Showing papers on "Lamb waves published in 1993"
TL;DR: In this paper, the S0 Lamb mode can propagate over distances of the order of 1 m in composite laminates and so has the potential to be used in long-range nondestructive inspection.
Abstract: The S0 Lamb mode can propagate over distances of the order of 1 m in composite laminates and so has the potential to be used in long‐range nondestructive inspection. This paper discusses the interaction of the S0 Lamb mode with delaminations. The dispersion curves and the corresponding stress and displacement mode shapes of the lower order Lamb modes are obtained analytically and the interaction of the S0 mode with delaminations at different interfaces in a composite laminate is then studied both by finite element analysis and by experiment. It is shown that the amplitude of the reflection of the S0 mode from a delamination is strongly dependent on the position of the delamination through the thickness of the laminate and that the delamination locations corresponding to the maximum and minimum reflectivity correspond to the locations of maximum and minimum shear stress across the interface in the S0 mode.
367 citations
TL;DR: In this paper, the authors studied the propagation of non-plane acoustic waves in homogeneous and inhomogeneous media and found that their phase velocity depends not only on the medium but also on the local geometry of the wavefield.
Abstract: SUMMARY
As a model for the 2-D horizontal propagation of seismic surface waves, we study the propagation of non-plane acoustic waves in homogeneous and inhomogeneous media. We find that their phase velocity depends not only on the medium but also on the local geometry of the wavefield, especially on the distribution of amplitudes around the point of observation. the phase velocity of a wave is therefore conceptually and in most cases numerically different from the phase velocity parameter in the wave equation, which is determined by the elastic properties of the medium. the same distinction must be made for seismic surface waves. Although it is a common observation that waves of the same period can propagate with different phase velocities over the same path, the fundamental character of this observation has apparently not been recognized, and the two phase velocities are frequently confused in the seismological literature. We derive a local relationship between the two phase velocities that permits a correct structural interpretation of acoustic waves in inhomogeneous media, and also of non-plane seismic surface waves in laterally homogeneous parts of the medium.
111 citations
TL;DR: In this paper, the Lamb wave experiments were performed in thin aluminium sheets using a pulsed laser source and an electromagnetic acoustic transducer (EMAT) detector, which can be used for tomographic imaging of changes in the structure of thin sheet material.
80 citations
TL;DR: In this article, it was shown that the coupling causes a repulsion of the dispersion curves, similar to that encountered in atomic physics for the energy levels of atoms combining into molecules, which prevents their intersection and at the same time exchanges the nature of the underlying vibrations.
Abstract: The Lamb waves propagating in an elastic plate in vacuo generate compressional (L) and shear type (T) plate vibrations that are coupled due to the boundary conditions. Without such coupling, their phase‐velocity dispersion curves would form two intersecting families, which at high frequency tend towards the elastic‐wave speeds CL and CT, respectively. It is shown that the coupling causes a repulsion of the dispersion curves, similar to that encountered in atomic physics for the energy levels of atoms combining into molecules, which prevents their intersection and at the same time exchanges the nature (L↔T) of the underlying vibrations. However, in the repulsion regions a succession of dispersion curves combines to asymptotically approach the uncoupled L or T dispersion curves, respectively. For the case of a plate bounded by fluid on one side, and vacuum on the other, the dispersion curves of the fluid‐borne (Stoneley–Scholte type) wave, which is known from the studies of Grabovska and Talmant to be prese...
62 citations
TL;DR: In this paper, an exact solution for diffraction of short-crested waves incident on a circular cylinder is presented, and the wave load increases as the wave number in the direction perpendicular to the direction of the wave propagation increases, or as the incident waves become shorter.
60 citations
TL;DR: In this paper, a technique which enables a dispersive Lamb wave to be propagated over a considerable distance to give a simple waveform at the receiver position is described, which works by launching a signal which, by superposition of its frequency components, will recombine to form a signal with a simple shape (a pulse or tone burst) at the measurement position.
