In der Arbeit werden spezielle Kupferlote im MIG- und Laserlötverfahren an Stählen mit unterschiedlichem Festigkeitsverhalten untersucht. Die Ergebnisse sollen dazu beitragen, den Lötprozess durch den Einsatz spezieller Kupferbasislote zu optimieren und durch reduzierten Energieeintrag ein homogeneres Eigenschaftsfeld im Bereich der Fügestelle zu erzeugen. Den Verarbeitern dieser Werkstoffe soll die Möglichkeit gegeben werden, diese Werkstoffe rationeller und mit höherer Effektivität zu verarbeiten. Im Ergebnis der Arbeit sollen Verbesserungen der Eigenschaften der Lötnähte erzielt werden, die besonders in der Dünnblechverarbeitung mit Schwerpunkt Karosseriebau Anwendung finden. Wesentliche Ziele sind die Erhöhung der Festigkeitseigenschaften, eine Erhöhung der Fügegeschwindigkeit, die Verbesserung des Phosphatierungsverhaltens sowie eine Reduzierung der eingebrachten Wärmeenergie. Die Vielfältigkeit dieser Anforderungen macht es notwendig, die Versuche sowohl im Laser- als auch im MIG-Lötverfahren durchzuführen. Die Lötverfahren werden in der Praxis für unterschiedliche Anforderungen innerhalb der Karosserie eingesetzt. Das Fügen von hochfesten Strukturelementen oder Außenhautbauteilen erfordert in Abhängigkeit von den Anforderungen die Verwendung ausgewählter Zusatzwerkstoffe. Die Vielfältigkeit der Werkstoffe und der Anforderungen spiegelt sich in den Untersuchungen der vorliegenden Arbeit wieder. Für weitergehende Untersuchungen, speziell im hochfesten Blechbereich, soll die Arbeit entsprechende Grundlagen bieten. Als Vorlage für die Erarbeitung von experimentellen und theoretischen Methodiken der Prozessbetrachtung werden neben typischen Kupferloten neu entwickelte Lotlegierungen verwendet. Bei der Betrachtung der Kupferlegierungen werden die unterschiedlichen Einflüsse auf den Fügeprozess definiert und beschrieben. Es wird festgestellt, dass niedrig schmelzende Lote mit ausgewählter Legierungszusammensetzung im Gegensatz zu Eisenbasis-Schweißdrähten einen geringeren negativen Einfluss auf das Gefüge der Bleche im Nahtbereich ausüben. Um die thermische Beanspruchung, besonders in der Wärmeeinflusszone, während des Fügeprozesses gering zu halten, kann zusätzlich eine geeignete Stromquellentechnik zum Einsatz kommen. Mit Hilfe des „kalten“ Lichtbogens ist es möglich, die eingebrachte Streckenenergie weiter zu reduzieren. Faktoren, die den Energieeintrag beeinflussen, werden in der vorliegenden Arbeit in experimentellen und theoretischen Untersuchungen hinsichtlich ihrer Wirkung auf das Festigkeitsverhalten betrachtet. Es werden durch geeignete Legierungskombinationen die Einflüsse auf die Steigerung der Lötgeschwindigkeit und auf eine Verbesserung des Phosphatierungsverhaltens untersucht. Die Ergebnisse dieser Untersuchungen liefern die Informations- und Beweisbasis für die erarbeiteten Legierungssysteme und ermöglichen es, den optimierten Lötprozess an hochfesten Stahlblechen wissenschaftlich zu betrachten. Die Auswertung der wissenschaftlichen Experimente, dargestellt in den angefügten ausführlichen Tabellen, stellen die Zusammenhänge zwischen der Legierungsauswahl und der eingebrachten Streckenenergie dar. Die Erkenntnisse aus der vorliegenden Arbeit sollen für das Fügen von hochfesten Blechen die Entscheidung über die Auswahl geeigneter Zusatzwerkstoffe erleichtern. Die Ergebnisse der theoretischen Untersuchungen anhand mathematischer Modelle zur Beschreibung der physikalischen Prozesse der Wärmezufuhr durch Verwendung eines ausgewählten Lotes in Kombination mit geeigneter Stromquellentechnologie sind die Grundlage für die Optimierung des Lötprozesses. Die vorgeschlagenen Modelle zur Entwicklung und Optimierung von Lichtbogenlötprozessen mit neu entwickelten Lotlegierungen wurde im Rahmen der vorgelegten Arbeit an realen Blechqualitäten angewendet und überprüft. In den Ergebnissen hat sich bestätigt, dass die Verwendung spezieller Kupferlote zu verbesserten Verarbeitungseigenschaften führen, und damit Konzepte zum wirtschaftlich verbesserten Fügen angeboten werden.

