Thermo-mechanically Coupled Cyclic Deformation Of Ultra-high Molecular Weight Polyethylene And Its Constitutive Model

Ultra-high molecular weight polyethylene(UHMWPE)is a kind of semi-crystalline thermoplastic polymers,which has been widely used in the areas of aerospace,mechanical engineering structures and biomedical devices due to its low friction coefficient,high specific strength and good biocompatibility.In the practical application,the polymeric components and devices are often subjected to a cyclic loading.The accumulated deformation occurred during cyclic loading would cause the failure of the components and devices or reduce their fatigue lives,which should be considered in designing them.In recent decades,experimental and theoretical studies on the cyclic deformation of polymers have been widely performed by many researchers.However,they mainly focus on the uniaxial cyclic deformation of polymers.The studies on the non-proportional multiaxial cyclic deformation of polymers are very rare.Meanwhile,the effect of crystalline content on the cyclic deformation of semi-crystalline polymers has not been reported in the existing literature.The existing constitutive models are all not able to describe the cyclic deformation of semi-crystalline polymers(including UHMWPE)comprehensively and reasonably,due to the lacking of systematical experimental study on the effect of crystalline content on the cyclic deformation of UHMWPE.Therefore,it is necessary to perform the uniaxial and multiaxial cyclic tests on the UHMWPEs with different crystalline contents at various temperatures to reveal their cyclic softening/hardening features and uniaxial and multiaxial ratchetting as well as the dependences of cyclic deformation on the loading time,temperature and crystalline content and its thermo-mechanically coupled characteristics.Finally,based on the experimental observations,a thermo-mechanically coupled constitutive model of UHMWPE by considering the influence of crystalline content is developed.To systematically experimental and theoretical studies on the uniaxial and multiaxial cyclic deformation of UHMWPEs with different crystalline contents and at different temperatures,the following contents are included in this thesis:(1)Uniaxial and non-proportional multiaxial cyclic tests with different loading modes,loading levels,loading rates,peak/valley stress holding times and ambient temperatures were performed on the UHMWPEs with different crystalline contents.The effects of crystalline content and above-mentioned factors on the cyclic softening/hardening features and ratchetting of UHMWPE were discussed.The mechanism of thermal dissipation and the interaction between temperature rise and inelastic deformation during the cyclic deformation were revealed.These results are helpful to construct a thermo-mechanically coupled cyclic constitutive model of UHMWPE.(2)Based on the experimental observations,a viscoelastic-viscoplastic constitutive model was firstly constructed to describe the cyclic deformation of UHMWPE.In the newly developed constitutive model,the viscoelastic deformation was described by adopting the modified Schapery non-linearly viscoelastic model.While,the viscoplastic strain was assumed to be composed of two parts,i.e.,the one related to cyclic viscoplasticity and the other caused by creep strain.The cyclic viscoplasticity was described by the modified Armstrong-Frederick kinematic hardening model and the creep deformation was depicted by the creep equations in a power-law mode.Meanwhile,a non-proportional factor related to the multiaxial loading path and some evolution equations related to the crystalline content were introduced into the developed constitutive model.The model was then verified by comparing its predictions with the corresponding experimental results of UHMWPEs.It is demonstrated that the newly developed model can reasonably predict the uniaxial and non-proportional multiaxial ratchetting of UHMWPEs with different crystalline contents at room temperature.(3)Furthermore,a thermo-mechanically coupled viscoelastic-viscoplastic constitutive model was developed by considering the mechanism of thermal dissipation and the relation between internal heat generation and external heat exchange during the cyclic deformation,and also by introducing some evolution equations related to the temperature.Comparing the predictions by the proposed model with the corresponding experimental results,it demonstrates that the proposed thermo-mechanically coupled constitutive model describes the thermo-mechancially coupled cyclic deformation of UHMWPE and related temperature variation reasonably.