Fatigue crack initiation site transition of high‐strength steel under very high‐cycle fatigue.

The very high‐cycle fatigue (VHCF) fractographies of high‐strength steel AISI 4340 with tensile strength ranging from 1285 to 2363 MPa fabricated by tempering were systematically observed and quantitatively analyzed. It is found that the fatigue crack initiation sites were gradually transferred from the specimen surface to the interior with increasing the tensile strength or decreasing the stress amplitude. Such phenomenon has become common rule in the high‐cycle fatigue (HCF) or VHCF regimes of high‐strength steels. Based on the fracture mechanics, a simplified transition mechanism for fatigue crack initiation sites is established in terms of the influence of applied stress amplitude and microstructure, which is beneficial to enhance our understanding on the variation of fatigue strength increasing the tensile strength. Highlights: The effects of tempering temperature on fatigue crack initiation are studied.The sites of fatigue crack initiation is counted, and influence mechanism is discussed.A model is proposed to explain the relationship between tensile and fatigue strength. [ABSTRACT FROM AUTHOR]