Modelling of the effect of thermal and material properties on rail wear

Repeated wheel passes on rail can cause the accumulation of plastic strain in a thin subsurface layer of material through a process called ratcheting. If the accumulated plastic strain reaches the critical strain to failure material fails by ratcheting failure. This failure takes the form of either generating wear debris or causing rolling contact fatigue crack initiation. The severity of wear and the initiation of rolling contact fatigue crack depend on the contact conditions during service and the properties of the rail material. A computer simulation has been developed to simulate the effect of contact and thermal stresses and different material properties to predict the wear rate and the rolling contact fatigue initiation. The operating conditions, such as the maximum contact pressure, the friction coefficient, the slip/roll ratio and the train speed will determine the severity of the contact stress in the rail material. These contact conditions will also influence the amount of frictional heating that occurs at the interface during the rolling/sliding. The heat generated is transferred into both the wheel and the rail and increases their temperature. The temperature rise in the rail material can be higher if the wheel bulk temperature also increases due to continuous heating during rolling/sliding contact. When the hot wheel touches the rail, there will be additional heat flows in the rail in addition to the heat from frictional heating. The temperature rise in the rail material causes the development of thermal stress and can also cause thermal softening if the temperature is high enough. This thermal effect increases the accumulated plastic strain and leads to more severe wear and the initiation of rolling contact fatigue cracks. However, the greater wear rate may reduce the length of the crack-like flaw and prevent a crack being initiated at the surface, which brings the subsurface flaws closer to the surface. In addition to the contact conditions, different rail materials with different initial hardness, strain hardening behaviour, critical strain to failure, and thermal softening behaviour have shown different results in wear and the initiation of rolling contact fatigue. The material with greater initial hardness has been found to have a higher resistance to failure. However, during contact the initial hardness is altered by the hardening and softening process. As the material hardens, the plastic deformation reduces and hence reduces the failure. Likewise greater resistance to thermal softening results in material having a greater final hardness, and this can resist further failure. The material with the greatest final hardness was found to have better resistance to wear. The severity of rolling contact fatigue crack initiation depends on the dominancy of the wear rate over the growth of failures. If the wear rate is relatively low, then the crack may be initiated sooner as the contact loading becomes severe. On the other hand, a greater wear rate may truncate the length of a crack-like flaw and lead to later crack initiation.