Presented is an overview of a semi-analytical solution which was developed to study the response of internally pressurized elliptical composite cylinders with clamped boundaries. Using a geometrically linear analysis and the solution scheme, the response of a quasi-isotropic elliptical cylinder is compared with the response of a quasi-isotropic circular cylinder in order to study the effects of elliptical geometry. The distinguishing features of the response of an elliptical cylinder are the inward normal displacement of the cross section at the ends of the major diameter that occur despite the outward force of the internal pressure, the presence of circumferential displacements, and the presence of inplane shear strains. These effects lead to spatial variations, including sign reversals, of a number of displacement, strain, and curvature responses. The responses of a quasi-isotropic elliptical cylinder evaluated using a geometrically linear analysis are then compared to the responses evaluated using a geometrically nonlinear analysis. It is shown that geometric nonlinearities tend to flatten certain responses at the ends of the minor diameter, and reduce the magnitude of certain responses in the boundary region. To study the influence of material orthotropy, the responses of axially-stiff and circumferentially-stiff elliptical cylinders evaluated using geometrically nonlinear analyses are examined. It is shown that in some instances material orthotropy can be used to mitigate the influence of the elliptical geometry and make particular responses look like those of a circular cylinder. An evaluation of failure using the maximum stress and Hashin failure criteria and geometrically linear and nonlinear analyses is presented for elliptical cylinders. These failure criteria involve interlaminar shear stresses which are computed by integrating the equilibrium equations of elasticity through the thickness of the cylinder wall. The failure criteria are used to assess the mode of failure (e.g., tensile or compressive fiber or matrix modes), the location of failure, and the pressure at failure. Both criteria predict first failure to occur at the clamped boundaries because of matrix cracking. The predicted failure pressures and circumferential locations are very similar for the two criteria, and the nonlinear analyses predict slightly higher pressures at somewhat different circumferential locations. First fiber failure is also considered. For this failure the two criteria predict similar failure scenarios for the linear analyses, but they differ in their predictions for the nonlinear analyses. Specifically, using the maximum stress criterion, the circumferentially-stiff elliptical cylinder is predicted to fail due to fiber compression, but the Hashin criterion predicts failure to be due to fiber tension, and at a different circumferential location. Also, first fiber failure pressures are at least a factor of two greater than the first matrix failure pressure. / Master of Science

All satellites need a propulsion system for orbit correction maneuvers. Electric Hall effect thrusters are an interesting technology for space applications. The big advantage compared to chemical propulsion devices is the higher specific impulse Isp, a higher ejection speed and thus a substantial gain in terms of propellant consumption. In a Hall effect thruster the ions are created and accelerated in a low pressure discharge plasma in a magnetic field. The first part of the work concerns scaling laws for Hall effect thrusters. A semi-empirical scaling model based on analytical laws and relying on simplifying assumptions is developed. This scaling model can be used to extrapolate existing thruster technologies in order to meet new mission requirements. In a second part, the influence of the channel width on the thruster performance level is investigated. It has been demonstrated that enlarging the channel width of a low power Hall effect thruster leads to an increase in thruster efficiency. Finally, electron properties are measured by means of electrostatic probes in the plume of different Hall effect thrusters. Experimental data on electron properties is of great interest for the validation of numerical plume models that are essential for the integration of the thruster on the satellite. Time-averaged and timeresolved measurements of the electron properties have been carried out for different operating conditions of the thruster. A fast-moving probe system has been developed in order to perform measurements of the electron properties close to the thruster exit plane.

Chaque satellite nécessite un système de propulsion pour des corrections d’orbite. Les propulseurs électriques à effet de Hall sont une technologie intéressante pour des applications spatiales. Le grand avantage par rapport à la propulsion chimique est une impulsion spécifique Isp plus élevée, une vitesse d’éjection plus élevée et donc un gain substantiel en termes de consommation de carburant. Dans un propulseur à effet Hall les ions sont créés et accélérés dans une décharge plasma à basse pression dans un champ magnétique. La première partie de ce travail concerne les lois d’échelle pour les propulseurs à effet Hall. Un modèle de dimensionnement semi-empirique basé sur des lois analytiques et reposant sur des hypothèses simplificatrices a été développé. Ce modèle de dimensionnement peut être utilisé pour une extrapolation des propulseurs existants afin de répondre aux exigences pour de nouvelles missions. Dans une deuxième étape, l’influence de la largeur du canal sur les performances d’un propulseur est étudiée. Il a été démontré qu’augmenter la largeur du canal conduit à une amélioration de l’efficacité du propulseur. Finalement, les propriétés électroniques ont été mesurées à l’aide de sondes électrostatiques dans la plume de différents propulseurs à effet Hall. Des données expérimentales concernant les propriétés électroniques sont très intéressantes pour la validation des modèles numériques de la plume indispensables pour l’intégration du propulseur sur le satellite. Des mesures moyennées et résolues en temps des propriétés électroniques ont été réalisées pour différents points de fonctionnement du propulseur. Un système de déplacement rapide pour les sondes a été développé afin de pouvoir faire des mesures des propriétés électroniques dans la zone proche du plan de sortie du propulseur. / All satellites need a propulsion system for orbit correction maneuvers. Electric Hall effect thrusters are an interesting technology for space applications. The big advantage compared to chemical propulsion devices is the higher specific impulse Isp, a higher ejection speed and thus a substantial gain in terms of propellant consumption. In a Hall effect thruster the ions are created and accelerated in a low pressure discharge plasma in a magnetic field. The first part of the work concerns scaling laws for Hall effect thrusters. A semi-empirical scaling model based on analytical laws and relying on simplifying assumptions is developed. This scaling model can be used to extrapolate existing thruster technologies in order to meet new mission requirements. In a second part, the influence of the channel width on the thruster performance level is investigated. It has been demonstrated that enlarging the channel width of a low power Hall effect thruster leads to an increase in thruster efficiency. Finally, electron properties are measured by means of electrostatic probes in the plume of different Hall effect thrusters. Experimental data on electron properties is of great interest for the validation of numerical plume models that are essential for the integration of the thruster on the satellite. Time-averaged and timeresolved measurements of the electron properties have been carried out for different operating conditions of the thruster. A fast-moving probe system has been developed in order to perform measurements of the electron properties close to the thruster exit plane.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.