Afrl-rx-wp-tp-2008-4052 Random Heterogeneity Scales and Probabilistic Description of the Long- Lifetime Regime of Fatigue (preprint)

The long-lifetime fatigue regime is suggested to be a probabilistic realization of sequentially occurring mechanisms. We associate these mechanisms with the development of a ranking of heterogeneity scales in the material, with decreasing probability of occurrence in the order of increasing scale, at any given loading condition. The underlying drivers for these heterogeneity levels are an array of randomly occurring microstructural configurations. With respect to the α+β titanium alloy, Ti-6Al-2Sn-4Zr-6Mo (Ti-6-2-4-6), we identify four microstructural configurations producing different degrees of heterogeneous behavior. At lower stress levels, these configurations present probabilities of failure by a crack-growth-controlled, life-limiting mechanism, and a group of long-lifetime mechanisms. This description of the long-lifetime regime seems to explain the increased incidence of subsurface failures with decreasing stress level, as well as the microstructural neighborhoods involving crack initiation in short and longlifetime mechanisms and those related to surface and subsurface failures.