A finite element model with discrete and explicit representation of steel fibers is applied for modeling the post-cracking behavior of Steel Fiber Reinforced Concrete (SFRC) in order to contribute on the design of beams with combined reinforcement of steel fibers and rebars (RC-SFRC beams). In this numerical approach, concrete and fibers are initially discretized in finite elements in an independent way, avoiding high computational costs due to conforming meshes. Then, coupling finite elements are introduced to describe the concrete-fiber interaction. The steel fibers are discretized using truss finite elements and their behavior described by an elastoplastic constitutive model. The position of each fiber is defined into the specimen by an uniform isotropic random distribution using as reference the concrete finite element mesh. Concrete and concrete-fiber interface are represented using three and fournoded triangular finite elements, respectively, and their behavior represented by appropriate continuum damage models integrated using an implicit-explicit scheme to enhance the robustness and to reduce the expense of computation. Firstly, the numerical tool is applied in the simulation of three-point bending tests according to EN 14651 to verify its ability to obtain the performance parameters of SFRC and for calibrating the material parameters that describe the concrete-fiber interface. Secondly, both numerical and experimental performance parameters of SFRC are used on the design of RC-SFRC beams according to fib Model Code 2010 to study their influence on the amount of bending and shear reinforcements required. Thirdly, the RC-SFRC beams designed are numerically simulated and the results are compared to the designed ones in terms of crack width, mean crack spacing, deflection and ultimate and service loads. Finally, the numerical results of small scale beams are compared to the experimental and the fib Model Code 2010 predictions to study the capability of the numerical tool to simulate the behavior of structural members. The results demonstrated that computational simulations with an appropriated approach to represent the composite may be an important tool to contribute to better understanding its behavior, extrapolating the conditions considered in laboratory and contributing on the design of SFRC structural members. / Um modelo em elementos finitos com representação discreta e explícita de fibras de aço é utilizado para modelar o comportamento pós-fissuração do Concreto Reforçado com Fibras de Aço (CRFA) com objetivo de contribuir para o dimensionamento de vigas com reforço combinado de fibras e armadura convencional (vigas de CACRFA). Na abordagem numérica utilizada para modelagem de CRFA o concreto e as fibras são inicialmente discretizados em elementos finitos de forma independente, evitando altos custos computacionais devido às malhas conformes. Então, elementos finitos de acoplamento são introduzidos para descrever a interação concreto-fibra. As fibras de aço são discretizadas utilizando elementos finitos de treliça e seu comportamento é descrito por um modelo constitutivo elastoplástico Um algoritmo para distribuição isotrópica randômica é utilizado para gerar e distribuir fibras de aço com base na malha de elementos finitos do concreto. O concreto e a interface concreto-fibra são representados utilizando elementos finitos triangulares de três e quatro nós, respectivamente, e seus comportamentos representados por uma modelos apropriados de dano contínuo integrados utilizando um esquema implícito-explícito com objetivo de aumentar a robustez a reduzir o custo computacional. Primeiramente, a ferramenta numérica é aplicada na simulação de ensaios de flexão de três pontos de acordo com EN 14651 para verificar sua capacidade de obter os parâmetros de desempenho do CRFA e para calibrar os parâmetros do material que descrevem a interface concreto-fibra. Em segundo lugar, os parâmetros de desempenho numéricos e experimentais do CRFA são usados no vigas de CA-CRFA de acordo com o fib Model Code 2010, a fim de estudar sua influência na quantidade de armadura de flexão e cisalhamento necessárias. Em terceiro lugar, as vigas de CA-CRFA são numericamente simuladas e os resultados são comparados com os dimensionados em termos de largura de fissura, espaçamento médio entre fissuras, flecha e cargas últimas e de serviço. Finalmente, os resultados numéricos de vigas de pequena escala são comparados com aqueles obtidos experimentalmente e pelo fib Model Code 2010 para estudar a capacidade da ferramenta numérica em simular o comportamento de elementos estruturais. Os resultados demonstraram que a utilização de simulações computacionais com uma abordagem apropriada para representar o compósito podem ser uma importante ferramenta para contribuir para um melhor entendimento do seu comportamento, extrapolando as condições consideradas em laboratório e contribuindo para o dimensionamento de elementos estruturais de CRFA.

