NBSIR 75-671 ^ Examination of Piston Rod and

Rod End From DC-8 Main Landing

Gear Retract Cylinder Assembly

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

February, ,1975

Failure Analysis Report

Prepared for Bureau of Avicilion Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

NBSIR 75-671

EXAMINATION OF PISTON ROD AND

ROD END FROM DC-8 MAIN LANDING

GEAR RETRACT CYLINDER ASSEMBLY

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

February, 1975

Failure Analysis Report

"This document has been prepared for the use of the Bureau of Aviation Safety, National Transportation

Safety Board, Department of Transportation,

Washington, DC. Responsibility for its further use

rests with that agency. NBS requests that if release to the public is contemplated, such action be taken only

after consultation with the Office of Public Affairs at the National Bureau of Standards."

Prepa red for Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington, DX. 20591

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

TABLE OF CONTENTS

SUMMARY Page

1. GENERAL INFORMATION 1

1.1 Reference 1

1.2 Accident Information 1

1.3 Parts Submitted 1

2. PURPOSE 1

3. RESULTS OF EXAMINATIONS AND TESTS 1

3.1 Visual and Macroscopic Examination 1

3.2 Chemical Analysis of the Corrosion Product. ... 2

3.3 Measurement of Thread Dimensions 3

3.4 Fractographic Examination with the Scanning

Electron Microscope 3

3.5 Metallographic Examination 4

3.6 Hardness Measurements 5

4. DISCUSSION. . 6

5. CONCLUSIONS 7

6. ACKNOWLEDGEMENT 8

TABLE 1. Thread Dimensions

FIGURES

1. Piston rod and rod end as received at NBS

2. View of the rod end as received

3. View of the piston rod as received

4. View of the piston rod as received

5. Scanning electron fractograph showing part of the

undamaged fracture surface

i
FIGURES (continued)

6. Scanning electron fractograph showing a typical area

of the undamaged part of the fracture surface of

the piston rod

7. Scanning electron fractograph showing a typical area

of the piston rod fracture surface

8. Scanning electron fractograph where the fracture

V- intersects the threads

9. As-polished longitudinal sections through the piston

rod showing the thread profile

10. Longitudinal section through the piston rod showing

the roots of two threads in areas not opposite the

keyway in the mating rod end

11. Etched longitudinal section through piston rod

away from the fracture showing part of one thread . . . .

12. Etched longitudinal section through piston rod away

from the fracture

13. Longitudinal section through the piston rod showing

the fracture profile in an undamaged region of the

fracture

14. Longitudinal section through the piston rod showing

the fracture profile in a mechanically damaged region

of the fracture

15. As-polished longitudinal section through the piston

rod showing an average inclusion content

16. As-polished longitudinal sections through the rod end

shov;ing the thread profile

17. As-polished longitudinal section through the rod end. . .

18. Etched longitudinal section through the rod end

showing part of one of the threads

19. Etched longitudinal section through the rod end

showing typical microstructure

20. As-polished longitudinal section through the rod end

showing an average inclusion content
SUMMARY

A failed piston rod and rod end from a DC-8 aircraft main

landing gear retract cylinder assembly was submitted to the

NBS Mechanical Properties Section for examination. The threads

of both of the mating parts were severely corroded and had

suffered rather severe mechanical damage. The mechanical

damage of the threads appeared to be the result of the failure

rather than the cause of the failure. A large portion of the

fracture surface had been damaged by an apparent rubbing action.

Dimpled rupture was the primary feature exhibited by the

undamaged portion of the fracture surface. An anti-seize

compound with a molybdenum disulfide base was reportedly used

on the threads when the components were in service, but no

molybdenum was detected in a chemical analysis of the corrosion

product. Of those thread dimensions checked, the root radius

of the rod end did not appear to meet specifications.

Examination of Piston Rod and Rod End from DC-8

Main Landing Gear Retract Cylinder Assembly

1. GENERAL INFORMATION

1.1 Reference

Bureau of Aviation Safety, National Transportation Safety

Board, Department of Transportation, Washington, DC. This

investigation was requested by Mr. Jerry A. Houck in a letter

dated July 26, 1974.

1 . 2 Accident Information

The submitted parts were from a Braniff DC-8 aircraft

involved in an accident at Tocumen, Panama, on May 19, 1974.

