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Brake squeal is an uncomfortable noise that occurs while braking. It is an important issue in automobile quality to prevent brake products from squealing. Brake shims are widely used to reduce squeal occurrence rate. The anti-squeal effect of shims is quantified as damping properties measured with a bending mode tester, instead of repeating many dynamometer tests. However, there are cases where measurement results have less correlation to actual squeal suppression rate. Therefore, the evaluation of the anti-squeal effect with a dynamometer or on an actual car is needed until the best shim can be selected. To improve the predicted accuracy of the anti-squeal effect, the difference between measurement conditions and actual braking conditions of shims, was focused on. The bending mode tester measures loss factor under pressure-free conditions, even though shims are compressed by pistons or cylinders towards the backplate of the pad.

Brake pads are composite materials made from dozens of ingredients intended to simultaneously satisfy various performances such as brake effectiveness, wear, noise and vibrations. For this reason, the friction phenomena that occur during braking are complicated. It is important to clarify the friction phenomena, but that is not easy because the associated complexities as mentioned above. We looked to acoustic emission (AE) as an online evaluation method of friction phenomena. AE is a non-destructive testing method that measures elastic stress waves caused by the deformation and fracturing of materials. In fact, it has been reported that the difference between abrasive wear and adhesive wear of a metal can be identified from the change in the frequency spectrum of AE signals. In this study, we verify whether differences in the friction phenomena of brake pads are detectable by the AE method. Three kinds of brake pads were used in the experiments.