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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
111

Fundamental study of underfill void formation in flip chip assembly

Lee, Sangil 06 July 2009 (has links)
Flip Chip in Package (FCIP) has been developed to achieve the assembly process with area array interconnects. Particularly, a high I/O count coupled with finer pitch area array interconnects structured FCIP can be achieved using no-flow underfill assembly process. Using the assembly process, a high, stable yield assembly process recently reported with eutectic lead-tin solder interconnections, 150 µm pitch, and I/O counts in excess of 3000. The assembly process reported created a large number of voids among solder interconnects in FCIP. The voids formed among solder interconnections can propagate, grow, and produce defects such as solder joint cracking and solder bridging. Moreover, these voids can severely reduce reliability performance. Indeed, many studies were conducted to examine the void formation in FCIP. Based on the studies, flip chip geometric design, process conditions, and material formulation have been considered as the potential causes of void formation. However, the present research won't be able to identify the mechanism of void formation, causing a lot of voids in assembly process without consideration of chemical reaction in the assembly process with a fine-pitch, high I/O density FCIP. Therefore, this research will present process technology necessary to achieve high yields in FCIP assemblies using no-flow underfills and investigate the underlying problem of underfill void formation in these assemblies. The plausible causes of void formation will be investigated using experimental techniques. The techniques will identify the primary source of the void formation. Besides, theoretical models will be established to predict the number of voids and to explain the growth behavior of voids in the FCIP. The established theoretical models will be verified by experiments. These models will validate with respect to the relationship between process parameters to achieve a high yield and to minimize voids in FCIP assemblies using no-flow underfill materials regarding process as well as material stand points. Eventually, this research provides design guideline achieving a high, stable yield and void-free assembly process.
112

La plénitude ontologique du vide / The ontological fullness of the void

Kopacz, André 17 April 2015 (has links)
Ce titre de plénitude ontologique du vide signifie que le vide n’est pas un néant, un rien ou une absence définitive mais, au contraire, l’affirmation en son autosuffisance de la présence à l’état pur. Le vide est plein de la présence pure, mieux il s’identifie à elle. De l’atomisme antique à la Révolution scientifique en passant par la Renaissance, de Démocrite à Newton via Bruno, d’Aristote à Descartes en tant que ses plus éminentes adversaires, le vide n’a cessé de hanter tant le champ de la métaphysique que celui de la science, celui de l’expérience comme celui de l’imaginaire. Après avoir tenté de l’expulser de la physique au moyen de la notion d’éther, la cosmologie contemporaine redécouvre le rôle essentiel du vide dans le devenir et peut-être l’origine de l’univers. Aujourd’hui, c’est au tour de la métaphysique de soupeser à nouveaux frais le poids du vide dans le cadre de la problématique qui lui est propre. Si le vide, en tant que lieu par excellence de l’exposition de la présence, projette l’ontologie fondamentale vers sa véritable dimension, la question de l’être telle que l’entendait Heidegger prend alors toute son amplitude, non plus mondaine mais cosmologique. C’est alors tous les concepts fondamentaux heideggériens (monde, temporalité, Dasein) qui doivent être réévalué à l’aune de cette nouvelle image de la pensée imposée par la nécessité du vide. L’heideggerianisme indique-t-il ainsi une avancée ou marque-t-il une régression par rapport aux pensées cosmologiques de la métaphysique classique ? Car c’est bien une toute autre image de la pensée que celle des déconstructions qu’il s’agit de fonder, celle où les concepts de présence, de substance, de fondement et même d’Absolu recouvrent toute leur actualité. / The title of ontological fullness of the void means that the void is not a nothingness, a mere nothing or a definitive absence but, on the contrary, an assertion in its self-sufficiency of the presence in the pure state. The void is full of the pure presence, better it is identified with it. From ancient atomism to the scientific Revolution including the Renaissance, from Democritus to Newton via Bruno, from Aristotle to Descartes as their most eminent opponents, the void did not stop haunting both the field of the metaphysics and that of the science, that of the experiment as that of the imagination. Having tried to expel it from the physics by means of the notion of ether, the contemporary cosmology rediscovers the essential role of the void in the future and maybe the origin of the universe. Today, it is in the turn of the metaphysics to feel the weight at new expenses of the weight of the void within the framework of the problem of its own. If the void, as pre-eminently place of the exhibition of the presence, throws the fundamental ontology towards its real dimension, the question of the being such as heard it Heidegger takes then all its amplitude, either worldly but cosmological. It is then all the fundamental heideggerian concepts (world, temporality, Dasein) that must be revalued in the alder of this new image of the thought been imperative by the necessity of the void. Does the heideggerianism so indicate an advance or does it mark a regression with regard to the cosmological thoughts of the classic metaphysics? Because it is well very another image of the thought which that of the deconstructions which it is a question of basing, that where the concepts of presence, substance, foundation and even Absolute recover all their present situation.
113

