As ligas de alumínio são aplicadas em cerca de 70% dos componentes estruturais dos aviões e o processo de fadiga e o modo de falha predominante em estruturas aeronáuticas, para a maioria das quais a presença de concentradores de tensão e inevitável. O comportamento em fadiga e as propriedades mecânicas das ligas de alumínio endurecíveis por precipitação são fortemente influenciadas por parâmetros como tamanho, espaçamento e densidade dos precipitados endurecedores. Neste contexto, pesquisas anteriores tem mostrado que o envelhecimento interrompido (T6I4) pode proporcionar melhores combinações de propriedades mecânicas para estas ligas. O presente trabalho tem como objetivo o estudo das propriedades mecânicas e do comportamento em fadiga de alto ciclo das ligas de alumínio AA6351 e AA7050 nas condições de tratamento térmico convencionais (T6 e T7451, respectivamente) e na condição T6I4, bem como da influência das características microestruturais e do tratamento térmico sobre estas propriedades. Dentro deste contexto, analises de Microscopia Eletrônica de Transmissão (MET) da liga AA6351 mostraram que a condição T6I4 resultou em uma maior densidade de precipitados endurecedores com tamanho heterogêneo, quando comparada com a condição T6. Para esta liga, a condição T6I4 também resultou em menores valores de tensão limite de escoamento, resistência a tração, resistência a fadiga e sensibilidade ao entalhe, com maior ductilidade e tenacidade. Para a liga AA7050, as análises de MET mostraram que a condição T6I4 resulta em uma maior densidade de precipitados endurecedores com menor tamanho, promovendo um melhor impedimento ao movimento de discordâncias durante a deformação por fadiga, quando comparada com a condição T7451. Esta alteração microestrutural proporcionou a condição T6I4 valores de resistência ao escoamento e resistência a tração similares a condição T7451, com maior ductilidade e tenacidade. A resistência a fadiga da condição T6I4 foi similar a condição T7451, entretanto o envelhecimento interrompido resultou em um melhor comportamento em sensibilidade ao entalhe. / Aluminum alloys are applied in approximately 70% of the aircraft structural components and the fatigue process is the dominant failure mode in aeronautical structures, for the most of which, the presence of stress concentrators is unavoidable. The fatigue behavior and the mechanical properties of the age hardenable aluminum alloys are strongly influenced by parameters including the size, spacing and density of strengthening precipitates. Within this context, previous researches have shown that the interrupted ageing (T6I4) could provide an improved combination of mechanical properties for these alloys. This work aims to study the mechanical properties and the high cycle fatigue behavior of AA6351 and AA7050 aluminum alloys in the conventional heat treatment conditions (T6 and T7451, respectively) and in the T6I4 condition, as well as the influence of the microstructural characteristics and of the heat treatment on these properties. Within this context, Transmission Electron Microscopy (TEM) analyzes of the AA6351 alloy showed that T6I4 condition resulted in higher density of hardening precipitates with heterogeneous size compared to T6 condition. For this alloy, the T6I4 condition resulted in lower values of yield stress, ultimate tensile strength, fatigue strength and notch sensitivity, with higher ductility and toughness. For the AA7050 alloy, TEM analyses showed that T6I4 condition presented a higher density of strengthening precipitates with smaller size promoting an improved dislocation pinning effect during the fatigue deformation compared to T7451 condition. This microstructural change provided to T6I4 condition yield stress and ultimate tensile strength similar to T7451, with higher ductility and toughness. The fatigue strength of T6I4 condition was also similar to T7451, however the interrupted ageing provided a better notch sensitivity behavior.

