Influence of heat treatment-induced residual stress on residual fatigue life of railway axles

Assessment of residual fatigue life of railway axles commonly does not include effect of residual stress. This paper presents advanced methodology for estimation of residual fatigue life of railway axles considering not only external loading but also internal residual stresses. The studied axles made of the EA4T steel were treated by induction hardening in order to generate very high compressive residual stress in the surface layer of the axle. Such residual stress has positive effect on behaviour of surface defects and leads to fatigue crack retardation or even crack arrest and, consequently, to immense prolongation of residual fatigue life. Experimentally measured data of residual stress were implemented in a numerical model in order to determine the true stress state in the axle. The model included a crack and took the effects of bending, press fit and residual stress into account. Residual fatigue life was calculated for various starting crack lengths based on the experimentally determined da/dN-ΔK curves for various load ratios. Finally, the results for axles hardened by standard method and by induction hardening were compared with residual fatigue lives obtained experimentally from fatigue tests on real railway axles with artificial cracks. The calculated values were conservative with good agreement with experimental data