Abstract
In order to understand a constitutive equation of solid polymers, volume expansion and stress-strain behavior after strain reversal were investigated due to compression using amorphous polymers and crystalline polymers. The following are very interesting results for studying the constitutive law obtained from the present study: (1) For crystalline polymers, volume expansion may occur even at uniaxial compression where the first stress invariant is negative, and (2) at a stress relaxation (or creep) test after strain reversal, the stress rate (or the strain rate) may change its sign during the test depending on the test condition.