Made available in DSpace on 2014-06-11T19:28:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-02-07Bitstream added on 2014-06-13T18:57:57Z : No. of bitstreams: 1 gallo_gb_me_guara.pdf: 1706567 bytes, checksum: 597ea250f08f8ddc714ee7df53e7829b (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Estudos de novos materiais, mais resistentes, econômicos e de menor densidade, são de grande interesse para aplicações estruturais em projetos de Engenharias. Propriedades mecânicas antes obtidas principalmente por meio de adição de elementos de ligas, como por exemplo elevada dureza, tenacidade e resistência mecânica, podem ser atingidas atualmente com variações no processamento térmico e termomecânico. Estas alterações têm proporcionado a formação de estruturas multifásicas nos aços, e como elemento deste estudo, os aços bifásicos. Neste trabalho, buscou-se entender os efeitos que processamentos térmicos possuem sobre as microestruturas dos aços de médio teor de carbono (SAE 1045), assim como a sua influência sobre as propriedades mecânicas, especialmente sobre a tenacidade, evidenciada por meio de ensaios de impacto. O material tratado termicamente obteve, sua estrutura transformada, visando à formação de estrutura bifásica (ferrita e martensita) e de estrutura formada por martensita revenida, sendo comparadas com a condição do material como fornecido (ferrita e perlita). A caracterização microestrutural foi realizada para determinação das fases presentes, assim como das frações volumétricas respectivas. Ensaios de tração e de impacto foram realizados visando a determinação das propriedades mecânicas do aço. Os resultados mostraram que a interação entre ferrita e martensita, gerada na condição bifásica, possue forte influência nos resultados referentes à tenacidade, assim como a temperatura em que o material foi ensaiado. Comparando-se três estruturas, comprova-se que a estrutura bifásica possui resistência mecânica e tenacidade mais elevadas que os aços temperados e revenidos, e tenacidade proxima a do aço com médio teor de carbono, sem tratamento térmico. / Studies of new materials, more resistance, cheaper costs and lower density, have high interest for structural use in engineering projects. Mechanical properties adquired in the past by alloyes adition, for example igher hardness, toughness and mechanical strenght, nowadays can be reached using controlled thermo mechanical process. These changes have provided the multiphase structures formation in steels, for example the dual phase steel, subject of the present study. In this work it was researched the effects of heat process in microestructure of medium carbon steels (SAE 1045) and the influence over mechanical properties, specially in toughers, realized by impact analysis. The heat treatment material has its structure transformed in dual-phase structure (ferrite and martensite) and compared with supplied material (ferrite and perlite). The microestructural characterization was done to determine the present phase, and the respective volumetric fraction of martensite. Tension and impact analysis were done to determine the steel mechanical properties. Results have shown that the interaction between ferrite and martensite generated in dual-phase condition, has higher influence, related the materials toughness, so the temperature that material was analysed comparing the three structures, verifies that the dual-phase struture has level of mechanical resistence and toughness, higher than quenched and tempered steels, and toughness near of medium carbon steel that has not received heat treatment in higher or lower temperatures.

