Asperity and Material Parameters in Thermomechanical Cracking due to Moving Friction Load

摘要

When an asperity traverses at a relatively high speed over the surface of a medium, the resulting friction force in the contact zone may cause a 'hot spot' that would lead eventually to 'heat-checking' or thermomechanical cracking in the medium. The asperity traversing speed, the load distribution of the asperity over the contact area and the relative dimensions of the contact area all affect the stress state in the medium, thus the initiation of fracture therein. Moreover, the mechanical and the thermal properties of the medium also play significant roles in the susceptibility of thermomechanical cracking, thus wear of the friction surface. This paper shows that, by assuming a uniform distribution of the load over the contact zone, the predicted stress level could be 40% less than that of a non-uniform distribution of an equal total load. While the shape of the contact area does not affect the stress level, its size does. For the same average pressure, increasing the size in the direction of asperity travel raises the stress level; while increasing the aspect ratio (relatively increasing the length normal to the traversing direction) lowers the stress level. In the limit of an infinite aspect ratio (a plane strain model), less than 20% of the stress level at the aspect ratio of one results. The multifacet influence of asperity speed manifests in rate of thermal energy input and in the thermal layer thickness, which determines the magnitude of thermal stress and the location of fracture initiation.