Vazio intervalar : o espaço do corpo e da roupa

Sinzato, Alice Yumi 28 July 2015 (has links)
Made available in DSpace on 2016-12-08T16:18:56Z (GMT). No. of bitstreams: 1 123646.pdf: 12876862 bytes, checksum: 19128eff46cba6292e5b1fdedd5770d7 (MD5) Previous issue date: 2015-07-28 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / An outlook on the relationship between body, clothing and the world, using ma, interstitial space, as dispositif. This condition of existence is present in Japanese life and art, and is within this research as an activator of poetics for creation and reflection. Ma is an empty interval full of possibilities, void as potentiality, that activates frontiers, hallways and dividers between art, fashion, clothing, performance and architecture. The text os accompanied by sliding images that act as fissures, bodies that are adjacent to what is written. The point of view of ma is that of a foreigner, of someone who is located between one thing and another, and from this position we observe the wearable works of Laura Lima, tracing relations between body, skin, costume and surfaces. / Um olhar sobre a relação entre corpo, roupa e mundo, utilizando como dispositivo o ma, vazio intervalar. Esta condição de existência presente na arte e na vida japonesa se insere na pesquisa como acionador poético para a criação e reflexão. Ma é um intervalo vazio pleno de possibilidades, um vazio como potencialidade, que aciona as fronteiras, corredores e divisórias entre arte, moda, roupa, performance e arquitetura. O texto é acompanhado de imagens deslizantes que agem como frestas, corpos adjacentes à escrita. O olhar do ma é um olhar de estrangeiro, de quem se localiza no intervalo entre uma coisa e outra, e a partir deste posicionamento observamos as obras vestíveis de Laura Lima, traçando relações entre corpo, pele, vestimenta e superfícies.
114

Otimização da determinação de vazios em compósitos híbridos processados por RTM / Voids measurement optimization in hybrid composites processed by RTM

