Time-frequency representations (TFR) are a widely used tool to analyze signals of guided waves such as Lamb waves. As a consequence of the uncertainty principle, however, the resolution in time and frequency is limited for all existing TFR methods. Due to the multi-modal and dispersive character of Lamb waves, displacement or energy related quantities can only be allocated to individual modes when they are separated in the time-frequency plane. The chirplet transform has been introduced as a generalization of both the wavelet and Short-time Fourier transform. It offers additional degrees of freedom to adjust time-frequency atoms which can be exploited in a model-based approach to match the group delay of individual modes. The objective of the current thesis is to apply the algorithm proposed by Kuttig to a series of candidate nondestructive evaluation problems. The accuracy and robustness of the CT based procedure is examined for each of these example problems and is benchmarked against analytical solutions (if available) and to the conventional STFT.

Independent sets of reflection seismic and seismoelectric data were collected, processed, and interpreted with the aim of generating and studying guided waves within a fault zone. While seismic surveys have recently been utilized to investigate fault zones, past and current seismoelectric experiments have been more focused on identifying lithological interfaces and the presence of fluids within the shallow subsurface. The utilization of a fault structure to study seismoelectric conversions associated with guided waves has not hitherto been reported in the literature. The purpose of this research is to investigate the capabilities of the seismoelectric geophysical method to image fault structures, and to compare these to the capabilities of the conventional reflection seismic technique. I hypothesize that the presence of subsurface fluids will enhance seismoelectric imaging of a fault system. My results show that seismoelectric data contribute significant new for fault zone characterization and subsurface. I collected seismic and seismoelectric data sets across a fault system in the Llano Uplift of central Texas. The seismic reflection data were collected with a Geometrics Strataview seismograph equipped with 36 geophones. The seismoelectric data utilized three Geometrics Geode seismographs, with electric fields recorded by stainless steel dipole pairs instead of geophones. A sledgehammer and an accelerated weight drop provided the seismic energy sources throughout the experiment. Elementary processing techniques were applied to both data sets to enhance the signal to noise ratio. Seismic reflection studies previously have been shown capable of identifying fault zones through the characterization of guided waves. The seismoelectric phenomenon has not yet been utilized for this purpose. Identification of fault-zone trapped waves within each data set was attempted separately before the two data types were qualitatively compared as to their relative capabilities for illuminating the fault zone. The seismic data revealed dispersive energy packets, indicative of guided waves, within the fault zone and absent in the surrounding lithologies. The seismoelectric data was able to produce comparable signals in the fault zone showing guided waves.

This thesis is study about the capability of guided waves in the inspection of cracks in pipelines, and studying about the guided waves of mode conversion phenomenon caused by cracks. The generally used inspection methods in industry are all localized area inspection. It will cost expensively and spend lots of human resource and time consuming, if we want to inspect the whole area in pipelines in factory. Thus, guided waves are used to improve these shortcomings of traditional inspection methods. Guided Waves can propagate fast and long range along the pipelines without decay. With the ability that guided waves can incident at a single location then inspect the whole region of pipelines under efficiently propagating distance. In this thesis, with using laboratory equipment, 3 Toneburst Cycles, 400¡B500¡B600 kHz, and L(0,1)¡BL(0,2) incident mode set up for crack detective sensitivity experiment. By calculating the group velocity of the signals of crack, it is known that there are three separated modes L(0,1)¡BL(0,2) and F(1,2) from reflected waves by cracks. These modes are exactly mode converted by cracks. The results show that in the same crack circumferential length or crack depth, the longer length or the deeper depth cause the higher reflection coefficient. It means they are more sensitive on the inspection of cracks. Also the reflection coefficient increases with the raising cross section area loss of cracks. In two dimensional fast Fourier transform experiment (2-D FFT), with 3 Toneburst Cycles, 400¡B500¡B600 kHz, L(0,1)¡BL(0,2) incident mode, and 0.5 cm interval set up to gather 40-point signals for proceeding the experiment. It can separate signals with different group velocity which are mixing together in time domain. From results, there are three different modes L(0,1)¡BL(0,2) and F(1,2) mode separated at three-dimensional picture. Finally, using the guided waves inspection system with 3 toneburst, 90 kHz and T(0,1) incident mode set up to proceed the experiment as before. In inspecting crack sensitive experiment, the use of this system has the similar result with the use of laboratory equipment. It proves that the excellent capability of guided waves in inspection of cracks in pipelines. While in mode identification, this system can identify there is only T(0,1) and F(1,2) mode with the same group velocity exist. Then the same result is verified by 2-D FFT experiment. It shows that this system generate or receive the specific mode in detection cracks by phase construction or phase destruction of phase interference.

