Effect of Microtextured Region Stereology on Crack Growth Lifetime Predictions in Ti-6Al-4V

The fatigue life of near-alpha titanium alloys in service can be reduced by the presence of clusters of alpha phase with similar c -axis orientations, known as micro-textured regions (MTRs) or macrozones. MTRs provide a local environment that facilitates initiation and growth of subsurface cracks. Models exist for the prediction of fatigue crack growth rate, taking into account both dwell time and microtextured region parameters, such as size; however, to date, only 2-dimensional measurements have been used as inputs to this inherently 3-dimensional problem. In the present work, the MTR regions are assumed to have the shape of prolate spheroids, and the major and minor axis lengths are measured in cross-section. Then, the 3D size and shape distributions are estimated using the expectation maximization and the Cruz-Orive spheroid unfolding algorithm. The magnitude of uncertainty in the unfolding results and implications for fatigue life models for Ti-6Al-4V are discussed.