Fibre Reinforced Polymer Composites are extensively used in aircraft structures because of its high specific stiffness, high specific strength and tailorability. Though Fibre Reinforced Polymers offer many advantages, they are not free from problems. The damage of different nature, e.g., service mechanical damages, fatigue damage or environmental damage can be observed during operating conditions. Among all the damages, manufacturing or service induced, delamination related damage is the most important failure mechanisms of aircraft-composite structures and can be detrimental for safety. Delamination growth under fatigue loading may take place due to local buckling, growth from free edges and notches such as holes, growth from ply-drops and impact damaged composites containing considerable delamination. Delamination growth can also occur due to interlaminar stresses, which can arise in complex structures due to unanticipated loading. The complex nature of composite failure, involving different failure modes and their interactions, makes it necessary to characterize/identify the relevant parameters for fatigue damage resistance, accumulation and life prediction. An effort has been made in this thesis to understand the fatigue behavior of carbon fibre reinforced epoxy laminates under aircraft wing service loading conditions. The study was made on laminates with different lay-up sequences (quasi-isotropic and fibre dominated) and different geometries (plain specimen, specimen with a hole and ply-drop specimen). The fatigue behaviour of the composite was analyzed by following methods: . Ultrasonic C-Scan was used to characterize the delamination growth. . Dynamic Mechanical Analysis (DMA) was done to study the interfacial degradation due to fatigue loading. In this analysis, the interfacial strength indicator and interfacial damping were calculated. The DMA also provides the storage modulus degradation under fatigue loading. . Scanning electron microscope examination was carried out to understand the fatigue damage mechanisms. . A semi-empirical phenomenological model was also used to estimate the residual fatigue life. This research work reveals that the Carbon Fibre Reinforced Polymer laminates are in the safe limit under service loading conditions, except the specimen with a hole. The specimen with a hole showed delaminations around the hole due to stress concentration and higher interlaminar stresses at the hole edges and this delamination is found to be associated with fibre breakage and fibre pullout. The quasi-isotropic laminate is found to show poorer fatigue behaviour when compared to fibre dominated laminate and ply-drop also shows poor performance due to high stress concentration in the ply-drop region.

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre debonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance for application of such materials. This PhD project is aimed at developing numerical models to analyze the effect of material randomness on delamination damage in CFRP materials by the implementation of the cohesive-zone model (CZM) within the framework of the finite-element (FE) method. Both the unidirectional and cross-ply laminates subjected to quasi-static loading conditions were studied. The initiation and propagation in delamination of unidirectional CFRP laminates were analyzed. The CZM was used to simulate the progress of that failure mechanism in a pre-cracked double-cantilever beam (DCB) specimen loaded under mode-I employing initially, a two-dimensional FE model. Model validation was then carried out comparing the numerical results with experimental data. The inherent microstructural stochasticity of CFRP laminates was accounted for in the simulations, and various statistical realizations for a half-scatter of 50% of fracture energy were performed, based on the approximation of that parameter with the Weibull s two-parameter probability density function. More detailed analyses were undertaken employing three-dimensional DCB models, and a number of statistical realizations based on variation of fracture energy were presented. In contrast to the results of two-dimensional analyses, simulations with 3D models demonstrated a lower load-bearing capacity for most of the random models as compared to the deterministic model with uniform material properties. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. The effect of material randomness on delamination in CFRP cross-ply laminates was also investigated. Initially, two-dimensional finite-element analyses were carried out to study the effect of microstructural randomness in a cross-ply laminate under bending with the direct introduction of matrix cracks with varying spacings and delamination zones. A considerable variation in the stiffness for cases with different crack spacings suggested that the assumption of averaged distributions of defects can lead to unreliable predictions of structural response. Three-dimensional uniform, deterministic cross-ply laminate models subjected to a tensile load were analyzed to study the delamination initiation and propagation from the tips of a pre-existing matrix crack. The material s stochasticity was then introduced, and a number of random statistical realizations were analyzed. It was observed that by neglecting the inherent material randomness of CFRP laminates, the initiation conditions for delamination as well as the character of its propagation cannot be properly detected and studied. For instance, the delamination crack length value for all the simulated random statistical realizations predicted its higher magnitudes compared to the uniform (deterministic) case for the same value of applied strain. Furthermore, the location of delamination initiation was shown to be different for different random statistical realizations. Another aspect, emphasizing the importance of microstructural randomness, was the scatter in the magnitudes of global strain at the instance of initiation and subsequent propagation of delamination. In summary, the material randomness in CFRPs can induce randomness in localised damage and it can affect the global properties of laminates and critical failure parameters. These effects can be investigated computationally through the use of stochastic cohesive-zone elements.

