Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 214-234).

Thesis (Ph. D.)--West Virginia University, 2005. / Title from document title page. Document formatted into pages; contains xxiv, 371 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 343-367).

Thesis (M.S.)--Ohio University, November, 2004. / Title from PDF t.p. Includes bibliographical references (p. 11-113)

Resumo: Os aços multifásicos são de grande interesse comercial e têm atraído a atenção, principalmente, da indústria automobilística devido a combinação de alta resistência e ductilidade. A fase austenita retida, presente nestes aços, tem sido objeto de especial interesse devido ao efeito TRIP, que contribui para a melhoria da tenacidade. Neste trabalho é utilizado o aço aeronáutico 300M e, com o objetivo de estudar a influência das fases sobre as propriedades mecânicas são realizadas as seguintes etapas: a otimização de rotas de tratamentos térmicos para a obtenção de uma microestrutura multifásica; caracterização microestrutural por microscopia óptica, microscopia eletrônica de varredura e microscopia de força atômica e avaliação do envelhecimento de deformação à temperatura ambiente. A principal contribuição é a determinação da fração volumétrica da austenita retida por meio de medidas de saturação magnética através de curvas de histerese magnética. As frações volumétricas de austenita retida obtidas por medidas de magnetização são comparadas com as obtidas por difratometria de raios X e microscopia óptica após ataques químicos de metabissulfito de sódio. Os resultados obtidos por microscopia óptica e pelo método magnético apresentam boa concordância com os valores encontrados na literatura. A alteração microestrutural devido aos tratamentos térmicos aplicados permite melhorias no limite de escoamento e resistência. Analisa-se, ainda, o efeito do envelhecimento por deformação à temperatura ambiente. Os resultados são surpreendentes, ocorrendo um aumento no limite de escoamento em até 87%, devido à associação do envelhecimento por deformação com o efeito TRIP. / Abstract: The multiphase steels are of great commercial interest and have attracted attention mainly of the automobile industry due to the combination of their high strength and ductility. The retained austenite phase present in these steels has been object of special interest due to TRIP effect contribution for the improvement of the toughness. In this work the influence of the phases of the aircraft 300M steel is carried out by the optimization of heat treatments routes to obtain multiphase microstructure. Microestructural characterization is evaluated by optical microscopy, scanning electronic microscopy, atomic force microscopy and analysis of the deformation ageing at room temperature. The main contribution is the determination of retained austenite volume fractions from magnetic hysteresis curves. These values are compared with x-ray diffraction data and optical microscopy after sodium metabisulfite etching, presenting good agreement with the literature. The microstructural alterations due the applied heat treatments also affect the yield strength and strength. The association of the deformation ageing and the TRIP effect resulted in a surprising increase of up to 87% in the yield strength. / Orientador: Antonio Jorge Abdalla / Coorientador: Tomaz Manabu Hashimoto / Banca: Marcelo dos Santos Pereira / Banca: Sergio João Crnkovic / Banca: Carlos de Moura Neto / Banca: Getulio de Vasconcelos / Doutor

Two streamline, ordinary differential equation (ODE) models for detonation, the Chan-Kirby model and the straight streamline approach of Watt et al., are extended to a multiphase system of equations. These multiphase equations, with realistic equations of state, are used to better capture the heterogeneities in non-ideal explosives used in mining applications. Streamline ODE multidimensional models are normally obtained by reducing the partial differential equations (PDEs) describing the motion of the material to ODEs by making approximations about some of the physics of the problem. These models are referred to as reduced ODE models in this work and are the primary focus of this research into fast, efficient solutions of non-ideal explosives. In the development of these reduced order forms, some terms in the full equations have been removed for analytical convenience. Although this is not always the result of a formal order of magnitude analysis, this somewhat empirical approach is justified by simulation studies. In particular, by demonstrating that in a variety of benchmark problems, the reduced order ODEs give similar results to those obtained from the much more complex, full order PDE models. Further support is obtained by comparing the reduced order solution with experimental results. Comparisons with multiphase direct numerical simulations and experiments are undertaken to investigate the effect of the approximations and assumptions made in the derivation of the models. Both models produce comparable diameter effect curves for two different non-ideal explosives, EM120D and ANFO, in unconfined conditions. Empirical assumptions in the Chan-Kirby model can be eliminated but investigation shows that the straight streamline multiphase extension is based on better approximations for non-ideal explosives. This latter approach also gives better prediction of the diameter effect curve and detonation driving zone shape. The multiphase straight streamline model is then extended to model confined multiphase detonations, with realistic equations of state for the confining material, and predicts most strong confinement examples well. Future work of extending to curved streamlines and including confinement other than strong or weak is discussed.

