The structural response of unreinforced masonry elements strengthened with hybrid elastomeric/fiber materials was investigated through material characterization and flexural experiments. Material characterization tests were performed on various unreinforced and reinforced elastomeric materials to identify those materials that were best suited for use as structural retrofits. After material characterization was completed, the three most promising material systems were selected for further investigation, including one unreinforced elastomer film and two reinforced elastomer films with fiber orientations at 0/90° and +/- 45° relative to the major axis of the masonry elements. A series of four-point bending tests were performed on the selected masonry and epoxy bonded elastomer/fiber hybrid retrofits to determine the structural response of the composite systems. The experimental load-deformation response was used, along with material characterization results, in the development of a semi-empirical model to predict the static moment capacity of the strengthened masonry system. This model will be used in the development of reliable design criteria for masonry walls strengthened with these advanced materials.

Oxide dispersion strengthened (ODS) alloys have been developed with unique mechanical properties. However, in order to achieve commercial application an appropriate joining process is necessary which minimizes disruption to the alloy microstructure. Transient liquid phase (TLP) bonding is a promising joining method, but previous work has shown that the segregation of dispersoids within the joint region results in bonds with poor mechanical strengths. This research work was undertaken to further explore particulate segregation at the joint region when TLP bonding and to develop bonding techniques to prevent it. A Ni-Cr-Fe-Si-B interlayer was used to bond an alloy MA 758. The effects of parent alloy grain size, bonding temperature, and external pressure on the TLP bonding process were investigated. Three melting stages were identified for the interlayer, and the bonding temperature was chosen so that the interlayer was in the semi-solid state during bonding. This novel bonding mechanism is described and applied to counteract the segregation of Y203 dispersoids. The grain size of the parent alloy does not alter the particulate segregation behaviour. It is concluded that a low bonding temperature with moderate pressure applied during bonding is preferable for producing bonds with less disruption to the microstructures of the parent alloy. Joint shear tests revealed that a near parent alloy strength can be achieved. This study also shed some light on choosing the right bonding parameters suitable for joining the complicated alloy systems. A Ni-P interlayer was also used to bond the ODS alloy. Microstructural examination indicated that a thin joint width and less disruption to the parent grain structure were achieved when bonding the alloy in the fine grain state. The time for isothermal solidification was found to be shorter when compared with bonds made with the parent alloy in the recrystallized state. All these observations were attributed to the greater diffusivity of P along the grain boundaries than that of the bulk material. A high Cr content within the parent alloy changes the mechanism of the bonding process. The diffusion of Cr into the liquid interlayer has the effect of raising the solidus temperature, which not only accelerates the isothermal solidification process, but also reduces the extent of parent alloy dissolution.

The constant gamma in the strengthened Cauchy-Bunyakowskii-Schwarz inequality is a basic tool for constructing of two-level and multilevel preconditioning matrices. Therefore many authors consider estimates or computations of this quantity. In this paper the bilinear form arising from 3D linear elasticity problems is considered on a polyhedron. The cosine of the abstract angle between multilevel finite element subspaces is computed by a spectral analysis of a general eigenvalue problem. Octasection and bisection approaches are used for refining the triangulations. Tetrahedron, pentahedron and hexahedron meshes are considered. The dependence of the constant $\gamma$ on the Poisson ratio is presented graphically.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第17914号 / エネ博第286号 / 新制||エネ||59(附属図書館) / 30734 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 小西 哲之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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Les alliages ODS (Oxide Dispersion Strengthened) sont des matériaux candidats pour la réalisation du gainage combustible des réacteurs nucléaires de Génération IV de type RNR-Na. Leurs propriétés mécaniques à haute température sont assurées par une dispersion d'oxydes nanométriques qui peut être modifiée sous l'effet de la température et notamment lors du passage par l'état liquide. Les procédés d'assemblage en phase solide sont donc préconisés. La méthode de soudage étudiée dans cette thèse est le soudage par résistance en bout. Une double approche simulation numérique-expérimentation a été mise en place pour étudier l'influence des paramètres opératoires sur le soudage et la soudure. L'ensemble des essais (expérimentaux et numériques) est réalisé selon la méthode des plans d'expériences factoriels complets. Un alliage ODS base fer à 20 % de chrome a été utilisé comme alliage d'essai.L'analyse de l'influence des paramètres opératoires sur le soudage est réalisée dans un premier temps. Les résultats montrent que l'étape de soudage peut se découper en trois phases. Dans une première phase, la température augmente au niveau des interfaces. Dans une seconde phase, le procédé de soudage est gouverné par le changement de section macroscopique lorsque le courant passe du bouchon massif à la gaine plus étroite. La température augmente alors dans la partie de gaine dépassant de l'électrode. Lorsque la température atteint une valeur suffisamment élevée, la partie de gaine dépassant de l'électrode se déforme, entrainant un affaissement général des pièces et constituant la troisième phase de l'étape de soudage. L'influence des paramètres opératoires sur les phénomènes physiques lors de l'opération d'assemblage est évaluée. Il est alors possible d'influer sur les sollicitations thermiques et mécaniques à l'aide des paramètres opératoires. L'analyse de l'influence des sollicitations thermomécaniques sur la soudure est réalisée dans un second temps. Sur les plages de paramètres opératoires étudiées, les résultats révèlent sur certaines soudures la présence de défauts de compacité ainsi qu'une modification de la microstructure et de la dispersion des oxydes. Les défauts de compacité peuvent être liés à des phénomènes thermiques et mécaniques au contact entre pièces. Les modifications de la microstructure sont liées à des phénomènes de recristallisation dynamique ou de fusion locale et donc à des phénomènes thermiques mais aussi mécaniques avec la présence de déformations importantes. Les modifications de la microstructure sont alors reliées à une modification de la dispersion d'oxydes. A l'aide des paramètres opératoires il est possible d'influer sur les températures et les déformations afin de limiter les modifications de la structure de l'alliage ODS ainsi que l'apparition de défaut de compacité. A l'aide de l'ensemble de ces résultats, la procédure de soudage est adaptée sur un alliage ODS à 9 % de chrome, nuance envisagée pour la réalisation des futurs gainages combustibles. L'influence des propriétés matériaux sur le soudage et la soudure est alors discutée en comparant les deux nuances ODS de compositions différentes mais aussi en comparant les résultats obtenus sur l'alliage ODS à 20 % de chrome avec un alliage non renforcé de composition semblable.