Este trabalho procura avaliar o retorno elástico em aços avançados de alta resistência utilizados em processo de dobramento. Para isso, foram usadas chapas de aço bifásico DP 600 e DP 800, ambas revestidas. Foram empregados dois métodos de medição a fim de abordar e avaliar o retorno elástico. O primeiro método abordado foi à conformação de tiras em U e o segundo método foi à realização do ensaio Dobramento Sob Tensão, aos quais possibilitaram a coleta de dados para aplicação em cálculo de coeficiente de atrito no processo de dobramento. Em ambas as avaliações foram utilizadas duas diferentes condições de lubrificação para o processo de dobramento, visando variar a condição de atrito no processo. A primeira foi à aplicação de um lubrificante em forma de pasta, de nome comercial Clarus Desmoldax DCP 35 e a segunda foi à aplicação em forma líquida, denominado Clarus Desmoldax DCP 35X. Para analisar o retorno elástico, foram utilizadas como critério de avaliação as medições de abertura da parede da tira conformada, permitindo desta forma considerar o atrito por intermédio da variação do tipo lubrificante. As análises realizadas nos corpos de prova conformados em formato U evidenciaram um retorno elástico menor para as amostras do aço DP 600, se comparadas com as amostras do aço DP 800. Não existiu relevância na variação dos lubrificantes quanto ao retorno elástico nas amostras de um mesmo tipo de aço. As análises realizadas nos corpos de prova submetidos ao ensaio de dobramento sob tensão, também evidenciaram um retorno elástico menor para as amostras do aço DP 600 se comparadas com as amostras do aço DP 800. Comparando as amostras com base nos dados coletados e aplicados ao cálculo de coeficiente de atrito, percebeu-se que as amostras que tiveram um retorno elástico menor, também possuíam coeficientes de atrito menor, constatando assim que quanto menor o coeficiente de atrito, menor é o retorno elástico. / This piece of work seeks to evaluate the springback effect on advanced high strength steel used in the bending process. For this, biphasic steel plates DP 600 and DP 800, both coated, were used. Two methods of measurement were used to approach and asses the springback effect. The first method approached was the conformation of strips in U and the second method was the preformance of the Bending Under Tension test, which enabled the data collect for the application in calculation of the friction coefficient in the bending process. In both assessments two different lubricant conditions for the bending process were used with the aim of varying the condition of friction in the process. The first one was the application of a paste lubricant, with the brand name Clarus Desmoldax DCP 35 and the second one was the application of a liquid lubricant, called Clarus Desmoldax DCP 35X. To analyze the springback effect the measurements of the opening of the wall of the conformed strip were used, thus allowing to consider the friction through the variation of the type of lubricant. The assessments carried out in the specimens in U shape showed a smaller springback effect for the steel samples DP 600 in comparison with the DP 800 ones. There was no relevance in the range of lubricants as the springback effect in the samples of the same type of steel. The analyzes carried out in the specimens subjected to bending under pressure test also showed a smaller springback effect for the steel samples DP 600 compared with the DP 800 ones. Comparing the samples based on the data collected and applied to the calculation of the friction coefficient, it was noticed that the samples that had a smaller springback effect also had lower friction coefficients, therefore verifying that the lower the friction coefficient is, the lower the springback effect.

Este trabalho procura avaliar o retorno elástico em aços avançados de alta resistência utilizados em processo de dobramento. Para isso, foram usadas chapas de aço bifásico DP 600 e DP 800, ambas revestidas. Foram empregados dois métodos de medição a fim de abordar e avaliar o retorno elástico. O primeiro método abordado foi à conformação de tiras em U e o segundo método foi à realização do ensaio Dobramento Sob Tensão, aos quais possibilitaram a coleta de dados para aplicação em cálculo de coeficiente de atrito no processo de dobramento. Em ambas as avaliações foram utilizadas duas diferentes condições de lubrificação para o processo de dobramento, visando variar a condição de atrito no processo. A primeira foi à aplicação de um lubrificante em forma de pasta, de nome comercial Clarus Desmoldax DCP 35 e a segunda foi à aplicação em forma líquida, denominado Clarus Desmoldax DCP 35X. Para analisar o retorno elástico, foram utilizadas como critério de avaliação as medições de abertura da parede da tira conformada, permitindo desta forma considerar o atrito por intermédio da variação do tipo lubrificante. As análises realizadas nos corpos de prova conformados em formato U evidenciaram um retorno elástico menor para as amostras do aço DP 600, se comparadas com as amostras do aço DP 800. Não existiu relevância na variação dos lubrificantes quanto ao retorno elástico nas amostras de um mesmo tipo de aço. As análises realizadas nos corpos de prova submetidos ao ensaio de dobramento sob tensão, também evidenciaram um retorno elástico menor para as amostras do aço DP 600 se comparadas com as amostras do aço DP 800. Comparando as amostras com base nos dados coletados e aplicados ao cálculo de coeficiente de atrito, percebeu-se que as amostras que tiveram um retorno elástico menor, também possuíam coeficientes de atrito menor, constatando assim que quanto menor o coeficiente de atrito, menor é o retorno elástico. / This piece of work seeks to evaluate the springback effect on advanced high strength steel used in the bending process. For this, biphasic steel plates DP 600 and DP 800, both coated, were used. Two methods of measurement were used to approach and asses the springback effect. The first method approached was the conformation of strips in U and the second method was the preformance of the Bending Under Tension test, which enabled the data collect for the application in calculation of the friction coefficient in the bending process. In both assessments two different lubricant conditions for the bending process were used with the aim of varying the condition of friction in the process. The first one was the application of a paste lubricant, with the brand name Clarus Desmoldax DCP 35 and the second one was the application of a liquid lubricant, called Clarus Desmoldax DCP 35X. To analyze the springback effect the measurements of the opening of the wall of the conformed strip were used, thus allowing to consider the friction through the variation of the type of lubricant. The assessments carried out in the specimens in U shape showed a smaller springback effect for the steel samples DP 600 in comparison with the DP 800 ones. There was no relevance in the range of lubricants as the springback effect in the samples of the same type of steel. The analyzes carried out in the specimens subjected to bending under pressure test also showed a smaller springback effect for the steel samples DP 600 compared with the DP 800 ones. Comparing the samples based on the data collected and applied to the calculation of the friction coefficient, it was noticed that the samples that had a smaller springback effect also had lower friction coefficients, therefore verifying that the lower the friction coefficient is, the lower the springback effect.

