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1
Aumaitre, Elodie. « Viscoelastic properties of hydrophobin layers ». Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607660.
Texte intégral
2
Ravish, Masti Sarangapany. « Vibration damping analysis of cylindrical shells partially coated with constrained visco-elastic layers ». Hong Kong : University of Hong Kong, 2001. http://sunzi.lib.hku.hk/hkuto/record.jsp?B23000867.
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3
Ravish, Masti Sarangapany. « Vibration damping analysis of cylindrical shells partially coated withconstrained visco-elastic layers ». Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B31242169.
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4
Slanik, Marta. « A numerical and experimental investigation of steel beams damped with constrained viscoelastic layers ». Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0027/MQ50664.pdf.
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5
Sher, Branca Rosa Ribeiro Leite de Sousa. « Optimisation of viscoelastic treatments using genetic algorithms ». Doctoral thesis, Universidade de Aveiro, 2013. http://hdl.handle.net/10773/12431.
Texte intégralRésumé :
Doutoramento em Engenharia MecânicaViscoelastic treatments are one of the most efficient treatments, as far as passive damping is concerned, particularly in the case of thin and light structures. In this type of treatment, part of the strain energy generated in the viscoelastic material is dissipated to the surroundings, in the form of heat. A layer of viscoelastic material is applied to a structure in an unconstrained or constrained configuration, the latter proving to be the most efficient arrangement. This is due to the fact that the relative movement of both the host and constraining layers cause the viscoelastic material to be subjected to a relatively high strain energy. There are studies, however, that claim that the partial application of the viscoelastic material is just as efficient, in terms of economic costs or any other form of treatment application costs. The application of patches of material in specific and selected areas of the structure, thus minimising the extension of damping material, results in an equally efficient treatment. Since the damping mechanism of a viscoelastic material is based on the dissipation of part of the strain energy, the efficiency of the partial treatment can be correlated to the modal strain energy of the structure. Even though the results obtained with this approach in various studies are considered very satisfactory, an optimisation procedure is deemed necessary. In order to obtain optimum solutions, however, time consuming numerical simulations are required. The optimisation process to use the minimum amount of viscoelastic material is based on an evolutionary geometry re-design and calculation of the modal damping, making this procedure computationally costly. To avert this disadvantage, this study uses adaptive layerwise finite elements and applies Genetic Algorithms in the optimisation process.
Os tratamentos viscoelásticos permitem amortecer estruturas finas e leves de uma forma bastante eficiente. Neste tipo de amortecimento passivo, parte da energia de deformação é dissipada pelo material viscoelástico sob a forma de calor. O material viscoelástico é aplicado à superfície de uma estrutura e pode ser, ou não, restringido por uma camada de restrição. Dentro destas duas possibilidades, o tratamento com restrição é o que apresenta maior eficiência. Isto deve-se ao facto de que o movimento relativo das camadas adjacentes impõe uma elevada deformação de corte ao material viscoelástico. De um modo geral, a minimização da extensão da aplicação do material viscoelástico sob a forma de tratamentos parciais localizados torna-se benéfico em termos de custo, quer económico, quer qualquer outra forma de custo associado à aplicação do tratamento. A aplicação de pequenas porções de material sobre áreas específicas e selecionadas torna o tratamento igualmente eficiente, segundo estudos e resultados apresentados por vários autores. Como mencionado anteriormente, o mecanismo de amortecimento do material viscoelástico baseia-se na dissipação de parte da energia de deformação. Este facto permite relacionar a eficiência do tratamento parcial com a energia de deformação modal da estrutura para cada um dos modos naturais. Não obstante os bons resultados obtidos na abordagem desta técnica, este método requer a aplicação de um processo de otimização que conduza a uma solução ótima. Todavia, a simulação numérica deste processo de otimização, exige um elevado custo computacional pois é baseado num processo evolutivo de redesenho da geometria e cálculo do amortecimento modal por forma a utilizar o mínimo de material possível. Baseado nestes pressupostos, este estudo utiliza elementos finitos de camada discreta adaptativos associados a um processo de otimização com base em Algoritmos Genéticos. Este procedimento permite desenvolver um método de otimização de baixo custo computacional e objetivo.
6
Hall, Braydon Day. « The Dynamic Analysis of a Composite Overwrapped Gun Barrel with Constrained Viscoelastic Damping Layers Using the Modal Strain Energy Method ». DigitalCommons@USU, 2013. https://digitalcommons.usu.edu/etd/1972.
Texte intégralRésumé :
The effects of a composite overwrapped gun barrel with viscoelastic damping layers are investigated. Interlaminar stresses and constrained layer damping effects are described. The Modal Strain Energy method is developed for measuring the extent to which the barrel is damped. The equations of motion used in the finite element analysis are derived. The transient solution process is outlined. Decisions for selected parameters are discussed. The results of the finite element analyses are presented using the program written in FORTRAN. The static solution is solved with a constant internal pressure resulting in a calculated loss factor from the Modal Strain Energy Method. The transient solution is solved using the Newmark-Beta method and a variable internal pressure. The analyses conclude that strategically placed viscoelastic layers dissipate strain energy more effectively than a thick single viscoelastic layer. The optimal angle for maximizing the coefficient of mutual influence in a composite cylinder is not necessarily the optimal angle when viscoelastic layers are introduced between layers.
7
Lázaro, Navarro Mario. « The Eigenvalue Problem in Linear Viscoelastic Structures : New Numerical Approaches and the Equivalent Viscous Model ». Doctoral thesis, Universitat Politècnica de València, 2013. http://hdl.handle.net/10251/30062.
Texte intégralRésumé :
El análisis y el control de las vibraciones cobra especial importancia en muchas
ramas de la ingeniería, en especial la ingeniería mecánica, civil, aeronáutica y
automovilística. Tal es así que prácticamente se identi¿ca como un área independiente dentro del análisis dinámico de estructuras. Desde los comienzos de esta
teoría, las fuerzas disipativas o de amortiguamiento han sido uno de los fenómenos
más difíciles de modelizar. El modelo viscoso, por su sencillez y versatilidad ha
sido y sigue siendo el gran paradigma de los modelos de amortiguamiento. Sin embargo, como consecuencia de la aparición de materiales con memoria se introdujo
el fenómeno de la viscoelasticidad; Esta, si bien está también 'íntimamente ligada '
a la velocidad de la respuesta, necesito de la introducción de las denominadas funciones hereditarias, que permiten poner a las fuerzas disipativas como función no
solo de la velocidad instantánea sino de la historia de velocidades desde el comienzo
del movimiento, de ahí el termino memoria. De forma natural, el avance teórico
introducido en el modelo supone también una complicación computacional, pues
donde antes teníamos un sistema lineal de ecuaciones diferenciales ahora tenemos
un sistema de ecuaciones integro-diferenciales.
El análisis de las vibraciones libres de los sistemas con amortiguamiento viscoelástico conduce a un problema nolineal de autovalores donde la característica
principal es una matriz de amortiguamiento que depende de la frecuencia de excitación. El estudio de la solución de autovalores y autovectores de este problema
es importante si se desean conocer los modos de vibración de la estructura o si se
pretende obtener la respuesta en el dominio de la frecuencia del sistema. El objetivo fundamental de esta Tesis Doctoral es doble: Por un lado, profundizar en el
conocimiento del problema de autovalores de sistemas viscoelásticos proponiendo
para ello nuevos métodos numéricos de resolución. Por otro, desarrollar un nuevo
modelo viscoso que, bajo ciertas condiciones, reproduzca la respuesta del modelo
viscoelástico con su¿ciente aproximación.
La Tesis se divide en ocho capítulos, de ellos el cuerpo principal se encuentra en
los seis centrales (Capítulos 2 a 7. Todos ellos son artículos de investigación que,
o bien han sido publicados, o bien están en proceso de revisión en revistas contenidas en el Journal Citation Reports (JCR). Por esta razón, todos los capítulos
conservan la estructura intrínseca de un artículo, incluidas una introducción y una
bibliografía en cada uno.
Los cuatro primeros capítulos (Capítulos 2 a 5) se centran en el estudio del problema no lineal de autovalores. Se proponen dos metodologías de resolución: la
primera es un procedimiento iterativo basado en el esquema del punto-¿jo y desarrollado para sistemas proporcionales o ligeramente no-proporcionales (aquellos
en los que los modos se presentan desacoplados o casi desacoplados). La segunda
metodología (presentada en dos capítulos diferentes), denominada paramétrica,
permite obtener soluciones casi-analíticas de los autovalores, tanto para sistemas
de un grado de libertad como para sistemas de múltiples grados de libertad y
dentro de 'estos, para sistemas proporcionales y no proporcionales. El estudio del
problema de autovalores se completa con un capítulo dedicado a los autovalores
reales, también denominados autovalores no viscosos. En 'él se demuestra una
nueva caracterización maten ática que deben cumplir dichos autovalores y que
permite proponer un nuevo concepto: el conjunto no-viscoso.
