Na busca contínua de atender as exigências de um mercado cada vez mais competitivo, as empresas devem buscar diminuir todas as suas perdas em seus processos produtivos, reduzindo custos e ganhando margem em seus produtos. Este trabalho teve como objetivo investigar os fenômenos presentes na ocorrência de quebras dos produtos derivados do aço SAE 1057B trefilado e galvanizado utilizado em culturas aéreas. O estudo utilizou bibliografia documental, pesquisa de laboratório e uma amostra composta por trinta e seis relatórios de análises sobre o desempenho do produto intermediário trefilado, ou do produto final galvanizado. Os dados referem-se à eventos de falhas do produto em quaisquer etapas do processo produtivo e mesmo de aplicação em campo. Juntamente com ensaios efetuados especificamente para amparar as conclusões destes relatórios, efetuou-se pesquisa bibliográfica para dar suporte às conclusões. Identificaram-se similaridades entre estudos de alguns autores e análises amostrais de testes padronizados de tração e enrolamento sobre o próprio eixo de outubro 2010 a setembro de 2011. Os resultados mostraram que o fenômeno que induz a um elevado índice de quebra do arame Cultura Aérea é o envelhecimento que ocorre posteriormente à sua passagem por elevadas reduções percentuais de área no processo de trefilação, que encrua o material, e em função de elevadas temperaturas encontradas no processo de galvanização, tendo, ainda, suas consequências amplificadas por pontos de concentração de tensões. Algumas soluções para amenizar a ocorrência do envelhecimento foram propostas, como o aumento do número de passes, o que reduz o encruamento, e a substituição do material por outro menos suscetível ao fenômeno, como um com menor percentual de carbono. / In the ongoing pursuit to meet demands of an increasingly competitive market, companies have to reduce all their losses in the production process, therefore reducing costs and adding margin to their products. The main goal of this study was to investigate the phenomena present in the occurrence of ruptures of products originated from drawn and galvanized SAE 1057B steel, used in aerial cultivation. The study uses literature and laboratory research and a sample of thirty-six report concerning galvanized final and drawn intermediate product behavior. The data reference to product failure in any stage of the productive process or in field application. Along with testing a bibliographical research was made for conclusion support . Similarities were found between some authors studies and sample analysis of standardized tensile tests and winding over own axis tests from October 2010 to September 2011. The results shown that the primary phenomenon inducing wire failure in aerial cultivation wire is alignment of the hardening due to the high percentage area reductions in the drawing process that allows the development of the strain aging phenomenon after the hot dip galvanizing process witch has it´s effects enhanced by stress concentration points. Some solutions to mitigate the strain aging were proposed, like increasing the number of reduction passes for hardening reduction and the use of less susceptible material to the phenomena as a lower percentage carbon steel.

Na busca contínua de atender as exigências de um mercado cada vez mais competitivo, as empresas devem buscar diminuir todas as suas perdas em seus processos produtivos, reduzindo custos e ganhando margem em seus produtos. Este trabalho teve como objetivo investigar os fenômenos presentes na ocorrência de quebras dos produtos derivados do aço SAE 1057B trefilado e galvanizado utilizado em culturas aéreas. O estudo utilizou bibliografia documental, pesquisa de laboratório e uma amostra composta por trinta e seis relatórios de análises sobre o desempenho do produto intermediário trefilado, ou do produto final galvanizado. Os dados referem-se à eventos de falhas do produto em quaisquer etapas do processo produtivo e mesmo de aplicação em campo. Juntamente com ensaios efetuados especificamente para amparar as conclusões destes relatórios, efetuou-se pesquisa bibliográfica para dar suporte às conclusões. Identificaram-se similaridades entre estudos de alguns autores e análises amostrais de testes padronizados de tração e enrolamento sobre o próprio eixo de outubro 2010 a setembro de 2011. Os resultados mostraram que o fenômeno que induz a um elevado índice de quebra do arame Cultura Aérea é o envelhecimento que ocorre posteriormente à sua passagem por elevadas reduções percentuais de área no processo de trefilação, que encrua o material, e em função de elevadas temperaturas encontradas no processo de galvanização, tendo, ainda, suas consequências amplificadas por pontos de concentração de tensões. Algumas soluções para amenizar a ocorrência do envelhecimento foram propostas, como o aumento do número de