Cette étude est consacrée à la modélisation numérique de l'endommagement pour des matériaux poreux ductiles et à des applications aux géomatériaux. On rappelle et analyse d'abord au chapitre 1 différentes approches d'endommagement pour la classe de matériaux étudiés. L'implantation numérique du modèle basé sur un critère macroscopique de type Gurson, récemment proposé par Monchiet et al.(2007) est effectué au chapitre 2. La bonne performance de ce nouveau modèle est vérifiée en comparant ses prédictions numériques à celles d'autres modèles de type Gurson. Dans le chapitre 3, les modélisations numériques sont étendues aux géomatériaux en implantant dans un code éléments finis un nouveau modèle formulé sur la base du critère de Guo et al.(2008) qui prend en compte de la compressibilité de la matrice. L'adéquation du modèle développé aux géomatériaux est démontrée à travers l'étude du comportement d'une craie. L'implantation numérique et l'analyse du modèle basé sur une version anisotrope du critère proposé par Monchiet et al.(2007) sont présentées au chapitre 4. Les résultats numériques issus de cette implantation sont discutés en les comparant aux ceux des autres modèles existants. Ils apparaissent tout à fait satisfaisants / This study is devoted to the numerical modeling of damage for ductile porous materials and its applications to geomaterials. We first recall and analyze in chapter 1 different approaches of ductile damage. We then propose in chapter 2 a model based on the isotropic Gurson-type macroscopic criterion established by Monchietet al.(2007). This model is implemented numerically in ABAQUS Finite Element software. The good performance of this new model is checked by comparing its numerical predictions with the ones provided by other Gurson-type models. In chapter 3, numerical modelings are performed for geomaterials by implementing an appropriate model (based on Guo et al.(2008) criterion) in ABAQUS. The adequacy of the considered model to geomaterials is demonstrated through the study of a chalk. The numerical implementation and analysis of an anisotropic model based on the anisotropic version of the criterion proposed by Monchiet et al.(2007) are presented in chapter 4. The numerical results derived from this implementation are discussed with the results of others existing models. They appear to be quite satisfactory.

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In this work, the simulation of monotonic fracture in ductile metals was studied and a method of predicting damage-based fracture propagation was introduced. Traditional methodologies for predicting stable crack growth were investigated, and an error analysis was performed to show the suitability of the fracture simulation method chosen for this study. J2 plasticity was investigated for its applicability in predicting crack advance direction for mode-I and mixed-mode simulations. A two parameter crack advance criterion was introduced, and crack propagation simulations were performed to show the suitability of the new fracture criterion that is dependent on damage. J2 plasticity was modified in an attempt to capture the damage mechanisms occurring in front of the crack tip. The end result of this research is a computational tool that is capable of predicting the crack propagation path based on physical and measurable material parameters without knowledge of the crack trajectory a priori while also allowing the constitutive model for the material response to be readily changed. An error analysis was also performed on the existing equations of crack surface displacements for symmetric cracks emanating from a circular hole in an infinite plate subjected to remote stress and stress applied to a segment of the crack surface. New equations were developed for crack surface displacements for symmetric cracks emanating from the circular hole in an infinite plate subjected to a remote stress.

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The aim of this thesis is to propose a more general understanding of the influence of the thickness of the plate and of the microstructural and mechanical properties of the material on the resistance to ductile tearing in thin metallic plates. The objective is to attempt unifying different observations made in the literature together with the results of a new extensive experimental campaign. The final goal is to develop predictive simulation tools with a micromechanics-based foundation. In order to reach this objective, a detailed experimental campaign has been performed concerning the fracture behavior of the aluminium alloy 6082, complemented by experiments on a stainless steel A316L and on a set of 14 other materials. In a first modelling effort, we propose very simple closed-form models in order to separate the different contributions to the total work of fracture in thin plates: the work of necking and the work of damage and material separation. The respective contributions are compared and an unique explanation of the different behaviors observed experimentally is proposed. In a second modelling step, we develop a full 3D numerical tool based on cohesive elements for simulating crack propagation in thin ductile plates. Three different methods are proposed to calibrate the parameters of the model in order to reproduce the experimental data and to extrapolate the results to other material properties or geometric conditions. Finally, the parameters of the cohesive zone model are justified using micromechanics-based arguments. / Le but de cette thèse est de proposer un modèle général à base micromécanique permettant de comprendre l’influence de l’épaisseur de la tôle ainsi que de la microstructure et des propriétés mécaniques du matériau sur la résistance à la rupture ductile de plaques minces métalliques. L’objectif est d’essayer d’unifier les différentes observations de la littérature ainsi que les résultats d’une nouvelle campagne expérimentale afin d’aboutir au développement d’outils numériques prédictifs. Pour atteindre cet objectif, nous avons réalisé une campagne d’essais concernant le comportement à la rupture de différents matériaux. Cette campagne a été menée en profondeur sur l’alliage d’aluminium 6082 et de manière moins approfondie sur un acier inoxydable A316L ainsi que sur 14 autres matériaux. Dans un premier temps, nous présentons une série de modèles semi-analytiques simples dont le but est de séparer les différentes contributions au travail de rupture total dans les tôles minces : le travail de striction et le travail d’endommagement du matériau. Ces deux contributions sont ensuite comparées et nous proposons une explication qui reprend les différents comportements observés expérimentalement. Dans un deuxième temps, nous développons un outil numérique 3D complet destiné à simuler la propagation de fissures dans les tôles minces ductiles et qui utilise des éléments cohésifs. Trois méthodes différentes sont proposées pour calibrer les paramètres du modèle de manière à reproduire les données expérimentales et à permettre l’extrapolation des résultats à d’autres matériaux ou d’autres épaisseurs de tôles. Finalement, les paramètres du modèle de zone cohésive sont justifiés grâce à des arguments à fondement micromécanique.

