Lithium-ion batteries with higher capacity and longer cycle life than that available today are required as secondary energy sources for a wide range of emerging applications. In particular, the cycling performance of several candidate materials for lithium-ion battery electrodes is insufficient because of the fast capacity fading and short cycle life, which is mainly a result of mechanical degradation. This dissertation mainly focuses on the issue of mechanical degradation in advanced lithium-ion battery electrodes. Thin films of tin electrodes were studied where we observed whisker growth as a result of electrochemical cycling. These whiskers bring safety concerns because they may penetrate through the separator, and cause short-circuit of the electrochemical cells. Cracking patterns generated in amorphous silicon thin film electrodes because of electrochemical cycling were observed and analyzed. A two-dimensional spring-block model was proposed to successfully simulate the observed cracking patterns. With semi-quantitative study of the cracking pattern features, two strategies to void cracking in thin-film electrodes were proposed, namely reducing the film thickness and patterning the thin-film electrodes. We also investigated electrodes consisting of low melting point elements and showed that cracks can be self-healed by the solid-to-liquid phase transformation upon cycling. Using gallium as an example, mechanical degradation as a failure mechanism for lithium-ion battery electrodes can be eliminated. In order to quantitatively understand the effect of surface modification on electrodes, we analyzed diffusion equations with boundary conditions of finite interfacial reactions, and proposed a modified potentialstatic intermittent titration technique (PITT) as an electro-analytical technique to study diffusion and interfacial kinetics. The modified PITT has been extended to thin-film geometry and spherical geometry, and thus can be used to study thin-film and composite electrodes consisting of particles as active materials.

In line with its commitment to safe nuclear power generation, the Koeberg Nuclear Power Station (KNPS) replaced the outdated vibration monitoring system with a modern on-line vibration monitoring system. This will allow plant personnel to monitor components on a continuous basis which will provide faster response time in the scenario of excessive vibrations of the primary components. This study focuses on the analysis of the vibration of the primary components of the KNPS by analysing the frequency spectra of the vibration signals of the primary components and comparing these to reference signatures obtained during similar operating conditions. The condition of the vibration sensors will also be evaluated. In order to obtain a deeper understanding of the vibration behaviour and hence vibration signatures of the KNPS primary reactor components, a simplified mathematical model of the primary components is developed, based on the system of elasto-dynamic equations. The equations are solved numerically and used to simulate the KNPS vibration monitoring system. The mechanical system is modelled. Time series are generated and Fast Fourier Transforms (FFT) are calculated to simulate the new KNPS monitoring system. In the simulation mechanical degradation of the primary components as well as sensor degradation is simulated. The purpose of this study is to indicate whether mechanical degradation has occurred in the primary components of the plant and to validate the vibration signals. At the same time the study aims to lay a foundation for future monitoring and interpretation of vibration signatures by simulating the vibration and the monitoring signals. It was found that the primary components had not been affected by mechanical degradation as no deviations in resonances were detected in the frequency signatures. A small number of vibration sensors were found to have deteriorated; hence replacement / maintenance was proposed. The mechanical model and the simulation of the monitoring signals proved to be useful to understand and interpret the vibration of the KNPS primary components. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.

