published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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The behaviour of heated structures is strongly governed by thermal induced deformation and degradation of material properties. This thesis presents an augmentation of the software framework OpenSees to enable thermomechanical analysis of structures. The developments contributed to OpenSees are tested by series of benchmark cases and experimental results. OpenSees is an object-oriented, open source software framework developed at UC Berekeley for providing an advanced computational tool to simulate non-linear response of structural frames to earthquakes. OpenSees was chosen to be extended to enable the modelling of structures in fire. The development of this capability involved creating new thermal load classes to define the temperature distribution in structural members and modifying existing material classes to include temperature dependent properties according to Eurocodes. New functions were also added into the existing corotational beam/column element (2D and 3D) to apply temperature related loads. A new geometrically nonlinear shell element was created (based on the existing linear MITC4 shell element in OpenSees) using total Lagrangian formulation. Appropriate thermal load, material and section classes were also developed for enabling thermomechanical analysis using the nonlinear shell element. A number of benchmark tests were carried out to verify the performance of the new developments implemented in OpenSees. The benchmark tests involved subjecting beams and plates to a range of through depth temperature gradients with OpenSees results compared against closed form solutions. Further verification was also carried out by comparing OpenSees results with ABAQUS results. The extended OpenSees framework was also used to model experiments such as two plane steel frames at elevated temperatures, the Cardington Restrained Beam Test and the Cardington Corner Test and an earthquake damaged reinforced concrete (RC) frame subjected to a subsequent fire. The existing DruckerPrager material class in OpenSees was used to the model concrete in the composite floor in the Cardington tests and in the RC frame. The pinching material available in OpenSees was used to model the beams and columns in the RC frame to consider the cyclic degradation of strength and stiffness during the increasing cyclic displacements imposed on the RC frame before the fire. In all cases the results from OpenSees show good agreement with test data.

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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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Combustion gas turbine components designed for application in power generation equipment are subject to periodic replacement as a result of cracking, damage, and mechanical property degeneration that render them unsafe for continued operation. In view of the significant costs associated with inspecting, servicing, and replacing damaged components, there has been much interest in developing models that not only predict service life, but also estimate the evolved state of the material. This thesis explains manifestations of microstructural damage mechanisms that facilitate fatigue crack nucleation in directionally-solidified (DS) Ni-base superalloy components exposed to elevated temperatures and high stresses. In this study, models were developed and validated for damage and life prediction using DS GTD-111 as the subject material. This material has a chemical composition and grain structure designed to withstand creep damage occurring in blades of gas-powered turbines. The service conditions in these components, which generally exceed 600C, facilitate the onset of one or more damage mechanisms related to fatigue, creep, or environment. The study was divided into an empirical phase, which consisted of experimentally simulating service conditions in fatigue specimens, and a modeling phase, which entailed numerically simulating the stress-strain response of the material. Tests have been carried out to simulate a variety of thermal, mechanical, and environmental operating conditions endured by longitudinally (L) and transversely (T) oriented DS GTD-111. In some cases, tests in extreme environments/temperatures were needed to isolate one or at most two of the mechanisms causing damage. Microstructural examinations were carried out via microscopy. A continuum crystal plasticity model was used to simulate the material behavior in the L and T orientations. The constitutive model was implemented in ABAQUS and a parameter estimation scheme was developed to obtain the material constants. A physically-based model was developed for correlating crack initiation life based on the experimental life data. Assuming a unique relationship between the damage fraction and cycle fraction with respect to cycles to crack initiation for each damage mode, total crack initiation life has been represented in terms of the individual damage components observed at the end state of crack initiation.