Crack branching and deflection in AISI 4340 steel under cyclic torsional loading

Abstract

The high-cycle torsional fatigue crack propagation behaviors of the martensitic AISI 4340 steel were investigated in this work. Quantitative analyses of fracture processes were conducted to establish the branching mechanisms in the early crack propagation stage and the deflection in the later crack propagation stage. It was found that when the mode I stress intensity factor was greater than the threshold value KI,th, the initiated crack would branch off to four cracks in mode I. Furthermore, there is a competitive relationship among different pairs of branches. Firstly, the backward pair of branches encountered more boundary barriers. Secondly, their driving force was reduced by the stress redistribution caused by the leading pair of branches. In addition, due to the restriction of the grips, the crack would gradually turn into mode II from mode I. When KII/KI > 1.5, the cracks growth type became complete mode II.