South Africa, with its world leadership in the mining sector, and well-developed industrial sector, understandably has many tailings dams of various types. South Africa’s tailings dams are among the largest in the world in terms of delivered tailings tonnages, plan size and height. Obviously tailings disposal from the mining and industrial sectors in South Africa can have a major impact on the environment and the safety of human life if the dam design and tailings deposition process are not properly controlled. In South Africa there is a growing awareness of the importance of the environment and of the safety of the tailings dams. Catastrophes like the Merriespruit Gold Tailings dam failure in February 1994, where 17 people died and widespread devastation and environmental damage was caused, has sparked the renewal of research into tailings dam stability and safety. The rate of rise of tailings dams has an influence on the safety and stability of a tailings dam. If rate at which a tailings dam is built is too high, the dam may become unstable and be at risk of failure. There are many factors that control the rate of rise of tailings dams that are not very well understood. This research deals with sedimentation and desiccation of gold mine tailings. Sedimentation and desiccation are factors that influence the rate of rise. This research looked at how the gold mine tailings behave when sedimentation and desiccation occur. This was achieved through laboratory experiments, which consisted of column settling tests and drying box tests, and field tests. A model that predicts the behaviour of sedimentation and desiccation of tailings was also analysed. It was found that tailings sedimentation occurs very quickly. It was also found that suctions play an important role during the desiccation of the tailings. / Dissertation (MEng (Geotechnical Engineering))--University of Pretoria, 2007. / Civil Engineering / MEng / unrestricted

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Le processus d'étalement et d'évaporation d'une goutte de suspensions de particules sur une surface solide est très intéressant permettant la formation de motifs. Une étude expérimentale à été effectuée avec du sang total humain et avec des fluides purs dans une chambre sous atmosphère contrôlée en humidité relative. Pour des angles de contact faible, le processus d'étalement/évaporation peut être divisé en deux régimes. Un premier régime rapide gouverné par un équilibre entre les forces visqueuses et les forces capillaires et un deuxième régime plus lent dominé par la cinétique d'évaporation. Nous montrons que les bio-colloïdes jouent un rôle significatif sur la dynamique de la ligne de contact. La vitesse moyenne de la ligne de contact suit la même dynamique d'étalement que le modèle de Tanner, où le temps d'étalement et les paramètres géométriques de la goutte sont fonctions de l'humidité. Dans cette étude, nous montrons que l'humidité relative influence les paramètres géométriques de la goutte et par conséquent le motif a la fin du processus d'évaporation. Un modèle purement diffusif pur a été obtenu dont le diamètre de mouillage et l'angle de contact sont fonction de l'humidité. Pour l'analyse morphologique des motifs de craquelures, une méthode de segmentation manuelle a été utilisée comme une méthode de référence pour la validation de la méthode de segmentation automatique développée dans "iBlood". Par cette méthode, nous montrons que la cinétique d'évaporation influence la distribution structurelle et morphologique des cellules de forme trapézoïdale, et par conséquent, l'espacement des fractures moyenne finale. / Spreading/evaporation process of droplets over solid surfaces is a fundamental process and a wide research field because of number of applications in printing, micro-electronics, DNA analysis and even in biomedical. This experimental work aims to investigate the effect of relative humidity on the contact line dynamics, on the evaporation dynamics and on the final pattern of a drop of whole human blood. The spreading of a pure fluid model that has the same physical properties as human blood was studied and compared to the blood. We showed that bio-colloids play significant effect on the dynamics of contact line and the pinning effect of the drop. For low contact angles, we showed that the spreading/evaporation process could be divided into two regimes. A fast first regime determined by a balance between viscous forces and capillary forces and a second slower regime dominated by the evaporation rate. Physical mechanisms that are responsible for the spreading enhancement are proposed and discussed. The average velocity of the contact line was found to follow the same behaviour as Tanner's model, where the spreading dynamics and geometrical parameters of the droplet are function of relative humidity. The experimental measurements are in a good agreement with the purely diffusive model where the equilibrium wetting radius and contact angle are function of relative humidity. For the morphological analysis of crack patterns, a manual segmentation method was used as a reference for the validation of the automatic developed segmentation method. We showed that the evaporation rate influences structural distribution of plaques in the corona region and hence, the mean crack spacing.

