Several active methods, which requires external control systems and moving parts, have been developed to control the fiber orientation during 3D printing. Active mechanisms like rotating nozzle, impeller, and magnetic field have been integrated to realize complex internal fiber structures. In this study, instead of using active methods, I investigate a passive method for controlling the fiber orientation without any moving parts or additional mechatronics added in the printing process. Composites of polydimethylsiloxane (PDMS) and glass fibers (GF) are 3D printed. Channels, such as helicoid, are designed and integrated to guide the ink flow and passively result in different pre-alignment of fibers before the ink flow into narrow nozzle space. While passing through the designed channels, the fibers orient due to the shear between channel walls and the ink. The effect of helicoids with different pitch sizes are investigated via mechanical experiments, microstructural analysis, and numerical simulations. The results show that both surface to volume ratio and helix angle of the channel affect pre-alignment of fiber orientation at the entry of nozzle. The internal fiber structures lead to enhanced and tunable mechanical properties of printed composites. Pitch size 7-9 mm (helix angle of 7.92- 10.15o) is found to be optimal for the 3D printed PDMS-GF composites. Stiffness and strength can be tuned up to 77.6% and 47.8%, respectively, compared with the case without helicoid channel. Channels of parallel holes, parallel holes with taper end and gradually changing pitch helicoids are experimentally tested, showing further enhancement in mechanical properties.

No description available.

Traditional dynamic vibration absorbers (DVAs) consist of a mass-spring-damper system and are an effective means of attenuating structural vibration over a narrow frequency band. The effective bandwidth of the DVA can be increased by the addition of an externally controlled force, generally applied between the reaction-mass and the primary structure. Such devices are known as hybrid DVAs. This thesis presents a new hybrid DVA configuration which utilizes two reaction-masses in parallel. On this proposed hybrid dual-mass (DM) DVA, the control force is applied between the reaction-masses. It is shown that in broadband control applications, the proposed DM-DVA requires less control force to achieve the same primary attenuation as the traditional hybrid single-mass (SM) DVA. The hybrid DM-DVA was compared to the hybrid SM-DVA with two tests. A numerical simulation of the hybrid DVAs attenuating a single-degree-of-freedom structure was performed. To achieve an equal amount of primary attenuation, the hybrid SM-DVA required 65% higher root-mean-square (RMS) control effort than the hybrid DV-DVA. The numerical model also demonstrated that the hybrid DM-DVA was less sensitive to changes in the system as compared to the hybrid SM-DVA. Additionally, a prototype hybrid DVA was built which could be configured as either the hybrid SM or DM-DVA. The prototype hybrid DVA was used with the feedforward Filtered-X LMS algorithm to control the vibration of a fixed-free beam. The hybrid SM and DM-DVAs attenuated the primary response by a factor of 11.5 and 12.3, while requiring control efforts of 4.9 and 2.7 V/N RMS, respectively. Thus, the hybrid DM-DVA required 45% less control effort while yielding a higher attenuation ratio in this experiment. These results demonstrate the superior performance of the proposed DM-DVA for broadband control applications as compared to the traditional SM-DVA. / Master of Science

Passive control techniques to minimize structural vibrations are limited with respect to the amount of attenuation obtained especially in the low-frequency region but do not require adding any power. Active control methods are effective for reducing structural vibrations, especially at low frequencies, but may require significant control effort. Thus, passive and active control methods have complementary frequency ranges of application. This research consists of combining active and passive control techniques to simultaneously attenuate extensional and flexural power flows in infinite thin beams and determine the advantages and disadvantages of such a combination. An analytical model is developed for an infinite beam with a passive insert of high damping placed at some distance from a point force excitation (passive approach). The passive control of vibrations results in a reduction of both extensional and flexural power flows downstream of the passive material discontinuity. The simultaneous active control of extensional and flexural waves, using two co-located independent piezoceramic actuators bonded to the surface of the beam, is theoretically studied. The active control model shows that the use of two independent piezoceramic actuators allows complete cancellation of the total power flow (sum of the extensional and flexural power flows) downstream of the actuators. The combination of passive and active control methods for three different configurations (actuators located upstream of, downstream of, and on the passive insert) is investigated and complete control of the total power flow is again achieved. The results demonstrate that in the case of the actuators bonded to the passive material discontinuity, the active/passive combination has great potential for reducing the control effort required for the active controller. Finally, an approximation of the influence of heavy fluid flanking paths on the optimal active/passive system is developed by simulation of these flanking paths using axial and torsional springs. This last study shows that both axial and torsional springs will result in modification of the control effort required by the actuators if their respective stiffness is greater than the equivalent stiffness of the section in parallel with the springs. / Master of Science

