Subsurface inclusion-induced crack nucleation and growth behaviors of high strength steels under very high cycle fatigue: Characterization and microstructure-based modeling

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.