NBSIR 75-686 ^)

Examination of Failed Pylon

Components

T. Robert Shives

Mechanical Properties Section

Metal lurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

March 1975

Failure Analysis Report

Prepared for Naval Air Test Center

Patuxent River, Maryland 20670

NBSIR 75-686

EXAMINATION OF FAILED PYLON

COMPONENTS

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington. D. C. 20234

March 1975

Failure Analysis Report

"This document has been prepared for the use of the Naval Air Test Center, Patuxent River, Maryland.

Responsibility for its further use rests with that agency. NBS requests that if release to the general

public is contemplated, such action be taken only

after consultation with the Office of Public Affairs

at the National Bureau of Standards."

Prepared for Naval Air Test Center Patuxent River, Maryland 20670

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

TABLE OF CONTENTS

Page

SUMMARY

1. INTRODUCTION ' 1

1 . 1 Reference 1

1.2 Parts Submitted 1

1.3 Parts Examined 1

2. PURPOSE 1

3. RESULTS OF EXAMINATIONS, ANALYSES, AND TESTS 2

3.1 Bolts 2

3.1.1 Fractured Bolts from Pylon No. 3 2

3.1.1.1 Visual and Macroscopic

Examination 2

3.1.1.2 Fractographic Examination 2

3.1.1.3 Metal lographic Examination 3

3.1.1.4 Chemical Analysis 3

3.1.1.5 Hardness Measurements 4

3.1.2 Unbroken Bolts Used at Normal Torque 4

3.1.2.1 Visual and Macroscopic

Examination 4

3.1.2.2 Inspection for Cracks 5

3.1.3 Unbroken Bolts Used at Higher Torque 5

3.1.3.1 Visual and Macroscopic

Examination 5

3.1.3.2 Inspection for Cracks 5

3.2 Spar Subassembly Lugs 5

3.2.1 Visual and Macroscopic Examination 5

i
Paae

3.2.2 Fractographic Examination 6

'

. 3.2.3 Metallographic Examination 7

3.2.4 Hardness Measurements 7

4. DISCUSSION AND CONCLUSIONS 7

5. ACKNOWLEDGEMENT 10

REFERENCES 10

FIGURES

1. Significant pylon parts as received.

2. Intact bolts used at the normal torque as received.

3. Intact bolts used at the new higher torque as

received.

4. Parts of the two fractured bolts from pylon no. 3.

5. Scanning electron photomicrograph showing secondary

cracks in the bolt shank.

6. Fracture surfaces of the bolts shown in figure 4.

7. Scanning electron fractographs from the bolt shown

in figure 4b.

8. Scanning electron fractograph from the overload

portion of the fracture of the bolt shown in figure 4b.

9. As-polished longitudinal section from the bolt shown

in figure 4b.

10. Part of the area shown in figure 9.

11. As-polished longitudinal section through bolt shown

in figure 4b.

12. Etched longitudinal section from the bolt shown in

figure 4b.

13. Part of the shank of one of the unbroken bolts from

the LQ-2 group.

'

'

'

' ii
FIGURES (continued)

14. Separated piece from the aft spar subassembly lug from

pylon no. 3.

15. Aft spar subassembly lug from pylon no. 4.

16. Forward spar subassembly lug from pylon no. 4.

17. Part of the lug in figure 16 after being cut out.

18. The bolt hole surface of the part of the lug shown in

figure 17.

19. Part of the surface of the other bolt hole from the

forward lug from pylon no. 4.

20. Scanning electron fractograph from the fatigue portion

of the fracture through the aft spar subassembly lug

from pylon no. 3.

21. Scanning electron fractograph from the fatigue portion

of the fracture through the aft spar subassembly lug

from pylon no. 3.

22. Scanning electron fractograph from the overload portion

of the fracture through the forward spar subassembly

lug from pylon no. 4.

23. An as-polished section through the aft spar subassembly

lug from pylon no. 3.

24. Etched section through the aft spar subassembly lug from

pylon no. 3. ,

25. Etched section through the aft spar subassembly lug from

pylon no. 3.

26. Etched section through the intact hole in the forward spar

subassembly lug from pylon no. 4.

SUMMARY

At the request of the Naval Air Test Center, Patuxent,

Maryland, the NBS Mechanical Properties Section examined a

number of components from failed aircraft pylons. It was

found that fractured bolts and spar subassembly lugs had

failed due to the initiation and propagation of fatigue

cracks. The cracks had initiated in regions of mechanical

damage or fretting corrosion damage, or where there was a

rather abrupt change in cross section thickness. There

was evidence that the systems had been subjected to

vibrations which caused the cyclic loading necessary for

fatigue cracking.

