Shear Mode Growth and Threshold of Small Fatigue Cracks in SUJ2 Bearing Steel

Abstract

The shear mode propagation and threshold behaviors of small, semi-elliptical fatigue cracks were investigated in the JIS-SUJ2 bearing steel. Fully-reversed torsional loading was combined with a static axial compressive load to obtain crack growth in the longitudinal direction of cylindrical specimens. Non-propagating cracks smaller than ≅1mm in size were obtained (i) by decreasing the stress amplitude in tests using notched specimens, and (ii) by using smooth specimens in constant stress amplitude tests. The Mode II and Mode III threshold stress intensity factor ranges, ΔK_IIth and ΔK_IIIth, were estimated from the shapes and dimensions of non-propagating cracks. The threshold values decreased with a decrease in crack size, and there was no significant difference between ΔK_IIth and ΔK_IIIth. Wear on the crack faces was inferred by a powder and also by changes in microstructure in the wake of a crack. The ΔK_IIth in the absence of interference of crack faces was estimated to be approximately 13MPa·m^1/2 for a 1mm long crack.