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Mr. John Smith

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Organic friction linings of passenger car brake systems are complex and highly sophisticated composite materials which own a wide spectrum of characteristics. Meaningful testing methods to determine their physical and chemical characteristics are crucial for the systematic design of the complete brake system. The necessity of effort optimized testing methods which are at the same time informative in terms of several characteristic parameters - so called "fingerprints" - increases regarding time constraints during the design process. The present article presents a new method that allows three dimensional capturing and analysing of a well-known characteristic of linings - namely the density and porosity distribution respectively. It is well-known that linings own a porous structure, which lies in the material compound. The lining matrix consists of numerous different ingredients that are mixed and grouted subsequently. As a result not only anisotropic but also high porosity values can be stated. Another reason means the production process itself and its combination with special geometric pad dimensions. Different production methods, especially cold and hot pressing procedures, influence the porosity behaviour in variable ways. This is additionally interesting when certain pad dimensions have to be realized. Particular big brake pads (high length-width-ratio) imply the risk of inhomogeneous porosity distributions. Several aspects can have an influence on the porosity even after the production. The thermal and mechanical loads which are caused by the brake application let expect a porosity gradient in axial direction (i.e. between backing plate and friction zone). Furthermore water and salt input mean a change of porosities. Also the underlayer (regarding material composition and connection to backing plate and lining matrix respectively) might have an influence on it. The consequences can be described tribologically by the formation of the friction layer and friction coefficient and physically by compressibility and damping respectively. Diverse porosities can also affect the wear behaviour. Especially inhomogeneous porosity distributions favour angular wear. In addition the comfort is sensitive regarding material damping (especially when it shows spatially differences) that is directly influenced by the defect volume. Previous approaches tend to determine global specific values by the determination of density and porosity respectively that do not reflect local specifics. Phenomena like angular wear, low and frequency disturbances and varying contact pressure distributions cannot be explained and described. The authors follow a novel approach by the use of a 3D computed tomography device. Complete linings and adapted specimen are investigated in the range of microns. Subsequently analyses of cross sections in axial, radial and tangential direction are executed. Green and used linings as well as several production lots are compared. The comparison of complete pads and test samples reveals the resolution limits. The presented method shall enhance the already existing methods of porosity and compressibility determination and offer information about spatial porosity distributions. Since this novel approach owns a high sensitivity and spatial resolution it is likely that it is also interesting to determine differences in production lots of brake pads.

Augsburg, Klaus; Gramstat, Sebastian; Thomas, Bernard; Schellakowsky, Benjamin; - Ilmenau University of Technology

New Characterization Methods Of Spatial Porosity Distributions Of Friction

EB2013-FMC-022 • Paper • EuroBrake 2013 • Friction Material Characterisation (FMC)


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