A complete solution of thermoelastic dilatational waves in an elastic, heat conducting, homogeneous, and isotropic medium has been obtained for both the steady and unsteady states. Discussions of the solution for a wide range of frequencies and various values of the coefficient of rise (or decay) have been made and the result has been applied to the explanation of the unusual seismic waves recorded from an underground atomic explosion of September 19, 1957, in Nevada. Reflection and retraction of plane waves at plane boundary and plane interface between two media have been studied. The surface waves have also been studied. The solutions of spherical and cylindrical dilatational waves have been found. / Ph. D.

In seismic field operations complex problems often arise which cannot be solved mathematically. In recent years investigators in both the commercial and academic fields have begun to approach the problems of elastic wave propagation by the use of seismic scale models. This thesis discusses the results measured from simulated seismic activity on a scale model built by the researcher.

An investigation is presented of the behavior of time-reversal focusing in soils. Initial numerical models demonstrate time-reversal focusing to be effective in elastic media, including when a large number of scattering objects were present in the medium. When scattering objects are present, time-reversal focusing demonstrates superior focusing ability when compared to other excitation methods such as uniform excitation or time-delay focusing. Multiple experimental investigations of experimental time-reversal focusing performed in sand evaluate time-reversal focusing effectiveness when multiple near-surface scattering objects are present in the medium. Experimental results demonstrate that time-reversal focusing is effective in the experimental context as well as the numerical models. Further experiments examine time-reversal focusing in more extreme cases where the entire ballistic wave is blocked, and the only energy reaching the focus point is reflected from scattering objects in the medium. A comparison to other focusing methods demonstrates that under these conditions, most focusing attempts with traditional methods will fail completely while time-reversal focusing does not. Additional configurations of time-reversal focusing examine its effectiveness when scattering is caused by an asymmetrical surface layers. The impact of an asymmetrical or non-uniform excitation array is also examined for time-reversal focusing in the presence of scattering objects. An investigation of the effects of scattering object geometry on focusing resolution in time-reversal focusing is also presented. Scattering object field density is found to have a strong, but diminishing effect on focusing resolution as the scattering object field density increased. Loss of surface wave energy available for focusing due to mode-conversion is found to be correlated with the density of the scattering object field. The impact of the weak non-linear nature of the soil on time-reversal focusing is examined through a study of time-reversal focusing behavior for a variety of amplitudes that generate different levels of non-linearity in the soil. This study of nonlinearity is coupled with a study of the impact of noise on time-reversal focusing. It appears that both non-linearity and noise have an impact on time-reversal focusing effectiveness. Further, the loss from these mechanisms seems to be interrelated. Noise seems to enhance non-linear loss in the soil.

by Andrew Loris Kurkjian. / Originally published as thesis (Dept. of Electrical Engineering and Computer Science, Ph.D., 1982). / Bibliography: p. 186-189. / Supported in part by the Advanced Research Projects Agency monitored by ONR under Contract N00014-81-K-0742 NR-049-506 Supported in part by the National Science Foundation under Grant ECS80-07102

The analysis of wave propagation in solids with complex microstructures, and local heterogeneities finds extensive applications in areas such as material characterization, structural health monitoring (SHM), and metamaterial design. Within continuum mechanics, sources of heterogeneities are typically associated to localized defects in structural components, or to periodic microstructures in phononic crystals and acoustic metamaterials. Numerical analysis often requires computational meshes which are refined enough to resolve the wavelengths of deformation and to properly capture the fine geometrical features of the heterogeneities. It is common for the size of the microstructure to be small compared to the dimensions of the structural component under investigation, which suggests multiscale analysis as an effective approach to minimize computational costs while retaining predictive accuracy. This research proposes a multiscale framework for the efficient analysis of the dynamic behavior of heterogeneous solids. The developed methodology, called Geometric Multiscale Finite Element Method (GMsFEM), is based on the formulation of multi-node elements with numerically computed shape functions. Such shape functions are capable to explicitly model the geometry of heterogeneities at sub-elemental length scales, and are computed to automatically satisfy compatibility of the solution across the boundaries of adjacent elements. Numerical examples illustrate the approach and validate it through comparison with available analytical and numerical solutions. The developed methodology is then applied to the analysis of periodic media, structural lattices, and phononic crystal structures. Finally, GMsFEM is exploited to study the interaction of guided elastic waves and defects in plate structures.

