The objectives of the thesis work were to study theinteraction between embedded piezoceramic transducers andcomposite structures as well as determine techniques tosimplify the Lamb waves analysis. Firstly, this studyconsidered the design of the embedded piezoceramic transducers.Secondly, the effect of the embedded transducer on thecomposite strength as well as the influence of the mechanicallyloaded composite on the characteristics of the embeddedtransducer were investigated. Finally, to simplify the analysisof such complex Lamb wave responses, two techniques weredeveloped. They were based on the wavelet technique and amodelling technique, respectively. The design of the embedded piezoceramic transducers wasimproved by reducing the stress concentrations in the compositeas well as in all components constituting the piezoceramictransducer, that is, the piezoceramic element, interconnectorand conductive adhesive. The numerical analysis showed that thethickness of the interconnector had no significant influence onthe stress state of the piezoceramic transducer. It was alsofound that a compliant conductive adhesive reduced the stressconcentration located at the edge of the piezoceramic element.The structural integrity of composites embedded with theimproved piezoceramic transducer was investigated. Theexperiments, performed in tensile and compressive staticloading, indicated that the strength of the composite was notsignificantly reduced by the embedded piezoceramic transducer.Further investigations were conducted to evaluate theperformance of the improved piezoceramic transducer used as aLamb wave generator embedded in composites subjected tomechanical loading. The tests were conducted in tensile andcompressive static loading as well as fatigue loading. Thestudy showed a large working range of the embedded piezoceramictransducer. A post processing technique based on the waveletswas further assessed in the detection of damage and in thedamage size evaluation. A new wavelet basis was developedspecially for processing the Lamb wave response. This method,focused on the wavelet coefficients from the decomposition Lambwave response, showed promising results in evaluating thedamage size. The wavelets offered a sensitive tool to detectsmall damage, compared to other detection methods, improvingthe damage detection capabilities. The other technique wasdevoted to the simplification of the generated Lamb waves bythe use of multi-element transducers. The transducers weredesigned using both a normal-mode expansion and a FE-method.This technique allowed reducing the effect of a Lamb wave modetowards another. This technique was successfully implemented ina damage detection system in composites. <b>Keywords:</b>Embedded piezoceramic, transducer, composite,structural integrity, health monitoring, damage detection, Lambwaves, wavelets, normal-mode expansion, FE-method

La técnica de las ondas de Lamb acopladas por aire representa un importante avance en el área de los Ensayos No Destructivos (END) de materiales laminares.Sin embargo la compleja naturaleza de las vibraciones mecánicas encontradas en acústica, hacen que el análisis y el estudio de esta área del conocimiento sea un tema muy complejo. De allí que la posibilidad de contar con una herramienta de simulación de software que permita la evaluación y prueba de diferentes configuraciones de excitación y recepción acústica utilizando la flexibilidad de un modelo de computadora sea de una gran utilidad y ayuda.El objetivo de la presente tesis es proveer al área de los END con un software de simulación gratuito: The LAMB Matlab® toolbox basado en el modelo del software libre de la GNU.El software es capaz de simular el comportamiento de sistemas de END basados en ondas de Lamb acopladas por aire en láminas isótropas simples utilizando transductores tipo array.El programa se basa en un arreglo tipo C-scan de un sistema de END y está compuesto por tres bloques principales: 1) Excitación, 2) Propagación y 3) Recepción.La verificación individual del funcionamiento de dichos módulos se presenta a lo largo de la tesis mediante una serie de comparaciones entre simulaciones y datos experimentales provenientes de diferentes pruebas. Por otro lado, la validación del programa completo se llevo a cabo por medio de experimentos en láminas de cobre y aluminio; utilizando un sistema real de END por ondas de Lamb acopladas en aire mediante arrays cóncavos.La influencia negativa en el desempeño general de dicho sistema de END real basado en este tipo de transductores se comprobó efectivamente mediante el simulador desarrollado. Esto se debió fundamentalmente al efecto de directividad de los sensores individuales en los transductores y a la simetría cóncava de los arrays.Para emular este comportamiento la tesis presenta un modelo geométrico bidimensional simple de un filtro espacial, junto a las simulaciones de un nuevo tipo de array plano propuesto.El programa desarrollado comprobó así mismo la naturaleza coherente de los campos acústicos emitidos en aire por las láminas sujetas a vibraciones de Lamb. Esto se realizó mediante la implementación de un conformador de haz simple de suma y demora; constituyéndose así la etapa inicial de procesamiento de señal del bloque de recepción del programa.El objetivo principal del presente trabajo fue contribuir con un modelo operativo