[pt] Simula-se numericamente o fenômeno de estricção em um bloco sob tração. Supõe-se que o material seja hiperelástico, isotrópico e que tenha uma lei de comportamento descrita pelo modelo de Blatz e Ko para espuma de borracha de poliuretano. Discretiza-se o problema de problema de equilíbio utilizando o Método dos Elementos Finitos clássico. A pesquisa das soluções é realizada por meio da implementação de uma Técnica de Continuação de Euler-Newton, que permite abordar situações onde instabilidades mecânicas, tais como a localização de deformações e o amolecimento, estejam presentes. Finalmente, identifica-se as regiões do material onde estas instabilidades ocorrem. / [en] The phenomenon of necking is numerically simulated for a block under traction tensile test. A hyperelastic, isotropic constitutive model, developed by Blatz and Ko for the description of some foamed polyurethane elastomers, is considered. An Euler-Newton Continuation Algorithm is employed in the search for solutions of the equilibrium problem, which is discretized by means of a classical Finite Element Method. This algorithm makes possible tha treatment of situations where mechanical instabilities, such as localization and strain-softening behaviors, are present. Finally, the regions of material, where these instabilities appear, are identified.

Le secteur aéronautique ayant des exigences toujours plus contraignantes, l’emploi de nouveaux matériaux pour les structures a été incontournable. C’est pourquoi le composite à fibre de carbone a fait son apparition dans les structures d’avions, épaulé par des pièces en alliage de titane, majoritairement du Ti6Al4V. Ces matériaux sont alors disposés en empilages et percés en une seule opération pour être ensuite assemblés par des rivets ou des boulons. Un certain nombre de problèmes, d’un point de vue industriel, sont soulevés par cette opération, générant des surcoûts importants. Les verrous technologiques et scientifiques liés à cette opération seront donc analysés au cours de cette étude.La connaissance des efforts locaux exercés par l’outil sur la matière usinée constitue un premier point permettant l’amélioration de la compréhension des phénomènes survenant lors de la coupe. Pour cela plusieurs procédés seront expérimentés pour déterminer avec précision les efforts subis par l’outil tout au long du perçage. Afin de compléter les informations relatives aux efforts sur l’outil, l’analyse de la température au sein du foret est nécessaire. Plusieurs dispositifs expérimentaux de mesure de la température in-situ sont développés.Ces dispositifs ont permis l’usinage instrumenté de trous à la fois dans chacun des deux matériaux composant l’empilage et dans l’empilage afin de comprendre au mieux les phénomènes survenant lors de ces opérations. / The aeronautic field has stronger and stronger requirements so the use of materials for structures has been unavoidable. That’s why carbon fiber composite is appeared in plane structures, used with elements in titanium alloy, and most part of them in Ti6Al4V. These materials are also dispatched in stacks and are drilled in a “one shot” operation, in the aim of being assembled with rivets or bolts. Through the industrial point of view, many problems are highlighted by this operation, generating several over costs. Technologic and scientific locks linked with this operation will be analyzed in this study.The knowledge of local stresses exerted by the tool on the machined matter is the first point allowing the improvement of the understanding of phenomenon that occurs during cutting. Few processes will be experimented to determine stresses submitted by the tool during drilling with accuracy. In the aim of completing the knowledge of the cutting operation, the analysis of the temperature inside the drill is needed. That’s why different experimental devices of thermal measurement are developed.These devices have allowed instrumented machining of holes in each material separately and in the stack of Ti6Al4V/CFRP in order to understanding phenomenon that occur during this operations.

No description available.