The guided wave technique is an ultrasonic technique which is used to monitor large structures in a variety of industry sectors to safeguard against catastrophic failure. The guided wave technique for pipeline inspection has been commercially used since the early 2000s and this facilitates rapid inspection where from a single location over 100 metres of pipeline can be inspected. This technique is currently being used in pipeline infrastructure across the globe. For the technique to be successful it is highly dependent on a numerous of factors including, frequency selection, array designs and pipeline geometries. The transducers used on pipeline are dry coupled and the magnitude of the signal transmitted is dependent on the normal force applied to it. If this force is not controlled the signal being transmitted can degrade and lead to the difficult analysis of a complex signal. In this thesis studies have been undertaken to understand the relationship between dry force coupling of the transducer and the signal received, aligning this connection to classical contact theory. This is then further to extended to the influence surface contact conditions have on the transmission of signal from the transducer. Analysis of the results detected a peak in the operational frequency response which in turn initiated electrical impedance and structural resonance measurements to identify the presence of resonances which are induced by dry coupling. This behaviour was then modelled in FEA software and the validity of the FEA approach was tested against several prototype transducers. This thesis has been funded in joint collaboration between the Engineering Physics and Science Research Council and TWI ltd.

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The waveguiding effect of spatial solitary waves in nonlinear optical media has been suggested as a potential basis for future all-optical devices, such as optical interconnects. It has been shown that low power (∼ mW) beams, which can encode information, can be optically steered using external electric fields or through interactions with other beams. This opens up the possibility of creating reconfigurable optical interconnects. Nematic liquid crystals are a potential medium for such future optical interconnects, possessing many advantageous properties, including a “huge” nonlinear response at comparatively low input power levels. Consequently, a thorough understanding of the behaviour of spatial optical solitary waves in nematic liquid crystals, termed nematicons, is needed. The investigation of multiple beam interaction behaviour will form an essential part of this understanding due to the possibility of beam-on-beam control. Here, the interactions of two nematicons of different wavelengths in nematic liquid crystals, and the optical steering of nematicons in dye-doped nematic liquid crystals will be investigated with the aim of achieving a broader understanding of nematicon interaction and steering. The governing equations modelling nematicon interactions are nonintegrable, which means that nematicon collisions are inelastic and radiative losses occur during and after collision. Consequently numerical techniques have been employed to solve these equations. However, to fully understand the physical dynamics of nematicon interactions in a simple manner, an approximate variational method is used here which reduces the infinite-dimensional partial differential equation problem to a finite dynamical system of comparatively simple ordinary differential equations. The resulting ordinary differential equations are modified to include radiative losses due to beam evolution and interaction, and are then quickly solved numerically, in contrast to the original governing partial differential equations. N¨other’s Theorem is applied to find various conservation laws which determine the final steady states, aid in calculating shed radiation and accurately compute the trajectories of nematicons. Solutions of the approximate equations are compared with numerical solutions of the original governing equations to determine the accuracy of the approximation. Excellent agreement is found between full numerical solutions and approximate solutions for each physical situation modelled. Furthermore, the results obtained not only confirm, but explain theoretically, the interaction phenomena observed experimentally. Finally, the relationship between the nature of the nonlinear response of the medium, the trajectories of the beams and radiation shed as the beams evolve is investigated.

The possibility and feasibility of experimental detection of localized defects in tubes using laser-induced ultrasonic wave approach through Thermo Acousto Photonic Non Destructive Evaluation (TAP-NDE) and Signal processing through wavelet transform is examined in this research. Guided waves in cylindrical surfaces provide solutions for detection of different defects in the material. Several experiments were conducted to this respect. Wave propagation in both axial and circumferential directions was studied. The dispersive wave propagation of ultrasonic waves in hollow cylinders has been investigated experimentally, primarily for use in non-contact and nondestructive inspections of pipes and tubes. The laser ultrasonic waves propagated in cylindrical waveguides are particularly attractive because of their unique characteristics in the applications of nondestructive evaluation (NDE). Contrary to studies making use of only axially symmetric guided waves in hollow cylinders, here are analyzed also nonaxisymmetric waves. The analysis of data is made by using the Gabor wavelet transform. The capability of modeling the guided wave dispersion in hollow cylinders is used in developing guided wave experimental techniques for flaw detection. Good agreement was obtained when comparing the dispersion spectra between theory and experimentation. Measurement of group velocities of guided waves, which are obtained directly from the wavelet transform coefficients, can be used to determine allocation and sizing of flaws.

