NBSIR 83-2774 (NTSB) (R)

Examination of Failed Tension-Torsion Strap

from Be!! 206B, IM90071 , Helicopter

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

'J.S. Department of Commerce

National Bureau of Standards

Washington, D.C. 20234

April 1983

Failure Analysis Report

Prepared for National Transportation Safety Board

Washington, IkC. 20594

NBSIR 83-2774 (NTSB) (R)

EXAMINATION OF FAILED TENSION-TORSION STRAP

FROM BELL 206B, N90071, HELICOPTER

T. Robert Shives

Fracture and Deformation Division

Center for Materials Science

National Measurement Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, D.C. 20234

April 1983

Failure Analysis Report

Prepared for National Transportation Safety Board

Washington, D.C. 20594

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

Examination of Failed Tension-Torsion Strap from Bell 206B, N90071, Helicopter

1.

INTRODUCTION

1.1 Reference

National Transportation Safety Board, Washington, DC 20594. This

investigation was conducted at the request of Mr. Jerry A. Houck of the

National Transportation Safety Board in a letter received at NBS on October

17, 1977.

1.2 Parts Submitted

Two tension-torsion straps from Bell 206B, N90071 helicopter were submitted to

the National Bureau of Standards for examination. Essentially all of the

wires in one of the straps had fractured, and much of the polyurethane molding

had been removed. This strap is shown as received at NBS in figure 1. The

wires in the other strap were essentially intact, but the polyurethane molding

had been removed from one side exposing the wires. This strap is shown as received at NBS in figure 2.

1.3 Background Information

The information in this section was furnished by the National Transportation

Safety Board. The tension-torsion straps reportedly consist of more than

eight thousand 0.0058 inch diameter AM 355 stainless steel wires wrapped

around 17-7PH stainless steel spools at either end of the assembly. The parts

of the spools in contact with the wires were coated with teflon before the

wires were attached. Each of the individual wires was coated with a polyurethane batter before being wound onto the spools. After assembly, the

whole unit except for the faces of the spools was molded in a polyurethane

covering cured with Cature 21.

The aircraft to which the failed tension-torsion strap had been attached had

been used in off-shore oil well drilling rig service. Thus, the

tension-torsion straps were subjected to a marine environment.

2.

PURPOSE

It was requested by the National Transportation Safety Board that NBS perform

a failure analysis of the fractured wires in the failed tension-torsion strap.

3.

PREPARATION OF WIRES FOR EXAMINATION

In order to characterize the wire fractures, it was necessary to clean the

fracture surfaces and surfaces of the wires in the vicinity of the fractures

of foreign material, including the polyurethane coating. In several cases,

what appeared to be the polyurethane batter used to coat the wires before

assembly protruded over part of the fracture surface. Several different

cleaning procedures were employed for different groups of wires. These

procedures are as follows:
(1) Cleaning ul trasonical ly in petroleum ether.

(2) Cleaning ul trasonically successively in (1) petroleum ether,

(2) amyl acetate, and (3) commercial paint remover followed by

rinsing in petroleum ether.

(3) Cleaning by soaking in a commercial polyurethane solvent followed by

rinsing in petroleum ether.

(4) Cleaning by soaking in the same polyurethane solvent as in

(3) followed by rinsing in ethanol.

(5) Cleaning by soaking in another commercial polyurethane solvent

followed by rinsing in petroleum ether.

None of the above procedures was able to remove all of the foreign material

from the fracture surfaces or from the surfaces of the wires. The above

procedures seemed to work well on some wires but not on others. In some instances, a considerable amount of debris remained on the wires after

cleaning. Much of this debris may have been the partially loosened

polyurethane batter that had been used to coat the wires before assembly of

the tension-torsion strap.

4. RESULTS OF THE EXAMINATION

4.1 Fractographic Examination

In order to have a fracture produced under known conditions for comparison

with the field fractures, a length of wire was taken from the region of the

failed strap identified by the letter A in figure 1 and fractured in the NBS

laboratory by pulling in tension with pliers. Scanning electron microscope

(SEM) fractographs of the fracture are shown in figure 3. The fracture is

quite ductile with dimpled rupture being the predominant fracture mode.