passes, o que reduz o encruamento, e a substituição do material por outro menos suscetível ao fenômeno, como um com menor percentual de carbono. / In the ongoing pursuit to meet demands of an increasingly competitive market, companies have to reduce all their losses in the production process, therefore reducing costs and adding margin to their products. The main goal of this study was to investigate the phenomena present in the occurrence of ruptures of products originated from drawn and galvanized SAE 1057B steel, used in aerial cultivation. The study uses literature and laboratory research and a sample of thirty-six report concerning galvanized final and drawn intermediate product behavior. The data reference to product failure in any stage of the productive process or in field application. Along with testing a bibliographical research was made for conclusion support . Similarities were found between some authors studies and sample analysis of standardized tensile tests and winding over own axis tests from October 2010 to September 2011. The results shown that the primary phenomenon inducing wire failure in aerial cultivation wire is alignment of the hardening due to the high percentage area reductions in the drawing process that allows the development of the strain aging phenomenon after the hot dip galvanizing process witch has it´s effects enhanced by stress concentration points. Some solutions to mitigate the strain aging were proposed, like increasing the number of reduction passes for hardening reduction and the use of less susceptible material to the phenomena as a lower percentage carbon steel.

Na busca contínua de atender as exigências de um mercado cada vez mais competitivo, as empresas devem buscar diminuir todas as suas perdas em seus processos produtivos, reduzindo custos e ganhando margem em seus produtos. Este trabalho teve como objetivo investigar os fenômenos presentes na ocorrência de quebras dos produtos derivados do aço SAE 1057B trefilado e galvanizado utilizado em culturas aéreas. O estudo utilizou bibliografia documental, pesquisa de laboratório e uma amostra composta por trinta e seis relatórios de análises sobre o desempenho do produto intermediário trefilado, ou do produto final galvanizado. Os dados referem-se à eventos de falhas do produto em quaisquer etapas do processo produtivo e mesmo de aplicação em campo. Juntamente com ensaios efetuados especificamente para amparar as conclusões destes relatórios, efetuou-se pesquisa bibliográfica para dar suporte às conclusões. Identificaram-se similaridades entre estudos de alguns autores e análises amostrais de testes padronizados de tração e enrolamento sobre o próprio eixo de outubro 2010 a setembro de 2011. Os resultados mostraram que o fenômeno que induz a um elevado índice de quebra do arame Cultura Aérea é o envelhecimento que ocorre posteriormente à sua passagem por elevadas reduções percentuais de área no processo de trefilação, que encrua o material, e em função de elevadas temperaturas encontradas no processo de galvanização, tendo, ainda, suas consequências amplificadas por pontos de concentração de tensões. Algumas soluções para amenizar a ocorrência do envelhecimento foram propostas, como o aumento do número de passes, o que reduz o encruamento, e a substituição do material por outro menos suscetível ao fenômeno, como um com menor percentual de carbono. / In the ongoing pursuit to meet demands of an increasingly competitive market, companies have to reduce all their losses in the production process, therefore reducing costs and adding margin to their products. The main goal of this study was to investigate the phenomena present in the occurrence of ruptures of products originated from drawn and galvanized SAE 1057B steel, used in aerial cultivation. The study uses literature and laboratory research and a sample of thirty-six report concerning galvanized final and drawn intermediate product behavior. The data reference to product failure in any stage of the productive process or in field application. Along with testing a bibliographical research was made for conclusion support . Similarities were found between some authors studies and sample analysis of standardized tensile tests and winding over own axis tests from October 2010 to September 2011. The results shown that the primary phenomenon inducing wire failure in aerial cultivation wire is alignment of the hardening due to the high percentage area reductions in the drawing process that allows the development of the strain aging phenomenon after the hot dip galvanizing process witch has it´s effects enhanced by stress concentration points. Some solutions to mitigate the strain aging were proposed, like increasing the number of reduction passes for hardening reduction and the use of less susceptible material to the phenomena as a lower percentage carbon steel.

