This thesis describes the measurement technique of fast-neutron tomography for assessing spatial distributions of steam and water in two-phase flows. This so-called void distribution is of importance both for safe operation and for efficient use of the fuel in light water reactors, which compose the majority of the world’s commercial nuclear reactors. The technique is aimed for usage at thermal-hydraulic test loops, where heated two-phase flows are being investigated under reactor-relevant conditions. By deploying portable neutron generators in transmission tomography, the technique becomes applicable to stationary objects, such as thermal-hydraulic test loops. Fast neutrons have the advantage of high transmission through metallic structures while simultaneously being relatively sensitive to the water/void content. However, there are also challenges, such as the relatively low yield of commercially available fast-neutron generators, the tendency of fast neutrons to scatter in the interactions with materials and the relatively low efficiency encountered in fast-neutron detection. The thesis describes the design of a prototype instrument, FANTOM, which has been assembled and demonstrated. The main design parameters have been optimized to achieve maximal signal count rate in the detector elements, while simultaneously reaching an image unsharpness of ≤0.5 mm. Radiographic projections recorded with the assembled instrument are presented, and the performance parameters of FANTOM are deduced. Furthermore, tomographic reconstruction methods for axially symmetric objects, which is relevant for some test loops, have been developed and demonstrated on measured data from three test objects. The attenuation distribution was reconstructed with a radial resolution of 0.5 mm and an RMS error of 0.02 cm-1, based on data recorded using an effective measurement time of 3.5 hours per object. For a thermal-hydraulic test loop, this can give a useful indication of the flow mode, but further development is desired to improve the precision of the measurements. Instrument upgrades are foreseen by introducing a more powerful neutron generator and by adding detector elements, speeding up the data collection by several orders of magnitude and allowing for higher precision data. The requirements and performance of an instrument for assessment of arbitrary non-symmetric test loops is discussed, based on simulations.

During this thesis work a coupled thermo-mechanical finite element model (FEM) was builtto simulate hot rolling in the blooming mill at Sandvik Materials Technology (SMT) inSandviken. The blooming mill is the first in a long line of processes that continuously or ingotcast ingots are subjected to before becoming finished products. The aim of this thesis work was twofold. The first was to create a parameterized finiteelement (FE) model of the blooming mill. The commercial FE software package MSCMarc/Mentat was used to create this model and the programing language Python was used toparameterize it. Second, two different pass schedules (A and B) were studied and comparedusing the model. The two pass series were evaluated with focus on their ability to healcentreline porosity, i.e. to close voids in the centre of the ingot. This evaluation was made by studying the hydrostatic stress (σm), the von Mises stress (σeq)and the plastic strain (εp) in the centre of the ingot. From these parameters the stress triaxiality(Tx) and the hydrostatic integration parameter (Gm) were calculated for each pass in bothseries using two different transportation times (30 and 150 s) from the furnace. The relationbetween Gm and an analytical parameter (Δ) was also studied. This parameter is the ratiobetween the mean height of the ingot and the contact length between the rolls and the ingot,which is useful as a rule of thumb to determine the homogeneity or penetration of strain for aspecific pass. The pass series designed with fewer passes (B), many with greater reduction, was shown toachieve better void closure theoretically. It was also shown that a temperature gradient, whichis the result of a longer holding time between the furnace and the blooming mill leads toimproved void closure.

