CoordenaÃÃo de AperfeÃoamento de Pessoal de NÃvel Superior / Materiais compÃsitos reforÃados por fibras tÃm sido cada vez mais empregados devido a suas altas relaÃÃes rigidez/peso e resistÃncia/peso, alÃm de outras vantagens, como alta resistÃncia à corrosÃo, bom isolamento tÃrmico, excelente amortecimento e resistÃncia à fadiga. Por isso, estes materiais tÃm sido aplicados na fabricaÃÃo de tubos para o transporte de fluidos em diversas indÃstrias, como por exemplo, a petroquÃmica. A exploraÃÃo e produÃÃo de petrÃleo e gÃs em Ãguas profundas tornam necessÃrio o uso de plataformas flutuantes conectadas ao poÃo por dutos (flowlines) e risers, que quando feitos de aÃo, se tornam menos atrativos devido ao seu elevado peso. Nessas condiÃÃes, requerem-se mecanismos capazes de suportar as altas tensÃes desenvolvidas no topo, sendo que tais mecanismos sà podem ser acomodados por plataformas maiores e mais caras. AlÃm disso, torna-se necessÃrio tambÃm o uso de mais flutuadores visando suportar o peso total do sistema, encarecendo tambÃm o projeto. Diante de tais fatos, tem-se estudado a viabilidade do emprego de risers de materiais compÃsitos na exploraÃÃo em Ãguas ultra-profundas. Devido ao grande nÃmero de parÃmetros envolvidos, tais como o nÃmero de camadas e o material, a espessura e o Ãngulo de orientaÃÃo das fibras de cada camada, o projeto de estruturas de compÃsitos laminados à mais complexo que o de aÃo. No caso dos risers, acrescentam-se ainda as vÃrias condiÃÃes de carregamento e ambientais que tornam o processo de projeto tradicional de tentativa e erro inadequado. Uma soluÃÃo apenas satisfatÃria nÃo explora, em geral, o potencial que os compÃsitos apresentam de se adaptarem Ãs solicitaÃÃes da forma mais eficiente possÃvel. Esse trabalho tem como objetivo desenvolver uma metodologia e ferramentas computacionais para o prÃ-dimensionamento de risers de material compÃsito via tÃcnicas de otimizaÃÃo. SÃo consideradas como vÃriaveis de projeto as espessuras, as orientaÃÃes das fibras e os materiais de cada lÃmina. Peso, custo e um fator de seguranÃa sÃo adotados como funÃÃo objetivo. RestriÃÃes de resistencia, estabilidade e fabricaÃÃo avaliadas. O riser à anÃlisado usando um modelo de catenÃria inextensÃvel. Um Algoritmo GenÃtico com operadores especÃficos para estruturas laminadas à empregado. / The depletion of oil and gas reserves has increasingly led to the search of deepwater fields. Most of recent oil and gas discoveries in Brazil occurred in deepwater fields. However, using steel risers for deepwater application is not always feasible, due to its high weight. In such enviroments, the traditional extraction and production risers made of steel tend to become expensive, due to increased top tension which overburdens the plataform. Fiber reinforced composite materials, being lighter, present interesting characteristics for offshore applications, such as high specific strength and stiffness, high corrosion resistance, good thermal insulation, high structural damping properties, and fatigue resistance. Thus, the application of composite risers is an interesting alternative to deepwater oil fields. The design of laminated composite risers is very difficult since the strength and stiffness of these components depend on the number of layers and the material, thickness, and orientation of each layer. Thus, the use of the conventional trial-and-error strategy is not adequate and it is necessary to apply optimization techniques. In this work, optimization techniques are applied to optimize composite catenary risers. the design variables are the thickness, the orientation and material of each layer. A multi-objective formulation is adopted to minimize the weight, cost and maximize the buckling safety factor of the composite riser. The optimization model includes strength and stability constraints and considers multiple load cases. The global analysis of the riser is carried out using the catenary equations and the stress computation in the critical locations is performed using the Classical Lamination Theory (CLT) and the theory of thin-walled tubes. It is important to note that, due to manufacture constraints, the design variables can only assume discrete values. Therefore, a genetic algorithm is used for optimization since it can easily handle discrete variables. In addition to classical genetic operators, as crossover and mutation, this algorithm also includes operators specially designed to handle laminate structures, such as layer swap and layer deletion. The proposed formulation is applied in the design optimization of composite catenary risers with different water depths and top angles. Numerical examples show that the proposed methodology is very robust.

