Du fait de leurs bonnes propriétés physiques et thermiques et de leur faible coût, les aciers ferrito-martensitiques à 9%Cr sont envisagés comme matériaux de circuits et comme tubes des échangeurs et des générateurs de vapeur pour le prototype de Réacteur Rapide au sodium (RNR-Na) ASTRID. Plusieurs mécanismes existent dans la littérature pour expliquer la formation de la couche d’oxyde duplexe spinelle Fe-Cr/magnétite formée lors de l’oxydation des aciers Fe-9Cr dans différents environnements. Par ailleurs, un endommagement partiel ou total de la couche d’oxyde apparait systématiquement en environnement eau ou vapeur d’eau. Aussi, afin d’apporter des éléments pour une évaluation robuste de ces alliages sur de longues durées, ce travail a eu pour but d’une part d’étudier les cinétiques d’oxydation et d’autre part de comprendre l’ensemble des mécanismes conduisant à l’écaillage de la couche d’oxyde. La première partie de l’étude a consisté à mener des essais d’oxydation à 550°C en milieu vapeur d’eau pure et sous Ar/D2O/H2, avec différentes teneurs en hydrogène et vapeur d’eau. Une simulation, basée sur une résolution analytique, a permis de montrer que le modèle de l’espace disponible proposé dans la littérature pour la croissance de la couche duplexe permet de représenter quantitativement les résultats expérimentaux (cinétique, stœchiométrie, proportion des phases). Le deuxième volet de cette étude a été consacré à l’étude de l’endommagement de la couche d’oxyde lors de sa croissance en milieu vapeur d’eau pure. La rupture de la couche d’oxyde par cloquage puis écaillage a systématiquement lieu au sein de la couche de magnétite. Les pores observés au sein de cette couche apparaissent être des sites d’initiation du phénomène de décohésion. Un modèle en cohérence avec le modèle cinétique a été proposé pour anticiper le lieu de formation des pores. Il repose sur le calcul du flux de fer au sein de la couche d’oxyde en croissance. Il permet d’expliquer quantitativement le lieu de formation des pores par l’accumulation de lacunes de fer au sein de la couche de magnétite dans une zone de plus fort gradient de potentiel chimique. Pour évaluer les contraintes présentes au sein de la couche d’oxyde et impliquées dans l’endommagement de celle-ci, des essais de déflexion d’une lame mince asymétrique ont été réalisés sous différents environnements oxydants à 550°C. Les essais réalisés en milieu humide sous Ar/H2O/(H2) et sous CO2 ont mis en évidence la présence de contraintes de croissance en compression de l’ordre de -150 MPa au sein de la couche d’oxyde lors de l’isotherme. Par ailleurs, la présence d’hydrogène dans le milieu oxydant apparait favoriser la fragilisation de la couche d’oxyde. Une approche numérique a été développée pour simuler les essais de déflexion, en prenant en compte les phénomènes de relaxation par fluage et le caractère multi-strates de la couche d’oxyde. Les principales formes de relaxation des contraintes, viscoplasticité de la couche, signes d’endommagement macroscopique (fissurations), ont ainsi été quantifiées. / In the framework of the development of Generation IV reactors and specifically in the new Sodium Fast Reactor (SFR) project, Fe-9Cr ferritic-martensitic steels are candidates as structural materials for steam generators. Indeed, Fe-9Cr steels are already widely used in high temperature steam environments – like boilers and steam turbines- for their combination of creep strength and high thermal properties. Many studies have been focused on Fe-9Cr steels oxidation behavior between 550°C-700°C.Depending on the oxidizing environment, formation of a triplex (Fe-Cr spinel/magnetite/hematite) or duplex (Fe-Cr spinel/magnetite) oxide scales are reported.. Besides, for long time exposure in steam, the exfoliation of oxide scales can cause serious problems such as tube obstruction and steam turbine erosion. Consequently, this work has been dedicated to study, on the one hand the oxidation kinetics of T91 steel in water vapor environments, and on the other hand, the mechanisms leading to the spallation of the oxide scale. Oxidation tests have been carried out at 550°C in pure water vapor and in Ar/D2O/H2 environments with different hydrogen contents. Based on an analytical resolution, a quantitative modeling has shown that the “available space model” proposed in the literature for duplex oxide scale formation well reproduces both scales growth kinetics and spinel oxide stoichiometry. Then, oxidized samples have been precisely characterized and it turns out that buckling then spalling of