In line with its commitment to safe nuclear power generation, the Koeberg Nuclear Power Station (KNPS) replaced the outdated vibration monitoring system with a modern on-line vibration monitoring system. This will allow plant personnel to monitor components on a continuous basis which will provide faster response time in the scenario of excessive vibrations of the primary components. This study focuses on the analysis of the vibration of the primary components of the KNPS by analysing the frequency spectra of the vibration signals of the primary components and comparing these to reference signatures obtained during similar operating conditions. The condition of the vibration sensors will also be evaluated. In order to obtain a deeper understanding of the vibration behaviour and hence vibration signatures of the KNPS primary reactor components, a simplified mathematical model of the primary components is developed, based on the system of elasto-dynamic equations. The equations are solved numerically and used to simulate the KNPS vibration monitoring system. The mechanical system is modelled. Time series are generated and Fast Fourier Transforms (FFT) are calculated to simulate the new KNPS monitoring system. In the simulation mechanical degradation of the primary components as well as sensor degradation is simulated. The purpose of this study is to indicate whether mechanical degradation has occurred in the primary components of the plant and to validate the vibration signals. At the same time the study aims to lay a foundation for future monitoring and interpretation of vibration signatures by simulating the vibration and the monitoring signals. It was found that the primary components had not been affected by mechanical degradation as no deviations in resonances were detected in the frequency signatures. A small number of vibration sensors were found to have deteriorated; hence replacement / maintenance was proposed. The mechanical model and the simulation of the monitoring signals proved to be useful to understand and interpret the vibration of the KNPS primary components. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.

The use of carbon fibre-reinforced plastic technology is steadily gaining traction in the modern automotive industry as a lightweight alternative to conventional materials. The versatile chemical resistance of polyamide resins combined with the high strength properties of carbon fibre filler content aims to meet this growing need in the industry. By employing a number of accelerated and amplified ageing techniques, this work hopes to assess the resilience of carbon fibre-reinforced polyamide composites in a variety of foreseeable chemical, temperature, moisture, and stress environments. The resins included in this characterization study include polyamide-6 (PA6) and polyamide-6,6 (PA6/6). The carbon fibre-reinforced composite specimens are subject to long term immersion in commercial automotive fluids at room and elevated temperatures. Results show that the mechanical properties of both polyamide resins are sensitive to windshield washer fluid exposure, regardless of temperature. The significant drop in glass transition temperature and greater elongation at break confirmed a plasticization effect. The Young’s modulus and tensile strength experienced a loss of approximately 40% at saturation. Elevated temperatures resulted in increased fluid sorption rates of antifreeze and E-20 gasoline into the PA6 composite specimens. Likewise, a corresponding drop in PA6 composite mechanical properties was noted for the antifreeze and E-20 gasoline at elevated temperatures. The mechanical properties of the PA6/6 composite were largely retained in all tested automotive fluids, except windshield washer fluid, at elevated temperatures. The effect of absorbed fluid on mechanical properties tended to increase with higher fibre loadings for the PA6/6 composite and lower fibre loadings for the PA6 composite. Finally, a single parameter acoustic emission testing technique was employed to assess internal damage of stressed PA6 composite specimens exposed to different temperature and humidity levels. However, there was no discernible correlation between environmental stress conditions and internal damage for short term exposure times. / Thesis / Master of Applied Science (MASc) / The use of carbon fibre-reinforced plastic technology is steadily gaining traction in the modern automotive industry as a lightweight alternative to conventional materials. The versatile chemical resistance of polyamide resins combined with the high strength properties of carbon fibre filler content aims to meet this growing need in the industry. By employing a number of accelerated and amplified ageing techniques, this work assessed the resilience of carbon fibre-reinforced polyamide composites in a variety of foreseeable chemical, temperature, moisture, and stress environments. The composite only showed significant sensitivity to windshield wiper fluid in the tests.

