O polietileno de média densidade (PEMD) é um polímero termoplástico parcialmente cristalino, cujo uso tem crescido bastante em aplicações de engenharia, como em tubos plásticos para sistemas de distribuição de água e de gás. Sob carga constante, entretanto, este material pode eventualmente apresentar fratura por fluência, por meio de um mecanismo de crescimento lento de trincas, provocando acentuadas perdas por vazamento nos sistemas. Os métodos atuais empregados pela indústria para estimar a durabilidade dos tubos são caros, demorados, pouco práticos e imprecisos. Busca-se o desenvolvimento de técnicas mais eficientes, sendo que os métodos da mecânica da fratura são promissores no sentido de descrever a etapa de propagação da trinca. Ensaios de mecânica da fratura, entretanto, requerem a introdução de pré-trincas nos corpos de prova. Sabe-se que as técnicas artificiais de introdução de pré-trincas causam algum tipo de dano na matriz polimérica, que pode ou não afetar os resultados dos ensaios, dependendo dos eventos que ocorrerem após o carregamento inicial. A principal propriedade requerida de um método de introdução de pré-trinca, portanto, é reprodutibilidade. Neste estudo foram aplicados três importantes métodos de mecânica da fratura ao PEMD e investigados os efeitos de diferentes técnicas de introdução de pré-trincas sobre os resultados dos ensaios e sobre as estruturas de deformação na matriz do material na ponta da trinca. Os ensaios de tenacidade à fratura no estado plano de deformação - KIc - foram realizados em condições criogênicas, em vista do comportamento dúctil do material à temperatura ambiente, sendo que a estratégia mostrou-se satisfatória. Os resultados dos ensaios de Integral-J sugerem que o método pode não ser aplicável ao PEMD, devido ao peculiar mecanismo de fratura do material. O método do trabalho essencial de fratura - EWF - se aplica bem ao PEMD. Os resultados dos ensaios de KIc e EWF demonstraram que diferentes técnicas de introdução de pré-trincas provocam diferentes estruturas de deformação no material na ponta da trinca e afetam os resultados de ensaios de fratura no PEMD. Os resultados de EWF demonstraram ainda que o processamento também afeta as propriedades de fratura do material. É proposta uma nova técnica para a introdução de pré-trincas, com características mais naturais, a qual requer estudos complementares para seu aperfeiçoamento. / Medium density polyethylene (MDPE) is a semicrystalline thermoplastic polymer that has been increasingly used in engineering applications, as plastic pipes for water and gas distribution systems. Under constant load, however, this material may occasionally present creep failure, by means of a mechanism of slow crack growth, leading to leakage losses in the systems. Current methods used by industry to estimate durability of pipes are expensive, time consuming, non practical and inaccurate. The development of more efficient methods is a common target and fracture mechanics methods are promising in describing the crack propagation stage. Fracture mechanics testing methods, however, require the introduction of pre-cracks into the specimens. It is known that artificial methods of introducing pre-cracks produce some damage on the polymeric matrix, which may or not affect the results of tests, depending on the events that occur after the initial loading. Main propriety required of a pre-crack introducing method, therefore, is reproducibility. In this study three important fracture mechanics methods were applied to MDPE and the effects of different methods of pre-cracking over test results and over deformation structures of material matrix at the crack tip were investigated. Plane-strain fracture toughness - KIc tests were performed under cryogenic conditions, in view of the ductile behavior of material at room temperature and this strategy was well succeed. Results of Integral-J tests suggest that this method may not be applicable to MDPE, due to the peculiar fracture mechanism of the material. The essential work of fracture method EWF is well suited to study MDPE. The results of KIc and EWF tests showed that different pre-cracking methods cause different deformation structures in the material at the crack tip and affect the fracture tests with MDPE. EWF results showed also that the processing affect the fracture properties of materials too. It is proposed a new method for introducing pre-cracks, with more natural characteristics that requires complementary studies for its improving.

