NBSIR 73-150

Examination of Compressor Parts from

Allison Engine from Fairchild

Hiller Helicopter

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

Na:tional Bureau of Standards

Washington, D. C. 20234

March 30, 1973

Failure Analysis. Report

Prepared for Bureau of Aviation Safety

National Transportation Safety Board

Department of Transportation

Washington. D. C. 20591

NBSIR 73-150

EXAMINATION OF COMPRESSOR PARTS FROM

ALLISON ENGINE FROM FAIRCHILD

HILLER HELICOPTER

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

March 30, 1973

Failure Analysis Report

"This document has been prepared for the use of 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."

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

IMATIOMAL BUREAU OF STANDARDS, Richard W. Roberts. Director

TABLE OF CONTENTS

Page

SUMJyiARY iii

1. GENERAL INFORMATION 1

1.1 Reference 1

1.2 Accident Identification 1

1.3 Parts Identification 1

1.4 Background Information 1

2. PURPOSE ,

..

2

3. RESULTS OF EXAMINATIONS 2

3.1 Visual Examination 2

3.2 Fractographic Examination 4

3 . 3 Metallographic Examination 4

3.4 Qualitative Chemical Analyses 5

3.5 Hardness Measurements 6

4. DISCUSSION OF RESULTS 6

5. CONCLUSIONS 7

6. ACKNOWLEDGEMENT " 8

REFERENCE

.

8

FIGURES

1. Compressor case, fifth and sixth stage wheel assembly,

and a fragment of a compressor blade and a stator

blade as received 9

2. Open compressor case as received 10

3. Half of open compressor case shown in figure 2b 11

4. Fifth and sixth stage compressor wheel assembly

as received 12

i

Page

FIGURES (Continued)

5. The separated portions of a stator blade and a compressor blade 13

6. Fracture surfaces of the three fractured

compressor blades 14

7. Scanning electron photomicrographs of parts of

fracture surfaces 15

8. SEM of fracture surface of blade F in slowly

propagating crack region • 16

9. SEM of fracture surface of blade F in slowly

propagating crack region 17

10. SEM of fracture surface of blade G in slov/ly

propagating crack region 18

11. SEM of fracture surface of blade G in slowly

propagating crack region 19

12. SEM in overload region of fracture surface of

blade F 20

13. SEM of fracture surface of blade H 21

14. Unetched microstructure of cross section through

blade G 22

15. Unetched microstructure of cross section through

blade F 23

16. Unetched microstructure of cross section through

blade G 24

17. Etched microstructure of cross section through

blade F 25

18. Etched microstructure of cross section through

blade G 26

19. Etched microstructure of cross section through

blade H 27

20. Etched microstructure of cross section through blade

G 28

ii

SUMMARY

The compressor case and the fifth and sixth stage

compressor wheel assembly from an Allison helicopter engine

were submitted for examination. The compressor case lining

had been wiped in areas opposite the first five stages of

the compressor blades. The compressor case lining was

severely damaged in the areas opposite the fifth and sixth

compressor wheel stages and in the fifth and sixth stator

blade stages. The lining was cracked in places; wiped,

gouged, or galled in others; and in some areas, pieces of the

lining were missing. The fifth and sixth stage compressor blades

and stator blades were badly damaged. Two of the sixth stage

and one of the fifth stage compressor blades had completely

fractured. The fifth stage blade and one of the sixth stage

blades had failed due to the propagation of fatigue cracks.

The other fractured sixth stage compressor blade exhibited

considerable deformation adjacent to the fracture surface and

had apparently failed from tensile overload. The extensive

dam.age to the case lining and to the blades in and around the

fourth, fifth, and sixth stage of the case and the fifth and

sixth stage of the wheel may have been caused by motion of one

or more of the fractured blades or a foreign object in the

operating compressor. The microstructure of the compressor

blade material appeared to be normal, except perhaps for a

high inclusion content. The hardness of the compressor

blade material was close to, and in some areas possibly

below, the specified minimum.

iii

Examination of Compressor Parts from Allison

Engine from Fairchild Hiller Helicopter

1. GENERAL INFORMATION

1 . 1 Reference

Bureau of Aviation Safety, National Transportation Safety

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

Request by Mr. William L. Holshouser, dated October 4, 1972.

