We numerically analyze three mechanics problems described below. For each problem, the developed computational model is verified by comparing computed results for example problems with those available in the literature. Effective utilization of single wall carbon nanotubes (SWCNTs) and single layer graphene sheets (SLGSs) as reinforcements in nanocomposites requires their strong binding with the surrounding matrix. An effective technique to enhance this binding is to functionalize SWCNTs and SLGSs by covalent attachment of appropriate chemical groups. However, this damages their pristine structures that may degrade their mechanical properties. Here, we delineate using molecular mechanics simulations effects of covalent functionalization on elastic moduli of these nanomaterials. It is found that Young's modulus and the shear modulus of an SWCNT (SLGS), respectively, decrease by about 34% (73%) and 43% (42%) when 20% (10%) of carbon atoms are functionalized for each of the four functional groups of different polarities studied. A shell theory that gives results close to the solution of the corresponding 3-dimensional problem depends upon the shell geometry, applied loads, and initial and boundary conditions. Here, by using a third order shear and normal deformable theory and the finite element method (FEM), we delineate for a doubly curved shell deformed statically with general tractions and subjected to different boundary conditions effects of geometric parameters on in-plane and transverse stretching and bending deformations. These results should help designers decide when to consider effects of these deformation modes for doubly curved shells. Composite laminates are usually fabricated by curing resin pre-impregnated fiber layers in an autoclave under prescribed temperature and pressure cycles. A challenge is to reduce residual stresses developed during this process and simultaneously minimize the cure cycle time. Here, we use the FEM and a genetic algorithm to find the optimal cycle parameters. It is found that in comparison to the manufacturer's recommended cycle, for a laminate with the span/thickness of 12.5, one optimal cycle reduces residual stresses by 47% and the total cure time from 5 to 4 hours, and another reduces the total cure time to 2 hours and residual stresses by 8%. / Ph. D.

Cylindrical shells are widely used in civil engineering. Examples include cooling towers, pipelines, nuclear containment vessels, steel silos and tanks for storage of bulk solids and liquids, and pressure vessels. The loading condition for these shells is quite varied depending on the function of the shell. Axial compression, global bending, external or internal pressure and wind loading are some of the most common loading forms for realistic structures. The failure of these cylindrical shell structures is often controlled by elastic or elastic-plastic buckling failure. Yield failure may occur in thick cylinders in some situations. A cylindrical shell under different loading conditions may display quite different buckling behaviour. The objective of this thesis is to investigate the characteristics of different buckling behaviours of cylindrical shell structures under axial compression, global bending, uniform external pressure and wind pressure. Some challenging practical problems in the design of these shell structures are explored. This thesis is expected to have some far-reaching impacts in defining how to design cylindrical shell structures to give them adequate strength to resist extreme events. Many aspects will be based on the latest Eurocode (EN 1993-1-6, 2007) and Recommendations (ECCS EDR5, 2008). The results show both some strength and some weaknesses in the Eurocode in design of shell structures. New methods are proposed for some practical problems. Some new conclusions and suggestions are derived and are expected to provide some useful knowledge for the improvement of the Eurocode in cylindrical shell design in general.

The problem with which this investigation is concerned is that of making experimental measurements of proton-induced K-shell x-ray production cross sections and to study the dependence of these cross sections upon the energy of the incident proton. The measurements were made by detection of the characteristic x-rays emitted as a consequence of the ionization of the K-shell of the atom. The method for relating this characteristic x-ray emission to the x-ray production cross section is discussed in this work.

