Failure Investigation of an Aircraft Crankshaft Gear Connection … · 2017-08-23 · Failure...

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CASE HISTORY—PEER-REVIEWED Failure Investigation of an Aircraft Crankshaft Gear Connection Michael Stevenson David Klepacki Jeff McDougall Dale Alexander Submitted: 4 October 2012 / Published online: 7 November 2012 Ó ASM International 2012 Abstract Improper assembly of an aircraft crankshaft can have serious consequences. If an adequate joint clamping force is not applied to the connection between the crank- shaft and crankshaft gear during assembly, relative motion in the system could create flexural loads on connection components, and cause damage such as cyclic fatigue cracking, shear overstress fracture, and plastic deformation. Many factors can contribute to insufficient joint clamping, including poor joint seating, the presence of a foreign object on the faying surface, and failure to apply proper torque during assembly. This paper reviews a case involving a crankshaft gear connection, which separated while the subject aircraft was in flight, causing the engine to fail and the aircraft to crash. To determine the root cause of the failure, a metallurgical analysis was performed. Keywords Crankshaft gear connection Cyclic fatigue cracking Joint clamping Background A metallurgical failure analysis was conducted on a sepa- rated crankshaft gear connection from an aircraft engine involved in a non-fatal crash into a river. At the request of the owner/pilot, the aircraft engine, which was modified for increased horsepower, had been recently disassembled and inspected. The inspection was prompted by a propeller strike that occurred when the landing gear failed. The aircraft had logged *10 h of flight time since the engine teardown/inspection. Introduction Components of the subject aircraft available for examina- tion included the subject crankshaft, the crankshaft gear, the lockplate, the crankshaft bolt, and the fractured remnant of the crankshaft dowel. A photograph of the components in the as-received condition is shown in Fig. 1. The investi- gation was limited to non-destructive analyses, including visual examination and light microscopy, dimensional evaluation (which confirmed critical dimensions), scanning electron microscopy (SEM), and energy dispersive spec- troscopy (EDS). Visual and Light Microscopic Examination The following observations were made during visual and stereomicroscopic examination of the components: The crankshaft bolt showed extensive damage, includ- ing plastic deformation of the body, shearing of the apparently engaged threads, and damage to the wrench flats (Figs. 2, 3). The lock plate appeared distorted (Fig. 4) and was fractured in two locations (Figure 5). The end of the crankshaft contained a fractured section at the large end of the dowel (Fig. 6). The large end of the crankshaft dowel was apparently recessed below the crankshaft counterbore surface (Fig. 7). M. Stevenson (&) D. Klepacki J. McDougall D. Alexander Engineering Systems Inc., 4215 Campus Drive, Aurora, IL 60504, USA e-mail: [email protected] 123 J Fail. Anal. and Preven. (2012) 12:617–623 DOI 10.1007/s11668-012-9625-6

Transcript of Failure Investigation of an Aircraft Crankshaft Gear Connection … · 2017-08-23 · Failure...

CASE HISTORY—PEER-REVIEWED

Failure Investigation of an Aircraft Crankshaft Gear Connection

Michael Stevenson • David Klepacki •

Jeff McDougall • Dale Alexander

Submitted: 4 October 2012 / Published online: 7 November 2012

� ASM International 2012

Abstract Improper assembly of an aircraft crankshaft can

have serious consequences. If an adequate joint clamping

force is not applied to the connection between the crank-

shaft and crankshaft gear during assembly, relative motion

in the system could create flexural loads on connection

components, and cause damage such as cyclic fatigue

cracking, shear overstress fracture, and plastic deformation.

Many factors can contribute to insufficient joint clamping,

including poor joint seating, the presence of a foreign

object on the faying surface, and failure to apply proper

torque during assembly. This paper reviews a case

involving a crankshaft gear connection, which separated

while the subject aircraft was in flight, causing the engine

to fail and the aircraft to crash. To determine the root cause

of the failure, a metallurgical analysis was performed.

Keywords Crankshaft gear connection �Cyclic fatigue cracking � Joint clamping

Background

A metallurgical failure analysis was conducted on a sepa-

rated crankshaft gear connection from an aircraft engine

involved in a non-fatal crash into a river. At the request of

the owner/pilot, the aircraft engine, which was modified for

increased horsepower, had been recently disassembled and

inspected. The inspection was prompted by a propeller

strike that occurred when the landing gear failed. The

aircraft had logged *10 h of flight time since the engine

teardown/inspection.

