Mesomechanics of Metal-Polymeric Pairs in Friction Units under Low-Cycle Fatigue

The stress-strain state of metal-polymeric friction pairs for vehicle brakes under low-cycle fatigue is examined from the standpoint of mesomechanics. The concepts of metal fatigue are developed in the scope of physical mesomechanics associated with studies on the damage accumulation, primarily in the field of multi-cycle fatigue. This has led to the understanding that low-cycle fatigue should correspond to a much higher level of damage accumulation in metal. A model of unsteady-state friction and fatigue modes of contacting elements is developed, wherein thermal explosions are the most characteristic features of the friction in the metal-polymeric friction pairs. It is shown that at the first stage, the internal chemical energy of the subsurface layer of the polymeric patch is converted into heat. Novel regularities concerning the onset and development of microcracks on the metal elements of friction pairs and on their operational surfaces in the presence of mechanical stress concentrators taking into account thermal stresses are established. The proposed model explains changes in the mechanism of crack propagation under the conditions of low-cycle fatigue in metal-polymeric friction pairs when air enters the crack and leads to the oxidation of fracture surfaces with the formation of a film. It is proposed that in this case the formation of surfaces with a subgrain structure occurs. The rate of the changes can be used to study the effect of cooling conditions on the kinetics of thermal processes in such friction pairs.