L'énergie nucléaire est l'une des plus propres en matière d'émission de gaz à effet de serre et, malgré ses atouts, elle n'est développée que dans quelques pays du monde. La sûreté reste une question ouverte pour l'avenir de cette énergie après l'accident de Fukushima. En France, la loi de 2006 sur la gestion des déchets soutient le développement d'une nouvelle génération de réacteurs nucléaires et du prototype de Réacteur Technologiquement Avancé au Sodium pour la Démonstration Industrielle (projet ASTRID) qui vise à apporter une réponse industrielle et technologique à de nombreux enjeux de ce siècle. L'une des préoccupations de la technologie du Réacteur à Neutrons Rapide et caloporteur sodium (RNR Na) est la perte de ce dernier car elle pourrait entraîner un emballement de la réaction en chaîne si l'effet en réactivité de vidange sodium (SVRE) est positif. Lorsque le sodium est retiré du cœur, deux effets antagonistes se produisent qui affectent l'équilibre neutronique: l'un augmente la réactivité du cœur et est appelé la composante centrale (CC) et l'autre est la composante de fuite (LC) avec un effet négatif sur la réactivité. Maximiser la dernière composante est l'une des réponses pour augmenter la sûreté inhérente aux RNR-Na. C'est pourquoi le CEA a développé un concept de cœur innovant: le «Cœur à Faible Vidange» (CFV) qui donne une SVRE négatif. Cependant, de telles innovations doivent être validées expérimentalement et l'incertitude sur cet effet en réactivité doit être maîtrisée. En soutien au développement des RNR Na : la base de données expérimentale existante est assez importante (PRE-RACINE, CIRANO, BFS). / The nuclear energy is one of the cleanest energy in regard of greenhouse gas emission and despite its assets is only developed in few countries in the world. Safety remains an open issue for the future of this energy after the Fukushima accident . In France the 2006 law on the waste management ensures the development of a new generation of nuclear reactor and has lead to the Advanced Sodium Technology Reactor for Industrial Demonstration (ASTRID) which aims to bring an industrial and technological advanced answer to many issues of this century. One of the concerns in the sodium cooled fast reactor (SFR) technology is the loss of sodium coolant accident because it might lead to a snowball effect in the chain reaction if the sodium void reactivity effect (SVRE) is positive. When the sodium is removed from the core, two antagonistic effects arise that affect the neutron balance: one increases the reactivity of the core and is called the central component (CC) and the other is the leakage component (LC) with a negative feedback on the reactivity. Maximizing the last component is one of the answer to increase the inherent safety of the SFRs. That is why the CEA has developed an innovative core design: the “Cœur à Faible Vidange” (CFV : Core with low void effect) which exhibits a negative SVRE. However, such innovations have to be experimentally validated and the uncertainty on this reactivity effect has to be mastered. In support of SFRs the existing experimental data base is quite large (PRE-RACINE, CIRANO, BFS).

Encontramos-nos em um momento histórico que privilegia a rediscussão de certos conceitos referentes a diversas áreas do conhecimento, indo da arte à biologia, da filosofia à matemática, passando pela arquitetura. Entre estas diversas questões atuais, este trabalho escolhe a do vazio como assunto central, tratado em duas instâncias: uma, as suas configurações urbanas; a outra, seu papel como elemento conceitual na arquitetura contemporânea. Escolhendo projetos de arquitetura que incluem tais questionamentos entre suas preocupações centrais, quer-se evidenciar a relação de interdependência e de enriquecimento mútuo entre dois tipos de vazio: os resultantes da dinâmica urbana tipicamente metropolitana, e aqueles concebidos no processo projetual de arquitetura. Através de cinco projetos, assinados por cinco arquitetos contemporâneos em quatro diferentes metrópoles, mostra-se o papel específico de cada um destes vazios. Além disso, a primeira parte aborda a questão da memória e da temporalidade, em São Paulo. A segunda parte trata do fim da história única, em Berlim, segundo a definição hegeliana. Finalmente, a terceira parte, relativa às cidades japonesas de Osaka e Tokyo, procura discutir a concepção japonesa do vazio, ligada a noção da efemeridade, e o conceito japonês de superposição de tempo e espaço.O trabalho é apresentado sob forma de uma narrativa interrompida, resultante do percurso pessoal da autora, e ligada por uma trama invisivél que seria, em última instância, o próprio vazio. O trabalho porcura também entender os elementos específicos de nossa época, atraves da arquitetura e do urbanismo, em relação com a literatura, a filosofia e a arte. Busca ainda, esboçar representações da situação selvagem das metrópoles contemporâneas com a ajuda de projetos emblemáticas. / We are living today a moment that favours the rediscussion of some concepts concerning several domains of knowledge, ranging from art to biology, from philosophy to mathematics, to architecture. Amidst these questions, this work chooses that of the void´s role as its central one, which will be treated in two instances: as an urban configuration, and as a conceptual contemporary architectural element. Selecting architectural projects, in which one of the basic criteria is the void´s problematic, this work wants to make clear the relationship in a form of interdependence and mutual enrichment between voids, either ebing a result of an urbain metropolitain dynamic, or those architecturaly conceived. Throughout five projects, signed by five contemporary architects within four metropolises, we try to show the specific role of each void. Furthermore, the first part will approach the questions of memory and temporality at São Paulo. The second part treats the end of the unique history as defined by Hegel, in Berlin. Finaly the third part, concerning the cities of Osaka and Tokyo aimes the discussion on the japanese conception of void, related to the notion of ephemerality in the metropolis, and presents the japanese concept of superposition of time and space. This work is presented as an interrupted narrative, as a result of the author´s own journey and connected by an unvisible frame, which is eventually the void itself. It tries to understand the elements specific to architetcure and urbanism, related to literature, philosophy and art. At the same time, it aims sketching representations of the selvage situation of contemporary metropolises through emblematic architectural projects.