the oxide scale is always located in the magnetite layer. Voids observed in the magnetite layer are major initiation sites of decohesion of the outer oxide scale. A mechanism of formation of these voids has been proposed, in accordance with the mechanism of duplex scale formation. The derived model based on the assumption that vacancies accumulate where the iron vacancies flux divergence is maximal gives a good estimation of the location of pores inside the magnetite layer. Then, in order to evaluate stresses involved in the spallation of the oxide scale, deflection tests have been performed in different oxidizing environments at 550°C. Tests carried out in Ar/H2O/(H2) and in CO2 have highlighted the presence of compressive growth strains (around -150 MPa) during isothermal oxidation. Moreover, hydrogen seems to promote the oxide scale embrittlement. A numerical approach has also been developed in order to simulate the deflection test experiment, taking into account different relaxation phenomenon and considering a triplex oxide scale. Hence, stress relaxation by oxide scale viscoplasticity and microfissuration have been quantified.

[pt] A Mecânica do Dano Contínuo é uma ferramenta promissora para a análise de vida residual em componentes de máquinas e de estruturas. Todavia, não é uma tarefa simples a de se obter uma descrição fisica realística, associada a uma descrição matemática correta, do acoplamento entre a deformação e o amolecimento causado pela degradação da microestrutura. No caso de barras metálicas, a deformação plástica cíclica causa um endurecimento junto com uma degradação na estrutura (dano de fadiga). Por outro lado, a degradação da estrutura induz o amolecimento observado na curva tensão de engenharia vs. deformação. Logo, torna-se importante a modelagem do acoplamento entre plasticidade e dano para que se possa prever de maneira adequada o tempo de vida (ciclos), de um componente estrutural. Muitas tentativas feitas para descrever este tipo de comportamento mostraram-se insatisfatórias. O problema matemático é, em geral, mal posto e uma aproximação numérica da solução é incorreta do ponto de vista fisico. Nestes casos, o fenômeno de localização da deformação é malha-dependente. No presente trabalho, propõe-se uma nova teoria de dano para materiais elasto-plásticos que supera este problema. A teoria tem uma forte base termodinâmica e leva em conta o fenômeno de amolecimento. Uma diferença básica em relação a outros modelos consiste no fato de que a variável escalar D, associada ao dano, é considerada não apenas uma variável de estado mas também uma variável cinemática independente, com abordagem semelhante à apresentada nas teorias de contínuo com microestrutura. As possibilidades de utilização da teoria apresentada são verificadas através da comparação de simulações numéricas com resultados experimentais, para solicitações cíclicas uniaxiais, em barras de almnínioestrutural e em barras de aço austenitico AISI 316 L. / [en] Continuum Damage Mechanics is a promising tool for the failure prediction of structural components. Nevertheless, it is not a simple task to do a mathematically correct and physically realistic description of the strain-softcning behavior due to the degradation of the microstructure. In the case of metallic bars, the cyclic plastic deformation induces a strain-hardening and also a degradation of the structure (fatigue damage). In the other hand, the degradation of the structure induces a softening behavior in the engineering stress-strain curve. Hence, it is very important to model the coupling between plasticity and damage in order to perform an adequate lifetime prevision. Many attempts to describe this type of behavior have been unsatisfatory. The mathematical problem is, in general, ill posed and a numerical approximation of the solution is incorrect from the physical point of view. In this cases the phenomenon of strain localization due to strain-soflzening is mesh dependent. In the present work a new Damage theory for elasto-plastic materials that overcome this problem is proposed. The theory has a strong thermodynarnic basis and take into account the softening behavior. One basic difference from the others models is that the scalar variable D related with damage is taken as an independent kinematic variable, similarly as in the theories of continua with microstructure. The effectiveness and usefulness of the theory is checked by comparing numerical simulations of cyclic uniaxial tests in Aluminiun bars and 316L stainless steel bars with experimental results.

