STRESS-DEPENDENT DYNAMIC COMPLIANCE SPECTRA APPROACH TO THE NONLINEARVISCOELASTIC RESPONSE OF POLYMERS

The present work reports a discrete, stress-dependent dynamic complian ce spectra method which may be used to predict the mechanical response of nonlinear viscoelastic polymers during strain-defined processes. T he method is based on the observation that the real and complex parts of the discrete dynamic compliance frequency components obtained from creep measurements are smooth, easily fit functions of stress. Compari sons between experimental measurements and model calculations show tha t the model exhibits excellent quantitative agreement with the basis c reep measurements at all experimental stress levels. The model exhibit s good quantitative agreement with stress relaxation measurements at m oderate levels of applied strain. However, the model underestimates th e experimental stress relaxation at an applied strain of 3.26%. The st ress relaxation error appears to be a real material effect resulting f rom the different strain character of creep and stress relaxation test s. The model provides a good quantitative agreement with experimental constant strain rate measurements up to approximately 4% strain, after which the model underestimates the experimental flow stress. This eff ect is explained by the time dependence of the stress-activated config urational changes necessary for large strains in glassy polymers. (C) 1998 John Wiley & Sons, Inc.