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Handling and making decisions based on data taken at different scales is a critical issue in the design of exploration and production tasks in the oil industry. Acoustic data is the classical example of the integration of dissimilar scales (i.e. seismic, well logs, lab data) where there is a scale dependent velocity. An understanding of the acoustic dispersion phenomenon in granular samples is needed. A detailed numerical work was conducted in order to establish the relationship between frequency and propagation speed for an acoustical pulse induced in simulated granular materials. The granular samples were generated with different grain size distributions while porosity and pressure were targeted and kept invariant using the grain radii expansion method. A sinusoidal burst with frequencies from 10Hz to 1MHz was applied and the corresponding acoustical speeds were estimated for each frequency. A coherent sigmoid dispersion relationship was obtained for each granular sample. The asymptotic boundaries for the dispersion function reflect the limiting cases for the wavelength/heterogeneity ratio in the granular pack. The lower speed asymptote was explained as the mean field value while upper speed asymptote can be understood based on a ray theory approximation scaled by a parameter we defined as the “acoustic tortuosity factor”. This factor reflects the intricate acoustical path due to the texture of the stress network developed in the granular samples and can be used together with the sigmoid dispersive relationship to describe and clarify the scale discrepancy between different source acoustic data in granular materials.

In this paper we study time dependent problems, like the propagation of sound waves or the behavior of small local wave disturbances induced by spontaneous internal fluctuations, in a binary mixture undergoing a chemical reaction of type A+A= B+B. The study is developed at the hydrodynamic Euler level, in a chemical regime of fast reactive process in which the chemical reaction is close to its final equilibrium state. The hydrodynamic state of the mixture is described by the balance equations for the mass densities of both constituents A and B, together with the conservation laws for the momentum and total energy of the mixture. The progress of the chemical reaction is specified by an Arrhenius-type reaction rate which defines the net balance between production and consumption of each constituent. Assuming that the considered time dependent problems induce weak macroscopic deviations, the hydrodynamic equations are linearized through a normal mode expansion of the state variables around the equilibrium state. From the dispersion relation of the normal modes, we determine the free and forced phase velocities as well as the attenuation coefficients of the waves. We show that the dispersion and absorption of these waves depend explicitly on the heat of the chemical reaction, the concentrations of the constituents and the activation energy through the exponential factor of Arrhenius law., Fundação para a Ciência e a Tecnologia, Brazilian Research Council (CNPq)

To improve the results of the Delay-And-Sum (DAS) beamformer a weighting factor based on a measurement of the phase dispersion of the signals in the image point under consideration has been proposed. With this objective, the spectral analysis of the phase dispersion is used here to obtain a new descriptor that can be easily computed and introduced as a ponderation factor in the beamforming process. Theoretical results show that, for a linear array of 32 elements operating in synthetic aperture technique with dynamic focussing in emission and reception (Total Focussing Method) the improvement can reach up to 24dB. In order to validate the theoretical hypothesis, experimental results obtained using a linear array of 64 elements (2,4MHz) and a CIRS 040GSE phantom also are presented., This work has been supported by the Spanish Ministry of Science and Innovation under the project DPI2010-19376

It is well known that the dispersion relations for guided acoustic modes in free standing plates commonly display backward propagation regions, where the group velocity V = ∂ω/∂k is opposite in direction to the phase velocity V = ω/k At the boundaries of such regions there occur zero group velocity (ZGV), V = 0, modes. Accurate depiction of wave packet propagation and spread in this situation requires that account be taken also of the curvature α = ∂2ω/∂k2 of the dispersion relation. The generic types of wave packet behavior, dependent on the signs of V, ν and α are illustrated in the first part of this paper. In the second part, the existence of backward propagation in supported solid layers is explored. It is shown that for a single layer on an elastic substrate, backward propagation is a rare phenomenon, because it requires that the transverse velocity of the layer be very small compared to that of the substrate. On the other hand, in certain bi-layer structures of importance in the microelectronics in...

