Ce travail est consacré à l'étude de l'ingénierie des joints de grains dans les superalliages à base de Nickel pour disques de turbines aéronautiques. L'ingénierie des joints de grains a montré son efficacité dans l'amélioration des propriétés en fatigue et en fluage dans certains alliages de cuivre et de nickel. Cette amélioration est obtenue en réalisant une succession de déformations à température ambiante séparées par des traitement thermiques. Ce traitement a pour objectif de modifier la distribution du réseau de joints de grains. L'objectif de l'étude est d'adapter ces traitements au procédé du forgeage à haute température des superalliages. Une étude expérimentale est menée pour mettre en évidence l'influence des paramètres de forgeage sur les caractéristiques du réseau de joints de grains. La caractérisation de ce réseau est faite grâce à la réalisation de cartographies d'orientations cristallographiques par EBSD. Une méthode de caractérisation innovante basée sur la discrétisation des cartographies en réseaux de points triples est proposée. Cette méthode permet d'analyser la connectivité du réseau de joints de grains en vue d'une corrélation avec les propriétés mécaniques.

Cette thèse porte sur l'évolution de la microstructure de superalliages à base de Nickel au cours de traitements thermomécaniques, avec une attention particulière portée sur la taille de grains et la quantité de macles thermiques. Ce travail a été réalisé dans l'optique d'évaluer la possibilité d'optimiser les propriétés de service par le principe de l'ingénierie de joints de grains. Les matériaux étudiés sont principalement l'alliage PER®72, et de manière exploratoire l'alliage N19. Ces deux matériaux appartiennent à la famille des superalliages à base de nickel à durcissement structural élaborés par voie conventionnelle (coulé et forgé) et par métallurgie des poudres respectivement. Deux modes de déformation, la torsion et la compression, ont été testés. Nous avons démontré que le maclage thermique est favorisé par les grandes vitesses de migration des joints de grains, essentiellement induites par (1) leur courbure qui diminue lorsque la taille de grains augmente et (2) les gradients de densité de dislocations produits par les faibles déformations appliquées à grande vitesse qui peuvent maintenir des vitesses de migration élevées même lorsque la taille de grains augmente. La force motrice associée à la courbure est en outre sensible à la distribution de taille de grains, qui s'est avéré être un paramètre microstructural important. La force motrice associée aux gradients de densité de dislocations peut par ailleurs permettre le franchissement des précipités primaires malgré la force de freinage de Zener qui s'exerce sur les joints de grains, ceci au cours de la déformation comme au cours d'un recuit subsolvus. L'occurrence ou la non-occurrence de la recristallisation dynamique au cours de la phase de déformation s'est ainsi avérée être une condition décisive pour l'évolution de la microstructure au cours d'un recuit ultérieur dans le domaine sub- ou super-solvus. Les conditions thermomécaniques de mise en forme permettent donc de moduler la taille de grains et la quantité de macles, même à l'issue d'un traitement supersolvus final.

La fatigue des joints brasés de composants électroniques est étudiée dans le cas des environnements sévères rencontrés dans les applications aéronautiques, spatiales et militaires. L'objectif de la thèse est de proposer un modèle de fatigue générique adapté aux joints brasés de type Sn-Ag-Cu soumis à des chargements thermomécaniques et vibratoires. Une étude bibliographique critique la pertinence des modèles et principalement des procédures expérimentales d'identification des paramètres des modèles en loi puissance. La modélisation continue de l'endommagement est également abordée car elle est plus performante lorsqu'elle est appliquée à des chargements complexes combinés. La microstructure triphasée des alliages Sn-Ag-Cu est présentée ainsi que ses différents paramètres microstructuraux assez dispersés selon le mode d'assemblage et la composition des alliages. Le deuxième chapitre présente une étude expérimentale et numérique sur la fatigue oligocyclique de l'alliage Sn- 3,0Ag-0,5Cu en cisaillement. Une loi de comportement viscoplastique à écrouissage est établie ainsi qu'un modèle d'endommagement continu isotherme. Des résultats en fatigue à 25 °C et à 75 °C mettent en évidence l'influence de la tempéra ture sur le comportement et la fatigue de l'alliage. Une étude du comportement en fatigue vibratoire des assemblages est ensuite menée au moyen de cartes de test. Les temps à défaillance des assemblages donnent des tendances en fatigue pour des composants de différentes technologies (BGA, PLCC...). Les modes de rupture des brasures sans plomb semblent similaires à ceux des alliages standards Sn-Pb. Enfin une méthodologie de simulation par éléments finis des cartes et assemblages électroniques en vibration est proposée. Celle-ci permet sur un exemple concret de calculer un indicateur du dommage accumulé dans les joints lors d'un chargement vibratoire aléatoire de la carte. Les deux approches, en fatigue oligocyclique et en vibration, permettent d'initier la proposition d'un modèle générique adapté aux sollicitations combinées pour les joints brasés.

