Ce travail concerne l’étude et la modélisation du phénomène de multifissuration transversedes fils dans les CMC tissés. Sa connaissance est fondamentale pour déterminer soneffet sur les champs de contraintes, la progression des endommagements et la durée de viedu matériau.À partir d’observations sur des coupes de CMC, des matériaux virtuels sont développéset des essais virtuels réalisés. Différentes séquences de fissuration transverse sont simuléessur diverses microstructures de CMC. Ces simulations se substituent à des observations expérimentalesimpossibles à réaliser.Un modèle statistique de multifissuration est développé sur la base du principe dumaillon faible appliqué à une distribution ponctuelle de Poisson. Les singularités micostructurellessont représentées par des défauts dans un milieu homogène équivalent (MHE).Les modifications des fonctions de distribution au cours de la multifissuration sont modélisées.Le modèle statistique permet de réaliser un changement d’échelle à la suite duquel lamultifissuration transverse est simulée dans le MHE avec une réduction des temps de calculde l’ordre de 90%. / This work deals with the study and modeling of multiple crakcing of tows in wovenCMCs. Its understanding is fundamental to determine the effect on stress fields, the evolutionof damage and the lifetime of material.From observations on real CMC pieces, virtual materials are developed and multiplecracking virtual testing is achieved. Different scenarii are simulated on various CMC microstructures.These simulations are a substitute for impossible experimental observations.A statistical model for multiple cracking based on the weakest link principle applied to adistribution of Poisson is developed. Micostructural singularities are represented by defectsin a homogeneous medium equivalent (EHM). Modifications of distribution functions duringthe multicracking are modeled.The statistical model realizes a scale changing : transverse multicracking is simulated inthe EHM with a reduction of almost 90% for computational time.

L'enjeu de cette thèse était de proposer un modèle de prévision de la fissuration (trajet et vitesse) par fatigue sous chargement complexe, afin de définir les pas d’inspection de structures industrielles. Dans le cas des métaux, il est reconnu que la plasticité au voisinage de l'extrémité de la fissure a des effets majeurs sur la vitesse et la direction de fissuration. La simulation par éléments finis non-linéaire permet de comprendre et de modéliser ces effets, mais le traitement de problèmes industriels par cette méthode (fissures tridimensionnelles se propageant pendant plusieurs millions de cycles en non-linéaire) reste toujours hors de portée. Pour réduire les temps de calcul, une démarche de changement d'échelle a été mise en place, afin d'identifier un modèle de comportement élasto-plastique en mode mixte non-proportionnel, non pas local, mais relatif à la région en pointe de fissure. Ce modèle, qui condense tous les effets liés à la plasticité confinée en pointe de fissure, peut alors être utilisé dans un calcul de structure linéaire pour simuler la fissuration par fatigue en mode mixte, à amplitude variable, sous des millions de cycles en quelques minutes. La méthode et les hypothèses de changement d'échelle, d'abord appliquées à des simulations numériques, ont ensuite été validées expérimentalement sur des éprouvettes cruciformes fissurées en s’appuyant sur la mesure optique du champ de déplacement à l’extrémité de la fissure. Enfin, une campagne d’essais, visant à caractériser le comportement en fissuration de l’acier S355NL en fatigue, a été réalisée. Les résultats expérimentaux et de simulation sont en bon accord en mode I et en mode mixte. / Predicting the growth of fatigue cracks under multi-axial loading conditions still remains very difficult because of the complexity of the crack path and because of history effects induced by plasticity. This thesis is devoted to the modeling of the effect of mixed mode plasticity on fatigue crack growth. A model was developed using a multi-scale approach from elastic-plastic finite element computations, it was validated using experimental measurements of the velocity field at crack tip by digital image correlation in I+II mixed mode loading conditions. The mode I and mode II components of the velocity field (experimental or from FE simulations) were extracted using symmetry and partitioned into elastic and plastic parts. With this approach, the velocity field is described using only four degrees of freedom, 2 for elasticity and 2 for plasticity. A criterion was proposed to determine the yield surface, and it was shown that the experimental yield surface agrees well with the theoretical one. The plastic flow direction was also measured in non-proportional loading conditions are consistent with the hypotheses of the model. The crack growth rate and direction is then calculated as function of the plasticity rate, which makes possible the use of this simplified model in a linear finite element code to predict 3D fatigue crack growth in mixed mode non-proportional conditions. In mode I, the model was successfully compared with a set of fatigue crack growth experiment under variable amplitude conditions. In mixed mode conditions, the crack growth direction is correctly predicted.