Vibrações estruturais constituem uma das principais fontes de ruído e de desconforto em diversas aplicações, tais como nos produtos das indústrias automotiva, aeroespacial e naval, que, em grande parte, são compostos por estruturas flexíveis semelhantes a placas. Ao mesmo tempo, a demanda crescente por aumento de eficiência energética e por redução de emissão de gases nocivos ao meio ambiente tem levado os fabricantes de todos os segmentos para a utilização de materiais cada vez mais leves. Um desafio importante relacionado ao uso dessas soluções diz respeito ao seu comportamento vibroacústico, já que estruturas leves tendem a transmitir e emitir ruídos de forma mais intensa, particularmente em baixas frequências. Em decorrência disso, a obtenção de uma boa relação entre peso e desempenho vibroacústico usualmente demanda algumas iterações de projeto. Nesse contexto, este trabalho tem por objetivo estabelecer um procedimento numérico para avaliar as propriedades vibroacústicas de placas flexíveis excitadas por um campo gerado no interior de uma cavidade acústica, bem como investigar a influência de parâmetros estruturais que definem a placa sobre o seu desempenho vibroacústico. O escopo aqui é a faixa de baixas frequências, na qual o comportamento do sistema é descrito de forma determinística através de características modais. Foi desenvolvido um modelo numérico em elementos finitos de uma cavidade acústica retangular de paredes rígidas acoplada a uma placa flexível. Uma fonte acústica posicionada no interior da cavidade produz um campo de pressão sonora que induz a placa à vibração e, consequentemente, a irradiar potência sonora para o campo livre exterior. Para realização das análises de sensibilidade, desenvolveu-se um conjunto de rotinas de modo a permitir o gerenciamento automatizado de simulações. Resultados numéricos são apresentados para avaliar a influência de diversos parâmetros estruturais sobre o desempenho vibroacústico da superfície estrutural. O procedimento proposto pode ser aplicado de forma eficiente para a análise de sensibilidade da transmissão de ruído em sistemas vibroacústicos sujeitos à variação de parâmetros estruturais e pode também ser utilizado para avaliar diversos tipos de materiais, como isotrópicos, ortotrópicos e até metamateriais. / Structural vibrations are the major causes of noise and people discomforts in several applications, such as in the products of the automotive, aerospace and naval industries, which are mainly composed of flexible plate-like structures. At the same time, the ever-increasing demands for energy efficiency and for reduced emissions of harmful gases into the environment lead product manufacturers of all industrial sectors towards an increased use of lighter materials. However, these solutions are most sensitive to issues related to vibroacoustic behavior, since the lightweight structures tend to present a high sound transmission, particularly at low frequencies. Consequently, the optimal design of lightweight and noise insulation properties of a structure usually require some design trade-offs. In this context, this dissertation aims to propose a numerical procedure to analyze the vibroacoustic performance of flexible plates subject to an interior acoustic field, as well as to investigate the influence of structural parameters on the sound insulation properties of the plate. The scope here is the low frequency range, in which the response of the coupled system is described in a deterministic way in terms of a mode set. A finite element model of a rigid rectangular acoustic cavity coupled to a flexible plate was developed. An acoustic source inside the cavity produces an interior sound pressure field that forces the flexible plate to vibrate and, consequently, to radiate sound power to the external free field. In order to enable the automatic management of the variability simulations, a set of routines was also developed. Numerical results are presented in order to evaluate the influence of several structural parameters on the vibroacoustic properties of the plate. The proposed methodology can be largely used in sensitivity analysis of noise transmission in vibroacoustic systems subject to the variation of structural parameters and can be used to evaluate many material types, as isotropic, orthotropic and even metamaterials.

