The growing usage and demand for microwave communications has led to the increase in system density, particularly in urban areas, and consequently to the increase in the probability of mutual interference between systems sharing the same frequency band. Satellite and point to point microwave communication systems might have to rely on site shielding to reduce the level of this co-channel interference. In urban areas, there is a great possibility of a building obstructing the interference path, thus providing protection. However, there could be more than one interference path. Scattered interference from other buildings in the vicinity of the site is one which can be found to be almost as harmful as the direct interference. Although site shielding has long been identified as an interference reduction technique, most studies have been devoted to the protection obtained from the obstruction of interference paths, namely diffraction. There is little information available regarding the effects of building scatter in site shielding. The work reported in this thesis was set to carry out theoretical and experimental investigations and characterisation of building scatter. The studies are aimed at the effects of building scatter on site shielding at microwave frequencies. Building scatter prediction models are developed based on Fresnel-Kirchhoff diffraction theory and verified against results obtained from measurement campaigns conducted at a frequency of 11.2 GHz using firstly a perfectly conducting reflector and later a number of buildings in urban environments as the scattering obstacles. Scattering prediction models are developed as extending to that originally applicable to the far field for use in the near and very near field regions of the scattering surface. The very near field model is found to be particularly useful in site shielding applications in urban environments. Furthermore, it has been possible to establish the scattering characteristics in terms of angular and distance dependence of the scattering coefficient using the expansions of the Fresnel integral with appropriate approximations. The effects of building features and surface variations, e.g. protruding and recessed features, windows and surface deviations, are analysed and characterised. These effects are particularly significant in the interpretation of measurement results obtained from buildings in typical urban environments. The thesis provides a prediction procedure which radio system planners and design engineers can use for determining the effects of building scatter on the site shielding factor for specified radio path geometries. The procedure is expected, through UK study group 3, to add considerably to a revised ITU-R (CCIR) procedure and recommendations for building scatter effects in radiowave propagation. The work has also contributed regularly to the technical output of European COST project 235.

Les vibrations dans les sols constituent un problème environnemental de plus en plus important. Cette étude adresse plusieurs aspects des vibrations du sol. Les conditions locales du site et l'amplification des ondes sismiques représentent un sujet largement étudié en sismologie et en ingénierie sismique. Bien que la plupart des études soient consacrées à la sismologie d'ingénierie et à l'ingénierie sismique, des approches similaires pour la classification des sols et l'amplification du site n'ont pas encore été pleinement établies dans le domaine des vibrations urbaines.D'abord, une stratégie d’optimisation pour la méthode des couches absorbantes a été développée afin d'améliorer la précision des modèles par éléments finis. La conception de couches absorbantes simples en éléments finis par l'annulation de la partie réelle des ondes réfléchies dans le domaine du nombre complexe s'est révélée efficace lorsqu'elle est couplée à la réduction de la rigidité de la couche absorbante. Une réduction excessive de la rigidité ainsi que l'augmentation excessive du facteur de rigidité dans la matrice d'atténuation ont permis une grande réduction de la taille de la couche absorbante et par suite la conception d'une couche absorbante moins coûteuse.Ensuite, une partie importante du travail a été consacrée à la dérivation d'un nouvel ensemble de paramètres du type gradient de vitesse qui contrôle le transfert des vibrations à travers des sols multicouches. L'absence d'une approche compréhensive et bien structurée pour la prédiction et la classification des sites pour les problèmes de vibration laisse le problème large et compliqué. Différents sites ayant différentes propriétés mécaniques et géométriques ont été examinés à l'aide des éléments finis. Les réponses des sites ont été formulées dans des approches spectrales et temporelles simples. Les paramètres nouvellement dérivées ainsi que les lois spectrales servent de moyen de classification des sols multicouches pour les problèmes de vibrations et peuvent même être utilisées à des fins de conception.D’ailleurs, un outil d'intelligence artificielle pour prédire la réponse du sol en utilisant les paramètres précédemment dérivées associées aux propriétés géométriques de la couche de surface a été développé. L'outil des