de simulación y prueba de nuevos diseños de arrays e implementación de estrategias de procesado de señal útiles en sistemas de END basados en ondas de Lamb acopladas por aire.Finalmente, si bien el objetivo de la calibración del programa no se pudo conseguir; si se logró efectivamente un notable grado de similitud con un sistema de END real. / Air-coupled ultrasonic Lamb waves represent an important advance in Non- Destructive Testing and Evaluation (NDT & NDE) techniques of plate materials and structures. Examples of these advances are the characterization and quality assessment of laminate materials in manufacturing processes, the location of damaged parts in aircrafts and structure monitoring in the aerospace industry.However the rich and complex nature of mechanical vibrations encountered in acoustics make the subject of analysis and study of these systems a very complex task. Therefore a simulation tool that permits the evaluation and testing of different configuration scenarios using the flexibility of a computer model is an invaluable aid and advantage.The objective of this thesis is to provide the field of NDT with free open source software i.e. the LAMB Matlabrtoolbox. The toolbox is capable of simulating the behaviour of Lamb wave based NDE systems for single ideal isotropic laminates using air-coupled ultrasonic arrays. The programme usesa pitch-catch type of a Cscan NDE arrangement and is composed of three integrated sections each individually modelling a feature in the system: 1) Excitation, 2) Propagation, and 3) Reception.For assessment of the individual modules of the toolbox the thesis presents comparisons between each section simulations and the data obtained from different acoustic experiments. The validation of the complete simulator was carried out by evaluation tests on the copper and aluminium plates by use of a real hardware prototype of a Lamb wave based NDE system with aircoupled concave arrays.The negative impact on the performance of the real air-coupled NDE systembased on concave arrays was effectively confirmed by the programme. This was produced by the inherent directivity of the individual sensors as well as their concave arrangement. To emulate this behaviour the thesis introduces a simple two-dimensional geometric model for the inclusion of the spatial filtering effect of the sensors plus a group of simulations for a new proposed air-coupled plane array transducer.The software also verified the spatial coherent nature of the Lamb wave fields emitted by a plate in air. This was demonstrated by the implementation of a delay and sum beamformer to constitute an initial signal processing stage in the reception section.

Structural health monitoring (SHM) of aerospace components is a rapidly emerging field due in part to commercial and military transport vehicles remaining in operation beyond their designed life cycles. Damage detection strategies are sought that provide real-time information of the structure's integrity. One approach that has shown promise to accurately identify and quantify structural defects is based on guided ultrasonic wave (GUW) inspections, where low amplitude attenuation properties allow for long range and large specimen evaluation. One drawback to GUWs is that they exhibit a complex multi-modal response, such that each frequency corresponds to at least two excited modes, and thus intelligent signal processing is required for even the simplest of structures. In addition, GUWs are dispersive, whereby the wave velocity is a function of frequency, and the shape of the wave packet changes over the spatial domain, requiring sophisticated detection algorithms. Moreover, existing damage quantification measures are typically formulated as a comparison of the damaged to undamaged response, which has proven to be highly sensitive to changes in environment, and therefore often unreliable. As a response to these challenges inherent to GUW inspections, this research develops techniques to locate and estimate the severity of the damage. Specifically, a phase gradient based localization algorithm is introduced to identify the defect position independent of excitation frequency and damage size. Mode separation through the filtering technique is central in isolating and extracting single mode components, such as reflected, converted, and transmitted modes that may arise from the incident wave impacting a damage. Spatially-integrated single and multiple component mode coefficients are also formulated with the intent to better characterize wave reflections and conversions and to increase the signal to noise ratios. The techniques are applied to damaged isotropic finite element plate models and experimental data obtained from Scanning Laser Doppler Vibrometry tests. Numerical and experimental parametric studies are conducted, and the current strengths and weaknesses of the proposed approaches are discussed. In particular, limitations to the damage profiling characterization are shown for low ultrasonic frequency regimes, whereas the multiple component mode conversion coefficients provide excellent noise mitigation. Multiple component estimation relies on an experimental technique developed for the estimation of Lamb wave polarization using a 1D Laser Vibrometer. Lastly, suggestions are made to apply the techniques to more structurally complex geometries.

