抄録
In order to investigate a nonlinear constitutive equation of polymers at finite strain for polymer solids, tension, torsion and combined tension-torsion tests under constant strain rate, stress relaxation and creep have been conducted to provide experimental data required for constructing the equation. Although the stress response under strain paths with its reversal is very important to understand fatigue behavior, there has been few experiments for polymers. Hence, the effect of strain history on the constitutive equation still remains unclear. Furthermore. the constitutive equation still remains unclear. Furthermore, the pressure dependent behavior on the exact stress-strain curves has not been observed in spite of many experiments on the pressure dependence of elastic modulus and flow stress. In this paper, cyclic torsional tests under hydrostatic pressures of 1 to 2000kgf/cm2 were carried out to describe both pressure and cyclic dependent behavior for crystalline polymers of polyethylene (PE), polypropylene (PP) and polyoximethylene (POM). It has been shown that (1) the shape of the stress-strain curves is not significantly affected by applied pressures, (2) cyclic softening occurs for the materials tested, but the degree of softening is dependent on materials, (3) the stress responses under partly reversed cyclic loading are differnt from the ones expected from fully reversed cyclic loading and (4) the stress responses for POM and PP are very sensitive to strain history, while the current stress-strain curve for PE is little affected by the previous strain paths.
