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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.

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.

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.

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.

The kinetics of moisture adsorption is studied for copper-free brake pads. The pad weight gain is found to increase linearly with the square root of exposure time to humidity at a given temperature in the initial stage of adsorption - the higher the humidity, the higher the weight gain. Pads cured at 150°C adsorb less moisture than pads cured at 220°C. As the moisture content in the pad increases, the tangent modulus increases while the secant modulus decreases, resulting in decreasing compressibility associated with the tangent modulus of compression and increasing compressibility associated with the secant modulus of compression - compressibility defined as a reciprocal of compression modulus. Static modulus of compression, dynamic modulus of compression and hardness measurements are compared, and they all show the same trend. A rate constant of adsorption is proposed to define and compare moisture sensitivity of friction material

Friction materials for automotive brakes are known to exhibit a time-dependent tribological behavior. When examining these dynamic effects special demands are made on the measurement device: The influences of the brake system should be minimized and parameters like velocity, contact pressure and temperature should be controlled closely and independently. Furthermore, special test procedures need to be designed. This can ideally be achieved using a scaled tribometer like the High-Load-Tribometer at the Institute of Dynamics and Vibrations in Braunschweig. Former investigations [1] have shown that a kind of memory effect can occur for a low-met brake pad rubbing on a cast iron disk. A variation of the initial disk temperatures has revealed that a temporary increase of the coefficient of friction can occur at slightly elevated temperatures. This effect is memorized by the material as a certain procedure needs to be performed in order to achieve a regeneration.

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.

The influence of processing conditions on Low-Copper NAO disc pads were investigated as part of an effort to develop Low-Copper disc pad formulations as this kind of information is not readily available in open literature. Processing conditions as well as formulation modifications are found to influence friction, pad wear, disc wear and brake squeal. Low-Copper disc pads for pick-up trucks, equivalent to an OE pad, are developed. It is also found that brake squeal measured during the SAE J2522 (AK Master) Performance testing is related to the combined total wear rate of the disc plus the inner/outer pads or the disc wear rate alone, and that there is a threshold wear rate, above which brake squeal increases rapidly.

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.

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.

Dynamic aspects in the understanding of friction are increasingly coming into focus. Therefore, test arrangements are required, which allow dynamic measurements as well, especially with defined and reproducible sliding speeds. In addition, investigations of vehicle brakes require high power. A simple, reliable measurement device, based on the pin-on-disc principle, that meets these requirements was developed and will be presented here. This high load tribometer is based on a specially altered lathe. One key benefit of a lathe as a basic unit is the fact that the main mechanical part is standardized industrial equipment. Additional components and measuring devices can be integrated with little effort and great flexibility. The built-in 15kW AC motor and the embedded gearbox permit speeds and torques to the service areas of vehicle brakes, also of heavy duty vehicles and sports cars.

The current investigation was undertaken to find out if lighter-weight passenger car disc pads would exhibit wear behaviors similar to pickup truck pads and commercial heavy truck drum linings in terms of the permanent volume expansion of the friction material contact surface region. 2 high-copper NonAsbestos Organic formulations and 3 copper-free LowMet formulations were tested according to the SAE J2522 test procedure. In all cases, the measured pad thickness loss was found to be less than the thickness loss calculated from the weight loss, indicating pad volume expansion in the pad surface region, in full agreement with the results from the pickup truck and heavy trucks. The heataffected swollen/expanded layer ranges from 0.27 to 0.61 mm in thickness depending on the formula and test conditions. Due to the expansion, pad durability projections made from test results based on high temperature city traffic tests can result in underestimating the actual durability.

Heavy truck brake blocks are found to swell (or expand) permanently during testing or usage, especially so at high temperatures, thus leading to longer durability as measured by thickness loss, similar to light vehicle disc pads. This swelling phenomenon occurs continuously in the layer adjacent to the friction surface during testing or usage; not a one time event. The thickness loss estimated from the weight loss is always greater than measured thickness loss. Brake block wear does not increase linearly with increasing normal load, and the load-sensitivity of block wear is very much dependent on the products. A new test procedure has been developed for generating friction-vs.-temperature and wear-vs.-temperature data at a constant temperature employing intermittent braking on the Chase Brake Lining Quality Tester (SAE J661) and friction material wear can be compared on equivalent-work basis.