Significant necking is evident adjacent to the fracture. There is a considerable amount of debris on this fracture surface, especially for a fresh, laboratory-produced fracture. Most of this debris is probably from the

polyurethane coating from the wire surface that stretched and loosened when

the wire was pulled in tension.

A number of randomly selected field wire fractures from two separate locations

on the failed strap (identified by letters A and B in figure 1) were taken for

examination. As received at NBS, all the wires in the strap had failed at

location B, whereas only some of the wires had fractured at location A. Each

fracture was cleaned by one of the procedures listed in Section 3 above and

then examined with the SEM in order to characterize the fractures. Fracture

profiles and fractographs of many of these wires are shown in figures 4

through 25. Each of the wires examined was given a number designation which

appears in the figure legend (such as 4-13 in figure 3). These designations

are only for convenience during the examination and have no significance with

respect to location in the assembly. The fractures shown in these figures

have been classified into several groups according to fracture features.

Eight of the fractures (figures 4 through 11) have characteristics of cup-cone

or a combination of cup-cone and shear fracture. The predominant fracture

mode in at least seven of these eight fractures is dimpled rupture which is

indicative of ductile overload failure. The fracture mode for the eighth

specimen was unclear due to the presence of foreign material on the fracture

surface. Although there is considerable variation in the amount of necking

associated with the fractures of these eight specimens, there was significant

necking associated with all of them. Necking is another indication of ductile

overload failure.

2
One wire fracture (figure 12) is relatively flat with a small amount of

necking. There are numerous small holes on the fracture surface. The

fracture mode is unclear.

Eight of the wires (figures 13 through 20) exhibit apparent shear fractures.

For two of these fractures (figures 13 and 14), there was considerable

necking. Dimpled rupture was the predominant fracture mode for at least one of the fractures. The fracture mode for the other one is unclear. There was

some necking associated with the fractures shown in figures 15 through 19,

although less than with the fractures shown in figures 13 and 14. Dimpled

rupture is the predominant fracture mode for at least three and possibly four

of these five specimens. The fracture mode is unclear for the fifth specimen

(figure 17), although there is some evidence of striations. Striations

normally are an indication of a time-dependent fracture mechanism such as fatigue or stress corrosion cracking. The fracture shown in figure 20

exhibits little if any necking, but the predominant fracture mode is still

dimpled rupture. There is a large longitudinal crack intersecting the

fractures shown in figures 17 through 20.

The fracture shown in figure 21 appears to be a combination of cup-cone and

shear, but it has a configuration dissimilar to those of the other fractures

examined. The predominant fracture mode is dimpled rupture, although there is some evidence of striations.

In four cases (figures 22 through 25), the fractures have substantial

longitudinal components. These fractures may be somewhat similar to those

shown in figures 17 through 20 where a longitudinal crack intersected the

fracture surface. In the two cases where the mode could be determined

(figures 24 and 25), the predominant fracture mode of the transverse

components of these fractures is dimpled rupture. There is also some evidence

of striations on the transverse components of the two fractures shown in

figures 24 and 25. The fracture mode(s) of the longitudinal components of the

fractures shown in figures 22 through 25 is unclear. These components have a "woody" appearance that may indicate intergranular fracture along the

boundaries between the elongated grains of the wire.

A number of other fractured wires from the failed tension-torsion strap were

examined. All of these fractures fit into one of the classifications

described above.

4.2 Metal! ographic Examination

An etched longitudinal section through wire 1-11 at the fracture shown in

figure 23a is shown in figure 26. Part of the fracture is shown at higher

magnification in figure 27. The fracture is "stepped", being comprised of a series of alternate transverse and longitudinal segments. The microstructure

appears to be highly worked with the grains elongated in the direction

parallel to the longitudinal axis of the wire. The transverse components of

the fracture appear to be transgranular. Even with the high magnification

photomicrograph shown in figure 27, it is not clear whether the fracture path

of the longitudinal components is transgranular or intergranul ar. No

secondary cracking is evident in the section shown in figures 26 and 27.

There are some stringer type inclusions evident. Such inclusions could act as initiation sites for longitudinal cracking. In general, the microstructure

appears typical for a highly worked austenitic stainless steel.