Los dos 'últimos capítulos (Capítulos 6 y 7) analizan el Modelo Viscoso Equivalente
como propuesta para la modelización de la respuesta de sistemas viscoelásticos.
El análisis se realiza desde el dominio de la frecuencia estudiando la función de
transferencia. En una primera etapa (pen último capítulo), de naturaleza más
maten ática, se demuestra que la función de transferencia exacta de un modelo viscoelástico se puede expresar como suma de una función de transferencia
propia de un modelo viscoso más un término denominado residual, directamente
dependiente del nivel de amortiguamiento inducido y del acoplamiento modal (noproporcionalidad de la matriz de amortiguamiento). En una segunda etapa ('ultimo
capítulo), se desarrolla una aplicación para estructuras reales formadas por entramados planos de elementos 1D amortiguados con capas de material visco elástico.
Este tipo de estructuras ha permitido usar una variante mejorada del método
paramétrico para la obtención de los autovalores, de forma que en este 'ultimo
capítulo ha servido como nexo de unión de las metodologías más importantes desarrolladas en la Tesis.Lázaro Navarro, M. (2013). The Eigenvalue Problem in Linear Viscoelastic Structures: New Numerical Approaches and the Equivalent Viscous Model [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/30062
TESIS
8
Sibley, David N. « Viscoelastic flows of PTT fluid ». Thesis, University of Bath, 2010. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.518114.
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9
Spears, Mark William. « Microgel-based coatings and their use as self-healing, dynamic substrates for bioapplications ». Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/53060.
Texte intégralRésumé :
Microgels are solvent swollen, cross-linked polymer macromolecules of micro or nanoscale dimensions. In this work, microgels are used as versatile building blocks in layer-by-layer assemblies to form thin coatings. While conceptually simple materials, these microgel-based films actually possess extremely complex behavior as evidenced by two particular areas. First, microgel films have self-healing properties, allowing them to rapidly recover from damage in the presence of solvent. The healing step requires rearrangement of film components, demonstrating the dynamic and mobile nature of the films. Second, fibroblasts display complex behavior on microgel films arising from the properties of the coating. A chemical crosslinking treatment of the film affects the film network structure in a concentration-dependent manner. These network changes result in altered mechanical properties that are the primary controlling factor in determining cell behavior at the interface. These data suggest that fibroblasts are not solely controlled by the film elasticity, but rather by the viscoelasticity, and there is a viscoelastic range that results in maximal cell spreading.
10
Jeung, Yeun S. « Finite element analysis for sandwich structures with a viscoelastic-constrained layer / ». Thesis, Connect to this title online ; UW restricted, 1998. http://hdl.handle.net/1773/9999.
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11
Bateman, Michael John. « Constrained viscoelastic layer damping of thick aluminum plates : design, analysis, and testing ». Thesis, Monterey, California. Naval Postgraduate School, 1990. http://hdl.handle.net/10945/30674.
Texte intégralRésumé :
Approved for public release, distribution is unlimitedModern naval warfare has been increasingly dependent upon the acoustic silencing of the participants. Constrained viscoelastic layer damping of vibrating elements is one method which can be used to meet acoustic silencing goals. This paper considers constrained viscoelastic layer damping treatments applied to a thick aluminum plate, including single layer, double layer, a milled pocket plate, and a milled 'floating element' configuration. High modal damping values were obtained for each configuration. The Modal Strain Energy method, using finite element analysis to estimate modal loss factors, was investigated for use as a tool in constrained viscoelastic layer damping design. A comparison of experimentally measured frequency response and modal loss factors with those predicted by the modal strain energy method is presented to confirm the possible use of the modal strain energy method as a design tool.
12
Gallimore, Craig Allen. « Passive Viscoelastic Constrained Layer Damping Application for a Small Aircraft Landing Gear System ». Thesis, Virginia Tech, 2008. http://hdl.handle.net/10919/35350.
Texte intégralRésumé :
The main purpose of this report was to test several common viscoelastic polymers and identify key attributes of their applicability to a small aircraft landing gear system for improved damping performance. The applied viscoelastic damping treatment to the gear was of a constrained layer type, promoting increased shear deformation over free surface treatments, and therefore enhanced energy dissipation within the viscoelastic layer. A total of eight materials were tested and analyzed using cyclic loading equipment to establish approximate storage modulus and loss factor data at varying loading frequencies. The three viscoelastic polymers having the highest loss factor to shear modulus ratio were chosen and tested using a cantilever beam system. A Ross, Kerwin, and Ungar analysis was used to predict the loss factor of the cantilever beam system with applied treatment and the predictions were compared to experimental data.
Customer requirements often govern the scope and intensity of design in many engineering applications. Limitations and constraints, such as cost, weight, serviceability, landing gear geometry, environmental factors, and manufacturability in regards to the addition of a viscoelastic damping treatment to a landing gear system are discussed.
Based on results found from theoretical and experimental testing, application of a damping treatment to a small aircraft landing gear system is very promising. Relatively high loss factors were seen in a cantilever beam for simple single layer constrained treatments for very low strain amplitudes relative to strains seen during loading of the landing gear. With future design iterations, damping levels several times those seen in this document will be seen with a constrained treatment applied to a landing gear system.Master of Science
13
Ping, Hsin-Chih. « Passive vibration control of thick aluminum plates using viscoelastic layered damping ». Thesis, Monterey, California : Naval Postgraduate School, 1990. http://handle.dtic.mil/100.2/ADA243990.
Texte intégralRésumé :
Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, December 1990.Thesis Advisor(s): Shin, Y.S. ; Kim, K.S. "December 1990." Description based on title screen as viewed on March 30, 2010. DTIC Identifier(s): Acoustics, damping, ship silencing, MSE (modal strain energy), constrained viscoeleastic layer method, aluminum plates, milled pocket plate, floating element plate. Author(s) subject terms: Vibration, damping, viscoelastic material, constrained layer damping. Includes bibliographical references (p. 149). Also available in print.
14
Ahmad, Naveed. « Passive Damping in Stiffened Structures Using Viscoelastic Polymers ». Diss., Virginia Tech, 2016. http://hdl.handle.net/10919/79566.
Texte intégralRésumé :
Noise and vibration suppression is an important aspect in the design process of structures and machines. Undesirable vibrations can cause fatigue in a structure and are, therefore, a risk to the safety of a structure. One of the most effective and widely used methods of mitigating these unwanted vibrations from a system is passive damping, by using a viscoelastic material. This dissertation will primarily focus on constrained layer passive damping treatments in structures and the investigation of associated complex modes. The key idea behind constrained damping treatment is to increase damping as affected by the presence of a highly damped core layer vibrating mainly in shear. Our main goal was to incorporate viscoelastic material in a thick stiffened panel with plate-strip stiffeners, to enhance the damping characteristics of the structure.
First, we investigated complex damped modes in beams in the presence of a viscoelastic layer sandwiched between two elastic layers. The problem was solved using two approaches, (1) Rayleigh beam theory and analyzed using the principle of virtual work, and (2) by using 2D plane stress elasticity based finite-element method. The damping in the viscoelastic material was modeled using the complex modulus approach. We used FEM without any kinematic assumptions for the transverse shear in both the core and elastic layers. Moreover, numerical examples were studied, by including complex modulus in the base and constraining layers. The loss factor was calculated by modal strain energy method, and by solving a complex eigenvalue problem. The efficiency of the modal strain energy method was tested for different loss factors in the core layer. Complex mode shapes of the beam were also examined in the study, and a comparison was made between viscoelastically damped and non-proportionally damped structures.
Secondly, we studied the free vibration response of an integrally stiffened and/or stepped plate. The stiffeners used here were plate-strip stiffeners, unlike the rib stiffeners often investigated by researchers. Both plate and stiffeners were analyzed using the first-order shear deformation theory. The deflections and rotations were assumed as a product of Timoshenko beam functions, chosen appropriately according to the given boundary conditions. Unlike Navier and Levy solution techniques, the approach used here can also be applied to fully clamped, free and cantilever supported stiffened plates. The governing differential equations were solved using the Rayleigh-Ritz method. The development of the stiffness and the mass matrices in the Ritz analysis was found to consume a huge amount of CPU time due to the recursive integration of Timoshenko beam functions. An approach is suggested to greatly decrease this amount of CPU time, by replacing the recursive integration in a loop structure in the computer program, with the analytical integration of the integrand in the loop. The numerical results were compared with the exact solutions available in the literature and the commercially available finite-element software ABAQUS. Some parametric studies were carried out to show the influence of certain important parameters on the overall natural frequencies of the stiffened plate.
