Inhomogeneous deformation fields arising from the grain-grain interactions in polycrystalline materials have been evaluated using a crystal plasticity finite element method and extensively compared to neutron diffraction measurements under fatigue crack growth conditions. The roles of intergranular deformation anisotropy, grain boundary damage, and non-common deformation mechanisms (such as twinning for hexagonal close packed crystals) are systematically evaluated. The lattice misorientation field can be used to determine the intragranular deformation behavior in polycrystals or to describe the deformation inhomogeneity due to dislocation plasticity in single crystals. The study of indentation-induced lattice misorientation fields in single crystals sheds lights on the understanding of the scale-dependent plasticity mechanisms. A two-scale micromechanical analysis is performed to study the lattice strain distributions near a fatigue crack tip. The experimental finding of vanishing residual intergranular strain in polycrystals as the increase of the fully reversed loading cycles suggests the intergranular damage be the dominant failure mechanism. Our model predictions are compared to in situ neutron diffraction measurements of Ni-based superalloys under fatigue crack growth conditions. Predicted and measured lattice strains in the vicinity of fatigue crack tips illustrate the important roles played by the intergranular damage and the surrounding plasticity in fatigue growth. Motivated by the synchrotron x-ray measurements of lattice rotation fields in single crystals under indentation, the effect of the orientation of slip systems on the 2D wedge indentation of a model single crystal is investigated. Furthermore, the crystallographic orientations of the indented solids are gradually rotated, resulting changes of lattice misorientation patterns under the indenter. These 2D simulations, as well as a 3D Berkovich indentation simulation, suggest a kinematic relationship between the lattice misorientation and crystalline slip fields. Advanced structural materials such as light-weighted materials, nanocrystalline metals/alloys, and hierarchically structured alloys often encounter unconventional deformation mechanisms. The convolution of crystalline slip and deformation twin are considered in the hexagonal close packed polycrystals. Specifically, we have determined the lattice strain distributions near fatigue crack tips in Zircaloy-4, and the role of tensile-twins on intergranular strain evolution in a wrought Mg alloy, which compare favorable to available neutron diffraction measurements.

Ti-6Al-4V, known for high strength-to-weight ratio and good resistance to corrosion, has been widely used in aerospace, biomedical, and high-performance sports applications. A wide range of physical and mechanical properties of Ti-6Al-4V can be achieved by varying the microstructures via deformation and recrystallization processes. The aim of this thesis is to establish a microstructure-sensitive fatigue analysis approach that can be applied in engineering applications such as fretting fatigue to permit explicit assessment of the influence of microstructure. In this thesis, crystal plasticity constitutive relations are developed to model the cyclic deformation -TiAl has beenabehavior of Ti-6Al-4V. The development of the slip bands within widely reported and has been found to play an important role in deformation and fatigue behaviors of Ti-6Al-4V. The shear enhanced model is used to simulate the formation and evolution of slip bands triggered by planar slip under static or quasi-static loading at room temperature. Fatigue Indicator Parameters (FIPs) are introduced to reflect driving force for the different crack formation mechanisms in Ti-6Al-4V. The cyclic stress-strain behavior and fretting fatigue sensitivity to microstructure and loading parameters in dual phase Ti-6Al-4V are investigated.

In this dissertation the work done in the projects KME-410/502 will be presented.The overall objective in these projects is to evaluate and develop tools for designingagainst fatigue in single-crystal nickel-base superalloys in gas turbines. Experimentshave been done on single-crystal nickel-base superalloy specimens in order toinvestigate the mechanical and fatigue behaviour of the material. The constitutivebehaviour has been modelled and veried by FE-simulations of the experiments.Furthermore, the microstructural degradation during long-time ageing has been investigatedwith respect to the material's yield limit. The eect has been includedin the constitutive model by lowering the resulting yield limit. Moreover, the fatiguecrack initiation of a component has been analysed and modelled by using acritical plane approach in combination with a critical distance method. Finally, asan application, the derived single-crystal model was applied to all the individualgrains in a coarse grained specimen to predict the dispersion in fatigue crack initiationlife depending on random grain distributions. This thesis is divided into three parts. In the rst part the theoretical framework,based upon continuum mechanics, crystal plasticity, the critical plane approachand the critical distance method, is derived. This framework is then used in thesecond part, which consists of six included papers. Finally, in the third part, detailsof the used numerical procedures are presented.