A finite element model with discrete and explicit representation of steel fibers is applied for modeling the post-cracking behavior of Steel Fiber Reinforced Concrete (SFRC) in order to contribute on the design of beams with combined reinforcement of steel fibers and rebars (RC-SFRC beams). In this numerical approach, concrete and fibers are initially discretized in finite elements in an independent way, avoiding high computational costs due to conforming meshes. Then, coupling finite elements are introduced to describe the concrete-fiber interaction. The steel fibers are discretized using truss finite elements and their behavior described by an elastoplastic constitutive model. The position of each fiber is defined into the specimen by an uniform isotropic random distribution using as reference the concrete finite element mesh. Concrete and concrete-fiber interface are represented using three and fournoded triangular finite elements, respectively, and their behavior represented by appropriate continuum damage models integrated using an implicit-explicit scheme to enhance the robustness and to reduce the expense of computation. Firstly, the numerical tool is applied in the simulation of three-point bending tests according to EN 14651 to verify its ability to obtain the performance parameters of SFRC and for calibrating the material parameters that describe the concrete-fiber interface. Secondly, both numerical and experimental performance parameters of SFRC are used on the design of RC-SFRC beams according to fib Model Code 2010 to study their influence on the amount of bending and shear reinforcements required. Thirdly, the RC-SFRC beams designed are numerically simulated and the results are compared to the designed ones in terms of crack width, mean crack spacing, deflection and ultimate and service loads. Finally, the numerical results of small scale beams are compared to the experimental and the fib Model Code 2010 predictions to study the capability of the numerical tool to simulate the behavior of structural members. The results demonstrated that computational simulations with an appropriated approach to represent the composite may be an important tool to contribute to better understanding its behavior, extrapolating the conditions considered in laboratory and contributing on the design of SFRC structural members. / Um modelo em elementos finitos com representação discreta e explícita de fibras de aço é utilizado para modelar o comportamento pós-fissuração do Concreto Reforçado com Fibras de Aço (CRFA) com objetivo de contribuir para o dimensionamento de vigas com reforço combinado de fibras e armadura convencional (vigas de CACRFA). Na abordagem numérica utilizada para modelagem de CRFA o concreto e as fibras são inicialmente discretizados em elementos finitos de forma independente, evitando altos custos computacionais devido às malhas conformes. Então, elementos finitos de acoplamento são introduzidos para descrever a interação concreto-fibra. As fibras de aço são discretizadas utilizando elementos finitos de treliça e seu comportamento é descrito por um modelo constitutivo elastoplástico Um algoritmo para distribuição isotrópica randômica é utilizado para gerar e distribuir fibras de aço com base na malha de elementos finitos do concreto. O concreto e a interface concreto-fibra são representados utilizando elementos finitos triangulares de três e quatro nós, respectivamente, e seus comportamentos representados por uma modelos apropriados de dano contínuo integrados utilizando um esquema implícito-explícito com objetivo de aumentar a robustez a reduzir o custo computacional. Primeiramente, a ferramenta numérica é aplicada na simulação de ensaios de flexão de três pontos de acordo com EN 14651 para verificar sua capacidade de obter os parâmetros de desempenho do CRFA e para calibrar os parâmetros do material que descrevem a interface concreto-fibra. Em segundo lugar, os parâmetros de desempenho numéricos e experimentais do CRFA são usados no vigas de CA-CRFA de acordo com o fib Model Code 2010, a fim de estudar sua influência na quantidade de armadura de flexão e cisalhamento necessárias. Em terceiro lugar, as vigas de CA-CRFA são numericamente simuladas e os resultados são comparados com os dimensionados em termos de largura de fissura, espaçamento médio entre fissuras, flecha e cargas últimas e de serviço. Finalmente, os resultados numéricos de vigas de pequena escala são comparados com aqueles obtidos experimentalmente e pelo fib Model Code 2010 para estudar a capacidade da ferramenta numérica em simular o comportamento de elementos estruturais. Os resultados demonstraram que a utilização de simulações computacionais com uma abordagem apropriada para representar o compósito podem ser uma importante ferramenta para contribuir para um melhor entendimento do seu comportamento, extrapolando as condições consideradas em laboratório e contribuindo para o dimensionamento de elementos estruturais de CRFA.