Monticeli, Francisco Maciel [UNESP] 04 July 2017 (has links)
Submitted by FRANCISCO MACIEL MONTICELI null (francisco_monticelli@hotmail.com) on 2017-08-15T16:18:26Z No. of bitstreams: 1 Dissertação Francisco_M_Monticeli.pdf: 22030385 bytes, checksum: 6b57793ae92e8109811a1ce2e8318122 (MD5) / Approved for entry into archive by Monique Sasaki (sayumi_sasaki@hotmail.com) on 2017-08-22T17:27:29Z (GMT) No. of bitstreams: 1 monticeli_fm_me_guara.pdf: 22030385 bytes, checksum: 6b57793ae92e8109811a1ce2e8318122 (MD5) / Made available in DSpace on 2017-08-22T17:27:29Z (GMT). No. of bitstreams: 1 monticeli_fm_me_guara.pdf: 22030385 bytes, checksum: 6b57793ae92e8109811a1ce2e8318122 (MD5) Previous issue date: 2017-07-04 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O compósito híbrido surgiu com o objetivo de reduzir a quantidade de materiais de elevado custo e, ao mesmo tempo, manter as elevadas propriedades mecânicas. Além disso, viu-se a possibilidade de, em se usando reforços diferentes, obter um novo material que evidenciasse as vantagens dos reforços e diminuísse as desvantagens simultaneamente. Um importante fator durante o processamento de compósitos poliméricos para aplicação estrutural é o controle da formação de vazios, pois estes atuam como concentradores de tensão. O objetivo deste trabalho foi produzir compósitos híbridos utilizando diferentes disposições de tecidos de fibra de vidro e carbono, sem perda significativa de propriedades mecânicas. Para a produção do compósito, a pré-forma foi inicialmente caracterizada quanto à impregnação, sendo, para isto desenvolvido um modelo analítico geral que determina o parâmetro de permeabilidade dos compósitos híbridos. O modelo foi validado através de teste de permeabilidade e a qualidade dos laminados (híbridos, e não híbridos) foi certificada pelo processamento dos compósitos e pela quantificação da fração volumétrica de vazios. Este projeto propôs, ainda, a melhoria da análise de vazios pela técnica de porosimetria de Hg com auxílio do planejamento de experimentos. Assim foi possível determinar a fração volumétrica de poros abertos e fechados, a distribuição do diâmetro dos poros e a distância entre os poros dos compósitos, em uma análise conjunta com as técnicas de digestão ácida e microscopia óptica. A pré-forma de carbono apresentou elevada taxa de resistência ao fluxo; por outro lado, observou-se um comportamento oposto para a pré-forma de vidro. Contudo, as pré-formas híbridas apresentaram um efeito híbrido positivo, resultado de uma sinergia que proporcionou um maior valor de permeabilidade. Consequentemente, pode-se conseguir uma otimização do tempo de injeção, considerando uma combinação de tecidos de vidro e carbono equilibrados. O modelo analítico foi capaz de prever o comportamento da frente de fluxo mostrando um valor superestimado de 10%. A técnica de porosimetria de Hg foi validada para análise de poro em compósito avançados com valores próximos obtidos pelas técnicas de digestão ácida e de microscopia óptica. Em função dos resultados obtidos dos valores de diâmetro dos poros, que foi semelhante para todos os compósitos, concluí-se que este ocorre em função do tipo de processo e da resina. Do mesmo modo, a distância e a fração de poros abertos dependem diretamente da quantidade de poros presente no material. Os resultados encontrados indicaram que os compósitos híbridos estudados neste trabalho são materiais promissores para a aplicação aeronáutica, combinando as excelentes propriedades mecânicas da fibra de carbono com a viabilidade do ciclo de injeção da fibra de vidro. Com isso, o laminado híbrido 2 foi o compósito ideal para um processamento com fração de vazios próximo ao uso aeronáutico, resultando em uma redução de custo de matéria prima e tempo de processamento. / Hybrid composite arose with the aim of reducing high cost materials and, at the same time, maintaining mechanical properties suitable for use. In addition, using different reinforcements, a new material could be obtained which would evidence the advantages of the reinforcements and decrease the disadvantages simultaneously. An important factor, during the processing of polymeric composites for structural application, is the voids formation control, since they act as stress concentrators. The aim of this work was to produce hybrid composites using different stacking of glass and carbon fabrics without significant loss of mechanical properties. For the composite manuftacturing, the preform was initially characterized as the impregnation, for which a general analytical model was developed that determines the permeability parameter of the hybrid composites. In addition, the model was validated by conducting permeability test and the quality of several laminates (hybrid, and non-hybrid) was certified by processing them, and voids were quantified thereof. This project also proposed the improvement of voids analysis by the Hg porosimetry technique with the support of design of experimental. Therefore, it was possible to determine the volumetric fraction of open and closed voids, pore diameter distribution and the distance between voids, in an ensemble analysis with acid digestion and optical microscopy. Carbon preform presented high flow resistance; on the other hand, an opposite behavior was observed for the glass preform. The hybrid architecture presented a positive hybrid effect, which means a synergy that provided a higher permeability value. Therefore, optimization in injection time can be achieved, considering a combination of balanced glass and carbon fabrics. The analytical model was enabled to predict the flow front behavior by showing an overestimated value of 10%. The Hg porosimetry technique was validated for advanced composite voids analysis with similar results obtained by acid digestion techniques and optical microscopy. Based on the results obtained from pore diameter values, which were similar for all composites, it was concluded that this occurs as a function of the type of process and the resin. Although, the distance and the open pores fraction depend directly on the amount of pores along the laminate. Hybrid composites have proven to be a promising material in which it combines the excellent mechanical properties of carbon fiber with the viability of the fiberglass injection cycle. With that, hybrid 2 laminate is the ideal composite for a processing with voids fraction close to the aeronautical use requirement with reduction of high-cost material and processing time. / FAPESP: 2015/19967-4
115

