Alguns métodos aproximados são sugeridos na literatura relacionada para encontrar a resposta de sistemas com amortecimento não-proporcional. Muitas vezes procura-se estabelecer um critério para aproximar o amortecimento não-proporcional por um modelo de amortecimento proporcional. Neste trabalho foram utilizadas simulações de modelos dinâmicos com três graus de liberdade, com amortecimento não proporcional, a fim de analisar os valores obtidos para as freqüências naturais, estimados a partir dos autovalores resultantes desses modelos. Os cálculos das freqüências naturais e dos amortecimentos modais foram feitos admitindo-se a validade das relações entre estes parâmetros e os autovalores do problema como são bem conhecidas no caso do amortecimento proporcional. Observa-se que, para o caso de amortecimento não-proporcional, este procedimento pode levar a erros significativos na avaliação destes parâmetros. Nos problemas simulados é possível quantificar os erros nas avaliações das freqüências naturais, sendo significativos para fatores de amortecimentos altos. Observa-se que para os fatores de amortecimento não é possível quantificar estes erros, sendo que seus valores são apenas aproximações baseadas na teoria de amortecimento proporcional. Este trabalho apresenta dados que possibilitam uma discussão sobre as diferenças encontradas entre os valores das freqüências naturais e os valores estimados pelas expressões clássicas do amortecimento proporcional. / Some approximate methods are suggested in the related literature to find the output of mechanical systems with non-proportional damping. Often they try to establish a criterion that approximates the non-proportional damping to proportional damping model. In this work, stimulations of dynamics models of three degree of freedom with non-proportional damping were used to examine the values of natural frequencies, estimated from the eigenvalues obtained by these models. The calculations of natural frequencies and modal damping ratio were performed assuming the validity of the relationship between these parameters and the well known eigenvalues of the problem in the proportional damping case. In the simulated problems is possible to quantify the errors in the evaluations of the natural frequencies and this errors are significant for the case where the damping factors are high. It is observed that for the damping factors it is not possible to quantify these errors, and their magnitudes are only approximations based on the theory of proportional damping. This work presents data which enables a discussion about the differences between the magnitude of natural frequencies and the magnitude estimated by the classic equations of proportional damping.

Consideram-se neste trabalho seções bidimensionais oscilando em balanço. Adotam-se modelos representativos do movimento segundo este único grau de liberdade. Estimam-se os parâmetros de inércia e de amortecimento hidrodinâmicos por meio de experimentos de decaimento. Simulações numéricas incorporando os parâmetros estimados experimentalmente, bem como previsões empíricas de resultados da literatura, são confrontadas à registros experimentais para verificação da adequação das previsões, e para avaliação da adequação dos modelos à representação da dinâmica da oscilação. Com a metodologia de análise escolhida, avalia-se a influência de determinados parâmetros sobre as propriedades hidrodinâmicas; consideram-se variações: de proporções da seção, de posição vertical do centro de massa, de inércia própria em balanço, de forma do bojo, e quanto a presença de bolina e suas proporções. Também é considerada a influência do ângulo inicial de banda nas propriedades hidrodinâmicas, bem como a participação da energia irradiada pela onda na energia acumulada pelo sistema anterior ao início do movimento. Os elementos anteriores levam à proposição de orientações para avaliação da influência das propriedades geométricas e inerciais no comportamento do casco em balanço. / The scope of this work is the verification of hydrodynamic properties of two dimensional rolling sections. Analytical models of a single degree-of-freedom movement are adopted. The added inertia and damping are estimated by free decay tests. Numerical simulations with the obtained experimental parameters as well as empirical literature results are compared to the data tests in order to verify the adequacy of the procedures. With the chosen method of analysis, the influence of the hydrodynamic parameters are verified. The following parameters are considered: the beam draft ratio, vertical position of center of gravity, moment inertia, bilge shape and the addition or not of bilge keels. The initial angle of roll is also considered as well as the transport of the energy accumulated before the decay by the wave generated by the movement of the model. These elements lead to the proposition of orientations for the verification of the influence of geometric and inertial properties in the behave of rolling two-dimensional sections.

