Dissertations / Theses on the topic 'Defects propagation'

Failure in composite structures due to low-velocity impact damage raises a significant maintenance concern because it can lead to a barely visible and difficult-to-detect damage. Depending on the severity of the impact, fiber and matrix breakage or delaminations can occur, reducing the load carrying capacity of the structure. Efficient structural health monitoring (SHM) of composite structures can be achieved by using low- frequency guided ultrasonic waves as they have advantages of propagating over large structure and being sensitive to defects located at any thickness position. This work focuses on the use of first antisymmetric guided wave mode (A0) for health monitoring in laminated composite plates. The first part of this work is to investigate the propagation of A0 mode in undamaged composite plates experimentally and compare the results to Finite Element simulations and semi-analytical analysis. This study is essential in order to improve understanding of the guided waves behavior in composite plates and would benefit the interpretation of received signals particularly for defect characterization. To gain a good understanding of the A0 mode interaction with defects in composites, a full three- dimensional (3D) Finite Element (FE) analysis is used. A systematic study of the influence of defect geometry and range of situations on guided wave scattering is demonstrated. Combined delamination with material degradation to simulate mixed- modes defect is shown. Two dimensional FE simulations used for analysis of large delamination are also presented. The final part of this thesis presents the scattering of guided waves at the impact damage using a non-contact laser interferometer. In this study, the results were quantified and compared to baseline measurements on undamaged composite panels. Significant scattering activities were observed, allowing for the detection of impact damage in composite plates. The impact damage was further characterized using standard ultrasonic C-scans. Good agreement between experiments and predictions was found.

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 12 Dec 2003.
Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2048" Stavroula Foteinopoulou. 12/12/2003. Report is also available in paper and microfiche from NTIS.

Le nouveau procédé d’élaboration par solidification dirigée de lingots de Si quasi-monocristallin (« monolike ») offre une alternative séduisante à l’utilisation plus coûteuse de monocristaux pour la fabrication de cellules photovoltaïques à architecture avancée et haut rendement. Toutefois, la présence locale de zones de fortes densités de dislocations entraîne une dispersion des rendements obtenus.Cette Thèse présente une analyse détaillée des mécanismes de formation des structures de dislocations lors de la cristallisation monolike et de leur influence sur les propriétés électriques du matériau. Des conclusions pratiques en sont tirées pour l’amélioration du procédé.Des expériences de cristallisation en four pilote ont été réalisées en faisant varier les paramètres tenant à la mise en œuvre du pavage de germes et à la réutilisation de ceux-ci, à l’orientation cristallographique de croissance, et au dopage en élément durcissant (Ge). Des tests complémentaires de recuit et de flexion 4 points à haute température ont été utilisés pour analyser l’influence du niveau de contrainte et du temps.Une caractérisation avancée des structures de dislocations a été réalisée par imagerie X synchrotron. En arrière du front de croissance, les dislocations s’organisent en structures cellulaires qui correspondent à l’état final de fluage stationnaire. Les dislocations qui émergent au front peuvent, par accumulation locale, générer des domaines désorientés de forme conique, qui présentent des angles de rotation croissants autour de la direction de solidification, et s’étendent latéralement lors de la progression du front. Les fortes activités recombinantes de ces défauts ont été caractérisées par LBIC et Photoluminescence. Un choix approprié des orientations et des conditions de mise en œuvre des germes permet de s’affranchir des défauts initiés à l’interface germes/lingot. Toutefois, de tels défauts peuvent aussi être générés par accumulation locale de dislocations en partie supérieure des lingots sous l’effet de contraintes élevées.Ces derniers défauts n’ont pas été observés dans les lingots cristallisés dans les directions <110> et <112>, ce qui constitue un avantage par rapport aux lingots <100>. Par contre, des macles et sous-joints se sont propagés à partir des joints de grains de rotation créés volontairement, de sorte que l’effet de l’angle de rotation reste à analyser. Enfin, l’addition de germanium s’est révélée très efficace pour ralentir la multiplication des dislocations lors de tests de flexion sous faibles contraintes. Toutefois, son application à la cristallisation nécessitera une meilleure planéité du front de cristallisation et un brassage forcé du bain pour éviter une ségrégation radiale de Ge. L’utilisation d’autres éléments durcissants est également envisagée
The new generation of directionally solidified “monolike” Si ingots presents an attractive alternative to high-cost monocrystals for the manufacture of high performance solar cells with advanced architecture. However, local zones with high densities of dislocations still affect the overall solar cell efficiency.In the present work, the mechanisms of formation of dislocations structures during monolike growth and their influence on the electrical properties of the material were analyzed, and practical conclusions were drawn for the improvement of the process.Pilot scale crystal growth experiments were performed with varying parameters related to seed pavement and seed recycling, crystallographic orientation of the growth, and doping with a strengthening element (Ge). Complementary annealing and 4-point bending tests at high temperature were used to analyze the influence of stress level and time under stress.Advanced structural characterization of dislocations structures was performed by synchrotron X-ray imaging. Behind the growth front, dislocations organize in cellular patterns which correspond to a quasi-stationary creep stage, reached in the solid after long time under stress at high temperature. Dislocations that emerge at the growth front develop, from local sources, cone-shaped misoriented domains, which present increasing tilt around the growth axis and expand laterally as growth proceeds. Characterization by LBIC and Photoluminescence showed that these defects have the highest recombination activities. The sources of these defects located at the seed ingot interface can be suppressed by proper choice of seeds orientations and arrangement. However, another source is bunching of dislocations at the growth front under the higher stresses upper in the ingot.In <110> and <112> grown ingots, dislocation bunching was not observed inside the monocrystalline parts, which shows an advantage of these orientations over <100>. On another hand, twins and sub-grain boundaries propagated from higher angle grain boundaries with these growth directions, and further studies are needed to prevent the generation of such defects. Finally, Ge doping was effective to reduce dislocations multiplication in bending under low stresses. However, its application to crystal growth will require a planar growth interface, and forced melt mixing to avoid Ge radial segregation. New researches inspired by the addition of strengthening elements are now in development

Mechanical damages, like scratches, are commonly detected on the surfaces of aircraft components and structures. They can be accidentally introduced during machining or maintenance operations or be the result of wear and impacts during aircraft service. Under the action of service loads, such mechanical damage can generate fatigue cracks reducing the component fatigue life and compromising the aircraft structural integrity. The evaluation of the effect of scratches and other small mechanical defects on the structure and component fatigue lives is therefore necessary to define an inspections programme and ensure the structural safety. Conventional fatigue life prediction methods generally consider scratches tents of microns deep too shallow to appreciably affect the fatigue performances of structural components. However the discovery of the scribe marks on fuselage joints disproved that prediction. In fact several commercial airlines discovered during inspections that aircraft which have been repainted showed multiple scratches on the fuselage skin along longitudinal and circumferential joints. Those scratches, referred to scribe marks, appear to have been caused by use of sharp tools during sealant removal process prior repainting. Scratches less than 200 μm deep were capable of severely reducing the fatigue life performance under service load rendering some aircraft beyond economical repair. This thesis investigates the fatigue performances of 2024-T531 aluminium alloy sheets weakened by mechanically machined scratches. 2 mm thick clad and unclad samples were scribed at their gauge section using a diamond tipped tool. The scribing process produced very regular rounded V-shaped notches with an included angle of 60° across the sample width. Scratches from 25 μm to 185 μm deep, with 5 μm, 25 μm and 50 μm root radii were cut on the sample surfaces. Scribed sample were subsequently fatigue tested under constant amplitude tensile and bending load with a stress ratio of R=0.1 at a maximum stress of 200 MPa. Scribes were found to reduce the fatigue life of tension and bending samples up to 97% compared to that of smooth unscribed specimens. Both scribe shape and size affected the fatigue life of tensile and bending aluminium samples. The sharper and the larger the notch, the shorter the fatigue life. Post failure fractography investigations were performed on sample fracture surfaces by means of optical and scanning electron microscope. Crack nucleation sites, fracture morphology and peculiar features left during crack propagation were analysed. Finally crack propagation data under different loading conditions were obtained by striation counting performed on fracture surfaces. Multiple crack initiation occurred at scribe roots usually from inclusions, defects or weak points along the root. The number and density of crack nucleation sites appeared to be determined by the scribe; increasing for notches with larger stress concentrations. Scribe geometry did not affect the fatigue growth rate but the propagation life for cracks deeper than 50-100 μm was influenced. Cracks nucleated from scribe marks showed a typical short crack behaviour growing faster than long cracks with the same linear elastic stress intensity factor. Finite element calculations were performed on scribed samples evaluating how the presence of scribes altered the local stress and strain fields. Monotonic elastic and elastic-plastic and cyclic elastic-plastic analyses were carried out under tensile and bending loads. Local elastic-plastic stress and strain fields in the neighbourhood of different scribes were determined by the notch shape and size. According to the occurrence of mechanical similitude conditions, scribes with the same shape but different size showed similar plastic zone and stress and strain distributions. A stabilised cyclic plastic zone was developed just at the root of scribes with a ratio between the root radius and depth ρ/d≤0.2. No correlations were observed between the occurrence of a stabilised cyclic plastic zone and the sample nucleation lives defined as the number of cycles to obtain an initial crack 50 μm deep from the notch root. Traditional fatigue life prediction methods, based on the notch sensitivity factor, were not able to correctly characterise the effect of scratches few tens of microns deep on the fatigue life of 2024-T351 aluminium alloy components. An approach based on the critical distance theory was developed to characterise the total fatigue life reduction produced by the introduction of scribes relating the fatigue live to a critical stress range Δσlm. The critical stress range was capable of describing the effect of the elastic stress distribution produced by dissimilar notches on the nucleation and propagation of fatigue crack considering also the effect of the variation of the fatigue load nominal applied stress.

