance and overhaul, must consider

inspection and maintainability at the concept stage of the system.

Provision for access to critical load carrying parts and components

for inspection, primarily nondestructive inspection, must be

considered at the design stage rather than an afterthought to a fixed

configuration or design. This consideration demands continual

dialogue and discussions among the designer, stress analyst, materials

and quality assurance people. With such rapport, certain parts may

even be redesigned without loss of efficiency to facilitate and

accomodate nondestructive inspection. Nondestructive inspection

during overhaul or periodic maintenance has been found to maximize

safety and enhance reliability with the added feature of being cost

effective. There is ample evidence and examples in the aircraft field

to support this claimc Therefore, in the interest of reliability,

safety, and reduced costs during operation and maintenance of an

important system, it behooves us to take advantage of the advancing

technology in the nondestructive testing and inspection discipline by

consideration of its subsequent use at the earliest stage of system

design.

*Letter from Mr. Erthal read by A. Wolff

222
MECHANICAL FAILURES PREVENTION GROUP

21st MEETING

Success by Design: Progress Through Failure Analysis

List of Registrants

Harold Ascher

Naval Research Laboratory

Code 4023

Washington, DC 20390

J. K. L. Bajaj

Envirotech Corp.

9501 Allen Drive

Cleveland, OH 44125

William F. Bangs

NASA-GSFC, Code 321

Greenbelt, MD 20771

Bruce P. Bardes

Bimba Mfg. Company

101 E. Main Street

Monee, IL 60449

Chuck Baskin

Bell Helicopter Company

Ft. Worth, TX 76101

Michael Boucher

Dept. of Energy, Mines &

Resources

Ottawa, Ontario, Canada

H. C. Burnett

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

George E. Butterworth

U.S. Atomic Energy Commission

Division RRD, F-309

Washington, DC 20545

R, A. Coulombe

Naval Ship Engineering Center

3700 East West Highway

Hyattsville, MD 20782

R. M. Davison

NAVWESA

ESA 742

9205 W. Stayman Drive

Ellicott City, MD 21043

Evan J. Deemer

Naval Air Systems Command

Representative, Atlantic

NAS Norfolk, Bldg. R-52

Norfolk, VA 23511

Paul De Jong

Iowa State University

Department of Eng. Graphics

Ames, lA 50010

M. G. Dhawan

Naval Ship Engineering Center

Code 6147; Center Building

Prince Georges Center

Hyattsville, MD 20782

Thomas J. Dolan

Engineering Consultant

510 South Highland Avenue

Champaign, IL 61820

Stanley W. Doroff

Office of Naval Research

Arlington, VA 22217

225
Robert J. Dudash

American Telephone & Telegraph

Company

P. 0. Box 665

Herndon, VA 22070

Henry R. Hegner

General American Trans. Corporation

Research Division

7449 North Natchez Avenue

Niles, IL 60648

R. M. Evan-Iwanowski

Syracuse University

139 Link Hall

Syracuse, NY 13210

Hal Ferris

Hughes Helicopters

Culver City, CA 90230

Paul Fleming

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Jeffrey T. Fong

Applied Mathematics Division

National Bureau of Standards

Washington, DC 20234

Henry 0. Fuchs

Stanford University

Mechanical Eng. Department

Stanford, CA 94305

D. D. Fuller

Department of Mechanical Eng.

Columbia University

New York, NY 10027

Dr. L. M. Gillin

Embassy of Australia

1601 Massachusetts Avenue, NW

Washington, DC 20036

Richard Grassier

FAA

800 Independence Avenue, SW

Washington, DC 20591

Peter Hal lick

FAA

800 Independence Avenue, SW

Washington, DC 20591

J. D. Hoffman

Director, I MR

National Bureau

Washington, DC

of Standards

20234

E. Horowitz

Deputy Director, IMR

National Bureau of Standards

Washington, DC 20234

Jerry Houck

National Transportation Safety Board

Department of Transportation

Washington, DC 20591

Jack Hubacher

Value Engineering Laboratory

2550 Huntington Avenue

Alexandria, VA 22303

George Hurt

Detroit Diesel, Allison Division

P. 0. Box 894

Indianapolis, IN 46201

R. B. Johnson

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

David T. Jones

U.S. Army Ballistic Research Labs.

Aberdeen Proving Ground, MD 21005

Geza Kardos

Carleton University

College of Engineering

Ottawa, Ontario, Canada

Brian Keegan

Structural Dynamics Branch

Goddard Space Flight Center, Code 321

Greenbelt, MD 20771

22h
E. E. Klaus

Pennsylvania State University

133 Chemical Eng. Bldg.

University Park, PA 16802

Rudolph Lenich

Caterpillar Tractor Company

100 NE Adams Street, AB6D

Peoria, IL 61602

James Lesko

Purdue University

Lafayette, IN 47905

F. F. Ling

Rensselaer Polytechnic Institute

Mechanics Division

Troy, NY 12181

Harold S. Link

U.S. Steel Research Lab.

Monroeville, PA 15146

Major David W. Logan

U.S. Army Agency for Aviation

Safety

Dept. of the Army (IGAR-TS)

Fort Rucker, AL 36360

Rodney Lord

The Boeing Company

P. 0. Box 58747

Houston, TX 77058

Robert L. Lundeen, P.E.

Boeing Commercial Airplane Co.

12449 - 20th Avenue South

Seattle, WA 98168

Guy Mangano

Naval Air Propulsion

Trenton, NJ 08628

Test Center

Robert Maringer

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

R. Martin

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Michael Marx

National Transportation Safety Board

Department of Transportation

Washington, DC 20591

Vaughn Matney

General Dynamics

Mail Zone 5921, P. 0.