1 . 2 Accident Identification

Fairchild Hiller FHllOO, N590FH, helicopter. North Bend,

Washington, July 19, 1972. NTSB accident no. SEA 73-A-SOOl.

1 . 3 Parts Identification

The following parts were submitted for examination:

1. Compressor case.

2. Fifth and sixth stage compressor wheel assembly.

3. Plastic envelope containing fragments of a stator

blade and a compressor blade.

The parts are shown as received in figure 1.

1 . 4 Background Information

The information in this section was taken from documents

furnished by the Bureau of Aviation Safety.

According to drawing no. 6873965 of the Allison Division

of the General Motors Corporation, the fifth and sixth stage

wheel assem.bly was to be cast from AI^^S 5355 CRES , which is a precipitation hardening alloy steel. The material is

described in Aerospace Material Specification (AMS) 5355B, a

copy of which was furnished to NBS by the Bureau of Aviation

Safety

.

The aircraft had suffered engine failure while being used

in a timber survey. Upon examination after the failure, it

was noted that the first stage compressor vanes could not be

turned by hand. The compressor was disassembled, revealing

extensive damage to the fifth and sixth stage wheel blades,

and to the case lining and the stator blades. The engine had

logged a total of 1206.5 hours, and had been in service 318

hours since a new fifth and sixth stage wheel assembly and a new case assembly had been installed.

- 1 -

2 . PURPOSE

The submitting agency requested an analysis of the

failure of the missing blades and docum.entation of the other

damage to the compressor parts.

3 . RESULTS OF EXAMINATIONS

3 . 1 Visual Examination

There was no apparent damage to the outside of the compressor case. The case had been painted and the paint appeared

to be in good condition. Part of the outside of the case can

be seen in figure 1 where the case is shown closed.

There was considerable damage inside the compressor case

both to the case lining and to some of the stator blades.

The two halves of the open compressor case are shown in

figures 2a and 2b. The lining on the inside of the case

appears to have been wiped betv/een the end of the case and

the first stator blade stage, and between the first and second,

second and third, third and fourth, and fourth and fifth

stator blade stages. These regions would be opposite the

first, second, third, fourth, and fifth stages, respectively,

of the compressor blades when the compressor is assembled for

operation. In figure 2, wiped areas generally appear darker

than adjacent unwiped areas. Some examples of wiped areas

are indicated by arrows A in figures 2a and 2b. One wiped

area beitween stator blade stages one and two is shown at

higher magnification in figure 2c.

Between the third and fourth stator blade stages, the

lining was cracked and had begun to pull away from the case

(arrow B, figure 2b) . On the half of the case shov/n in

figure 2a, there are cracks in the lining at the edges of

some of the stator blades, and there are two cracks parallel

to the fourth stage (arrows C, figure 2a) . One of these

cracks spans seven blades. Varying amounts of the lining are

missing between several pairs of the fourth stage blades

exposing the metal case (arrows D, figure 2b)

.

Severe damage is exhibited by the case lining between the

fourth and fifth, and the fifth and sixth stator stages. In

some areas, the bare metal case was exposed (arrows E, figure

2b) . Where the bare metal is not exposed, there are a num.ber

of cracks (arrows F, figure 2a) in the lining adjacent to the

fifth stage blades , and the lining had been gouged and/or

galled in a num.ber of places (arrov^s G, figures 2a and 2b) .

- 2 -

There appears to have been no visible damage to any of

the stator blades in the first three stages. The fourth

stage blades in the half of the case shown in figure 2a did

not appear to be damaged, but at least eleven of the fourth

stage blades in the half of the case shown in figure 2b were

either bent or partially torn loose or both near the blade

base on the side closest to the fifth stage. Typical

examples are indicated by arrows H in figures 3a and 3b.