La réduction de la consommation en carburant des voitures est une préoccupation importante. L'objectif est de réduire les frottements entre pièces mobiles du moteur à travers la formulation du lubrifiant.Un lubrifiant est composé d'une huile de base et d'additifs afin d'ajuster les propriétés de l'huile moteur. En particulier, des polymères sont utilisés pour limiter la perte de viscosité avec la température afin de protéger au mieux les pièces du moteur. Le fonctionnement de ce type d'additifs, appelés améliorants de viscosité (AVI), est basé sur le gonflement avec la température des chaines polymères. Le paramètre le plus important à prendre en compte dans la formulation d'un lubrifiant est sa viscosité ainsi que sa dépendance en température. Le but de ce travail a été d'étudier l'influence de polymères AVI récemment commercialisés afin de sélectionner le plus efficace dans le cadre d'une application moteur, tout en éclaircissant les mécanismes d'action en jeu à l'échelle moléculaire dans ce type de systèmes. Les propriétés rhéologiques de poly(alkyle méthacrylates) en solution ont été étudiées à l'aide de tests standards dans le but de comprendre l'influence des lubrifiants sous pression et dans des conditions confinées. La dégradation de ce type de polymère a également été considérée à travers un test standard afin de vérifier si les performances du polymère sont conservées dans le temps. Les comportements en dégradation et en tribologie ont pu être rationalisés par des considérations de volume occupé par le polymère. / The reduction of car fuel consumption is one of the biggest concerns for oil companies. It involves the reduction of friction between moving parts through the formulation of lubricants.A lubricant is composed of base oil and several additives used to improve the properties of engine oils. Polymeric compounds are used to limit the decrease of viscosity with temperatures to efficiently protect the motor. This type of additives, called Viscosity Index Improver (VII), is based on the swelling of polymer chains with temperature. The most important parameter in the lubricant formulation is the viscosity and its dependance on temperature. The aim of the work was to study the influence of recently developed families of VII additives in different base oils in ordrer to select the most efficient type of polymer for engine lubricant uses, by understanding the molecular mechanisms responsible of the VII behavior. Various polyalkylmethacrylates were investigated to establishing the rheological properties of the solutions, namely as a function of temperature (dependence of intrinsic viscosity). For a given base oil, the efficiency of a VII varies with its chemical structure, its molecular weight and its concentration. Then, tribological properties were investigated using standard tests with the aim to obtain an overview of the lubricant's behavior under pressure. We also considered the degradation of this kind of polymers in a representative test in order to check the performance's durability. Degradation and tribological behaviors were rationalized in terms of volume occupied by polymer chains.

There is an intense, worldwide effort to develop durable lithium ion batteries with high energy and power densities for a wide range of applications, including electric and hybrid electric vehicles. For improvement of battery technology understanding the capacity fading mechanism in batteries is of utmost importance. Novel electrode material and improved electrode designs are needed for high energy- high power batteries with less capacity fading. Furthermore, for applications such as automotive applications, precise cycle-life prediction of batteries is necessary. One of the critical challenges in advancing lithium ion battery technologies is fracture and decrepitation of the electrodes as a result of lithium diffusion during charging and discharging operations. When lithium is inserted in either the positive or negative electrode, there is a volume change associated with insertion or de-insertion. Diffusion-induced stresses (DISs) can therefore cause the nucleation and growth of cracks, leading to mechanical degradation of the batteries. With different mathematical models we studied the behavior of diffusion induces stresses and effects of electrode shape, size, concentration dependent material properties, pre-existing cracks, phase transformations, operating conditions etc. on the diffusion induced stresses. Thus we develop tools to guide the design of the electrode material with better mechanical stability for durable batteries. Along with mechanical degradation, chemical degradation of batteries also plays an important role in deciding battery cycle life. The instability of commonly employed electrolytes results in solid electrolyte interphase (SEI) formation. Although SEI formation contributes to irreversible capacity loss, the SEI layer is necessary, as it passivates the electrode-electrolyte interface from further solvent decomposition. SEI layer and diffusion induced stresses are inter-dependent and affect each-other. We study coupled chemical-mechanical degradation of electrode materials to understand the capacity fading of the battery with cycling. With the understanding of chemical and mechanical degradation, we develop a simple phenomenological model to predict battery life. On the experimental part we come up with a novel concept of using liquid metal alloy as a self-healing battery electrode. We develop a method to prepare thin film liquid gallium electrode on a conductive substrate. This enabled us to perform a series of electrochemical and characterization experiments which certify that liquid electrode undergo liquid-solid-liquid transition and thus self-heals the cracks formed during de-insertion. Thus the mechanical degradation can be avoided. We also perform ab-initio calculations to understand the equilibrium potential of various lithium-gallium phases.