Ce travail comporte deux volets. Le premier concerne l'étude de défauts locaux dans des matériaux cristallins. Le chapitre 1 donne un bref panorama des principaux modèles utilisés en chimie quantique pour le calcul de structures électroniques. Dans le chapitre 2, nous présentons un modèle variationnel exact qui permet de décrire les défauts locaux d'un cristal périodique dans le cadre de la théorie de Thomas-Fermi-von Weiszäcker. Celui-ci est justifié à l'aide d'arguments de limite thermodynamique. On montre en particulier que les défauts modélisés par cette théorie ne peuvent pas être chargés électriquement. Les chapitres 3 et 4 de cette thèse traitent du phénomène de pollution spectrale. En effet, lorsqu'un opérateur est discrétisé, il peut apparaître des valeurs propres parasites, qui n'appartiennent pas au spectre de l'opérateur initial. Dans le chapitre 3, nous montrons que des méthodes d'approximation de Galerkin via une discrétisation en éléments finis pour approcher le spectre d'opérateurs de Schrödinger périodiques perturbés sont sujettes au phénomène de pollution spectrale. Par ailleurs, les vecteurs propres associés aux valeurs propres parasites peuvent être interprétés comme des états de surface. Nous prouvons qu'il est possible d'éviter ce problème en utilisant des espaces d'éléments finis augmentés, construits à partir des fonctions de Wannier associées à l'opérateur de Schrödinger périodique non perturbé. On montre également que la méthode dite de supercellule, qui consiste à imposer des conditions limites périodiques sur un domaine de simulation contenant le défaut, ne produit pas de pollution spectrale. Dans le chapitre 4, nous établissons des estimations d'erreur a priori pour la méthode de supercellule. En particulier, nous montrons que l'erreur effectuée décroît exponentiellement vite en fonction de la taille de la supercellule considérée. Un deuxième volet concerne l'étude d'algorithmes gloutons pour résoudre des problèmes de propagation d'incertitudes en grande dimension. Le chapitre 5 de cette thèse présente une introduction aux méthodes numériques classiques utilisées dans le domaine de la propagation d'incertitudes, ainsi qu'aux algorithmes gloutons. Dans le chapitre 6, nous prouvons que ces algorithmes peuvent être appliqués à la minimisation de fonctionnelles d'énergie fortement convexes non linéaires et que leur vitesse de convergence est exponentielle en dimension finie. Nous illustrons ces résultats par la résolution de problèmes de l'obstacle avec incertitudes via une formulation pénalisée
The contributions of this thesis work are two fold. The first part deals with the study of local defects in crystalline materials. Chapter 1 gives a brief overview of the main models used in quantum chemistry for electronic structure calculations. In Chapter 2, an exact variational model for the description of local defects in a periodic crystal in the framework of the Thomas-Fermi-von Weisz"acker theory is presented. It is justified by means of thermodynamic limit arguments. In particular, it is proved that the defects modeled within this theory are necessarily neutrally charged. Chapters 3 and 4 are concerned with the so-called spectral pollution phenomenon. Indeed, when an operator is discretized, spurious eigenvalues which do not belong to the spectrum of the initial operator may appear. In Chapter 3, we prove that standard Galerkin methods with finite elements discretization for the approximation of perturbed periodic Schrödinger operators are prone to spectral pollution. Besides, the eigenvectors associated with spurious eigenvalues can be characterized as surface states. It is possible to circumvent this problem by using augmented finite element spaces, constructed with the Wannier functions of the periodic unperturbed Schr"odinger operator. We also prove that the supercell method, which consists in imposing periodic boundary conditions on a large simulation domain containing the defect, does not produce spectral pollution. In Chapter 4, we give a priori error estimates for the supercell method. It is proved in particular that the rate of convergence of the method scales exponentiall with respect to the size of the supercell. The second part of this thesis is devoted to the study of greedy algorithms for the resolution of high-dimensional uncertainty quantification problems. Chapter 5 presents the most classical numerical methods used in the field of uncertainty quantification and an introduction to greedy algorithms. In Chapter 6, we prove that these algorithms can be applied to the minimization of strongly convex nonlinear energy functionals and that their convergence rate is exponential in the finite-dimensional case. We illustrate these results on obstacle problems with uncertainty via penalized formulations

Dans un contexte de forte compétitivité économique et de respect de l’environnement, de nombreuses actions sont entreprises chaque année dans le but d’améliorer la performance énergétique des bâtiments. En phase de conception, le recours à la simulation permet d’évaluer les différentes alternatives au regard de la performance énergétique et du confort des occupants et constitue ainsi un outil d’aide à la décision incontournable. Toutefois, il est courant d’observer des écarts entre les performances énergétiques réelles et celles prévues lors de la conception. Cette thèse porte sur l’amélioration du processus de conception de l’isolation dans le but de limiter les déviations des consommations réelles par rapport à celles prévues lors des simulations. Dans un premier temps, nous situons le contexte énergétique actuel puis nous présentons les différentes sources d’incertitudes qui affectent les résultats des simulations. Dans ce travail, nous nous intéressons particulièrement à la variabilité des propriétés thermophysiques des isolants, au caractère variable de la mise en œuvre et à l’impact du changement climatique. Des études expérimentales ont permis de quantifier l’incertitude associée à la performance des matériaux sains d’une part, et celle associée à des isolants défectueux d’autre part. Un couplage entre des techniques de thermographie infrarouge et des modèles éléments finis ont permis de proposer des modèles paramétriques permettant d’évaluer la performance effective d’un isolant défectueux, en fonction du type et de la taille du défaut dans l’isolant. Pour une bonne estimation à long terme de la performance de l’isolation, il est nécessaire d’intégrer les prévisions météorologiques. Ces dernières sont généralement estimées sur la base des données historiques de la région. Toutefois, il est encore difficile de prévoir avec exactitude l’évolution climatique car elle dépend de nombreux facteurs socio-économiques, démographiques et environnementaux. Dans ce travail, nous proposons de considérer les différents scénarios climatiques proposés par les climatologues et de prendre en compte leur variabilité de manière à vérifier la fiabilité de l’isolation. Enfin, nous proposons d’utiliser des approches probabilistes pour intégrer ces différents types d’incertitudes dans le processus de simulation. Pour optimiser le dimensionnement de l’isolation, nous proposons une méthodologie de conception basée sur la fiabilité. Un nouveau modèle de coût est également proposé dans le but d’améliorer l’aide à la décision, en considérant les pertes indirectes, jusqu’à présent négligées dans la conception
In the context of growing world energy demand and environmental degradation, many actions are undertaken each year to improve the energy performance of buildings. During the design stage, the use of building energy simulations remains a valuable tool as it evaluates the possible options in terms of energy performance and comfort. However, as precision requirements increase, it becomes essential to assess the uncertainties associated with input data in simulation. This thesis focuses on the insulation design process under uncertainty, in order to limit gaps between real and predicted performance for better control of energy consumptions. This work firstly presents the current alarming energy context. We consider the main uncertainties that affect the insulation, mainly the variability of the thermophysical properties, the uncertainty on climate and the uncertainties due to workmanship defects. Experimental studies were carried out to evaluate the uncertainty associated to the intrinsic performance of healthy insulation materials on one hand, and those associated to defects in insulations on the other hand. A coupling between thermography techniques and finite element models was used to provide analytical models that assess the effective thermal performance of a defective insulation, according to the type and size of the defect. As the performance of insulation also depends on climate, it is necessary to integrate future weather data to evaluate the energy consumption. These weather data are generally estimated based on the historical climatic data of the region. However, it is still difficult to predict climate change as it depends on many uncontrollable factors. In this work, we consider the different climate scenarios proposed by climate expert groups, and the uncertainty associated to each scenario to evaluate the reliability of the insulation and to improve the decision making process. Finally, we propose a probabilistic approach to integrate uncertainties in simulation and an optimization methodology based on reliability. A new cost formulation is also proposed to improve the decision-making, through indirect losses related to comfort, pollution and living space losses

Le contrôle non destructif par ondes guidées générées et détectées par des transducteurs ultrasonores à couplage par air, présente deux avantages majeurs. Le premier réside dans la capacité des ondes guidées à transporter l’information sur la qualité du milieu sur une grande distance. De plus, l’absence d’un milieu de couplage liquide entre les capteurs et le milieu à tester, rend le contrôle plus commode. Ce travail consiste d’abord à développer un procédé de simulation numérique qui prend en considération de nombreux paramètres du système de contrôle. Dans une optique de réduire le nombre de degrés de liberté, un modèle hybride a été développé qui consiste en une combinaison entre un modèle analytique basé sur l’intégrale de Kirchhoff pour la propagation des ultrasons dans l’air et un modèle éléments finis de la propagation des ondes guidées dans le matériau. La mesure des caractéristiques du transducteur à couplage par air (efficacité de l’émetteur et sensibilité du récepteur) permet, d’une part, de calculer la valeur exacte de la pression dans l’air et les valeurs exactes des champs de contraintes et de déplacements dans la structure, pour une tension et une fréquence d’excitation, et d’autre part, de remonter à la tension électrique aux bornes de ce récepteur pour une pression rayonnée par le matériau. Par suite, cette caractérisation rend possible la comparaison entre les prédictions numériques de la réponse (en niveau de tension) du système et les mesures expérimentales correspondantes. A la lumière du modèle numérique développé, une optimisation des paramètres du système de contrôle (angle, fréquence,diamètre, direction de propagation, champ proche et champ lointain) a été effectuée pour améliorer la pureté des modes guidés par le matériau. Une manipulation expérimentale, basée sur un transducteur à couplage par air pour l’émission et une sonde laser pour la réception, a été alors mise en place pour valider quelques prédictions numériques. Ensuite, on a étudié l’interaction des ondes guidées ultrasonores avec des défauts de type délaminage enfouis dans une plaque composite à symétrie quadratique. Pour cela, on a analysé la sensibilité des deux modes fondamentaux A0 et S0 au délaminage en terme de détectabilité. En parallèle, on a traité un problème inverse qui consiste à dimensionner un délaminage par le calcul du spectre fréquentiel du coefficient de réflexion. Enfin, on a mis en évidence le potentiel des transducteurs à couplage par air à ausculter des pièces aéronautiques impactées
Non-destructive testing (NDT) using guided waves generated and detected by air-coupled ultrasonic transducers have two main advantages. First, this non-contact technique without coupled medium allows obvious convenience of use. Moreover, the ability of guided waves to carry information about medium quality over long distance. In this context, a numerical model has been developed, which takes into account many parameters of the control system. In order to reduce the number of degrees of freedom, a hybrid model has been developed which consists of a combination between an analytical model, based on the Kirchhoff integral for the propagation of ultrasound in air and a finite element model for the propagation of guided waves in the material. The measured characteristics (efficiency and sensitivity) of two air-coupled transducers allow the prediction of the accurate values of the pressure of bulk waves generated in air and the measurement of the pressure of the radiated field in air by guided waves propagating in a structure. This process enables the comparison between predicted and measured guided waves modes. Based on the hybrid model, an optimization of the parameters of the control system (angle, frequency, diameter, direction of propagation, near and far field) was performed to improve the purity of guided modes along the material plate. To validate some numerical predictions, an aircoupled ultrasonic transducer is used and oriented at a specific angle chosen for generating one specific Lambmode guided along a composite plate sample, and a laser probe measures the normal velocity at different locations on the surface of the plate. Then, the interaction of ultrasonic guided waves with delamination in acomposite plate was studied. In particular, the sensitivity of the two fundamental modes A0 and S0 was analyzed in order to predict the detectability of the defect. In parallel, the inverse problem is solved and the defect size is quantified by calculating the spectrum of the reflection coefficient. Finally, the potential of air-coupled transducers to examine an aircraft structure, has been demonstrated

