Sound can propagate for long distances with a low loss of intensity in objects whose geometry acts as a guide for the sound waves; a phenomenon that can be utilised for long range testing of structures. The guided sound waves can be used to conduct materials evaluation or to detect flaws, which can be done for a relatively large region of coverage from a relatively small region of access. In particular this technology can be used to inspect or monitor large engineering structures whose structural integrity is critical for safety and the environment, such as wind turbine towers, ship hulls, and pipelines. The use of guided waves for structural inspection is complicated by the existence of many wave modes. In this thesis, the Macro Fiber Composite (MFC) is characterised for its frequency, wavelength, wave mode and direction dependent sensitivity. These devices are flexible, light and thin, and, here have been shown to have wave mode sensitivity characteristics that are favourable for some applications. The MFC is a piezoelectric actuator that can be used to excite and sense in-plane vibrations at a structures surface. The surface area of an MFC is significantly large with respect to typical wavelengths used in ultrasonic guided wave applications, which combined with their in-plane extensional nature gives rise to a significantly wave mode, frequency and direction dependent sensitivity. This can limit their application, but can also potentially be exploited for greater wave mode control. A method for simulating the output from hypothesised transducer behaviour is shown and validated for the MFC. This allows their behaviour to be predicted for new structures. It is shown that their frequency response can depend on the waveguide and can vary with direction, which can lead to wave mode transmission and reception characteristics that may be advantageous for some methods of application and detrimental to others. A novel method of adapting a flexible transducer, such as the MFC, has been developed and its characterisation is given. It is shown that through the use of a decoupling membrane, an MFC can be caused to have very different wave mode sensitivity characteristics whilst retaining their light and flexible nature. These altered characteristics are favourable for applications where shear horizontal wave modes are required. Both fully coupled MFC transducers and the adapted MFC transducers are considered for application to pipeline testing. Fully coupled MFC transducers are used for inspection using longitudinal waves, whilst the adapted MFC transducers are used with torsional waves. These arrays are compared to a current commercial tool.

Le but de la thèse est de modéliser la génération d’ondes élastiques ultrasonores émises par EMATs dans une pièce ferromagnétique, modélisation appliquée au domaine du contrôle non destructif (CND). Les traducteurs EMATs combinent deux physiques différentes : électromagnétisme et élastodynamique. L’enjeu est d’intégrer dans la plateforme de simulation CIVA, dédiée notamment aux CND par courant de Foucault et par ultrasons, les éléments de modélisation inhérents à la problématique multi-physique posée. Ces éléments sont multiples et concernent premièrement la modélisation des forces électromagnétiques créées par un EMAT : la force de Lorentz, existant dans tous milieux conducteurs, et les forces d’aimantation et de magnétostriction, spécifiques aux milieux ferromagnétiques. Deuxièmement, la modélisation proposée prend aussi en compte le phénomène de création d’harmoniques, phénomène dû aux différentes forces et traduisant le fait que les fréquences de l’onde ultrasonore émise peuvent être des harmoniques des fréquences du signal d’excitation de l’EMAT. Le modèle de forces et de création d’harmoniques permet de modéliser des situations de contrôle non prises en compte par les modèles de la littérature, notamment lorsque les champs statiques créés par l’EMAT sont faibles ou lorsque le courant d’excitation possède une forte intensité. Enfin, les forces électromagnétiques sont transformées en contraintes surfaciques équivalentes pour correspondre au formalisme des données d’entrée des modèles de rayonnement des ondes élastiques implémentés dans CIVA. L’outil informatique développé permet donc de traiter toute configuration et condition d’utilisation des EMATs, constituant un outil d’optimisation de leur conception. Les caractéristiques électromagnétiques de différents matériaux, données par l’expérience, sont utilisées pour mener diverses études paramétriques. / The present study aims at modelling the ultrasonic wave generation by EMATs into a ferromagnetic part in the context of non-destructive testing (NDT). Wave radiation by EMATs combines two different kinds of physical phenomena: electromagnetism and elastodynamics. The issue is to implement into the NDT simulation platform CIVA, dedicated in particular to eddy current and ultrasonic NDT, the models developed in the present work. These developments are of various nature and firstly concern the modelling of the electromagnetic forces created by an EMAT: the Lorentz force, existing in any conductive media, and the magnetization and the magnetostriction forces, which add up when the medium is further ferromagnetic. Secondly, the proposed models take into account the generation of harmonics due to the three forces. Ultrasonic frequencies of the generated ultrasonic waves can be harmonics of the frequencies of the excitation electrical signal. The model of force and harmonic generation allows us to deal with configurations not treated in the literature, namely, when the static field created by the EMAT is low or when the excitation current intensity is high. Finally, the three electromagnetic forces are transformed into equivalent surface stresses readily usable as entries of an existing model of ultrasonic wave radiation already implemented in CIVA. Thus, the combination of all these models and their translation as a numerical tool running within the CIVA platform can be used for optimizing the design of EMAT. Experimentally measured electromagnetic characteristics of different materials are used to perform various parametric studies.

