Este trabalho apresenta uma metodologia de análise da heterogeneidade do concreto a partir de modelos computacionais desenvolvidos com o método de Galerkin livre de elementos. Esse método se caracteriza pela discretização de um domínio de interesse por um conjunto de partículas sem que exista explicitamente uma malha de elementos no sentido convencional. O objetivo é a previsão das propriedades mecânicas macroscópicas do material resultante a partir das fases individuais e do arranjo geométrico. O concreto foi admitido, na escala mesoscópica, como um composto formado por inclusões (agregado graúdo) imersas em uma matriz (argamassa). Para a simulação foi desenvolvida uma formulação multiregiões onde se admitiu que cada agregado e a argamassa são domínios distintos interligados nas suas interfaces. Para isto foram utilizadas técnicas de subdivisões do domínio (elemento representativo) ao ponto que os seus comportamentos mecânicos não foram comprometidos. Para simular o processo das perdas de rigidez com a formação da fissuração no concreto foi admitido o efeito da mecânica do dano contínuo através do modelo de Mazars. Para as análises foram desenvolvidos modelos computacionais bidimensionais e tridimensionais da heterogeneidade do concreto. A geometria dos agregados foi aproximada por circunferências e elipses no caso 2D e por esferas e elipsoides no caso 3D. Como conclusão a metodologia de multiregiões com o método de Galerkin livre de elementos foi satisfatória e os modelos apresentaram caminhos preferenciais de ruptura adequados durante a evolução da danificação. / This thesis presents a methodology for analyzing the heterogeneity of concrete from computational models developed with the element free Galerkin method. This method is characterized by discretization of a domain of interest by a set of particles with no explicit mesh in the conventional sense. The goal is to predict the macroscopic mechanical properties of the material resulting from the individual phases and the geometric arrangement. The concrete was assumed, in the mesoscopic scale, as a compound formed by inclusions (coarse aggregate) embedded in a matrix (mortar). For the simulation, a formulation was developed where multi regions were admitted, assuming that each aggregate and mortar are distinct domains connected by their interfaces. For this we used techniques of subdivisions of the domain (representative elements) to the point that their mechanical behaviors were not compromised. To simulate the process of loss of stiffness with the formation of cracks in the concrete, continuum damage mechanics was admitted through Mazars model. For the analysis, two-dimensional and three-dimensional computer models of the heterogeneity of the concrete were developed. The shape of the aggregates was approximated by circles and ellipses in the two-dimensional case, and by spheres and ellipsoids for the 3D problems. In conclusion the multi region methodology with the element free Galerkin methods was satisfactory and the models presented suitable preferred paths for the rupture during the evolution of damage.

Este trabalho investiga o comportamento dúctil de aços microligados classe API utilizado em tubulações na indústria do petróleo, e apresenta um estudo exploratório da aplicação da abordagem local e do modelo micromecânico de células computacionais para modelar a extensão estável de trinca em Modo I de abertura em aço ARBL. Ensaios laboratoriais na temperatura ambiente do aço API 5L X70 (utilizando corpos-de-prova normalizados) fornecem a curva de resistência à fratura (curva-R) do material. Esta curva foi utilizada para calibrar os parâmetros micromecânicos de células computacionais empregados no modelo. Este modelo foi utilizado para prever a pressão de colapso de dutos de paredes finas utilizados no transporte de gás, que apresentam defeitos longitudinais de diferentes razões entre profundidade de trinca e espessura de parede (a/t). As análises numéricas realizadas demonstram a capacidade da metodologia de células computacionais 2D em simular o rasgamento dúctil e o crescimento estável de trincas em corpos-de-prova de mecânica da fratura, assim como prever a pressão de colapso de estruturas tubulares contendo defeitos (trincas). / This study presents the experimental investigation of the ductile behaviour of microalloyed pipeline steel. Additionally, it extends the computational cell methodology to model Mode I crack extension in a high strength low alloy HSLA steel. Laboratory testing of an API 5L X70 steel at room temperature using standard, deep crack C(T) specimens provides the crack growth resistance curve to calibrate the micromechanics cell parameters for the material. The cell model incorporating the calibrated material-specific parameters is then applied to predict the burst pressure of a thin-walled gas pipeline containing longitudinal cracks with varying crack depth to thickness ratios (a/t). The numerical analyses demonstrate the capability of the computational cell approach to simulate ductile crack growth in fracture specimens and to predict the burst pressure of thin-walled tubular structures containing crack-like defects.

