Experimental Investigation on Fatigue Crack Propagation in Surface Hardened Layer of High-Speed Train Axles

This paper presents an experimental investigation on fatigue crack propagation in the surface hardened layer of high-speed train axles made of S38C steel. Three-point bending specimens were extracted from CRH2 EMU axles, with the hardened surface layer (0–2 mm depth) and core matrix (beyond 6 mm) characterized via microstructure and microhardness analysis. Fatigue tests were conducted using a PLG100 machine, monitoring crack growth under stress ratio R = 0.1. Results show that the hardened layer exhibits lower fatigue crack growth rate (da/dN) than the transition layer (2–6 mm) and core matrix. The da/dN-ΔK relationships were fitted following Paris Law, revealing the hardened layer’s higher resistance to crack propagation, attributed to its tempered martensite microstructure. This study provides insights into how gradient microstructures affect fatigue behavior in axle materials.