Ft. Worth, TX 76101

Box 748

William R. McWhirter, Jr.

Naval Ship Research & Development

Center, Code 2762. 3B

Annapolis, MD 21402

Joseph Messina

Frankford Arsenal

Philadelphia, PA 19137

Lt. Richard Miller

Office of Naval Research (Code 211)

Department of the Navy

Arlington, VA 22217

R. Misialek

Naval Ship Eng. Center (6762)

Philadelphia Div., Naval Base

Philadelphia, PA 19112

Simone Mola

Prof, of Mechanical Engineering

General Motors Institute

1700 W. Third Avenue

Flint, MI 48502

Ernest J. Mosher

NASA - GSFC

Code 328

Greenbelt, MD 20771

225
John C. New

NASA/Goddard Space Flight Ctr.

Code 320

Greenbelt, MD 20771

Bryan R. Noton

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

Jesse Oroshnik

U. S. Navy

(ESA-74D) Bldg. 200

Washington Navy Yard

Washington, DC 21374

E. Passaglia

Chief, Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Byron J. Pel an

College of Engineering

Busch Campus

Rutgers University

New Brunswick, NJ 08903

Robert B. Pond, Jr.

Johns Hopkins University

Baltimore, MD 21218

John D. Preston

Consumer Product Safety Comm.

5401 Westbard Avenue

Bethesda, MD 20016

J. Rabinow

Room A- 349, Bldg. 224

National Bureau of Standards

Washington, DC 20234

G. Sapy

Electricite de France

17 Ave du General de Gaulle

92 Clamart, France

Alan H. Schoem

Consumer Product Safety Commission

Office of the General Council

Washington, DC 20207

Harry Selden

Bell Helicopter Company

Ft. Worth, TX 76101

Peter Senholzi

Naval Air Engineering Center

Philadelphia, PA 19112

Nelson N. Shapter

Federal Aviation Administration

800 Independence Avenue, SW

Washington, DC 20591

T. R. Shives

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Charles 0. Smith

University of Detroit

4001 W. McNichols

Detroit, MI 48221

R. W. Staehle

Ohio State University

116 W. 19th Avenue

Columbus, OH 43210

Jesse E. Stern

NASA/Goddard Space Flight Center

Greenbelt, MD 20771

William C. Stitzel

ARMCO Steel Corporation

Research and Technology

Middletown, OH 45043

Keith E. Sturgill

Naval Air Rework Facility

13051 Warwick Boulevard

Newport News, VA 23602

226
Charles Suggs

Agriculture Experiment Station

North Carolina State Univ.

Raleigh, NC 27607

Victor T. Sweet

General Electric Company

Valley Forge Space Tech. Center

Philadelphia, PA 20850

Robb M. Thomson

National Bureau of Standards

Washington, DC 20234

Scott Thornton

Tracor Inc.

6500 Tracor Lane

Austin, TX 78721

A. R. Timmins

11010 Bucknell Drive

Silver Spring, MD 20902

John Torgesen

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

George Uriano

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Gerald Wagner

1320 Hornbake Drive

Clairton, PA 15025

James R. Ward

Value Engineering Lab.

2550 Huntington Avenue

Alexandria, VA 22303

John M. Ward

Naval Air Systems Command

Washington, DC 20360

David C. Watson

Naval Engineering Test Establishment

161 Wanklyn Street

LaSalle, Quebec

H8R 1Z2 Canada

L. Wechsler

Hydronautics , Inc.

7210 Pindell School Road

Laurel, MD 20810

W. Willard

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Will iam F. Wilson

Bell Helicopter Company

P. 0. Box 482

Fort Worth, TX 76101

Alan K. Wolff

Massachusetts Materials Research, Inc.

55 Mi 11 brook Street

Worcester, MA 01613

G. D. Xistris

Naval Engineering Test Establishment

161 Wanklyn Street

LaSalle, Quebec

HSR 1Z2 Canada

227
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. PUBLIC ATION OR Rl{POR r NO. 2. Gov't Acccs.sion

BIBLIOGRAPHIC DATA No. SHEET NBS SP 433

3. Recipient's Accession No.

4. TITLt- AND SUBTlTI.Ii

Success By Design: Progress Through Failure Analysis--

Proceedings of the 21st Meeting of the Mechanical Failures

Prevention Group

5. Publication Date July 1976

6. Performing Organization Code

7.Mmmm editors

T. Robert Shives and William A. Willard

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DFPARTMENT OF rOMMFRTF

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street. City, State. ZIP)

Institute for Materials Research, NBS, Washington, DC 20234;

ONR, Dept. of the Navy, Arlington, VA 22217; Frankford Arsenal,

Phila., PA 19137; NADC, Dept. of the Navy, Warminster, PA 18974;

FAA, Dept. of Transportation, Washington, DC 20591

13. Type of Report & Period

Covered

Fiaal

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 2 00- word or less factual summary ot most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

These Proceedings consist of a group of twenty two submitted papers and discussions

from the 21st meeting of the Mechanical Failures Prevention Group which was held at

the National Bureau of Standards in Gaithersburg, Maryland on November 7-8, 1974.

The central theme of the Proceedings was improvement in design through failure analysis.

Emphasis was on design philosophy, the use of failure analysis case studies as an educational tool, successful redesign through failure analysis, and design oversights.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Design; engineering education; failure analysis; failure analysis case histories;

failure prevention; reliability; safety

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