<p>The objectives of the thesis work were to study theinteraction between embedded piezoceramic transducers andcomposite structures as well as determine techniques tosimplify the Lamb waves analysis. Firstly, this studyconsidered the design of the embedded piezoceramic transducers.Secondly, the effect of the embedded transducer on thecomposite strength as well as the influence of the mechanicallyloaded composite on the characteristics of the embeddedtransducer were investigated. Finally, to simplify the analysisof such complex Lamb wave responses, two techniques weredeveloped. They were based on the wavelet technique and amodelling technique, respectively.</p><p>The design of the embedded piezoceramic transducers wasimproved by reducing the stress concentrations in the compositeas well as in all components constituting the piezoceramictransducer, that is, the piezoceramic element, interconnectorand conductive adhesive. The numerical analysis showed that thethickness of the interconnector had no significant influence onthe stress state of the piezoceramic transducer. It was alsofound that a compliant conductive adhesive reduced the stressconcentration located at the edge of the piezoceramic element.The structural integrity of composites embedded with theimproved piezoceramic transducer was investigated. Theexperiments, performed in tensile and compressive staticloading, indicated that the strength of the composite was notsignificantly reduced by the embedded piezoceramic transducer.Further investigations were conducted to evaluate theperformance of the improved piezoceramic transducer used as aLamb wave generator embedded in composites subjected tomechanical loading. The tests were conducted in tensile andcompressive static loading as well as fatigue loading. Thestudy showed a large working range of the embedded piezoceramictransducer. A post processing technique based on the waveletswas further assessed in the detection of damage and in thedamage size evaluation. A new wavelet basis was developedspecially for processing the Lamb wave response. This method,focused on the wavelet coefficients from the decomposition Lambwave response, showed promising results in evaluating thedamage size. The wavelets offered a sensitive tool to detectsmall damage, compared to other detection methods, improvingthe damage detection capabilities. The other technique wasdevoted to the simplification of the generated Lamb waves bythe use of multi-element transducers. The transducers weredesigned using both a normal-mode expansion and a FE-method.This technique allowed reducing the effect of a Lamb wave modetowards another. This technique was successfully implemented ina damage detection system in composites.</p><p><b>Keywords:</b>Embedded piezoceramic, transducer, composite,structural integrity, health monitoring, damage detection, Lambwaves, wavelets, normal-mode expansion, FE-method</p>

Submitted by MARLON RODRIGUES GARCIA null (marlon92319@aluno.feis.unesp.br) on 2016-04-18T18:04:02Z No. of bitstreams: 1 Dissertação Final - Marlon.pdf: 6766003 bytes, checksum: 4849566caadd0c8935bb400de40fb690 (MD5) / Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-04-19T14:54:06Z (GMT) No. of bitstreams: 1 garcia_mr_me_ilha.pdf: 6766003 bytes, checksum: 4849566caadd0c8935bb400de40fb690 (MD5) / Made available in DSpace on 2016-04-19T14:54:06Z (GMT). No. of bitstreams: 1 garcia_mr_me_ilha.pdf: 6766003 bytes, checksum: 4849566caadd0c8935bb400de40fb690 (MD5) Previous issue date: 2016-02-23 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho aborda-se a utilização de um sensor de deslocamento angular em fibra óptica para a caracterização de líquidos utilizando-se ondas de Lamb e o modo quase Scholte (QSCH) propagados em placa metálica. O sensor reflexivo compreende duas fibras ópticas, uma emissora e outra receptora, uma lente positiva, uma superfície reflexiva, um laser e um fotodetector. Primeiramente, realiza-se um breve estudo sobre a propagação de ondas mecânicas guiadas em placas metálicas imersas em fluidos. Nesse