Investigation of the weld unzipping failure mode during collisions of welded aluminium rail vehicles

Abstract

The performance of aluminium welds in rail vehicles under high dynamic loading conditions has been investigated. Aluminium alloy welded joints subjected to dynamic loading can fail by a mechanism of unstable crack growth along the heat affected zone/weld metal interface, a phenomenon commonly known as ‘weld unzipping’. Modelling of weld unzipping in large rail structures is a challenging task since it requires knowledge of material tearing instability and material fracture parameters, as well as addressing the problem of mesh resolution, which together pose severe challenges to computability. The proposed methodology for the prediction of weld failure is based on the validation of the numerical models through correlation with laboratory scale tearing tests. The tearing tests were conducted on samples taken from real rail extrusions with the purpose of obtaining the failure parameters under dynamic loading and understanding the effect of weld material composition on joint behaviour. The validated material models were used to construct a finite-element analysis simulation of the collision of an aluminium rail car and investigate the effect of joint geometry on the failure mechanism. Comparisons of the model with the failure observed in an aluminium rail vehicle that was involved in a high-speed collision have shown that it is possible to model the phenomenon of weld unzipping with good accuracy. The numerical models have also been used as a tool for the optimization of joint design to improve crashworthiness. A proposed thickening of the aluminium sheet at the weld region is shown to eliminate the weld unzipping failure mode with the impact energy absorbed by controlled buckling of the structure.