Este trabalho procura avaliar o retorno elástico em aços avançados de alta resistência utilizados em processo de dobramento. Para isso, foram usadas chapas de aço bifásico DP 600 e DP 800, ambas revestidas. Foram empregados dois métodos de medição a fim de abordar e avaliar o retorno elástico. O primeiro método abordado foi à conformação de tiras em U e o segundo método foi à realização do ensaio Dobramento Sob Tensão, aos quais possibilitaram a coleta de dados para aplicação em cálculo de coeficiente de atrito no processo de dobramento. Em ambas as avaliações foram utilizadas duas diferentes condições de lubrificação para o processo de dobramento, visando variar a condição de atrito no processo. A primeira foi à aplicação de um lubrificante em forma de pasta, de nome comercial Clarus Desmoldax DCP 35 e a segunda foi à aplicação em forma líquida, denominado Clarus Desmoldax DCP 35X. Para analisar o retorno elástico, foram utilizadas como critério de avaliação as medições de abertura da parede da tira conformada, permitindo desta forma considerar o atrito por intermédio da variação do tipo lubrificante. As análises realizadas nos corpos de prova conformados em formato U evidenciaram um retorno elástico menor para as amostras do aço DP 600, se comparadas com as amostras do aço DP 800. Não existiu relevância na variação dos lubrificantes quanto ao retorno elástico nas amostras de um mesmo tipo de aço. As análises realizadas nos corpos de prova submetidos ao ensaio de dobramento sob tensão, também evidenciaram um retorno elástico menor para as amostras do aço DP 600 se comparadas com as amostras do aço DP 800. Comparando as amostras com base nos dados coletados e aplicados ao cálculo de coeficiente de atrito, percebeu-se que as amostras que tiveram um retorno elástico menor, também possuíam coeficientes de atrito menor, constatando assim que quanto menor o coeficiente de atrito, menor é o retorno elástico. / This piece of work seeks to evaluate the springback effect on advanced high strength steel used in the bending process. For this, biphasic steel plates DP 600 and DP 800, both coated, were used. Two methods of measurement were used to approach and asses the springback effect. The first method approached was the conformation of strips in U and the second method was the preformance of the Bending Under Tension test, which enabled the data collect for the application in calculation of the friction coefficient in the bending process. In both assessments two different lubricant conditions for the bending process were used with the aim of varying the condition of friction in the process. The first one was the application of a paste lubricant, with the brand name Clarus Desmoldax DCP 35 and the second one was the application of a liquid lubricant, called Clarus Desmoldax DCP 35X. To analyze the springback effect the measurements of the opening of the wall of the conformed strip were used, thus allowing to consider the friction through the variation of the type of lubricant. The assessments carried out in the specimens in U shape showed a smaller springback effect for the steel samples DP 600 in comparison with the DP 800 ones. There was no relevance in the range of lubricants as the springback effect in the samples of the same type of steel. The analyzes carried out in the specimens subjected to bending under pressure test also showed a smaller springback effect for the steel samples DP 600 compared with the DP 800 ones. Comparing the samples based on the data collected and applied to the calculation of the friction coefficient, it was noticed that the samples that had a smaller springback effect also had lower friction coefficients, therefore verifying that the lower the friction coefficient is, the lower the springback effect.

