En contrôle non destructif par ultrasons, l'utilisation de la génération et de la détection par laser présente l'avantage de ne pas nécessiter de contact mécanique. La source laser crée à la surface du matériau un échauffement qui engendre dans celui-ci un champ de contraintes. Ce champ est la source d'ondes élastiques de surface et de volume. En régime thermoélastique (régime non destructif) et dans le cas d'une source linéique, les ondes de surface sont prépondérantes. Nous avons étudié la propagation de ces ondes sur des pièces cylindriques. Dans le cas d'un cylindre, nous avons développé un modèle analytique permettant d'analyser l'influence de la dispersion due aux variations de la vitesse de groupe sur la forme de l'onde de Rayleigh. Nous avons également constaté expérimentalement la déformation de l'onde de Rayleigh selon la position du point d'observation déterminée par son angle par rapport aux pôles d'émission. Ces résultats ont été corroborés par une simulation aux différences finies. Dans une approche plus industrielle, nous avons analysé les différents éléments pouvant perturber la détection des ondes ultrasonores (la sonde optique, la configuration de contrôle, la dimension des échantillons ...). En présence d'un défaut, nous avons montré, dans le domaine temporel et dans le domaine spectral, que la détection de fissures débouchantes de profondeur 0,2 mm à 1 mm est réalisable sur le premier tour de propagation. En raison des propriétés géométriques de l'échantillon, nous avons constaté un effet cumulatif sur l'énergie des ondes réfléchies par la fissure en fonction du nombre de tour de propagation. A l'aide d'une méthode de calcul du degré de ressemblance et en décomposant le signal ultrasonore temporel en fenêtres centrées sur chaque onde transmise, nous avons mis en évidence des défauts de profondeur 0,08 mm, quelle que soit leur position sur le cylindre. Dans le cas d'un tube, nous avons comparé la méthode sans contact mécanique à une méthode en immersion, développée au laboratoire, basée sur la décomposition de l'opérateur de retournement temporel (DORT). Nous avons mis en évidence la présence d'une fissure de profondeur égale à la moitié de l'épaisseur du tube par ces deux techniques de contrôle et probablement des conversions de mode au passage de la fissure à l'aide de la méthode DORT.

<p>Today wireless transmission of data is becoming more and more popular and the need for faster transmission rates is increasing. Since the bandwidth is limited it is important to try to use it to the fullest. CDMA is a technology that allows multiple accesses on the same frequency and time thus making it very bandwidth efficient. The CDMA technology was first introduced in the second generation’s cellular systems but has since then been improved and is reused in today’s 3G systems as Wideband CDMA. ISY is interested in getting a behavioural model of a W-CDMA system since they had developed a DSP processor called BBP1 and were thinking about adding W-CDMA support for it. Even though our system is not implemented on the BBP1 it should provide a good base for future implementations. This thesis project will describe the construction of a behavioural model of a W-CDMA system following the standard specified by 3GPP. The system simulates W-CDMA transmission and reception and has an optional channel used to simulate real world interference. The receiver uses a rake combiner to improve the performance of the system.</p>

Conventional contact ultrasonic methods suffer from large variability, which is known to originate from a number of sources such as coupling variability, and the surface roughness at the transducer/specimen interface. The inherently small higherharmonic signals can be significantly influenced by the changes in contact conditions, especially in nonlinear ultrasonic measurements. For this reason, the noncontact generation and detection techniques are very attractive. This research first focuses on the optical generation of tone-burst surface acoustic waves in a metallic specimen. Two methods that use laser light as an optical source are compared for generating surface acoustics waves in the 5 MHz range. Both the shadow mask and diffraction grating are used to convert a laser pulse to a tone-burst signal pattern on the specimen. The generated signals are detected by a wedge transducer at a fixed location and then the harmonic contents in the generated signals and the repeatability of the methods are evaluated. Finally, the developed method is used to characterize the material nonlinearity of aluminum (Al 6061) and steel (A36). The results showed repeatable measurements for ablative signal excitation on aluminum.