La modélisation numérique du comportement des matériaux cimentaires sous l’action des sollicitations complexes constitue un point de vue alternatif pour identifier et évaluer les mécanismes internes qui ne peuvent être étudiés directement à travers les essais expérimentaux. A cet effet, le développement des outils de modélisation permettant la prise en compte des interactions entre la microstructure hétérogène de la pâte de ciment et le comportement macroscopique des matériaux cimentaires est fortement apprécié. Une telle approche numérique donne une meilleure description des processus physiques et évite la calibration répétitive des paramètres lorsque la microstructure change.Ce travail de thèse a pour objet de mettre au point une approche de modélisation de l’endommagement des matériaux cimentaires compte tenu des mécanismes physiques qui se produisent à l’échelle microscopique. Dans l’approche proposée, les principes de la construction d’une microstructure virtuelle de la pâte de ciment sont présentés et les paramètres micromécaniques des phases cimentaires sont identifiés. La capacité prédictive de l’approche est testée en comparant les résultats numériques aux résultats des essais expérimentaux réalisés dans ce travail et aux résultats tirés de la littérature. L’application de cette approche est ensuite illustrée à travers des simulations de la pâte de ciment sous fluage et cycles de gel-dégel. Cette approche ouvre la voie à une multitude d’applications, comme l’étude de l’effet du retrait, du fluage, des cycles de gel-dégel, de la fissuration thermique, de l’auto-cicatrisation et de la carbonatation sur les propriétés thermomécaniques des matériaux cimentaires. / Numerical modelling of the constitutivebehaviour of cementitious materials exposed to aggressive environment offers an alternative point of view for the identification and assessment of internal mechanisms which cannot be explicitly explored using standard experimental techniques. In this regard, the development of advanced modelling tools that take into account the interactions between the heterogeneous microstructure of cement paste and the macroscopic behaviour cementitious materials is highly valued. Such modelling approaches give a much better description of the physical processes and avoid recurrent parameter calibration when dealing with a different microstructure.The work presented in this PhD thesis proposes a numerical modelling approach of damage in cement based materials taking heed of the physical mechanics that can only be characterized at the microscopic level. In the proposed approach, the principles of constructing a virtual microstructure of cement paste are laid out and the micromechanical parameters of cement phases are identified. The predictive capacity of the micromechanical approach is put to the test by a comparison of numerical results with experimental data determined in the present study and found in the literature. Finally, the power of the approach is illustrated through simulations of creep and freeze-thaw behaviour at the microscopic scale of cement paste.This approach paves the way for a multitude of applications, such as the study of the effect of shrinkage, creep, freeze-thaw cycles, thermal cracking, self-healing and carbonation on the thermomechanical properties of cement-based materials.

<p>The objective of this thesis was to improve a paraffin actuated micropump design, to be able to pump against extreme pressures (above 100 bar). This was accomplished by initially studying the membrane activation, using video capturing. The micropump has been improved to withstand pressures high enough, to enable use in an high-performance liquid chromatography (HPLC) system. The micropump has been shown to pump against back pressures up to 150 bar, with a positive net-flow. This should be compared with the previously recorded maximum back pressure of 50 bar. The pumping against high back pressures was possible due to an increased understanding of the sealing of the membranes. This resulted in a new design that was manufactured and characterised. Without clamping the pump was measured to manage back pressures of 10 bar, and then starting to leak in a bond at the flow channel. With supporting clamping, the managed back pressures increased ten folded.</p><p>When measured on the different valves, pressure above 200 bar has been possible to withhold. Although the valves were below their maximum limit, the pressure was not possible to be further increased due to a limitation in the equipment, i.e. risk of damaging the connections. When examined after pressurised at extreme pressures (above 100 bar) several times, no signs of fatigue or damage of the membrane was seen.</p><p>A new behaviour of the valves was discovered. Above certain pressures some designs self sealed, i.e. withholding the pressure after the voltage was turned off. For these valves the pressure had to be released by some other means.</p>

