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• A baseline-free defect localization method based on the mode conversion and reciprocity principle. • A model function with five parameters to model the conversed wave packets. • Verify the feasibility on aluminum plates via the numerical and experiment methods. Guided wave (GW) plays an important role in nondestructive testing and structural health monitoring. In the general Lamb wave inspection, difference between the baselines and the measured signals are used for structural damage evaluation. However, the baselines are unavoidably influenced by operational and environmental conditions leads to an increased error during inspection. To deal with this issue, this paper presents a base-line free method based on the mode conversion and reciprocity principle of Lamb wave. Once the defect is asymmetrical with respect to the transducer pair (two identical piezoelectric transducers used as actuator and receiver respectively), the elastic wave path length of the converted mode will be different, resulting in failing of the reciprocity principle. By processing the signals received by the transducer pair and extracting the time parameters of the wave packets in mode conversion signals, the defect can be localized. Results from the signals generated by numerical simulation are presented to verify the feasibility of the method, and the effectiveness of the proposed baseline-free method is demonstrated with experimental results. [ABSTRACT FROM AUTHOR]

• A novel tomographic method for corrosion monitoring is proposed based on compressed sensing. • The compressed sensing-based tomographic method is presented to optimize the conventional Lamb wave tomography method. • The novel tomographic method is adopted to reduce the sensor number. • Real corrosion damage test of aluminum structure is performed for validation. Corrosion damage of aircraft structures can significantly reduce the structural performance and endanger flight safety. There is a pressing need for research on aircraft structural corrosion monitoring technology. Lamb wave tomography (LWT) can be used to evaluate structural corrosion. However, the conventional tomographic method needs sensors with dense array, which is not easy to be satisfied in practice and limits its application. Due to the sparsity of corrosion damage in aircraft structures, compressed sensing (CS), which is an emerging signal processing technique, can be employed to optimize LWT. This paper presents a novel CS-based tomographic method to map out the internal situation of aircraft structure. Compared to conventional LWT, the CS-based tomographic method requires fewer sensors to detect the same corrosion damage while the imaging quality still maintains. The experimental study is carried out to diagnose the real corrosion damage by the new approach. Results show the advantages of the proposed CS-based tomographic method. [ABSTRACT FROM AUTHOR]

• Single sided air coupled ultrasonic inspection setup with Lamb waves. • Defect detection and localization by Lamb mode conversion. • Signal analysis for dispersive Lamb waves. • Separation of overlapping Lamb modes and estimation of the dispersion relation. In this paper we demonstrate a non-destructive, non-contact detection method for small defects in thin polymer plates using an air coupled ultrasonic (ACUS) setup. There exist many applications for such methods, e.g. quality control in the manufacturing process or failure prevention by periodical inspections during the lifetime of a product. We demonstrate a setup for the inspection of plates together with signal analysis algorithms to process the measured data, meeting the challenges to handle the dispersive signals and establishing a robust failure criterion. Pressure waves from the transmitter excite different modes of Lamb waves inside the plate. These Lamb waves propagate in the plate and reradiate pressure waves into the air that are then detected by the receiver. Lamb mode conversion is used for defect detection. A numerical model allows the visualization of the propagating waves in the air as well as the Lamb waves inside the plate. Four key parameters of the setup are identified, two angles and two distances. The transmitter and the receiver angles are used to select which Lamb mode (anti-symmetric A 0 or symmetric S 0) is mainly excited and detected, respectively. For the acquisition of the Lamb wave signal the distance from the transmitter to the receiver should be as large as possible but is limited by the attenuation of the signal. Measurements for different values of this distance are essential for signal analysis. The distance between transducer and plate surface should be as small as possible even if it may introduce secondary Lamb waves due to reflections of the pressure wave between transmitter and plate surface. Two algorithms, a model based one and a data driven one, are presented to separate Lamb modes that overlap in time. In these separated signals, the Lamb mode conversion from A 0 to S 0 is shown, allowing a localization of the defect. We conclude that defect detection and localization with Lamb mode conversion is possible with an air coupled ultrasonic setup. Multiple measurements along the propagation direction of the Lamb waves are necessary to allow a thorough signal analysis and visualize the mode conversion. [ABSTRACT FROM AUTHOR]

