Les composites SiC/SiC à interphase pyrocarbone (PyC) sont des candidats prometteurs en tant que matériau de gainage du combustible et de structure des réacteurs à neutrons rapides, constituant une alternative aux alliages métalliques. Leur comportement sous irradiation neutronique et leur caractère réfractaire sont de sérieux atouts en milieu irradiant. Néanmoins, les fibres et la matrice en carbure de silicium (SiC) sont, individuellement, des céramiques fragiles. L’intégrité des structures ne peut donc être assurée que si le composite acquiert une tolérance aux déformations. Cette tolérance n’est possible que grâce à la présence d’une interphase de pyrocarbone, entre la matrice et les fibres, assurant le rôle de déviateur de fissures. La capacité des composites SiC/SiC à résister à l’endommagement est dictée par le couplage fibre/matrice (F/M). L’intensité de ce couplage peut être influencée par de nombreux paramètres, tels que la rugosité et la physicochimie de surface du renfort. Les travaux faisant l’objet de cette thèse ont mis en évidence une couche de carbone en surface des fibres par microscopie électronique en transmission à haute résolution (METHR) et via des analyses physicochimiques de surface. Les caractéristiques de cette couche de carbone varient avec le procédé de fabrication des fibres. Son impact sur le couplage F/M a été appréhendé par l’observation des mécanismes locaux d’endommagement. La décohésion fibre/matrice a été étudiée en analysant par METHR les régions interfaciales des composites SiC/SiC ayant subi un essai mécanique. La compréhension de l’origine de cette couche de surface de fibres a permis de mieux connaitre les mécanismes locaux d’interaction. Ces mécanismes dépendent de la structure du carbone de surface des fibres dont découle le mode d’adhésion entre ce carbone de surface et l’interphase de pyrocarbone. Un traitement de surface sur un type de fibres a alors été développé, suggérant une légère amélioration du comportement mécanique des composites SiC/SiC élaborés à partir de ces renforts fibreux. / SiC/SiC composites including the third generation SiC fibers with pyrocarbon interphase (PyC) are promising candidates to improve the safety of nuclear reactors, especially for core materials such as cladding and to replace metallic alloys for these applications. Their intrinsic refractory properties, their neutron transparency and their microstructural stability when irradiated or exposed to high temperatures make them attractive for nuclear applications. However SiC fibers and SiC matrix are brittle ceramics. The integrity of the structures can be fulfilled only if the composite is damage tolerant and can acquire a pseudo-ductile mechanical behavior. An interphase is deposited between the fibers and the matrix to provide this damage tolerance of SiC/SiC composites.The ability of SiC/SiC composites to sustain damage is dictated by the fiber/matrix (F/M) coupling mode. The intensity of this coupling can be related to many parameters such as the roughness and the chemistry of the surface of the reinforcement. A carbon layer on the fiberssurface was highlighted by High Resolution Transmission Electronic Microscopy (HRTEM) and by physico-chemical analyses. The characteristics of this carbon layer vary with the fabrication process of the fibers. The impact of this carbon layer on the F/M coupling was investigated by the observation of the local damage mechanisms. To elucidate the local bonding modes governing the damage mechanisms at the F/M interface of these SiC/SiC composites, macroscopic mechanical tests have been coupled with observations of structural modifications occurring in the interface region after loading. Understanding the origin of this carbon layer allowed elucidating the local interaction mechanisms according to these studied materials. These mechanisms depend on the carbon structure of the SiC fibers surface which in turn governs the adhesion between this carbon and the PyC interphase. Thanks to this study, a surface treatment on fibers was developed to optimize the mechanical behavior of SiC/SiC composites, whatever the fibrous reinforcement chosen.