Devido à necessidade de caracterizar as armadilhas magneto-ópticas (AMO), as quais fornecem amostras densas de átomos frios, e de extrema importância o conhecimento dos valores das constantes mecânicas k e &#946 inerentes ao movimento harmônico super amortecido dos átomos nestas armadilhas. Neste trabalho, obtivemos estas constantes usando a técnica que aplica uma oscilação forçada aos átomos aprisionados através de um pequeno campo magnético oscilante adicional de forma a não perturbar o aprisionamento. Fizemos um estudo destas constantes como função de alguns parâmetros inerentes ao aprisionamento, como por exemplo: intensidade dos feixes laser, gradiente de campo magnético e dessintonia. A manipulação de átomos aprisionados, obtida através da observação de diversas estruturas espaciais dos mesmos, mostra um certo controle sobre estes átomos e a obtenção de valores para a temperatura dos átomos nestas estruturas, permite uma maior compreensão do mecanismo de aprisionamento. Nos detivemos então, na medida da temperatura dos átomos aprisionados em estrutura espacial do tipo anel além de propor um modelo teórico simples capaz de justificar os resultados obtidos. / In order to have precise characterization of magneto-optical traps (MOT), which are used to obtain a dense sample of cold atoms, it is important to measure the mechanical constants k and &#946 of the over-damped harmonic motion that rule the motion of the atoms inside the MOT. In this work, we obtain these constants by applying a small additional a.c. magnetic field that does not perturb the trap d.c. field but forces the trapped atoms to oscillate. We studied the variation of the constants by changing laser intensities, trapping field gradient and detuning. The manipulation of trapped atoms and the observation of several kinds of spatial structures allow one to have a better comprehension of the trapping mechanism. We measured the temperature of the atoms in the ring type structure and a simple theoretical model that explains the results is proposed.

Les structures aérospatiales sont de plus en plus légères, ce qui les rend sensibles aux vibrations. Pour ces structures le frottement pourrait fournir plus d’amortissement que les polymères d’une part à cause des températures extrêmes de fonctionnement (<-20°C et >100°C) et d’autre part parce qu’il est plus efficace que les absorbeurs actifs ou passifs à masse ajoutée qui ne sont pas adaptés aux problèmes sur larges bandes de fréquence, qui comprennent plusieurs fréquences propres. Pour que l’amortissement induit par frottement soit efficace, il faut asservir la charge normale aux amplitudes vibratoires. Le premier objectif de la thèse est de définir un coefficient indicateur de la capacité du changement des forces normales de serrage de vis à contrôler les niveaux vibratoires. Ce coefficient sera un outil de dimensionnement des structures réelles à contrôler. Le deuxième objectif est de concevoir des lois de contrôle semi-actif de la force de serrage pour des régimes libre et forcé d’un problème modal à deux degrés de liberté. L’objectif final est de concevoir un dispositif pouvant s’intégrer à une structure assemblée pour contrôler la charge normale dans les liaisons et de tester expérimentalement l’efficacité des lois de contrôle conçues pour une structure académique ou industrielle. Le contrôle des forces de serrage s’effectue avec l’utilisation des patchs piézoélectriques. / Because of their lightness the aerospace structures are vulnerable to vibrations whose the amplitudes need to be mitigated through the active and the semi active methods. For extreme temperatures (<20oC and > 100oC), Coulomb friction might provide more structural damping than polymers. Nevertheless, there is a consensus to say that these friction-damping values are often lower than 5%. Moreover friction damping is amplitudedependent. Therefore, engineers have to tune the normal load in order to get the maximum damping for the average vibration level. The first objective of the thesis is to define a coefficient indicator of the ability of the tightening force to control the vibration levels. This coefficient is a design tool of real structures witch need controlling. The second objective is to design semi-active control laws of the clamping force for free and forced modal problem of two degrees of freedom. The ultimate goal is to design a device that can be integrated to an assembly structure for controlling the normal load in the joints and then experimentally test the effectiveness of the laws control for an academic or an industrial structure. The tightening control forces is effected with the use of piezoelectric patches.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Robert Bradlee Campbell. / Ph.D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Bibliography: leaf 65. / by Mark Howard Sloan. / B.S.