The diploma thesis is focused on determining mechanical properties of alloys produced by powder metallurgy methods. Two alloys were studied, namely a highly entropic CrMnFeCoNi alloy and an oxide dispersion-strenthened FeAlO alloy. Both alloys were made in laboratory amount of test material and miniature test specimens were used to measure their mechanical properties. The main goal of the diploma thesis was to develop a crack preparation methodology, to examine and to evaluate the properties of miniature test specimens for materials with low levels of resilience. In the analytical part of the thesis, tensile and deformation properties and fracture toughness of both alloys were measured. For the highly-entropic CrMnFeCoNi alloy, the thesis focused on the effect of the strain rate on the properties of the alloy. For the oxide dispersion-strenthened FeAlO, the effect of temperature in the 23-600 °C temperature range on its mechanical properties was studied.

The constant gamma in the strengthened Cauchy-Bunyakowskii-Schwarz inequality is a basic tool for constructing of two-level and multilevel preconditioning matrices. Therefore many authors consider estimates or computations of this quantity. In this paper the bilinear form arising from 3D linear elasticity problems is considered on a polyhedron. The cosine of the abstract angle between multilevel finite element subspaces is computed by a spectral analysis of a general eigenvalue problem. Octasection and bisection approaches are used for refining the triangulations. Tetrahedron, pentahedron and hexahedron meshes are considered. The dependence of the constant $\gamma$ on the Poisson ratio is presented graphically.

Through a series of carefully designed experiments, characterization and some modeling tools, this work is aimed at studying the role of thermal profiles on different microstructural zones and associated properties like strength and corrosion through a variation of weld parameters, thermal boundary conditions and material temper. Two different alloys belonging to the Al-Cu and Al-Cu-Li system in different temper conditions- peak aged (T8) and annealed (O) were used. A 3D-thermal pseudo mechanical (TPM) model is developed for the FSW process using heat transfer module in COMSOL Multiphysics and is based on a heat source wherein the temperature dependent yield shear stress is used for the heat generation. The precipitation and coarsening model is based on the Kampmann and Wagner theoretical framework and accounts for the competition between the various nucleation sites for both metastable and equilibrium precipitates. The model predicts different precipitate mean radius and volume fraction for the various zones in the friction stir welded material. A model for the yield strength is developed which considers contributions from different strengthening mechanisms. The predictions of the each models have been verified against experimental data and literature. At constant advance per rotation, the peak temperature decreases with a decrease in traverse speed and increases with an increase in tool rotation. Weld properties were significantly affected by choice of thermal boundary conditions in terms of backing plate diffusivity. Weld conditions with a higher peak temperature and high strain rate results in more dissolution of precipitates and fragmentation of constituent particles resulting in a better corrosion behavior for the weld nugget. For a peak aged temper of 2XXX alloys, the weld nugget experiences dissolution of strengthening precipitates resulting in a lower strength and the Heat affected zone (HAZ) experiences coarsening of precipitates. For an annealed material, both the weld nugget and HAZ experiences dissolution of precipitates with an increase in strength in the weld nugget.

Al₃Ti, Al₅CuTi₂, and Al₆₆Mn₁₁Ti₂₃, intermetallic matrices reinforced with 30, 40, and 50 volume % TiB, have been produced by XD_™ processing and densified into bulk form by powder metallurgy techniques. The compressive flow behavior of the Al₃Ti and Al₅CuTi₂, composites have been evaluated at temperatures ranging from 1000°C to 1200°C and strain rates of 0.0001 and 0.001 sec⁻¹. The results have been analyzed by a correlation to the resulting microstructure, especially with regards to the intermetallic matrix composition, TiB₂ size, and interparticle spacing. The results of these analyses are applied to a constitutive equation based upon the flow stress equation. It has been shown that at in this temperature regime, all deformation occurs within the intermetallic matrix. This is substantiated by the lack of TiB₂-dependent behavior with variations in volume percentage of reinforcement. In all cases, the composites are readily deformed at relatively low loads. The average strain-rate sensitivity values were 0.349 for the Al₃Ti-based composites, and 0.247 for the Al₅CuTi₂, variants. The average activation energies were calculated to be 485 kJ/mol and 920 kJ/mol for the Al₃Ti-and Al₅CuTi₂-matrices, respectively. The structure constant was calculated based upon these values. The Zener-Hollomon parameter was used to plot the flow stress along lines of constant temperature and strain rate, and serves as one type of constitutive equation. In addition, the flow stress was evaluated as a function of the strain rate and temperature. These models were found to provide adequate correlation to the measured flow behavior. / Master of Science