Made available in DSpace on 2016-03-15T19:36:41Z (GMT). No. of bitstreams: 1 Emerson Bertaglia de Paula.pdf: 2673349 bytes, checksum: 7ced38cf4db0e9c40ec1a57cbdf6bf07 (MD5) Previous issue date: 2014-02-13 / Universidade Presbiteriana Mackenzie / McPherson shock absorber is a metallic structure that is used to link body to wheels of vehicles, most part of shock absorbers are produced using carbon steel low alloy of medium strength, because in this way it has a lowest cost. Due to the continuous change of market requirement, it is necessary that it be created an alternative to performance improvement and reduce fuel consumption and one possibility, it is reduce the weight of shock absorbers. There are many type of materials, metallic and no metallic that can reduce the weigh, however, dual phase steel shows an interesting alternative to substitute carbon steels, because dual phase steel does not change or demand new investments to current production process. Dual-phase steel has high mechanical properties that allow significant weight reductions. In this study there is a comparison between carbon steel and dual-phase steels, performed with current loads case used for approve McPherson shock absorbers and components. Components of shock absorber was analyzed by analytical and finite element methods. Result of this study showed that it is possible use dual-phase steel with a weight reduction of 10% to 24% for each component studied and a reduction of 7,23% for strut shock absorber. / Amortecedores McPherson são estruturas metálicas que servem de ligação entre a carroceria e as rodas do veículo e a grande maioria dos amortecedores produzidos mundialmente utilizam aço carbono de baixa liga e média resistência, pois desta forma apresentam um menor custo. Com as contínuas mudanças das exigências do mercado consumidor, é necessário que seja apresentado uma alternativa para a redução de massa dos amortecedores, pois somente desta forma tem-se um melhor desempenho com uma redução no consumo de combustíveis. Há diversos tipos de materiais metálicos e não metálicos que podem proporcionar esta redução de massa, porém o aço dual-phase apresenta uma interessante alternativa para substituição dos aços carbono, pois não gera modificações e investimentos significativos no atual processo produtivo. O aço dual-phase possui elevadas propriedades mecânicas possibilitando reduções significativas de massa. Neste estudo foi realizado um comparativo entre os aços carbono e o aço dual-phase em condições de carregamento normalmente utilizada para aprovação dos amortecedores McPherson. Os componentes estudados, foram analisados pelos métodos analítico e de elementos finitos, o resultado deste estudo mostra que é possível utilizar o aço dual-phase com uma redução de massa de 10% a 24% para cada componente estudado e uma redução no amortecedor estrutural de 7,23%.

L’acier à double phase (DP) a été développé par l'industrie automobile pour le but de réduire le poids, l'amélioration de la performance de la sécurité et l'efficacité énergétique. Habituellement, l'acier DP contient des îlots de martensite dure noyée dans une matrice de ferrite doux. La synergie entre ces deux phases avec la microstructure inhomogène présente d'excellentes propriétés mécaniques. Les propriétés mécaniques (comportements de plasticité et de dégâts) d'acier DP sont principalement dérivés de sa microstructure, par exemple, la fraction de volume, la taille, la distribution et la morphologie de chaque phase constituante. Les approches micromécaniques sont largement appliquées pour prédire la plasticité et d'autres propriétés mécaniques de l'acier DP selon divers scénarios de chargement. Dans ce travail, la modélisation micromécanique de l'acier DP a été réalisée en utilisant des microstructures réelles ou artificielles. Une véritable microstructure est obtenue à partir de l'image métallographique, tandis qu'un générateur de microstructure artificielle à l'aide d'un algorithme d'affectation de phase améliorée basée sur l'optimisation de la topologie matériau est proposé d'étudier les propriétés mécaniques. Dans ce générateur artificiel, un processus d'affectation de phase est réalisé sur une mosaïque de Voronoï modifié pour obtenir une mesure représentative de l'élément de volume (VER) avec une bonne convergence. La méthode proposée comprend également une réduction appropriée décomposition orthogonale (POD) des courbes de débit (instantanés), qui sont calculés en utilisant le schéma asymptotique homogénéisation d'extension (AEH), pour identifier le contrôle des paramètres optimaux pour l'acier DP. Cette méthode numérique est vérifiée en utilisant DP590 et DP980 aciers qui indiquent un bon accord avec la contrainte d'écoulement à partir de mesures et prédiction de RVE basés sur de vraies microstructures. Les prédictions des modèles de déformation plastique, y compris des bandes de