Centrée sur la matérialité de la contrainte littéraire en tant que programme heuristique, cette thèse propose une analyse de cinq thèmes majeurs de La Disparition de Georges Perec : celui du travail « artisanal » régi par des règles conscientes, s’opposant aux explorations dadaïstes et surréalistes du hasard ou de l’inconscient ; celui de la dignité accordée au ludique, voire au « gratuit », dans le droit fil de la pensée pataphysique ; l’introduction volontaire d’erreurs dans l’application de la contrainte, autrement dit du « clinamen », dont il n’est pas juste de faire remonter l’origine, ainsi qu’on le dit souvent, à La Vie mode d’emploi ; la genèse de La Disparition, éclairant l’élaboration lipogrammatique à partir de l’écriture de soi ; l’application « sociologique » de la contrainte, enfin, dont la fonction heuristique se réoriente vers l’exploration du monde, dans le cadre de nouvelles tentatives de Perec étroitement liées aux recherches alphabétiques du roman sans e. Nous établirons ainsi une comparaison entre La Disparition et d’autres œuvres ou projets perecquiens, et introduirons une perspective historique fondée sur une réflexion sur plusieurs mouvements d’avant-garde du vingtième-siècle (Dada, Surréalisme, Internationale Situationniste, Tel Quel, etc.), pour démontrer l’originalité de ce roman fondé sur les principes de l’Oulipo / Focusing on materiality of literary constraint as a heuristic program, the following dissertation offers an analysis of five major themes of Georges Perec’s A Void: the notion of “craftsmanship” governed by conscious rules, as opposed to Dadaist and surrealist explorations of chance or of the unconscious; the dignity granted to playfulness, even to “gratuitousness”, in line with pataphysical thought; the voluntary introduction of errors in the application of constraints, in other words the “clinamen”, whose origin is often mistakenly located in Life A User’s Manual; the genesis of A Void, whose study will shed new light on the link between lipogrammatic constraint and the writing of the self; lastly, the “sociological” application of constraint, whose heuristic function is reoriented towards the exploration of the world, in the context of Perec’s later experiments, which derive from the alphabetical structure of the novel without e. In this way, we will compare A Void with other Perecquian works and projects and introduce a historical perspective based on a reflection on several avant-garde movements of the twentieth century (Dada, Surrealism, Situationist International, Tel Quel, etc.), in order to demonstrate the radical originality of this novel based on the principles of the Oulipo

Many legacy nuclear facilities exist with the number of such facilities due to increase in the future. For a variety of reasons, some of these facilities have poorly documented blueprints and floor plans. This has led to many areas within such facilities being left unexplored and in an unknown state for some considerable time. The risk to health that these areas might pose has in some cases precluded human exploration and facilities have been maintained in a containment state for many years. However, in more recent years there has been a move to decommission such facilities. The change of strategy from containment to decommissioning will require knowledge of what it is that needs to be decommissioned. It is hoped that an autonomous or semi- autonomous robotic solution can satisfy the requirement. For successful mapping of such environments, it is required that the robot is capable of producing complete scans of the world around it. As it moves through the environment the robot will not only need to map the presence, type and extent of radioactivity, but do so in a way that is economical from the perspective of battery life. Additionally, the presence of radioactivity presents a threat to the robot electronics. Exposure to radiation will be necessary but should be minimised to prolong the functional life of the robot. Some tethered robots have been developed for such applications, but these can cause issues such as snagging or the tether inadvertently spreading contamination, due to being dragged along the floor. Nuclear environments have very unique challenges, due to the radiation. Alpha and beta radiation have a short emission distance and therefore cannot be detected until the robot is in very close proximity. Although the robot will not become disabled by these forms of radiation, it may become contaminated which is undesirable. Radiation from gamma sources can be detected at range, however pinpointing a source requires sensors to be taken close to the emitter, which has adverse effects on the robot's electronics, for example gamma radiation damages silicon based electronics. Anything entering these environments is deemed to be contaminated and will eventually require disposal. Consequently the number of entries made should ideally be minimised, to reduce the production and spread of potential