No description available.

The use of sandwich structures in automobile body panels is investigated in this thesis. The applications on vehicles such as trains, aeroplanes and automobiles, advantages, isadvantages and modelling of sandwich structures are discussed and studies about static, vibrational and acoustic benefits of sandwich structures by several authors are presented. The floor, luggage, firewall and rear wheel panels in sheet metal form is replaced with panel made from sandwich materials in order to reduce the weight obtained by a trial and error based optimization method by keeping the same bending stiffness performance. In addition to these, the use of sandwich structures over free layer surface damping treatments glued on floor panel to decrease the vibration levels and air-borne noise inside the cabin is investigated. It has been proven that, the same vibration performance of both flat beam and floor panel can be obtained using sandwich structures instead of free layer surface damping treatments with a less weight addition. Furthermore, the damping effect of sandwich structures on sound transmission loss of complex shaped panels like floor panel is investigated. A 2D flat and curved panel representing the floor panel of FIAT Car model are analysed in a very large frequency range. Four different loss factors are applied on these panels and it is seen that, until it reaches damping controlled region, damping has a very little effect on TL of flat panels but has an obvious damping effect on TL of curved panels. However in that region, damping has an increasing effect on TL of both flat and curved panels.

Viscoelastic materials can today be found in a wide range of practical applications. In order to make efficient use of these materials in construction, it is of importance to know how they behave when subjected to dynamic load. Characterization of viscoelastic materials is therefore an important topic, that has received a lot of attention over the years. This thesis treats different methods for identifying the complex modulus of an viscoelastic material. The complex modulus is a frequency dependent material function, that describes the deformation of the material when subjected to stress. With knowledge of this and other material functions, it is possible to simulate and predict how the material behaves under different kinds of dynamic load. The complex modulus is often identified through wave propagation testing, where the viscoelastic material is subjected to some kind of load and the response then measured. Models describing the wave propagation in the setups are then needed. In order for the identification to be accurate, it is important that these models can describe the wave propagation in an adequate way. A statistical test quantity is therefore derived and used to evaluate the wave propagation models in this thesis. Both nonparametric and parametric identification of the complex modulus is considered in this thesis.  An important aspect of the identification is the accuracy of the estimates.  Theoretical expressions for the variance of the estimates are therefore derived, both for the nonparametric and the parametric identification. In order for the identification to be as accurate as possible, it is also important that the experimental data contains as much valuable information as possible. Different experimental conditions, such as sensor locations and choice of excitation, can influence the amount of information in the data. The procedure of determining optimal values for such design parameters is known as optimal experiment design. In this thesis, both optimal sensor locations and optimal excitation are considered.