This thesis addresses the problem of damage at elevated temperature with a view to analysing transverse cracking during the continuous casting of microalloyed steels. Based on the results of a previous project undertaken at the University of Liège to simulate the continuous casting process at the macroscopic level, the present research aims at studying the damage growth using a finite element mesoscopic approach that models the grains structure of the material. The developments are done at the mesoscopic scale using information from both the microscopic and macroscopic levels. In order to determine the constitutive laws governing the damage process at the mesoscopic scale, the physical mechanisms leading to the apparition of cracks during steel continuous casting are first investigated. It is acknowledged that in the studied temperature range (800 to 1200 °C), the austenitic grain boundary is a favourable place for cracks to initiate and propagate. The mechanisms of voids nucleation, growth and coalescence are established, the cavities evolving under diffusion and creep deformations. Having identified the damage mechanisms occurring under continuous casting conditions, a numerical approach for the modelling of these phenomena at the grain scale is proposed. The mesoscopic model, which is implemented in the Lagrangian finite element code LAGAMINE developed at the University of Liège, is built on the basis of a 2D mesoscopic cell representative of the material. The finite element discretization comprises solid elements inside the grains and interface elements on the grains boundaries. An elastic-viscous-plastic law of Norton-Hoff type, which represents the thermo-mechanical behaviour of the material, is associated to the solid elements for the modelling of the grains; and a damage law accounting for cavitation and sliding is linked to the interface elements for the modelling of the damage growth at the grains boundaries. The transfer between the macroscopic and mesoscopic scales is realised by imposing the stress, strain and temperature fields, collected during the parent macroscopic simulation, as boundary conditions on the mesosopic cell. Macroscopic experiments, analytical computations and finite element simulations, as well as literature review and microscopic analyses, are used to define the parameters of the material laws. The experimental results and the identification methodology leading to the definition of the set of parameters specific to the studied steel are described. Finally, the influence of oscillation marks and process defects on cracks formation during the industrial process of continuous casting is analysed. The results are compared with in-situ observations and cracking risk indicators computed by the macroscopic model.

The industrial application of high strength steels in structural components has increased the demand on understanding the ductile failure behavior of this type of materials. In practical situations the loading experienced on components made out of these materials can be very complex, which may affect the failure behavior. The objective of this work is to study the effect of stress state on ductile failure and the mechanisms leading to rupture in high strength steels. The stress state is characterized by the stress triaxiality T and the Lode parameter L, which is a deviatoric stress state parameter that discriminates between axisymmetric or shear dominated stress states. For this purpose experiments on two different specimen configurations are performed; a double notched tube (DNT) specimen tested in combined tension and shear and a round notched bar (RNB) specimen tested in uniaxial tension. The two specimens give rise to different stress states at failure in terms of T and L. The failure loci for the DNT specimen show an abrupt change in ductility, indicating a transition between the rupture mechanisms necking of intervoid ligaments and shearing of intervoid ligaments. A clear difference in ductility between the two specimen configurations is also observed, which is closely associated with the difference in stress state at failure. A micromechanical model is developed, which assumes that ductile material failure occurs when the deformation becomes highly non-linear and localizes into a band. The model, which is applied to analyze the experiments, consists of a band with a square array of equally sized cells, with a spherical void located in the center of each cell. The model, extended with a shear criterion, captures the experimental trend rather well. The model also shows that the effect of the deviatoric stress state (L) on void growth, void shape evolution and coalescence is significant, especially at low levels of T and shear dominated stress state. / QC 20100621

The objective of this thesis was to improve a paraffin actuated micropump design, to be able to pump against extreme pressures (above 100 bar). This was accomplished by initially studying the membrane activation, using video capturing. The micropump has been improved to withstand pressures high enough, to enable use in an high-performance liquid chromatography (HPLC) system. The micropump has been shown to pump against back pressures up to 150 bar, with a positive net-flow. This should be compared with the previously recorded maximum back pressure of 50 bar. The pumping against high back pressures was possible due to an increased understanding of the sealing of the membranes. This resulted in a new design that was manufactured and characterised. Without clamping the pump was measured to manage back pressures of 10 bar, and then starting to leak in a bond at the flow channel. With supporting clamping, the managed back pressures increased ten folded. When measured on the different valves, pressure above 200 bar has been possible to withhold. Although the valves were below their maximum limit, the pressure was not possible to be further increased due to a limitation in the equipment, i.e. risk of damaging the connections. When examined after pressurised at extreme pressures (above 100 bar) several times, no signs of fatigue or damage of the membrane was seen. A new behaviour of the valves was discovered. Above certain pressures some designs self sealed, i.e. withholding the pressure after the voltage was turned off. For these valves the pressure had to be released by some other means.

