The performance of some polymeric materials is profoundly affected by long-term exposure to moisture during service. This poses problems for high precision and/or load bearing components in engineering applications where moisture-induced changes in mechanical properties and dimensional stability could compromise the reliability of the device or structure. In addition to external factors such as moisture, the material properties are also evolving due to inherent structural relaxation within the polymeric material through a process known as physical aging. Based on the current knowledge of both mechanisms, they have opposite effects on material properties. The common approach to studying the effects of moisture is to expose the polymeric material to combined moisture and heat, also referred to as hygrothermal conditions. The application of heat not only increases the rate of moisture diffusion but also accelerates physical aging processes which would otherwise be very slow. In spite of this coupled response, nearly all hygrothermal studies ignore physical aging in their investigations due to the complexity of the coupled problem. The goal of this work is to develop a numerical model for simulating the interactive effects of moisture diffusion and physical aging in a glassy polymer. The intent is to develop a capability that would also allow one to model these effects under various mechanical loading and heat transfer conditions. The study has chosen to model the response of polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS), which is a glassy polymer blend that has very similar behaviour to polycarbonate. In this study, a comprehensive approach which considers four physical mechanisms – structural mechanics, moisture diffusion, heat conduction, and physical aging – has been applied. The most current analytical models in the literature usually attempt to model two or three coupled physical phenomena. To develop the four coupled phenomena model, the current work has undertaken an extensive scope of work involving experimental characterization and finite element modeling. In the experimental part of this work, seven sets of different tests were conducted to characterize the behaviour of PC/ABS exposed to moisture diffusion and accelerated physical aging. These experiments provided a comparative study between the effects of physical aging and moisture diffusion on the material’s behaviour; and at the same time, provided data for the numerical modeling. The dual glass transition temperatures (Tg) of the material were determined using two techniques: dynamic mechanical analysis (DMA) and thermo-mechanical analysis (TMA). The DMA tests provided data for studying the effects of hygrothermal aging on the Tg’s of the material and were also useful for mechanical tests such as creep and stress relaxation performed using the DMA. The Tg’s obtained using the TMA were also required for physical aging experiments using the dilatometry mode of TMA. Structural relaxation of the blend was studied by aging the material at 80 °C for 7 aging times in the TMA. These experiments gave an insight into the volume relaxation behaviour of the blend at a constant temperature. Specific heat capacity of the PC/ABS blend was also measured using another thermal analysis technique; i.e., differential scanning calorimeter (DSC), before and after test specimens were exposed to hygrothermal aging for 168 hours. The interactive effects of physical aging and moisture diffusion on the moisture uptake of the material were studied using gravimetric experiments performed at 5 different hygrothermal conditions. The experimental results were used to determine the coefficient of diffusion as well as the equilibrium moisture uptake of the samples. Furthermore, the effects of both moisture diffusion and physical aging on the mechanical behaviour of the polymer blend were investigated using stress relaxation tests. The comparison of the results of the tests performed on un-aged specimens with those of thermally and hygrothermally aged samples showed how physical aging effects competed with moisture diffusion. Also, the coefficient of hygroscopic expansion of the PC/ABS blend was determined using a so-called TMA/TGA technique. The numerical modeling of the four-coupled physics was achieved using the governing equations in the form of partial differential equations. Modeling was performed using the commercial finite element software package, COMSOL Multiphysics®. First, the uncoupled physical mechanisms of structural mechanics, moisture diffusion, and heat conduction were modeled separately to investigate the validity of the PDEs for each individual phenomenon. The modeling of the coupled physics was undertaken in two parts. The three coupled physics of structural mechanics, moisture diffusion, and heat conduction was first simulated for a gas pipe having a linear elastic behaviour. The comparison of the results with similar analysis available in the literature showed the capability of the developed model for the analysis of the triple coupled mechanisms. The second part modeled the four coupled phenomena by incorporating the experimentally determined coupling coefficients. In the developed numerical model, the material behaviour was considered to be linear viscoelastic, which complicated the model further but provided more realistic results for the behaviour of the polymer blend. Moreover, an approximation method was proposed for estimating the coupling coefficients that exist between different coupled physics in this study. It was also suggested that the anomalous moisture diffusion in the material can be modeled using a time varying boundary condition. Finally, the model was successfully verified and demonstrated using test case studies with thin thermoplastic plates. The proposed four-coupled physics model was able to predict with good accuracy the deflection of thin thermoplastic plates under bending for a set of hygorthermal test condition.