<p>Rolling contact fatigue (RCF) is a common cause of failure in tribological machine components such as rolling-element bearings (REBs). Steels selected for RCF applications are subject to various material processes in order to produce martensitic microstructures. An effect of such material processing is the retention of the austenitic phase within the steel microstructure. Retained austenite (RA) transformation in martensitic steels subjected to RCF is a well-established phenomenon. In this investigation, a novel approach is developed to predict martensitic transformations of RA in steels subjected to RCF. A criteria for phase transformations is developed by comparing the required thermodynamic driving force for transformations to the energy dissipation in the microstructure. The method combines principles from phase transformations in solids with a damage mechanics framework to calculate energy availability for transformations. The modeling is then extended to incorporate material alterations as a result of RA transforming within the material. A continuum damage mechanics (CDM) FEM simulation is used to capture material deterioration, phase transformations, and the formation of internal stresses as a result of RCF. Crystal lattice orientation is included to modify energy requirements for RA transformation. Damage laws are modified to consider residual stresses and different components of the stress state as the drivers of energy dissipation. The resulting model is capable of capturing microstructural evolution during RCF.</p> <p>The development and stability of internal stresses caused by RA transformation in bearing steel material was experimentally investigated. Specimens of 8620 case carburized steel were subjected to torsional fatigue at specific stress levels for a prescribed number of cycles. X-ray diffraction techniques were used to measure residual stress and RA volume fraction as a function of depth in the material. A model is set forth to predict compressive residual stress in the material as a function of RA transformation and material relaxation. Modeling results are corroborated with experimental data. In addition, varying levels of retained austenite (RA) were achieved through varying undercooling severity in uniformly treated case carburized 8620 steel. Specimens were characterized via XRD and EBSD techniques to determine RA volume fraction and material characteristics prior to rolling contact fatigue (RCF). Higher RA volume fractions did not lead to improvement in RCF lives. XRD measurements after RCF testing indicated that little RA decomposition had occurred during RCF. The previously established RCF simulations were modified to investigate the effects of RA stability on RCF. The results obtained from the CDM FEM captured similar behavior observed in the experimental results. Utilizing the developed model, a parametric study was undertaken to examine the effects of RA quantity, RA stability, and applied pressure on RCF performance. The study demonstrates that the energy requirements to transform the RA phase is critical to RCF performance.</p>

<p dir="ltr">Rolling contact fatigue (RCF) is one of the leading causes of failure in critical tribological components such as rolling element bearings (REBs), gears, cam and followers, etc. This is especially paramount for advanced aerospace applications where REB components need to operate for billions of RCF cycles before routine maintenance or inspection is performed. The rolling motion between the rolling elements and raceway produces RCF, wherein a complex, non-proportional, alternating contract stress is applied over a small material volume. Moreover, the highly localized stress occurs on the same length scale as microstructural features such as carbides, inclusions, grain size, hardness gradients from carburization, surface roughness, thereby amplifying their effect on fatigue performance. Therefore, the objective of this dissertation is to investigate critical microstructural modifications and their effects on RCF performance via experiments and computational modeling.</p><p dir="ltr">Initially, an investigation was undertaken to investigate surface roughness effects on RCF. The surface roughness of various REBs was measured through optical surface profilometry and used to construct rough surface pressure distributions, which were then used in a continuum damage mechanics (CDM) finite element (FE) framework. The results demonstrated that life is reduced as lambda ratio decreases. It was also observed that a 2-parameter Weibull cumulative distribution function can describe the relationship between the near surface orthogonal shear stress concentration and ratio of surface failures.</p><p dir="ltr">Next, the enhancement to RCF life from grain size refinement of through hardened bearing steels was studied. To capture the effects of grain refinement, torsion stress-life data of various grain size were used in the RCF model. A predictive life equation for different grain sizes was constructed based on the exponential trend observed between grain size and life from the simulation data. The life equation was then used to calculate the quotient of RCF at two different grain sizes. This quotient was defined as the life improvement ratio and it was observed that this investigation’s ratios compared well with existing life improvement ratios from RCF experiments.