Un mur de soutènement en pierre sèche (MSPS) est un ouvrage vernaculaire construit par empilement de blocs rocheux sans liant. Ce type de mur a fortement sculpté les campagnes françaises mais a aussi été utilisé par le passé dans des ouvrages de soutènement que ce soit routier ou ferroviaire. Pourtant, l’utilisation de cette technologie a disparu au cours du 20e siècle au profit de solutions industrialisées comme le béton armé. Face à ce patrimoine vieillissant, il est très difficile de proposer des protocoles de réparation alors que la réglementation associée à ces ouvrages est inexistante. Pour les mêmes raisons, alors que cette technologie répond parfaitement aux questions soulevées par le développement durable, elle est rarement envisagée ou retenue par les prescripteurs. La rupture des MSPSs neufs peut apparaître à cause de : (1) une poussée excessive du remblai soutenu par le mur, (2) un effort concentré excessif présent sur la surface du remblai et proche de la tête du mur. Ces deux causes induisent deux types de rupture très différents, respectivement : (1) rupture en déformation plane, (2) rupture par apparition d’un ventre. Trois expériences récentes à l’échelle 1 des MSPSs chargés par : (1) une poussée hydrostatique, (2) un remblai, (3) un remblai avec une force concentrée sur la surface de ce dernier, ont permis d’identifier certains phénomènes mis en jeu dans ces ruptures. Ce travail de thèse contribuera alors au développement de nouveaux outils scientifiques mais aussi à valider des outils existants pour dimensionner les MSPSs. Les expériences réalisées à l’échelle 1 serviront alors de cadre à ce travail pour valider les outils développés. Nous avons choisi d’utiliser une approche aux éléments discrets (MED) où chaque bloc de pierre du mur est modélisé individuellement, respectant ainsi la nature-Même de l’ouvrage réel. Dans un premier temps, une rupture de type déformation plane est modélisée par une approche discrète pure et une approche discrète-Continue. Les atouts et inconvénients de chacune des méthodes sont mis en avant. Une validation quantitative des modèles numériques est faite en comparant la hauteur critique de chargement avec les expériences à l’échelle 1. Ensuite, on étudie la sensibilité de plusieurs paramètres géométriques et mécaniques sur la hauteur critique de chargement, tout comme l’influence de la cohésion du remblai. Enfin, une première modélisation 3D de type qualitative sur la rupture des MSPS par un chargement concentré en surface du remblai est présentée. Les problèmes de modélisation sont soulevés et l’influence de la forme des blocs dans le processus de rupture est étudiée. / A dry-Stone retaining wall (DSRW) is a vernacular structure constructed by stacking the stone blocks without using any binder. In the past, this wall has been extensively used, shaping the French countryside area, built either for road or railroad retaining wall. However, the use of this technology has disappeared during 20th century, due to the emergence of more industrialized materials such as reinforced concrete. Confronted by these ageing heritage structures, it is very difficult to propose a proper reparation procedure since the building codes associated with this structure are nonexistent. For the same reason, though this technology perfectly answers the questions raised by the concept of sustainable development, in practice this structure is rarely considered or used by the engineering advisors. The failure of the newly built DSRWs can be attributed to the following reasons: (1) an excessive pressure of backfill retained by the wall, (2) a presence of an excessive concentrated load on the backfill surface, close to the top part of the wall. These two types of loading lead to two very different types of failure, respectively: (1) plane strain failure, (2) bulging failure. Three recent experimental campaigns of DSRWs loaded with: (1) hydrostatic pressure, (2) backfill, (3) backfill with a concentrated on its surface, provided a better understanding of phenomena involved in these failures. The work presented in this PhD thesis contributes to the development of new scientific tools capable to help design the DSRWs, as well as validating existing tools. The recent full-Scale experimental campaign will serve as a basis to the validation of the developed numerical tools. We have chosen to use a discrete element method (DEM) where each stone block of the wall is modeled individually, complying with the nature of real DSRW. In the first place, the plane strain failure is modeled by a purely discrete approach and a discrete-Continuum approach. The advantage and inconvenient of each method will be presented in advance. A quantitative validation of numerical models is provided by comparing the critical height of loading with results derived from the full-Scale experimental campaign. Thereafter, a 3D qualitative model of DSRWs loaded with a concentrated load on the backfill surface is presented. The modeling problem is noted and the influence of the block form in regards of the related failure mechanism is studied.

