Master of Science / Industrial & Manufacturing Systems Engineering / Shuting Lei / Transparent brittle materials such as glass and silicon dioxide have begun to replace the conventional materials due to the advantageous properties including high strength and hardness, resistance to corrosion, wear, chemicals and heat, high electrical isolation, low optical absorption, large optical transmission range and biocompatibility. However because these materials are extremely hard and brittle, development of an ideal machining process has been a challenge for researchers. Non-traditional machining processes such as abrasive jet and ultrasonic machining have improved machining quality but these processes typically results with issues of poor surface integrity, high tool wear and low productivity. Therefore a machining technique that overcomes the disadvantages of existing methods must be developed. This study focused primarily on improving the machinability and attaining crack-free machined surfaces on transparent brittle materials by inducing micro cracks or seed damages on the subsurface of the materials. The hypothesis was that micro-cracks induced by femtosecond laser would synergistically assist the material removal process by a cutting tool by weakening or softening the material, followed by conventional machining process. Laser induced damages due to varying laser intensities and at different depths in bulk BK7 glass was studied in order to select the optimal laser machining conditions for the experiments. Dimensional and structural profiles of laser cracks are observed using an optical microscope. A comparative study of machined untreated BK7 samples and damage induced BK7 samples was conducted. Due to its simple process kinematics and tool geometry, orthogonal machining is used for the study. Results showed that machining laser-treated samples caused an average 75% force reduction on comparison to machining of untreated samples. Laser treated machined samples were produced without subsurface damages, and reduced tool wear was noted. Overall improved machinability of BK7 glass samples was achieved.

A formação e propagação de fissuras é um fenômeno observado em diversos materiais utilizados na engenharia, como concreto, metais, cerâmicas e rochas. Tendo em vista a grande influência que fissuras têm no comportamento global da estrutura o objetivo deste trabalho consiste na implementação de um modelo de fissura com descontinuidades fortes incorporadas a fim de analisar o processo de fratura em materiais quase-frágeis. A descontinuidade no campo de deslocamentos (descontinuidade forte) é representada através da introdução de graus de liberdade adicionais no interior do elemento finito, sendo esta abordagem denominada enriquecimento elementar (E-FEM). Nestes modelos a fissura pode se propagar em qualquer direção dentro do elemento finito, evitando a necessidade de redefinição da malha em cada etapa, além de fornecer resultados relativamente independentes da malha de elementos finitos utilizada. Por serem internos a cada elemento finito, os graus de liberdade adicionais podem ser eliminados da solução global por condensação estática. Desta forma as descontinuidades são definidas em nível de elemento e o modelo pode ser facilmente implementado em códigos computacionais existentes. O modelo implementado foi proposto por Dvorkin, Cuitiño e Gioia (1990), o qual pertence à classe de modelos com formulação assimétrica estaticamente e cinematicamente consistente (SKON). Esta formulação é caracterizada por garantir o movimento de corpo rígido entre as partes do elemento além de assegurar a continuidade de tensões na linha de fissura, resultando numa matriz de rigidez assimétrica. Diferentes relações constitutivas podem ser utilizadas para descrever o comportamento das regiões com e sem fissura. Portanto, para a região não fissurada, utilizouse um modelo constitutivo elástico linear e para a região fissurada foi analisada a performance de dois modelos constitutivos distintos: linear e exponencial. A capacidade de representar o comportamento de elementos estruturais fissurados foi ilustrada através de exemplos de tração e flexão comparados com outros modelos de fissura existentes e com resultados experimentais. Em relação aos modelos constitutivos para a linha da fissura, o modelo linear não se mostrou adequado por superestimar as tensões de pico além de apresentar um ramo de amolecimento mais frágil. Já o modelo exponencial mostrou-se bastante eficiente representando de forma correta o comportamento de materiais quase-frágeis. / The formation and propagation of cracks is a phenomenon observed in many materials used in engineering, such as concrete, metals, ceramics and rocks. In view of the influence of cracks in the global