The compressor half shown in figure 2b is also shown in

figure 3a, photographed at a different angle.

Every stator blade in the fifth and sixth stages was

damaged. All were bent and nearly all exhibited tears.

One blade from the sixth stage had broken off (arrow I,

figure 3a)

.

All of the compressor blades of the fifth and sixth

stages of the wheel assembly were damaged. Much of this

damage can be seen in figure 4. The ends of the fifth stage

blades were wiped or "smeared" (arrows A, figure 4) , and

about 1/16 inch of the sides of the blades adjacent to the

blade ends and facing the sixth stage was smeared. This can

not be seen in figure 4, but arrov; B indicates the location

of the damage on one of the blades. The damage was more

severe to the sixth stage blades than to the fifth stage

blades. The ends were smeared (arrows C, figure 4) . There

was smearing and tearing on the sides of the blades facing

the fifth stage (arrov7s D, figure 4) , and there was some

smearing and bending on the other sides of the blades (arrovj^s

E, figure 4). In addition, there was one completely fractured

blade on the fifth stage, and two completely fractured blades

on the sixth stage. Two of these are indicated by the letters

F and G in figure 4. One fractured blade from the sixth stage

cannot be seen in the figure. It is designated H.

The separated parts of one of the fractured compressor

blades and the fractured stator blade are shown in figure 5.

The compressor blade fragm.ent was severely damaged. The

stator blade appeared to have damage similar to that sustained

by other stator blades from the sixth stage. There was a small spot on each, indicated by arrows in figure 5, which

was yellowish-orange in color. Similar spots were found on

other blades.

- 3 -

.3 . 2 Fractographic Examination

The fracture surfaces of the three completely fractured

compressor blades were examined. The fracture surfaces of

blades F and G (location indicated in figure 4) , which are

shown in the photomacrographs of figures 6a and 6b, were

relatively smooth but showed definite evidence of slowly

propagating cracks. The fracture surface of blade H, shown

in figure 6c and not visible in figure 4, exhibited severe

mechanical damage but no evidence of slowly propagating

cracks . The locations of the apparent primary crack origins

are indicated by arrows in figures 6a and 6b.

The fracture surface features of blades F and G can be

more readily seen, on a macro scale, in the scanning electron

photomicrographs (SEM) in figures 7a and 7b, respectively.

The apparent primary crack origins are again indicated by

arrows. There appears to be a number of crack arrest lines

on both fracture surfaces. These are quite evident in

figure 7a.

A scanning electron microscope (SEM) examination at

higher magnifications revealed evidence of apparent fatigue

striations in the slowly propagating regions of the fracture

surfaces of both blades F and G. Examples are shown in

figures 8 and 9 for blade F, and in figures 10 and 11 for

blade G. Features indicating the cast structure can also

be seen in these four figures.

In the overload regions, the fracture surfaces of blades

F and G exhibited dimpled rupture. An example of this is

shown in figure 12 for the fracture surface of blade F.

No evidence of a slowly propagating crack was found on

the fracture surface of blade H. Although mechanical damage

obscured the fracture features in some areas, dimpled rupture

was exhibited over most of the fracture surface (figure 13)

.

3 . 3 Metallographic Exam.ination

Cross sections intersecting the fracture surfaces v/ere

taken through the remaining part of each of the three fractured

compressor blades. The unetched microstructure appeared

similar for the material from all three blades. An unetched

field from the cross section through blade G considered to

have a typical inclusion content for the three cross sections

is shown in figure 14. Two areas judged to have the highest

inclusion contents for the cross sections through blades F

and G are shown in figures 15 and 16, respectively. Nearly

- 4 -

all of the inclusions appear to be of the oxide type. A total

of six fields, including those shown in figures 14 through 16,

plus one typical field each from the cross sections through

blades F and G, and one "worst" field from the cross section

through blade H were rated for inclusion content according to

the inclusion charts given in ASTM Standard E-45. All six

fields were rated 5 for the D thin series, and the field

shown in figure 16 was rated between 1 and 2 for the D heavy

series. There is a relatively large inclusion in the field

shown in figure 15 which does not appear to be of the oxide

type

.