O Método dos Elementos Finitos é a forma mais difundida de análise estrutural numérica, com aplicações nas mais diversas teorias estruturais. Contudo, no estudo das cascas e alguns outros usos, suas deficiências impulsionaram a pesquisa em outros métodos de resolução de Equações Diferenciais Parciais. O presente trabalho utiliza uma dessas alternativas, o Método de Galerkin Livre de Elementos (Element-Free Galerkin) para estudar as cascas. Inicia com a observação da aproximação usada no método, os Moving Least Squares e os Multiple-Fixed Least Squares. A seguir, estabelece uma formulação que combina a teoria de placas moderadamente espessas de Reissner-Mindlin à teoria da Elasticidade Plana e se utiliza da aproximação estudada para analisar placas e chapas deste tipo. Depois, expõe uma teoria geometricamente exata de cascas inicialmente curvas onde as curvaturas iniciais são impostas como deformações livres de tensão a partir de uma configuração de referência plana. Tal teoria exclui a necessidade de coordenadas curvilíneas e consequentemente da utilização de objetos como os símbolos de Cristoffel, já que todas as integrações e imposições são feitas na configuração plana de referência, em um sistema ortonormal de coordenadas. A imposição das condições essenciais de contorno é feita por forma fraca, resultando em um funcional híbrido de deslocamentos que permite a maleabilidade necessária ao uso dos Moving Least Squares. Esse trabalho se propõe a particularizar tal teoria para o caso de pequenos deslocamentos e deformações (linearidade geométrica), mantendo a consistência das definições de tensões e deformações generalizadas enquanto permite uma imposição da forma fraca resultante, depois de discretizada, por um sistema linear de equações. Por fim, exemplos numéricos são usados para discutir sua eficácia e exatidão. / The Finite Element Method is the most spread numerical analysis tool, applied to a wide range of structural theories. However, for the study of shells and other problems, some of its deficiencies have stimulated research in other methods for solving the derived Partial Differential Equations. The present work uses one of those alternatives, the Element Free Galerkin Method, for the study of shells. It begins with the observation of the approximation used in the method, Moving Least Squares and Multiple-Fixed Least Squares. Then, it establishes a formulation that combines the Reissner-Mindlin moderately thick plate theory with plane elasticity, and uses the proponed approximation to analyze such plates and stabs. Afterwards, it demonstrates a geometrically exact shell theory that accounts for initial curvatures as a stress-free deformation from a flat reference configuration. Such theory precludes the use of curvilinear coordinates and, subsequently, the use of objects such as Cristoffel symbols, as all integrations and impositions are done in the flat reference configuration, in an orthogonal frame. The essential boundary conditions are imposed in a eak statement, rendering a hybrid displacement functional that provides the necessary conditions for the use of Moving Least Squares. This works main objective is the particularization of this theory for the small displacement and strains assumption (geometrical linearity), keeping the consistent definition of generalized stresses and strains, while allowing the imposition of the discretized weak form through a system of linear equations. Lastly, numerical simulations are carried out to assess the methods efficiency and accuracy.

Coberturas em casca de concreto apresentam bom desempenho estrutural, desde que a forma de sua superfície média seja adequada para resistir predominantemente com esforços de membrana ao carregamento atuante. Atualmente, é possível utilizar-se métodos numéricos para determinar a forma mais adequada de uma casca para carregamentos de naturezas diversas e várias condições de apoio, projetando-se cascas com forma livre. Uma das grandes dificuldades para aplicação dessas estruturas consiste em compatibilizar um processo executivo adequado para moldagem de formas livres, que têm características geométricas muito complexas. A utilização de processo executivo de construção tipo sanduíche, com concreto ou argamassa projetados, é vista neste trabalho como uma alternativa para a viabilização dessas estruturas. Propõe-se definir procedimentos para o projeto de cascas com forma livre, utilizando-se recursos computacionais para definição da forma e análise da estrutura, e um método construtivo adequado a esse tipo de estrutura. Apresentam-se os resultados de geração de formas livres com métodos numéricos disponíveis, bem como parâmetros que podem auxiliar na aplicação dessas formas livres em projetos de coberturas em casca, baseados em resultados numéricos de análise dessas estruturas. São descritas as experiências de execução de dois protótipos de cobertura em casca, com aplicação de argamassa em núcleo de material leve, posicionado entre telas de aço moldadas na forma de membranas pênseis invertidas. Apresentam-se os resultados obtidos em prova de carga de um dos protótipos executados, bem como comparações desses resultados com os obtidos em modelos numéricos, comprovando-se o bom desempenho da estrutura, mesmo sendo a seção formada por duas faces finas sem conexão rígida. Acredita-se que o procedimento executivo proposto para moldagem de cascas com forma livre pode, com pequenas melhorias, ser utilizado na execução de estruturas semelhantes, mesmo com dimensões maiores, compondo-se, dessa maneira, um processo tecnológico viável. / Concrete shell roofs are very efficient structures when designed to support the loads mainly by membrane forces. Nowadays, there are numerical methods that can be used to determine the suitable shell shape, once it is given the loading and the boundary conditions, with a great variety of these parameters, thus creating the so called free form shells. When applying these shell shapes at practical situations, using conventional concrete construction technology, problems appear because of their very complex geometrical characteristics. Sandwich type construction and shotcrete have been seen in this work as alternatives to solve many construction problems involved with free form shell construction. It is propposed that conceptual design of free form shells, with basis on computational methods, be applied together with a adequate construction method to define a design methodology. Results of the application of numerical methods for shell shape generation are presented, as well as data that can be used when apllying these shell shapes in roof design. Two prototypes were built by applying mortar on ligthweight curved panels fixed between two steel meshs. The results of a loading test performed on one of these prototypes are described and compared with those obtained in a numerical simulation. The prototype proved to be so stiff, beware the fact that the thin faces of the sandwich section did not have rigid shear connections. One believes that there are sufficient data to ensure that the proposed construction method is very suitable to cast free form shell roofs and so it can be used, with few improvements, even on large span structures.