Le dioxyde d'uranium (UO2) est le combustible de référence pour les réacteurs nucléaires à eau pressurisée. Notre étude traite de la compréhension et de la modélisation du comportement mécanique, dans les domaines basse température (rupture fragile) et haute température (déformation viscoplastique), à l'échelle de la microstructure. Dans un premier temps est présentée une étude des propriétés géométriques des polycristaux en général et du polycristal d'UO2 en particulier. Nous montrons que nous pouvons reproduire des agrégats polycristallins réalistes et économes en nombre d'éléments. Pour améliorer les connaissances du comportement de ce matériau dans le domaine de rupture fragile, nous avons développé une méthode expérimentale permettant de mieux comprendre le phénomène de rupture fragile à l'échelle du grain. Nous montrons que la rupture est entièrement intragranulaire et que les plans {100} semblent être les plans préférentiels pour cette rupture. Les résultats expérimentaux obtenus sont directement utilisés pour formuler une loi de comportement de rupture fragile intragranulaire à l'échelle du cristal, utilisée ensuite dans des calculs de rupture fragile sur un polycristal tridimensionnel. Le calcul est réalisé en champ complet, donnant ainsi accès à l'amorçage et à la propagation de la fissure à travers les grains. Enfin, nous avons développé une modélisation du comportement de l'UO2 dans le domaine viscoplastique. Nous présentons tout d'abord une loi de comportement à l'échelle macroscopique qui inclut un effet de vieillissement par migration de défauts vers les dislocations. Dans un second temps, nous avons développé une loi de comportement de type plasticité cristalline adaptée à l'UO2, incluant les effets de rotation de réseau. Nous présentons des exemples de calculs sur polycristaux. / Uranium dioxide (UO2) is the reference fuel for pressurized water nuclear reactors. Our study deals with understanding and modeling of mechanical behavior at the microstructure scale at low temperatures (brittle fracture) and high temperature (viscoplastic strain). We have first studied the geometrical properties of polycrystals at large and of UO2 polycrystal more specifically. As of now, knowledge of this behavior in the brittle fracture range is limited. Consequently, we developed an experimental method which allows better understanding of brittle fracture phenomenon at grain scale. We show that fracture is fully intra-granular and {100} planes seem to be the most preferential cleavage planes. Experimental results are directly used to deduce constitutive equations of intra-granular brittle fracture at crystal scale. This behavior is then used in 3D polycrystal simulation of brittle fracture. The full field calculation gives access to the initiation of fracture and propagation of the crack through the grains. Finally, we developed a mechanical behavior model of UO2 in the viscoplastic range. We first present constitutive equations at macroscopic scale which accounts for an ageing process caused by migration of defects towards dislocations. Secondly, we have developed a crystal plasticity model which was fitted to UO2. This model includes the rotation of the crystal lattice. We present examples of polycrystalline simulations.

