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One of the critical characteristics in a brake pad is its deformation when subjected to compressive loads. This deformation is called compressibility, which can be obtained quantitatively through compression cycles performed by a specific test bench. However, such testing is costly and long enough not to meet the demand of manufacturers of friction materials. To overcome this difficulty, this work presents regressions for the estimation of compressibility through the natural frequencies of the pads, since they can be obtained quickly and at low cost through a simple modal analysis. The theoretical basis for the correlation between compressibility and natural frequencies includes the theory of elasticity - which defines an inverse relationship between strain and stiffness - and the theory of vibrations, which defines the natural frequencies of a mechanical system as a function of its mass matrices and stiffness.

The paper proposes a method to qualify the occurrence and behavior of noise in a braking or a set of brakings, resulting in a concept from 0 to 10. For this qualification, analytical equations consider intensity, frequency and duration effects of noise, operation temperature of brake, speed and deceleration of vehicle, besides weighting the number of occurrences, when dealing with a set of brakings. For the validation of the proposal, some noise tests conducted with an inertial dynamometer are subjected to the method, presenting results that shall be exposed.

Several studies on acoustics and vibration have been encouraged due to the automotive industry's interest in projecting less noisy braking systems. In the interest of furthering these kinds of studies, understanding one of the main vibration excitation phenomena in friction systems, the stick-slip, is very important. Stick-slip is an auto-excited vibration that might occur in a fixed body that frictions with another moving body, based on the difference between the static and kinetic friction force. For this reason, characterizing the friction material regarding the static and kinetic coefficients seems to be essential for developing simulations and experiments. On the other hand, the characterization of the static coefficient is very complex due to its dependence on time while the surfaces were in stationary contact.