Estudo teórico e experimental sobre padrões de escoamento, fração de vazio e perda de pressão durante escoamento bifásico água-ar cruzado ascendente externo a banco de tubos / Theoretical and experimental study on flow pattern, void fraction and pressure drop during air-water two-phase upward crossflow through tube bundles

Fábio Toshio Kanizawa 21 November 2014 (has links)
O presente trabalho envolve um estudo teórico e experimental do escoamento bifásico externo a banco de tubos. Inicialmente, apresenta-se uma ampla revisão da literatura sobre padrões de escoamento, fração de vazio e perda de pressão, durante escoamentos monofásicos e bifásicos externos a banco de tubos. Nesta análise são também descritos os métodos de previsão destes parâmetros. Verificam-se diferenças significativas entre as estimativas proporcionadas por eles, fato que indica a inexistência de métodos generalizados. Posteriormente é apresentada uma descrição detalhada da bancada experimental projetada e construída durante o doutoramento. O aparato completo compõe-se da seção de testes, circuito de água, sistema de compressão e condicionamento de ar, e seções de injeção dos fluxos e condicionamento do escoamento. A seção de testes consiste em um banco de tubos distribuídos segundo configuração triangular normal, com os tubos apresentando diâmetro externo de 19,1 mm, comprimento de 381 mm, e espaçamento transversal de 24 mm. Os experimentos foram realizados para escoamento vertical ascendente de misturas água-ar e velocidades superficiais da fase líquida e gás de 0,020 a 1,500 m/s e de 0,10 a 10,00 m/s, respectivamente. Neste estudo foram desenvolvidas técnicas inéditas para determinação experimental da fração de vazio superficial no interior do banco de tubos baseadas em sistemas óptico e de sensoriamento capacitivo. Os padrões de escoamento foram identificados subjetivamente através de visualização de imagens e vídeos do escoamento, e objetivamente com o auxílio do método de agrupamento de dados k-means utilizando parâmetros baseados no sinal de perda de pressão e do sensoriamento capacitivo. Identificou-se subjetivamente os padrões de escoamento bolhas, bolhas dispersas, bolhas grandes, agitante, intermitente e anular. Constatou-se equivalência entre os padrões de escoamento identificados através dos métodos objetivo e subjetivo. Resultados experimentais para fração de vazio foram obtidos através de técnicas óptica e capacitiva. Constatou-se que o traçador rodamina B utilizado no método óptico altera as condições do escoamento, ainda que em concentrações reduzidas. A partir dos resultados obtidos com o sensoriamento capacitivo estimou-se a fração de vazio para o padrão bolhas. Resultados para a parcela friccional da perda de pressão também foram levantados. Constata-se o incremento da fração de vazio e da parcela friccional da perda de pressão com as velocidades superficiais das fases líquida e gás. Os resultados para fração de vazio foram comparados com métodos de previsão da literatura, e de maneira geral os métodos preveem as tendências dos resultados experimentais apenas para vazões de líquido reduzidas. Analogamente, os resultados para perda de pressão foram comparados com estimativas segundo métodos da literatura, concluindo que os métodos não preveem satisfatoriamente os resultados obtidos. Desta forma, foram propostos novos métodos de previsão para padrões de escoamento, fração de vazio e parcela friccional da perda de pressão, desenvolvidos a partir de análises dos mecanismos dominantes do escoamento, e adotando parâmetros adimensionais para correlacionar os dados. Os métodos propostos preveem satisfatoriamente os resultados experimentais deste estudo e da literatura para escoamentos bifásicos água-ar. / The present thesis concerns a theoretical and experimental study of external two-phase flows across tube bundles. Initially, a comprehensive literature review covering flow patterns, void fraction and pressure drop for single and two-phase flows across tubes bundle is presented. The review also describes predictive methods for these parameters. A comparison of these methods reveals reasonable disagreement among their predictions, indicating the absence of generalized methods. Subsequently, the apparatus and instrumentation designed and built to obtain the experimental data are described. The experimental apparatus comprises the test section, a water loop, air compression and conditioning systems, and sets for fluid flow injections and conditioning. The test section is a normal triangular tube bundle, with 19.1 mm OD tubes, 381 mm long and transversal pitch of 24 mm. The experiments were performed for air-water upward vertical flow, for superficial liquid and gas velocities ranging from 0.020 to 1.500 m/s and 0.10 to 10.00 m/s, respectively. Innovative techniques to evaluate the void fraction within the bundle were developed based on capacitive and optical methods. The flow patterns were identified subjectively and objectively by k-means clustering method, using as clustering parameters the pressure drop and the capacitive signals. Bubbles, dispersed bubbles, large bubbles, churn, intermittent and annular flow patterns were identified subjectively. The data groups identified by the objective method are representative of the flow patterns. Void fraction measurements were obtained for bubbly flow using both techniques (optical and capacitive). The void fraction data based on the optical method had its experimental range limited due to changes in the flow characteristics caused by the addition of the fluorescent dye Rhodamine B. The experimental results indicate that the void fraction increases with increasing the superficial velocities of both phases. In general, the void fraction predictive methods available in the literature capture the trends of the experimental results only for reduced liquid flow rates. According to the experimental results, the frictional pressure drop increases asymptotically with increasing the flow rates of both phases. None of the predictive methods from literature evaluated in the present study predicted satisfactorily the experimental results. Methods for prediction of flow patterns, void fraction and frictional pressure drop parcel were also developed in the present study. These methods provided reasonable predictions of the experimental results obtained in the present study, and also from the literature for air and water flows across tube bundles.
116