The accurate description of the nonlinear structural behaviour is an important issue in engineering science. Usually, classic nonlinear theories, such as fracture and damage mechanics, applied to finite element programmes are used to fulfil that purpose. Classic fracture mechanics describes the structural deterioration process by a few discrete cracks. This theory presents good precision for structures with simple geometries, few cracks and homogeneous materials. Classic damage mechanics measures the deterioration process by an internal variable called damage. This theory has been successful in the description of several deterioration mechanisms in continuum media. Despite their accuracy, classic fracture and damage mechanics present some drawbacks. Firstly, regarding civil engineering problems, both theories are not suitable for some practical applications. Secondly, fracture mechanics demands the consideration of initial cracks to begin the analysis. Lastly, classic damage models may present an issue known as localisation, what essentially leads to ill-posed problems and mesh-dependent numerical algorithms. Alternatively, a recent theory, called lumped damage mechanics, was proposed in order to achieve good accuracy in actual engineering problems. Such theory applies key concepts from fracture and damage mechanics in plastic hinges. In the light of the foregoing, the main goal of this thesis is the extension of the lumped damage mechanics framework to analyse different engineering problems. So far, lumped damage mechanics was characterised as a simplified methodology to analyse reinforced concrete frames under seismic and monotonic loadings; even with a few contributions on the analysis of local buckling in metallic structures. Therefore, this work extends the lumped damage mechanics framework to analyse reinforced concrete arches, unreinforced concrete structures, high cycle fatigue and continuum problems. The application examples show the accuracy of the proposed methodologies. / A descrição acurada do comportamento não linear de estruturas é um problema importante na engenharia. Usualmente, teorias não lineares clássicas, tais como as mecânicas da fratura e do dano, aplicadas a programas de elementos finitos são utilizadas a fim de cumprir aquele propósito. A mecânica da fratura clássica descreve o processo de deterioração estrutural por meio de um pequeno número de fissuras discretas. Esta teoria apresenta boa precisão para estruturas com geometrias simples, poucas fissuras e materiais homogêneos. A mecânica do dano clássica tem sido exitosa na descrição de diversos mecanismos de deterioração em meios contínuos. Apesar de precisas, as abordagens clássicas em fratura e dano apresentam alguns entraves. Primeiramente, tratando-se de problemas da engenharia civil, ambas teorias não são adequadas para aplicações práticas. Em segundo lugar, os modelos clássicos de fratura demandam a consideração de fissuras iniciais para iniciar a análise. Por fim, os modelos clássicos de dano podem apresentar um problema conhecido como localização, o que essencialmente implica em problemas mal colocados e algoritmos com dependência de malha. Alternativamente, uma teoria recente, chamada teoria do dano concentrado, foi proposta a fim de obter boa precisão em problemas reais de engenharia. Tal teoria aplica conceitos-chave das mecânicas da fratura e do dano em rótulas plásticas. À luz do exposto, o principal objetivo desta tese é a extensão da teoria do dano concentrado para analisar diferente problemas da engenharia. Até então, a teoria do dano concentrado era caracterizada como uma metodologia simplificada para analisar pórticos de concreto armado sob solicitações monotônicas ou sísmicas; mesmo com algumas poucas contribuições na análise de instabilidade local em estruturas metálicas. Desta forma, este trabalho estende a teoria do dano concentrado a fim de analisar arcos de concreto armado, estruturas de concreto simples, fadiga de alto ciclo e problemas contínuos. Os exemplos de aplicação mostram a acurácia das metodologias propostas.