behavior of the structure, the aim of this work is the implementation of an embedded strong discontinuity model in order to analyze the fracture process in quasi-brittle materials. The discontinuity in the displacement field (strong discontinuity) is represented by the introduction of additional degrees of freedom within the finite element. This approach is called elemental enrichment (E-FEM). The embedded models allow the propagation of crack in any direction within the finite element, avoiding the need of remeshing and providing objective results (mesh independent). The additional degrees of freedom are introduced into the finite element, then these degrees can be eliminated from the global solution by static condensation and the model can be easily implemented in existent computational codes. The model used here was proposed by Dvorkin, Cuitiño and Gioia (1990), which belongs to the statically and kinematically optimal non-symmetric (SKON) formulation. In this formulation, the kinematics that allows for relative rigid body motion and the enforcement of the traction continuity are introduced at element level, resulting a non-symmetric formulation. Different constitutive relations can be used to describe the behavior of the zones with and without cracks. For the zone without cracks it was used a linear elastic model and for the cracked zone it was analyzed the behavior of two different constitutive models: linear and exponential. The ability of the model to represent the behavior of cracked structural elements was illustrated by bending and tensile tests and the results were compared with numerical and experimental data. Regarding the constitutive models for the fracture zone, it was concluded that the linear model was not suitable because it overestimated the maximum stress and promoted a britller softening. In contrast, the exponential model proved to be very efficient to represent the behavior of quasi-brittle materials.

O estudo da hidrogeologia em aqüífero cristalino fraturado apresenta como maior desafio a imprevisibilidade do meio, uma vez que o fluxo ocorre exclusivamente pelas fraturas. Neste sentido informações de atitude, abertura e conectividade de fraturas tornam-se essenciais no entendimento do fluxo da água subterrânea. O presente trabalho busca aplicar o conhecimento da tectônica rúptil para caracterizar o meio fraturado, com intuito de auxiliar o estudo hidrogeológico do aqüífero. A área de estudo está localizada na porção leste do Município de Cotia, SP, no contexto do Complexo Embu, de idade proterozóica, na porção central da Faixa Ribeira. Ocorrem predominantemente rochas granitóides, com permeabilidade primária desprezível, rochas cataclásticas relacionadas às grandes zonas de cisalhamento de Taxaquara e Caucaia do Alto, metassedimentos e coberturas aluviais cenozóicas, estas ao longo das principais drenagens. Os principais eventos tectônicos de caráter rúptil da região estiveram relacionados à reativação mesozóico-cenozóica, inicialmente com a ruptura continental e abertura do Oceano Atlântico Sul e, posteriormente, a formação do Rift Continental do Sudeste do Brasil (RCSB), e seus principais eventos de deformação. Os estudos foram desenvolvidos em duas escalas de trabalho. A primeira de semi-detalhe, em 1:50.000, e a segunda de detalhe, em 1:5.000, esta última em duas porções distintas dentro da área de semi-detalhe. Os resultados obtidos em escala de semi-detalhe, a partir da análise da tectônica rúptil (morfometria, fotogeologia e levantamentos de campo), serviram para direcionar as investigações em escala de detalhe, onde foram usados dados provenientes de sondagens, métodos BHTV (Bore Hole Television), levantamento de perfis de eletrorresistividade e dados hidrogeológicos de poços de monitoramento instalados no aqüífero em meio fraturado. Os estudos de escala de semi-detalhe permitiram delinear lineamentos de direções N-S, E-W, NE-SW e NW-SE, originados pelas diversas fases de deformação rúptil. Esses dados de lineamentos foram correlacionados com dados de atitude de falhas e juntas medidas em afloramento, e, posteriormente, associados às diferentes fases de deformação descritas para o RCSB. Foram definidas seis famílias de juntas: a) família NNW-SSE, com mergulho alto para NE); b) família NE-SW com mergulho sub-vertical; c) família E-W com mergulho alto para NW); d) família WNW-ESE com mergulho alto para NE); e) família NESW com mergulho médio para NW; e f) família NE-SW com mergulho médio para SE. Os resultados dos estudos desenvolvidos em escala de semi-detalhe, somados aos condicionantes geológicos e geomorfológicos das áreas selecionadas para estudos de detalhe, orientaram a instalação de poços de monitoramento verticais e inclinados nessas últimas. Foram instalados 11 poços na