The ultrasonic nondestructive technique is mainly used to evaluate interior defect, material properties and outside dimensions by measuring the transmitting and reflecting sound waves. Generally, the evaluation of the ultrasonic testing depends on the amplitudes and delay time of the received signals; however, this research is focused on the analysis of the phase differences of the received signals. The leakage phenomenon of surface waves propagating at the liquid-solid interface has been studied for more than fifty years. The main characteristic of this phenomenon is the 180-degree phase difference between the reflected and leaky ultrasound when a bounded ultrasonic beam is incidented. And the null zone caused by the interference of these two waves is appearing in the reflected field. The phase difference is changed as the surface condition altered, including surface roughness and layered structures. The normal-mode theory is used at this research to analyze the analytical model of the leaky surface wave in thin layered structures. In experiments, the measurements of the reflected field are proceeded by the scanning system, so as to analyze the phase difference between the reflected and leaky ultrasound and calculate the layer thickness by the phase difference. As a result of the surface roughness of the thin layers, the leakage is more serious when the ultrasound propagates with lower frequency. For the consideration of locating the null-zone in the reflected field, this research prefers using lower frequency as an initial frequency at the beginning of the testing, then increasing the frequency to achieve a better sensitive of the thickness.

A36 steel is a commonly used material in civil engineering structures where fatigue damage can lead to catastrophic failure. In this research, nonlinear Rayleigh surface waves are used to characterize damage in A36 steel specimens caused by monotonic tension and low cycle fatigue. Fatigue damage produces the increased acoustic nonlinearity that leads to the generation of measurable higher harmonics in an initially monochromatic Rayleigh wave signal. One specimen is subjected to static tension and four specimens are used for low cycle fatigue tests in the tension-tension mode with a constant stress amplitude. The fatigue tests are interrupted at different numbers of cycles for the nonlinear ultrasonic measurements. Tone burst Rayleigh wave signals are generated and detected using a pair of oil coupled wedge transducers. The amplitudes of the first and second harmonic are measured at varying propagation distances to obtain the nonlinearity parameter for a given damage state. The experimental results show an increase of acoustic nonlinearity in the early stages of fatigue life. Furthermore, a close relationship between plastic deformation and the acoustic nonlinearity is found, which indicates that the acoustic nonlinearity is indeed a measure of microplasticity in this material.

Tez (Yüksek Lisans) - Süleyman Demirel Üniversitesi, Fen Bilimleri Enstitüsü, Jeofizik Mühendisliği Anabilim Dalı, 2008. / Kaynakça var.

Nonlinear mixing of two collinear, initially monochromatic, Rayleigh waves propagating in the same direction in an isotropic, nonlinear elastic solid is investigated: analytically, by finite element method simulations and experimentally. In the analytical part, it is shown that only collinear mixing in the same direction fulfills the phase matching condition based on Jones and Kobett 1963 for the resonant generation of the second harmonics, as well as the sum and difference frequency components caused by the interaction of the two fundamental waves. Next, a coupled system of ordinary differential equations is derived based on the Lagrange equations of the second kind for the varying amplitudes of the higher harmonic and combination frequency components of the fundamentals waves. Numerical results of the evolution of the amplitudes of these frequency components over the propagation distance are provided for different ratios of the fundamental wave frequencies. It is shown that the energy transfer is larger for higher frequencies, and that the oscillation of the energy between the different frequency components depends on the amplitudes and frequencies of the fundamental waves. Furthermore, it is illustrated that the horizontal velocity component forms a shock wave while the vertical velocity component forms a pulse in the case of low attenuation. This behavior is independent of the two fundamental frequencies and amplitudes that are mixed. The analytical model is then extended by implementing diffraction effects in the parabolic approximation. To be able to quantify the acoustic nonlinearity parameter, β, general relations based on the plane wave assumption are derived. With these relations a β is expressed, that is analog to the β for longitudinal waves, in terms of the second harmonics and the sum and the difference frequencies. As a next step, frequency and amplitude ratios of the fundamental frequencies are identified, which provide a maximum amplitude of one of the second harmonics as well as the sum or difference frequency components to enhance experimental results. Subsequently, the results of the analytical model are compared to those of finite element method simulations. Two dimensional simulations for small propagation distances gave similar results for analytical and finite element simulations. Consquently. this shows the validity of the analytical model for this setup. In order to demonstrate the feasibility of the mixing technique and of the models, experiments were conducted using a wedge transducer to excite mixed Rayleigh waves and an air-coupled transducer to detect the fundamentals, second harmonics and the sum frequency. Thus, these experiments yield more physical information compared to the case of using a single fundamental wave. Further experiments were conducted that confirm the modeled dependence on the amplitudes of the generated waves. In conclusion, the results of this research show that it is possible to measure the acoustic nonlinearity parameter β to quantify material damage by mixing Rayleigh waves on up to four ways.