A indústria automotiva tem mostrado um crescente interesse pelas ligas de alumínio, em especial as ligas de alumínio da série 6xxx. Esta classe é amplamente empregada nas áreas de construção e de transporte, devido a sua boa resistência mecânica e excelente resistência à corrosão. No setor automobilístico sempre houve a necessidade de aprimoramento dos estudos do comportamento em fadiga, pois os componentes estruturais são submetidos a carregamentos vibratórios, esforços e tensões cíclicas e, como consequência, podem trincar e finalmente fraturar. A presença de um entalhe geralmente diminui a vida em fadiga, criando regiões de altas tensões triaxiais localizadas que restringem a deformação plástica, fragilizando o material. A resposta mecânica depende das solicitações, microestrutura, componentes da liga e propriedades do material. Neste trabalho foi realizado o estudo do comportamento em fadiga de alto ciclo e da sensibilidade ao entalhe de três ligas de alumínio da série 6xxx destinadas à fabricação de componentes de carroçarias para caminhões e ônibus: AA 6005, AA 6063 e AA 6351, todas na condição T6. As curvas S/N foram obtidas por meio de ensaios de fadiga em flexão rotativa (R = -1). Ensaios com peças entalhadas (Kt ? 3,0) permitiram determinar e comparar o fator de concentração de tensão em fadiga e a sensibilidade ao entalhe das ligas estudadas. Também foi estudada a influência da microestrutura e das partículas intermetálicas sobre as propriedades de fadiga. As superfícies das peças fraturadas foram observadas ao MEV e verificou-se que a maioria dos sítios de nucleação de trincas por fadiga ocorreram próximos a partículas de segunda fase que atuam como concentradores de tensão. / The automotive industry has shown a growing interest in aluminum alloys, particularly the AA 6xxx series. This class of alloys is widely used in construction and transportation because of their good mechanical strength, easy fabrication and excellent corrosion resistance. In the automotive sector there was always a need for improved studies of the fatigue behavior, because the structural components are subjected to vibratory loads and cyclic stresses and, as a consequence, may eventually crack and fracture. The presence of a notch generally decreases the fatigue life, creating regions of high triaxial stresses which restrain plastic deformation, weakening the material. The response depends on the mechanical solicitations, microstructure, the alloy components and the material properties. In this work it was conducted a study of the high cycle fatigue behavior and the notch sensitivity of three aluminum alloys used in components of truck and bus bodies: AA 6005, AA 6063 and AA 6351, all provided in T6 condition. The S/N curves were obtained by tests in rotating bending fatigue (R = -1). Tests with notched samples (Kt?3.0) allowed to determine and compare the fatigue concentration factor and the notch sensitivity of the studied alloys. It was also studied the influence of microstructure and the intermetallic particles on the fatigue properties. The surfaces of the fractured samples were observed via SEM and showed that most of the nucleation sites of fatigue cracking occurred near the second phase particles, which act as stress concentrators.

Neste trabalho, foram determinadas as propriedades mecânicas de fadiga de baixo e alto ciclo e de tenacidade à fratura JIC de um aço inoxidável austenítico do sistema Cr-Mn-N, utilizado na confecção de colares para perfurações pela indústria de exploração de petróleo. Foi também avaliada a influência da temperatura de serviço nas propriedades mecânicas do aço, que apresentou um decréscimo significativo nos valores dos limites de escoamento e de resistência e do alongamento com o aumento da temperatura. Na temperatura ambiente, os ensaios de fadiga de baixo e alto ciclo foram realizados segundo as normas ASTM E606 e ASTM E466. Em temperaturas superiores, as propriedades de fadiga foram estimadas utilizando alguns métodos atualmente disponíveis, que utilizam somente as propriedades mecânicas de tração. Os resultados obtidos mostraram uma pequena influência da temperatura na vida à fadiga na região de baixo ciclo e uma grande influência na região de alto ciclo. A curva tensão-deformação cíclica do material foi obtida pelo método convencional e pelo método do passo incrementai, que utiliza somente um corpo de prova. Os ensaios de JIC foram realizados nas temperaturas ambiente e 150°C, segundo a norma ASTM E813, utilizando a técnica de variação da flexibilidade elástica. Na temperatura ambiente, foi observada uma grande dispersão nos valores obtidos de JIC, atribuída à presença de uma fase frágil, identificada como precipitados de carbonitretos mistos de ferro, cromo, manganês e nióbio. Foi também observada uma diminuição no valor médio de JIC com o aumento da temperatura. / In the present work, the low and high cycle fatigue and fracture toughness properties of a Cr-Mn-N austenitic stainless steel used for application in drill collars were investigated. Also, the temperarure dependence of the tensile properties was determined, and it was found that the yield strength, ultimate strength and elongation values decrease significantly with the temperature. The low and high cycle fatigue testing were carried out at room temperature according to ASTM E606 and ASTM E466 standards. At high temperarure, the fatigue properties were estimated from monotonic propetties obtained in the tensile testing. The results showed small influence of temperature in the fatigue life in the short-life range and great influence in the long-life range. The cyclic stress-strain curve was obtained by companion specimen tests and incremental step tests methods. The last method showed to be advantageous because only one specimen and short testing time were required. The fracture toughness was detennined at room temperature and at 150°C applying elastic-plastic fracture mechanics concepts. The single specimen elastic compliance technique for crack length determination was employied. At room temperature a great scattering of the JIC values was observed due the presence of the brittle phase identified as a precipitates of carbonitride content Fe, Cr, Mn, Nb elements. Also a decreasing of average value o f the JIC was observed with the increasing of temperature.

