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Une approche combinant simulation numérique et expérimentation est développée pour modéliser la microfissuration complexe dans des matériaux hétérogènes cimentaires. Le modèle numérique proposé a permis de prévoir précisément en 3D l'initiation et la propagation des microfissures à l'échelle de la microstructure réelle d'un échantillon soumis à un chargement de compression. Ses prévisions ont été validées par une comparaison directe avec le réseau de fissures réel caractérisé par des techniques d'imagerie 3D. Dans une première partie, nous développons et testons les outils de simulation numérique. Plus précisément, la méthode de champ de phase est appliquée pour simuler la microfissuration dans des milieux fortement hétérogènes et ses avantages pour ce type de modélisation sont discutés. Ensuite, une extension de cette méthode est proposée pour tenir compte d'un endommagement interfacial, notamment aux interfaces inclusion/matrice. Dans une deuxième partie, les méthodes expérimentales utilisées et développées au cours de cette thèse sont décrites. Les procédures utilisées pour obtenir l'évolution du réseau de fissures 3D dans les échantillons à l'aide de microtomographie aux rayons X et d'essais mécaniques in-situ sont présentées. Ensuite, les outils de traitement d'image utilisant la corrélation d'images volumiques, pour extraire les fissures des images en niveaux de gris avec une bonne précision, sont détaillés. Dans une troisième partie, les prévisions du modèle numérique sons comparées avec les données expérimentales d'un matériau modèle en billes de polystyrène expansé intégrées dans une matrice de plâtre dans un premier temps, et, dans un second temps, d'un béton léger plus complexe. Plus précisément, nous utilisons les données expérimentales pour identifier les paramètres microscopiques inconnus par une approche inverse, et utilisons les déplacements expérimentaux déterminés par corrélation d'images volumiques pour définir des conditions limites à appliquer sur les bords de sous-domaines dans l'échantillon pour les simulations. Les comparaisons directes de réseaux de microfissures 3D et de leur évolution montrent une très bonne capacité prédictive du modèle numérique / An approach combining numerical simulations and experimental techniques is developed to model complex microcracking in heterogeneous cementitious materials. The proposed numerical model allowed us to predict accurately in 3D the initiation and the propagation of microcracks at the scale of the actual microstructure of a real sample subjected to compression. Its predictions have been validated by a direct comparison with the actual crack network characterized by 3D imaging techniques. In a first part, the numerical simulation tools are developed and tested. More specifically, the phase-field method is applied to microcracking simulations in highly heterogeneous microstructures and its advantages for such simulations are discussed. Then, the technique is extended to account for interfacial cracking, possibly occurring at inclusion/matrix interfaces. In a second part, the experimental methods used and developed in this work are described. The procedures to obtain the evolution of the 3D crack network within the samples by means of X-rays computed microtomography and in-situ mechanical testing are presented. Then, we focus on the developed image processing tools based on digital volume correlation to extract with good accuracy the cracks from the grey level images. In a third part, we compare the predictions of the numerical model with experimental results obtained, first, with a model material made of expanded polystyrene beads embedded in a plaster matrix, and second, to a more complex lightweight concrete. More precisely, we use the experimental data to identify by inverse approaches the local microstructural parameters, and use the experimental displacements measured by digital volume correlation to define boundary conditions to be applied on sub-domains within the sample for the simulations. The obtained direct comparisons of 3D microcrack networks and their evolutions demonstrate the very good predictive capability of the numerical model

Improved fracture toughness at a given strength level is one of the most important properties of cemented carbides. Large number of different testing methods (both theoretical and experimental) has been proposed to determine fracture toughness of cemented carbides and it has been always a topic of interest to relate fracture toughness with mechanical properties. In this report method such as Palmqvist toughness, chevron notch, toughness determination through Hertzian indentation and different theoretical toughness models have been utilized in order to investigate fracture characteristics and is also compared with other mechanical properties by varying cobalt content and grain size. This study has provided some new and interesting results as well as new information about hardmetals in different loading conditions.

No description available.