The employment of composite materials in the aerospace industry has been gradually considered due to the fundamental lightweight and strength characteristics that these type of materials offer. The science material and technological progress that has been reached, matches perfectly with the requirements for high-performance materials in aircraft and aerospace structures, thus, the development of primary structure elements applying composite materials became something very convenient. It is extremely important to pay attention to the failure modes that influence composite materials performances, since, these failures lead to a loss of stiffness and strength of the laminate. Delamination is a failure mode present in most of the damaged structures and can be ruinous, considering that, the evolution of interlaminar defects can carry the structure to a total failure followed by its collapse. Different techniques are usually adopted to accurately predict the behavior of damaged structures but, due to the complex nature of failure phenomena, there is not an established pattern. The present research project aims to develop a delamination propagation model to estimate a progressive interlaminar delamination failure in laminated composite materials and to allow the prediction of material\'s degradation due to the delamination phenomenon. Experimental tests assisted by ASTM Standards were performed to determine material\'s parameter, like the strain energy release rate, using GFRPs laminated composites. The delamination propagation model proposed was implemented as subroutines in FORTRAN language (UMAT-User Material Subroutine) with formulations based on the Fracture Mechanics. Finally, the model was compiled beside with the commercial Finite Element program ABAQUSTM. / O emprego de materiais compósitos na indústria aeroespacial tem sido gradualmente utilizado devido às suas características fundamentais, como peso leve e alta rigidez, que este tipo de material oferece. Tanto a ciência do material como o desenvolvimento tecnológico que se tem logrado, possibilitaram que estes materiais cumprissem com os requisitos de desempenho para aplicações em estruturas aeronáuticas e aeroespaciais, por tanto, o desenvolvimento de elementos de estruturas primárias usando materiais compósitos, passou a ser muito conveniente. É de extrema importância prestar atenção aos modos de falha que comprometem a performance dos materiais compósitos, uma vez que, estas falhas levam a uma perda de resistência e rigidez do laminado. A delaminação é um modo de falha presente na maioria de estruturas danificadas e pode ser desastroso, considerando que, a evolução dos defeitos interlaminares podem levar a estrutura a falhar seguido pelo colapso estructural. Diferentes técnicas são geralmente adotadas para prever, de maneira correta, o comportamento de estruturas danificadas, porém, devido à natureza complexa do fenômeno de falha, não existe um padrão estabelecido. O presente trabalho de pesquisa visa desenvolver um modelo de delaminação e de propagação da delaminação para estimar a evolução da falha interlaminar em materiais compósitos laminados e permitir a predição do comportamento do material com a evolução da delaminação. Ensaios experimentais auxiliados por normas ASTM foram realizados para determinar parâmetros do material, tais como, as taxas de liberação de energia de deformação, usando materiais compósitos laminados de matriz polimérica reforçada com fibra de vidro. O modelo de propagação da delaminação proposto, foi implementado como uma sub-rotina em linguagem FORTRAN (UMAT – User Material) com formulações baseadas na Mecânica da Fratura. Finalmente, o modelo foi compilado com o software comercial de Elementos Finitos, ABAQUSTM.

Carbon nanotube (CNT)/polymer composite sheets can be extremely high strength and lightweight, which makes them attractive for fabrication of mechanical structures. This thesis demonstrates a method whereby smooth, thin CNT/polymer composite sheets can be fabricated and patterned on the microscale using a process of photolithography and plasma etching. CNT/polymer composites were made from CNTs grown using chemical vapor deposition using supported catalyst growth and floating catalyst growth. The composite sheets had a roughness of approximately 30nm and were about 61¼m or 261¼m depending on whether they were made from supported catalyst grown or floating catalyst grown CNTs. The composites were patterned using an oxygen plasma as the etchant and a hard mask of silicon nitride.