L’Inconel 718 est utilisé dans les disques de turbine des moteurs d’avion, de par sa haute résistance à la corrosion, à l’oxydation, au fluage et sa haute résistance mécanique à très haute température. Le nombre total de cycles de ces composants mécaniques s’élève à 109~1010 durant sa vie. Ils subissent des chargements de grande amplitude à faible fréquence, comme les forces centrifuges ou les contraintes thermiques mais aussi des chargements de faibles amplitudes à très haute fréquence, du aux vibrations des pales. Dans ce travail, on se propose d’étudier la fatigue à très grand nombre de cycles (VHCF) de l’Inconel 718 en utilisant des machines de fatigue ultrasonique, fonctionnant à 20KHz. Le système d’acquisition utilise des cartes NI et le logiciel LabView pour superviser la fréquence, la température, les déplacements durant toute la durée des tests. Des capteurs laser Keyence utilisant deux sondes pour les faces supérieure et inférieure de l’éprouvette permettent de capturer la fréquence et les modes de vibration. La différence entre les valeurs moyennes mesurées permet d’accéder à l’allongement de l’éprouvette, dû à l’auto-échauffement.3 types de matériaux avec différents traitements thermiques, AR, DA et DAHQ de l’ONERA et SAFRAN sont comparés. La différence au niveau de la taille de grain, de la phase, des précipités, … est analysée par micrographie métallographique en utilisant un microscope optique (MO) et un microscope électronique à balayage (MEB). Le comportement en traction quasi-statique et sous chargement cyclique contrainte-déformation est aussi proposé. La transition entre durcissement et adoucissement cyclique apparait à l’issue du traitement thermique. Finalement, les surfaces de ruptures sont observées en utilisant des caméras optiques et un MEB afin d’identifier les mécanismes de ruptures de l’Inconel 718 dans le domaine de la fatigue à très grand nombre de cycles. / Inconel 718 is widely used in turbine disk of aeronautic engines, due to its high resistance to corrosion, oxidation, thermal creep deformation and high mechanical strength at elevated temperature. The total cycle of these mechanical components is up to 109~1010 during its whole lifetime. It endures high-amplitude low-frequency loading including centrifugal force or thermal stress, and also low-amplitude high-frequency loading came from vibration of blade.In this work, the very high cycle fatigue (VHCF) behaviour of Inconel 718 with self-heating phenomenon without any cooling is studied using ultrasonic fatigue system at 20KHz. Acquisition system is improved using NI capture card with LabView for monitoring the frequency, temperature, displacement and so on during all the tests. Keyence laser sensor with two probes at the top and bottom surfaces of the specimens is used to reveal the frequency and vibration mode. The difference of mean values between these two probes is the elongation of the specimen caused by self-heating phenomenon.Three sets of materials with different heat treatment, As-Received (AR), Directly Aged (DA) and Directly Aged High Quality (DAHQ) from ONERA and SAFRAN are compared. The difference of grain size, phase, precipitate particle, etc. is investigated by metallographic micrograph using optical microscope (OM) and scanning electron microscope (SEM). Quasi-static uniaxial tensile property and cyclic stress-strain response is also proposed. The transition from cyclic hardening to cyclic softening appears after aged heat treatment. Finally, fracture surfaces are observed using optical camera and scanning electron microscope in order to identify the mechanism of fracture of Inconel 718 in the VHCF domain.

