Uniaxial Strain and Stress-Controlled Cyclic Responses of Ultrahigh Molecular Weight Polyethylene: Experiments and Model Simulations

Abstract

Thermoplastics such as uitrahigh molecular weight polyethylene (UHMWPE) are used for a wide variety of applications, such as bearing material in total replacement of knee and hip components, seals, gears, and unlubricated bearing. Accurate prediction of stresses and deformations of UHMWPE components under service conditions is essential for the design and analysis of these components. This, in turn, requires a cyclic, viscoplastic constitutive model that can simulate cyclic responses of UHMWPE under a wide variety of uniaxial and multiaxial, strain, and stress-controlled cyclic loading. Such a constitutive model validated against a broad set of experimental responses is not available mainly because of the lack of experimental data of UHMWPE. Toward achieving such a model, this study conducted a systematic set of uniaxial e.xperiments on UHMWPE thin-walled, tubular specimens by prescribing strain and stress-controlled cyclic loading. The tubular specimen was designed so that both uniaxial and biaxial experiments can be conducted using one type of specimen. The experimental responses developed are presented for demonstrating the cyclic and ratcheting responses of UHMWPE under uniaxial loading. The responses also are scrutinized for determining the applicability of the thin-walled, tubular specimen in conducting large strain cyclic experiments. A unified state variable theory, the viscoplasticity theory based on overstress for polymers (VBOP) is implemented to simulate the recorded uniaxial responses of UHMWPE. The state of the VBOP model simulation is discussed and model improvements needed are suggested.

Modeling of multi‐cycle deformation of polymers with various deformation programs

“…This may be explained by the fact that conventional stress-strain relations in viscoelastoplasticity of polymers and polymer composites (Yakimets et al, 2007;Avanzini, 2008Avanzini, , 2011Dusunceli and Colak, 2008;Bles et al, 2009;Mizuno and Sanomura, 2009;Spieckermann et al, 2009;Zrida et al, 2009;Ayoub et al, 2010;Buckley et al, 2010;Drozdov, 2010;Ramkumar et al, 2010;Silberstein and Boyce, 2010;Hassan et al, 2011;Hizoum et al, 2011;Launay et al, 2011;Tang et al, 2012) are confined to the analysis of observations along the first cycle of loading-unloading only. The first constitutive equations able to describe the mechanical response of polymers subjected to severe (hundreds of cycles) deformations have appeared in the past couple of years (Ayoub et al, 2011;Drozdov, 2011b, c).…”

An internal-state-variable based viscoelastic-plastic model for polymers

“…Drozdov and Christiansen (2007) developed a viscoplastic constitutive model aimed at predicting the stresses in high density polyethylene (HDPE) subjected to the cyclic loadings in tension. Hassan et al (2011) utilized an overstress-based viscoplasticity theory to model the hysteresis loops of ultra high molecular weigth polyethylene (UHMWPE) for the maximum strains equal to 10%. Ben Hadj Hamouda et al (2007) developed a viscoplastic model to capture the nonlinear stress-strain relation, creep and relaxation of the medium density polyethylene (MDPE).…”

Cooperative viscoplasticity theory based on the overstress approach for modeling large deformation behavior of amorphous polymers

“…The VBO model was initially developed for metallic materials . This theory was later modifed for modeling polymeric materials . For modeling the mechanical behavior of polymeric materials, two main mechanisms are needed: molecular interaction (stretching and orientation of the molecular network) and intermolecular interaction.…”

“…To macroscopically model the behavior of polymers, a different approach based on state variable theory was also developed and used for the simulation of the deformation response of amorphous and semicrystalline polymers. This theory is known as the viscoplasticity theory based on overstress (VBO) model . Note that the VBO model is a macroscopic approach based on the unified state variable theories.…”