réseaux neurone a été utilisé pour analyser les effets paramétriques des paramètres de gradient de vitesse par rapport à la profondeur de la couche de surface. Des conclusions importantes ont été tirées de l'analyse concernant les propriétés mécaniques et géométriques des couches multiples et leurs effets variantes avec la distance de la source.Enfin, les enregistrements de sources multiples ont été étudiés en les comparants aux réponses spectrales des différents sites définis dans les sections précédentes. Le taux d'appariement entre le contenu spectral d'une source particulière et un site donné sert à évaluer l’aléa vibratoire causé par cette source au site correspondant. L'évaluation des risques de vibrations conduit à un lien de classification entre les sources d'une part et les sites caractérisés par des paramètres à gradient de vitesse d'autre part / Ground vibration is an increasingly important environmental problem. This study investigates multiple aspects of ground vibration. Local site conditions and the related amplification of seismic waves represent a widely studied topic in seismology and earthquake engineering. While most of the studies are dedicated to engineering seismology and earthquake engineering, similar approaches for soil classification and site amplification have not been yet fully established in the field of urban vibrations.First an improvement strategy for absorbing layer method was developed in order to enhance precision of the FEM models. The design of simple absorbing layers in FEM through the nullification of the real part of reflected waves in the complex number domain proved to be efficient when coupled with the stiffness reduction of the absorbing layer. Excessive reduction of the stiffness along with the excessive increase of the stiffness factor in the attenuation matrix enabled a large reduction in the size of the absorbing layer and therefore the design of an inexpensive absorbing layer.Afterwards an important part of the work was dedicated to the derivation of a new set of parameters of the velocity-gradient type that controls the vibration transfer through multilayered soil. The absence of a well-structured comprehensive approach for prediction and site classification for vibration problems leaves the problem broad and complicated. Different sites with different mechanical and geometrical properties were examined using FEM and their surface response was studied. Sites responses were formulated in simple time domain and spectral approaches. The newly derived proxies along with the spectral laws serve as a classification mean for multilayered soils in the vibration problem and may even be used for design purposes.An artificial intelligence tool for predicting soil response using the previously derived proxies coupled with the geometrical properties of the surface layer was later developed. The neural networks tool was used to analyze the parametric effects of the velocity-gradient proxies versus that of the surface layer’s depth. Important conclusions were derived from the analysis regarding the mechanical and geometrical properties of multiple layers and their varying effects with distance from the source.Finally multiple sources recordings were studied through comparing them with the spectral responses of different sites defined in the previous sections. The rate of matching between the spectral content of a particular source and a given site serves as a mean to assess the vibration hazard caused by this source to the corresponding site. The vibration hazard assessment leads to a classification link between sources in one hand and sites characterized by velocity-gradient proxies on the other hand

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: Luiz Carlos Pereira da Silva / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-18T01:45:50Z (GMT). No. of bitstreams: 1 Villar_FernandaSpada_M.pdf: 4737992 bytes, checksum: 3ce0cbb4f82fc2a7fd54013a9d3724f1 (MD5) Previous issue date: 2011 / Resumo: Neste trabalho a propagação de transitórios de tensão dentro de uma rede coletora de parques eólicos marítimos foi estudada. Foram analisados especificamente os transitórios de tensão originados durante a energização do parque eólico, e foi modelado o efeito dos múltiplos pontos de reflexão na composição da forma de onda da sobretensão transitória. O momento de energização desperta interesse pois todos os transformadores localizados nos topos das torres, por estarem descarregados, se comportam como um ponto de circuito aberto, refletindo de volta para a rede 100% de qualquer onda de tensão viajante que incide sobre ele. As configurações de rede coletora escolhidas foram as comumente utilizadas em Parques Eólicos Marítimos atualmente pela Europa. Ainda, uma topologia alternativa que emprega a rede de cabos em estrela foi estudada. Para validação dos estudos analíticos e da modelagem desenvolvida foi utilizado o software PSCAD. A análise numérica dos coeficientes de reflexão e refração nos nós da rede coletora e de seu efeito na propagação dos transientes em questão mostrou que o efeito atenuador resultante de refrações consecutivas dentro da rede, no caso do coeficiente de refração menor