The use of geophysical testing methods has considerable potential to be a cost effective and accurate technique to assess near-surface soil conditions. Multi channel analysis of surface waves (MASW) test is a geophysical non-intrusive test that uses the dispersive characteristic of Rayleigh waves to estimate low strain shear modulus and damping coefficient of near-surface soil. Also, this technique is used to detect underground voids. Recently, MASW technique has gained more attention, partly because of its ease of use and partly because of the significant improvements in data acquisition systems. The theories of MASW test consider the effect of horizontal soil layering, though the effect of lateral inhomogeneities (i. e. cavities and voids), inclined layering and inverse layering (i. e. a layered system in which the top layers are stiffer than the bottom ones) are not addressed properly in these theories. <br /><br /> The objective of this dissertation is to investigate the effect of lateral inhomogeneities on the propagation of Rayleigh waves in an elastic half-space excited by a transient loading. The results can be applied to locate underground cavities using MASW test and to improve the MASW analysis techniques. In lieu of theoretical solutions, two and three dimensional numerical models are constructed to simulate the MASW test. To assure the quality of the obtained data, numerical models are calibrated with Lamb solution. Voids with different sizes and embedment depths are inserted in the medium. Responses along the surface as well as inside the medium are recorded and analyzed in time, frequency, spatial and frequency-wave number domains. Different material types and sources are used to generalize the results. Afterwards, the combined effect of void and layered systems on the surface responses are studied. To verify the results, experimental field and laboratory data are presented and the trends are compared to the numerical results. <br /><br /> It is found that the void starts to vibrate in response to the Rayleigh wave excitation. Due to the vibration of the void energy partitioning occurs. Part of the incident energy is reflected in the form of Rayleigh wave. Another part is converted to body waves, and spread into the medium. The transferred part of the energy is attenuated and has smaller amplitudes. Finally, a part of energy is trapped in the void region and bounces back and forth between the void boundaries, until it damps. The trapped energy is associated to higher modes of Rayleigh waves and excited Lamb waves. The effect of trapped energy is seen as a region in the vicinity of the void with concentrated energy, in frequency domain. The extents of this region depends on the void size, and the frequency content of the incident energy. Thus, in some cases it is possible to correspond the size of the model to the extents of the region with energy concentration. <br /><br /> A new technique is proposed to determine the location of a void, and estimate its embedment depth. The technique is called Attenuation Analysis of Rayleigh Waves (AARW), and is based on the observed damping effect of the void on the surface responses. For verification, the results are compared to experimental field and laboratory data. The observations are in good agreement with the observed numerical results. Further, the AARW technique showed to be a promising tool for void detection.

The use of geophysical testing methods has considerable potential to be a cost effective and accurate technique to assess near-surface soil conditions. Multi channel analysis of surface waves (MASW) test is a geophysical non-intrusive test that uses the dispersive characteristic of Rayleigh waves to estimate low strain shear modulus and damping coefficient of near-surface soil. Also, this technique is used to detect underground voids. Recently, MASW technique has gained more attention, partly because of its ease of use and partly because of the significant improvements in data acquisition systems. The theories of MASW test consider the effect of horizontal soil layering, though the effect of lateral inhomogeneities (i. e. cavities and voids), inclined layering and inverse layering (i. e. a layered system in which the top layers are stiffer than the bottom ones) are not addressed properly in these theories. <br /><br /> The objective of this dissertation is to investigate the effect of lateral inhomogeneities on the propagation of Rayleigh waves in an elastic half-space excited by a transient loading. The results can be applied to locate underground cavities using MASW test and to improve the MASW analysis techniques. In lieu of theoretical solutions, two and three dimensional numerical models are constructed to simulate the MASW test. To assure the quality of the obtained data, numerical models are calibrated with Lamb solution. Voids with different sizes and embedment depths are inserted in the medium. Responses along the surface as well as inside the medium are recorded and analyzed in time, frequency, spatial and frequency-wave number domains. Different material types and sources are used to generalize the results. Afterwards, the combined effect of void and layered systems on the surface responses are studied. To verify the results, experimental field and laboratory data are presented and the trends are compared to the numerical results. <br /><br /> It is found that the void starts to vibrate in response to the Rayleigh wave excitation. Due to the vibration of the void energy partitioning occurs. Part of the incident energy is reflected in the form of Rayleigh wave. Another part is converted to body waves, and spread into the medium. The transferred part of the energy is attenuated and has smaller amplitudes. Finally, a part of energy is trapped in the void region and bounces back and forth between the void boundaries, until it damps. The trapped energy is associated to higher modes of Rayleigh waves and excited Lamb waves. The effect of trapped energy is seen as a region in the vicinity of the void with concentrated energy, in frequency domain. The extents of this region depends on the void size, and the frequency content of the incident energy. Thus, in some cases it is possible to correspond the size of the model to the extents of the region with energy concentration. <br /><br /> A new technique is proposed to determine the location of a void, and estimate its embedment depth. The technique is called Attenuation Analysis of Rayleigh Waves (AARW), and is based on the observed damping effect of the void on the surface responses. For verification, the results are compared to experimental field and laboratory data. The observations are in good agreement with the observed numerical results. Further, the AARW technique showed to be a promising tool for void detection.