This thesis deals with the problem of hydrogen explosions in nuclear power plants, and evaluates if the reactor hall is to be seen as a safety barrier for such events. Today, the reactor hall is not seen as a safety barrier that is able to withstand an internal explosion. In the analysis Abaqus was used for the FEM calculations, where a main scenario of a wall subjected to a hydrogen explosion was used. In conclusion, the results showed that a reactor hall with the assumed dimensions cannot be seen as a safety barrier, since the deformation after a hydrogen explosion near the LEL was too great. However, it is also concluded that with increased wall and rebar dimensions it is possible to construct a wall of this kind that fulfills the requirements of a safety barrier.

No description available.

<p>  </p> <p>Many engineering structures are designed to withstand a critical mechanical load before failure. When a load greater than the critical load is encountered, the manner of structural failure is important. Nature has been a source of technical inspiration for centuries, and the power of modern scientific investigative techniques has enhanced engineers’ abilities to learn from millennia of evolutionary mechanical refinement. </p> <p>Chitons, a family of marine mollusks, feed on algae attached to rocky substrates, and parts of their feeding organs are subjected to varied loads in the process. In this work, the manner of failure of a chiton’s tooth and supporting structure is investigated, and it is suggested that mechanical details of the structure enable load-limiting and fail-safe performance that protects the animal from potentially dangerous overloading.</p>

Ventiler utgör en avgörande roll i det industriella samhället och används flitigt inom områden som säkerhet, filtrering och reglering av flöden. Säkerställandet av hållbarhet och drift av ventilen är därför ytters viktigt. Ventilen som undersöks har fyra packningsringar gjord av grafit. Grafitringarna leder till ett enklare underhåll, där tidsbesparingen vid isärplockningen av ventilen är 5 timmar jämfört med utan. Andra fördelar med grafitringarna är den starka motstånden mot höga temperaturer, rost och kemikalier. Nackdelen är dess svaga motstånd mot drag- och skjuvspänning. Syfte med studien är att få en ökad förståelse av värmebildningen över ventilen samt den medförande spänning som förekommer. En ökad förståelse av hur spänning bildas i ventilen kan leda till åtgärder som leder till minskad sprickbildning. Målen med studien är att med numeriska metoder och modeller bestämma spänningen vid tre driftfall, min, normal samt max. Spänningen vid grafitpackningen undersöks samt den maximala och minimala spänningen för hela ventilen. Ett kompletterande mål till studien är att undersöka värmeövergångstalen för simuleringar sedan jämföra dessa med empiriska ekvationer. Resultatet visade att den maximala spänningen över grafitområdet ligger på cirka 600 MPa, medan borttagningen av grafitringarna leder till en minskning på 9%. Den maximala spänningen för hela ventilen är lokaliserad på kopplingen mellan inloppet och skalet. Spänningen vid kopplingen är 6000 MPa, vilket är ett orimligt högt värde, då liknande studier visar värden på 200-600 MPa. Borttagning av grafitringarna leder inte till någon skillnad i maximalspänningen för hela ventilen. Från en spänningsaspekt kan ingen fördel med grafitringarna ses. Värmeöverföringstal från COMSOL-simuleringar, vilket använder sig av Navier-Stokes ekvationer, är markant högre vid höga tryck än värmeöverföringstal från empiriska ekvationer. Detta kan tyda på att empiriska ekvationer inte är tillämpbara för de höga tryck och temperaturer som studien undersöker. / Valves play an important role in the industrial society and are widely used in areas such as safety, filtration, and flow regulation. Ensuring the sustainability and operation of the valve is therefore of incredible importance. The studied valve contains four graphite packing rings. The use of graphite rings ensures easier maintenance, resulting in a time saving of 5 hours during disassembly compared to the removal of the graphite rings. Other advantages of graphite rings include their strong resistance to high temperatures, rust, and chemicals. However, their drawback lies in their weak resistance to tensile and shear stress. The purpose of the study is to gain a better understanding of the heat transport across the valve and the accompanying stress that occurs. Increased knowledge of stress formation in the valve can lead to ways to reduce crack formation in the future. The objectives of the study are to use numerical methods and models to determine the stress at three operating conditions, min, normal and max. The stress at the graphite gasket is investigated as well as the maximum and minimum stress for the entire valve. A complementary goal of the study is to investigate the heat transfer coefficients for simulations and compare these with empirical equations. The results show that the maximum stress over the graphite area is approximately 600 MPa, while the removal of graphite rings leads to a 9% reduction. The maximum stress for the entire valve is located at the connection between the inlet and the shell. The stress at the connection is 6000 MPa, which is unreasonably high compared to similar studies that report values of 200-600 MPa. The removal of graphite rings does not affect the maximum stress for the entire valve. From a stress perspective, no advantage of the graphite rings can be found. Heat transfer coefficients from COMSOL simulations, which use Navier-Stokes equations, are significantly higher at high pressures compared to those from empirical equations. This suggests that empirical equations are not applicable for the high pressures and temperatures investigated in the study.

