Foreign object damage tolerance and fatigue analysis of induction hardened S38C axles

Highlights: • Impact damages under different velocities and incident angles on S38C axle specimens are fabricated. • Morphologies of artificial impact damages are explored for S38C axle. • Influences of impact velocity and incident angle on fatigue strength of S38C axle specimens are studied. • Fatigue properties of specimens with artificial defects are analyzed based on K-T diagram. Flying ballast impact damage on railway axles poses a threat to structural integrity and safety of high-speed trains. A comprehensive evaluation on the fatigue strength debit of axles with foreign object damage (FOD) is vital for their safe operation and maintenance under damage tolerance philosophy. In this study, FODs were simulated by shooting tungsten steel balls to the specimens extracted from surface induction hardened S38C axles. Specifically, tests were performed under three impact velocities, i.e. 200, 300 and 400 m/s, and five incident angles for each impact velocity. Morphologies of impact damage were observed by scanning electron microscope (SEM) as well as the cross-sections. Fatigue strength of impacted specimens were then evaluated. Results show that depth and volume of damaged zone increases with the incident angle at the velocity of 200 m/s. Fatigue strength losses of specimens impacted at velocities of 300 and 400 m/s is around 50% compared with that of smooth ones. Fatigue crack originated from the exit rim for 200 m/s glanced impacted specimens, while multiple cracks propagated from both exit rim and bottom of crater impacted at 300 and 400 m/s. Finally, fatigue strength of damaged specimens was evaluated based on the defect depth concept and Kitagawa–Takahashi (K–T) diagram.