O presente trabalho visa avaliar a microestrutura em chapas de um aço de alta resistência e baixa liga após soldagem por centelhamento e quantificar a resistência ao crescimento de trincas por fadiga ao longo do cordão de solda e da zona termicamente afetada, comparando-a ao comportamento do material base. O aço em estudo, recentemente desenvolvido pela Companhia Siderúrgica Nacional sob a designação RD480, foi fornecido na forma de chapas com 5,0 mm de espessura e soldado pelo processo de centelhamento na divisão de rodas e chassis da IOCHPE-MAXION. A avaliação microestrutural do aço após o processo de soldagem por centelhamento foi realizada nas regiões do metal de base, zona termicamente afetada e metal de solda, utilizando microscopia ótica com diferentes ataques químicos e microscopia eletrônica de varredura. A caracterização mecânica foi realiza com ensaio de tração, teste de dureza Vickers e ensaio de propagação de trincas por fadiga. Para este último foram adotados corpos-de-prova do tipo compacto C(T) e carregamento senoidal de amplitude constante com frequencia 10 Hz e razão de tensão R= 0,1 e as curvas obtidas deste ensaio foram avaliadas em relação a dois modelos matemáticos, o de Paris-Erdogan e o exponencial. Após o ensaio de propagação de trincas por fadiga, as superfícies dos corpos-de-prova foram avaliadas por microscopia eletrônica de varredura. Os resultados encontrados permitiram correlacionar a resistência mecânica e a taxa de crescimento da trinca com as características microestruturais resultantes do processo de soldagem. / This study aims to evaluate the microstructure of a high strength, low alloy (HSLA) steel after flash welding and to quantify the resistance to fatigue crack growth along the weld and heat affected zone, comparing it to the behavior of the base material. The steel under study, recently developed by the Companhia Siderúrgica Nacional under the designation RD480, was provided in the form of plates with 5.0 mm in thickness and welded by IOCHPE-MAXION, division of wheels and chassis. The microstructural evaluation of the steel after the flash welding process was performed at the base metal, heat affected zone and weld metal, using optical microscopy with different chemical attacks and scanning electron microscopy (SEM). The mechanical characterization was performed by means of tensile test, Vickers hardness measurement and fatigue crack growth (FCG) test. Compact Tension C(T) specimens were chosen for the fatigue tests, and the loading was sinusoidal with constant amplitude, frequency of 10 Hz and stress ratio R = 0.1. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The fracture surfaces of the fatigued specimens were examined via SEM in the secondary electrons regime. The results allowed correlating the mechanical strength and crack growth rate with the microestrutural characteristics resulting from the welding process.

Aços microligados pertencem à classe dos aços ARBL contendo baixa ou média quantidade de carbono e pequena adição de elementos de liga tais como Mn, Nb, Mo, V e Ti. A variedade microestrutural desses aços pode ser obtida dependendo da temperatura de conformação, taxa de resfriamento e composição química. Os tratamentos intercríticos e isotérmicos produzem microestruturas multifásicas com diferentes quantidades de ferrita, martensita, bainita e austenita retida. A presença de diferentes fases nestes materiais, com morfologias distintas, pode afetar de modo significativo seu comportamento mecânico, afetando, por exemplo, o fechamento da trinca e resultando em mudanças na taxa de crescimento da mesma. O objetivo deste trabalho é avaliar as propriedades de tração e a resistência ao crescimento da trinca por fadiga de um aço microligado RD 480 com 0.08%C-1, 5%Mn (p), correlacionando-as com suas características microestruturais. Esse aço, desenvolvido recentemente pela CSN (Companhia Siderúrgica Nacional), é considerado promissor como alternativa para substituir o aço de baixo carbono utilizado em componentes de rodas na indústria automotiva. Distintas condições microestruturais foram obtidas por meio de tratamentos térmicos seguidos de resfriamento em água. As condições de tratamento intercrítico e têmpera simples foram escolhidas para se avaliar a resistência à propagação da trinca por fadiga. Os resultados dos ensaios foram sintetizados em termos da taxa de crescimento da trinca (da/dN) versus a variação do Fator Intensidade de Tensão (_K) no ciclo de carregamento. Para descrever o comportamento das trincas foram utilizados dois modelos: a equação convencional de Paris e um novo modelo exponencial que mostra o comportamento não linear das curvas de fadiga. Os resultados mostraram que uma microestrutura combinando ferrita de aspecto acicular e fases duras (martensita/bainita) resultou em menores taxas de crescimento da trinca. No entanto, a melhor combinação entre as propriedades de tração (limite de escoamento, resistência e ductilidade) e fadiga foi obtida com uma microestrura bifásica contendo martensita dispersa em uma matriz ferrítica. Observou-se uma transição nas curvas de crescimento da trinca para todas as condições tratadas termicamente e, por conta disto, as curvas das condições microestruturais bifásicas e multifásicas foram