De par leur diversité et leur large gamme d’applications, les polymères se sont imposés dans notre environnement. Dans le cas d’applications techniques ces matériaux peuvent être exposés à des environnements agressifs conduisant à une altération de leurs propriétés. Les effets de cette dégradation sont reliés à la notion de durée de vie, c'est-à-dire au temps nécessaire pour qu’une propriété atteigne un seuil en dessous duquel le matériau devient inutilisable. Le suivi du vieillissement des matériaux polymères présente donc des enjeux importants. La spectroscopie de fluorescence est une technique qui permet d’apporter certaines réponses à ce problème. Dans le cadre de cette étude, l’accent a été porté sur l’utilisation de la spectroscopie de fluorescence pour l’étude des phénomènes intervenant lors de l’irradiation UV ou de la sollicitation mécanique d’un polymère. Dans le cas du polyéthylène haute densité, l’absence de signal fluorescent intrinsèque impose l’ajout d’un colorant. Ce colorant donnant une réponse en fluorescence dépendant de son microenvironnement, toutes modifications des chaînes du polymère engendrent un déplacement du pic de fluorescence de la sonde. Ce travail peut être séparé en deux grandes parties indépendantes, d’un coté l’influence du vieillissement UV sur la réponse fluorescente et de l’autre l’influence d’une sollicitation mécanique. Dans la première partie, l’utilisation de techniques complémentaires telles que l’IRTF ou l’AED a permis de corréler les différents résultats avec les mécanismes de vieillissement connus du polyéthylène. Les résultats obtenus dans cette partie montrent la grande sensibilité de la spectroscopie de fluorescence aux réarrangements microstructuraux intervenant dans le matériau. Dans la seconde partie, la dépendance entre la contrainte appliquée au matériau et la longueur d’onde de fluorescence a permis a partir de modèles simples d’évaluer les contraintes internes qui se développent au cours d’une sollicitation cyclique. / Due to their diversity and their wide range of applications, polymers have emerged in our environment. For technical applications, these materials can be exposed to aggressive environment leading to an alteration of their properties. The effects of this degradation are linked to the concept of life duration, corresponding to the time required for a property to reach a threshold below which the material becomes unusable. Monitoring the ageing of polymer materials constitute a major challenge. Fluorescence spectroscopy is a technique able to provide accurate information concerning this issue. In this study, emphasis was placed on the use of fluorescence spectroscopy to study the phenomena involved in either the UV radiation or mechanical stresses of a polymer. In the case of high density polyethylene, the lack of intrinsic fluorescent signal leads to the use of a dye. This dye gives a fluorescent response depending on its microenvironment. All modifications in the macromolecular chain generate a shift of the fluorescent peak. This work can be dissociated in two major parts, on one hand the influence of UV aging on the fluorescent response and in another hand the influence of mechanical stresses. In the first part, complementary analyses like FTIR or DSC are used to correlate fluorescent results with known photo degradation mechanisms. The results show the great sensibility of the technique to the microstructural rearrangement in the polymer. In the second part, the dependence between the stress and the fluorescence emission gives opportunity to evaluate internal stresses in the material during cyclic solicitations.

Understanding the relationships between microstructures and properties of materials is a key to developing new materials with more suitable qualities or employing the appropriate materials in special uses. In the present world of material research, the main focus is on microstructural control to cost-effectively enhance properties and meet performance specifications. This present work is directed towards improving the fundamental understanding of the microscale deformation mechanisms and mechanical behavior of metallic alloys, particularly focusing on face centered cubic (FCC) structured metals through a unique computational methodology called three-dimensional dislocation dynamics (3D-DD). In these simulations, the equations of motion for dislocations are mathematically solved to determine the evolution and interaction of dislocations. Microstructure details and stress-strain curves are a direct observation in the simulation and can be used to validate experimental results. The effect of initial dislocation microstructure on the yield strength has been studied. It has been shown that dislocation density based crystal plasticity formulations only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable. The evolution of the flow stress for grain sizes ranging from 0.5 to 10 µm under uniaxial tension was simulated using an improvised model by integrating dislocation pile-up mechanism at grain boundaries has been performed. This study showed that for a same initial dislocation density, the Hall–Petch relationship holds well at small grain sizes (0.5–2 µm), beyond which the yield strength remains constant as the grain size increases.