Examination of Failed Pylon Components

1 . INTRODUCTION

1 . 1 Reference

Naval Air Test Center, Patuxent River, Maryland.

This investigation was conducted at the request of

Dr. Joseph G. Hoeg under Job Order No. P79 8426WS dated

June 20, 1974.

1.2 Parts Submitted

Parts of four separate pylons made from an aluminum

alloy were submitted for examination. The significant

parts from each of the pylons are shown as received at

NBS in figure 1. In addition, twelve unbroken cadmium

plated steel bolts (shown in figure 2) that had been used

in the pylons in the "normal" torque condition were

submitted at a later date. Still later, eleven unbroken

cadmium plated steel bolts (shown in figure 3) that had

been used in the pylons at a new higher torque condition

were submitted for examination.

1 . 3 Parts Examined

Although there are fractures, cracks, and regions of

deformation and other mechanical damage in many of the

aluminum alloy sheet metal components of all four pylons,

this examination, with some exceptions, has been concentrated

on the aluminum spar subassembly lugs from all four pylons,

the two fractured bolts from pylon no. 3, and the two sets

of unbroken bolts that were submitted.

2 . PURPOSE

The Naval Air Test Center requested that the NBS

Mechanical Properties Section characterize the fractures

of the two submitted failed bolts from pylon no. 3 and

the fractures of the failed spar subassembly lugs. In

addition, it was requested that the intact lugs be examined

for cracks and that the unbroken bolts be examined for

general condition and cracks. A determination, if possi-ble,

of the sequence of events during the failure process was

desired

- 1 -
3 . RESULTS OF EXAMINATIONS, ANALYSES, AND TESTS

3 . 1 Bolts

3.1.1 Fractured Bolts from Pylon No. 3

3.1.1.1 Visual and Macroscopic Examination

Fracture of the two failed cadmium

plated steel bolts from pylon no. 3 occurred in the shank

in each case and not in the threaded area. There was considerable mechanical damage - largely in the form of circumferential grooves - and what appeared to be fretting corrosion

damage on the bolt shanks. The fracture cracks initiated in

or adjacent to damaged areas. One view of each bolt shank

is shown in figure 4. (The parts of the bolts not shown were

lost at the time of failure and therefore were not available

for examination.)

There was some deformation in the form

of necking and bending near the fracture of the bolt shown

in figure 4b. The change in the diameter of the bolt shank

from 0.4 36 inch in the undeformed regions away from the

fracture to 0.4 30 inch adjacent to the fracture gives an

indication of the amount of necking.

There were also a number of secondary

cracks in this same bolt in the vicinity of, and essentially

parallel to, the fracture. Two of these cracks are indicated

by the arrows on the scanning electron photomicrograph shown

in figure 5, and others can be seen in this same figure.

3.1.1.2 Fractographic Examination

The fracture surfaces of the bolts

shown in figure 4 appear in figure 6 - on the left is the

bolt in figure 4a, and on the right is the bolt in figure 4b.

Approximate locations of the crack origins are indicated by

arrows in figure 6. These fracture surfaces were examined

with the scanning electron microscope. The fracture cracks

in both bolts appear to have propagated initially in fatigue.

When the cross sections of the bolts had been reduced by the

advancing fatigue cracks so that the applied load could no

longer be sustained, the cracks propagated to fracture in

ductile overload.

- 2 -
Representative scanning electron

fractographs exhibiting fracture modes typical of the

apparent fatigue portion of the fracture of the bolt shown

in figure 4b appear in figure 7. A suggestion of fatigue

striations appears in figure 7a. An area representative

of the overload portions of the bolt fractures is shown in

figure 8, where the primary fracture mode is dimpled rupture.

3.1.1.3 Metallographic Examination

A longitudinal section intersecting the

fracture was prepared for metallographic examination from

the bolt exhibiting secondary cracks (bolt shown in figures

4b and 5) . An as-polished field showing the fracture profile

in the fatigue portion of the fracture appears in figure 9.

The fracture profile is relatively smooth. The material

appears to be quite clean and relatively free of inclusions.

