Thermoelastic instabilities in the sliding of conforming solids

Abstract

If two sliding solids are nominally in contact over a large area, the inevitable irregularities in the surfaces will cause the pressure distribution to be non-uniform. The generation of heat due to friction at the interface will also be non-uniform and the solids will be distorted by thermal expansion. The highest parts of the surface will carry the greatest pressure, reach the highest temperature and consequently expand more than the surroundings. Thus the thermal distortion tends to exaggerate the initial irregularity of the surface. The wear at the interface has the opposite effect, but under suitable conditions the process can be unstable. Experiments are described in which the effects of this instability are observed: the load is concentrated into a few small parts of the surface causing high local temperatures. After a few seconds, the load is transferred to a new part of the surface and, when the original contact area has returned to the temperature of the surroundings, it contracts leaving an observable depression in the surface. The scale of this process is large in comparison with the size of the surface asperities. An experimental model has been produced which demonstrates the characteristics of the instability in a simplified form. The thermal expansion drives the surfaces apart at the beginning of the cycle and this movement has been observed experimentally. An analysis of the instability is produced and a good correlation is obtained with experiment.