NBSIR 74-441

Fractographic Examination of Failed

Helicopter Tail Rotor Hub Yoke

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington. D. C. 20234

January 1974

Failure Analysis Report

Prepared for Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington. D. C. 20591

NBSIR 74-441

FRACTOGRAPHIC EXAMINATION OF FAILED

HELICOPTER TAIL ROTOR HUB YOKE

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1974

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

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

Fractographic Examination of Failed

Helicopter Tail Rotor Hub Yoke

Reference : Bureau of Aviation Safety, National Transportation

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

Request by Mr. Michael L. Marx, dated August 29, 1973.

Accident Information ; Brantly Helicopter Model 305,

registration number N2203V. The accident occurred on

May 22, 1973, in Denver, Colorado.

Part Submitted : One fractured tail rotor hub yoke as shown

In figure T~. Purpose : The Bureau of Aviation Safety requested a fractographic examination of the failed yoke in order to determine

the mode of fracture.

Results of Examinations : Visual and macroscopic examinations

of the fracture surface indicated that two crack fronts had

formed, and had originated almost diametrically opposite each

other at the outside surface of the yoke. "Ratchet marks"

on the fracture surface indicated that each crack front had

formed by the joining of several separate cracks. The

general regions of crack initiation are indicated by A and B

in figure 2, which shows one of the fracture faces. The

crack front originating in region A propagated slowly to a depth of about one millimeter, while the one originating in

region B propagated slowly to a much greater depth before

final rapid failure occurred in region C (figure 2)

.

Examination of the fracture surface with the scanning electron

microscope revealed strong evidence of fatigue striations in

one area of the fracture crack originating in region B. An

example is shown in figure 3. Other evidence of fatigue

striations, although not as strong as that shown in figure 3,

was found in other areas of the fracture crack originating

in region B. Examples are shown in figures 4 and 5.

Weak evidence of fatigue striations was found in areas of the

fracture crack originating in region A, an example of which

is shown in figure 6. Several large inclusions were found

on the fracture surface near region A, but none of those

observed appeared to be associated with a crack origin.

One such inclusion is shov^n in figure 7.

Figures 8 and 9 show areas in the final fracture region

(region C, figure 2) . The principal fracture feature is

dimpled rupture, but there appears to be a significant

amount of porosity-. There also appears to be a number of

"holes" where inclusions may have been.

Discussion and Conclusions : This tail rotor hub yoke

appears to have failed due to the propagation of two almost

diametrically opposed crack fronts to sufficient depths so

that the reduced cross section of the part could no longer

sustain the applied load, resulting in final fracture due

to overload. There is evidence that the deeper crack

front propagated in fatigue, and, while the evidence is

less strong, the shallower crack probably also propagated

in fatigue. A number of large inclusions were found on

the fracture surface, but none of these appeared to be

associated with a crack initiation site. A significant

amount of porosity was evident, particularly in the

overload region of the fracture.

Figure 1. Tail rotor hub yoke as received.

Figure 2. One of the fracture faces. A and B

are regions of crack initiation. C

is region of overload. X 4

Figure 3. Scanning electron photomicrograph

showing strong evidence of fatigue

striations on the fracture crack

initiating in region B. X 1000

Figure 4. Scanning electron photomicrograph

showing evidence of fatigue

striations (arrow) on the fracture

crack initiating in region B. X 475

Figure 5. Scanning electron photomicrograph

showing evidence of fatigue

striations (arrows) on the fracture

crack initiating in region B. X 1800

Figure 6. Scanning electron photomicrograph

showing weak evidence of fatigue

striations (arrow) on the fracture

crack initiating in region A. X 1100

Figure 7 . Scanning electron photomicrograph

showing large inclusion on the

fracture surface near region A.

X HOO

Figure 8. Scanning electron photomicrograph

of overload area of the fracture

showing porosity and "holes" where

inclusions may have been. Dimpled

rupture is the primary fracture

feature. X 100

Figure 9. Scanning electron photomicrograph

of overload area of the fracture

showing porosity and "holes" where

inclusions may have been. Dimpled

rupture is the primary fracture

feature. X 200

NBS-1 14A iREv. ~- -• ,1

U DLPT . C^' CCMM. BIBLIOGRAPHIC DATA

SHEF.T

1. l-l \ ! ION I )i< Kl- i'OK r NBSIR 74-441

2. Cuiv 't Accession

No.

3. Kccipunt's Accession N.i.

4. "IT 1T.1-: A\P -Li 1 1 IT.I FRAuTOGRAPHIC EXA.^INATION Ox' r AILED HELICOPTER

TAIL ROTOR HUB YOKE

5. I'uhiicition Date January, 1974

6. Performing Drgan i ;'.ii ion Loie'

7. AU I HOK Si T. Robert Shives

8. Performing Organ. I<e;>;'rt No. NBSIR 74-441

9. PKRF Oi.Mi.W, Oi^.v: AN riON XASU: AND AlMM<i:SS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project Task \\ork L'n;'. No. 3120412

11. ^-Ontract Gtcint No

12. >ponsormj; (^rg.ui kTi X.iir.i .irij C .-v.ir-ktc AJJress (Street. City. Stare, ZIP)

Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washinqton , D. C. 20591

13. 1 vpe of Report t-v: Period Covered Failure

Analysis Report

1 14. Sponsoring .^genc\ <.-i;Je

15. SL ;n^i.i:N!!:\ 1 .\K\ xori-.s

16. Ali>T i\.A(, 1 fA J00-:vord or /ess tactual summary olm,ost significant infonnation. If document includes a significant

bibliographv or literature survey, mention it here.) A fractographic examination of a failed helicopter tail rotor

hub yoke revealed that apparent fatigue crack fronts had initiated

at the surface of the yoke at two nearly diametrically opposed

regions. These cracJc fronts propagated until the cross section

of the part was reduced sufficiently so that it: could no longer

sustain the load to which it v.-as subjected and final failure

occurred due to ductile overload. Some large inclusions, none

of v/hich appeared to be associated with a crack origin, were found

on the fracture surface. Porosity was also found on the fracture

surface

17. Kh"l \XOI\!)> (s;x to twelve entries; alphabetical order, capitalize only the first letter of ;/ic> first key word unless a proper name: Si r.a rated by semicolons)

Dimpled rupture, fatigue crack, f ractographic examination, metal

fatigue, striaticns.

18. A\- Ml-AiTl-ITY I ; n 1 1 m 1 1 e d

19. Si^CLRIT Y CLASS

(THIS REPORT)

21. NO. Ot- l^AcFS

-X F.r DiTTcial Dis- rilnition. Oo .Not Kcleaso to N'TIS UNCLASSIFIi-:n

9

i u

'A Jer I roir. Sup.

isi;:p.,-'on, H.l

of Hoc, I'.S. ('i>'Vernment Printiiii; Clffice 20. Si:c LRIT ^- CLASS

( T MIS IWOL)

22. Price

O: ,:er 1 -o,n Na-.• nal lec'inical Inr^irm ition Service (N'T IS)

.\- 1 1 UNf I.ASSlI'U-.n USCOMM-DC