Etched cross sections intersecting the fracture surfaces

of each of the three fractured compressor blades are shown in

figures 17, 18, and 19 for blades F, G, and H, respectively.

Each of the fractures is transgranular in nature. Considerable

deformation is exhibited by blade H adjacent to the fracture.

Part of the cross section through blade G is shown at a higher

magnification in figure 20. The microstructure appears to

consist primarily of tempered martensite with stringer-shaped

patches of ferrite. This would be the expected microstructure

for an austenitized and aged material of the composition

specified in AMS Specification 5355B.-'-

3 . 4 Qualitative Chemical Analyses

Qualitative chemical analyses were performed on the

fracture surface of blade G, on the etched cross section

through blade G, and on one of the yellowish-orange spots on

the end of one of the compressor blades in order to determine

if any significant amounts of elements foreign to this alloy

could be detected. The analyses were carried out with the

non-dispersive X-ray analyzer used in conjunction with the

scanning electron microscope. AMS Specification 5355B

calls for the following chemical composition limits for the

material used in the fifth and sixth stage compressor wheel

assembly:

- 5 -

Element Min % Max %

Carbon 0.06

Manganese 0.7 0

Silicon 0.5 1.0

Phosphorus 0.04

Sulfur 0.03

Chromium 15.5 16.7

Nickel 3.6 4.6

Columbium+Tantalum 0.15 0.4 0

Copper 2.8 3.5

Nitrogen 0.05

No element was detected that was not included in the

specification, and there were no significant differences among

the various analyses.

3 . 5 Hardness Measurements

Knoop microhardness measurements (KHN) at a load of 500

grams were made on cross sections through blades F and H.

For blade F, the average of eight measurements was KHN 3 98

(Rq 40), and the values ranged from KHN 420 (Rc 41.5) adjacent

to the fracture to KHN 373 (Rc 37.5) at the blade base. The

average KHN for six measurements on blade H was 393 (R^ 39)

and the values ranged from KHN 402 {Rq 40) adjacent to the

fracture to KHN 386 (R^ 38.5) at the blade base.

The specified hardness for this material after

precipitation heat treatment is Rq 4 0 minimum or equivalent.

4. DISCUSSION OF RESULTS

The apparent wiping of the compressor case lining

opposite the first, second, third, fourth, and fifth compressor v/heel stages appears to have been caused by contact

with the blades of the rotating compressor wheel. Several

conditions may have permitted the wiping action, such as 1)

the compressor wheel was oversize, 2) the compressor wheel

bearings were worn or off center, allowing the wheel to

wobble, 3) the axes of the case and the wheel were not

coincident, or 4) the compressor case was under size, or not

truly cylindrical. It would seem that the m.ost probable of

these four conditions would be wobbling of the wheel.

- 6 -
I
The severe damage suffered by the compressor case lining

and to both the stator and compressor blades in the area of

the fifth and sixth stages might have been caused by motion

in the assembly of one or m.ore of the fractured blades, or by

motion of a foreign object in the assembly.

Compressor blades F (sixth stage) and G (fifth stage)

appear to have failed due to the propagation of fatigue cracks.

Compressor blade H (sixth stage) appears to have failed from

overload as evidenced by the fracture surface features and the

deformation exhibited in the microstructure near the fracture

surface. The blade (H) also appeared to have been bent.

Although no specification was submitted for inclusion

content, there appeared to be a rather large number of

inclusions in the compressor blade material. Otherwise, the

microstructure of the compressor blade material appeared to

be normal.

The Rockwell C hardness values given are approximate

equivalents converted from the measured Knoop values. The

conversion from KHN to Rq is not exact, and although some

of the converted values were below the specified Rq minimum,

it can not be stated with certainty that the material does

not meet hardness specifications. The lov;est converted value

was 2.5 Rq points below the minimum allowed. The hardness

is generally close to the minimum specified.

