Study of wet friction material test under severe thermal and mechanical loading ("bump test")

One of the fatigue tests used for friction materials is the so called ''bum p test". In this lest one of the reaction disks in a pack of disks has mach ined bumps across the contact surface. These bumps generate locally high co ntact pressure and temperature. In this way, severe contact conditions like that arising in service can be reproduced. The major difficulty in utilizi ng the "bump test" is estimation of the local pressure and temperature. In this paper, a model is developed and experimental verification of thermoela stic contact is presented. The model represents a multi-surface contact pro blem for a pack of disks. Dice to the presence of blimps, contact surfaces are not conforming. To solve tills strongly nonlinear contact problem, a ro bust finite element code is used The model also includes thermal effects ge nerated by frictional heating. The coupled elastic contact and thermal prob lems are solved sequentially. Contact pressure at interfaces is measured in static conditions (without sliding) using pressure sensitive film. Good ag reement between the monel and the measurement is observed in sense of conta ct length and magnitude of pressure. In order to verify the full thermoelas tic model, the transient temperature inside one of the disks is measured by thermocouples. The results agree with model predictions. The study shows t hat the local contact pressure is an order of magnitude higher than,nominal pressure. Also temperatures are much higher than that predicted for unifor m contact conditions. The developed model helps to understand complex therm oelastic effects accompanying the test and to control the test parameters.