1 . 3 Parts Submitted

Portions of a piston rod and the mating rod end from a

main landing gear retract cylinder assembly from the above

mentioned aircraft were submitted to the NBS Mechanical

Properties Section for examination. A nut was attached to

the rod end. The parts are shown as received in figure 1.

The piece of material missing from the piston rod and a key

fitted to the rod end were not submitted.

2. PURPOSE

The Bureau of Aviation Safety requested that the NBS

Mechanical Properties Section perform a failure analysis of the

submitted parts with emphasis on (1) an analysis of the corrosion

product in the threaded regions and (2) a measurement of the

thread dimensions.

3 . RESULTS OF EXAMINATIONS AND TESTS

3 . 1 Visual and Macroscopic Examination

The threaded regions of both the piston rod and the rod end

were covered with what appeared to be a rather heavy corrosion

product. Both sets of threads had suffered extensive mechanical

damage. Part of the threaded region of the rod end is shown in

figure 2. In addition to the corrosion product and the

mechanical damage, parts of some of the threads from the mating

piston rod can be seen still attached to the rod end (arrows,

figure 2).

- 1 -
Part of the threaded region of the piston rod is shown in

figure 3. The fracture surface and the damaged and corroded

threads can be seen. A piece of the rod which had separated

at the fracture was not submitted for examination. Portions

of several threads are missing. The arrows in figure 3 indicate

the location of the transition from corroded to essentially

non-corroded regions. Another view of the piston rod is shown

in figure 4. The fracture surface had been subjected to severe

mechanical damage except for the area indicated by arrow A

where the fracture path was parallel to the longitudinal axis

of the rod. Discoloration of the fracture surface in some

areas of the damaged region indicates that the fracture surface

may have been subjected to temperatures high enough to produce

an oxide with a straw or blue color. In many places, the

fracture surface is smooth and somewhat shiny and has the

appearance of having been rubbed. Where it had not been damaged,

the fracture surface has a "woody" or "layered" appearance.

The piston rod was deformed in the vicinity of the fracture.

This deformation can be seen in figure 4 in the region designated

by arrow B. Another indication of the deformation is the cracked

chromium plating on the outside surface of the rod in the

vicinity of the fracture. This is especially evident in the

area designated C in figure 4.

In the region from arrow C to approximately the location

of arrow D, there is a pronounced lip, as if the outside portion

of the fracture surface had been subjected to a compressive

deformation or a pronounced downward wiping.

3 . 2 Chemical Analysis of the Corrosion Product

A chemical analysis of the corrosion product on the threads

of the piston rod was made by the NBS Analytical Chemistry

Division using electron microprobe techniques. The semiquantitative results are as follows:

Element Percent (weight)

Iron

Chromium

Sulfur

Silicon

Aluminum

Nickel

Carbon

The remaining major constituent was oxygen. Carbon and

oxygen were the only elements with atomic numbers less than

that of sodium that were checked. Elements with atomic numbers

higher than that of sodium were analyzed with a sensitivity of

about 0.1%.

50

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- 2 -
3.3 Measurement of Thread Dimensions

According to Douglas Aircraft Company drawings, the

threads of the piston rod and the rod end were to be rolled.

The dimensions of the 1 3/8 - 12 threads were to be in

conformance with Military Specification MIL-S-8879A. The

dimensions and permissible deviations for the pitch, half angle,

and root radius as given in MIL-S-8879A are listed in part "a"

of Table 1.

The corroded and damaged condition of the threads of both

the piston rod and the rod end made accurate measurement of

the thread dimensions very difficult. The threads of the

piston rod appeared to be in the best condition in a region

that had apparently been opposite a keyway in the mating rod

end. A longitudinal section was taken through this region of

the rod so that the thread profile was as nearly perpendicular

to the tangent to the rod at this point as practicable. The

pitch was measured for a series of seven consecutive threads.

The results of these measurements are given in part "b" of

Table 1. The average pitch based on the total accumulated

distance across the seven threads was 0.0843 inch. Because

of the condition of the threads, the half angle was measured

for only one thread and the root radius was determined for

only three threads. The results of the half angle and root

radius measurements are also given in part "b" of Table 1.

