NBSIR 74-468

Examination of Failed Aircraft

Rudder Torque Tube Assembly

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

April 1974

Failure Analysis Report

Prepared for

Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

NBSIR 74-468

Examination of Failed Aircraft Rudder

Torque Tube Assembly

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

April 197 4

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, D. C. 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."

Prepared for

Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

•/ \^ \

'• \W\ I

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

NATIONAL BUREAU OF STANDARDS, RJehard W. ftotorts. Oiractor

TABLE OF CONTENTS

Page

SUMMARY

1. GENERAL INFORMATION 1

1.1 Reference 1

1.2 Accident Identification 1

1.3 Part Identification 1

2. PURPOSE 1

3. RESULTS OF EXAMINATIONS, ANALYSES, AND

MEASUREMENTS 1

3.1 Visual Examination 1

3.2 Fractographic Examination 2

3.3 Thickness and Diameter Measurements 3

3.4 Chemical Analyses 4

3.5 Metallographic Examination 5

3.5.1 As polished cross sections 5

3.5.1.1 Left assembly 5

3.5.1.2 Right assembly 6

3.5.2 Etched cross sections 7

3.5.2.1 Left assembly 7

3.5.2.2 Right assembly 7

3.6 Hardness Measurements 8

4. DISCUSSION 9

5. CONCLUSIONS 11

6. ACKNOWLEDGEMENT 13

REFERENCES 13

i
FIGURES

1. Failed left and intact right rudder torque tube

assemblies as received.

2. Opposing fracture surfaces of the left torque

tube and the left pedal connecting tube.

3. Corrosion product in the right pedal connecting tube

4. SEN fractograph from the back part of the fracture

showing dimpled rupture.

5. SEM fractograph from the front part of the fracture

showing dimpled rupture.

6.. SEM fractograph from the side of the fracture

surface showing a mixture of dimpled rupture and

quasi-cleavage

7. SEM fractograph from the back part of the fracture

showing an intergranular appearance.

8. SEM fractograph from the front part of the fracture

showing an intergranular appearance.

9. Longitudinal cross sections through left pedal

connecting tube.

10. Longitudinal cross section through left torque

tube assembly showing field v;ith a typical

inclusion content.

11. Area of longitudinal cross section through left

torque tube assembly.

12. Longitudinal cross sections through right pedal

connecting tube.

13. Longitudinal cross section through right torque

tube assembly showing crack in heat affected zone.

14. Longitudinal cross section through right torque

tube assembly showing large stringer-type

inclusion, area of gas porosity-slag inclusion

in the heat affected zone, and apparent crack.
FIGURES (Continued)

15. Longitudinal cross section through right torque

tube assembly showing gas porosity-slag

inclusion and apparent crack in heat affected

zone

16. Etched longitudinal cross sections through left

pedal connecting tube.

> 17. Etched longitudinal cross section showing

microstruc ture of the tube material.

18. Etched longitudinal cross section showing

decarburized layer adjacent to the outside

surface of the tube.

19. Etched longitudinal cross section showing

microstructure of the weld metal.

20. Etched longitudinal cross section showing the

microstructure of the heat affected zone

adjacent to the weld.

21. Etched longitudinal cross section showing the

microstructure of the heat affected zone

adjacent to the parent tube material.

22. Etched longitudinal cross section through left

torque tube assembly showing part of

fracture profile.

23. Etched longitudinal cross section through left

torque tube assembly showing part of

fracture profile.

24. Etched longitudinal cross section through right

torque tube assembly showing location of gas

porosity, v/elding slag inclusion, and crack

in the heat affected zone.
SUMMARY

The left rudder torque tube assembly from a Nieuport 28

Replica had failed vrhen fracture occurred through the heat

affected zone on the torque tube side of the welded joint

between the torque tube and the pedal connecting tube. The

fracture appeared to have been caused by tensile overload

in bending, probably due to a force applied to the left

pedal

The primary feature exhibited by the fracture surface

v;as dimpled rupture indicating ductile fracture. There

were some areas of the fracture surface having an intergranular appearance. These were probably regions of gas

porosity through which the fracture passed.

