Papers by Author: Kazuaki Shiozawa

Abstract: In order to investigate the fatigue behaviors in very high cycle fatigue regime of bearing steel GCr15 (Chinese standard), cantilever-type rotary bending fatigue test was carried out, by using an hourglass-shaped specimen. As a result, the S-N data were classified into surface grinding flaw or inclusion induced fracture mode, subsurface inclusion without ‘granular-bright-facet’ (GBF) area induced fracture mode and subsurface inclusion with GBF area induced fracture mode. Formation of a GBF area was clearly observed in the vicinity of subsurface inclusion on the fracture surface ruptured at high- cycle fatigue regime. The cause of the S-N curves of GCr15 obtained by this experiment continuously decreased, which is different from the common ‘duplex S-N’ curve. Experiments on fracture surface was carried out based on the ‘dispersive decohesion of spherical carbide’ model aimed to clarify the mechanism of the formation of GBF area.

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.

Abstract: Gigacycle fatigue behavior in high-strength steels tested under rotary bending fatigue was summarized in this paper. Characteristic of the very high cycle fatigue is to be caused the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced one. In the vicinity of an inclusion at the origin of internal crack, granular-bright-facet (GBF) area was formed during extremely long fatigue cycles. It was pointed out that the formation of GBF area was an important factor for the control of the internal fatigue fracture in gigacycle regime. The GBF area revealed a very rough granular morphology compared with the area outside the GBF inside the fish-eye zone, and was related to the carbide distribution in the microstructure of the matrix. From the detailed observation of fracture surface and computer simulation by FRASTA method, the GBF area formation mechanism in a gigacycle fatigue regime was proposed as the ‘dispersive decohesion of spherical carbide model’.