<p> Small scale tests were utilized in order to obtain characteristic fracture mechanics parameters such as the crack opening displacement (C.O.D.) and the J-integral. Two main types of steels were used (H.S.L.A. and AISI 4340) in obtaining data over a wide range of yield strengths and ductilities. Tests were done to see the effect of both notch geometry and sample geometry·in one of the H.S.L.A. steels, and it is verified that the minimum value of C.O.D., at crack initiation in plane strain, is independent of the geometry and plastic zone size. Further the development of a simple single specimen J-integral test method is done during this work. In terms of the data obtained both the minimum C.O.D.i values and the Jlc values increase with increasing ductility of the materials tested. In the attempt to relate the magnitude of the fracture toughness with microstructural parameters and the limiting processes occuring at the crack tip prior to fracture, the concept of the process zone is discussed. For this study a simple plain carbon steel spheroidized with different carbon contents was examined in addition to the H.S.L.A. and 4340 steels. From the data obtained the minimum C.O.D.i at crack initiation was found to be approximately equal to the product of the materials plain strain ductility and a characteristic distance scaling with the spacing between large non metallic inclusions or the spacing between the bands of the sulphides. </p> / Thesis / Master of Engineering (MEngr)

Lower urinary tract disorders are significant prognostic indicators of institutionalisation and lower quality of life in the elderly and their incidence increases with age. Urodynamics, the gold standard in diagnosis, replicates symptoms to assess functionality through controlled filling and voiding of the bladder but its interpretation is subjective and may be inconclusive; often requiring further testing or leading to inappropriate treatment. Normal filling and voiding biomechanics of the bladder relate directly to the structural composition of the bladder wall. Alterations to tissue composition in aging and pathology have significant impacts on biomechanics but are yet to be fully described. The aim of this thesis was to gain insight into the individual microstructural components of the bladder wall and how they relate to the gross mechanical response. Additionally, representation of these observations in a mathematical model that can be used to improve our understanding of urodynamic data. This aim was achieved through a combination of in situ mechanical testing and the development of a microstructural constitutive model, which was then included within an overall micturition framework to simulate filling and voiding functions, and evaluated with clinical data. Coupled systems of multiphoton microscopy and uniaxial, biaxial and inflation testing were used to correlate extra cellular matrix interactions with the mechanical response of young and aged murine bladder. Wall-layer specific collagen fibre orientation, dispersion and recruitment were quantified and implemented into a novel microstructural constitutive model. The bladder was modelled as a nonlinear elastic, constrainedmixture planar membrane with contribution from smooth muscle and collagen fibres in the detrusor. Collagen recruitment in the detrusor was observed to occur at a finite stretch; correlated with a steep increase in stiffness of the tissue, while collagen of the lamina propria plays a capacitance role. Collagen recruitment was modelled using a triangular probability density function; quantified from sequential microscopy images and fitted to mechanical data. Increased collagen area fraction and changes in dominant fibre orientation were attributed to reduced compliance in aged bladder. This behaviour was captured by the model. The microstructural model was modified to an isotropic thin-walled spherical membrane for the filling phase of a micturition model framework, consisting of a bladder outlet relation and urethral resistance relation. A contractile smooth muscle element was included in the active response. In the first steps towards clinical application the model was applied to male and female 'normal' patient urodynamic data to observe quality of model fit and estimate baseline parameter values. The model simulated key filling and voiding features seen in normal male and female clinical data. Mechanobiological modelling combined with clinically relevant micturition modelling has the potential to quantify bladder dysfunction. Moreover, improved understanding of how the microstructure influences macroscopic mechanics will yield improved understanding of how changes to the bladder impair its functionality. We predict that modelling will become a clinically relevant tool in urodynamics; leading to new options for diagnosis and management of patients with bladder dysfunction.