melhores modeladas quando divididas em duas regiões. As superfícies de fratura dessas amostras, bem como o caminho percorrido pela trinca, foram analisados via MEV e MO. / Microalloyed steels are a class of HSLA steels with low or medium carbon content and small additions of alloy elements such as Mn, Nb, Mo, V and Ti. A variety of microstructures in microalloyed steels can be obtained depending on the deformation temperature, cooling rate and chemical composition. Heat treatments and isothermal transformation on these materials, with various temperatures and holding times, produce multiphase microstructures with different amounts of ferrite, martensite, bainite and retained austenite. These different phases, with distinct morphologies, are determinant of the mechanical behavior of the steel and can, for instance, affect crack closure or promote crack shielding, thus resulting in changes on its propagation rate under cyclic loading. The aim of this study is to evaluate the tensile properties and resistance to fatigue crack growth in a microalloyed steel RD 480 with 0.08%C-1, 5% Mn (wt), correlating with their microstructural characteristics. This steel, recently developed by CSN (Companhia Siderurgica Nacional), is being considered as a promising alternative to replace low carbon steel in wheel components for the automotive industry. Distinct microstructural conditions were obtained by means of heat treatments followed by water quench. The intercritical treatment and quenching conditions were chosen to evaluate the strength to crack propagation. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The results showed that a microstructure combining aspect acicular ferrite and hard phases (martensite / bainite) resulted in lower rates of crack growth. However, the best combination between the tensile properties (yield stress, tensile strength and ductility) and fatigue was obtained with a dual phase steel microstructure containing martensite dispersed in a ferrite matrix. It was observed a transition in the crack growth curves for all heat treated conditions, so the curves of the dual and multiphase microstructural conditions were better modeled by dividing them in two regions. The fracture planes of the fatigued specimens, as well as the crack path, were examined using a scanning electron microscope (SEM) and optical micrography (OM).

O presente trabalho visa avaliar a microestrutura em chapas de um aço de alta resistência e baixa liga após soldagem por centelhamento e quantificar a resistência ao crescimento de trincas por fadiga ao longo do cordão de solda e da zona termicamente afetada, comparando-a ao comportamento do material base. O aço em estudo, recentemente desenvolvido pela Companhia Siderúrgica Nacional sob a designação RD480, foi fornecido na forma de chapas com 5,0 mm de espessura e soldado pelo processo de centelhamento na divisão de rodas e chassis da IOCHPE-MAXION. A avaliação microestrutural do aço após o processo de soldagem por centelhamento foi realizada nas regiões do metal de base, zona termicamente afetada e metal de solda, utilizando microscopia ótica com diferentes ataques químicos e microscopia eletrônica de varredura. A caracterização mecânica foi realiza com ensaio de tração, teste de dureza Vickers e ensaio de propagação de trincas por fadiga. Para este último foram adotados corpos-de-prova do tipo compacto C(T) e carregamento senoidal de amplitude constante com frequencia 10 Hz e razão de tensão R= 0,1 e as curvas obtidas deste ensaio foram avaliadas em relação a dois modelos matemáticos, o de Paris-Erdogan e o exponencial. Após o ensaio de propagação de trincas por fadiga, as superfícies dos corpos-de-prova foram avaliadas por microscopia eletrônica de varredura. Os resultados encontrados permitiram correlacionar a resistência mecânica e a taxa de crescimento da trinca com as características microestruturais resultantes do processo de soldagem. / This study aims to evaluate the microstructure of a high strength, low alloy (HSLA) steel after flash welding and to quantify the resistance to fatigue crack growth along the weld and heat affected zone, comparing it to the behavior of the base material. The steel under study, recently developed by the Companhia Siderúrgica Nacional under the designation RD480, was provided in the form of plates with 5.0 mm in thickness and welded by IOCHPE-MAXION, division of wheels and chassis. The microstructural evaluation of the steel after the flash welding process was performed at the base metal, heat affected zone and weld metal, using optical microscopy with different chemical attacks and scanning electron microscopy (SEM). The mechanical characterization was performed by means of tensile test, Vickers hardness measurement and fatigue crack growth (FCG) test. Compact Tension C(T) specimens were chosen for the fatigue tests, and the loading was sinusoidal with constant amplitude, frequency of 10 Hz and stress ratio R = 0.1. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The fracture surfaces of the fatigued specimens were examined via SEM in the secondary electrons regime. The results allowed correlating the mechanical strength and crack growth rate with the microestrutural characteristics resulting from the welding process.

