Brake pad compressibility is a parameter which is commonly utilized within the braking industry for design and production considerations. With a number of procedures relying on the accurate measurement of this metric, Greening Testing Laboratories, Inc. undertook to examine the extent to which different intrinsic or extrinsic factors may affect these measurements. This paper is a status update on continuing efforts by our laboratory in this area, first reported in early 2022.
The primary goal of this study was to measure how brake pad compressibility changed as a function of the environmental humidity in which they were stored in prior to testing, due to the absorption of moisture. To properly evaluate these deviations, it was also necessary to evaluate and compensate for how compressibility values may change over the course of repeated testing due to material characteristics alone - a parameter which may be of significance in its own right. Two axle sets (eight pads) of five different brands of ceramic style material were purchased and tested over the course of five testing sessions lasting eight days. Four of these pads alternated between storage at high or low humidity levels prior to testing while four control samples from each type were kept at constant humidity over the same period. Results of compressive testing revealed considerable variation in deflection across samples of the same material, and a small decrease in deflection was observed with repeated testing. Humidity-induced changes were about 2 - 10% of total deflection and showed repeatability for the same sample, but with both intra- and inter- manufacturer/type differences. These observations may play a role in the procedural standardization of brake compressive testing and may also be a valuable tool in enhancing our understanding, leading to improvement of brake performance.