Wasserstoff-induzierte Schäden sind ein verbreitetes Problem in verschiedenen Anwendungen von Metallen. In dieser Arbeit wurde Wasserstoff-induzierte Rissbildung in Eisen untersucht. Die Proben wurden elektrochemisch mit Wasserstoff beladen. Diese Beladung führt zu Rissen in den Eisenproben und Blistern auf deren Oberfläche, wenn Risse oberflächennah lokalisiert sind. Als Triebkraft der Rissbildung wurde der hohe Wasserstoffdruck in den Rissen gefolgert. Dieser Druck wurde durch eine Kombination aus Ausgasexperimenten und Dichtemessungen bestimmt. Die Mikrostruktur, die Risse und Blister umgibt, wurde mit Elektronenmikroskopie untersucht. Dafür wurden Rissflächen durch Zugversuche freigelegt. Oxidische Einschlüsse konnten als Ausgangspunkt für Risse ausgemacht werden. Mit Transmissionselektronenmikroskopie wurden duktile Merkmale in der Nähe von Rissen sichtbar. Ein Mechanismus für die Riss- und Blisterbildung wurde vorgeschlagen.

Les structures en béton armé sont amenées à répondre à différentes exigences pouvant dépasser la simple résistance mécanique. Pendant le processus de fissuration, les contraintes dans le béton armé sont progressivement redistribuées entre l’acier et le béton via l’interface entre ces deux matériaux. Cette redistribution de contraintes a un impact direct sur l’état de fissuration final et doit donc être prise en compte dans la modélisation. Il existe différents modèles numériques capables de représenter les effets de la liaison acier-béton. Cependant, leur usage est, pour l’instant, incompatible avec les applications concernant les structures de grandes dimensions (difficultés de maillage, coût de calcul…). Dans ce cadre d’application, l’hypothèse de liaison parfaite entre l’acier et le béton (déplacement identique) est donc toujours utilisée. On se propose ici de développer un nouveau modèle éléments finis de liaison acier-béton qui soit à la fois représentatif des phénomènes physiques se produisant à l’interface entre les deux matériaux et compatible avec les contraintes de modélisation des structures de grandes dimensions. Ce travail de thèse se découpe en trois grandes parties : - le développement d’un modèle élément fini de liaison acier-béton adapté aux contraintes de modélisation des structures de grandes dimensions. Ce modèle numérique permet ainsi de tenir compte des interactions mécaniques entre le béton et les armatures d’acier représentées à l’aide d’éléments barres. - la caractérisation du comportement de la liaison acier-béton. Un modèle de loi d’adhérence (évolution de la contrainte d’adhérence en fonction du glissement) basé sur des observations expérimentales (campagne expérimentale de pull-out menée au cours de la thèse et données bibliographiques) est proposé. Il permet en particulier de différencier le cas d’une rupture par arrachement, d’une rupture par éclatement en tenant compte des caractéristiques matériaux et géométriques de la structure. - l’application du modèle proposé à un élément structurel (poutre). Un essai de poutre en flexion quatre points visant à caractériser l’évolution de la fissuration (évolution de l’ouverture de fissure mesurée à l’aide de la technique de corrélation d’images notamment) a ainsi été proposé. Ces résultats ont ensuite été comparés à ceux de simulations numériques tenant compte de la liaison acier-béton d’une part ou de l’hypothèse de liaison parfaite d’autre part. Les deux modélisations donnent une bonne approximation du comportement extérieur de la structure (comportement global et ouvertures de fissure des surfaces extérieures de la poutre). Le modèle de liaison acier-béton apporte cependant une meilleure caractérisation de la phase de fissuration active (apparition des fissures) et modifie plus particulièrement le comportement local de la structure à proximité directe des armatures (limitant le développement de l’endommagement du béton le long des renforts). / Reinforced concrete structures may have to fulfill functions that go beyond their simple mechanical resistance. During the cracking process, stresses are progressively transferred from steel to concrete through the steel-concrete interface. This stress transfer has a direct impact on the crack properties. Taking into account these effects seems thus essential to predict correctly the cracking of reinforced concrete structures. Different models exist to represent the steel-concrete bond behavior. However, these models are rarely compatible with large scale simulations (meshing difficulties, heavy computational cost…). To overcome these difficulties, a perfect relation between steel and concrete (same displacements) is generally considered for structural applications. In this contribution, a new finite element approach is proposed to represent the steel-concrete bond effects in a context adapted for large scale simulations. This thesis is divided in three parts: - the development of a finite element steel-concrete bond model adapted for large scale structural applications . This model takes into account mechanical interactions between concrete and steel reinforcement represented by truss elements. - the characterization of the steel-concrete bond behavior. A model for the bond stress-slip law based on experimental observations (experimental campaign on pull-out test carried out during the thesis and data of literature) is proposed. This model differentiates the case of a pull-out failure and of splitting failure and takes into account the material properties and the geometric characteristics of the structure. - an application of the proposed model on a structural element (beam). A four point bending beam is experimentally tested. This test aims to characterize the crack evolution (in particular the crack opening using the image correlation technique). Experimental results are then compared with numerical simulations taking into account the bond–slip effect between steel and concrete or considering the perfect relation hypothesis. The two simulations give a good approximation of the external behavior of the structure (global behavior and crack opening on the external face of the beam). Nevertheless, the bond model improves the cracking description during the active cracking phase (beginning of crack apparition) and influences the local behavior of the structure especially near the steel bars (avoiding the propagation of the damage of concrete along the steel reinforcement).

