Extension of viscoplasticity based on overstress to capture the effects of prior aging on the time dependent deformation behavior of a high-temperature polymer: Experiments and modeling

The inelastic deformation behavior of PMR-15 neat resin, a high-temperature thermoset polymer, was investigated at 288° C. The experimental program was designed to explore the influence of strain rate on tensile loading, unloading, and strain recovery behaviors. In addition, the effect of the prior strain rate on the relaxation response of the material, as well as on the creep behavior following strain controlled loading were examined. The material exhibits positive, nonlinear strain rate sensitivity in monotonic loading. Nonlinear, "curved" stress-strain behavior during unloading is observed at all strain rates. The recovery of strain at zero stress is strongly affected by prior strain rate. The prior strain rate also has a profound influence on relaxation behavior. The rest stresses measured at the termination of relaxation tests form the relaxation boundary which resembles a nonlinear stress-strain curve. Likewise, creep response is significantly influenced by prior strain rate. The experimental results suggest that the inelastic behavior of the PMR-15 solid polymer at 288°C can be represented using a unified constitutive model with an overstress dependence of the inelastic rate of deformation. The experimental data were modeled with the Viscoplasticity Based on Overstress (VBO) theory. A systematic procedure for determining model parameters was developed and the model was employed to predict the response of the material under various test histories. Additionally the effects of prior aging at 288° C in argon on the time (rate)-dependent behavior of the PMR-15 polymer were evaluated in a series of strain and load controlled experiments. Based on experimental results, the VBO theory was extended to capture the environmentally induced changes in the material response. Several of the VBO material parameters were expanded as functions of prior aging time. The resulting model was used to predict the high-temperature behavior of the PMR-15 polymer subjected to prior aging of various durations.