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The rising popularity of EVs has led to a resurgence of interest in drum brakes. Drum brakes benefit from less complex mechanical design, have no residual brake drag, and the enclosed design is less susceptible to corrosion and debris emission. For the commercial EVs, the elimination of engine noise makes brake noise a major contributor to vehicle noise. With the renewed interest in drum brakes, there is an increased need for property data for NVH simulations to optimize noise performance. Similar to disc brakes, the modeling of drum brake performance requires a complete set of friction material engineering properties determined over the pre-loads and temperatures encountered in brake applications. Results are presented for eight different drum brake formulations and platforms. The measurement approach and data analysis parallels that used for the elastic property measurements of disc pad friction materials, SAE J2725.

Friction material manufacturing is a complex process where numerous raw materials are mixed, pressed, and cured to make brake pads. It is important to have a consistent manufacturing process that can produce a brake pad that satisfies the vehicle braking requirements. A basic and critical requirement for any brake pad is structural integrity with no internal cracks. In this work a series of processing changes were made to intentionally produce internal cracks in the friction material. Various pad crack detection methods were studied, and their advantages and disadvantages are discussed in detail. One of the crack detection methods used an ultrasonic measuring instrument which gives objective data in the form of calculated modulus of elasticity and signal loss. The details of the machine and how the measurements are obtained are discussed. The modulus calculation is also described.

In this study several non-destructive test methods have been applied to as-manufactured automotive brake pads. The primary emphasis of our study is the formulation and development of ultrasonic methods where four independent velocity modes are measured on each pad. For two of the measurements, the ultrasound is propagated in-the-plane of the pad, while in two other measurements the ultrasound is propagated through-the-thickness (out-of-plane). Over 300 pads from five different manufacturers have been tested. In many cases, the ultrasonic data is compared with other testing methods including conventional compressibility tests, modal analysis, and hardness testing. In some cases, measurements have been made of several different batches of materials to test long term consistency of the material properties in the production environment. In other studies the production process has been deliberately altered to help establish specific cause and effect relationships.

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.