Vertical drop testing of transport aircraft fuselage sections indicates that the frames play a major role in the process of absorbing the impact energy in the crushing of the substructure below the main passenger deck. Hence, static tests are performed on individual circumferential frames under a radially inward load to assess their progressive failure response and energy absorption characteristics. The test articles in the first series of tests are six-foot diameter, semicircular, I-section frames fabricated from graphite-epoxy unidirectional tape. The test articles in the second series of tests are J-section frames subtending a forty-eight degree circular arc, having an inside radius of 118 inches, a depth of 4.8 inches, and manufactured by resin transfer molding into a 2x2 2D triaxial braided composite preform made of AS4 graphite yarns. Frames of both materials exhibit fractures at the pint of load application and at selected locations around the circumference, but the delamination prevalent in the tape layup frames is not evident in the textile frames. A mathematical model developed to optimize open section curved composite frames for improved energy absorption is used to redesign the I-section frames by resizing the flanges. The test results of the redesigned frames show that the mathematical model predicted the correct sequence and locations of the failure events. However, the mathematical model does not predict the magnitude of the force and displacement at the first major failure event, which maybe due to the fact that delamination is not included in the progressive failure model Tests results from two of the J-section frames are compared with a beam finite element analysis using the computer code ABAQUS. Effective elastic moduli for the textile material are obtained from the computer code TEXCAD. The ABAQUS results correlate reasonably well with the experimental results prior to the first major failure event. / Master of Science

Textile composites can be tailored to meet specific thermo-mechanical requirements for structural applications. The focus of this research is on 2x2 biaxial braided composites since they have good stiffness and strength properties. Moreover, they have potentially better impact and fatigue resistance than laminated composites. Along with good properties, they have a reduced manufacturing cost because much of the fabrication can be automated. In order to exploit these benefits, thorough understanding of the effect of various factors on their material behavior is necessary. Obtaining effective mechanical properties is the first order of concern in any structural analysis. This work presents an investigation of the effect of various parameters like braid angle, waviness ratio, stacking sequence and material properties on the effective engineering properties of the 2x2 braids. To achieve this goal, three dimensional finite element micromechanics models were developed first. Extensive parametric studies were conducted for two material systems: 1). Glass (S2) fiber / epoxy (SC-15) matrix and 2). Carbon (AS4) fiber / Vinyl Ester (411-350) matrix. Equivalent laminated materials with angle plies and a resin layer were also analyzed to compare the difference in predictions from the full three dimensional finite element analysis of the 2x2 braided composites. A full three-dimensional stress state exists in braids even for very simple loading. In order to locate the potential damage spots, the stress distributions in both the matrix and the tows were predicted. The effect of braid angle on location and magnitude of peak stresses was determined.

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Du fait de leurs propriétés physiques et chimiques exceptionnelles à haute température par rapport aux métaux, les composites de carbure de silicium (SiC) sont étudiés comme éventuel matériau de gainage du combustible nucléaire dans les réacteurs de fusion ou fission avancée futurs, ainsi que, depuis plus récemment, dans les réacteurs à eau légère existants. Les tubes composites SiC/SiC tressés en 2D, fabriqués par procédé d'infiltration chimique en phase vapeur (CVI), présentent un comportement mécanique anisotrope, faiblement déformable (~ 1%). La maîtrise des relations entre la microstructure, l’endommagement et le comportement macroscopique est essentielle pour optimiser précisément le dimensionnement structurel de ce matériau pour les applications envisagées. Un paramètre de fabrication important est l'angle de tressage, angle entre les torons de fibres et l'axe du tube. L'objectif de ce travail est de fournir une compréhension détaillée de la relation endommagement-microstructure, en particulier des effets de l'angle de tressage sur les mécanismes d’endommagement. Dans ce but, une étude combinant observations expérimentales à macro et micro-échelle et simulations numériques est menée. Les tubes composites sont d’abord étudiés par des essais de traction in situ sous tomographie par rayons X. Les expériences ont été réalisées sur la ligne PSICHE du synchrotron SOLEIL sous faisceau rose polychromatique. Les images tridimensionnelles sont analysées par la technique de corrélation d’image volumique (DVC), complétée par une série d'algorithmes de traitement d'image originaux, développés spécifiquement pour analyser les microstructures 3D, mesurer les déformations à travers l'épaisseur du tube, détecter et caractériser quantitativement le réseau de microfissures créées par le chargement mécanique. De plus, les microstructures réelles, décrites par les images de haute résolution issues des tests in situ, sont utilisées dans les simulations numériques multi-échelle. Les champs de contrainte à l’échelle microstructurale sont calculés en régime élastique par une technique utilisant la transformée de Fourier rapide (FFT). Ils permettent de mieux comprendre l'initiation des fissures et d’interpréter les observations expérimentales par une comparaison directe. Ces approches expérimentales et numériques sont appliquées à trois tubes présentant différents angles de tressage (30 °, 45 ° et 60 °). L’influence de l'angle de tressage sur l'initiation et l'évolution de l’endommagement à cœur des composites est ainsi mise en évidence / Because of their outstanding physical and chemical properties at high temperature, in comparison with metals, silicon carbide (SiC) composite materials are studied as possible nuclear fuel cladding materials either for future advanced fission/fusion reactors, or more recently, for the currently existing light water reactors. 2D-braided SiC/SiC composite tubes, manufactured by chemical vapor infiltration (CVI), exhibit an anisotropic, hardly deformable (~1%) mechanical behavior. Understanding the relations between the microstructure, the damage mechanisms and the macroscopic behavior is essential to optimize the structural design of this material for the considered applications. One important manufacturing parameter is the braiding angle, i.e. the angle between the fiber tows and the tube axis. The objective of this work is to provide a comprehensive understanding of the damage-microstructure relations, in particular of the effects of the braiding angle on the damage mechanisms. For this purpose, an investigation combining experimental observations at macro and micro-scale and numerical simulations is developed. The composite tubes are first studied through in situ tensile testing under X-ray computed tomography. Experiments were carried out on the PSICHE beamline at synchrotron SOLEIL using a pink polychromatic beam. The recorded 3D images are processed using the digital volume correlation (DVC) technique, extended by a series of advanced image processing algorithms specifically developed in order to analyze the 3D microstructures, to measure the deformations through the tube thickness, and to detect and quantitatively characterize the network of micro-cracks created by the mechanical loading. In addition, numerical simulations are performed on the real microstructures as observed in the high-resolution images recorded during the in situ tests. Stress fields are calculated at the microstructural scale in the elastic regime using a numerical tool based on the Fast Fourier Transform (FFT). They help to better understand crack initiation and interpret the experimental observations within one-to-one comparisons. Both the experimental and numerical approaches are applied to three tubes with different braiding angles (30°, 45° and 60°). The effect of the braiding angle on the initiation and evolution of damage in the bulk of the composite materials can thus be highlighted

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