Probabilistic micromechanical modeling of fatigue-life variability in an α+β Ti alloy

Abstract

The fatigue-life variability in an α+β Ti alloy (Ti-6Al-4V) has been examined through a probabilistic micromechanical model that treats the crack-initiation and growth processes at the grain-size level. First, a physics-based crack-initiation model is described. This is followed by a summary of a physics-based fatigue-crack-growth model. The combined model is applied to predict the variability of crack initiation and growth lives due to microstructural variations in Ti-6Al-4V. Finally, possible fatigue mechanisms or scenarios that can lead to the worst-case fatigue life are elucidated via probabilistic modeling of the fatigue-crack-initiation process, the driving force of the grain-sized cracks, as well as the intrinsic (closure-free) threshold and the closure-affected threshold of the small cracks. In the absence of preexisting cracks, the worst-case total fatigue life is obtained when two conditions are met: (1) the crack size at initiation is on the order of 1 to 2 times the grain size, and (2) the driving force (applied ΔK) exceeds the intrinsic threshold of the small cracks. The probabilistic results are also used to elucidate the conditions for the occurrence of dual fatigue limits in high-cycle fatigue (HCF) or giga-cycle fatigue.