Thesis: Ph. D., Joint Program in Biological Oceanography (Massachusetts Institute of Technology, Department of Biology; and the Woods Hole Oceanographic Institution), 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 159-168). / Thecosome pteropods are planktonic mollusks that form aragonite shells and that may experience increased dissolution and other adverse effects due to ocean acidification. This thesis focuses on assessing the possible biological effects of ocean acidification on the shells and locomotion of pteropods and examining the response of a local pteropod population to environmental change over time. I analyzed shell condition after exposing pteropods to elevated CO₂ as well as in natural populations to investigate the sensitivity of the shells of different species to aragonite saturation state ([omega][subscript A]). The pteropods (Limacina retroversa) from laboratory experiments showed the clearest pattern of shell dissolution in response to decreased [omega][subscript A], while wild populations either had non-significant regional trends in shell condition (Clio pyramidata) or variability in shell condition that did not match expectations due to regional variability in [omega][subscript A] (Limacina helicina). At locations with intermediate [omega][subscript A] (1.5-2.5) the variability seen in L. helicina shell condition might be affected by food availability more than tA. I examined sinking and swimming behaviors in the laboratory in order to investigate a possible fitness effect of ocean acidification on pteropods. The sinking rates of L. retroversa from elevated CO₂ treatments were slower in conjunction with worsened shell condition. These changes could increase their vulnerability to predators in the wild. Swimming ability was mostly unchanged by elevated CO₂ after experiments that were up to three weeks in duration. I used a long-term dataset of pteropods in the Gulf of Maine to directly test whether there has been a population effect of environmental change over the past several decades. I did not observe a population decline between 1977 and 2015, and L. retroversa abundance in the fall actually increased over the time series. Analysis of the habitat use of L. retroversa revealed seasonal associations with temperature, salinity, and bottom depths. The combination of laboratory experiments and field surveys helped to address gaps in knowledge about pteropod ecology and improve our understanding of the effects of ocean acidification on pteropods. / by Alexander Bergan. / Ph. D.

Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERONAUTICS. / Includes bibliographical references. / M.S.