Vibrações mecânicas oferecem grande limitação para a produtividade, qualidade ou mesmo viabilidade das operações de usinagem, especialmente quando se trata das autoexcitadas (chatter). Neste trabalho, foram desenvolvidas estratégias que tem como objetivo diminuir a intensidade de vibrações em processos de torneamento por meio do acoplamento de material piezelétrico ao suporte de ferramenta em conjunto com uso de shunts resistivo, indutivo e resistivo-indutivo em série ou em paralelo, criando assim estruturas eletromecânicas passivamente amortecidas. Para tanto, foram construídos modelos eletromecânicos de parâmetros distribuídos para mostrar a capacidade que tais estruturas eletromecânicas possuem em oferecer um maior amortecimento quando comparadas com estruturas mecânicas convencionais. Com os modelos construídos, foi possível verificar a influência causada pela espessura da camada de material piezelétrico bem como a influência dos shunts no comportamento da estrutura, sendo constatado que camadas mais espessas aumentam a capacidade de amortecimento da estrutura e que os shunts resistivo-indutivo, tanto em série quanto em paralelo, funcionam como um amortecedor dinâmico de vibrações amortecido e oferecem o melhor desempenho. A seguir, construiu-se o diagrama de lóbulos de estabilidade para comparar as estruturas com e sem shunts e observou-se que as estruturas com shunts resistivo-indutivo possuem um melhor desempenho. Também foram executados testes de impacto (tap tests) para a verificação experimental do comportamento da estrutura quando conectadas aos shunts e os resultados mostraram que há um maior amortecimento. Considerando os resultados obtidos, acredita-se que seja possível melhorar o desempenho de processos de torneamento usando material piezelétrico. / Mechanical vibrations offer great limitation for the productivity, quality or even feasibility of the machining operations when chatter is present. In this work it was developed strategies aiming to diminish the intensity of the vibration in turning processes. By coupling a piezoelectric material with a turning tool and by using different associations of resistive and inductive shunt (series or parallel) it was created electromechanical structures passively damped. Electromechanical models of distributed parameters were developed in order to show the capacity that these structures has to offer a greater dumping when compared with conventional mechanical structures. By using these constructed models it was possible to verify the influence of the thickness of the piezoelectric material as well as the influence of shunts in the behavior of structure. It was observed that thicker layers increase the damping capacity of the structure that resistive-inductive shunt (series or parallel) works as a damped dynamic vibration absorber which offer better performance. Latter was developed a stability lobes diagram in order to compare the structures with and without shunts and it was observed that structures connected to resistiveinductive shunt has a better performance. Tap tests were performed for the purpose of study the experimental behavior of the structure connected to shunt and results showed that there is a better damping in this situation. Considering the results obtained, is fair to believe that is possible to improve turning process by using piezoelectric materials.

