Collaborative crack initiation mechanism of 25CrMo4 alloy steels subjected to foreign object damages

High-speed railway axles are seriously threaten by external flying objects. Foreign object damage (FOD) on 25CrMo4 or EA4T axle steel specimens was then prepared by a compressed-gas gun. The microstructure and micro-damage at the crater were investigated by scanning electron microscopy, electron backscattered diffraction, transmission electron microscopy and nano-indentation. To correlate the FOD to the fatigue crack resistance, residual stress distribution due to FOD was also simulated by calibrating the Johnson-Cook constitutive equation and associated failure criterion in experiments. Besides the significant uncertainty of fatigue crack initiations from the FOD-induced micro-damage, residual stress and microstructure change, those FODed specimens also present a significantly lower fatigue strength than those at the absence of FOD. Furthermore, a higher impact velocity on the specimens appears to produce lower fatigue endurance. Finally, a modified Kitagawa-Takahashi diagram was established to combine the fatigue strength with FOD defects, which can be used in the fatigue resistance evaluation of high-speed railway axles.