A study of loading path influence on fatigue crack growth under combined loading

Single edge specimens made of 1070 steel were subjected to combined axial–torsion loading at room temperature. Three loading paths were designed to study the loading path influence on the crack growth behavior. Path I was proportional loading and Path II was the loading case, where the torque decreased/increased with increasing/decreasing axial load. Path III followed a circular path in the axial load–torque coordinates. With identical load amplitudes, the crack growth behavior was found to be critically dependent on the loading path. Path III resulted in the fastest crack growth rate and Path II produced the least crack extension. Furthermore, the crack profile was dependent on the loading path. Path III resulted in a crack profile similar to that observed under Mode I loading. Significant branching and slant angles were observed for Paths I and II, with Path II in particular. Two existing models were discussed with respect to their capabilities to predict the crack growth rate and cracking direction for combined loading.