L’objectif de ces travaux de thèse est d’étudier l’influence du rapport entre taille de défaut et longueur caractéristique de la microstructure, sur la limite de fatigue pour une durée de vie fixée d’un matériaux métallique,dans le cadre de la fatigue à grand nombre de cycle (FGNC). Le fer pur Armco est choisi comme matériau d’étude, car sa microstructure simple présente une seule longueur caractéristique à l’échelle mésoscopique (échelle des grains) : la taille de grain. Le but de l’étude revient ainsi à étudier la compétition entre un effet de structure (défaut surfacique)et un effet matériau (taille de grain) dans le cadre de sollicitations mécaniques en FGNC.Afin d’obtenir une taille de défaut comparable mais aussi inférieure à la taille de grain du matériau, un protocole thermomécanique a été élaboré pour augmenter la taille de grain. Des essais de FGNC, utilisant des éprouvettes issues des deux tailles de microstructures (matériau initial et celui écroui traité) dans lesquelles sont introduits des défauts hémisphériques de tailles différentes, ont été effectués pour estimer les limites de fatigue pour différents rapports taille de défaut / taille de grain. Lorsque les diagrammes de Kitagawa sont présentés en valeurs relatives(limite de fatigue / celle du matériau sans défaut en fonction de la taille de défaut / taille de grain), il existe une seule courbe qui combine les deux microstructures. Ce diagramme de Kitagawa sans dimension permet ainsi d’analyser la réduction de la limite de fatigue causée par un défaut. L’utilisation de la taille relative du défaut par rapport à la dimension microstructurale caractéristique apparaît comme plus pertinente que l’emploi de la taille physique réelle du défaut.Ces résultats expérimentaux sont utilisés pour reproduire les essais en FGNC avec des simulations Éléments Finis sur des microstructures 3D représentatives du fer Armco. La compétition existant entre la concentration de contrainte induite par le défaut géométrique et les régions fortement sollicitées de la microstructure engendrées parl’anisotropie du comportement mécanique des grains est étudiée. Un critère mésoscopique (à partir des grandeurs mécaniques moyennées par grain) basé sur une approche statistique permet de retrouver l’allure du diagramme de Kitagawa adimensionné, c’est-à-dire la taille relative du défaut critique à partir de laquelle ce dernier prend le passur l’hétérogénéité de la réponse de la microstructure et conditionne ainsi la tenue en fatigue du polycristal. La modification du critère mésoscopique par la prise en compte des hétérogénéités intragranulaires (via l’écart-type par grain des grandeurs mécaniques) est discutée. / The objective of this thesis is to study the influence of the ratio between the defect size and themicrostructure characteristic length on the fatigue limit (for a fixed fatigue life) of a metallic material, under highcycle fatigue (HCF). Pure Armco iron is chosen because its simple microstructure has a single characteristic lengthat the mesoscopic scale (grain scale) : the grain size. The aim of the study is thus to study the competition betweena structural effect (surface defect) and a material effect (grain size) in the context of mechanical stresses in HCF.In order to obtain a comparable different grain size, a thermomechanical protocol has been developed. HCF tests,using specimens from both microstructure sizes (initial material and processed hardened material) in which hemisphericaldefects of different sizes were introduced, were performed to estimate the fatigue limits for different defectsize / grain size ratios. When Kitagawa diagrams are presented in relative values (fatigue limit / fatigue limit ofdefect free material versus defect size / grain size), there is a single curve that combines the two microstructures.This dimensionless Kitagawa diagram thus makes it possible to analyze the reduction of the fatigue limit inducedby a defect. The use of the relative size of the defect with respect to the characteristic microstructural dimensionappears to be more relevant than the use of the physical size of the defect.These experimental results are used to reproduce the HCF tests with Finite Element simulations on 3D microstructuresrepresentative of Armco iron. The competition existing between the stress concentration induced by thegeometrical defect and the highly stressed regions of the microstructure generated by the anisotropy of the mechanicalbehavior of the grains is studied. A mesoscopic criterion (involving mechanical quantities averaged by grain)based on a statistical approach allows to find the evolution of the dimensionless Kitagawa diagram, ie the relativesize of the critical defect from which it predominates over the response heterogeneity of the microstructure and thusgoverns the fatigue behavior of the polycrystal. The modification of the mesoscopic criterion by taking into accountintragranular heterogeneities (with the standard deviation per grain of mechanical quantities) is discussed.