Metamaterials are materials with artificial properties defined by their sub-wavelength structure rather than their chemical composition. With the arrival of photonic crystals and phononic crystals, the fabrication of metamaterials which do not exist in nature become a reality. We discovered parity invariance in acoustical field equation. We also show that negative refraction is a special case of coordinates transformation (used in acoustical cloaking) when the determinant of the direction cosines matrix equals −1 and we develop a unifed theory for negative refraction and cloaking. Gauge invariance approach also removes the ambiguity problem of positive sign and negative sign when using the dispersion relation for the negative refractive index. Lefthanded materials produce Poynting vector in opposite direction to wave propagation. This gives rise to new phenomena in refraction, diffraction, and scattering of acoustic waves in the material‥ These three are the basic mechanisms of sound propagation in medium...

The algorithm for dispersion curves calculation in radially inhomogeneous isotropic elastic waveguides is presented. Main attention is directed at helical waves, which propagate along cylindrical surface under arbitrary angle to its generatrix. They can be described by noninteger azimuthal wavenumbers. Such waves find expanding application in non-destructive testing, acoustic tomography, etc. Provided physical parameters of the medium are piecewise continuous functions of the radial coordinate, one can tackle the problem under consideration by matrix Riccati equation technique for elastic impedance matrix. It can be solved by parametric continuation method. Cayley transform was applied to avoid singularities in the course of computations.

The emission of sound waves resulting from mode conversion of Lamb waves propagating on thin glass plates submerged into water was investigated by a PVDF membrane hydrophone. The antisymmetrical zero order Lamb wave mode was excited by a piezoelectric single phase transducer operated at a frequency of 1 MHz. With the hydrophone the emission pressure distribution of the emitted sound waves in the liquid was recorded and the phase velocities of the Lamb waves were calculated from the direction of the emitted sound waves, i. e. the Lamb angles. Due to the dispersion of Lamb waves the velocity changes with the plate thickness. The measured phase velocities coincide well with the calculated values using the global matrix method for plate thicknesses between 0.5 and 4 mm.

Nondestructive testing for multilayered structures is challenging because of increased numbers of layers and plate thicknesses. In this paper, ultrasonic guided waves are applied to detect delamination defects inside a 23‐layer Alcoa Advanced Hybrid Structural plate. A semi‐analytical finite element (SAFE) method generates dispersion curves and wave structures in order to select appropriate wave structures to detect certain defects. One guided wave mode and frequency is chosen to achieve large in‐plane displacements at regions of interest. The interactions of the selected mode with defects are simulated using finite element models. Experiments are conducted and compared with bulk wave measurements. It is shown that guided waves can detect deeply embedded damages inside thick multilayer fiber‐metal laminates with suitable mode and frequency selection.

Lamb waves are frequently used in nondestructive evaluation and are proposed for structural health monitoring applications. While many numerical methods to obtain dispersion curves for Lamb waves have been developed and implemented, in general they are time consuming and do not lend themselves to analyses involving the perturbation of parameters such as the small thickness and wave speed changes caused by temperature variations. Presented here are two methods of approximating the solutions of the Rayleigh‐Lamb equations under a small perturbation assumption. The first method is a gradient‐based linear approximation to the Raleigh‐Lamb equations, and the second is a linear approximation method based on a number of pre‐computed solutions to the Rayleigh‐Lamb equations. Also presented is a simple algorithm to efficiently compute Lamb wave dispersion curves, which is used to study the approximation methods.

Ultrasonic guided waves in elastic bare pipes are studied from a theoretical point of view. Dispersion curves of both axisymmetric and non‐axisymmetric waves are presented. The parameters of 4‐channel focusing are calculated by a phased array focusing algorithm. Experimentally four defects are detected in a cased pipe inspection. The defect probability of detection is highly improved by applying the phased array focusing technique. Guided wave dispersion curves in coated pipes are calculated using a 1‐D Semi‐Analytical Finite Element (SAFE) technique. This theoretically driven hybrid SAFE method has perfectly tackled the guided wave problem in coated pipes, including dispersion curves (phase velocity dispersion curves and attenuation dispersion curves) and wave structures. The time delays and amplitudes of the 8‐channel focusing in an 8 in sch40 coated pipe are calculated by the SAFE algorithm and input to the FEM simulation in ABAQUS®. Focusing performance is improved up to 67% by applying the coated pip...