There was one small area of the fracture well removed

from the fracture origin that exhibited a mixture of dimpled

rupture and quasi-cleavage , indicating a somewhat brittle

fracture in this area. This may indicate an unusually hard

place in the heat affected zone. The heat affected zones

adjacent to the welds in both the right and left assemblies

were harder than desirable relative to the hardness of the

parent tube material and the weld metal.

The prior austenite grain size in the heat affected

zones adjacent to the welds of both assemblies was larger

than desirable. Otherwise, the microstructures of the

various regions appeared to be satisfactory. The weld metal

and the parent tube material were relatively free of

inclusions except for a few areas in the parent tube material

in the right assembly which contained stringers.

There were several undesirable conditions associated

with the welded joints, particularly that of the right

assembly, such as apparent undercutting, a sharp angle

betvreen the two tubes, and gas porosity, a slag inclusion,

and cracking in the heat affected zone. These conditions

were much less severe in the welded joint of the left

assembly, even though the left assembly was the one that

failed

iv
Examination of Failed Aircraft Rudder

Torque Tube Assembly

1. GENERAL INFORMATION

1 . 1 Reference

Bureau of Aviation Safety, National Transportation

Safety Board, Department of Transportation, Washington, D.C.

Request by Mr. Jerry A. Houck dated February 11, 1974.

1 . 2 Accident Identification

Nieuport 28 Replica, registration number N2SR. The

accident occurred on January 21, 1974 at Indian Dunes,

California

1 . 3 Part Identification

Left and right rudder torque tube assemblies were

submitted for examination. The left assembly had failed

when the tube connecting the left pedal to the torque tube

had separated from the torque tube. The parts are shown

as received in figure 1.

2. PURPOSE

The Bureau of Aviation Safety requested a failure

analysis of the failed part.

3. RESULTS OF EXAMINATIONS, ANALYSES, AND M_EASUREMENTS

3 . 1 Visual Examination

The tube connecting the left pedal to the left torque

tube (referred to as the left pedal connecting tube in this

report) had separated from the torque tube where the two

tubes had been welded together. Fracture occurred adjacent

to and on the torque tube side of the weld. Assuming the

pedal connecting tube to be vertical (or nearly so) in

service, the fracture apparently started at the back of the

welded joint and propagated across both sides toward the

front, with respect to the fore and aft directions of the

aircraft. The region of fracture initiation is indicated

by the arrows in the left part of figure 1. The front

of the welded joint appeared to be the last part to fail.

- 1 -
The fracture surface on the left pedal connecting tube side

of the failure is shown in the lower part of figure 2. A

curved disk, part of the torque tube, can be seen attached

to the pedal connecting tube at the welded joint. The

location of this disk is indicated by the letter A in

figure 2. The presence and location of this portion of the

torque tube relative to the fracture indicates that the

wall of the torque tube had been melted through when the

welded joint was made. In addition, the presence and

location of what appears to be weld metal around the circumference of the joint also indicates that the weld had

penetrated the wall of the torque tube.

There was considerable deformation in the torque tube

in the vicinity of the fracture, as can be seen in figure 2.

Near the front of the fracture, the torque tube had been

"pushed" inward. This was apparently caused by the application of a bending load on the welded joint between the

pedal connecting tube and the torque tube after it had

been partially fractured. The bending load was likely

caused by a force exerted on the pedal. There was some

"necking" or thinning of the torque tube wall in the

vicinity of the fracture origin.

Both the left and right pedal connecting tubes were

sectioned longitudinally, through the welded joint for the

right tube and intersecting the fracture for the left tube.

While the inner wall surface of the failed left pedal connecting tube was relatively clean, there was a considerable

amount of corrosion product found inside the intact right

pedal connecting tube. Some of this corrosion product can

be seen in figure 3.

