Thermal Stress Analysis of Automotive Disc Brakes

A brake disc rotor forms pan of a foundation brake and rotates with the wheel hub assembly. The main function of a foundation brake is to generate a retarding torque by converting mechanical energy to thermal energy by virtue of the frictional work done in relative sliding at the rotor-pad interface. In practice, most brake discs arc made from
cast iron and in use arc sometimes subjected to high thermal stresses which can lead to permanent plastic deformation and occasionally rotor cracking. The aim of the present work is to investigate the thermal response of such a cast iron brake disc using the finite element (FE) method. One particular existing brake disc design for a medium passenger car was chosen for the investigation. This is a "back-vented" disc designed to minimise disc coning but the present work also includes an investigation of a conventional "frontvented" disc similar in dimension to the back-vented disc but with different detailed geometry. Experimental work was undertaken to derive the rotor material properties in tension and compression as a function of temperature. This data was used to generate
suitable FE material model routines which accurately allow for the different temperature-dependent yield properties of cast iron in tension and compression. The modelling work highlights the role of the rotor geometry in controlling the thermal response of the structure but. more importantly, the necessity for an accurate material
model is illustrated, particularly with regard to its ability to predict the accumulation of plastic strain which may lead to rotor cracking. Using the most accurate user developed material subroutine, the thermal response of the back-and front-vented disc designs are compared: the back-vented disc suffers lower thermal distortion but at the expense of higher plastic strain accumulation, particularly near the point of attachment of the vanes.