Abstract
In order to study the fatigue properties of low alloy steel, JIS SNCM439, under axial loading condition in very high cycle regime, fatigue test under the stress ratio of -1 was carried out in air at room temperature using the hourglass shaped specimen. From the experimental results, clear duplex S-N curve existed in this material ; that is, there were two curves composed of a surface inclusion-induced fracture mode and a subsurface inclusion-induced fracture mode with granular bright facet (GBF) area in the vicinity of inclusion. Roughness of the GBF area was about 2 times larger than the area outside the GBF in the fish-eye zone relating to the size and distribution of carbide particle in the matrix. An appearance of the dual-stage S-N behavior was related to the difference of threshold for crack initiation and crack growth rate between surface inclusion and subsurface inclusion. Stress amplitude for transition of the fracture mode was explained by considering an effect of compressive residual stress on the surface layer. From the evaluation of fatigue life based on the estimated subsurface crack growth rate from the S-Ndata, 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.
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