A technique that allows for nonparallel interfaces and lateral inhomogeneities in an irregular layered medium is described. The formulation combines a semidiscrete finite-element technique with a perturbation method, providing an approximate treatment of wave propagation in irregular layered media. The procedure treats the irregularities as perturbations with respect to a reference, horizontally-layered, laterally-homogeneous medium and produces approximations of the perturbed wave motion with little additional computation effort. Within the framework of the method, consistent transmitting boundaries and other semidiscrete hyperelements as well as Green’s functions, already available for regular layered media, can be reformulated. The method is relevant in problems of foundation dynamics, ground response to seismic waves and site characterization. Example problems are presented toward evaluation of the effectiveness of the method. / text

No description available.

L’objectif de ce travail de thèse est de prendre en compte à la fois l’hétérogénéité, l’anisotropie et des incertitudes dans la simulation 3D de la propagation d’ondes élastiques. Pour ce faire, dans un premier temps, on modélise le champ de propriétés mécaniques, ici le champ de tenseur d’élasticité, par un modèle de champ stochastique 3D des matrices définie-positives. La construction de ce modèle de champ est essentiellement fondée sur celle de Soize [2008]. Notre modèle conserve ainsi les propriétés principales du modèle de Soize comme le paramétrage minimal contrôlant l’amplitude de la fluctuation et la taille caractéristique de la variabilités patiale, le comportement local a priori arbitrairement anisotrope (anisotropie triclinique) et les propriétés mathématiques fondamentales. De plus, un nouveau paramètre est introduit dans ce modèle pour imposer un niveau d’anisotropie moyen souhaité. Dans un deuxième temps, on effectue des adaptations du code de calcul d’éléments finis spectraux, à savoir le code parallèle SPEC3D, afin d’une part de générer les réalisations du champ stochastique du tenseur d’élasticité et d’autre part de prendre en compte l’anisotropie dans la résolution numérique du problème élastodynamique. Des études paramétriques utilisant SPEC3D sont ensuite réalisées mettant en évidence les influences de l’anisotropie et des paramètres d’hétérogénéité sur la propagation d’ondes sismiques. En particulier, elles démontrent une dépendance directe entre la longueur de corrélation du champ de propriétés et le temps caractéristique d’apparition de la diffusion. Ce régime se manifeste par l’équipartition d’énergie entre les mouvements irrotationnels et rotationnels. / The aim of this thesis is to take into account the heterogeneity, the anisotropy and the uncertainties within 3D numerical simulation of elastic waves propagation. Firstly, the elasticity tensor field is modeled by means of a stochastic tensor-valued field. Its construction is generalized from the model of Soize [2008]. Hence, our model preserves principle properties of the former : a small set of parameters controlling the whole dispersion and the characteristic size of spatial variability, a local behavior being a priori arbitrary anisotropic (triclinic anisotropy) andothers essential mathematical properties. Moreover, a new parameter is added in order to impose a desired anisotropy mean level. Secondly, we carry out adaptations of an existing spectral finite elements-based elastic waves simulation software, namely the SPEC3D parallel computing code. On the one hand a sample generator of the elasticity random field model is implemented and on the other hand anisotropic material behavior is introduced in the elastodynamic solver. Finally, numerical parametric studies are performed using SPEC3D highlighting influences of heterogeneity and anisotropy on elastic waves behavior. In particular, it is observed that the characteristic time beyond which a multiple scattering pattern can be approximated by a diffusion regime directly depends on the correlation length of elasticity tensor field model. This time is detected by an energy equipartition between rotational and irrotational movements.