Les défauts connus pour réduire la durée de vie des piles à combustible à membrane électrolyte polymère (PEMFC) peuvent apparaître sur différents composants de l'assemblage membrane-électrode (AME) et sous différentes formes en raison des procédés de fabrication ou du vieillissement -en fonctionnement- de la pile à combustible. Ce travail concerne l’étude de l’apparition, la détection et la propagation de défauts dans les AME de PEMFC, et plus spécifiquement à l’anode. À cet effet, un protocole de vieillissement accéléré (AST) combinant des cycles de potentiel et d'humidité -induits par les variations de courant-, et de maintien en circuit ouvert est appliqué à des AME standard ou avec défauts initiaux. Les AME avec défauts ont été fabriqués avec un manque de couche active à l’anode, ce défaut pouvant être localisé près de l'entrée ou bien près de la sortie d’hydrogène. Des caractérisations électrochimiques sont réalisées périodiquement à l'aide d'une cellule linéaire instrumentée et segmentée, permettant de suivre les performances de la cellule à travers les courants, les potentiels d’électrode, les impédances locales, ainsi que l’évolution de la surface active électrochimique (ECSA) à l’anode et à la cathode pendant le test de vieillissement, avec une résolution spatiale le long des canaux. Une étude par spectroscopie d’impédance a été menée conjointement, basée sur une interprétation par circuits électriques équivalents et se focalisant sur la détection de la contribution anodique à l’impédance globale de la cellule. Les résultats mettent en évidence une dégradation accélérée de l’AME ainsi que les premières preuves de propagation de défauts, en termes de perte de l’ECSA à l’anode. Cette propagation se produit dans le sens du flux d'hydrogène. L’ECSA de la cathode semble également impactée, bien qu'apparemment de manière homogène. Un amincissement important de la membrane a également été observé dans les segments défectueux, avec propagation probable aux segments adjacents, mais à plus long terme
Defects known to shorten the lifetime of polymer electrolyte membrane fuel cells (PEMFC) can appear on different membrane electrode assembly (MEA) components and under different forms due to manufacturing processes or operational aging of the fuel cell. This work concerns the occurrence, detection, and propagation of defects in PEMFC MEA, and more specifically at the anode. To this end, an accelerated stress test (AST) combining potential and humidity cycles -induced by load variations-, and open-circuit hold is applied to standard MEA, and to MEA with initial defects. Those customized MEA were intentionally prepared with a lack of active layer at the anode, the defect being located either near the hydrogen inlet or near the hydrogen outlet. Periodic electrochemical characterizations were carried out using a segmented instrumented linear cell, allowing to monitor the cell performance through the currents, electrode potentials, and local impedance, as well as the evolution of the electrochemical active surface (ECSA) at the anode and cathode during the ageing test, with a spatial resolution along the channels. An electrochemical impedance spectroscopy study was conducted jointly, using equivalent electrical circuits, and focusing on the detection of the anodic contribution to the global impedance of the cell. Results showed an accelerated degradation of the MEA and the first evidence of defect propagation, in terms of loss of ECSA at the anode. This propagation occurred in the direction of the hydrogen flow. The ECSA at the cathode also appeared to be impacted, although apparently homogeneously. Significant membrane thinning was also observed in the defective segments, with probable propagation to adjacent segments, but over a longer time period

In microwave frequency band, the planar technology is mainly used to fabricate electronic circuits. Propagation of surface waves belongs to the significant problem of this technology. Surface waves can cause unwanted coupling among particular parts of the structure and can degrade its parameters. The problem can be solved using an electromagnetic band gap structure (EBG). These periodic structures are able to suppress surface waves in different frequency bands. This thesis is focused on the modeling of these structures in the program COMSOL Multiphysics.

In dieser Arbeit wurde eine neue fehleranalytische Methode zur industriellen Anwendung an neuen Technologien der Aufbau- und Verbindungstechnik entwickelt. Das Verfahren basiert auf der Wechselwirkung von thermischen Wellen und Defekten. Die Besonderheit ist dabei die Zerstörungsfreiheit, die Geschwindigkeit, das Auflösungsvermögen und die durch neueste IR-Detektoren erreichte Temperaturempfindlichkeit. Es wurden grundlegende Studien bezüglich Auflösung und parasitären Effekten bei der Anwendung unter industriellen Randbedingungen durchgeführt. Dabei wurde eine systematische Vorgehensweise bezüglich der Komplexität gewählt. Dies ermöglicht nun u.a. eine Vorhersage der zu erwartenden Prüfdauer zur Auflösung vergrabener Defekte, der Begrenzung der maximalen Anregungsimpulsbreite (bei gegebener Defekttiefe) und die quantitative Ermittlung des Einflusses einer Lackschicht. Methodisch kamen grundsätzlich Simulationen und vergleichende Experimente zum Einsatz. Es wurden spezielle Proben zur Isolierung und Klärung parasitärer Effekte verwendet. Letztlich konnte das Messsystem erfolgreich an industriellen Problemstellung demonstriert werden. Das entwickelte Messsystem zeichnet sich durch hohe Flexibilität aus. Verschiedene problemangepasste Anregungsquellen (interne und externe Anregung durch zahlreiche physikalische Effekte) kommen zum Einsatz. Das Messsystem besteht aus vier Hauptmodulen, der Differenzbild-Methode, der Impulsthermografie, und zwei Varianten der LockIn-Thermografie. Zusammen ist das System in der Lage, Voids, Delaminationen und Risse in verschiedenen Bereichen auch der modernen AVT sicher zu erkennen. Es werden dabei Temperaturauflösugnen bis zu 5 mK und laterale Auflösungen bis 17 µm erreicht. Diese Arbeit legt einen Grundstein für die Anwendung der thermischen Fehleranalytik in der Industrie, indem hier die Grenzen der IR-Messtechnik aufgezeigt und charakterisiert werden
In this work a new failure analytical method for the industrial application of new technologies in electronic packaging has been developed. The developed method is based on the interaction of the thermal waves and defects. The special fature is non-destructive, speed, resolution and high temperature sensitivity due to latest IR-detectors. It fundamental studies regarding resolution and parasitic effects in the application were carried out cinsidering industrial conditions. Here, a systematic approach regarding the complexity has been selected. This now enables a prediction of the expected test period for detecting buried defects, limits for excitation pulse width (for a given defect depth) and the quantitative determination of the influence of parasitic paints. Methodically always simulations and comparative experiments were used. Simple samples for the isolation and purification of parasitic effects has been used. Finally, the measurement system has been successfully demonstrated on an industrial applications. The developed measurement system is characterized by high flexibility. Different problem-adapted excitation sources (internal and external excitation by numerous physical effects) are used. The measurement system currently consists of four main modules, the difference image method, the pulse thermography, and two variants of LockIn-thermography. Together, the system is capable of detecting voids, delaminations and cracks in various fields of electronic packageing. It will reach temperature resolutions up to 5 mK and lateral resolutions down to 17 µm. This work stes a foundation for the application of thermal failure analysis for industry by showing and charcterization the limits of IR imaging