que 1, tem grande importância, chegando a anular o efeito amplificador da interferência construtiva, decorrente da existência de múltiplos pontos de reflexão / Abstract: In this dissertation, the propagation of voltage transients inside a collection grid of Offshore Wind Parks (composed by cables) is studied. Mainly the transients generated during the WP energizing are analyzed, and the effect of the multiple reflection points in the composition of the overvoltage is modeled. The energizing moment is a special case to study because all the transformers at the towers top, being unloaded, behave like an open circuit and reflect 100% of the incident voltage travelling waves back to the grid. The grid configurations chosen are the commonly used in modern Offshore Wind Parks in Europe. Also an alternative layout, that uses the cables in a star grid, is studied. In order to validate the theory developed, simulations were made using the software PSCAD. The numerical analysis of the reflection and refraction coefficients in every node of the collection grid and its effect in the voltage transient's propagation showed that the attenuation effect, as a result of the consecutive refractions inside the grid in the cases where the refraction coefficient is smaller than 1, is important enough to cancel the amplification effect of the constructive interference, that happens due to the multiple reflection points. / Mestrado / Energia Eletrica / Mestre em Engenharia Elétrica

Em telecomunicações é utilizada uma grande variedade de modelos de propagação para prever a intensidade do sinal de recepção num enlace de rádio. Os resultados dos modelos empíricos e dos modelos baseados em simplificações do problema eletromagnético apresentam, em geral, grande discrepância entre si, na análise do fenômeno de propagação. Por exemplo, o modelo de dois raios, que prevê atenuação de enlace com variação de 40 dB/década, na região de campo distante da antena de transmissão, quando colocada a uma certa altura da superfície de um solo condutor perfeito. Já o modelo empírico de Okumura-Hata exibe, para as mesmas condições geométricas, mas para solo de condutividade finita, uma variação da atenuação de enlace de 30 dB/década. Neste trabalho propõe-se uma calibração do modelo de dois raios para aumentar sua aderência aos resultados obtidos por Okumura-Hata num cenário real na faixa de frequências de 450MHz a 1900MHz para comprimentos de enlace entre 1 km e 20 km. Os resultados obtidos com o modelo proposto são comparados com os obtidos através de outros modelos apresentando boa aderência verificada através de uma comparação numérica particularmente com o modelo Okumura-Hata. / In telecommunications, there is a large variety of propagation models in order to predict the intensity of the receiving signal in a communication link. The empirical models and the models based on simplifications of the electromagnetic problem with theoretically obtained results show, in general, a significant disagreement, for the same propagation phenomenon. For example two-ray model predicts a path-loss of 40 dB/decade in the far-field region of a transmission antenna deployed at a certain height of the surface of a perfect conducting soil. On the other hand, the Okumura-Hata empirical model shows, for the same geometrical conditions, bur for a real soil, a path-loss of 30 dB/decade. This work proposes a calibration of the Two-Ray model for better adherence to the results obtained by Okumura-Hata in a real scenario for the frequency range 450MHz to 1900MHz and for a radio link length of 1 km to 20 km. The results obtained with the proposed model are compared with other model results showing good adherence through numeric comparison, in particular with the Okumura - Hata model.

L’industrie pétrolière s’intéresse désormais à des régions de la terre de plus en plus difficiles d’accès et il est essentiel de proposer des techniques permettant de garantir l’efficaité d’un forage. Parmi ces techniques, la Reverse Time Migration (RTM) est connue pour sa précision. Elle utilise les ondes réfléchies pour reconstruire une carte du sous-sol représentant les interfaces géophysiques. Elle peut être décrite en trois étapes : (i) propager le champs émis par les sources durant la campagne d’acquisition; (ii) pour chaque source, propager le champ enregistré par les récepteurs; (iii) obtenir une image du sous-sol en appliquant une condition d’imagerie à chaque pas de temps et pour chaque source. Cette technique requiert de très grosses capacités de calcul et il est encore difficile d’imager des milieux réalistes 3D, même avec l’aide du calcul haute performance. Nous avons choisi la méthode de Galerkine discontinue pour modéliser la partie propagation car elle permet d’obtenir des solutions précises et est adaptable au calcul parallèle. La quantité d’information à sauvegarder pour faire une corrélation étant importante, on se doit de trouver un algorithme de calcul d’images du sous-sol réduisant ce coût. Nous avons utilisé l’algorithme de Griewank, appelé “Optimal Checkpointing”. Ce problème de coût étant réglé, on se doit de considérer l’efficacité des ondes élastiques incluant des champs multiples pour