Non-proportional multiaxial ratchetting of ultrahigh molecular weight polyethylene polymer: Experiments and constitutive model

Non-proportional multiaxial ratchetting of ultrahigh molecular weight polyethylene (UHMWPE) polymer was observed under the stress-controlled cyclic tension-torsion loading condition and at room temperature at first. The dependence of the multiaxial ratchetting of the UHMWPE on the non-proportional loading paths (e.g., butterfly-type, hourglass-type, square, rhombic and circular paths), stress level, stress rate and loading history was investigated. Based on the viscoelastic-viscoplastic cyclic constitutive model for the uniaxial ratchetting of the UHMWPE proposed by Chen et al. (2016a) and from the experimental observations in this work, a new multiaxial cyclic constitutive model was developed by introducing a non-proportional loading path factor into the existing uniaxial model so that the remarkable dependence of the multiaxial ratchetting of the UHMWPE on loading path could be reflected. The capability of the extended constitutive model to predict the multiaxial ratchetting of the UHMWPE was verified by comparing the predictions with the corresponding experiments. It is concluded that the predictions are in good agreement with the experiments.