Elektronentomographie erlaubt die dreidimensionale (3D) Charakterisierung von Kristalldefekten auf der Nanometerskala. Die Anwendung in der Forschung an epitaktischen Halbleiterheterostrukturen ist bisher nicht durchgesetzt worden, obwohl kleiner werdende Bauteile mit zunehmend dreidimensionaler Struktur entsprechende Untersuchungen verlangen, um die Beziehung von Struktur und physikalischen Eigenschaften in entsprechenden Materialsystemen zu verstehen. Die vorliegende Arbeit demonstriert die konsequente Anwendung der Elektronentomographie auf eine III-Sb basierte Laser- und eine 3D (In,Ga)N/GaN Nanosäulenheterostruktur. Die unerlässliche Zielpräparation von Proben mittels FIB-SEM-Zweistrahlmikroskops wird herausgestellt. Die kontrollierte Orientierung der Probe während der Präparation und die sorfältige Auswahl eines Abbildungsverfahrens im STEM werden detailliert beschrieben. Die umfassende räumliche Mikrostrukturanalyse einer antimonidbasierten Schichtstruktur folgt der Dimensionalität von Kristalldefekten. Die Facettierung und Lage einer Pore (3D Defekt), deren Auftreten in der MBE gewachsenen GaSb-Schicht untypisch ist, werden bestimmt. Das Zusammenspiel von anfänglich abgeschiedenen AlSb-Inseln auf dem Si-Substrat, der Ausbildung eines Fehlversetzungsnetzwerkes an der Grenzfläche der Heterostruktur (2D Defekt) und dem Auftreten von Durchstoßversetzungen wird mit Hilfe der Kombination tomographischer und komplementärer TEM-/STEM-Ergebnisse untersucht. Die räumliche Anordnung von Versetzungen (1D Defekte), die das ganze Schichtsystem durchziehen, wird mit Elektronentomographie offenbart. Die Wechselwirkung dieser Versetzungen mit Antiphasengrenzen und anderen Liniendefekten sind ein einzigartiges Ergebnis der Elektronentomographie. Abschließend sind Unterschiede im Indiumgehalt und in der Schichtdicke von (In,Ga)N-Einschlüssen auf verschiedenen Facetten schief aufgewachsener GaN-Nanosäulen einmalig per Elektronentomographie herausgearbeitet worden. / Electron tomography exhibits a very poor spread in the research field of epitaxial semiconductor heterostructures in spite of the ongoing miniaturization and increasing three-dimensional (3D) character of nano-structured devices. This necessitates a tomographic approach at the nanometre scale in order to characterize and understand the relation between structure and physical properties of respective material systems. The present work demonstrates the rigorous application of electron tomography to an III-Sb based laser and to an (In,Ga)N/GaN nanocolumn heterostructure. A specific target preparation using a versatile FIB-SEM dual-beam microscope is emphasized as indispensable. The purposeful orientation of the specimen during preparation and the careful selection of an imaging mode in the scanning-/transmission electron microscope (S/TEM) are regarded in great detail. The comprehensive spatial microstructure characterization of the antimonide based heterostructure follows the dimensionality of crystal defects. The facetting and position of a pore (3D defect) which is unexpected in the MBE grown GaSb layer, is determined. The interplay of the initially grown AlSb islands on Si, the formation of a misfit dislocation network at the heterostructure interface (2D defect) and the presence of threading dislocations is investigated by the correlation of tomographic and complementary S/TEM results. The spatial arrangement of dislocations (1D defects) penetrating the whole stack of antimonide layers is revealed by electron tomography. The interaction of these line defects with anti-phase boundaries and with other dislocations is exclusively observed in the 3D result. The insertion of (In,Ga)N into oblique GaN nanocolumns is uniquely accessed by electron tomography. The amount of incorporated indium and the (In,Ga)N layer thickness is shown to vary on the different facets of the GaN core.

