Ing. Alessandro Lazzari, University of Rome 'La Sapienza', ITALY
Ms. Simona Totaro, University of Rome 'La Sapienza', ITALY
Dr.-Ing. Davide Tonazzi, University of Rome 'La Sapienza', ITALY
Prof. Dr.-Ing. Aurélien Saulot, INSA-Lyon, FRANCE
Prof. Dr.-Ing. Francesco Massi, University of Rome 'La Sapienza', ITALY
-Research and /or Engineering Questions/Objective:
In brake industries, one main concern regards friction-induced vibration originating from the brake frictional interface. Specific braking conditions, in fact, can lead to dynamic instabilities, which are the result of the coupling between the system dynamics and the frictional response of the materials in contact. These forms of vibration are often undesirable and can cause excessive wear of components, surface damage, fatigue failure, and noise. Nevertheless, even if several excitation mechanisms of friction-induced vibrations are known, not all the phenomena observed in practice can be explained or fully characterized. Moreover, when dealing with commercial brake systems, several parameters could affect the material propensity to destabilize the system dynamics and it is not possible to have a reliable estimation of the material vibrational response only by tests performed on a full brake system. The present work is developed within this framework and aims to the understanding and characterization of vibrations induced by the sliding of carbon/carbon (C/C) materials, widely adopted in brake lining applications.
C/C materials have been tested by means of a tribometer, which allows studying the frictional and vibrational response of the investigated materials by imposing the relative motion between C/C specimens, under well controlled boundary conditions, and without introducing parasitic noise. As a result, tribological and dynamical information on the tested materials have been retrieved, while the material propensity to generate dynamic instabilities is investigated as a function of different contact parameters. The results are then discussed and the correlation between the frictional response and the onset of the instabilities, under specific braking conditions, is analysed. Moreover, a two degrees of freedom lumped-parameter model has been developed, in order to investigate numerically the observed vibrational phenomena and provide a further characterization of the C/C dynamical response.
The experimental and numerical analysis, carried out on C/C materials, has allowed retrieving meaningful information on the operating conditions leading to unstable friction-induced vibrations and on the main features of the observed dynamic instabilities. Furthermore, the C/C tribological and dynamical behaviours have been investigated under a wide range of temperatures and a variable imposed velocity profile, providing information so far missing in the literature.
-Limitations of this study:
Even if the test bench has been developed with a proper thermal insulation system, the high temperature reached during the tests limits the exposure time of the specimens to so high temperature conditions. Nevertheless, the successfully designed thermal shield allows for a sufficient retention time of the samples in order to obtain the desired information in terms of dynamical and frictional response.
-What does the paper offer that is new in the field in comparison to other works of the author:
The work provides new outcomes on the frictional response of C/C materials and on the propensity to generate dynamical instabilities when varying the main contact parameters. Thanks to the developed numerical model and to the experimental campaigns, a further characterization of the unstable dynamical response of C/C materials is achieved, as well. Moreover, tests carried out under controlled boundary conditions, on a specifically designed test bench, provide information on the C/C material behaviour even when severe temperature conditions occur during the braking.
The experimental campaign carried out on tested C/C specimens pointed out the onset of dynamic instabilities arising when varying the main contact parameters. The possibility of studying the vibrational and tribological response of the tested materials, numerically and experimentally, provided new information on the C/C material response and allowed a characterization of the vibrational phenomena originating from the sliding of the contact interfaces. Overall, this work provides a wider understanding of the conditions under which friction-induced vibrations lead to dynamic instabilities and explores the frictional and dynamical response of C/C materials under diverse boundary conditions.