Les méthodes de contrôle de vibrations passives basées sur des absorbeurslinéaires ont été largement étudiées et ils ont aujourd'hui une vaste gammed'applications. Cependant, les absorbeurs linéaires n’étant efficaces quelorsqu'ils sont accordés à la fréquence que l'on veut contrôler, ils présententdes limites considérables quand ils sont appliqués à des systèmes possédantdes incertitudes sur les paramètres modaux ou ayant une fréquence propredépendante de la force extérieure. Dans cette thèse la réduction des vibrations dans les systèmes mécaniques à l'aide d'un absorbeur Nonlinear Energy Sink est étudiée. Le phénomène qui gouverne la physique de ce dispositif est appelé pompage énergétique (Targeted Energy Transfer) et il consiste en un transfert irréversible d'énergie du système principal vers le NES, où l’énergie est dissipée. Ce transfert d'énergie peut se produire pour une large gamme de fréquences et sans besoin que le NES ne soit accordé _a une fréquence spécifique.La dynamique d'un premier type de NES appelé Vibro-Impact NonlinearEnergy Sink (VI-NES) est investiguée expérimentalement grâce à unoscillateur linéaire (OL) à un degré de liberté forcé harmoniquement auquelle VI-NES est attaché. Le pompage énergétique du OL vers le VI-NESest observé expérimentalement, ce qui a permis d'obtenir une importanteréduction du pic de résonance du système principal. Le système est étudiéanalytiquement à l'aide de la méthode Multi-Echelles et le comportementnon-linéaire observé est expliqué théoriquement. Le deuxième type de NES présenté est le Magnetic-Strung NES avec récupération d'énergie. Cette étude ajoute l'aspect lié à la récupération d'énergie au domaine de recherche des absorbeurs non-linéaires. Le système consiste en un oscillateur linéaire (OL) à un degré de liberté forcé harmoniquement auquel le MS-NES est appliqué. La force non-linéaire de rappel peut être modulée grâce à une force magnétique introduite judicieusement, ce qui permet au NES d'avoir plusieurs configurations possibles. Lesystème résultant est un système électromécanique où l'énergie vibratoire dusystème principal est absorbée par le NES et est ensuite dissipée en partiepar l'amortissement visqueux et convertie en partie en puissance électrique.Les études numérique et expérimentale analysent les performances du MSNESen tant qu'absorbeur d'énergie et en tant que récupérateur d'énergie.Finalement, les idées et les perspectives issues de cette étude sont traitéeset les directions pour les travaux futurs sont fournies. / Passive vibration control methods using linear dampers have been largelystudied and investigated, and they have nowadays a broad range of applications.However, linear dampers are efficient when tuned to the specificfrequency to control but present substantial limitations when applied to primarysystems with uncertainties on the modal parameters or to systemshaving a natural frequency that may vary with external forcing.In this thesis the vibration mitigation in mechanical systems by meansof a Nonlinear Energy Sink absorber is studied. The phenomenon governingthe physics of this kind of device is referred to as Targeted Energy Transferand it consists in an irreversible energy transfer from the primary systemto the NES where the energy is then dissipated. This energy transfer mayoccur over a broad range of frequencies with no need for the NES to betuned to a specific one.The dynamics of a first type of NES called Vibro-Impact Nonlinear EnergySink (VI-NES) is experimentally investigated via a harmonically forcedsingle-degree-of-freedom linear oscillator to which a VI-NES is attached. ATargeted Energy Transfer from the LO towards the VI-NES is experimentallyobserved and a significant reduction of the primary system's resonancepeak is obtained. The system is analytically studied by means of the MultipleScales method and the nonlinear behavior experimentally observed istheoretically explained. The second type of NES presented is the Magnetic-Strung NES withenergy harvesting. This study adds the energy harvesting aspect to the researchon nonlinear vibration absorbers. The system consists in a harmonicallyforced single-degree-of-freedom linear oscillator to which the MS-NESis applied. The type of nonlinearity used can be shaped thanks to a magneticforce aptly introduced, allowing the NES to have several possible configurations.The resulting system is an electro-mechanical system in which thevibration energy of the primary system is absorbed by the NES and subsequentlypartially dissipated by the viscous damping and partially convertedinto electrical power. The numerical and experimental studies analyze theperformances of the MS-NES both as an energy absorber and as an energyharvester.

Vibrações mecânicas oferecem grande limitação para a produtividade, qualidade ou mesmo viabilidade das operações de usinagem, especialmente quando se trata das autoexcitadas (chatter). Neste trabalho, foram desenvolvidas estratégias que tem como objetivo diminuir a intensidade de vibrações em processos de torneamento por meio do acoplamento de material piezelétrico ao suporte de ferramenta em conjunto com uso de shunts resistivo, indutivo e resistivo-indutivo em série ou em paralelo, criando assim estruturas eletromecânicas passivamente amortecidas. Para tanto, foram construídos modelos eletromecânicos de parâmetros distribuídos para mostrar a capacidade que tais estruturas eletromecânicas possuem em oferecer um maior amortecimento quando comparadas com estruturas mecânicas convencionais. Com os modelos construídos, foi possível verificar a influência causada pela espessura da camada de material piezelétrico bem como a influência dos shunts no comportamento da estrutura, sendo constatado que camadas mais espessas aumentam a capacidade de amortecimento da estrutura e que os shunts resistivo-indutivo, tanto em série quanto em paralelo, funcionam como um amortecedor dinâmico de vibrações amortecido e oferecem o melhor desempenho. A seguir, construiu-se o diagrama de lóbulos de estabilidade para comparar as estruturas com e sem shunts e observou-se que as estruturas com shunts resistivo-indutivo possuem um melhor desempenho. Também foram executados testes de impacto (tap tests) para a verificação experimental do comportamento da estrutura quando conectadas aos shunts e os resultados mostraram que há um maior amortecimento. Considerando os resultados obtidos, acredita-se que seja possível melhorar o desempenho de processos de torneamento usando material piezelétrico. / Mechanical vibrations offer great limitation for the productivity, quality or even feasibility of the machining operations when chatter is present. In this work it was developed strategies aiming to diminish the intensity of the vibration in turning processes. By coupling a piezoelectric material with a turning tool and by using different associations of resistive and inductive shunt (series or parallel) it was created electromechanical structures passively damped. Electromechanical models of distributed parameters were developed in order to show the capacity that these structures has to offer a greater dumping when compared with conventional mechanical structures. By using these constructed models it was possible to verify the influence of the thickness of the piezoelectric material as well as the influence of shunts in the behavior of structure. It was observed that thicker layers increase the damping capacity of the structure that resistive-inductive shunt (series or parallel) works as a damped dynamic vibration absorber which offer better performance. Latter was developed a stability lobes diagram in order to compare the structures with and without shunts and it was observed that structures connected to resistiveinductive shunt has a better performance. Tap tests were performed for the purpose of study the experimental behavior of the structure connected to shunt and results showed that there is a better damping in this situation. Considering the results obtained, is fair to believe that is possible to improve turning process by using piezoelectric materials.