L'objectif de cette thèse est de concevoir et élaborer des outils innovants, baptisés nano-antennes (NAs), pour sonder le champ électromagnétique optique au voisinage des structures de la nanooptique. Une NA est ici une nanostructure métallique placée à l'extrémité d'une sonde locale fonctionnant en mode collection. Une telle structure joue le rôle de relais entre le champ électromagnétique optique à la surface de l'échantillon et la sonde locale, attribuant à la sonde une sensibilité spécifique vis-à-vis du champ électromagnétique vectoriel. Les nano-antennes seront conçues de manière à fournir une information optique encore inaccessible par le biais de la microscopie en champ proche conventionnelle, pour répondre aux besoins actuels et futurs de la nano-optique. Dans la première partie, nous proposons une nouvelle configuration de sonde locale la nano-ouverture papillon complément d'une nano-antenne papillon et nous exposons sa fabrication aux dimensions spécifiques, à l'extrémité d'une fibre optique en polymère. Dans la deuxième partie, nous présentons une nouvelle configuration de microscopie en champ proche permettant l'accès aux lignes de champs électriques et magnétiques optiques 3D au dessus d'un échantillon, avec une résolution sub-longueur d'onde. Le microscope proposé permet de cartographier simultanément et indépendamment les distributions des amplitudes complexes des deux composantes du champ électrique transverse. La dernière partie est consacrée à l'application d'une méthode numérique dite FDTD afin d'étudier le comportement spectral et l'origine de l'exaltation au niveau de la partie centrale d'une nano-ouverture papillon.

Eine ortsauflösende Spannungs- und Dehnungssensortechnik soll durch die Nutzung magnetostriktiver Materialien auf der Oberfläche von Kohlenstoffeinzelfasern (C-Fasern) und Mikrofeinstrukturierung dieser Schichten erzeugt und zur elektronischen Überwachung des Belastungszustandes von sicherheits- oder servicerelevanten Faserverbundbauteilen eingesetzt werden. Eine auf lokaler Gasphasenabscheidung und Mikrostrukturierung mittels der Focused Ion Beam (FIB)-Technik beruhende Sensorfabrikationsmethode wurde gemeinsam mit dem Institut für Mikrotechnologie Hannover (imt) entwickelt. Mehrschichtig mittels CVD und PVD bedampfte und zusätzlich galvanisch beschichtete C-Fasern weisen neuartige Eigenschaften auf, die im vorgestellten Vorhaben am Lehrstuhl für Verbundwerkstoffe (LVW) charakterisiert wurden. Insbesondere die Untersuchung der verschiedenen Schichten sowie deren Interfaces nehmen eine bedeutende Rolle ein.