• Lamb mode amplitude response provides method of source depth characterization. • Mid-plane, quarter-plane, and surface sources are experimentally distinguished. • Approach has applications from geometrically simple to complex plate-like structures. • Algorithm provides fast computation for real-time applications. An inversion algorithm (termed AEDep) is proposed for estimating the depth of acoustic emission (AE) sources in plate-like structural components. The work is motivated by the need for characterizing early-stage fatigue crack growth in such components. The algorithm achieves depth estimation by automatically extracting the depth-dependent amplitude ratio between the fundamental Lamb modes which comprise the AE signals. A finite element model is designed to study the frequency-dependent forward problem of Lamb wave motion due to a given source, from which the relation between source depth and amplitude ratio is established. Elastodynamic theory is used to validate the model in the frequency domain, as well as to derive a sensor tuning factor which may be incorporated into the solution. The proposed algorithm was tested on two plate-like specimens: a 6061-T6 aluminum plate and a 2025-T6 aluminum aircraft fuselage panel. Validation of the algorithm was achieved by generating controlled AE sources at various depths along the edges of the specimens, in the form of Hsu-Nielsen pencil lead breaks. Good agreement was found in the aluminum plate between the true and estimated source depths. A slight decrease in accuracy was found in the fuselage panel between the true values and their estimations. However, both experimental cases demonstrated the ability to distinguish between sources originating near the mid-plane of a plate-like structure from those near the surface. Lastly, the fast computation of the inversion algorithm shows strong potential for real-time monitoring applications. [ABSTRACT FROM AUTHOR]

• A nearly pure SH1 wave in a plate is generated by an MPT. • Two optimally configured annular patches are designed for the MPT. • A high ratio of the SH1 mode to the SH0 mode was achieved. The use of a specific wave mode is critical in ultrasonic non-destructive evaluations but it is difficult to generate a specific mode, especially a higher mode at a frequency where there exist multiple wave modes. Here, we propose a compact omnidirectional shear-horizontal wave MPT (magnetostrictive patch transducer) having two annular magnetostrictive patches for the generation of a nearly pure SH1 (second shear-horizontal) mode in a plate for frequencies above the first cutoff frequency. While a common wavelength-matching approach would typically require the use of several patches and does not appear completely to eliminate unwanted omnidirectional wave modes, the proposed MPT, with only two annular patches, generates the desired SH1 mode predominantly with the unwanted SH0 (first shear-horizontal) mode nearly eliminated. For the design, the geometries of the annular patches are optimally configured to maximize the ratio of the SH1 mode to the SH0 mode. Numerical simulations and experiments confirm the effectiveness of the proposed bi-annular shear-horizontal wave MPT. Because the SH1 mode near the first cutoff frequency is highly dispersive, the developed transducer is expected to be critically useful in various applications, such as ultrasonic inspections of wall thinning. [ABSTRACT FROM AUTHOR]

• "Light refractive vibrometry" was employed for the visualization of GAW. • The propagation of the quasi-Scholte plate mode was observed. • The wavelengths of the QSP mode were determined at different frequencies. • The Lamb angles were determined at different frequencies. • A comparison with numerical simulations has taken place. Guided acoustic waves, such as Lamb waves, are widely applied for material characterization, sensing of liquids and the generation of streaming in liquids. There are numerical simulation tools for the prediction of their propagation near a solid-liquid boundary but a demand for complementary measurement techniques for the validation of the simulation results remains. In this contribution it is demonstrated that light refractive vibrometry is a suitable approach for the visualization of the interaction of guided acoustic waves with liquids. For this purpose Lamb waves were excited by piezoelectric transducers on copper plates partially immersed in water. There the fundamental symmetric and antisymmetric modes are converted to compressional waves and quasi-Scholte plate waves below a frequency-thickness product of 1 MHz mm. From the vibrometry scans the wavelengths, radiation angles and pressure amplitudes of the involved modes could be determined and thus theoretical predictions of the attenuation of the Lamb modes and the energy distribution of quasi-Scholte plate waves between the solid substrate and the liquid environment could be confirmed. [ABSTRACT FROM AUTHOR]

