Ozgen U. Colak

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

Thermoplastics such as ultrahigh 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 experiments 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.

Biaxial Ratcheting of Ultra High Molecular Weight Polyethylene: Experiments and Constitutive Modeling

Responses of ultra-high molecular weight polyethylene (UHMWPE) under biaxial cyclic loading were investigated through systematically conducting experiments. Biaxial experiments on UHMWPE tubular specimens were conducted first by prescribing a steady internal pressure followed by a symmetric axial-strain controlled cycle. The steady internal pressure induced a steady nominal circumferential stress, which under the application of the axial strain-controlled cycle, induced circumferential strain ratcheting in the UHMWPE tubular specimens. Experimentally observed ratcheting responses of UHMWPE under biaxial cyclic loading was simulated using one of the unified state variable theories, the viscoplasticity theory based on overstress for polymers (VBOP). To improve the circumferential strain ratcheting simulation of the VBOP model, the Chaboche kinematic hardening rule was implemented in the model. The simulation of the VBOP model with the classical kinematic hardening model was also carried out to demonstrate the current state of the modeling for UHMWPE. Improvement of the circumferential strain ratcheting simulation by the modified VBOP model is demonstrated; however, simulations also indicate that further model modification will be needed.

Mechanical Characterization and Modeling of Cyclic Behavior of Ultra High Molecular Weight Polyethylene (UHMWPE)

The objective of this work is to study the stress-strain responses of ultra high molecular weight polyethylene (UHMWPE) under uniaxial and biaxial cyclic loading through systematically conducting experiments and model simulations. Experiments involved prescribing axial, strain and stress controlled, cycles to the specimens of UHMWPE. Since the ratcheting strain and its accumulation rate are sensitive to the mean (or steady) and amplitude stresses of the prescribed loading cycles, these parameters were varied in the experiments conducted. The viscoplasticity theory based on overstress (VBO) [ was implemented to simulate the cyclic and ratcheting responses of UHMWPE. Kinematic stress is the main state variable in constitutive models which affect cyclic behavior and the ratcheting. Therefore, different kinematic hardening laws such as Prager, Frederick-Armstrong, Burlet-Cailletaud, Ohno-Wang and Chaboche, are used to investigate ratcheting behavior of UHMWPE. The experimental and VBO simulated responses are compared to demonstrate the current state of the simulations and future model development needs.

Synthesis and characterization of graphene-epoxy nanocomposites

Abstract Graphene, a monolayer of carbon atoms arranged in a two dimensional lattice, has attracted great attention in recent years due to its extra ordinary properties and potential applications. One obvious application of graphene is in the field of nano-composites. In this work, graphene platelets (GPL) reinforced with epoxy nano-composites were fabricated by using soft molding technique in two different ratios with two different solvents. Raman spectroscopy and scanning electron microscopy (SEM) were used to investigate the structure of graphene and graphene reinforced composites. Tensile and dynamic mechanical analysis (DMA) in three point bending mode were used to investigate the mechanical properties of the composites. The tensile strength and strain to failure of nanocomposites of GPL reinforced epoxy nanocomposite were enhanced by 9.31 % and 34.78 %, respectively, while small improvement is observed in the elasticity modulus. Dynamic mechanical analysis has shown that with the addition of 0.1 and 0.5 wt.-% graphene nanoplatelets, storage modulus has increased by 20 and 46 % on the glassy region, respectively. The glass transition temperature is not affected with addition of graphene.

Investigation of Hygro-Thermal Cycle Effects on the Membranes of Proton Exchange Membrane Fuel Cells

Nafion is the most commonly used membrane material in proton exchange membrane fuel cells. New and used membrane electrode assemblies (MEAs) are subjected to dynamic mechanical analysis (DMA) tests in temperature scan mode to determine the effect of catalyst coating and hygro-thermal aging. DMA tests are also performed using specimens obtained from the rolling and transverse directions to investigate the anisotropic effects of the membrane and the coating. The catalyst coating increases the stiffness and brittleness of the membrane. Increased stiffness is observed in used MEAs relative to new MEAs, which indicates that hygro-thermal aging increases the brittleness of the material. A slight dependence on direction—that is, anisotropy—is also determined.

Modeling of Storage Modulus of Graphene Epoxy Nanocomposites

In this work, the storage modulus of epoxy and graphene-epoxy nanocomposites are modeled using a temperature and rate dependent modulus formulation. The stiffening effect of reinforcements is included to the elastic modulus formulation for the modeling of nanocomposite materials. Simulation results are compared to experimental data from Acar et al. [1]. Keywords—graphene, epoxy, storage modulus.

Fabrication of Graphene Platelet (GPL)-Epoxy Nanocomposites and Characterization by Nanoindentation

Main objective of this work is to manufacture the graphene platelet (GPL)-epoxy nanocomposite and to characterize the nanocomposite using nanoindentation technique. Thermal reduction of graphite oxide is the method used to obtain bulk quantities of graphene platelets (GPL) which comprise multiple graphene sheets. Dispersion of GPL in epoxy matrix is done with sonication and high speed shear mixing is used for mixing curing agent and resin. Following the manufacturing of graphene platelet-epoxy nanocomposites, characterization of the material was performed by nanoindentation. Nanoindentation experiments are performed under load or displacement control at different load/displacement rates to investigate rate dependent behavior of the nanocomposite. The primary mechanical properties obtained from the nanoindentation tests which are the hardness and the elasticity modulus are determined.