Submitted by Geandra Rodrigues (geandrar@gmail.com) on 2018-10-16T10:52:39Z No. of bitstreams: 1 tamaradecarvalhofreitas.pdf: 4928276 bytes, checksum: 9de5596b41afe271cb80f81cfe1197a6 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2018-10-16T14:32:24Z (GMT) No. of bitstreams: 1 tamaradecarvalhofreitas.pdf: 4928276 bytes, checksum: 9de5596b41afe271cb80f81cfe1197a6 (MD5) / Made available in DSpace on 2018-10-16T14:32:24Z (GMT). No. of bitstreams: 1 tamaradecarvalhofreitas.pdf: 4928276 bytes, checksum: 9de5596b41afe271cb80f81cfe1197a6 (MD5) Previous issue date: 2018-09-05 / Sistemas passivos de controle para a atenuação de vibrações em estruturas apresentam grande diversidade de concepções, pois resultam de projetos criativos voltados para cada problema específico. Em geral, são mecanicamente robustos e se mostram como alternativas mais eficientes, sob o ponto de vista dinâmico estrutural, do que as técnicas usuais e conservadoras de enrijecimento da estrutura. Dentre estes sistemas, podem-se destacar aqueles que utilizam materiais viscoelásticos (MVE) como núcleo amortecedor, como por exemplo os sistemas tipo sanduíche. Estes materiais têm propriedades mecânicas dependentes da temperatura e, principalmente, da frequência de vibração, trazendo dificuldades adicionais às já complexas formulações teóricas do problema. Neste trabalho, as formulações usadas para modelar sistemas sanduíches viscoelásticos GHM (Golla-Hughes-MacTavish); ADF (Anelastic Displacement Field); e DF (Derivadas de Ordem Fracionária) são tomadas como base para as simulações computacionais analisadas. Partindo de experimentos de caracterização de materiais desenvolvidos, as formulações supracitadas foram adotadas para estimar o comportamento dinâmico de estruturas sanduíche. Por fim, estratégias voltadas para o projeto de estruturas sanduíches com MVE são propostas, baseadas no desempenho de cada uma das formulações avaliadas no que se refere às suas respectivas capacidades de simular os experimentos realizados. / Passive control systems for vibration control in structures demonstrate a wide variety of conceptions, as they are results of creative projects focused on specific problems. Usually, they are robust mechanisms and are shown as more efficient alternatives than ordinary and conservative techniques of structural stiffening. Among these systems, it is possible to highlight the ones that use viscoelastic materials (VEM) as damping core, such as sandwich systems. These materials have mechanical properties depending on temperature and, mainly, on the vibration frequency, introducing additional difficulties to the already complex theoretical formulations of the problem. In this work, formulations used to model viscoelastic sandwich systems, such as GHM (Golla-Hughes-MacTavish); ADF (Anelastic Displacement Field); and DF (Fractional Order Derivatives) are taken as basis for the computational simulations considered. Based on the experimental characterization of the VEM’s, the formulations previously mentioned were used to estimate the dynamic behavior of sandwich structures. Finally, strategies aimed at design of sandwich VEM structures are indicated, based on the performance of each of the evaluated formulations, regarding to their ability to simulate the experiments performed.

Orientador: Marcos Akira d'Ávila / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-22T01:11:51Z (GMT). No. of bitstreams: 1 Lima_NicolaoCerqueira_M.pdf: 2608433 bytes, checksum: 1f09fc99d86a78b7861be6972463c29c (MD5) Previous issue date: 2013 / Resumo: A eletrohidrodinâmica trata basicamente dos efeitos de um campo elétrico em meios contínuos. Um fluido sob o efeito de um campo elétrico tende a se deformar devido a uma força elétrica que age sobre ele. Essa força elétrica é consequência de alguns fatores que têm como base as próprias propriedades do fluido. Entre elas estão à condutividade elétrica (capacidade do fluido de ionizar-se ou de conduzir corrente elétrica) e a permissividade elétrica (capacidade do fluido de polarizar-se). Diversos processos que utilizam efeitos elétricos em fluidos foram desenvolvidos nas últimas décadas. O fato de estes processos envolverem uma alta complexidade de parâmetros faz com que seja consumido bastante tempo e material durante a fase de testes. Por esse motivo, as simulações numéricas passaram a ser uma boa alternativa para otimizar tais processos, além de aumentar o conhecimento sobre eles. Nesse contexto, no presente trabalho foi implementado um código numérico (solver) no pacote de CFD OpenFOAM baseado no modelo para fluidos pouco condutores (leaky dieletric model) no intuito de descrever escoamentos eletrohidrodinâmicos. Entre eles estão o efeito de um campo elétrico em uma gota condutora e o afinamento de um jato na saída de um tubo capilar. Eventualmente, em processos eletrohidrodinâmicos, são utilizados polímeros. Entretanto, não há na literatura muitos estudos sobre a simulação de escoamentos eletrohidrodinâmicos utilizando fluidos viscoelásticos. Assim, um segundo código foi implementado, baseado em um código já existente. Este é capaz de simular efeitos elétricos em fluidos viscoelásticos, utilizando a equação constitutiva de Giesekus como modelo viscoelástico. Os resultados obtidos para a deformação da gota condutora foram comparados com resultados analíticos para fluidos newtonianos e com observações experimentais para fluidos viscoelásticos. Para o caso do jato na saída de um tubo capilar, ambos os tipos de fluidos (newtonianos e viscoelásticos) foram comparados com resultados experimentais e teóricos / Abstract: Electrohydrodynamics deals basically on the effects of an electric field on a continuum media. A fluid under the effect of an electric field tends to deform due to an electric force that acts on it. This electric force is a consequence of some factors which are based on the fluid properties, including the electric conductivity (ability of the fluid to ionize or to conduct electrical current) and the permittivity (ability of the fluid to polarize). Many processes using electrical effects in fluids have been developed in recent decades. The fact that these processes involve a high complexity of parameters, it consumes time and materials during the test phase. For that reason, the numerical simulations start being a good alternative to optimize such processes, and also to increase the knowledge about them. In this context, on the present work was implemented a solver on the open CFD software OpenFOAM, based on the leaky dielectric model, in order to describe electrohydrodynamic flows. Among them, are the effect of an electric field on a conducting droplet and the thinning of a jet on the exit of a nozzle. Eventually, in electrohydrodynamic processes, polymers are used. However, there aren't many studies about electrohydrodynamic flow simulation using viscoelastic fluids. So, a second solver was made, based on another existing solver. This second solver is able to simulate electric effects on viscoelastc fluids, using the Giesekus model as a constitutive equation. The results of the deformation of a conducting droplet were compared to analytical results, for Newtonian fluids, and with experimental observations, for viscoelastic ones. As for the jet, both types of fluids, Newtonians and viscoelastics, were compared to experimental and theoretical results / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

