•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.(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.Download high-res image (177KB)Download full-size image

Very high cycle fatigue (VHCF) tests for three kinds of high-strength low-alloy steels were performed to clarify the subsurface inclusion-induced crack nucleation and growth behaviors accompanied with the presence of fine granular area (FGA) and fisheye. Based on the computer-aided reconstruction of fatigue process and the evaluation of stress intensity factor at crack tip, the whole crack nucleation and growth processes can be divided into the following stages: (i) crack nucleation around the inclusion and within the FGA, (ii) microcrack growth within the FGA, (iii) stable macrocrack growth outside the FGA and within the fisheye and (iv) unstable macrocrack growth outside the fisheye. A crack nucleation life model for stage (i) and a crack growth life model for stages (ii) and (iii) were established respectively. The predicted crack nucleation life related to the FGA size is almost equivalent to the total fatigue life, whereas the predicted crack growth life only occupies a tiny fraction of the total fatigue life. In view of the good agreement between the predicted and experimental results, the theoretically modeling method that involves crack nucleation and growth can be well used to predict the VHCF life of high-strength steel with the subsurface failure induced by inclusion-FGA-fisheye.