Crack Paths 2012

Small CrackPropagation in Multiaxial Notch Fatigue

K. Tanaka1

1 Department of Mechanical Engineering, Meijo University, Nagoya 468-8502, Japan

E-mail: ktanaka@meijo-u.ac.jp

ABSTRACT.The present paper deals with torsional fatigue of circumferentially

notched round bars of austenitic stainless steel and carbon steel without and with static

tension. The torsional fatigue life of stainless steels was found to increase with

increasing stress concentration under the same nominal shear stress amplitude. The

electrical potential monitoring revealed that the crack initiation life decreased with

increasing stress concentration, while the crack propagation life increased. The

anomalous behavior of the notch-strengthening effect was ascribed to the larger

retardation of fatigue crack propagation by crack surface contact. The superposition of

static tension on cyclic torsion causes notch weakening. The notch-strengthening effect

in torsional fatigue was not found in carbon steels. The difference in the crack path of

small cracks near notch root between stainless steel and carbon steel gives rise to the

difference in the notch effect in torsional fatigue.

I N T R O D U C T I O N

Fatigue fracture of several engineering components such as transmission shafts, pipes

and springs occurs under combined torsional and axial loading. Notches or stress

concentrations are the commonsite of crack initiation.

The assessment of the notch

effect on the fatigue strength and life is essential in fatigue designs. In comparison with

axial fatigue, torsional fatigue studies have been relatively limited, and the combination

of torsional and axial loading further complicates the assessment of the fatigue strength

and life. Since fatigue fracture results from the initiation and propagation of fatigue

cracks, the understanding of small crack behavior greatly helps to improve the accuracy

of the fatigue life prediction of notched components.

Ananomalous phenomenon of the notch-strengthening effect was found in torsional

fatigue of circumferentially notched round bars of austenitic stainless steels [1,2]. The

fatigue life of notched bars was found to be longer than that of smooth bars, and to

increase with increasing stress concentration under the same amplitude of the nominal

shear stress. The anomalous notch-strengthening was not found in carbon steels [3]. In

the present paper, the propagation behavior of small cracks was observed by the

scanning electron microscopy (SEM) and continuously monitored by the electrical

potential method. The difference in the notch effect between stainless steel and carbon

steel is discussed on the basis of the crack path morphology of small cracks near the

notch root observed by SEM.

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