Global increase in energy consumption and global warming require more energy production but less CO2emission. Increase in efficiency of energy production is an effective way for this purpose. This can be reached by increasing boiler temperature and pressure in a biomass power plant. By increasing material temperature 50°C, the efficiency in biomass power plants can be increased significantly and the CO2emission can be greatly reduced. However, the materials used for future biomass power plants with higher temperature require improved properties. Austenitic stainless steels are used in most biomass power plants. In austenitic stainless steels a phenomenon called dynamic strain aging (DSA), can occur in the operating temperature range for biomass power plants. DSA is an effect of interaction between moving dislocations and solute atoms and occurs during deformation at certain temperatures. An investigation of DSA influences on ductility in austenitic stainless steels and nickel base alloys have been done. Tensile tests at room temperature up to 700°C and scanning electron microscope investigations have been used. Tensile tests revealed that ductility increases with increased temperature for some materials when for others the ductility decreases. This is, probably due to formation of twins. Increased stacking fault energy (SFE) gives increased amount of twins and high nickel content gives a higher SFE. Deformation mechanisms observed in the microstructure are glide bands (or deformations band), twins, dislocation cells and shear bands. Damage due to DSA can probably be related to intersection between glide bands or twins, see figure 6 a). Broken particles and voids are damage mechanisms observed in the microstructure.

Static strain aging is an important concept in metalurgy that refers to the hardening of a material that has undergone plastic deformation and then is aged for a certain period of time. A theory proposed in the late 1940s by Cottrell and Bilby explains this phenomenon as being caused by the pinning of dislocations by impurities (e.g., carbon atoms in solid solution) that migrate to the vicinity of the line defect. In the course of this PhD work, the atomistic mechanism behind the static strain aging phenomenon in bcc iron has been studied by means of computer simulations. Given the fact that diffusion in the solid state proceeds slowly, thus preventing the use of molecular dynamics at low temperatures (when the effect of the dislocation stress field on carbon diffusion is more pronounced), we have preferentially employed a method coupling molecular statics with atomistic kinetic Monte Carlo. Three major points have been addressed by this thesis: (i) the effect of the stress field of an edge or screw dislocation on a carbon atom diffusing nearby; (ii) the diffusion of a carbon atom in the tight channel found in the dislocation core (pipe diffusion); and (iii) the equilibrium carbon distribution in a Cottrell atmosphere. The main effect of the dislocation stress field outside the dislocation core consists of biasing carbon diffusion, such that some transitions become more likely than others. This effect is expected to drive the early stages of Cottrell atmosphere formation, when the mutual interaction between carbon atoms is negligible. Right in the dislocation core, as expected, carbon was seen to diffuse faster than in the bulk. Carbon concentration in the neighborhood of an edge or a screw dislocation was modeled by an approach based in statistical physics using the binding energies calculated by molecular statics, revealing a good agreement with experimental data obtained by atom probe techniques.