Naturen inspirerar mig, i detta projekt jobbar jag med strukturen hos trädstammar och bark, och översätter det jag ser till vävda textiler. När jag befinner mig i naturen känner jag mig ofta lugn och kan koppla bort allt annat i min omgivning. Det lugnet vill jag återskapa och föra in i en rumslighet. I barkens olika strukturer såg jag intressanta varierande ytor som jag ville jobba med i textil. I mitt examensarbete har jag designat textilier med uppmätt akustiska egenskaper för offentlig miljö. Genom att testa materialen på SP Sveriges Tekniska Forskningsinstitut, har jag undersökt hur akustiskt dämpande de är. Resultatet har blivit en kollektion med tre vävda textilier med olika utseende och ljudpåverkande funktioner.Nature inspires me, in this project I am working with the structure of tree trunks and bark, and the translation of things I see into woven fabrics. When I am in the nature I often feel calm and relaxed. I can ignore everything else in my environment. I would like to re-create this calm feeling and bring it into a room. In the different bark structures can I see an interesting variety in the shapes witch I would like to work with in textile. In my bachelor degree work I have designed textiles with measured acoustical effects for public spaces. By making samples and test them at SP Technical Research Institute of Sweden, I have examined the acoustic damping of the materials. The result is a collection of three woven fabrics with different appearances and sound affecting functions. / Program: Textildesignutbildningen

Déterminer la durée de vie d'une pièce sous excitation vibratoire est l'un des enjeux majeurs dans l'ingénierie mécanique. Afin de donner une estimation fiable de la durée de vie, un calcul correct de champ de contrainte, qui est fortement lié à la forme et l'amplitude modale est obligatoire. Cependant, le calcul de réponse d'une structure assemblée est difficile, surtout avec la présence de frottement sec aux interfaces de liaisons qui entraîne un amortissement non linéaire. La recherche de la thèse commence par une identification phénoménologique de l'amortissement induit par le frottement sec avec 3 modèles analytiques, notamment la plaque sandwich, la rotule frottante et la plaque von Kármán. Après la caractérisation de l'amortissement structural au 1er et 2ème ordre, les influences des paramètres comme la pression de serrage, le rapport d'épaisseurs ainsi que le nombre de soudures sont analysés. La deuxième partie des travaux traite les problèmes d'amortissement avec une géométrie de dimension finie. La plaque de von Kármán est reprise dans l'étude et une méthode hybride de différences finies et d'éléments finis est adopté pour résoudre le couplage entre la flexion et les efforts internes. Une attention particulière est portée à la notion de modes non linéaires dans le cas de la plaque von Kármán. Les conditions nécessitant l'introduction de modes non linéaires sont identifiées. Le champ de force fictif qui est proposé dans l'étude sur la plaque von Kármán est ensuite introduit dans le cas de la plaque sandwich. Puis la propagation de glissement et l'influence de champ fictif sont étudiées. Afin de vérifier les phénomènes trouvés dans les études théoriques, une installation expérimentale est conçue et montée pour l'amortissement structural au premier ordre. Dans la dernière partie qui est basée sur les observations faites dans les études précédentes, une méthode de calcul de l'amortissement dans un environnement industriel est proposée. Cette méthode permet de donner un amortissement pour chaque mode isolé. / The determination of a component's lifetime under vibrational excitation is one of the most difficult challenges in mechanical engineering. In order to provide a reliable estimation of lifetime, a correct calculation of stress field, which depends on the modal form and its amplitude, is needed. However, the vibrational response calculation on an assembled structure is not easy, especially with the nonlinear structural damping induced by frictional contact surface. The research in the current thesis starts from the phenomenological identification of damping with the help of 3 analytical models, in particular the sandwich plate, rotational joint and von Karman plate. Structural damping of 1st and 2nd order are identified. The influence of parameters like clamping pressure, thickness ratio and number of welding points are also analyzed. The second part of the research focuses on problems with finite dimensions. The von Karman plate serves as the subject of the study and a hydride method which combines FDM and FEM is proposed to solve the coupling between defection and in-plane force field. A special attention is paid to nonlinear mode theory, the conditions under which the nonlinear mode is necessary are identified. The fictive force field proposed in the study of von Kármán plate is then applied to the sandwich plate model. The slipping's propagation as well as the influence of fictive force field are studied. To verify the existence of the found phenomena in the previous studies, an experimental setup is designed and mounted for the 1st order structural damping. In the last part of the research, which is based on the observations previously obtained from academic models, a calculation method of friction-induced damping in structures with complex geometries is proposed for the application in the industrial environment. This methods enables the estimation of damping for each isolated mode.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Bibliography: leaf 60. / by Susan Russell Stapleton. / B.S.

