The Boltzman and the Schapery equations of a time-strain reduced type are used for the linear and the nonlinear models, respectively. The predicted results from both models have experimentally been discussed by two kinds of polyethylene films at room temperature.
The fractional elastic recovery (U.E.R.) of the linear model is constant regardless of the initial strain ε1; when holding time tr; =0, and increases with ε1; when tr; >0. U.E.R, of the nonlinear model is smaller than that of the linear model and the difference between them is larger at higher ε1; . If a solid shows more stress relaxation and stronger nonlinearity, its U.E.R. is smaller. The experimental results of U.E.R. are qualitatively similar to U.E.R. predicted by using the nonlinear model.
The fractional delayed elastic recovery (D.E.R) can only be calculated from the linear model. D.E.R. of the model is constant regardless of ε1; when tr; =0, and decreases with ε1; when tr; >0. A solid exhibiting a remakable stress relaxation has higher rate of delayed recovery and larger D.E.R. The experimental results of D.E.R. show remarkable nonlinearlity and are inconsistent with the linear theory.