Finally, we investigated the damped response of an adhesively bonded plate employing plate-strip stiffeners, using FSDT for both the plate and stiffeners. The problem was analyzed using the principle of virtual work. At first, we did not consider damping in the adhesive in order to validate our code, by comparing our results with those available in the literature as well as with the results obtained using ABAQUS 3D model. The results were found to be highly satisfactory. We also considered the effect of changing the stiffness of the adhesive layer on the vibration of the bonded system. As a second step, we included damping in the stiffened structure using complex modulus approach, a widely used technique to represent the rheology of the viscoelastic material. We observed an overall increase in the natural frequencies of the system, due to the damping provided by the viscoelastic material. Moreover, it was noticed that when the thickness of the adhesive layer is increased, the natural frequencies and loss factor of the stiffened structure decrease. A viscoelastic material with high loss factor and small thickness will be a perfect design variable to obtain overall high damping in the structure.Ph. D.
15
Meddeb, Firas. « Caractérisation du comportement d'un composite biosourcé intégrant une couche viscoélastique ». Electronic Thesis or Diss., Le Mans, 2024. http://www.theses.fr/2024LEMA1032.
Texte intégralRésumé :
Les matériaux composites biosourcés sont de plus en plus utilisés dans différents secteurs pour leurs intérêts, notamment, environnementaux. L’objectif de cette thèse est l'étude de l'effet de l'intégration d'un matériau fonctionnel, une couche viscoélastique sur le comportement mécanique (en statique, en fatigue, en vibration et à l’impact ; température) d'un composite biosourcé. Les matériaux considérés sont constitués de fibres courtes de lin associées à un biopolymère Acide Poly-Lactique (PLA). Une couche viscoélastique en polyuréthane thermoplastique (TPU) est intégrée à ces composites. Ils sont élaborés par la fabrication additive en impression 3D. Tout d’abord, les caractéristiques élastiques et à la rupture du composite Lin/PLA et des composites avec couches viscoélastiques sont déterminées en traction et en flexion. Ensuite, une analyse approfondie de ses caractéristiques en fonction de l’épaisseur et de la séquence d’empilement des couches viscoélastiques est réalisée en statique, en fatigue et en vibration. Le comportement en statique est ici analysé, en mettant l’accent sur l’efficacité de l’intégration de la couche viscoélastique. Par la suite, des essais de fatigue jusqu'à rupture et des essais cycliques avec paliers de chargement sont réalisés. L'évolution de la raideur et du coefficient d'amortissement en fonction du nombre de cycles et des conditions de chargement est analysée pour les composites avec et sans couche viscoélastique. Les essais vibratoires sont réalisés afin d’analyser les propriétés modales des composites. Le module de Young et le facteur de perte sont déterminés aux pics de résonance en fonction de la fréquence et ont permis d’évaluer l’efficacité de la couche viscoélastique en termes d’amortissement des vibrations. L’effet de la variation de la température sur le comportement mécanique de tous les composites est réalisé en statique, en fatigue et vibration. Les résultats obtenus des différents essais montrent que les caractéristiques mécaniques dépendent fortement de la température. Cet effet est beaucoup plus prononcé dans les composites en présence de couches viscoélastiques. Une dernière partie de ce travail est relative à l’étude de l’effet de la résistance à l'impact des composites sous différents niveaux d’énergies. L'analyse des dommages internes et externes montre que les composites avec couches viscoélastiques présentent une meilleure capacité d'absorption de l'énergie malgré une déformation plus importante que celle des composites sans couches viscoélastiquesBio-based composites are increasingly used in various sectors due to their environmental benefits. The objective of this thesis is to study the effect of integrating a functional material, a viscoelastic layer, on the mechanical behavior (in static, fatigue, vibration, and impact; temperature) of a bio-based composite. The materials considered consist of short flax fibers combined with a polylactic acid (PLA) biopolymer. A viscoelastic layer of thermoplastic polyurethane (TPU) is integrated into these composites. They are produced through additive manufacturing using 3D printing. First, the elastic and fracture characteristics of the flax/PLA composite and the composites with viscoelastic layers are determined in tension and bending. Then, an in-depth analysis of their characteristics based on the thickness and stacking sequence of the viscoelastic layers is conducted in static, fatigue, and vibration scenarios. The static behavior is analyzed, focusing on the effectiveness of integrating the viscoelastic layer. Next, fatigue tests up to failure and cyclic tests with loading increments are performed. The evolution of stiffness and damping coefficient as a function of the number of cycles and loading conditions is analyzed for composites with and without viscoelastic layers. Vibration tests are conducted to analyze the modal properties of the composites. The Young's modulus and loss factor are determined at resonance peaks based on frequency, allowing evaluation of the effectiveness of the viscoelastic layer in damping vibrations. The effect of temperature variation on the mechanical behavior of all composites is studied in static, fatigue, and vibration scenarios. The results of the various tests show that the mechanical characteristics are highly dependent on temperature. This effect is more pronounced in composites with viscoelastic layers. The final part of this work examines the impact resistance of the composites under different energy levels. Analysis of internal and external damage shows that composites with viscoelastic layers have better energy absorption capacity but higher deformation levels compared to composites without viscoelastic layers
16
Mu, Rui Lin. « Improvement of Sound Insulation Performance of Multi-layer Structures in Buildings ». 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/174914.
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John, Jo-Anne Louise. « An investigation into compliance and the rotating disc ». Thesis, University of Birmingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.369721.
Texte intégral
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Austin, Eric M. « Influences of Higher Order Modeling Techniques on the Analysis of Layered Viscoelastic Damping Treatments ». Diss., Virginia Tech, 1998. http://hdl.handle.net/10919/29307.
Texte intégralRésumé :
Much of the work done on active and passive constrained layer beams is done with mathematical models proposed by Kerwin and extended by DiTaranto, Mead and Markus, and others. The mathematics proposed by these early researchers was tailored to fit the damping treatments in use at that time: thin foil damping tapes applied to panels for noise reduction. A key assumption was that all layers had identical transverse displacements. While these assumptions are reasonable when the core layer, normally a soft viscoelastic material(VEM), is thin and the constraining layer is weak in bending, there are many situations in industry and in the literature where the ``Mead and Markus'' (MM) assumptions should be questioned. An important consequence of the MM modeling assumptions is that the strain energy in the VEM core is dominated by shear strain, and this in turn means that only the shear modulus is of primary importance. This is fortunate since only the shear modulus is available to engineers for viscoelastic materials used for layered damping treatments. It is a common practice in industry and academia to simply make an educated guess of the value of Poisson's ratio. It is shown in the dissertation that this can result in erroneous predictions of damping, particularly in partial-coverage configurations. Finite element analysis is used to model both the MM assumptions and a less-restrictive approach commonly used in industry. Predictions of damping from these models are compared against models with elements from C0 elements and a C1-capable element that matches tractions at material interfaces. It is shown that the time-honored modal strain energy method is a good indicator of modeling accuracy. To assess the effects of the MM assumptions on an active PZT used as a constraining layer, closed-loop damping versus gain is determined using both the MM and higher order elements. For these analyses, the time-dependent properties of the viscoelastic material are represented by a Maxwell model using internal variables. Finally, the basic MM premise that all layers share the same transverse displacement is disproved by experiment.Ph. D.
19
Verstappen, André Paul. « Passive damping treatments for controlling vibration in isotropic and orthotropic structural materials ». Thesis, University of Canterbury. Mechanical Engineering, 2015. http://hdl.handle.net/10092/10197.
Texte intégralRésumé :
The structural vibration damping behaviour of plates and beams can be improved by the application of viscoelastic passive damping materials. Unconstrained layer damping treatments applied to metal plate systems were studied experimentally. Design and modelling of novel fibre reinforced constrained layer damping materials was performed, and implementation of these composite damping materials into laminated composite sandwich constructions commonly used as structural elements within large composite marine vessels was explored. These studies established effective methods for examining, designing and applying damping materials to metal and composite marine structures.
Two test fixtures were designed and constructed to facilitate testing of viscoelastic material damping properties to ISO 6721-3 and ASTM E756. Values of material damping made in accordance with ASTM E756 over a range of temperatures were compared to values produced by a Dynamic Mechanical Analyser (DMA). Glass transition temperatures and peak damping values were found to agree well, although results deviated significantly at temperatures above the glass transition temperature.