Les aciers inoxydables ferritiques sont actuellement les meilleurs candidats pour répondre au cahier des charges des matériaux destinés aux interconnexions de piles à combustibles à oxyde solide (SOFC acronyme anglo-saxon pour Solid Oxide Fuel Cell). Cependant, du coté du compartiment cathodique de ces piles, le phénomène de sublimation du chrome à haute température conduit à des dégradations importantes, réduisant ainsi la durée de vie en service des SOFC. Les objectifs de ce travail de thèse sont (i) d'étudier le phénomène de sublimation du chrome sur l'acier inoxydable ferritique AISI 441 et (ii) de rechercher le ou les moyen(s) d'atténuer cette même sublimation. Sur le deuxième point, des méthodes de protection ont été mises en œuvre, en particulier avec le dépôt de films minces de spinelle Mn-Co obtenues via un procédé de galvanoplastie ou bien encore en procédant à des pré-oxydations des alliages dans des atmosphères contrôlées de type argon impur ou mélanges gazeux équimolaires CO / CO2 (250 ° C et 850 ° C, pour une durée totale de 3 h).Concernant l'étude de la sublimation du chrome, des essais d'oxydation simulant les conditions de service ont été conduites dans des mélanges gazeux : 5% de O2 dans H2O à 800 °C pendant 96 h. A vitesse de gaz oxydants faible, de 1 à 3 cm / s, la sublimation du chrome est limitée par un phénomène de diffusion dans la phase gazeuse de l'espèce volatile oxo-hydroxyde de chrome. A vitesse plus élevée, de 3 à 10 cm / s, la sublimation du chrome semble limitée par un phénomène interfacial. La vitesse de sublimation apparait indépendante de la rugosité de surface des échantillons, elle même variant selon la préparation des échantillons. Du point de vue de la caractérisation morphologique des couches d'oxydation thermiques obtenues, sur les surfaces de plus forte rugosité (surface industrielle), nous avons noté le développement de nodules riches en titane et en niobium avec une partie externe et une autre interne. Dans le substrat, des phases de lave de type Fe2Nb ont été observées le long des joints de grain métalliques.Dans une deuxième partie, la sublimation du chrome a été étudiée sur échantillons revêtus ou pré-oxydés. Les films minces de spinelle Mn-Co ne conduisent pas à l'abaissement de la vitesse de sublimation du chrome et sont donc pas protecteurs. L'adhérence de ces revêtements est de mauvaise qualité. Les couches présentent de nombreuses fissures. La pré-oxydation conduit quant à elle à une réduction considérable (jusqu'à un facteur 10) de la vitesse de sublimation du chrome (sauf dans le cas de la pré-oxydation dans l'argon à 850 °C). La formation d'un film riche en fer obtenu à basse température (250 °C) peut expliquer cette réduction par l'établissement d'une barrière de diffusion. Cette même réduction est cependant surprenante sur les films riches en chrome obtenus par pré-oxydation à 850 °C dans CO / CO2. Nous proposons dans ce travail une interprétation originale basée sur les différences de nature semi-conductrice des films de chromine formées à haute pression d'oxygène (proche de l'atmosphère) et identifiés comme étant de type p par rapport à ceux obtenus à basse pression (dans CO/CO2) qui sont connu pour être de type n. Après l'oxydation à haute température de 96 heures, tous les échantillons préalablement pré-oxydés ont été observés comme étant composés d'oxyde de chrome et d'un spinelle Mn-Cr.Au terme de ce travail de thèse, il peut être conclu que la pré-oxydation de l'alliage 441 à basse température (250 °C) dans l'argon ou le mélange CO/CO2 ou encore à plus haute température (850 °C) dans le mélange CO/CO2 sont des traitements qui conduisent à l'abaissement de la vitesse de sublimation du chrome et par voie de conséquence à l'amélioration de la tenue de cet alliage en condition d'oxydation dans le compartiment cathodique des SOFC. / Ferritic stainless steels are the most attractive materials that are able to fulfil SOFC(Solid Oxide Fuel Cell) interconnect properties. However, in cathodic SOFC condition, Crvolatilisation from ferritic steels leads to degradations of interconnect and cathode materialsand shorter service lifetime of SOFC. The objectives of this work are (i) to study Crvolatilisation from AISI 441 ferritic stainless steel and (ii) to find the way out to suppress Crvolatilisation. The Cr volatilisation protective methods used in this work were coating withMn-Co spinel by an electroplating method and preoxidation in the condition of Ar or CO/CO2at either 250°C or 850°C, for 3 h. Cr volatilisation experiments were performed in 5%H2O inO2 at 800°C for 96 h. According to the results, it can be concluded that the preoxidation ofthe AISI 441 in Ar or CO/CO2 at low temperature (250°C) or in the CO/CO2 at a highertemperature (850°C) are treatments that lead to lowering the sublimation rate of the Cr andimprovement in the oxidation resistance of this alloy in the cathodic compartment of theSOFC.