A guided wave technique for measurement of remaining thickness distribution is presented for the purpose of maintenance of large structures like oil storage tanks and pipe networks. This study used a laser source to excite the guided wave at many locations on aluminum plates with defect that have high ratio of A0 mode transmission and small mode conversions. The A0 mode was used to interrogate the through‐thickness over the rastered areas in the test plates. It was expected that many modes of Lamb wave were excited in intact and defect regions of the plates through the thermoelastic stresses from the laser source. The fundamental antisymmetric mode A0 was extracted to indicate the thickness distribution in aluminum test plates with the defect. The results exhibit greater amplitude distributions of A0 mode over thinner regions around defects and smaller amplitudes distributions over the intact regions.

A new method for detecting cracked, thick‐walled, cylindrical geometries using statistical pattern recognition is described. A defective cylinder is identified by monitoring the fundamental modes of vibration that arise following an impulse load. A new model was developed to illustrate the effects of a crack on these modes using the dispersion characteristics of tangentially guided and standing waves. These effects are inherently incorporated in a feature that is used by a statistical pattern classification algorithm to identify a defective sample.

Solid state detonation (SSD) refers to nonclassical supersonic reactive wave phenomena in energetic materials that are not typically considered explosives. Reactive energetic materials include both metal/metal oxide and metal oxide/polymer systems with thermitic reaction. Like conventional solid explosives, the materials are manufactured composites with a well‐defined microstructure. Ingredients include nano‐engineered energetic materials with novel surface and reaction properties. The manufactured materials are still described by a continuum limit informed by the microstructural properties. We consider limit model formulations that include acoustic dispersion phenomena, void effect, macroscopic ignition and extinction of steady traveling reactive waves, in a modeling framework that can aid the design of new materials, which will be the basis for our continued work on SSD including the inter‐material heat transfer and kinetics.

The direct simulation Monte Carlo (DSMC) method describes gas dynamics through direct physical modeling of particle motions and collisions, DSMC is based on the kinetic theory of gas dynamics, where representative particles are followed as they move and collide with other particles. DSMC provides a useful tool for capturing all physical properties of interest for nonlinear acoustic problems, such as dispersion, attenuation, absorption, harmonic generation and nonequilibrium effects. The validity of DSMC for the entire range of Knudsen numbers (Kn), where Kn is defined as the mean free path divided by the wavelength, allows for the exploration of sound propagation at low Kn (low frequency, atmospheric conditions) as well as sound propagation at high Kn (high frequency, dilute gases, or in microdevices). For low Kn, nonlinear effects play an important role in waveform evolution. For high Kn, nonequilibrium effects are strong and increased absorption cancels out nonlinearity effects.

This paper reviews the progress of air‐coupled surface wave sensing methods in NDT of concrete structures. The theoretical background of air‐coupled sensing and its applications to surface wave measurement are discussed. The theoretical analysis shows that air‐coupled sensors are able to detect leaky surface waves that are excited by a transient point load in concrete. The air‐coupled sensing technique can be used to measure surface wave dispersion curves with SASW and MASW approaches, and to image defects in a concrete slab by measuring surface wave transmission ratio. Experimental results show that air‐coupled sensors are able to replace contact sensors in traditional surface wave tests, and therefore greatly improve test efficiency.

A dust crystal consisting of charged dust grains of alternating charge sign (…/+/−/+/−/+/…) and mass is considered. Considering the equations of longitudinal motion, a linear dispersion relation is derived from first principles, and then analyzed. Two modes are obtained, including an acoustic mode and an inverse‐dispersive optic‐like one. The nonlinear aspects of longitudinal dust grain motion are also briefly addressed, via a Boussineq and Korteweg‐ de Vries description.