área de detalhe A e quatro poços na área de detalhe B. Os poços foram perfilados por televisionamento acústico e as fraturas identificadas nesses poços foram separadas por famílias e subfamílias: Família 1 (sub-horizontal), Família 2A (N-S com mergulho médio para W) e 2B (N-S com mergulho alto para E), Família 3A (NE-SW com mergulho médio para NW) e 3B (NE-SW com mergulho médio para SE) e Família 4 (WNW-ESE sub-vertical). Na maioria dos poços foram realizados ensaios hidráulicos para determinação da condutividade hidráulica (K), que variou de 1,55x10-8 a 2x10-5 cm/s, com média de 2,98x10-6 e desvio padrão de 5,60 x10-6 cm/s. Ensaios de bombeamento indicaram fluxo de água subterrânea através das estruturas N-S, com boa conexão com as estruturas NE-SW. A abertura média das fraturas foi de 1,13x10-3 cm, calculada com base nos valores de condutividade hidráulica correlacionados aos dados estruturais. A partir dos dados hidrogeológicos e estruturais foi proposto um método para se estimar a transmissividade e condutividade hidráulica de um poço instalado no aqüífero fraturado. / The main challenge in a hydrogeological study of a fractured-bedrock aquifer is the unpredictability of media, because the ground-water flows only through the fractures. Informations about strike, dip, opening and connectivity among fractures become essential in order to understand the ground-water flow. This work is an application of the concepts of the brittle tectonics to characterize a fractured media in order to help the hydrogeological study of a fractured-bedrock aquifer. The study area is located at the eastern part of the City of Cotia, State of São Paulo, Southeastern Brazil, in the context of the Proterozoic Embu Complex, in the central part of Ribeira Belt. This area encompass predominantly granitoid rocks (with negligible primary permeability), cataclastic rocks related to the Taxaquara and Caucaia do Alto shear zones, and metasedimentary rocks. Quaternary alluvial deposits occur along the main drainages. The main brittle tectonic events in the region are related to the Mesozoic-Cenozoic reactivation, associated with the continental break-up, the opening of the South Atlantic Ocean, and the formation and deformation of the basins of the Continental Rift of the Southeastern Brazil (CRSB). The study was carried out in two working scales. First in a semi-detail scale, at 1:50,000, of a large area, were brittle tectonic analysis (morphometry, photogeology and field survey in outcrops) was performed. The achieved results were used to select two small areas, within the semi-detail area, for a detailed investigation, on scale of 1:5,000, in which data from rock-boring, BHTV (Bore Hole Television), eletroresistivity and hydrogeology from bedrock monitoring wells were obtained. At a semi-detail scale it was possible to delineate lineaments of N-S, E-W, NE-SW and NW-SE-trending directions, probably originated during several brittle deformational events. Data from lineaments were first correlated with data of fault and joint measured in outcrops and latterly associated with different deformation phases described in CRSB. Six joint families were defined: a) steeply northeastward dipping NNW-SSE-trending family; b) sub-vertically dipping NE-SW trending family; c) steeply northwestward dipping E-W-trending family; d) steeply northeastward dipping WNW-ESE-trending family); e) NE-SW-trending family with moderate dipping towards the NW; and f) NE-SW-trending family with moderate dipping towards the SE. Results of studies in the semi-detail investigation area allowed the selection of sites for drilling of 11 wells in detail area A and 4 wells in the detail area B. All the wells were surveyed by acoustic teleview and the identified fractures were classified in four families and two subfamilies: family 1 (sub-horizontal to gently dipping); family 2A (N-S-trending with moderate dipping towards the W) and 2B (NS-trending with steep dipping towards the E); family 3A (NE-SW-trending with moderate dipping towards the NW) and 3B (NE-SW-trending with moderate dipping towards the SE); and family 4 (WNW-ESE-trending with sub-vertical dipping). Hydraulic conductivity (K) measured in the wells ranged from 1.55x10-8 to 2x10-5 cm/s, with a mean of 2.98x10-6 and standard deviation of 5.60 x10-6 cm/s. Pumping tests showed the groundwater flow through N-S-trending structures and good connection with NE-SW-trending structures. The average opening of fractures attained 1.13x10-3 cm and was calculated on the basis of the amount of hydraulic conductivity correlated with structural data. Based on structural and hydrogeological data it was proposed a method for estimating the transmissivity and hydraulic conductivity of a well installed in the fractured-bedrock aquifer.

