The Role of Microstructural Variability on the Very High Cycle Fatigue Lifetime Variability of an

Abstract

The fatigue behavior of structural components in the regime of very high cycle fatigue (VHCF) (10^6-10^9 cycles) has been attracting increased commercial interest as components are increasingly being called upon to perform in this regime of lifetimes. In VHCF, the applied stresses relative to the yield stress are very low. Therefore, it is presumed that a substantial portion of fatigue lifetime is consumed by the fatigue crack initiation process, and that cyclic plasticity only accumulates in microstructural neighborhoods that are susceptible to fatigue damage accumulation. Thus, microstructural heterogeneity is believed to significantly effect the fatigue lifetime variability. The very high cycle fatigue behavior of Ti-6246 has been investigated using ultrasonic fatigue techniques, and lifetimes ranging from 10^6-10^9 cycles have been observed. Fatigue cracks initiate by facet formation within αp grains. It has been found that the facets form in grains that are slightly larger than average and that the facets appear to form by a process of slip since the facet plane normals are oriented approximately 35-55° with respect to the tensile axis. Three characteristic fatigue crack initiation sites have been identified. These initiation sites, ranked in order of increasing fatigue resistance are: surface, subsurface with isolated facets, and subsurface with contiguous transgranular faceting. The texture of the material in these initiation regions is favorable for basal and prism slip. This texture is derived from the prior β texture, and a mechanism of fatigue crack initiation resulting from strain incompatibility has been proposed. Fatigue crack growth experiments have been completed to measure the influence of local texture on the ease of fatigue crack initiation and the resulting fatigue crack growth rates. These experiments have found that the controlling microstructural dimension is on the order of 20-25 µm. The texture appears to affect initiation of fatigue cracks from notches as well. For notches placed within neighborhoods favorably oriented for basal slip, fatigue crack initiation will be encouraged. However, for notches placed within microstructural regions unfavorably oriented for basal slip, fatigue crack initiation will be inhibited.