Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Ume, Charles; Committee Member: Chen, Ye-Hwa; Committee Member: Michaels, Jennifer; Committee Member: Sadegh, Nader; Committee Member: Vachtsevanos, George. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph. D.)--University of Alberta, 2010. / Title from pdf file main screen (viewed on July 8, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Engineering, Department of Chemical and Materials Engineering, University of Alberta. Includes bibliographical references.

Thesis (MScIng)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Thèse (M.Sc.)--Université Laval, 2005. / Bibliogr. Publié aussi en version électronique.

Thesis (M. Sc.)(Electronic Engineering)--University of Pretoria, 2006. / Includes bibliographical references. Available on the Internet via the World Wide Web.

Les arcs impulsionnels sont des arcs électriques établis d'une façon générale lors de la séparation entre deux contacts électriques. Ces arcs indésirables mais inévitables peuvent causer des dégâts en raison d'un important transfert d'énergie vers les électrodes. Lors de l'interaction entre l'arc et l'électrode, cette dernière s'érode entraînant une contamination plus ou moins importante du plasma par les vapeurs des éléments issues de cette érosion et pouvant conduire à une modification notable de ses propriétés. Le travail présenté dans ce mémoire consiste à étudier par une approche expérimentale une configuration d'arc impulsionnel. La première étape indispensable à cette étude est la conception et la réalisation d'un dispositif spécifique permettant de simuler et de caractériser ce type d'arc. Des techniques de diagnostic adaptées au dispositif utilisé ont été développées dans ce travail. Les caractéristiques physiques du plasma (température, densité électronique...) déterminées par spectroscopie optique d'émission associées aux caractéristiques électriques de l'arc ont été corrélées avec les résultats de la visualisation effectuée par imagerie rapide de l'érosion de l'électrode et de la géométrie de l'arc durant son extinction. Des travaux spécifiques sur l'évolution de la tension totale d'arc en régime transitoire ont été réalisés en détaillant les chutes de tension aux électrodes et à la colonne. Par ailleurs des mesures spectroscopiques ont montré l'influence de divers paramètres (nature de l'anode, amplitude et forme de l'onde de courant) sur la température du plasma. Nous avons ainsi mis en évidence que les propriétés du plasma pour une configuration donnée, ne dépendent pas uniquement de la valeur instantanée du courant mais sont également fonctions de l'état antérieur de l'arc / The impulse arcs are electric arcs established generally during the separation between electrical contacts. These undesirable and unavoidable arcs can cause damage because of an important energy transfer to the electrodes. During the arc-electrodes interaction, electrodes erosion may involve an important contamination of the plasma by the vapors of the materials resulting from this erosion and may lead to a notable modification of its properties. The work presented in this thesis is an experimental approach of impulse arc configuration. The first essential stage to this study is the design and the realization of a specific device to simulate and characterize this kind of arc. Techniques of diagnostic adapted to the device were developed in this work. The plasma characteristics (temperature, electron number density...) determined by optical emission spectroscopy associated with the electrical characteristics of the arc were correlated with the results of the fast imaging visualization of the electrode erosion and of the arc geometry during its extinction. A specific study was developed to precisely determine the evolution of the arc voltage in transient state, separating the electrode voltage drop from the plasma column voltage. Spectroscopic measurement showed the influence of various parameters (nature of the anode, current pulse) on the plasma properties. In particular we put in evidence that these properties are not only dependent on the instantaneous value of the current, but depend also on the previous arc story