Civil Engineering thin cylindrical shells such as silos and tanks are normally subjected to axial compression that arises from a stored solid, wind, earthquake, self-weight or roof loads. The walls of these shells are very thin, generally of the order of 6 to 25 mm, and massively less than the radius, which is typically 5 to 30 m. They are thus very thin shell structures, like those of rockets, spacecraft, motor vehicles and aircraft. The commonest failure mode is elastic buckling under axial compression. It has long been known that the buckling strength of a thin cylindrical shell under axial compression is very sensitive to tiny deviations of geometry, reducing the buckling strength to perhaps 10 or 20% of the value for the perfect structure. A normal internal pressure usually accompanies the axial compression, caused by stored granular solids or fluids. At relatively low pressures, the elastic buckling strength under axial compression rises, but an elastic-plastic buckling phenomenon intervenes at higher pressures, causing a dramatic decrease in buckling resistance associated with an elephant’s foot collapse mode. To construct such large shells, the fabrication technique is generally the assembly of many rolled plates or panels, joined by short longitudinal welds and continuous circumferential welds. The process of welding produces a distinctive geometric imperfection form at each weld joint, which in turn is extremely detrimental to the shell axial buckling carrying capacity. The strength may be further reduced by slight misalignments between adjacent panels, or in bolted construction, by vertical and horizontal lap splices. Due to the pattern of loading, both the axial compression and internal pressure increase progressively down the wall. Accordingly, practical construction usually uses a stepped wall, formed from panels of uniform thickness, but with larger thicknesses at lower levels. Since the loading varies smoothly, but each panel has constant thickness, the critical location for buckling lies at the base of a panel. But the greater thickness of the lower panel can usefully enhance the buckling strength of the critical panel above it. This thesis presents an extensive computational study that examines all the above influences, divided into chapters that are outlined here. A full exploration of the effect of the cylinder length on the perfect and imperfect elastic buckling strength is presented in Chapter 3. In Chapter 4, the elastic-plastic buckling resistance of imperfect cylinders is described, including strain hardening. These lead to many capacity curves, for which the key parameters are extracted. The effect of internal pressure on the buckling resistance of imperfect elastic cylinders is explored in Chapter 5. Chapter 6 studies the effect of high pressures that produce elastic-plastic elephant’s foot buckling at circumferential welds in imperfect shells. Next, a step change in plate thickness is studied in Chapter 7 for imperfect butt jointed cylinders with and without the internal pressure. Chapter 8 presents an exploration of the effect of plate misalignments at a circumferential joint, as well as the full misalignment of a circumferential lap joint in bolted construction. These are investigated in both the elastic and elastic-plastic domains. The entire thesis is conceived in the context of EN 1993-1-6 (2007) and the ECCS Recommendations on Shell Buckling (2008). This research has shown significant weaknesses in both the concepts and the detailed rules of these standards. Many conditions are found where either the standard is unnecessarily conservative, or its safety margin may be too low. Thus, some new provisions are proposed for each of the above practical problems. These are expected to provide useful knowledge for the design of such structures against buckling in the future.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERONAUTICS. / Vita. / Includes bibliographical references. / Ph.D.

Several problems must be solved in the construction, design, and operation of a nuclear fusion reactor. One of the chief problems in the manufacture of high-powered copper/polymer composite magnets is the difficulty to precisely control the fiber volume fraction. In this thesis, the effect of variations in fiber volume fraction on thermal stresses in copper/cyanate ester composite cylinders is investigated. The cylinder is a composite that uses copper wires that run longitudinally in a cyanate ester resin specifically developed by Composite Technology Development, Inc. This composite cylinder design is commonly used in magnets for nuclear fusion reactors. The application of this research is for magnets that use cylindrical coil geometry such as the Mega Amp Spherical Tokamak (MAST) in the UK. However, most stellarator magnet designs use complex geometries including the National Compact Stellarator Experiment (NCSX), and the Quasi-Poloidal Stellarator (QPS). Even though the actual stresses calculated for the cylindrical geometry may not be directly applicable to these projects, the relationship between fiber volume fraction and stresses will be useful for any geometry. The effect of fiber volume fraction on stresses produced by mechanical, thermal and magnetic loads on cylindrical magnet coils is studied using micromechanics with laminate plate theory (LPT) and finite element analysis (FEA). Based on the findings of this research, variations in volume fraction do significantly affect the stress experienced by the composite cylinder. Over a range of volume fractions from 0.3 to 0.5, the LPT results demonstrate that thermally induced stresses vary approximately 30% while stresses due to pressure vary negligibly. The FEA shows that magnetic stresses vary much less at around only 5%. FEA results seem to confirm the LPT model. It was also concluded that the stress in the insulation layers due to all types of loadings is significant and must be considered when using this system in fusion applications.