This study investigates the utilization of guided waves for inspecting structures that consist of multiple layers. Advances have been made in recent years using guided waves to inspect single layer structures, such as pipes, tubes, and aircraft structures. Multi‐layered structures present many new aspects to guided wave propagation. A theoretical understanding of what modes exist, how do the modes behave, and what factors influence them needs to be acquired for many applications. Experiments must be carried out to evaluate potential for practical applications. Examples of practical applications include coated pipes, composites, diffusion bonded aircraft structures, and microelectronic structures. This work is a fundamental study of ultrasonic guided waves in multi‐layered plates. Experiments were conducted on multi‐layered plates to demonstrate defect detection in layer of interest of a multi‐layered structure by preferentially exciting modes with sufficient energy in that layer. Analysis of the dispersion curves show that some modes are more attractive candidates than others based on their displacements and energy distribution across the structure. Experimental results show that sweeping frequency and phase velocity can be performed to find suitable modes for inspecting a layer of interest for a given multi‐layered structure.This study investigates the utilization of guided waves for inspecting structures that consist of multiple layers. Advances have been made in recent years using guided waves to inspect single layer structures, such as pipes, tubes, and aircraft structures. Multi‐layered structures present many new aspects to guided wave propagation. A theoretical understanding of what modes exist, how do the modes behave, and what factors influence them needs to be acquired for many applications. Experiments must be carried out to evaluate potential for practical applications. Examples of practical applications include coated pipes, composites, diffusion bonded aircraft structures, and microelectronic structures. This work is a fundamental study of ultrasonic guided waves in multi‐layered plates. Experiments were conducted on multi‐layered plates to demonstrate defect detection in layer of interest of a multi‐layered structure by preferentially exciting modes with sufficient energy in that layer. Analysis of the dispersio...

Functionally graded materials (FGMs) have spatial variations in composition and structure resulting in corresponding changes in material properties. In this work, a model for the laser generation of ultrasound in FGMs is used to explore the dependence of the generated acoustic waves on the compositional profile, and to evaluate the feasibility of using laser generated surface acoustic waves (SAWs) to determine the depth dependent properties of graded coating materials. The materials studied are coatings which exhibit elastic property gradations from the coating surface to the interface with the substrate. The graded coatings are modeled as multilayer systems and the number of layers required to achieve a desired level of convergence of the model over a given frequency range is determined. For an assumed compositional profile, the elastic properties of the respective layers are calculated using a modified rule of mixtures formulation. Numerical results showing the sensitivity of SAW dispersion behavior to various compositional profiles are presented. In addition, an inversion algorithm is developed to extract position dependent elastic properties from measured SAW dispersion behavior and preliminary results are obtained using simulated experimental data.

Laser-ultrasonics has been used with success for the microstructure evaluation of metals by measuring ultrasonic attenuation and velocity. Extension of this technique to fiber reinforced polymer composites could provide useful information such as fiber contents and porosity. Ultrasonic generation in composite materials is usually performed by using a pulsed TEA-CO2 laser operated on several transverse modes. The intensity profile of this laser could change from shot-to-shot, thus affecting the ultrasonic generation pattern and the accuracy of attenuation measurement. Two solutions are possible to overcome this problem. The first reported approach consists in numerically correcting for the diffraction effect by using a Monte Carlo integration after simultaneous acquisition of the ultrasonic signal and the laser mode pattern. The second reported approach consists in modifying the laser resonator to get a single transverse mode while maintaining high energy per pulse, and to make the diffraction correction much simpler. Both approaches have been tested on a bare carbon epoxy plate with laser generation in the thermoelastic regime. The results obtained from using either approaches show a significant improvement and are compared to those obtained by conventional ultrasonics., Quantitative Nondestructive Evaluation (QNDE 2003), Green Bay, WI, USA, 27 July-1 August 2003, 27 July-1 August 2003, Green Bay, WI, USA, Series: AIP conference proceedings

The possibility of using long range ultrasonic guided wave non‐destructive testing for the inspection of metallic structures coated with attenuative layers is extremely attractive. However, the material damping may cause severe attenuation of the acoustic signal, resulting in reduction of the test range. This paper addresses the dispersion of shear horizontal waves propagating in metallic plates coated with viscoelastic layers and the implications for long range testing.