Guided wave techniques have great potential for the structural health monitoring of plate-like components. Previous research has demonstrated the effectiveness of combining laser-ultrasonic techniques with time-frequency representations to experimentally develop the dispersion relationship of a plate; the high fidelity, broad bandwidth and point-like nature of laser ultrasonics are critical for the success of these results. Unfortunately, laser ultrasonic techniques are time and cost intensive, and are impractical for many in-service applications. Therefore this research develops a complementary digital signal processing methodology that uses mounted piezoelectric elements instead of optical devices. This study first characterizes the spatial and temporal effects of oil coupled and glued piezoelectric sources, and then develops a procedure to interpret and model the distortion caused by their limited bandwidth and finite size. Furthermore, it outlines any inherent difficulties for time and frequency domain considerations. The deconvolution theory for source function extraction in the time - and frequency domain under the presence of noise is provided and applied to measured data. These considerations give the background for further studies to develop a dispersion relationship of a plate with the fidelity and bandwidth similar to results possible with laser ultrasonics, but made using mounted piezoelectric sources.

The possibility and feasibility of experimental detection of localized defects in tubes using laser-induced ultrasonic wave approach through Thermo Acousto Photonic Non Destructive Evaluation (TAP-NDE) and Signal processing through wavelet transform is examined in this research. Guided waves in cylindrical surfaces provide solutions for detection of different defects in the material. Several experiments were conducted to this respect. Wave propagation in both axial and circumferential directions was studied. The dispersive wave propagation of ultrasonic waves in hollow cylinders has been investigated experimentally, primarily for use in non-contact and nondestructive inspections of pipes and tubes. The laser ultrasonic waves propagated in cylindrical waveguides are particularly attractive because of their unique characteristics in the applications of nondestructive evaluation (NDE). Contrary to studies making use of only axially symmetric guided waves in hollow cylinders, here are analyzed also nonaxisymmetric waves. The analysis of data is made by using the Gabor wavelet transform. The capability of modeling the guided wave dispersion in hollow cylinders is used in developing guided wave experimental techniques for flaw detection. Good agreement was obtained when comparing the dispersion spectra between theory and experimentation. Measurement of group velocities of guided waves, which are obtained directly from the wavelet transform coefficients, can be used to determine allocation and sizing of flaws.

Composite materials have increasingly been used as an alternative to metals and other isotropic materials for primary structural components in aerospace industries. Unlike traditional isotropic materials, composite materials are known to have complex internal microstructures. Therefore, it is essential to develop methods for the inspection, evaluation, and monitoring of composite materials. Ultrasonic-guided waves and, more precisely, Lamb waves have proven to be an efficient and accurate technique for the non-destructive testing. Since guided waves are dispersive and multimodal, it is important to develop a practical method to manipulate Lamb waves to achieve better structural health monitoring and non-destructive inspection results. There are minimal studies involving manipulating guided waves for the inspection of composite materials. Moreover, the currently proposed methods to manipulate Lamb waves are complex and costly. The objective of this dissertation research is to offer practical and straightforward methods with a simple design to control Lamb waves using additively manufactured lenses used as superstrates on composite plates. This dissertation is organized in three major parts. Part I focuses on the Lamb wave propagation in composite plates with different lay-up and plate orientations. Finite element simulations were performed to investigate the behavior of Lamb wave propagation in different plates. A semi-finite element approach was used to derive the dispersive curves in each plate. In Part II, a lap-joint study was conducted to investigate the interaction of Lamb waves in the lap joint regions. Two different lap joints were considered, composite-aluminum and composite-plastic. In each lap joint the thickness of the top surface (aluminum or plastic) is continuously increased. In Part III, additively manufactured lenses are designed to modulate the wavefront of Lamb waves in thick composite plates. The first design is a prism-shaped lens proposed to steer Lamb waves to a targeted direction. Multiple prism designs are considered to offer a flexible steering direction by either changing the prism thickness or the wedge angle. The second design is a plano-concave shaped lens designed to focus the Lamb wave at a targeted focal point. This dissertation research will provide a clear understanding of Lamb wave propagation in anisotropic material, anisotropic-isotropic lap joints, and wavefront modulation on anisotropic material using additively manufactured lenses. This approach contributes to the development of better quality SHM for online monitoring systems.