Influence of inclusion size on S‐N curve characteristics of high‐strength steels in the giga‐cycle fatigue regime

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Abstract
Fatigue fracture of high‐strength steels often occurs from small defect on the surface of a material or from non‐metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S‐N curve of high‐strength steels consists of two curves corresponding to surface defect‐induced fracture and internal inclusion‐induced fracture. The surface defect‐induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion‐induced fracture occurs at low stress amplitude levels and high‐cycle region of more than 106 cycles (giga‐cycle fatigue life). There is a definite stress range in the S‐N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S‐N curve or a duplex S‐N curve. On the other hand, under cyclic axial loading, the S‐N curve of high‐strength steels displays a continuous decline and surface defect‐induced or internal inclusion‐induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S‐N curve characteristics of high‐strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga‐cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S‐N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S‐N curve for subsurface inclusion‐induced fracture depends on the inclusion size.

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