Fatigue Crack Propagation in Circumferentially Pre-Cracked Steel Bars under Torsional and Axial Loadings

The propagation behaviour of a circumferential fatigue crack in a cylindrical bar of medium carbon steel was studied under various combinations of cyclic torsional and axial loadings. The fatigue fracture surface was macroscopically flat under the conditions of excessive plasticity, and the J integral was applied to fatigue crack propagation under mixed mode loading. The propagation rate of fatigue cracks was expressed as a power function of the J integral range for both single and mixed mode cases. When compared at the same J intergal range, the propagation rate was highest for the mode I case and the lowest for the mode III case. The simple addition of the propagation rates of mode I and III gives the lower bound of the propagation rate under mixed mode loading. Striations were observed on the fatigue fracture surface made under mixed mode loading, and their spacing was equal to the crack propagation rate as in the case of mode I loading.