| The inelastic deformation of six engineering polymers has been investigated with the desideratum being a thorough mapping of the mechanical response characteristics and the subsequent application of a state-variable based constitutive material model to the data. Materials included in the investigation were polycarbonate (PC), Nylon 66, high-density polyethylene (HDPE), polyethylene-terephthalate (PET), polyethersulfone (PES) and polyphenylene oxide (PPO). Cylindrical specimens were machined from as-received rod stock. The use of a servo-hydraulic test frame with control mode switching capability has permitted data collection under strain and load controlled test configurations. In the region of homogeneous deformation with strain typically less than 10%, during loading all materials have been found to exhibit, (i) positive non-linear rate sensitivity in loading, (ii) the magnitude of the response in creep, relaxation and recovery tests varies non-linearly with changes in the prior loading rate, and (iii) in the inelastic flow region the stress drop in relaxation has been found to be independent of the test strain value. In addition to these findings, perhaps the most singular deformation response has been in the instance of relaxation (creep) during unloading when the rate of change of stress (strain) may undergo a change in sign. This phenomenon has been labeled 'rate reversal' and has surfaced in tensile and compression load conditions. The preponderance of data, therefore, suggests that the amorphous versus crystalline distinction does not largely manifest itself in the qualitative nature of the deformation behavior.{09}This finding endorses the competence of macro-based models to undertake the task of polymer deformation modeling.; Common response characteristics such as positive strain rate sensitivity, monotonic decreases in the stress magnitude in a relaxation test (strain hold), and response during creep have been modeled well with the existing viscoplasticity theory based on overstress (VBO), a state variable model consisting of a set of non-linear differential equations. Material constants and model predictions for HDPE and PPO have been generated. Curved unloading and the aforementioned rate reversal behavior, however, appear to fall beyond the purview of the existing formulation. Potential modifications to the model are discussed. |