Chování laminátových desek namáhaných na tlak či na smyk může být výrazně ovlivněno přítomností delaminací, tedy oblastí, kde je porušena vazba mezi sousedními vrstvami. Cílem této práce je rozšířit znalosti o chování delaminovaných desek, a to především o chování desek s větším počtem delaminací a desek s delaminacemi libovolného tvaru, neboť taková podoba porušení laminátu více odpovídá poškození vznikajícího v důsledku nízkorychlostního dopadu cizího tělesa na laminátovou desku. Disertační práce se skládá ze tří hlavních částí. V první části jsou stručně nastíněny postupy využívané při analýze boulení delaminovaných desek a jsou diskutována omezení těchto analýz. Dále jsou v této části shrnuty hlavní poznatky o boulení delaminovaných desek. V druhé části práce je popsán výpočtový model použitý v rámci disertační práce pro analýzu boulení delaminovaných desek. Schopnost modelu předpovědět chování delaminovaných desek je pak dokumentována na několika ověřovacích úlohách. Třetí část disertační práce se skládá ze tří samostatných studií chování desek s několika delaminacemi eliptického či kruhového tvaru a jedné studie zabývající se možností náhrady obecného tvaru delaminace kruhem či elipsou. Je probírán vliv řady parametrů na chování delaminovaných desek, konkrétně vliv orientace vrstev laminátu a dále vliv počtu, tvaru, orientace a umístění delaminací. Na základě těchto studií jsou pak zformulována doporučení ohledně postupu při posuzování únosnosti delaminovaných konstrukcí.

Cette thèse résulte d’une collaboration entre le Pr Ayadi de l’Institut National Polytechnique de Lorraine et le Pr Varna de l’Université Technologique de Luleå. Depuis 2005, le Professeur Varna a développé une méthode originale qui permet d’obtenir l’ensemble des constantes thermoélastiques d’un stratifié endommagé à partir de deux paramètres : l’ouverture moyenne et le glissement moyen des lèvres des fissures, normalisés par rapport à la contrainte appliquée. Dans cette thèse, nous analysons le potentiel de l’interférométrie de speckle pour obtenir la mesure de ces deux grandeurs. Plus généralement, nous montrons l’utilité de la mesure du champ de déplacement pour caractériser le développement de l’endommagement dans les stratifiés. Le premier chapitre décrit l’endommagement qui peut affecter les matériaux composites stratifiés. L’objectif est de mettre en évidence les points particuliers pour lesquels la mesure du champ de déplacement pourrait apporter des informations originales. Dans le deuxième chapitre, le principe de l’interférométrie de speckle est exposé. Nous avons particulièrement insisté sur les raisons qui nous on conduit à choisir cette technique pour réaliser cette étude. Dans le troisième chapitre, nous présentons une étude expérimentale du potentiel de l’interférométrie de speckle pour caractériser les différentes formes d’endommagement qui peuvent apparaître dans un stratifié verre/époxyde . Enfin, dans le dernier chapitre, nous analysons expérimentalement le développement de l’endommagement dans un stratifié carbone/époxyde / This work results from a collaboration between Pr Ayadi (Institut Polytechnique de Lorraine) and Pr Varna (Technical University of Luleå). Since 2005, Pr Varna has developed an original method that allows for the determination of all the thermoelastic constants of a damaged laminate. The stiffness reduction is governed by two parameters: the average value of the opening and the average value of the sliding of the crack surfaces. These values are normalized with respect to the applied loading. In this work, the potential of speckle interferometry is analysed to obtain these two parameters. More generally, we show the usefulness of displacement field measurements to characterize damage development in laminates. The first chapter is dedicated to the damage that occurs in laminates. The main objective is to highlight the points for which displacement field measurements could bring interesting information. In the second chapter, the principle of speckle interferometry is described. The choice of this technique for the considered application is discussed and justified. In the third chapter, an experimental study of the potential of speckle interferometry is proposed to characterize the damage forms that occur in a glass/epoxy laminate. In the last chapter, the development of damage is experimentally analysed in a carbon/epoxy laminate