Les aciers ferritiques pour structures industrielles présentent une transition fragile-ductile de ténacité et de résilience avec leur température. Leur résistance à la rupture fragile joue un rôle essentiel dans la certification de la sécurité des structures industrielles importantes. De nos jours, le souci de performance et de longévité sont des points clés pour des acteurs majeurs comme EDF.Dans ces études de transition ductile-fragile, de ténacité et de résilience, la ténacité est prédite à partir de la résilience. Plusieurs travaux antérieurs ont déjà montré que la probabilité de rupture par clivage peut être correctement décrite dans le palier fragile par une approche locale de la rupture. Mais ces études supposent que le matériau ne subit pas de vieillissement sous déformation, ce qui est en fait rarement pertinent pour les aciers bas carbone et peu calmés. Le travail a consisté d'une part à caractériser le comportement et d'autre part à en proposer une modélisation robuste et explicite des phénomènes observés. La caractérisation a consisté en la réalisation d'essais de traction entre -150°C et 20°C à plusieurs vitesses de déformation. Un modèle capable de simuler le vieillissement statique est identifié en mettant en place une stratégie adéquate et systématique. Des essais de résilience permettent de construire la courbe de transition ductile-fragile du matériau pour différentes conditions afin de comprendre et d'observer l'influence du vieillissement statique sur la rupture. Enfin, la modélisation de la rupture fragile a été décrite pour toutes les conditions expérimentales testées en utilisant le modèle de comportement développé et identifié dans la partie précédente afin de prédire numériquement la transition pour les différentes conditions du matériau. / Ferritic steels for industrial structures have a brittle-ductile transition toughness and impact energy with temperature. Their resistance to the brittle fracture plays an essential role in the safety certification of industrial structures. Nowadays, the performance and the durability are key issues for major players such as EDF. In these approaches ductile-to-brittle transition toughness and impact energy, toughness is predicted from resilience. Several previous studies have shown that the probability of cleavage fracture can be adequately described in brittle plateau by a local approach to fracture. However, these studies assume that the material does not undergo strain aging, which is rarely relevant for low carbon steels and low calmed down. The work consisted firstly to characterize the behavior and secondly to propose a robust and explicit modeling of the observed phenomena. Characterization consisted of performing tensile tests between -150degreC~and 20degreC for several strain rates. A model able to simulate the static aging is identified by implementing an appropriate and systematic strategy. Impact resistance test allows us to build the curve of ductile-to-brittle transition of the material for different conditions to understand and observe the influence of static strain aging on the failure. Finally, the modeling of the brittle fracture has been described for all experimental conditions tested using the model developed and identified in the previous section to predict the transition for different material conditions.

No description available.

Le vieillissement dynamique se manifeste en particulier par le phénomène de Portevin-Le Chatelier (PLC). Il se produit dans les aciers aux environs de 200°C pour des sollicitations quasi-statiques. Dans les aciers au C-Mn, il conduit à une chute de ductilité et de ténacité qui doit être prise en compte dans le dimensionnement des structures de sûreté. L’objectif de la thèse consiste à modéliser le comportement mécanique des aciers au C-Mn en tenant compte du vieillissement dynamique et à prédire leur rupture ductile en présence de ce phénomène. Le comportement mécanique du matériau étudié, un acier au C-Mn, a été caractérisé par des essais de traction simple. Le modèle KEMC implémenté dans le code de calculs par éléments finis Zébulon, a été identifié sur ces essais : l’effet de Portevin Le-Chatelier (PLC) a été correctement simulé sur les éprouvettes lisses, entaillées et CT. Nous avons montré l’importance des conditions aux limites dans la manifestation du PLC. Pour la rupture ductile, l’application du critère de Rice et Tracey (identifié à 20°C) sur les éprouvettes entaillées AE4 montre que la prise en compte du vieillissement dynamique dans le comportement ne suffit pas pour avoir une bonne prédiction de la rupture. Des études micromécaniques de croissance de cavité indiquent que les localisations de PLC peuvent favoriser la croissance et la coalescence de cavité. L’écrouissage apparent, qui dépend du durcissement par la déformation mais aussi du durcissement provenant du vieillissement dynamique, modifie la vitesse de croissance de cavité, mais pas le taux critique de croissance de cavité. On identifie une loi d’endommagement dont les paramètres dépendent de la température à partir des calculs