Mode II fatigue failures at rail butt-welds

Some typical fatigue failures of butt-welded rails consist of fractures of the web. In the early stage, the crack propagates parallel to the rail surface but, after a while, it tends to propagate with a slant surface. The aim of the present work is to analyse the mechanisms involved in crack propagation in rail webs and the crack path, which are the two essential elements for a future determination of the rail inspection intervals. At first, a fractographic analysis of a typical failure revealed the typical features of mode II and showed that mode I is involved only after the final crack kinking. Then, FEM analyses of an observed fracture were carried out in order to determinate the KI and KII history at different positions of the crack tip during the passage of the typical loads induced by trains. The results have shown that the initial flaw tends to follow a path where ΔKII is close to maximum values and a small superimposed KI traction is present and after the kinking, the flaw tends to maximise mode I crack propagation.