56 citations
TL;DR: In this article, a three-layered laminated composite plate with different anomaly content is studied, focusing on the detection of interlaminar bond layer imperfections and cracks normal to the interfaces.
Abstract: Overall philosophy of composite material inspection is discussed with a sample problem presentation to illustrate the utility of guided wave analysis and the use of field distributions across the thickness of a specimen to predict its sensitivity to specific anomalies in the material. Guided Lamb-type waves in a three-layered laminated compos ite plate with different anomaly content is studied in this article. Our work is focused pri marily on the detection of interlaminar bond layer imperfections and cracks normal to the interfaces. A Lamb wave mode selection criteria is discussed with respect to defect sen sitivity analysis. Selection of appropriate modes and frequencies is suggested by an ap proach based on an analysis of the modal displacement and power distribution across the multilayered structure as opposed to the previously utilized approach based on a numerical analysis of the dispersion relations for different boundary conditions and effective material properties.
55 citations
31 Oct 1993
TL;DR: In this article, the authors focused on the study of a pressure sensor using an AlN film over a silicon membrane, which was micromachined using both anisotropic and isotropic chemical etchings.
Abstract: Among sensors using a thin membrane covered with a piezoelectric layer, we focused on the study of a pressure sensor using an AlN film over a silicon membrane The membrane was micromachined using both anisotropic and isotropic chemical etchings The AlN thin film was deposited by a sputtering method with very good stochiometric results Elastic waves were generated and detected through IDT transducers calculated at a frequency of 88 MHz for the SO Lamb wave mode We studied the velocity and the electromechanical coupling factor of the first symmetric and antisymmetric Lamb modes according to the thickness of the piezoelectric layer The pressure and the temperature behavior of such a sensor using the SO Lamb mode are presented We have analysed the pressure sensitivity and we propose some solutions to obtain a well temperature compensated structure Especially, we were thinking to achieve a self-compensated structure by using the opposite temperature behavior of silicon and AlN
54 citations
TL;DR: In this article, the Lamb wave propagation in composite laminates both with and without defects is investigated both numerically and experimentally, and the factors which lead to the poor reproducibility of acousto-ultrasonic results are discussed and possible improvements are proposed.
Abstract: The acousto-ultrasonic technique has many potential applications in the NDE of composite materials. However, problems of poor reproducibility and of the sensitivity of the results to precise instrument settings have restricted its application in industry The waves employed in acousto-ultrasonics are chiefly Lamb waves which propagate in the plane of the laminate. In this paper, Lamb wave propagation in composite laminates both with and without defects is investigated both numerically and experimentally. Acousto-ultrasonic parameters based on both predicted and measured responses are calculated, and tests are carried out using a commercially available instrument. The factors which lead to the poor reproducibility of acousto-ultrasonic results are discussed and possible improvements to the technique are proposed.
44 citations
TL;DR: In this article, a quantitative optical beam deflection (OBD) technique was used to measure the Lamb wave in terms of the total light intensity falling on the photodetector, assumed to have a square-law voltage response.
TL;DR: In this paper, the vanishing of the normal component of the particle displacement vector on the free surfaces of an isotropic, homogeneous plate for nonzero-order symmetric Lamb waves is given.
Abstract: Proof of the vanishing of the normal component of the particle displacement vector on the free surfaces of an isotropic, homogeneous plate for nonzero‐order symmetric Lamb waves is given. Such a conclusion was stated without proof in the book by Viktorov [A. I. Viktorov, Rayleigh and Lamb Waves: Physical Theory and Applications (Plenum, New York, 1967), 1st ed., Chap. II, p. 121]. An expression for the frequency‐thickness products where this occurs is also given. This enables practical (e.g., experimental) selection of such modes. Other interesting features of such modes, such as the independence of their group velocity on the mode’s order or its nondispersitivity, are also addressed. The features discussed here are of great practical importance, e.g., in the nondestructive testing (NDT) of plates loaded by liquids and/or in the ultrasonic tensometry of a thin plate.
