Residual stress induced granular bright facets around inclusions in high-strength steels under high-cycle fatigue

Granular bright facets (GBFs) are frequently observed adjacent to inclusions and within fish-eye areas in the fatigue fractures of high-strength steels under very-high-cycle fatigue (VHCF), considered as a characteristic fracture feature of VHCF. Previous understanding on GBF formation emphasizes the occurrence of nanocystallization in microstructure. In this study, however, the same morphology can also be observed under high-cycle fatigue (HCF). GBF, although closer to fatigue crack initiator and experiencing more stress cycles than the peripheral part of fish-eye (PPFE), exhibits rougher surface morphology in the absence of accumulated plastic strain in microstructure, manifesting relieved crack surface wear. This indicates that the traditional theories on GBF formation for VHCF becomes invalid for HCF. The root cause for GBF formation under HCF is analyzed to be the presence of residual stress around inclusions, which reduces the contact pressure of fatigue crack surfaces inducing wear relief. Accordingly, an analytical model capable of predicting GBF thickness with the effects of HCF conditions and steel properties taken into consideration is established. The model yields accurate predictions of GBF thickness with various loading stresses and inclusion diameters, validated by experimental observations. This study provides theoretical guidance for HCF fracture analysis and fatigue life prediction.