Introduction

Components of the subject aircraft available for examina-

tion included the subject crankshaft, the crankshaft gear, the

lockplate, the crankshaft bolt, and the fractured remnant of

the crankshaft dowel. A photograph of the components in

the as-received condition is shown in Fig. 1. The investi-

gation was limited to non-destructive analyses, including

visual examination and light microscopy, dimensional

evaluation (which confirmed critical dimensions), scanning

electron microscopy (SEM), and energy dispersive spec-

troscopy (EDS).

Visual and Light Microscopic Examination

The following observations were made during visual and

stereomicroscopic examination of the components:

• The crankshaft bolt showed extensive damage, includ-

ing plastic deformation of the body, shearing of the

apparently engaged threads, and damage to the wrench

flats (Figs. 2, 3).

• The lock plate appeared distorted (Fig. 4) and was

fractured in two locations (Figure 5).

• The end of the crankshaft contained a fractured section

at the large end of the dowel (Fig. 6).

• The large end of the crankshaft dowel was apparently

recessed below the crankshaft counterbore surface

(Fig. 7).

M. Stevenson (&) � D. Klepacki � J. McDougall � D. Alexander

Engineering Systems Inc., 4215 Campus Drive,

Aurora, IL 60504, USA

e-mail: [email protected]

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J Fail. Anal. and Preven. (2012) 12:617–623

DOI 10.1007/s11668-012-9625-6

• The dowel fracture surface was consistent with unidi-

rectional bending fatigue [1], with the origin of the

fracture oriented near the outer diameter of the

crankshaft (Fig. 8).

• The surface of the dowel associated with the crankshaft

gear dowel hole appeared battered from repetitive

contact loading (Fig. 9).

• One of the crankshaft gear dowel holes appeared

battered in a way that is consistent with the damage

observed on the dowel, indicating that this hole was

Fig. 2 Comparison of subject and exemplar crankshaft bolts

Fig. 3 Stereomicroscope photographs of subject crankshaft gear wrench flats

Fig. 4 Photograph of subject lockplate and comparison to exemplar

lockplate

Fig. 1 Components received for inspection

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engaged with the dowel prior to the separation of the

crankshaft gear from the crankshaft (Fig. 10).

• The forward surface of the crankshaft gear exhibited a

burr/lip near the lobe containing the apparently unused

dowel hole (Fig. 11).

Scanning Electron Microscopy and Energy

Dispersive Spectroscopy

SEM and EDS were performed on the crankshaft gear bolt,

the lock plate, the dowel fragment fracture surface, and the

Fig. 5 Satellite view of

fractured locations on lockplate

Fig. 7 Stereomicroscope photograph of dowel fracture surface

remnant contained in crankshaft

Fig. 6 Annotated photograph of crankshaft end

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crankshaft gear. The following observations were made

during the SEM and EDS examinations:

• The dowel fracture mechanism, macroscopically iden-

tified as unidirectional bending fatigue, exhibited

fatigue striations when examined at magnifications of

25–5,0009. These features (Fig. 12) are consistent with

high-cycle fatigue cracking in steel components. The

high-cycle fatigue cracking is consistent with a total

engine operating time of 10 h at 2,500 RPM or 1.5

million cycles.

• The fracture surfaces on the lockplate exhibited

features consistent with high-cycle fatigue (Fig. 13).

Fig. 10 Satellite view of

crankshaft gear dowel holes

Fig. 8 Stereomicroscope photograph of dowel fracture surface on

dowel fragment Fig. 9 Representative stereomicroscope photograph of dowel surface

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Fig. 13 Representative SEM micrograph of lockplate fracture surface

Fig. 12 Representative SEM micrographs of dowel fracture surface

Fig. 11 Satellite view of burr/lip on crankshaft gear forward surface

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• The thread fractures on the crankshaft bolt were

consistent with shear overstress (Fig. 14).

• The observed burr/lip on the forward surface of the

crankshaft gear was measured using iterative SEM

focusing. The approximate height of the burr/lip was

0.00200 (Fig. 15).

Analysis

The damage pattern displayed on the crankshaft/crank-

shaft gear connection components indicates multiple

instances of cyclic fatigue loading, including fractures of

the dowel and lockplate and cyclic contact damage of the

dowel and crankshaft gear dowel hole. This damage could

only occur in the absence of appropriate clamping force in

the bolted joint [2–4].

