Effect of defect shape on rolling contact fatigue crack initiation and propagation in high strength steel

The objective of the present paper is to clarify the effect of shapes of circular hole defects, orientated perpendicular to the surface, on rolling contact fatigue (RCF) crack initiation and propagation in high strength steel. RCF test and synchrotron radiation micro computed tomography (SR micro CT) imaging were conducted. In the case of a 15-μm-diameter defect, the number of cycles of rolling contact to flaking occurring (flaking life) decreased with increasing defect length. In a comparison of the CT image and the SEM view, the shapes of defects and the locations of the horizontal cracks were almost the same respectively. The mechanism of RCF crack propagation was discussed by finite element (FE) analysis. Defects led to higher tensile residual stress than that without defects in the region where the defect exists. The shear stress range at 0.1 mm in depth on the middle line of the defect and the range of mode II stress intensity factor at the bottom of a vertical crack increased with increasing defect length. In the case of a defect 50 μm in diameter, defect length does not affect flaking life. The reason for this is probably that the horizontal cracks form and propagate before vertical cracks grow to an effective size.