<p><strong>Abstract</strong></p> <p>The objective of this research is to develop an understanding of the mechanical behavior, failure and microstructure evolution of aluminum tubes under internal pressure loading, and to delineate the physical and mechanical origins of spatially-localized plastic deformation. Traditional approaches to the study of plastic instabilities, necking and failure have either been based on kinematic considerations, such as finite strain effects and geometric softening, or physics-based concepts. In this study, we develop a framework that combines both approaches to investigate the tube deformation and failure behavior at various loading conditions.</p> <p>A rate-dependent dislocation-based MTS model has been developed to study the tube hydro-forming process at high temperatures and at various strain rates. The development and application of the MTS model led to an advanced industrial application of PRF bottle forming, which has been fully investigated. This simulation shows a good agreement between experimental results and prediction. The model has been used extensively throughout the PRF bottle development, with several patent applications.</p> <p>The crystal plasticity based finite element model is selected to simulate surface roughening and localized necking in aluminum alloy tubes under internal pressure. The measured electron backscatter diffraction (EBSD) data are directly incorporated into the finite element model and the constitutive response at an integration point is described by the single crystal plasticity theory. The effects of the spatial grain orientation distribution, strain rate sensitivity, work hardening, and initial surface topography on surface roughening and necking are discussed. It is demonstrated that while localized necking is very sensitive to both the initial texture and its spatial orientation distribution, the initial surface topography has only a small influence on necking, but a large influence on surface roughness of the formed product.</p> <p>An elastic-viscoplastic based finite element model has been developed to study the necking behavior of tube expansion for rate dependent monolithic materials and laminated materials during dynamic loading. Numerical study shows that a high strain rate sensitivity can significantly delay the onset of necking for both monolithic and laminated sheets, and affect the multiple-neck formation in high speed dynamic loading. The model also shows that higher volume fractions of a clad layer with positive rate sensitivity material in laminated sheet could improve the sheet ductility as well.</p> <p>A commercial FE package, ABAQUS, is employed as a finite element method solver in this research work, and several user subroutines were developed to model various hydro-forming processes. Interfaces between the ABAQUS user subroutine UMAT and the ABAQUS main code are developed to allow further extension of the current method.</p> / Doctor of Philosophy (PhD)

L’objectif de ce sujet de thèse est de prédire l’apparition des instabilités plastiques (striction diffuse et striction localisée) dans les matériaux métalliques. Ces matériaux sont décrits par des modèles de comportement élasto-plastique couplés à l’endommagement. L'approche de Lemaitre, reliant l'endommagement à la déformation plastique équivalente et au taux de restitution de la densité d'énergie élastique, est adoptée. Parmi les différents critères et indicateurs qui sont considérés pour la prédiction des instabilités matériau, la théorie de bifurcation et les critères de type force maximum sont tout particulièrement analysés et comparés. Un objectif important de cette étude consiste à déterminer les mécanismes déstabilisants clés associés à cette modélisation du comportement, ainsi que l’impact des différents aspects physiques et des paramètres matériau sur l’apparition de la striction. Les développements résultants sont appliqués à une sélection représentative de matériaux métalliques afin prédire leurs limites de formabilités. Cette approche combinant des lois de comportement et critères de striction peut être utilisée comme outil théorique et numérique d’aide à la conception de nouveaux matériaux à ductilité améliorée / The aim of the present work is to predict the occurrence of plastic instabilities (diffuse and localized necking) in thin sheet metals. The prediction of these plastic instabilities is undertaken using an elastic–plastic model coupled with ductile damage, which is then combined with various plastic instability criteria theory. The bifurcation-based criteria and the maximum force criterion used in this work are formulated within a general three-dimensional modeling framework, and then applied for the particular case of plane-stress conditions for sheet metals. Some theoretical relationships or links between the different investigated necking criteria are established, which allows a hierarchical classification in terms of their conservative character in predicting critical necking strains. The resulting numerical tool is implemented into the finite element code ABAQUS/Standard to predict forming limit diagrams, in both situations of a fully three-dimensional formulation and a plane-stress framework. This approach, that combines constitutive equations to necking criteria, serves as a useful tool in the design of new materials with improved ductility

No description available.

