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.

This thesis presents the results of an experimental investigation into the behavior of a pultruded E-glass/polyester fiber reinforced polymer (FRP) composite under sustained loads at elevated temperatures in the range of those that might be seen in service. This investigation involved compression creep tests of material coupons performed at a constant stress level of 33% of ultimate strength and three temperatures levels; 23.3°C (74°F), 37.7°F (100°F), and 54.4°C (130°F). The results of these experiments were used in conjunction with the Findley power law and the Time- Temperature Superposition Principle (TTSP) to formulate a predictive curve for the longterm creep behavior of these pultruded sections. Further experiments were performed to investigate the effects of thermal cycles in order to better simulate service conditions.

The barrier properties and long term strength retention of polymers are of significant importance in a number of applications. Enhanced lifetime food packaging, substrates for OLED based flexible displays and long duration scientific balloons are among them. Higher material requirements in these applications drive the need for an accurate measurement system. Therefore, a new system was engineered with enhanced sensitivity and accuracy. Permeability of polymers is affected by permeant solubility and diffusion. One effort to decrease diffusion rates is via increasing the transport path length. We explore this through dispersion of layered silicates into polymers. Layered silicates with effective aspect ratio of 1000:1 have shown promise in improving the barrier and mechanical properties of polymers. The surface of these inorganic silicates was modified with surfactants to improve the interaction with organic polymers. The micro and nanoscale dispersion of the layered silicates was probed using optical and transmission microscopy as well as x-ray diffraction. Thermal transitions were analyzed using differential scanning calorimetry. Mechanical and permeability measurements were correlated to the dispersion and increased density. The essential structure-property relationships were established by comparing semicrystalline and amorphous polymers. Semicrystalline polymers selected were nylon-6 and polyethylene terephthalate. The amorphous polymer was polyethylene terphthalate-glycol. Densification due to the layered silicate in both semicrystalline and amorphous polymers was associated with significant impact on barrier and long term creep behavior. The inferences were confirmed by investigating a semi-crystalline polymer - polyethylene - above and below the glass transition. The results show that the layered silicate influences the amorphous segments in polymers and barrier properties are affected by synergistic influences of densification and uniform dispersion of the layered silicates.

A novel microstructure-sensitive extreme value probabilistic framework is introduced to evaluate material performance/variability for damage evolution processes (e.g., fatigue, fracture, creep). This framework employs newly developed extreme value marked correlation functions (EVMCF) to identify the coupled microstructure attributes (e.g., phase/grain size, grain orientation, grain misorientation) that have the greatest statistical relevance to the extreme value response variables (e.g., stress, elastic/plastic strain) that describe the damage evolution processes of interest. This is an improvement on previous approaches that account for distributed extreme value response variables that describe the damage evolution process of interest based only on the extreme value distributions of a single microstructure attribute; previous approaches have given no consideration of how coupled microstructure attributes affect the distributions of extreme value response. This framework also utilizes computational modeling techniques to identify correlations between microstructure attributes that significantly raise or lower the magnitudes of the damage response variables of interest through the simulation of multiple statistical volume elements (SVE). Each SVE for a given response is constructed to be a statistical sample of the entire microstructure ensemble (i.e., bulk material); therefore, the response of interest in each SVE is not expected to be the same. This is in contrast to computational simulation of a single representative volume element (RVE), which often is untenably large for response variables dependent on the extreme value microstructure attributes. This framework has been demonstrated in the context of characterizing microstructure-sensitive high cycle fatigue (HCF) variability due to the processes of fatigue crack formation (nucleation and microstructurally small crack growth) in polycrystalline metallic alloys. Specifically, the framework is exercised to estimate the local driving forces for fatigue crack formation, to validate these with limited existing experiments, and to explore how the extreme value probabilities of certain fatigue indicator parameters (FIPs) affect overall variability in fatigue life in the HCF regime. Various FIPs have been introduced and used previously as a means to quantify the potential for fatigue crack formation based on experimentally observed mechanisms. Distributions of the extreme value FIPs are calculated for multiple SVEs simulated via the FEM with crystal plasticity constitutive relations. By using crystal plasticity relations, the FIPs can be computed based on the cyclic plastic strain on the scale of the individual grains. These simulated SVEs are instantiated such that they are statistically similar to real microstructures in terms of the crystallographic microstructure attributes that are hypothesized to have the most influence on the extreme value HCF response. The polycrystalline alloys considered here include the Ni-base superalloy IN100 and the Ti alloy Ti-6Al-4V. In applying this framework to study the microstructure dependent variability of HCF in these alloys, the extreme value distributions of the FIPs and associated extreme value marked correlations of crystallographic microstructure attributes are characterized. This information can then be used to rank order multiple variants of the microstructure for a specific material system for relative HCF performance or to design new microstructures hypothesized to exhibit improved performance. This framework enables limiting the (presently) large number of experiments required to characterize scatter in HCF and lends quantitative support to designing improved, fatigue-resistant materials and accelerating insertion of modified and new materials into service.