waste/contamination. This thesis presents results from an investigation of ways to provide complete scans of an environment with novel algorithms which take advantage of common features found in industrial environments and thereby allow for gaps in the data set to be detected. From this data it is then possible to calculate a minimum set of way points required to be visited to allow for all of the gaps to be filled in. This is achieved by taking into account the sensor's parameters such as minimum and maximum sensor range, angle of incidence and optimal sensor distance, along with robot and environmental factors. An investigation into appropriate exploration strategies has been undertaken looking at the ways in which gamma radiation sources affect the coverage of an environment. It has discovered undesired behaviours exhibited by the robot when radiation is present. To overcome these behaviours a novel movement strategy has been presented, along with a set of linear and binary battery modifiers, which adapt common movement strategies to help improve overall coverage of an unknown environment. Collaborative exploration of unknown environments has also been investigated, looking into the specific challenges radiation and contamination offer. This work has presented new ways of allowing multiple robots to independently explore an environment, sharing knowledge as they go, whilst safely exploring unknown hazardous space where a robot may be lost due to contamination or radiation damage.

Encontramos-nos em um momento histórico que privilegia a rediscussão de certos conceitos referentes a diversas áreas do conhecimento, indo da arte à biologia, da filosofia à matemática, passando pela arquitetura. Entre estas diversas questões atuais, este trabalho escolhe a do vazio como assunto central, tratado em duas instâncias: uma, as suas configurações urbanas; a outra, seu papel como elemento conceitual na arquitetura contemporânea. Escolhendo projetos de arquitetura que incluem tais questionamentos entre suas preocupações centrais, quer-se evidenciar a relação de interdependência e de enriquecimento mútuo entre dois tipos de vazio: os resultantes da dinâmica urbana tipicamente metropolitana, e aqueles concebidos no processo projetual de arquitetura. Através de cinco projetos, assinados por cinco arquitetos contemporâneos em quatro diferentes metrópoles, mostra-se o papel específico de cada um destes vazios. Além disso, a primeira parte aborda a questão da memória e da temporalidade, em São Paulo. A segunda parte trata do fim da história única, em Berlim, segundo a definição hegeliana. Finalmente, a terceira parte, relativa às cidades japonesas de Osaka e Tokyo, procura discutir a concepção japonesa do vazio, ligada a noção da efemeridade, e o conceito japonês de superposição de tempo e espaço.O trabalho é apresentado sob forma de uma narrativa interrompida, resultante do percurso pessoal da autora, e ligada por uma trama invisivél que seria, em última instância, o próprio vazio. O trabalho porcura também entender os elementos específicos de nossa época, atraves da arquitetura e do urbanismo, em relação com a literatura, a filosofia e a arte. Busca ainda, esboçar representações da situação selvagem das metrópoles contemporâneas com a ajuda de projetos emblemáticas. / We are living today a moment that favours the rediscussion of some concepts concerning several domains of knowledge, ranging from art to biology, from philosophy to mathematics, to architecture. Amidst these questions, this work chooses that of the void´s role as its central one, which will be treated in two instances: as an urban configuration, and as a conceptual contemporary architectural element. Selecting architectural projects, in which one of the basic criteria is the void´s problematic, this work wants to make clear the relationship in a form of interdependence and mutual enrichment between voids, either ebing a result of an urbain metropolitain dynamic, or those architecturaly conceived. Throughout five projects, signed by five contemporary architects within four metropolises, we try to show the specific role of each void. Furthermore, the first part will approach the questions of memory and temporality at São Paulo. The second part treats the end of the unique history as defined by Hegel, in Berlin. Finaly the third part, concerning the cities of Osaka and Tokyo aimes the discussion on the japanese conception of void, related to the notion of ephemerality in the metropolis, and presents the japanese concept of superposition of time and space. This work is presented as an interrupted narrative, as a result of the author´s own journey and connected by an unvisible frame, which is eventually the void itself. It tries to understand the elements specific to architetcure and urbanism, related to literature, philosophy and art. At the same time, it aims sketching representations of the selvage situation of contemporary metropolises through emblematic architectural projects.