estudo, são obtidas as curvas de dispersão para a velocidade de fase e para a velocidade grupo em função do produto frequência-semiespessura da placa, tanto para as ondas de Lamb quanto para o modo QSCH, considerando diferentes fluidos. Posteriormente, aborda-se o modelamento matemático do sensor reflexivo, obtendo-se as curvas características estáticas para quatro diferentes configurações. Em seguida, descreve-se o processo de montagem do sensor reflexivo, com todos os seus detalhes construtivos. Já na parte experimental, aplicando-se um pulso senoidal de envoltória gaussiana em um transdutor, investiga-se primeiramente a excitação do modo fundamental simétrico da onda de Lamb, S0. Captando-se simultaneamente os sinais da onda propagada na placa pelo sensor reflexivo e por uma piezocerâmica em modo de recepção, obtêm-se os valores experimentais da velocidade de grupo. Comparando-se os valores experimentais com os valores teóricos previstos pelas curvas de dispersão, percebe-se que o sensor reflexivo pode ser utilizado para se medir modos simétricos da onda de Lamb.Visando-se a caracterização de líquidos, imerge-se parcialmente uma placa de alumínio em um fluido sob teste, e excita-se o modo fundamental antissimétrico da onda de Lamb, A0. O modo A0 converte-se em modo QSCH na interface ar-fluido, o qual sofre reflexão na borda da placa e se reconverte em modo A0 na interface fluido-ar. Utilizando-se o sensor reflexivo, pode-se medir a velocidade de propagação do modo QSCH. Sabendo-se que a velocidade do modo QSCH é diferente para diferentes fluido, verifica-se que o sensor pode ser utilizado para medir porcentagens de misturas de fluidos. Adicionalmente, medem-se também os deslocamentos lineares gerados pelo modo A0 na placa de alumínio na direção out-of-plane, comparando-se os resultados com as medições realizadas por um interferômetro de Michelson. / In this work one approaches the use of an angular displacement optical fiber sensor for characterization of liquids using Lamb waves and quasi-Scholte waves (QSCH) propagated in a metal plate. The reflective sensor comprises two optical fibers, one emitting and another receiving, a reflective surface, a laser and a photodetector. Firstly, one develops a brief study about the propagation of mechanical waves in metal plates imbibed in fluids. In this study, one obtains the dispersion curves for the phase velocity and the group velocity as a function of the frequency-halfwidth product for both Lamb waves and QSCH mode, considering different fluids. Subsequently, it one approaches the mathematical modeling of the reflective sensor, obtaining the static characteristic curves for four different configurations. Next, one describes the assembly process of the reflective sensor, with all the construction details. In the experimental part, one investigates the excitement of the symmetrical fundamental mode of the Lamb wave applying a sinusoidal pulse of gaussian envelope. Catching up simultaneously the wave propagated signals by the reflective sensor and by a piezoceramic, one obtains the experimental values of the group velocity. Comparing the experimental values with the values proposed by the dispersion curves, one notices that the reflective sensor can be used to measure the symmetric modes of the Lamb waves. Aimed at the characterization of liquids, one immerges partially an aluminum plate in a fliud under test, and one excites the fundamental antisymmetric mode of the Lamb wave, A0. The A0 mode becomes QSCH in the air-fluid interface, which reflects in the edge of the plate and is reconverted into A0 mode in the fluid-air interface. Using the reflective sensor, one can measure the QSCH mode propagation velocity. Knowing that the QSCH velocity is different for different fluids, one verifies that the sensor can be used to measure percentages of fluid mixtures. One measures also the linear displacements generated by the A0 mode in the aluminum plate in the out-of-plane direction, comparing the results with the measurements made by a Michelson interferometer.