The relative influence of damping layer thickness, ambient temperature, edge conditions, plate dimensions and substrate material on the system damping performance of metal plates treated with an unconstrained viscoelastic layer was investigated experimentally. This investigation found that substrate material had the greatest influence on system damping performance, followed by damping layer thickness and plate size. Plate edge conditions were found to have little influence on the measured system damping performance. These results were dependent on the values of each variable used in the study.
Modal damping behaviour of a novel fibre reinforced composite constrained layer damping material was investigated using finite element analysis and experimental methods. The material consisted of two carbon fibre reinforced polymer (CFRP) layers surrounding a viscoelastic core. Opposing complex sinusoidal fibre patterns in the CFRP face sheets were used to achieve stress-coupling by way of orthotropic anisotopy about the core. A finite element model was developed in MATLAB to determine the modal damping, displacement, stress, and strain behaviour of these complex patterned fibre constrained layer damping (CPF-CLD) materials. This model was validated using experimental results produced by modal damping measurements on CPF-CLD beam test specimens. Studies of multiple fibre pattern arrangements found that fibre pattern properties and the resulting localised material property distributions influenced modal damping performance.
Inclusion of CPF-CLD materials in laminated composite sandwich geometries commonly used in marine hull and bulkhead constructions was studied experimentally. Composite sandwich beam test specimens were fabricated using materials and techniques frequently used in industry. It was found that the greatest increases in modal damping performance were achieved when the CPF-CLD materials were applied to bulkhead geometries, and were inserted within the sandwich structure, rather than being attached to the surface.
20
Fares, Nabil F. (Nabil Fares). « Green's functions for plane-layered elastostatic and viscoelastic regions with application to 3-D crack analysis ». Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/14632.
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Coggins, M. Denise. « Bi-layered viscoelastic model for a step change in velocity and a constant acceleration stimulus for the human otolith organs ». Thesis, This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-02132009-172020/.
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Nair, Aravind R. « Characterization of thermo-mechanical and long-term behaviors of multi-layered composite materials ». [College Station, Tex. : Texas A&M University, 2006. http://hdl.handle.net/1969.1/ETD-TAMU-1821.
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Votsios, Vasilis. « Contact mechanics and impact dynamics of non-conforming elastic and viscoelastic semi-infinite or thin bonded layered solids ». Thesis, Loughborough University, 2003. https://dspace.lboro.ac.uk/2134/11815.
Texte intégralRésumé :
The thesis is concerned with the contact mechanics behaviour of non-conforming solids. The geometry of the solids considered gives rise to various contact configurations, from concentrated contacts with circular and elliptical configuration to those of finite line nature, as well as those of less concentrated form such as circular flat punches. The radii of curvature of mating bodies in contact or impact give rise to these various nonconforming contact configurations and affect their contact characteristics, from those considered as semi-infinite solids in accord with the classical Hertzian theory to those that deviate from it. Furthermore, layered solids have been considered, some with higher elastic modulus than that of the substrate material (such as hard protective coatings) and some with low elastic moduli, often employed as tribological coatings (such as solid lubricants). Other bonded layered solids behave in viscoelastic manner, with creep relaxation behaviour under load, and are often used to dampen structural vibration upon impact. Analytic models have been developed for all these solids to predict their contact and impact behaviour and obtain pressure distribution, footprint shape and deformation under both elastostatic and transient dynamic conditions. Only few solutions for thin bonded layered elastic solids have been reported for elastostatic analysis. The analytical model developed in this thesis is in accord with those reported in the literature and is extended to the case of impact of balls, and employed for a number of practical applications. The elastostatic impact of a roller against a semi-infinite elastic half-space is also treated by analytic means, which has not been reported in literature. Two and three-dimensional finite element models have been developed and compared with all the derived analytic methods, and good agreement found in all cases. The finite element approach used has been made into a generic tool for all the contact configurations, elastic and viscoelastic. The physics of the contact mechanical problems is fully explained by analytic, numerical and supporting experimentation and agreement found between all these approaches to a high level of conformance. This level of agreement, the development of various analytical impact models for layered solids and finite line configuration, and the development of a multi-layered viscoelastic transducer with agreed numerical predictions account for the main contributions to knowledge. There are a significant number of findings within the thesis, but the major findings relate to the protective nature of hard coatings and high modulus bonded layered solids, and the verified viscoelastic behaviour of low elastic modulus compressible thin bonded layers. Most importantly, the thesis has created a rational framework for contact/impact of solids of low contact contiguity.
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Lee, Hsing-Juin. « Determination of the complex modulus of a solid propellant and random vibration analysis of the layered viscoelastic cylinders with finite element method ». Diss., Virginia Polytechnic Institute and State University, 1987. http://hdl.handle.net/10919/77816.
Texte intégralRésumé :
Aeronautical structures, such as aircraft or missiles, are usually highly sophisticated systems often subjected to random vibration environment. Thus, in various design, development, and production stages, laboratory random vibration testing of sampled solid rocket motors on electromagnetic or hydraulic shakers are routinely performed as an important experiment-oriented quality control strategy. Nevertheless, it is crucial to understand the dynamic structural behavior of these layered viscoelastic cylinders such as solid rocket motors under random vibration tests analytically.
In this study, a methodology has been developed to deal with the random vibration of a general class of composite structures with frequency-dependent viscoelastic material properties as represented by the example of solid rocket motors. The method combines the finite element method, structural dynamics, strain energy approach, and random vibration analysis concepts. The method is a more powerful technique capable of treating sophisticated random vibration problems with complicated geometry, nonhomogeneous materials, and frequency-dependent stiffness and damping properties.
Before the random vibration analysis could proceed, a microcomputer-based dynamic mechanical analyzer system was used together with time-temperature superposition principle to obtain the frequency-dependent dynamic viscoelastic properties of the solid propellant. The strain energy approach has been used to calculate the frequency-dependent equivalent viscoelastic damping which is in turn judiciously represented by a combination of viscous damping and structural damping to accommodate this frequency dependent material property.
Modal analysis data together with half power band width calculated at each natural frequency are highly useful guides in the harmonic analysis to achieve computational efficiency. On one hand, the technique used in this study has a hybrid taste in the sense that it makes use of best features and capabilities of both modal analysis and harmonic analysis to achieve the goal of random vibration analysis in addition to the power of finite element technique. The displacement, acceleration and stress power spectra have been obtained for significant points on the rocket model together with their root mean square values. These data can be used for various analyses, testing, design, and other purposes as discussed in later sections of this study.Ph. D.
25
Nguyen, Trung Dung. « Experimental and numerical investigation of strain-rate dependent mechanical properties of single living cells ». Thesis, Queensland University of Technology, 2015. https://eprints.qut.edu.au/82791/1/Trung%20Dung_Nguyen_Thesis.pdf.
Texte intégralRésumé :
The objective of this project is to investigate the strain-rate dependent mechanical behaviour of single living cells using both experimental and numerical techniques. The results revealed that living cells behave as porohyperlastic materials and that both solid and fluid phases within the cells play important roles in their mechanical responses. The research reported in this thesis provides a better understanding of the mechanisms underlying the cellular responses to external mechanical loadings and of the process of mechanical signal transduction in living cells. It would help us to enhance knowledge of and insight into the role of mechanical forces in supporting tissue regeneration or degeneration.
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Akoussan, Komlan. « Modélisation et conception de structures composites viscoélastiques à haut pouvoir amortissant ». Thesis, Université de Lorraine, 2015. http://www.theses.fr/2015LORR0188/document.