L’ image espace : pour une géopoétique du cinéma / Image-Espace : essay on a geopoetic of film

Gaudin, Antoine 05 December 2011 (has links)
L'image-espace constitue un concept pour l'exploration d'une qualité-puissance du cinéma jusqu'ici peu prise en compte. L'espace n'est plus seulement considéré comme un fond, un motif ou un actant, mais, à la fois, comme un problème existentiel primordial et un matériau essentiel de la composition filmique. Il faut pour cela concevoir de nouveaux outils théoriques, moins dépendants de l'héritage pictural ou scénographique comme de la syntaxe propre au découpage classique, et plus étroitement accordés aux puissances fondamentales – cinéplastiques et rythmiques – de l'image filmique. Ce paradigme phénoménologique du film en fonctionnement a pour conséquence que l'espace cinématographique n'est jamais donné comme un objet stable, une forme arrêtée, et qu'il fait au contraire l'objet d'un modelage perpétuel. Cette étude est donc conçue comme une invitation à appréhender autrement les images de films, en fonction d'une construction esthétique dont le principe s'accorderait le mieux possible avec le mode d'être de son objet : c'est-à-dire avec la spécificité du cinéma en tant que mode d'approfondissement sensible de la spatialité de notre existence. Notre proposition de re-spatialisation des procédés expressifs du cinéma sera l'occasion d'articuler un questionnement esthétique (sur la spécificité du médium) à un problème philosophique né de l'expérience sensible (la structure spatiale de l'être-dans-le-monde). La "géopoétique" du film constitue justement une proposition particulière de mobilisation du matériau plastique de l'image-espace en direction d‟une intensification du rapport primordial de l'homme à la Terre qu'il habite et qu'il parcourt. / The issue of space constitutes a fertile starting point for exploring a power of film so far neglected by the classical theory. It seems indeed that one of the major current tasks of film Aesthetics is to construct new concepts in order to think an "image-espace cinema": a cinema that would make the space not just a background, a pattern or an actant, but at the same time, an important existential issue and a critical material of its plastic composition. This requires developing new theoretical tools, less dependent on the pictorial or theatrical categories as on the "grammar" specific to the classic narrative and editing, and more closely linked to the basic powers – cineplastic and rhythmic – of moving pictures. This new paradigm has the consequence that the space film has no permanent substance: it is never given as a fixed form. On the contrary, space is the subject of constant shaping. Therefore this study is designed as an invitation to a different understanding of moving pictures, based on an aesthetic construction whose principle would accord the best with the "mode of being" of its object: that is to say with the specificity of film as a way of deepening our understanding of the spatiality of existence. Our proposition to re-spatialize the traditional means of film expression will just constitute the opportunity to articulate an aesthetic question (on the specificity of film) to a philosophical issue derived of sensory experience (the spatial structure of being-in-the-world). A geopoetic of film consists precisely of a particular way to engage the plastic material of image-espace towards an intensification of the relationship between man and Earth that he roams.
117