Para a certificação de um procedimento de soldagem, quer seja de alta responsabilidade na soldagem de oleodutos e gasodutos em tubulações de aço carbono ou para um uso rotineiro e diário em fábricas e oficinas, deve ser realizada uma análise profunda dos fenômenos ocorridos no material de base (MB), no metal de adição (MA) e na zona afetada pelo calor (ZAC). Por meio desta análise criteriosa na qualificação de um procedimento de soldagem, o procedimento é liberado para ser utilizado, seja qual for o objetivo final da soldagem. Apenas com a Especificação do Procedimento de Soldagem (EPS) qualificada em mãos é possível iniciar os trabalhos de soldagem, e caso esta seja seguida integralmente, o sucesso do processo está a um passo de se concretizar. Para demonstração desta interação existente entre qualificação, análise dos resultados, aprovação dos ensaios e aprovação da especificação, foi desenvolvido este trabalho utilizando o material de base na forma tubular API 5L Gr. B soldado com os processos eletrodo revestido (SMAW) e arame tubular autoprotegido (FCAW-S). Foi utilizada uma junta tubular como exemplo de estudo e qualificação de um procedimento de soldagem baseado na documentação contida na norma da Petrobrás N-2301 e em outros códigos e normas, que regem a qualificação do processo de soldagem utilizado em refinarias e plantas de produção brasileiras. Foram estudados em conjunto os resultados obtidos por meio de ensaios mecânicos, ensaios não destrutivos e microestruturais. Ambas as Especificações dos Procedimentos de Soldagem preliminares (pEPS), estabelecidas antes do início da soldagem dos dois processos de soldagem, foram aprovadas e qualificadas, satisfazendo as características macro e microscópicas, metalúrgicas e mecânicas da junta soldada. / A comprehensive analysis of the phenomena occurring in the base metal (MB), filler metal (FM) and the heat affected zone (HAZ) should be conducted to provide the qualification of the welding procedure, either in high-demand carbon steel oil & gas pipelines or standard factory & workshop daily, usual operation. By means of this thorough welding procedure qualification, the process will be released, whatever the ultimate goal of welding. Only after the welding procedure specification (WPS) is qualified, is possible to start any welding work, and being the norm fully observed, the process is nearly to start successfully. The present work has been elaborated to demonstrate the interaction of qualification, result analysis, tests and specification approval, by applying the following elements: tubular base metal API 5L grade B welded with Shielded Metal Arc Welding process (SMAW) and self-shielded Flux Cored Arc Welding process (FCAW-S). A tubular joint was used as sample for a welding procedure qualification studies based on technical documentation specified on Petrobras N-2301, a Brazilian standard, and any others codes and standards that rule welding qualification on refineries & production plants of this company. The results obtained through mechanical testing, non-destructive testing and microstructure were studied together. Both preliminary welding procedure specifications (pWPS) established before the welding of these two welding processes, have been approved and qualified. The support given by welding qualification record (WQR) allows the joints to be welded in production, and result in reliable weld bead that satisfy the macro & microscopic, metallurgical and mechanical characteristics.