Knowledge of acoustic arrival angle can be useful for studying penetration mechanisms and for estimating sediment sound speed dispersion. The arrival angle of the acoustic field penetrating the water‐sediment interface, however, is difficult to measure using sparsely distributed pressure sensors. The arrival angle, as well as amplitude information, can be unambiguously obtained by measuring particle motion with directional receivers. An experiment was conducted off of Elba Island, Italy, to assess the feasibility of a novel technique that uses high‐frequency accelerometers to measure the directionality of acoustic arrivals. Measurements of acoustic penetration into a sandy seafloor were obtained over a wide frequency range (2.5 to 29 kHz) using off‐the‐shelf accelerometers, adapted for use in marine studies. The sensors that were developed are suitable for the penetration studies for which they were devised, but their angular resolution would limit their application in dispersion studies. Insights from th...

Recent interest in applications like the measurement of compressive surface residual stresses due to shot peening using Rayleigh type surface acoustic waves have brought into the spot light the adverse effects surface roughness might have on such measurements. Surface roughness produces significant dispersion of the Rayleigh wave used in the measurements. Therefore an accurate understanding of the effect of surface roughness on the dispersion of Rayleigh waves is of crucial importance for this application and others in which surface roughness is a natural by‐product of the manufacturing processes used to produce the part. In this paper, the dispersion of Rayleigh surface wave on a 2D‐randomly‐rough surface is studied numerically using Wave2000Pro, finite difference wave propagation simulation software. First the dispersion of the wave due to the discretization of the model is investigated to establish the accuracy of the model. Wave 2000Pro is then used to analyze the dispersion of Rayleigh wave propagating on the rough surface. The results of the numerical analysis are then compared to the calculations of Kosachev and Shchegrov.

Transient ultrasonic guided waves in anisotropic bi‐layered plates with finite‐width are investigated in this paper. Composite bi‐layered plates consisting of GaAs substrate coated with Nb sheath is considered as an example because of its application to electronics and calorimetry. The purpose is to investigate the acoustic mode coupling (“pinching”) phenomena for phonon transport. A semi‐analytical finite element (SAFE) method is adopted to study the guided wave dispersion behavior in finite‐width elastic plates. Nine‐noded quadrilateral elements are used to model the cross section of the finite‐width plate. Propagation in the axial direction is modeled by analytical wave functions. Elastodynamic Green’s functions are derived using modal summation in the frequency‐wavenumber and time‐space domains. Results for dispersion and transient analysis of guided waves in finite‐width plates are presented and compared for different aspect ratios. Group velocities are calculated and wave arrival times are computed for different plate cross sections as well as different excitation frequency. Numerical results show significant influence of the plate aspect ratio on the dispersion and transient wave response. Complex nature of quasi‐mode dispersion and propagation due to pinching phenomena in anisotropic plates require such quantitative analysis to afford easy interpretation. These results would be important for nondestructive material evaluation and for characterization of phonon transport in anisotropic bi‐layered plates.

Presented in this paper is a guided wave focusing feasibility study in the thickness direction to precisely tune modes for monitoring layered materials. The in‐plane energy distributions through the thickness of multi‐layered waveguide models are numerically calculated. The experiments were also done with a pulse echo test using an angle beam transducer. The experimental results clearly show that the defect detection in a particular layer of multi‐layered specimens can indeed be established by appropriate manipulation of the phase velocity and frequency values based on the information from the wave structure profiles.

Guided waves are potentially very useful for the rapid NDE of plate structures. In the aerospace industry in particular, they have been proposed for the structural health monitoring of carbon fibre skin panels, in either an active or a passive configuration. In an active configuration a guided wave is deliberately generated and then later received after it has travelled through the structure; in a passive configuration an acoustic emission sensor is used to listen to sound created by the structure itself. The successful development of these ideas depends on a good knowledge of the properties of the guided waves, yet it has been found that the properties of guided waves in such materials are very much more complicated than those in a simple metal skin. The complications include steering of the direction of the group velocity by the anisotropy of the plate, and attenuation because of damping of the matrix material and scattering by the fibres. The authors have studied these phenomena both analytically and experimentally. This paper presents an overview of the findings and identifies the key implications for practical developments of structural health monitoring techniques.

