Hysteretic friction of rubber in tribological tests

Abstract

The goal of this paper is to summarize different investigations of FE modelling of rubber hysteretic friction which was produced in the last 4 years in the KRISTAL project (Knowledgebased Radical Innovation Surfacing for Tribology and Advanced Lubrication, EU Project Reference CT-2005-515837). László PálfiDepartment of Machine and Product Design, BME, H-1111 Budapest, Müe-gyetem rkp. 3., Hungary e-mail: palfi.laszlo@gt3.bme.hu Károly VáradiDepartment of Machine and Product Design, BME, H-1111 Budapest, Müe-gyetem rkp. 3., Hungary e-mail: varadi@eik.bme.hu IntroductionDuring the design process of different mechanical engineering applications wherein components are sliding on each other it is important to know the mechanical behaviour and tribological properties of the components. In many technical applications one of the sliding surfaces is made of rubber or a rubber-like material (such as seals contacting with a metal or polymer surface, wiper blades-wind screen and tires-road contact etc.). In case of clean surfaces and in the absence of lubricant when contacting surfaces are dry the main source of rubber friction is the adhesion and hysteresis [1][2][3]. The joint effect of them generates frictional resistance when an elastomer part is pressed and rubbed against a rough surface.If the rubber slides on a rough surface which is rigid compared to the rubber, the asperities of the surface repeatedly deform the surface of the rubber. Due to the viscoelastic material behaviour of rubber, during deformation, part of the strain energy is transformed to heat as a result of the hysteresis [3]. Generally, rubber components operate under lubricated conditions, to separate rubbing surfaces thus reducing the impact of adhesion, as well as by filling up the valleys with the lubricant to reduce the exciting effect of asperities. The shearing of the fluid film also acts as a source of friction. In general, the fluid film is extremely thin, therefore many times boundary lubrication occurs.Numbers of articles deal with the analytic and semi-analytic determination of friction force induced by from hysteresis. Grosch [3] performs pioneering work by revealing that the two main sources of the friction force are adhesion between the surfaces and the energy loss generated in the material, that is, hysteresis. In addition, he stated that both physical phenomena are closely connected to internal friction (hysteresis) of the rubber. In the papers of Persson and Klüppel [4,5] they investigated the rubber when it is sliding on a hard, rough substrate and energy dissipated inside the rubber due to the surface asperities of the substrate exert oscillating forces on the rubber surface. Hysteretic friction was determined and the results were compared with the experimental data of Grosch. Persson stated that rubber friction on rough surfaces in presence of lubricant is mainly due to the viscoelastic deformations of rubber (hysteresis).

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