Dr. JeanFrancois Brunel, LaMCube Univ Lille, FRANCE
Dr. Van-Vuong Lai, LaMCube, FRANCE
Ing. Igor Paszkiewicz, Paszkiewicz, FRANCE
Prof. Maxence Bigerelle, Université Polytechnique Hauts-de-France (UPHF), FRANCE
Prof. Philippe Dufrénoy, LaMCube, FRANCE
Context and objectives
It is now accepted that squeal is associated to flutter instabilities related to mode coupling of the braking system components in frictional contact. Many models are devoted to determine the possible instabilities associated to these mode coupling, considering ideal friction contact areas. Some recent works have shown that the effective rubbing surface are different than the apparent one with an influence on instabilities [1-3]. These contact localizations modify mode coupling.
Such considerations are based on post-mortem surface analysis, as effective contact variation is a difficult to measure experimentally. However tribological considerations indicate that the contact area should be seen as a dynamic situation with always moving bearing areas. It is then difficult to verify if the effective contact area distribution and its variation during braking is directly linked to squeal occurrence.
This purpose is the subject of this paper which proposes to find a methodology to track the contact areas during braking and to investigate the consequence on squeal emission.
Identifying effective rubbing areas presents some difficulties not only due to the closed contact but also to various contact scales: here we consider 2 scales: macroscopic and mesoscopic i.e. at the scale of global contact area and at the scale of the tribological layer made of powders and plateaus.
Tracking effective contact areas is done using multi-physics measurements and post-mortem surface observations:
- Various measurements are done of mechanical data (forces, displacements, accelerations) and thermal ones of the disc surface, by infrared camera, and inside the pin, by inserted thermocouples. With the thermal measurements an inverse method is used to identify the heat dissipated areas which are associated to effective contact zones.
- Complementary analysis of the surface are done, by optical measurements, with test interruptions. These measurements have been submitted to a careful calibration in order to get information of the tribological mechanisms and topology. The optical measurements are made at very high resolution with specific lighting. They were calibrated with topography measurements allowing to obtain the surface topology and with SEM analysis to be able to distinguish the material components and the different stages of third body evolution. These measurements give information of contact localizations and surface evolution in terms of topography and tribolayer.
Noise and vibrations are recording during the tests and superposed to contact surface evolution.
Experiments are done on a simplified geometry to better identify the dynamic behavior and to reduce the contact area. The set-up is a pin on disc, with automotive brake dimensions and a realistic friction pair. The friction material formulation has been simplified (reduced number of components) to facilitate the understanding of tribological mechanisms at the contact interface. The tests are done with drag braking sequences, varying pressure and speed (with initial bedding-in). Loading level is limited to avoid too strong material modifications and too fast surface evolutions.
In order to understand the dynamic behavior of the system and to investigate the effect of contact localization, the system has been modelled numerically by the FEM (stability and time-domain analysis). The model has been developed to introduce a non-uniform contact at the macro and meso scale.
Main results / innovative aspects
Results show that squeal frequency variations can be linked with contact evolutions, especially the contact area. This can be clearly seen thanks to the thermal and optical measurements, and these dependency between squeal frequencies and contact localization has been confirmed with the numerical model. This phenomenon is qualified as macroscopic.
The other main result concerns noise occurrence associated to the surface evolution and to the associated tribological mechanisms. A tribological stage appear to fit with squeal occurrence in addition to the macroscopic stage previously described. This phenomenon is qualified as mesoscopic.
The innovative aspect of this work concerns the link between contact surface evolution and squeal occurrence with a superposition of two scale levels (macro-meso) related to squeal frequency and squeal occurrence. These scales concerns the global contact areas and the tribological mechanisms (topography and third body nature). Thanks to the improved instrumentation these information can be obtained quantitatively.
The main limitation is the difficulty of transposing the results to another scale or a more complex system that presents a different dynamic behavior and additional non-linearities (complex assembly). Nevertheless general indications can be extracted and the methodology can be extended to more complex systems.