On Numerical Modeling of Equal Channel Angular Extrusion of Ultra High Molecular Weight Polyethylene

Ultra high molecular weight polyethylene (UHMWPE) is widely used in biomedical applications, e.g. as a bearing surface in total joint arthroplasty. Recently, equal channel angular extrusion (ECAE) was proposed as a processing method to achieve higher molecular entanglement and superior mechanical properties of this material. Numerical modeling can be utilized to evaluate the influence of such important manufacturing parameters as the extrusion rate, temperature, geometry of the die, back pressure and friction effects in the ECAE of polyethylenes. In this paper we focus on the development of efficient FE models of ECAE for UHMWPE. We study the applicability of the available constitutive models traditionally used in polymer mechanics for UHMWPE, evaluate the importance of the proper choice of the friction parameters between the billet and the die, and compare the accuracy of predictions between 2D (plane strain) and 3D models. Our studies demonstrate that the choice of the constitutive model is extremely important for the accuracy of numerical modeling predictions. It is also shown that the friction coefficient significantly influences the punch force and that 2D plane strain assumption can become inaccurate in the presence of friction between the billet and the extrusion channel

Shear enhancement of mechanical and microstructural properties of synthetic graphite and ultra-high molecular weight polyethylene carbon composites

Ultra-high molecular weight polyethylene (UHMWPE) has a variety of industrial and clinical applications due to its superb mechanical properties including ductility, tensile strength, and work-to-failure. The versatility of UHMWPE is hindered by the difficulty in processing the polymer into a well consolidated material. This study presents on the effects of shear imparted by equal channel angular pressing (ECAP) on UHMWPE composites containing Nano27 Synthetic Graphite (N27SG). Ductility and work-to-failure improvements up to ~60–80% are obtained in sheared N27SG-UHMWPE composites as compared to non-sheared N27SG-UHMWPE controls of the same composition. Microscopy reveals increased fusion at particle boundaries and smaller voids in the sheared materials. Micro-computed tomography results indicate different distribution of N27SG particulates in ECAP samples as compared to CM indicating enhanced grain boundary interactions. Tradeoffs are not avoided as ECAP samples were lower in conductivity as compared to compression molded (CM) billets of the same weight percent. However, ECAP samples were able to be doped with more N27SG allowing for an ~170% increase in conductivity over CM samples of the same work-to-failure. This work shows that ECAP is a viable processing method for obtaining stronger, more ductile conductive composite materials

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

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

Experimental Studies and Numerical Modeling of Copper Nets in Marine Environment

abstract Copper alloy netting is increasingly used for offshore aquaculture, harbor protection and other marine applications. Its advantageous characteristics include high resistance to biofouling and increased strength compared to polymer nets. However, the hydrodynamic properties of copper nets are not well studied. In this paper, the results of experimental studies of drag forces on copper alloy net panels are reported. Based on these studies, empirical values for drag coefficients are proposed for various types of copper nets, and compared to the corresponding data for polymer netting. It is shown that copper nets exhibit significantly lower resistance to the current flow which corresponds to lower values of drag coefficient. Coefficients obtained from the experiments are incorporated into the finite element program Aqua-FE, developed at the University of New Hampshire for analysis of flexible structures subjected to waves and currents in marine environment. The results of the numerical simulations for a small volume fish cage, subjected to two different sets of environmental conditions, are analyzed to compare how introduction of copper netting instead of traditional nylon nets affects the dynamic response of the system

Development of a porous media model with application to flow through and around a net panel

Abstract The flow characteristics through and around a net panel have been investigated through computational fluid dynamics (CFD) and measurements. A finite volume approach was used for solving the Reynolds averaged Navier–Stokes equations combined with a k–ε turbulence model for describing the flow. For computational efficiency, the net was modeled as a sheet of porous media rather than a large number of cylinders connected by knots. The model resistance coefficients needed for the porous media equations were found by optimizing the fit between computed lift and drag forces on the net panel and lift and drag measured in tow tank experiments. Lift and drag acting on a flat panel of knotless nylon net (2.8 mm twine thickness and 29 mm mesh size) stretched on a frame were measured at different speeds and angles of attack, and fluid velocity was recorded in the region behind the net. The optimization process used to obtain the best fit porous media coefficients was simplified through the use of an analytical model. Final comparisons between CFD predictions and measurements of lift and drag coefficients and velocity reduction behind the net panel were made for two of the speeds and angles of attack. The agreement between measured and modeled data was good with a mean normalized absolute error of 6.2%