Etude de la porosité dans les matériaux composites stratifiés aéronautiques / Voids in aeronautical composite laminates

Ledru, Yohann 14 December 2009 (has links)
La mise en œuvre des préimprégnés unidirectionnels à fibre de carbone et résine époxyde est une étape fondamentale pour la conception de pièces structurales aéronautiques. Elle influe directement sur la qualité de la pièce et notamment sur la quantité de défauts poreux qu’elle renferme. Ainsi, deux phénomènes de création de la porosité ont été identifiés expérimentalement : l’un mécanique, l’autre thermodynamique. Une fois créés, ces défauts peuvent être extraits hors de la pièce si le marouflage et le cycle de polymérisation favorisent cette évacuation gazeuse. Les effets sur la porosité de la température et de la pression imposées pendant la réticulation de la résine sont modélisés en prenant en compte les phénomènes thermo-diffuso-mécaniques entre la résine et la bulle de gaz. Un protocole expérimental reposant sur l’analyse d’image a été développé afin de quantifier le plus précisément possible le taux volumique de porosité, ainsi que leurs tailles et leurs localisations. / Long fiber reinforced epoxy matrix composite laminate manufacturing process is divided into several stages. The most critical one is the polymerization stage. If not optimized, defects in the bulk material such as voids can occur. The aim of this work is to investigate the void formation and evolution processes in order to improve the thermoset laminates quality in minimizing the void ratio. Two phenomena causing void formation have been identified. The first is the mechanical entrapment of gas bubbles between prepreg plies during the lay up. Second is a thermodynamical one. Solvents and humidity absorbed by the prepreg during its manufacturing can be evaporated by increasing the temperature. Then, it has been shown that the vaccum bag lay up permeability in combination with the vaccum pressure could favour the gas washing out. In parallel, thermo-mechanical and diffusion models are coupled to obtain an accurate void size prediction along temperature and pressure applied during the polymerisation. In fact, these two parameters induce variations of the gas bubble radius inside resin. The first experimental results seem to validate qualitatively the calculated void size behaviour. Indeed, hydrostatic pressure imposed during polymerization plays a very important role on gas bubble shrinkage. Finally, a new experimental setup using image analyses has been developed to measure as accurate as possible the volume void ratio. Under specific conditions, stereology allows to extrapolate 2D results to 3D ones. Void ratios obtained with this method are in good agreement with acid digestion results. Complementary morphometric studies on void shapes have given new information about the heterogeneous void distribution in the specimen and also on the statistical void size distribution versus polymerization conditions.
118

The relationship between void ratio and shear wave velocity of gold tailings

Chang, Hsin-Pei Nicol 07 June 2005 (has links)
South Africa, as one of the world’s largest gold producing countries, also generates large amounts of tailings. These tailings are disposed in tailings dams, which pose great threat to the environment in the case of failure, in particular, liquefaction. In order to evaluate the potential of liquefaction, the void ratio of the tailings is required and is often impossible to obtain. Seismic methods allow an indirect method to estimate void ratio of in situ deposits of which tailings are examples of. Currently, the use of seismic methods to estimate void ratio of tailings rely on shear wave velocity – void ratio relationships derived for sands. It is thus uncertain whether this relationship holds for gold tailings, which is classified as a sandy silt or silt. The measurement of shear wave velocity of tailings is done in the laboratory using a triaxial apparatus modified to accommodate bender element. Shear wave velocities are measured using wide square pulses and continuous sinusoidal waves. The results show that there is a near linear relationship between void ratio and shear wave velocity normalized against effective stress. The position of this relationship lies below the previously published results for sands. Shear wave velocity of gold tailings is more sensitive to changes in effective stress than changes in void ratio or over-consolidation ratio. Furthermore, using phase sensitive detection of continuous waves, we can conclude that shear wave velocity of gold tailings is also frequency dependent. / Dissertation (MEng)--University of Pretoria, 2006. / Civil Engineering / MEng / Unrestricted
119