Vibrações estruturais constituem uma das principais fontes de ruído e de desconforto em diversas aplicações, tais como nos produtos das indústrias automotiva, aeroespacial e naval, que, em grande parte, são compostos por estruturas flexíveis semelhantes a placas. Ao mesmo tempo, a demanda crescente por aumento de eficiência energética e por redução de emissão de gases nocivos ao meio ambiente tem levado os fabricantes de todos os segmentos para a utilização de materiais cada vez mais leves. Um desafio importante relacionado ao uso dessas soluções diz respeito ao seu comportamento vibroacústico, já que estruturas leves tendem a transmitir e emitir ruídos de forma mais intensa, particularmente em baixas frequências. Em decorrência disso, a obtenção de uma boa relação entre peso e desempenho vibroacústico usualmente demanda algumas iterações de projeto. Nesse contexto, este trabalho tem por objetivo estabelecer um procedimento numérico para avaliar as propriedades vibroacústicas de placas flexíveis excitadas por um campo gerado no interior de uma cavidade acústica, bem como investigar a influência de parâmetros estruturais que definem a placa sobre o seu desempenho vibroacústico. O escopo aqui é a faixa de baixas frequências, na qual o comportamento do sistema é descrito de forma determinística através de características modais. Foi desenvolvido um modelo numérico em elementos finitos de uma cavidade acústica retangular de paredes rígidas acoplada a uma placa flexível. Uma fonte acústica posicionada no interior da cavidade produz um campo de pressão sonora que induz a placa à vibração e, consequentemente, a irradiar potência sonora para o campo livre exterior. Para realização das análises de sensibilidade, desenvolveu-se um conjunto de rotinas de modo a permitir o gerenciamento automatizado de simulações. Resultados numéricos são apresentados para avaliar a influência de diversos parâmetros estruturais sobre o desempenho vibroacústico da superfície estrutural. O procedimento proposto pode ser aplicado de forma eficiente para a análise de sensibilidade da transmissão de ruído em sistemas vibroacústicos sujeitos à variação de parâmetros estruturais e pode também ser utilizado para avaliar diversos tipos de materiais, como isotrópicos, ortotrópicos e até metamateriais. / Structural vibrations are the major causes of noise and people discomforts in several applications, such as in the products of the automotive, aerospace and naval industries, which are mainly composed of flexible plate-like structures. At the same time, the ever-increasing demands for energy efficiency and for reduced emissions of harmful gases into the environment lead product manufacturers of all industrial sectors towards an increased use of lighter materials. However, these solutions are most sensitive to issues related to vibroacoustic behavior, since the lightweight structures tend to present a high sound transmission, particularly at low frequencies. Consequently, the optimal design of lightweight and noise insulation properties of a structure usually require some design trade-offs. In this context, this dissertation aims to propose a numerical procedure to analyze the vibroacoustic performance of flexible plates subject to an interior acoustic field, as well as to investigate the influence of structural parameters on the sound insulation properties of the plate. The scope here is the low frequency range, in which the response of the coupled system is described in a deterministic way in terms of a mode set. A finite element model of a rigid rectangular acoustic cavity coupled to a flexible plate was developed. An acoustic source inside the cavity produces an interior sound pressure field that forces the flexible plate to vibrate and, consequently, to radiate sound power to the external free field. In order to enable the automatic management of the variability simulations, a set of routines was also developed. Numerical results are presented in order to evaluate the influence of several structural parameters on the vibroacoustic properties of the plate. The proposed methodology can be largely used in sensitivity analysis of noise transmission in vibroacoustic systems subject to the variation of structural parameters and can be used to evaluate many material types, as isotropic, orthotropic and even metamaterials.

No description available.