Part of the area shown in figure 9

appears at higher magnification in the etched condition in

figure 10. (The specimen orientation is the same in both

figures 9 and 10.) The microstructure consists primarily of

tempered martens ite. Several transverse cracks can be seen

in the cadmium plating, and one crack (arrow) can be seen in

the basis steel material. In figure 11, two more cracks in

the basis steel are shown where there are no cracks in the

cadmium plating. Indeed, the cadmium plating had substantially

filled the cracks. The cracks shown in figure 11 are about 2 mm

from the fracture.

The fracture profile in the overload

portion of the fracture from the same bolt is shown in

figure 12. The fracture is horizontal at the top of the

figure. The rougher, more jagged appearance of the overload

fracture profile - where the fracture mode is dimpled rupture - is contrasted with the relatively smooth appearance of the

fracture profile in the fatigue portion of the fracture as

shown in figures 9 and 10.

3.1.1.4 Chemical Analysis

A qualitative chemical analysis was made

of the product of the fretting corrosion on the shanks of the

two failed bolts and on the overload region of the fracture

surface of the bolt shown in figure 4b. The analyses were

made using a non-dispersive X-ray analyzer in conjunction

with the scanning electron microscope.

- 3 -
The most strongly detected element in

the fretting corrosion product was aluminum. The only

element found in detectable quantities other than the aluminum

and iron from the bolt was sulfur. The amount of sulfur was much

less then the amount of aluminum. (The equipment employed

in this analysis does not permit the determination of

elements above ruthenium in the Periodic Table unless they

are present in substantial amounts; hence the cadmium plating

was not detected.)

On the overload region of the fracture

surface, iron and silicon were the only elements detected in

significant quantities.

3.1.1.5 Hardness Measurements

Rockwell C hardness measurements were

made on a cross section through each of the fractured bolts.

Five measurements were made on each bolt and the average

hardness value for each was 42. This corresponds

approximately to an ultimate tensile strength of 194 ksi.

3.1.2 Unbroken Bolts Used at Normal Torque

3.1.2.1 Visual and Macroscopic Examination

" rj^he twelve unbroken bolts that had been

used in the normal torque condition are shown as received

at NBS in figures 2a, b, and c. They were in three groups

of four each labeled as indicated in the legend of figure 2.

Each of the bolts had suffered mechanical

damage and apparent fretting corrosion damage. Some of this

damage can be seen in figure 2. The damage to the LQ-6 bolts

and to two of the LQ-9 bolts was concentrated in a 1.9 inch

length along the shank, starting at about 0.4 inch from the

underside of the bolt head and extending to a location just

short of the threaded region. For the other two LQ-9 bolts

and the LQ-2 bolts, the damage extended essentially from the

underside of the bolt head to just short of the threaded

region. The damage adjacent to the bolt heads was primarily

fretting corrosion. Most of the mechanical damage was

circumferential in nature. In many cases there were areas

where the cadmium plating was ruptured due to abrasion - and

consequently was either missing or peeling away from the bolt.

One example showing the absence of cadmium plating appears in

figure 13, which is higher in magnification than figure 2.

- 4 -
3.1.2.2 Inspection for Cracks

All twelve of the unbroken bolts used

at the normal torque were cleaned and then examined for

cracks with a dye penetrant technique. No cracks were found.

3.1.3 Unbroken Bolts Used at Higher Torque

3.1.3.1 Visual and Macroscopic Examination

The eleven unbroken bolts that had been used

in the higher torque condition are shown as received at NBS in

figure 3. These bolts had suffered some circumferential

mechanical damage, but in general, this damage was much less

severe than that sustained by the unbroken bolts used in the

normal torque condition.

3.1.3.2 Inspection for Cracks

The unbroken bolts used at the higher

torque level were examined for cracks in the same way that

the other unbroken bolts were examined. Evidence suggesting

a crack in one of the bolts was detected, but when the bolt

was sectioned through the apparent crack, it was found to be

a mechanically produced gouge. Thus, as in the case of the

unbroken bolts used in the normal torque condition, no cracks

were found in the bolts used in the higher torque condition.

3 . 2 Spar Subassembly Lugs

3.2.1 Visual and Macroscopic Examination

Both the forward and aft lugs from pylons number

2 and 4 were received and examined. Only the aft lugs from

pylons 1 and 3 were received. There were complete fractures

in the aft lugs from pylons 3 and 4. These fracture surfaces

are shown in figures 14 and 15, respectively. In both cases,

the fracture cracks appear to have started (arrows A) at the

edge of the flat area where the bolt head would seat. A

secondary fatigue crack (arrow B) can also be seen in figure 14.