In the first part of this thesis, the FE program MSC.Marc is applied for coupled thermomechanical simulations of wire-rod rolling. In order to predict material behaviour of an AISI 302 stainless steel at high strain rates generated during wire-rod rolling, a material model based on dislocation density is applied. Then, the evolution of temperature, strain rate and flow stress is predicted in the first four rolling passes of a wire block. In the second part of the thesis, an alternative approach to simulation of shape rolling is evaluated. The approach is applied in order to save the computational time in cases where many shape-rolling passes are to be simulated. The approach is a combination of the slab method and a 2D FEM with a generalized plane-strain formulation. A number of various isothermal shape-rolling passes are simulated applying the simplified approach. The simulations are carried out using an in-house 2D FE code implemented in Matlab. The results are compared to fully 3D FE analyses. The comparison shows that the simplified approach can predict roll forces and roll torques with a fair accuracy, but the predicted area reductions are a bit underestimated. The reasons for the deviations between the simplified approach and the 3D FEM are discussed. / QC 20101123

No description available.

The Two-Parameter-Fracture-Criterion (TPFC) was validated using an elastic-plastic two-dimensional (2D) finite-element code, ZIP2D, with the plane-strain-core concept. Fracture simulations were performed on three crack configurations: (1) middle-crack-tension, M(T), (2) single-edge-crack-tension, SE(T), and (3) single-edge crack-bend, SE(B), specimens. They were made of 2014-T6 (TL) aluminum alloy. Fracture test data from Thomas Orange work (NASA) were only available on M(T) specimens (one-half width, w = 1.5 to 6 in.) and they were all tested at cryogenic (-320oF) temperature. All crack configurations were analysed over a very wide range of widths (w = 0.75 to 24 in.) and crack-length-to-width ratios ranged from 0.2 to 0.8. The TPFC was shown to fit the simulated fracture data fairly well (within 6.5%) for all crack configurations for net-section stresses less than the material proportional limit. For M(T) specimens, a simple approximation was shown to work well for net-section stresses greater than the proportional limit. Further study is needed for net-section stresses greater than the proportional limit for the SE(T) and SE(B) specimens.

No presente trabalho produziu-se amostras de flexão em três pontos, SENB, a partir de placas moldadas por compressão feitas de uma resina de Poli(metacrilato de metila) comercial. Dois conjuntos contendo 6 amostras cada foram então pré-trincados por meio de uma lamina afiada de aço adotando-se dois métodos diferentes propostos pela literatura técnica: através de batidinhas (gentle tapping) e através da inserção controlada a velocidade constante da lâmina, ambas usando um dispositivo especialmente desenhado para este fim. Estas amostras foram então submetidas ao ensaio de tenacidade à fratura em deformação plana, KIc, com o intuito de se testar a hipótese de que os diferentes métodos introduzem diferentes padrões de dano na matriz próxima à região da ponta da pré-trinca, o que, por sua vez, influência os resultados obtidos. Adotou-se a norma ASTM D 5045 como padrão do ensaio. Todas as amostras produziram resultados válidos para KIc, considerando-se os critérios da norma. Os resultados confirmam a hipótese inicial mostrando que os diferentes métodos de introdução da pré-trinca influenciam o valor medido de KIc a um nível de significância de 5% para a diferença das médias obtidas nos dois grupos, sendo que o valor mais conservador é obtido quando as pré-trincas são introduzidas pelo método das batidinhas (KIc = 1,15 ± 0,11 MPa m1/2). Com base nestes resultados novas amostras com geometria auto-similar às anteriores foram produzidas e usadas para a investigação do estado de tensão desenvolvido nas amostras pelo método da fotoelasticidade. Os resultados confirmam que os diferentes métodos de pré-trincamento produzem diferentes padrões de Moiré próximos à ponta da trinca, o que prova que os estados de tensão são efetivamente afetados e que o método da inserção controlada produz a maior perturbação no campo de tensão próximo à ponta da pré-trinca. / Single-Edge-Notch Bending samples produced out of compression-molded plates obtained from a commercial Poly(methyl metacrylate), PMMA, resin were investigated concerning the influence of the adopted pre-cracking methodology on plane strain fracture toughness results according to ASTM D 5045 standard. Two sets of 6 samples each were pre-cracked using a sharp-edge scalpel steel blade using two methodologies proposed in the technical literature: by gentle tapping on the blade and by controlled insertion at constant velocity, both using a device specially designed for these purposes. These sample were, then, submitted to the plane strain