Texte intégralRésumé :
L’objectif de ce travail est de développer des outils numériques utilisables dans la détermination de manière exacte des propriétés modales des structures sandwichs viscoélastiques composites au vue de la conception des structures sandwichs viscoélastiques légères mais à haut pouvoir amortissant. Pour cela nous avons tout d’abord développé un outil générique implémenté en Matlab pour la détermination des propriétés modales en vibration libre des plaques sandwichs viscoélastiques dont les faces sont en stratifié de plusieurs couches orientées dans diverses directions. L’intérêt de cet outil, basé sur une formulation éléments finis, réside dans sa capacité à prendre en compte l’anisotropie des couches composites, la non linéarité matérielle de la couche viscoélastique traduit par diverses lois viscoélastiques dépendant de la fréquence ainsi que diverses conditions aux limites. La résolution du problème aux valeurs propres non linéaires complexes se fait par le couplage entre la technique d’homotopie, la méthode asymptotique numérique et la différentiation automatique. Ensuite pour permettre une étude continue des effets d’un paramètre de modélisation sur les propriétés modales des sandwichs viscoélastiques, nous avons proposé une méthode générique de résolution de problème résiduel non linéaire aux valeurs propres complexes possédant en plus de la dépendance en fréquence introduite par la couche viscoélastique du coeur, la dépendance du paramètre de modélisation qui décrit un intervalle d’étude bien spécifique. Cette résolution est basée sur la méthode asymptotique numérique, la différentiation automatique, la technique d’homotopie et la continuation et prend en compte diverses lois viscoélastiques. Nous proposons après cela, deux formulations distinctes pour étudier les effets, sur les propriétés amortissantes, de deux paramètres de modélisation qui sont importants dans la conception de sandwichs viscoélastiques à haut pouvoir amortissement. Le premier est l’orientation des fibres des composites dans la référence du sandwich et le second est l’épaisseur des couches qui lorsqu’elles sont bien définies permettent d’obtenir non seulement des structures sandwichs à haut pouvoir amortissant mais très légères. Les équations fortement non linéaires aux valeurs propres complexes obtenues dans ces formulations sont résolues par la nouvelle méthode de résolution d’équation résiduelle développée. Des comparaisons avec des résultats discrets sont faites ainsi que les temps de calcul pour montrer non seulement l’utilité de ces deux formulations mais également celle de la méthode de résolution d’équations résiduelles non linéaires aux valeurs propres complexes à double dépendanceModeling and design of composite viscoelastic structures with high damping powerThe aim of this thesis is to develop numerical tools to determine accurately damping properties of composite sandwich structures for the design of lightweight viscoelastic sandwichs structures with high damping power. In a first step, we developed a generic tool implemented in Matlab for determining damping properties in free vibration of viscoelastic sandwich plates with laminate faces composed of multilayers. The advantage of this tool, which is based on a finite element formulation, is its ability to take into account the anisotropy of composite layers, the material non-linearity of the viscoelastic core induiced by the frequency-dependent viscoelastic laws and various boundary conditions . The nonlinear complex eigenvalues problem is solved by coupling homotopy technic, asymptotic numerical method and automatic differentiation. Then for the continuous study of a modeling parameter on damping properties of viscoelastic sandwichs, we proposed a generic method to solve the nonlinear residual complex eigenvalues problem which has in addition to the frequency dependence introduced by the viscoelastic core, a modeling parameter dependence that describes a very specific study interval. This resolution is based on asymptotic numerical method, automatic differentiation, homotopy technique and continuation technic and takes into account various viscoelastic laws. We propose after that, two separate formulations to study effects on the damping properties according to two modeling parameters which are important in the design of high viscoelastic sandwichs with high damping power. The first is laminate fibers orientation in the sandwich reference and the second is layers thickness which when they are well defined allow to obtain not only sandwich structures with high damping power but also very light. The highly nonlinear complex eigenvalues problems obtained in these formulations are solved by the new method of resolution of eigenvalue residual problem with two nonlinearity developed before. Comparisons with discrete results and computation time are made to show the usefulness of these two formulations and of the new method of solving nonlinear complex eigenvalues residual problem of two dependances
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Wallace, Efoé Rodrigue. « On the rolling contact between multi-layered bodies, application to tire-pavement modelling ». Thesis, Lyon, 2022. http://theses.insa-lyon.fr/publication/2022LYSEI014/these.pdf.
Texte intégralRésumé :
Cette thèse a été consacrée au développement d’un outil numérique de dimensionnement des chaussées. En effet, la récurrence des dégradations observées à la surface des chaussées, pousse la recherche scientifique à s’intéresser à leur compréhension. Le but est donc de mieux cerner l’origine de ces dégradations, mieux les prédire afin de mieux les maîtriser dans le processus de dimensionnement des chaussées. C’est dans cette optique que s’inscrit la présente étude. Pour ce faire, une étude numérique est réalisée. Elle est basée sur une modélisation semi-analytique du contact. La méthode couple des solutions analytiques et des méthodes d’accélération numériques telles que la méthode du gradient conjugué (CGM) et les algorithmes de transformée de Fourier rapide (FFT). Pour atteindre une modélisation plus réaliste du contact pneu-chaussée, trois principaux aspects sont modélisés dans ce travail. Dans un premier temps, il est important de prendre en compte l’aspect multicouches de la structure de la chaussée. On utilise les coefficients de Papkovich-Neuber qui donnent des formes pour les contraintes et déplacements élastiques. En faisant une transformée de Fourier de ces dernières et en résolvant les conditions aux limites, on obtient les coefficients d’influence d’un espace semi-infini multi-couches avec un nombre quelconque de couches. Deuxièmement, la viscoélasticité est intégrée. Il s’agit de mettre en exergue le comportement viscoélastique des chaussées dû principalement à l’utilisation significative de l’asphalte dans la composition des matériaux. Afin d’intégrer ce comportement viscoélastique, une méthode de correspondance Elastique/Viscoélastique est proposée. Cette méthode impose de recalculer les coefficients d’influence à chaque pas de temps, occasionnant ainsi une augmentation des temps de calcul. Toutefois, les simulations restent très rapides par comparaison avec la méthode des Eléments Finis. Troisièmement, le contact roulant est modélisé avec la prise en compte des effets tangentiels et du coefficient de frottement statique. Le but est de pouvoir analyser les régimes transitoires (accélération, freinage, virage, etc.) pendant le roulement. Pour ce faire, il faut prendre en compte non seulement l’effort normal appliqué au contact, mais aussi le frottement à l’interface et les efforts tangentiels et éventuellement un moment de torsion. Les différents aspects de la modélisation sont validés avec des modèles de la littérature. De plus, différentes études paramétriques permettent de mieux saisir les aspects sus cités. L’application du modèle au contact pneu-chaussée a permis de montrer que dans les cas transitoires tels que l’accélération, le freinage, le virage, le dérapage, les effets tangentiels entrainent un accroissement significatif des contraintes dans la structure chaussée, surtout près de la surfaceThe purpose of this thesis has been the development of a dimensioning tool for pavement design. In order to better understand their surface degradations (mainly rutting and cracking), a modelling study is carried out. This modelling task has been performed with contact mechanics tools. Particularly, a semi-analytical model has been developed, based on Fast Fourier Transform (FFT) and Conjugate Gradient Method (CGM) algorithms. With view to achieve a more realistic modelling of the tire-pavement contact, the focus has been put on three aspects of the contact problem. Firstly, the multi-layered aspect of the pavement has been considered. Using the Papkovich-Neuber potentials, the influence coefficients have been found in the Fourier frequency domain. A numerical inversion using FFT algorithms has allowed to find the influence coefficients in the space domain. Secondly, the viscoelastic behaviour of asphaltic materials, used in roads construction, has been accounted. To this aim, an Elastic/Viscoelastic correspondence has been proposed. This correspondence imposes to recalculate the influence coefficients at every time step. These additional calculations imply an increase of the computation time; however, the simulations remain straight and fast. In addition, the proposed correspondence is exact in some cases (especially in steady-state regime); and it is an approximation in the other cases where the committed error has been shown to be marginal. Thirdly, the effects of the tangential overall forces have been integrated to the rolling contact. The goal is that the present tool can be able to simulate acceleration, braking, turnaround, etc. cases where tangential forces and/or moment are applied on the wheel in addition to the normal force (which is generally the weight of the car or truck). This tractive rolling contact has been solved between elastically dissimilar bodies submitted not only to tangential forces but also to a spinning moment. All these aspects, introduced together in the model, have allowed to perform various parametric analyses for a better grasp of their influence on general contacts. Furthermore, an application of the developed model has allowed to simulate realistic cases of rolling contact between the tire and the pavement. From examples such as acceleration, turnaround and drift, it has been proven that the tangential forces increase significantly the overall stresses
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Daoud, Hajer. « Contribution à l'étude du comportement mécanique et vibratoire des composites biosourcés incorporant des matériaux fonctionnels ». Thesis, Le Mans, 2018. http://www.theses.fr/2018LEMA1017/document.