• Detection of evanescent waves is possible when real part of their velocity is more than imaginary one. • Detection of evanescent waves is possible by using set of IDT with various spatial periods. • The frequency range of detectable evanescent waves is more for stronger piezoelectric materials. • Evanescent waves could be used for high sensitive sensors. This paper presents the results of the theoretical and 2D FEM modeling excitation and detection of evanescent acoustic waves in piezoelectric plates. By application of 2D ordinary differential equations derived by Auld, we obtained the dispersion curves of A 1 and SH 1 waves in YX LiNbO 3 and YX KNbO 3 plates in proximity to a zero group velocity point. The branches corresponding to evanescent acoustic waves are distinguished. A frequency range where real part of evanescent wave velocity is more than imaginary one was found. In this region evanescent mode is characterized by opposite directions of the phase and group velocities, i.e. this is backward wave. The theoretical analysis was verified in commercial 2D FEM software COMSOL 5.3. By modeling a set of interdigital transducers placed on the surface of Y-cut LiNbO 3 wafer with various values of the spatial period we found the resonant frequencies corresponding to evanescent A 1 mode. Due to proximity of this wave to a zero group velocity point its properties should be extremely sensitive to change of waveguide quality and ambient air. This is open the possibility to use these waves for development of high sensitive sensors and for nondestructive waveguide analysis. [ABSTRACT FROM AUTHOR]

• Selective generation of a desired Lamb mode by a moving laser has been demonstrated. • Switching between forward and backward symmetric Lamb mode by tuning the speed of the laser source. • Existence criteria for the NGV (and ZGV) for different modes of Lamb waves were considered. Analytical existence conditions for negative group velocity (NGV) of an arbitrary mode Lamb waves in elastic plate was obtained. Experimentally, NGV waves were excited by means of a thermoelastic laser source moving across the sample plate with a controllable velocity. Such source is coupled with modes which phase velocity component parallel to the surface coincides with the source speed. Additional mode selection was performed by choosing a quasiperiodic shape of the laser spot, which specifies the range of the wavenumber k. This technique was successfully applied for generation of backward (NGV) waves. By tuning the speed of the source the propagation direction was switched from forward to reversal. [ABSTRACT FROM AUTHOR]

Abstract: It is well known that harmonic bounded Gaussian beams undergo a transformation into two bounded beams upon reflection on a solid immersed in a liquid. The effect is known as the Schoch effect and can be found at the Rayleigh angle for thick plates and at the different Lamb angles for thin plates. Here, a study is made on the effect of pulsed Gaussian beams reflected on solids. It is found experimentally that the Rayleigh wave phenomenon still generates two reflected bounded beams, whereas Lamb wave phenomena do not generate this effect. This fact may be explained intuitively by realizing that the Rayleigh phenomenon is a coincidental phenomenon that is generated in situ, whereas the Lamb wave phenomenon is a non-coincidental phenomenon that is generated only after incident sound is influenced by both sides of a thin plate. Another explanation is the fact that Rayleigh waves are not dispersive, whereas stimulation and propagation of Lamb waves is frequency dependent. A pulse contains many frequencies and therefore only a fraction of the incident pulse is transformed into a Lamb wave. In this paper, numerical simulations are performed that show that actually the Schoch effect does occur neither for Rayleigh waves, nor for Lamb waves. As a matter of fact, a pulse, incident at the Rayleigh angle, generates two reflected lobes with a null zone of a different kind. The null zone is beating several times during the passage of each pulse. This results in a ‘null zone’ having a lower mean intensity than any of the two lobes, still less outspoken than for the case of harmonic incident bounded beams. This effect does only occur for Rayleigh wave generation and is much less outspoken for Lamb wave generation. [Copyright &y& Elsevier]

Abstract: The crystalline texture of a sheet metal strongly affects its formability, so having knowledge of this texture is of great industrial relevance. The texture of rolled sheet metals, such as aluminium and steel, may be determined by ultrasonic measurement of the velocity of the zero order symmetric (S 0) Lamb wave as a function of angle to the rolling direction. Electromagnetic acoustic transducers (EMATs) may perform this measurement without contacting the sample, therefore reducing perturbation to the plate wave system, as they are electromagnetically coupled to the sheet. The EMAT system measurements are non-destructive and may be made in real time, therefore offering advantages over the conventional techniques such as X-ray and neutron diffraction. It has been noticed that in the two EMAT pitch–catch system, the apparent arrival times of the ultrasonic waves change with variation in lift-off (distance between sample and transducer) due to impedance and aperture effects. For precise and accurate texture parameters to be obtained, accurate absolute ultrasonic velocity measurement is required and hence lift-off must be compensated for. This is of particular importance to online inspection systems where constant lift-off may be difficult to maintain. The impedance behaviour of various coil geometries has been investigated as a function of lift-off and frequency and compared to the received ultrasonic signal and the drive current pulse profile. Theoretical models have been used to explain the observed behaviour, and hence a scheme has been proposed for the compensation of lift-off effects in real time. [Copyright &y& Elsevier]

