Student: Sofia Arevalo
Professor/Sponsor: Professor Lisa Pruitt
Research Project Title: Nanoindentation of Ultra High Molecular Weight Polyethylene (UHMWPE) infused with alpha-tocopheral, UHMWPE Cross-Linked and UHMWPE 1050 and Nanoidentation Troubleshooting
Ultra high molecular weight polyethylene (UHMWPE) has been the main material utilized in joint arthroplasty because of its biocompatibility and desirable mechanical properties. However, wear has been a large problem that decreases the life of the joint replacement. Wear can result in failure of the implant and particles may be released into the bloodstream. The body’s response to remove the debris may lead to osteolysis and aseptic loosening of the device.
Characterization of surface mechanical properties may offer new insights to wear behavior and fracture mechanisms. The surface’s of Ultra High Molecular Weight Poly-Ethylene (UHMWPE) infused with Vitamin E, UHMWPE 1050, UHMWPE Cross-linked re-melted were indented to characterize the surface mechanical properties and correlate the relationship between the bulk and surface mechanical properties and its behavior in vivo.
The nanoindentations were performed with a conospherical tip with a loading range of 150-600 µN at a loading/unloading rate of 30 µN per second. The parameters were chosen such that the indentations remain within the surface regime of the material.
Student: Bernard Kim
Professor/Sponsor: Professor Paul Wright
Research Project Title: A Fully Printed, Integrated Supercapacitor with an Ionic Liquid Electrolyte
Stencil casting provides a highly rheologically-tolerant and scalable printing method for fabrication of electrochemical energy storage devices. The supercapacitor is designed to augment onboard batteries in small wireless sensors by providing the large power draws over short periods of time that would reduce the cycle life and health of the battery. The effects of different carbon-based materials of varying particle size on device performance and cyclability are investigated. In addition, the device is fabricated on a printed nickel-based current collector to eliminate the need for conductive substrates. The supercapacitor electrodes are composed of mesocarbon microbeads or activated carbon particles suspended in a poly(vinylidene fluoride-cohexafluoropropene) binder, and the current collector is composed of ball-milled nickel particles in the same binder. The electrolyte is composed of the same binder and 1-butyl-3- methylimidazolium tetrafluoroborate, a room temperature ionic liquid. All layers are printed successively on top of each other, with the binder providing enough mechanical support to separate each layer and hold the device together. The electric conductivities of the current collector and carbon electrodes with respect to mechanical stability were optimized to be 308.9 S/cm and 0.3660 S/cm respectively, and the fully assembled device demonstrated adequate preliminary cyclability.
Student: Cynthia Tan
Professor/Sponsor: Professor George Johnson
Sub Area: Design Research Project Title: Energy absorption properties of a regular Weaire-Phelan open-cell foam under compression
This study investigates the mechanical behaviors and energy absorption properties of a regular open-cell foam under quasi-static and dynamic impact loads. The main motivation for this research is to provide an alternative approach to foam design and to the manufacturing process of protective gear and impact-resistant parts. In this study, the foams are comprised of a periodic lattice of tessellated cells that use the Weaire-Phelan structure as the primitive cell. The geometry of the foams, or the thickness of the edges, was changed to vary relative density, all designed to be less than 30 %. Foams of different relative densities were fabricated through selective laser sintering (SLS) of nylon powder. Compressive behaviors of the foam was modeled through simulations using LS-DYNA and experimentally tested in the Werner Goldsmith Impact Lab. Finite element analysis of the Weaire-Phelan foam, meshed through MATLAB, provided a predictive model as to how the part would respond experimentally. Simulated results showed layer-by-layer collapse of the cells during deformation, which was also observed during quasi-static compression in experiments. For each relative density and strain rate, numerical results provided responses that quantitatively matched those of the experiments in the elastic regime, but predicted higher stiffness for the foam. To improve simulations, coupon testing of the SLS nylon material for different build orientations was performed to collect more information on the material's strain-rate dependencies and on the effects of the printing parameters on the SLS material properties. Impact loading of the foam will be conducted by shooting a projectile at speeds of around 30 m/s with a high-pressure pneumatic gas gun. The higher the relative density, the more energy the foam will absorb. Hence, experiments will provide a model of the absorbed energy as a function of relative density and geometry. It should be noted that initial stages of this research project are purely basic, but better understanding of the material properties of foam as a function of geometry may lead to improvements in both functional foam designs and the manufacturing process of cellular parts.