Le travail présenté dans ce mémoire est consacré à l'application des méthodes de changement d'échelles pour la modélisation et le calcul des ouvrages construits en milieux rocheux fracturés. Après avoir présenté les deux approches usuellement adoptées, l'approche discontinue et l'approche continue, et leurs limites en termes de pertinence, dans une première partie essentiellement bibliographique (chap.1), nous avons formulé à partir des hypothèses sur les caractéristiques géométriques et mécaniques des deux constituants (matrice et joints), le comportement macroscopique en élasto plasticité du milieu homogénéisé aussi bien dans le cas périodique que dans le cas des milieux aléatoirement hétérogènes (chap.2). Les deux traits importants de ce comportement sont l'absence d'écruissage et l'absence de terme rendant compte de l'interaction entre les joints se coupant. Afin de valider le code d'éléments finis en déplacement que nous avons mis au point (chap.4), nous avons construit une solution analytique (convergence du tunnel circulaire) pour servir de référence (chap.3). Pour terminer, nous avons procédé à l'application du code numérique construit sur deux problèmes typiques de la Mécanique des Roches (chap.5).

The Interstate Highway System plays a vital role in our economic development by providing a continuous corridor for transporting goods and services. Currently, there is a need for repair and expansion of the existing highways, which include all bridges along its path. Because of the high demand for the highway system, repair and expansion must occur rapidly and efficiently. In recent years, precast bridge deck systems have become an efficient way to reduce construction time during repair. This thesis presents the experimental research of the behavior of the U-Bar joint detail used in precast bridge deck systems. This detail consists of staggered reinforcement extending beyond the precast deck portion into the joint. Six specimens utilizing the U-Bar detail were constructed and tested. Three specimens were tested in flexure to simulate the forces applied in a longitudinal deck joint, while three specimens were tested in pure tension to simulate the forces experienced in a transverse deck joint located over an interior pier. A tight 180° bend at 3db was desired in order to minimize the thickness of the deck. To achieve this tight bend, deformed wire reinforcement was chosen for the U-Bar detail due to the favorable material properties of deformed wire reinforcement. The purpose of the testing was to determine if the joint details could generate a precast deck system that could emulate the monolithic cast-in-place deck systems already in use. For monolithic behavior in a precast deck system, the joints must be able transfer shear, tension and moments. In this research, the joint overlap length was the most dominant variable, and should not be less than 152.4 mm (6”). The precast bridge deck joint should consist of high strength concrete with f’c of at least 68.9 MPa (10 ksi). The longitudinal reinforcement spacing should be no greater than 152.4 mm (6”).

Dans cette thèse on s'est intéressé aux problèmes tridimensionnels de Laplace et de stokes dans des domaines axisymétriques. Ces problèmes sont réduits, sans approximation et par des développements en coefficients de Fourier en une famille dénombrable de problèmes bidimensionnels. Les domaines qu'on a considéré présentent des singularités géométriques et sont décomposés de façons non nécessairement conformes. Les non conformités sur les interfaces entre les sous domaines sont traités par la méthode des joints. La méthode de base de discrétisation est la méthode spectrale. On a montre alors des résultats d'approximation optimaux, proches de ceux trouves lors de l'approximation conformes avec des contraintes de continuités sur les interfaces. Ceci prouve encore une fois l'efficacité de la méthode des joints.

