Changes in fatigue life mechanism due to soft grains and hard particles

Soft surface grains and hard interior particles are favored sites for fatigue crack initiation, which subsequent growth can lead to component failure. This paper examines the processes of crack initiation at soft primary alpha grains and hard alpha particles in two-phase Ti alloys. Existing microstructure-based fatigue models are utilized to treat fatigue crack initiation and to delineate the effects of microstructure on fatigue life variability. Specifically, competing fatigue crack initiation processes are analyzed to determine how they affect the shape of the stress-fatigue life (S–Nf) curves. Soft primary alpha grains in Ti-alloys with a duplex microstructure concentrate plastic strains and raise hydrostatic stresses, leading to bilinear stress amplitude and mean stress curves that deviate substantially from the Haigh or Goodman relation. Similarly, residual tensile stresses at matrices adjacent to hard particles can change the mean stress dependence and reduce the fatigue limit in the high-cycle fatigue regime and beyond.