TL;DR: In this article, the integration of laser and fiber optic methods for generating and detecting broadband, transient Lamb waves in a steel plate is described, and the generated Lamb waves are shown to be a superposition of different Lamb modes including the zeroth-order symmetric together with the first-order antisymmetric modes.
Abstract: This Letter describes the integration of laser and fiber optic methods for generating and detecting broadband, transient Lamb waves in a steel plate. The generated Lamb waves are shown to be a superposition of different Lamb modes including the zeroth‐order symmetric together with the zeroth‐ and first‐order antisymmetric modes. Signal processing techniques are used to extract the phase velocities for each of these modes over a continuous, broadband frequency spectrum from the transient time histories of the waves recorded at two locations on a steel plate. The results are found to be in excellent agreement with theoretical phase velocities for each of the three modes evaluated.
TL;DR: In this article, an electromechanical coupling constant is defined on the basis of group velocities of the acoustic waves, and the material properties of the plate change gradually with the thickness.
Abstract: The material properties of the plate change gradually with the thickness. To evaluate the properties of the energy transfer between the electrical and mechanical energies during acoustic wave propagation in dispersive media, an electromechanical coupling constant is defined on the basis of group velocities of the acoustic waves
TL;DR: In this paper, it was shown that finite-amplitude ion-acoustic waves in a collision-dominated plasma are described by a Korteweg-de Vries-Burger equation, which has the same form, but with very different scaling and parameter dependence from that describing ionacoustic wave in a weakly collisional plasma.
Abstract: It is shown that finite-amplitude ion-acoustic waves in a collision-dominated plasma are described by a Korteweg--de Vries--Burger equation, which has the same form, but with very different scaling and parameter dependence from that describing ion-acoustic waves in a weakly collisional plasma.
TL;DR: In this article, the dispersion of Lamb waves in solid bilayers obtained by coating a 0.61mm-thick aluminum plate with a polymer of thickness between 180 and 500 μm was analyzed.
Abstract: This paper deals with the dispersion of Lamb waves in solid bilayers obtained by coating a 0.61‐mm‐thick aluminum plate with a polymer of thickness between 180 and 500 μm. The theoretical dispersion curves of the bilayers in air are analyzed in terms of the dispersion in the decoupled layers, the aluminum substratum, and the polymer coating, respectively. Because of the smaller acoustic impedance of the polymer coating, it behaves in the bilayer as if it were rigidly held by the aluminum substratum. The side of the polymer facing air is considered to be stress‐free. The decoupled aluminum plate is modeled as if it were stress‐free on both sides. The approximate boundary conditions for the coating are verified by a separate experiment. The experimental data obtained by means of a double transmission immersion technique, confirm the numerical analysis. Not all theoretically predicted modes are observed. The results show how coupling between the Lamb modes of the aluminum substratum, and those of the polymer coating, makes the bilayer switch between modes. Displacement and stress profiles are used to further elucidate the analysis. In the last paragraph a simple mechanical model composed of two masses and three springs, is analyzed. It exhibits mode coupling and mode switching in a way which is very similar to that found for these solid bilayers.
TL;DR: In this article, a new way of reconstructing wave fronts was developed for testing the validity of the great circle approxima- tion in dispersion analysis of surface waves, where phase and time delays between pairs of stations of an array were used to re- construct the wave fronts or surfaces of equal phase.
Abstract: A new way of reconstructing wave fronts is de- veloped for testing the validity of the great circle approxima- tion in dispersion analysis of surface waves. Phase and time delays between pairs of stations of an array are used to re- construct the wave fronts or surfaces of equal phase. This techque is applied to fundamental mode Rayleigh waves recorded at the NARS stations dudlag the ILIHA (Iberian Lithosphere Heterogeneity and Anisotropy) project. The re- sulting wave fronts are compared with the theoretical ones, i.e. calculated assuming that the waves have traveled along the great circle path with a constant phase velocity, and the angle between both fronts is calculated. Waves traveling mosfiy through a laterally homogeneous oceanic path before arriving to the Iberian Peninsula are analyzed for the fre- quency range of 10 to 60 mHz. These show angles up to 8 degrees between the theoretical wave fronts and the wave fronts reconstructed using the delay data. This implies that we make a relative error less than 1% when calculating path- averaged phase velocities ignoring the deviation of the ar- rival of the wave from the great circle direction.