Chaque faisceau d'un laser de puissance, tel que le Laser MégaJoule, est mis en forme et voit son énergie amplifiée à l’aide d’une centaine de composants optiques tels que des plaques de verre amplificateur, des lentilles, des miroirs, des lames de phase, des réseaux... 'Evidemment ces composants ne sont pas parfaitement réalisés et ils présentent à leurs surfaces des défauts de fabrication. Ces imperfections peuvent être dues aux outils de polissage des composants, aux dépôts antireflet ou même apparaitre au cours de la phase d’exploitation ou de remédiation du composant. Ces défauts contribuent à une baisse des performances du Laser MégaJoule que ce soit au niveau de l’énergie déposée au centre de la chambre d’expériences ou à l’endommagement accéléré des composants optiques placés en aval. Actuellement la norme ISO 10110-7 est utilisée pour spécifier les défauts d’aspect. Cependant elle manque de justification pour les besoins d’une chaine laser de puissance. Dans le cadre de la thèse nous nous intéressons exclusivement à l’endommagement fratricide local des composants optiques. Un défaut présent à la surface du composant module l’onde lumineuse par phénomène de diffraction. Un << point chaud >> peut apparaitre dans le faisceau laser augmentant alors la densité locale surfacique d’énergie appliquée aux composants suivants. La loi d’endommagement prédit une augmentation de la probabilité d’endommagement puisque la densité d’énergie est modifiée. Une première étude, fondée sur les équations de Fresnel, met en évidence les paramètres intéressants à spécifier pour prédire les intensifications engendrées par des défauts typiques. Le lien entre paramètres du défaut et intensifications diffractées est, ensuite, validité expérimentalement sur des cas réels de défauts. Une seconde étude établit un seuil exprimé en puissance en deçà duquel l'hypothèse d'une propagation linéaire, selon les équations de Fresnel, est valide. La cohérence des résultats donnés par le seuil en puissance et par la simulation numérique renforce l’idée voulant que la propagation d’un << point chaud >> en présence d'effet Kerr soit sensiblement différente de celle d’un faisceau gaussien. Fort de ces deux résultats nous sommes en capacité d'établir une spécification des défauts d’aspect en ayant une meilleure compréhension de l’endommagement fratricide local
Each beam of a high power laser facility, such as the Laser MégaJoule, is shaped and amplified thanks to hundreds optical components such as amplifier slabs at Brewster’s angle, lenses, mirrors, phase plates, diffraction gratings... Of course, all these components cannot be perfect; there are some defects on their surface. These imperfections appear at each stage of the life of the component, during polishing, coating, or mitigation process or when the component is used on the facility. They have a huge impact on the energy losses delivered on the target and they decrease the resistance of downstream components to intense light. The ISO 10110-7 standard is currently used to specify the visual defects. However, this standard is poorly justified and do not fit a high power laser needs. In this thesis, we are focused on the fratricide effect. Light propagates through a defect, then, some intensity modulations appear along the propagation. The damage law states that high energy density leads to an increase of the damage probability. Firstly, we investigate the characteristic parameters of the defect morphology linked to the formation of downstream << hot spots >>. Then, the link between these typical parameters and the high intensifications are confirmed by experiments on real defects. Secondly, a power criterion is demonstrated to guarantee the linear propagation hypothesis. This criterion is compared to numerical simulations and it is shown how the nonlinear propagation, induced by Kerr effect, can be different between the << hot spot >> formed by a defect and a Gaussian beam. Finally, the results are used to improve the visual defect specification thanks to a better understanding of the fratricide effect

Les miroirs multicouches trouvent de nombreuses applications utilisant les rayons X produits par les synchrotrons et les lasers à électrons libres et doivent relever de nouveaux challenges apparus avec l'amélioration de ces sources de rayonnement. Pour étudier les causes des contrastes d'intensité produits dans le faisceau réfléchi, des mesures expérimentales et une modélisation numérique ont été effectuées.Plusieurs méthodes d’analyse de la structure des multicouches et du front d’onde réfléchi par un miroir multicouches ont été mises en œuvre sur la ligne de lumière BM05 de l'ESRF. Les méthodes de mesure de 'Rocking Curve Imaging' et 'theta/2theta Imaging' ont été appliquées et adaptées pour la première fois aux miroirs multicouches. Des mesures du front d'onde réfléchi par une multicouche ont aussi été effectuées à partir des techniques d’holographie et de tavelure en champ proche. Les résultats obtenus permettent de mieux comprendre les effets de phase produits par les multicouches ainsi que leurs origines et de reconstruire la topographie de défauts de hauteur d'un miroir multicouches par l'utilisation du modèle numérique développé durant cette thèse.Un modèle numérique basé sur les équations de Takagi-Taupin a été modifié pour tenir compte de défauts dans la structure multicouches d'un miroir. Des simulations de défauts simples ont été effectuées afin de caractériser les performances et les limites du modèle numérique. La propagation et la cohérence de faisceau réfléchi ont été simulées. Les résultats expérimentaux et simulés ont alors permis de montrer que les défauts de hauteur dans la structure multicouches des miroirs constituent la cause principale des contrastes d'intensité. Les simulations effectuées conduisent à conclure que les effets induits par les défauts de hauteur dans une multicouche sont analogues aux effets de phase produits par ces même défauts de hauteur sur une surface réfléchissante.Ces modèles et ces simulations peuvent être utiliser pour spécifier les caractéristiques des défaut de hauteur qui minimise la production du contraste d'intensité. Les techniques expérimentales développées permettront de caractériser avec les rayons X les futures multicouches
Multilayer mirrors find numerous X-ray applications in synchrotron and X-rays free electron lasers. These multilayers optical devices must take up new challenges raised with the upgrade these radiation sources. To study the origin of intensity contrast in reflected beam, experimental measurements and numerical modeling were performed.Several techniques for multilayer structure charactérization have been implemented at the ESRF beamline BM05. Measurements methods based on 'Rocking Curve Imaging' and 'theta/2theta' Imaging were performed and applied for the first time to multilayer mirrors. Measurements of the wavefront reflected by multilayers were performed using holography and near field speckle techniques. The results obtained allowed a better understanding of the phase effects induced by multilayer reflection of their origin and to reconstruct the topography of the height defects within a multilayer mirror using the numerical model developped in the course of this PhD thesis.A numerical model based on Takagi-Taupin equations was modified to account for defects present in the multilayer mirror structure. Simulations for simple defects were performed to evaluate performance and limits of the numerical model. The propagation and the coherence of the reflected beam were simulated. The measurement and simulation results show the main influence of defect heights on the generation of the intensity contrast observed. The simulations also lead to conclude the equivalency phase effect resulting of the same height defects in multilayers mirror structure and reflecting surface.These modelization and simulations results can be usefull to specify defect feature which minimise reflected intensity contrast. The new developped experimental technics will allows X-rays caracterization for next multilayer mirrors

Le collage structural est une technique d’assemblage de plus en plus demandée aujourd’hui dans beaucoup de domaines comme l’automobile, l’aéronautique, l’aérospatial et dans d’autres domaines comme la construction, le sport et les loisirs. Cette technique très avantageuse, permet l’assemblage de matériaux semblables ou différents à l’aide d’un adhésif, la réduction importante du poids et la répartition uniforme des charges sur l’assemblage. Malgré ses avantages, le collage souffre encore de quelques inconvénients liés à l’existence de défauts dans les joints de colle. Parmi eux, il existe des défauts qui sont situés à l’interface colle/substrat comme un « kissing bond » ou un mauvais état de surface, qui restent indétectables ou difficilement détectables utilisant les techniques de contrôle non destructives. Donc, afin de prendre en compte l’existence des défauts d’adhésion dans les assemblages collés lors de la phase de conception, il est nécessaire de fournir un modèle analytique capable de prédire la propagation de fissure. Dans cette thèse, un modèle analytique qui prédit la propagation de fissure et qui évalue la résistance effective d’un assemblage collé contenant des défauts d’adhésion a été développé. Un défaut a généralement une géométrie complexe, et une étude générique est difficilement réalisable ce qui nous amène à considérer des géométries de défauts idéales. Le modèle a été vérifié par des expériences réalisées sur des éprouvettes DCB. Des simulations numériques utilisant la méthode de zone cohésive ont été réalisées également pour décrire plus complètement le processus de décohésion et simuler les essais expérimentaux. La dernière partie de ce travail a été dédiée à l’étude de la fissuration des éprouvettes en alliage de titane. Profitant de la collaboration avec Safran et Alphanov, les substrats ont subi un traitement de surface laser en laissant des zones non traitées. Le but de cette partie était de vérifier le modèle analytique proposé avec des configurations plus complexes
Structural adhesive bonding has known an increasing use in many fields like aeronautics, aerospace and automotive and other fields like construction and sports. This very advantageous technique allows the assembly of similar or different materials using an adhesive, the significant reduction in weight and a uniform distribution of loads on the assembly. Despite its advantages, the bonding still suffers from some disadvantages related to the existence of defects in the bonded joints. Among them, there are defects that are located at the interface glue / substrate as "kissing bond" or poor surface due to bad surface treatment, which remain undetectable or hardly detectable using non-destructive control techniques. Therefore, in order to take into account the existence of adhesion defects in bonded assemblies during the design phase, it is necessary to provide an analytical model capable of predicting crack propagation and estimate the criticality of a defect. In this thesis, an analytical model that predicts crack propagation and evaluates the effective strength of a bonded assembly containing adhesion defects has been developed. A defect usually has a complex geometry, and a generic study is difficult to achieve, which leads us to consider ideal defect geometries. The model was verified by experiments performed on DCB specimens. Numerical simulations using the cohesive zone method were also performed to more fully describe the decohesion process and to simulate the experimental tests. The last part of this work was devoted to the study of titanium alloy assembly containing patterns. Taking advantage of the collaboration with Safran and Alphanov, the substrates underwent a laser surface treatment leaving untreated areas. The purpose of this part was to check the proposed analytical model with more complex configurations