Os aços de alta resistência e baixa liga (ARBL) utilizados em tubos para linhas de transmissão, ou seja, transferência e transporte de petróleo e gás, são especificados pela American Petroleum Institute (API) através da norma API 5L. Neste trabalho foi realizado um estudo da influência da têmpera e da nitretação no comportamento mecânico do aço API 5L X56. A microestrutura, os compostos formados com a nitretação e os mecanismos de fratura foram analisadas usando microscopia óptica, difração de raios X e microscopia eletrônica de varredura respectivamente. As amostras foram caracterizadas mecanicamente por testes de microdureza, tração, impacto e fadiga. Os resultados revelaram que apenas a têmpera promoveu modificações severas na microestrutura em relação à como recebida, formando bainita, martensita e constituinte M-A. A camada nitretada apresentou profundidade de 25 ?m e formação dos nitretos Fe3N e Fe4N. Houve aumento da microdureza, resistência mecânica com os tratamentos, juntamente com a diminuição da tenacidade e também houve um melhoramento na vida em fadiga para as duas amostras tratadas termicamente e superficialmente. Os mecanismos de fratura variaram de dúctil à frágil e em fadiga foi observado que as trincas nuclearam a partir da superfície para todas as amostras. / The high strength and low alloy steel (HSLA) used in pipeline for line transmission, there is transference and transportation of oil and gas, they are specified by American Petroleum Institute (API) through norm API 5L. In this work it was conducted a study of the nitriding and quenching influence on the mechanical behavior of API 5L X56. The microstructure, compounds made with the nitriding and the mechanisms failure were analyzed using optical microscopy, X-ray diffraction and scanning electron microscopy respectively. The samples were mechanical characterized by microhardness, tensile, impact and fatigue test. The results revealed that only quenching treatment promoted severe modifications in microstructure in relation to as received, making up bainite, martensite and M-A constituent. The nitriding layer showed depth of 25 ?m and formation of Fe3N e Fe4N nitrides. There was an increasing of microhardness and mechanical strength with the treatments, along with the decreasing of toughness and there was also an improvement in fatigue life for the two samples heat e surface treated. The failure mechanisms varied from ductile to brittle and in fatigue was observed that the cracks nuclear from the surface to all samples.

Aços microligados pertencem à classe dos aços ARBL contendo baixa ou média quantidade de carbono e pequena adição de elementos de liga tais como Mn, Nb, Mo, V e Ti. A variedade microestrutural desses aços pode ser obtida dependendo da temperatura de conformação, taxa de resfriamento e composição química. Os tratamentos intercríticos e isotérmicos produzem microestruturas multifásicas com diferentes quantidades de ferrita, martensita, bainita e austenita retida. A presença de diferentes fases nestes materiais, com morfologias distintas, pode afetar de modo significativo seu comportamento mecânico, afetando, por exemplo, o fechamento da trinca e resultando em mudanças na taxa de crescimento da mesma. O objetivo deste trabalho é avaliar as propriedades de tração e a resistência ao crescimento da trinca por fadiga de um aço microligado RD 480 com 0.08%C-1, 5%Mn (p), correlacionando-as com suas características microestruturais. Esse aço, desenvolvido recentemente pela CSN (Companhia Siderúrgica Nacional), é considerado promissor como alternativa para substituir o aço de baixo carbono utilizado em componentes de rodas na indústria automotiva. Distintas condições microestruturais foram obtidas por meio de tratamentos térmicos seguidos de resfriamento em água. As condições de tratamento intercrítico e têmpera simples foram escolhidas para se avaliar a resistência à propagação da trinca por fadiga. Os resultados dos ensaios foram sintetizados em termos da taxa de crescimento da trinca (da/dN) versus a variação do Fator Intensidade de Tensão (_K) no ciclo de carregamento. Para descrever o comportamento das trincas foram utilizados dois modelos: a equação convencional de Paris e um novo modelo exponencial que mostra o comportamento não linear das curvas de fadiga. Os resultados mostraram que uma microestrutura combinando ferrita de aspecto acicular e fases duras (martensita/bainita) resultou em menores taxas de crescimento da trinca. No entanto, a melhor combinação entre as propriedades de tração (limite de escoamento, resistência e ductilidade) e fadiga foi obtida com uma microestrura bifásica contendo martensita dispersa em uma matriz ferrítica. Observou-se uma transição nas curvas de crescimento da trinca para todas as condições tratadas termicamente e, por conta disto, as curvas das condições microestruturais bifásicas e multifásicas foram melhores modeladas quando divididas em duas regiões. As superfícies de fratura dessas amostras, bem como o caminho percorrido pela trinca, foram analisados via MEV e MO. / Microalloyed steels are a class of HSLA steels with low or medium carbon content and small additions of alloy elements such as Mn, Nb, Mo, V and Ti. A variety of microstructures in