TL;DR: In this article, a new method for the complete characterization of an elastic plate in terms of resonances and, on the other hand, of Lamb wave propagation is proposed, which is based upon the study of the derivatives of the reflection coefficient phase, considered either as a function of a frequency variable or an angular variable.
Abstract: This article deals with the introduction of a new method which allows the complete characterization of an elastic plate, on the one hand, in terms of resonances and, on the other hand, in terms of Lamb wave propagation. It is based upon the study of the derivatives of the reflection coefficient phase, considered either as a function of a frequency variable or as a function of an angular variable. When the Breit–Wigner approximation is valid, i.e., when the poles of the reflection coefficient are close to the real axis, the comparison with the resonant scattering theory (RST) leads to the characterization of frequency or angular resonances in terms of positions and widths, by means of the location and of the magnitude of the phase derivative peaks. Moreover, this method allows one to establish the important link between this resonant theory derived from the RST and the normal mode theory of propagation. So, the width of a resonance is explicitly related to the imaginary part of the wave vector of the associated Lamb wave.
TL;DR: A numerical inversion procedure based on the simplex algorithm was implemented and tested for the dispersion relation of generalized Lamb waves propagating in a thin film on a semi-infinite substrate.
Abstract: A numerical inversion procedure based on the simplex algorithm was implemented and tested for the dispersion relation of generalized Lamb waves propagating in a thin film on a semi‐infinite substrate. This procedure allows for the determination of up to three parameters of the thin film (transverse elastic wave velocity, density, and thickness). Continuous wave scanning acoustic microscopy was used to measure the dispersion curves of surface waves in a gold film sputtered on a fused silica substrate. The inversion of the data yielded the thickness of the gold film, a Young’s modulus of 80.55 GPa and a Poisson coefficient of 0.418.
TL;DR: New contactless techniques using laser-generated ultrasound have been applied to the inspection of composite materials and it is shown with this point-source measurement technique that the wavefront arrival times agree with the energy velocities rather than with the phase Velocities.
Abstract: New contactless techniques using laser-generated ultrasound have been applied to the inspection of composite materials. Transient elastic waves were launched thermoelastically in half cylindrical composite samples using a long pulse dye laser or a Q-switched Nd-YAG laser. The waves were detected with piezoelectric transducers or with an optical heterodyne interferometer. The measurements have been carried out on two different lay up design composites: carbon/epoxy: undirectional and cross-ply 0 degrees /45 degrees /90 degrees /-45 degrees . Quasi-longitudinal, quasi-shear, and shear bulk waves and head waves are clearly discerned in the stacking of a large number of waveforms. Velocities of the different types of waves simultaneously generated are compared to the phase and group velocities computed using Christoffel equations and an hexagonal model. It is shown with this point-source measurement technique that the wavefront arrival times agree with the energy velocities rather than with the phase velocities. A pronounced anisotropy is observed in the amplitudes of the wave arrivals. Angular directivity patterns of quasi-longitudinal, quasi-shear, and transverse bulk waves are plotted. >
TL;DR: In this paper, the eigenfunction expansions method is used to study the incidence of monochromatic waves in coastal areas and it is shown that maximum short wave reflection is associated with maximum long wave transmission.
TL;DR: In this paper, it was shown that the crossing points are points where the dispersion curves are discontinuous so that no distinct Lamb mode exists for these particular velocity-fd combinations, and procedures were given to predict how many such points exist and where they are located in a given range of fd.
