Failures of engineering components caused by rolling contact fatigue, such as flaking in bearings, are closely related to the initiation and growth of share-mode, i.e. Modes II and III, fatigue cracks. In order to evaluate quantitatively the fatigue strength of those components, it is necessary to elucidate the propagation and threshold behaviors of share-mode cracks, particularly for small cracks, on the basis of fracture mechanics. In this study, fatigue tests of fully-reversed cyclic torsion superposed upon static compression were carried out using SAE52100 bearing steel shafts into which semi-elliptical cracks smaller than 1 mm in size were initially introduced in the axial direction. Propagation and nonpropagation of shear-mode fatigue cracks were controlled by changing the torsional stress amplitude. The threshold stress intensity factor (SIF) range for a share-mode crack was defined at the minimum stress required for crack propagation. Crack face interference was responsible for the reduction in crack driving force. An intrinsic value of threshold SIF range that does not include the effect of crack face interference was obtained to be approximately 13 MPa + m 0.5 .