This research project primarily focuses on three major aspects: synthesis and inclusion of silver microparticles and nanowires within a polymer matrix, extrusion of composite filaments and, three-dimensional (3D) printing of multifunctional polymer composites. Since very few studies have explored the inclusion of silver nanoparticles in 3D printing materials, the findings from this study can be significant for additive manufacturing technology. Over the past few decades, the applications of additive manufacturing has been expanding considerably in several industries like automobile, biomechanics, aerospace, hardware engineering, to name a few. We are particularly interested in silver particles and nanowires because of their enhanced antimicrobial, mechanical and optical properties. The unique antimicrobial properties of the silver-polymer composite will especially be applicable in the food and meat industry, where microbial infection is a major concern because of the exposure of microbes in the polymer belts that are used to transfer and package the items in the factory. It costs the industries a considerable amount of time, money and labor to regularly clean and sanitize those belts. If we are able to develop polymer belts with embedded antimicrobial properties, it could have tremendous applications in the food and meat industries. The morphology of the particles will be studied using scientific techniques like Transmission electron microscopy (TEM) and Scanning Electron Microscopy (SEM). The idea is then to nanoparticles will be incorporated into PLA polymer pellets and extruded into composite filaments that can be used for 3D printing of dog-bone test structures. After the fabrication process, tensile tests and fracture surface analysis will be conducted to study the extent of enhancement of the mechanical properties as compared to neat polymer 3D printed specimens. The critical challenge in this project would be to ensure homogenous distribution of the nanoparticles throughout the polymer filaments. This project will integrate concepts and applications from three different fields: nanotechnology, material science, and additive manufacturing.

Les structures aéronautiques sont aujourd’hui largement réalisées à partir de matériaux composites, permettant ainsi d’en diminuer la masse. L’impact sur structure composite est une des sources de dommage la plus courante et la plus pénalisante. L’objectif de cette thèse est de développer un modèle prédictif d’impact sur stratifiés unidirectionnels de faible épaisseur afin, dans un premier temps, de mieux comprendre les mécanismes d’endommagement mis en jeu et, dans un deuxième temps, de pouvoir proposer des pistes d’amélioration de la tenue de ces structures. La première étape de ces travaux a consisté à développer un modèle éléments finis explicite de stratifiés composite unidirectionnel, en s’appuyant sur les travaux d’impact sur pale d’hélicoptère réalisés par F. Pascal. Ainsi, un nouvel élément capable de représenter le comportement à l’impact des plis unidirectionnels a été formulé. Le comportement des paquets de fibres est représenté par des éléments barres stabilisés grâce à des éléments 2D spécifiques. La dégradation de ces éléments est pilotée par des lois d’endommagement déduites d’observations expérimentales. La stratégie de modélisation proposée a été identifiée sur la base d’essais expérimentaux pour deux matériaux : T700/M21 et HTA7/913. Les résultats donnés par le modèle ont ensuite été validés par une comparaison avec des essais d’impact à basse vitesse. Enfin, dans le but d’optimiser la tenue des stratifiés, des impacts à basse et moyenne vitesse sur des stratifiés hybrides unidirectionnels/tissus composites ont été étudiés expérimentalement et numériquement grâce à la stratégie de modélisation proposée. / Today, composite materials are largely used in the manufacturing of aeronautical structures, in order to lighten their weight. Impact on composite structures is one of the most detrimental loading. The present study aims to develop a predictive modelling of impacts on thin unidirectional laminates in order to, firstly, better understand the damage mechanisms involved, and, secondly, be able to provide guidance for a strength improvement of these structures. First, an explicit finite element modelling of unidirectional laminates is developed based on the work of F Pascal dealing with impact modeling on helicopter blades. Thus, a new element that can represent the impact response of unidirectional plies is formulated. The behavior of the bundle of fibers is modeled with rod elements stabilized with specific 2D elements. The degradation is managed using damage based on experimental observations. The parameters of the proposed modelling strategy are identified on the basis of experimental tests carried out on T700/M21 and HTA7/913 composites. The results provided by the modeling are then validated by a comparison with experimental low velocity impact results. Finally, in order to improve the laminate strength, low velocity and medium velocity impacts on hybrid unidirectional/woven composite laminates are studied experimentally and with the developed modelling strategy.