The failure of longitudinal construction joints is one of the critical factors causing accelerated pavement deterioration. Poor-quality longitudinal construction joints are often characterized by a difference in elevation between adjacent lanes or by unraveling of the hot mix asphalt (HMA). Current wave-based non-destructive testing (NDT) methods are efficient and economical for the evaluation of material properties. In this thesis, an innovative surface wave testing technique for condition assessment of longitudinal construction joints in asphalt pavements is presented. This method enables reduction of the number of cores required for large-strain testing and provision of a more uniform quality assessment of longitudinal joints as well as the relative condition of the asphalt pavements. The technical development is based on a theoretical study of the wave attenuation mechanisms and on the reported deficiencies in current seismic wave-based methods. Traditionally, the use of ultrasonic testing to determine small-strain elastic property for asphalt concrete was uniquely based on the measurement of wave velocity. However, isolated use of wave velocity does not provide complete information of the materials strength because of the different variables that affect the strength-velocity relationship. Therefore, it is necessary to complement velocity data with independent information such as the change in attenuation and frequency content of the propagating pulse. The existing deficiencies in current seismic wave-based methods were addressed to improve reliability, accuracy and consistency for asphalt concrete material characterization in the laboratory and in the field. Refined and improved signal processing techniques were used to overcome the shortcomings in the existing wave-based methods that contribute to uncertainties in the interpretation of test results. To capture more information from a wave, the signal was analyzed in the time and frequency domains. The basic analyses included a simple method such as peak to peak amplitude of the first cycle of arriving wave, and complex methods such as maximum magnitude and area of corresponding frequency spectrum through Fourier transform. A novel approach based on wavelet transform of the signal was presented, which provides an alternative method to determine wave characteristics. Material characterization tests (experiment Phase I) were carried out to study the relationship between the wave characteristics obtained from UPV test and the quality of the asphalt concrete specimens prepared in the laboratory. The specimens were identical in terms of aggregate and asphalt binder ratio, but varying in volumetric properties produced by different compaction efforts. The specimen quality was determined using two methods: 1) the traditional method using density measurement and 2) an innovative approach using dynamic modulus which is recommended by the Federal Highway Administration (FHWA) for use as a fundamental material property for characterizing Superpave mixes (Witczak, et al., 2002; Bonaquist et al., 2003; Christensen et al., 2004). The measured wave characteristics showed excellent correlations (R2 > 0.9) with the fundamental properties of the mix. The results revealed that the wave amplitude parameters as the condition index have the potential to provide a reliable assessment of the quality of HMA mixtures. The finding is very critical to moving the technology forward in the right direction, and form an important basis for the experiment Phase II. Experiment Phase II investigated the feasibility and effectiveness of using the wave characteristics identified in experiment Phase I to assess the condition of longitudinal construction joints. Particular attention was given to examining the sensitivity of the wave-based technique to different types of construction joints. For this purpose, three types of construction joints (good, fair and poor) were fabricated in the laboratory and identified using the wave-based technique. The research was intended to develop a suitable test procedure for condition assessment the longitudinal joints in asphalt pavement in the field. The data from experiment Phase II revealed that the wave-based technique enables assessment of not only the quality of different types of longitudinal construction joints but also the relative condition of asphalt pavements.

The failure of longitudinal construction joints is one of the critical factors causing accelerated pavement deterioration. Poor-quality longitudinal construction joints are often characterized by a difference in elevation between adjacent lanes or by unraveling of the hot mix asphalt (HMA). Current wave-based non-destructive testing (NDT) methods are efficient and economical for the evaluation of material properties. In this thesis, an innovative surface wave testing technique for condition assessment of longitudinal construction joints in asphalt pavements is presented. This method enables reduction of the number of cores required for large-strain testing and provision of a more uniform quality assessment of longitudinal joints as well as the relative condition of the asphalt pavements. The technical development is based on a theoretical study of the wave attenuation mechanisms and on the reported deficiencies in current seismic wave-based methods. Traditionally, the use of ultrasonic testing to determine small-strain elastic property for asphalt concrete was uniquely based on the measurement of wave velocity. However, isolated use of wave velocity does not provide complete information of the materials strength because of the different variables that affect the strength-velocity relationship. Therefore, it is necessary to complement velocity data with independent information such as the change in attenuation and frequency content of the propagating pulse. The existing deficiencies in current seismic wave-based methods were addressed to improve reliability, accuracy and consistency for asphalt concrete material characterization in the laboratory and in the field. Refined and improved signal processing techniques were used to overcome the shortcomings in the existing wave-based methods that contribute to uncertainties in the interpretation of test results. To capture more information from a wave, the signal was analyzed in the time and frequency domains. The basic analyses included a simple method such as peak to peak amplitude of the first cycle of arriving wave, and complex methods such as maximum magnitude and area of corresponding frequency spectrum through Fourier transform. A novel approach based on wavelet transform of the signal was presented, which provides an alternative method to determine wave characteristics. Material characterization tests (experiment Phase I) were carried out to study the relationship between the wave characteristics obtained from UPV test and the quality of the asphalt concrete specimens prepared in the laboratory. The specimens were identical in terms of aggregate and asphalt binder ratio, but varying in volumetric properties produced by different compaction efforts. The specimen quality was determined using two methods: 1) the traditional method using density measurement and 2) an innovative approach using dynamic modulus which is recommended by the Federal Highway Administration (FHWA) for use as a fundamental material property for characterizing Superpave mixes (Witczak, et al., 2002; Bonaquist et al., 2003; Christensen et al., 2004). The measured wave characteristics showed excellent correlations (R2 > 0.9) with the fundamental properties of the mix. The results revealed that the wave amplitude parameters as the condition index have the potential to provide a reliable assessment of the quality of HMA mixtures. The finding is very critical to moving the technology forward in the right direction, and form an important basis for the experiment Phase II. Experiment Phase II investigated the feasibility and effectiveness of using the wave characteristics identified in experiment Phase I to assess the condition of longitudinal construction joints. Particular attention was given to examining the sensitivity of the wave-based technique to different types of construction joints. For this purpose, three types of construction joints (good, fair and poor) were fabricated in the laboratory and identified using the wave-based technique. The research was intended to develop a suitable test procedure for condition assessment the longitudinal joints in asphalt pavement in the field. The data from experiment Phase II revealed that the wave-based technique enables assessment of not only the quality of different types of longitudinal construction joints but also the relative condition of asphalt pavements.