Abstract: Standard dispersion curves relating the phase velocity of Lamb waves to the frequency‐plate thickness parameter fd indicate that in some cases symmetrical and antisymmetrical mode velocity curves cross each other. Viktorov’s equations are used to show that the crossing points are points where the dispersion curves are discontinuous so that no distinct Lamb mode exists for these particular velocity‐fd combinations. Procedures are given to predict how many such points exist and where they are located in a given range of fd.
TL;DR: In this article, a decoupling method for the efficient inversion of the measured LLW data to determine the unknown elastic wave speeds of coatings is presented, based on the effects of the elastic wave speed in the substrate and coating on the frequency difference between adjacent modes known as the modal frequency spacing (MFS) at high frequency.
Abstract: This study applies the leaky Lamb wave (LLW) technique to the quantitative characterization of coatings. A new decoupling method for the efficient inversion of the measured LLW data to determine the unknown elastic wave speeds of coatings is presented. The method is based on the effects of the elastic wave speeds in the substrate and coating on the frequency difference between adjacent modes known as the modal frequency spacing (MFS) at high frequency. LLW measurements were carried out on titanium and aluminum samples with 0.2–0.3 mm plasma‐sprayed coating. The measured data were analyzed and inverted with the help of the proposed MFS method and excellent agreement between measurements and predictions of coating is observed. The LLW technique is shown to be potentially viable for real‐time inspection of coating quality.
TL;DR: In this paper, the authors evaluated the performance of three shear-horizontal (SH) wave devices in terms of the mass-to-frequency sensitivity of each sensor, including the bulk acoustic wave (BAW) device, the acoustic plate mode (PM) device and the Love waveguide device.
Abstract: Acoustic sensors based on shear-horizontal (SH) waves have become very popular in recent years due to their ability to operate in liquid media. SH waves can propagate in the presence of aqueous solutions with minimum losses, since they do not possess a vertically polarized displacement component. Three acoustic geometries based on pure SH waves have appeared in the literature: the bulk acoustic wave (BAW) device, the acoustic plate mode @PM) device and the Love waveguide device [l] (Fig. 1). The BAW device utilizes a crystal resonator to generate shear acoustic waves, confined between the vibrating surfaces in the bulk of the crystal. APMs and Love waves are surface-generated bulk acoustic waves, since they utilize a surface acoustic wave (SAW) device to excite mainly SH bulk waves. APMs exist in thin piezoelectric plates and energy is confined mainly in the bulk of the plate as the wave propagates through multiple reflections, while generating a displacement at the upper and lower surface. The Love plate sensor is a waveguide structure where the SH wave is confined within an elastic layer deposited on a SAW substrate that supports SH waves. Due to the excitation and propagation differences of the above SH waves, the sensitivity of each device to mass deposition is expected to vary considerably, making some geometries more attractive than others for sensing applications. In this paper the performance of the three SH wave devices is evaluated in terms of the mass-tofrequency sensitivity of each sensor. Calculations are
Journal Article•
TL;DR: In this paper, a plane strain finite element model has been developed to study the elastic wave propagation and scattering in general anisotropic media, and the interaction of pulsed compression waves and Rayleigh waves with cracks in isotropic material has been studied.
Abstract: Defects such as cracks and inclusions act as sources of wave scattering when illuminated by an incident ultrasonic pulse through reflection, diffraction, and mode conversion. Interaction of elastic waves with cracks provides all the necessary information regarding the inverse characterization of the defects which has not yet been thoroughly solved. In the present work, a plane strain finite-element model has been developed to study the elastic wave propagation and scattering in general anisotropic media. The interaction of pulsed compression waves and Rayleigh waves with cracks in isotropic material has been studied. The wave interaction with an interface crack in a layered graphite/epoxy composite structure has been investigated. It predicts the mode-conversion and diffraction of waves by cracks. It is found that for quasi-longitudinal waves the displacement along the fiber is prominent, whereas for quasi-transverse, that displacement perpendicular to the fiber is prominent.
