Abstract
Previously, we considered the stress relaxation properties of polyethylene under relatively small deformation( 1-5 %) and showed that relaxation stress σr (ε0, t) could be represented by this equation:
σr (ε0, t)=α Ε (1-β1 ε0+β2ε20) t-n…………………………………(1)
By applying the superposition principle, stress-strain relation under a constant strain rate can be derived from the above equation as follows:
σ(ε)=α epsilon;/1-n{1-2β epsilon;/2-n + 6β epsilon;2/2(2-n) (3-n)}t-nααα(2)
In other words, the strain and time dependence of stress are separable in the same way as stress relaxation behavior.
The tensile properties of polyethylene were measured with Instron tester at strain rates between 1.67 X 10-4/sec and 3.33 X 10-2/sec. The experimental equation of stress-strain curve agreed with eq. (2), except that the constants differed.
The fact that stress, like relaxation behavior, can be resolved into strain and time factors suggests non-linearity of both properties.