[pt] Geralmente, o ambiente marinho é o ambiente natural mais agressivo para tubulações metálicas, promovendo corrosão, levando a falhas catastróficas. O método de reparo tradicional metálico soldado é um método de reparo inconveniente e custoso para a indústria. Compósitos poliméricos (FRP) são um material com potencial para reparo devido a sua alta razão resistência/peso e alta resistência à corrosão e degradação ambiental. Porém, os compósitos poliméricos também são suscetíveis a degradação severa quando expostos a condições ambientais agressivas, incluindo absorção de água, temperatura, UV e pressão. Para um melhor entendimento do mecanismo de degradação, amostras de compósito de matriz polimérica reforçado por fibras de vidro e resina epóxi pura foram fabricados e analisados em função do tempo e temperatura de envelhecimento em uma atmosfera salina. Além disso, um grupo de compósitos tiveram suas bordas cobertas com resina epóxi para similar práticas reais comuns no campo, denominado coated FRP. Uma resina epóxi DGEBA bicomponente e um tecido bidirecional de fibras de vidro foram usados como matriz e reforço, respectivamente. O tecido de fibra de vidro detém de uma razão de fibras longitudinais por transversais de 2:1. Os materiais foram sujeitos a envelhecimento de névoa salina em três câmaras à 35, 55 e 70 Celsius por aproximadamente 15171 horas. A concentração de sal na solução usada foi 5.0 por cento por massa. As amostras foram periodicamente retiradas das câmaras de envelhecimento para terem seus ganhos de massa mensurados pelo método gravitacional. Suas mudanças dimensionais também foram capturadas para avaliação do comportamento de inchamento dos materiais. Análises térmicas com DSC e DMTA foram feitas para avaliar o grau de cura dos materiais poliméricos e os efeitos da temperatura na pós-cura do material. Análises químicas com testes FTIR foram feitas para investigar a ocorrência de processos de pós-cura, hidrólise e termo-oxidação durante o envelhecimento. Amostras de compósito para testes destrutivos de flexão e resistência ao cisalhamento (ILSS) foram testados periodicamente para terem suas degradações monitoradas com o tempo. Testes de Excitação por Impulso (IET) e Colorímetro foram realizados como testes não-destrutivos (NDT) complementares. A primeira parte deste trabalho foca na avaliação do comportamento de absorção de umidade nos compósitos e resina epóxi pura. A relação entre a capacidade de absorção de umidade dos materiais, considerando a fração volumétrica de fibras, foi investigada. Além disso, modelos de absorção não-Fickianos também foram aplicados aos dados experimentais do FRP e resina epóxi para considerar desvios do modelo Fickiano padrão. Com isso, relaxações poliméricas e interações polímero-água, assim como mudanças na rede polimérica induzidas por umidade e temperatura, foram investigados. Uma modificação ao modelo não-Fickiano de Berens-Hopfenberg (BH) foi proposta para incluir os efeitos de pós-cura na absorção de umidade; tal modificação aplicada a pós-cura não foi encontrada na literatura. Na segunda parte deste trabalho, o comportamento de inchamento da resina epóxi pura e compósitos revestidos e não-revestidos foi avaliado e correlacionado com a absorção de umidade. A ortotropia do compósito ficou evidente, visto que o material apresentou maior capacidade de inchamento na direção da espessura. Um modelo do tipo Fickiano foi implementado ao inchamento na espessura com o intuito de investigar deformação de inchamento de saturação e a frente de inchamento. A terceira parte deste trabalho foca nos testes destrutivos e na avaliação dos efeitos hidrotérmicos na degradação do material. A temperatura se mostrou um fator acelerador para degradação de propriedade. Além disso, uma metodologia para estimar a curva de serviço de sistemas de reparo e extrapolá-la para temperaturas mais baixas foi elaborada. Metodologias de extrapolação de propriedades para exposição hidrotérmica de longa duração não foram encontradas na literature. Parâmetros como platô de retenção de propriedade e taxa de degradação foram estimados para temperatura ambiente. Curvas de Arrhenius também foram plotadas para avaliar o tempo requerido para alcançar os níveis de retenção em cada temperatura testada. Por fim, duas técnicas não-destrutivas foram utilizadas no FRP e resina epóxi pura como testes complementares como forma de validar os resultados encontrados em outras técnicas. A Técnica de Excitação por Impulso (IET) foi realizada para obter o módulo de Young e boa correlação foi obtida entre o teste IET e o ensaio destrutivo de flexão. Através dos testes colorimétricos, os efeitos de umidade e temperatura foram visíveis, já