Le travail présenté dans cette thèse s’inscrit dans le contexte du développement de nouveaux procédés pour la mise en forme de matériaux composites pour répondre aux contraintes industrielles de gain de temps et d’énergie. Le procédé de chauffage par micro-onde reposant sur un principe de chauffage volumique permet de garantir ces avantages. L’inconvénient majeur est que le comportement du champ électrique lors de l’interaction entre matériaux composites et micro-ondes est peu connu. L’objectif de cette thèse est donc de proposer des solutions numériques pour approfondir la connaissance du procédé et la physique associée afin de mettre en avant ses capacités quant à un développement industriel. Pour ce faire, le travail réalisé est axé autour de trois verrous. En premier lieu, une nouvelle méthode numérique est proposée afin de résoudre les équations de Maxwell en 3D dans un stratifié composite. Prendre en compte le comportement 3D est essentiel afin de décrire avec précision l’impact des différentes couches du matériau et de leur épaisseur sur la propagation du champ électrique. Dans un second temps, la simulation électromagnétique est couplée à une résolution thermique afin de simuler le procédé de chauffage d’une pièce composite. Différents paramètres du procédé sont étudiés afin de mettre en avant les plus sensibles pour l’utilisation du chauffage micro-onde pour les matériaux composites. Enfin, l’attention du lecteur est portée sur le développement d’une méthode de simulation pour un contrôle en temps réel du procédé en comparaison avec des essais expérimentaux. / The context of the present work is the development of new processes for the heating and forming of composite materials in order to provide an answer to the industrials needs for less energy and less time-consuming processes. In that sense, microwave heating is perfect match as it relies on volumetric heating. The major drawback is that the behaviour of the electric field while interacting with composite material is poorly known. Therefore, the main objective of this thesis is to provide numerical solutions to go more deeply in the understanding of such process and put forward its capabilities for an industrial development.To fulfil this objective, the work is oriented over three main axes. First, an innovative simulation tool is presented in order to solve the Maxwell’s equations in a thin multi layered domain. Taking into account the 3D behaviour of the electric field is a major issue in order to describe precisely the impact of the different plies of the laminate on the propagation of the electric field.Then, the electromagnetic simulation is coupled with a thermal simulation in order to simulate the full heating process of a composite part. Parameters of the process are investigated to bring forward the most crucialones. Finally, real-time control of the process is tackled using a model order reduction simulation technique. These results are compared to experimental work on two sets of samples.