An experimental study of the effect of initial conditions on the development of Rayleigh Taylor Instabilities (RTI) at low Atwood numbers (order of 10-4) was performed in the water channel facility at TAMU. Initial conditions of the flow were generated using a controllable, highly reliable Servo motor. The uniqueness of the study is the system’s capability of generating the required initial conditions precisely as compared to the previous endeavors. Backlit photography was used for imaging and ensemble averaging of the images was performed to study mixing width characteristics in different regimes of evolution of Rayleigh-Taylor Instability (RTI). High-speed imaging of the flows was performed to provide insights into the growth of bubble and spikes in the linear and non-linear regime of instability development. RTI are observed in astrophysics, geophysics and in many instances in nature. The vital role of RTI in the feasibility and efficiency of the Inertial Confinement Fusion (ICF) experiment warrants a comprehensive study of the effect of mixing characteristics of RTI and its dependence on defining parameters. With this broader objective in perspective, the objectives of this present investigation were mainly threefold: First was the validation of the novel setup of the Water channel system. Towards this objective, validation of Servo motor, splitter plate thickness effects, density and temperature measurements and single-mode experiments were performed. The second objective was to study the mixing and growth characteristics of binary and multi-mode initial perturbations seeking an explanation of behavior of the resultant flow structures by performing the first ever set of such highly controlled experiments. The first-ever set of experiments with highly controlled multi-mode initial conditions was performed. The final objective of this study was to measure and compare the bubble and spike velocities with single-mode initial conditions with existing analytical models. The data derived from these experiments would qualitatively and quantitatively enhance the understanding of dependence of mixing width on parametric initial conditions. The knowledge would contribute towards a generalized theory for RTI mixing with specified dependence on various parameters, which has a wide range of applications. The system setup was validated to provide a reliable platform for the novel multi-modal experiments to be performed in the future. It was observed that the ensemble averaged mixing width of the binary system does not vary significantly with the phase-difference between the modes of a binary mode initial condition experiment, whereas it varies with the amplitudes of the component modes. In the exponential and non-linear regimes of evolution, growth rates of multi-mode perturbations were found to be higher than the component modes, whereas saturation growth rates correspond to the dominant wavelength. Quadratic saturation growth rate constants, alpha were found to be about 0.07 ± 0.01 for binary and multi modes whereas single-mode data measured alpha about 0.06 ± 0.01. High-speed imaging was performed to measure bubble and spike amplitudes to obtain velocities and growth rates. It was concluded that higher temporal and spatial resolution was required for accurate measurement. The knowledge gained from the above study will facilitate a better understanding of the physics underlying Rayleigh-Taylor instability. The results of this study will also help validating numerical models for simulation of this instability, thereby providing predictive capability for more complex configurations.