Les aciers austénitiques et alliages à base de nickel sont des matériaux de choix pour leurs propriétés mécaniques à haute température. L'enrichissement en chrome améliore leur durabilité de part la formation d'une couche d'oxyde protectrice à l'exemple de la chromine (Cr2O3).Il est néanmoins établi, par des essais mécaniques sous vide, que l'oxydation réduit de manière notable leur durée de vie en fatigue.En effet, la croissance d'oxyde peut être accompagnée d'une introduction de défauts tels que l'injection de lacunes, d'éléments délétères comme l'hydrogène mais également de contraintes résiduelles, etc., dans le métal.Les micromécanismes de fissuration sont ainsi régis par des interactions complexes entre l'environnement et la surface du métal, faisant intervenir composition chimique et microstructure.Aujourd'hui, les enjeux de sécurité et de compétitivité font de la prévision de la durée de vie de ces alliages une nécessité pour l'industrie nucléaire.L'augmentation de la dimension des modèles permet de prendre en compte de manière explicite les interactions multiphysiques du couple oxyde/métal sous l'action d'un chargement mécanique.La thèse s'inscrit dans cette démarche et propose une formulation d'un modèle de champ de phases couplé avec la mécanique et fondé sur les principes de la thermodynamique des milieux continus.Le comportement effectif de l'interface est présentement obtenu via des méthodes d'homogénéisation permettant de combiner des comportements mécaniques dissemblables, à l'image d'un substrat ductile et de son oxyde fragile.Les contraintes induites par la formation d'oxyde et également par le chargement mécanique peuvent être relaxées viscoplastiquement, de manière isotrope et anisotrope, respectivement dans l'oxyde et dans le substrat.Des simulations par éléments finis de l'oxydation généralisée ainsi que de l'oxydation intergranulaire sous chargement mécanique sont effectuées.Ces dernières mettent en évidence la possibilité d'un phénomène d'oxydation catastrophique par la génération de contraintes de tensions dans l'oxyde fragile, lesquelles peuvent être localisées le long des intrusions d'oxyde dans les joints de grains. / Austenitic stainless steels and nickel based alloys are widely used for their mechanical properties at high temperatures.Their durability can be increased by the addition of chromium resulting in the formation of a protective oxide layer such as chromia (Cr2O3).Nevertheless, it is established from vacuum mechanical tests that oxidation significantly decreases their fatigue life.In fact, oxide growth can be followed with the injection of defects such as vacancies, deleterious chemical elements and residual stresses, etc., into the metal.The resulting cracking micromechanisms are therefore governed by complex interactions between the environment and the metal surface, implying the chemical composition and the microstructure of the metal.To date, materials life prediction is a necessity for the nuclear industry due to safety and economic issues.The enhancement of the model dimensionality allow to explicitly account for multi-physics interactions between oxide and metallic phases under mechanical loads.The thesis is in line with it and relies on the development of a phase field model coupled with mechanics that heavily relies on the principles of continuum thermodynamics.The effective behaviour of the interface is obtained by homogenisation methods allowing the mixture of separate behaviours, as it is the case on a ductile metallic substrate and its fragile oxide.Oxide growth residual stresses and mechanical load induced stresses can be relaxed by viscoplasticity, which is isotropic and anisotropic respectively for the oxide and the substrate.Full field finite element simulations are performed to study both generalised and intergranular oxidation under mechanical loads.The simulations highlight the possibility of triggering breakaway oxidation by the generation of tensile stresses in the fragile oxide, which can be localised along oxide intrusions at grain boundaries.

L'application d'un temps de maintien, même de faible durée, lors d'un chargement cyclique, modifie de façon très sensible à la fois le comportement contrainte-déformation et le nombre de cycles à amorçage dans l'alliage base titane TI-6242. Ceci est lié à un régime de fluage cyclique, conduisant à de la déformation progressive d'une part, et à une forte interaction fatigue-temps de maintien pour ce qui concerne le nombre de cycles à amorçage. Les différents phénomènes sont pour le moment assez mal analysés, si bien qu'il n'est pas possible d'effectuer une conception optimale des pièces, de larges marges de sécurité étant nécessaires. Le but du travail est de mieux comprendre les mécanismes locaux qui régissent le comportement et l'amorçage des fissures, dans le but de suggérer des microstructures optimales, et de calibrer des modèles macroscopiques utilisables en bureau d'études. En s'appuyant sur une base expérimentale fournie par Snecma et l'ENSMA, une approche multiéchelles a été mise en place pour représenter les hétérogénéités locales qui ont un rôle significatif sur les comportements observés. Dans les calculs des microstructures, faisant intervenir une étape d'évaluation statistique, on se focalise sur la représentation explicite des ”plumes”, grains de taille exceptionnelle, qui sont à l'origine des premières microfissures en raison du contraste cristallin qu'ils introduisent avec l'environnement. Une revue des différentes configurations de plumes, afin de retenir celles qui sont le plus critique, a été établie. Cette analyse a permis de mettre en