fracture toughness (KIc) test based on the hypothesis that different pre-cracking methods result in different damage levels of the matrix near the pre-crack tip and, hence, in different measured values for KIc. All samples produced valid results for KIc based on the geometric criteria adopted by the standard. The results confirm the original hypothesis showing that the two pre-cracking methods influence the measured value at a significance level of 5% for the comparison of the averages obtained in both sets. The most conservative value for plane strain fracture toughness of the investigated resin is obtained by pre-cracks introduced by gentle tapping on the blade (KIc = 1.15 ± 0.11 MPa m1/2). Based on these results new samples with a self-similar geometry were produced and submitted to photoelasticity tests to investigate the influence of the pre-cracking methods on the strain-field developed during three-point bending loading. The results confirm that the different methods produce different Moiré patterns close to the pre-crack tip, proving that the developed stress fields are different and that pre-cracks introduced by controlled insertion of the blade produce the largest perturbations of the strain field close to the pre-crack tip.

Le forgeage multi-matériaux est un procédé permettant la mise en forme et l’assemblage simultanés de matériaux différents. Ce procédé permet d’obtenir des pièces multi-matériaux avec le « bon matériau placé au bon endroit ». L’objectif des travaux de thèse est de définir les conditions nécessaires à l’établissement de la liaison métallurgique par forgeage à l’interface d’un couple d’aciers. Dans un premier temps, l’état de l’art a servi à l’identification les phénomènes physiques accompagnant le forgeage multi-matériaux et les paramètres clés pilotant l’établissement de la liaison métallurgique. Le principe de base de l’établissement d’une liaison passe par la fragmentation des oxydes en surface des matériaux et par l’application d’une pression de contact favorisant le contact entre les matériaux nus et la diffusion. Les deux paramètres clés identifiés sont donc la pression normale de contact et l’expansion de surface. Le protocole de caractérisation du co-forgeage mis en place comporte trois essais « simples » permettant de solliciter les interfaces avec des pressions et des expansions différentes. Ces dernières, estimées par simulation numérique de l’essai, sont mises en relation avec la qualité des liaisons obtenues évaluée, quant à elle, au travers d’observations métallographiques. Les premières simulations permettent de dimensionner les campagnes expérimentales. Celles-ci sont ensuite conduites sur les moyens de mise en forme de la plateforme VULCAIN. Les efforts de mise en forme et la géométrie globale des pièces et la répartition de matière servent de base à l’identification des paramètres de la simulation. La simulation ainsi obtenue et les observations métallographiques aux interfaces sont ensuite mises en lien. Cette démarche a permis de confirmer l’importance du rôle joué par la pression de contact et l’expansion de surface sur l’établissement d’une liaison au cours de la mise en forme du multi-matériaux. La répartition et la forme des particules d’oxydes semblent liées au chemin thermomécanique subi par l’interface. / The multi-material forging is a forming process allowing, simultaneously, the welding and shaping of multi-material parts with the right material at the right place. The purpose of the presented work is to identify the necessary conditions to obtain a metallurgical bond during forming between two different grades of steel. First, the state of the art allowed the identification of the physical phenomena occurring during multi-material forging and the determination of the key parameters of the bonding which are the contact pressure and the surface expansion at the both sides of the interface. The mechanisms to establish metallurgical bond by forging are based on the breaking and the dispersion of the oxide layer at the interface then the extrusion of the soft material through the voids generated between the oxide fragments. Second, the characterization methodology of this work is presented. It consists of three “simple” forming tests leading to different interface conditions (contact pressure and surface expansion). The first simulations allow the design of the experimental plan for each test. The comparison between simulations and experiments allows the identification of physical parameters of the simulation. Then, the contact pressure and the surface expansion of the identified simulations are used to analyze the metallographic structure and the bonding at the interface.The developed work confirms the major effect of the contact pressure and the surface expansion on the establishment of a metallurgical bond during multi-material forming. The size and the shape of the oxide particles seem to depend on the thermomechanical path at the interface.

