Squeal vibration properties of braking systems have been investigated for decades in the field of NVH studies. Thanks to the expansion of powerful and rich measurement techniques like 3D-laser vibrometers, it is now possible to describe with good quality the vibration frequencies and 3D operational deflection shapes (ODS) of braking systems. That kind of measurement is usually performed in frequency-domain and gives access to steady-state vibration data while the brake squeal instability has already reached its limit-cycle.
The study of brake squeal phenomenon is much less investigated during the unstable vibration onset because of its transient and complex nature. However, this vibration onset phase allows also observing an interesting brake squeal feature like the brake squeal instability index or the so-called negative damping of the system, which can be compared to the operational damping properties of the brake system before the squeal onset. This specific damping feature can be used to correlate Finite Element Models Complex Eigenvalue Analysis through the focus on unstable modes negative real parts.
The present investigation presents a laser-based measurement campaign. Several techniques borrowed to time- and frequency- analysis are described and damping properties from an operational brake system are derived before/during squeal vibration onset as a result. A methodology is described for identifying the squeal instability index from test-data. The onset of vibration is observed and discussed in terms of modal participation or time-delay on several components of the braking system like the pad, the disc, the caliper and the anchor bracket. The damping property results are discussed as global system properties contrary to local sub-system damping properties. The discussion on global to local braking system damping properties set a new point of view on localizing the source-location of squeal vibration instability onset. Finally, an example of Finite Element Model Complex Eigenvalue Analysis illustrates the possibility of a correlation between the experimental squeal instability index and the real part results of unstable system.
Gabriel Rejdych, Thierry Chancelier, Thierry Pasquet - Chassis Brakes International