Non-linear viscoelastic behaviour of polymers: An implicit equation approach

Abstract

I t is proposed that a comprehensive description of the non-linear viscoelastic behaviour of polymers can be most readily obtained using an implicit form of the constitutive relation. Extensive creep, constant strain-rate and stress relaxation measurements on oriented polyethylene terephthalate and creep measurements on isotropic polypropylene and polymethylmethacrylate have tested the usefulness of the proposed implicit constitutive equation. First, this equation has been shown to describe accurately the stress dependence of the relaxed and unrelaxed creep response. Secondly, it enables the strain dependence of the stress relaxation response to be accurately predicted from the observed creep behaviour. Finally, making simplifying assumptions regarding the form of the two response functions involved, good first-order predictions were obtained for the creep curves at any stress, the stress relaxation behaviour at any strain and the constant strain-rate behaviour at any strain-rate using only six parameters.