Durante os últimos anos tem-se observado um aumento crescente no uso de materiais compósitos. Isto se deve principalmente devido às vantagens que os mesmos apresentam em relação aos materiais convencionais tais como: alta resistência e rigidez associado à elevado desempenho estrutural, baixo peso e boa resistência à corrosão e oxidação, dentre outros. Contudo, o uso de materiais compósitos em projetos estruturais exige um bom conhecimento de suas propriedades elásticas e ferramentas adequadas para análise de falhas destes materiais. Algumas ferramentas comerciais já foram desenvolvidas, entretanto estas ferramentas apresentam arquitetura fechada que, na maioria das vezes, não permitem implementações específicas para as necessidades de cada usuário. Este trabalho desenvolve um sistema computacional para cálculo de propriedades elásticas e estadas de tensão - deformação nas diferentes camadas de laminados poliméricos, utilizando plataforma JAVA em código aberto e de baixo custo. Após a entrada dos dados de materiais e seqüência das lâminas, o sistema calcula as propriedades elásticas bem como o estado de tensões e deformações nas diferentes camadas, propiciando assim a aplicação de diferentes critérios de falhas. O sistema foi validado em estudos de caso da literatura com resultados satisfatórios. / During the past years a growing increase in the use of composite materials has been observed. This is due mainly to the advantages they present in relation to conventional materials such as: high strength and stiffness associate with low weight, good resistance to corrosion and oxidation and others. However, the use of composites in structural applications demands a good knowledge of their elastic properties as well as computational tools for failure analysis of these materials. Some commercial tools have been already developed, however these packages do not allow specific users implementations. This work develops a computational system for calculation of elastic properties and stress analysis along the different layers of a polymeric laminate, using JAVA platform in open code with low cost. After the material data input and laminate sequence, the system calculates the elastic properties as well as the stress state in the different layers, thus allowing the application of different failure criteria.

Laminados compósitos com matrizes poliméricas, respectivamente termorrígida e termoplástica, reforçadas com fibras contínuas de carbono foram submetidos a impacto único transversal com diferentes níveis de energia. Os danos imprimidos aos materiais estruturais foram avaliados por termografia ativa infravermelha na modalidade transmissão. Em geral, os termogramas do laminado termoplástico apresentaram indicações mais claras e bem definidas dos danos causados por impacto, se comparados aos do compósito termorrígido. O aquecimento convectivo das amostras por fluxo controlado de ar se mostrou mais eficaz que o realizado por irradiação, empregando-se lâmpadas incadecentes. Observou-se também que tempos mais longos de aquecimento favoreceram a visualização dos danos. O posicionamento da face impactada do espécime, relativamente à câmera infravermelha e à fonte de calor, não afetou a qualidade dos termogramas no caso do laminado termorrígido, enquanto que influenciou significativamente os termogramas do compósito termoplástico. Os resultados permitiram concluir que a termografia infravermelha é um método de ensaio não-destrutivo simples, robusto e confiável para a detecção de danos por impacto inferior à 5 Joules em laminados compósitos poliméricos reforçados com fibras de carbono. / Continuous carbon fiber-reinforced thermosetting and thermoplastic composite laminates were exposed to single transversal impact with different energy levels. The damages marked to the structural materials were evaluated by active infrared thermography in transmission mode. In general, the thermoplastic laminate thermograms showed more clear and delineated damage indications when compared to the ones from thermosetting composite. The convective heating of the samples by controlled hot air flow was more efficient than via irradiation using lamp. It was also observed that longer heating times improved the damage visualization. The positioning of the specimen´s impacted face regarding the infrared camera and the heating source did not affect the thermo-imaging of thermosetting specimens, whereas it substantially influenced the thermograms of thermoplastic laminates. The results allow concluding that infrared thermography is a simple, robust and trustworthy methodology for detecting impact damages as light as 5 Joules in carbon fiber composite laminates.