que a mudança de cor da resina foi mais forte para tempos longos e temperaturas mais altas. / [en] Generally, the marine environment is the most aggressive natural environment for metallic pipelines, promoting corrosion, leading to catastrophic failures. The traditional welded metallic repairs are high-cost and inconvenient repair methods for the industry. Polymeric composites (FRP) are a potential repair material due to their high resistance/weight ratio and high resistance to corrosion and environmental degradation. However, polymeric composites are also susceptible to severe degradation when exposed to harsh environment conditions, including water absorption, temperature, UV and pressure. To better understand the degradation mechanism, glass-fiber reinforced polymer matrix composite (GFRP) and neat epoxy samples were fabricated and analyzed as a function of aging time and temperature in a saline atmosphere. In addition, a group of composites had their exposed edges coated with epoxy resin to simulate common real-life practices in the field, namely coated FRP. A two-component DGEBA epoxy resin and a bidirectional glass-fiber woven fabric were used as matrix and reinforcement, respectively. The fiberglass fabric had a longitudinal to transverse fiber ratio of 2:1. The materials were subjected to salt spray aging in three chambers at 35, 55 and 70 Celsius for approximately 15171 hours. The salt concentration in the solution used was 5.0 per cent by weight. Samples were periodically removed from the chambers to have their mass gain measured by the gravitational method. Their dimensional changes were also measured to evaluate the swelling behavior of the materials. Thermal analyses with DSC and DMTA were performed to evaluate the curing degree of the polymeric materials tested and the effects of temperature on the material’s post-curing. Chemical analyses with FTIR tests were performed to investigate the occurrence of post-curing, hydrolysis and thermo-oxidation processes during aging. Composite samples for bending and interlaminar shear strength (ILSS) destructive tests were periodically tested to have their degradation monitored over time. Impulse Excitation Technique and colorimetry tests were also performed as complementary non-destructive tests (NDT). The first part of this work focuses on the assessment of the moisture absorption behavior of both composite and neat epoxy resin. The relationship between the moisture gain capacity of the materials, considering the fiber volume fraction, was investigated. Besides, non-Fickian absorption models were also applied to the experimental data of FRP and neat epoxy resin to account for deviations from the standard Fickian model. Then, polymeric relaxations and polymer-water interactions, as well as network changes induced by moisture and temperature, were investigated. A modification to the Berens-Hopfenberg (BH) non-Fickian model was proposed to account for post-curing effects on the moisture absorption; such modification applied for post-curing was not found in literature. In the second part of this work, the swelling behavior of neat epoxy resin, uncoated and coated composites was evaluated and correlated with moisture absorption. The orthotropy of the composite was evident, since this material showed greater swelling capacity in the thickness direction. A Fickian-like model was implemented to the thickness swelling to investigate swelling strain saturation and swelling front. The third part of this work focuses on the destructive tests and evaluation of the hygrothermal effects on the material degradation. Temperature proved to be an accelerating factor for property degradation. In addition, a methodology to estimate the service-life curve of the repair systems and extrapolate it to lower temperatures was elaborated. The methodology of property extrapolation for long-term hygrothermal exposure in salt spray environments could not be found in literature. Parameters like property retention plateau and degradation rate were estimated for room temperature (25°C). Arrhenius curves were also plotted to evaluate the time required to reach the properties’ retention levels for each temperature. Finally, two non-destructive techniques were performed on the uncoated FRP and on neat epoxy resin as complementary tests in order to validate the results found in other techniques. The Impulse Excitation Technique (IET) was performed to evaluate the Young modulus and good correlation was found between IET and destructive bending tests. From the colorimetry tests, the effects of moisture and temperature were visible, as the resin s color change was stronger at longer aging time and higher temperatures.