Depuis plusieurs années, la société MATIERE développe un nouveau type de renforcement des structures en béton reposant sur l'utilisation d'aciers plats crantés en substitution des aciers ronds à haute adhérence habituellement utilisés. Ce travail entre dans le cadre du programme de Recherche - Développement des techniques couvertes par les brevets de M. Marcel MATIERE. L'intérêt de ces nouveaux aciers plats crantés réside principalement dans leur géométrie qui permet d'envisager de nouvelles dispositions constructives associées à un gain sur l'épaisseur de béton, notamment au niveau de l'enrobage. Ces aciers sont principalement destinés aux éléments de type dalle ou aux voiles minces où ils permettront de réaliser les économies de béton les plus significatives. Cependant, aucune norme ou règlement ne prend en compte, à ce jour, ces nouveaux aciers. Une étude scientifique validée, principalement basée sur la modélisation numérique, s'est avérée nécessaire pour d'une part modéliser et comprendre l'interaction entre l'acier plat et le béton et, d'autre part, pour justifier que les méthodes de calcul traditionnelles restent applicables à ce genre de renforcement. Dans le cadre de la problématique de la fissuration, une stratégie de modélisation reposant sur une approche probabiliste multi-échelles a été développée. Cette approche multi-échelles ne consiste pas à développer une modélisation qui inclut, dans son formalisme, toutes les échelles, depuis l'échelle très locale jusqu'à l'échelle globale (une structure complète), mais à développer une panoplie de modélisations qui apportent des informations pertinences à l'échelle d'analyse choisie. Quelle que soit l'échelle considérée, la modélisation est succeptible de donner des informations sur l'ouverture et l'espacement de fissures. L'aspect probabiliste est essentiellement lié à l'hétérogénéité du matériau béton. Les modèles développés permettent aussi de tenir compte des effets d'échelle, propres aux matériaux hétérogènes, qui jouent un rôle prépondérant dans le comportement des structures en béton. Le travail de recherche a donc consisté à développer des outils de modélisation du comportement d'interface en parfaite cohérence avec l'échelle de modélisation des phénomènes envisagés, notamment au regard des processus de fissuration des structures renforcées par aciers plats. La démarche scientifique s'est appuyée sur une identification des paramètres de la modélisation par analyse inverse effectuée sur la base de résultats d'essais expérimentaux réalisés sur de grands tirants en béton armé par aciers plats. Les outils de modélisation ont ensuite été validés sur des modélisations du comportement en flexion de poutres-dalles de grandes dimensions comparées à des résultats d'essais expérimentaux. L'ensemble des essais expérimentaux, nécessaires à cette étude, ont été réalisés par Polytech' Clermont à la demande de l'entreprise MATIERE / For several years, the MATIERE company has developed a new type of reinforcement based on the use of ribbed flat steel in substitution of high-adhesion round steel used for precast products. This work is a part of the Technological Research and Development program implemented in the frame of Marcel MATIERE patents. The interest of these new ribbed carbon flat steel lies in their geometry that allows to consider a new structural disposition associated with a gain on the thickness of concrete, particularly at the coating level. These steels are mainly used for thin elements where they will achieve the savings of the most significant concrete. However, no standard or regulation take into account in this time for these new steels. A scientific study validated, mainly based on numerical modeling, is so necessary, firstly, to model and understand the interaction between the flat steel and the concrete, and secondly, to justify the traditional calculation methods are applicable to such reinforcement. In this frame, a modeling strategy based on a probabilistic multiscale approach was developed. This multiscale approach is not to develop a model that includes, in its formalism, all modeling scales, from the very local to the global scale (complete structure), but to develop a panoply of models that provides the relevant informations on the scale of analysis chosen. Whatever the scale considered, the modeling can provide the information relative to the cracks opening and cracks spacing. The probabilistic aspect is mainly due to the heterogeneity of the concrete. The developed models can take into account the scale effects, specific heterogeneous materials, which play an important role in the behavior of concrete structures. The research was therefore to develop a modeling tools of the interface behaviour which is perfectly consistent with the scale modeling of phenomena considered, particularly with regard to the cracking process of structure reinforced by steel plates. The scientific approach was based on a parameters identification of the modeling by an inverse analysis performed on the results of experimental tests carried out on the tie-beam reinforced by flat steel. Modeling tools were then validated on the modeling of the beams-slabs in flection and compared to experimental results. All experimental tests for this study were made by Polytech Clermont Ferrand to the request of the MATIERE company