évidence la présence de plumes simples, doubles ou triples, les domaines se présentant sous formes de bandes. Les configurations à étudier comportent comme paramètres critiques l'orientation géométrique de la bande par rapport à la direction du chargement macroscopique, mais surtout l'orientation cristallographique au sein de cette (ces) bande(s). Des calculs systématiques ont été effectués afin de mener une étude statistique et de déterminer les configurations les plus sensibles. / The application of a dwell period, even of short length, during a cyclic loading, simultaneously changes the stress-strain behaviour and the number of cycles to failure in a very sensitive way. This phenomenon is connected to a cyclic creep regime, generating progressive deformation, and to a strong interaction between the fatigue process and dwell periods for the number of cycles to failure. All these phenomena are poorly analysed nowadays, so that engineers hardly perform optimal design of the components, since large security margins are necessary. The aim of the work is to better understand the local mechanisms which govern both behaviour and crack initiation, having in view optimal microstructures, and to calibrate manageable macroscopic models for the design department. Using an experimental data set given by Snecma and ENSMA, a multiscale approach has been developed to represent the local heterogeneities that play a significant role on observed behaviour. In the calculations of microstructures that are performed for a statistical evaluation, the focus is made on the explicit representation of the so called "plumes", that are grains of exceptional size, which are at the origin of the first microcracks due to crystal contrast they introduce with the environment. A review of various "plume" configurations is made, in order to investigate the most critical ones. This analysis allowed to shed the light on the presence of simple, double or triple "plumes", the domains being in band shapes. The critical parameters are the geometric arrangement of the band with respect to the direction of the macroscopic loading, but essentially the crystal orientation within this (these) band(s). Systematic calculations were carried out in order to do a statistical study and to determine the most critical configurations.

La course à la miniaturisation entraine une forte hausse de la demande encomposants aux dimensions submillimétriques et donne un essor considérable aux procédés de micro-formage. Cependant le comportement mécanique des tôles ultrafines, employées dans ces procédés présente des singularités liées à la réduction du nombre de grains. Cette thèse a eu pour objet de mettre en place un outil d’aide à la prédiction du comportement mécanique des tôles ultrafines.Expérimentalement, le comportement de deux alliages de cuivre, le CuBe2 et le CuFe2P, a été caractérisé sous divers types de chargement. Diverses caractéristiques ont été mises en évidence, notamment l’anisotropie de la réponse mécanique, l’effet Bauschinger ou encore ladégradation du module de Young.Afin d’obtenir un cadre de modélisation apte à la description de tôles présentant un comportement plus ou moins homogène, deux approches ont été retenues. La première consiste en une modélisation phénoménologique inspirée des observations macroscopiques. La seconde est une description, en plasticité cristalline, à l’échelle du grain du comportement mécanique, basée sur les mécanismes physiques de déformation. Les modèles retenus ont été intégrés dansles logiciels ABAQUS et SiDoLo dans le formalisme des grandes transformations. Des stratégies d’identification paramétrique des différents modèles sont développées et une analyse comparative de l’impact de l’identification sur les prévisions des modèles est proposée.Enfin les approches développées sont mises en oeuvre sur des procédés industriels et des tests académiques. Une étude sur des facteurs influençant la prédiction du retour élastique estréalisée. Elle a montré qu’une attention particulière doit être portée à la modélisation de l’élasticité. / The on-going trend on device miniaturization has increased the demand forminiature parts and boosted micro forming processes. However, the mechanical behavior of ultra-thin sheet metals is subjected to certain peculiarities which are driven from the reduced number of grains in the sheets. The present work aimed to provide a numerical tool for the prediction of the mechanical behavior of ultra-thin sheet metals. The mechanical behavior of two copper alloys, CuBe2 and CuFe2P, was experimentally characterized through several strain paths. Various characteristics have been revealed, such as the anisotropic response, Bauschinger effect and the decrease of the Young modulus.In order to build a modeling frame capable of describing thin metal sheets which exhibit a highly heterogeneous behavior and those whose response is more homogeneous, two modeling approaches were considered. On one hand, a phenomenological model based on the experimental results is chosen. On the other hand, a crystal plasticity based model, which takes into account the physical deformation mechanisms, is adopted. Both models were implementedin ABAQUS and SiDoLo softwares, under the finite strain formalism. Parametric identification strategies are devised and the influence of calibration on models performance is assessed.Ultimately, the modeling approaches were applied to the simulation of industrial processes and academic tests. A numerical study on relevant parameters for the prediction of springback has been performed. The accurate modeling of elasticity proved highly influential.

