Slip dominated planar anisotropy of low cycle fatigue behavior of commercially pure titanium

Abstract

Low cycle fatigue (LCF) behavior of commercially pure titanium (CP-Ti) was investigated under stress cycling and strain cycling along the rolling direction (RD), 30° to the rolling direction (RD-30°), 60° to the rolling direction (RD-60°) and the transverse direction (TD), respectively. The loading direction brings about eminent differences in the fatigue properties, which is summarized as follows;Cyclic strain hardening exponent: RD＞RD-30°＞RD-60°＞TDCyclic stress amplitude: TD＞RD-60°＞RD-30°＞RDRatcheting effect: TD＞RD-60°＞RD-30°＞RDFatigue life: RD＞RD-30°＞RD-60°＞TDBased on the comprehensive analysis of Schmid factor, texture evolution, twinning activities, in-grain misorientation axis and geometrically necessary dislocation density, it is concluded that the anisotropic slip behavior characterizes the planar anisotropy of LCF behavior. The promotion of dislocation activities of non-prismatic slip with higher activation stress from the RD to the TD leads to the higher cyclic stress amplitude, lower cyclic strain hardening and more significant ratcheting effect, though prismatic slip always acts as the primary slip mode. The observation of fracture surfaces also confirms the critical role of dislocation activities on fatigue crack initiation. Introducing yield ratio parameter into the asymmetry coefficient related model, the fatigue lives in both the fatigue were accurately predicted.