Joints represent a design challenge, especially for composite structures. Among the available joining methods, co-curing is an efficient way to integrate parts for some applications. Coates and Armanios have proposed a Single Nested Overlap (SNO) co-cured joint configuration, obtained from a single lap joint through the overlap/interleafing of the adjoining top/bottom adherend plies, respectively. Through a comparative investigation, they have demonstrated joint strength and fatigue life improvements over the single lap joint counterparts for unidirectional and quasi-isotropic adherend lay-ups. This research extends the comparative investigation of Coates and Armanios by focusing upon characterizing and differentiating the damage initiation and progression mechanisms under quasi-static loading. Six specimen configurations are manufactured and tested. It is confirmed that single nested overlap joints show 29.2% and 27.4% average improvement in strength over single lap counterparts for zero-degree unidirectional and quasi-isotropic lay-ups, respectively. Several nondestructive evaluation techniques are used to observe and analyze damage initiation, damage progression and failure modes of the studied specimens and to monitor their mechanical response. Using X-ray Radiography and Optical Microscopy techniques during quasi-static loading, a physical characterization of damage and failure mechanisms is obtained. The acoustic emission data acquired during monotonic loading could reveal the overall picture of AE activities produced by the damage initiation, development and accumulation mechanisms within the specimen via parametric analysis. Further AE analysis by a selected supervised clustering method is carried out and shown successful in differentiating and clustering the AE data. Correlation with physical observations from other techniques suggests that the resulting clusters may be associated to specific damage modes and failure mechanisms.

The Georgia Department of Transportation has been installing steel pedestals on bridges, ranging in height up to 33½” (0.85m) to increase the vertical clearance of many multi-span simply-supported and multi-span continuous bridges in Georgia. But there is a concern about the performance of these steel pedestals as they are designed without seismic consideration and may perform poorly compared to high-type steel “rocker” bearings, which were found to be unstable supports in previous earthquakes. This research models a candidate bridge using experimental data that captures the force-displacement hysteretic behavior of the steel pedestals. The results show how these steel pedestals behave when subjected to a range of ground motions. Nonlinear time history analysis is conducted using SAP 2000 software on a three-dimensional model of the candidate bridge. In addition, parametric studies of various critical parameters that can affect the seismic performance of the bridge are investigated, such as 1) varying the mass of the structure, 2) varying the stiffness of the deck joint, 3) varying column heights, and 4) seismic retrofitting using cable restrainers. The results show that these pedestals should not be used in regions of high seismicity, and in regions of low seismicity, it is likely that they need to be retrofitted. They can, although, be used safely in regions of low seismicity. In addition, it was shown that the mass of a superstructure and height of the columns significantly affect the behavior of these steel pedestals, and should be given a careful consideration before usage. It was also shown that the stiffness of the expansion joints does not significantly affect the displacement of the steel pedestals and the forces transmitted to them. However, if the expansion joints are too stiff compared to the adjacent bridge components, then the forces transferred during pounding of superstructure is increased significantly.