Submitted by Cláudia Bueno (claudiamoura18@gmail.com) on 2016-01-07T16:28:03Z No. of bitstreams: 2 Dissertação - Dannilo Carvalho Borges - 2015.pdf: 3073251 bytes, checksum: 5efdd805f96baaf801696402a7ed039b (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2016-01-08T11:35:34Z (GMT) No. of bitstreams: 2 Dissertação - Dannilo Carvalho Borges - 2015.pdf: 3073251 bytes, checksum: 5efdd805f96baaf801696402a7ed039b (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2016-01-08T11:35:34Z (GMT). No. of bitstreams: 2 Dissertação - Dannilo Carvalho Borges - 2015.pdf: 3073251 bytes, checksum: 5efdd805f96baaf801696402a7ed039b (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-08-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This work deals with a proposed numerical modeling of mechanical behavior in brittle heterogeneous materials, such as concrete. For this propose, a two-dimensional meso-scale model is presented. The material is considered as a three-phase material consisting of interface zone, matrix and inclusions – each constituent modeled by an appropriate constitutive model. The Representative Volume Element (RVE) consists of inclusions idealized as circular shapes randomly placed into the RVE. The interface zone is modeled by means of cohesive contact finite elements the inclusions are considered as linear elastic media and matrix region is considered as elastoplastic material. The main goal here is to show that simple constitutive modeling of the microstructure can be employed with a computational homogenization-based approach as an alternative to complex macroscopic constitutive models for the mechanical behavior of the heterogeneous materials using a finite element procedure within a purely kinematical multi-scale framework. A set of numerical examples, involving the microcracking and plasticity processes, is provided in order to illustrate the qualitative performance of the proposed modeling. / Este trabalho apresenta uma proposta para a modelagem numérica do comportamento mecânico da microestrutura de materiais heterogêneos e frágeis, tal como o concreto. Para este fim, apresenta-se um modelo 2D de escala mesoscópica. O material é considerado como composto por três fases consistindo de zona de interface, matriz e inclusões, onde cada constituinte é modelado adequadamente. O Elemento de Volume Representativo (EVR) consiste de inclusões idealizadas de forma circular aleatoriamente dispostas no EVR. A zona de interface é modelada por meio de elementos finitos coesivos e de contato e as inclusões são modeladas como meio elástico linear. Já a matriz é considerada como material elastoplástico. O principal objetivo é mostrar que modelos constitutivos simples empregados na microestrutura em conjunto com uma formulação baseada numa homogeneização computacional são uma alternativa aos modelos constitutivos macroscópicos complexos para o comportamento mecânico de materiais heterogêneos usando um procedimento baseado no Método dos Elementos Finitos no âmbito de uma teoria Multiescala. Uma série de exemplos envolvendo processos de microfissuração e plasticidade é apresentada de modo a ilustrar qualitativamente a performance da modelagem proposta.

