Effect of Non-Metallic Inclusion Size and Residual Stresses on Gigacycle Fatigue Properties in High Strength Steel

Abstract

Fatigue fracture of some high-strength steels occurs at small defect in the subsurface zone of a material at low stress amplitude level and in a high-cycle region of more than 106 cycles (gigacycle fatigue life), whereas surface fatigue crack initiation occurs at high-stress amplitude and low cycles. There is a definite stress range where the crack initiation site changes from a surface to a subsurface defect, giving a step-wise S-N curve or a duplex S-N curve. From the experimental results, fatigue fracture mode was classified into three types, such as, surface inclusion induced fracture mode, subsurface inclusion induced fracture mode without granular bright facet (GBF) area and that with the GBF, depending on stress amplitude level and stress ratio. The GBF area was observed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye in gigacycle fatigue regime. It was made clear from the discussion with fracture mechanics that the transition of fracture mode was affected by compressive residual stresses on the specimen surface. Fracture-mode transition diagram was proposed through the experimental and theoretical investigation. Also, from the evaluation of the fatigue life 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 S-N curve for subsurface inclusion-induced fracture depended on the inclusion size was provided.