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Boundary Layer Dynamics and Sound Generation

TU Braunschweig-Bastian Recke, Georg Ostermeyer
Published 2018-10-05 by SAE International in United States
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 Sprag-Slip elements are suitable for describing the basic physics in the boundary layer between brake pad and rotor. Previous works have established their ability to depict principle macroscopic brake friction curves. In this paper, it is shown that characteristic boundary layer oscillations can represent characteristic braking behaviors, such as…
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On the Wear Dependence of Low-Frequency and High-Frequency Brake Squeal

SKR Consulting LLC-Seong Rhee
TU Braunschweig-Johannes Otto, Georg Ostermeyer
Published 2018-10-05 by SAE International in United States
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. With the aid of the Complex Eigenvalue Analysis (CEA), the real part of the eigenvalue is used as a quantification measure in order to investigate the dependence between wear…
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Effective Simulation of the Boundary Layer of an Entire Brake Pad

Tu Braunschweig-Georg Ostermeyer, Joshua Merlis
Published 2015-09-27 by SAE International in United States
The dynamic friction behavior of automotive brakes is generated by the boundary layer dynamics between pad and disk [OST01]. A key component of the Friction Interface is the influence of mesoscopic surface contact structures known as patches, upon which the friction power is concentrated, and whose sizes vary with time. Through this dynamic process, time and load history-dependent effects come about, which cause, for example, the brake moment behavior commonly observed in an AK-Master test.In recent years, several simulation tools have been developed in order to predict the complex friction behavior caused by the patch dynamics in the friction boundary layer. Such simulations are often based on a two or three-dimensional spatial grid, where the explicit physical phenomena at all locations in the boundary layer are modeled by time-consuming calculations of local material dependent balance equations.A new abstract Cellular Automata simulation tool is introduced, which reduces the necessary computation to the patches in the friction boundary layer. Rather than making use of a spatial grid, each patch is considered a single cell, whose size can…
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The Effect of Metal Pickup to the Friction Interfaces

TMD Friction-Jürgen Lange
Univ Of Braunschweig-Georg Ostermeyer
Published 2011-09-18 by SAE International in United States
Metal pickup is a phenomenon that can be observed during dynamic braking with automotive disc brakes. Hard metallic particle agglomerations embedded in the friction materials rubbing surface can lead to severe disc scoring, accelerated wear and deterioration of the friction surface of either brake disc or brake pad. Such kind of surface conditions are also suspected for generating brake squeal, even if a direct root cause effect had not been proven so far. Disc scoring effects have been reported for all kinds of applications, reaching from small passenger cars up to commercial vehicles as well as railroad brakes. Although such phenomena are known since long, they still appear causing problems in brake systems of today.Some recent papers have described the effect and mentioned preferable conditions for the appearance of metal pickup. Specific procedures have been proposed for provocation of the effect to allow a better and more systematic investigation of the influencing parameters. Observations on results of such experiments were described and several working hypotheses put forward with influences going back to the brake rotors…
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Dynamic Friction Laws and Their Impact on Friction Induced Vibrations

Technische Universität Braunschweig-Georg Ostermeyer
Published 2010-10-10 by SAE International in United States
There are a 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 ]. 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. Eigenvalues of the linearized models are computed in the frequency domain, and often used to judge the stabilities of the models. Industry applications typically use FEA programs in the modeling, which allows detailed descriptions of the geometry of the brake system, except the friction layer. In the friction layer between brake disk and pad, simple friction laws are used, which allows a modal analysis similar to that of the minimal models. 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…
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