Mean stress dependence of fatigue strength and subsurface crack initiation in Ti–15Mo–5Zr–3Al alloy

This paper describes the effect of mean stress on fatigue strength and subsurface crack initiation in β Ti–15Mo–5Zr–3Al alloy. Axial fatigue tests were performed at stress ratios, R, of −1, 0 and 0.5 using smooth specimens of two materials solution treated at different temperatures of 735°C (STA735) and 850°C (STA850). Fatigue strength decreased with increasing stress ratio in both materials and at a given stress ratio, STA735 exhibited a higher fatigue strength than STA850. The crack initiation site was the specimen surface at all stress ratios in STA735 and at R=−1 and 0.5 in STA850, while the interior at R=0 in the latter material, thus subsurface crack initiation was found to be both microstructure and stress ratio related phenomenon. The mean stress dependence of fatigue strength could be evaluated by the Goodman relationship, except for unconservative estimation of the fatigue strength at R=0 in STA850, which was attributed to the different crack initiation mode. A smooth facet was always seen at the crack initiation site in STA850 regardless of whether cracks were initiated at the specimen surface or internally. It was found that there was a good correlation between the initial stress intensity factor range calculated using the facet size and fatigue life.