A review of the overhaul manual and associated ser-

vice bulletins revealed explicit warnings about the critical

nature of developing an appropriate clamping force in this

connection. The service bulletin also indicates that

improper assembly, including the use of worn or damaged

parts, can result in damage to the crankshaft gear and

counterbored recess, and badly worn or broken gear

alignment dowels. The service bulletin warns that proper

inspection and reassembly of these parts is mandatory,

since a failure of the gear or gear attaching parts would

result in engine failure.

Logbook entries for the subject aircraft, specifically

those addressing inspections and maintenance performed

just prior to the accident do not indicate compliance with

the warnings in the service bulletin or the overhaul manual.

Fig. 15 SEM micrographs of burr/lip observed on crankshaft gear rear surface

Fig. 14 Representative SEM micrograph montage of bolt thread

damage

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Absent from the journal entry was any specific information

about the final bolt torque procedures and disposition of the

crankshaft bolt and lock plate after the initial gear

installation.

Potential causes of inadequate clamping force develop-

ment in the crankshaft/gear connection include but are not

limited to:

• Presence of foreign debris, burrs, or other asperities on

the faying surface during installation.

• Failure to apply proper torque during installation,

below or above the manufacturers torque of 204 in-lbf.

• Possibility that the bolt was used a second time after the

crankshaft gear was re-indexed. The re-indexing was

done with the engine installed in the airframe, which

would have made it difficult to inspect the reassembly

and ensure that the gear was seated properly. In

addition, reuse of the bolt compromises the cadmium

plate and reduces the clamp up force by increasing the

tightening friction, giving an artificial clamping force

for the indicated torque wrench reading.

• Failure to properly seat the crankshaft gear against the

crankshaft counterbore surface prior to torque application.

Discussion

Based on the available information, it was not possible to

determine the exact nature of the discrepancy that led to

the loss of an appropriate clamping force between the

crankshaft components in this aircraft. However, the evi-

dence of a burr/lip on the crankshaft gear forward surface,

and the interference fit between the crankshaft and the

crankshaft gear both implicate the final torque procedure

applied when the crankshaft gear was installed.

The presence of the burr/lip would result in a reduction

of the clamping load because of the additional force

required to compress the burr against the crankshaft. In

addition, as the burr frets the initial joint, clamp up loading

will be significantly and rapidly reduced.

The existence of an interference fit magnifies the impor-

tance of proper seating before final tightening, because

torques applied prior to seating of the gear against the

crankshaft would act to seat the gear instead of developing an

adequate joint clamping force. Any failure to confirm that the

crankshaft gear was properly and fully seated during the

torque-up process would result in a lower clamp load and

potentially, in a false bolt torque.

It is important to note that the observed damage to the

connection components could only occur if the joint

clamping force is lost and relative motion in the system is

possible. The physical evidence clearly indicates that this

relative motion took place prior to the separation of the

joint. Further, the observed fatigue fractures could not

have occurred unless relative motion between the compo-

nents (allowing flexural loads to be applied to both the

lockplate and the dowel) existed in the system.

Conclusions

Results of the metallurgical failure analysis of the aircraft

engine components were as follows:

(1) The subject crankshaft dowel failed as a result of

cyclic fatigue cracking, which resulted from appli-

cation of unidirectional bending.

(2) The subject crankshaft bolt lockplate exhibited

damage consistent with cyclic fatigue cracking.

(3) The subject crankshaft bolt exhibited gross plastic

deformation and shear overstress fracture of the

engaged threads.

(4) The damage pattern observed on components com-

prising the subject crankshaft/crankshaft gear con-

nection could only occur if there was loss of proper

joint preload. The probable cause of the preload loss

was improper assembly of the connection.

This case study clearly highlights the importance of

developing an appropriate clamping force when assem-

bling the crankshaft/crankshaft gear connection. Though it

was not possible to determinke the exact reason for the loss

of clamping force that led to the failure in this aircraft, the

observed damage on the crankshaft components was con-

sistent with the damage that would result from flexural

loads caused by relative motion between the components in

the system.

References

1. ASM: Fatigue Fracture Appearance, ASM Handbook, vol. 11,

pp. 627–640. ASM, Washington (2002)

2. O’Brien, M.J., Metcalfe, R.G.: High strength engineering fasteners:

design for fatigue resistance. J Fail Anal Prev 9, 171–181 (2009)

3. Wulpi, D.: Understanding How Components Fail, 2nd edn,

pp. 141–145. ASM International, Materials Park (1999)

4. Bickford, J.: An Introduction to the Design and Behavior of Bolted

Joints, 3rd edn, pp. 3–13. Marcel Dekker, New York (1990)

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