Texte intégralRésumé :
Cette thèse porte sur l’étude du comportement mécanique et vibratoire d’un composite biosourcé incorporant un matériau viscoélastique. Les matériaux étudiés sont des stratifiés en composite lin/greenpoxy et des stratifiés viscoélastiques composés d'un noyau viscoélastique en caoutchouc naturel confiné entre deux composites. La première partie du travail a été consacrée à l’étude de l’influence de l’intégration de la couche viscoélastique sur le comportement mécanique des composites. L’analyse des résultats expérimentaux et l’observation des signaux d’émission acoustique obtenus dans ces composites soumis à différentes sollicitations mécaniques en statique et en fatigue ont permis d’identifier les signatures acoustiques des mécanismes d’endommagement prépondérants dans les deux matériaux. Dans un deuxième temps, les propriétés dynamiques de ces composites ont été déterminées à partir des essais de vibration. Les résultats obtenus ont montré que la couche viscoélastique a joué un rôle majeur dans l'amortissement et la dissipation d'énergie des composites. Suite à cette analyse, nous avons mis en place une procédure, utilisant la méthode des éléments finis, pour calculer l’amortissement de ces matériaux. Dans le but de mettre en évidence l’influence des caractéristiques de la couche viscoélastique, une étude paramétrique a été menée sur le composite viscoélastique, permettant d’optimiser l’amortissement de ce matériau en faisant varier divers paramètres. Enfin, le comportement visqueux des composites a été caractérisé par la méthode de résonance non linéaire en faisant varier l’amplitude d’excitationThis thesis focuses on the study of the mechanical and vibration behaviour of a flax fibre reinforced composites with and without an interleaved natural viscoelastic layer. The composite materials have been characterized experimentally using different mechanical and vibrational tests. First, both types of composites were studied using uni-axial tensile and three-points bending tests. Acoustic emission (AE) has been often used for the identification and characterization of micro failure mechanisms in composites. The results showed that these composites have very high specific characteristics. It can be used for applications currently using composites reinforced with synthetic fibres such glass, carbon…. Next, experimental and finite element vibration analyses were carried out on the composites with and without an interleaved natural viscoelastic layer. A good agreement between the two methods was obtained. It has been shown that the viscoelastic layer plays a major role in damping because it has a high level of energy dissipation. Therefore, it improves with a significant way the modal properties of the composite. Finally, nonlinear resonance tests were performed on the composites. It has been shown that the viscoelastic layer generates a nonlinear behaviour in the material. The linear and nonlinear, elastic and dissipative parameters have been calculated to deduce finally that nonlinear parameters are more sensitive to heterogeneities than those derived from linear vibration tests
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Yao-Ju, Lu, et 呂耀如. « Damping Property and Vibration Analysis of Blades with Viscoelastic Layers ». Thesis, 2001. http://ndltd.ncl.edu.tw/handle/46710017942918321807.
Texte intégralRésumé :
碩士國立臺灣科技大學
機械工程系
89
This thesis shows the damping property and the vibration analysis of blades with viscoelastic layers. The topics of the research include the vibration behavior of shaft-disk-blades system, the torsional vibration of shaft and the bending vibration of blades due to the loads at blades. As to the mathematical method, the equations of motion via the energy approach are derived. This model, unlike the previous, uses only two displacement functions for layered blades: one is the axial displacement, and the other is the transverse displacement. Then, the assumed-modes method is employed to discretize the equations. The analyses of the relations between natural frequency and rotation speed and damping property are discussed afterward. The vibration reduction of shaft’s torsional response and blades’ bending response is looked into as well. The numerical results show the effects of constrained layer (CL) thickness and stiffness, and viscoelastic material (VEM) thickness for blades. Besides, the research also compares natural frequency and stability of the blades with viscoelastic layers to those without viscoelastic layers. According to these results, the engineers would gain very useful information in evaluating vibration reduction for different constrained layer (CL) and viscoelastic matrial (VEM) thickness.
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Dewangan, Palash. « Passive Viscoelastic Constrained Layer Damping for Structural Application ». Thesis, 2009. http://ethesis.nitrkl.ac.in/1374/1/MTECH_THESIS_PALASH_DEWANGAN(207ME109).pdf.
Texte intégralRésumé :
The purpose behind this study is to predict damping effects using method of passive viscoelastic constrained layer damping. Ross, Kerwin and Unger (RKU) model for passive viscoelastic damping has been used to predict damping effects in constrained layer sandwich cantilever beam. This method of passive damping treatment is widely used for structural application in many industries like automobile, aerospace, etc.
The RKU method has been applied to a cantilever beam because beam is a major part of a structure and this prediction may further leads to utilize for different kinds of structural application according to design requirements in many industries. In this method of damping a simple cantilever beam is treated by making sandwich structure to make the beam damp, and this is usually done by using viscoelastic material as a core to ensure the damping effect. Since few years in past viscoelastic materials has been significantly recognized as the best damping material for damping application which are usually polymers. Here some viscoelastic materials have been used as a core for sandwich beam to ensure damping effect.
Due to inherent complex properties of viscoelastic materials, its modeling has been the matter of talk. So in this report modeling of viscoelastic materials has been shown and damping treatment has been carried out using RKU model. The experimental results have been shown how the amplitude decreases with time for damped system compared to undamped system and further its prediction has been extended to finite element analysis with various damping material to show comparison of harmonic responses between damped and undamped systems.
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Lin, Yen Shiu, et 林彥旭. « Frequency Response Function of composite beam with viscoelastic layer ». Thesis, 1998. http://ndltd.ncl.edu.tw/handle/90167963988653947510.
Texte intégralRésumé :
碩士國立臺灣大學
機械工程學系研究所
86
In this thesis, we investigate the dynamic characteristics of an elastic bea m completely covered by a viscoelastic layer both theoretically and experiment ally. The fourth-order differential equations of motion are derived for the tr ansverse vibration of thecomposite beam. Not only the transverse strain and st ress of the elastic and viscoelastic layer are included in the equation of mot ion,but also the longitudinal strain and stress are included. An eigenvaluepro blem corresponding to the differential equation is solved to obtainthe require d dynamic characteristics. Numerical analysis and experimental measurements i s illustrated by a cantilever beam in transverse vibration. Galerkin Method is used to find the eigen solution. The dynamic behavior of the composite beam is illustrated with frequency response functions.
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Wang, Wene Yi, et 王文毅. « Viscoelastic Analysis for Adhesive Layer of Composite Sandwich Structure ». Thesis, 2002. http://ndltd.ncl.edu.tw/handle/91005433688521485029.
Texte intégralRésumé :
碩士國立中正大學
機械系
90
Composite materials exhibit high mechanical strength, corrosion resistance and fatigue life endurance. Therefore, the composite materials gradually due to the wide use capability in the engineering applications replace typical metallic materials. The sandwich structure became a major component used to control the mechanical vibration in the smart material because of the Viscoelasticity behavior in the recent years. The denatured property of the sandwich structure also inspired the interests of the scholar and became a popular research topic as well. However, it is rare to find the literatures studied the sandwich structure (Five Layered) with the adhesive layers, especially using the Viscoelasticity analysis. In this research, we considered a sandwich structure, which added two Viscoelasticity adhesive layers on both sides of the core and face sheets. The analytical formulations of the displacement, stress and strain on the interfaces of the structure were derived in present work under proposed geometric assumption. A referred numerical case was adopted to conduct the validation by comparing the results obtained form the analytical solutions and the finite element solutions. It can prove that the analytical or the approximated solutions without the adhesive layers or in elastic assumption cannot perform realistic responses of the sandwich structure. The Viscoelasticity adhesive layers must be involved into the analysis consideration according to our studying.
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LIN, JUN-YING, et 林君穎. « Structural-model tests of viscoelastic-supported and layered beams ». Thesis, 1992. http://ndltd.ncl.edu.tw/handle/09440770084656794163.
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Horne, Charles T. « Passive vibration control using viscoelastic and constrained layer beam waveguide absorbers ». Thesis, 1988. http://hdl.handle.net/10945/23345.
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Peng, Yu-Fang, et 彭于芳. « Viscoelastic Analysis of Flexible Organic Light-Emitting(FOLED) with Barrier Layer ». Thesis, 2006. http://ndltd.ncl.edu.tw/handle/13069005244241727030.
Texte intégralRésumé :
碩士國立成功大學
工程科學系碩博士班
94
The purpose of this paper is to execute the viscoelastic analysis of flexible polymer layer with hard film under externally bending couple or curvature and the influence of the residual strain induced during the process. The analytical results are then discussed against the conditions of causing crack onset strain of FOLED (Flexible and Rollable Organic Light-Emitting Diode). From the elastic analysis, it knows that curvature of flexible substrate with hard film must be modified from the Stoney formula by a correction factor. Furthermore, the curvature of multilayered thin-film structure can be modified by adding up correction factor of each layer. From viscoelastic analysis, the results are the same, but the correction factor must depend on time. In the analysis of FOLED, the top surface strain is smaller with the reduction of ITO film thickness. Also the analytical results can provide critical radii of curvature for ITO stiffness from 6 to 24 KN/m. Under constant bending moment condition, because of the relaxation in the PET substrate, the surface strain is increased with time, and may have maximum value over a long time and may cause fracture. This conclusion can not be drawn from the elastic analysis. Under cyclic bending moment condition, the lower frequency strain amplitude is larger than that of higher frequency, and may lead to crack. Adding a barrier layer can improve the resistance of water vapor and oxygen transmission into FOLED. In the analysis, it is concluded that higher stiffness of barrier layer induces higher surface strain, and may result in cracking. Therefore, this paper provides a methodology of choosing barrier layer based on the structure and its purpose and then estimate the time of crack forming.
