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.
Whereas many of the other testing methods measure the properties of the brake pad component, the ultrasonic technique uniquely measures only the friction material properties. Furthermore, the ultrasonic methods have spatial resolution on the order of one square centimeter, thus it is possible to make multiple measurements within a single pad. Data will be presented comparing the variation observed within pads, to the pad-to-pad variations and batch-to batch variations. These ultrasonic data will be correlated with compressibility, modal data, and hardness data.
The development of the ultrasonic measurement technique applicable to as-manufactured components will be described in detail. The reproducibility and repeatability of the methods are quantified by comparing repeat measurements on the same samples as well as comparing measurement made by different operators/instruments. Lastly, the ultrasonic measurement process is intrinsically fast and can be readily automated. We will discuss how these methods can be automated in order provide in-line quality assurance measurement capability at the point of manufacture.