In recent work at Northwestern University, Multiplexed Two‐Wave Mixing Interferometers (MTWM) have been developed. These systems are able to perform optical detection of ultrasonic motion over an array of points simultaneously. Optical phase gratings are used to create a detection‐array of laser beams that are directed to the specimen. The detection array can be arranged in several ways on the test object. The scattered beams from the detection‐array are collected and combined with a single reference beam in a photorefractive crystal to form a multiplexed two‐wave mixing configuration. Each of the output beams from the photorefractive crystal is imaged on to a separate element of a photodetector array. The resulting MTWM system is capable of providing simultaneous optical detection (with high spatial resolution and sub‐nanometer displacement sensitivities) at several points on a test object. The MTWM system can be used in several modes for laser ultrasonic NDE of flaws and materials characterization. In t...

A finite element (FE) technique for computing the properties of guided waves that can exist in an isotropic beam of arbitrary cross section is presented. The FE model uses a two-dimensional mesh to represent a cross section through the beam and cyclic axial symmetry conditions to prescribe the displacement field perpendicular to the mesh. FE results are presented for plate and angle sections. Excitability functions are calculated and implications for transducer placement are considered.

This paper focuses on the improvement of the understanding of guided wave propagation through pipes with bends. A Finite Element modal solution method was used in order to derive the dispersion curves and mode shapes for waves travelling along a curved pipe. The main characteristics of the dispersion curves in toroidal structures have also been highlighted.

The results of an experimental study of wideband sound propagation along shallow water traces in the Barents Sea during the transmission of pulse signals (with frequency band 5–140 Hz) by a pneumoacoustical source towed over range‐varying bottom bathymetry and sediment structure are submitted. Several autonomous bottom stations were used for the reception and the recording of signals. Measurements of energy spatial‐frequency distributions, mode composition, and dispersive characteristics of waveguides at distances of up to 200 km were performed. The spectral density modulation of the received signal energy in the frequency‐range plane essentially differs from interference patterns in analogous layered inhomogeneous waveguides. The regions of variations of the interference patterns and the waveguide mode spectrum depending on the length of the propagation path with various seafloor structure are shown. The frequency‐spatial dependence of transmission losses obtained from the measured data is researched. Th...

We explore the possibility of using the dispersion of Rayleigh waves for nondestructive inspection of the layer of inhomogeneous residual stress induced by low plasticity burnishing (LPB) on Ti-6Al-4V samples, which inherit mirror-smooth surfaces from the LPB treatment. Our findings suggest that, while the acoustoelastic effect is very small in Ti-6Al-4V, the magnitude of the stress gradient involved still leads to a measurable Rayleigh-wave dispersion, from which information on the stress present could be inferred.

Research is being conducted at IC to investigate the characteristics of acoustic wave propagation in buried iron water mains. The hope is that a better understanding will lead to improvements in leak location techniques. This paper describes a novel technique to evaluate in situ acoustic properties of soils that surround a pipe. Predicted wave propagation characteristics incorporating our measured soil properties are presented and compared to experimental results.

The relative generation efficiencies of the A0 and S0 modes in a steel plate excited by a piezoelectric disc bonded to the plate surface is investigated as a function of the thickness of the bond between the disc and the plate. Finite element predictions are confirmed by experimental measurements.

The motivation of this work is to study the group velocity of Lamb waves in anisotropic plates in order to understand the characteristics of the propagation of wave packets in arbitrary directions. The group velocity equations have been derived, taking into account the spatial dispersion (variation of the velocity with the direction of propagation) as well as the frequency dispersion. For bulk waves, it is already well known that anisotropy implies a difference between group velocity and phase velocity. It is shown here that this phenomenon occurs for Lamb waves as well, affecting both the velocity and the direction of propagation. An experimental demonstration is also given, using a plate composed of a unidirectional Glass Epoxy composite. The obtained experimental curves are in good agreement with the theoretical calculations.