O presente trabalho envolve um estudo teórico e experimental do escoamento bifásico externo a banco de tubos. Inicialmente, apresenta-se uma ampla revisão da literatura sobre padrões de escoamento, fração de vazio e perda de pressão, durante escoamentos monofásicos e bifásicos externos a banco de tubos. Nesta análise são também descritos os métodos de previsão destes parâmetros. Verificam-se diferenças significativas entre as estimativas proporcionadas por eles, fato que indica a inexistência de métodos generalizados. Posteriormente é apresentada uma descrição detalhada da bancada experimental projetada e construída durante o doutoramento. O aparato completo compõe-se da seção de testes, circuito de água, sistema de compressão e condicionamento de ar, e seções de injeção dos fluxos e condicionamento do escoamento. A seção de testes consiste em um banco de tubos distribuídos segundo configuração triangular normal, com os tubos apresentando diâmetro externo de 19,1 mm, comprimento de 381 mm, e espaçamento transversal de 24 mm. Os experimentos foram realizados para escoamento vertical ascendente de misturas água-ar e velocidades superficiais da fase líquida e gás de 0,020 a 1,500 m/s e de 0,10 a 10,00 m/s, respectivamente. Neste estudo foram desenvolvidas técnicas inéditas para determinação experimental da fração de vazio superficial no interior do banco de tubos baseadas em sistemas óptico e de sensoriamento capacitivo. Os padrões de escoamento foram identificados subjetivamente através de visualização de imagens e vídeos do escoamento, e objetivamente com o auxílio do método de agrupamento de dados k-means utilizando parâmetros baseados no sinal de perda de pressão e do sensoriamento capacitivo. Identificou-se subjetivamente os padrões de escoamento bolhas, bolhas dispersas, bolhas grandes, agitante, intermitente e anular. Constatou-se equivalência entre os padrões de escoamento identificados através dos métodos objetivo e subjetivo. Resultados experimentais para fração de vazio foram obtidos através de técnicas óptica e capacitiva. Constatou-se que o traçador rodamina B utilizado no método óptico altera as condições do escoamento, ainda que em concentrações reduzidas. A partir dos resultados obtidos com o sensoriamento capacitivo estimou-se a fração de vazio para o padrão bolhas. Resultados para a parcela friccional da perda de pressão também foram levantados. Constata-se o incremento da fração de vazio e da parcela friccional da perda de pressão com as velocidades superficiais das fases líquida e gás. Os resultados para fração de vazio foram comparados com métodos de previsão da literatura, e de maneira geral os métodos preveem as tendências dos resultados experimentais apenas para vazões de líquido reduzidas. Analogamente, os resultados para perda de pressão foram comparados com estimativas segundo métodos da literatura, concluindo que os métodos não preveem satisfatoriamente os resultados obtidos. Desta forma, foram propostos novos métodos de previsão para padrões de escoamento, fração de vazio e parcela friccional da perda de pressão, desenvolvidos a partir de análises dos mecanismos dominantes do escoamento, e adotando parâmetros adimensionais para correlacionar os dados. Os métodos propostos preveem satisfatoriamente os resultados experimentais deste estudo e da literatura para escoamentos bifásicos água-ar. / The present thesis concerns a theoretical and experimental study of external two-phase flows across tube bundles. Initially, a comprehensive literature review covering flow patterns, void fraction and pressure drop for single and two-phase flows across tubes bundle is presented. The review also describes predictive methods for these parameters. A comparison of these methods reveals reasonable disagreement among their predictions, indicating the absence of generalized methods. Subsequently, the apparatus and instrumentation designed and built to obtain the experimental data are described. The experimental apparatus comprises the test section, a water loop, air compression and conditioning systems, and sets for fluid flow injections and conditioning. The test section is a normal triangular tube bundle, with 19.1 mm OD tubes, 381 mm long and transversal pitch of 24 mm. The experiments were performed for air-water upward vertical flow, for superficial liquid and gas velocities ranging from 0.020 to 1.500 m/s and 0.10 to 10.00 m/s, respectively. Innovative techniques to evaluate the void fraction within the bundle were developed based on capacitive and optical methods. The flow patterns were identified subjectively and objectively by k-means clustering method, using as clustering parameters the pressure drop and the capacitive signals. Bubbles, dispersed bubbles, large bubbles, churn, intermittent and annular flow patterns were identified subjectively. The data groups identified by the objective method are representative of the flow patterns. Void fraction measurements were obtained for bubbly flow using both techniques (optical and capacitive). The void fraction data based on the optical method had its experimental range limited due to changes in the flow characteristics caused by the addition of the fluorescent dye Rhodamine B. The experimental results indicate