On the fatigue improvement of railways superstructure components due to cold expansion – Part II: Finite element prediction

This paper is the second of a two-part series dealing with the study of the residual stress field induced by cold expansion (CE) in rail-end-bolt holes. In the aeronautical field, cold expansion is a consolidated practice adopted to induce beneficial residual compressive stresses around holes of aluminium parts, with the aim to improve the fatigue strength. However, in the literature few experimental or numerical studies are proposed on the application of this technique to structural steels. In Part I, an in-depth experimental investigation was carried out on railway steel, in particular on rail-end-bolt holes, with the aim to better understand the full non-linear response of the material during the whole process. In this paper, finite element (FE) analyses simulating CE process are presented, and the experimental results of Part I have been used to validate the FE model. The strain-time history acquired during the entire cold expansion process allowed the comparison with FE-predicted strains, both in terms of residual and maximum strains. This approach is not present in literature, neither for aluminium nor for steel. The results, in terms of trend and magnitude, show that strains in both the experiments and the FE simulations are generally consistent, confirming the reliability of the FE model. In addition, a sensitivity study is presented for different levels of cold expansion. The results can be exploited to develop an a priori prediction of the residual stresses near the hole surface, aiming to an improvement of fatigue strength.