This research investigates how ultrasonic damage monitoring in civil structures can be implemented on a small, battery-powered, self-contained device. The device is intended for the continuous monitoring of surface breaking cracks in steel using Rayleigh waves. This study in detail presents the challenges that are to be considered for the intended ultrasonic monitoring, with the objective to provide a foundation for the future development of a fully autonomously operating device. The study proposes a suitable hardware and software layout, and a prototype device is built using a digital signal processor, a commercial wireless transceiver, and custom amplification circuits. With the help of two narrowband ultrasonic contact transducers in a pitch-catch setup and appropriate contact wedges, the wave field that arises from scattering of an incident tone burst wave at a crack is measured. A data analysis algorithm extracts wave burst signals from the acquired output in order to minimize the data that is to be transmitted. Additional compression of the data and the implementation of a communication protocol allow for a reliable and efficient wireless transmission. In order to demonstrate the feasibility of the proposed approach, measurements of notches in a steel plate with different depths are taken. Measurement results from experiments with commercial ultrasonic equipment are compared to measurements taken with the prototype device. The influence of the sampling distortions on the signals are analyzed. The scope of this study is limited to a qualitative analysis of the experimental results; quantitative methods to determine the dimensions of a crack or notch from the measured data are not included. The research conducted demonstrates that taking ultrasonic measurements with a small, self-contained device is feasible. Comparison of frequency-based to time-based signal analysis methods yields that frequency-based methods are preferable, as they are affected less by sampling effects. The experimental results show that the intended ultrasonic examination technique can be used for qualitative damage assessment. The knowledge gained in this study contributes to improving the safety of civil infrastructure. Continuous local damage monitoring as proposed helps to detect critical conditions in-time, and to take countermeasures to avoid catastrophic failures.

High explosive charges when detonated ensue in a flow field characterized by several physical phenomena that include blast wave propagation, hydrodynamic instabilities, real gas effects, fluid mixing and afterburn effects. Solid metal particles are often added to explosives to augment the total impulsive loading, either through direct bombardment if inert, or through afterburn energy release if reactive. These multiphase explosive charges, termed as heterogeneous explosives, are of interest from a scientific perspective as they involve the confluence and interplay of various additional physical phenomena such as shock-particle interaction, particle dispersion, ignition, and inter-phase mass, momentum and energy transfer. In the current research effort, chemical explosions in multiphase environments are investigated using a robust, state-of-the-art Eulerian-gas, Lagrangian-solid methodology that can handle both the dense and dilute particle regimes. Explosions into ambient air as well as into aluminum particle clouds are investigated, and hydrodynamic instabilities such as Rayleigh- Taylor and Richtmyer-Meshkov result in a mixing layer where the detonation products mix with the air and afterburn. The particles in the ambient cloud, when present, are observed to pick up significant amounts of momentum and heat from the gas, and thereafter disperse, ignite and burn. The amount of mixing and afterburn are observed to be independent of particle size, but dependent on the particle mass loading and cloud dimensions. Due to fast response times, small particles are observed to cluster as they interact with the vortex rings in the mixing layer, which leads to their preferential ignition/ combustion. The total deliverable impulsive loading from heterogeneous explosive charges containing inert steel particles is estimated for a suite of operating parameters and compared, and it is demonstrated that heterogeneous explosive charges deliver a higher near-field impulse than homogeneous explosive charges containing the same mass of the high explosive. Furthermore, particles are observed to introduce significant amounts of hydrodynamic instabilities in the mixing layer, resulting in augmented fluctuation intensities and fireball size, and different growth rates for heterogeneous explosions compared to homogeneous explosions. For aluminized explosions, the particles are observed to burn in two regimes, and the average particle velocities at late times are observed to be independent of the initial solid volume fraction in the explosive charge. Overall, this thesis provides useful insights on the role played by solid particles in chemical explosions.

The effect of stress on Lamb wave propagation is relevant to both nondestructive evaluation and structural health monitoring because of changes in received signals due to both the associated strain and the acoustoelastic effect. A homogeneous plate that is initially isotropic becomes anisotropic under biaxial stress, and dispersion of propagating waves becomes directionally dependent. The problem is similar to Lamb wave propagation in an anisotropic plate, except the fourth order tensor in the resulting wave equation does not have the same symmetry as that for the unstressed anisotropic plate, and the constitutive equation relating incremental stress to incremental strain is more complicated. Here we review the theory of acoustoelastic and develop theory for acoustoelastic Lamb wave propagation and show how dispersion curves shift anisotropically for an aluminum plate under biaxial tension. We also develop an approximate method using the effective elastic constants (EECs) and show that existing commercial tools to generate dispersion curves can be used under restricted conditions to describe wave propagation in biaxially stressed plates. Predictions of changes in phase velocity as a function of propagation direction using theory and the EEC method are compared to experimental results for a single wave mode.