No description available.

This thesis examines the stress relaxation phenomenon that may occur in stainless steel rods that are used as part of an all-stainless steel externally post-tensioned rod system for strengthening reinforced concrete pier caps in shear. An overview of the stress relaxation phenomenon and its modeling is presented. Previous work on stress relaxation of stainless steel is reviewed and used to select an appropriate stress relaxation model to analyze the stress relaxation data obtained from experiments performed during this research. An experimental program utilizing six specimens of Type 304/304L stainless steel stainless steel rods similar to those likely to be candidates for pier cap rehabilitation is presented, and the data are analyzed using the model identified from the review. This model reasonably captures the behavior of the resulting stress relaxation with time, and calculates stress values that are fairly close to those obtained experimentally. Using this model to predict long-term stress losses in a service environment, guidelines for design and installation of the post-tensioned stainless steel rods are proposed.

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / This work is dedicated to the development of a strategy for numerical-computational modeling and experimental verification of the self-heating phenomenon in viscoelastic materials with emphasis on the thermal runaway phenomenon taking into account the combined effects of dynamic loads and static preloads. The methodology of modeling by finite element allows us to consider the influence of frequency, temperature and static preload on the self-heating phenomenon of the linear viscoelastic materials. For this purpose, modifications are made that allow thermomechanical analysis of more complex viscoelastic structures, in addition the evaluation of introducing metal inserts in bulk material for reducing effects of self-heating. The validation of the proposed model and the identification of the physical parameters of thermal efficiency and heat transfer by natural convection, initially unknown, are obtained by comparison of the results of numerical simulations with the corresponding obtained through experimental tests for a specimen formed by a translational viscoelastic joint. The curve-fitting procedure is formulated as an inverse optimization problem through use of the Firefly Algorithm for minimizing the objective function defined as the square difference between the temperatures obtained from the simulations and the corresponding generated by the tests for each time instant. The accuracy and limitations of the model are evaluated by comparing the experimental and simulated temperature profile, allowing to verify the numerical evidence and the qualitative consistence of the results obtained with reported in the literature for the thermal runaway phenomenon for simple devices without effect preload. / Este trabalho é dedicado ao desenvolvimento de uma estratégia de modelagem numéricocomputacional e verificação experimental do fenômeno do autoaquecimento de materiais viscoelásticos com ênfase no fenômeno da fuga térmica levando-se em conta os efeitos combinados de cargas dinâmicas e pré-cargas estáticas. A metodologia de modelagem por elementos finitos permite considerar a influência da frequência, da temperatura e da pré-carga estática no fenômeno do autoaquecimento de materiais viscoelástico lineares. Para tanto, são feitas modificações que permitem a análise termomecânica de estruturas viscoelásticas mais complexas, além da avaliação da introdução de insertos metálicos no volume do material para a redução dos efeitos do autoaquecimento. A validação do modelo proposto e a identificação dos parâmetros físicos de rendimento térmico e de transferência de calor por convecção natural incialmente desconhecidos, são obtidos através da confrontação dos resultados das simulações numéricas com os correspondentes obtidos via ensaios experimentais para um corpo de prova formado por uma junta viscoelástica translacional. O procedimento de ajuste de curvas é formulado como um problema inverso de otimização via emprego da técnica Colônia de Vagalumes para a minimização da função objetivo definida como sendo a diferença quadrática entre as temperaturas obtidas das simulações e as correspondentes geradas pelos ensaios para cada instante de tempo. A precisão e as limitações do modelo são avaliadas pela comparação dos perfis simulados e experimentais de temperatura, possibilitando confirmar as evidências numéricas e a consistência qualitativa dos resultados obtidos com o reportado na literatura para o fenômeno da fuga térmica para dispositivos mais simples e sem o efeito da pré-carga. / Mestre em Engenharia Mecânica

