NONLINEAR VISCOELASTIC RESPONSE OF CARBON-BLACK REINFORCED RUBBER DERIVED FROM MODERATELY LARGE DEFORMATIONS IN TORSION

The response of Butyl rubber loaded with carbon black is investigated experimentally, by subjecting it to a finite twist. The experimental set-up and the measurement procedure are described. The material shows a highly nonlinear behaviour for relatively small shear strains. Constitutive relations corresponding to the data are derived from a theory based on a nonlinear tensorial generalization of a three-parameter model. The sample stiffness and normal force for a cylindrical sample subjected to simple torsion are derived for the quasistatic case. To explain the marked drop in stiffness of the material beyond the linear range, a damage and rehealing model is introduced. It consists basically of an evolution equation for the model parameters. With the theory proposed here the measured response for torque and normal force corresponding to a given deformation history can be confirmed satisfactorily.