Der Bericht beschäftigt sich mit spontaner Grenzflächenkonvektion und -turbulenz beim Stoff- und Wärmeübergang an fluiden Phasengrenzen zwischen zwei nicht mischbaren Phasen. Solche Effekte sind von großer industrieller Bedeutung, da die erzielten Stoffübergangsraten um ein Vielfaches über den bei gewöhnlicher Diffusion auftretenden liegen. Zwei unterschiedliche Mechanismen sind der "Motor" für die Instabilitäten: Marangoni-Instabilität: Die Grenzflächenspannung ist eine Funktion der Temperatur und der Grenzflächenkonzentration des ausgetauschten Stoffes. Schwankungen der Temperatur und der Konzentration entlang der Phasengrenze führen folglich zu Grenzflächenspannungsgradienten. Grenzflächenspannungsgetriebene Instabilitäten äußern sich durch rollenförmige oder polygonale Konvektionszellen, Eruptionen oder Turbulenz an der Phasengrenze. Schwerkraftgetriebene Instabilität: Die Dichte ist ebenfalls eine Funktion der Temperatur und der Konzentration des gelösten Stoffes. Der Transport eines Stoffes über eine fluide Phasengrenze verändert die Zusammensetzung und die Dichte der angrenzenden Flüssigkeitsschichten, sodass instabile Dichteschichtungen auftreten können. Temperaturgradienten entstehen dabei durch Freisetzung von Reaktions- und/oder Lösungsenthalpie. Auftriebsbewegungen haben die Form von Thermiken (engl. plumes, thermals). Die Phänomene der Grenzflächenkonvektion werden in einer vertikalen Kapillarspaltgeometrie untersucht. Neben Stoffsystemen mit reaktivem Stoffübergang (Neutralisation von Karbonsäuren, Hydrolyse und Veresterung von Alkanoylhloriden) kamen auch solche mit reaktionsfreiem Stoffübergang (Karbonsäuren, Tensid) zur Anwendung. Die instabile Dichteschichtung, die durch den Konzentrationsgradienten infolge der Stoffdiffusion erzeugt wird, führt zu Auftriebskonvektion in Form von Thermiken. Die Anwesenheit einer exothermen Reaktion bewirkt eine Vergrößerung des Längenwachstums der Thermiken in der oberen Phase durch Aufprägung eines zusätzlich destabilisierenden Temperaturgradienten. In der unteren Phase kommt es dagegen zum Entstehen des doppeldiffusiven Fingerregimes bei Überlagerung des destabilisierenden Konzentrationsgradienten durch den stabilisierenden Temperaturgradienten. Beim Übergang eines Tensids konnten die für diese Stoffklasse charakteristischen Rollzellen, die durch Grenzflächenspannungsgradienten angetrieben werden, beobachtet werden. Diese Konvektionsstrukturen bleiben auf einen schmalen Bereich ober- und unterhalb der Phasengrenze beschränkt. Die Transportgleichungen für Impuls, Stoff und Wärme wurden in ihrer 2-dimensionalen Form in einen Rechenkode umgesetzt und der Übergang einer einzelnen Komponente simuliert. Die hydrodynamischen Bedingungen an der Phasengrenze wurden so formuliert, dass lokale Änderungen der Zusammensetzung und der Temperatur zu Grenzflächenspannungsgradienten führen und die Phasengrenze damit dem Marangonieffekt unterliegt. Die Stoffeigenschaften wurden mit Ausnahme der Dichte im Volumenkraftterm der Impulsgleichung als konstant angenommen, sodass dichtegetriebene Konvektionen simuliert werden können. Die verschiedenen Konvektionsformen werden durch die Simulation qualitativ gut wiedergegeben. Bei Marangonikonvektion kommt es zu einer Verschiebung des steilen Konzentrationsgradienten von der Phasengrenze in die Kerne der Phasen, was zum schnellen Absterben der Marangonikonvektion führt. Die Wiedergabe des Längenwachstums der Thermiken durch Simulation eines realen Stoffsystems ist zufriedenstellend. Ebenso gibt die Simulation eine realistische Abschätzung zu erwartender Stoffströme bei Anwesenheit hydrodynamischer Instabilitäten. Größere Abweichungen zwischen Simulation und Experiment sind jedoch bei der horizontalen Größenskala der Fingerstruktur festzustellen, die wahrscheinlich auf die Boussinesq-Approximation zurückzuführen sind.