3 . 2 Fractographic Examination

Both of the opposing fracture faces were examined

macroscopically , and the fracture surface on the pedal connecting tube side of the failure was examined with the

scanning electron microscope (SE^O . There was a significant

amount of mechanical damage on the fracture surface at both

the back and front. The fracture surfaces appeared to have

been smeared as though the pedal connecting tube fracture

surface had rubbed across the torque tube fracture surface.

The appearance of the fracture surfaces indicated that the

fracture had been completed by bending the pedal connecting

tube forward relative to the torque tube (assuming the pedal

connecting tube to be in a nearly vertical position in

service) . This is consistent with the deformation observed

in the torque tube.

- 2 -
The primary feature found in essentially all areas

of the fracture surface examined with the SEM was dimpled

rupture, indicating a ductile failure. Typical examples of

this are shown in figures 4 and 5. In one area, however,

there was evidence of quasi-cleavage (figure 6) which

indicates brittle fracture. The quasi-cleavage was found

near one side of the fracture surface and does not appear

to be related to the apparent origin of the fracture.

In several isolated areas of the fracture surface,

there were small regions having an intergranular appearance.

Two examples of this are shown in figures 7 and 8. Dimpled

rupture is exhibited along the sides of many of the grains

shown in these figures. As indicated by an examination of

the microstructure , which will be discussed later, the

fracture passed through an area of the heat affected zone

adjacent to the weld where the microstructure is martensitic.

The sizes and shapes of the grains shown in figures 7 and 8

indicate that the fracture surface is probably following

prior austenite grain boundaries in these regions. Some

secondary cracking and porosity are evident in figures 6,

7, and 8. Mechanical damage is evident in figure 7.

3 . 3 Thickness and Diameter Measurements

After chemically removing the paint on the surface of

the tubes, wall thickness measurements were made on the left

pedal connecting tube about 1/4 inch from the fracture and

near one end of each of the torque tubes. Ten measurements

were made in each of these three areas at approximately

equal circumferential intervals. The results are as follows:

Fractured left pedal connecting tube 0.0356 0.034 - 0.037

Component Thickness (in.)

Average Range

Left torque tube 0. 0348 0.034 - 0.036

Right torque tube 0. 0356 0.035 - 0.036

Minimum and maximum outside diameter measurements were made

on the same three tubes in the same areas that the wall

thickness measurements were made with the following results:
Component

Fractured left pedal connecting tube

Left torque tube

Right torque tube

Outside Diameter (in.)

J^iinimum Maximum Average

0.731 0.769 0.750

0.751 0.753 0.752

0.748 0.752 0.750

The left pedal connecting tube was somewhat out of round, but

the nominal diameter of all three tubes was essentially the

same.

3 . 4 Chemical Analyses

In order to classify the steels from which the tubes had

been fabricated, semi-quantitative chemical analyses by emission spectroscopy were performed by the NBS Analytical

Chemistry Division on samples from the left torque tube and

the left pedal connecting tube. Similar amounts of those

elements detected were found in the material from both tubes.

The results are as follows:

Element

Nickel

Molybdenum

Chromium

Manganese

Per Cent (weight)

0.1 - 1.0

0.03 - 0.3

0.1 - 1.0

0.1 - 1.0

Silicon

Copper

Vanadium

0.03 - 0.3

0.03 - 0.3

< 0.01

Samples from these same two tubes were analyzed by combustion

techniques for carbon with the following results:

- 4 -
Component

Left pedal connecting tube

Per Cent Carbon (weight)

0.27

Left torque tube 0.31

No analyses for phosphorus or sulfur were made. Based on the

analyses performed, the material from both tubes appears to

be essentially the same low alloy, medium carbon steel,

probably AISI 8627-8630l.

A qualitative chemical analysis by electron microprobe

techniques was performed by the NBS Analytical Chemistry

Division on the weld material. . In addition to iron, chromium

and manganese were found in amounts of less than two percent.

No major differences were found between the weld metal and

the tube material. The system did not allow the detection

of nickel, and the samples were not analyzed for elements

with atomic numbers lower than that of sodium.

