Crack initiation mechanisms under two stress ratios up to very-high-cycle fatigue regime for a selective laser melted Ti-6Al-4V

Crack initiation mechanisms under two stress ratios (R = − 1 and 0.5) and up to very-high-cycle fatigue (VHCF) regime of a selective laser melted (SLMed) Ti-6Al-4V were investigated. Type I lack-of-fusion defects (almost equiaxed) induced crack initiation except for the cases of VHCF under R = 0.5 in which type II defects (large aspect ratio) caused facet mode crack initiation. A nanograin layer formed underneath the crack initiation region of rough area for the cases of VHCF at R = − 1, which was explained by the numerous cyclic pressing (NCP) model. A P-S-N approach was introduced to well describe the fatigue life up to VHCF regime under R = − 1 and 0.5 for the SLMed titanium alloy.