améliorer la précision de l’image. La condition traditionnelle de J. Claerbout ne prend pas en compte les conversions d’ondes, et n’est alors surtout utile que dans le cas acoustique. De plus, les ondes P et S interagissant entre elles, il est intéressant de trouver une condition d’imagerie utilisant ce fait. Cela a été abordé dans le cadre de la méthode de l’état adjoint dans les travaux de A. Tarantola et J. Tromp et ce travail en propose utilisation dans le cadre de la RTM. Nous proposons une nouvelle condition d’imagerie prenant en compte les paramètres élastiques du milieu considéré et permettant de supprimer les artefacts numériques. Nous illustrons les images sur des cas industriels / Since a large number of sedimentary basins have been explored, oil exploration is now interested in investigating regions of the Earth which are hostile. Among existing methods for seismic imaging, Reverse Time Migration (RTM) is a technique known by industry to be efficient. The RTM uses reflected waves and is able to construct a map of the subsurface which is depicted by the interfaces limiting the geophysical layers. The algorithm of RTM can be described as a three-step procedure: (i) compute the wavefields emitted by the sources used during the seismic acquisition campaign; (ii) for each source, compute the so-called “backpropagated wavefield”, which is the wavefield obtained by using as sources the signals recorded at the receivers during the acquisition campaign and by reversing the time; (iii) get an image of the subsurface by applying an imaging condition combining the propagated and the backpropagated wavefields at each time step of the numerical scheme and for each source. This technique is computationnaly intensive and it is still difficult to image realistic 3D elastic media, even with the help of HPC. We have thus chosen to consider high-order Discontinuous Galerkin Methods which are known to be well-adapted to provide accurate solutions based upon parallel computing. As we need to correlate a lot of wavefields, we need to find an algorithm reducing the CPU time and the storage : this is the Griewank’s algorithm, so-called “Optimal Checkpointing”. The traditional imaging condition, proposed by J. Claerbout, does not take wave conversions into account and since P-wave and S-wave interact with each other, it might be relevant to use an imaging condition including these interactions. In fact, this has been done successfully by A. Tarantola and J. Tromp for seismology applications based upon the inversion of the global Earth. In this work, we propose a new imaging condition using the elastic parameters which attenuates numerical artifacts. We illustrate the properties of the new imaging condition on industrial benchmarks like the Marmousi model. In particular, we compare the new imaging condition with other imaging conditions by using as criteria the quality of the image.

Em telecomunicações é utilizada uma grande variedade de modelos de propagação para prever a intensidade do sinal de recepção num enlace de rádio. Os resultados dos modelos empíricos e dos modelos baseados em simplificações do problema eletromagnético apresentam, em geral, grande discrepância entre si, na análise do fenômeno de propagação. Por exemplo, o modelo de dois raios, que prevê atenuação de enlace com variação de 40 dB/década, na região de campo distante da antena de transmissão, quando colocada a uma certa altura da superfície de um solo condutor perfeito. Já o modelo empírico de Okumura-Hata exibe, para as mesmas condições geométricas, mas para solo de condutividade finita, uma variação da atenuação de enlace de 30 dB/década. Neste trabalho propõe-se uma calibração do modelo de dois raios para aumentar sua aderência aos resultados obtidos por Okumura-Hata num cenário real na faixa de frequências de 450MHz a 1900MHz para comprimentos de enlace entre 1 km e 20 km. Os resultados obtidos com o modelo proposto são comparados com os obtidos através de outros modelos apresentando boa aderência verificada através de uma comparação numérica particularmente com o modelo Okumura-Hata. / In telecommunications, there is a large variety of propagation models in order to predict the intensity of the receiving signal in a communication link. The empirical models and the models based on simplifications of the electromagnetic problem with theoretically obtained results show, in general, a significant disagreement, for the same propagation phenomenon. For example two-ray model predicts a path-loss of 40 dB/decade in the far-field region of a transmission antenna deployed at a certain height of the surface of a perfect conducting soil. On the other hand, the Okumura-Hata empirical model shows, for the same geometrical conditions, bur for a real soil, a path-loss of 30 dB/decade. This work proposes a calibration of the Two-Ray model for better adherence to the results obtained by Okumura-Hata in a real scenario for the frequency range 450MHz to 1900MHz and for a radio link length of 1 km to 20 km. The results obtained with the proposed model are compared with other model results showing good adherence through numeric comparison, in particular with the Okumura - Hata model.

