Browsing by Subject "composite"
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Item Development of a composite repair system for reinforcing offshore risers(2009-05-15) Alexander, Christopher RichardA research program was conducted to investigate the application of composite materials in repairing corroded offshore risers, leading to the development of an optimized repair using a hybrid carbon/E-glass system. The objective of this research program was to investigate the feasibility of extending onshore composite repair techniques to offshore risers by developing integrated analytical and experimental methods. The study considered loads typical for offshore risers including internal pressure, tension, and bending. To fulfill this objective efforts included a state of the art assessment of current composite repair technology, designing a carbon-based composite repair system optimized by numerical simulation with prototype testing, and providing guidelines for industry in repairing and reinforcing offshore risers using composite materials. Research efforts integrated numerical modeling, as well as full-scale testing that included four composite repair manufacturers to assess the current state of the art on pipe samples with simulated corrosion reinforced with composite materials. Analysis and testing were also performed on the optimized carbon/E-glass system. The results of this program demonstrated that composite materials are a viable means for repairing corroded offshore steel risers as adequate reinforcement ensures that the steel risers are not loaded beyond acceptable design limits. For corroded risers, the results demonstrated through analysis and full-scale testing efforts that properly designed composite repair systems can provide adequate structural reinforcement to ensure that excessive strains are not induced in the steel when subjected to internal pressure, axial tension, and bending design loads. This was verified experimentally using strain gages placed beneath the composite repair. This program is the first of its kind and is thought to contribute significantly to the future of offshore riser repairs. It is likely that the findings of this program will foster future investigations involving operators by integrating their insights regarding the need for composite repair based on emerging technology. One of the most significant contributions to the existing body of work is the use of limit analysis in developing design limits for the repair of steel pipes using composite materials.Item Evaluation of Quasicrystal Al-Cu-Fe Alloys for Tribological Applications(2013-07-22) Nabelsi, NezarThis research investigated the tribological performance of a composite material, formed from an ultra high molecular weight polyethylene (UHMWPE) matrix and quasicrystalline Al-Cu-Fe alloy powders. An evaluation was conducted for the microstructure, material properties, and tribological performance of quasicrystalline materials formed from Al-Cu-Fe alloys. Arc melting was used as the fabrication technique for these alloys, and some samples were additionally heat treated in an argon environment. Vickers microhardness testing was done to make comparisons to wear rate behavior of the various alloys. Tribological studies were conducted using a linear pinon- desk configuration to evaluate friction and wear. Research indicated the annealed samples of Al-Cu-Fe that formed icosahedral quasicrystalline phases, where the quasicrystalline phase was most dominant of the observed alloys, displayed the greatest wear resistance and hardness. Abrasive wear was observed in each of the samples, as the brittle, hard nature of the quasicrystalline phase would not allow for the ductile adhesion. The addition of small amounts of Al-Cu-Fe quasicrystalline particles, crushed and pulverized from the arc-melted ingots, reduced the coefficient of friction and wear rate of UHMWPE, when added to the polymer.Item Improved performance of ultra-high molecular weight polyethylene for orthopedic applications(2009-05-15) Plumlee, Kevin GrantA considerable number of total-joint replacement devices used in orthopedic medicine involve articulation between a metallic alloy and ultra-high molecular weight polyethylene (UHMWPE). Though this polymer has excellent wear resistance, the wear particulate produced leads to the limited lifetime of the devices ? osteolytic bone loss. Crosslinking has been shown to reduce the wear rate of UHMWPE, but can cause a reduction in various mechanical properties such as impact toughness. This study presents two alternate approaches to improving the wear performance of UHMWPE in orthopedic applications Previous work has shown that UHMWPE-based composites have wear resistance comparable to the irradiation-crosslinked polymer. Zirconium has been shown to have excellent corrosion resistance and biocompatibility, and the authors have used the material as reinforcing filler in UHMWPE with promising results. Compression-molded UHMWPE composites with up to 20 weight percent (wt%) of micro-sized zirconium particles were investigated with regards to wear behavior and impact toughness. These composites showed a significant reduction in wear compared to unfilled polymer while still maintaining impact toughness. These results reinforce the paradigm of using polymer composites for orthopedic applications and may provide a viable alternative to the property tradeoffs encountered with irradiation crosslinking. Apart from UHMWPE, novel materials including hydrogels and bio-derived polymers show great potential in orthopedics, but such materials require the development of innovative fixation techniques [1-3]. The development of controlled porous UHMWPE morphologies offers the opportunity to utilize and expand these developing technologies. Interconnected porous structures were prepared by dry mechanical mixing of NaCl particles and UHMWPE powders followed by compression molding. Samples were soaked in water to remove the embedded salt, leaving a porous UHMWPE structure. Computational simulations of porogen distribution and leaching predicted leaching to be 95% effective when initial salt concentrations were 60wt% and higher, which was found to match very well with the experimental data. It was found that varying the concentration and particle size of the porogen can tailor the final pore morphology to a specific application, while DMA results showed that storage and loss moduli depend greatly on porosity, but not on pore size. Finally, porous UHMWPE scaffolds were successfully impregnated with gelatin, confirming the compatibility of UHMWPE with hydrogel-based fillers.Item Prediction of Damage Zone Growth in Composites Using Continuum Damage Mechanics(2010-07-14) McLendon, Wesley R.The continuum damage mechanics (CDM) approach is widely used to model damage in polymer matrix composite materials which are represented using the homogenized properties of the fiber and matrix constituents. CDM simplifies the problem of accounting for a large number of defects in a material by considering the homogenized effect of the defects as a change in constitutive properties of the material. However, recent investigations of textile composites have shown that CDM inaccurately predicts the direction of damage zone growth for some composite architectures which fail under shear load, tending to predict failure transverse to the fibers. This behavior is fundamentally attributable to the fact that shear failure in textiles results in large (tow-scale) matrix cracks, while CDM is intended to model distributed micro-cracks. It is shown that when CDM is used to model shear failure in anisotropic continua, material anisotropy tends to cause CDM to predict failure contrary to what is expected for these structures. An approach is presented that may allow CDM to better predict damage growth for shear failure in composites by encouraging the creation of an intial damage zone with sufficient directional bias to overcome the effect of material anisotropy.Item The effect of irregular fiber distribution and error in assumed transverse fiber CTE on thermally induced fiber/matrix interfacial stresses(Texas A&M University, 2006-08-16) Zu, Seung-DonThermally induced interfacial stress states between fiber and matrix at cryogenic temperature were studied using three-dimensional finite element based micromechanics. Mismatch of the coefficient of thermal expansion between fiber and matrix, and mismatch of coefficient of thermal expansion between plies with different fiber orientation were considered. In order to approximate irregular fiber distributions and to model irregular fiber arrangements, various types of unit cells, which can represent nonuniformity, were constructed and from the results the worst case of fiber distributions that can have serious stress states were suggested. Since it is difficult to measure the fiber transverse coefficient of thermal expansion at the micro scale, there is an uncertainty problem for stress analysis. In order to investigate the effect of error in assumed fiber transverse coefficient of thermal expansion on thermally induced interfacial stresses, systematic studies were carried out. In this paper, the effect of measurement errors on the local stress states will be studied. Also, in order to determine fiber transverse CTE values from lamina properties, a back calculation method is used for various composite systems.