Influence of artificial defects on fatigue strength of induction hardened S38C axles

Impact damage are critical for damage tolerance assessment of high-speed train axles. Flying ballast impact damage on railway axles produce geometry discontinuities, which become preferential sites for fatigue crack initiation and usually lead to premature failure. According to this, the formation of surface impact damage and its influence on structural integrity of induction hardened S38C axles are herein explored. In particular, the morphology of impact damage on induction hardened S38C axles were analyzed as well as fatigue properties of specimens with artificial defects, electronic discharge machine (EDM) defects and impact defects fabricated by shooting metallic balls to the surface with compressed-gas gun. Results show that surface impact damage of the axles include two types, namely scratches and notches. Fatigue strength of the specimens with EDM defects are larger than the Murakami's model predictions due to existence of compressive residual stresses. The influence of shallower impact damage (less than 200 mu m) on the fatigue strength is negligible. With a larger depth, the rim is extracted out of the original surface and fatigue crack starts from this point with a decline of fatigue strength. Microcracks and loss of materials appear at the rim of impact damage, and microcracks along adiabatic shear bands lies beneath the surface when the impact depth is about 300 mu m resulting in 50% reduction of fatigue strength. Fatigue strength of impacted specimens are larger than that of EDM specimens by 200 MPa with the equivalent root area due to the local residual stresses.