Entre os processos de fabricação mecânicos atuais destacam-se os processos de conformação de chapas metálicas devido à sua produtividade e confiabilidade e a seus baixos custos de produção, baixo consumo de material e produtos nos formatos bem próximos dos esperados nas fases de projeto. Isto só é possível se todas as variáveis do projeto e do processo estiverem conforme as especificações prévias, exigidas principalmente em setores da indústria tais como o aeronáutico e o automobilístico. Um recurso útil para se alcançar estas expectativas é o estudo da estricção e da conformabilidade em chapas metálicas, representadas em diagramas de limite de conformação (DLC), os quais, aliados à análise numérica através do Método dos Elementos Finitos, permitem predições a respeito das variáveis de projeto. Este trabalho tem como objetivos rever e aplicar conceitos relativos a estes aspectos. O objetivo principal deste trabalho é de dispor uma metodologia de avaliação e otimização para o processo de estampagem, utilizando-se elementos finitos e DLC. Uma modificação no método de Marciniak-Kuczynski é utilizada para a obtenção do DLC. Busca-se com isto uma contribuição para a redução de custos em aplicações industriais com o uso desta metodologia através da diminuição das fases de tentativas e erros em projeto e reforma de estampos (try-outs). / Nowadays, sheet metal forming processes have an important role amongst the mechanical manufacturing processes, because they are characterized by high productivity and reliability at low cost, low material waste and near net shapes from design. This is achieved by controlling the design and process parameters according to initial constraints. This scenario is common at aeronautic and at automobilistic productions . These results are obtained by studies of necking and formability in sheet metals and by the use of forming limit diagrams (FLD) and finite element numerical analysis to predict design parameters. This work reviews these concepts and presents a methodology for optimization of deep drawing processes, by using finite element method and FLD. A modification of the Marciniak-Kuczynski method has been introduced to obtain the FLD. Once this methodology has been performed, it can be possible to reduce try-outs in sheet metal dies and costs reduction can also be obtained.

L’Analyse Linéaire de Stabilité a été largement utilisée afin de décrire l’évolution du mode dominant des instabilités pour différentes configurations. Toutefois, plusieurs observations expérimentales et numériques ont démontré l’existence de distributions de longueurs inter-strictions, prémices aux distributions de tailles de fragments. Une extension de cette utilisation classique de l’Analyse Linéaire de Stabilité dédiée à l’extension d’un barreau en traction a été proposée afin de rendre compte de la contribution de l’ensemble des modes de perturbation sur le profil final des strictions. Cette approche, correspondant physiquement au cas des anneaux fins en expansion, est à même de déterminer une distribution de longueurs inter-strictions. En se basant sur les mêmes outils mathématiques que ceux ayant permis d’étendre l’analyse linéaire de stabilité, une nouvelle approche numérique a été suivie afin de déterminer le temps d’apparition et le nombre de strictions en fonction de la vitesse de chargement, de l’amplitude et de la taille de cellules de perturbations. Une comparaison, en termes de distribution de longueurs inter-strictions, a été par la suite obtenue entre résultats analytiques et numériques. De bonnes corrélations ont été observées. Le développement d’un montage expérimental d’expansion d’anneaux par forces électromagnétiques a été entamé au cours de ce travail de thèse. Lors de sa finalisation, il devrait permettre une validation expérimentale des approches développées dans les domaines numériques et analytiques / Linear Stability Analysis has been widely used in order to describe the evolution of the dominant necking pattern in different configurations. However, experimental and numerical observations have demonstrated that a distribution of internecking lengths is obtained instead of a unique dominant pattern. In this work, an extension of the classical Linear Stability Analysis applied to the dynamic extension of a round bar case has been developed to take into account the contribution of all perturbation modes on the final necking pattern. This approach, corresponding physically to the case of thin ring expansion, is able to determine a distribution on internecking distances. Based on the same mathematical tools that was developed for the extension of the linear stability analysis, a new numerical approach has been followed to determine the number and the onset time of necking with respect to loading velocity, amplitude and cell size of the perturbations. A comparison, in terms of distributions of inter-necking distance, has been carried out between analytical and numerical results. Good correlations have been observed. An electromagnetic ring expansion experimental device has been proposed during this work. Whith some additional development, it should enable to validate experimentally the working assumptions developed in the numerical and analytical studies