The estimated accuracy of the measurements presented in part

"b" of Table 1 is as follows: pitch, + 0.003 inch; half angle,

no better than + 1"; root radius, + 0.002 inch.

The tnreads of the rod end appeared to be in the best

condition under the nut that was attached to it when it was

submitted for examination. In a manner similar to that

described above, a longitudinal section was taken through the

rod end where the nut was attached. The pitch, half angles,

and root radii were measured for eight consecutive threads, six

of which were under the nut. The results of these measurements

are given in part "c" of Table 1. The estimated accuracy of

these measurements is as follows: pitch, + 0.002 inch; half

angle, no better than + 2**; root radius, + 0.002 inch. The

pitch based on the total accumulated distance across the eight

threads was 0.0835 inch.

3 . 4 Fractographic Examination with the Scanning Electron

Microscope

Part of the relatively undamaged portion of the fracture

surface in the region indicated by arrow B in figure 4 was

examined with the scanning electron microscope. A low magnification fractograph with the longitudinal direction of the piston

- 3 -
rod in the horizontal position is shown in figure 5. The

"woody" or "layered" appearance of the fracture is evident.

At higher magnifications, it is seen that this region of the

fracture surface exhibits dimpled rupture as the primary

fracture mode. Fractographs showing typical areas of the

relatively undamaged portion of the fracture surface appear

in figures 6 and 7. There may be some corrosion product on

the fracture surface in the area shown in figure 7.

In figure 8, part of the fracture surface intersecting

the threads is shown. Mechanical damage and the generally

deteriorated condition of the threads can be seen.

3.5 Metallographic Examination

As-polished longitudinal sections through the piston rod

showing the thread profile appear in figures 9a and 9b. The

profile shown in figure 9a is in a region that (apparently)

had been opposite the keyway in the mating rod end. The

threads appeared to be in better condition in this region

than elsewhere on the rod. Even so, the threads exhibited

rather severe deterioration. In figure 9b, a thread profile

exhibiting the representative condition of the threads away

from the region opposite the rod end keyway is shown. The

threads here have suffered extensive corrosive attack and

mechanical damage. In figure 10, the roots of two of the

threads in a region not opposite the keyway are shown at higher

magnification. There is a considerable amount of corrosion

product in evidence.

A field from an etched longitudinal section through the

rod showing part of one thread in an area away from the fracture

appears in figure 11. The results of mechanical damage as

manifested in microscopic deformation at the tip of the thread

is evident. The results of corrosive attack can also be seen.

In the particular field shown in figure 11, several stringer

type inclusions are present. The inclusion content in this

field appeared to be higher than the average for the material.

The microstructure of the rod material away from the fracture

shown in figure 11, and at higher magnification in another

area adjacent to the outside surface of the rod in figure 12,

consists primarily of tempered martensite.

The fracture profile in a relatively undamaged region of

the fracture (indicated by arrow A, figure 4) and in a mechanically smoothed region of the fracture (close to arrow

D, figure 4) are shown in figures 13 and 14, respectively.

The profile in the undamaged region is jagged, and there is no

clear evidence of deformation. The steel has been smeared over

the chromium plating at the left of the figure. The microstructure appears to be somewhat finer adjacent to the fracture.

- 4 -
A field from an as-polished longitudinal section through

the piston rod in an area that had been opposite the rod end

keyway exhibiting an average inclusion content for the material

examined is shown in figure 15.

As-polished longitudinal sections showing the thread profile

of the rod end in an area under the nut and in an area not under

the nut appear in figures 16a and 16b, respectively. It is

quite evident that the threads under the nut were in better

condition than those not under the nut. The threads not under

the nut had suffered both mechanical damage and rather severe

corrosive attack. The threads under the nut had suffered less

severe corrosive attack and essentially no mechanical damage.

Examples at higher magnification of a thread under the nut and

one not under the nut are shown in figures 17a and 17b, respectively. The results of corrosive attack are evident in

different degrees of severity in both figures, and rather

severe mechanical damage can be seen in figure 17b. Part of

a thread not under the nut in an etched longitudinal section

of the rod end is shown in figure 18. Microscopic deformation

is evident. A field from an etched longitudinal section is

shown at higher magnification in figure 19. The microstructure

consists primarily of tempered martensite.

