Granular materials utilised in the construction of highway foundation layers are currently specified on the basis of index tests. As a consequence, the material acceptability criteria, although developed from many years' experience, do not directly measure a fundamental performance parameter. Once the granular materials are placed and compacted they are rarely checked and as such no assurance can be given to their likely engineering performance in situ. An important performance parameter, the stiffness modulus, describes the ability of the constructed layer(s) to spread the construction (and in-service) vehicle contact pressures and reduce the stresses, and hence strains, transmitted to the lower weaker layers. A significant improvement upon current practice would be to include the specification of 'end product' testing and to include the direct measurement in situ of stiffness modulus to assure performance. A prerequisite of this is suitable site equipment to measure such a parameter, and a sound basis upon which to interpret and utilise such data. Tests do exist that measure stiffness modulus in situ, although in general they measure a 'composite' stiffness, i.e. a single transducer infers the surface strain, under controlled loading, for the construction as a whole and the region affecting the measurement is not precisely known. Currently then, no routine portable device exists for the direct stiffness modulus assessment of the near surface or last layer applied. This would not only provide for consistency of construction, but avoid burying poor or weaker layers. This thesis describes the evaluation of a portable impact test device and research into the behaviour of granular soils subject to rapid transient loads. The requirements for the assessment of pavement granular foundation layers are reviewed, followed by a critical appraisal of current devices that measure the stiffness modulus of material in situ. The prototype impact device, known as ODIN, comprising an accelerometer instrumented swinging hammer, is described. A selection of field data, demonstrating the primary soil influencing factors and correlations with other devices, is presented. Controlled laboratory testing is also described, comprising impact testing with free-falling masses in addition to the ODIN device and for tests on foundations instrumented with pressure cells, that further explains the dynamic behaviour of the material under test. Problems with both hardware and software, associated with high-frequency impact testing are highlighted. In particular, the restraint of the impact mass by the swinging arm mechanical component is observed to lead to a proportion of the impact energy being channelled back into the apparatus during a test. The channelled energy is shown to produce resonance of the apparatus, which in turn leads to problems in interpretation of the accelerometer signal. Numerical methods are then explored and it is demonstrated that the predictions approximated well to the free-falling weights experimental data. Discussion of the research findings concludes with a model for soil behaviour under impact testing, requirements for an improved impact device and the further research work required to realise the potential of such equipment.

No description available.

The master thesis deals with computational modeling of a turbocharger vibrations and and assessment of influnce of passive dynamic vibration absorber on vibrations of actuator bracket. The use of dynamic vibration absorber was summarized in the research study. The analysis were performed using finite element method in ANSYS. Several computational models of turbocharger were created with different ways of modeling bolted joints between turbocharger parts. Modal analysis of each model was performed and the results were compared. For the selected model, the response to the kinematic excitation from the internal combustion engine for two load conditions was calculated using harmonic analysis. A simple model of vibration dynamic absorber was applied to the turbocharger model with reduced degrees of freedom and its influnce on vibrations of actuator bracket was investigated. Significant decrease of the maximum acceleration amplitude was achieved in a given frequency range when absorber parameters were optimized.