La recherche en biomécanique des chocs est une nécessité pour améliorer la sécurité dans les transports. Pour une meilleure évaluation des critères lésionnels lors des simulations de crash, le manque de représentativité des modèles EF du thorax humain pourrait être comblé par une démarche de personnalisation aussi bien au niveau géométrique que mécanique. Cette thèse se base sur l’étude de 18 sujets humains post-mortem. A partir des données d’imagerie, les différentes dimensions des côtes sont analysées. La corrélation de ces paramètres aboutit à la prédiction de 192 dimensions à partir d’un unique paramètre d’entrée. A une échelle inférieure, un protocole innovant a permis de coupler des informations microstructurales issues d’un μCT avec la forme extérieure des côtes. 2 hémi-thorax ont été micro-scannés afin de générer une cartographie complète des épaisseurs d’os cortical. Une stratégie a été mise en place pour proposer un algorithme prédisant l’intégralité de cette géométrie locale d’après un seul tronçon de côte. La pertinence de cette personnalisation a été évaluée par une étude de sensibilité sur des modèles EF. Les résultats d’essais de traction sur os cortical montrent un comportement différent entre les éprouvettes prélevées sur la table interne ou externe des côtes. Une caractérisation précise de la structure interne de l’os cortical, couplé à des essais de micro-traction in-situ, a pu apporter des éléments de réponse sur cette différence. Unalgorithme de personnalisation a été aussi proposé pour les propriétés mécaniques, complétant ainsi la démarche d’adapter les modèles EF du thorax à chaque individu afin d’améliorer leur biofidélité. / For a better assessment of injury criteria on the human thorax, realistic numerical simulations need accurate geometrical characterization and an understanding of the mechanical behavior of the rib. Thelack of representation of the FE models of the human thorax could be filled by a personalization of these two aspects. This thesis is based on the study of 18 post-mortem human subjects. From medical data (CT-scans), the different dimensions of the ribs were analyzed. The correlation of the measurements led to the prediction of 192 dimensions from a single input parameter. At a lower scale, an innovative protocol enabled us to combine microstructural information obtained from a μCT with the external shape of the ribs. 2 hemi-thoraxes were scanned to generate a complete map of the thickness of cortical bone and cross-section area evolution. A strategy was implemented to provide an algorithm, predicting this entire local geometry from a single rib’s sample. The relevance of this customization was evaluated by a sensitivity analysis on FE models. The results of tensile tests on cortical bone showed different behaviors between the samples harvested from the inner or outer side of the rib. A precise characterization of the internal structure of the cortical bone, coupled with in-situ micro-tensile device, revealed certain answers about this difference. An algorithm is also proposed topersonalize the mechanical properties, completing the approach of adapting the FE models of the thorax of each individual to improve their biofidelity.

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[pt] Neste estudo diferentes concretos a base de cimento aluminoso foram desenvolvidos e caracterizados em termos de seu comportamento termomecânico para uma ampla faixa de temperaturas (25-1200 graus Celsius). Primeiramente, três refratários com diferentes teores de alumina (51, 71 e 90 porcento em peso) foram estudados para caracterizar seus comportamentos químico e mecânico em diferentes temperaturas. Para isso vários tipos de ensaios experimentais, após aquecimento e resfriamento, foram realizados: ensaios microestruturais, químicos, de compressão uniaxial, tração direta e flexão em três pontos. Com esses resultados foi desenvolvido, através de simulações numéricas, um novo tipo de revestimento combinando os três refratários em camadas. A solução mais eficiente em termos de gradiente térmico foi a combinação que usou o refratário com 90 porcento de alumina como camada interna e os refratários de 71 porcento e 51 porcento como as camadas média e externa, respectivamente. Um material compósito refratário reforçado com fibras de aço inoxidável foi também desenvolvido. Para isso o concreto refratário com 51 porcento de alumina foi selecionado, pois apresentou o melhor comportamento mecânico nas temperaturas analisadas (até 1200 graus Celsius). Três tipos de fibras foram consideradas: lisa, ondulada e recartilhada. Além da realização dos mesmos tipos de ensaios feitos para a matriz refratária, foram também realizados ensaios cíclicos, de arrancamento de fibras e estruturais (painéis circulares). Os ensaios forneceram os parâmetros necessários para o modelo constitutivo de dano plástico disponível no programa de elementos finitos ABAQUS. O modelo constitutivo foi validado através de simulações termomecânicas do ensaio do painel circular. Concluiu-se que o comportamento termomecânico dos compósitos refratários com a adição de fibras de aço inoxidável foi significativamente melhor do que do refratário sem reforço. O compósito com a fibra recartilhada foi o que teve o melhor desempenho devido à forte aderência da fibra com a matriz. Os resultados experimentais e as simulações numéricas mostraram que o efeito de reforço beneficia o