THERMO-MECHANICAL CRACKING OF RAILWAY WHEEL

Övrigt konferensbidrag, 2011

Thermo-mechanical wheel tread damages are common in railway wheels. While the damage magnitude is limited this is a rather benign phenomenon. However under harsher operational conditions (winter conditions, poorly tuned damping/suspension, poorly matched wheel–rail contact profiles etc) the extent of the problem may increase dramatically and lead to epidemics of wheel damages. Since this calls for wheel re-profiling, the result may be extensive operational disturbances. The presentation deals with thermo-mechanical damage. In practice one of the failure modes, thermal cracking or rolling contact fatigue, usually dominates. However, it is most likely that the combined thermal and rolling contact loading will have an influence in increasing the resulting damage as compared to both phenomena acting separately. In the literature there are a multitude of studies on both thermal and rolling contact loading. analyses of the combined load case are however scarce. One major reason for this is that a combined loading makes a simplification to 2D very cumbersome (not to say futile). Here, a numerical study of the impact of simultaneous thermal and mechanical loading on a railway wheel tread as imposed by braking and rolling contact is presented. 3D finite element (FE) simulations of the thermo-mechanical problem featuring a material model which accounts for thermal expansion and plastic deformations are carried out. Both pure rolling and tractive rolling are considered. The results indicate a significant influence of the thermal loading on the resulting stress/strain response also in cases of relatively moderate temperature increases. In particular, a combination of thermal loading and high traction rolling is found to be very detrimental. The resulting damage of the wheel tread is manifested by the formation of small surface cracks. In cases of high thermal loading, these cracks will evolve to deep radial cracks that can, in a worst-case scenario, cause catastrophic wheel failures. Since this is a potential safety problem it is important to understand the driving mechanisms behind these cracks. To this end, a numerical study of thermal cracking of a wheel subjected to high thermal loading was carried out. The analysis features a computationally efficient approach where 2D FE stress analysis owing to thermal loading during braking and subsequent cooling is combined with an analytical evaluation of resulting stress intensity factors of a radially oriented surface crack in the wheel tread. The analysis identifies critical sizes for when existing surface cracks are prone to propagate under thermal loading and resulting crack lengths after propagation. The results imply that fully functional brake systems are not likely to induce thermal crack propagation under normal stop braking, but that with pre-existing defects a severe drag braking due to malfunctioning brakes may cause very deep cracking. Further the analysis concludes the thermal cracking to be a (more or less) static phenomenon related to the most severe brake cycle. In other words: later brake cycles of similar or lower severity will not cause any significant propagation of existing cracks. To further validate the analysis of the semi-analytical analysis, preliminary 3D FE-simulations have been performed.