Prediction of Fatigue Crack Propagation Path from a Pre-Crack under Combined Torsional and Axial Loading

Abstract

Fatigue tests of crack propagation from a pre-crack in thin-walled tubular specimens made of a medium-carbon steel were performed under cyclic torsion with and without superposed static or cyclic axial loading. The stress ratio of cyclic loading was-1. The experimental path of fatigue crack propagation from a pre-crack was compared with the predictions based on the maximum tangential stress criterion. The direction of fatigue crack propagation was perpendicular to the direction of the maximum of the range of the tangential stress, Δσ*θmax, near the crack tip determined from the stress intensity factor which was calculated by considering the contacts of crack faces at the minimum load. The stress intensity factor calculated from the actual crack path by using the body force method showed that the mode II stress intensity factor range quickly got close to zero after a small amount of crack extension. In other words, fatigue cracks propagate to satisfy the local symmetry of the cyclic components of crack-tip deformation.