published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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Carbon Nanotubes are thought to have tremendous potential as reinforcements for the next generation of composite materials. In the past decade, the enhancing effects of the nanotubes on the mechanical, electrical as well as multi-functional properties of polymer composites have been reported. However, the same nanotubes/polymer composites investigated by different research groups, in many cases, do not show a good agreement with one another. The root cause of this variability is believed to lie in the processing methodology employed to prepare the composites. Before one can propose an ideal and systematic processing condition, it is imperative to have a fundamental understanding of the effect of these nanotubes on the processing of the nanotube-based composites. In this study, the effect of 0.2wt.% functionalized single walled carbon nanotubes on the various thermo mechanical and thermo chemical properties of aerospace grade epoxy was investigated. Namely, the thermal degradation, rheological behavior, cure kinetics as well as the thermal expansion behavior of the epoxy were addressed. In addition, the effect of the application small quantity of nanotube/epoxy composite film on the interlaminar shear strength (ILSS) of a conventional laminated carbon fibre/epoxy prepregs was also investigated. The characterization results show that the presence of the nanotubes has a very significant effect on some of the inherent physical and chemical properties of the epoxy. The presence of these nanotubes leads to a delay in the degradation temperature of the epoxy. The viscosity sees a seven-fold increase at room temperature and the resin also gels at a lower temperature in the presence of the nanotubes. At the same time, the total heat of reaction is also lowered on addition of the nanotubes. The mechanical test, however, shows that the addition of the nanotube/epoxy film does not affect the ILSS of the laminated carbon fibre/epoxy composite. This ILSS value is also found to be dependant on the controlled alignment of the nanotubes and the method of application of the film at the interfaces of the laminates. Finally, it was observed that the nanotubes, when used in such low contents, also had no effect on the thermo mechanical properties of the epoxy. / Les nanotubes de carbone sont considérés comme ayant un potentielénorme pour assurer le rôle de renforts dans la prochaine génération de matériauxcomposites. Dans les décennies précédentes, les effets des nanotubes surl'amélioration des propriétés mécaniques, électriques et multi-fonctionnelles despolymères ont été révélés. Par contre, dans la plupart des cas, les études réaliséespar différents groupes de recherche et portant sur les mêmes composites faits depolymère renforcé de nanotubes ne présentent pas toutes des résultatscomparables. La cause majeure de cette variabilité est la méthodologie utiliséelors du procédé de fabrication de ces composites. Avant que quiconque ne suggèreune méthodologie idéale et systématique, il est impératif de comprendre les basesfondamentales de l'effet des nanotubes sur le procédé de fabrication. Dans cetterecherche, les effets des nanotubes de carbone à paroi simple sur les propriétésthermo mécaniques et thermo chimiques d'une résine époxy de gradeaéronautique ont été investigués. Les caractéristiques étudiées comprennent ladégradation thermique, le comportement rhéologique, la cinétique depolymérisation, ainsi que l'expansion thermique. L'effet de l'application d'unfilm de nanotube/époxy sur la résistance interlaminaire au cisaillement aégalement été étudié avec un préimprégné conventionel de fibre de carbone etd'époxy. Les résultats de caractérisation montrent que les nanotubes ont un effetsignificatif sur certaines propriétés physiques et chimiques inhérentes à l'époxy.La présence des nanotubes crée un délai dans la température de dégradation del'époxy. La viscosité de la résine est 7 fois plus élevée à la température de la pièceet sa température de gélification est inférieure. De plus, la chaleur totale deréaction est diminuée. Par contre, les tests mécaniques montrent que l'applicationd'un film de nanotube/époxy ne cause pas de changement dans la résistanceinterlaminaire au cisaillement d'un préimprégné de fibre de carbone et d'époxy.Par ailleurs, cette valeur de résistance est dépendante de l'alignement desnanotubes et de la méthode d'application du film sur les interfaces du laminé.Finalement, il a été observé que les nanotubes n'ont aucun effet sur les propriétésthermo mécaniques.

Low profile additives (LPA) are thermoplastics that are incorporated to unsaturated polyester (UP) resins in order to improve the surface finish of UP/fibreglass composites, widely used in automotive applications. The effect of using LPA on the thermo-mechanical properties of resin transfer moulded UP/fibreglass composites is investigated. The flexural and shear properties are measured by three-point bending tests. The trend of these mechanical properties is identified for 0% to 40% LPA content. All the mechanical properties like flexural strength, flexural modulus and short beam strength reduce upon addition of LPA. The specimens fail by tension in the flexural test and show a mixed shear/tension failure mode in case of short beam tests. From scanning electron microscopy, morphological change of the fractured surface is observed with an LPA-rich phase. Glass transition temperature (Tg) measured by thermal mechanical analysis (TMA) and dynamic mechanical analysis (DMA) show reproducible data and compare well with each other. Tg is improved by LPA addition due to the development of a more compatible system compared to neat resin. Differential scanning calorimetry (DSC) is also performed to detect Tg, which gives unreliable results.

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Automobile manufacturers are interested in lightweight materials, including magnesium, to increase vehicle fuel economy, improve performance and reduce emissions. In this work the deformation behavior of as-cast and rolled magnesium AZ31 alloy has been studied. In as-cast material, it was found that reheating at 400°C and above for 60 minutes increased the homogeneity of the as-cast structure and gave rise to repeatable deformation. At compression temperatures above 300°C dynamic recrystallization occurred; below 200°C, there was significant twinning. Annealing completely recrystallized the structure deformed below 200°C, but did not change the dynamically recrystallized structure. AZ31 alloy was also rolled at temperatures of 350, 400 and 450°C and rolling speeds of 20 and 50 rpm for 15 and 30% reduction in thickness to produce sheet. Before rolling, the alloy was preheated for I and 10 hours at the rolling temperatures. The sheets were then tensile tested at 300, 400 and 450°C with strain rates of 0.1, 0.01 and 0.001s-1. The flow curves and microstructures indicated that the tensile deformation mechanism changed with processing conditions. Two deformation mechanisms were present in the magnesium sheet depending on the strain rate and grain size. At slow strain rates and small grain size, the active deformation mechanism was grain boundary sliding. As grain sizes increased there was also a component of dislocation creep. At the fast strain rate, the deformation mechanism, regardless of grain size, was dislocation creep. At a true strain rate of 0.001s-1, it was found that rolling at 350°C with 30% reduction per pass yielded the finest microstructure and subsequently, the best hot deformation characteristics. At a true strain rate of 0.1s-1, rolling at 450°C with 30% reduction per pass yielded a coarser, more recrystallized microstructure with best hot deformation characteristics.