Finite Element Model of Equal Channel Angular Extrusion of Ultra High Molecular Weight Polyethylene

Abstract

Ultra-high molecular weight polyethylene (UHMWPE) used in biomedical applications, e.g., as a bearing surface in total joint arthroplasty, has to possess superior tribological properties, high mechanical strength, and toughness. Recently, equal channel angular extrusion (ECAE) was proposed as a processing method to introduce large shear strains to achieve higher molecular entanglement and superior mechanical properties of this material. Finite element analysis (FEA) can be utilized to evaluate the influence of important manufacturing parameters such as the extrusion rate, temperature, geometry of the die, back pressure, and friction effects. In this paper, we present efficient FEA models of ECAE for UHMWPE. Our studies demonstrate that the choice of the constitutive model is extremely important for the accuracy of numerical modeling predictions. Three considered material models (J2-Plasticity, Bergstrom-Boyce, and the three-network model) predict different extrusion loads, deformed shapes, and accumulated shear strain distributions. The work has also shown that the friction coefficient significantly influences the punch force and that the two-dimensional (2D) plane strain assumption can become inaccurate in the presence of friction between the billet and the extrusion channel. Additionally, a sharp corner in the die can lead to the formation of the so-called “dead zone” due to a portion of the material lodging into the corner and separating from the billet. Our study shows that the presence of this material in the corner substantially affects the extrusion force and the resulting distribution of accumulated shear strain within the billet.