La thèse, dont le sujet est "Modélisation de la propagation électromagnétique en milieux à gradient d'indice basée sur les faisceaux gaussiens - Application à la propagation en atmosphère réaliste et à la radio-occultation entre satellites" a été commencée le 2 décembre 2013, au Département ÉlectroMagnétisme et Radar (DEMR) de l'Onera de Toulouse et avec le laboratoire LAPLACE de l'Université Paul Sabatier. Elle est co-financée par l'ONERA et par la Région Midi-Pyrénées. L'encadrement a été assuré par Jérôme Sokoloff (Laplace/UPS, directeur de thèse), Alexandre Chabory (ENAC, co-directeur) et Vincent Fabbro (ONERA). L'École Doctorale est l' "École Doctorale Génie Électrique, Électronique, Télécommunications : du système au nanosystème". Le faisceau gaussien a été principalement utilisé dans la recherche scientifique afin d'étudier les systèmes optiques tels que les lasers. Des études plus rares et plus récentes ont proposé de l'utiliser pour modéliser la propagation des ondes sismiques. Ses propriétés spatiales et spectrales ont amené certains auteurs à étudier son utilisation dans des modèles de propagation atmosphériques. Cette thèse a consisté à développer un modèle, appelé GBAR (Gaussian Beam for Atmospheric Refraction), de propagation troposphérique réaliste et déterministe en utilisant le formalisme des faisceaux gaussiens. La démarche adoptée a consisté à reprendre les équations fondamentales introduites par Cerveny et Popov décrivant de façon itérative la propagation d'un faisceau gaussien en milieu inhomogène, sous hypothèse de haute fréquence (modèle asymptotique). De nouvelles équations ont été développées à partir d'elles pour obtenir une description analytique de la propagation d'un faisceau gaussien dans un milieu troposphérique décrit par les variations spatiales de l'indice de réfraction. L'hypothèse de base pour l'obtention de la formulation analytique est que le gradient de l'indice de réfraction peut être considéré vertical et constant au voisinage du faisceau. Les équations analytiques pour la description de la propagation d'un seul faisceau ont ensuite été étendues à la modélisation d'un champ quelconque dans un milieu troposphérique pouvant contenir de fortes variations du gradient d'indice, y compris des inversions de gradient. Ceci a été réalisé en couplant les équations analytiques avec la procédure de décomposition multi-faisceaux développée dans sa thèse pas Alexandre Chabory. Le modèle GBAR a été validé dans des milieux troposphériques réalistes issus de simulations du modèle météo méso-échelle WRF (Weather Research and Forecasting). Dans un troisième temps, le modèle a été utilisé pour simuler des inversions de données de radio-occultation. Des outils existent pour fournir un modèle d'interprétation de ces données pour estimer les propriétés physiques de l'atmosphère à partir des mesures en phase, amplitude, Doppler et délai des signaux GNSS transmis entre satellites en orbite autour de la Terre / The subject of this PhD thesis is " Electromagnetic propagation modeling in inhomogeneous media with refractive index gradients based on Gaussian beams - Application to realistic atmospheric propagation and radio occultation between satellites ". The study started on december 2nd, 2013 at the DEMR (Département Électromagnétisme et Radar) department of the ONERA research laboratory, in Toulouse, France. It was funded both by the ONERA and Région Midi-Pyrénées. It was supervised by Jérôme Sokoloff (LAPLACE/UPS, thesis director), Alexandre Chabory (ENAC, thesis co-director) and Vincent Fabbro (ONERA). The doctoral school was "École Doctorale Génie Électrique, Électronique, Télécommunications : du système au nanosystème ". The Gaussian beam was mostly used in scientific investigations to study optical systems such as lasers. Rarer and more recent works suggested the use of the Gaussian beam formalism in order to model the propagation of seismic waves. The properties of the Gaussian beam also led some authors to develop models for atmospheric propagation. In this thesis a model based on Gaussian beams called GBAR (Gaussian Beam for Atmospheric Refraction) was developped for tropospheric propagation in realistic and deterministic conditions. The scientific approach consisted in rewritting the fundamental equations introduced by Cerveny and Popov describing iteratively the propagation of a Gaussian beam in inhomogeneous media, under the high-frequency assumption (asymptotic model). New equations were derived from them in order to get analytical equations of the propagation of a Gaussian beam in inhomogeneous media described by the variations of the refractive index. The basic assumption under to get the analytical equations is to consider that the refractive index gradient is vertical and constant around the beam axis. The analytical equations that describe the propagation of a Gaussian beam were extended to model the propagation of an arbitrary field in a tropospheric medium with strong variations and inversions of the refractive index. This was done by coupling the analytical equations with the multibeam expansion procedure developped by Alexandre Chabory in his PhD thesis. The GBAR model was validated in tropospheric conditions, using refractive index grids from the WRF (Weather Research and Forecasting) mesoscale meteorological model. In the third and final phase, the GBAR model was used to simulate Radio Occultation data inversions. Tools exist to allow for interpretations of Radio Occultation data in order to estimate the physical properties of the atmosphere from measured phased, amplitude, Doppler shift and delay of GNSS signals transmitted between satellites orbiting around the Earth