Durante os últimos anos tem-se observado um aumento crescente no uso de materiais compósitos. Isto se deve principalmente devido às vantagens que os mesmos apresentam em relação aos materiais convencionais tais como: alta resistência e rigidez associado à elevado desempenho estrutural, baixo peso e boa resistência à corrosão e oxidação, dentre outros. Contudo, o uso de materiais compósitos em projetos estruturais exige um bom conhecimento de suas propriedades elásticas e ferramentas adequadas para análise de falhas destes materiais. Algumas ferramentas comerciais já foram desenvolvidas, entretanto estas ferramentas apresentam arquitetura fechada que, na maioria das vezes, não permitem implementações específicas para as necessidades de cada usuário. Este trabalho desenvolve um sistema computacional para cálculo de propriedades elásticas e estadas de tensão - deformação nas diferentes camadas de laminados poliméricos, utilizando plataforma JAVA em código aberto e de baixo custo. Após a entrada dos dados de materiais e seqüência das lâminas, o sistema calcula as propriedades elásticas bem como o estado de tensões e deformações nas diferentes camadas, propiciando assim a aplicação de diferentes critérios de falhas. O sistema foi validado em estudos de caso da literatura com resultados satisfatórios. / During the past years a growing increase in the use of composite materials has been observed. This is due mainly to the advantages they present in relation to conventional materials such as: high strength and stiffness associate with low weight, good resistance to corrosion and oxidation and others. However, the use of composites in structural applications demands a good knowledge of their elastic properties as well as computational tools for failure analysis of these materials. Some commercial tools have been already developed, however these packages do not allow specific users implementations. This work develops a computational system for calculation of elastic properties and stress analysis along the different layers of a polymeric laminate, using JAVA platform in open code with low cost. After the material data input and laminate sequence, the system calculates the elastic properties as well as the stress state in the different layers, thus allowing the application of different failure criteria.

"In this work we consider an optimal design problem formulated on a two dimensional domain filled with two isotropic dielectric materials. The objective is to find a design that supports an electric field which is as close as possible to a target field, under a constraint on the amount of the better dielectric. In the case of a zero target field, the practical purpose of this problem is to avoid the so called dielectric breakdown of the material caused due to a relatively large electric field. In general, material layout problems of this type fail to have an optimal configuration of the two materials. Instead one must study the behavior of minimizing sequences of configurations. From a practical perspective, optimal or nearly optimal configurations of the two materials are of special interest since they provide the information needed for the manufacturing of optimal designs. Therefore in this work, we develop theoretical and numerical means to support a tractable method for the numerical computation of minimizing sequences of configurations and illustrate our approach through numerical examples. The same method applies if we were to replace the electric field by electric flux, in our objective functional. Similar optimization design problems can be formulated in the mathematically identical contexts of electrostatics and heat conduction."

Made available in DSpace on 2017-07-21T20:42:40Z (GMT). No. of bitstreams: 1 JULIANO Swiech Ciesielski.pdf: 2201955 bytes, checksum: acf9e0e88f80e1076d0bd050b2d0f0a7 (MD5) Previous issue date: 2012-07-31 / Many of the compounds multilayers are manufactured from advanced ceramic compositions, which seeks higher mechanical performance, the manufacture of these compositions based materials, ceramic structural has been little studied. Aiming to increase the fracture energy in ceramics traditional was investigated in this study the mechanical behavior of a multilayer system formed by stacking mullite and monoporosa. The mullite is an important raw material used in the manufacture of refractory ceramic for structural, because the physical and mechanical properties at high temperatures as high melting point, low expansion, fracture toughness and thermal shock that, high resistance fluency and low dielectric constant. The monoporosa has a high resistance which gives excellent mechanical and chemical properties and is widely used as coating mass on floors and re-investments outer. The increase in fracture energy was determined by comparing the curves of three point bending of the multilayer system with its monolithic references. The fractures were characterized by optical microscopy and MEV for microstructure analysis was carried out X-ray diffraction. Samples developed were evaluated multilayer reference to the materials employed in the formation of each layer in relation to these reference strains were obtained averages to 2.58 times larger and fracture energies of up to 7 times larger. / Muitos dos compostos multicamadas são fabricados a partir de composições de cerâmica avançada, em que se busca elevado desempenho mecânico, a fabricação destes materiais a base de composições de cerâmicas estruturais tem sido muito pouco estudado. Com o objetivo de aumentar a energia de fratura em cerâmicas tradicionais, foi pesquisado nesse trabalho o comportamento mecânico de um sistema multicamadas formado pelo empilhamento de mulita e monoporosa. A mulita é uma importante matéria-prima utilizada na fabricação de refratário para cerâmicas estruturais, devido as propriedades físicas e mecânicas a altas temperaturas como alto ponto de fusão, baixa expansão, resistência a fratura e ao choque térmico, alta resistência a fluência e baixa constante dielétrica. A monoporosa possui uma alta resistência o que confere excelentes propriedades mecânicas e químicas, sendo muito utilizado como massa de revestimento em pavimentos e revestimentos exteriores. O aumento na energia de fratura foi determinado comparando as de curvas de flexão em três pontos do sistema multicamadas com as de suas referências monolíticas. As fraturas foram caracterizadas por microscopia óptica e MEV, para análise da microestrutura realizou-se difração de raios X. As amostras multicamadas desenvolvidas foram avaliadas tomando como referência os materiais empregados na constituição de cada camada, em relação a estas referencias foram conseguidas deformações médias de até 2,58 vezes maiores e energias de fratura de até 7 vezes maiores.