</p><p dir="ltr">Hardness gradient is a common microstructural modification to improve RCF life of tribo-components. Variation of hardness gradients is prevalent in case hardened (i.e. case carburized) bearing materials. Therefore, the CDM-FE RCF model was modified to investigate the effects of various hardness gradient types and depths on fatigue life improvement. The simulation results enabled the identification of potentially optimal gradients aimed to mitigate manufacturing challenges and provided the foundation for the construction of a general fatigue life equation.</p><p dir="ltr">A fundamental study to understand the impact various common RCF failure criteria have on RCF life estimation was then conducted using computational modeling. To capture the variation of a material’s resistance to fatigue, the critical CDM damage parameters were assumed to follow a probabilistic distribution instead of a singular value. The CDM-FE model was modified to consider the shear reversal, the octahedral shear stress, the maximum shear stress, the Fatemi-Socie criteria, and the Dang Van multi-axial fatigue parameter as failure criteria. Simulation life results revealed that the CDM-FE model with shear reversal and Fatemi-Socie criteria best match empirical predictions from well-established RCF life theory. Notably, the Fatemi-Socie exhibited the best agreement over all operating conditions.</p><p dir="ltr">The next investigation focused on the cleanliness of aerospace-quality bearing steels. Torsion fatigue experiments established the stress-life (S-N) relation for three common aerospace quality bearing steels. The S-N data was later used to calibrate the RCF model’s damage equation, which considered the Fatemi-Socie criteria following conclusions from a previous investigation. Simulation results were observed to corroborate well with RCF experiments that were conducted for all three materials, while noting the simulations offered a significant time saving. As a result, a subsequent investigation focused on establishing the stress-life relationship for one of the aerospace quality bearing steels through a combined experimental and analytical approach. Good corroboration was observed between simulations and experiments at three contact pressures. This finding is particularly significant as it strengthens the reliability of computational RCF model as an efficient means to assess the RCF performance of bearing materials.</p><p dir="ltr">Furthermore, the detailed investigation on RCF performance of each critical microstructural modifications and their respective effect greatly improves the state-of-the-art. The findings emanating from the various investigations offer informed fatigue design recommendations that aid in the selection of rolling element bearings for critical tribological and aerospace applications.</p>

No description available.

<p>Carbon-fiber reinforced polymers (CFRPs) are often used in the aerospace and automotive  industries for their high strength-to-weight ratios and corrosion resistance. A new class of  composites – known as Prepreg Platelet Molded Composites (PPMCs) – offers further  advantageous such as high forming capabilities with modest compromises in strength and stiffness.  One such property of PPMCs that have garnered interest over the years is their apparent  insensitivity to notches. Previous studies have researched the effect of specimen size and platelet  length on its effect on the open-hole performance of PPMCs. Research however has focused on  thinner samples with smaller hole sizes and neglected thicker samples with larger holes.  Additionally, while platelet sizes have been investigated for unnotched samples, platelet width on  notched samples is less clear from the literature. The present thesis offers some investigations to  aid in filling this knowledge gap. </p> <p><br></p> <p>The objective of this work is to study two parameters that could influence the performance of PPMCs under open-hole tension. First, thick (7.6 mm) specimens are subjected to large hole  sizes (up to 19.08 mm) to investigate their behavior in comparison to the smaller sample sizes  previously investigated in the literature. Through-thickness DIC measurements are taken to  investigate strain gradients in these thicker specimens. Second, various platelet widths are tested  to research their influence on notch insensitivity of open-hole tensile PPMC specimens. Lastly, a  finite element based continuum damage mechanics model is implemented to predict macro-level  structural properties using only material properties of the parent prepreg. It is found that large holes  in thick samples increase notch sensitivity compared to other samples of similar diameter-to-width  ratios. Narrower platelets were found to produce higher unnotched strengths, while wider platelets  offered more notch insensitivity. Lastly, the finite element model developed was found to  qualitatively replicate features and failure modes that are exhibited by PPMCs, though strength  predictions became inaccurate at larger specimen sizes. Recommendations are made for future  work on the basis of these findings.   </p>