31 Oct 1993
TL;DR: In this article, amplitude and time-of-flight measurements using Lamb waves were performed on laboratory-fabricated aluminum specimens consisting of: single and double layers of various thickness, 5% to 22% of machine-milled and chemically-induced material loss.
Abstract: Corrosion is one of the major causes for the structural failure of an airframe. Assessment of corrosion in layered aircraft structures is thus an important issue in nondestructive evaluation for the airworthiness of aircraft. Advantage can be taken of the thickness-dependent propagation properties of Lamb waves to detect and characterize material thinning due to corrosion. Amplitude and time-of-flight measurements using Lamb waves were performed on laboratory-fabricated aluminum specimens consisting of: single and double layers of various thickness, 5% to 22% of machine-milled and chemically-induced material loss. Analysis and comparison of the results of these tests have led to further understanding of the wave propagation behavior in corroded areas and allow the estimation of the percentage of corrosion-induced material loss
02 Jun 1993
TL;DR: In this article, the influence of external static forces on SAW (surface acoustic wave) velocity is computed for circular plates (membranes) and the pressure sensitivity of the SAW velocity as a function of /spl alpha/quartz crystal anisotropy is studied by sensitivity contour mapping from two independent variables: direction of SAW propagation and cut angles of/spl alpha/-quartz.
Abstract: The influence of external static forces on SAW (surface acoustic wave) velocity is computed for circular plates (membranes). The pressure sensitivity of the SAW velocity as a function of /spl alpha/-quartz crystal anisotropy is studied by sensitivity contour mapping from two independent variables: direction of SAW propagation and cut angles of /spl alpha/-quartz. Theoretical sensitivity values are compared with experimental ones for circular (membrane) plates, and good agreement is found. >
TL;DR: In this article, a fully automatic ultrasonic instrument to measure texture and formability parameters on metal sheet is described, where an array of EMAT transducers are used to transmit and receive S o Lamb waves propagating at 0°, 45°, and 90° with respect to the rolling direction.
Abstract: A fully automatic, ultrasonic instrument to measure texture and formability parameters on metal sheet is described. Arrays of EMAT transducers are used to transmit and receiveS o Lamb waves propagating at 0°, 45°, and 90° with respect to the rolling direction. By analyzing the frequency dependence of the phase of the received signals, the long wavelength limit of the velocities is obtained. Included is a discussion of this algorithm, and subsequent processing steps to predict the ODC'sW400,W420, andW440. On steel, the prediction of drawability parametersr and Δr based on a correlation developed previously by Mould and Johnson is also discussed. Results of blind field trials at facilities of three suppliers/users of steel sheet for automotive applications and one supplier of aluminum sheet for beverage can production are reported. The former confirmed the Mould-Johnson correlation for lowr material but indicated that refinements are needed for modern steels with highr. The aluminum data suggest a correlation between W440 and the degree of four-fold earing.
TL;DR: In this article, the authors apply the Wigner distribution function to the mean square temporal spread (as a function of frequency) of the impulse response of a scatterer in water.
Abstract: The time‐frequency analysis of the transient response of elastic objects has been asserted to be useful for relating features of the scattering to physical parameters of the scatterer. [N. Yen, L. R. Dragonette, and S. K. Numrich, J. Acoust. Soc. Am. 87, 2359–2370 (1990)]. In this article, the Wigner distribution function is applied to the mean‐square temporal spread (as a function of frequency) of the impulse response of a scatterer. The application is illustrated for a thin stainless steel shell in water. The emphasis is on resonance frequencies, where the temporal spread can be large, and locally the frequency response is taken to have a Breit–Wigner form. That approximation yields a simple relationship between the measure of the temporal spread, Q(ka), and the width of the resonance when evaluated at the resonance frequency. The frequency dependence is expressed in terms of the wave number–radius product ka for the shell. The analysis is supported by calculation of resonance widths from ray theory and by ray approximations of form function contributions that dominate Q(ka) near certain resonances.