Master of Science / Department of Civil Engineering / Kyle A. Riding / When transverse cracks meander there is a high possibility for transverse cracks to meet at a point and connect to another transverse crack, creating a Y-crack. Y-cracks have been blamed for being the origin of punchouts and spallings in CRCPs. When the direction of maximum principal stress changes, it could cause a change in the crack direction, potentially forming a Y-crack. Finite Element Models (FEMs) were run to model the change in principal stress direction based on design and construction conditions. The finite element model of CRCP using typical Oklahoma CRCP pavement conditions and design was assembled. The model included the concrete pavement, asphalt concrete subbase, and soil subgrade. The effect of areas of changed friction on the direction of principal stress was simulated by considering a patch at the pavement-subbase interaction. Investigated factors related to this patch were location of patch, friction between patch and subbase, and patch size. Patches were placed at two different locations in the pavement: a patch at the corner of the pavement and a patch at the longitudinal edge between pavement ends. A change in the friction at the corner had a large effect on the stress magnitude and direction of principal stress, while a patch in the middle did not significantly change the stress state. Also, patch size had a noticeable effect on stress magnitude when the patch was at the corner. Another model was developed to understand the effect of jointed shoulder on direction of maximum principal stress. Analysis of this model showed that the stresses were not symmetric and changed along the width of the pavement. This meandering pattern shows a high potential for Y-cracking. Also, several finite element models were run to understand the effects of different shrinkage between mainline and shoulder. In order to simulate the effects of the differential drying shrinkage between the hardened mainline concrete and the newly cast shoulder, different temperature changes were applied on the mainline and shoulder. For these models, the orientation of the maximum principal stress was not significantly changed from different amounts of temperature decreases between mainline and shoulder. Also, effect of different longitudinal steel percentages was investigated by comparing two finite element models with different steel percentage. The model with higher steel percentage (0.7%) indicated more variation in stress, potentially leading to more crack direction diverging.

Master of Science / Department of Biological and Agricultural Engineering / Wenqiao Yuan / Gasification is an efficient way to produce energy from biomass, which has significant positive impacts on the environment, domestic economy, national energy security, and the society in general. In this study, a lab-scale updraft biomass gasifier was designed, built, and instrumented for stable gasification using low-bulk density biomass. Related accessories, such as a biomass feeder, inlet air temperature controller, air injection nozzle, and tar cracking system, were also developed to enhance gasifier performance. The effect of operation parameters on gasifier performance was studied. Two operational parameters, including air flow rate and feed-air temperature, were studied on three sources of biomass: prairie hay, sorghum biomass, and wood chips. Results showed that higher air flow rate increased tar contents in syngas for all three types. It was also found that different biomasses gave significantly different tar contents, in the order of wood chips>sorghum biomass>prairie hay. Feed-air temperature did not have a significant effect on tar content in syngas except for prairie hay, where higher feed air temperature reduced tar. A statistical model was implemented to study differences on syngas composition. Results showed that different biomasses produced syngas with different high heating value, e.g., wood chips > prairie hay > sorghum biomass. CO composition also showed differences by feed air temperature and biomass, e.g. prairie hay>wood chips>sorghum biomass, but H[subscript]2 did not show significant differences by either biomass type or operating conditions. Moreover, because of the downstream problems caused by tars in syngas such as tar condensation in pipelines, blockage and machinery collapse, an in-situ tar cracking system was developed to remove tars in syngas. The tar cracking device was built in the middle of the gasifier’s combustion using gasification heat to drive the reactions. The in-situ system was found to be very effective in tar removal and syngas enhancement. The highest tar removal of 95% was achieved at 0.3s residence time and 10% nickel loading. This condition also gave the highest syngas HHV increment of 36% (7.33 MJ/m[superscript]3). The effect of gas residence time and Ni loading on tar removal and syngas composition was also studied. Gas residence of 0.2-0.3s and Ni loading of 10% were found appropriate in this study.