cisaillement en utilisant la microstructure artificielle ressemblent étroitement le comportement mécanique réel dans des conditions de chargement similaires. En outre, une interpolation a été adoptée pour obtenir une corrélation entre ces paramètres de contrôles basés sur l'identification des différents aciers DP. En outre, un modèle de substitution bi-niveau réduit est élaboré et présenté pour identifier les paramètres matériels du critère de rupture de Mohr-Coulomb (MMC). En utilisant cette méthode, le processus d'identification devient possible avec un nombre limité de tests Expérimentaux. La méthode combine des éléments critiques locaux associés à des modèles globaux. Le modèle de substitution de la souche de fracture construit en utilisant l'approximation diffuse et les éléments locaux, réduit le coût de calcul pour la recherche des paramètres matériels. Des simulations de fracturation sont effectuées globales pour mettre à jour la déformation à la rupture de la cible et pour calculer le déplacement de l'apparition de la panne correspondante. Des résultats probants sont obtenus par application successive de la conception de l'expérience (DOE) et l'amélioration des algorithmes de transformation de l'espace de conception. Le protocole d'identification proposée est validé avec de l'acier DP590. Robustesse de la méthode est confirmée par des valeurs initiales différentes. Ces investigations numériques fournissent nouvelle direction pour les simulations multi-échelles de la plasticité et de dégâts des comportements d'acier DP. De plus, ils contribuent efficacement à combler le fossé entre la recherche scientifique et à l'application de l'ingénierie des matériaux hétérogènes. / Dual-phase (DP) steel has been developed by automotive industry for the purpose of weight reduction, improvement in safety performance and fuel efficiency. Usually, DP steel contains hard martensite islands embedded in a soft ferrite matrix. Synergy between these two phases with the inhomogeneous microstructure exhibits excellent mechanical properties. The mechanical properties (plasticity and damage behaviors) of DP steel are mostly derived from its microstructure, e.g., volume fraction, size, distribution and morphology of each constituent phase. Micromechanical approaches are vastly applied to predict plasticity and other mechanical properties of DP steel under various loading scenarios. In this work, micromechanical modelling of DP steel has been performed using real or artificial microstructures. A real microstructure is obtained from metallographic image, while an artificial microstructure generator with an enhanced phase assignment algorithm based on material topology optimization is proposed to investigate the mechanical properties. In this artificial generator, phase assignment process is performed on a modified Voronoï tessellation to achieve the tailored representative volume element (RVE) with a good convergence. The proposed method also includes a proper orthogonal decomposition (POD) reduction of flow curves (snapshots), which are computed using the asymptotic extension homogenization (AEH) scheme, to identify the optimal controlling parameters for DP steel. This numerical method is verified using DP590 and DP980 steels that indicate a good agreement with the flow stress from measurements and RVE prediction based on real microstructures. Predictions of plastic strain patterns including shear bands using the artificial microstructure closely resemble the actual mechanical behavior under similar loading conditions. Moreover, an interpolation has been adopted to obtain a correlation between these controlling parameters based on the identification for various DP steels. Additionally, a bi-level reduced surrogate model is developed and presented to identify the material parameters of the Mohr-Coulomb (MMC) fracture criterion. Using this method, the identification process becomes feasible with a limited number of experimental tests. The method combines local critical elements associated with global models. The surrogate model of fracture strain constructed using the diffuse approximation and the local elements, reduced the computational cost for searching material parameters. Global fracture simulations are performed to update the target fracture strain and to compute the corresponding failure onset displacement. Convincing results are obtained via successive application of design of experiment (DOE) and enhanced design space transformation algorithms. The proposed identification protocol is validated with DP590 steel. Robustness of the method is confirmed with different initial values. These numerical investigations provide new direction for multiscale simulations of the plasticity and damage behaviors of DP steel. Moreover, they efficiently contribute to bridge the gap between scientific research and engineering application of heterogeneous materials.