3 . 5 Metallographic Examination

3.5.1 As polished cross sections

3.5.1.1 Left assembly

Two longitudinal cross sections were

taken through the left pedal connecting tube intersecting

the fracture at the weld between the left torque tube and

the pedal connecting tube. One cross section, shown in

figure 9a, was in a plane perpendicular to the longitudinal

axis of the torque tube. The other, shown in figure 9b,

was in a plane parallel to the longitudinal axis of the

torque tube

The tube material in both cross sections was relatively

free of inclusions, and no areas containing large inclusions

were found. Figure 10 shows a field with a typical inclusion

content. No areas of gas porosity or welding slag were found

in the material, although the SEM fractographs indicated that

the fracture had passed through regions of gas porosity.

The angle between the tubes at the arrow in figure 9a

is rather severe with essentially no fillet, although this

is on the inside of the v/elded joint. There is what appears

to be a large deposit of welding flux or slag at this same

location, which is shovm at higher magnification in figure 11.

There is little or no evidence of undercutting in these two

cross sections.

- 5 -
3.5.1.2 Right assemblyTwo longitudinal cross sections were

taken through the right pedal connecting tube in locations

similar to those taken through the left torque tube assembly.

A photomacrograph of one, in a plane perpendicular to the

longitudinal axis of the torque tube, is shown in figure 12a.

The other cross section, in a plane parallel to the longitudinal axis of the torque tube, is shown in figure 12b.

An accumulation of corrosion product on the inside of

the right pedal connecting tube was shown in figure 3. As

can be seen in figure 12, the interior wall surface of the

pedal connecting tube and the outside wall surface of the

torque tube enclosed by the connecting tube at the weld

have suffered corrosive attack. Arrows A, figure 12,

indicate examples of corroded areas. The results of corrosive attack were much more severe on the interior wall

surface of the right pedal connecting tube than in the left

pedal connecting tube, which is consistent with the visual

appearance of the inside wall surfaces of the tubes.

A small crack starting at the inner vJ^all surface of

the right pedal connecting tube was found in an area of the

heat affected zone and is indicated by arrow B in figure 12b.

This is shown at higher magnification in figure 13.

Two areas of large, stringer type inclusions in the

torque tube material are indicated by arrows C in figure 12b.

One of these is shown at higher magnification (arrow C) in

figure 14. Except for a few isolated areas, the overall

inclusion content was essentially the same as that found in

the tube material in the left assembly.

There are some undesirable conditions associated with

the welded joint in the right assembly which can be seen in

figure 12. Arrows D, figure 12b, indicate an area in the

heat affected zone which appears to be a combination of gas

porosity and a slag inclusion. A crack (arrow E, figure 12b)

appears to have propagated between the porosity-slag region

and the surface. There is corrosion product in the crack

as can be seen in figure 15.

There are areas (indicated by arrows F in figure 12)

of what appear to be undercutting^ . Arrow G, figure 12a,

indicates a condition which, if on the outside of the joint,

would appear to be an overlap^ . The angle between the two

tubes at this point is rather severe and there is essentially

no fillet.

- 6 -
3.5.2 Etched cross sections

3.5.2.1 Left assembly

Photomacrographs of the two cross

sections shown as polished in figure 9 are shown in the etched

condition in figure 16. The microstructure of the parent

tube material away from the heat affected zones (in areas

indicated by arrows A, figure 16) is similar for both tubes.

It consists primarily of pearlite and ferrite with some

spheroidized cementite-^ . A typical field is shown in figure

17. There is some decarburization adjacent to the outside

surface of the tube material as can be seen in figure 18.

The microstructure of the weld metal (areas indicated by

arrows B, figure 16) appears to consist of coarse tempered

martensite^. A typical field in the weld is shown in

figure 19.

There are two distinctly different microstructures in

different areas of the heat affected zones. Adjacent to the

weld (locations indicated by arrows C, figure 16) , the

microstructure consists of coarse grained martensite outlined by the grain boundaries of prior austenite grains.

An example of this microstructure is shown in figure 20.