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

O presente trabalho consiste no projeto de um modelo em elementos finitos de um absorvedor dinâmico rotativo utilizando-se um material viscoelástico como componente dissipador do sistema. O absorvedor é composto por um anel de material viscoelástico interposto entre dois anéis de aço, o qual é fixado na extremidade livre de um sistema rotativo representado por um eixo flexível, suportado por dois rolamentos, no qual estão fixos dois discos igualmente espaçados do centro entre os dois mancais. O modelo em elementos finitos do sistema rotativo é validado com os dados experimentais do modelo real e suas velocidades críticas são determinadas baseadas no diagrama de Campbell e na resposta ao desbalanço em um dos discos. O modelo inicial do absorvedor dinâmico rotativo é replicado de um modelo da literatura e as respostas a uma excitação na forma de impulso são comparadas. O modelo desenvolvido equipara-se ao da literatura para frequências até 600 Hz, intervalo que compreende as velocidades críticas a serem amortecidas. A otimização do absorvedor é realizada através de variações da geometria do mesmo e são traçadas curvas de influência de cada parâmetro sobre suas frequências naturais. Com base nestas curvas são realizadas análises de influência conjunta dos parâmetros geométricos sobre tais frequências. Pelos resultados obtidos verifica-se a possibilidade da obtenção de um modelo que atue sobre modos de flexão específicos do sistema rotativo, atenuando as amplitudes de vibração das velocidades críticas correspondentes a cada modo. Uma vez obtidos os modelos de absorvedores dinâmicos correspondentes aos dois primeiros modos de flexão do sistema rotativo, ambos são acoplados ao sistema e se observa grande redução dos picos de amplitude do primeiro modo de flexão, enquanto os picos do segundo modo sofrem baixa alteração. / The present work concerns the development, optimization and validation of a finite element model of a dynamic vibration absorber using a viscoelastic material as the damping component. The dynamic absorber consists of a ring of viscoelastic material interposed between two rings of steel, which is fixed to the free end of a rotary system represented by a flexible shaft supported by two bearings, on which are fixed two discs equally spaced in the center of both bearings. The finite element model of the rotating system is validated with experimental data from the actual model and its critical speeds are determined based on the Campbell diagram and in its response to the imbalance. The initial model of the dynamic absorber is replicated from a model of the literature and the responses to an impulse excitation are compared. The developed model matches the literature one for frequencies up to 600 Hz, range comprising the critical speeds to be damped. The absorber\'s optimization is accomplished through variations of its geometry and influence curves of each parameter over its natural frequencies are drawn. Based on these curves, combined influence analyzes of the geometrical parameters over such frequencies are performed. From the results obtained, it can be seen the possibility of achieving a model that acts on specific bending modes of the rotation system, reducing the vibration amplitudes of the critical speeds corresponding to each mode. Once obtained the dynamic absorbers models corresponding to the first two modes of vibration of the rotatative system, both models are coupled to the system and it is observed great reduction of the amplitude of the first bending mode peaks, while the second mode suffer low peaks reduction.

Absorvedores de vibração são comumente usados em aplicações com intuito de reduzir indesejadas amplitudes de vibração de estruturas e maquinas vibrantes. O conceito de um absorvedor de vibração linear consiste na ideia de projetar um subsistema com frequência de ressonância coincidente com uma dada frequência de interesse, tal que a amplitude de vibração do sistema primário e significativamente reduzida quando comparada a situação original, sem o absorvedor de vibração. Porem, uma deficiência dos absorvedores de vibração lineares típicos e sua estreita faixa de frequência de operação. Para superar essa deficiência, muitas tentativas de solução usando subsistemas não lineares tem sido propostas na literatura, ja que se apropriadamente projetados, eles podem aumentar a faixa de frequência de absorção de vibração e/ou melhorar a redução das amplitudes de vibração do sistema primário. Contudo, a síntese e o projeto de tais absorvedores não lineares não e tão simples e direta como no caso linear. Baseado na geometria de uma topologia proposta e encontrada na literatura, que compreende a inclusão de uma montagem do tipo snap through truss no lugar da mola linear do absorvedor de vibração, este trabalho tem intenção de apresentar um estudo sobre o projeto e otimização de um absorvedor dinâmico de vibrações não linear. Portanto, o efeito dos parâmetros do absorvedor e analisado quanto as perspectivas de redução das amplitudes de vibração do sistema principal