Characterization and modeling of PEEK in histories with reverse loading

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.