Orientador: Prof. Dr. João Batista de Aguiar / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Mecânica, 2014. / Neste trabalho é desenvolvida a estrutura de uma torre eólica destinada ao território brasileiro em uma montagem com dimensões compatíveis com as realidades técnicas, físico ambientais e sócios econômicas brasileiras. Inicialmente a escolha da potencia eólica é considerada, estudando a mecânica do vento, a influência da topologia, a escolha de materiais disponíveis no Brasil, os processos de fabricação possíveis dentro do território nacional, a escolha de um fabricante de gerador, a escolha de pás ou aerofólios de acordo com a necessidade de projeto. O sistema modular da torre fornece algumas vantagens e benefícios no processo de fabricação, transporte, montagem, manutenção e projeto. No projeto a torre eólica é projetada para dar o máximo de estabilidade em condições extremas. Diferentes critérios de estabilidade são abordados e analisados tanto analiticamente quanto numericamente com a ajuda de diferentes normas (EN, AISC). Para a análise numérica todas as tarefas foram executadas com o pacote de elementos finitos do programa Abaqus¿. O modelo de torre tubular foi primeiramente analisado e verificou-se satisfazer os critérios de estabilidade (flambagem global e a flambagem local). Verificou-se que a torre é segura contra flambagens local e global e permanece estável sob a carga de vento extremo. As metodologias adotadas na concepção da torre eólica buscaram um resultado otimizado com minimização da massa, maximização da rigidez e maximização da frequência natural. Durante a otimização observou-se que o modelo de torre dá um resultado mais econômico se o seu projeto for com espessura variável, módulo a módulo, ou seja, menos material é necessário para a fabricação o qual reduz o custo, porém, o projeto requer a aplicação de amortecedores. Um segundo modelo de torre com espessura constante apesar de usar mais material traz o benefício de não usar amortecedores. / This work addresses the design and analysis of a wind tower destined to the Brazilian territory in an "onshore" assembly, compatible with the physical reality and environmental conditions of Brazilian territory and based in a modular concept. Wind power unit is discussed in the design process from studies of mechanics of wind, influence of topology, choice of materials available in Brazil and manufacturing processes possible within the national territory, leading to the choice of a generator manufacturer, of blades and airfoils according to need for the project. Other issues, such as design of bolt connections plus flanges to assemble the modules are also discussed. The modular system of the tower leads to some advantages and benefits in the manufacturing, transportation, installation, maintenance and design process. In this project the wind tower is first designed to give maximum stability in extreme conditions. Different stability criteria are addressed, both analytically and numerically, with the help of different standards (EN, AISC). Numerical analyses for all tasks were performed with Abaqus¿ finite element package. The tower model satisfied the stability criteria (bending, global buckling and local buckling). The methodologies adopted in the design of the wind tower sought as outcome an optimized, minimum weight, maximum stiffness and higher first natural frequency tower. During the optimization it is observed that the tower model gives a better economical result if variable thicknesses among moduli are allowed, with less need for manufacturing, which reduces material cost. However in this case the project requires the application of dampers. A second tower model with constant thickness even though using more material, has the benefit of dispensing the use of dampers.