micromécaniques. Le nouveau modèle donne une meilleure prédiction que le modèle de Rice et Tracey sur les éprouvettes entaillées AE4 et a permis de prédire un creux de ténacité sur les éprouvettes CT. Pour améliorer les prédictions, la loi d’endommagement doit dépendre de la vitesse de déformation. / The dynamic strain aging is manifested especially by the Portevin-Le Chatelier (PLC) phenomenon. It appears in steels around 200°C at quasi static solicitation conditions. In C-Mn steels, it leads to a drop of ductility and of toughness which should be taken into account in the design of safety structures. The thesis aims to model the mechanical behavior of the C-Mn steels taking into account the dynamic strain aging and to predict the ductile fracture of these steels in the presence of the phenomenon. The mechanical behavior of the material studied, a C-Mn steel, was characterized using tensile tests. The KEMC model, which was implemented in the Finite Element program Zébulon, was identified using these tests: the Portevin-Le Chatelier (PLC) effect was correctly simulated on the tensile specimens, the notched specimens and the CT specimens. We showed the importance of boundary conditions in the occurrence of the PLC effect. As far as the ductile fracture is concerned, the application of the Rice and Tracey’s criterion (identified at 20°C) on the notched specimens AE4 showed that the consideration of the dynamic strain aging in the behavior was insufficient to give a good prediction of the fracture. Micromechanical studies of the growth of voids showed that PLC localizations can facilitate the growth and the coalescence of voids. The nominal strain hardening, which depends on the strain hardening and the hardening due to the dynamic strain aging, does not modify the critic growth ratio of voids but the growth rate of voids. We identified a damage model using micromechanical simulations. The parameters of this damage model depend on temperature. The new model gave a better prediction than the Rice and Tracey model on the notched specimens AE4 and it also allowed predicting the drop of toughness on the CT specimens. To improve the prediction of fracture, le damage law must depend on strain rate.

Les aciers de construction au carbone manganèse (C-Mn) sont largement utilisés pour diverses applications mécaniques, et en particulier pour les tuyauteries de circuit secondaire des centrales nucléaires de type Réacteurs à Eau sous Pression (REP). La robustesse des composants des circuits sous pression des REP vis-à-vis de la fissuration doit être démontrée, tant au niveau de la conception que de l’exploitation. Les aciers au C-Mn sont sensibles au vieillissement sous déformation qui entraîne une chute importante de ductilité et de ténacité entre 150 et 350°C,températures de service des tuyauteries du circuit secondaire. Ce phénomène est dû à une interaction entre les atomes de solutés et les dislocations, et peut se traduire entre autres par une sensibilité négative de la contrainte à la vitesse de déformation, et des localisations de la déformation plastique (Lüders, Portevin – Le Chatelier). L’origine physique du vieillissement sous déformation a été beaucoup étudiée, surtout dans les métaux purs, en relation avec le phénomène Portevin-Le Chatelier (PLC), mais son influence sur les propriétés mécaniques et notamment la rupture reste très controversée. L’objectif de la thèse est de modéliser le comportement et la rupture d’un acier au C-Mn dans un large domaine de température compris entre 20 et 350°C, en tenant compte des phénomènes de vieillissement sous déformation, et en particulier des localisations de déformation. Le comportement et la rupture de l’acier au C-Mn étudié ont été caractérisés expérimentalement dans le domaine 20-350°C à l’aide d’essais de traction sur éprouvettes lisses, sur éprouvettes axisymétriques entaillées, et d’essais de déchirure sur éprouvettes CT. Le modèle d’Estrin Kubin McCormick, prenant en compte le vieillissement sous déformation, a été identifié dans cette même gamme de température et la plupart des effets du vieillissement sous déformation ont pu être simulés numériquement : sensibilité négative de la contrainte d’écoulement à la vitesse de déformation, bandes de Lüders, effet PLC, modification des propriétés mécaniques de traction… Le modèle ainsi identifié a été appliqué à l’étude de la rupture d’éprouvettes lisses, entaillées et CT. La baisse de l’allongement réparti est bien décrite en traction sur éprouvettes lisses. Pour prévoir la rupture des éprouvettes entaillées, l’approche locale de la rupture a été appliquée (modèle de Rice et Tracey). Cette étude a donc permis de disposer d’un modèle prenant en compte le vieillissement sous déformation de 20°C à 350°C et décrivant les localisations de déformation plastique de type Lüdersou PLC, pour différentes géométries d’éprouvettes. Ce modèle a été utilisé pour simuler la rupture des aciers au C-Mn, suscitant ainsi une vision nouvelle pour comprendre la baisse de ductilité associée au vieillissement dynamique. / Pas de résumé en anglais disponible.