El control de las propiedades acústicas de los cristales de sonido (CS) necesita del estudio de la distribución de dispersores en la propia estructura y de las propiedades acústicas intrínsecas de dichos dispersores. En este trabajo se presenta un estudio exhaustivo de diferentes distribuciones, así como el estudio de la mejora de las propiedades acústicas de CS constituidos por dispersores con propiedades absorbentes y/o resonantes. Estos dos procedimientos, tanto independientemente como conjuntamente, introducen posibilidades reales para el control de la propagación de ondas acústicas a través de los CS. Desde el punto de vista teórico, la propagación de ondas a través de estructuras periódicas y quasiperiódicas se ha analizado mediante los métodos de la dispersión múltiple, de la expansión en ondas planas y de los elementos finitos. En este trabajo se presenta una novedosa extensión del método de la expansión en ondas planas que permite obtener las relaciones complejas de dispersión para los CS. Esta técnica complementa la información obtenida por los métodos clásicos y permite conocer el comportamiento evanescente de los modos en el interior de las bandas de propagación prohibida del CS, así como de los modos localizados alrededor de posibles defectos puntuales en CS. La necesidad de medidas precisas de las propiedades acústicas de los CS ha provocado el desarrollo de un novedoso sistema tridimensional que sincroniza el movimiento del receptor y la adquisición de señales temporales. Los resultados experimentales obtenidos en este trabajo muestran una gran similitud con los resultados teóricos. La actuación conjunta de distribuciones de dispersores optimizadas y de las propiedades intrínsecas de éstos, se aplica para la generación de dispositivos que presentan un rango amplio de frecuencias atenuadas. Se presenta una alternativa a las barreras acústicas tradicionales basada en CS donde se puede controlar el paso de ondas a su través. Los resultados ayudan a entender correctamente el funcionamiento de los CS para la localización de sonido, y para el guiado y filtrado de ondas acústicas.
Romero García, V. (2010). On the control of propagating acoustic waves in sonic crystals: analytical, numerical and optimization techniques [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/8982
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La fatigue est le principal phénomène causant la rupture de structures mécaniques. Pour les très grandes durées de vie, l’amorçage des fissures menant à la rupture des pièces en service passe de la surface au volume. Du fait des difficultés expérimentales, la propagation des fissures internes n’a que très peu été étudiée in situ et en 3D. Il est cependant crucial du point de vue du dimensionnement de connaître le comportement de telles fissures. Dans ces travaux, le suivi in situ de l’amorçage et de la propagation de fissures internes a été réalisé par tomographie aux rayons X (source synchrotron). Premièrement, une méthode de fabrication d’éprouvettes a été élaborée pour permettre de visualiser de manière systématique, non destructive et en 3D la propagation à 20 Hz de fissures internes dans un alliage de titane (Ti-6Al-4V). Un défaut artificiel a pour cela été usiné à la surface d’une tôle laminée de ce matériau. Une seconde tôle, placée sur la première, a été soudée par diffusion au frittage flash, permettant ainsi de rendre le défaut volumique. Des essais de fatigue in situ ont permis de montrer le rôle crucial de l’environnement (vide) ainsi que, dans une moindre mesure, celui de la texture cristallographique sur les vitesses de propagation des fissures internes. Deuxièmement, une machine de fatigue ultrasonique (fréquence de cyclage de 20 kHz) permettant de réaliser des essais in situ au synchrotron a été utilisée pour étudier la propagation 3D de fissures internes dans des éprouvettes d’alliage d’aluminium A357-T6 contenant un défaut interne artificiel. L’obtention préalable de la distribution 3D des grains par tomographie en contraste de diffraction (DCT) a permis d’étudier les mécanismes d’interaction entre la microstructure et le chemin de fissuration en 3D
Fatigue is the main phenomenon causing the failure of mechanical structures. For very long service lives the initiation of cracks leading to the failure of in service mechanical parts changes from surface to volume. Due to experimental difficulties the propagation of internal cracks has only been studied to a limited extent in situ and in 3D. However, it is crucial from a dimensional point of view to know the behaviour of such cracks. In this work, in situ monitoring of the initiation and propagation of internal cracks was carried out by X-ray tomography (synchrotron source). Firstly, a method for manufacturing specimens was developed to enable systematic, non-destructive, and 3D visualization of the propagation of internal cracks in a titanium alloy (Ti-6Al-4V) at 20 Hz. For this purpose, an artificial defect was drilled on the surface of a rolled sheet of this material. A second sheet, placed on top of the first one, was diffusion bonded by spark plasma sintering, making it possible to make the defect volumetric. In situ fatigue tests showed the crucial role of the environment (vacuum) and, to a lesser extent, that of the crystallographic texture on the internal crack propagation rates. Secondly, an ultrasonic fatigue machine (cycling frequency of 20 kHz) allowing in situ testing at the synchrotron was used to study the 3D propagation of internal cracks in A357-T6 aluminium alloy specimens containing an artificial internal defect. The 3D grain distribution obtained by Diffraction Contrast Tomography (DCT) was used to study the interaction mechanisms between the microstructure and the 3D crack path

Etude des proprietes de fatigue des superalliages contenant des inclusions de type ceramique. Determination, a temperature ambiante, des cinetiques de debut de propagation de fissures a partir des defauts presents dans la structure. Mise en evidence, d'un effet de "fissure courte". Il est demontre que deux parametres jouent un role, essentiel dans la differenciation des cinetiques de propagation des fissures courtes et longues : le caractere tridimentionnel des fissures naturelles et le phenomenes de fermeture de fissures

Le contrôle de la propagation des ondes élastiques repose principalement sur la conception de milieu artificiel à base de matériaux structurés pour obtenir une ingénierie avancée de la dispersion de la propagation. Au cours de la thèse, la dispersion du mode (S0) dans des membranes micro-structurées à base d’AlN a été numériquement investiguée et les applications qui en découlent explorées. Il est mis en évidence le lien fort entre la dispersion du mode et la sensibilité aux perturbations externes en combinant la membrane d’AlN avec une couche de SiO2 structurée en rubans. En particulier, il est montré qu’il est possible d’obtenir un TCF=0 pour les résonateurs sans presque aucune dégradation du coefficient K2. Il est montré qu’il est possible d’ouvrir des bandes interdites avec une largeur de l’ordre de 50% en structurant l’AlN sous forme de rubans ou en utilisant des piliers pour former un PhnC. Sur cette base, des designs de cavités et de guides d’ondes sont proposés et leurs performances sont étudiées en fonction des paramètres géométriques. Il est également proposé un nouveau design de cavité basé sur l’introduction d’un défaut résonant dans le PhnC sous forme de disque de dimension très petite par-rapport à la taille de la cellule élémentaire. Le défaut permet d’introduire des modes quasi-plats dans le diagramme de bande et permet en conséquence la conception d’une nouvelle génération de dispositifs phononiques robustes pour des applications en traitement du signal et capteurs. Les structures optimales sont utilisées pour la conception de capteur de champs magnétiques, une sensibilité de 5% est obtenue pour le mode localisé dans le cas d’un disque magnéto-élastique
The control of elastic wave propagation relies mainly on the design of artificial media based on structured materials to achieve advanced propagation dispersion engineering. During the thesis, the dispersion of the mode (S0) in micro-structured membranes based on AlN was numerically investigated and the resulting applications explored. The strong link between mode dispersion and sensitivity to external disturbances is highlighted by combining the AlN membrane with a layer of SiO2 structured into strips. In particular, it is shown that it is possible to obtain a TCF = 0 for the resonators without any degradation of the K2 coefficient. It is shown that it is possible to open wide band-gaps of 50% by structuring the AlN in the shape of strips or using pillars to form a PhnC. On this basis, designs of cavities and waveguides are proposed and their performances are studied according to the geometrical parameters. It is also proposed a new cavity design based on the introduction of a resonant defect with a disc shape in the PhnC and presenting very small size in comparison to the unit cell. The defect makes it possible to introduce quasi-flat modes in the band diagram and consequently allows the design of a new generation of phononic devices for signal processing and sensor applications. The optimal structures are used to design a magnetic field sensor design, a sensitivity of 5% is obtained for the localized mode in the case of defect based on magneto-elastic thin film

Cette étude porte sur la compréhension des mécanismes de la corrosion par piqûres susceptible d’affecter les aciers au carbone prévus pour le stockage des déchets radioactifs. Des modèles de transport/réaction ont été développés pour simuler, par la méthode des éléments finis, la propagation d’une piqûre à la surface du fer en milieu chloruré. Pour les modèles conservatifs, la piqûre est activée par un potentiel imposé au métal. A partir de différentes études paramétriques (géométrie, environnement…), les simulations ont permis d’identifier les facteurs critiques responsables d’un accroissement ou d’une inhibition de la vitesse de corrosion localisée. Par ailleurs, les essais sur des électrodes occluses, dites « lead-in-pencil », confirment que le confinement favorise la précipitation d’un film salin en fond de piqûre. Enfin, la simulation de situations de couplage entre la piqûre et la surface cathodique externe a permis de décrire l’évolution réaliste de la vitesse de corrosion localisée en milieu aéré et en potentiel libre. L’analyse des résultats de ces trois axes démontre que la chute ohmique dans la piqûre est le facteur contrôlant sa progression. Dans la majorité des cas, toute augmentation de cette chute ohmique (due à la précipitation, au confinement ou à l’augmentation du rapport des surfaces cathodique/anodique), conduit à une vitesse de corrosion plus rapide des surfaces externes. Ce phénomène est qualifié « d’évasement ». La croissance en profondeur, pour une vitesse d’oxydation dépendante du pH, reste à confirmer. Pour simuler le comportement à long terme du fer vis-à-vis de la piqûration, dans les conditions de stockage, les nouveaux modèles devront reposer sur la détermination plus précise de l’environnement et des lois de dissolution et de précipitation
This study deals with the understanding of the mechanisms of pitting corrosion susceptible to occur on carbon steel selected for high level nuclear waste containers. Transport/reaction models have been developed in order to simulate, by finite element method, the propagation of a pit on iron in chlorinated medium. For conservative models, pit is activated by imposing a potential on metal. Thanks to different parametric studies (geometry, environment…) simulations have permitted to classify, by severity order, the factors responsible of the increase or the inhibition of the localized corrosion rate. Otherwise, experiments on occluded (called « lead-in-pencil ») electrodes confirm that confinement favors the formation of a salt film at the bottom of the pit. Finally, the simulation of galvanic coupling between the pit and the surrounding cathodic surface has permitted to describe the realistic evolution of the localized corrosion rate in aerated medium and for free corroding conditions. The analysis of the results from these three ways of investigation has demonstrated that the ohmic drop inside the pit is the most important factor controlling his growing. In most study cases, any increase of this ohmic drop (due to precipitation, confinement or augmentation of the cathodic /anodic surfaces ratio), leads to a faster corrosion rate of the surrounding surface. This phenomenon is called “opening” of the pit. The progression at the pit bottom for a pH-dependant oxidation rate is still to confirm. In order to simulate the long term behavior of iron during pitting, in storage application, new models require a more accurate determination of the environment conditions, dissolution and precipitation laws

