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Widely known is the fact that friction and wear characteristics of disk brakes are subject to pronounced temperature dependencies. For systems with organically bound brake pads, many thermally induced material changes can occur, ranging from degassing of the phenolic resin binder up to degradation of fibers and melting of metallic components. All these effects modulate the surface structure between pad and disk. They are a major contributor to friction layer dynamics [1] and directly influence the system's performance. Concerning the calculation of contact temperatures in disk brakes, several attempts have been made in the past. Most of them, however, use drastic assumptions (e.g. homogenous materials and ideal contact), which limit the results to qualitative approximations [2]. Recent studies already include the multi-material structure of brake pads. These give indications on how material mixtures must be changed, in order to modify contact temperatures into a certain direction [3].

Shorter product cycles, a high awareness for comfort properties on the customer side and an increasingly strict legislation with respect to environmental issues make the development of friction materials more and more challenging. In contrast to many other engineering tasks, nearly no software tools are available to the material developer, which systematically support the development process with simulations. This work focuses on the time-dependent three-dimensional heat distribution inside a pad material and shows, how heterogeneous material mixtures can be modeled, such that a prediction of non-stationary temperature fields becomes possible. In this context, a strategy is suggested, how the thermal interaction between pad and disk and the aspects of energy dissipation and heat partitioning can be described appropriately.

Brake pad materials in today's commercially marketed vehicles are usually complex phenolic resin based composites with numerous ingredients. Since the abandonment of asbestos fibers, different material classes evolved in Europe (low steel), North America (semimet) and Asia (NAO), which specifically meet the requirements of the respective market [ 1 ]. For these complex materials, no a-priori prediction of friction and wear performance is possible today [ 2 ]. Research over the past decade revealed that friction power and wear debris are interrelated [ 3 ] and that the topography of the friction layer shows a very rich dynamic [ 4 ]. The respective processes can be well described with a family of dynamic friction laws, which is suitable for the description of AK-Master test results [ 5 ], as well as for the understanding of history dependent high frequency effects.

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 recent years, characteristic structures in the boundary layer of high-load contacts such as brakes have been reported, which have an important impact on the dynamics of the tribological contact. Usually, local assumptions concerning the friction of these patches are used to reach global conclusions about the brake system. Several numerical methods (e.g. Cellular Automata) have been developed which make use of such assumptions. The validation of these methods through measured data tends to be laborious and costly. Sprag-Slip elements are friction elements which are typically considered to exclusively undergo static friction. Such elements have been sporadically utilized towards describing friction in brake applications. In this paper, many locally distributed Sprag-Slip elements are used to model the global dynamics of braking friction. The results show good agreement with the measured characteristics of brakes.

For brake performance measurements, many standardized test procedures and machines were developed. A characteristic problem of these highly complex machines is the occurrence of measurement uncertainties, which usually are not predictable and rather difficult to explain. One part of explanations is driven by the huge but not yet complete list of influencing factors for the high load friction processes. Another part of explanations is given by f.i. manufacturing tolerances, load histories of machines and specimens and so on. A systematic investigation of these influences is very time-consuming and could be difficult to realize on professional test-machines [1]. On the other hand, simple laboratory tribometers are rather good for sensitivity investigations in principle but the friction systems are too far from the real system.

Due to the increase in public attention in the analysis of non-exhaust emission sources because of the growing electrification of vehicles, measurements have been performed in recent years to develop a consistent test standard. In particular, the consideration of tyre and brake abrasion took a predominant position due to the small particle sizes. With measurements under controlled and laboratory-like athmosphere, for example for brakes on dynamometers, attempts have been made to create a uniform test standard according to the Worldwide harmonized Light vehicles Test Procedure (WLTP). However, a transfer to the real driving environment is not yet feasible because of many external disturbance variables, such as the wheel housing or atmospheric variables. Typical reference measurement sensors in the vehicle are only suitable to a limited extent for mobile operation due to their size and the necessary measurement infrastructure.

The Automated Universal Tribotester (AUT) represents a fully automated reduced scale brake dynamometer and was developed by the Institute of Dynamics and Vibrations at TU Braunschweig. The setup is based on the pin-on-disc principle. The downscaled test specimen is brought to contact to the disc, loaded and guided via the load unit, which was specifically designed for this purpose. It is laid out as a combination of parallel and serial leaf springs, facilitating a friction free and horizontal motion. The stiffness in radial and tangential directions are much higher than in normal orientation. If the test specimen has a two-dimensional, flat contact surface, the misalignment of the pin in a Pin-on-Disc setup is a phenomenon that appears even in very stiff and robust Pin-on-Disc testers. The measured friction forces are not sensitive to the tilting movement, but the wear processes that take place in the contact zone are.

In recent years, characteristic structures in the boundary layer of high-load contacts such as brakes have been reported, which substantially influence the dynamics of the tribological contact. Usually, local assumptions concerning the friction behavior of these patches are used to reach global conclusions about the brake system. Several numerical methods (e.g. Cellular Automata) have been developed which employ such assumptions. The validation of these methods through measurement data tends to be laborious and costly. Sprag-Slip elements are friction elements that are exclusively subjected to static friction, never undergoing sliding friction. Implementing such elements on a mesoscopic scale, it is possible to generate global descriptions of macroscopic stick and slip friction phenomena. Locally, these Sprag-Slip elements are similar to the patch structures in the tribological boundary layer of brakes.

The dynamics of disc brakes, and in particular their NVH behavior, have long been the focus of research. Measurements by Rhee et al. show that brake pad wear has a significant influence on the occurrence of low and high frequency squealing [1]. It is suspected that low frequency squealing is more likely to occur when the wear difference between the inner and outer brake pads is high. If the two pads incur comparable wear, however, the prevalence of high frequency squealing increases. In order to investigate this hypothesis, this paper focuses on a simplified model of a commercial brake system. First, the friction force between the inner pad and the disc is iteratively adjusted, while the force between the outer pad and the disc is held constant. In a second step, the inner pad’s wear is iteratively increased, while the wear on the outer pad remains unaffected.

Friction in tribological systems can lead to significant energy consumption and wear. While there are several dissipation mechanisms in the frictional boundary layer, the role of chemical processes is not fully understood. The aim of this study is to investigate the influence of chemical reactions on the tribological behavior of sliding friction pairs. In order to carry out initial analyses, minimal mixtures with a few simple components and epoxy resin as a binder are developed, produced and used. A series of experiments are performed on a pin-on-disc tribometer with different minimal mixtures. Temperature and friction coefficient are measured throughout the friction process, and the rubbed surface of the samples is measured in situ. Three types of chemically inert minimal mixtures are developed in the first phase of the experiment.