Entre os processos de fabricação mecânicos atuais destacam-se os processos de conformação de chapas metálicas devido à sua produtividade e confiabilidade e a seus baixos custos de produção, baixo consumo de material e produtos nos formatos bem próximos dos esperados nas fases de projeto. Isto só é possível se todas as variáveis do projeto e do processo estiverem conforme as especificações prévias, exigidas principalmente em setores da indústria tais como o aeronáutico e o automobilístico. Um recurso útil para se alcançar estas expectativas é o estudo da estricção e da conformabilidade em chapas metálicas, representadas em diagramas de limite de conformação (DLC), os quais, aliados à análise numérica através do Método dos Elementos Finitos, permitem predições a respeito das variáveis de projeto. Este trabalho tem como objetivos rever e aplicar conceitos relativos a estes aspectos. O objetivo principal deste trabalho é de dispor uma metodologia de avaliação e otimização para o processo de estampagem, utilizando-se elementos finitos e DLC. Uma modificação no método de Marciniak-Kuczynski é utilizada para a obtenção do DLC. Busca-se com isto uma contribuição para a redução de custos em aplicações industriais com o uso desta metodologia através da diminuição das fases de tentativas e erros em projeto e reforma de estampos (try-outs). / Nowadays, sheet metal forming processes have an important role amongst the mechanical manufacturing processes, because they are characterized by high productivity and reliability at low cost, low material waste and near net shapes from design. This is achieved by controlling the design and process parameters according to initial constraints. This scenario is common at aeronautic and at automobilistic productions . These results are obtained by studies of necking and formability in sheet metals and by the use of forming limit diagrams (FLD) and finite element numerical analysis to predict design parameters. This work reviews these concepts and presents a methodology for optimization of deep drawing processes, by using finite element method and FLD. A modification of the Marciniak-Kuczynski method has been introduced to obtain the FLD. Once this methodology has been performed, it can be possible to reduce try-outs in sheet metal dies and costs reduction can also be obtained.

La striction diffuse et localisée sont des phénomènes précurseurs à la rupture ductile et représentent l'une des principales causes de rebut des pièces métalliques au cours de leur mise en forme. La mise en œuvre d'outils théoriques et numériques capables de prédire l'apparition de ces défauts s'avère nécessaire pour des raisons économiques et environnementales. Ces outils nécessitent en partie l'intégration d'un modèle de comportement adéquat permettant de reproduire les phénomènes physiques mis en jeu. Un tel modèle de comportement est ensuite couplé à un indicateur d'instabilité plastique offrant la possibilité de prédire de manière fiable les phénomènes de striction diffuse et localisée. Dans ce travail de thèse, nous avons considéré certains modèles d'endommagement micromécaniques basés sur l'approche de Gurson, et qui ont été couplés à différents critères d'instabilités plastiques reposant sur l'Analyse de Bifurcation. L'implantation numérique des modèles retenus a été réalisée dans le code de calcul par éléments finis Abaqus/Standard. En ce qui concerne les modèles d'endommagement, plusieurs schémas d'intégrations ont été testés afin d'analyser leur performance et leur robustesse lorsque le comportement présente un effet adoucissant. L'approche combinant le modèle de comportement et les critères de striction a été utilisée pour prédire les limites de formabilité en striction diffuse et localisée de plusieurs matériaux métalliques. Les résultats obtenus ont permis de dresser une hiérarchisation théorique et numérique des critères de striction utilisés. / Diffuse and localized necking are precursors to ductile fracture, and represent one of the main causes of metal parts rejection during forming operations. The implementation of theoretical and computational tools to predict the occurrence of these defects turns out to be necessary for economic and environmental reasons. These tools require in part the introduction of an appropriate behavior model in order to reproduce the physical phenomena involved during forming operations. Such a behavior model is then coupled to a plastic instability indicator providing the ability to reliably predict diffuse and localized necking. In the present work, we considered a micromechanical damage models based on Gurson's approach, which were coupled to different plastic instabilities criteria, based on Bifurcation Analysis. The numerical implementation of these models was carried within the implicit finite element code Abaqus/Standard. With regard to the damage models, several integration schemes were tested to analyze their performance and robustness when the behavior exhibits softening effect. The approach combining the Gurson's damage model and necking criteria has been applied for the prediction of formability limits of several metallic materials. The obtained results allowed establishing a theoretical and numerical classification between the necking criteria used in this work.