Close to the parent tube material (areas indicated by

arrows D, figure 16) , the microstructure appears to consist

of martensite and some ferrite. An example of this is shown

in figure 21.

The fracture profiles of the cross sections shown in

figures 9a and 9b are shown in figures 22 and 23, respectively. It can be seen that the fracture passed through

the heat affected zone. There is some secondary cracking

evident in figure 22, and these cracks appear to be

following the grain boundaries of the prior austenite. The

fracture path appears to be transgranular for the most part.

3.5.2.2 Right assembly

The microstructures of the parent tube

material, the weld metal, and the material in the heat

affected zones in the cross sections from the right assembly

shown in figure 12 are essentially the same as in corresponding regions of the cross sections through the left

torque tube assembly as discussed in section 3.5.2.1.

- 7 -
The area containing the gas porosity and the slag

inclusion (arrow D, figure 12b) and the crack connecting

them to the surface of the tube (arrow E, figure 12b) is

shown in the etched condition in figure 24. It can be seen

that these defects are in the heat affected zone.

3 . 6 Hardness Measurements

Knoop microhardness measurements at a load of 500 grams

were made on the same cross sections used for the metallographic examination. The range of Knoop hardness numbers (KHN)

for the various regions of the cross sections, and the approximate Rockwell C equivalent hardness numbers, are given below:

Region 500 C_

Left pedal connecting tube

Left torque tube

Left assembly heat affected zones

Adjacent to the weld

Adjacent to the parent tube

Right pedal connecting tube

Right torque tube

Right assembly heat affected zones

Adjacent to the weld

Adjacent to the parent tube

265 -291 23 -27

264 23

441 -551 43 -51

359 -420 36 -42

267 -300 23 -28

240 -279 20 -24

420 -574 42 -52

394'-444 39 -43

The hardness of the tube material and the weld metal in both

assemblies was very similar, but some areas of the heat

affected zones, particularly adjacent to the welds, were

considerably harder.

- 8 -
4. DISCUSSION

The fracture in the left rudder torque tube assembly

occurred in the heat affected zone adjacent to the weld

joining the left pedal connecting tube to the left torque

tube. The fracture was on the torque tube side of the weld.

The primary feature found on the fracture surface was

dimpled rupture (ductile fracture) , indicating that the

fracture was probably due to tensile overload. The appearance

of the fracture surface and the deformation in the torque

tube indicates that the overload was caused by a bending

stress, probably induced by a force applied to the left

pedal. The pedal connecting tube acted as a bending moment

arm which amplified the load received at the point of

failure

Assuming that the pedal connecting tube was in a

vertical (or nearly so) position in service relative to the

longitudinal axis of the aircraft, the appearance of the

fracture and the deformation in the torque tube indicate

that the fracture initiated at the back (relative to the

back and front of the aircraft) and propagated toward the

front. This is consistent with the bending load having

been applied at the pedal.

The significance of the one small area of the fracture

surface exhibiting a mixture of dimpled rupture and quasicleavage is unclear. The quasi-cleavage does not appear to

be associated with the fracture origin, but the presence of

this feature indicates a somewhat brittle fracture area.

This may be an unusually hard region in the heat affected

zone

There were several small, isolated areas of the

fracture surface that had an intergranular appearance. This

probably indicates regions of gas porosity in the heat

affected zone through which the fracture passed. The microstructure in this part of the heat affected zone consisted

of martensite outlined by the grain boundaries of large

prior austenite grains. The intergranular appearance

indicates that the fracture probably followed these grain

boundaries in these areas.

The parent tube material in both assemblies appeared

to be clean, except for a few isolated areas in the right

assembly, and the weld material appeared to be free of

inclusions. The etched microstructures in both assemblies

appeared to be satisfactory except possibly for that of the

heat affected zones adjacent to the welds. As mentioned

- 9 -
above, the martensite in these zones was outlined by grain

boundaries of large prior austenite grains. A finer grained

microstructure would be more desirable.

