Fatigue Crack Growth Behavior of Ti-6Al-4V Alloy with Bimodal Microstructure under Repeated Two-step Load Sequences

Abstract

The effect of microstructure on fatigue crack growth behavior under repeated two-step load sequences was investigated using compact tension specimens of Ti-6A1-4V alloy with bimodal microstructure. Fatigue crack growth rate and macroscopic crack closure were measured by means of an unloading elastic compliance method. The da/dn-ΔKeff relationship exhibited a trilinear form in so-called Paris region corresponding to the change of growth mechanism from the intergranular fracture in the low ΔKeff region to the transgranular one in the high ΔKeff region. The crack opening point, Kop, was constant during one block of varying loading in the case that the crack extension during one block loading was relatively short. On the other hand, in the case that the crack grew beyond the cyclic plastic zone introduced by ΔKH, Kop under low level loading decreased from the initial high value to the stationary low one as the crack grew. Depending on the test conditions, the fatigue crack growth rate under varying loading was found to be higher or lower than that estimated by da/dn-ΔKeff relation under constant amplitude loading, which indicates that the load variation resulted in both crack grown acceleration and deceleration in terms of ΔKeff. The crack growth acceleration was attributed to the change of growth mechanism due to the load variation, while the crack arrest during one block loading brought about the crack growth deceleration.