como também de aumento da faixa de frequência de operação. A analise paramétrica do absorvedor foi promovida para responder questões sobre as variáveis de projeto, tanto físicas como geométricas. Realizou-se otimização do absorvedor com objetivo de sintoniza-lo a frequência de trabalho desejada, através de busca extensiva e algoritmos genéticos. Os resultados mostram que o absorvedor não linear proposto pode ser mais efetivo que seu correspondente linear em ambos os aspectos, na redução da máxima amplitude de vibração e no aumento da faixa de frequência de absorção. Portanto, apesar da dificuldade inicial de projeto, esse tipo de absorvedor representa uma alternativa interessante na atenuação das amplitudes de vibração ao longo de uma extensa faixa de frequência. / Dynamic vibration absorbers are commonly used in several applications in order to reduce undesired vibration amplitudes of vibrating machinery and structures. The concept of a linear vibration absorber is based on the idea of designing a subsystem with a resonance frequency coincident with a given frequency of interest such that the vibration amplitude of the primary system is significantly reduced when compared to the original situation (without the vibration absorber). But one of the known handicaps of typical linear vibration absorbers is their narrow frequency range of operation. To overcome this handicap, a number of tentative solutions have been proposed in the literature using nonlinear subsystems. If properly designed, they could enlarge the frequency range of vibration absorption and/or improve vibration reduction of the primary system. However, the synthesis and design of such nonlinear absorbers are not as straightforward as for their linear counterpart. A proposed design found in the open literature consists of replacing the linear spring of the vibration absorber by a nonlinear snap-through truss. This work aims to present a study on the design and optimization of a nonlinear dynamic vibration absorber based on snap-through absorber geometry. The effect of the absorber parameters was analyzed on both, the primary system vibration amplitude reduction and the frequency range of operation. Parametric analyses of the absorber were carried out to answer questions about the physical and geometric design variables. The absorber optimization was performed in two different ways, by extensive search and genetic algorithms, in order to tune it in the desired working frequency. The results show that the proposed nonlinear vibration absorber may be more effective than its linear counterpart both in terms of maximum vibration amplitude reduction and absorption frequency-range. Therefore, despite the increased design complexities such an absorber is an interesting alterna- tive in attenuating vibration amplitudes over a wide frequency range.

This study investigated the vibration serviceability of timber structures with dowel-type connections. It addressed the use of such connections in cutting-edge timber structures such as multi-storey buildings and long-span bridges, in which the light weight and flexibility of the structure make it possible that vibration induced by dynamic forces such as wind or footfall may cause discomfort to occupants or users of the structure, or otherwise impair its intended use. The nature of the oscillating force imposed on connections by this form of vibration was defined based on literature review and the use of established mathematical models. This allowed the appropriate cyclic load to be applied in experimental work on the most basic component of a dowel-type connection: a steel dowel embedding into a block of timber. A model for the stiffness of the timber in embedment under this cyclic load was developed based on an elastic stress function, which could then be used as the basis of a model for a complete connector. Nonlinear and time-dependent behaviour was also observed in embedment, and a simple rheological model incorporating elastic, viscoelastic and plastic elements was fitted to the measured response to cyclic load. Observations of the embedment response of the timber were then used to explain features of the behaviour of complete single- and multiple-dowel connections under cyclic load representative of in-service vibration. Complete portal frames and cantilever beams were tested under cyclic load, and a design method was derived for predicting the stiffness of such structures, using analytical equations based on the model for embedment behaviour. In each cyclic load test the energy dissipation in the specimen, which contributes to the damping in a complete structure, was measured. The analytical model was used to predict frictional energy dissipation in embedment, which was shown to make a significant contribution to damping in single-dowel connections. Based on the experimental results and analysis, several defining aspects of the dynamic response of the complete structures, such as a reduction of natural frequency with increased amplitude of applied load, were related to the observed and modelled embedment behaviour of the connections.