3
4.3 Hardness Measurements

Knoop hardness measurements were made on longitudinal sections through two of

the fractured wires. Measurements were made at loads of 500 and 1000 grams

force. Average hardness values are 663 HK.5 and 592 HK.l. These Knoop values

correspond approximately to Rockwell hardness values of 57 HRC and 53 HRC,

respectively. Approximately equivalent tensile strength values range from

slightly in excess of 300,000 psi to about 273,000 psi.

5. DISCUSSION AND CONCLUSIONS

Of the field fractures described in this report, seven are from location A and

fifteen are from location B, as shown in figure 1. There does not appear to

be any relationship between fracture configuration and location of the

fracture within the tension-torsion strap.

Eight of the 22 wire field fractures shown in this report have substantial

longitudinal cracks or longitudinal fracture components. The longitudinal

component of the four fractures, where it was exposed (figures 22 through 25),

had a "woody" appearance suggesting the possibility of intergranular fracture.

It could not be determined from the metal lographic examination whether the

fracture path was primarily intergranular or transgranular. The transverse

components of these fractures as well as of the four fractures where

longitudinal cracks intersect the fracture surface exhibit dimpled rupture as the predominant fracture mode where the mode can be determined. Even though

dimpled rupture is the predominant fracture mode, there is evidence of

striations on at least four of the eight fracture surfaces. Striations

suggest a time-dependent mechanism such as fatigue or stress corrosion

cracking. Stress corrosion cracking is often accompanied by secondary

cracking, no evidence for which was found in the metal lographic study.

Without discounting the possibility of stress corrosion cracking, fatigue or corrosion fatigue appears more likely to be the time-dependent mechanism

involved, if indeed a time-dependent mechanism is involved at all. Because of

the large longitudinal components of these fractures, it is possible that

torsional stresses may be responsible for these longitudinal components.

Dimpled rupture is the predominant fracture mode for all the wires examined in

this investigation where ever the fracture mode could be determined. Many of

the fractures are similar in appearance to the fracture produced in the N3S

laboratory by essentially uniaxial tensile overload. These fractures had a cup-cone appearance. A number of the other wire fractures appeared to have

occurred in shear. Many of the wires exhibited considerable necking in the

vicinity of the fracture. Both dimpled rupture and necking are indicative of

ductile overload fracture.

A very small percentage of the total wire fractures was examined in this

investigation, but the following hypothetical scenario appears reasonable

based on the results obtained. In certain of the wires, cracks initiated and

propagated essentially longitudinally by a time-dependent mechanism such as fatigue due to the application of torsional stresses. Although the principal

directional component of these cracks is longitudinal, they gradually

propagated transversely in small steps. Eventually, the transverse direction

of propagation dominated. After the crack had propagated far enough in the

transverse direction to reduce the effective cross sectional area of the wire

4
sufficiently so that it could no longer support the applied load, final

failure occurred in ductile overload. Whenever a wire fractured in this

manner, the effective cross sectional area of the whole assembly was decreased, although very slightly. As more and more wires fractured in this

way, however, the applied load eventually became too great for the reduced

cross sectional area to support and the remaining wires fractured in ductile

overload -- hence the large number of ductile overload failures.
I

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!

I
Figure

1.

Failed

tension-torsion

strap

with

the

fractured

wires

as-received

at

NBS.

The

letters

A

and B

indicate

locations

where

wire

fracture

samples

were

taken

for

examination.

X

3/5

Figure

2.

Tension-torsion

strap

with

the

wires

essentially

intact,

as-received

at

NBS.

Much

of

the

polyurethane

molding

has

been

removed.

X

3/5

a. X 210 b. X1050

Figure 3. Wire no. 4-13, location A. This fracture was produced in the

NBS laboratory by pulling the wire in tension. The fracture is

a cup-cone type with significant necking. The primary fracture

mode is dimpled rupture.

a. X 180 b. X 900

Figure 4. Wire no. 4-3, location A, cleaned in petroleum ether. The

fracture is a cup-cone type with significant necking. The

primary fracture mode is dimpled rupture.

- <

N

a. X 190 b. X 950

Figure 5. Wire no. 4-6, location A, cleaned in petroleum ether. The

fracture is primarily cup-cone with considerable necking.