This thesis deals with the design process of a tensile test specimen geometry with the intention that the specimen will show failure in a shear band during a tensile test. The triggered shear band is linked to a characteristic length lc, which is required for a nonlocal approach to continuum damage mechanics that predicts the life expectancy of a combustion chamber independent of the FEM mesh size. To predict if a specimen will fail in the preferred manner, numerical simulations have been performed and were analysed with the newly defined failure-in-shear-band indicator. Ductile failure modes and the fracture process depend strongly on the stress state. Therefore the indicator is formulated as a function of the Lode parameter and the stress triaxiality. Several double-notched bar specimens have been designed with different notch radii and notch depths. The failure-in-shear-band indicator implies promising values for a small notch radius and larger notch depth. Tensile tests were performed on four specimens which successfully failed in a shear band. Furthermore, a first statement on the magnitude of the characteristic length of CuAgZr is given. / Detta arbete behandlar designprocessen för en dragprovstavskonfiguration framtagen för att uppvisa brott i ett skjuvband under draghållfasthetsprovning. Initiering av skjuvbandet är kopplat till en karakteristisk längd lc, som krävs för att kunna använda en icke lokal metod för att analysera kontinuerlig skademekanik oberoende av maskstorleken i den numeriska modellen. Metoden är utvecklad för att kunna uppskatta den förväntade livslängden för en förbränningskammare. För att förutsäga om ett provobjekt kommer att gå sönder på det sätt som önskashar datorsimuleringar utförts och analyserats med den nyligen definierade indikatorn för skjuvbrott. Plastisk deformation, och så småningom brott, är starkt beroende avspänningstillståndet. Indikatorn är därför formulerad som en funktion av en s.k. Lode parametern och det treaxliga spänningstillståndet. Flera provstavsgeometrier har utformats med dubbla brottanvisningar vars radie och storlek varierats. Indikatorn för skjuvbrott ger lovande värden för små radier och ett större anvisningsdjup. Draghållfasthetsprovning utfördes på fyra provkroppar som uppvisade önskat skjuvbrott. Dessutom erhölls en första indikation om storleken på den karakteristiska längden för CuAgZr.

Styrene-Butadiene Rubber (SBR) is a form of rubber compound that is widely used in the tire industry. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging. In this study, a material aging model that can predict the hyperplastic response of styrene-butadiene rubber (SBR) was mathematically developed using an integrated testing and continuum damage model framework. Coupling between the mechanical changes of SBR to the change in the chemical properties, specifically crosslink density (CLD) was also investigated. SBR dogbone shaped samples were accelerated aged in an aging oven at various temperatures and aging periods. Subsequently, hyperelastic tests were conducted to obtain the high strain response taking the 'Mullin's effect' into consideration. These responses were calibrated to different hyperelastic material models and the Arruda-Boyce model was chosen, due to its stable behavior and optimal fit. An aging evolution function was developed based on the variation in the model coefficients. This damage model is able to predict the hyperelastic response of SBR as it ages. A user material subroutine (UMAT) was also implemented in Abaqus based on the obtained aging evolution function to predict the stress response of SBR for varied applications. Additionally, to couple the chemical variations with the hyperelastic response, the rubber structure and composition was probed using Fourier-transform infrared spectroscopy (FTIR). The degradation of additives and SBR polymer chains were analyzed microscopically to explain the impact on the macroscopic properties. This study helps to correlate the change in crosslink density to ameliorate mechanical properties, such as strain at break, modulus, and stiffness. The effects of aging on the viscoelastic properties of SBR were also studied. Dynamic Mechanical Analysis (DMA) was used to characterize the viscoelastic response. Master curves of storage and loss modulus were generated using the time-temperature superposition principle (TTSP). The friction coefficient was estimated from the storage and loss modulus using a simplified form of the Persson equation [1]. CLD was also estimated from DMA data. Wear experiments were conducted on the Dynamic Friction Tester (DFT) for various aging conditions. The estimated friction coefficient was compared to the one from the experiments. Archard's