The main objective of this dissertation is to quantify petrophysical properties of conventional and unconventional reservoirs using a mechanistic approach. Unconventional transport mechanisms are described from the pore to the reservoir scale to examine their effects on macroscopic petrophysical properties in hydrocarbon-bearing organic shale. Petrophysical properties at the pore level are quantified with a new finite-difference method. A geometrical approximation is invoked to describe the interstitial space of grid-based images of porous media. Subsequently, a generalized Laplace equation is derived and solved numerically to calculate fluid pressure and velocity distributions in the interstitial space. The resulting macroscopic permeability values are within 6% of results obtained with the Lattice-Boltzmann method after performing grid refinements. The finite-difference method is on average six times faster than the Lattice-Boltzmann method. In the next step, slip flow and Knudsen diffusion are added to the pore-scale method to take into account unconventional flow mechanisms in hydrocarbon-bearing shale. The effect of these mechanisms is appraised with a pore-scale image of Eagle Ford shale as well as with several grain packs. It is shown that neglecting slip flow in samples with pore-throat sizes in the nanometer range could result in errors as high as 2000% when estimating permeability in unconventional reservoirs. A new fluid percolation model is proposed for hydrocarbon-bearing shale. Electrical conductivity is quantified in the presence of kerogen, clay, hydrocarbon, water, and the Stern-diffuse layer in grain packs as well as in the Eagle Ford shale pore-scale image. The pore-scale model enables a critical study of the [delta]LogR evaluation method commonly used with gas-bearing shale to assess kerogen concentration. A parallel conductor model is introduced based on Archie's equation for water conductivity in pores and a parallel conductive path for the Stern-diffuse layer. Additionally, a non-destructive core analysis method is proposed for estimating input parameters of the parallel conductor model in shale formations. A modified reservoir model of single-phase, compressible fluid is also developed to take into account the following unconventional transport mechanisms: (a) slip flow and Knudsen diffusion enhancement in apparent permeability, (b) Langmuir desorption as a source of gas generation at kerogen surfaces, and (c) the diffusion mechanism in kerogen as a gas supply to adsorbed layers. The model includes an iterative verification method of surface mass balance to ensure real-time desorption-adsorption equilibrium with gas production. Gas desorption from kerogen surfaces and gas diffusion in kerogen are the main mechanisms responsible for higher-than-expected production velocities commonly observed in shale-gas reservoirs. Slip flow and Knudsen diffusion marginally enhance production rates by increasing permeability during production. / text

Grâce à ses propriétés d'antioxydant, d'anti-usure et éventuellement d'extrême pression le dithiophosphate de zinc (ZDDP) fait partie des additifs les plus utilisés dans les lubrifiants pour moteurs thermiques. De nos jours, dans un souci de respect de l'environnement, de nouveaux lubrifiants possédant de bonnes performances en lubrification (frottement faible et usure limitée) sont développés en prenant compte des limitations d'utilisations actuelles du ZDDP. L'idée étant de réduire dans ceux-ci les teneurs en phosphore et soufre (Normes euros VI), éléments essentiels du ZDDP qui endommagent les pots catalytiques. L'objectif de cette thèse est l'étude du comportement tribologique du phosphate de zinc di alkyl (ZP) en comparaison avec le ZDDP. La méthodologie expérimentale étudiée pour comprendre le mécanisme d'action de ces additifs, associe des essais de frottement à descaractérisations physico-chimiques des surfaces frottantes après essais.Cette comparaison a été effectuée en fonction de la température (25°C et 100°C), la vitesse de glissement (25, 50 et 100 mm/s) et la concentration en additif (200 et 600 ppm dephosphore). Les meilleures actions anti-usure sont obtenues avec le ZDDP pour une température de 100°C et une vitesse de glissement de 100 mm/s et le ZP pour une température de 25°C et une vitesse de glissement de 25 mm/s. Les analyses de surface XPS, AES, XANES et MET-EDX ont permis de mettre en évidence la présence d'un film protecteur constitué principalement de phosphate de zinc, ceci pour les deux additifs.Une synergie de comportement tribologique a été mise en évidence avec un lubrifiantconstitué de ZP (usure faible) et d'oléate d'urée (frottement faible). Des essais complémentaires sur un tribomètre dynamique ont permis d'étudier le niveau de frottement du tribofilm formé à partir du ZDDP. Le caractère visqueux du tribofilm de ZDDP a été mis en évidence.