A large crack was also found in the forward lug

from pylon no. 4 as shown in figure 16. This crack had

traversed the entire distance between the hole and the outside

of the lug on one side. This crack was opened by a saw-cut

in the approximate location of the dotted line in figure 16.

The exposed fracture surface is shown in figure 17. This

crack initiated at the edge of the bolt hole at the bottom

- 5 -
of the lug as indicated by the arrow in figure 17. The

other spar subassembly lugs were examined macroscopically

for cracks and none were found. The surfaces of many of

the holes in the lugs exhibited evidence of mechanical and

fretting corrosion damage. The surfaces of the two bolt

holes in the forward lug from pylon no. 4 appeared to have

suffered more damage than the surfaces of the holes in the

other submitted lugs. Part of the surface of the bolt hole

intersected by the fracture (hole on the right in figure 16)

is shown in figure 18. This same lug was sectioned through

the other bolt hole, and part of the surface of that hole is

shown in figure 19. There appeared to be cracks in the

surface (arrows, figure 19).

There was

the bottom of some of

with the aluminum all

both where they were

store. An example of

a lug can be seen in

lug from pylon no. 4.

damage on the hole su

contact was made with

the bolt heads for al

also some fretting c the lugs where they

oy lower rib, and on

in contact with the 1

the fretting damage

figure 16 which shows

A summary of mechan

rfaces, on the bottom

the lower rib, and i

1 of the submitted lu

orrosion damage on

were in contact

the lower ribs

ugs and with the

on the bottom of

part of the forward

ical and fretting

of the lugs where

n the areas under

gs is as follows

Amount of Mechanical and Fretting Corrosion Damage

Holes Bottom Under Bolt Head

Pylon no. 1, aft lug

Pylon no. 2, aft lug

fwd lug

Pylon no. 3, aft lug

Pylon no. 4, aft lug

' fwd lug

slight in one slight

none, paint some

not damaged

slight severe

slight in one slight

some some

severe some

none, paint not

damaged

none, paint not

damaged

insignificant

slight

some

some

3.2.2 Fractographic Examination

The fracture surfaces of the three fractured lugs

were examined with the scanning electron microscope. Examples

of fractographs taken in the apparent fatigue portion of the

- 6 -
fracture in the lug from pylon no. 3 are shown in figures 20

and 21. Ther§ is evidence of fatigue striations in each of

these figures. A representative fractograph from the overload

portion of one of the lug fractures from pylon no. 4 is

shown in figure 22. The mode of fracture in the overload

portion of the fractures appears to be dimpled rupture.

3.2.3 Metallographic Examination

An as-polished section intersecting the fatigue

portion of the fracture in the failed lug from pylon no. 3 is

shown in figure 23. Inclusions and second phase particles

can be seen. Another field from this same section is shown

in the etched condition in figure 24. It is evident that the

relatively smooth fracture profile is transgranular and thus

is consistent with fatigue cracking. The fracture profile in

the overload portion of the fracture (figure 25) is much less

smooth than that in the fatigue portion.

A section was taken through the intact hole in

the forward lug from pylon no. 4 parallel to the longitudinal

axis of the hole. This is the hole where apparent cracks

could be seen in the hole surface (figure 19). When this

section was polished, several cracks were revealed (figure 26).

In some areas, they tend to follow forging flow lines.

3.2.4 Hardness Measurements

Rockwell B hardness measurements were made on the

aft lugs from pylons 1 and 4. Five measurements were made for

each lug resulting in the following hardness values:

Lug from pylon no. 1: Average 90 1/2, range +1/2

Lug from pylon no. 4: Average 8 9 1/2, range +0

The acceptable hardness range for aluminum alloy 7075 in the

T6 temper is R^ 85 to 95 (1).

4 . DISCUSSION AND CONCLUSIONS

Both of the fractured bolts, the two completely fractured

spar subassembly lugs, and the one partially fractured spar

subassembly lug all failed due to the initiation and propagation of fatigue cracks. The fatigue cracks in the two failed

bolts, both of which were from pylon no. 3, initiated in

regions of the shank that had been damaged both mechanically

and by fretting corrosion. The fatigue cracks in the lugs

- 7 -
initiated either at a change in cross section thickness or

at a mechanically damaged area.