Ligas de titânio do tipo ??compostas de elementos não tóxicos são materiais com potencial para aplicações biomédicas por apresentarem baixo módulo de elasticidade, efeito de memória de forma, biocompatibilidade satisfatória e boa conformabilidade. Para os biomateriais, o conhecimento das propriedades de fadiga é essencial para garantir uma alta confiabilidade para implantes ortopédicos e odontológicos. As propriedades mecânicas dependem diretamente do processamento termomecânico, da taxa de resfriamento imposta à liga e da composição química, que são fatores responsáveis pela determinação de sua microestrutura. Neste contexto, neste trabalho foi avaliado o comportamento em fadiga da liga Ti-35Nb-7Zr (% em p.) para aplicações biomédicas. Trata-se de uma liga de titânio do tipo ??e a motivação desse estudo baseou-se em complementar resultados de suas propriedades microestruturais e mecânicas obtidos em estudos anteriores realizados no DEMAR - EEL/USP. A liga foi produzida por fusão a arco a partir de materiais (Ti, Nb, Zr) de pureza comercial. A rota de processamento termomecânico envolveu as etapas de tratamento térmico de solubilização, forjamento rotativo a frio, tratamento térmico de recristalização. Um tratamento térmico ultrarrápido (Flash) foi adicionado para promover o refino de grãos e aumento da resistência mecânica. A caracterização microestrutural foi realizada por técnicas de microscopia óptica, microscopia eletrônica de varredura, microscopia eletrônica de transmissão, difratometria de raios X e medidas de dureza Vickers. As propriedades mecânicas foram investigadas por ensaios de tração uniaxial e ensaios de fadiga de alto ciclo por flexão rotativa de corpos de prova lisos e entalhados. Com relação ao conjunto de propriedades, os melhores resultados podem ser considerados para a condição recristalizada à 1000ºC/2h + flash. Os resultados obtidos neste trabalho confirmam a possibilidade de uso da liga Ti-35Nb-7Zr para aplicações biomédicas. / ?-type titanium alloys composed of non-toxic elements are materials with potential for biomedical applications because they have low elastic modulus, shape memory effect, satisfactory biocompatibility and good workability. For biomaterials, knowledge of fatigue properties is essential to ensure high reliability for orthopedic and dental implants. The mechanical properties depend directly on thermomechanical processing, alloy cooling rate and chemical composition, which are responsible for the determination of its microstructure. In this context, this work was evaluated the fatigue behavior of the Ti- 35Nb-7Zr alloy (wt. %) for biomedical applications. It is a ?-type alloy and the motivation of this study was based on complementing the results of its microstructural and mechanical properties previously obtained at DEMAR-EEL/USP. The alloy was produced from materials of commercial purity (Ti, Nb and Zr) by arc meling. The thermomechanical processing route consisted the steps of solubilization heat treatment, cold rotary forging, recrystallization heat treatment. A ultrafast thermal treatment (Flash) was additioned to promote the grain refinement and to increase mechanical strength. The microstructural characterization was done by optical microscopy, scanning electron microscopy, transmission electron microscopy, X ray diffraction techniques and Vickers microhardness tests. The mechanical properties was investigated by uniaxial tensile tests and rotary bending high cycle fatigue tests of smooth and notched specimens. With respect to the set of properties, the best results can be considered for recrystalized condition at 1000ºC/2h + flash. The results obtained in this work confirm the possibility of using the Ti-35Nb-7Zr alloy for biomedical applications.