Made available in DSpace on 2014-06-11T19:27:14Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-11-24Bitstream added on 2014-06-13T18:55:43Z : No. of bitstreams: 1 franco_vr_me_ilha.pdf: 5148348 bytes, checksum: 722b347f89e5e9a0aa5c379afe0dadba (MD5) / Financiadora de Estudos e Projetos (FINEP) / Este trabalho apresenta o estudo e desenvolvimento de uma técnica de monitoramento da integridade estrutural, para identificação e caracterização de falhas estruturais através da metodologia das ondas de Lamb utilizando materiais piezelétricos como sensores e atuadores. Ondas de Lamb são uma forma de perturbação elástica que se propaga guiada entre duas superfícies paralelas livres. Ondas de Lamb são formadas quando o atuador excita a superfície da estrutura com um pulso depois de receber um sinal. Quando uma onda propaga na superfície de uma placa, ela chega em um PZT sensor por diferentes caminhos. Um caminho é quando a onda atinge o sensor diretamente, ou seja, sem obstáculos no caminho em que ela se propaga. Outro caminho possível é quando a onda chega ao sensor após se propagar sobre descontinuidades existentes na superfície da estrutura. Com as várias características dos sinais recebidos, e com o uso de certas técnicas de processamento de sinais, essas falhas podem ser identificadas, realizando-se a ação correta tentando evitar a total falha da estrutura. Nesse contexto, diferentes testes experimentais foram realizados em diferentes tipos de estruturas. Redes de sensores e atuadores piezelétricos foram acopladas na superfície dessas estruturas, a fim de se fazer a configuração das ondas de Lamb. Os PZTs atuadores excitaram a estrutura em altas faixas de frequência. Diferentes tipos de falhas estruturais foram simuladas, através do aumento de massa, alteração de rigidez e através de cortes na borda das estruturas. Quatro índices de falha foram utilizados para detectar a presença da falha na estrutura, são eles: Root- Means-Square Deviation (RMSD), Índice de Falha Métrica (IFM), Norma H2 e Correlation Coefficient Deviation Mean (CCDM). Estes índices foram computados através dos sinais de entrada e de saída no domínio da frequência... / This work presents the study and development of a Structural Health Monitoring technique for identification and characterization of structural damages based on Lamb waves methodology using piezoelectric materials as actuators and sensors. Lamb waves are a form of elastic perturbation that remains guided between two parallel free surfaces. Lamb waves are formed when the actuator excites the structure’s surface with a pulse after receiving a signal. When the wave propagates on the structure, it comes in a PZT sensor from different paths. One path is when the wave reaches the sensor directly, i.e. without obstacles in the path in which it propagated. Another possible path is when the wave reaches the sensor after spreads on discontinuities in the structure’s surface. Damages can be detected and located through several features of the received signals and with the use of certain techniques of signal processing. In this context, several experimental tests were performed on different kinds of structures. Piezoelectric actuators and sensors networks were attached on the surface of these structures in order to make the Lamb waves configuration. The PZTs actuators excited the structure in high frequency ranges. Different kinds of structural damages were simulated by increasing mass, reduction of stiffness and cuts through the edge of the structures. Four damage-sensitive indexes were used to detect the presence of the damage in the structure: Root-Means-Square Deviation (RMSD), Metric Damage Index (MDI), H2 Norm and Correlation Coefficient Deviation (CCDM). These indices were computed in the frequency domain. The results showed the viability of the Lamb waves methodology for Structural Health Monitoring system using smart materials as actuators and sensors

Orientador: Cláudio Kitano / Resumo: Neste trabalho aborda-se a utilização de um sensor de deslocamento angular em fibra óptica para a caracterização de líquidos utilizando-se ondas de Lamb e o modo quase Scholte (QSCH) propagados em placa metálica. O sensor reflexivo compreende duas fibras ópticas, uma emissora e outra receptora, uma lente positiva, uma superfície reflexiva, um laser e um fotodetector. Primeiramente, realiza-se um breve estudo sobre a propagação de ondas mecânicas guiadas em placas metálicas imersas em fluidos. Nesse estudo, são obtidas as curvas de dispersão para a velocidade de fase e para a velocidade grupo em função do produto frequência-semiespessura da placa, tanto para as ondas de Lamb quanto para o modo QSCH, considerando diferentes fluidos. Posteriormente, aborda-se o modelamento matemático do sensor reflexivo, obtendo-se as curvas características estáticas para quatro diferentes configurações. Em seguida, descreve-se o processo de montagem do sensor reflexivo, com todos os seus detalhes construtivos. Já na parte experimental, aplicando-se um pulso senoidal de envoltória gaussiana em um transdutor, investiga-se primeiramente a excitação do modo fundamental simétrico da onda de Lamb, S0. Captando-se simultaneamente os sinais da onda propagada na placa pelo sensor reflexivo e por uma piezocerâmica em modo de recepção, obtêm-se os valores experimentais da velocidade de grupo. Comparando-se os valores experimentais com os valores teóricos previstos pelas curvas de dispersão, percebe-se que... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In this work one approaches the use of an angular displacement optical fiber sensor for characterization of liquids using Lamb waves and quasi-Scholte waves (QSCH) propagated in a metal plate. The reflective sensor comprises two optical fibers, one emitting and another receiving, a reflective surface, a laser and a photodetector. Firstly, one develops a brief study about the propagation of mechanical waves in metal plates imbibed in fluids. In this study, one obtains the dispersion curves for the phase velocity and the group velocity as a function of the frequency-halfwidth product for both Lamb waves and QSCH mode, considering different fluids. Subsequently, it one approaches the mathematical modeling of the reflective sensor, obtaining the static characteristic curves for four different configurations. Next, one describes the assembly process of the reflective sensor, with all the construction details. In the experimental part, one investigates the excitement of the symmetrical fundamental mode of the Lamb wave applying a sinusoidal pulse of gaussian envelope. Catching up simultaneously the wave propagated signals by the reflective sensor and by a piezoceramic, one obtains the experimental values of the group velocity. Comparing the experimental values with the values proposed by the dispersion curves, one notices that the reflective sensor can be used to measure the symmetric modes of the Lamb waves. Aimed at the characterization of liquids, one immerges partially an alumi... (Complete abstract click electronic access below) / Mestre

Resumo: Este trabalho apresenta o estudo e desenvolvimento de uma técnica de monitoramento da integridade estrutural, para identificação e caracterização de falhas estruturais através da metodologia das ondas de Lamb utilizando materiais piezelétricos como sensores e atuadores. Ondas de Lamb são uma forma de perturbação elástica que se propaga guiada entre duas superfícies paralelas livres. Ondas de Lamb são formadas quando o atuador excita a superfície da estrutura com um pulso depois de receber um sinal. Quando uma onda propaga na superfície de uma placa, ela chega em um PZT sensor por diferentes caminhos. Um caminho é quando a onda atinge o sensor diretamente, ou seja, sem obstáculos no caminho em que ela se propaga. Outro caminho possível é quando a onda chega ao sensor após se propagar sobre descontinuidades existentes na superfície da estrutura. Com as várias características dos sinais recebidos, e com o uso de certas técnicas de processamento de sinais, essas falhas podem ser identificadas, realizando-se a ação correta tentando evitar a total falha da estrutura. Nesse contexto, diferentes testes experimentais foram realizados em diferentes tipos de estruturas. Redes de sensores e atuadores piezelétricos foram acopladas na superfície dessas estruturas, a fim de se fazer a configuração das ondas de Lamb. Os PZTs atuadores excitaram a estrutura em altas faixas de frequência. Diferentes tipos de falhas estruturais foram simuladas, através do aumento de massa, alteração de rigidez e através de cortes na borda das estruturas. Quatro índices de falha foram utilizados para detectar a presença da falha na estrutura, são eles: Root- Means-Square Deviation (RMSD), Índice de Falha Métrica (IFM), Norma H2 e Correlation Coefficient Deviation Mean (CCDM). Estes índices foram computados através dos sinais de entrada e de saída no domínio da frequência... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This work presents the study and development of a Structural Health Monitoring technique for identification and characterization of structural damages based on Lamb waves methodology using piezoelectric materials as actuators and sensors. Lamb waves are a form of elastic perturbation that remains guided between two parallel free surfaces. Lamb waves are formed when the actuator excites the structure's surface with a pulse after receiving a signal. When the wave propagates on the structure, it comes in a PZT sensor from different paths. One path is when the wave reaches the sensor directly, i.e. without obstacles in the path in which it propagated. Another possible path is when the wave reaches the sensor after spreads on discontinuities in the structure's surface. Damages can be detected and located through several features of the received signals and with the use of certain techniques of signal processing. In this context, several experimental tests were performed on different kinds of structures. Piezoelectric actuators and sensors networks were attached on the surface of these