The effect of roll-forming onto fatigue life of railway axles

In the last years, roll-forming has been adopted as a technique to improve the strength against crack propagation in railway axles, but very few scientific analyses and studies of the phenomenon are available in the literature. For this reason, the Maraxil Project was started in order to give the opportunity of experimentally and theoretically investigating some aspects related to this technological process in the case of railway axles made of EA4T, a typical steel for manufacture of high speed axles. Firstly, an experimental campaign on small-scale SE(B) specimens was carried out for determining the crack propagation behavior at stress ratios below -1, which correspond to the ones in presence of the high compressive residual stresses due to the roll-forming procedure. At the same time, the compressive residual stress field along the radial direction and due to the rollforming process was evaluated from axles coming from the production using both hole-drilling and x-ray diffractometry methods. The results obtained from crack propagation tests and residual stress field measurements were used together to build up a predictive crack growth model in presence of very high compressive stresses. This model was, then, validated through dedicated experimental crack propagation tests carried out on full-scale roll-formed axles made of EA4T; good agreement was found between the model and the experiments.