El objetivo de esta tesis es la investigación de estructuras y técnicas de acoplo para minimizar las pérdidas de acoplo entre guías dieléctricas y cristales fotónicos planares. En primer lugar se ha estudiado el modelado del acoplo entre guías dieléctricas y guías en cristal fotónico así como la influencia de los principales parámetros del cristal en la eficiencia de acoplo. Se han obtenido expresiones cerradas para las matrices de reflexión y transmisión que caracterizan totalmente el scattering que ocurre en el interfaz formado entre una guía dieléctrica y una guía en cristal fotónico. A continuación y con el fin de mejorar la eficiencia de acoplo desde guías dieléctrica de anchura arbitraria, se ha propuesto como contribución original una técnica de acoplo basada en la introducción de defectos puntuales en el interior de una estructura de acoplo tipo cuña realizada en el cristal fotónico. Diferentes soluciones, incluida los algoritmos genéticos, han sido propuestas con el objetivo de conseguir el diseño óptimo de la configuración de defectos. Una vez conseguido un acoplo eficiente desde guías dieléctricas a guías en cristal fotónico, se ha investigado el acoplo en guías de cavidades acopladas. Como contribución original se ha propuesto una técnica de acoplo basada en la variación gradual del radio de los defectos situados entre cavidades adyacentes. Además, se ha realizado un riguroso análisis en el dominio del tiempo y la frecuencia de la propagación de pulsos en guías acopladas de longitud finita. Dicho estudio ha tenido como objetivo la caracterización de la influencia de la eficiencia del acoplo en los parámetros del pulso. Finalmente, se han presentado los procesos de fabricación y resultados experimentales de las estructuras de acoplo propuestas.
Sanchis Kilders, P. (2005). Coupling techniques between dielectric waveguides and planar photonic crystals [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/1854
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En contrôle non destructif, dans la perspective de l’amélioration des images de défauts mais également dans le but de rendre leur interprétation plus simple par des opérateurs non spécialisés,de nouvelles méthodes d’imagerie ultra-sonore telle que l’imagerie TFM (Total Focusing Method)sont apparues depuis quelques années comme une alternative aux méthodes d’imageries conventionnelles.Elles offrent des images réalistes des défauts et permettent à partir d’une même acquisition d’avoir un nombre important d’images chacune pouvant porter des informations différentes et complémentaires sur les caractéristiques d’un même défaut.Lorsqu’elles sont correctement sélectionnées,ces images sont plus faciles à analyser, elles présentent moins de risques de mauvaise interprétation et permettent d’envisager des caractérisations de défauts plus rapides par des opérateurs moins spécialisés.Pour une exploitation industrielle, il reste cependant nécessaire de renforcer la robustesse et la facilité de mise en oeuvre de ces techniques d’imagerie.L’ensemble des travaux réalisés durant la thèse a permis de développer de nouveaux outils capables d’améliorer la caractérisation des défauts par les techniques d’imagerie TFM en termes de position,d’orientation et de dimensionnement
In non-destructive testing, with a view to improving defect images but also to simplify their interpretation by non-specialized operators,new ultrasonic imaging methods such as TFM imaging (Total Focusing Method ) have appeared for some years as an alternative to conventional imaging methods. They offer realistic images of defects and allow from the same acquisition to have a large number of images each that can carry different and complementary information on the characteristics of the same defect. When properly selected, these images are easier to analyze, they present less risk of misinterpretation and allow to consider faster fault characterizations by less specialized operators.However, for an industrial operation, it remains necessary to strengthen the robustness and ease of implementation of these imaging techniques. All the work carried out during the thesis allowed to develop new tools to improve the characterization of defects by TFM imaging techniques in terms of position,orientation and sizing

La reconstruction passive des fonctions de Green par corrélation de bruit ambiant suscite aujourd’hui un grand intérêt en contrôle santé intégré (CSI). Dans ce manuscrit, nous proposons une méthode originale reposant sur l’application de cette approche pour détecter et localiser des défauts (fissures, trous, rainures) dans des plaques minces réverbérantes avec un faible nombre de capteurs. Les ondes de flexion qui se propagent sur la plaque sont engendrées soit par un ensemble de sources aléatoirement réparties sur la surface ou un bruit ambiant. Un réseau de capteurs sensibles au déplacement normal permet d’estimer la matrice de corrélations inter-éléments avant et après l’apparition d’un défaut. Un critère d’évaluation de la qualité des corrélations est proposé sous forme d’un niveau de bruit relatif entre les résidus de reconstruction et les fonctions de Green. La matrice différentielle de corrélations avant et après défaut est utilisée pour l’imagerie de défaut. En dépit de la reconstruction imparfaite des réponses impulsionnelles, la technique proposée s’avère comparable aux méthodes actives avec une excellente résolution. On a proposé ensuite une extension de la méthode passive par corrélation de champs pour l’identification des zones de bruit. Un filtrage basé sur la technique de décomposition en valeurs singulières (DORT) est tout particulièrement utilisé pour améliorer les images de localisation. Des sources acoustiques secondaires ont été développées pour la translation du bruit ambiant basses fréquences en composantes hautes fréquences, utilisées pour localiser des défauts dans des plaques. Enfin, on a montré que ce type de méthode pourrait être également utilisé pour caractériser un défaut dans une structure réverbérante, en particulier, il a été souligné que l’intensité des images de localisation obtenues est liée à la section de diffusion de celui-ci
Green’s functions retrieval from ambient noise correlation has recently drawn a new interest in structural health monitoring. In this manuscript, we propose an original method based on this approach to detect and locate defects (cracks, holes, grooves) in a reverberant thin plate with a limited number of sensors. Flexural waves that propagate on the plate are generated by either a set of sources distributed randomly on the surface or an ambient noise. Covariance matrices are estimated from the sparse array after damage and compared to baseline-correlation matrix recorded from the healthy plate. An evaluation criterion has developed in the form of relative noise level to predict the quality of the GF reconstruction. The differential correlation matrix w/o defect is used to localize the defect. We have shown numerically and experimentally that this technique is exploitable for defect detection and localization, despite a non-perfect estimation of the GF. We have also proposed a passive technique to identify the regions of noise. A filtering technique based on the singular value decomposition is shown to improve the detection. A secondary acoustic sources have been developped to harvesting the LF ambient noise to HF field, used to localize defects in platelike structures. Finally, it was shown that such method could also be used to characterize a defect in a reverberant structure, in particular, it has been drawn that the obtained images intensity is related to the defect cross-section

碩士
國立臺北科技大學
機電整合研究所
98
Wedge wave is a kind of elastic wave propagat along the tip of wedges. Its energy is concentrated on the range of about one wavelength of tip. A combined experimental / numerical investigation employing a laser ultrasound technique and finite element analysis is used to characterize the propagation behaviors of ASF modes propagating along wedge tips with defects. More specifically, reflection coefficient and transmission coefficients are modeled and measured for several of defect depths on the wedge tip. Good agreement is obtained between the modeled and measured reflection and transmission coefficients. And this study discovers wedge wave has mode conversion phenomenon similar to Lamb wave.

碩士
國立臺北科技大學
製造科技研究所
106
Wedge Wave (WW) is a kind of elastic wave propagate along the tip of a wedge that energy is concentrated near wedge tip to in 1 wavelength. According to the study, when the wedge tip has no truncation, there is no dispersion relationship that changes its corresponding velocity with frequency. In the literatures have been many related studies on wedge-shaped guided waves, However, the study of combining wedges with phononic crystals has not yet been studied. Therefore, this article will explore it. Phononic Crystal, a study extended by Photonic Crystal, is characterized in that when sound waves pass through a periodic arrangement structure, the acoustic wave will cause interference of the penetrating wave and the reflected wave due to the structure. We call this Band Gap, which blocks the passage of acoustic wave with certain frequency oscillations. This study is divided into two parts, experimental measurement and finite element analysis method are used to investigate the wave propagation behavior of regular periodic defects at wedge tip, and to explore the influence of different periodic defect parameter changes on the guided wave of the wedge body. In this study, a brass wedge with a apex angle of 60° was used as the substrate test piece, the periodic defects were designed at the wedge tips, the ratio of the defects width and spacing width was 1:1,.Explore its influence after obtaining the frequency-domain by laser ultrasonic technique and finite element analysis. Laser ultrasonic technique and finite element analysis were used to investigate the influence of periodic defects on wedges, although the results couldn’t be observed band gap. However, in the experimental results, it is found that the frequency domain signals would be effected by periodic defects on wedge tips, basic frequency would be downgraded by increase in defect width or increase in number of defects.

abstract: Aging-related damage and failure in structures, such as fatigue cracking, corrosion, and delamination, are critical for structural integrity. Most engineering structures have embedded defects such as voids, cracks, inclusions from manufacturing. The properties and locations of embedded defects are generally unknown and hard to detect in complex engineering structures. Therefore, early detection of damage is beneficial for prognosis and risk management of aging infrastructure system. Non-destructive testing (NDT) and structural health monitoring (SHM) are widely used for this purpose. Different types of NDT techniques have been proposed for the damage detection, such as optical image, ultrasound wave, thermography, eddy current, and microwave. The focus in this study is on the wave-based detection method, which is grouped into two major categories: feature-based damage detection and model-assisted damage detection. Both damage detection approaches have their own pros and cons. Feature-based damage detection is usually very fast and doesn’t involve in the solution of the physical model. The key idea is the dimension reduction of signals to achieve efficient damage detection. The disadvantage is that the loss of information due to the feature extraction can induce significant uncertainties and reduces the resolution. The resolution of the feature-based approach highly depends on the sensing path density. Model-assisted damage detection is on the opposite side. Model-assisted damage detection has the ability for high resolution imaging with limited number of sensing paths since the entire signal histories are used for damage identification. Model-based methods are time-consuming due to the requirement for the inverse wave propagation solution, which is especially true for the large 3D structures. The motivation of the proposed method is to develop efficient and accurate model-based damage imaging technique with limited data. The special focus is on the efficiency of the damage imaging algorithm as it is the major bottleneck of the model-assisted approach. The computational efficiency is achieved by two complimentary components. First, a fast forward wave propagation solver is developed, which is verified with the classical Finite Element(FEM) solution and the speed is 10-20 times faster. Next, efficient inverse wave propagation algorithms is proposed. Classical gradient-based optimization algorithms usually require finite difference method for gradient calculation, which is prohibitively expensive for large degree of freedoms. An adjoint method-based optimization algorithms is proposed, which avoids the repetitive finite difference calculations for every imaging variables. Thus, superior computational efficiency can be achieved by combining these two methods together for the damage imaging. A coupled Piezoelectric (PZT) damage imaging model is proposed to include the interaction between PZT and host structure. Following the formulation of the framework, experimental validation is performed on isotropic and anisotropic material with defects such as cracks, delamination, and voids. The results show that the proposed method can detect and reconstruct multiple damage simultaneously and efficiently, which is promising to be applied to complex large-scale engineering structures.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2019