structures in order to make the Lamb waves configuration. The PZTs actuators excited the structure in high frequency ranges. Different kinds of structural damages were simulated by increasing mass, reduction of stiffness and cuts through the edge of the structures. Four damage-sensitive indexes were used to detect the presence of the damage in the structure: Root-Means-Square Deviation (RMSD), Metric Damage Index (MDI), H2 Norm and Correlation Coefficient Deviation (CCDM). These indices were computed in the frequency domain. The results showed the viability of the Lamb waves methodology for Structural Health Monitoring system using smart materials as actuators and sensors / Orientador: Vicente Lopes Junior / Coorientador: Michael J. Brennan / Banca: Gilberto Pechoto de Melo / Banca: José Roberto de França Arruda / Mestre

Ce travail de thèse adresse les phénomènes en jeux dans les interfaces tactiles par ondes de Lamb avec, d’une part, l’influence des perturbations engendrées par les touchers d’utilisateurs et, d’autre part, l’influence des conditions environnementales sur les matériaux et l’algorithme de localisation.Dans une première partie, des mesures expérimentales du comportement viscoélastique de l’index ont été réalisées et relient l’effort et l’angle d’appui à la superficie de contact elliptique par une loi en puissance. Les impédances acoustiques caractéristiques du doigt et de divers matériaux ont été mesurées pour les ondes longitudinales et transversales. Ces paramètres sont représentatifs de la capacité à perturber la propagation d’ondes dans une cavité acoustique. En s’appuyant sur ces résultats, un doigt artificiel a été conçu, permettant des mesures automatisées ou des calibrations répétables de surfaces tactiles.Dans une seconde partie, l’influence des conditions de température et d’humidité relative sur les propriétés mécaniques a été étudiée par l’analyse des vitesses et des atténuations des ondes de volumes dans deux thermoplastiques différents. Alors que l’ABS injecté présente des vitesses de propagation linéairement dépendantes avec la température, elles ont une dépendance quadratique dans le cas d’un polyamide fritté. La sensibilité des ondes de Lamb aux conditions climatiques est estimée à partir des propriétés des matériaux et permet de déduire les modifications des spectres de réponse d’un toucher sur une coque, autorisant une compensation de l’algorithme de localisation. Enfin, différentes pistes d’amélioration du contrôle des ondes guidées et de l’algorithme sont proposées en s’appuyant sur l’utilisation de matériaux viscoélastiques et de représentations temps-fréquence. / This work addresses the phenomena taking place in Lamb wave tactile interfaces by studying the perturbation due to a user touch, and the influence of environmental conditions on materials and localization algorithm.In the first part, viscoelastic measurements of the index finger show a power law relation between force, angle and elliptic area of the contact. The acoustic impedances of index finger and different materials have been measured for longitudinal and transversal waves. These parameters are representatives of the ability to perturb the propagation of waves in the acoustic cavity. From these results, an artificial finger has been designed and allows automated measurements and repeatable calibration of tactile surfaces to be carried out.In the second part, the influence of temperature and humidity on mechanical properties is investigated by analyzing velocities and attenuations of bulk waves in two different thermoplastics. While velocities are linearly dependent on temperature for injected ABS, they are quadratically dependent for the sintered polyamide. Sensibility of Lamb waves to the environmental conditions is estimated with material properties and allows the modifications of the spectral response of a touch to be calculated. That enables compensations in the localization algorithm.Finally, various improvements in the control of guided waves and localization algorithm are proposed. They rely on viscoelastic materials and time-frequency representations.

Structural Health Monitoring (SHM) techniques are widely used in a number of Non – destructive Evaluation (NDE) applications. There is a need to develop effective techniques for SHM, so that the safety and integrity of the structures can be improved. Two most widely used SHM methods for plates and rods use either the spectrum of the impedances or monitor the propagation of lamb waves. Piezoelectric wafer – active sensors (PWAS) were used for excitation and sensing. In this study, surface response to excitation (SuRE) and Lamb wave propagation was monitored to estimate the integrity of the round objects including the pipes, tubes and cutting tools. SuRE obtained the frequency response by applying sweep sine wave to surface. The envelope of the received signal was used to detect the arrival of lamb waves to the sensor. Both approaches detect the structural defects of the pipes and tubes and the wear of the cutting tool.