Micro cleavage cracking during fatigue crack propagation in low strength steel

Studies have been made of the mechanisms of fatigue crack propagation in a high-nitrogen mild steel tested near its ductile/brittle transition temperature (T_T), for a range of mean stresses. No significant effect of mean stress was observed for fine-grained (28 μm) steel, tested at room temperature (above T_T), where the mechanism of crack growth was almost entirely striation growth. In contrast, segments of cleavage fracture were observed in steel of 60 μm grain size, tested at room temperature (below T_T), resulting in a marked effect of mean stress on the overall propagation rate. A tentative model is proposed to account for the occurrence of major bursts of cleavage fracture during fatigue, based on the attainment of sufficient tensile stress to cause propagation from a cracked carbide particle situated in a grain boundary ahead of the crack tip.