Aluminium sheet production is a multi-stage process in which altering processing conditions can drastically alter the size and type of second phase particles found in the final product. The properties of these second phase particles also affects deformation and annealing processes, meaning that any attempt to create a through process model would require the ability to predict both how the particles would develop in the material, and how these particles then affect the alloy moving forward. This project first focuses on gaining insight into how the particles in a model aluminium alloy change during homogenisation heat treatment and hot rolling. This has been accomplished by utilising serial block face scanning electron microscopy (SBF-SEM), a technique which allows the capture of 3D data sets at sub micron resolutions. This has allowed the populations of primary (constituent) and secondary (dispersoid) particles to be analysed at different stages of sheet production, and thus allowing the effects of homogenisation and hot rolling on particle populations to be quantified. To discover how the particles would go on to affect further processing, digital image correlation has been used to examine the localised strain in the alloy near to a selection of particle configurations. This highlighted the heterogeneity in slip behaviour within the alloy and illustrated that plumes of rotation develop near to non deformable regions. Rotation plumes have previously been modelled using a crystal plasticity model, and so further work is also presented expanding upon this model to simulate a variety of particle configurations. This has shown that in the case of single particles, local deformation is dependent on both the aspect ratio of the particle and how it is aligned to the active slip system. With the incorporation of a second particle, the interparticle spacing must also be considered.

The ductile fracture process, which consists of the nucleation, growth and coalescence of microvoids, was extensively studied for materials deforming homogeneously. For materials with a non-homogeneous deformation behavior, such as those having hexagonal closed packed (HCP) crystal structure, experimental and numerical data is lacking. Therefore, the fracture properties of materials with such HCP structure, like titanium (used in aerospace and biomedical applications), zirconium (nuclear industry) and magnesium (manufacturing industry) are not well understood. The main research objective of this Ph.D. thesis is to better understand the mechanisms governing fracture in commercially pure (CP) titanium. In particular, the effect of grain orientation on void growth is investigated. The fracture process of CP titanium was visualized in model materials containing artificial holes. These model materials were fabricated using a femtosecond laser coupled with a diffusion bonding technique to obtain voids in the interior of titanium samples. Diffusion bonding was carried out either above or below the phase transformation temperature resulting in different microstructures. Changes in void dimensions during in-situ straining were recorded in three dimensions using x-ray computed tomography. Void growth obtained experimentally was compared with the Rice and Tracey model which predicted well the average void growth. However, a large scatter in void growth was observed experimentally and was explained in terms of differences in grain orientation which was confirmed by crystal plasticity simulations. It was also shown that grain orientation has a stronger effect on void growth than intervoid spacing and material strength. Intervoid spacing, however, appears to control whether the intervoid ligament failure is ductile or brittle. While this study showed a good agreement between experiments and simulations on average, there is no direct void growth comparison for particular grain orientations. In a follow-up study, an experimental approach was developed to directly relate the growth of a void to its underlying grain orientation. This is achieved by first annealing CP titanium samples below the α-β phase transformation temperature, then performing electron backscatter diffraction iii (EBSD) and finally diffusion bonding the samples together. Samples were then tested in x-ray tomography. This study showed the importance of the local state of strain on void growth. Crystal plasticity simulations that take into account the particular grain orientation and the local state of strain were found to predict well experimental void growth. Crystal plasticity simulations confirmed that the orientation of the voidcontaining grain is more important than the orientation of surrounding grains and more important than the volume fraction of voids, in order to determine void growth. This thesis on the growth and coalescence of voids is important to validate and improve the predictions of ductile fracture models and to design new materials with improved fracture properties.