A crescente necessidade por parte da indústria óptica e eletrônica por componentes que possuam características como elevada precisão de forma e acabamento em materiais frágeis, vem despertando um grande interesse nas pesquisas dos processos de usinagem de ultraprecisão. Neste sentido, a retificação e o torneamento de materiais frágeis com ferramentas de diamante têm se destacado como substitutos dos processos tradicionais de fabricação de precisão como a lapidação e polimento. Propõe-se o estudo da retificação de ultraprecisão de materiais frágeis através da realização de ensaios de corte em uma retificadora de ultraprecisão com rebolo de diamante de forma a estabelecer os parâmetros de corte compatível com a remoção dúctil de material. Foram usinadas amostras de silício monocristalino através de retificação tangencial de mergulho plana sob diferentes condições de usinagem para posteriormente avaliar a integridade superficial da peça. A amostra foi montada sob um aparato especial com uma inclinação conhecida a fim de se realizar um corte em cunha. Com isso, três situações distintas de remoção de material foram investigadas: totalmente dúctil, dúctil-frágil e totalmente frágil. O parâmetro de corte variado foi a profundidade de corte; o avanço e velocidade de corte foram mantidos constantes. / The requirements of components that demand high precision finishes and form in brittle materials has roused a huge interest in the ultraprecision machining process in the optical and electronic industries. The diamond grinding and turning of brittle materials have been used more and more as replacements to the tradicional lapping and polishing. The study of ultraprecision grinding of brittle materials is proposed. Cutting experiments on an ultraprecision grinding machine using a diamond wheel were carried out stablish cutting parameters compatible with ductile material removal. Silicon samples were machined through flat plunging tangencial grinding under different machining conditions and the sample surface integrity was assessed. The sample was mounted on a special apparatus with a known inclination so as to carry out a edge cutting. Thereby, three different material removal situations were investigated: whole ductile, ductile/brittle, whole brittle. The depth of cut was the parameter varied. Feedrate and cutting speed of the wheel were held constant.

Les aciers ODS (Oxide Dispersion Strengthened) sont envisagés comme matériau de gainage du combustible pour les futurs réacteurs nucléaires au sodium de Génération IV. Ces matériaux présentent une excellente résistance au fluage à haute température et au gonflement sous irradiation, mais des interrogations subsistent quant à leur propriété de résilience. L'objectif de ce travail de thèse est alors de comprendre l'effet de différents paramètres (composition chimique, texture, vieillissement thermique…) sur le comportement à l'impact des aciers ODS et sur l'apparition de la rupture de type fragile. L'objectif à terme est d'appréhender l'apparition de ce type de rupture afin d'assurer l'intégrité des composants en acier ODS en conditions normales ou incidentelles.Dans un premier temps, cette étude a permis d'évaluer la stabilité de deux nuances d'acier ODS contenant 14%Cr et 18%Cr dans des conditions de vieillissement thermique. La nuance à 18%Cr a été écartée à cause de la formation d'une phase intermétallique fragilisante de type sigma à 600°C. Par contre, la nuance à 14%Cr est plus prometteuse puisque son comportement est resté stable entre 400°C et 600°C pour une durée de vieillissement maximale de 10000 heures, et ce même si des précipités de phases alpha', de Laves Fe2W et des carbures de chrome ont été observés. D'autre part, la texture morphologique, caractérisée par des grains allongés selon la direction de filage, est propice à la propagation de fissures intergranulaires selon cette direction. La texture cristallographique régit, quant-à elle, les micromécanismes de clivage. En effet, les entités microstructurales qui contrôlent la propagation du clivage sont des groupes de grains faiblement désorientés les uns des autres d'un point de vue cristallographique, et qui sont associés à la notion de grains effectifs. Enfin, le comportement en traction et en flexion d'une nuance d'acier ODS à 14%Cr a été modélisé, et un critère de rupture fragile basé sur une valeur de contrainte critique a été proposé. Ce modèle nous a alors permis de simuler des essais sur différentes géométries et de prédire l'apparition de la rupture fragile sur cette nuance. / The Oxide Dispersion Strengthened (ODS) steels have been identified as potential materials for fuel cladding in Generation IV nuclear reactors. They are characterized by a very good resistance to swelling under irradiation and to high temperature creep, but questions still remain about the impact toughness of these materials. The first aim of this work is to understand the effects of different parameters (chemical composition, texture, thermal ageing...) on the impact behaviour of ODS steels. The final objective is to predict the occurrence of the brittle failure on the ODS steel components in normal or incidental conditions.Firstly, this study assesses the stability of two ODS steel grades containing either 14%C or 18%Cr during thermal ageing. The 18%Cr steel grade has been discarded due to the growing of the brittle intermetallic sigma phase at 600°C. However, the 14%Cr steel grade showed a promising behaviour given the stability of its mechanical properties after thermal ageing between 400°C and 600°C, for a maximum duration of 10000 hours, and regardless of the presence of chromium carbides, alpha' and Laves phases. Then, the morphological texture characterized by elongated grains along the extrusion direction enhances the propagation of intergranular cracks along this direction. Meanwhile, the crystallographic texture controls the cleavage micromechanisms. Indeed, the microstructural entities that control the propagation of the cleavage crack are groups of grains which are characterized by low crystallographic misorientations and referred to as effective grains. At last, the tensile and bending behaviour of a 14%Cr ODS steel grade has been modelled. A brittle failure criterion based on a critical stress value had also been suggested. This model allowed us to simulate mechanical tests on different geometries and to predict the occurrence of the brittle failure.