comportamento mecânico pré-pico e pós-pico dos compósitos refratários. A influência do endurecimento por deformação observada na fase inicial das curvas e do abrandamento (softening) pós-pico indicam a importância desses parâmetros para projetos estruturais. O dano do compósito refratário foi analisado através da análise de correlação digital de imagem para estudar a propagação da fissura nas várias temperaturas. Observou-se que com o aumento de temperatura a propagação da fissura fica mais lenta e a abertura da fissura fica menos acentuada. Conclui-se então que esses materiais são apropriados para utilização em aplicações que envolvem temperaturas elevadas. / [en] In this study different concretes based on aluminous cement were developed and characterized in terms of their thermomechanical behavior over a wide temperature range (25-1200 Celsius degrees). First, three refractories with different alumina contents (51, 71 and 90wt. percent) were studied to characterize their chemical and mechanical behavior at different temperatures. For this, several types of experimental tests, after heating and cooling, were carried out: microstructural, chemical, uniaxial compression tests, direct tensile tests and three-point bending test. With these results, a new type of coating was developed through numerical simulations combining the three refractories in layers. The most efficient solution in terms of thermal gradient was the combination that used the refractory with 90 percent of alumina as the inner layer and the refractories of 71 percent and 51 percent as the middle and outer layers, respectively. A refractory composite material reinforced with stainless steel fibers was developed. For this, the refractory concrete with 51 percent of alumina was selected, since it presented the best mechanical behavior at the analyzed temperatures (up to 1200 Celsius degrees). Three types of fibers were considered: straight, wavy and knurled. In addition to performing the same types of tests for the refractory matrix, cyclic, fiber pullout and structural tests (round panels) were also performed. The tests provided the parameters for the Damage Plasticity constitutive model available in the finite element software ABAQUS. The constitutive model was validated through thermomechanical simulations of the round panel test. It was concluded that the thermo-mechanical behavior of the refractory composites with the addition of stainless steel fibers was significantly better than the non-reinforced refractory. The composite with the knurled fiber was the one that had the best performance due to the strong bond of the fiber to the matrix. The experimental results and numerical simulations showed that the reinforcement effect benefits the pre-peak and post-peak mechanical behavior of the refractory composites. The influence of the strain-hardening observed in the initial phase of the curves and the softening post-peak indicated the importance of these parameters for structural projects. The damage of the refractory composite was analyzed through digital image correlation to study the crack propagation at various temperatures. It has been observed that with the increase in temperature the propagation of the crack becomes slower and the opening of the crack becomes less pronounced. It is therefore concluded that such materials are suitable for use in applications involving high temperatures.

The thermophysical properties and deformation behavior of a systematic series of model metallic glasses was investigated. For Zr-based metallic glasses with all metallic constituents, the activation energy of glass transition was determined to be in the range of 74-173 kJ/mol while the activation energy of crystallization was in the range of 155-170 kJ/mol. The reduced glass transition temperature was roughly the same for all the alloys (~ 0.6) while the supercooled liquid region was in the range of 100-150 K, indicating varying degree of thermal stability. In contrast, the metal-metalloid systems (such as Ni-Pd-P-B) showed relatively higher activation energy of crystallization from short range ordering in the form of triagonal prism clusters with strongly bonded metal-metalloid atomic pairs. Deformation mechanisms of all the alloys were investigated by uniaxial compression tests, strain rate sensitivity (SRS) measurements, and detailed characterization of the fracture surface morphology. For the metal-metal systems, plasticity was found to be directly correlated with shear transformation zone (STZ) size, with systems of larger STZ size showing better plasticity. In metal-metalloid amorphous alloys, plasticity was limited by the distribution of STZ units, with lower activation energy leading to more STZ units and better plasticity. The alloys with relatively higher plasticity showed multiple shear bands while the brittle alloys showed a single dominant shear band and vein-pattern on the fracture surface indicating sudden catastrophic failure. The effect of chemistry change on thermodynamics, kinetics, and deformation behavior was investigated for the model binary NixP100-x and CoxP100-x metallic glasses. Alloys with higher phosphorous content showed greater activation energy of crystallization, indicating better thermal stability. In addition, metallic glasses with higher % P showed greater hardness, modulus, and serrated flow behavior during indentation that is characteristic of inhomogeneous deformation.