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The Automated Universal Tribotester (AUT) is developed by the Institute of Dynamics and Vibrations (TU Braunschweig) and represents a reduced scale brake dynamometer. The setup is based on the pin-on-disc principle and the down-scaled test specimen are brought to contact to the disc and loaded via the specifically designed load unit. The AUT’s load unit is designed as a combination of parallel and serial leaf springs, resulting in a friction free motion. The stiffnesses in radial and tangential directions are much higher than in normal orientation. For the investigation of wear debris over time, changes in loads (e.g. forces, speeds, temperatures) are applied. Those varying loads result in tilting of the contact surface of the test specimen due to small elastic deformations. A change of the contact area is inevitable, and long time periods are needed to adopt the contact area to the new conditions. This prevents from investigating fast changes in the above mentioned loads.

The dynamics and, in particular, the NVH phenomena in brakes are still in the focus of research. Recent investigations of for example Rhee et al. show two principal vibrational forms of the linings on the rotor [1]. The first form is characterized by vibrations where both linings are in-phase (minimal differential torque between the inner pad and the outer pad). This produces in-plane vibrations of the rotor and results in high-frequency squealing events in the brake. The second form is an antiphase vibration of the brake linings with respect to each other (increased differential torque between the inner pad and the outer pad). This produce directly out-of-plane vibrational modes of the disc, which results in lower-frequency caliper and rotor oscillations. One hypothesis is that different wear densities of the linings essentially characterize the two vibrational modes. The wear behavior is not taken into consideration of this paper as it will be discussed in further publications.

In today’s research and development of brake systems the model-based prediction of complex vibrations and NVH phenomena plays an important role. Despite the efforts, the high dimensional computational simulation models only provide a limited part of the results gained through experimental measurements. Several reasons are discussed by the industry and academic research. One potential source of these inadequacies is the very simple formulation of the friction forces in the simulation models. Due to a significant shorter computation time (by orders of magnitude), the complex eigenvalue analysis has been established, in comparison to the transient analysis, as the standard method in the case of industrial research, where systems with more than one million degrees of freedom are simulated.

Inertia dynamometers are commonly used to determine the friction coefficient of brake assemblies. Dynamometers are a well-established platform, allow testing under controlled conditions, exhibit a good correlation to many situations encountered in real driving, and are comparatively economical and less time-consuming than full vehicle test. On the other side of the spectrum is the use of scaled tribometer. These test systems make possible a test without the entire brake corner. This separation allows the investigation of the frictional-contact only (frictional boundary layer) speedily and independently of a given brake system or vehicle configuration. As the two test systems (inertia dynamometers and tribometers) may have different users with possibly different tasks, the question remains regarding how comparable the two systems are. These issues provide incentives to better define the fields of investigations, correlation, and applicability for the two systems.

There are few principal excitation mechanisms that brake system NVH simulations are based on, especially the high frequency squeal simulations. These mechanisms can be described by some simple mechanical models that exhibit excitation or self-excitation effects induced by friction [1, 2]. These models use very simple friction laws of Coulomb type, described by a friction coefficient that is either a constant or simple functions of some state variables, taking into account a Stribeck characteristic. Measurements from the AK-Master or SAE J2521, however, show that the friction coefficient is not a simple function of some state variables, describing a steady state behavior of friction. In the past several years, material dependent descriptions of the frictional brake interface have started attracting attention [3]. These aspects are greatly influenced by the tribological effects at the frictional interface, which can be characterized by typical wear patterns.

Although the radial migration of hot bands has been frequently observed, a systematic investigation of this phenomenon has not yet been performed. The ring-shaped temperature maximum, which occurs on the brake disk, is undesirable because the focused temperatures destroy the local materials in contact. Moreover, a hot band carries a dominant portion of the frictional load. If a hot band moves radially, the braking torque is directly influenced. It is supposed that material wear influences the radial hot band migration. New models demonstrate that wear is indeed the mechanism that triggers hot band migration. First, a minimal model including thermal expansion and a load-dependent loss of material is introduced. The simplicity of the model allows for an understanding of the impact of wear, as well as the mechanisms that lead to a periodic load distribution. This model can be analyzed in terms of complex eigenvalues, showing a periodic load distribution in the sliding plane.