Stress ratio effect on notched fatigue behavior of a Ti-8Al-1Mo-1V alloy in the very high cycle fatigue regime

In order to investigate the stress ratio effect on notched fatigue behavior of a turbine engine blade titanium alloy (Ti-8Al-1Mo-1V), notched specimens were employed to perform ultrasonic fatigue experiments up to 1010 cycles at the stress ratios (R) of −1, 0.1 and 0.5. Three typical shapes of S-N curves were clearly observed at three different stress ratios. Moreover, the unique morphology of “surface and subsurface crack initiation” appeared in the very high cycle fatigue (VHCF) regime at R = 0.1 and 0.5. With the increase of stress ratios, the occurrence probability of multi-points surface crack initiation became lower. The fatigue crack originates from the preferred micro-texture, rather than cluster, of two types of αp grains. The quantitative feature of crack initiation region was presented by means of the facets density, and it was used to explain the life variability at the nominal maximum stress of 333.3 MPa. Furthermore, the varying dominance among cyclic creep and fatigue on the failure of specimens was proposed to expound the variation of S-N curve shapes at different stress ratios.