碩士
龍華科技大學
機械系碩士班
94
This study is mainly utilizing Back-Propagation Neural Network technology to identify the shape of defective tin ball and to promote the accuracy of the inspection. By developing two dimensional BGA optical inspecting system, incorporate with Halcon which developed by Visual Basic as developing tool of function database for image process. Inspecting system is capable of detecting the shape of tin ball and its location precisely and classifying its quality under the condition of BGA board offsetting and rotating at any angle. Inspecting items include, ball offset, ball presence, over size, under size, ball deformation, ball bridging and ball absence. 　　For the study and processing procedure developments of the automatic inspecting system, image is processed by preprocess and sub-pixel basing on its grey value to identify the edge accuracy in order to acquire the coordinate of the tin ball precisely, the nearly ellipse axis scale, compactness, radius of tin ball, BGA board bias angle relevant information. Based on the predefined criteria, defines the dimension and location defects of the tin ball and utilizes Back-Propagation Neural Network technology identifying and classifying the shape defects of the tin ball. Experiments within this study prove that Back-Propagation Neural Network technology can correctly identify and classify the shape defects of the tin ball which also can achieve and contribute the requirements for the automatic inspection and high efficiency of identification capabilities.

Granular crystals are compact periodic assemblies of elastic particles in Hertzian contact whose dynamic response can be tuned from strongly nonlinear to linear by the addition of a static precompression force. This unique feature allows for a wide range of studies that include the investigation of new fundamental nonlinear phenomena in discrete systems such as solitary waves, shock waves, discrete breathers and other defect modes. In the absence of precompression, a particularly interesting property of these systems is their ability to support the formation and propagation of spatially localized soliton-like waves with highly tunable properties. The wealth of parameters one can modify (particle size, geometry and material properties, periodicity of the crystal, presence of a static force, type of excitation, etc.) makes them ideal candidates for the design of new materials for practical applications. This thesis describes several ways to optimally control and tailor the propagation of stress waves in granular crystals through the use of heterogeneities (interstitial defect particles and material heterogeneities) in otherwise perfectly ordered systems. We focus on uncompressed two-dimensional granular crystals with interstitial spherical intruders and composite hexagonal packings and study their dynamic response using a combination of experimental, numerical and analytical techniques. We first investigate the interaction of defect particles with a solitary wave and utilize this fundamental knowledge in the optimal design of novel composite wave guides, shock or vibration absorbers obtained using gradient-based optimization methods.

Ultrasonic guided wave-based detection of structural defects or damages is one of the promising methods for structural health monitoring (SHM). A significant proportion of structural elements in an aircraft structure and other civil and marine structures are thin structures and those components can be monitored using ultrasonic guided wave. Piezoelectric transducers have been extensively applied to ultrasonic wave based non-destructive inspection (NDI). SHM is considered an extension of NDI adding a new dimension to the way future structures will be designed. In recent years, research on the behavior of guided wave in structures, especially targeted for aerospace application, received enormous significance. Guided wave interaction with structural features and defects/damages creates complicated wave field patterns. Experimental observation is limited to the availability of sophisticated instrumentation and tomography techniques to visualize the wave pattern even on the surface of a structure and simulation is the only way various details regarding internal patterns of the waves in a complex geometry can be analyzed. Analytical approach to deal with the wave propagation in a structure is limited to simple geometries and simple forms of damage. Guided wave in a simple geometry like beam/plate is modeled using analytical as well as semi-analytical methods. But when it comes to complicated problems like structures with actual damage/fracture or geometrical complexity which includes curvature and components with joints etc., one requires efficient computational schemes to simulate the behavior. Contents x There are various numerical tools developed in last few decades, such as finite difference, finite element, boundary element methods etc. In high frequency guided wave propagation problems, higher-order modes participate. Moreover, to detect a small damage, a high-frequency wave is needed. To deal with such kind of problems incorporating standard polynomial incorporation based finite element schemes need very fine mesh, which makes the computations of such problem enormously expensive, sometimes prohibitive. Time domain spectral element (TSFE) method is an efficient numerical method that can capture higher order field and therefore can deal with wave propagation problems efficiently and with better accuracy. TSFE uses higher order highly convergent interpolation functions based on the Chebyshev or Lobatto nodal distributions. TSFE based on the Lobatto nodal distribution and Legendre-Lobatto quadrature rule makes the mass matrix diagonal, which reduces the computer memory requirement to a great extent. As the number of nodes increases, accuracy increases exponentially and hence the term spectral finite element. The present thesis incorporates this idea and formulates a TSFE scheme to simulate guided wave propagation in laminated composite materials with damages such as material uncertainty/degradation, micro-cracks, and delaminations. Various benchmark problems are solved to validate the simulated results and establish superior convergence properties. Because of anisotropy of composite laminate and direction dependent properties of its constituent, composite laminate has various damage modes including matrix crack, fiber break, delamination etc. Among those damage modes, in this thesis, a special focus is given on delamination detection problem as it grows in the interfacial plane under repeated loading and reduces load-carrying capability to a great extent. Those damage modes are internal hence invisible. In wave propagation based detection methods, delamination can be identified and localized from the wave scattering from it. But it is of great interest to quantify the damage in terms of various parameters such as delamination length as well as the thickness and position of the laminate. Delamination scatters an incident wave and the strength of the reflection depends on the frequency/wavelength, length and thickness position of the delamination for a given structure. Simulation results show that the near-field effect of the damaged region provides crucial information about the scattering and reflected wave characteristics. Delamination in a laminate divides the damaged region into sub-laminates which are thinner compared to the base laminate. Each sub-laminate behaves like a separate waveguide. The vibration of the sub-laminates during the propagation of the wave through the damaged region Contents xi is of great interest. The energy of scattered waves and dissipation/conversion of energy due to the damage depends on the resonance characteristics of the sub-laminates. Therefore, the resonance phenomenon is correlated to the damage quantification problem with the help of simulation. Detection of damage near the structural boundary is one of the most challenging tasks as the scattering from the damage is overlapped by the strong reflection from the boundary. Effect of incident wave frequency/wavelength on the delamination near a structural boundary is studied. Damaged region behaves like the material degradation and in some frequency range the energy is trapped inside the damaged region and slow dissipation/conversion of the trapped energy into other forms of vibration creates a significant difference in the reflected wave and the simulated results help to identify the presence of damage. Impact-induced damage in composite has a great influence on the integrity and life of a composite structure. In most cases, initially, it develops material degradation in terms of matrix cracks at micro-scale. Although in composite structural design, micro-scale matrix cracks are not considered, however, as these micro-cracks coalesce, it gives initiate delamination. Under a severe dynamic impact loading, such small size delamination can grow and can lead to catastrophic failure of the structure. Ultrasonic wave propagation in composite with matrix crack is one of the major subjects of study which can predict the delamination onset in the composite. In the present thesis, wave scattering due to matrix cracks is studied and behavior of wave reflection from matrix cracking zone is investigated for various damage severity, which is expressed in terms of matrix crack density. Moreover, the matrix cracks along with delamination initiated from the zone, which is a kind of mixed-mode damage zone, appears more commonly in a composite structure than an ideal single-mode damage like a sharp crack or delamination. A mixed-mode damage complicates the modeling problem to be dealt with considering complex nature of near-field scattering of the incident wave. In the present thesis, this aspect is studied in details and damage severity effects are correlated to the scattered wave packet properties. Guided wave has a special characteristic that it is guided by the material media geometrically even when the structure is curved. This advantage can be exploited in developing damage detection scheme, for example, by bringing the scattered wave field located behind the curvature in a structural component without direct access to the surface where inspection cannot be carried out using local methods. Another important aspect of Contents xii guided wave is that propagation through a curved region not only produces reflection, it also generates mode converted waves which appear in both the reflected and transmitted waves. In the present thesis, wave transmissibility and signal loss at various frequencies and the effect of the radius of curvature is studied in detail. The simulation results provide a new insight regarding the wave mode and frequency for inspection for a given curved structure. Delamination near the curved region and T-joint is modeled and simulation shows correlation where the wave scattering due to delamination is possible to discriminate from that due to curved junctions. In composite, material uncertainty is inherent because of limited control over the fabrication processes, which in turn affects the proportion of the constituent materials or the fiber orientation. Significant material property variation can take place due to the variability in fiber volume fraction or the distortion in the stacking angle. The location of a damage and various other parameters are directly influenced by the material property variations. Therefore, the deterministic study is not sufficient to deal with these problems. In the present thesis, a Monte-Carlo method based simulation of wave scattering is carried out. The study primarily focuses on the problem of quantification of uncertainty in various damage detection parameters such as wave scattering coefficient and variation in the time of flight of the scattered packet, wave velocity etc. Detailed analysis is carried out regarding how the simulation based inspection method can be developed that gives better insight on the probabilistic distribution of the detection parameters of interests.