O presente trabalho trata da análise numérica de cascas abatidas com o uso do método dos elementos de contorno (MEC). A formulação é desenvolvida a partir do acoplamento entre as equações integrais para flexão de placas delgadas e para estado plano de tensão. No esquema implementado, os termos sobre o contorno são avaliados a partir de processos analíticos e numéricos de integração. No caso das integrais de domínio, aplica-se um procedimento semi-analítico de cálculo sobre células discretas. A validação do modelo computacional desenvolvido é feita com base em resultados da literatura, obtidos com uso do método dos elementos finitos e dos elementos de contorno, além de soluções analíticas. / The present work deals with the numerical analysis of shallow shells using boundary element method (BEM). The formulation is developed by coupling integral equations of plate bending and plane stress elasticity. In the implemented scheme, the boundary terms are evaluated with analytical and numerical processes of integration. In the case of domain integrals, a semi-analytical calculation procedure is applied on discrete cells. The validation of developed computational model is made with results from other works, obtained by use of BEM or finite element method, besides analytical solutions.

Les paliers à feuilles sont des organes de guidage en rotation adaptés pour des rotors légers fonctionnant à des grandes vitesses de rotation. Leur fonctionnement ne nécessite aucun apport d’huile ni de graisse. Ces paliers sont réalisés avec une structure compliante. La présence de frottements dans la structure apporte l’amortissement nécessaire au système rotor-palier.Après une présentation de la technologie, un tour d’horizon des modèles existants et des limitations de chacun d’eux est exposé. Par la suite, un modèle non-linéaire du coussinet compliant est introduit. Dans ce modèle les feuilles sont considérées comme des solides élastiques. Les forces de frottement et les jeux entre les feuilles sont pris en compte par l’utilisation de la condition de non-interférence de Moreau-Signorini. Les efforts normaux et les forces de frottement sont calculés respectivement par la méthode des multiplicateurs de Lagrange augmentés et la méthode des pénalités. Ensuite, cette structure compliante est couplée au film fluide, traité par la résolution de l’équation de Reynolds en conditions de lubrification mixte.L’intérêt est ensuite accordé au palier dans sa globalité : structure déformable, film fluide, rotor. En régime statique, cette étude passe par l’analyse des démarrages du rotor puis de son fonctionnement à des hautes vitesses de rotation et sous de très fortes charges statiques. L’effet des erreurs d’usinage est mis en évidence. En régime dynamique, l’étude se fait par l’analyse non-linéaire d’un rotor de Jeffcott supporté par des paliers à feuilles. Les résultats montrent les limites de stabilité du système rotor-paliers et l’influence du balourd. / The foil bearings are used for guiding and supporting small rotors with high rotational speeds. Their operating functioning does not require any oil or grease. These bearings have a compliant structure. The presence of friction in the compliant structure brings damping, which is necessary for the rotor-bearing system operating at high speeds.After a presentation of the technology, an overview of the existing models and the limitations of each of them is exposed. Subsequently, a nonlinear model of the compliant structure is introduced. In this model, foils are considered as elastic solids. The friction forces and the gaps between the foils are taken into account by using the Moreau-Signorini non-interference condition. Normal forces and friction forces are calculated by using the augmented Lagrange multiplier method and the penalty method. Then, the compliant structure is coupled to the fluid film, dealt with the Reynolds equation for mixed lubrication conditions.The study is then focused on the bearing as a whole : compliant structure, fluid film, rotor. The start-up torque, the lift-off speed as well as operating conditions at high speeds and important static loads are part of the steady regime analyses. The effect of machining errors is highlighted. For the dynamic regime, the study consists of the non-linear analysis of a Jeffcott rotor supported by foil bearings. The results show the stability limits of the rotor-bearing system and the influence of unbalance.