A field from an as-polished longitudinal section showing

the average inclusion content for the rod end material appears

in figure 20. This steel appears to be somewhat cleaner than

the steel of the piston rod.

3 . 6 Hardness Measurements

Rockwell hardness measurements were made on transverse

sections through the piston rod and the rod end. Based on four

measurements each, the average hardness values for the piston

rod and for the rod end were R^-. 38 and 42 1/2, respectively.

These hardness values are approximately equivalent to ultimate

tensile strengths of 170 ksi for the piston rod and 197 ksi

for the rod end. Douglas Aircraft Company drawings call for

tensile strengths of 150 to 180 ksi for the rod and 180 to 200

ksi for the rod end.

Knoop microhardness measurements at a load of 5 00 grams

were made on the piston rod in a longitudinal section that

intersected the fracture in a damaged region. The average

hardness at locations 0.003 inch and further from the fracture

was KHN 410, which corresponds approximately to R^-. 41. Adjacent

to the fracture, however, the material was much softer. At

distances of 0.002 and 0.001 inch from the fracture, the hardness

values were KHN 239 and 264, respectively. These values

correspond approximately to Rg 98 at 0.002 inch and R^ 23 at

0.001 inch from the fracture.

- 5 -
4. DISCUSSION

Where the threads of the piston rod and the rod end had

been in contact, the threads were generally badly corroded and

exhibited severe mechanical damage. There was a considerable

amount of corrosion product in the threads of both parts. This

corrosion product extended only slightly beyond the threaded

regions. The corrosion of the rod end threads under the nut

was much less severe than that of the threads not under the nut.

On the other hand, the nut was solidly "frozen" in place,

which implies that before failure, the rod end was also frozen

into the piston rod. The mechanical damage incurred by the

threads appears to be the result, rather than the cause, of the

failure

.

It was reported that, in service, a molybdenum disulfide

base anti-seize compound was supposed to have been applied to

the threaded joint between the piston rod and the rod end. It

may be significant that molybdenum was not detected in the

chemical analysis of the corrosion product on the threads.

Sulfur in amounts of 1% or less was detected.

The deteriorated and damaged condition of the threads made

the accurate measurement of thread dimensions very difficult.

In Table 1, all pitch measurements except one for the piston

rod appear to meet specifications, when considering the estimated

accuracy of the measurements. The apparent discrepancy for the

one measurement (no. 7, Table 1, part b) may be due to both

corrosion and mechanical damage and therefore may not be real.

There is no sound evidence to indicate that the pitch of any of

the threads failed to meet specifications at the time the parts

were put into service.

Again, considering the permitted deviation and the estimated

accuracy of measurement, all of the half angles measured except

one (no. 1, Table 1, part c) appeared to meet specifications.

The one which did not appear to meet specifications measured

about one degree greater than acceptable, but the thread

conditions may be responsible for this apparent discrepancy.

There does appear to be a real discrepancy in the root

radii dimensions of the threads of the rod end. In every case

where this dimension was checked, the measured value of the root

radius is not within the specified range, even when considering

the estimated accuracy of measurement.

The features of the undamaged portion of the fracture

indicated that the fracture was ductile. The damaged portion

- 6 -
of the fracture surface appeared to have rubbed against

something which made it relatively smooth. This part of

the fracture was discolored in places and had the appearance

of having been subjected to temperatures high enough to form

straw or bluish colored oxides. Hardness measurements

indicated that the material adjacent to the fracture was

softer than that away from the fracture. This change in

hardness also indicates a possible heat effect.

The general appearance of the fracture surface, the

overall shape of the fracture path, and particularly the

lip under the lowest portion of the fracture surface, are

all consistent with the failure having occurred by the rod

end breaking out through the wall of the piston rod. This

is visualized to have occurred by a rotating motion of the

rod about a transverse axis. However, the mode of failure

is not consistent with the lack of serious mechanical damage

on the bottom portion of the rod end. In addition, there is

no obvious relation between this failure mode and the corroded

condition of the threads. The cause of failure is not clear.

Based on hardness values from measurements away from the

fracture, the ultimate tensile strength of the material from

both the piston rod and the rod end met specifications.

The material from both parts was, on the average,

relatively clean with few inclusions, but there was at least

one area in a longitudinal section through the piston rod

that contained a significantly higher concentration of

inclusions.

