Strain-Induced Variation of Relaxation Time in Crosslinked Epoxy Glass Evaluated by a Simple Nonlinear Mechanical Model

Abstract

The single relaxation time of epoxy glass, a thermoset having crosslinked molecular structures, was evaluated during tensile yielding process by using a nonlinear single relaxation model. The model consisted of two elastic springs expressing linear viscoelastic behavior and a dashpot with variable viscosity as a single parameter representing strain-induced structural change. We calculated the strain-dependent relaxation time τ_SS by fitting the model to experimental stress-strain curves observed at various strain rates and temperatures. The relaxation time τ_SS steeply decreased with increasing strain at the beginning of stretching, and then attained to low steady values at strains slightly larger than the yield strain. The steady values of τ_SS were almost inversely proportional to the strain rate, and slightly shorter at a higher stretching temperature. These dependences of τ_SS for epoxy glass on strain, strain rate and temperature were qualitatively identical to those for thermoplastic glassy polymers without crosslinked molecular structures. Strain-dependent relaxation times for thermoplastic glassy polymers are known as the result of change in glassy structures. Thus, it is concluded that the nonlinear viscoelastic behavior of crosslinked epoxy glass arises from strain-induced structural change.