There is a significant transverse component. The predominant

fracture mode is dimpled rupture.

a. X 300 b. X 1400

Figure 6. Wire no. 2-2, location B, cleaned ul trasonically in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint remover

followed by rinsing in petroleum ether. The fracture appears

to be basically cup-cone with some necking. Dimpled rupture is

the predominant fracture mode. There is a considerable amount

of foreign material adhering to the surface of the wire.

Figure 7. Wire no. 1-2, location B, cleaned ul trasonical ly in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint remover

followed by rinsing in petroleum ether. The fracture is

basically cup-ccne with significant necking. Dimpled rupture

is the predominant fracture mode. The fractograph shown in

figure 7c was taken in the area indicated by the arrow in 7a.

There appear to be imperfections or mechanical damage in the

coating or surface of the wire as can be seen in figure 7a.

c. X 2000

Figure 8. Wire no. 2-3, location B, cleaned by soaking in a commercial

polyurethane solvent followed by rinsing in petroleum ether.

The fracture is basically cup-cone. Some necking is evident.

What appears to be the polyurethane batter used to coat the

wire is covering part of the fracture adjacent to the wire

surface (figure 8b). The fractograph shown in figure 8c was taken near the center of the fracture. There appears to be

foreign material on the fracture surface and the fracture mode

is unclear.

b. X 1680

Figure 9. Wire no. 1-6, location B, cleaned ul trasonically in petroleum

ether. The fracture appears to be primarily cup-cone with

considerable necking. Dimpled rupture is the predominant

fracture mode.

a. X 190 b. X 1520

Figure 10. Wire no. 1-5, location B, cleaned ul trasonical ly in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint

remover followed by rinsing in petroleum ether. The fracture

appears to be primarily cup-cone with significant necking

adjacent to the fracture. The primary fracture mode is

dimpled rupture.

a. X 300 b. X 2000

Figure 12. Wire no. 2-1, location B, cleaned by soaking in a commercial

polyurethane solvent followed by rinsing in petroleum ether.

The fracture is relatively flat and is about 90° to the

longitudinal axis of the wire. There are numerous small

holes in the fracture surface. The fractograph shown in

figure 12b was taken at the lower right of the fracture as shown in figure 12a where the concentration of holes appeared

to be greatest. There is very little necking and the

fracture mode is unclear.

Figure 11. Wire no. 2-4, location B, cleaned by soaking in a commercial

polyurethane solvent followed by rinsing in petroleum ether.

The fracture appears to be a combination of cup-cone and

shear with significant necking. The predominant fracture

mode is dimpled rupture. There is a considerable amount of

debris on both the fracture surface and on the surface of the

wi re.

a. X 300 b. X 2000

a. X 180 b. X 900

Figure 13. Wire no. 3-5, location B, cleaned by soaking in a commercial

polyurethane solvent followed by rinsing in ethanol.

Apparent shear fracture with small amount of necking. The

fracture mode is unclear, but the necking indicates some ductility. There is considerable debris on both the fracture

surface and on the surface of the wire.

Figure 14. Wire no. 3-2, location B, cleaned by soaking in a commercial

polyurethane solvent followed by rinsing in ethanol.

Primarily shear fracture with some necking. Dimpled rupture

is the predominant fracture mode. There is considerable

debris on both the fracture surface and on the surface of the

wire.

Figure 15. Wire no. 2-5, location B, cleaned by soaking in a commercial

polyurethane solvent followed by rinsing in petroleum ether.

The fracture is primarily shear with a small amount of

necking. The predominant fracture mode is dimpled rupture.

There is a considerable amount of debris on both the fracture

surface and the surface of the wire.

Figure 16. Wire no. 1-1, location B, cleaned ultrasonical ly in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint

remover followed by rinsing in petroleum ether. Shear

fracture with little or no necking. The fracture mode is

unclear, although there is some evidence to indicate dimpled

rupture.

a. X 160 b. X 1050

Figure 17. Wire no. 1-3, location B, cleaned ul trasonical ly in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint

remover followed by rinsing in petroleum ether. The shear

fracture follows two different planes over about the upper

2/3 of figure 15a. There is a longitudinal crack

intersecting the fracture at the dividing line between the

two fracture planes. The fracture mode is unclear, but there

is some evidence of striations.

a. X 215 b. X 1050

Figure 18. Wire no. 1-7, location B, cleaned ul trasonical ly in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint

remover followed by rinsing in petroleum ether. This is a shear fracture with some necking. There is a longitudinal

crack intersecting the fracture. The predominant fracture

mode is dimpled rupture.