microalloyed steels can be obtained depending on the deformation temperature, cooling rate and chemical composition. Heat treatments and isothermal transformation on these materials, with various temperatures and holding times, produce multiphase microstructures with different amounts of ferrite, martensite, bainite and retained austenite. These different phases, with distinct morphologies, are determinant of the mechanical behavior of the steel and can, for instance, affect crack closure or promote crack shielding, thus resulting in changes on its propagation rate under cyclic loading. The aim of this study is to evaluate the tensile properties and resistance to fatigue crack growth in a microalloyed steel RD 480 with 0.08%C-1, 5% Mn (wt), correlating with their microstructural characteristics. This steel, recently developed by CSN (Companhia Siderurgica Nacional), is being considered as a promising alternative to replace low carbon steel in wheel components for the automotive industry. Distinct microstructural conditions were obtained by means of heat treatments followed by water quench. The intercritical treatment and quenching conditions were chosen to evaluate the strength to crack propagation. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The results showed that a microstructure combining aspect acicular ferrite and hard phases (martensite / bainite) resulted in lower rates of crack growth. However, the best combination between the tensile properties (yield stress, tensile strength and ductility) and fatigue was obtained with a dual phase steel microstructure containing martensite dispersed in a ferrite matrix. It was observed a transition in the crack growth curves for all heat treated conditions, so the curves of the dual and multiphase microstructural conditions were better modeled by dividing them in two regions. The fracture planes of the fatigued specimens, as well as the crack path, were examined using a scanning electron microscope (SEM) and optical micrography (OM).

O uso das ligas de alumínio em aplicações estruturais tem crescido consideravelmente nas últimas décadas. Nos transportes, a baixa massa específica do alumínio resulta em uma alta razão resistência/ peso, favorecendo a fabricação de aviões, trens e automóveis. Com a crescente preocupação em reduzir a emissão de gases poluentes, vem tornando-se promissora a alternativa de reduzir o peso dos veículos substituindo peças convencionalmente produzidas com outros materiais por peças de alumínio. As ligas tratáveis termicamente da série 6xxx são frequentemente escolhidas para estas aplicações. Assim, para que o emprego dessas ligas seja otimizado, é importante um estudo mais detalhado de suas propriedades mecânicas, principalmente sob solicitações cíclicas. Neste trabalho foram estudadas ligas de Al-Si-Mg que são amplamente utilizadas nas indústrias automotivas, em especial na fabricação de componentes de carroçarias para caminhões e ônibus. Foi realizado o estudo do comportamento em fadiga de baixo ciclo e fadiga multiaxial das ligas AA6005 T6, AA6063 T6 e AA6351 T6, fornecidas pela CBA (Companhia Brasileira de Alumínio) visando caracterizar e comparar essas ligas em sua microestrutura, propriedades de tração e fadiga. As propriedades básicas de fadiga foram determinadas por meio do método ε-N (fadiga de baixo ciclo) e os ensaios foram realizados com controle de deformação total, onda triangular e taxa de deformação 0,005 seg-1. As análises dos laços de histerese elasto-plástica permitiram inferir sobre aspectos microestruturais relacionados às propriedades mecânicas das ligas estudadas. O comportamento em fadiga multiaxial foi avaliado por meio de carregamentos combinados axial-torcional em fase e fora de fase. Para ajustar os dados experimentais obtidos, foram testados alguns dos modelos encontrados na literatura. Os cálculos baseados no modelo de plano crítico, proposto por Fatemi e Socie, apresentaram resultados satisfatórios. Também foram realizadas análises microestruturais e fractográficas para as três ligas. As superfícies de fratura dos ensaios de fadiga multiaxial mostraram resultados diferentes de acordo com o carregamento adotado. A avaliação comparativa das três ligas estudadas fornece subsídios para fundamentar a seleção de materiais para a fabricação de componentes estruturais para o setor automotivo. / The use of aluminum alloys in structural applications has grown considerably in recent decades. In transportation, the low density of aluminum results in a high strength-to weight ratio, proving attractive for production of aircrafts, trains and automobiles. With a growing concern for the reduction of pollutant gas emissions, aluminum alloys are becoming a promising alternative to diminish vehicle weight through the replacement of conventionally produced parts made from other heavier materials for aluminum parts. The heat treatable alloys from the 6xxx series are often chosen for these applications. Therefore, to optimize the employment of these alloys, a detailed study of their mechanical properties, primary under cyclic solicitations is necessary For the present study Al-Mg-Si alloys were chosen, which are widely used in automotive industries, particularly in the manufacturing