The design parameters axial gap and stator count of highpressure turbine stages are evaluated numerically towards theirinfluence on the unsteady aerodynamic excitation of rotorblades. Of particular interest is if and how unsteadyaerodynamic considerations in the design could reduce the riskofhigh cycle fatigue (HCF) failures of the turbine rotor. A well-documented 2D/Q3D non-linear unsteady code (UNSFLO)is chosen to perform the stage flow analyses. The evaluatedresults are interpreted as aerodynamic excitation mechanisms onstream sheets neglecting 3D effects. Mesh studies andvalidations against measurements and 3D computations provideconfidence in the unsteady results. Three test cases areanalysed. First, a typical aero-engine high pressure turbinestage is studied at subsonic and transonic flow conditions,with four axial gaps (37% - 52% of cax,rotor) and two statorconfigurations (43 and 70 NGV). Operating conditions areaccording to the resonant conditions of the blades used inaccompanied experiments. Second, a subsonic high pressureturbine intended to drive the turbopump of a rocket engine isinvestigated. Four axial gap variations (10% - 29% ofcax,rotor) and three stator geometry variations are analysed toextend and generalise the findings made on the first study.Third, a transonic low pressure turbine rotor, known as theInternational Standard Configuration 11, has been modelled tocompute the unsteady flow due to blade vibration and comparedto available experimental data. Excitation mechanisms due to shock, potential waves andwakes are described and related to the work found in the openliterature. The strength of shock excitation leads to increasedpressure excitation levels by a factor 2 to 3 compared tosubsonic cases. Potential excitations are of a typical wavetype in all cases, differences in the propagation direction ofthe waves and the wave reflection pattern in the rotor passagelead to modifications in the time and space resolved unsteadypressures on the blade surface. The significant influence ofoperating conditions, axial gap and stator size on the wavepropagation is discussed on chosen cases. The wake influence onthe rotorblade unsteady pressure is small in the presentevaluations, which is explicitly demonstrated on the turbopumpturbine by a parametric study of wake and potentialexcitations. A reduction in stator size (towards R≈1)reduces the potential excitation part so that wake andpotential excitation approach in their magnitude. Potentials to reduce the risk of HCF excitation in transonicflow are the decrease of stator exit Mach number and themodification of temporal relations between shock and potentialexcitation events. A similar temporal tuning of wake excitationto shock excitation appears not efficient because of the smallwake excitation contribution. The increase of axial gap doesnot necessarily decrease the shock excitation strength neitherdoes the decrease of vane size because the shock excitation mayremain strong even behind a smaller stator. The evaluation ofthe aerodynamic excitation towards a HCF risk reduction shouldonly be done with regard to the excited mode shape, asdemonstrated with parametric studies of the mode shapeinfluence on excitability. <b>Keywords:</b>Aeroelasticity, Aerodynamics, Stator-RotorInteraction, Excitation Mechanism, Unsteady Flow Computation,Forced Response, High Cycle Fatigue, Turbomachinery,Gas-Turbine, High-Pressure Turbine, Turbopump, CFD, Design

Microelectronic systems are subjected to thermal cycling, power cycling, and vibration environments in various applications. These environments, whether applied sequentially or simultaneously, affect the solder joint reliability. Literature is scarce on predicting solder joint fatigue failure under such multiple loading environments. This thesis aims to develop a unified modeling methodology to study the reliability of electronic packages subjected to thermal cycling, power cycling, and vibration loading conditions. Such a modeling methodology is comprised of an enriched material model to accommodate time-, temperature-, and direction-dependent behavior of various materials in the assembly, and at the same time, will have a geometry model that can accommodate thermal- and power-cycling induced low-cycle fatigue damage mechanism as well as vibration-induced high-cycle fatigue damage mechanism. The developed modeling methodology is applied to study the reliability characteristics of ceramic area array electronic packages with lead-based solder interconnections. In particular, this thesis aims to study the reliability of such solder interconnections under thermal, power, and vibration conditions individually, and validate the model against these conditions using appropriate experimental data either from in-house experiments or existing literature. Once validated, this thesis also aims to perform a design of simulations study to understand the effect of various materials, geometry, and thermal parameters on solder joint reliability of ceramic ball grid array and ceramic column grid array packages, and use such a study to develop universal polynomial predictive equations for solder joint reliability. The thesis also aims to employ the unified modeling methodology to develop new understanding of the acceleration factor relationship between power cycling and thermal cycling. Finally, this thesis plans to use the unified modeling methodology to study solder joint reliability under the sequential application of thermal cycling and vibration loading conditions, and to validate the modeling results with first-of-its-kind experimental data. A nonlinear cumulative damage law is developed to account for the nonlinearity and effect of sequence loading under thermal cycling, power cycling, and vibration loading.

