Research and/or engineering questions/objectives: Dynamic contact instabilities represent a common issue in brake industry, causing noise, structural vibrations and wear of surfaces, resulting in large warranty cost for brake producers. Different dynamic phenomena have been individuated to be at the origin of unstable vibrations, such as negative friction-velocity slope and mode coupling, commonly accepted to be the cause of brake squeal. Several research works investigated the effect of contact parameters on the occurrence of mode coupling, but some apparently random variability is still associated with squeal occurrence. In this optic, this works aims to investigate the relationship between friction noise, intended as the dynamic excitation coming from interface interactions, and the system unstable dynamic response. Methodology: An experimental test bench, specifically designed to reproduce and measure friction induced vibrations, has been used for the experimental campaign. The test bench, by the use of air bearings, isolates the parasitic external noise and maintains controlled boundary conditions. The developed setup allows to impose different motion and temperature conditions, in order to reproduce, by the variation of this parameters, both stable and unstable dynamic responses. Accelerometers, placed on the test bench structure, and a LASER vibrometer, focused as close as possible to the contact interface, allow to measure the dynamic response at different scales, the structural and the contact one, respectively. Results: The performed tests allowed to investigate for the material frictional response under different imposed conditions. It has been observed that, even in presence of similar frictional responses, the dynamic response was stable, in some cases, and unstable, in others. This variability of the vibrational response has been correlated with the spectral energy content (PSD frequency distribution) of friction noise, suggesting, for the dynamic excitation coming from the contact interface, a possible role in triggering the instability. Limitations of this study: The measure of the interface dynamic, intended as the dynamic response due to physicochemical interaction between asperities, ideally dependent only on material pair and contact conditions, is difficult to be decoupled from the system dynamics, dependent on the specific setup geometry. For this reason, from the obtained results, only general trends can be transposed on different geometries. Despite this limitation, it is worth to note that the correlation between friction noise and unstable vibrations is not specifically linked to the specific dynamic system, and can be qualitatively generalized. Original content of the work: The occurrence of mode coupling has been extensively studied in the literature, however, the focus has always been centered on the dynamics of the system. The role of the dynamic excitation (friction noise) coming from the contact interface has rarely been considered [1]. This work investigates for a correlation between dynamics at the mesoscopic scale, at the level of contact interface, and dynamics at the macroscopic scale, at the system level. Conclusions: The present work aims to provide a possible explication for the apparently random variability that is often associated with the occurrence of brake squeal. An experimental campaign, by the use of a LASER vibrometer, evidenced the correlation between the spectral energy distribution of the excitation coming from the contact interface, namely, the friction noise, and the occurrence of mode coupling unstable vibrations. [1] A. Lazzari, D. Tonazzi, e F. Massi, «Squeal propensity characterization of brake lining materials through friction noise measurements», Mech. Syst. Signal Process., vol. 128, pp. 216–228, ago. 2019, doi: 10.1016/j.ymssp.2019.03.034.

Ing. Simone Ciprari, PhD Student, Sapienza University of Rome; Dr.-Ing. Davide Tonazzi, Researcher / Assistant Professor, Sapienza University of Rome, Department of Mechanical and Aerospace Engineering, Via Eudossiana 18, I-00184 Rome, Italy; Prof. Dr.-Ing. Francesco Massi, Full Professor, Sapienza University of Rome, Department of Mechanical and Aerospace Engineering, Via Eudossiana 18, I-00184 Rome, Italy; Prof. Dr.-Ing. Aurélien Saulot, Full Professor, Univ Lyon, INSA-Lyon, CNRS UMR5259, LaMCoS, F-69621, France