A NONLINEAR VISCOELASTIC MODEL FOR PREDICTING THE YIELD STRESS OF AMORPHOUS POLYMERS

A non-linear viscoelastic model is introduced by employing a new formu lation where the relaxation time of rate processes is directly depende nt on the induced deformation state applied to the material. The plast ic flow of amorphous polymers is considered as a continuous structural change, and the yield phenomenon is modelled through the correspondin g viscoelastic constitutive equations. All material constants necessar y to predict the yield stress, the rate and temperature dependence are non-adjustable parameters depending on the particular deformation mod e. To verify the results of the proposed model, two independent series of experiments have been performed on specific polymeric materials, a nd have been proved to satisfy adequately the proposed analysis. (C) 1 997 Elsevier Science Ltd.