Indentation/Scratch Simulation And Hip Contact Mechanics Of UHMWPE Based On Viscoplastic Constitutive Model

Ultra-High Molecular Weight Polyethylene(UHMWPE)is a typical thermoplastic polymer whose mechanical properties are dependent on the loading conditions and it has long been used as a load bearing component in artificial hip and knee joints.During human service,complex movement loads between joints and ambient temperatures can lead to increased failure of UHMWPE.To solve this problem,researchers have often used methods such as finite elements to simulate the stress or wear behaviour of actual prostheses.However,the modelling of the intrinsic behaviour of UHMWPE in the current literature has mainly relied on constitutive models such as elasticity and J2 plasticity,which can lead to large errors in the prediction results of the models when applied to real loading environments.Therefore,in this thesis,the experimental curves of UHMWPE in compression and tension at different temperatures and rates are calibrated by genetic algorithms based on the Three Network model and Three Network viscoplastic constitutive models to obtain a material model that can reflect the viscoplastic behaviour of UHMWPE under complex loading conditions.In addition,an indentation model and a scratch model for the study of UHMWPE surface tribology and a hip joint motion model reflecting the temperature variation were developed by this constitutive model.It provides a reference for exploring the variation law of thermoplastic response of indentation or viscoplastic response of scratch on the UHMWPE surface and subsequently establishing a thermodynamic coupled hip joint contact model.The main findings are as follows.(1)The viscoplastic indentation response of UHMWPE at different temperatures and rates was investigated,and the relationship between indentation depth and contact force,contact area and mean contact pressure during indentation loading and unloading was analysed by a dimensionless parameter method and quantified by a power-law equivalence function.The results show that an increase in the indentation loading rate leads to a corresponding increase in contact force,mean contact pressure and contact area,and conversely,an increase in the indentation ambient temperature decreases the corresponding contact force,mean contact pressure and contact area;the network flow resistance in the Three Network model is the key factor affecting the indentation contact;under the coupling effect of loading rate and ambient temperature,the maximum surface stress and maximum viscoplastic strain show a Under the coupling effect of loading rate and ambient temperature,the maximum surface stress and maximum viscoplastic strain show a regular pattern: an increase in loading rate or a decrease in ambient temperature increases the equivalent force on the surface of UHMWPE and decreases the viscoplastic strain;the Three Network model is effective in simulating the creep of the material.(2)The effect of different factors on the unidirectional scratch response of UHMWPE was investigated,and the mechanical experimental curves of different materials were used to predict the scratch response of different polyethylene,providing a theoretical basis for subsequent experiments.The results show that: in the unidirectional scratch model,the high speed of scratching causes higher positive/lateral friction and energy loss on the UHMWPE surface,and reduces the residual strain on the scratch surface;the effect of scratch depth is more decisive than that of scratch speed;the difference between the dynamically decaying Coulomb friction model and the traditional static Coulomb friction model increases with the increase of scratch distance.The difference between the dynamic decaying Coulomb friction model and the conventional static Coulomb friction model increases with the increase of the scratch distance,which is mainly reflected in the scratch forward friction force and friction energy loss;the mechanical experimental curves of different molecular weight polyethylene were obtained experimentally and their constitutive model parameters were obtained after calibration.The relationship between Three Network viscoplastic constitutive model parameters and the scratch model was established.(3)The effects of different constitutive models on the contact mechanics of hip prostheses under human gait loading and the effects of different temperatures on the contact mechanics of prostheses under gait loading were investigated.The results showed that the differences between the different instantiation models were not significant under walking gait,because the hip motion under walking gait exerted less load on the UHMWPE,and the differences between the instantiation models could be seen under extreme gait;the influence of ambient temperature on the contact mechanics of the whole hip prosthesis liner was relatively large,and the increase in ambient temperature led to a slight increase in the maximum slip of the surface,an increase in the contact area,a decrease in the Mises stress on the liner and a decrease in the maximum Mises stress.The Mises stress on the liner decreases and the maximum true principal strain increases significantly.