A new approach to enhancement of fatigue life of rail-end-bolt holes

The object of this paper is bolted joint railroads as the accent is put on the material behaviour around the bolted holes. The fatigue failure around rail-end-bolt holes is particularly dangerous, since it leads to derailment of trains and consequently, to inevitable accidents. Moreover, the cracking at rail-ends, which starts from bolt hole surface, causes premature rails replacement. It is well-known that the presence of residual compressive hoop stresses around the bolted holes closes the existing first mode cracks and impedes the formation of new ones and thereby extends the fatigue life of the holed components. In this article a new approach to enhancement of fatigue life of rail-end-bolt holes has been developed on the basis of a novel method and tool for cold expansion (CE) of holes, patented by the authors. The major advantage of the method is in imparting around the holes of beneficial residual compressive hoop stresses which are symmetric toward the middle longitudinal plane of the rail and thus the axial stress gradient is minimum. The developed approach consists in setting and solving a multi-objective optimization task of the CE of the rail-end-bolt holes. Because of the specificity of the studied problem, the optimal solution has been found by finite element (FE) simulations. For that purpose generalized FE model of the object has been developed. Through this model the critical point around the outside holes has been localized and the cycle of variation of hoop stresses has been determined taking into consideration assembly stresses. On the basis of adapting the generalized FE model of the rail joint to simulating the CE process, a comparison between the cases with and without CE of rail-end-bolt holes has been made. On this basis the optimal degree of CE and the successive hole treatment have been found. After simulating the CE process by the determined optimal degree of CE, the beneficial effect of implementing the new approach has been proved. (C) 2012 Elsevier Ltd. All rights reserved.