Le massif de marbres de Saint-Béat se situe dans la Zone Interne Métamorphique des Pyrénées centrales françaises. Il est formé de sédiments mésozoïques métamorphisés lors de l'événement métamorphique extensif Haute température - Basse pression classiquement décrit dans les Pyrénées. L'objet de cette thèse est de caractériser la déformation ductile et la fracturation qui affectent ce massif, et de déterminer dans quelle mesure les processus responsables de la fracturation peuvent être influencés par l'état de déformation. Les caractéristiques de la déformation des marbres et de ses roches encaissantes seront déterminées essentiellement grâce aux orientations préférentielles des axes de la calcite (marbre) et du quartz (socle paléozoïque), déterminées à l'aide de mesures à l'EBSD. L'observation en lame mince des textures de la calcite permet de quantifier les taux de déformation et d'estimer la température de cette déformation. Combinés aux observations de terrain, ces résultats permettent de proposer un modèle structural retraçant l'évolution du bassin depuis l'extension aptio-albienne jusqu'à son état actuel. La fracturation du massif est ensuite regardée à différentes échelles, depuis la photo aérienne jusqu'à l'affleurement. Quatre zones seront étudiées en détail afin de classifier les fractures par familles de mêmes caractéristiques et de reconstituer localement les paléo-régimes de contraintes. On comparera les tenseurs obtenus avec les régimes tectoniques décrits dans la littérature. Trois faciès légèrement différents de marbre servent de base à une étude visant à discuter de l'état d'anisotropie de la roche. Ainsi, des essais en compression simple, en traction indirecte et des mesures de vitesses acoustiques sont pratiqués sur des carottes orientées réalisées dans ces trois faciès ; leurs résultats donnent accès aux caractéristiques internes de la roche. On comparera et discutera les résultats d'un faciès à l'autre, et au sein du même faciès, d'une orientation à l'autre. Les résultats obtenus sur la fracturation servent de guide à la classification des fractures observées sur les fronts de tirs, lors du creusement du tunnel routier de Saint-Béat, recoupant le massif sur plus d'un kilomètre du Nord au Sud. / The Saint-Béat massif, composed of different marble facies, is part of the Internal Metamorphic Zone of the French Central Pyrenees. It is formed by Mesozoic sediments metamorphosed by the High Temperature – Low Pressure extensional event, classically described in the Pyrenees. The aim of this work is to characterize the deformation and fractured state of the massif, and to understand how the former can constrain the latter. The ductile deformation state is recorded in calcite grains for Mesozoic rocks, or in quartz grains for Paleozoic rocks. Their preferential lattice orientations have been measured with the EBSD method. Calcite texture observations give indications about deformation rates and temperatures. These results, in addition with field observations, allow us to rebuild the structural evolution of the massif, from the Albian extension. Fracturing along the massif is described at different scales. Four outcrops are selected in order to characterize and to classify fractures, and locally reconstruct the paleo stress tensor. The obtained tensors are presented and compared to those already published. Three slightly different marble facies are used to discuss the degree of anisotropy of the rock. Mechanical experiments such as compression tests, tensile tests and velocity measurements of elastic waves are carried out on oriented cores within these three facies. These results provide internal rock characteristics which are discussed and compared for the three facies, and for different orientations of the cores.