law was used to correlate the frictional energy to the volume loss during wear experiments. Correlation between the wear and the viscoelastic properties of SBR is also studied. Finally, the lifetime of SBR for various aging temperatures is predicted using various models. [1] M. Ciavarella, "A Simplified Version of Persson's Multiscale Theory for Rubber Friction Due to Viscoelastic Losses," J. Tribol., vol. 140, no. 1, 2018, doi: 10.1115/1.4036917. / Master of Science / Elastomers or rubbers are they are generally referred to are an indispensable part of human life. They are made up of long-chain polymer units linked to one other through crosslinks. This peculiar morphology of rubbers is what gives them their unique characteristics. There are as many as 40,000 known products that use some form of rubber as the primary raw material. Apart, from this, they are also widely used in aviation and aerospace, automobiles, dampers and absorbers, civil engineering, electronics, medical, toys, clothing, sports, footwear, and so on. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. Over the last couple of years, it has also played a pivotal role in personal protective equipment (PPE) and masks worn by billions of people and frontline workers all over the globe. The fact that rubber is included in the EU's list of critical raw materials highlights its global importance. However, over the past several years, the rubber supply has dwindled. COVID-19 also caused disruptions in the supply chain of rubber. As the effects of COVID-19 are fading, there has been a spike in the demand for rubber; the primary reason being automotive tires! Even though substantial research is being conducted to try and replace rubber as a raw material with synthetic alternatives such as polyurethane, the excellent blend of damping, friction and wear characteristics, heat dissipation provided by natural rubber cannot be replicated by any of these laboratory compounds. Hence, at this time, there is an increased need to conserve and improve the longevity of rubber compounds. Styrene-Butadiene Rubber (SBR) is a form of a rubber compound that is widely used in the tire industry. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging. The present study aims to model changes in the hyperelastic (large stretching) behavior of SBR using a Continuum Damage Mechanics (CDM) approach. This mathematical model is translated into ABAQUS, a finite element analysis software to study the mechanical response of components with various geometries and loading conditions. Secondly, the effects of aging on the viscoelastic behavior of SBR is studied. This helps us to estimate the cross-link density (CLD) as well as the friction coefficient of SBR as it is aged. The impact of aging on the wear and friction properties of SBR is studied experimentally. Finally, using various mechanical and chemical models the lifetime of SBR is estimated for various aging temperatures. Thus, the end goal of the study is to drive the development of new rubber compounds that will help improve the service life of rubbers and also have a positive impact on the environment.

Lors de leur construction, les structures massives (ex. les enceintes de confinement des centrales nucléaires) sont soumises à des déformations dues à l’hydratation du béton. En effet, d’une part la réaction chimique du ciment avec l’eau est exothermique et thermo activée, ce qui induit des déformations de dilatation puis de contraction. D’autre part une dépression capillaire étant créée par la consommation d’eau due à l’hydratation du ciment, des déformations de contraction se produisent. Lorsqu’elles sont empêchées par la partie de la structure déjà construite, ces déformations volumiques induisent des contraintes de compression puis de traction pouvant causer une fissuration traversante augmentant alors sensiblement la perméabilité du béton. Mon travail de thèse a consisté à caractériser le comportement au jeune âge du matériau que l’on utilise (basé sur la formulation utilisée lors de la construction d’une enceinte) puis à mettre au point un essai permettant d’étudier la fissuration d’une éprouvette de béton soumise à des déformations empêchées lors de son durcissement. Ce nouvel essai est en fait une évolution de l’essai à l’anneau de retrait gêné permettant de prendre également en compte les déformations d’origine thermique. Les essais de caractérisation concernent essentiellement, d’un point de vue macroscopique, le retrait (endogène et thermique), le fluage (propre et thermique transitoire) et l’évolution des caractéristiques mécaniques (résistance à la compression, à la traction et module d’élasticité). La campagne d’essais réalisée avec ce nouveau dispositif, appelé essai à l’anneau thermique actif, et l’analyse numérique de ces essais par des simulations aux éléments finis (basées sur un modèle viscoélastique endommageable introduisant un couplage entre le fluage et l’endommagement) a permis d’évaluer ce couplage, d’identifier la diminution de résistance en traction due à une reprise de bétonnage et de quantifier l’effet