L'assiette de fibrage utilisée pour la production de la laine de verre d'isolation subit à haute température de nombreuses contraintes (chimiques, mécaniques et thermiques). La ruine des assiettes peut être due à l'oxydation par les gaz chauds, à la corrosion par le verre fondu ou à la déformation par fluage. La première partie des travaux de thèse a porté sur l'amélioration des propriétés mécaniques des alliages pour le fibrage à 1200°C. Dans un premier temps, la teneur en chrome au coeur d'alliages à base de cobalt a été réduite afin d'augmenter la réfractarité en conservant la microstructure. Cependant, cette réduction a dégradé le comportement en oxydation de ces alliages. Afin de conserver un bon niveau en oxydation, un enrichissement en chrome de la surface a été réalisé à l'aide de la technique de pack-cémentation. Le comportement en fluage de ces alliages a été maintenu à un niveau correct. Dans un second temps, de nouveaux systèmes métallurgiques ont été explorés, basés sur le nickel et le fer, et ont été renforcés mécaniquement par des éléments lourds en solution solide ou par des précipités intermétalliques. La réfractarité et le comportement en oxydation se sont révélés intéressants mais la tenue en fluage a été décevante. La seconde partie de ces travaux a consisté à améliorer le comportement en oxydation de l'alliage utilisé lors du fibrage à 1000-1050°C. Deux voies ont été explorées : un enrichissement en chrome de la surface de l'alliage par pack-cémentation ou l'addition d'un élément réactif, l'yttrium. La seconde solution a apporté des résultats intéressants avec une amélioration significative du comportement en oxydation cyclique / The spinner used to product glass wool for thermal building insulation undergoes several stresses (chemical, mechanical and thermal) at high temperature. The lifetime of the spinner is limited by oxidation by hot gases, corrosion by molten glass or creep deformation. The first part of this thesis has concerned the improvement of the mechanical properties of the alloys for the fiberizing at 1200°C. First, the bulk chromium content of the usual cobalt-base alloys has been decreased to increase their refractoriness at constant microstructure. However, this reduction has degraded the oxidation behavior of these alloys. To keep a good oxidation behavior, a surface chromium enrichment of the Cr-impoverished alloys is achieved by pack-cementation technique. The creep behavior of these alloys is kept at a good level. Secondly, several new metallurgical systems were explored: alloys based on nickel and iron and reinforced by heavy elements in solid solution or by precipitates inter metallic particles. The refractoriness and the oxidation behavior appeared to be interesting but the creep deformation was disappointing. The second part of this work has concerned the improvement of the oxidation and corrosion properties of the alloys for the process at 1000-1050°C. Two ways have been explored: a chromium enrichment on the sub-surface or an addition of a reactive element, as yttrium. The second solution has given interesting results with a significant improvement of the oxidation behavior, and particularly in cyclic oxidation