L'objet de cette thèse est l’étude du comportement des fissures dans le verre. En particulier, l’évaluationde l’effet à long terme des contraintes en compression, est étudié par des essais de fluage. Le dispositifexpérimental utilisé est une presse triaxiale qui permet de confiner des échantillons cylindriques et de leurappliquer des contraintes axiales, une pression de pore ainsi qu’une variation de température. Un réseaude capteurs fixés sur chaque échantillon permet de mesurer la déformation, les vitesses de propagation desondes élastiques et les émissions acoustiques. Les échantillons de verre fournis pour cette étude sont fabriquéspar le CEA de Marcoule. Nous avons observé que le verre sain présente un comportement élastiquefragileet une grande résistance mécanique. Un réseau de fissures est ensuite introduit par choc thermique.L’étude de la microstructure du réseau initial a révélé que le choc thermique induit des contraintes detension aux bords de l’échantillon qui nucléent les fissures. Le réseau introduit est homogène et présenteune symétrie transverse isotrope. La densité de fissure du réseau peut être mesurée à partir d’images demicroscopie et aussi à partir des mesures de vitesses. L’étude de fluage a mis en évidence la propagationsouscritique des fissures. Un modèle théorique, reliant une loi de propagation des fissures à une descriptionen “wing-crack”, décrit bien le comportement expérimental. Une dernière étude est abordée qui met enévidence le rôle des fluides dans la propagation des fissures. À l’échelle du temps du laboratoire on montrealors que l’eau peut accentuer la propagation des fissures et qu’une corrosion chimique est aussi mesurable. / The purpose of this thesis is the study of crack behavior in glass. In particular, the long term effects ofcompressive stresses are observed through creep tests. The used experimental apparatus is a triaxial celldesigned to investigate the mechanical and physical properties of rocks under conditions of effective pressure,deviatoric stress, pore pressure and temperature. A set of sensors allows to measure radial and axialstrain, elastic wave velocities and acoustic emissions. The original boro-silicate glass samples are producedby the French Atomic Energy Commission (CEA). We observe that original glass has an elastic-brittlebehavior and a high mechanical strength. Then, the studied crack network is introduced with a thermalshock. The microstructural study of the initial crack network reveals that the thermal shock induces tensilestresses at sample edges which nucleate cracks. The network is homogeneous and has a transverseisotropic symmetry. The crack density can be measured from Scanning Electron Microscopy images andcan also be inferred from elastic wave velocities. Creep experiments evidenced the sub-critical crack propagation.A theoretical model, that links a crack propagation law to a “wing-crack” geometry, describesthe experimental behavior. A last point is reported that investigates the fluid role in crack behavior. Weobserve that the crack propagation is enhanced by the presence of a pore fluid. Chemical corrosion canalso be measured at our laboratory scale.

Por vezes o comportamento vinculado a fadiga é a principal fonte de dúvidas em projetos de engenharia. Aqui, o crescimento de trincas por fadiga é analisado numericamente utilizando uma plataforma de elementos finitos, onde o problema da fratura é modelado de forma discreta através do uso de elementos coesivos de interface. Os casos estudados abordam estruturas que apresentam uma trinca inicial de tamanho conhecido que propaga por um caminho condicionado. Os problemas são restritos ao estado plano de deformações e sujeitos em sua grande maioria a abertura normal entre as superfícies da trinca. O processo de separação das interfaces da trinca é descrito pelo uso de dois modelos de zona coesiva irreversível. Os modelos possuem relações de tensão-separação que não seguem um caminho pré-definido, sendo dependentes da evolução do dano ligado as propriedades da zona coesiva. Inicialmente são mostradas respostas básicas uniaxiais para os elementos coesivos, que provam a existência de curvas S-N e que o acumulo de dano ocorre de forma não linear. Em seguida, analisa-se crescimento de trincas por fadiga em uma viga do tipo double cantilever beam, onde a estrutura observada é formada pela união de duas chapas metálicas através de um adesivo coesivo que direciona o crescimento da trinca Casos com carga de amplitude única são computados, resultando na representatividade da propagação por uma lei de Paris. Além disso, uma análise de fadiga transiente é realizada através de respostas geradas pela aplicação de sequências de carregamentos em blocos, que mostram a dependência entre o formato do bloco e a maneira a qual ocorre a propagação da trinca. Na sequência, observa-se o efeito escala no crescimento de trincas por fadiga em uma placa submetida a tração com comportamento quase-frágil. Obtém-se uma conexão entre a altura da placa e o formato da ruptura. Por fim, o efeito da plastificação sobre a propagação de trincas, incluindo a captura do efeito crack closure, é comprovado em uma estrutura multicamada que emprega uma malha refinada na ponta da trinca. Os resultados mostram que o modelo computacional implementado reproduz comportamentos semelhantes atingidos nos trabalhos de referência para os problemas analisados. / Sometimes fatigue-related behavior is the main source of doubt in engineering projects. Here, the fatigue crack growth is analyzed numerically using a finite element platform, where the fracture problem is discretely modeled through the use of cohesive interface elements. The cases studied deal with structures that present an initial crack of known size that propagates through a conditioned path. The problems are restricted to plane strain and are mostly subject to the normal opening between the surfaces of the crack. The process of separating the interfaces of the crack is described by the use of two models of irreversible cohesive zone. The models have stress-separation relationships that do not follow a predefined path, being dependent on the evolution of the damage linked to the properties of the cohesive zone. Initially basic uniaxial responses are shown for the cohesive elements, which prove the existence of S-N curves and that the accumulation of damage occurs non-linearly. Then, fatigue crack growth is analyzed in a double cantilever beam, where the observed structure is formed by the union of two metal plates through a cohesive adhesive that directs the growth of the crack Cases with single amplitude loading are computed, resulting in the representativeness of propagation by a Paris law. In addition, a transient fatigue analysis is performed through responses generated by the application of block loading sequences, which show the dependence between the block shape and the manner in which crack propagation occurs. In the sequence, the scale effect on the growth of fatigue cracks in a plate subjected to traction with quasi-fragile behavior is observed. A connection is obtained between the height of the plate and the rupture format. Finally, the plastification effect on crack propagation, including crack closure capture, is demonstrated in a multilayer structure that employs a refined mesh at the crack tip. The results show that the computational model implemented reproduces similar behaviors reached in the reference works for the analyzed problems.

