Master curves of viscoelastic behavior in the plastic region of a solid polymer



Stress relaxation and creep tests following strain ramps were made on Mylar, both above and below the yield stress. The ramp velocity was varied over a 40-fold range. All data exhibit nonlinear viscoelastic behavior. However, those obtained above the yield point, i.e., in the plastic region, could be reduced to single master curves for both the creep and the relaxation tests by means of a simple time shift factor. This factor is inversely proportional to the strain rate existing just prior to the test.