Liquid sloshing in a 2D tank applied on a wave energy capture system and reducing the oscillation of an offshore platform are discussed in this study. A fully nonlinear time-independent finite difference method and the forth-order Runge-Kutta method are implemented to solve the coupled motions of liquid sloshing in a 2D tank with a floating platform. When the external forcing frequency of the Dynamic Vibration Absorber System composed by a tuned liquid damper and a tuned mass damper is identical to the fundamental frequency of the tank, the external force can be effectively diminished by the sloshing-induced force. In the meantime, the maximum effect of tuned mass damper on reducing the amplitude of the floating platform appears. When the frequency of external forcing is close to the first natural frequency of the liquid tank, the coupled effect between the motions of both the tank and the platform can effectively reduce the vibration of the platform and the total energy of the whole system. The Eigenfrequency of a wave capture system is formed by the coupled effect of a liquid tank and a wave capture system. When the excitation frequency of the wave capture system is near its Eigenfrequency, the sloshing-induced force is much larger than that of external and the maximum displacement of the wave energy capture system occurs. As a result, the wave energy capacity of the wave capture system can be averagely increased to 150% by the influence of liquid sloshing in the tank.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / This work is dedicated to the design of a vibrating blade dynamic vibration absorber (ADVLV), which is composed by a blade that is subjected to an initial traction T , and contains a concentrated mass m that is fixed at a given position d along the blade. These three parameters can be adjusted so that the ADVLV is tuned. For this aim, a finite element model of the system was built, leading to a design methodology for the absorber. Also, design of experiment techniques were performed to obtain the most interesting configurations for the system, both for the computational and experimental models. Special care was taken with respect to the boundary conditions for the finite element model, so that the dynamic responses could correspond to the physical aspects of the problem, accordingly. Besides, an experimental prototype was constructed and tested under laboratory conditions. The experimental results were compared with those obtained from mathematical simulation. From this comparison, it was concluded that the finite element model had to be updated in such a way that experimental results could match. A vibrating string dynamic vibration absorber (ADVCV) was also studied. However, this DVA configuration presented two anti-resonant frequencies due to the coupling of the first vibration mode along the horizontal and vertical directions with a concentrated mass. Another phenomenon that was observed is the tridimensional motion of the vibrating string around its equilibrium position, leading to an ellipsoid-shape movement when a harmonic excitation whose frequency coincides with the primary system resonance frequency is applied to the system. This way, the ADVCV is not able to attenuate the vibration amplitude of the primary system satisfactorily. It is worth mentioning that the proposed ADVLV presents a good dynamic behavior besides a wide frequency range along which the DVA can be tuned. Besides, the present vibration absorbing device is simple and can be easily connected to the primary system both to mechanical and civil engineering structures. / Este trabalho aborda o projeto de um absorvedor dinâmico de vibrações do tipo lâmina vibrante (ADVLV), sendo este constituído por uma lâmina sujeita a uma tração inicial T com uma massa concentrada m que pode ser fixada em uma posição d da lâmina. Este três parâmetros podem ser alterados a fim de se obter a sintonia do ADVLV. Para realizar o estudo deste, foi elaborado um modelo de elementos finitos do sistema, permitindo assim obter a metodologia para seu projeto. Também foram usadas técnicas de planejamento de experimento para obter as melhores configurações, tanto para os ensaios computacionais como experimentais. Foram tomados cuidados na criação das condições de contorno do modelo de elementos finitos, a fim de se obter respostas que representem adequadamente os aspectos físicos do problema. Também foi construído um protótipo e este foi ensaiado no laboratório. Os resultados obtidos foram comparados com os obtidos através da simulação computacional. A partir desta comparação verificou-se a importância de realizar ajustes no modelo de elementos finitos para adequar este à realidade. Também foi estudado o absorvedor dinâmico de vibração do tipo corda vibrante. Entretanto este ultimo ADV apresentou duas freqüências de anti-ressonância devido ao acoplamento do primeiro modo de vibrar nas direções horizontal e vertical da corda vibrante com uma massa concentrada. Outro fenômeno observado foi o movimento tridimensional da corda vibrante em torno da sua posição de equilíbrio, resultando uma forma semelhante a um elipsóide de revolução quando uma excitação harmônica com freqüência igual à freqüência de ressonância do sistema primário é aplicada sobre o sistema. Desta forma, o ADVCV não consegue cumprir a sua função de atenuar a amplitude de vibração da estrutura primária, sendo, portanto, completamente ineficiente neste caso. O ADVLV, proposto neste trabalho, apresentou comportamento dinâmico satisfatório, além de uma grande faixa de freqüências na qual o ADV pode ser sintonizado. Este dispositivo é de fácil construção e acoplamento, tanto a sistemas mecânicos, como a estruturas de construção civil. / Mestre em Engenharia Mecânica