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Diversas pesquisas em processos de soldagem para construção naval concentram-se em reduzir peso, aumentar a eficiência de energia, melhorar a resistência à corrosão e à tenacidade, bem como reduzir custos e tempo na construção dos navios. O aço naval microligado, AH36, apresenta boa correlação entre estrutura, propriedades mecânicas e soldabilidade. Estas características se devem principalmente a redução dos teores de carbono em função do uso de elementos microligantes como V, Nb e Ti, e do tipo de processo de obtenção das chapas pelo processo termomecânico seguido de resfriamento acelerado (em inglês: Thermomechanical control process -TMCP). Assim, o objetivo deste trabalho é avaliar a soldagem do aço naval AH 36 pelo processo de arco submerso com um e dois arames. Para tanto foram empregados os ensaios mecânicos de tração, de dobramento e de dureza. A tenacidade foi determinada pelo ensaio de Charpy com entalhe em V. Para caracterização metalográfica foram aplicadas as seguintes técnicas: Microscopia óptica (MO) e Microscopia eletrônica de varredura (MEV) e difração de raios X. Os resultados mostraram que as juntas soldadas apresentaram limite de resistência máxima de 561 MPa, com rompimento localizado no metal de base. No ensaio de microdureza foi observado que a região de crescimento de grão da zona afetada pelo calor, no experimento com a técnica Tandem, apresentou-se a região mais rígida das juntas analisadas, também o valor de microdureza no metal de solda foi 10% maior que no metal de base. Os resultados dos ensaios de impacto Charpy V mostraram que a temperatura de transição dúctil frágil do metal de base é de -30ºC. Da mesma maneira, o menor valor de energia absorvida foi para região do metal de solda. Através da análise da micrografia foi possível identificar diferentes morfologias de ferrita, a presença de perlita e pequenas regiões de martensita, bem como a presença de agregados MA. / Several researches in welding processes for shipbuilding has been focusing on reducing weight, increasing energy efficiency, improving corrosion resistance and toughness as well as reducing costs and time in the construction of ships. The microalloyed steel AH36 shipbuilding presents a good correlation among structure, mechanical properties and weldability. These features are mainly because reduction in carbon content due to the use of microalloying elements such as V, Nb and Ti, together with the process of steel plates by thermomechanical control process (TMCP), Therefore, the objective of this study is to evaluate the welding of shipbuilding steel AH 36 by submerged arc process with one and two wires. It was utilized tests such as: tensile, bending and hardness tests. The toughness was determined by Charpy V-Notch test. The metallographic characterization was carried out by the following techniques: optical microscopy (OM) and scanning electron microscopy (SEM) and X-Rays diffraction. The results showed that the tensile test for welded joints presented maximum resistance limit of 561 MPa, and the rupture was located in the base metal. Microhardness test showed that the region of coarse grain of heat affected zone, in the tandem submerged arc welding (SAW), presented the hardest region of the welded joint, also the microhardness value in the weld metal was 10% greater than the metal base. Charpy V notch test tests depicted a ductile brittle transition temperature at about -30 ºC. In the same way, the lowest absorbed energy was identified in the weld metal region. Through microstructure characterization it was possible to identify different morphologies of ferrite, pearlite and the small presence of martensite, as well as, the presence of aggregates MA.

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Understanding the controls on the distribution and type of hydrothermal venting in modern oceanic spreading environments is key to developing tools for exploration and understanding the metallogeny of ancient massive sulfide deposits. Compared to mid-ocean ridges, subduction zones are characterized by additional tectonic complexities, including arc-ridge collisions, arc rotations, pre-existing structures, and variable distances to the arc. This thesis addresses the question, “How do tectonic complexities associated with subduction influence the structure and volcanic evolution of a back-arc basin, and how do they affect the distribution and type of hydrothermal venting?” A multi-scaled approach was used to address this question in the nascent back-arc region of the New Hebrides and in the more advanced stages of opening of the Mariana back-arc basin. In the New Hebrides, an arc-ridge collision segmented the volcanic front and affected the southern and northern back-arc regions in different ways. In the southern Coriolis Troughs (CT), voluminous eruptions are closely linked to the ridge collision, forming a large shield volcano in the near-arc region (Nifonea Volcano). The caldera-hosted eruptions produced high-temperature but short-lived magmatic-hydrothermal activity restricted to the shield volcano. In the northern Jean Charcot Troughs (JCT), ridge collision caused a reversal in the rotation of the arc, reducing extension in the south and increasing extension in the north. Unlike the CT, extension in the JCT is strongly affected by pre-existing structures, which form irregular widely-spaced grabens and volcanic ridges and magmatism in the central part of the back-arc. Here, hydrothermal venting is focused along deeply penetrating faults, associated with widespread tectonic extension. Detailed studies of the mineralogy and geochemistry of the ore and alteration at the Tinakula deposit reveal that massive sulfide accumulation in the region dominated by tectonic extension is characterized by longer-lived, lower-temperature venting than at Nifonea. Hydrothermal activity in the JCT at Tinakula is dominated by (1) long-lived heat from an underlying magma source; (2) fluid circulation along a fissure with long-lived or reactivated permeability; (3) enrichment in fluid-mobile elements such as Ba that are transported at low temperature; (4) mixing of cold seawater with hydrothermal fluids within the permeable volcaniclastic substrate and at the seafloor; (5) water depth controls on maximum hydrothermal vent temperatures; and (6) reduced permeability of the host volcaniclastic succession at the site of mineralization caused by precipitation of alteration minerals and sulfates, focusing fluid flow. The different styles of volcanic and hydrothermal activity closely resemble those of mid-ocean ridge environments in areas that are dominated by tectonic rather than magmatic extension. A comparison with the more advanced stages of rifting and segmentation of the Mariana back-arc demonstrates that Mid-Ocean Ridge (MOR)-type structural and magmatic controls on hydrothermal activity are important during all stages of back-arc basin evolution. This work highlights the diversity of volcanic eruption styles and hydrothermal venting from the earliest stages of back-arc rifting to the advanced stages of basin opening and shows that processes normally associated with MOR-type spreading are directly analogous to back-arc basin systems. However, additional tectonic complexities (e.g., ridge-arc collisions) have a major impact on the location and type of magmatic and hydrothermal activity at back-arc spreading centers, with important implications for understanding ancient volcanic-hosted massive sulfide deposits that mainly formed in back-arc basins.