It is the study of banded iron formations (BIFs) that provides understanding into the conditions of the Earth's oceans and atmosphere during the Archean and Early Proterozoic. The aim of this thesis is to provide a detailed mineralogical and geochemical understand of BIFs from two separate localities separated by the Archean Proterozoic boundary. Close attention is paid to their carbonate and iron oxide mineralogy.The BIFs of the 2.7 Ga Manjeri Formation, Zimbabwe and 2.4 Ga Itabira Group, Brazil were both precipitated from oxygenated mixed marine-hydrothermal fluids. This is demonstrated by the presence of nano-hematite inclusions in the chert (Itabira and Manjeri) and dolomite (Itabira only) laminae, which is interpreted as the oldest mineral phase within the samples. Additionally, focused ion beam transmission electron microscopy (FIB-TEM) reveals the presence of nano ferrihydrite platelets within the dolomitic BIFs (carbonate itabirite). The dolomite is interpreted to be a primary phase precipitated at higher temperatures (~100°C) from CO2-rich hydrothermal fluids. Positive Eu anomalies in both formations indicate a hydrothermal component, likely to be the source of the reduced iron. Facies changes in both units are the result of transgression/regression and post depositional hydrothermal events mask primary conditions. Iron-rich carbonates in both facies have different origins; diagenetic (Itabira) and post depositional hydrothermal (Manjeri). However, the iron-rich carbonates of both formations have negative ∂13C values, indicating that at least part of the carbon in the carbonates is of organic origin. Curie Balance analyses into the carbonate itabirite reveals that maghemite is the transformation product of the ferrihydrite when dolomite decomposes at ~790°C. The maghemite has a Curie temperature between 320 and 350°C and is stable up to temperatures of 925°C.FIB-TEM investigations into the martitisation process revealed two possible mechanisms from two martite samples, from Brazil and India. Depending of the cause of the martitisation, here found to be deformation and hydrothermalism, the martitisation occurs respectively via either: 1. Ordering of point defects caused by vacancies in the spinel structure of maghemite, due to the removal of excess Fe3+ ions during the oxidation of magnetite, to form twins. It is in this twinning that the martitisation mechanism occurs.2. Grain boundary migration by hematite at the expense of magnetite is due to the presence of fluid along the crystal interfaces, where maghemite forms due to excess Fe3+ produced during martitisation of the magnetite, moving towards the surface of the magnetite crystals.

A combined experimental and computational investigation of the deformation behavior of pure magnesium single crystal at the micron length scale has been carried out. Employing the recently exploited method of microcompression testing, uniaxial microcompression experiments have been performed on magnesium single crystals with [0001], [2-1-12], [10-11], [11-20] and [10-10] compression axes. The advantage of the microcompression method over conventional mechanical testing techniques is the ability to localize a single crystalline volume which is characterizable after deformation. The stress-strain relations resulting from microcompression experiments are presented and discussed in terms of orientation dependent slip activity, twinning mechanisms and an anisotropic size effect. Such a mechanistic picture of the deformation behavior is revealed through SEM, EBSD and TEM characterization of the deformation structures, and further supported by 3D discrete dislocation dynamics simulations. The [0001], [2-1-12], and [10-11] compression axes results show dislocation plasticity. Specifically, the deformation due to [0001] compression is governed by pyramidal slip and displays significant hardening and massive unstable shear at stresses above 500MPa. In the case of the two orientations with compression along an axis 45 degrees to the basal plane, unsurpringly it is found that basal slip dominates the deformation. In contrast, compression along the [11-20] and [10-10] directions show deformation twinning in addition to dislocation plasticity. In the case of compression along [11-20], the twinning leads to easy basal slip, while the twin resultant during compression along [10-10] does not lead to easy basal slip. In all cases, a size effect in the stress-strain behavior is observed; the flow stress increases with decreasing column diameter. Furthermore, the extent of the size effect is shown to depend strongly on the number of active slip systems; compression along the [0001] axis is associated with 12 slips systems and displays a saturation of the size effect at a diameter of 10μm, while the other orientations still show a significant size effect at this diameter. The experimental evidence of an orientation-dependent deformation behavior in flow stress has been investigated by 3D discrete dislocation dynamics simulations. Here, the code TRIDIS was modified for hcp structure and c/a ratio of Mg. By matching the simulation results to experimental results, some proper constitutive material parameters such as initial dislocation density, dislocation source length, the critical resolved shear stress were suggested. For the case of [0001] and [2-1-12] orientation, dislocation feature in the pillar during the deformation was exhibited and strain burst was discussed.

