23,529 research outputs found
Perturbation theory and the Rayleigh quotient
No full textThe characteristic frequencies ? of the vibrations of an elastic solid subject to boundary conditions of either zero displacement or zero traction are given by the Rayleigh quotient expressed in terms of the corresponding exact eigenfunctions. In problems that can be analytically expanded in a small parameter ?, it is shown that when an approximate eigenfunction is known with an error O(?N), the Rayleigh quotient gives the frequency with an error O(?2N), a gain of N orders. This result generalizes a well-known theorem for N=1. A non-trivial example is presented for N=4, whereby knowledge of the 3rd-order eigenfunction (error being 4th order) gives the eigenvalue with an error that is 8th order; the 6th-order term thus determined provides an unambiguous derivation of the shear coefficient in Timoshenko beam theor
A nonlocal sinusoidal shear deformation beam theory with application to bending, buckling, and vibration of nanobeams
No full textThis paper presents a nonlocal sinusoidal shear deformation beam theory for the bending, buckling, and vibration of nanobeams. The present model is capable of capturing both small scale effect and transverse shear deformation effects of nanobeams, and does not require shear correction factors. Based on the nonlocal differential constitutive relations of Eringen, the equations of motion as well as the boundary conditions of the beam are derived using Hamilton’s principle. Analytical solutions for the deflection, buckling load, and natural frequency are presented for a simply supported beam, and the obtained results are compared with those predicted by the nonlocal Timoshenko beam theory. The comparison firmly establishes that the present beam theory can accurately predict the bending, buckling, and vibration responses of short nanobeams where the small scale and transverse shear deformation effects are significant
Analise de tensoes e deslocamentos atraves de series de Fourier em reservatorios cilindricos apoiados em diafragmas
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina. Centro TecnologicoEste trabalho apresenta uma solução analítica para o problema de cascas cilíndricas finas apoiadas em diafragmas, com carregamentos radiais, usando a formulação mostrada por Timoshenko. São determinadas expressões para calcular deslocamentos e tensões, objetivando possibilitar a análise do comportamento dc cilindros com carregamentos radiais
Shear deformable beams in contact with an elastic half-plane
No full textThe present work deals with the contact problem of a Timoshenko beam bonded to an elastic semi-infinite substrate under different loading conditions. The analysis allows investigating the effects induced by shear compliance of the beam, the stress intensity factors ad the beam edges as well as the singular nature of the interfacial stresses
Square Dancing with the Stars to Enhance Dynamic Hirschman Linkages?
No full textIn this Presidential Address, the author takes the reader on a reconnaissance of his life and time as a regional scientist. He points out scenery he found scintillating along the way, hoping that some may pick up the banner and chew on a few of the ideas for a while. He suggests a revisit to Albert O. Hirschman’s notion of key sectors and more empirical analysis related to Marcus Berliant’s and Masahisa Fujita’s notion of knowledge creation and transfer.Presidential Address, San Antonio, Texas, March 29, 2014 (53rd Meetings of the Southern Regional Science Association
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Comparative analysis between Euler-Bernoulli and Timoshenko beam theories
No full textThis paper seeks to analyze the structural behavior of Euler-Bernoulli and Timoshenko beams, with the main focus of comparing the transverse displacements for the two theories when applied to isostatic beams. The Euler-Bernoulli beam theory assumes that the cross-section remains flat after bending, and remains perpendicular to the longitudinal axis of the beam, while the Timoshenko beam theory considers both bending and shear deformation in the cross-section. For the comparison, the analytical formulations governing the two beam theories were described. With the formulations deduced, analytical results were obtained for Euler-Bernoulli and Timoshenko beams. The transverse displacement values of both theories were compared, analyzing parameters such as percentage difference and length to height ratio. It was found that in simple bi-axial beams with rectangular and T sections, the application of the Euler-Bernoulli theory is satisfactory when the L/h ratio is greater than 10, with percentage differences of 1.40% and 1.36% for each configuration. Similarly, for cantilevered beams, this ratio is limited to 7.58 and 9.09, with differences of 1.497% and 1.571%, respectively. In addition, two formulations of shear