Qucilitative chemical analyses (by X-ray emission) of the

material in some of the compressor blades detected nothing

abnormal 1 , all elements detected being included in the

specifications for the material.

5. CONCLUSIONS

1. Two of the three fractured compressor wheel blades (one

from the fifth stage and one from the sixth stage) failed

due to the propagation of fatigue cracks

.

2. The other fractured compressor wheel blade (from the

sixth stage) apparently failed from tensile overload.

3. Wiping of the compressor case lining may have been caused

by worn or off-center compressor wheel bearings.

- 7 -

4. The extensive damage to the fifth and sixth stage

regions of the compressor case lining, and the damage

to the fifth and sixth stage compressor blades and to

the fourth, fifth, and sixth stage stator blades, may

have been caused by motion of either one or more of

the pieces of the fractured blades or a foreign object

in the operating compressor assembly.

5. The inclusion content in the compressor blade material

appears to be somewhat high, though there is no

specification requirement. Otherwise, the microstructure appeared to be normal.

6. The hardness of the compressor blade material was close

to, and perhaps in some instances below, the specified

minimum.

6. ACKNOWLEDGEMENT

Messrs L. C. Smith and C. H. Brady performed the

metallographic specimen preparation, and Mr. Smith assisted

in other areas of the examination.

REFERENCE

1. Metals Handbook, 8th edition, volume 7, American Society

for Metals, 1972.

- 8 -

I

Figure 2. Open compressor case as received.

a. Half of open compressor case. X 1

b. Second half of open compressor case. X 1

c. Example of wiped area of case lining between

the first and second stator stages at higher

magnification than in figure 2b. X 4

The numbers to the left of the case in figures 2a

and 2b designate the positions and numbers of the

mating compressor stages.

Damage to the compressor case lining and to the

stator blades can be seen. Specific locations of

damage are indicated as follows

:

Arrows A indicate examples of areas where the case

lining had apparently been wiped by the

compressor blades.

Arrow B, figure 2b, indicates where the lining is

cracked and is pulling away from the case.

Arrows C, figure 2a, indicate cracks in the

lining parallel to and adjacent to the fourth

stage stator blades.

Arrows D, figure 2b, indicate examples of areas

where the lining is missing between the blades

of the fourth stator stage.

Arrows E, figures 2a and 2b, indicate examples of

areas between the fourth and fifth stator stages

where the lining is missing.

Arrows F, figures 2a and 2b, indicate examples of

cracks in the lining adjacent to the fifth

stage stator blades.

Arrows G, figures 2a and 2b, indicate examples of

gouged or galled areas in the case lining

between the fifth and sixth stator stages.



Figure 2b. Second half of open compressor case. X 1
I
Example of wiped area of case lining between

the first and second stator stages at higher

magnification than in figure 2b. X 4
I
Figure 3a. Half of open compressor case shown in figure 2b.

Examples of damaged fourth stage stator blades

are indicated by arrows H. A missing sixth

stage stator blade is indicated by arrow I.

Damage to the fifth and sixth stage blades can

be seen. X 1
-

1
Figure 3b. Part of the half of the compressor case shown

in figure 3a, but at higher magnification.

Arrows H indicate examples of the damaged

fourth stage stator blades. X 4

Figure 4. Fifth and sixth stage compressor wheel assembly as

received. X 1

The numbers in the upper left indicate the stage number.

Arrows A indicate examples of wiping or smearing of

the fifth stage blades.

Arrow B indicates the location of smearing on the side

of one of the fifth stage blades.

Arrows C indicate examples of smearing of the ends of

the sixth stage blades.

Arrows D indicate examples of tearing on the sides of

the sixth stage blades.

Arrows E indicate examples of bending and smearing on

the sides of the blades opposite to that indicated

by arrows D.

F indicates a fractured sixth stage blade.

G indicates a fractured fifth stage blade.
1

f
Figure 5. The separated portions of a stator blade

(left) and a compressor blade (right)

.