Today’s aerospace industry continues to exploit liquid hydrocarbon fossil fuels. Motivated by operational considerations, continued availability and cost, this is likely to be the case for many years, despite the obvious environmental concerns. The interplay of liquid atomisation, spray vaporisation and the combustion process are intricately linked. However, the physical process of fuel injection and its atomisation into tiny droplets prior to combustion remains poorly understood. Because atomisation governs the size of the fuel droplets, and therefore their subsequent evaporation rate, adjusting the injection sequence is of paramount importance and will have far-reaching repercussions on many aspects of the combustion process, for example pollutant formation. In the context of jet engines, kerosene is usually injected in its liquid form via an airblast-type fuel injector. A coflowing high-speed airstream destabilises the liquid fuel, which is thus sprayed into fine droplets into the combustion chamber. The prediction of this phenomenon for various operating conditions relevant to the aeronautical industry requires a deeper understanding of the mechanisms involved in the interaction of the two fluids. A key element in predicting the complex behaviour of spray formation and evolution in jet engines is accurate modelling of fuel atomisation. Atomisation represents one of the key challenges that remains to be undertaken to make predictive computational simulations possible. However, the inherent multi-physics and multi-scale nature of this process limits numerical investigations. Thanks to the steady progress in computer power and Computational Fluid Dynamics (CFD) methods, computational modelling of injection systems emerges as a promising tool that can drive the design of future devices. This research project sets out to investigate the atomisation process in detail, in particular in providing physical insight into the fundamental physics of the phenomenon, in conjunction with an analysis on wetting behaviours and liquid droplet tracking. High-fidelity numerical simulations are performed using a novel in-house state-of-the-art multiphase flow modelling capability, RCLSFoam. The performance of the numerical scheme is demonstrated on typical two-dimensional and three-dimensional benchmark test cases relevant to both multiphase flow modelling and atomisation, and validated against other computational methods. An informed and systematic qualitative assessment of the topological variations of the phase interface during primary atomisation of a liquid film is made through dynamical analysis, while investigating an extensive domain of operating conditions at ambient and aero-engine injection conditions relevant to industry. This analysis demonstrated the influence of shear-driven instabilities on the atomisation process. The shear stress and difference in inertia between liquid and gas are observed to play a significant role in the atomisation process. In addition, the key physical mechanisms and their competing effects have been mapped out in order to predict the evolution of the process according to the operating conditions of the injection system. The proposed cartography gathers four different atomisation mechanisms. In particular, for sufficiently high liquid injection speeds, three-dimensional wave modes were observed to co-exist (the “3-D wave mode” regime). For very low liquid flow rates, accumulated liquid at the atomising edge undergoes deformation by which droplets are generated (the “accumulation” regime). For an increasing gas injection speed and a fixed liquid velocity, the effects of surface tension were observed to result in the generation of streamwise ligaments only, which tend to pair up (the “ligament-merging” regime). Finally, “vortex action” is another observed mechanism by which the liquid film is fragmented. Overall, this research project culminated in (i) the study of dynamic wetting behaviours, with the implementation and validation against experimental data of the Kistler dynamic contact model; and (ii) the demonstration of an algorithm for droplet capture and subsequent post-processing analysis of the droplet characteristics.