This work analyzes partial slip contact problems in the theory of linear viscoelasticity using both the semi-analytical method and nite element method. Such problems arise in metal-polymer contacts in orthopedic implants and similar applications. The boundary conditions of such problems are inherently mixed and vary with time, thus restricting the use of classical correspondence principle, which have been the basic approach for most of the solved problems in viscoelasticity. In the present semi-analytical approach, the governing equations for the vis-coelastic partial-slip contact are formulated as a pair of coupled Singular Integral Equations (SIEs) for a pin-plate geometry using the viscoelastic analogues of Green's functions. The formulation is entirely in the time-domain, avoiding Laplace transforms. Both Coulomb and hysteretic e ects are considered, and arbitrary load histories, including the bidirectional pin loads and remote plate stresses, are allowed. Moreover, the contact patch is allowed to advance and recede with no restrictions. The presence of viscoelastic behavior necessitates the application of the stick zone boundary condition in convolved form, and also introduces additional convolved gap terms in the governing equations, which are not present in the elastic case. Transient, as well as steady-state contact tractions, are obtained under load-hold, unload-hold, unload-reload, cyclic bidirectional (fretting) and remote plate loading for a three-element delayed elastic solid. A wide range of loads, loading rates, friction coeficients and the conforming nature of the contact are considered. The contact size, stick-zone size, indenter approach, maximum pressure, Coulomb energy dissipation are tracked during fretting. The edge-of-contact stresses and the subsurface stresses for the viscoelastic plate due to the contact tractions are determined by solving an equivalent traction boundary value problem. It is found that the viscoelastic fretting contact tractions for materials with delayed elastic nature shakedown just like their elastic counterparts. However, the number of cycles to attain shakedown states is strongly dependent on the ratio of the load cycle time to the relaxation time constant of the viscoelastic material. In monotonic load-hold case, the viscoelastic steady-state tractions agree well with the tractions from an equivalent elastic analysis using the shear modulus at infinite time. Whereas, the viscoelastic fretting tractions in shakedown differ considerably from their elastic counterparts. This is due to the fact that the contact patch does not increase monotonically in fretting-type(cyclic) loading. Hence, an approximate elastic analysis misleads to an incorrect edge-of-contact stresses. During fretting, the edge-of-contact hoop stress also shakedown and reaches its peak value at the trailing edge-of-contact when the horizontal pin load reaches its maximum. Moreover, the peak tensile of the edge-of-contact hoop stress increases with the increase in the Coulomb friction coefficient. In cyclic loading, Coulomb dissipation in a cycle at steady-state is almost independent of the rate at which the load is cycled. However, the viscous energy dissipated in a cycle is a strong function of the ratio of the load cycle time to the relaxation time constant. The steady-state cyclic hysteretic energy dissipation typically dominates the cyclic Coulomb dissipation, with a more pronounced difference at slower load cycling. However, despite this, it is essential to model an accurate viscoelastic fretting contacts including the effects of both viscous and Coulomb friction dissipation to obtain accurate contact stresses. A 12-element generalized Maxwell solid with long time scales representing a well characterized viscoelastic material like PMMA is also studied. The material chosen is of slowly relaxing nature and the ratio of the instantaneous shear modulus(G0) to the modulus at the infinite time(G1) is almost equal to 1000. In such cases, the material is effectively always in a transient state, with no steady edge-of-contact. As a consequence, the location of the peak hoop stress keeps on shifting when the load cycle is repeated. Interestingly, the rate at which the viscoelastic material relaxes affects the contact tractions. It is observed that the rapidly relaxing materials show qualitatively different tractions in the partial slip, with local traction spikes close to the edges-of-contact and concomitant high-stress gradients. On the other hand, finite element method is also used to analyze the partial slip viscoelastic contacts. In FEA, the pin-plate geometry is modeled using a custom mesh maker, where a 2D-continuum plane strain element is used for the plate and rigid element for the pin. The technique uses 'ABAQUS Standard' solver to solve the contact problem. Finite element analysis for a wide range of loads comparable with the SIE technique is performed. The tractions and contact sizes for various load cases such as unload-reload, fretting-type cyclic loads from both SIE and FEA agrees well. In certain conditions, there exist multiple contact arcs or stick zones that are currently difficult to solve with SIE's. However, such problems are treated using FEA and one such problem is illustrated.