A theoretical model for the analysis of ultrasonic guided wave mode excitation of a comb transducer with time delay features was developed. Time delay characteristics are included via a Fourier transform into the frequency domain. The phase velocity spectrum can be used to determine the mode excitation on the phase velocity dispersion curves for a given structure. Experimental and theoretical results demonstrate the tuning of guided wave modes using a time delay comb transducer.

Numerical simulations are presented for laser-generated guided waves in hollow cylinders. The laser pulses are assumed to be radial incident on outer surface of hollow cylinder with axis-symmetry in order to restrain generation of torsional modes. Instead of using classical integral transform, the method of eigen-function expansion is employed, by which time-dependent displacement at the outer surface of hollow cylinders will be expressed by summation of longitudinal and flexural modes. This formula is particularly suitable for analyses of generation efficient of each guided mode.

The motivation of this work is to investigate the simulation of a beam which is bounded both spatially and temporally and to look at the response of the plate either near or far from the specular reflection. Using the model it is demonstrated that the dispersion curves built up from a two dimensional Fourier transform correspond either to the leaky Lamb waves dispersion curves (with real frequency and complex slowness) or to the transient Lamb waves dispersion curves (with complex frequency and real slowness) according to the location of the receiver.

Plate wave motions excited by a point surface load on a finite plate were analyzed using the Mindlin plate theory. Both the flexural and extensional wave motions were considered. To validate the Mindlin plate solutions for finite plates, plate waves in a 3 mm thick aluminum plate were generated by a point load and the wavelet transform method was applied to the displacements of each mode. The Mindlin solution of dispersion curves for both modes agreed very well with the exact 3-D elastic solutions for plates in the frequency range of 0–500 kHz.

An initial study of the interaction of Lamb waves with circular defects in plates is presented. The Lamb wave mode chosen for this paper was the S0 mode in the low frequency-thickness regime. The geometrical properties of this mode allow it to be analyzed using a 2D membrane finite element model which is computationally simpler than the alternative 3D case. A mesh was generated representing an area of the plate with a circular through-thickness hole. The reflection and scattering of an incident S0 wave at different monitoring angles are predicted showing that the scattered field contains the SH0 mode as well as S0. The amplitude of the reflected field is verified experimentally.

In this paper we apply time-varying matched filters to detect and characterize reflections in guided wave inspection data. The results have wide application for detection of defects in objects such as piping, rail, stranded cable, and plate. Due to the dispersive nature of guided waves in these objects, the signature of a reflection is a function of the distance (or time) that the wave travels and the wave mode. The optimal detector for these nonstationary reflections is a time-varying matched filter.

Reflection of the fundamental modes a0 and s0 from the end of a plate is studied, in order to identify the extent to which the generation of non-propagating modes influences the field and could therefore affect Lamb wave NDE of the plate. Analytical predictions, Finite Element simulations and experimental measurements are presented for frequencies below the a1 cut-off. It is shown that reflection of the a0 mode is delayed by 90 degrees in phase and that there is significant additional motion due to non-propagating modes within about 5 plate thicknesses of the end. Reflection of the s0 mode occurs without significant evidence of non-propagating modes or phase delay.

In the past few years, PVDF line-focus acoustic transducers have been proven to be a useful and convenient tool for accurately measuring surface wave velocity. The transducer is very easy to construct and the measurement system can be readily established with conventional ultrasonic instruments. In this investigation, however, the capability of PVDF line-focus transducers will be further extended to the measurement of dispersion relation of surface acoustic waves of a layered medium. To achieve this, a number of line-focus transducers are first fabricated with PVDF films of various thickness so that they can operate at different frequencies. Experimental testing on these transducers shows that surface acoustic waves of frequency ranging from 2 MHz to 20 MHz can be effectively generated and detected. For the determination of surface wave velocity as a function of frequency, a new method of processing the measured waveforms during a z-direction defocusing measurements is developed. A mathematical model is given to explain how this method works. With the transducers and the analyzing method, the surface wave dispersion relation of a layer/substrate configuration have been experimentally determined. Samples include thick polymeric films as well as metal films deposited on glass, aluminum, and silicon crystal. Possibility of determining material properties of the layers from the measured dispersion curves will be discussed.