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Kuo, Chi-Wei 1982. « Guided Wave Propagation in Tubular Section with Multi-Layered Viscoelastic Coating ». Thesis, 2012. http://hdl.handle.net/1969.1/148398.
Texte intégralRésumé :
Three kinds of propagating waves physically admissible in a tubular section are derived to establish their dispersion characteristics in response to the presence of multi-layered viscoelastic coatings. One is the longitudinal wave that propagates in the axial direction. The other two are shear and longitudinal waves along the circumferential direction. To characterize the hollow cylinder with coating layers, wave dispersion and attenuation are studied using the “global matrix” technique. Since each layer is considered to be perfectly bonded to each other, displacement and strain continuity are imposed as the interfacial boundary conditions. Viscoelastic coating materials such as bitumen and epoxy serve to improve pipeline reliability, but they also dampen and dissipate wave energy. The viscoelastic materials are studied as well. By replacing the real material constants with complex material constants in the characteristic equation, the impact of the viscoelastic coatings on wave dispersion is established. Bisection method is followed to find the real and complex roots of the three characteristic equations derived. Roots thus obtained are manipulated to allow the phase velocity and attenuation dispersion to be plotted against frequency. The dispersion of phase velocity and wave attenuation for coated pipes are evaluated against a baseline model which is the bare, uncoated tubing to establish the propagation characteristics of the guided shear and longitudinal waves in the presence of multiple coating layers. The effects of increasing attenuation parameter and coating thickness are also investigated.
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Nhan, Phan-Thien, et Xi-Jun Fan. « Viscoelastic Mobility Problem Using A Boundary Element Method ». 2002. http://hdl.handle.net/1721.1/4021.
Texte intégralRésumé :
In this paper, the complete double layer boundary integral equation formulation for Stokes flows is extended to viscoelastic fluids to solve the mobility problem for a system of particles, where the non-linearity is handled by particular solutions of the Stokes inhomogeneous equation. Some techniques of the meshless method are employed and a point-wise solver is used to solve the viscoelastic constitutive equation. Hence volume meshing is avoided. The method is tested against the numerical solution for a sphere settling in the Odroyd-B fluid and some results on a prolate motion in shear flow of the Oldroyd-B fluid are reported and compared with some theoretical and experimental results.Singapore-MIT Alliance (SMA)
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Nault, James Robert. « Analytical and experimental investigation of constrained viscoelastic layer damping for a plate and shell model ». Thesis, 1988. http://hdl.handle.net/10945/22906.
Texte intégral
39
Chen, Yu-Ren, et 陳煜仁. « Vibration and Dynamic Stability Analysis of Polar Orthotropic Annular Plates with a Viscoelastic Core Layer ». Thesis, 2005. http://ndltd.ncl.edu.tw/handle/54405262671377679053.
Texte intégralRésumé :
博士國立成功大學
機械工程學系碩博士班
93
ABSTRACT Vibration, damping, and dynamic stability of polar orthotropic sandwich annular plates with a viscoelastic core layer are investigated. The governing equations of a polar orthotropic sandwich annular plate system are derived by a discrete layer annular finite element method. The transverse shear effects are included in the finite element, which conveniently handles the thick and thin plate problems. The material properties of the face layers are elastic, polar orthotropic and homogeneous. The material properties of isotropic, linear and incompressible viscoelastic materials in the damping layer are described by complex representations. The free vibration and dynamic stability of stationary and rotational sandwich annular plates are focused. In the mathematical modeling, initial stress distribution induced by rotational and external load effects are obtained from the solutions of static problems and are taken into account in the strain energy expression to calculate the geometry stiffness matrices. The governing equations with complex coefficients are developed, and natural frequencies, modal loss factors and boundaries between dynamic stability region and instability region are solved. The effects of many design parameters, including stiffness and thickness of the viscoelastic core layer and face layers, inner ratios and rotational speeds are discussed. Numerical results show that the thicker damping layer or the larger treatment size does not always provide better damping properties of annular plate systems. The modal loss factors of systems are decreased with increasing of rotational speeds. Moreover, increasing the modulus ratios of the face layers tends to stabilize annular plate systems.
40
Yeh, Chang-Hsin, et 葉昌鑫. « Nanoindentation and micro-compression studies of viscoelastic properties of nano-layered metallic thin films ». Thesis, 2008. http://ndltd.ncl.edu.tw/handle/32606165445965561244.
Texte intégralRésumé :
碩士國立成功大學
土木工程學系碩博士班
96
The aim of this study is to perform nanoindentation and micro-compression experimental tests to study the mechanical and viscoelastic properties of nano-layered metallic thin films. The nano-layered thin films were prepared with RF magnetron sputtering technique, and consisted of copper (Cu) and “native” copper oxide. For comparisons, single-layer copper and multilayers that contain copper and “deposited” copper oxide were tested. In addition to metallic multilayers, other materials, such as PMMA, 304 stainless steel, epoxy, metallic glass and silicon, are also studied for verification of testing methodology and comparison in their time-dependent mechanical properties. Two testing techniques are adopted. One is the conventional nanoindentation test, established by Oliver and Pharr, and the other is the micro-compression test on micropillars. Micropillars are milled with the focus ion beam (FIB) technique, and are subsequently compressed by nanoindenter with a flat tip for introducing uniform compressive stress. It is found that the nanoindentation hardness of Cu/Cu multilayers is larger than that Cu single layer, as well as the modulus of multilayers. For the indentation modulus and hardness of the tested materials, the Cu single layer (1 micron thick) exhibits a modulus of 150 GPa and hardness of 2.5 GPa, Cu/Cu multilayers shows a modulus of 155 GPa and hardness of 3 GPa, Cu/Cu2O a modulus of 50 ~60 GPa and hardness of 4 ~ 4.5 GPa, PMMA a modulus of 5.5 GPa and hardness of 0.3 GPa, silicon (100) modulus of 200 GPa and hardness of 14.5 GPa, steel a modulus 200 GPa and hardness of 6 GPa, epoxy a modulus 3 ~ 4 GPa and hardness of 0.1 ~ 0.2 GPa, Cu-based metallic glass a modulus of 150 GPa and hardness of 6 GPa. The indentation creep exponent n for Cu/Cu multilayers is to be 4.6, and is 0.7 for Cu single layer at tip load being 3 mN. In addition, the indentation creep exponent for Cu/Cu multilayers is 16.9, and for Cu single layer it is found n is equal to 1.0 at 10 mN tip load. Furthermore, the indentation creep exponent for Cu/Cu multilayers is 835.3 and that of Cu single layer is 1.1 at 25 mN tip load. As for Cu/Cu2O micropillars, their uniaxial compression creep exponent is found to be 7.5 (minimum) and 154.1 (maximum) at 0.01 mN tip load. Overall, it is found that, for all the materials tested, including Cu single layer, Cu/Cu multilayers, Cu/Cu2O multilayers, silicon (100) and PMMA, their creep exponents increase with tip load.
41
Joseph, Thomas K. « Dynamic Response Of A Satellite With Flexible Appendages And Its Passive Control ». Thesis, 2009. https://etd.iisc.ac.in/handle/2005/943.
Texte intégralRésumé :
Most present day spacecrafts have large interconnected solar panels. The dynamic behavior of the spacecraft in orbit can be modeled as a free rigid mass with flexible elements attached to it. The natural frequencies of such spacecrafts with deployed solar panels are very low. The low values of the natural frequencies pose difficulties for maneuvering the spacecraft. The control torque required to maneuver the spacecraft is influenced by the flexibility of the solar arrays. The control torque sets up transient oscillations in the flexible solar panels which in turn induces disturbances in the rigid satellite body and the payload within. Therefore the payload operations can be carried out only after the disturbances die out. For any reduction of the above disturbances it is necessary to understand the dynamic behavior of such systems to an applied torque. The present work first studies the nature of the disturbances. The influence of structural parameters on these disturbances is then investigated. Finally, the use of passive damping treatment using viscoelastic material is investigated for the reduction of the disturbances.
In order to understand the nature of vibrations induced in the flexible appendages of a satellite during maneuvers, we model the maneuver loads in terms of applied angular acceleration as well as varying torque. The transient decay of the disturbance of the rigid element is characterized by the dynamic characteristics of the flexible panels or appendages. It is shown that by changing the stiffness of the panel the response of the rigid element can be modified.
A simple model consisting of an Euler-Bernoulli beam attached to a free mass is next considered. The influence of various parameters of the EulerBernoulli beam in mitigating vibration and thereby the disturbance in the rigid mass is investigated. As the response of the rigid system mounted with the large flexible panels are influenced by the dynamics of the flexible panels, reduction of these disturbances can be achieved by reducing the vibration in the flexible panels. Therefore application of viscoelastic materials for passive damping treatment is investigated.
