Analysis of Fatigue Indicator Parameters for Ti-6Al-4V microstructures using extreme value statistics in the transition fatigue regime

摘要

Extreme value (EV) statistics have been previously utilized to model the tails of distributions of computed Fatigue Indicator Parameters (FIPs) to provide a method of ranking microstructures in terms of resistance to fatigue crack formation under identical loading conditions. FIPs that capture the two primary transgranular mechanisms for fatigue crack formation and early stages of growth in bimodal Ti-6A1-4V are computed for cyclic strain-controlled loading conditions corresponding to transition fatigue where there is perceptible macroscopic plastic cyclic strain, but the polycrystal cyclic elastic strain range dominates the cyclic plastic strain range. A statistical volume element (SVE) ensemble approach is employed, in which a large number of microstructure samples are constructed for simulation. The peaks-over-threshold method is used to identify extreme values within the FIP distributions which are then fit to the Generalized Pareto Distributions, enabling rank-ordering of various microstructures with regard to resistance to fatigue crack formation. Microstructure attributes including the grain size, orientation, and nearest neighbor arrangements are quantified in the neighborhood of FIP 'hot spots'.