Non-linear Aspects of Friction Material Elastic Constants

In this report, we describe the ultrasonic measurement process applied to a series of three brake pad materials with varying degrees of load-dependent, non-linear behavior. For each material type, we measure the spatial variation of the ultrasonic velocity and the spatial variation of ultrasonic attenuation on several, as-manufactured, brake pads. We correlate these ultrasonic test results with those obtained by conventional compressibility tests. Subsequently, we destructively analyze each friction material. The load-dependence of the through-the-thickness longitudinal and shear velocity is measured using static loads ranging from 0.5 MPa to 8.0 MPa. For selected friction materials exhibiting significant variations in velocity with load, we compute the full set of engineering constants. Our analysis includes computation of the Young's modulus, shear modulus, and Poisson's ratios as a function of load. We discuss the sensitivity and repeatability of the methods and their application to both material formulation development and improved material uniformity.