Thermal cracking is an industrial process sensitive to both temperature and pressure operating conditions. The use of internally ribbed reactors is a passive method to enhance the chemical selectivity of the process, thanks to a significant increase of heat transfer. However, this method also induces an increase in pressure loss, which is damageable to the chemical yield and must be quantified. Because of the complexity of turbulence and chemical kinetics, and as detailed experimental measurements are difficult to conduct, the real advantage of such geometries in terms of selectivity is however poorly known and difficult to assess. This work aims both at evaluating the real benefits of internally ribbed reactors in terms of chemical yields and at proposing innovative and optimized reactor designs. This is made possible using the Large Eddy Simulation (LES) approach, which allows to study in detail the reactive flow inside several reactor geometries. The AVBP code, which solves the Navier-Stokes compressible equations for turbulent flows, is used in order to simulate thermal cracking thanks to a dedicated numerical methodology. In particular, the effect of pressure loss and heat transfer on chemical conversion is compared for both a smooth and a ribbed reactor in order to conclude about the impact of wall roughness in industrial operating conditions. An optimization methodology, based on series of LES and Gaussian process, is finally developed and an innovative reactor design for thermal cracking applications, which maximizes the chemical yield, is proposed

Titanium alloys are highly resistant to all types of corrosion due to their excellent ability to form an oxide film on the surface. However, under certain circumstances, these alloys may experience an environmental degradation which could potentially, under the application of mechanical stress, lead to a complete failure of the material. One of these cracking processes is stress-corrosion cracking (SCC). SCC has an embrittling effect on otherwise ductile materials under tensile stress. Since titanium alloys are frequently used in the aerospace industry and it is therefore of interest to test these alloys in different environment in order to prevent any future accidents. SCC testing is frequently tested at GKN Aerospace and a new testing method is of interest. The main objective with this work was to gain knowledge of the testing method. Bent-beam testing method has been used to investigate stress-corrosion cracking (SCC) of titanium alloys in a laboratory based experiment. The bent-beam testing method was of type 2-point bent beam test, where a saline solution was applied at the apex of the specimen. The specimens were loaded to a range of stresses from 40%, to 95% of the materials yield strength and the salt concentration in the saline solution was 1wt% and 3wt%. By doing so, a relative susceptibility of the different alloys could be established. Three different titanium alloys were tested: Ti-6Al-4V, Ti-8Al-1Mo-1V, and Ti-6Al-2Sn-4Zr-2Mo. The testing method was able to cause cracking on all titanium alloys, where Ti-6Al-4V was found to be the least susceptible to SCC. Ti-8Al-1Mo-1V, and Ti-6Al-2Sn-4Zr-2Mo showed an overall high susceptibility to SCC as cracking occurred in all testing configurations. Cracking was observed on both the surface of the specimen as well as in the cross sections, where the cracks grew perpendicular to the surface. SEM was also used to evaluate the crack propagation in Ti-8Al-1Mo-1V, and Ti-6Al-2Sn-4Zr-2Mo, and it was found that the cracks grew mostly along the grain boundaries.

This paper presents the comparison of shrinkage and corrosion characteristics of optimized hybrid Rice Husk Ash (RHA)/Fly Ash (FA)-modified Concrete, with those of normal concrete in the marine environment. Uses of both FA and RHA have numerous environmental benefits. Shrinkage performance was determined by subjecting the mixes to restrained shrinkage testing per ASTM C1581. The time to cracking of the specimens improved an average of 18% with the hybrid mixes. Corrosion testing of reinforced columns was performed in a simulated tidal cycle Marine Environment. Corrosion potential improved by as much as 35% for the mix with the highest FA/RHA replacement, and corrosion activity as measured with potentiostat equipment improved by an average of 34% . These results indicate a clear performance improvement of the modified concrete that is proportional to the percent replacement of cement. / by Diana Arboleda. / Thesis (M.S.C.S.)--Florida Atlantic University, 2010 / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2010. Mode of access: World Wide Web.