<p>Energy storage devices have become an integral part of the digital infrastructure of the 21st century. Li-ion batteries are a widely used chemical form of energy storage devices comprising components with varied chemical, mechanical and electrochemical properties. Over long-term usage, the anode and cathode experience spatially heterogeneous Li reaction, mechanical degradation, and reversible capacity loss. The small particle size and environmental sensitivity of materials used in Li-ion battery materials make investigating electrodes' electrochemical and mechanical properties an arduous task. Nevertheless, understanding the effect of electrochemical fatigue load (during the battery's charging and discharging process) on composite electrodes' mechanical stability is imperative to design and manufacture long-lasting energy storage devices.</p><p>Due to the low-symmetry lattice, Lithium Nickel Manganese Cobalt Oxide (NMC) cathode materials exhibit direction-dependent (anisotropic) mechanical properties. In this Dissertation, we first measure the anisotropic elastic stiffness of NMC cathode material using nano-indentation. We also determine the effect of Ni stoichiometry on the indentation modulus, hardness, and fracture toughness of NMC materials. The complete information on the mechanical properties of cathode materials will enable accurate computational results and the design of robust cathodes.</p><p>Further, using operando optical experiments, we report that NMC porous composite cathode experiences asynchronous reactions only during the 1st charging process. Non-uniform carbon binder network coverage across the cathode and Li concentration-dependent material properties of NMC results in the initial asynchronous phenomenon. The information on the degree of electrochemical conditioning of Li-ion battery cathode obtained from optical microscopy can test the consistency of product quality in the industrial manufacturing process. We also investigate the effects of non-uniform reactions on active material’s local morphology change and study the evolution of particle network over long-term cycling. Reported data from experiments depicts that in the early cycles, individual particles’ characteristics significantly influence the degree of damage across the cathode.</p><p>However, the interaction with neighboring particles becomes more influential in later cycles. Computational modeling uses a multiphysics-based theoretical framework to explain the interplay between electrochemical activity and mechanical damage. The methodology, theoretical framework, and experimental procedure detailed here will enable the design of efficient composite electrodes for long-lasting batteries.</p>

[pt] A presente tese de doutorado tem como origem o projeto de pesquisa Aneel PD-0394-1905/2019, realizado a partir de uma colaboração entre Furnas e PUC-Rio. A principal meta desse grande projeto de pesquisa está no desenvolvimento do concreto reforçado com fibras visando sua aplicação em elementos estruturais que estão submetidos à fadiga na flexão ao longo de toda sua vida útil, como torres eólicas, pavimentos e elementos de pontes. Dessa maneira, ao longo de todo essa tese doutorado, a degradação mecânica do concreto reforçado com fibras sob fadiga é analisada em detalhe desde a escala da fibra em ensaios de arrancamento até a escala estrutural através de testes mecânicos de fadiga em larga escala. A primeira etapa desse estudo traz uma análise do comportamento à fadiga do concreto reforçado com fibras. A vida à fadiga desse material é estudada através de diferentes modelos estatísticos, que garantem avaliar a falha do material baseada em uma probabilidade falha. Já os ensaios de fadiga no arrancamento ajudam a explicar na escala interface fibra-matriz como ocorre a ruptura dos prismas sob carregamentos cíclicos. Uma segunda fase desse trabalho mostra a degradação mecânica de vigas armadas sob fadiga e o impacto da adição de fibras nos principais parâmetros de interesse. A adição do reforço fibroso é responsável por causar uma redistribuição de tensões na zona tracionada