that the void fraction increases with increasing the superficial velocities of both phases. In general, the void fraction predictive methods available in the literature capture the trends of the experimental results only for reduced liquid flow rates. According to the experimental results, the frictional pressure drop increases asymptotically with increasing the flow rates of both phases. None of the predictive methods from literature evaluated in the present study predicted satisfactorily the experimental results. Methods for prediction of flow patterns, void fraction and frictional pressure drop parcel were also developed in the present study. These methods provided reasonable predictions of the experimental results obtained in the present study, and also from the literature for air and water flows across tube bundles.

De nos jours, le domaine des implants est un des enjeux important pour notre civilisation pour permettre d’améliorer notre quotidien. Pour ce faire, une large offre de matériaux et de technologies existe offrant de nombreuses possibilités afin de répondre aux attentes chirurgicales. Plusieurs familles de matériaux coexistent : les polymères, les céramiques et les matériaux métalliques ainsi que différents procédés de mise en forme. Parmi ceux-ci, le procédé de micro fusion laser sur lit de poudre est un procédé prometteur permettant de réaliser des pièces de géométries complexes. C’est précisément cette technologie qui a été retenue. Pour cela, afin d’approfondir la connaissance du procédé et évaluer l’impact des paramètres sur les structures métallographiques, une orientation se tournant vers des matériaux métalliques tels que le CoCrMo et le titane T40 a été envisagée.Ainsi, les objectifs de la thèse ont été de générer un matériau possédant de bonnes caractéristiques mécaniques ainsi qu’en faciliter son intégration dans un milieu biologique (implants). Pour ce faire, une structure de porosité contrôlée (pour faciliter le développement tissulaire) avec des propriétés mécaniques adaptées aux sollicitations est nécessaire. Le premier travail effectué fut donc une recherche préliminaire afin d’approfondir la connaissance du procédé, en particulier d’un point de vue énergétique et thermique. Afin d’identifier et d’évaluer l’impact des paramètres sur le taux de porosité et donc les propriétés mécaniques de la pièce, une méthode statistique de type Taguchi a été utilisée. Au travers de cette analyse, il est apparu que 3 paramètres inhérents au procédé (la distance entre tache laser, temps d’exposition et le pas de balayage) expliquent prêt de 80% des résultats. De plus, il est mis en évidence que les propriétés mécaniques d’une structure (module de Young et résistance à la rupture) peuvent être maitrisées grâce au taux de porosité de cette dernière et permettre ainsi un rapprochement des propriétés mécaniques de l’os cortical. Pour ce faire, des caractérisations mécaniques ont donc été réalisées pour évaluer le module de Young et la résistance à la rupture des pièces avec différentes structures. Une maîtrise des propriétés peut donc être envisagée et peut même être adaptée en réalisant des structures mixtes alliant partie dense et partie poreuse. / Nowadays, the field of implants is one of the major challenges for our civilization to help improve our lives. To do this, a wide range of materials and technologies are offering many opportunities to meet the surgical needs. Several types of materials exist: polymers, ceramics and metal as well as different methods of shaping materials. Among them , the process of micro fusion laser powder bed is a promising method for producing parts with complex geometries. It is this technology that has been used. To do this, in order to deepen the knowledge of the process and evaluate the impact of parameters on the metallographic structures , guidance , turning to metallic materials such as CoCrMo and titanium T40 was considered.Thus, the objectives of the thesis were to generate a material with good mechanical properties as well as ease of integration in a biological medium ( implants) . To do this, a controlled pore structure (to facilitate tissue development ) with properties adapted to mechanical stress is required. The first work was therefore a preliminary research to deepen understanding of the process , particularly an energy and thermal point of view . To identify and assess the impact of parameters on the porosity and therefore the mechanical properties of the part , a statistical method of Taguchi type was used. Through this analysis, it appeared that three parameters inherent to the process ( the distance between laser spot exposure and no scanning time ) explain 80% loan results. Moreover, it is highlighted that the mechanical properties of a structure ( Young's modulus and tensile strength ) can be controlled through the porosity of the latter and thus permit reconciliation of the mechanical properties of cortical bone . To do this , mechanical characterizations were therefore conducted to evaluate the Young's modulus and tensile strength of parts with different structures. A control properties can be considered and can even be adapted by making composite structures combining dense part and porous part .

