An analysis of the effect of compressive residual stresses due to roll-forming onto fatigue crack propagation in railway axles

Over the last years, deep rolling has been adopted to improve the fatigue strength of railway axles, but very few scientific analyses and studies of this phenomenon are available in the literature. For this reason, a funded research project was started in order to experimentally and theoretically investigate some aspects related to the application of this technological process to railway axles made of EA4T grade. Firstly, the compressive residual stress field, due to the deep rolling process, was experimentally evaluated by x-ray diffraction. An experimental crack growth campaign on small-scale SE(B) and SE(T) specimens was then carried out for investigating the behavior of the material at very negative stress ratios. The results obtained from crack propagation tests and residual stress field measurements were used together to build up a predictive crack growth model which was then validated through dedicated experimental crack propagation tests carried out on full-scale deep rolled axles made of EA4T grade.