Muitas edificações têm apresentado degradação ao longo dos anos em todo o mundo, gerando situações de risco e causando acidentes, de modo que se têm estudado alternativas para se recuperar ou reforçar estruturalmente suas vigas, pilares e lajes. Entre as técnicas desenvolvidas para essas finalidades, destacam-se, atualmente, as que utilizam compósitos de polímeros reforçados com fibras (PRF), de modo que, desde as últimas décadas do século passado, desenvolveram-se os sistemas de reforço estrutural por colagem externa de tecidos e laminados de PRF de carbono (PRFC). Isto deve-se a este tipo de fibra apresentar o melhor conjunto de propriedades necessárias para se reforçarem as estruturas de concreto armado, como altas resistência à tração e módulo de elasticidade longitudinal. Na última década, surgiu o sistema de inserção de laminados de PRFC em entalhes executados no concreto de cobrimento de elementos estruturais preenchidos com resina epóxi. Embora haja resultados de outros pesquisadores, há a necessidade de maiores investigações no Brasil sobre o desempenho dessa nova técnica. Assim se propôs o planejamento experimental desta dissertação, cujo objetivo principal foi a análise experimental do desempenho de sistemas de reforço estrutural à flexão com laminados de PRFC aplicados a vigas de concreto armado ensaiadas com carregamento estático. De um total de dez vigas pré-moldadas, quatro foram testemunhos e seis foram reforçadas à flexão com dois laminados de PRFC através de três sistemas: colagem externa, inserção em entalhes longitudinais preenchidos com resina epóxi e com argamassa com sílica ativa. Analisa-se comparativamente o seu desempenho quanto às cargas máximas, aos deslocamentos verticais no centro do vão e às aberturas de fissuras. Constata-se que as reforçadas com laminados de PRFC inseridos em entalhes longitudinais preenchidos com resina epóxi apresentam os maiores valores de carga máxima e de rigidez. Entretanto as preenchidas com argamassa com sílica ativa obtêm desempenho inferior ao das vigas testemunhos por falta de aderência. Também se conclui que as reforçadas com colagem externa de laminados de PRFC têm os menores deslocamentos verticais no centro do vão. / Many buildings have shown deterioration over the years around the world, creating a hazardous situation and causing accidents, so that they have studied alternatives to recover or strengthen their structural beams, columns and slabs. Among the techniques developed for these purposes, stand out, currently, those using fiber reinforced polymer (FRP) composites. Since the last decades of the last century, it has been developed structural reinforcement systems of externally bonded carbon FRP (CFRP) textiles and laminates. This is due to the type of fiber having the best set of properties that are necessary to strengthen reinforced concrete structures, such as high tensile strength and longitudinal elastic modulus. In the last decade, the near-surface mounted CFRP laminates system executed in grooves in the concrete cover of structural elements filled with epoxy resin came up. Although there are results from other researchers, there is a need for further investigation in Brazil on the performance of this new technique. Thus, the experimental program of this thesis has been proposed, which main objective was the experimental analysis of the performance of bending structural reinforcement systems with CFRP laminates applied to reinforced concrete beams tested under static loading. From a total of ten precast beams, four were for control and six were strengthened in bending with two CFRP laminates composites through three systems: externally bonded, near-surface mounted insertion into longitudinal grooves filled with epoxy resin and mortar with silica fume. Their performance is comparatively analyzed for maximum loads, vertical displacements at mid-span and crack openings. It is concluded that those reinforced with near-surface mounted CFRP laminates inserted into longitudinal grooves filled with epoxy resin shows the highest maximum load and stiffness. However those filled with mortar with silica fume obtained lower performance than the control beams for lack of bonding. In addition, it is concluded that the ones strengthened with externally bonded CFRP laminates have the lowest mid-span vertical displacements.