Cho, J., Jeong, J., Kim, H., Lee, H. et al., "Development of Noise Propensity Index (NPI) for Robust Brake Friction," SAE Int. J. Commer. Veh. 10(2):589-595, 2017.
A semi-empirical index to evaluate the noise propensity of brake friction materials is introduced. The noise propensity index (NPI) is based on the ratio of surface and matrix stiffness of the friction material, fraction of high-pressure contact plateaus on the sliding surface, and standard deviation of the surface stiffness of the friction material that affect the amplitude and frequency of the stick-slip oscillation. The correlation between noise occurrence and NPI was examined using various brake linings for commercial vehicles. The results obtained from reduced-scale noise dynamometer and vehicle tests indicated that NPI is well correlated with noise propensity. The analysis of the stick-slip profiles also indicated that the surface property affects the amplitude of friction oscillation, while the mechanical property of the friction material influences the propagation of friction oscillation after the onset of vibration. An additional case study for noise reduction was carried out using commercial brake friction materials before and after the modification of NPI. The results of the case study based on in-vehicle noise tests indicate that a robust friction material can be designed by reducing the stiffness ratio and large contact plateaus on the sliding surface of brake friction materials.