Nous avons étudié la sensibilité du T91 à la fragilisation par le sodium liquide. Une procédure expérimentale a été mise en place afin de procéder à des essais mécaniques en sodium sous atmosphère inerte. Grâce à l’introduction d’une étape préliminaire d’exposition au sodium, la mouillabilité du T91 par le sodium liquide et la structure de l’interface sodium/acier ont pu être étudiés en fonction des différents paramètres d’exposition. Une réduction significative des propriétés mécaniques est observée quand le mouillage de l’acier par le sodium est bon. L’utilisation de différentes teneurs en oxygène et en hydrogène suggère que l’oxygène joue un rôle primordial dans l’amélioration de la mouillabilité du T91. La sensibilité du phénomène de fragilisation à la vitesse de sollicitation et à la température a été caractérisée. A partir de ces résultats, l’existence d’une transition fragile/ductile, fonction de ces deux paramètres a été mis en évidence. L’étude de cette transition suggère qu’une étape de diffusion du sodium dans les joints de grains du T91 est limitante pour la fragilisation. Des analyses en microscopie électronique en transmission et par cartographies d’orientation de fissures arrêtées ont également permis de constater que le mode de rupture est interlatte ou intergranulaire, fournissant ainsi une image cohérente du processus de fragilisation. La même méthodologie a été appliquée à l’acier non allié XC10. Les résultats montrent un comportement en tous points similaire à celui de l’acier T91 et suggèrent donc un mécanisme commun pour les aciers cubiques centrés. De plus, ils confirment que la transition fragile ductile observée semble la conséquence d’une vitesse de propagation de fissure fragile relativement limitée. Cette propagation est activée thermiquement avec une énergie d’activation d’environ 50 kJ/mol. Enfin, il a été montré que l’acier austénitique 304L est également sensible à la fragilisation par le sodium liquide. Certaines surfaces de rupture témoignent clairement d’une rupture intergranulaire, cependant des interrogations persistent à propos du chemin de fissuration. / We studied the sensitivity of T91 steel to embrittlement by liquid sodium. An experimental procedure was set up to proceed to mechanical testing in sodium under an inert atmosphere. The introduction of a liquid sodium pre-exposure step prior to mechanical testing enabled the study of both the wettability of T91 by sodium and the structure of the sodium steel/interface as a function of the exposure parameters. The mechanical properties of T91 steel are significantly reduced in liquid sodium provided the wetting conditions are good. The use of varying oxygen and hydrogen concentrations suggests that oxygen plays a major role in enhancing the wettability of T91. The sensitivity of the embrittlement to strain rate and temperature was characterized. These results showed the existence of a ductile to brittle transition depending on both parameters. Its characterization suggests that a diffusion step is the limiting rate phenomenon of this embrittlement case. TEM and EBSD analysis of arrested cracks enabled us to establish that the fracture mode is interlath or intergranular. This characteristic is coherent with the crack path commonly reported in liquid metal embrittlement. A similar procedure was applied to the unalloyed XC10 steel. The results show a behavior which is similar to the one of T91 steel and suggest a common mechanism for liquid sodium embrittlement of body centered cubic steels. Moreover, they confirm that the ductile to brittle transition seems associated with a limited crack propagation rate. The propagation is thermally activated with activation energy of about 50 kJ/mol. Finally, it was shown that 304L austenitic steel is sensitive to liquid sodium embrittlement as well. Some fracture surfaces testify of an intergranular fracture mode, but some questions still remain about the crack path.

