14 th International Conference on Deformation, Yield and Fracture of Polymers Kerkrade, Holland, Apr 6-9 2009. Viscoelastic deformation of carbon-black filled EPDM rubber D.S.A. De Focatiis , F. Abraham and C.P. Buckley, 2009. Viscoelastic deformation of carbon-black filled EPDM rubber Davide S.A. De Focatiis 1,2 , Frank Abraham 3 and C. Paul Buckley 2 1 Department of Chemical and Environmental Engineering, University of Nottingham, UK 2 Department of Engineering Science, University of Oxford, UK 3 Mechanical & Design Engineering, University of Portsmouth, UK davide.de.focatiis@nottingham.ac.uk; frank.abraham@port.ac.uk; paul.buckley@eng.ox.ac.uk Cyclic tensile stress-strain data for a cross-linked EPDM rubber are presented. Data are modeled using the Edwards-Vilgis strain energy function with parameters evolving with the maximum stretch, in parallel with viscoelastic anisotropic flow units. The crosslink density reduces with increasing stretch, but the slip-link density and viscous contribution are invariant. Introduction Filled elastomers are increasingly important industrial materials because of their unique flexibility and damping properties, and are used in a range of applications such as seals, dampers, transmission belts and automotive tyres. They exhibit a range of complex phenomena when subjected to repeated loadings: (1) the stress-softening phenomenon known as the Mullins effect; (2) complex pre-conditioning dependent viscoelasticity; and (3) a small degree of permanent set. The generation of constitutive models able to accurately predict the mechanical response of such components forms an essential part of their design, and can also contribute to the understanding of the mechanisms underpinning such a response. In this paper we present a series of experimental observations aimed at shedding light on these phenomena and at supporting the implementation of a new constitutive model able to capture these features. Experimental method The material studied in this work is an accelerated sulphur cross-linked carbon-black filled (50phr) oil extended ethylene-propylene-diene (EPDM) rubber. Sheets of material approximately 0.5mm thick were compression-moulded for 13 minutes at 160ºC using a heated press. Specimens for tensile testing were cut from the sheet using a dog-bone cutter. Tensile testing was performed in an Instron tensile testing machine fitted with a counterbalanced elastomer extensometer, at room temperature at a constant true strain rate of 0.03 s -1 , as measured in the gauge length by the extensometer, using a feedback loop. The tests consisted of 4 load-unload cycles, loading to a specified maximum stretch, and unloading to a tensile force of 0.1N to avoid buckling. A range of 10 maximum stretches from λ = 1.5 to λ = 6 were used. Representative stress-strain curves for four such tests are shown in Figure 1(a).