do elemento estrutural, diminuindo as deformações da armadura longitudinal e amenizando a degradação mecânica do concreto armado em termos de curvatura, deslocamento e rigidez. Além disso, a adição de fibras também é responsável por incrementar significativamente a aderência da barra de aço ao redor da matriz de concreto. Fator chave para explicar a melhora da resposta mecânica da estrutura sob fadiga e estudada nessa tese de doutorado através dos ensaios de arrancamento da barra aço. Por fim, uma nova solução analítica foi desenvolvida para avaliar a degradação mecânica dos prismas de concreto reforçado com fibras sob fadiga. As curvas analíticas propostas se adequaram de forma bem sucedida os resultados experimentais analisados nesse trabalho. A adição de fibras apresentou grande potencial visando uma diminuição da degradação mecânica das estruturas de concreto armado submetidas a carregamentos cíclicos. A redistribuição de tensões na zona tracionada devido às fibras promove uma maior rigidez da estrutura sob fadiga, uma melhora da aderência da armadura e uma maior capacidade de resistir aos ciclos de fadiga ao longo do tempo. Esse ganho mecânico com o reforço fibroso, portanto, pode garantir maior vida útil das estruturas em concreto armado. / [en] This doctoral thesis originates from the research project Aneel PD-0394- 1905/2019, carried out through a collaboration between Furnas and PUC-Rio. The main objective of this extensive research project is the development of fiber reinforced concrete for distinct structural application which are subjected to continuous flexural fatigue loading along their useful life, such wind tower endeavors, concrete pavements and bridge elements. The addition of fibers in the concrete mix has the potential to mitigate the mechanical deterioration along the continuous load cycles, enhancing, as a consequence, the durability and the fatigue life of the cited concrete structural elements. Throughout this doctoral thesis, the mechanical degradation of fiber reinforced concrete under fatigue is carefully analyzed, starting from the fiber scale with pull-out tests and going up to the structural scale through large-scale fatigue mechanical tests. The first stage of this study involves an analysis of the mechanical behavior of fiber reinforced concrete under fatigue loading. The material fatigue life is examined using different statistical models, which allow evaluating material failure based on a failure probability. Fatigue pull-out tests help explain, at the fiber-matrix interface scale, how the prisms rupture under cyclic loading. A second phase of this work demonstrates the mechanical degradation of reinforced structural beams under fatigue and the impact of fiber addition on key concerned parameters. The addition of fiber reinforcement causes a redistribution of stresses in the tension zone of the structural element, reducing the deformations of the longitudinal rebar and mitigating the mechanical degradation of reinforced concrete in terms of curvature, displacement and stiffness. Furthermore, fiber addition significantly improves the bond between the steel bar and the surrounding concrete matrix, a key factor in explaining the enhanced mechanical response of the structure under fatigue, as studied in this doctoral thesis through rebar pull-out tests. Finally, a new analytical solution was developed to assess the mechanical degradation of fiber reinforced concrete prisms under fatigue loads. The proposed analytical curves successfully fit the experimental results analyzed in this work. The addition of fibers showed great potential in reducing the mechanical degradation of reinforced concrete structures subjected to cyclic loading. The stress redistribution in the tension zone, caused by the fibers, promotes greater stiffness of the structure under fatigue, improves the bond with the reinforcement and enhances the ability to withstand fatigue cycles over time. Therefore, the observed enhancement of mechanical properties through fiber reinforcement can ensure a longer service life for reinforced concrete structures.