Made available in DSpace on 2014-06-11T19:32:50Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-04-13Bitstream added on 2014-06-13T20:24:12Z : No. of bitstreams: 1 delsant_r_dr_guara.pdf: 3721030 bytes, checksum: 88232a756e66c084fa98967c48ab9853 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Os aços multifásicos constituídos de ferrita, bainita, austenita retida e martensita apresentam combinações muito atrativas de resistência e tenacidade. Há ainda um potencial adicional de melhorias de propriedades mecânicas quando a fração de austenita retida for alta, conferindo alta conformabilidade pelo efeito TRIP. Neste contexto, é fundamental a análise qualitativa e quantitativa das fases, especialmente de austenita retida e sua transformação em martensita induzida por deformação. Este trabalho enfoca a transformação da austenita retida em martensita por deformação em tração monotônica em um aço AISI 4340 com estrutura multifásica. Os resultados confirmam a transformação da austenita retida em martensita atingindo cerca de 80% de transformação. As frações volumétricas de austenita retida antes e após a deformação foram estimadas por duas técnicas. A primeira foi feita por análise de imagens em microscopia óptica e a segunda por magnetização de saturação, tendo em vista o caráter paramagnético desta fase. As frações estimadas pelas duas técnicas foram comparadas, concluindo-se que o método magnético deve ser reavaliado, tendo sido proposto um fator de correção na equação básica deste processo e presenta na literatura. / The multiphase steels made of ferrite, bainite, retained austenite and martensite present very attractive combinations and toughness. There is still an additional potential of improvement of the mechanical properties when the fraction of retained austenite is high allowing high conformability by the TRIP effect. In this context the qualitative and quantitative analysis of the phases is essentual specially from retained austenite and its transformation in martensite induced by deformation. This work focus on the transformation of retained austenite in martensite by the deformation in monotonic traction in one steel AISI 4340 with multiphasic structure. The results confirm the transformation of retained in martensite reaching about 80% of transformation. The volumetric of retained austenite before and after the deformation were estimated by two technique: the first was made by the anllysis of images in optic microscopy and the second by magnetization of saturation taking into consideration the paramagnetica character of this phase. The estimated fraction by the two technique were compared leading to the conclusion that the magnetic method must be evaluated again using a proposed factor of correction in the basic equation of this process and present in the literature.

Made available in DSpace on 2014-06-11T19:34:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-12Bitstream added on 2014-06-13T19:44:43Z : No. of bitstreams: 1 silva_ap_dr_guara.pdf: 3551125 bytes, checksum: 3f75e8c0e99826910556f8cd772c1403 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho analisa a transformação da austenita retida induzida por deformação, em um aço AISI 4340 com estruturas multifásicas. O aço foi submetido a duas rotas de tratamentos isotérmicos selecionadas e foi analisado quanto à fração volumétrica das fases e características mecânicas através da valiação dos limites de escoamento. Os corpos de prova foram submetidos a três diferentes níveis de tensões de tração, acima do limite de escoamento, para que se produzissem deformações plásticas determinadas. A influência destas deformações, na microestrutura e nos novos limites de escoamento, foram avaliadas. Mostrando variações significativas nas frações volumétricas das fases, com diminuição das quantidades de austenita, indicando uma transformação, por efeito TRIP (Transformation Induced Plasticity), em martensita o que provocou um aumento nos valores dos limites de escoamento e envelhecimento por deformação. / The Transformation induced plasticity of austenite on the multiphase microstructure in a AISI 4340 steel was studied. Two sequences of isothermal treatments were performed. The volume fractions of the phases were analyzed and evaluation of mechanical characteristics through the yield point measurements. The samples were submitted at three different levels of traction above of the yield strength, so that definitive plastic deformations were produced. The influence of these deformations in the microstructure and in the new yield strength was evaluated. Significant variations were showed in the phase volume fraction, with reduction of the amounts of austenite indicating a martensitic transformation, by effect TRIP (Transformation Induced Plasticity) which results increases in the values of the yield strength and deformation ageing.

