Very high cycle fatigue of TC4 titanium alloy under variable stress ratio: Failure mechanism and life prediction

(a) Duplex S–N curves associated with surface and interior failures under R=−0.3, 0.1, 0.3 and 0.5, and only related to surface failure under R=−1; (b) Surface crack nucleation area; (c) Facets with slip lines at the surface crack nucleation area for R>0; (d) Interior failure associated with GBF and fisheye in the VHCF regime; (e) Facets with slip lines at the GBF area. [Display omitted] •The interior failure mechanism involving facets→GBF→fish-eye→final failure was elucidated.•An energy-based crack nucleation life model associated with failure mechanism was established.•A VHCF life prediction approach involving crack nucleation and growth behavior was developed. Axial loading tests with variable stress ratio were performed to investigate the very high cycle fatigue property of TC4 titanium alloy, and a life prediction approach associated with failure mechanism was proposed. The interior failure process is characterized as: (1) occurrence of slip lines or bands on partial α grains, (2) nucleation of micro-cracks within some α grains, (3) coalescence of micro-cracks and formation of granular bright facets, (4) stable macro-crack growth within fisheye (5) unstable crack growth outside fisheye and (6) momentary fracture. The validity of approach is confirmed by the good agreement between predicted and experimental results.