Abstract: In order to investigate the stress softening-stiffening effect and the rate-dependent properties caused by the hyper-viscoelasticity of epoxy shape memory polymer (ESMP) in large strain regime, this work makes attempt to reveal the softening and stiffening effect of ESMP near and above glass transition temperature (Tg). Firstly, to account the stiffening effect in large strain, a hyper-viscoelastic model is established by substituting the second-order polynomial hyper-elastic model to the linear Hooke’s law in generalized Maxwell model. Meanwhile, the material constants calibration equations which are convenient for engineering application are derived for the case of constant true strain rate in analytical form. Then, the tensile tests in various strain rates are carried out at temperature above Tg in rubbery state and 7℃ blow Tg, where the exponential tensile displacement curve are approached by finite pieces of linear displacement curve. The material constants are calibrated by the tests respectively. The numerical simulation of each test is carried out using the corresponding parameters, where the simulation results are found to have high consistency with the test results. The results indicate that ESMP behaviors significant rate-dependence. Meanwhile, the softening effect and the stiffening effect increase with increasing strain rate; the tangential modulus in a large strain rate is always greater than the tangential modulus in a small strain rate.

Key words: epoxy shape memory polymer (ESMP); large strain; hyper-viscoelasticity; rate-dependence; stiffening effect