There was a considerable amount of fretting corrosion

damage on the spar subassembly lugs in some of the bolt

holes, in areas where the bolt heads had seated, and in

areas where the lugs had contacted the aluminum alloy lower

ribs. Evidence of fretting damage was also found on the

lower ribs both where these members contacted the lugs and

where they contacted the store.

Fretting corrosion damage indicates relative oscillatory

motion between two parts in contact. The relative motion

resulting in fretting corrosion is usually very slight.

Fretting damage frequently leads to the development of stress

concentrators which increase the susceptibility of a part to

fatigue cracking. Mechanical damage such as that found and

changes in cross section area can also act as stress concentrators . , -

There were several cracks in the basis steel under the

cadmium plating on one of the failed bolts. These cracks

did not pass through the plating material; in fact, the

plating material had filled some of the cracks in the steel

indicating that the cracks were probably present before

the plating was deposited. It is not clear how these cracks

formed, but they could be detrimental in that they could act

as stress concentrators. One of the failed bolts exhibited

some necking, indicating that the yield strength of the material

had been exceeded. There were a number of secondary cracks

in the vicinity of the fracture in this same bolt.

In addition to the partial fatigue fracture at one bolt

hole in the forward lug from pylon no. 4, there were a number

of cracks in the vicinity of the other bolt hole. These

cracks looked similar to those that can form from cavities

that develop at the time of solidification (2). Such cracks

can remain in the material through the forging process.

It was interesting to note that in the aft lug from

pylon no. 1, the unmarred paint in the areas where the bolt

heads would normally seat suggested that the bolt heads had

not been seated. The bolt hole surfaces exhibited evidence

that bolts had been in the holes. Moreover, the appearance

of the lug surface that would have been in contact with the

lower rib indicated that the lug had been in contact with

the rib.

- 8 -
Nothing abnormal was noted in the hardness or microstructure of either the bolt or lug material. The limited

chemical analysis on the fracture surface and of the fretting

corrosion product on one of the bolt shanks revealed nothing

unusual except for sulfur, which appeared to be present in

small amounts. The significance of the sulfur is not clear.

No fatigue cracks were found in any of the unbroken bolts

used at either the normal torque or the high torque conditions.

The shanks of the bolts used at both torque conditions

exhibited some mechanical damage, but those used at the high

torque condition exhibited less damage than those used at

the normal torque. In addition to the mechanical damage,

rather severe fretting corrosion damage was exhibited on the

shanks of the bolts used at the normal torque condition.

Since the length of service for the various bolts used under

different conditions is not known, no definitive conclusion

can be drawn regarding the effect of changing the torque on

the life of the bolts and, subsequently, the integrity of

the pylons

There appears to be no clear pattern to the sequence of

events resulting in the pylon failures. Complete fractures

were found in two bolts from one pylon and in the aft spar

subassembly lugs from two pylons, but there was a partial

fracture in one of the forward lugs, and there were cracks

in the vicinity of a bolt hole from the same forward lug.

Failure of part of one lug would, of course, intensify the

applied stress on the remaining parts of the system. Bolt

failures would have the same effect. In the two cases where

there were complete fractures on one side of a lug, the

increased stress on the rest of the lug does not appear to

have initiated cracking. In the case of the partial

fracture, the cracks in the vicinity of the other bolt hole

did not appear to be fatigue cracks.

It would appear that the fine scale fretting corrosion

damage to the various components may be due to improper

torquing of the bolts and subsequent vibration in service.

The coarser circumferential damage to the bolts and the

bolt hole surfaces may be due to improper tightening of the

bolts at some point in time. Fatigue cracks initiated in the

pylons in areas where stress concentrators had developed due

to fretting corrosion damage or mechanical damage, or where

there was a rather abrupt change in cross section area. The

cyclic stressing necessary for the initiation and propagation

of the fatigue cracks apparently came from the vibrations in

the system in service.

- 9 -
5 . ACKNOWLEDGEMENT

Mr. L. C. Smith of the NBS Mechanical Properties

Section performed the metallographic specimen preparation

and the photographic work and assisted in other aspects of

this investigation.

REFERENCES

1. Metals Handbook, American Society for Metals, 8th

Edition, Volume 1, 1961.

2. Metals Handbook, American Society for Metals, 8th

Edition, Volume 7, 1972.

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Figure 6. Fracture surfaces of the bolts shown in figure 4.