Under loading conditions found in many geotechnical structures, it is common to observe failure in zones of high localized strain called shear bands. Existing models predict these localizations, but provide little insight into the micromechanics within the shear bands. This research captures the variation in microstructure inside and outside of shear bands that were formed in laboratory plane strain and two-dimensional discrete element method (DEM) biaxial compression experiments. Plane strain compression tests were conducted on dry specimens of Ottawa 20-30 sand to calibrate the device, assess global response repeatability, and develop a procedure to quantitatively define the onset of localization. A new methodology was employed to quantify and correct for the additional stresses imparted by the confining membrane in the vicinity of the shear band. Unsheared and sheared specimens of varying dilatancy were solidified using a two-stage resin impregnation procedure. DEM tests were performed using an innovative servo-controlled flexible lateral confinement algorithm to provide additional insights into laboratory results. The solidified specimens were sectioned and the resulting surfaces prepared for microstructure observation using bright field microscopy and morphological analysis. Local void ratio distributions and their statistical properties were determined and compared. Microstructural parameters for subregions in a grid pattern and along predefined inclined zones were also calculated. Virtual surfaces parallel to the shear band were identified and their roughnesses assessed. Similar calculations were performed on the DEM simulations at varying strain levels to characterize the evolution of microstructure with increasing strain. The various observations showed that the mean, standard deviation, and entropy of the local void ratio distributions all increased with increasing strain levels, particularly within regions of high local strains. These results indicate that disorder increases within a shear band and that the soil within the shear band does not adhere to the classical concept of critical state, but reaches a terminal void ratio that is largely a function of initial void ratio. Furthermore, there appears to be a transition zone between the far field and the fully formed shear block, as opposed to an abrupt delineation as traditionally inferred.

The difficulty in developing bolted connection designs lies in the limitations in existing methods to characterize their strength and typically nonlinear response due to the complex interaction of the bolts and structural components. Yet it is necessary for the engineer to be able to determine the three main connection response characteristics: stiffness, strength, and ductility to account for their influence on the overall structural response behavior. The need for better connection response characterization becomes even more crucial in a performance based design approach or when designing partially-restrained moment frames. Several welded moment resisting frame connections were found to have serious failures following the 1994 Northridge earthquake leading to more interest and research on bolted connections as an alternative. In this study a refined three dimensional nonlinear finite element modeling approach to accurately simulate the response of bolted connections is presented. Sensitivity studies of modeling parameters are also performed. A nonlinear response dataset of over 400 connection cases is generated using this approach with a parametric bolted angle connection model. The use of a parametric Richard-Abbott type function and a neural network, calibrated using the response dataset, as practical tool to model the nonlinear stiffness response of bolted connections under monotonic loading is demonstrated and assessed. Failure criteria that can be practically used in conjunction with the refined three dimensional finite element models without any additional modeling requirements are developed. The stress modified critical strain (SMCS) criterion based on the void growth and coalescence mechanism initiating ductile fracture in steel is used for determining failure in the connection member. The bolt failure criterion developed is a mechanics based model using the elliptical interaction of the tensile and shear capacity envelope. The failure criteria and bolted angle response dataset is combined to assess in detail the impact of geometry and topography of the bolted angle connections on the following response characteristics: strength, initial stiffness, plastic stiffness, and absolute ductility or the displacement capacity. Finally, using the dataset of bolted angle connection response, along with their capacities and failure modes determined using the developed failure criteria, the prying strength models in the AISC LRFD Specifications, Eurocode, and a hybrid model are assessed and found to be very conservative for some cases. Based on these results a modified Eurocode and hybrid prying strength model is proposed which greatly improves the prying strength prediction. These prying models are assessed and verified using experimental data found in literature.

Existing numerical techniques for modeling saturated deformable porous media are based on homogenization techniques and thus are incapable of performing micro-mechanical investigations, such as the effect of micro-structure on the deformational characteristics of the media. In this research work, a numerical scheme is developed based on the parallelized hybrid lattice-Boltzmann finite-element method, that is capable of performing micro-mechanical investigations through direct numerical simulations. The method has been used to simulate compression of model saturated porous media made of spheres and cylinders in regular arrangements. Through these simulations it is found that in the limit of small Reynolds number, Capillary number and strain, the deformational behaviour of a real porous media can be recovered through model porous media when the parameters porosity, permeability and bulk compressive modulus are matched between the two media. This finding motivated research in using model porous geometries to represent more complex real porous geometries in order to perform investigations of deformation on the latter. An attempt has been made to apply this technique to the complex geometries of ªfeltº, (a fibrous mat used in paper industries). These investigations lead to new understanding on the effect of fiber diameter on the bulk properties of a fibrous media and subsequently on the deformational behaviour of the media. Further the method has been used to investigate the constitutive relationships in deformable porous media. Particularly the relationship between permeability and porosity during the deformation of the media is investigated. Results show the need of geometry specific investigations.