Development of Correlation Between Experimental and Analytical Modal Analysis of Brake Pads

Disc brake squeal is caused by mechanical vibrations generated from friction force excitations exerted by the pad on the rotor. In an effort to understand the origin of these vibrations and speed the design of quiet brakes, proper characterization of the individual brake system components is desirable. In this paper we apply modal analysis, combining both experimental and analytical methods to measure and model the pad and rotor assemblies and compare the results to those obtained on vehicle and noise dynamometer tests.

Of particular importance, ultrasonic methods are used to measure the friction material elastic properties. The Young's moduli, shear moduli, and Poisson's ratios are essential for quantitative analytical modal analysis. We find that the friction materials are highly anisotropic. For example in some formulations we find more than a factor of five difference between the in-plane and out-of-plane Young's modulus. For shear moduli the differences are less pronounced, in-plane values are generally between 1 6 and 2 1 tunes the out-of plane values. All friction materials analyzed exhibit transversely isotropic symmetry and their mechanical properties can be adequately described by five independent elastic constants.

The ultrasonically measured friction material properties were used as input data for our analytical modal analysis model. With accurate property data, the error percentages comparing predictive versus measured pad resonant frequencies were in the 1-3% range. The low error percentages between analytical and experimental modal analysis made it possible to re-design the brake pad's backing plate in an effort to reduce noise and validate design iterations before building a prototype.