With increasing fuel costs and the current drive to reduce greenhouse gas emissions and fuel consumption, a need to reduce vehicle weight is apparent. Weight reduction can be achieved by replacing conventionally stamped structural members with hydroformed parts. The weight reduction can be further enhanced by reducing the thickness of the hydroformed members through the use of advanced high strength steel (AHSS). A primary limitation in hydroforming AHSS, is the limited ductility or formability of these materials. This limitation becomes acute in multi-stage forming operations in which strain path changes become large making it difficult to predict formability. Thus, the focus of the current work is to study the effects of pre-bending on the subsequent hydroformability of Dual-Phase DP600 steel tubes. As part of this effort, the effect of key bending and hydroforming process parameters, bending boost and hydroforming end-feed, have been studied in a parametric fashion. <br /><br /> Multi-step pre-bending and hydroforming experiments were performed on 76. 2 mm (3. 0") OD tubes with a wall-thickness of 1. 85mm (DP600). Experiments were also performed on 1. 74mm Interstitial Free (IF) steel tube, which provided a low strength, high formability baseline material for comparison purposes. A fully instrumented servo-hydraulic mandrel-rotary draw tube bender was used in the pre-bending experiments in which various levels of boost were applied. The results showed that increased boost reduced the major (tensile) strain and thinning at the outside of the bend. At the inside of the bend, the compressive minor strain became larger and thickening increased. <br /><br /> Hydroforming of the straight and pre-bent tubes was conducted using various levels of load-control end-feed (EF). For both straight and pre-bend tube hydroforming, an increase in hydroforming EF resulted in increased burst pressure and corner-fill expansion (CFE). The effect of bending boost on CFE was also measured. For a given hydroforming EF case, a tube bent with greater boost achieved a higher burst pressure and consequently a greater CFE which increased the hydroformability of the material. Pre-bending was shown to consume a considerable amount of the formability of the tube in the hydroforming experiments. For the same EF case, the pre-bent tubes could only achieve a fraction of the straight tube CFE at burst. <br /><br /> The pre-bending and hydroforming experiments were complimented by finite element simulation in the hope of providing additional insight into these processes. The finite element (FE) models were able to accurately predict the strain and thickness changes imposed during pre-bending. The models were able to accurately predict the CFE, EF displacement, and strain and thickness distributions after hydroforming. <br /><br /> The extended stress-based forming limit curve (XSFLC) failure criterion was applied to predict failure (onset of necking) during hydroforming, which was measured as the burst pressure in the experiments. For straight tube hydroforming, the XSFLC predicted the correct failure pressure versus hydroforming EF load trend, but over predicted the failure pressures. In pre-bend hydroforming, the models were able to capture the effect of bending boost and hydroforming EF on the hydroformability of the tubes. The XSFLC was able to capture the drop in formability for bending versus straight tube hydroforming, but was unable to capture the failure pressure versus hydroforming EF load trend or magnitude. Further work is required to make the XSFLC applicable to straight and pre-bend hydroforming.

Ce travail vise à améliorer notre compréhension du comportement en fatigue à grand et très grand nombre de cycles d’un acier ferrito-martensitique dual phase, notamment les effets de la température et de la vitesse de déformation résultant de chargements cycliques à haute fréquence. L'effet de la fréquence sur la réponse en fatigue de l'acier DP600 a été étudié en effectuant des essais de fatigue sur une machine ultrasonique travaillant à 20 kHz et sur une machine conventionnelle travaillant à des fréquences inférieures à 100 Hz. Des études de fractographie et des observations microscopiques à la surface des échantillons ont été effectuées pour étudier les mécanismes de déformation et de rupture. De plus, la thermographie infrarouge in situ a été utilisée pour étudier la réponse thermique et les mécanismes dissipatifs du matériau lors des essais de fatigue. Les courbes S-N ont été déterminées à partir de chargements de fatigue ultrasoniques à 20 kHz et d’essais conventionnels à 30 Hz. La durée de vie pour une amplitude de contrainte donnée est plus élevée dans le cas de la fatigue ultrasonique bien que la limite de fatigue soit identique dans les deux cas. L’augmentation inévitable de la température en fatigue ultrasonique à fortes amplitudes de contraintes, ainsi que le comportement dépendant de la vitesse de déformation de la ferrite, en tant que structure CC, ont été trouvés comme les paramètres clés expliquant le comportement observé en fatigue, et la réponse thermique sous les fréquences faibles et ultrasoniques. Les écarts observés entre l’essai de fatigue conventionnel et celui ultrasonique ont été évalués à travers les mécanismes de mobilité des dislocations vis dans la phase ferritique de structure cubique centrée (CC). La durée de vie plus élevée et l’amorçage de la fissure principale sur une inclusion observés en fatigue ultrasonique ont été attribués au vieillissement dynamique résultant du fort auto-échauffement du matériau aux fortes amplitudes de contraintes. L'existence d'une transition du régime thermiquement activé au régime athermique avec l’augmentation de l'amplitude de contrainte a été mise en évidence. Au-dessous de la limite de fatigue, la déformation a lieu dans un régime thermiquement activé alors qu'elle est dans un régime athermique au-dessus de la limite de fatigue. En fatigue conventionnelle, la déformation est athermique