The hardness of the tube material, in areas away from

the heat affected zones, was similar for both torque tubes

and both pedal connecting tubes. Based on the hardness,

the approximate ultimate tensile strength of the tube

material would be about 110 to 130 ksi. The hardness of the

weld metal was similar to that of the parent tube material.

The hardness of the material in the heat affected zones

adjacent to the parent tube material was greater than that

of the parent tube and the weld metal, and the hardness of

the heat affected zones adjacent to the welds was significantly greater than that of any of the other areas where

the hardness was determined. The fracture passed through

this relatively hard zone. Even though this fracture was

primarily ductile, a very hard heat affected zone could lead

to a region of brittle material. It would be desirable for

the hardness of all the regions in the welded joint to be

similar. Large differences in hardness could result in

unfavorable stress patterns at the boundaries between

adjacent regions.

The most detrimental single condition, in addition to

differences in hardness, found in either assembly was

associated with the heat affected zone adjacent to the

welded joint between the right torque tube and the right

pedal connecting tube. There was a large gas porosityslag inclusion combination which was connected to the

surface by an apparent crack. Corrosion product in the

crack indicates that the crack is not fresh. It may have

formed when the joint was cooled after welding. The cross

section of the material could be significantly reduced at

the location of the gas porosity-slag inclusion-crack,

which would in turn result in a reduced load carrying

capacity. The areas of intergranular features on the left

assembly fracture indicate that there probably was porosity

in the heat affected zone of this weld also.

Other undesirable features associated with the weld

areas such as undercutting and the results of corrosive

attack appeared to be worse in the intact right assembly

than in the failed left assembly. These features do not

appear to have contributed to the failure.

- 10 -
Chemical analyses indicate that the material in the

left torque tube assembly and the left pedal connecting

tube was essentially the same medium carbon, low alloy

nickel-chromium-molybdenum steel. The results of the

analyses of the tube material and the weld material do not

indicate incompatibility of materials at the welds.

The wall thicknesses and diameters of the tubes

measured were all nominally the same, although the left

pedal tube was considerably out of round at the place the

diameter was measured.

The inside wall surface of the right pedal connecting

tube exhibited corrosion product. The inside wall surface

of the left pedal connecting tube showed little evidence

of corrosive attack. Corrosion did not appear to be a significant contributing factor in the failure.

5. CONCLUSIONS

1. The left torque tube assembly failed due to a tensile

overload caused by a bending load probably applied

at the left pedal.

2. Fracture occurred through the heat affected zone on

the torque tube side of the welded joint between the

left pedal connecting tube and the left torque tube.

3* The primary feature exhibited by the fracture surface

was dimpled rupture indicating a ductile fracture.

4. Small areas of the fracture surface exhibited an

intergranular appearance indicating that the

fracture passed through regions of possible gas

porosity in the heat affected zone.

5. There were a number of undesirable conditions associated

with the welded joint between the right torque tube and

the right pedal connecting tube such as apparent undercutting, a sharp angle between the two tubes with

essentially no fillet, and gas porosity, a slag

inclusion, and cracking in the heat affected zone.

6. The undesirable conditions associated with the welded

joint in the right assembly (conclusion 5, above)

were much less severe in the welded joint in the failed

left assembly, and none except possibly gas porosity

appeared to have contributed to the failure.

- 11 -
7. The hardness of the heat affected zones adjacent to

the welds was undesirably high relative to that of

the other regions.

8. A small area of the fracture surface exhibited a mixture of dimpled rupture (ductile fracture) and

quasi-cleavage (brittle fracture) which may indicate

an unusually hard region of the heat affected zone.

9. The tube material was relatively free of inclusions

except for a fev7 isolated areas where there were

large stringers. These were not involved in the

fracture

10. The weld metal appeared to be clean and free of

inclusions.

11. The microstructures of the various regions appeared

to be satisfactory except possibly for that of the

heat affected zones and adjacent to the welds which

consisted of martensite outlined by the grain

boundaries of larger than desirable prior austenite

grains

12. The results of semi -quantitative chemical analyses

indicate that the left torque tube and the left pedal

connecting tube were both fabricated from essentially

the same medium carbon, low alloy steel, probably

AISI 8627 - 8630.

