A STATE-VARIABLE MODEL FOR HIGH-STRENGTH POLYMERS

Displacement controlled experiments on nylon 66, poly(etherether keton e), and poly(ether imide) at room temperature suggest that nonlinear e lasticity is not a good model for these polymers. Rather, qualitative evidence is presented that a state variable model shows promise. In th is model, the rate of deformation is the sum of the elastic and the in elastic rates of deformation. The elastic rate of deformation is given by an objective formulation of Hooke's law, and the inelastic deforma tion is an increasing function of the overstress, the difference betwe en the Cauchy stress and the equilibrium stress. The equilibrium stres s is a state variable, and represents the stress that can be sustained at rest following deformation. Load controlled tests, intended to ver ify or falsify the model, show that the creep rate at the same stress level can be different on loading and unloading, and that the creep ra te need not increase with an increase in creep stress level. These ano malous results can easily be explained by the introduction of the over stress concept, and by proper evolution of the equilibrium stress. The y confirm the usefulness of the overstress concept for the modeling of these polymers.