pour toutes les amplitudes de contrainte. Une carte de transition a été produite en utilisant les résultats expérimentaux pour l'acier DP600 ainsi que les données disponibles dans la littérature pour d'autres aciers à base de ferrite, montrant ainsi la corrélation entre le mouvement des dislocations vis thermiquement activé et l'absence de rupture en fatigue à très grand nombre de cycle. / This work is an attempt towards a better understanding of the high cycle and very high cycle fatigue behaviors of a ferritic-martensitic dual-phase steel, with a regard to temperature and strain rate effects, resulting from accelerated fatigue loading frequencies. The influence of frequency on fatigue response of DP600 steel was investigated by conducting ultrasonic and conventional low frequency fatigue tests. Fractography studies and microscopic observations on the surface of specimens were carried out to study the deformation and fracture mechanisms under low and ultrasonic frequencies. Moreover, in situ infrared thermography was carried out to investigate the thermal response and dissipative mechanisms of the material under fatigue tests. The S-N curves were determined from ultrasonic 20-kHz fatigue loadings and conventional tests at 30 Hz. Fatigue life for a given stress amplitude was found to be higher in the case of ultrasonic fatigue whereas the fatigue limit was the same for both cases. Moreover, crack initiation was always inclusion-induced under ultrasonic loading while under conventional tests it occurred at slip bands or defects on the surface. The inevitable temperature increase under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a body centered cubic (BCC) structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. The discrepancies observed between conventional and ultrasonic fatigue tests were assessed through the mechanisms of screw dislocation mobility in the ferrite phase as a BCC structure. The higher fatigue life and inclusion-induced crack initiations in the case of ultrasonic loading were attributed to the dynamic strain aging, which resulted from the high temperature increases at high stress amplitudes. The existence of a transition in deformation regime from thermally-activated to athermal regime under ultrasonic fatigue loading by increasing the stress amplitude was confirmed. Below the fatigue limit, deformation occurred in thermally-activated regime while it was in athermal regime above the fatigue limit. Under conventional loading deformation occurred in athermal regime for all stress amplitudes. From the analysis of the experimental data gathered in this work, guidelines were given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue.

Dans le cadre du développement de nuances d’aciers toujours plus performantes en termes de résistance à l’effort et à l’endommagement, les aciers Dual-Phase (DP) présentent un bon compromis résistance/ductilité. Cependant, il est nécessaire de disposer de meilleures connaissances concernant les mécanismes menant à la rupture de tels aciers. Les mécanismes d’endommagement ont ainsi été étudiés dans cette thèse à l’aide de la tomographie aux rayons X. Des essais de traction in-situ ont été réalisés sur plusieurs nuances d’aciers DP, un acier ferritique et un acier martensitique afin de caractériser chaque étape de l’endommagement ductile. Des observations qualitatives et des données quantitatives concernant la germination de l’endommagement, la croissance des cavités et la coalescence ont été recueillies lors de ces essais. Ces données quantitatives ont ensuite été utilisées pour le développement et/ou la validation de modèles d’endommagement. Une prédiction de la cinétique de germination a ainsi été proposée et la version du modèle de croissance de cavités de Rice et Tracey corrigée par Huang et prenant mieux en compte l’effet de la triaxialité a été validée expérimentalement. L’étape de coalescence des cavités menant à la rupture des matériaux a pour la première fois été caractérisée de façon quantitative dans un matériau industriel et des critères de coalescence ont été appliqués localement sur les couples de cavités présentes dans le matériau. L’utilisation de ces modèles analytiques a permis une meilleure compréhension des propriétés agissant sur les phénomènes mis en jeu. L’effet de la part cinématique de l’écrouissage sur la germination et la croissance de l’endommagement a notamment été souligné et validé par des essais de chargements complexes. / As part of the current context of requiring ever more efficient grades of steels in terms of resistance to stress and to damage, the Dual-Phase steels (DP) present an acceptable strength/ductility compromise. It is nevertheless necessary to have a better understanding of the mechanisms leading to the fracture of such steels. Damage mechanisms were studied in this PhD using X-ray tomography. In-situ tensile tests were carried out on several grades of DP steel, a ferritic steel and a martensitic steel in order to characterize each step of ductile damage. Qualitative observations and quantitative data on the nucleation of damage, the void growth and the coalescence of cavities were collected during these tests. This quantitative data was then used for the development and/or the validation of damage models. A prediction of the kinetic of nucleation was proposed and the Huang’s correction of the void growth model of Rice and Tracey accounting for the triaxiality was experimentally validated. For the first time, the step of void coalescence leading to fracture of materials was quantitatively characterized in an industrial material and coalescence criteria were locally applied on couples of neighboring cavities present in the studied specimen. The use of analytical models enabled a better understanding of the properties influencing the studied damage phenomena. The effect of the kinematic part of the strain hardening on void nucleation and void growth was notably emphasized and validated by performing complex loading tests.