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Traditional viscoelastic models for describing polymer response during large deformations are normally designed to capture the response during monotonic loading and typically have difficulty capturing the response after a reversal of the deformation process. In particular, most models pay little attention to capturing the equilibrium stress, the anisotropy developed after plastic flow in the elastic response, and the characteristics of the yield and subsequent flow after reversal of the loading. To characterize these events, the thermo-mechanical response of PEEK is studied during shear histories that have one or more points at which the strain rate is reversed. In particular, using digital image correlation (DIC) methods, the response of PEEK is captured during processes that subject the material to histories that reverse the straining direction one or more times. These studies show that the response of PEEK in monotonic loading is very different from that observed after reversing the loading, and also from that observed in further cycling. Yet, after multiple cycles of loading and reverse loading, if the loading is then continue beyond the point that loading reversal was initiated in the cycling, the response after this point returns to that of the initial monotonic loading.

With the development of new methods for the characterization of equilibrium stress through cyclic loading, it is now possible to follow the evolution of back stress during the nonlinear deformation of polymers. Experiments on PEEK and PC below the glass-transition temperature indicate a back stress that may evolve with plastic deformation, and which is substantially different from that seen during the response in the rubbery range. In particular, the back stress during the response of PC shows the characteristic post-yield softening, possibly indicating that the observed post-yield softening in the response comes from the back stress. This is not seen in PEEK, which also shows no substantial post-yield softening. The equilibrium stress plays a central role in modeling both the quasi-static and dynamic response of PEEK.

Poly-ether-ether-ketone (PEEK) is a high-performance semi-crystalline polymer with mechanical and thermal stability characteristics that are superior to most tough polymers. The mechanical characteristics of this polymer are modeled over a broad range of mechanical loading conditions using a thermodynamically consistent modeling process. This preliminary model, which ignores the thermal response and the possible recrystallization of this material during loading, shows an outstanding ability to capture the multidimensional nonlinear response of PEEK up to 60 % compression, with loading rates from 0.0001 to 3000 1/s at room temperature. The model includes the measured anisotropy in the wave response that develops with plastic flow, captures the evolution of the measured equilibrium stress, and correctly matches the evolution of the tangent modulus at equilibrium. This broad range of rates and experimental conditions are achieved by using a two-element nonlinear thermodynamically-consistent model.