coefficients were taken into account when calculating the transverse displacements of the Timoshenko beam. The choice between the formulations depends on the desired accuracy of the displacements. For a deflection check, it is recommended to opt for the simplified coefficient, which has proven to be more conservative compared to the more elaborate coefficient.Este trabalho busca analisar o comportamento estrutural de vigas de Euler-Bernoulli e de Timoshenko, com o principal foco de comparar os deslocamentos transversais para as duas teorias quando aplicadas em vigas isostáticas. Na teoria de vigas de Euler-Bernoulli assume que a seção permanece plana após a flexão, e permanece perpendicular ao eixo longitudinal da viga, enquanto a teoria de viga de Timoshenko considera tanto a deformação por flexão como a por cisalhamento na seção transversal. Para a comparação, realizou-se a descrição das formulações analíticas que regem as duas teorias de vigas. Com as formulações deduzidas, foram obtidos os resultados analíticos para vigas de Euler-Bernoulli e Timoshenko. Foram comparados os valores de deslocamento transversal de ambas as teorias, analisando parâmetros como diferença percentual e razão comprimento e altura. Verificou-se que, em vigas biapoiadas simples com seções retangular e T, a aplicação da teoria de Euler-Bernoulli é satisfatória quando a relação L/h é superior a 10, apresentando diferenças percentuais a 1,40% e 1,36%, para cada configuração. De forma análoga, para vigas engastadas em balanço, essa relação é limitada a 7,58 e 9,09, com diferenças de 1,497% e 1,571%, respectivamente. Além disso, destacou-se a consideração de dois formulações de coeficientes de cisalhamento nos cálculos de deslocamentos transversais da viga de Timoshenko. A escolha entre as formulações depende da precisão desejada nos deslocamentos. Para uma verificação de flecha, é recomendado optar pelo coeficiente simplificado, que se mostrou mais conservador em comparação com o coeficiente mais elaborado
Análise do comportamento dinâmico de rotores em eixos bobinados
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico.Este trabalho tem por objetivo analisar, utilizando o método dos elementos finitos, o comportamento dinâmico de rotores em eixos bobinados. Os rotores são formados por um ou dois discos acoplados à um eixo apoiado em mancais flexíveis. Os eixos são, neste caso, fabricados a partir do bobinamento de fibras impregnadas em resina sobre um mandril. Os discos são considerados rígidos e um elemento do tipo viga com módulo equivalente é utilizado para modelar o eixo do rotor. Um modelo de amortecimento interno é introduzido através de um módulo equivalente amortecido. Inicialmente, são determinadas a resposta a uma massa desbalanceada e as zonas de instabilidade para os modos em flexão de rotores apoiados em mancais com diferentes configurações. Nos modos em torção, são determinadas as freqüências naturais e os respectivos fatores de amortecimento, bem como a resposta em freqüência. Os resultados obtidos com o eixo em carbono/epóxi e em vidro/epóxi, mostraram que o ângulo de bobinagem do eixo pode ter forte influência na localização das freqüências naturais, tanto nos modos em flexão quanto em torção. O amortecimento interno, proveniente do material composto, pode reduzir consideravelmente os picos de amplitudes de vibrações, porém pode gerar instabilidade no sistema. E por fim, com o objetivo de ressaltar as vantagens e a flexibilidade oferecidas pelo uso de materiais compostos em rotores, um método de otimização foi utilizado para minimizar picos de amplitude, evitar zonas de instabilidade e determinar a posição ótima das velocidades críticas, tendo como variáveis de projeto o ângulo de bobinagem, as rigidezes dos mancais e a posição do disco no eixo
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Interaction curves for vibration and buckling of thin-walled composite box beams under axial loads and end moments
No full textInteraction curves for vibration and buckling of thin-walled composite box beams with arbitrary lay-ups under constant axial loads and equal end moments are presented. This model is based on the classical lamination theory, and accounts for all the structural coupling coming from material anisotropy. The governing differential equations are derived from the Hamilton’s principle. The resulting coupling is referred to as triply flexural–torsional coupled vibration and buckling. A displacement-based one-dimensional finite element model with seven degrees of freedoms per node is developed to solve the problem. Numerical results are obtained for thin-walled composite box beams to investigate the effects of axial force, bending moment, fiber orientation on the buckling loads, buckling moments, natural frequencies and corresponding vibration mode shapes as well as axial-moment–frequency interaction curves
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