抄録
Superposition principle of the Schapery type, discussed in a previous paper, is examined by the experiments of loading, unloading and stress relaxation after ceasing of loading, under constant rate of elongation, for low and high density polyethylene films.
In loading process, the calculated values from time-strain reduced model (model III) agree well with the experimental values to 5 and 3% in strain for the low and high density samples, respectively. But, the perfect time-strain factorized model (model I) can be applied only in the limited range of small strain.
In stress relaxation process for the low density sample, the rates of relaxation in the observed curves are a little higher than that in the predicted curves from model III. While for the high density sample, the theory of model III has good agreements with experiments.
In unloading process, the calculated values from both models are greater than the experimental values. In other words, the observed strain recovery in unloading delays in comparison with the predicted recovery. These tendencies are remarkable for the high density sample. The discrepancy between theory and practice is explained as follows.
It is considered that plastic strain develops in loading process, not recovers by unloading and would affect the reduced factor aε(ε). In the theory of model III, it is assumed that aε(ε) in unloading is equal to that in loading at the same strain. However, when plastic strain yields, the value of aε(ε) would decrease and the stress decay would be accelerated. Since, the unloading stress would be less than the expected value. These considerations are grounded on the view of Ferry's free volume theory.
If aε(ε+εb) is substituted for aε(ε) under unloading process in model III, the calculated values well agree with the experimental results, where εb is shift strain and should be determined by the method of trial and error, but depends on the value of plastic strain.