'

:

Figure 19. Wire no. 1-10, location B, cleaned ul trasonically in (1)

' petroleum ether, (2) amyl acetate, (3) commercial paint

remover followed by rinsing in petroleum ether. This is an apparent shear fracture with a small amount of necking. The

predominant fracture mode is dimpled rupture.

a. X 215 b. X 1125

Figure 20. Wire no. 4-7, location A, cleaned in petroleum ether. The

fracture is primarily shear with little necking. The

predominant fracture mode is dimpled rupture. A

longitudinal crack intersects the fracture. There is

considerable debris on the fracture surface.

Figure 21. Wire no. 4-5, location A, cleaned in petroleum ether. The

fracture appears to be a combination of cup-cone and shear

with some necking. The predominant fracture mode is dimpled

rupture, although there is evidence of striations. Some of

the apparent striations are actually elongated dimples.

a. X 190 b. X 950

Figure 22. Wire no. 4-2, location A, cleaned with petroleum ether. The

longitudinal part of the fracture has a woody appearance.

The nearly transverse part of the fracture is covered with

debris and the features can not be seen.

c. X 1200

Figure 23. Wire no. 1-11, location B, cleaned ul trasonical ly in (1)

petroleum ether, (2) amyl acetate, (3) commercial paint

remover followed by rinsing in petroleum ether. The

longitudinal part of the fracture has a woody appearance

(figure b). For the most part, the transverse portion of the

fracture (figure c) appeared to be covered with a foreign

substance; therefore, the fracture mode was not determined.

The foreign material is likely the polyurethane batter used

to coat the wires at the time of assembly during manufacture.

k

9 j ' /

/

i r f.

: r '

i , ^r ; i am , 5;

f f

' ••/r/

/;'Vf

/

/

;

, /' U ifc.lt

J

' f

wlfiu! Vi; ;/ k

a t'lKi k }. jit

c. X 1360

Figure 24. Wire no. 5-1, location A, cleaned in commercial polyurethane

solvent and rinsed in ethanol. The fracture mode of the

transverse part of the fracture in the vicinity of arrow b is

dimpled rupture (figure 24b). There is a hint of striations

in the left part of figure 24b. The longitudinal component

of the fracture has a woody appearance, (figure 24c)

c. X 940

Figure 25. Wire no. 5-2, location A, cleaned in commercial polyurethane

solvent and rinsed in ethanol. There is some dimpled rupture

on the nearly transverse component of the fracture (figure

25b). There is also evidence of striations in some areas of

this part of the fracture. The longitudinal component of the

fracture exhibits considerable mechanical damage and has a somewhat woody appearance where the features have not been

obi iterated.

Figure 26. Longitudinal section through wire shown in figure 23

exhibiting the fracture profile. The longitudinal axis of

the wire is vertical.

Etchant: 4% picral + 1% HCL X 500

Figure 27. Part of the field shown in figure 26 above, but at higher

magnification.

Etchant: 4% picral + 1% HCL X 2500

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 83-2774 November 1983

4. TITLE AND SUBTITLE

Examination of Failed Tension-Torsion Strap from Bell 206B, N90071 , Helicopter

5. AUTHOR(S)

T. Robert Shives

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instruction s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

National iransportation Safety Board

Washington, D.C. 20594

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The National Transportation Safety Board submitted a failed tension torsion strap

to the NBS Fracture and Deformation Division for examination. The strap consisted

of more than eight thousand 0.0058 inch diameter AM 355 stainless steel wires

wrapped around 17-7PH stainless steel spools at either end of the assembly. The

unit was molded in polyurethane. All of the wires had fractured. NBS was requested to characterize the fractures. It appeared that in certain of the

wires, cracks had initiated and propagated essentially longitudinally by a timedependent mechanism such as fatigue due to the application of torsional stresses.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

AM355 stainless steel; helicopter; tension-torsion strap; time-dependent fracture

mechanism.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

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