Cette thèse s'inscrit dans le cadre d'une collaboration avec EADS Innovation Works et AIRBUS. L'objectif des travaux est d'identifier les modes d'endommagements possibles d'éléments de structure métalliques d'aéronefs développés en service et d'en comprendre les mécanismes et les effets sur les propriétés des matériaux afin de contribuer au développement d'une méthode de contrôle non destructif innovante. Le matériau sélectionné est un alliage d'aluminium 2024-T351, l'un des matériaux constitutifs de la voilure et du fuselage d'avions civils. Les modes d'endommagement étudiés sont la corrosion et le vieillissement microstructural. La première partie de ces travaux est consacrée à l'analyse de l'influence des conditions d'exposition au milieu corrosif sur le développement de la corrosion intergranulaire et à l'identification des mécanismes de dégradation associés et de leurs cinétiques. Des conditions d'exposition originales alternant des phases d'immersion et d'émersion à différentes températures ont été explorées dans la mesure où elles semblent particulièrement représentatives des conditions d'exposition réelles. Les mécanismes proposés pour comprendre l'endommagement observé dans certaines de ces conditions d'exposition au milieu corrosif, impliquent un phénomène apparenté à de la fragilisation par l'hydrogène, phénomène qui n'est, à l'heure actuelle, pas encore reconnu pour les alliages d'aluminium de la série 2xxx. L'influence de l'hydrogène sur les propriétés physico-chimiques et mécaniques du matériau est donc étudiée dans la seconde partie de ces travaux. Enfin, l'influence d'un vieillissement microstructural sur les propriétés de l'alliage ainsi que les couplages possibles entre vieillissement microstructural et phénomènes de corrosion sont abordés dans une dernière partie. L'ensemble des résultats obtenus permet de révéler des pistes pour développer une méthode CND innovante permettant la caractérisation physique in-situ du niveau d'endommagement à l'échelle locale d'éléments de structures en alliages d'aluminium. / In the framework of a collaborative program research with EADS Innovation Works and AIRBUS, this work aims to identify the possible sources of in service damage of pieces of aircraft structure and the impact of these degradations on the mechanical properties of the materials. The results obtained allow describing the mechanisms involved and their kinetics, in order to contribute to the development of an innovative non destructive method. The material selected is the aluminum alloy 2024-T351, one of the constitutive materials for skin and wings of civil aircraft. For this study, the corrosion and the microstructural evolutions have been selected among the possible causes of degradation identified. The first part of this study is dedicated to the analysis of the influence of exposure conditions to the aggressive media on the development of intergranular corrosion and to the identification of the corrosion mechanisms involved and theirs kinetics. Original exposure conditions, alternating immersion steps in corrosive media and emersion steps in air at different temperatures, have been used insofar as these conditions have been estimated as representative of real exposure conditions. For some exposure conditions, the proposed mechanism to explain the damage observed implies a phenomenon related to hydrogen embrittlement which is, at the moment, not well recognized for aluminum alloys of the 2xxx series. The influence of hydrogen on the mechanical and physicochemical properties of the 2024 is so treated in the second part of this study. Finally, the impact of microstructural ageing as well as its possible coupling with corrosion is discussed in the last part. The whole results obtained allow the identification of leads to develop an innovative non destructive method allowing the physical characterization of local damage of aluminum alloys used to build civil aircrafts.

A good slag practice is essential for production of a high-quality stainless steel. In addition, the electrical and material efficiency of the electric arc furnace (EAF) can considerably be improved by a good slag practice. The metallurgical properties of the slag are strongly influenced by its high-temperature microstructure. Thus, characterization of the phases within the EAF slag as well as the determination of the amount of these phases is of high importance.In addition, the knowledge about the chemical composition of the liquid slag and solid phases at the process temperatures is instrumental in developing a good slag practice.In order to study the slag in EAF high-chromium stainless steelmaking, slag samples were collected from 14 heats of AISI 304L steel (two samples per heat) and 7 heats of duplex steel (three samples per heat).The selected slag samples were petrographically studied both using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) and light optical microscopy (LOM). In some cases, X-ray diffraction (XRD) analyses were also performed. Moreover, computational thermodynamics was used to determine the equilibrium phases in the EAF steelmaking slags at the process temperatures. In addition, parameter studies were performed on the factors influencing the equilibria.More specifically, a petrographical and thermodynamic characterization was performed on the EAF austenitic steelmaking slags. Thereafter, the microstructural evolution of the slag during the EAF duplex steelmaking process was investigated. Moreover, an investigation with focus on the total amount of precipitates within the high-chromium stainless steelmaking slags was done. Finally, the foamability of these slags was quantified and evaluated.The petrographic investigations showed that, during the refining stage, in both austenitic and duplex cases, the main constituent of the EAF slag is a melt consisting of liquid oxides. In addition, the slag samples contain solid spinel particles. However, before ferrosilicon-addition (FeSi), the slag may also contain solid stoichiometric calcium chromite. Moreover, depending on the slag basicity, the slag may contain solid dicalcium silicate at the process temperatures.The evolution of the slag during the refining stage of the EAF was graphically illustrated in the calculated isothermal phase diagrams for the slag system Al2O3-Cr2O3-CaO-MgO-SiO2-TiO2.It was found that the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium-oxide content. More specifically, it was shown that the amount of the spinel particles in the slag increases with an increased chromium-oxide content of the slag. It wasvialso shown that a higher basicity and a lower temperature of the slag contribute to the dicalcium silicate precipitation.In order to evaluate and quantify the foamability of the slags, the slag’s physical properties influencing its foaming index were determined. Computational thermodynamics was used as a tool to calculate the weight fractions of the solid phases within the slag at different EAF process stages. The computational thermophysics was used to estimate the viscosity of the liquid part of the slag samples at the process temperatures. The apparent viscosity of the samples was calculated by combining the above results. By estimating the density, surface tension and the foaming-gas bubble size, the foaming index of the slag samples were quantified. It could be shown that the foaming index of the EAF high-chromium stainless steelmaking slag may be on its minimum as the slag’s basicity takes a value in the range of 1.2 – 1.5. A basicity value of around 1.50 – 1.60 can be suitable for enhancing the foaming index of the slag, during the refining period in EAF high-chromium stainless steelmaking.

