Responses of ultra-high molecular weight polyethylene (UHMWPE) under biaxial cyclic loading were investigated through systematically conducting experiments. Biaxial experiments on UHMWPE tubular specimens were conducted first by prescribing a steady internal pressure followed by a symmetric axial-strain controlled cycle. The steady internal pressure induced a steady nominal circumferential stress, which under the application of the axial strain-controlled cycle, induced circumferential strain ratcheting in the UHMWPE tubular specimens. Experimentally observed ratcheting responses of UHMWPE under biaxial cyclic loading was simulated using one of the unified state variable theories, the viscoplasticity theory based on overstress for polymers (VBOP). To improve the circumferential strain ratcheting simulation of the VBOP model, the Chaboche kinematic hardening rule was implemented in the model. The simulation of the VBOP model with the classical kinematic hardening model was also carried out to demonstrate the current state of the modeling for UHMWPE. Improvement of the circumferential strain ratcheting simulation by the modified VBOP model is demonstrated; however, simulations also indicate that further model modification will be needed.

Biaxial ratcheting behaviour of ultrahigh-molecular-weight polyethylene (UHMWPE) has been modelled using the viscoplasticity theory based on overstress for polymers (VBOP) with the modified Chaboche kinematic hardening rule. Investigated loading condition is: axial strain-controlled cyclic loading of thin-walled tubular specimen in the presence of constant pressure. To improve the circumferential strain ratcheting response of UHMWPE, changes designed to account for kinematic hardening and tangent modulus effects are proposed. Numerical results are compared with previously obtained experimental data. It is shown that modified tangent modulus improves the model responses. The biaxial ratcheting behaviour of UHMWPE is modelled quantitatively with VBOP.

The objective of this work is to study the stress-strain responses of ultra high molecular weight polyethylene (UHMWPE) under uniaxial and biaxial cyclic loading through systematically conducting experiments and model simulations. Experiments involved prescribing axial, strain and stress controlled, cycles to the specimens of UHMWPE. Since the ratcheting strain and its accumulation rate are sensitive to the mean (or steady) and amplitude stresses of the prescribed loading cycles, these parameters were varied in the experiments conducted. The viscoplasticity theory based on overstress (VBO) [ was implemented to simulate the cyclic and ratcheting responses of UHMWPE. Kinematic stress is the main state variable in constitutive models which affect cyclic behavior and the ratcheting. Therefore, different kinematic hardening laws such as Prager, Frederick-Armstrong, Burlet-Cailletaud, Ohno-Wang and Chaboche, are used to investigate ratcheting behavior of UHMWPE. The experimental and VBO simulated responses are compared to demonstrate the current state of the simulations and future model development needs.

No abstract