Approximation of Irregular Loading by Intervals of Constant Stress Rate to Predict Creep and Relaxation of Polyurethane by Three Integral Representations

Three integral representations for nonlinear creep were employed to predict from constant-stress creep data the strain resulting from two irregular loading experiments. The stress history for these experiments was approximated by intervals of constant stress rate. By employing this method of approximation and a numerical procedure, the creep representations were inverted to permit predicting from the same constant-stress creep data the stress resulting from prescribed time-varying strains. This procedure was used to predict the tensile stress relaxation of two multiple-step tensile strain tests.