The loss factor of a structure is significantly improved by using constrained viscoelastic layer damping treatment. However providing a constrained layer damping treatment on the entire structure is very inefficient in terms of the additional mass involved. Therefore damping material is applied at suitable optimal locations. In previous studies reported in literature, modal strain energy distribution in the viscoelastic material as well as the base structure is used as a tool to arrive at the optimum location for the damping treatment. It is shown in this study that such locations selected are not the optimum.
A new approach is proposed in this study by which both the above shortcomings are overcome. It is shown that use of a parameter that is the ratio of the strain in the viscoelastic material to the angle of flexure is a more reliable measure in arriving at optimal locations for the application of constrained viscoelastic layers. The method considers the deformations in the viscoelastic material and it is shown that significant values of loss factors are achieved by providing material in a small region alone. We also show that loss factor can be improved by providing damping material near the interface region. The loss factor can be further improved by incorporating spacers by using spacer material having higher extensional modulus. Also shown is the fact that loss factor is unaffected by the shear modulus of the spacer material. Experiments have been conducted to validate these results.
In a related study we consider honeycomb type flexible structures since in most of the spacecraft applications honeycomb sandwich constructions are employed. But loss factors of sandwich panels with constrained layer damping treatment are seldom discussed in the literature. Use of viscoelastic layers to improve the loss factors of the honeycomb sandwich beams is explored. The results show that the loss factors are enhanced by increasing the inplane stiffness of the constraining layer. These conclusions too are validated by experimental results.
Finally a typical satellite with flexible solar panels is considered, and the use of the viscoelastic material for improving the damping is demonstrated.
42
Joseph, Thomas K. « Dynamic Response Of A Satellite With Flexible Appendages And Its Passive Control ». Thesis, 2009. http://hdl.handle.net/2005/943.
Texte intégralRésumé :
Most present day spacecrafts have large interconnected solar panels. The dynamic behavior of the spacecraft in orbit can be modeled as a free rigid mass with flexible elements attached to it. The natural frequencies of such spacecrafts with deployed solar panels are very low. The low values of the natural frequencies pose difficulties for maneuvering the spacecraft. The control torque required to maneuver the spacecraft is influenced by the flexibility of the solar arrays. The control torque sets up transient oscillations in the flexible solar panels which in turn induces disturbances in the rigid satellite body and the payload within. Therefore the payload operations can be carried out only after the disturbances die out. For any reduction of the above disturbances it is necessary to understand the dynamic behavior of such systems to an applied torque. The present work first studies the nature of the disturbances. The influence of structural parameters on these disturbances is then investigated. Finally, the use of passive damping treatment using viscoelastic material is investigated for the reduction of the disturbances.
In order to understand the nature of vibrations induced in the flexible appendages of a satellite during maneuvers, we model the maneuver loads in terms of applied angular acceleration as well as varying torque. The transient decay of the disturbance of the rigid element is characterized by the dynamic characteristics of the flexible panels or appendages. It is shown that by changing the stiffness of the panel the response of the rigid element can be modified.
A simple model consisting of an Euler-Bernoulli beam attached to a free mass is next considered. The influence of various parameters of the EulerBernoulli beam in mitigating vibration and thereby the disturbance in the rigid mass is investigated. As the response of the rigid system mounted with the large flexible panels are influenced by the dynamics of the flexible panels, reduction of these disturbances can be achieved by reducing the vibration in the flexible panels. Therefore application of viscoelastic materials for passive damping treatment is investigated.
The loss factor of a structure is significantly improved by using constrained viscoelastic layer damping treatment. However providing a constrained layer damping treatment on the entire structure is very inefficient in terms of the additional mass involved. Therefore damping material is applied at suitable optimal locations. In previous studies reported in literature, modal strain energy distribution in the viscoelastic material as well as the base structure is used as a tool to arrive at the optimum location for the damping treatment. It is shown in this study that such locations selected are not the optimum.
A new approach is proposed in this study by which both the above shortcomings are overcome. It is shown that use of a parameter that is the ratio of the strain in the viscoelastic material to the angle of flexure is a more reliable measure in arriving at optimal locations for the application of constrained viscoelastic layers. The method considers the deformations in the viscoelastic material and it is shown that significant values of loss factors are achieved by providing material in a small region alone. We also show that loss factor can be improved by providing damping material near the interface region. The loss factor can be further improved by incorporating spacers by using spacer material having higher extensional modulus. Also shown is the fact that loss factor is unaffected by the shear modulus of the spacer material. Experiments have been conducted to validate these results.
In a related study we consider honeycomb type flexible structures since in most of the spacecraft applications honeycomb sandwich constructions are employed. But loss factors of sandwich panels with constrained layer damping treatment are seldom discussed in the literature. Use of viscoelastic layers to improve the loss factors of the honeycomb sandwich beams is explored. The results show that the loss factors are enhanced by increasing the inplane stiffness of the constraining layer. These conclusions too are validated by experimental results.
Finally a typical satellite with flexible solar panels is considered, and the use of the viscoelastic material for improving the damping is demonstrated.
43
Benson, James. « A Study of the Structure and Dynamics of Smectic 8CB Under Mesoscale Confinement ». Thesis, 2012. http://hdl.handle.net/10012/6919.
Texte intégralRésumé :
The structure and dynamics of the smectic-A liquid crystal 8CB (4 cyano-4 octylbiphenyl) when sheared and confined to mesoscale gaps (with crossed cylindrical geometry and mica confining surfaces) were studied using a Surface Forces Apparatus (SFA). Triangular shear patterns with frequencies of 0.01, 0.1, 1.0 and 10 Hz, and amplitudes of 62.5 nm, 625 nm and 6.25 m were applied to samples at gap sizes of 0.5 and 5.0 m. The study was performed at room temperature (20.5C) and at two higher temperatures (22C and 27C). In order to minimize the thermal fluctuations within the test chamber and hence to allow for the rapid re-initialization of test runs, the SFA was modified to allow for quick, precise and remote control of the confining surfaces. The procedure maximized the number of tests that could be undertaken with a single pair of surfaces so that a single gap geometry could be maintained for the duration of the test run. In order to run the SFA remotely, scripts written with a commercial software package, LabVIEW, were used to control of the SFA components, its FECO-monitoring camera and all its peripheral electronic equipment as well. Samples were agitated to disrupt any shear-induced liquid crystal domain alignment from previous testing following each shear test, and methodologies were developed to ascertain the extent of confinement quickly and remotely following agitation. Separate methods were developed for gap sizes at each extreme of the mesoscale regime, where the transition from bulklike structure and dynamics to nano-confinement occurs (between 1 and 10 microns for smectic-A 8CB).
The results revealed that the greater amplitude-gap aspect ratio and surface-to-domain contact associated with smaller gaps facilitated reorientation of the domains in the shear direction. Evidence was also presented of domains at the higher end or outside of the mesoscale regime that, while straining and accreting, were unable to reorient and thereby led to an overall increase of viscoelastic response. The effective viscosity was found to obey a simple power law with respect to shear rate, , and the flow behaviour indices, n, slightly in excess of unity indicate shear thickening occurs with large enough shear amplitude, and that the viscosity reached a plateau near unity over shear rates of 0.005 to 500 s-1 within the mesoscale regime. Different K and n values were observed depending on the shear amplitude used.
Unlike bulk smectic 8CB, whose domains do not align well in the shear direction with large shear-strain amplitude, at mesoscale levels of confinement large amplitude shearing (up to 12.5 shear strain amplitude) was found to be very effective at aligning domains. In general domain reorientation is found to be much more rapid within the mesoscale regime than has been reported in bulk. Aggressive shearing was found to result in a complete drop in viscoelastic response within seconds, while gentler shearing is found to produce a very gradual increase that persists for more than six hours, with individual shear periods exhibiting frequent and significant deviations from the expected smooth shear path that may be a product of discrete domain reorientations.
From these findings, certain traits of the smectic 8CB domain structures under mesoscale confinement were deduced, including how they respond to shear depending on the level of confinement, and how their reorientation due to shear varies not only with shear rate but also independently with shear amplitude. An equation describing the viscosity change as a function of both shear rate and shear amplitude is proposed. The shear amplitude dependence introduces the notion of shearing beyond the proposed smectic 8CB “viscoelastic limit”, which was shown to exhibit behaviour in accordance with Large Amplitude Oscillatory Shear (LAOS) techniques developed for Fourier Transform rheology. The findings provided an understanding of the behavioural changes that occur as one reduces the level of confinement of smectic materials from bulk to nanoconfinement.