<p>In the first part of this thesis, the FE program MSC.Marc is applied for coupled thermomechanical simulations of wire-rod rolling. In order to predict material behaviour of an AISI 302 stainless steel at high strain rates generated during wire-rod rolling, a material model based on dislocation density is applied. Then, the evolution of temperature, strain rate and flow stress is predicted in the first four rolling passes of a wire block.</p><p>In the second part of the thesis, an alternative approach to simulation of shape rolling is evaluated. The approach is applied in order to save the computational time in cases where many shape-rolling passes are to be simulated. The approach is a combination of the slab method and a 2D FEM with a generalized plane-strain formulation. A number of various isothermal shape-rolling passes are simulated applying the simplified approach. The simulations are carried out using an in-house 2D FE code implemented in Matlab. The results are compared to fully 3D FE analyses. The comparison shows that the simplified approach can predict roll forces and roll torques with a fair accuracy, but the predicted area reductions are a bit underestimated. The reasons for the deviations between the simplified approach and the 3D FEM are discussed.</p>

No presente trabalho produziu-se amostras de flexão em três pontos, SENB, a partir de placas moldadas por compressão feitas de uma resina de Poli(metacrilato de metila) comercial. Dois conjuntos contendo 6 amostras cada foram então pré-trincados por meio de uma lamina afiada de aço adotando-se dois métodos diferentes propostos pela literatura técnica: através de batidinhas (gentle tapping) e através da inserção controlada a velocidade constante da lâmina, ambas usando um dispositivo especialmente desenhado para este fim. Estas amostras foram então submetidas ao ensaio de tenacidade à fratura em deformação plana, KIc, com o intuito de se testar a hipótese de que os diferentes métodos introduzem diferentes padrões de dano na matriz próxima à região da ponta da pré-trinca, o que, por sua vez, influência os resultados obtidos. Adotou-se a norma ASTM D 5045 como padrão do ensaio. Todas as amostras produziram resultados válidos para KIc, considerando-se os critérios da norma. Os resultados confirmam a hipótese inicial mostrando que os diferentes métodos de introdução da pré-trinca influenciam o valor medido de KIc a um nível de significância de 5% para a diferença das médias obtidas nos dois grupos, sendo que o valor mais conservador é obtido quando as pré-trincas são introduzidas pelo método das batidinhas (KIc = 1,15 ± 0,11 MPa m1/2). Com base nestes resultados novas amostras com geometria auto-similar às anteriores foram produzidas e usadas para a investigação do estado de tensão desenvolvido nas amostras pelo método da fotoelasticidade. Os resultados confirmam que os diferentes métodos de pré-trincamento produzem diferentes padrões de Moiré próximos à ponta da trinca, o que prova que os estados de tensão são efetivamente afetados e que o método da inserção controlada produz a maior perturbação no campo de tensão próximo à ponta da pré-trinca. / Single-Edge-Notch Bending samples produced out of compression-molded plates obtained from a commercial Poly(methyl metacrylate), PMMA, resin were investigated concerning the influence of the adopted pre-cracking methodology on plane strain fracture toughness results according to ASTM D 5045 standard. Two sets of 6 samples each were pre-cracked using a sharp-edge scalpel steel blade using two methodologies proposed in the technical literature: by gentle tapping on the blade and by controlled insertion at constant velocity, both using a device specially designed for these purposes. These sample were, then, submitted to the plane strain fracture toughness (KIc) test based on the hypothesis that different pre-cracking methods result in different damage levels of the matrix near the pre-crack tip and, hence, in different measured values for KIc. All samples produced valid results for KIc based on the geometric criteria adopted by the standard. The results confirm the original hypothesis showing that the two pre-cracking methods influence the measured value at a significance level of 5% for the comparison of the averages obtained in both sets. The most conservative value for plane strain fracture toughness of the investigated resin is obtained by pre-cracks introduced by gentle tapping on the blade (KIc = 1.15 ± 0.11 MPa m1/2). Based on these results new samples with a self-similar geometry were produced and submitted to photoelasticity tests to investigate the influence of the pre-cracking methods on the strain-field developed during three-point bending loading. The results confirm that the different methods produce different Moiré patterns close to the pre-crack tip, proving that the developed stress fields are different and that pre-cracks introduced by controlled insertion of the blade produce the largest perturbations of the strain field close to the pre-crack tip.

