Influence of complex LCF and dwell load regimes on fatigue of Ti–6Al–4V

Abstract Real components are usually subjected to variable amplitude fatigue, and yet the deformation micromechanisms that occur due to such load changes have barely been the subject of study. Here, unidirectionally rolled equiaxed Ti–6Al–4V plate was subjected to mixed dwell and variable amplitude low cycle fatigue (LCF), with the finding that overloads near the yield stress were found to retard subsequent fatigue crack growth, whilst elastic underloads were found to accelerate subsequent growth. Dwell intervals were found to be especially damaging, to a far greater extent than either dwell or LCF alone. Dwell facets were found to initiate subsurface and to be smoother than LCF facets, but were otherwise similar in orientation (∼30° to the loading axis) and crystallographic plane, 2–13° from (0002). However, no alteration of the slip bands underlying striations was observed at the point of load changes using TEM. In failure investigation, striation counting is an important tool; the loading changes used were not found to affect the number of striations formed. Dislocation networks were found between similarly oriented grains in the as-received material, which disintegrated under dwell loading and at high stresses.