Mechanical characterization and FEA modelling of cyclic stress-strain behaviour of UHMWPE

This paper is concerned with the application of advanced plasticity models to the prediction of cyclic stress-strain behaviour of ultra high molecular weight polyethylene (UHMWPE). The material was first tested under fully reversed symmetric load cycles: a steady-state condition was attained after a few hundreds of cycles and cyclic strain softening and stress-strain hysteresis were observed. Based on the experimental data material parameters were then obtained for classical plasticity models, with isotropic or kinematic hardening, and for an evolutionary plasticity model, with non-linear isotropic/kinematic hardening, in a standard form in which yield surface depends on equivalent plastic strain. Experimental findings and literature data suggest that a predictive model applied to cyclic loading of UHMWPE should incorporate some features to relate the change of the shape of hysteresis cycle to the reduction of elastic properties observed on a macroscopic scale. A modified evolutionary plasticity model is therefore proposed in which a further dependence of elastic and hardening properties on equivalent plastic strain is introduced, thus allowing significantly improvement of experimental data fitting. A procedure to implement this modification into FEM code via programmable user subroutine is finally described.