Three-dimensional correlation of damage criticality with the defect size and lifetime of externally impacted 25CrMo4 steel

Abstract

Modern railway axles frequently experience external damages, which results in deteriorated fatigue performance and a need for careful maintenance. Herein, the shot peening (SP)-induced strengthening of high-speed railway axle EA4T alloy or 25CrMo4 steel subjected to foreign object damage (FOD) and the effect of this strengthening on fatigue strength were elucidated using the electron backscattered diffraction, transmission electron microscopy, X-ray diffraction and nanoindentation techniques. An improved backward statistical inference method was employed to construct probabilistic fatigue S-N curves based on the results of high cycle fatigue test and thus derive reliable endurance limits for unFODed and FODed specimens subjected or not subjected to SP treatment. Eventually, El-Haddad model and the Wöhler curve were combined with the modified Miner damage accumulation theory to develop a three-dimensional Kitagawa-Takahashi diagram in the nominal stress framework. The above diagram was shown to well correlate the fatigue damage accumulation and the critical FOD defect size of externally impacted railway axles with knee-point life and fatigue S-N curve slope.