Absorvedores de vibração são comumente usados em aplicações com intuito de reduzir indesejadas amplitudes de vibração de estruturas e maquinas vibrantes. O conceito de um absorvedor de vibração linear consiste na ideia de projetar um subsistema com frequência de ressonância coincidente com uma dada frequência de interesse, tal que a amplitude de vibração do sistema primário e significativamente reduzida quando comparada a situação original, sem o absorvedor de vibração. Porem, uma deficiência dos absorvedores de vibração lineares típicos e sua estreita faixa de frequência de operação. Para superar essa deficiência, muitas tentativas de solução usando subsistemas não lineares tem sido propostas na literatura, ja que se apropriadamente projetados, eles podem aumentar a faixa de frequência de absorção de vibração e/ou melhorar a redução das amplitudes de vibração do sistema primário. Contudo, a síntese e o projeto de tais absorvedores não lineares não e tão simples e direta como no caso linear. Baseado na geometria de uma topologia proposta e encontrada na literatura, que compreende a inclusão de uma montagem do tipo snap through truss no lugar da mola linear do absorvedor de vibração, este trabalho tem intenção de apresentar um estudo sobre o projeto e otimização de um absorvedor dinâmico de vibrações não linear. Portanto, o efeito dos parâmetros do absorvedor e analisado quanto as perspectivas de redução das amplitudes de vibração do sistema principal como também de aumento da faixa de frequência de operação. A analise paramétrica do absorvedor foi promovida para responder questões sobre as variáveis de projeto, tanto físicas como geométricas. Realizou-se otimização do absorvedor com objetivo de sintoniza-lo a frequência de trabalho desejada, através de busca extensiva e algoritmos genéticos. Os resultados mostram que o absorvedor não linear proposto pode ser mais efetivo que seu correspondente linear em ambos os aspectos, na redução da máxima amplitude de vibração e no aumento da faixa de frequência de absorção. Portanto, apesar da dificuldade inicial de projeto, esse tipo de absorvedor representa uma alternativa interessante na atenuação das amplitudes de vibração ao longo de uma extensa faixa de frequência. / Dynamic vibration absorbers are commonly used in several applications in order to reduce undesired vibration amplitudes of vibrating machinery and structures. The concept of a linear vibration absorber is based on the idea of designing a subsystem with a resonance frequency coincident with a given frequency of interest such that the vibration amplitude of the primary system is significantly reduced when compared to the original situation (without the vibration absorber). But one of the known handicaps of typical linear vibration absorbers is their narrow frequency range of operation. To overcome this handicap, a number of tentative solutions have been proposed in the literature using nonlinear subsystems. If properly designed, they could enlarge the frequency range of vibration absorption and/or improve vibration reduction of the primary system. However, the synthesis and design of such nonlinear absorbers are not as straightforward as for their linear counterpart. A proposed design found in the open literature consists of replacing the linear spring of the vibration absorber by a nonlinear snap-through truss. This work aims to present a study on the design and optimization of a nonlinear dynamic vibration absorber based on snap-through absorber geometry. The effect of the absorber parameters was analyzed on both, the primary system vibration amplitude reduction and the frequency range of operation. Parametric analyses of the absorber were carried out to answer questions about the physical and geometric design variables. The absorber optimization was performed in two different ways, by extensive search and genetic algorithms, in order to tune it in the desired working frequency. The results show that the proposed nonlinear vibration absorber may be more effective than its linear counterpart both in terms of maximum vibration amplitude reduction and absorption frequency-range. Therefore, despite the increased design complexities such an absorber is an interesting alterna- tive in attenuating vibration amplitudes over a wide frequency range.