Dans le cadre de cette thèse, nous nous sommes intéressés à la caractérisation des mécanismes de déformation et de leurs interactions pour un zinc polycristallin pur, sous les conditions de chargement complexe et local qui caractérisent l'essai de nanoindentation. En effet, l'interaction des différents mécanismes mis en jeu a généralement été étudiée sur la base de sollicitations dites simples, telles que les essais de traction uniaxiale, biaxiale… Sous l'action d'un chargement uniforme, la prépondérance d'un système particulier sera conditionnée par l'orientation du cristal et par le sens du chargement par rapport à l'axe sénaire. La situation peut être rendue plus complexe dans le cas d'un chargement non simple, comme c'est le cas de l'essai d'indentation. Nous avons réalisé des essais de nanoindentation sur des grains de différentes orientations cristallographiques (mesurées par EBSD), et les résultats obtenus en termes de courbes "charge-profondeur de pénétration" et topographie des empreintes résiduelles ont été analysés. La complexité de l'état de contrainte qui se développe dans le matériau dépend des caractéristiques géométriques de l'indenteur et des orientations cristallographiques en présence, ce qui peut donner lieu à diverses interactions entre les modes de déformation. Ces interactions impacteront directement l'écoulement plastique local du matériau, et par voie de conséquence les propriétés mécaniques macroscopiques du matériau. En adoptant une loi de comportement en plasticité cristalline, nous avons ensuite procédé à la détermination des cissions résolues critiques et des paramètres d'écrouissage pour les différents mécanismes observés. Cette détermination s'est basée sur la résolution d'un problème inverse, au cours duquel nous avons couplé la simulation numérique 3D de l'essai de nanoindentation à l'identification des paramètres de la loi de comportement au moyen d'algorithmes génétiques. La confrontation des résultats expérimentaux et numériques en termes de courbes "charge-profondeur de pénétration" et profils de déformation montrent la bonne adéquation entre les données expérimentales et le modèle identifié. Les résultats obtenus ont ainsi permis de caractériser les mécanismes de déformation observés, et de proposer des perspectives à ce travail. / Within the scope of this thesis, we focused on the characterization of deformation mechanisms and their interactions for pure polycrystalline zinc under complex and local loading conditions such that those involved in a nanoindentation test. Indeed, the interaction between the different mechanisms involved has generally been studied on the basis of so-called simple tests, such as uniaxial or biaxial tensile tests ... Given uniform loading conditions, the predominance of a given deformation system depends on the crystal orientation and the loading direction relative to the crystal c-axis. The situation may be further complicated in case of a complex stress state, as it is the case of the indentation test. We performed nanoindentation tests on grains of different crystallographic orientations (measured by EBSD) and the results (curves "load-penetration depth" and topography of residual imprints) were analyzed. The complexity of the stress state that develops underneath the indenter depends on both the geometrical characteristics of the latter and the crystallographic orientations of the grains, which can give rise to different interactions between the deformation modes. Those interactions will directly affect the local plastic flow, and thus the mechanical properties of the macroscopic material.By using a crystal plasticity model, we have then determined the critical resolved shear stresses and hardening parameters for the observed deformation mechanisms. This determination is based on the solution of an inverse problem, in which we have coupled 3D numerical simulations of the nanoindentation test to genetic algorithms to solve an optimization problem. Comparison between experimental and numerical results in terms of "load-penetration depth" curves and penetration depth profiles show a good agreement between the experimental data and the identified model. The results enabled to characterize the observed deformation mechanisms, and to provide perspectives to this work.