Orientador: Auteliano Antunes dos Santos Junior / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-28T12:03:37Z (GMT). No. of bitstreams: 1 PereiraJunior_Paulo_D.pdf: 3933527 bytes, checksum: ce6440b45089eda939114fc7869bdc4d (MD5) Previous issue date: 2015 / Resumo: As técnicas não destrutivas que utilizam ondas ultrassônicas para avaliação de tensões mecânicas se destacam pela sua simplicidade e baixo custo. A medição de tensão por ultrassom utiliza o princípio da acustoelasticidade que relaciona a variação da velocidade de ondas elásticas com a variação da tensão no material. Entretanto, não apenas a tensão influencia a velocidade de onda, mas também a microestrutura do material inspecionado. A influência da microestrutura pode levar a erros de medição de tensão. O objetivo deste trabalho é verificar a viabilidade da utilização de imagens ultrassônicas para avaliar o efeito de variações microestruturais na velocidade da onda ultrassônica longitudinal criticamente refratada (Lcr) utilizada na medição de tensões. As imagens são geradas a partir de sinais obtidos por uma sonda de transdutores estratificados (array) ultrassônicos e aplicando a técnica Total Focusing Method (TFM). Os sinais obtidos com o array contêm reflexões causadas pela microestrutura granular do material que resultam em um ruído no sinal conhecido como ruído de grão. Por esta razão, os sinais e consequentemente a imagem formada contêm informação acerca da microestrutura do material. Neste trabalho foram verificadas as relações entre tamanho e forma de grão, velocidade de onda e intensidade de ruído em imagens de TFM para amostras de aço ASTM A36. A influência da tensão e temperatura nas imagens ultrassônicas também foi analisada. Os resultados mostraram que assim como a velocidade de onda, a intensidade do ruído de grão nas imagens de TFM também sofre influência do tamanho e forma do grão do material. Entretanto, não foram verificadas variações significativas no ruído de grão em função da tensão aplicada e temperatura. Sendo a intensidade do ruído de grão nas imagens sensível somente às variações microestruturais do material, a técnica pode ser empregada para corrigir valores de velocidade de onda para medições de tensão. No trabalho também é proposto um modelo matemático para obtenção de imagens de TFM para uma dada distribuição de grãos. Os resultados obtidos com o modelo mostraram boa concordância qualitativa com os dados experimentais para a frequência de 2,5 MHz / Abstract: Non-destructive techniques using ultrasonic waves to evaluate stresses excel for exhibiting simplicity and low cost. Stress evaluation by ultrasound applies the acoustoelastic theory that relates the variation in velocity of elastic waves with the variation in stress in the material. However, not only stress has influence in wave velocity, but also the microstructure of the specimen being evaluated. The microstructure influence can lead to errors in the stress measurement. The aim of this work is to verify the feasibility of using ultrasonic images to evaluate the effect of microstructural variations in the velocity of longitudinal critically refracted waves (Lcr) used in stress measurements. The images are generated from signals obtained with a probe of ultrasonic array applying the Total Focusing Method (TFM). The signals recorded using the array contain reflections caused by the granular structure of the material that creates a noise in the signal known as grain noise. For this reason, the signals and consequently the images formed carry information about the material¿s microstructure. In this work, it was verified the relations between size and shape of grains, wave velocity and noise intensity in TFM images for samples of ASTM A36 steel. The influence of stress and temperature in the ultrasonic images was also verified. The results showed that as for the wave velocity, the grain noise intensity in TFM images are also influenced by size and shape of grains that constitute the material under inspection. Nevertheless, no significant variations were seen in the noise intensity due to the applied stress or temperature changes. Being the noise intensity of the images sensitive only to microstructural variations, the technique presented can be used to correct values of ultrasonic wave speed in the stress evaluations. In this work, it is also proposed a mathematical model to obtain TFM images for a given distribution of grains. The results showed qualitative good agreement between simulated and experimental data for the frequency of 2,5 MHz / Doutorado / Mecanica dos Sólidos e Projeto Mecanico / Doutor em Engenharia Mecânica