Design adjustment factors and the economical application of concrete flat-slabs with internal spherical voids in South Africa

Marais, Corneille Charles 23 August 2010 (has links)
Long span flat slab systems with internal spherical void formers have been used in Europe for a decade now. Cobiax® is the brand name of a successful system, recently introduced in South Africa. It is a bi-axial reinforced concrete flat slab system, with a grid of internal spherical void formers. The main advantage is the possibility of long spans due to the significant reduction in own weight, as well as the fast construction sequence with the use of flat slab formwork systems. Design requirements of SANS 10100:2000 are affected. Vertical shear capacity is a concern due to loss of aggregate interlock. Research in Germany proved a factor of 0.55 to be a conservative shear resistance reduction factor for Cobiax slabs. Theoretical and preliminary laboratory South African research suggests that a greater factor of 0.85 might be used when considering the shear capacity of the steel cages. These cages’ vertical legs also cross the cold joint caused by the two concrete pours required for Cobiax slabs, and proved to provide sufficient horisontal shear resistance if the correct cage diameters are used. Laboratory tests in Germany supported by theoretical calculations further showed reduced deflections for Cobiax slabs. Although stiffness and own weight are reduced due to the voids, Cobiax slabs had smaller absolute deflections than solid slabs with the same thickness. Cobiax research factors are safe to apply to SANS 10100-01:2000. The economy of Cobiax slabs was tested against that of coffer and post-tensioned slabs. Different span lengths and loads were considered. Based on 2007 material costs in South Africa, Cobiax slabs subject to the same loads and span lengths will be slightly more expensive than that of coffer slabs and post-tensioned slabs when considering only direct slab construction costs. Cobiax will be most appropriate where a flat soffit is required for high multi-storey buildings, requiring large spans with a light load application. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Civil Engineering / unrestricted
120

Effect of Void Fraction on Transverse Shear Modulus of Advanced Unidirectional Composites

Tai, Jui-He 06 October 2016 (has links)
In composite materials, transverse shear modulus is a critical moduli parameter for designing complex composite structures. For dependable mathematical modeling of mechanical behavior of composite materials, an accurate estimate of the moduli parameters is critically important as opposed to estimates of strength parameters where underestimation may lead to a non-optimal design but still would give one a safe one. Although there are mechanical and empirical models available to find transverse shear modulus, they are based on many assumptions. In this work, the model is based on a three-dimensional elastic finite element analysis with multiple cells. To find the shear modulus, appropriate boundary conditions are applied to a three-dimensional representative volume element (RVE). To improve the accuracy of the model, multiple cells of the RVE are used and the value of the transverse shear modulus is calculated by an extrapolation technique that represents a large number of cells. Comparing the available analytical and empirical models to the finite element model from this work shows that for polymeric matrix composites, the estimate of the transverse shear modulus by Halpin-Tsai model had high credibility for lower fiber volume fractions; the Mori-Tanaka model was most accurate for the mid-range fiber volume fractions; and the Elasticity Approach model was most accurate for high fiber volume fractions. Since real-life composites have voids, this study investigated the effect of void fraction on the transverse shear modulus through design of experiment (DOE) statistical analysis. Fiber volume fraction and fiber-to-matrix Young’s moduli ratio were the other influencing parameters used. The results indicate that the fiber volume fraction is the most dominating of the three variables, making up to 96% contribution to the transverse shear modulus. The void content and fiber-to-matrix Young’s moduli ratio have negligible effects. To find how voids themselves influence the shear modulus, the transverse shear modulus was normalized with the corresponding shear modulus with a perfect composite with no voids. As expected, the void content has the largest contribution to the normalized shear modulus of 80%. The fiber volume fraction contributed 12%, and the fiber-to-matrix Young’s moduli ratio contribution was again low. Based on the results of this work, the influences and sensitivities of void content have helped in the development of accurate models for transverse shear modulus, and let us confidently study the influence of fiber-to-matrix Young’s moduli ratio, fiber volume fraction and void content on its value.

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