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Industry and academia agree that brake squeal is a nonlinear phenomenon. Consequently, using solely linear finite-element (FE) models and assessing the tendency of a brake system to squeal exclusively on the stability of the trivial solution is not appropriate. However, the latter approach - in the brake community known as complex eigenvalue analysis (CEA) - is extensively used in industry. Until now, nonlinear simulation approaches considering existence and stability of periodic solutions are mostly limited to minimal models. Among the variety of reasons for this the complexity of large-scale nonlinear models as well as the identification of nonlinear material and system parameters are crucial. This contribution discusses the relevance of nonlinearities in friction brake noise, vibration, harshness (NVH) and presents a novel simulation approach for brake squeal.

The development process of automotive brakes is known to be challenging and time-consuming. It is an iterative process consisting of interplay between brake squeal simulation and extensive experimental investigations of the brake system at the test rig and in the vehicle. In this context, the complex eigenvalue analysis (CEA) of linearized FE models is a part of standard development process of brake systems. Nevertheless this linear analysis has not reached the status of a predictive tool yet, remaining a tool accompanying experimental investigations of the brake system only. Possible reasons may be inadequate simplifications of frictional contact, damping effects and friction material modeling on one hand and insufficiencies of the mathematical mechanical models themselves, i.e. linear vs. nonlinear stability analyses on the other hand. The extensive experimental investigations apply time consuming standard test procedures and need efficiency improvement.

Friction material properties influence brake squeal simulation results decisively. It is well known that friction materials exhibit nonlinear and transversely isotropic characteristics dependent on the type and direction of loading. In order to improve brake squeal prediction reliability, friction material properties identified under squeal loading conditions have to be introduced to the simulation models. Because of this fact, the development of a measurement and identification method for friction material properties in context of brake squeal simulation is in progress. The present paper presents the further developed Dynamic Compression Test Rig (DCTR) and the enhanced evaluation method for the estimation of the normal dynamic component stiffness of friction material specimens under typical squeal conditions. In general, the development of testing procedures implies a set of influence and uncertainty factors, which may influence measurement results decisively.

Brake lining material is one of the main factors influencing brake squeal. Actual simulation of brake squeal suffers on the missing of correct material parameters identified under conditions relevant for squeal. The comparison of different measurement methods for friction material characterization, e.g. compressibility tests, modal analyses or ultrasonic measurements shows that the material properties strongly vary depending on the testing conditions which are static preload, dynamic amplitude, frequency range and the loading direction. The different results obtained from these various test procedures show a nonlinear and transversely isotropic material behavior of the brake lining. In order to identify the correct material parameters for successful brake squeal simulation it is necessary to reproduce the operating conditions during the squealing state as close as possible in experimental setups.

Brake squeal is usually investigated using linearized models and the eigenvalues of the linear equations of motion. Eigenvalues with positive real parts are interpreted as the onset of squeal. Nonlinearities are commonly neglected due to the high effort associated with the corresponding calculations. Following the linear theory, the vibration amplitude should increase exponentially. On the other hand experimental results and overall experience show, that brake squeal is a stationary or quasi-stationary vibration phenomenon with approximately constant amplitude. This can only be explained by introducing nonlinearities into the model. These nonlinearities are limiting the increasing vibration amplitudes to a stationary limit cycle. Considering experimentally identified material properties of the brake lining as the main source of nonlinearities in the system a nonlinear disk brake model is introduced.

The last decades have shown extensive efforts on the investigation of automotive disk brake squeal. The origin of brake squeal is seen in self-excited vibrations, caused by the friction forces transferring energy from the rotating disk into the brake system. Based on a very simple model, Popp et al. described in 2002 the conditions for positive work of the friction forces (i.e. excitation of squeal), which depends on the phase shift between the in-plane motion (with respect to the disk) of the brake pad and the friction forces. Experiments on active manipulation of this phase shift using pads with integrated piezoceramic actuators, performed by von Wagner et al. in 2004, resulted in successful suppression of disk brake squeal. The authors of the present paper used a variety of models for the investigation of the origin of the excitation mechanism by observing phase relations between the friction forces and the vibrations of the pads.

Considerable effort is spent in the design and testing of disk brakes of modern passenger cars. This effort can be reduced if refined mathematical-mechanical models and new experimental techniques are used for studying the dynamics of these brakes. The present paper is devoted to the modeling and experimental investigation of a floating caliper disk brake, special regard being given to the suppression of squeal using active elements. To actively suppress brake squeal, “smart pads” were designed and manufactured. These pads contain piezoceramic staple actuators, which can be independently driven at both pads and within the pads. In experiments they were successfully used for the active suppression of squeal via optimal control. As the piezoceramic elements can be used both as actuators as well as sensors, the “smart pads” are also useful in experimental investigations such as measuring transfer functions.