Changing Properties of Brake Pads and Discs during Brake Testing

Earlier publications show that brake pad physical properties such as hardness, modulus and natural frequencies continue to increase at room temperature over a period of 12 months and that the changes are faster during the first 3 - 6 months. The current investigation was undertaken to see how the properties might change during testing for the pads as well as for the discs. Low-copper and copper-free formulations were tested on pickup truck and passenger car brakes. In all cases, the dynamic modulus and natural frequencies are found to decrease (not increase) after the SAE J2522 performance testing, indicating that the stiffness of the pad and that of the disc decrease faster than the mass loss due to wear. Also, the inboard pad and the outboard pad change at two different rates. Questions arise on what properties to measure for quality assurance and when to measure in light of these changing properties as well as the fact that the relationship between the initial properties and the later properties is not well established and that the relationship between the properties and brake friction/NVH is also not well established.