Le polymer flooding est une des techniques de récupération assistée des hydrocarbures (RAH) ; elle consiste à injecter une solution de polymères de forte masse moléculaire afin de déplacer plus efficacement le pétrole emprisonné dans la roche. Cependant, une limite importante de cette technique est la possible dégradation mécanique des polymères au cours de l'injection et dans le réservoir, due à une scission des chaînes macromoléculaires induite par l’écoulement. Ce travail de thèse a pour objectif de mieux comprendre les mécanismes et scénarios de scission, mais aussi leur impact sur le procédé de polymer flooding. Nous nous sommes intéressés au seuil de dégradation mécanique de solutions de poly(oxyde d’éthylène) et de de polyacrylamide partiellement hydrolysé, pour différents régimes de concentration (solutions diluées et semi-diluées) en régime laminaire et inertiel, et pour des solvants de différentes qualités. L’étude de la dégradation mécanique des solutions et de leur impact sur les propriétés rhéologiques a été menée à l’aide de différents dispositifs de dégradation et de différents rhéomètres, dont un dispositif microfluidique en élongation ; ces techniques de mesure ont été combinées à des mesures de distribution de masses moléculaires par chromatographie d’exclusion stérique couplée à la diffusion de lumière. L’étude montre en premier lieu qu’une composante élongationnelle est indispensable pour dégrader les chaînes macromoléculaires en solution. Les résultats mettent aussi clairement en évidence que les mécanismes de dégradation sont très différents en régime dilué et semi-dilué. En régime dilué, la dégradation mécanique des solutions de polymères est indépendante du régime d’écoulement et affecte préférentiellement les macromolécules de fortes masses, avec une scission en milieu de chaîne. En revanche, en régime semi-dilué, la dégradation mécanique dépend du régime de l’écoulement : en écoulement laminaire, la dégradation est gouvernée par le réseau d’enchevêtrements et la scission des chaînes est aléatoire, tandis qu’en régime inertiel, les chaînes se dégradent comme en régime dilué, avec le même scénario de scission en milieu de chaîne. Par ailleurs, les résultats montrent que les propriétés rhéologiques en élongation peuvent être très fortement impactées par la dégradation mécanique. Enfin, les résultats de l’étude préliminaire des propriétés d’injectivité dans un milieu poreux d’une solution de polymère semi-diluée faiblement dégradée montrent que la dégradation mécanique améliore l’injectivité du polymère aux abords du puits. / Polymer flooding is a technique used in enhanced oil recovery; it consists in injecting high molecular weight polymer solutions in order improve oil sweep efficiency in the reservoir. However, polymer flooding is challenged by possible mechanical degradation of polymer solutions during injection and in the reservoir, due to the flow induced scission of macromolecules. This work aims at better understanding the scission mechanisms and scenarios, but also their impact on polymer flooding. We investigated the onset of mechanical degradation of poly(ethylene oxide) and partially hydrolysed polyacrylamide solutions, for different concentration regimes (dilute and semi-dilute regimes), under laminar or inertial conditions, but also under good or bad solvent conditions. The study of mechanical degradation of polymer solutions and their impact on the rheological properties was performed using different degradation devices and different rheometers, including a microfluidic extensional device; these investigation techniques were combined with measurements of the molecular weight distributions using size exclusion chromatography coupled with light scattering experiments. The study first shows that an extensional component is needed to get a mechanical degradation of polymer chains. The results also clearly show that the degradation mechanisms are very different in dilute and semi-dilute regime. In dilute regime, the mechanical degradation of polymer solutions does not dependent on flow regime and mainly affects the macromolecules with high molecular weights, with a mid-chain scission scenario. On the other hand, in semi-dilute regime, mechanical degradation depends on flow regime: in laminar flows, degradation is governed by the entanglement network and chain scission is random, whereas in inertial flows, chain degradation is similar to that observed in dilute regime, with the same mid-chain scission scenario. Besides, the results show that the extensional rheological properties can be very strongly affected by mechanical degradation. At last, the results of a preliminary study of the injectivity of a slightly degraded semi-dilute polymer solution in porous media show that mechanical degradation improves polymer injectivity near the wellbore.