The fracture surface on the left corresponds to the

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approximate locations of the crack origins. X 4

Figure 7. Scanning electron fractographs from the apparent

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bolt shown in figure 4b. A suggestion of fatigue

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b. X 1050

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Figure 12. Etched longitudinal section from the bolt shown in

figure 4b. The fracture profile in the overload

portion of the fracture is horizontal at the top.

Etchant: 4% picral X 100

Figure 13. Part of the shank of one of the unbroken bolts from

the LQ-2 group showing the mechanical damage and an

area where the cadmium plating is missing. X 16

Figure 14. Separated piece from the aft spar subassembly

lug from pylon no. 3 showing the fracture

surface. Arrow A indicates the approximate

location of the crack origin. Arrow B indicates

a secondary fatigue crack. X 2

Figure 15. Aft spar subassembly lug from pylon no. 4 showing

the fracture surface. Arrow A indicates the

approximate location of the crack origin. X 1

Figure 16. Forward spar subassembly lug from pylon no. 4.

Arrow indicates crack. A saw-cut was made at the

approximate location of the dotted line in order

to expose the fracture surface. X 1

Figure 17. Part of the lug shown in figure 16 after being cut

out. The fracture surface is at the left. The

arrow indicates the approximate location of the

point of crack initiation. X 4

Figure 18. The bolt hole surface of the part of the lug shown

in figure 17 is in the center of the photograph.

The fretted and mechanically damaged surface can be

seen. Part of the fracture surface is at the far

left. A saw-cut surface is at the right. X 8

Figure 19. Part of the surface of the other bolt hole from the

forward lug from pylon no. 4. This is the left bolt

hole in figure 16. The damaged surface can be seen.

Arrows indicate two apparent cracks. X 4

Figure 20. Scanning electron fractograph from the fatigue

portion of the fracture through the aft spar

subasseinbly lug from pylon no. 3. Fatigue

striations are evident. X 470

Figure 21. Scanning electron fractograph from the fatigue

portion of the fracture through the aft spar

subassembly lug from pylon no. 3. Fatigue

striations are evident. This fractograph was

taken in a different area than that shown in

figure 20. X 540

Figure 22. Scanning electron fractograph from the overload

portion of the fracture through the forward spar

subassembly lug from pylon no. 4. X 44 0

Figure 23. An as-polished section through the aft spar

subassembly lug from pylon no. 3 intersecting

the fatigue portion of the fracture. The

fracture profile is vertical at the right.

Second phase particles and inclusions can be

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Figure 26. Etched section through the intact hole in the

forward spar subassembly lug from pylon no. 4

revealing several cracks.

Etchant: 10% NaOH X 7

NBS.114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. IHIMI.K AH ION OR REPORT NO. NBSIR 75-686

2. (lov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLK

EXAMINATION OF FAILED PYLON COMPONENTS

5. Publication Date March. 1975

6. Performing Organization Code

7. AUTHOR(S)

T. R. Shives

8. Performing Organ. Report No. NBSIR 75-686

9. PERFORMING ORC.ANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120416

11. Contract/Grant No.

12. Sponsoring Organi/.atmn Name and < »)niplt ti- AJdrt-ss (Street, Ctty, State, ZIP)

Naval Air Test Center

Patuxent River, Maryland 20670

13. Type of Report & Period Covered pallure

Analysis Report

I4f Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRAC'T (A 200-word or less factual summary oi most sigftificant information. If document includes a significant

bibliography or literature survey, mention it here.)

At the request of the Naval Air Test Center, Patuxent, Maryland, the

NBS Mechanical Properties Section examined a number of components

from failed aircraft pylons. It was found that fractured bolts and

spar subassembly lugs had failed due to the initiation and propagation

of fatigue cracks. The cracks had initiated in regions of mechanical

damage or fretting corrosion damage, or where there was a rather

abrupt change in cross section thickness. There was evidence that the

systems had been subjected to vibrations which caused the cyclic loading

necessary for fatigue cracking.

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 semic olons ) Fatigue; fracture; fretting corrosion; stress concentrator

18. AVAILABILITY Unlimited

F^of Official Distribution. Do Not Release to NTIS

['7 Order Erom Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. CI3

[ J Order Erom National Technical Information Service (NTIS)

Springfield, Virginia 221')1

19. SECURITY CLASS 21. NO. OF PAGES

(THIS REPORT)

31

UNCI. ASSIEIED

20. SE( URITY CLASS 22. Price (THIS PAGE)

UNCLASSIFIED

USCOMM.OC 29042-P74