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  1. Ultra-High Molecular Weight Polyethylene (UHMWPE) is widely used as a bearing surface in total and partial joint arthroplasty. In addition to medical applications, this polymer is utilized in the fields of ballistic protection, sports, and industrial tribology. The addition of carbon allotropes, such as nanographite or carbon black powders, to UHMWPE offers potential benefits including added conductivity, increased wear resistance, and introduction of micro-tracers for understanding microstructural behavior and monitoring damage [1]. The mechanical properties of these Carbon/UHWPE nanocomposites can be enhanced by subjecting them to equal channel angular extrusion (ECAE) as a way to introduce large shear strains to achieve higher molecular entanglement of UHMWPE and better distribution of carbon nanoparticles [2, 3]. In this paper, micro-computed tomography (µCT) is used to characterize carbon black (CB) and nanographite (N27SG) reinforced UHMWPE polymers. It is shown that the procedure described in [1] results in almost uniform distribution of carbon inclusions around UHMWPE particles with both compression molding (CM), and ECAE processes. Multiscale numerical models of the composite are developed based on the µCT images, including mesoscale finite element (FE) models of representative volume element (RVE) on the mesoscale, and micromechanical predictions for carbon-rich interphase layers on the microscale. 
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  2. null (Ed.)
    3D woven composites are well known for their high strength, dimensional stability, delamination, and impact resistance. They are often used in aerospace, energy, and automotive industries where material parts can experience harsh service conditions including substantial variations in temperature. This may lead to significant thermal deformations and thermally-induced stresses in the material. Additionally, 3D woven composites are often produced using resin transfer molding (RTM) technique which involves curing the epoxy resin at elevated temperatures leading to accumulation of the processing-induced residual stress. Thus, understanding of effective thermal behavior of 3D woven composites is essential for their successful design and service. In this paper, the effective thermal properties of 3D woven carbon-epoxy composite materials are estimated using mesoscale finite element models previously developed for evaluation of the manufacturing-induced residual stresses. We determine effective coefficients of thermal expansion (CTEs) of the composites in terms of the known thermal and mechanical properties of epoxy resin and carbon fibers. We investigate how temperature sensitivity of the thermal and mechanical properties of the epoxy influences the overall thermal properties of the composite. The simulations are performed for different composite reinforcement morphologies including ply-to-ply and orthogonal. It is shown that even linear dependence of epoxy’s stiffness and CTE on temperature results in a nonlinear dependence on temperature of the overall composite’s CTE. 
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  3. null (Ed.)
    Manufacturing-induced residual stresses in carbon/epoxy 3D woven composites arise during cooling after curing due to a large difference in the coefficients of thermal expansion between the carbon fibers and the epoxy matrix. The magnitudes of these stresses appear to be higher in composites with high throughthickness reinforcement and in some cases are sufficient to lead to matrix cracking. This paper presents a numerical approach to simulation of development of manufacturing-induced residual stresses in an orthogonal 3D woven composite unit cell using finite element analysis. The proposed mesoscale modeling combines viscoelastic stress relaxation of the epoxy matrix and realistic reinforcement geometry (based on microtomography and fabric mechanics simulations) and includes imaginginformed interfacial (tow/matrix) cracks. Sensitivity of the numerical predictions to reinforcement geometry and presence of defects is discussed. To validate the predictions, blind hole drilling is simulated, and the predicted resulting surface displacements are compared to the experimentally measured values. The validated model provides an insight into the volumetric distribution of residual stresses in 3D woven composites. The presented approach can be used for studies of residual stress effects on mechanical performance of composites and strategies directed at their mitigation. 
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  4. null (Ed.)
    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 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 
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  5. null (Ed.)
    3D woven carbon/epoxy composites are often produced using resin transfer molding technique which includes epoxy curing at elevated temperatures. The process may lead to accumulation of the intrinsic residual stresses during cooling of the material caused by the mismatch between carbon and epoxy coefficients of thermal expansion. This paper deals with implementation of mesoscale finite element models to evaluate intrinsic residual stresses in 3D woven composites. The stresses are determined by correlation of the surface displacements observed after drilling 1-mm diameter blind holes with the corresponding predictions of the models. We investigated how a numerical representation of the composite plate surface affects the correlation between the experimental measurements and numerical predictions and how it influences the evaluation of the process-induced residual stresses. It has been shown for ply-to-ply woven composites with different pick spacing that the absence of the resin layer leads to more accurate interpretation of the experimental measurements. The prediction of the average residual stress in the matrix phase of the composite was found to be sensitive to the surface representation accuracy, however, the residual stress magnitude and distribution was not affected fundamentally. 
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  6. null (Ed.)
    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 molec ular 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 fricti on 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. 
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  7. null (Ed.)
    Three panels of 3D woven carbon fiber/RTM6 epoxy composites with a ply-to-ply weave with 12x12 (warp/weft) picks per inch (ppi), 10x12 ppi, and 10x8 ppi were fabricated by resin transfer molding. Realistic finite element models of each weave architecture were constructed using Dynamic Fabric Mechanics Analyzer. The resin properties were isotropic and linear elastic and dependent on temperature. The resin-infiltrated fiber tow properties were estimated using homogenization based on Hashin and Shapery formulas. The model was considered to be at zero stress at the 165C curing temperature. The stresses resulting from cooling the composite to 25C were estimated using the resin temperature-dependent properties and the temperature independent properties of the tows. The displacement fields resulting from holes drilled through the middle of the top warp or weft yarn were estimated by virtually drilling a hole in the finite element model and were measured on the specimens using electronic speckle pattern interferometry. In general, the measured displacements transverse to the yarn were lower than the predicted displacements. This suggests the resin in the infiltrated yarns relieves some of the stress by permanently deforming during cooling. The measured displacements along the yarn were approximately the same for the 12x12 ppi,, lower for the 10x12 ppi, and significantly higher for the 10x8 ppi. 
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