In the first section, the thickness of ice on the lakes and canals is estimated by analyzing the sound spectrum generated by dispersion of Lamb type waves propagating in ice. In winters when the lakes and canals freeze, it is important to know the thickness of the ice layer before setting foot on it. When a stone is thrown on the ice layer, a fluting sound can be heard. This is recorded for different thicknesses of ice, and the sound spectrum is compared with the results simulated using a parameterized model. This model is created using a combination of plane waves for different incident angles and frequencies to generate dispersion curves for different thicknesses of ice. The frequencies of the reflected sound are then compared with the frequencies of musical instruments in order to assign different musical notes to different thicknesses of ice. The technique enables thickness estimation without the use of specialized equipment or time consuming drilling and may therefore be of practical value in the preservation of the lives of ice skaters and playing children. In the second half of the study, high frequencies (400 MHz and 1 GHz) are used to investigate samples with micro structures. Acoustic microscopy is a well established technique as far as smooth surfaces are concerned. V (z) curves are obtained from which, through surface wave generation, important features concerning elasticity and related properties can be extracted. Recently, high resolution imaging using high frequency focused transducers, based on acoustic microscopy has appeared. The surface profiles of the samples used in this study, have periodic structures but lack smoothness. The periodicity causes sound diffraction and the roughness influences the acoustic microscopic investigation. The small acoustic contrast between the substrate and the periodic corrugation on the material, gives us information about the additional stresses which develop and affect the bonding between the two materials. In this study, experiments are conducted using samples with corrugations of different periodicity, and a comparison is made between the results for smoother surfaces and results for the periodic structures of the same material. An attempt is made to analyse the effects described above.

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Structural Health Monitoring (SHM) through adequate damage detection and prediction of the remaining useful life of structures is a major area of interest in the aerospace community, where the growing maintenance costs can reduce the operational life of flight vehicles. The objective of a SHM system with an advanced diagnostic capability is to gradually replace current schedule-based maintenance tasks, where components are inspected following a pre-established number of cycles using condition-based maintenance, or are maintained prior to attaining an insufficient remaining useful life, based on specified confidence bounds. The research challenge is to obtain a reliable method for determining damage existence and respective location during its initial growth state as a component of an early warning system. In this thesis, an SHM system based on Lamb waves is proposed. A damage detection algorithm based on the comparison between the damaged structural state and a reference state has been developed. The detection algorithm, based on discrete signals correlation, was tested and improved by incorporating statistical methods and domain division techniques. Two SHM system architectures, namely the sensor network and phased array system were designed, implemented and tested. A visualization method based on the superposition of solutions obtained from a test set was implemented. Tests executed with multiple damage, representing surface and through-the-thickness holes and cracks were performed. The proposed SHM systems using Lamb waves were able to reliably detect holes of 1 mm holes in aluminum and 1.5 mm in composite plates with great confidence. / Graduate

Thesis (M.S. in Mechanical Engineer and M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2009. / Thesis Advisor(s): Kwon, Young W. ; Pollak, Randall D. "September 2009." Description based on title screen as viewed on November 5, 2009. Author(s) subject terms: Lamb Waves, modeling, finite element, longitudinal, and shear mode, ANSYS, debonding, sensor, piezoelectric. Includes bibliographical references (p. 93-96). Also available in print.

Engineering industries usually require nondestructive evaluation (NDE) methods to ensure quality control, safety, and optimized use of resources. Among potential NDE techniques, ultrasonic wave methods are widely used because of their versatility and affordability. For applications to layered structures, ultrasonic guided waves are naturally excited and detected, so these guided waves are the preferred choice when compared to conventional bulk waves. The main advantage of guided waves over bulk waves for layered structures is that these guided waves can propagate a much farther distance, and thus they enable long range inspection. It is important to note that guided waves are multi-mode, so a preferred mode can be selectively used, although it is sometimes more efficient to use multiple wave modes. The characteristics of guided waves, namely dispersive propagation and attenuation, are directly related to the properties of the system in which they are propagating, so the measurement of these wave characteristics can be used for material characterization and condition monitoring. Despite a number of successful techniques to experimentally measure propagation characteristics of guided waves, there is a lack of a standard procedure to obtain attenuation characteristics. This research develops such a quantitative and systematic procedure to extract attenuation characteristics from real guided wave time-domain signals. This research considers multiple wave-modes, and focuses on broadband attenuation measurements with laser ultrasonic techniques. The analytical model of guided waves with attenuation is studied in general cases, and a numerical simulation is developed to model the point source/receiver laser measurement system. The attenuation extraction technique is developed using synthetic signals generated by the simulation. Finally, this research demonstrates the use of experimentally-measured attenuation data for material characterization and condition monitoring by developing an inversion scheme to back-calculate material properties for a number of practical cases.

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Thesis (M.Sc.)--York University, 2008. Graduate Programme in Earth and Space Science. / Typescript. Includes bibliographical references (leaves 108-115). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR51581

In this thesis ultrasonic Lamb wave measurements are performed to detect material nonlinearity in aluminum sheets. When a Lamb wave propagates, higher harmonic wave fields are generated and under certain conditions the second harmonic is cumulative. When these conditions hold the Lamb waves are serviceable for material nonlinearity measurements. For generation, a wedge transducer combination is used. The detection of the Lamb wave are performed with either a laser interferometer or a second wedge transducer combination and the results are benchmarked. A short time Fourier transformation (STFT) is applied to the detected signal to extract the amplitudes of the first and second harmonics. A relative ratio of the first and second harmonics is deduced from nonlinear wave theory to assign the nonlinearity of the material. To verify the capability of the measurement setup and to show that cumulative second harmonics are generated, measurements for different propagation distances are performed. Further measurements on plasticly deformed specimens are carried out to examine the change of the material nonlinearity as a function of plasticity.