Thermoelastic instabilities in automotive disk brakes and clutches

This dissertation presents theoretical and experimental studies on the phenomenon of frictionally excited thermoelastic instability or “TEI” in automotive disk brakes and clutches.; In this dissertation the finite element method is used to investigate the effect of geometry on TEI problems. Results are obtained for two-dimensional layer and three-dimensional geometries. The hot spots are generally focal in shape, though modes with several reversals through the width start to become dominant at small axial wavenumbers. The dominant wavelength and critical speed are not greatly affected by the three-dimensional effects, except for banding modes. Also, the three-dimensional solutions can be approximated as a monotonic interpolation between the two-dimensional critical speeds for plane stress and plane strain.; The method is then extended to the axisymmetric clutch or brake geometry. Linear perturbations are sought that vary sinusoidally in the circumferential direction and grow exponentially in time. These factors cancel in the governing equations, leading to a linear eigenvalue problem on the two-dimensional cross-sectional domain for the exponential growth rate for each Fourier wavenumber. The algorithm is tested against an analytical solution for a layer sliding between two half-planes. The method is then used to determine the unstable mode and critical speed in geometries approximating current multi-disk clutch practice.; The intermittent contact problem in caliper/disk brake systems is also solved numerically by setting up a frame of reference stationary with respect to the pad. An upwind scheme is introduced in the finite element formulation to overcome the numerical difficulties. The technique is first used to obtain solutions for the two-dimensional problem in which a layer slides between two blocks. It is then extended to a three-dimensional example representing a practical caliper/disk brake system.; A series of brake dynamometer drag were made to investigate experimentally the phenomenon of TEI in an automotive disk brake. The temperature field on the rotor surface was measured with infrared (IR) detectors and a high-speed data acquisition system. The Fast Fourier Transform (FFT) method was used to determine the exponential growth rate and critical speeds. Linear interpolation was then used to determine the speed for zero growth rate—i.e. the critical speed. The results for the critical speed and the number of hot spots show good agreement with the numerical predictions given by the TEI model with intermittent contact.