5. CONCLUSIONS

1. The mechanism of the piston rod fracture appeared to be

ductile overload.

2. The primary fracture mode was dimpled rupture.

3. Part of the fracture surface had suffered rather severe

mechanical damage and appeared to have been subjected

to a temperature sufficiently high to cause discoloration

and a decrease in hardness in adjacent material.

4. The threads of both the piston rod and the rod end

were severely corroded where the two parts had been

in contact. The threads were corroded elsewhere,

but less severely.

- 7 -
5. The threads of both the piston rod and the rod end had

suffered rather severe mechanical damage. This damage

appeared to be the result of the failure. The damage

was more severe in the piston rod than in the rod end.

6. Of those thread dimensions checked, only the root radii

of the rod end appeared not to meet specifications,

7. Molybdenum was not detected in a chemical analysis of

the corrosion product in the threads.

8. Based on hardness measurements, the ultimate tensile

strength of both the piston rod and the rod end met

specifications

.

9. At least one area of a longitudinal section through the

piston rod contained a significantly higher than average

concentration of inclusions.

6. ACKNOWLEDGEMENT

The specimen preparation work, photography, and hardness

measurements were performed by Mr. L. C. Smith of the NBS

Mechanical Properties Section.

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Figure 2. View of the rod end as received.

Thread damage is evident. Parts

of some of the threads of the mating

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indicate examples of regions where

parts of threads from the mating

piston rod are still attached. X2

Figure 3. View of the piston rod as received. The

fracture surface and portions of the

corroded and damaged threads can be seen.

The arrows indicate the approximate location of a transverse plane through the

rod beyond which there is essentially no

corrosion product. X2

view of the piston rod as received

showing most of the fracture surface.

Except for the longitudinal part of

the fracture (arrow A) , the fracture

surface was rather severely damaged.

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an example of cracks in the chromium

plating. There is a pronounced lip

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Figure 5. Scanning electron fractograph showing part of the

undamaged fracture surface in the area designated

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Figure 6. Scanning electron fractograph showing a typical

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Figure 7. Scanning electron fractograph showing a typical

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There may be some corrosion product on the surface.

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Figure 8. Scanning electron fractograph where the fracture

intersects the threads. Mechanical damage and

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Etchant: 4% picral X200

Figure 14. Longitudinal section through the piston rod

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outside surface in a mechanically damaged

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Etchant: 4% picral X200

Figure 15. As-polished longitudinal section through

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XlOO
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NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC Al ION OR REPORT NO.

NBSIR 75-671

2. Ciov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

EXAMINATION OF PISTON ROD AND ROD END FROM DC-8

MAIN LANDING GEAR RETRACT CYLINDER ASSEMBLY

5. Publication Date February 1975

6. Performing Organization Code

7. AUTHOR(S)

T, R. Shives

8. Performing Organ. Report No. NBSIR 75-671

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract /Grant No.

12. Sponsoring Organization Name and ( ompiete Address (Street, City, Stale, ZIP)

Bureau of Aviation Safety

National Transportation Safety Board, DOT

Washington, D.C. 20591

13. Type of Repqrt & Period Covered FailUrS

Analysis Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT" (A 200-word or less (actual summary of moat sigfiificant irttormation. If document includes a significant

bibliography or literature survey, mention it here.)

A failed piston rod and rod end from a DC-8 aircraft main landing gear

retract cylinder assembly was submitted to the NBS Mechanical

Properties Section for examination. The threads of both of the mating

parts were severely corroded and had suffered rather severe mechanical

damage. The mechanical damage of the threads appeared to be the result

of the failure rather than the cause of the failure. A large portion

of the fracture surface had been damaged by an apparent rubbing action.

Dimpled rupture was the primary feature exhibited by the undamaged

portion of the fracture surface. An anti-seize compound with a

molybdenum disulfide base was reportedly used on the threads when the

components were in service, but no molybdenum was detected in a

chemical analysis of the corrosion product. Of those thread dimensions

checked, the root radius of the rod end did not appear to meet

specifications.

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

)

Aircraft landing gear; corrosion; dimpled rupture; ductile fracture.

18. AVAILABILITY Unlimited

1 X ' For Official Distribution. Do Not Release to NTIS

|_ ' Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. CI 3

1

1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

29

20. SE( URITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74
I

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