Abstract
In this paper, the adaptability of multiple integral equation of the constituents to nonlinear viscoelastic behaviors in uniaxial stretching, for two kinds of polyethylene having different crystallinity, has been investigated. And the principle of a generalized superposition has been tested in respect of its stress relaxation, of its constant rate of elongation, and its two steps-stress relaxation experiments made by using only kernel functions G1(t), G2(t, t), G3(t, t, t), G4(t, t, t, t).
The first kernel G1(t) is obtained from the data on nonlinear stress relaxation test, which agrees fairly well with the stress relaxation modulus in linear range. The calculated value of G1(t) from the relation between the stress and the strain rate in the constant rate of elongation test, also agrees with both the above mentioned values. These result mean that the superposition in the linear terms in the integral equation of the constituent holds true.
The behaviors under the two steps-stress relaxation tests depend strongly on the additional strain, and the effects of the first strain are irregular. These tendencies differ from the analytical results given out in the fourth order theory. Only the dependencies of input time of additional strain is, but quantitatively, qualitatively similar to the theory. Then, the fourth order theory in the integral equation of its constituent is not efficient enough to describe the complex nonlinear viscoelastic behaviors of polyethylene.