The arrows indicate spots which were

yellowish-orange in color. X 3

•a. b c

.Figure 6- Fracture surfaces of the three fractured compressor

blades. Arrows indicate the apparent primary crack

origins in 6a and 6b. X 10

a. Blade F.

b. Blade G.

c. Blade H.

b

Figure 7 . Scanning electron photomicrographs of

parts of fracture surfaces. Apparent

primary crack origins are indicated

by arrows

.

a. Blade F. Crack arrest lines are

prominant. X 21.5

b. Blade G. Crack arrest lines are

less evident than in a. X 20

Figure 8. SEM of fracture surface of blade F in

slowly propagating crack region showing

evidence of apparent fatigue striations.

X 540

Figure 10. SEM of fracture surface of blade G in

slowly propagating crack region showing

evidence of apparent fatigue striations.

X 1000

Figure 11. SEM of fracture surface of blade G in

slowly propagating crack region showing

evidence of apparent fatigue striations.

X 1200



Figure 14. Unetched microstructure of cross section

through blade G. This field was considered to have an inclusion content

typical of the three cross sections

examined. X 100

*

Figure 15.

Figure 16. Unetched microstructure of cross section

through blade G. This field was considered to have the greatest inclusion

content for this cross section. X 100

Figure 17. Etched microstructure of cross section

through blade F showing fracture profile

at right. Fracture is transgranular

.

Microstructure consists primarily of

tempered martensite and "stringer" shaped

patches of ferrite. Etch: Acidified picral.

X 100
1.
Figure 18. Etched microstructure of cross section

through blade G showing fracture profile

at right. Fracture is transgranular

.

Microstructure consists primarily of

tempered martensite and "stringer" shaped

areas of ferrite. Etch: Acidified picral.

X 100

Figure 19. Etched microstructure of cross section

through blade H showing fracture profile at

right. Fracture is transgranular and there

is considerable deformation adjacent to the

fracture. Microstructure consists primarily

of tempered martensite and "stringer" shaped

areas of ferrite. Etch: Acidified picral.

X 100

•

/ /

o

1 \

>

Figure 20. Etched microstructure of cross section

through blade G at higher magnification

than figures 17 through 19. The elongated

"stringer" shaped areas appear to be

ferrite. The matrix is tempered martensite.

Etch: Acidified picral. X 1000

o=RM NBS-n4A (i.-'i) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

PUBLICATION OR REPOR"

NBSIR 73-150

NO. 2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBHTLE

EXAxMINATION OF COMPRESSOR PARTS FROM ALLISON

ENGINE FROM FAIRCHILD HILLER HELICOPTER

5. Publication Date March, 1973

6. Perfonning Organization Code

'7. AUTHOR(S)

T. Robert Shives

8. Performing Organization

NBSIR 73-150

9 PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projcct/Taslc/ Work Unu No. 3120412

11. Contract/Grant No.

12, Sponsoring Organization Name and Address

Bureau of Aviation SafetyNational Transportation Safety Board

Department of Transportation

Washington, D. C. 205 91

13. Type of Report & Period

Covered Failure

Analysis Report

14. Sponsoring Agency Code

nS. SUPPLEMENTARY NOTES

16. 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 compressor case and the fifth and sixth stage compressor wheel

assembly from an Allison helicopter engine which had failed were

examined. Parts of the compressor case lining had been wiped,

apparently by contact with the compressor blades. This may have

been caused by, among other things, worn or off center compressor

wheel bearings. The fifth and sixth stage regions of the case

lining were severely damaged as were the fifth and sixth stage

compressor and stator blades. The fourth stage stator blades were

damaged to a lesser extent. Two of the three completely fractured

compressor blades had failed in fatigue. The extensive damage to

the case and to the blades may have been caused by motion in the

operating compressor of one or more of the fractured blades or a

foreign object.

•7. KEY WORDS (Alphabetical order, separated by semicolons)

Compressor; fatigue; fatigue fracture; helicopter engine

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UNCLASSIFIED

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28

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