The aim of this work is to optimize the acoustic nondestructive method (NDT) Impact-Echo (IE) as a complementary diagnostic tool for the assessment of concrete structures exposed to high temperatures. Commonly used diagnostic procedures for concrete structures affected by fire are most often limited to in-situ core drilling and subsequent testing of the drilled holes by destructive testing. The Impact-Echo method allows non-destructive testing of variously degraded test specimens and concrete structures, but its use for the assessment of temperature-degraded cementitious composites is hardly ever used in practice. Many studies suggest that Impact-Echo is one of the most suitable non-destructive methods for assessing the rate of degradation by elevated temperature. The dissertation work consists of three main stages, where the first stage is devoted to the production of test specimens of different mixtures and their subsequent degradation by elevated temperature and testing by the IE method and by reference destructive and non-destructive methods. The second stage consists of manufacturing a reinforced concrete element with built-in thermocouples and its subsequent inhomogeneous degradation by a local source of elevated temperature. The controlled degraded element thus prepared contains all grades of thermally degraded concrete tested in the first stage. Subsequent testing consists of verification of standard procedures in the field of acoustic NDT with simultaneous use of experimental modal analysis. In the third stage, the objective is to interpret the measured data from both previous parts, to verify whether within the closed element temperature-degraded areas can be distinguished from one another, or the degree of thermal degradation of concrete. Based on these results, the optimal testing procedure was determined and the basic assumptions for the creation of degrees of damage of the concrete by increased temperature were determined.