des armatures sur le comportement du béton. Des mesures de transfert d’air sec à travers une éprouvette fissurée ont également été réalisées sur ce dispositif. Finalement, des simulations numériques d’ouvrages massifs ont mis en exergue l’influence des conditions de restriction du retrait sur le faciès d’endommagement et l’influence du couplage fluage fissuration sur le calcul des ouvertures de fissures. / At early-age, massive concrete structures (ex. nuclear power plant) are submitted to strains due to the hydration reaction. If they are restrained, crossing cracks can occurs. This cracking may increase significantly the concrete wall permeability. The objectives of this work was to characterize the early age concrete behavior (thermal and endogenous shrinkage, basic and thermal transient creep, mechanical characteristic evolution) as well as develop a new device to study the early age cracking of a concrete structure submitted to restrained shrinkage.The experimental campaign achieved with this new device (called thermal active ring test) and the numerical analysis of the test thanks to finite element simulations allows us to evaluate the coupling betwwen creep and damage, to identify the tensile strength decrease due to construction joints and to quantify the effect of reinforcement on the concrete behaviour. Moreover, with this device, permeability measurements have been performed on a cracked specimen. Finally, numerical simulations of massive structures highlight the influence of boundary conditions for restrained shrinkage and the influence of the coupling between creep and damage on the damage pattern.

A Mecânica do Dano Contínuo (MDC) teve importante desenvolvimento desde os trabalhos iniciais de Kachanov e Rabotnov sendo uma ferramenta prática para considerar processos de danificação em materiais e estruturas em nível de contínuo macroscópico. Neste trabalho, apresenta-se uma aplicação da teoria do dano anisotrópico baseada em teorias desenvolvidas a partir dos trabalhos de Murakami. Nas formulações apresentadas, o tensor de dano de quarta ordem M (que relaciona tensões aplicadas e tensões efetivas) é determinado com base no tensor Ω (densidade de área tridimensional devida ao dano) que, por sua vez, pode ser determinado com base em dados experimentais. São propostas três formulações teóricas que são transformadas em formulações incrementais e incorporadas em um programa computacional de elementos finitos (para placas e cascas laminadas em material compósito) que considera efeitos geométricos não-lineares. A primeira e segunda formulação são casos particulares da terceira formulação que é um modelo termodinâmico tridimensional. As forças termodinâmicas associadas à evolução do tensor de dano são deduzidas a partir da expressão da dissipação intrínseca. Um critério fenomenológico para o dano é proposto. Em consistência com a positividade da dissipação intrínseca é adotada uma regra de normalidade para a evolução da força termodinâmica. Também é proposta, baseada em dados experimentais, uma lei para o encruamento associada ao processo de dano. Os modelos são validados comparando resultados numéricos a soluções analíticas ou a resultados experimentais. A formulação viscoelástica é definida do dano elástico e por componentes viscoelásticas representadas no formato de variáveis de estado e, posteriormente, validadas através de resultados experimentais. / The Continuum Damage Mechanics (CDM) had important development since the initial works of Kachanov and Rabotnov and constitutes now a practical tool to account for macroscopic damage in materials and structures. In this work, an application of an anisotropic damage theory based in Murakami theory is presented. In the formulations presented here, the fourth order damage tensor M (that relates Cauchy stress and effective stress) is determined on the basis of the tensor Ω (damaged three-dimensional area density) that, can be determined through experimental data. The three theoretical formulations presented here are transformed into incremental formulations and implemented in a finite element program (for plates and shell structures in composite material) taking account of geometrically non-linear effects. The first and second formulations are particular cases of the third formularization that is a tridimensional model for continuous damage formulated. The thermodynamic force associated with the evolution of the damage tensor is deduced from the expression of the intrinsic dissipation. A phenomenological criterion for damage yielding is proposed. In consistence with the positivity of the intrinsic dissipation, a normality rule is adopted for the evolution of the thermodynamic force. In addition, a hardening law associated with the damage process is identified from available experiment results. The models are validated by comparison with closed-form solutions or with experiment results. The viscoelastic formulation is defined through damage elastic and viscous components and set in a state variables format and then validated by comparison with experimental creep tests.