of components for trucks and bus bodies. The low-cycle fatigue behavior and multiaxial fatigue of the three following aluminum alloys: AA6005 T6, AA6063 T6 and AA6351 T6, provided by CBA (Brazilian Aluminum Company), were assessed, with the aim of characterizing and comparing these alloys in their microstructure, tensile properties and fatigue. The basic properties of fatigue were studied by ε-N method (low cycle fatigue) and the experiments were performed with total strain control, triangular waveform and with a constant deformation rate of 5.0x10-3 s-1. The analyses of hysteresis loops elasto-plastic provided insight about microstructural aspects, related to mechanical properties of the studied alloys. Multiaxial fatigue behavior was assessed in combined axial-torsion loading in phase and out of phase. To adjust the experimental data, some models found in the literature were tested. Calculations based on critical plane model, proposed by Fatemi Socie, presented satisfactory results. Furthermore, microstructure analyses and fractography were performed for these three alloys. The fracture surface of multiaxial fatigue assays demonstrated different results according to the adopted loading. Comparative evaluation of the three studied alloys provides support for the selection of materials for manufacturing structural components of the automotive sector.

<div>Several truck fires have occurred in recent years involving bridges with reinforced and prestressed concrete components. If the fire burns for a significant period of time (15 minutes or more), bridge inspectors and engineers must determine if the exposure to elevated temperature has reduced the strength and serviceability of the concrete components. Little guidance is available, however, correlating the results of field inspections with the actual condition of the reinforced/prestressed concrete elements. This dissertation presents the results of a research program conducted to develop rational guidance for inspectors and engineers to evaluate concrete bridge elements after a fire event and help them make informed decisions regarding the future status of the bridge. <br></div><div><br></div><div>The research program includes tests on portions of a reinforced concrete deck and three full-scaled AASHTO Type I prestressed girders acquired from a decommissioned highway bridge. In addition, six pretensioned concrete prismatic beam specimens with varying levels of prestress were fabricated and tested. The specimens had cross-sectional dimensions of 8 in. by 8 in. and were designed to simulate the bottom flanges of common I-shaped prestressed concrete bridge girders. The deck specimens and four (of the six) concrete beam specimens were subjected to elevated temperatures using radiation-based heaters. Two (of the six) prismatic specimens built in the laboratory were subjected to a hydrocarbon pool fire test conducted in the field for using approximately 135 gallons of kerosene. The concrete temperature profiles and the deformations of the specimens were measured using thermocouple trees and displacement transducers, respectively. Concrete samples were also cored and examined using various methods (DSC and SEM) to correlate microstructure degradation (microcracking, dehydration of C-S-H, decomposition of calcium hydroxide, etc.) with the measured temperatures through the depth of the specimens. <br></div><div><br></div><div>To evaluate the residual loading-carrying capacities of prestressed concrete girders after being subjected to fire, a hydrocarbon pool fire test was performed on two decommissioned AASHTO Type I girders in the field. Load tests were then conducted on the prestressed girders under both ambient and post-fire conditions. After structural testing, material tests were also conducted on concrete cores taken from the girders to evaluate the post-fire concrete microstructure alteration. Furthermore, three-dimensional finite element models were developed to predict the residual load-carrying capacities and overall structural responses of prestressed concrete bridge girders after being exposed to fire. Results from the numerical models generally agree favorably with experimental observations and provide insights into the behavior of the specimens. A parametric study was performed using the benchmarked finite element models to expand the database and establish design recommendations further. Capacity influence lines for load-carrying capacities and structural stiffness were developed and discussed.</div><div><br></div><div>Based on the results from this research, guidelines for the post-fire assessment of prestressed concrete bridges are included in this dissertation along with a step-by-step checklist. Bridge inspectors can infer the extent of damage to prestressed concrete bridge girders in the event of a fire and develop a post-fire assessment plan cognizant of the findings. In most cases, no more than 1.0 in. of the concrete from the exposed surface undergoes material damage / deterioration due to loss of CH, cracking, and spalling. The impact on the strength of prestressed concrete girders is relatively minor based on experimental results. Their initial stiffness, however, will likely be reduced. <br> </div><br>