Fluid-structure interaction (FSI), occurrent in many areas of engineering and in the natural world, has been the subject of much research using a wide range of modelling strategies. However, problems with high levels of structural deformation are difficult to resolve and this is particularly the case for biomedical flows. A Lagrangian flow model coupled with a robust model for nonlinear structural mechanics seems a natural candidate since large distortion of the computational geometry is expected. Smoothed particle Hydrodynamics (SPH) has been widely applied for nonlinear interface modelling and this approach is investigated here. Biomedical applications often involve thin flexible structures and a consistent approach for modelling the interaction of fluids with such structures is also required. The Lagrangian weakly compressible SPH method is investigated in its recent delta-SPH form utilising inter-particle density fluxes to improve stability. Particle shifting is also used to maintain particle distributions sufficiently close to uniform to enable stable computation. The use of artificial viscosity is avoided since it introduces unphysical dissipation. First, solid boundary conditions are studied using a channel flow test. Results show that when the particle distribution is allowed to evolve naturally instabilities are observed and deviations are noted from the expected order of accuracy. A parallel development in the SPH group at Manchester has considered SPH in Eulerian form (for different applications). The Eulerian form is applied to the channel flow test resulting in improved accuracy and stability due to the maintenance of a uniform particle distribution. A higher-order accurate boundary model is developed and applied for the Eulerian SPH tests and third-order convergence is achieved. The well documented case of flow past a thin plate is then considered. The immersed boundary method (IBM) is now a natural candidate for the solid boundary. Again, it quickly becomes apparent that the Lagrangian SPH form has limitations in terms of numerical noise arising from anisotropic particle distributions. This corrupts the predicted flow structures for moderate Reynolds numbers (O(102)). Eulerian weakly compressible SPH is applied to the problem with the IBM and is found to give accurate and convergent results without any numerical stability problems (given the time step limitation defined by the Courant condition). Modelling highly flexible structures using the discrete element model is investigated where granular structures are represented as bonded particles. A novel vector-based form (the V-Model) is identified as an attractive approach and developed further for application to solid structures. This is shown to give accurate results for quasi-static and dynamic structural deformation tests. The V-model is applied to the decay of structural vibration in a still fluid modelled using Eulerian SPH with no artificial stabilising techniques. Again, results are in good agreement with predictions of other numerical models. A more demanding case representative of pulsatile flow through a deep leg vein valve is also modelled using the same form of Eulerian SPH. The results are free of numerical noise and complex FSI features are captured such as vortex shedding and non-linear structural deflection. Reasonable agreement is achieved with direct in-vivo observations despite the simplified two-dimensional numerical geometry. A robust, accurate and convergent method has thus been developed, at present for laminar two-dimensional low Reynolds number flows but this may be generalised. In summary a novel robust and convergent FSI model has been established based on Eulerian SPH coupled to the V-Model for large structural deformation. While these developments are in two dimensions the method is readily extendible to three-dimensional, laminar and turbulent flows for a wide range of applications in engineering and the natural world.

O presente trabalho trata da análise numérica de cascas abatidas com o uso do método dos elementos de contorno (MEC). A formulação é desenvolvida a partir do acoplamento entre as equações integrais para flexão de placas delgadas e para estado plano de tensão. No esquema implementado, os termos sobre o contorno são avaliados a partir de processos analíticos e numéricos de integração. No caso das integrais de domínio, aplica-se um procedimento semi-analítico de cálculo sobre células discretas. A validação do modelo computacional desenvolvido é feita com base em resultados da literatura, obtidos com uso do método dos elementos finitos e dos elementos de contorno, além de soluções analíticas. / The present work deals with the numerical analysis of shallow shells using boundary element method (BEM). The formulation is developed by coupling integral equations of plate bending and plane stress elasticity. In the implemented scheme, the boundary terms are evaluated with analytical and numerical processes of integration. In the case of domain integrals, a semi-analytical calculation procedure is applied on discrete cells. The validation of developed computational model is made with results from other works, obtained by use of BEM or finite element method, besides analytical solutions.