Le vieillissement statique est un concept important dans la métallurgie qui se réfère à un durcissement de la matière ayant subi une déformation plastique et est ensuite vieilli pendant une certaine période de temps. La théorie proposée dans les années 1940 par Cottrell et Bilby explique ce phénomène comme étant causé par l'épinglage des dislocations par les impuretés (par exemple, les atomes de carbone en solution solide) qui migrent au voisinage du défaut de ligne. Au cours de ce travail de thèse, le mécanisme atomistique responsable du phénomène du vieillissement statique dans le fer alpha a été étudié par des simulations numériques. Etant donné que la diffusion à l'état solide se déroule lentement, l'utilisation de la dynamique moléculaire à basse température (lorsque l'effet du champ de contraintes sur la dislocation de diffusion du carbone est plus prononcé) a été évitée, et nous avons utilisé préférentiellement le couplage de la statique moléculaire avec le Monte-Carlo cinétique atomistique. Trois points principaux ont été abordés dans cette thèse: (i) l'effet du champ de contraintes d'une dislocation coin ou vis sur un atome de carbone qui diffuse à proximité, (ii) la diffusion de l'atome de carbone dans le cour de la dislocation («pipe diffusion»), et (iii) la distribution d'équilibre des atomes de carbone dans une atmosphère de Cottrell. Le principal effet du champ de contrainte de la dislocation à l'extérieur du coeur est de biaiser la diffusion de l'impurité, de sorte que certains sauts (transitions) deviennent plus probables que d'autres. Cet effet va conduire aux premiers stades de la formation de l'atmosphère de Cottrell, lorsque l'interaction mutuelle entre atomes de carbone est négligeable. Au cœur de la dislocation, comme prévu, nos résultats indiquent que le carbone diffuse plus vite que dans le volume. La concentration de carbone dans le voisinage d'une dislocation coin ou vis a été modélisée par une approche de physique statistique en utilisant les énergies de liaison calculées par la statique moléculaire. Cette approche est en bon accord avec les données expérimentales. / Static strain aging is an important concept in metalurgy that refers to the hardening of a material that has undergone plastic deformation and then is aged for a certain period of time. A theory proposed in the late 1940s by Cottrell and Bilby explains this phenomenon as being caused by the pinning of dislocations by impurities (e.g., carbon atoms in solid solution) that migrate to the vicinity of the line defect. In the course of this PhD work, the atomistic mechanism behind the static strain aging phenomenon in bcc iron has been studied by means of computer simulations. Given the fact that diffusion in the solid state proceeds slowly, thus preventing the use of molecular dynamics at low temperatures (when the effect of the dislocation stress field on carbon diffusion is more pronounced), we have preferentially employed a method coupling molecular statics with atomistic kinetic Monte Carlo. Three major points have been addressed by this thesis: (i) the effect of the stress field of an edge or screw dislocation on a carbon atom diffusing nearby; (ii) the diffusion of a carbon atom in the tight channel found in the dislocation core (pipe diffusion); and (iii) the equilibrium carbon distribution in a Cottrell atmosphere. The main effect of the dislocation stress field outside the dislocation core consists of biasing carbon diffusion, such that some transitions become more likely than others. This effect is expected to drive the early stages of Cottrell atmosphere formation, when the mutual interaction between carbon atoms is negligible. Right in the dislocation core, as expected, carbon was seen to diffuse faster than in the bulk. Carbon concentration in the neighborhood of an edge or a screw dislocation was modeled by an approach based in statistical physics using the binding energies calculated by molecular statics, revealing a good agreement with experimental data obtained by atom probe techniques.