Characterization of the global deformation behaviour of engineering plastics rolls
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Characterization of the global deformation behaviour of engineering plastics rolls M. Berer • Z. Major
Received: 6 May 2009 / Accepted: 27 February 2010 / Published online: 12 March 2010 Ó Springer Science+Business Media, B.V. 2010
Abstract In real service application of polymer rolls as rolling elements, the time-dependent deformation behaviour leads to a negative influence on the running smoothness after a long-lasting static load. By means of two polymeric materials (POM and PEEK) with different viscoelastic properties over a wide temperature range the influence of material properties on the running smoothness was investigated. A combination of static and dynamic tests as well as finite element simulations were performed. Observed differences in the behaviour of both materials in the service could be reflected well by this work. POM revealed creep flattening and a running performance which could be modelled well by viscoelastic material laws. By contrast, PEEK showed a different performance and its behaviour could not be represented adequately by viscoelastic material models. Keywords FEM Engineering plastics Rolling elements Viscoelasticity Prony series
M. Berer (&) Polymer Competence Center Leoben GmbH, Roseggerstrasse 12, 8700 Leoben, Austria e-mail: [email protected] Z. Major Institute for Polymer Product Engineering, Johannes Kepler University of Linz, Altenberger Strasse 69, 4040 Linz, Austria
1 Introduction and objective Plastics are widely used in all areas of modern life. Due to the inherent viscoelasticity, plastics show particular properties such as the distinct creeping at room temperature. In the case of polymer rolls as rolling elements, this time-dependent deformation behaviour leads to a reduced running smoothness after a long-lasting static load. The intensity of time dependence of the mechanic parameters is different for each plastic and depends on the temperature and the magnitude of the load (at higher load magnitudes). The reason for the time dependence is the inner structure of polymers. The threadlike molecules are entangled and therefore cannot react instantaneously on loads put on these materials. The new equilibrium state can only be reached through disentanglement which happens through thermal movements of the molecules. Thus most of the thermal movements are frozen, it takes time to reach the new equilibrium state (Ferry 1980). The behaviour of viscoelastic materials can be described through differential equations that link stresses and their time-related derivatives with strains and the time-related strain derivatives. For the assumption of infinite small strains, the differential equations are linear with constant coefficients. The assumption of infinite small strains and the resulting linear differential equations with constant coefficients form the basis of the ‘‘linear viscoelastic theory’’. This theory indeed applies to polymers at small strains below the so-called
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‘‘linearity limit’’. The linearity limit is different for each plastic and depe
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