Shot peening is a classical mechanical surface treatment used to improve the properties of components, principally the fatigue and stress corrosion resistance. The possibility of characterization of shot peening treatment efficacy was assessed by leaky Rayleigh SAW generated and detected by a large aperture, broadband and lens-less focused transducer. The broadband pulses propagated in the material surface were processed in order to obtain the dispersion curves, and were compared for different samples that were submitted to different treatment conditions. The results showed that the dispersion curve is sensitive to these treatment parameters. The possibility of depth profiling of the elastic properties changes due the shot peening are discussed on base of the fact that the penetration of SAW depends on the frequency and so the dispersion curve should have information of the elastic properties in different depths. The source of these elastic changes is also analyzed in terms of plastic deformation, residual stresses, roughness and microstructural changes. A comparison of the dispersion curves with a laser based SAW system was also performed showing equivalence of the methods.

A time-localized frequency (TLF) analysis is employed for the guided wave mode identification and improved guided wave applications. For the analysis of time-localized frequency contents of digitized ultrasonic signals, TLF analysis consists of splitting the time domain signal into overlapping segments, weighting each with the hanning window, and forming the columns of discrete Fourier transforms. The result is presented by a frequency versus time domain diagram showing frequency variation along the signal arrival time. For the demonstration of the utility of TLF analysis, an experimental group velocity dispersion pattern obtained by TLF analysis is compared with the dispersion diagram obtained by theory of elasticity. Sample piping is carbon steel piping that is used for the transportation of natural gas underground. Guided wave propagation characteristic on the piping is considered with TLF analysis and wave structure concepts. TLF analysis is used for the detection of simulated corrosion defects and the assessment of weld joint using ultrasonic guided waves. TLF analysis has revealed that the difficulty of mode identification in multi-mode propagation could be overcome. Group velocity dispersion pattern obtained by TLF analysis agrees well with theoretical results.

Due to their improved mode selectivity and small geometrical size, linear-array transducers have recently found increasing application in guided wave NDT of metallic structures. In this paper, simple and flexible Time-Delay Periodic Linear Array (TDPLA) designs are described and used for guided wave inspection of different metallic structures. Using wave interference effect, unidirectional guided wave excitation and reception are achieved. Several TDPLA patterns are experimentally tested to produce different guided modes. The unidirectional TDPLA transducer has been applied to detect defects (cracks and notches) in metallic structures in a pulse-echo scheme. Experimental results from several specimens are also compared to those obtained from tests using conventional wedge transducers. Compared to bi-directional linear arrays (such as combs and interdigital arrays), which emit guided waves at both sides of arrays, time-delay periodic arrays generate efficiently guided modes with high wave energy concentration at one side and avoid the unnecessary wave emission at the other side.

Case hardening is widely applied to many critical parts in the automotive industry. Determining the hardened case depth is presently a time-consuming destructive process which would greatly benefit from an efficient nondestructive technique. It has been shown that ultrasonic surface-wave measurements are a candidate solution to the problem. However, their implementation and acceptance by industry has not taken place due to poor reproducibility and limited accuracy. We have developed an ultrasonic surface-microscopy technique based on a custom-designed broadband, lensless, line-focus PVDF transducer. This technique can be used to accurately measure leaky surface waves of a solid submerged in water, and has been demonstrated in many materials characterization applications. In this paper we show how the techniques can be used to determine hardened case depth in plasma-nitrided alloy steels. By operating in a time-resolved echo mode with the transducer defocused relative to the sample surface, a set of waveforms are obtained as a function of incremental changes in the defocus distance. The group velocity of the leaky surface wave can be accurately determined from the slope of path length versus echo arrival time. By straightforward manipulation of the data and subsequent signal processing, the phase velocity and other leaky-surface-wave information may be obtained. The hardened case depth can then be evaluated from these results.