Structural Health Monitoring (SHM) systems for future structures and vehicles would involve a process of damage identification and prediction of certain quantities of interest that concerns the function and safety. This process provides SHM systems the ability to not only save cost but also enhance the service life, safety and reliability of the structures and vehicles. Integrated SHM system (ISHM) is an advancement of SHM system that has additional capability of predicting the component life/failure. ISHM system development involves detailed understanding of diagnostic waves, hardware components, signal processing paradigms and intelligent use of algorithms. Diagnostic waves like the guided waves are the elastic waves that propagate in a direction defined by the material boundaries. These waves have the capability of traveling large distance probing the entire thickness in plates/shells. Thus, they are widely used by SHM systems in monitoring the plate structures. Piezoelectric transducers are often employed in the interrogation using guided waves. Most SHM systems employing guided waves are designed for specific structures. Current paradigms of SHM systems are unable to enable the transition from simple or ideal structures to realistic and complicated structures. This is due to the challenges at the fundamental level involving transducer, wave propagation and phenomena of guided wave scattering with damages to evaluate the possible solutions through mathematical modeling and signal analysis capability required by ISHM systems. This thesis aims to develop understanding of these problems at a fundamental level. Complex system level understanding is still needed which is left out as open problem. A primary requirement in designing SHM system is the proper understanding of wave characteristics such as number of modes, wavelength and dispersiveness. Although three-dimensional elasticity solution and simplified theories are available to understand them, their applicability in SHM problem requires a much more detailed look. Effort toward this direction has led to the development of simpler models. However, mathematical models are not available for understanding the wave characteristics in complex structures involving stiffeners and adhesive joints. This problem is addressed in this thesis. There is a fair amount of understanding developed regarding transducer characteristics. This is accomplished by analytical and finite element models of transducers in the past. However, simplified transducer model that are computationally fast to suit SHM system requirements needs to be developed. The development of such model is presented in this thesis. Apart from modeling the transducers and wave scattering due to damage, signal correlation and calibration are needed for practical implementation in SHM. Characterization studies reported in published literature are limited to quasi-static and low frequencies applications. However, SHM of aerospace structures employ guided waves typically in the frequency range of 100-500 kHz. Methods to characterize the transducers at this frequency range needs to be developed, which is addressed in this thesis. Another major requirement of SHM system is the design and development of sensor-actuator network and appropriate algorithm. Techniques developed earlier involving transducer arrays in this regard have limitation due to complexity of geometry and signal interpretation that needs to be addressed. The network with suitable algorithm should ideally monitor large area including the critical areas of failure with minimum number of transducers. ISHM systems further require some capability to estimate the useful life of the damaged structure in order to take suitable decisions. Efficient techniques to achieve these are not developed. Overall, there is a need to improve highly interdisciplinary areas involving mathematical modeling, transducer design, fabrication and characterization, damage detection and monitoring strategies. In this thesis, various novel techniques to combine mathematical model with experimental signals to enhance the damage detection capability are presented. In this thesis, developments in the three main aspects of SHM systems are focused upon. They are (1) development of mathematical models of sensors/actuators, wave propagation and scattering due to damage (2) characterization and calibration of transducers and (3) development of technique to monitor wide variety of damages within the scope of ultrasonic guided wave based SHM. The thesis comprises of ten chapters. First chapter is devoted to the background and motivation for the problem addressed in this thesis. In second chapter, brief overview of available mathematical models and conventional damage monitoring strategy is presented. The significant contributions reported in the subsequent chapters in this thesis are outlined below In chapter 3, a reduced-order model of guided wave propagation in thick structures with reduced-order approximation of higher-order elasto-dynamic field is formulated. The surface normal and shear tractions of the thick structure are satisfied in a closed form. The time-frequency Fourier spectral finite element is developed and is validated using detailed and computationally intensive finite element simulations. Natural frequencies obtained from the developed spectral finite element and the detailed finite element simulations are compared. Transient response due to broad frequency band and narrow frequency band excitations given in the form of surface tractions are validated by comparing with