La présence de porosités dans les lingots métalliques représente un problème majeur dans l’industrie des matériaux. En effet, ces porosités altèrent significativement les caractéristiques mécaniques du matériau (ductilité notamment), et sont des sources d’apparition de défauts en mise en forme ou en tenue en service. Pour éliminer ces porosités, les industriels utilisent souvent des procédés de mise forme à chaud tels que le forgeage ou le laminage, mais il est souvent difficile de définir le taux de déformation à appliquer pour refermer entièrement ces porosités. La modélisation numérique s’avère donc être un outil particulièrement intéressant afin d’étudier l’impact des paramètres procédé sur le taux de refermeture de porosités. Dans ce travail, nous avons développé une méthodologie de calibration basée sur des algorithmes d’optimisation et une base de données de 800 simulations à champ complet sur VER, où les paramètres influents sur la refermeture des porosités sont variés (mécaniques et géométriques). Le premier modèle proposé est un modèle scalaire qui s’affranchit de l’hypothèse de chargement axisymétrique, largement utilisée dans la littérature. Le paramètre de Lode a permis avec l’utilisation de la triaxialité des contraintes de définir l’état de contraintes d’une manière unique. Les comparaisons de ce nouveau modèle à trois autres modèles de refermeture de la littérature montrent le gain de précision de ce nouveau modèle scalaire de refermeture. Le deuxième modèle est un modèle tensoriel adapté aux procédés multipasses grâce à l’analyse de la matrice d’inertie de la porosité. Cette matrice sert pour calculer le volume, la forme et l’orientation de la porosité. Ce modèle a été calibré en utilisant une approche basée sur les réseaux de neurones artificiels. La comparaison avec le modèle scalaire et la modélisation en champ complet a montré un gain en précision jusqu’à 35%. Il s’agit là par ailleurs du premier modèle tensoriel proposé dans la littérature. / The presence of voids in ingots is a major issue in the casting industry. These voids decrease materials properties (in particular ductility) and may induce premature failure during metal forming or service life. Hot metal forming processes are therefore used to close these voids and obtain a sound product. However, the amount of deformation required to close these voids is difficult to estimate.Numerical modeling is an interesting tool to study the influence of process parameters on void closure rate. In this work, an optimization-based strategy has been developed to identify the parameters of a mean-field model based on a database of 800 full-field REV simulations with various loading conditions and voids geometry and orientations. The first void closure model is a scalar model that gets rid of the axisymmetric loading hypothesis considered in most models in the literature. The Lode angle, coupled with the stress triaxiality ratio enables to identify the stress state in a unique way. Comparisons of this new model with three other models fromthe literature show the accuracy increase for general loading conditions. In order to address multistages processes, a second model is defined in a tensor version. The ellipsoid void inertia matrix is used to define void’s morphology, orientation and volume. The tensor model predicts the evolution of the inertia terms and its calibration is based on the full-field REV database and on a new Artificial Neural Networks approach. Comparisons were carried out between this tensor model, the scalar model and full-field simulations for multi-stages configurations. These comparisons showed up to 35% accuracy improvement with the tensor model. It is worth mentioning that this is the first attempt to define a void closure tensor model in the literature.