Orientador: Jose Roberto de França Arruda / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-07-25T10:44:12Z (GMT). No. of bitstreams: 1 Pereira_AllanKardecAraujo_D.pdf: 9794175 bytes, checksum: 619e00345631f64039182d262ce4f5e0 (MD5) Previous issue date: 1999 / Resumo: O objetivo deste trabalho foi estudar a aplicabilidade de uma metodologia de controle de vibrações em estruturas usando uma abordagem de intensidade estrutural (SI). Tal metodologia (ASIC) foi então aplicada ao estudo de vigas e placas em flexão. Técnicas de estimativa da SI em estruturas foram revistas e foram utilizados acelerômetros e sensores cerâmicos piezelétricos para as medidas de vibração. Para fins de controle, as estruturas foram identificadas usando algoritmos Q-Markov Cover e os modelos obtidos foram utilizados em simulações numéricas. Experimentos também foram implementados. Para o caso do controle da SI em vigas, o ASIC foi comparado com outras metodologias quais sejam, o FXLMS, o FXLMS MIMO e o método de Gibbs-Fuller. O ASIC se mostrou em média ligeiramente mais efetivo que o Gibbs-Fuller para os casos de excitação tonal e bastante mais efetivo para os casos de multi-frequência. Já para o caso de controle de placas, foi utilizada uma configuração com um atuador e oito sensores para a estimativa do divergente da SI. Tal configuração só permitiu atenuações locais para os casos estudados. Modificações na metodologia foram propostas e o controlador se transformou em controlador geral de intensidade, tanto acústica quanto estrutural / Abstract: The main goal of this work was to study the aplicability of a new methodology to control structural vibrations using a structural intensity approach. This methodology was then used to control flexural vibration in beams and plates. Techniques to estimate the structural intensity were revised. Accelerometers and piezelectric sensors were used in order to measure the vibration. The structures were identified using a Q-Markov Cover algorithm and the identified models were used in numerical simulations. Experiments were made using a DSP board. In the case of the beam, the ASIC was compared to the FXLMS, the MIMO FXLMS and the Gibbs-Fuller methods. The ASIC method performed slightly better throughout the investigated frequency range when compared to the Gibbs-Fuller method in the case of tonal excitation. For the case of multiple frequencies, the ASIC performed much better than the Gibbs-Fuller. In the case of the plate, only one actuator and eight sensors were used. With this configuration, the ASIC only controlled the vibrations locally. Some changes in the ASIC were proposed which made it a general intensity controller / Doutorado / Mecanica dos Sólidos e Projeto Mecanico / Doutor em Engenharia Mecânica

Orientador: José Maria Campos dos Santos / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-27T02:09:23Z (GMT). No. of bitstreams: 1 Lucena_RaimundoLiberato_M.pdf: 3221588 bytes, checksum: 2a3c7aed85d815f7b04f953992d3ac0e (MD5) Previous issue date: 2015 / Resumo: O monitoramento da integridade e detecção de danos em estruturas tem recebido considerável atenção nas últimas décadas. Houveram progressos significativos no desenvolvimento de métodos para a detecção de danos estruturais, com base nas técnicas de análise modal e resposta dinâmica do sistema. Estas técnicas têm se mostrado bastante apropriadas para detectar danos de alta intensidade, mas não para danos menores, tais como trincas. Um método alternativo é a utilização de métodos de propagação de ondas elásticas. Uma das vantagens é que as ondas guiadas viajam ao longo do comprimento da estrutura, o que permite um teste rápido de longo alcance e elimina a necessidade de se analisar todas as partes da amostra. Neste trabalho será apresentado um método não destrutivo para a detecção de danos em estruturas tipo barra. A fim de evitar uma demorada análise de toda a estrutura, como é feito com métodos clássicos de ultrassom, guias de ondas elásticas são utilizados. Ondas são excitadas e se propagam ao longo da amostra. Estas ondas interagem com um possível dano o que resulta no espalhamento da onda. Os sinais das ondas geradas pelo espalhamento são medidos em algumas posições. Estes sinais são revertidos no tempo e reinjetados nos mesmos pontos de aquisição sobre o modelo numérico da barra, fornecendo uma indicação da posição da falha. Esta combinação de experimento, tempo reverso e simulação numérica fornece uma ferramenta para a detecção de danos em grandes estruturas. O método foi desenvolvido e simulado em um código numérico para guias de onda tipo barra utilizando o Método do Elemento Espectral (Spectral Element Method - SEM). Exemplos simulados de todo o processo feito para uma barra simples são mostrados e os resultados discutidos e comparados com aqueles encontrados na literatura / Abstract: The health monitoring and damage detection in structures have received considerable attention in recent decades. There have been significant advances in the development of methods for structural damage detection based on the modal analysis techniques and dynamic system response. These techniques have shown to be quite appropriate for high-intensity detecting damages but not for minor damages such as cracks. An alternative method is the use of elastic waves propagation methods. The advantage is that the guided waves travel along the length of the structure, which allows a quick test of long range and eliminates the necessity of analyzing all peer samples. In this work a non-destructive method for damage detection in rod-like structures will be presented. In order to avoid a time-consuming scan of the entire structure, as is done with traditional ultrasound methods, elastic wave guides are used. Waves are excited and propagate along the sample. These waves interact with a possible damage which results in the scattering of the wave. The signals generated by scattering of waves are measured in some positions. These signals are reversed in time and re-injected at the same points of acquisition over the numerical model of the rod, providing an indication of the damage position. This combination of experiment, time reversal and numerical simulation gives a tool for damage detection in large structures. The method was developed and simulated in a numeric code for a rod waveguide using the Spectral Element Method (SEM). Simulated examples of the entire process made for a single rod are shown and results discussed and compared with those found in the literature / Mestrado / Mecanica dos Sólidos e Projeto Mecanico / Mestre em Engenharia Mecânica