A primary motivation for this research comes from the need to improve the ability of polymer-matrix composites to withstand lightning strikes. In particular, we are concerned with lightning strike damage in composite wind turbine blades. The direct effects of lightning strike on polymer-matrix composites often include rapid temperature rise, melting or burning at the lightning attachment points, and mechanical damage due to lightning-induced magnetic force and acoustic shock wave. The lightning strike damage accumulation problem is essentially multiphysic. The lightning plasma channel discharges an electric current up to 200 kA, inducing a severe heat flux at the surface of the composite structure, as well as generating Joule heating through the composite structure. The resulting electro-thermo-mechanical response of the composite structure may include matrix degradation and decomposition, delamination, and fiber breakage and sublimation, thus leading to catastrophic failure. The existing studies related to the lightning strike damage in composites ignored the lightning channel radius expansion during the initial lightning discharge and lacked adequate treatment of material phase transitions. These assumptions significantly simplify the mathematical treatment of the problem and affect the predictive capabilities of the models. Another common feature of these limited studies is that they all focused on carbon-fiber-reinforced polymer-matrix (CFRP) composites, which are electrically conductive. In the present thesis, the thermal responses and thermal ablations in a non-conductive glass-fiber-reinforced polymer-matrix (GFRP) composite wind turbine blade and in a conductive CFRP composite wind turbine blade are studied, respectively. In the case of non-conductive GFRP composite wind turbine blade, prior to the thermal response and thermal ablation analysis, a finite element analysis is performed to calculate the electric field due to lightning stepped leader to estimate the dielectric breakdown of the non-conductive composite wind turbine blade. The estimation of dielectric breakdown is used to determine whether Joule heating needs to be included in the problem formulation. To predict the thermal response and thermal ablation in the composite structure due to lightning strike, a physics-based model describing surface interaction between the lightning channel and the composite structure has been developed. The model consists of: (i) spatial and temporal evolution of the lightning channel as a function of the electric current waveform; (ii) temporary and spatially non-uniform heat flux and current density (in the case of electrically conductive CFRP composite or if dielectric breakdown occurs in the case of non-conductive GFRP composite) generated at the composite structure; and (iii) nonlinear transient heat transfer problem formulation for layered anisotropic composites that includes the moving boundary of the expanding lightning channel and the phase transition moving boundary associated with instantaneous material removal due to sublimation. The model has been employed to investigate the thermal responses and thermal ablations in a GFRP composite laminated panel used in a Sandia 100-meter all-glass baseline wind turbine blade (SNL 100-00) and a typical CFRP composite laminated panel subjected to lightning strike. The temperature-dependent directional material properties for both the GFRP and CFRP composites have been determined in this thesis using a micromechanics approach based on the experimental data for fibers and resin. An integrated Matlab-ABAQUS numerical procedure features the aforementioned aspects (i), (ii), and (iii) of the developed model. The obtained results include the evolution of temperature fields in the composite laminated panel and the progressive shape change of the composite laminated panel due to thermal ablation. The predictions of thermal ablation in the CFRP composite laminated panel are validated by reported experimental results.

Abstract not available

Flush repairs to military aircraft are expected to become more prevalent as more thick skin composites are used, particularly on the surface of the fuselage, wings and other external surfaces. The use of these repairs, whilst difficult to manufacture provide an aerodynamic, ???stealthy??? finish that is also more structurally efficient than overlap repairs. This research was undertaken to improve the design methodology of scarf repairs with reduced material removal and to investigate the damage tolerance of scarf repair to low velocity impact damage. Scarf repairs involve shallow bevel angles to ensure the shear stress in the adhesive does not exceed allowable strength. This is important when repairing structures that need to withstand hot and humid conditions, when the adhesive properties degrade. Therefore, considerable amounts of parent material must be machined away prior to repair. The tips of the repair patch and the parent laminate are very sharp, thus a scarf repair is susceptible to accidental damage. The original contributions include: ??? Developed analytic means of predicting the stresses within optimised scarf joints with dissimilar materials. New equations were developed and solved using numerical algorithms. ??? Verified using finite element modelling that a scarfed insert with dissimilar modulus subjected to uniaxial loading attracted the same amount of load as an insert without a scarf. As such, the simple analytic formula used to predict load attraction/diversion through a plate with an insert may be used to predict the load attraction/diversion into a scarf repair that contains a dissimilar adherend patch. ??? Developed a more efficient flush joint with a doubler insert placed near the mid line of the parent structure material. This joint configuration has a lower load eccentricity than external doubler joint. ??? Investigated the damage tolerance of scarf joints, with and without the external doubler. The results showed that scarf joints without external doublers exhibited a considerable strength reduction following low velocity impact. Based on the observations, the major damage mechanics in the scarf joint region following impact have been identified. These results demonstrated that it is important to incorporate damage tolerance in the design of scarf repairs.

Analysis of calcium and oxygen atom arrays of known cement minerals and the structures of cellulose polymorphs were performed to see if it was possible to arrange a cellulose fibre on a cement mineral face such that the fibre is bonded by a repeating array of hydrogen or hydroxide coordination bonds for the full length of the attachment. Of the sixteen important cement minerals modelled, xonotlite, foshagite, tricalcium aluminate hydrate, chondronite and rosenhahnite could form such bonds to modified cellulose fibre. However, this was not the case for other cement minerals, especially tobermorite. Alumium hydroxide, when added to cement-quartz pastes and autoclaved at 180 degrees C, was found to improve the cement's ability to resist carbonation. / Doctor of Philosophy (PhD)