Fatigue and fracture behaviour of aluminothermic rail welds under high axle load conditions

Among different rail welding methods Aluminothermic welding (ATW) is the oldest and simplest procedure widely used for re-railing and replacement of defective rails. Since ATW is a cast welding process in which several aspects are operator-dependent it suffers from the variability of the produced weld quality, presence of casting defects and inconsistencies in the microstructure and mechanical properties. Previous observations have shown that field-welded ATWs have been major sources of fatigue and overload failures in Australian heavy haul railway systems. The most common failure modes are categorized into straight breaks (transverse fissure) and horizontal split webs (HSW). Straight breaks initiate from stress concentration sites at the edge of the weld collar, in the foot, lower web and underhead regions, and propagate in vertical direction under Mode I loading. HSW failures involve the development of a horizontal fatigue crack which initiates from a surface or near-surface gross defect in the weld collar, generally in the mid- or upper-web region. In this study analysis of fatigue crack initiation is performed at the edge of the weld collar of an aluminothermic weld, in order to examine the formation of straight break under high axle load conditions. The fatigue assessment is accomplished using a thermo-structural finite element simulation in ANSYS package followed by a shear based multi-axial fatigue critical plane criterion implemented in a MATLAB computer code. The influence of several parameters including wheel-rail contact patch eccentricity, contact tractions, residual stress distribution, seasonal temperature variation and track support condition is investigated. The analysis identifies the underhead region of a defect-free weld as the most critical location which is subject to severe fatigue damage under harsh curving and hunting behaviours.