A novel microstructure-sensitive extreme value probabilistic framework is introduced to evaluate material performance/variability for damage evolution processes (e.g., fatigue, fracture, creep). This framework employs newly developed extreme value marked correlation functions (EVMCF) to identify the coupled microstructure attributes (e.g., phase/grain size, grain orientation, grain misorientation) that have the greatest statistical relevance to the extreme value response variables (e.g., stress, elastic/plastic strain) that describe the damage evolution processes of interest. This is an improvement on previous approaches that account for distributed extreme value response variables that describe the damage evolution process of interest based only on the extreme value distributions of a single microstructure attribute; previous approaches have given no consideration of how coupled microstructure attributes affect the distributions of extreme value response. This framework also utilizes computational modeling techniques to identify correlations between microstructure attributes that significantly raise or lower the magnitudes of the damage response variables of interest through the simulation of multiple statistical volume elements (SVE). Each SVE for a given response is constructed to be a statistical sample of the entire microstructure ensemble (i.e., bulk material); therefore, the response of interest in each SVE is not expected to be the same. This is in contrast to computational simulation of a single representative volume element (RVE), which often is untenably large for response variables dependent on the extreme value microstructure attributes. This framework has been demonstrated in the context of characterizing microstructure-sensitive high cycle fatigue (HCF) variability due to the processes of fatigue crack formation (nucleation and microstructurally small crack growth) in polycrystalline metallic alloys. Specifically, the framework is exercised to estimate the local driving forces for fatigue crack formation, to validate these with limited existing experiments, and to explore how the extreme value probabilities of certain fatigue indicator parameters (FIPs) affect overall variability in fatigue life in the HCF regime. Various FIPs have been introduced and used previously as a means to quantify the potential for fatigue crack formation based on experimentally observed mechanisms. Distributions of the extreme value FIPs are calculated for multiple SVEs simulated via the FEM with crystal plasticity constitutive relations. By using crystal plasticity relations, the FIPs can be computed based on the cyclic plastic strain on the scale of the individual grains. These simulated SVEs are instantiated such that they are statistically similar to real microstructures in terms of the crystallographic microstructure attributes that are hypothesized to have the most influence on the extreme value HCF response. The polycrystalline alloys considered here include the Ni-base superalloy IN100 and the Ti alloy Ti-6Al-4V. In applying this framework to study the microstructure dependent variability of HCF in these alloys, the extreme value distributions of the FIPs and associated extreme value marked correlations of crystallographic microstructure attributes are characterized. This information can then be used to rank order multiple variants of the microstructure for a specific material system for relative HCF performance or to design new microstructures hypothesized to exhibit improved performance. This framework enables limiting the (presently) large number of experiments required to characterize scatter in HCF and lends quantitative support to designing improved, fatigue-resistant materials and accelerating insertion of modified and new materials into service.