Spring Fatigue Fractures Due to Microstructural Changes in ... · of deformation and fracture,’’ ASM Handbook Volume 11: Failure Analysis and Prevention (ASM International, Materials

CASE HISTORY—PEER-REVIEWED

Spring Fatigue Fractures Due to Microstructural Changesin Service

J. Maciejewski • B. Akyuz

Submitted: 22 November 2013 / Published online: 31 January 2014

� ASM International 2014

Abstract Multiple in-service fractures of torsion springs

were experienced in the same system, which was the sup-

port assembly to the electrical pickup for an electric-

powered vehicle, similar to a subway rail car or electric

trolley car. Scanning electron microscopy and metallo-

graphic examinations determined that the fractures initiated

due to electric arc damage. Intergranular quench cracks at

the spring surface through the transformed untempered

martensite provided crack initiations for fatigue that

propagated during operation.

Keywords Spring fracture � Fatigue � Stray current �Arc damage � Untempered martensite � Quench cracking �Torsion spring

Introduction

Multiple in-service fractures of torsion springs were

experienced in the same system, which was the support

assembly to the electrical pickup for an electric-powered

vehicle, similar to a subway rail car or electric trolley car.

The remainder of the system details is withheld to protect

client confidentiality. The springs were round-wire helical

construction, made from patented music wire (carbon steel)

that was subsequently electro-galvanized.

Analysis

The fracture surfaces consistently exhibited a flat fracture

transverse to the wire at the spring outer diameter (OD) that

extended across half the wire diameter, with the remainder of

the fracture approximately parallel to the wire longitudinal

direction (Fig. 1). This two zone fracture morphology is

typical of fatigue of a spring, wherein the transverse zone is

the fatigue zone and the longitudinal fracture is ductile

overload of the remaining section [1]. However, the origins

at the OD of the springs were unusual, since it is well known

that the highest stress location on a cycling spring is the inner

diameter (ID) [2, 3]. This is the reason fractures of springs at

the end of their fatigue life normally initiate on the ID of the

coil. Therefore, a surface defect or other feature was sus-

pected that would shift the fatigue initiation site to the OD.

A scanning electron microscope (SEM) was used to

examine the samples in more detail at up to 5,0009

magnification. The flat fracture zones at the OD of the

springs exhibited thumbnail-shaped origins with radial

marks extending into the wire, indicating the OD was

indeed the fracture origin (Fig. 2). Detailed examination of

the origins revealed intergranular fracture between equi-

axed, presumably prior austenite, grains that exhibited

decreasing grain size inward from the surface (Fig. 3).

At first, these features would suggest some form of

embrittlement (i.e., possibly hydrogen embrittlement due to

the electroplating operation); however, embrittlement of

prior austenite grain boundaries should not be possible with

cold-drawn music wire. In music wire the ferrite grains and

pearlite colonies are severely deformed, and any prior

austenite grain boundaries are destroyed during the draw-

ing operation. Indeed, longitudinal metallographic sections

of the wires revealed highly deformed ferrite and pearlite,

the normal and expected microstructure for cold-drawn

J. Maciejewski (&)

Materials Testing, Applied Technical Services, Inc., Marietta,

GA, USA

e-mail: [email protected]

B. Akyuz

Failure Analysis and Metallurgy, Applied Technical Services,

Inc., Marietta, GA, USA

123

J Fail. Anal. and Preven. (2014) 14:148–151

DOI 10.1007/s11668-014-9783-9

wire (Fig. 4). However, the longitudinal sections through

the fatigue cracks exhibited a white-etching microstructural

phase at the origins (Fig. 5).

This microstructural feature was untempered martensite,

which could only result from a highly localized, high tem-

perature event, and self-quenching. This is consistent with an

electric arc from stray current in the system. Examination of

the wire surfaces at the OD of the springs did show areas of

remelted material at the fatigue zone origins (Figs. 6, 7).

Stray current electrical discharge damage can occur due to

the component constituting a transmission pathway to

ground for welding currents, improperly grounded machin-

ery electrical energy, or even lightning strikes.

Untempered martensite is a hard, brittle phase with

significant residual stresses that can often lead to inter-

granular cracking immediately after transformation (i.e.,

Fig. 1 Typical spring fracture, showing flat transverse fracture at the

OD (arrow) and adjacent longitudinal fracture

Fig. 2 SEM image showing the thumbnail zone at the crack initiation

site on the OD

Fig. 3 Detail of a fracture origin, showing intergranular fracture and

decreasing grain size inward from the surface

Fig. 4 Longitudinal microstructure of a typical spring wire, exhib-

iting highly deformed ferrite (white) and pearlite (dark), Nital etch

Fig. 5 Longitudinal section through a fatigue crack origin, showing

white-etching untempered martensite at the initiation, as well as

highly deformed ferrite and pearlite. The longitudinal direction is

indicated

J Fail. Anal. and Preven. (2014) 14:148–151 149

123

quench cracking) and cracking from loads in service [4].

The reaustenitized volume caused by stray current arc

damage also explains the varying prior austenite grain size

observed in Fig. 3. During the event there would be a

significant temperature gradient in the arced zone resulting

in the most grain growth at the hottest point (the exterior

surface). The temperature gradient is estimated by: at least

1500 �C (melting) at the surface to \500 �C (no recrys-

tallization) at a point 0.081 mm deep beneath the surface

(from Fig. 5), which is [12,300 �C/mm.

The remainder of the transverse fracture zone area

exhibited a feathery or mottled morphology (Fig. 8),

commonly observed in fatigue of springs. This morphology

is a result of somewhat microstructure control of the

fracture [5], the transverse microstructure of the cold-

drawn wire being reproduced on the fracture surface

(compare Figs. 8 and 9).

Fig. 6 SEM image of a remelted, arc-damage area at one fracture

origin (arrow)

Fig. 7 Detail of one remelt area at a fatigue origin

Fig. 8 Detail of the flat fracture surface outside of the origin

thumbnail, exhibiting a mottled or feathery morphology

Fig. 9 Typical transverse microstructure of a patented music wire,

exhibiting folded and deformed ferrite grains (white) and pearlite

colonies (dark), Nital etch

Fig. 10 SEM image of typical longitudinal final overload region,

showing a ‘‘woody’’ dimpled morphology

150 J Fail. Anal. and Preven. (2014) 14:148–151

123

The longitudinal fracture zones on the springs exhibited

ductile microvoid dimples with a ‘‘woody’’ appearance

(Fig. 10), typical of ductile overload transverse to the

rolling direction of elongated structures [6]. This is also a

microstructure-controlled fracture morphology.

Conclusion

The fractures of the springs initiated in a brittle mode by

intergranular cracking through untempered martensite

volumes at the surfaces caused by in-service stray current

arcing. The fractures propagated by fatigue transverse to

the spring wire axes, due to the normal cyclic loading in

service. Finally, the remaining cross-sectional area failed in

overload, generating the longitudinal fracture planes.

It is known that a significant portion of the high cycle

fatigue life is spent in initiation [7, 8]. Therefore, the

springs experienced significantly reduced service lives due

to the instantaneous crack initiations by electric arc dam-

age-induced intergranular cracking.

Prevention of further failures and elimination of stray

currents required a structural and electrical assessment of

the design and as-assembled components.

References

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3. J. Landes, W.T. Becker, R. Shipley, J. Raphael, ‘‘Stress analysis

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