Mon travail de thèse porte sur la modélisation électromagnétique et l'imagerie de structures périodiques désorganisées. Un certain motif dans une subdivision élémentaire (une "cellule") est répété dans les autres cellules de la structure dans certaines directions de l'espace. Cette répétition est désorganisée par un changement des propriétés des matériaux et/ou géométries des parties constitutives, dans une ou plusieurs cellules. Au premier niveau de modélisation, ces panneaux sont une succession de plaques planes l'une sur l'autre. Chacun se compose d'un agencement linéaire régulier de longs cylindres avec mêmes sections circulaires finies, tous orientés dans la même direction: nous les appelons "fibres", chaque cylindre résultant de l'hypothèse d'un faisceau de fibres de petite taille. Le matériau constitutif des fibres est différent du matériau d'enrobage (matrice) et le renforce. Chaque plaque est constituée de fibres avec différents axes pour assurer la robustesse. Il y a peu ou beaucoup de plaques, avec la répétition d'une petite pile de plaques. Pour les panneaux conducteurs (à base de carbone), l'imagerie est MHz ; pour des panneaux sans pertes ou à faibles pertes (à base de verre), l'imagerie est micro-onde (quelques dizaines de GHz, voire plus, THz). Il pourrait y avoir des cylindres manquants ou déplacés à l'intérieur d'une plaque, avec des changements conséquents dans éventuellement plusieurs cellules, adjacentes ou non. Des dommages locaux peuvent également se produire, entraînant des changements dans la forme ou les propriétés électromagnétiques d'un ou plusieurs cylindres dans une ou plusieurs cellules dans une ou plusieurs plaques. Un caractère aléatoire de la distribution des inclusions pourrait tenir compte des incertitudes de positionnement par rapport aux géométries supposées. Illuminer correctement les structures et la collecte des champs résultant (dans le champ proche espérons-le, peut-être dans le champ lointain) devraient permettre leur imagerie et concourir à leur diagnostic. Ainsi, si une structure périodique sous interrogation est désorganisée, on souhaite imager cette structure tout en prenant soin au mieux de l'information préalable sur la périodicité et la désorganisation, sur les systèmes de détection, et, évidemment, à propos des besoins et des limites de l'essai. / My PhD work is about electromagnetic modeling and imaging of disorganized periodic structures. A certain pattern in an elementary subdivision (a “cell”) is repeated in the other cells of the structure into certain directions of space. This repetition is disorganized by a change of material properties and/or geometries of the constitutive parts, within one or more cells. At first level of modeling, these panels are a succession of planar plates one over the other. Each consists of a regular linear arrangement of long cylinders with same finite circular sections, all orientated into the same direction: we call them “fibers”, each cylinder resulting from the assumption of a bundle of small-size fibers. The constitutive material of the fibers differs from the embedding material (matrix) that they reinforce. Each plate is made of fibers with different axes for sturdiness. There are few or many plates, with repetition of a small stack of plates. For conductive panels (carbon-based), imaging is MHz; for lossless or weakly lossy panels (glass based), imaging is microwave (a few tens GHz, possibly more). There might be missing/displaced cylinders inside a plate, with consequent changes in possibly several cells, adjacent or not. Local damages might occur also, leading to changes in shape or electromagnetic properties of one or more cylinders in one or more cells in one or more plates. Randomness in distribution of the inclusions might account for uncertainties of positioning with respect to assumed geometries. Properly illuminating the structures and collecting the resulting fields (in the near-field hopefully, possibly in the far-field) should allow their imaging and concur to their diagnostics. So, a periodic structure under interrogation is disorganized. One wishes to successfully image the structure while taking care at best of prior information on periodicity and disorganization, on sensing systems, and obviously of needs and limitations of the testing. The PhD benefits from a grant from the Chinese Scholarship Council.