13. Based on limited semi -quantitative chemical analyses,

the tube material and the weld metal appear to be

compatible

14. The nominal wall thicknesses and diameters of both

torque tubes and both pedal connecting tubes were the

same

15. The inside wall surface of the right pedal connecting

tube exhibited results of corrosive attack,

16. The inside wall surface of the left pedal connecting

tube showed little evidence of corrosive attack, and

corrosion does not appear to have contributed to the

failure.

- 12 -
6 . ACKNOWLEDGEMENT

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

Section prepared the metallographic specimens, performed

the photographic work, and made the hardness and

dimensional measurements.

REFERENCES

1. Heat Treatment and Properties of Iron and Steel,

NBS Monograph 88, National Bureau of Standards,

1966.

2. Welding Handbook, 5th Edition, American Welding

Society, 1962.

3. Metals Handbook, American Society for Metals,

Volume 7, 8th Edition, 1972.

- 13 -

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Figure 3. Corrosion product in the right

pedal connecting tube adjacent to

the welded joint between the connecting tube and the torque tube.

X 2

Figure 4. SEM fractograph from the back part

of the fracture showing dimpled

rupture. X 1190

Figure 5. SEM fractograph from the

front part of the fracture

showing dimpled rupture. X 1150

Figure 6. SEM fractograph from the side

of the fracture surface showing

a mixture of dimpled rupture

and quasi-cleavage . Some

porosity is evident. X 510

Figure 7 . SEM f ractograph from the back part

of the fracture showing an intergranular appearance. Dimpled

rupture can be seen on the sides of

several of the grains. The arrow

indicates an area of mechanical

damage. Some porosity is evident.

X 260

Figure 8. SEM f ractograph from the front part

of the fracture showing an intergranular appearance . Some dimpled

rupture is evident and there is some

porosity. X 650

Figure 9. Longitudinal cross sections through left pedal

connecting tube. As polished. X 7

a. Section perpendicular to longitudinal

axis of torque tube.

b. Section parallel to longitudinal axis

of torque tube.

Arrow indicates location of flux deposit and

sharp angle between the tubes.

Figure 10. Longitudinal cross section through

left torque tube assembly showing

a field with a typical inclusion

content. As polished. X 100

Figure 11. Area of longitudinal cross section

through left torque tube assembly.

Arrow indicates location of heavy

deposit of welding flux or slag and

sharp angle between the two tubes

(arrow, figure 9a) . As polished.

X 40

Figure 12. Longitudinal cross s connecting tube. As

a

.

b.

Section perpendi

of torque tube.

Section parallel

torque tube.

Arrows A indicat

Arrow B indicate

affected zone.

Arrows C indicat

Arrows D indicat

inclusion in hea

Arrow E indicate

affected zone.

Arrows F indicat

Arrow G indicate

tubes

.

ections through right pedal

polished. X 10

cular to longitudinal axis

to longitudinal axis of

e examples of corroded areas,

s location of crack in heat

e locations of large inclusions.

e area of gas porosity-slag

t affected zone.

s apparent crack in heat

e examples of apparent undercutting,

s sharp angle between the two

Figure 13. Longitudinal cross section through

right torque tube assembly showing

crack in heat affected zone at location of arrow B in figure 12b.

As polished. X 500

Figure 14. Longitudinal cross section through

right torque tube assembly showing

large stringer type inclusion at

arrow C, area of gas porosity-slag

inclusion in heat affected zone at

arrows D, and apparent crack at

arrow E. Same arrow designations

indicate these features in figure 12b.

As polished. X 40

Figure 15. Longitudinal cross section through

right torque tube assembly showing

gas porosity-slag inclusion ar-ea (arrows D) , apparent crack in heat

affected zone between inclusion and

surface (arrow E) , and part of

stringer inclusion in tube (arrow C)

shown in figure 14. As polished.