This research uses nonlinear Rayleigh surface waves to characterize stress corrosion cracking (SCC) damage in sensitized 304 Stainless Steel (304 SS). 304 SS is widely used in reactor pressure vessels and fuel pipelines, where a corrosive environment in combination with applied stress due to high internal pressures can cause SCC. SCC poses great risk to these structures as it initiates cracks late in the lifetime and often unexpectedly. The initiated microcracks grow and accumulate very quickly to form macroscopic cracks that lead to material failure. Welds and the nearby heat affected zones (HAZ) in the vessels and pipework are particularly affected by SCC as welding induces sensitization in the material. SCC damage results in microstructural changes such as dislocation movement and microcrack initiation that in the long term lead to reduced structural integrity and material failure. Therefore, the early detection of SCC is crucial to ensure safe operation. It has been shown that the microstructural changes caused by SCC can generate higher harmonic waves when excited harmonically. This research considers different levels of SCC damage induced in samples of sensitized 304 SS by applying stress to a specimen held in a corrosive medium (Sodium Thiosulfate). Nonlinear Rayleigh surface waves are introduced in the material and the fundamental and the second harmonic waves are measured. The nonlinearity parameter that relates the fundamental and the second harmonic amplitudes, is computed to quantify the SCC damage in each sample. The results obtained are used to demonstrate the feasibility of using nonlinear Rayleigh waves to characterize SCC damage.

This research investigates second harmonic generation in Rayleigh surface waves propagating in 9%Cr ferritic martensitic steel. Previous experimental results show that the nonlinearity parameter is sensitive to certain changes in a material's properties such as thermal embrittlement and hardness changes. Therefore, the nonlinearity parameter can be used as an indicator of thermal damage due to changes in dislocation density and precipitations. The specimens are isothermally aged for different holding times to create progressive changes in the microstructure and obtain different levels of thermal aging damage. As aging progresses the dislocation density decreases and precipitations are formed; these microstructural evolutions lead to changes in the nonlinearity parameter β. Nonlinear ultrasonic experiments are conducted for each specimen using a wedge transducer for generation and an air-coupled transducer for detection of Rayleigh surface waves. The amplitudes of the first and second order harmonics are measured at different propagation distances, and these amplitudes are used to obtain the relative nonlinearity parameter for each specimen at different aging stages. Conclusions about microstructural changes are drawn based on the nonlinear Rayleigh surface wave measurement and complementary measurements including scanning electron microscopy (SEM) and Rockwell HRC hardness. The results indicate that the nonlinearity parameter is very sensitive to the dislocation density and precipitate formation, and thus can be used to track the microstructural change in this material during the process of thermal aging.

