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Multiscale carbon fiber-reinforced epoxy composites containing nanofillers

Authors Khan, Shafi Ullah
Issue Date 2011
Summary Carbon Fiber-reinforced polymer composites (CFRPs) have found widespread applications as structural material in many load bearing structures owing to their high specific strength and stiffness. However, CFRPs for advanced engineering applications often require the most demanding mechanical and multifunctional properties. The introduction of nanotechnology in the field of composite materials with nanoscale fillers offered new opportunities to improve these properties of CFRPs. This research aims at the enhancement of mechanical and multifunctional properties of epoxy nanocomposites and carbon fiber-reinforced epoxy composites containing nanoclay and carbon nanotubes. The influences of nanoclay on the fracture resistance, flexural properties and fatigue performance of epoxy-based nanocomposites and the corresponding carbon fiber-epoxy composites (CFRPs) have been studied. The presence of nanoclay enhances the impact and quasi-static fracture resistance, as well as the flexural strength and modulus of the composites. Microscopic examination based on the double-notch bending test identifies pertinent toughening mechanisms responsible for the enhanced toughness of clay nanocomposites, namely microcracking, crack pining, crack tip bifurcation and deflection, and microvoids along the clay galleries. Multilayer delaminations are among the key toughening mechanisms identified for the clay-CFRP hybrid composites. The tension-tension cyclic fatigue tests are conducted at various load levels to generate the S-N curve. The residual strength and modulus are measured at different stages of fatigue cycles. Nanoclay not only improves the mechanical properties of the CFRP composite in static loading, but also the fatigue life for a given cyclic load and the residual mechanical properties after cyclic fatigue. Nanoclay serves to suppress and delay delamination damage growth and eventual failure by improving the fiber/matrix interfacial bond and through the formation of nanoclay-induced dimples that are two of the most important underlying toughening mechanisms. Vibration damping characteristic of CFRPs containing CNTs have been investigated by performing the free and forced vibration tests. The damping ratio of the hybrid composites is enhanced with the addition of CNTs arising from the sliding at the CNT-matrix interfaces. The forced vibration test reveals that the hybrid composites have higher damping ratio than neat CFRP in both the 1st and 2nd vibration modes. The damping ratio of the CNT-epoxy nanocomposites show a similar increasing trend with CNT content, indicating that the enhanced damping property of CFRPs is due mainly to the improved damping property of the modified matrix. The dynamic mechanical analysis further confirms that the CNTs have a strong influence on the composites damping properties. CNTs are aligned in an epoxy matrix as a result of DC electric fields applied during composite curing. It is shown that the electrical and mechanical properties are significantly higher in composites containing aligned CNTs than those with random orientation. There is a saturation CNT content above which the improvements in these properties due to CNT alignment tend to decrease, indicating a limit in the efficiency of CNT alignment. This technique provides an ability to tailor physical, mechanical and fracture properties of bulk nanocomposites even at a very low CNT concentration. A technique is developed to integrate high CNT contents in to CFRP composites using bucky paper interleaves. The effects of CNF-bucky paper interleaves on mechanical properties of CFRPs have been studied. The mode-II fracture toughness of CFRP composites with CNF bucky paper interleaves increased by about 104% whereas the corresponding interlaminar shear strength improved by about 31% compared to the composites without interleaves. The CFRPs with bucky paper interleaves exhibited significant improvement in impact damage resistance in the form of higher initiation and maximum load, and smaller damage area. The technique developed here can be used to incorporate CNTs of higher contents to strengthen/toughen at failure prone locations in CFRP composites, which has not been possible previously because of the high viscosity caused by simple dispersion of randomly-oriented CNTs in the matrix material.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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