Experimental and numerical correlations between material and morphological parameters and vibration phenomena of a disc brake

Since nowadays the NVH performance of vehicles, train included, has become a priority, the noise radiating from brakes is considered a source of considerable passenger discomfort and dissatisfaction. Creep groan and squeal that show up with annoying vibrations and noise in specific frequency ranges are typical examples of self-excited brake vibration caused by, respectively, the stick-slip effect and the coupling of brake disc and friction pads or caliper. In both cases, the friction coefficient is a fundamental parameter for the occurrence of the vibratory phenomena. The knowledge of the friction coefficient of the pair of materials in contact, and also its possible dependence on the morphology of the surfaces and the operating conditions is therefore essential for predicting the behavior of the brake and supporting design choices. The present work presents the tests carried out with a pin-disc tribometer according to ASTM on specimens of brake friction material, with circular cross-section of different sizes while the disc was of cast iron. Tests were conducted imposing the normal load and the sliding speed and measuring the tangential force. The correlation between the coefficient of friction and the load, the nominal pressure and the sliding velocity, was calculated and compared with literature models. A negative correlation with the sliding speed and a positive correlation with applied load was found as well as a weak correlation with the nominal pressure. In parallel, a finite element (FE) complex eigenvalue parametric analysis was performed on the brake assembly to evaluate propensity to dynamic instability of brakes with multiple pads as a function of the number of pads, geometrical and material parameters obtaining useful indications.