Fatigue strength assessment of high-speed railway axle EA4T steel with foreign object damage

•The FOD crater characteristics were detected carefully based on EA4T axles.•The design fatigue P-S-N curves with 97.5% survival rate for damaged high-speed railway axle specimens with or without surface strengthened were obtained.•The modified Murakami equation for predicting the fatigue limits of full-scale FODed axles was proposed.•A safe assessment probabilistic Kitagawa-Takahashi diagram was established. Axle is easy to be hit by ballast and other foreign object rolled up due to aerodynamic effect to form the typical foreign object damage (FOD) on the surface, leading to deteriorate the axle structural integrity and reduction of fatigue performance. In order to evaluate the effects of surface damage and strengthening treatment on the fatigue performance, the EA4T axle steel specimens with or without shot peening (SP) were induced FOD by a compressed-gas gun. Meanwhile, the strengthening mechanism of shot peening were investigated by both nanoindentation technique and X-ray diffraction. Then the probabilistic fatigue S-N curves (P-S-N) were acquired for unSPed and SPed specimens subjected to FOD or not. In addition, the fatigue limit of FODed axle estimated by Murakami equation was corrected by size effect, surface quality, loading mode factors. Finally, the fatigue limits with 97.5% reliability were adopted to construct the modified Kitagawa-Takahashi (K-T) diagrams for FODed axle with or without shot peening treatment, which can be used in the service performance evaluation of foreign object damaged axle.