碩士
國立臺北科技大學
製造科技研究所
101
Wedge waves are guided acoustic waves along the wedge tip. Wedge wave is not only highly concentrated, but also difficult to decay characteristics. These reasons let it used in non-destructive evaluation. Laser beam excites ultrasound has advantages those are long-range detection and high-resolution. With the time-domain signal integral can calculate energy of wave. This study developed a method to analyze the defect depth and shape affect the transmission and reflection coefficient. This study use Laser ultrasonic technique (LUT) and finite element method (FEM) to characterize the anti-symmetrical flexural (ASF) modes propagation along wedge tips with a natural crack. The mode conversion (MC) will emerge when ASF wave encounter a defect on the wedge tip. Reflection coefficient (RC) and transmission coefficient (TC) are obtained from energy ratio that calculated from simulation and measurement results with different defect depths and different crack’s shape on the wedge tip. The results showed that when the defect depth increases, reflected wave energy will rise and transmitted wave energy decreased appears the phenomenon of high and low level mode conversion. Finite element analysis can show the behavior of wedge propagation, and thus to estimate the transmission coefficient and reflection coefficient.

碩士
國立清華大學
工業工程與工程管理學系
91
For semiconductor industry, the complicated chips manufacturing process, expensive raw materials and demanding production environment make the cost relatively high. Every fabrication company eagerly searches through different manufacturing control and analysis methods to reach the goals of production cost reduction and yield improvement. In a fabrication line, the most direct and efficient way to identify an abnormal process during manufacture is by analyzing the defect maps of semiconductor wafers. Hence it is really crucial to construct a defect inspection system for semiconductor chips, which can provide better judgment and response to the detection of faults. The outcome would be an improved yield rate, the timely identification and elimination of malfunctions and eventual cost reduction. Our current research proposed a defect detection system for semiconductor wafers based on Back-Propagation Network .The major purpose of the current study is to evaluate the characteristics of defects on observed defect maps or bin maps and, thus, obtain the most related defect characteristics through the network training process. Our research will show the effect of different data conversion on the defect detection rate and provide a better output model among the various combinations of parameters. Moreover, our detection system can differentiate between the distinguishable and indistinguishable situations. Based on the verification of real-time manufacturing data provided by online engineers, it is proven that our system can correctly diagnose and analyze the defects such that it fulfills the requirements of semiconductor industry. This system could help engineers find out the problems on the fabrication line in a more timely and efficient manner so that engineers can modify the process to prevent similar problems from recurring, which would consequently stabilize the production process and even improve the yield rate.

碩士
遠東科技大學
機械工程研究所
100
This thesis uses the back-propagation neural network to recognize two types defects of LED die automatically. First of all, a LED die recognition process will be developed. The Otsu's method and template-matching techniques are employed in this recognition process to find a single LED die from the LED wafer image. Through the multi-sample algorithm and the simple logic operators, the characteristics of LED die defects will be obtained quickly. These characteristics of LED die defects will be the training samples of the neural network. After the training process, the LED die defects detection system will be examined by a lot of the single LED die images. The defects detection system will identify whether the defect of the sample images. The results show that the proposed feature extraction method can increase the detection accuracy effectively. The detection accuracy of these two types of LED die are 99.23% and 84.81%. The time of detection for every single die is about 70 milliseconds on average.

碩士
中原大學
機械工程研究所
103
In order to meet the industry needs for high-speed high-precision automated detection and remote monitoring, a defective part that has undergone line inspection is subjected to a fast classification scheme consisting of an algorithm which can feed the results directly to a cloud storage server. The purpose of this study is to develop a combination of cloud storage and defect classification system where the detection results are stored to the cloud server and then accessed via web presentation to a remote user. The classification of defect profile is implemented by means of comparing a defect’s luminance characteristics to a target pixel value using the Hu set of invariant moments. The characteristic values normalization and the Back-Propagation Neural Network (BPN) model are implemented to train the algorithm to recognize defect information their characteristics. The classification results are saved to an established Mysql database and use PHP to communicate with the database side of the page. The user can view and access the test results via a website. Defect maps from the optical inspection of touch panel glass samples were used to evaluate experimentally the classification algorithm. Each map records a 350 * 350 pixel region of the glass sample, with each pixel equal to 3.5 m. Different iterations of the BPN were implemented to seek optimized classification by comparing the mean square error (MSE) between target output and defect profile of the test sample. Defect classification of microscopic scratches, bubbles, and dusts on 20 different defect maps yielded accuracy rates above 90%. Classification of each image was completed in 0.1 ms.

碩士
龍華科技大學
工程技術研究所
99
This research is conducted mainly by using the Auto Optical Inspection (AOI) in the fifth generation TFT-LCD factory. This machine is specifically fit the substrates with the size 1100*1300 mm. It performs image-scanned inspection in the active area and then obtains the photos by the 10X electronic microscope. Then, the classification of defects is made by using the back-propagation neural network. In the development of detect-classification system, we designed the back-propagation neural network which combined with Visual Basic as the interface and MATLAB as an image-processing tool. The system is able to determine and display the detected results. The defect classification mainly designed to detect and classify the following defects: the second layer of the photo resist residue (AS-Residue), the second layer of large-area photo resist residue (AS-BPADJ), and the third layer of photo resist residue (M2-residue) in the Array Photolithography Process. In the processing procedure, we first took four 640*480 pictures with three different types of defects and a normal one by using AOI Microscope camera. Secondly, we utilized the MATLAB for image processing and did Blob analysis and binary operation. After the training in the back-propagation neural network, we input all the defected images and data, such as weights and bias values of input layer, output layer, and hidden layer, into Visual Basic software, and the software will display the result of the judgments, which including the four classifications of defects in this experiment. Finally, the result shows the fact that without complicated processing procedure, the four defects in the TFT-LCD Array Photo Process can be precisely and quickly classified only by imaging processing and back-propagation neural network training. As result, it is feasible to reduce the costs and the risk of human judgments.

For successful identification of structural defects in plates and pipes, it is essential to understand structural wave propagation characteristics such as dispersion relations. Analytical approaches to identify the dispersion relations of homogeneous, simple plates and circular pipes have been investigated by many researchers. However, for plates or pipes with irregular cross-sectional configurations or multi-layered composite structures, it is almost impossible to obtain the analytical dispersion relations and associated mode shapes. In addition, full numerical modeling approaches such as finite element (FE) methods are not economically feasible for high (e.g., ultrasonic) frequency analyses where an extremely large number of discretized meshes are required, resulting in significantly expensive computation. In order to address these limitations, Hybrid Analytical/Finite Element Methods (HAFEMs) are developed to model composite plates and pipes in a computationally-efficient manner. When a pipe system is used to transport a fluid, the dispersion curves obtained from a “hollow” pipe model can mislead non-destructive evaluation (NDE) results of the pipe system. In this study, the HAFEM procedure with solid elements is extended by developing fluid elements and solid-fluid boundary conditions, resulting in the dispersion curves of fluid-filled pipes. In addition, a HAFEM-based acoustic transfer function approach is suggested to consider a long pipe system assembled with multiple pipe sections with different cross-sections. For the validation of the proposed methods, experimental and full FE modeling results are compared to the results obtained from the HAFEM models. In order to detect structural defect locations in shell structures from defect-induced, subtle wave reflection signals and eliminate direct-excitation-induced and boundary-reflected, relatively-strong wave signals, a time-frequency MUSIC algorithm is applied to ultrasonic wave data measured by using an array of piezoelectric transducers. A normalized, structurally-damped, cylindrical 2-D steering vector is proposed to increase the spatial resolution of time-frequency MUSIC power results. A cross-shaped array is selected over a circular or linear array to further improve the spatial resolution and to avoid the mirrored virtual image effects of a linear array. Here, it is experimentally demonstrated that the proposed time-frequency MUSIC beamforming procedure can be used to identify structural defect locations on an aluminum plate by distinguishing the defect-induced waves from both the excitation-generated and boundary-reflected waves.

碩士
國立雲林科技大學
環境與安全衛生工程系碩士班
99
In the industry, non-destructive testing was commonly used for detecting quality of steel materials or inner defects, defects might be located at the surface or inside the material, surface defects could easily be found to immediately eliminate or replace, But the material contained an internal defect, if not find it in time, when the stress concentrate on these defects, the defects grew up easily, and then damaged, even ruptured, affecting the facilities’ structure safety. Ultrasonic techniques are suitable for examining metals or nonmetallic materials, using the difference of acoustic properties between material and defects, that affecting the propagation of ultrasonic, it had a good ability for detecting inner or near-surface defects. However, on-site detection process, the measured error of low-frequency probe for near-surface defect’s detection was large, because the initial echo too large to hide defect echo. Then we could adopt simulation method to simulate wave propagation, to reduce uncertainty waste, and reduce experimental error because of human factor. In this study, we design and process nine defects which under surface 2 mm ~ 10 mm respectively. Each defect was carried out ultrasonic detection (Frequencies were 2, 4, 5, 10 MHz respectively) and ANSYS finite element analysis, then compared their results. The simulate results errors by four frequency probes were less than experimental errors. Hence, we could confirm that using ANSYS finite element analysis to simulate near-surface defects could improve the accuracy of detection.

碩士
國立臺灣科技大學
自動化及控制研究所
98
This thesis presents a recognition system that contains optical character recognition (OCR) and automatic optical inspection (AOI) proceedings for laser marking of IC system. The recognition system can be divided into three flows: IC location alignment, characters extraction and recognition, and defects inspection. For the OCR system, eighty image features are designed and applied to the Back-Propagation Neural Network (BPNN). The experimental rusts show that the OCR system can achieve 100 of correct recognition rate. For the AOI system, we use four image process methods to detect six kinds of defects for the IC laser marking. From the experimental result, the correct detection rate is 96.3% achieved. In addition, the time required for the OCR and AOI proceeding is averaged about 0.189 seconds.

碩士
國立臺北科技大學
材料科學與工程研究所
98
This study is focused on the propagation conducts of elastic wedge waves (WW) propagating along the tips. Employing a laser ultrasound technique (LUT) and finite element method (FEM) is used to characterize the more specifically, reflection coefficient and transmission coefficients is obtained between the modeled and measured coefficients are modeled and measured for several of defect depths on the wedge tip. In this study, mode conversion phenomenon commonly known in plates are also observed in ASF modes. When ASF modes are partially reflected and partially transmitted, with the RC and TC highly depending on the defect depth.