X 200

Figure 16. Etched longitudinal cross sections through left

pedal connecting tube. Etch: 1% nital. X 7

a. Section perpendicular to longitudinal axis

of torque tube.

b. Section parallel to longitudinal axis of

torque tube.

Arrow A indicates parent tube material.

Arrows B indicate weld metal.

Arrows C indicate heat affected zone

adjacent to the weld.

Arrows D indicate heat affected zone

adjacent to the parent tube material.

Figure 17. Etched longitudinal cross section

showing microstructure of the tube

material. Microstructure consists of

pearlite (dark) and ferrite (light)

and some spheroidized cementite.

Etch: 1% nital. X 1000

Figure 18. Etched longitudinal cross section

showing decarburized layer adjacent

to the outside surface of the tube.

The entire thickness of the tube is

shown. The decarburized layer is at

the right. Etch: 1% nital. X 100

Figure 19. Etched longitudinal cross section

showing microstructure of the weld

metal which consists of coarse,

tempered martensite. Etch: 1% nital.

X 1000

Figure 20. Etched longitudinal cross section

showing the microstructure of the heat

affected zone adjacent to the weld.

Microstructure consists of coarse

grained martensite outlined by grain

boundaries of prior austenite grains.

Etch: 1% nital. X 1000

Figure 21. Etched longitudinal cross section

showing the microstructure of the

heat affected zone adjacent to the

parent tube material. Microstructure

consists primarily of martensite with

some ferrite. Etch: 4% picral. X 1000

Figure 22. Etched longitudinal cross section

through left torque tube assembly

showing part of fracture profile

(upper left) and some secondary

cracking. Etch: 1% nital. X 200

Figure 23. Etched longitudinal cross section

through left torque tube assembly

showing part of fracture profile.

Etch: 1% nital. X 200

Figure 24. Etched longitudinal cross section

through right torque tube assembly

showing location of gas porosity,

welding slag inclusion, and crack

in the heat affected zone.

Etch: 1% nital. X 200

I,

I

NBS-114A (REV. 7-73)

U .5. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. I'LULK ATION OR RliPORT NO. NBSIR 74-468

2. Gov't Accession

No.

3. Recipient's Accession No.

4. T1T1.1-: .^^l•> si btitlh

laamination of failed aircraft rudder

torque tube assembly

5. Publ icai ion Date April 1974

6. Performing Organization Code

7. ArTI10R(S\

T. Robert Shives

8. Performing Organ. Report No. NBSIR 74-468

9. PKRFOR.MING ORGANIZATION NA.ME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Vlork Unit No. 3120412

11. Contract/ Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Bureau of Aviation SafetyNational Transportation Safety Board

Department of Transportation

Washington, D. C. 20591

13. Type of Report & Period Covered _ . Failure

Analysis Report

14. Sponsoring Agency Code

! 15. SUPPLE.MENTARY NOTES

16. .\BSTR.ACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A fractured rudder torque tube assembly from a Nieuport 28 Replica

y/as submitted for examination. Fracture had occurred through the

heat affected zone adjacent to a weld joining the left torque tube to the left pedal connecting tube. The primary feature exhibited by the fracture v/as dimpled rupture indicating that failure had

probably occurred by ductile overload. Certain areas of the heat

affected zones were quite hard relative to other regions of the

assembly, and there were certain other undesirable features

associated with the welds in both the right and left assemblies.

17. KE"! 'AORDS (six to twelve entries; alphabetical order; capitalize only the fin;! letter of the first key word unless a proper name; separated by semicolons)

Aircraft failure; dimpled rupture; f ractograohic examination; overload failure; weld heat affected zone.

18. AVAILABILITY r_ Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

.'^ For Official Distribution. Do Not Release to NTIS

UNCLASSIFIED

53

L_ Order From Sup. of Doc, U.S. Government Printing

Washington, D.C. 2040:. SO Cat. No. Cn

Office 20. SF.CURITY CLASS

(THIS PAGE)

22. Price

Order I- mm National Technical Information Service

Springfield, Virginia 22151

(NTIS)

UNCLASSIFIED

USCOMM-OC 29042-P74