­^¤åºK­n The purpose of this thesis is to propose the rules for designing a grinding system with nano-order of relative vibration. Such a system is aimed to perform the ductile-regime grinding for brittle materials. The study will examine the influence of the structure stiffness, damping, and mass of the grinding system on the relative vibration between grinding wheel and work. Accordingly, the design rules will be proposed. To analyze the relative vibration, the radial and axial models for a grinding system were built. The trends of relative vibration between grinding wheel and work were then analyzed under three kinds of vibration sources. Because of the nonlinearity of system equations, the 4th order Runge-Kutta numerical method was used to calculate the solutions. In addition, the genetic algorithm was used to search an optimum design. From the analysis, several design rules for grinding system, which can minimize the relative vibration, were proposed.

In this study the effects of Sn content and isothermal transformation temperature on the ultimate tensile strength (UTS), elongation and hardness of austempered ductile cast iron (ADI) was investigated. To determine the possible effect of Sn on these properties the Sn content of standard GGG30, GGG40 and GGG50 materials were taken as reference, whose chemical compositions vary from 0,016 to 0,050% in terms of Sn. However the Sn content was increased to a maximum of 0,26% for investigating the effect of Sn on mechanical properties. The test specimens were obtained from standard 1-in Y-blocks and austenitized at 925oC for 1 hour and austempered at 350oC,375oC, 400oC and 420oC for 60 minutes in a salt bath. Microstructural examination of the specimens were also done. To determine the amount of retained austenite of the specimens X-Ray analyses were used. As a result of the study The UTS increases up to 0,1% Sn and then levels off. A similar behavior is observed in the variation of hardness. Percent elongation decreases and falls to a minimum around 0,1% Sn, from this point on, a slight increase is observed. Strength and hardness increase while percent elongation decreases with decreasing temperatures.

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The main aim of this dissertation is to relate measurable and hopefully controllable features of a material's microstructure to its observed failure modes to provide a basis for designing better materials. The understanding of creep in materials used at high temperatures is of prime engineering importance. Single crystal Ni-based superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at very high temperatures. In contrast to current theories, creep tests on Ni-based superalloy specimens have shown size dependent creep response termed as the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep tests were performed on uncoated Ni-based single crystal superalloy sheet specimens with two thicknesses and under two test conditions: a low temperature high stress condition and a high temperature low stress condition. At the high temperature, surface oxidation induced microstructural changes near the free surface forming a layered microstructure. Finite element calculations showed that this layered microstructure gave rise to local changes in the stress state. The specimens also contained nonuniform distribution of initial voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This motivated a basic mechanics study of porosity evolution in single crystals subjected to creep for a range of stress states. The study was performed using three-dimensional finite deformation finite element analysis of unit cells containing a single initially spherical void in a single crystal matrix. The materials are characterized by a rate-dependent crystal plasticity constitutive relation accounting for both primary and secondary creep. The effect of initial void spacing and creep exponent was also explored. Based on the experimental observations and results of finite element calculations a quantitative mechanistic model is proposed that can account for both bulk and surface damage effects and assess their relative roles in the observed thickness debit effect. Another set of calculations aim at relating the crack growth resistance and fracture surface morphology to material microstructure for ductile structural metals. The process that governs the ductile fracture of structural materials at room temperature is one of nucleation, growth and coalescence of micron scale voids, and involves large plastic deformations. Experimental studies have shown that fracture surfaces in a wide variety of materials and under a wide variety of loading conditions have remarkable scaling properties. For thirty years, the hope to relate the statistical characterization of fracture surfaces to a measure of a material's crack growth resistance has remained unfulfilled. Only recently has the capability been developed to calculate sufficient amounts of three dimensional ductile crack growth in heterogeneous microstructures to obtain a statistical characterization of the predicted fracture surfaces. This development has enabled the exploration of the relation of both fracture toughness and fracture surface statistics to material properties and microstructure when the fracture mechanism is one of void nucleation, growth and coalescence. The relation of both toughness and the statistical properties of fracture surfaces in calculations of heterogeneous microstructures to various microstructural features is discussed and a remarkable correlation between fracture surface roughness and fracture toughness is shown for the first time.