Frictionally excited thermoelastic instability in automotive disk brakes.

Abstract

When two bodies slide against each other at sufficiently high speed, the interaction among the frictional heat, thermoelastic distortion and contact pressure becomes unstable, leading to the development of regions of very high local temperature, or hot spots. This phenomenon, known as frictionally-excited thermoelastic instability, or TEI, is of critical importance in the design of high performance brakes since TEI causes hot spot-related vibrations, fade and material degradation. The sliding speed at which instability occurs is referred to as the critical speed. It has been previously calculated for two half-planes using the perturbation method. The resulting critical speed was found to be considerably higher than the speeds at which hot spots are observed experimentally in practical sliding systems, such as automotive brakes. An important factor in this discrepancy is probably associated with the finite geometries of actual brake systems. In this dissertation, theoretical studies were conducted to predict a more accurate critical speed of the automotive disk brake system. The effects of the finite disk thickness on the stability was determined by analyzing a finite layer sliding between two stationary half-planes. Results show that the layer model predicts critical speeds of the order of those observed in automotive disk brakes. To increase the critical speed, it is necessary to increase the thermal conductivity of the brake pad, shorten the brake pad, and decrease Young's modulus of the brake pad. Additionally, experiments were performed with an actual disk brake system by measuring surface temperatures of the sliding pair. A circumferential perturbation (hot spots) is generated on the brake pad at relatively low sliding speeds and, as the pad temperature increases, this evolves into a radial perturbation (hot band) oscillating in time. A thermoelastic stability boundary is also established in a temperature and speed space for given braking materials.