Thesis (M. S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Jacobs, Laurence J.; Committee Co-Chair: Wang, Yang; Committee Member: Kim, Jin-Yeon. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Esta dissertação apresenta o método de análise espectral de ondas superficiais (SASW) para a obtenção das variações do módulo cisalhante (Gmáx) com a profundidade, no domínio das deformações muito pequenas (abaixo de 0,001%). O SASW é um método sísmico in situ, não destrutivo, baseado na geração e detecção de ondas Rayleigh e na natureza dispersiva desta onda. Pela aplicação de um impacto na superfície do solo e detecção da onda em vários pontos, através de dois receptores, é construída uma curva de dispersão (velocidade de fase versus comprimento de onda). Esta curva de dispersão é, então, invertida. A inversão é um processo analítico para a reconstrução do perfil de velocidade de onda de cisalhamento (VS), partindo-se da curva de dispersão experimental de campo. O módulo de cisalhamento máximo de cada camada é facilmente obtido a partir do perfil de VS. No conteúdo teórico da dissertação discutem-se propriedades dinâmicas dos solos e descrevem-se as equações que dominam a propagação das ondas elásticas, tanto em meios homogêneos como em meios estratificados. A metodologia desenvolvida para a obtenção das curvas de dispersão, através da realização de ensaios SASW, apresenta os resultados obtidos em ensaios realizados na Cidade Universitária em São Paulo, sendo esses resultados comparados com estimativas feitas a partir de correlações baseadas em ensaios SPT existentes. Essas comparações permitem concluir que a metodologia SASW é uma boa alternativa para a determinação do perfil de rigidez (Gmáx) do solo, concordando com o nível de deformação envolvido nos ensaios. São desenvolvidos estudos de sensibilidade do método para verificar a influência na mudança dos parâmetros assumidos (peso específico, coeficiente de Poisson e espessuras das camadas) no processo de redução de dados (inversão) sobre o perfil final de VS, concluindo-se que o parâmetro que apresenta maior influência é o coeficiente de Poisson. / This dissertation presents the spectral-analysis-of-surface-waves (SASW) method as a tool for obtaining the variations in the modulus shear (Gmax) with depth in the field of very small strains (below 0,001%). The SASW method is a nondestructive in situ seismic method, based on the generation and measurement of Rayleigh wave and on its dispersive characteristic nature. Throughout the implementation of an impact on the soil surface and the detection of the wave at various points by two receptors a dispersion curve is constructed (phase velocity versus wave-length). This dispersion curve is then inverted. Inversion is an analytical process for reconstructing the shear wave velocity profile from the experimental field. The shear modulus of each layer is readily obtained from the shear wave velocity profile. The theoretical content of the dissertation presents dynamic properties of the soils and is described in the equations that dominate the propagation of elastic waves, both in homogeneous media and in stratified media. The methodology developed to obtain the dispersion curves through the implementation of SASW test is defined, and results from tests carried out at the University Campus in São Paulo are presented and compared with values obtained from correlations based on SPT tests. These comparisons indicate that the SASW method is a good alternative to determine the profile of stiffness (Gmax) of the soil, agreeing with the level of deformation involved in the tests. Studies on the methods sensitivity are developed to verify the influence on the changing of the parameters given (natural unit weight, Poisson coefficient and thickness of layers) in reduction of data (inversion) on the final profile of VS. The conclusion is that the Poisson coefficient is the parameter with greater influence.

<div>This thesis compiles my work on three projects.</div><div>The first project studies the cancellation of singularities in the inversion of two X-ray type transforms in the presence of conjugate points. The second project studies the recovery of singularities for the weighted cone transform. The third project studies the phenomenon of Rayleigh waves and Stoneley waves in the isotropic elastic wave equation of variable coefficients with a curved boundary.</div>

With the recent emergence of the field of metamaterials, the study of subwavelength propagation of plane waves and the dissipation of their energy either in the form of Joule losses in the case of electomagnetic waves or in the form of viscous dissipation in the case of acoustic waves in different interfaced media assumes great importance. with this motivation, I have worked on problems in two different areas, viz., plasmonics and surface acoustics. the first part (chapters 2 & 3) of the dissertation deals with the emerging field of plasmonics. Researchers have come up with various designs in an efort to fabricate efficient plasmonic waveguides capable of guiding plasmonic signals. However, the inherent dissipation in the form of Joule losses limits efficient usage of surface plasmon signal. a dielectric-metal-¬dielectric planar structure is one of the most practical plasmonic structures that can serve as an efficient waveguide to guide electromagnetic waves along the metal-dielectric boundary. I present here a theoretical study of propagation of surface plasmons along a symmetric dielectric-metal-dielectric structure and show how proper orientation of the optical axis of the anisotropic substrate enhances the propagation length. an equation for propagation length is derived in a wide range of frequencies. I also show how the frequency of coupled surface plasmons can be modulated by changing the thickness of the metal film. I propose a Kronig-Penny model for the plasmonic crystal, which in the long wavelength limit, may serve as a homogeneous dielectric substrate with high anisotropy which do not exist for natural optical crystals. in the second part (chapters 4 & 5) of the dissertation, I discuss an interesting effect of extraordinary absorption of acoustic energy due to resonant excitation of Rayleigh waves in a narrow water channel clad between two metal plates. Starting from the elastic properties of the metal plates, I derive a dispersion equation that gives resonant frequencies, which coincide with those observed in the experiment that was performed by Wave Phenomena Group at Polytechnic University of Valencia, Spain. Two eigenmodes with different polarizations and phase velocities are obtained from the dispersion equation. at certain critical aperture of the channel, an interesting cutoff effect, which is unusual for an acoustic wave, is observed for one of the eigenmodes with symmetric distribution of the pressure field. the theoretical prediction of the coupling and synchronization of Rayleigh waves strongly supports the experimentally measured shift of the resonant frequencies in the transmission spectra with channel aperture. the observed high level of absorption may find applications in designing metamaterial acoustic absorbers.