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.

It is widely believed or speculated that higher pad compressibility leads to reduced brake squeal and that caliper design can affect brake squeal. After encountering anecdotal contradictory cases, this investigation was undertaken to systematically generate basic data and clarify the beliefs or speculations. In order to adjust pad compressibility, it is common to modify pad molding temperatures, pressures and times, which in addition to changing the compressibility, changes friction coefficient and physical properties of the pad at the same time. In order to separate these two effects, NAO disc pads were prepared under the same molding conditions while using different thicknesses of the underlayer to achieve different compressibilities, thus changing the compressibility only without changing the friction coefficient and physical properties of the pad.

Earlier publications show that brake pad physical properties such as hardness, modulus and natural frequencies continue to increase at room temperature over a period of 12 months and that the changes are faster during the first 3 - 6 months. The current investigation was undertaken to see how the properties might change during testing for the pads as well as for the discs. Low-copper and copper-free formulations were tested on pickup truck and passenger car brakes. In all cases, the dynamic modulus and natural frequencies are found to decrease (not increase) after the SAE J2522 performance testing, indicating that the stiffness of the pad and that of the disc decrease faster than the mass loss due to wear. Also, the inboard pad and the outboard pad change at two different rates.

The moisture adsorption kinetics of copper-free brake pads was studied to confirm an earlier finding that the adsorption weight gain follows a logarithmic relationship with respect to the square root of humidity exposure time and the relationship is linear in the beginning. When the pad cure temperature was raised from 120 to 180 and 240 °C, the adsorption rate increased. The 180 °C cure produced the highest pad modulus. With increasing moisture adsorption, the pad compression modulus increased just like the pad dynamic modulus, meaning decreasing compression/compressibility while the ISO ‘compressibility’ determined after 3 compressions under 160 bars increased in contradiction. It is concluded that the ISO ‘compressibility’ is a destructive hardness measurement like the Gogan or Rockwell hardness: the key difference is the indenter covers the entire surface of the pad. The true compressibility must be determined as an inverse function of bulk modulus.

Earlier publications have demonstrated that pad and disc properties change during storage and also during the SAE J2522 Brake Effectiveness Test Procedure. The current investigation was undertaken to find out how the properties change under milder braking conditions, using the SAE J2707 Wear Test Procedure. A copper-free formulation was selected for the investigation and tested on an inertia dynamometer using a front caliper designed for a passenger car. The pad dynamic modulus changed up or down throughout the test, depending on the test conditions. The pad dynamic modulus, the pad natural frequencies and the disc natural frequencies all decreased by the end of the test. Under high-speed, high-deceleration and high-temperature braking conditions, the pad surface region permanently expands, which results in reduced dynamic modulus and also leads to reduced pad thickness loss as compared with pad weight loss.

The moisture sorption-desorption kinetics of copper-free brake pads was studied in detail. The sorption-desorption behavior is dependent on the environmental temperature and humidity. At 24 °C under 54% RH, the sorption increases rapidly for a week or so identified as the first stage of sorption, enters the second stage of negligible weight gain for a month and then the third stage of rapid sorption again. With increasing moisture sorption, the pad thickness increases through the 3 stages and the dynamic modulus also increases through the 3 stages. Friction materials lose moisture rapidly at 130°C and behave like desiccants. The sorption-desorption phenomenon significantly influences the friction coefficient -- a higher moisture content leading to lower friction coefficients. It is demonstrated that the rising friction coefficient for the half a dozen braking stops at the beginning of every brake testing in general is due to moisture desorption caused by rising pad temperatures.

Copper-free disc pads of 9 different compositions were made using a traditional hot molding process and tested to study frictional behavior. It is found that the friction coefficient consists primarily of two parts; one part controlled by the plastic deformation of the friction surface region of the disc and pad, and the second part controlled by the total wear of the disc and pads. As the plastic deformation and the wear are non-linear with respect to the load and sliding speed, the friction coefficient becomes a non-linear function of the load and speed. Under moderate braking conditions, the plastic deformation part is more significant in determining the friction coefficient while under more severe braking conditions, the wear contribution becomes more significant. The frictional behavior of a fade cycle is explained, and the correlation between brake squeal and disc wear is confirmed.