This research aims to investigate the application of infrared thermography (IRT) as a method of nondestructive evaluation (NDE) for the detection of defects in the rail base area. Rails have to withstand harsh conditions during their application. Therefore, defects can develop in the base area of rails due to stresses such as bending, shear, contact, and thermal stresses, fatigue, and corrosion. Such defects can cause catastrophic failures in the rails, ultimately leading to train derailments. Rail base defects due to fatigue and corrosion are difficult to detect and currently there are no reliable or practical non-destructive evaluation (NDE) methods for finding these types of defects in the revenue service. Transportation Technology Center, Inc. (TTCI) had previously conducted a research on the capability of flash IRT to detect defects in rail base area based on simulation approach. The research covered in this thesis is the continuation of the same project.In this research, three rail samples were prepared with each containing a notched-edge, side-drilled holes (SDHs), and bottom-drilled holes (BDHs). Two steel sample blocks containing BDHs and SDHs of different sizes and depths were also prepared. Preliminary IRT trials were conducted on the steel samples to obtain an optimal IRT setup configuration. The initial inspections for one of the steel samples were outsourced to Thermal Wave Imaging (TWI) where they employed Thermographic Signal Reconstruction (TSR) technique to enhance the resulting images. Additional inspections of the steel samples were performed in the Southern Illinois University-Carbondale (SIUC) facility. In case of the rail samples, the SDHs and the notched-edge reflectors could not be detected in any of the experimental trials performed in this research. In addition, two more rail samples containing BDHs were prepared to investigate the detection capabilities for three different surface conditions: painted, unpainted, and rusted. The painted surface provided a best-case scenario for inspections while the other conditions offered further insight on correlating the application to industry-like cases.A 1300 W halogen lamp was employed as the heat source for providing continuous thermal excitation for various durations. Post-processing and analysis of the resulting thermal images was performed within the acquisition software using built-in analysis tools such as temperature probes, Region of Interest (ROI) based intensity profiles, and smoothing filters. The minimum defect diameter to depth (aspect) ratio detected in preliminary trials for the steel sample blocks were 1.0 at a diameter of 4.7625 mm (0.1875 in) and 1.5 at a diameter of 3.175 mm (0.125 in). For the inspection of painted rail sample, the longest exposure times (10 sec) provided the best detection capabilities in all sets of trials. The three holes having aspect ratio greater or equal to 1.0 were indicated in the thermal response of the painted and rusted samples while only the two holes having aspect ratio greater or equal to 1.5 were indicated in the unaltered sample. Indications of reflectors were identified through qualitative graphical analysis of pixel intensity distributions obtained along a bending line profile. The results obtained from the painted sample provided a baseline for analyzing the results from the unpainted and rusted rail samples. This provided an insight on the limitations and requirements for future development. The primary takeaway is the need for an optimized heat source. Poor contrast in the resulting image for the unpainted and rusted rail samples is experienced due to both noise and lack of penetration of the heat energy. This could have been due to decreased emissivity values. Moreover, the excitation method employed in this research does not comply with current industry standards for track clearances. Therefore, exploration of alternative excitation methods is recommended.

Portland cement concrete (PCC) structures deteriorate with age and need to be maintained or replaced. Early detection of deterioration in PCC (e.g., alkali-silica reaction, freeze/thaw damage or chloride presence) can lead to significant reductions in maintenance costs. Portland cement concrete can be nondestructively evaluated by electrically characterizing its complex dielectric constant in a laboratory setting. A parallel-plate capacitor operating in the frequency range of 0.1 to 40.1 MHz was developed at Virginia Tech for this purpose. While useful in research, this approach is not practical for field implementation. In this study, a capacitor probe was designed and fabricated to determine the in-situ dielectric properties of PCC over a frequency range of 2.0 to 20.0 MHz. It is modeled after the parallel-plate capacitor in that it consists of two conducting plates with a known separation. The conducting plates are flexible, which allows them to conform to different geometric shapes. Prior to PCC testing, measurements were conducted to determine the validity of such a system by testing specimens possessing known dielectric properties (Teflon). Portland cement concrete specimens were cast (of sufficient size to prevent edge diffraction of the electromagnetic waves) having two different air contents, two void thicknesses, and two void depths (from the specimen's surface). Two specimens were cast for each parameter and their results were averaged. The dielectric properties over curing time were measured for all specimens, using the capacitor probe and the parallel-plate capacitor. The capacitor probe showed a decrease in dielectric constant with increasing curing time and/or air content. In addition to measuring dielectric properties accurately and monitoring the curing process, the capacitor probe was also found to detect the presence and relative depth of air voids, however, determining air void thickness was difficult. / Master of Science

No description available.

Image quality metrics for several inverse scattering methods and algorithms are presented. Analytical estimates and numerical simulations provide a basis for poor image quality diagnostics. The limitations and noise behavior of reconstructed images are explored analytically and empirically using a contrast ratio. Theoretical contrast ratio estimates using the canonical PEC circular cylinder are derived. Empirical studies are conducted to confirm theoretical estimates and to provide examples of image quality vs SNR for more complex scatterer profiles. Regularized sampling is shown to be more noise sensitive than tomographic reconstructive methods.

No description available.