Il est difficile de quantifier les dissipations d'énergie se produisant dans les ouvrages lors d'un séisme de manière réaliste, en particulier dans le cas de structures en béton armé. Ceci s'explique par la diversité des sources de dissipations. Classiquement, elles sont introduites dans la modélisation des structures sous la forme d'un amortissement global peu physique. Le problème qui se pose est de savoir comment prendre en compte l'amortissement de manière plus physique. Ces travaux visent à apporter des éléments de réponse à cette problématique. Pour cela, deux objectifs ont été poursuivis : le premier consiste à qualifier et quantifier expérimentalement les sources d'amortissement dues au béton, le second vise à développer une méthode de calcul, peu couteuse et permettant de modéliser le comportement global mais aussi l'amortissement de manière réaliste. Une campagne d'essais de flexion 3 points alternée a été réalisée afin de quantifier les mécanismes contribuant à l'amortissement. Cette campagne est effectuée sur des poutres saines mais également sur des poutres pré-endommagées. L'analyse des expériences a permis d’identifier différentes grandeurs physiques influençant l’amortissement. Cette campagne a été modélisée à l'aide de trois lois de comportements, une loi s'est révélée pertinente. La loi de comportement ainsi identifiée a permis de développer une modélisation simplifiée, en vue d'études probabilistes. Cette dernière est fondée sur une loi de comportement simple couplée à une actualisation de l'amortissement visqueux. Trois actualisations de l'amortissement ont été développées et confrontées à une campagne d'essais sismiques mono axiaux sur un poteau. / It is hard to determine the energy dissipation that occurs during an earthquake, especially where reinforced concrete structures are concerned. The reason for this is the many different causes of energy dissipation, these dissipations typically creep into the essential pattern of the structures as a uniform, slight damping, and which is heavily quantify. The challenge is therefore to ascertain how to carry out damping in a way that relies more on the laws of physics themselves. This study aims at bringing some clarifications to this problem. In order to achieve this, two objectives were targeted during the case study: the first consisted in experimentally qualifying and quantifying the sources of damping in concrete, the second aims at developing a method which model both the overall behaviour and the damping in a realistic way with low computational costs. Reverse 3-point bending tests were carried out to determine and quantify the mechanisms responsible for damping. The tests were carried out on sound beams, and also on pre-damaged beams. It was possible to relate the damping to the damage, the intensity of the load and the erosion of the crack surfaces. These tests were modeled using three physical constitutive laws, one proved to be relevant. The behaviour law thus identified allowed us to develop a simplified model to be used during probabilistic assessments. This model is based on a simple constitutive law, coupled with the updating of viscous damping. It is carried out according to the evolutions of the mechanical properties of the structure and the load. Three updates of the damping were developed and subjected to a serie of mono axial seismic tests on a column.