the detailed finite element simulations. Using the developed spectral finite element, wave scattering from a free edge and a notch are simulated and validated by comparing with the detailed finite element simulations. In chapter 4, two-dimensional plane wave and flexural wave scattering models for more complicated features such as stiffener with delamination and stiffener with bolt failures in a stiffened panel are derived using ultrasonic ray tracing based approach combined with wave-field representation. Dispersion relations are reformulated for the base plate where it is bolted with the stiffener. Surface conditions due to contact stiffness and contact damping are modeled by introducing springs and dampers. Scattering coefficients for the bonded and bolted stiffeners are derived. The scattering coefficients are evaluated for various different frequencies. Results are compared for different stiffener parameters. In chapter 5, a simplified analytical model of a piezoelectric actuator with uniform electrodes is modeled. The problem is to determine the launched guided wave characteristics in the structure. The analytical model is derived considering two-dimensional elasticity based approach and Airy’s stress function. The actuator model is used to specify the displacement boundary conditions in the detailed finite element model. The radiated wave patterns in a plate due to actuation from transducers of different shapes are obtained and validated with experiments. Phased array actuators are modeled in the detailed finite element model using the displacements estimated from the actuator model. The radiated wave pattern from the detailed finite element simulations are validated with experiments. Chapter 6 is devoted to the design and characterization of transducers for ultrasonic guided wave applications. The characterization techniques involve the estimation of voltage response for the induced strain by the guided wave at various different frequencies. First, a novel removable bonding technique and a calibration technique are demonstrated and related advantages are discussed. Performance of the piezoelectric thin film under quasi-static, dynamic and transient impact loadings are analyzed first. Next, a guided wave technique is developed to characterize piezoelectric thin film sensors and actuators at ultrasonic frequencies. The transducers with inter digital electrodes are characterized for frequency tuning and directional sensitivity. This characterization study enables in the selection of optimal frequency bands for interrogation. Further, the characterization of transducers with thermal degradation is presented. In chapter 7, a novel guided wave technique to calibrate the thin film sensors for ultrasonic applications is presented. Calibration procedure involves the estimation of the piezoelectric coefficient at ultrasonic range of frequencies. Calibration is done by the measurement of voltage generated across thin films when guided waves are induced on them. With the proposed technique, piezoelectric coefficient can be estimated accurately at any frequency of the propagating wave. Similarly, the measurement of piezoelectric coefficient of thin films with inter digital electrodes is presented. The estimation of piezoelectric coefficient at various different directions using laser Doppler vibrometer is presented. Lastly, the degradation of piezoelectric coefficient is studied for increasing thermal fatigue. In chapter 8, toward SHM methodology development, a guided wave based technique to detect and monitor cracks in a structure is presented. To establish the methodology, a detailed study is carried out on the effect of crack and specimen size on the guided wave propagation characteristics. Using the wave characteristics, an analytical way of modeling Lamb wave propagation in the specimen with plastic zone is proposed. The feasibility to determine plastic zone and fatigue crack propagation with integrated piezoelectric transducers is demonstrated experimentally and the results are verified analytically. A method is further established to detect damage at initial stage and crack-tip plastic zone size along with crack length for a given stress amplitude or vice-versa. An approach to estimate fatigue life from the transducer signals is also proposed. In chapter 9, a compact circular array of sensor-actuator network and an algorithm is presented to monitor large plate structures. A method based on the wavelet transforms of transducer signals is established to localize and estimate the severity of damages. Experiments are conducted to demonstrate the capability of the circular array based method in the localization and quantification of various types of damages like debonding of stiffeners, failure of bolted joints, corrosion and hole-enlargement. A damage index is then computed from wavelet time-frequency map that indicates the severity of damage. Chapter 10 ends with the concluding remarks on the work done with simultaneous discussion on the future scope. The work reported in this thesis is interdisciplinary in nature and it aims to combine the modeling and simulation techniques with realistic data in SHM to impart higher confidence levels in the prediction of damages and its prognosis. The work also aims in incorporating various mathematical models of wave propagation and ray tracing based algorithm to optimize the detection scheme employed in SHM. The future direction based on this study could be aimed at developing intelligent SHM systems with high confidence levels so that statistical machine learning would be possible to deal with complex real-world SHM problems.