La thèse présente un ensemble de résultats expérimentaux sur des sols soumis à une pression interstitielle négative sur différents chemins : drainage- humidification, oedométrique, triaxial, etc... Ceux-ci mettent en évidence les différents domaines de comportement du sol et les aspects spécifiques du comportement dans chaque domaine : saturé, quasi-saturé faiblement saturé.<br />Parallèlement, une modélisation à partir d'arrangements réguliers de billes est proposée et validée par confrontation avec les résultats expérimentaux ; cette<br />approche permet d'expliquer un certain nombre de comportements observés dans la réalité et est à la base d'une modélisation élasto-plastique du comportement des sols partiellement saturés utilisant un concept de contraintes effectives généralisées.<br />L'étude est complétée par des mesures de perméabilités polyphasiques dans différents matériaux et la recherche d'une plus grande cohérence entre toutes les propriétés de ces matériaux.

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre debonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance for application of such materials. This PhD project is aimed at developing numerical models to analyze the effect of material randomness on delamination damage in CFRP materials by the implementation of the cohesive-zone model (CZM) within the framework of the finite-element (FE) method. Both the unidirectional and cross-ply laminates subjected to quasi-static loading conditions were studied. The initiation and propagation in delamination of unidirectional CFRP laminates were analyzed. The CZM was used to simulate the progress of that failure mechanism in a pre-cracked double-cantilever beam (DCB) specimen loaded under mode-I employing initially, a two-dimensional FE model. Model validation was then carried out comparing the numerical results with experimental data. The inherent microstructural stochasticity of CFRP laminates was accounted for in the simulations, and various statistical realizations for a half-scatter of 50% of fracture energy were performed, based on the approximation of that parameter with the Weibull s two-parameter probability density function. More detailed analyses were undertaken employing three-dimensional DCB models, and a number of statistical realizations based on variation of fracture energy were presented. In contrast to the results of two-dimensional analyses, simulations with 3D models demonstrated a lower load-bearing capacity for most of the random models as compared to the deterministic model with uniform material properties. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. The effect of material randomness on delamination in CFRP cross-ply laminates was also investigated. Initially, two-dimensional finite-element analyses were carried out to study the effect of microstructural randomness in a cross-ply laminate under bending with the direct introduction of matrix cracks with varying spacings and delamination zones. A considerable variation in the stiffness for cases with different crack spacings suggested that the assumption of averaged distributions of defects can lead to unreliable predictions of structural response. Three-dimensional uniform, deterministic cross-ply laminate models subjected to a tensile load were analyzed to study the delamination initiation and propagation from the tips of a pre-existing matrix crack. The material s stochasticity was then introduced, and a number of random statistical realizations were analyzed. It was observed that by neglecting the inherent material randomness of CFRP laminates, the initiation conditions for delamination as well as the character of its propagation cannot be properly detected and studied. For instance, the delamination crack length value for all the simulated random statistical realizations predicted its higher magnitudes compared to the uniform (deterministic) case for the same value of applied strain. Furthermore, the location of delamination initiation was shown to be different for different random statistical realizations. Another aspect, emphasizing the importance of microstructural randomness, was the scatter in the magnitudes of global strain at the instance of initiation and subsequent propagation of delamination. In summary, the material randomness in CFRPs can induce randomness in localised damage and it can affect the global properties of laminates and critical failure parameters. These effects can be investigated computationally through the use of stochastic cohesive-zone elements.