L’objectif de cette thèse est d’étudier le comportement à la rupture des matériaux fragiles avec une approche capable de relier l’amorçage de fissures à leurs propagations. On adopte la méthode des éléments discrets (DEM), avec le matériau composé d'un ensemble bidimensionnel régulier de particules en contact. Ceci qui nous permet de dériver une expression pour le facteur d'intensité de contraintes en fonction des forces et des déplacements relatifs des deux contacts adjacents à la pointe d’une fissure. Un critère de rupture classique, basé sur la ténacité du matériau, est ensuite adopté pour l'analyse de la propagation des fissures, représenté par la perte des forces de contact entre les particules. La vérification de la formulation est faite par la comparaison des simulations numériques à des solutions classiques de la mécanique de la rupture en mode I, mode II et mode mixte. Ensuite, on étend l’application du critère discret au comportement d’un matériau sain (non pré-fissuré), soumis à des contraintes homogènes, pour lequel on retrouve un critère de rupture du type Rankine. Le résultat final est un modèle discret simple totalement compatible avec les approches continues usuelles, et qui ne demande pas d’essais de calibration, typiques des approches discrètes conventionnelles. / The objective of this thesis is to study the fracture behavior of brittle materials by an approach which relates crack initiation to crack growth. We adopt the discrete element method (DEM) and we represent the material by a 2D regular set of particles in contact. This allows us to derive an expression for the stress intensity factor as a function of the forces and relative displacements of two adjacent contacts at the crack tip. A classical failure criterion, based on the material’s toughness, is then adopted for the analysis of crack propagation, represented by the loss of contacts forces between particles. The formulation is verified by the comparison of numerical simulations to classical solutions of fracture mechanics in mode I, mode II and mixed mode. Afterwards, we apply our discrete criterion to uncracked materials under homogenous stress conditions, obtaining a Rankine like behavior.. The work results in a simple discrete model which is totally compatible to continuum mechanics, where no calibration tests are required, in contrast to most of discrete approaches.

Nous proposons une nouvelle approche inspirée des modèles d'endommagement non-locaux pour décrire la ruine des matériaux quasi-fragiles désordonnés. Les hétérogénéités matériaux sont introduites à une échelle continue mésoscopique via des variations spatiales de la résistance à l'endommagement alors que le mécanisme de redistribution des contraintes est décrit à travers une fonction d'interaction que l'on peut faire varier. L'évolution spatio-temporelle de l'endommagement est déterminée à partir du principe de conservation d'énergie et caractérisée via une étude statistique des précurseurs à la rupture. Cette approche nous permet de prédire la valeur des seuils de localisation et de rupture en fonction de la nature des redistributions. A l'approche de la rupture, nous mettons également en évidence une augmentation en loi de puissance du taux d'énergie dissipée ainsi qu'une longueur de corrélation, supportant l'interprétation de la rupture quasi-fragile comme un phénomène critique. En effet, nous démontrons que notre model d'endommagement s'apparente à la loi d'évolution d'une interface élastique évoluant dans un milieu désordonné. Cette analogie nous permet d'identifier les paramètres d'ordre et de contrôle de cette transition critique et d'expliquer les invariances d'échelle des fluctuations dans la limite champ moyen. Enfin, nous appliquons ces concepts théoriques à travers l'étude expérimentale de la compression d'un empilement bidimensionnel de cylindres élastiques. Notre approche permet de décrire de façon quantitative la réponse mécanique non-linéaire du matériau, et en particulier la statistique des précurseurs ainsi que la localisation des déformations. / We propose a novel approach inspired from non-local damage continuum mechanics to describe damage evolution and quasi-brittle failure of disordered solids. Heterogeneities are introduced at a mesoscopic continuous scale through spatial variations of the material resistance to damage. The central role played by the load redistribution during damage growth is analyzed by varying the interaction function used in the non-local model formulation. The spatio-temporal evolution of the damage field is obtained from energy conservation arguments, so that the formulation is thermodynamically consistent. We analytically determine the onsets of localization and failure that appear controlled by the redistribution function. Damage spreading is characterized through a complete statistical analysis of the spatio-temporal organization of the precursors to failure. The power law increase of the rate of energy dissipated by damage and an extracted correlation length close to failure supports the interpretation of quasi-brittle failure as a critical phenomena. Indeed, we establish a connection between our damage model and the evolution law of an elastic interface driven in a disordered medium. It allows to identify the order and control parameters of the critical transition, and capture the scale-free statistical properties of the precursors within the mean field limit. Finally, we experimentally investigate the coaction of localized dissipative events and elastic redistributions in disordered media via compression experiments of two-dimensional arrays of hollow soft cylinders. Our experimental observations show a quantitative agreement with the predictions derived following our approach.