El Método de los Elementos Finitos (MEF) se ha afianzado durante las últimas décadas como una de las técnicas numéricas más utilizadas para resolver una gran variedad de problemas en diferentes áreas de la ingeniería, como por ejemplo, el análisis estructural, análisis térmicos, de fluidos, procesos de fabricación, etc. Una de las aplicaciones donde el método resulta de mayor interés es en el análisis de problemas propios de la Mecánica de la Fractura, facilitando el estudio y evaluación de la integridad estructural de componentes mecánicos, la fiabilidad, y la detección y control de grietas. Recientemente, el desarrollo de nuevas técnicas como el Método Extendido de los Elementos Finitos (XFEM) ha permitido aumentar aún más el potencial del MEF. Dichas técnicas mejoran la descripción de problemas con singularidades, con discontinuidades, etc., mediante la adición de funciones especiales que enriquecen el espacio de la aproximación convencional de elementos finitos. Sin embargo, siempre que se aproxima un problema mediante técnicas numéricas, la solución obtenida presenta discrepancias con respecto al sistema que representa. En las técnicas basadas en la representación discreta del dominio mediante elementos finitos (MEF, XFEM, ...) interesa controlar el denominado error de discretización. En la literatura se pueden encontrar numerosas referencias a técnicas que permiten cuantificar el error en formulaciones convencionales de elementos finitos. No obstante, por ser el XFEM un método relativamente reciente, aún no se han desarrollado suficientemente las técnicas de estimación del error para aproximaciones enriquecidas de elementos finitos. El objetivo de esta Tesis es cuantificar el error de discretización cuando se utilizan aproximaciones enriquecidas del tipo XFEM para representar problemas propios de la Mecánica de la Fractura Elástico Lineal (MFEL), como es el caso del modelado de una grieta. / González Estrada, OA. (2010). Estimación y acotación del error de discretización en el modelado de grietas mediante el método extendido de los elementos finitos [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/7203 / Palancia

This thesis deals with building survey and diagnostics of residential house in Brno, which since its construction showed extensive faults. The theoretical presents to the general issue of building technical surveys, diagnostics reinforced concrete structures and describes methods for diagnosis of reinforced concrete structures. Practical part deals with building survey and diagnostics of the object including static assessment. Overall assessment is given in the conclusion.

This Master’s thesis deals with the influence of process parameters in forward extrusion on the initiation of chevron cracks in material by numerical simulation. Solution was executed by ANSYS 11 and the influence of fiction, angle of reduction, reduction and numbers of extrusion operations was solved for the 12 050.3 steel by the chosen conditions.

The aim of this thesis was to perform the classification of defects of emerging in a specific type of steel castings made by company UNEX Foundry, Ltd. and then determine the conditions of their origin. This is mainly discontinuity of casting material. The next step was to propose the corrective measures eliminating creation of these defects.