abstract: This dissertation describes a process for interface capturing via an arbitrary-order, nearly quadrature free, discontinuous Galerkin (DG) scheme for the conservative level set method (Olsson et al., 2005, 2008). The DG numerical method is utilized to solve both advection and reinitialization, and executed on a refined level set grid (Herrmann, 2008) for effective use of processing power. Computation is executed in parallel utilizing both CPU and GPU architectures to make the method feasible at high order. Finally, a sparse data structure is implemented to take full advantage of parallelism on the GPU, where performance relies on well-managed memory operations. With solution variables projected into a kth order polynomial basis, a k+1 order convergence rate is found for both advection and reinitialization tests using the method of manufactured solutions. Other standard test cases, such as Zalesak's disk and deformation of columns and spheres in periodic vortices are also performed, showing several orders of magnitude improvement over traditional WENO level set methods. These tests also show the impact of reinitialization, which often increases shape and volume errors as a result of level set scalar trapping by normal vectors calculated from the local level set field. Accelerating advection via GPU hardware is found to provide a 30x speedup factor comparing a 2.0GHz Intel Xeon E5-2620 CPU in serial vs. a Nvidia Tesla K20 GPU, with speedup factors increasing with polynomial degree until shared memory is filled. A similar algorithm is implemented for reinitialization, which relies on heavier use of shared and global memory and as a result fills them more quickly and produces smaller speedups of 18x. / Dissertation/Thesis / Doctoral Dissertation Aerospace Engineering 2015

Orientador: Marcelo dos Santos Pereira / Banca: Herman Jacobus Cornelis Voorwlad / Banca: Antonio Jorge Abdalla / Resumo: Foram caracterizados os comportamentos eletroquímicos e avaliadas as resistências à corrosão das ligas aeronáuticas 2024-T351 e 7050-T7451 em soluções aquosas de cloreto contendo cromato, molibdato e tungstato. Foram realizados ensaios de corrosão não-eletroquímicos de imersão prolongada acompanhados de análise metalográfica de superfície por microscopia óptica e identificação dos produtos de corrosão por difratometria de raios-X. A análise quantitativa de superfícies das ligas após a imersão, indica que os pites formados têm áreas médias similares. Os pites são mais largos do que profundos e de geometria, predominantemente, cônica ou quase-cônica e irregular. Em todos os produtos de corrosão de cada liga foi encontrado hidróxido de alumínio, em suas diferentes formas cristalinas. Medidas de perda de dureza, como uma conseqüência da deterioração superficial, também foram determinadas. Além disso, ensaios eletroquímicos como medidas de potencial em circuito aberto, curvas de polarização e voltametria cíclica complementaram este estudo. Em meio aerado os resultados obtidos mediante medidas eletroquímicas são consistentes com aqueles obtidos nos ensaios de imersão, em particular o efeito do CrO42- e do MoO42-. O WO42- mostrou-se agressivo em períodos prolongados de imersão. Apesar dos ensaios revelarem uma redução parcial de MoO42- em ambas as ligas, o efeito desse oxi-ânion parece ser diferente sobre cada liga. Em meio desaerado as ligas apresentam passivação em todos os eletrólitos. A adição dos oxi-ânions não modificou significativamente o potencial de pite para a liga 7050, enquanto que para a liga 2024 ele foi deslocado levemente para valores mais positivos. / Abstract: It has been characterized the electrochemical behavior and evaluated the 2024-T351 and 7050-T7451 aircraft alloys corrosion resistance in chloride aqueous solutions containing chromate, molybdate and tungstate. It has been carried out non-electrochemical long immersion corrosion testings accompanied by surface metalography analysis achieved by light microscopy and corrosion products identification by X-ray difratometry. Surfaces quantitative analysis upon the alloys after immersion, indicates that formed pits have similar average area. Pits are widther than deeper and own predominantly a conical or quasi-conical and irregular geometry. In all corrosion products of each alloy it has been found aluminum hydroxide in its different crystalline ways. Hardness loss measurements have also been determined. In addition, electrochemical testings such as open circuit potential measures, polarization curves and cyclical voltammetry have completed this study. In aerated means the obtained results before electrochemical mesurements are similar to those obtained in the immersion tests, in particular CrO42- and MoO42- effects. WO42- has been found to be aggressive in very long immersion period. Though tests display a MoO42- partial reduction in both alloys, this oxi-anion effect seems to be different upon each alloy. In de-aerated means alloys present passivation in all eletrolytes. Oxi-anion addition has not changed significantly pit potential for 7050 alloy, while for 2024 alloy it has been dislocated, slightly, for more positive values. / Mestre