Application of local fatigue strength approach to assess and optimise the impact of deep rolling on the fatigue performance of railway axles

Deep rolling is a frequently used post-treatment process to increase the service life of railway axles. Through the process application, the surface roughness is reduced, the surface is work-hardened and compressive residual stresses are introduced in the near surface region in a controlled manner. These three effects have a positive impact on service life, and in the investigated case of railway axles, the introduced residual stress state constitutes the most significant potential. The compressive residual stresses introduced near the surface, which are responsible for the positive lifetime effect, are inevitably balanced by compensating tensile residual stresses below the surface. Therefore, it is generally challenging to estimate the actual potential of the process application properly. Classical design methods consider only the condition at the surface and are thus not suitable for a surface layer-based analysis. In the present publication, the concept of local fatigue strength is applied for the fatigue strength assessment of deep rolled railway axles. The locally permissible fatigue strength at each surface-layer depth of the railway axle is determined as a function of local properties and compared with the locally occurring load. This allows a proper fatigue design in depth and, at the same time, the estimation of the critical point at which the potential of crack initiation and thus failure occurs. Based on the methodology presented, it is possible to determine the fatigue benefit achieved by deep rolling railway axles considering the influence of various deep rolling parameters, and to further systematically optimise the significant process parameters. An increase in fatigue strength of 4.7 % can be proven by deep rolling the railway axle with reference parameters compared with the not deep rolled state. By applying the proposed optimisation, fatigue strength can be further increased up to 10.4 %.