[pt] A caracterização tridimensional tem sido mais utilizada na área da ciência dos materiais devido à necessidade de melhor compreensão e resultados mais precisos acerca da microestrutura dos materiais, que não são completamente revelados pelas técnicas tradicionais de microscopia bidimensional. No presente trabalho dois tipos de técnicas de caracterização 3D foram utilizadas: MicroTC - Microtomografia de Raios-X (com tomógrafos de bancada e baseados em fonte síncrotron) e FIB-SEM (feixe de íons focalizados acoplados a um MEV). Estas técnicas foram aplicadas a um sistema específico: descontinuidades em metal de solda subaquática molhada. Estas descontinuidades (poros, trincas e inclusões) apresentam tamanhos típicos variando de nanômetros a dezenas de micrômetros. Além disso, apresentam formas, distribuição espacial e orientação bastante variada e complexa. Assim, esta tese apresenta o desenvolvimento de metodologia de aquisição, processamento, análise e visualização 3D de poros, trincas e inclusões em solda subaquática molhada, a partir de imagens obtidas por MicroTC e FIB-SEM. As técnicas de aquisição foram otimizadas para os diferentes tipos de descontinuidades. Rotinas especializadas de processamento e análise de imagens foram criadas, sempre que possível utilizando um ambiente de software livre (FIJI/ImageJ). Diversas medidas foram automaticamente obtidas: número de objetos, volume, fração volumétrica, área superficial, diâmetro de Feret, espessura, esfericidade e compacidade. Além disso, a construção de imagens 3D permitiu observar a forma e a distribuição espacial das descontinuidades presentes. Visando avaliar a sensibilidade para detecção de trincas por MicroTC, um corpo de prova com seção variável foi submetido a um ensaio de tração, de forma que as diferentes seções sofressem diferentes valores de tensão. Foi verificada uma correlação positiva entre o valor de tensão e o número, comprimento e espessura das trincas detectadas. Este experimento também revelou o impacto da resolução espacial e do ruído sobre a possibilidade de detectar as trincas de forma acurada. / [en] 3D characterization is growing quickly in materials science due to the demands of better microstructural characterization, which cannot be fully achieved with the traditional 2D microscopy techniques. In this work, two types of 3D characterization techniques were employed: MicroCT – microcomputed x-ray tomography (with both bench top and synchrotron sources) and FIB-SEM (focused ion beam coupled to SEM). These techniques were applied to a specific system: discontinuities in underwater wet welds. These discontinuities (pores, cracks and inclusions) range in size from nanometers to tens of microns. Moreover, they present complex and varied shapes, spatial distribution and orientation. Thus, this thesis presents the development of methodology for the acquisition, processing, analysis and visualization of pores, cracks and inclusions in underwater wet welds, from images obtained by MicroCT and FIB-SEM. The acquisition techniques and conditions were optimized for the different kinds of discontinuities. Specialized routines for image processing and analysis were developed, employing a free software environment whenever possible (FIJI/ImageJ). Several measurements were automatically obtained: number of objects, volume, volume fraction, surface area, feret diameter, thickness, sphericity and compacity. Moreover, the rendering of 3D images allowed the observation of the shape and spatial distribution of the discontinuities in the weld metal. To evaluate the detection sensitivity of cracks by MicroCT, a specimen with varied cross-sections was submitted to a tensile test, so that the different sections were submitted to to different stress values. A positive correlation was observed between the stress value and the number, length and thickness of the detected cracks. This experiment also showed the influence of spatial resolution and noise upon the possibility of detecting cracks accurately.