Small kinematics assumption in classical engineering has been in the center of consideration in structural analysis for decennaries. In the recent years the interest for sustainable and optimized structures, lightweight structures and new materials has grown rapidly as a consequence of desire to archive economical sustainability. These issues involve non-linear constitutive response of materials and can only be accessed on the basis of geometrically and materially non-linear analysis. Numerical simulations have become a conventional tool in modern engineering and have proven accuracy in computation and are on the verge of superseding time consuming and costly experiments.\newlineConsequently, this work presents a numerical computational framework for modeling of geometrically non-linear large deformation of isotropic and orthotropic materials under static and dynamic loading. The numerical model is applied on isotropic steel in plane strain and orthotropic wood in 3D under static and dynamic loading. In plane strain Total Lagrangian, Updated Lagrangian, Newmark-$\beta$ and Energy Conserving Algorithm time-integration methods are compared and evaluated. In 3D, a Total Lagrangian static approach and a Total Lagrangian based dynamic approach with Newmark-$\beta$ time-integration method is proposed to numerically predict deformation of wood under static and dynamic loading. The numerical model's accuracy is validated through an experiment where a knot-free pine wood board under large deformation is studied. The results indicate accuracy and capability of the numerical model in predicting static and dynamic behaviour of wood under large deformation. Contrastingly, classical engineering solution proves its inaccuracy and incapability of predicting kinematics of the wood board under studied conditions. / Små kinematikantaganden inom klassisk ingenjörsteknik har varit centralt för konstruktionslösningar under decennier. Under de senaste åren har intresset för hållbara och optimerade strukturer, lättviktskonstruktioner och nya material ökat kraftigt till följd av önskan att uppnå ekonomisk hållbarhet. Dessa nya konstruktionslösningar involverar icke-linjär konstitutiv respons hos material och kan endast studeras baserad på geometriskt och materiellt olinjär analys. Numeriska simuleringar har blivit ett konventionellt verktyg inom modern ingenjörsteknik och visat sig ge noggrannhet i beräkning och kan på sikt ersätta tidskrävande och kostsamma experiment.\newlineDetta examensarbete presenterar ett numeriskt beräkningsramverk för modellering av geometrisk olinjäritet med stora deformationer hos isotropa och ortotropa material vid statisk och dynamisk belastning. Den numeriska modellen appliceras på isotropiskt stål i plantöjning och ortotropisk trä i 3D vid statisk och dynamisk belastning. I fallet med plantöjning jämförs och utvärderas den Totala Lagrangianen, Uppdaterade Lagrangianen, Newmark-$\beta$ och Energi Konserverings Algoritm metoderna. I 3D föreslås en statisk Total Lagrangian metod och en dynamisk Total Lagrangian-baserad metod med Newmark-$\beta$ tidsintegreringsmetod för att numeriskt förutse statisk och dynamisk deformation hos trä. Den numeriska modellens noggrannhet valideras genom ett experiment där en kvistfri furuplanka studeras under stora deformationer. Resultaten bekräftar noggrannhet och förmåga hos den numeriska modellen att förutse statiska och dynamiska processer hos trä vid stora deformationer. Däremot, visar klassisk ingenjörslösning brist på förmåga att förutse trä plankans kinematik under studerade förhållanden.