Rubber like materials find wide applications in damping treatment of structures, vibration isolations and they appear prominently in the form of hoses in many structures such as aircraft engines. The study reported in this thesis addresses a few issues in computational modeling of vibration of structures with some of its components made up of rubber like materials. Specifically, the study explores the use of fractional derivatives in representing the constitutive laws of such material and focuses its attention on problems of parameter identification in linear time invariant systems with fractional order damping models. The thesis is divided into four chapters and two annexures. A review of literature related to mathematical modeling of damping with emphasis on fractional order derivative models is presented in chapter 1. The review covers lternatives available for modeling energy dissipation that include viscous, structural and hybrid damping models. The advantages of using fractional order derivative models in this context is pointed out and papers dealing with solution of differential equations with fractional order derivatives are reviewed. Issues related to finite element modeling and random vibration analysis of systems with fractional order damping models are also covered. The review recognizes the problems of system parameter identification based on inverse eigensensitivity and inverse FRF sensitivity as problems requiring further research. The problem of determination of derivatives of eigensolutions and FRF-s with respect to system parameters of linear time invariant systems with fractional order damping models is considered in chapter 2. The eigensolutions here are obtained as solutions of a generalized asymmetric eigenvalue problem. The order of system matrices here depends upon the mechanical degrees of freedom and also somewhat artificially on the fractional order of the derivative terms. The formulary for first and second order eigenderivatives are developed taking account of these features. This derivation also takes into account the various orthogonality relations satisfied by the complex valued eigenvectors. The system FRF-s are obtained by a straight forward inversion of the system dynamic stiffness matrix and also by using a series solution in terms of system eigensolutions. As might be expected, the two solutions lead to identical results. The first and the second order derivatives of FRF-s are obtained based on system dynamic matrix and without taking recourse to modal summation. Numerical examples that bring out various facets of eigensolutions, FRF-s and their sensitivities are presented with reference to single and multi degree freedom systems. The application sensitivity analysis developed in chapter 2 to problems of system parameter identification is considered in chapter 3. Methods based on inverse eigensensitivity and inverse FRF sensitivity are outlined. The scope of these methods cover first and second order analyses and applications to single and multi degree freedom systems. While most illustrations are based on synthetic measurement data, limited efforts are also made to implement the identification methods using laboratory measurement data. The experimental work has involved the measurement of FRF-s on a system consisting of two steel tubes connected by a rubber hose. The two system identification methods are shown to perform well especially when information on second order sensitivity are included in the analysis. The method based on inverse eigensolution is shown to become increasingly unwieldy to apply as the order of the system matrices increases while the FRF based method does not suffer from this drawback. The FRF based method also has the advantage that the prior knowledge of order of fractional order derivative terms is not needed in its implementation while such knowledge is assumed in the method based on eigensolutions. While the methods are shown to perform satisfactorily when synthetic measurement data is used, their success is not uniformly good when laboratory measurement data are employed. Chapter 4 presents a summary of contributions made in the thesis and also enlists a few suggestions for further research. Annexure I provides a précis of elementary notion of fractional order derivatives and integrals. A case study on finite element analysis of aircraft engine component made up of metallic and rubber materials is outlined in Annexure II and the study points towards possible advantages of using fractional order damping models in the study of such structures.