Abstract: In order to investigate non-destructively the bonding between rough plates, the problem of Lamb waves propagating on a rough plate is addressed in this paper. Numerical analysis is performed on periodical gratings made of identical triangular grooves. If the surface profile is made up of grooves with one periodicity, then a mode conversion is observed. In the wave-number/frequency space, a phonon relation is written between phonons related to the grating and to the incident and reflected-converted modes. If the grooved surface is made up of several spatial periodicities, then the phonon relation is still verified. Signal processing allows us to give an interpretation of the results in the dual space (wave-number/frequency). An experimental study is also performed to corroborate the numerical predictions. [Copyright &y& Elsevier]

Abstract: The laser generation method of focused-Lamb wave is expected to have high defect-detection ability with advantages of non-contact testing. In this method, the laser beam is illuminated on the surface of the object through an arrayed-arc slit, and then the energy of the generated Lamb wave is concentrated on the focus point of arc. This focusing effect enables the concentration of higher wave intensity on the focus with better S/N ratio of signal, and has better spatial resolution compared to the conventional line arrayed method. This paper describes a 2-D imaging system using this laser generated, focused-Lamb wave combined with its detection by the air-coupled transducer. This technique is fully non-contact so it can be easily applied for the automatic inspection. The effectiveness of the proposed method was verified by experiments on a 1-mm thick aluminum plate with artificial drill-hole defect with diameters of 1mm. The 2-D image of was constructed by scanning and the result showed that the location and size of defects were clearly detected. [Copyright &y& Elsevier]

Abstract: On the one hand, it is well known that Gaussian beams posses the ability to stimulate Rayleigh waves, resulting in the Schoch effect, a lateral beam displacement. This effect, often characterized by a reflected sound pattern consisting of two anti-phase beams, is due to the re-radiation of sound because of the stimulation of leaky Rayleigh waves. On the other hand, fringed sound beams are characterized by the fact that they consist of a number of neighboring anti-phase narrow beams. They are a first approximation of a sound field originating from a phased array of harmonic vibrating crystals in which each crystal vibrates in anti-phase compared to its neighbor. The individual lobes within the fringed sound pattern diverge much less than standard Gaussian beams of the same size. The current study investigates the interaction of fringed beams with a liquid–solid interface. It is found that under certain conditions, a fringed beam, incident at the Rayleigh angle, produces a reflected sound pattern that contains a wide lobe that is not fringed. It is also shown that under other conditions, contrary to the famous forward displacement of the reflected sound for incident Gaussian beams, a strong backward displacement occurs for fringed beams. [Copyright &y& Elsevier]

Abstract: A Lamb wave interdigital transducer mounted on a layered substrate composed of two plates, a thin piezoelectric ceramic plate and an acrylic plate, operating at a liquid–solid boundary, is investigated for ultrasonic nondestructive testing of the layer thickness in human teeth. A higher-order mode having a phase velocity higher than the longitudinal wave velocity in the human teeth can be used for nondestructive testing. In the combination of the two layers, the fourth mode of leaky Lamb wave is most favorable for a frequency-controllable radiation angle of an ultrasound beam into a water layer as an acoustic coupler. In the configuration of an acoustic delay line, the layer-thickness measurement in vivo, evaluated from the time interval between two reflected ultrasound echoes, is successfully realized under a thin water layer as the acoustic coupler. [Copyright &y& Elsevier]

Abstract: The materials used for the manufacture of flexible printed circuits are selected according to various characteristics: thermal and electrical behavior, moisture absorption, flexibility… Those are determined by the basic materials of the three components of the circuit, which are the conducting layer, the adhesive layer and the dielectric film. Such circuits have a typical thickness of about 200μm and are therefore an interesting solution for a great number of electronic applications. However, these circuits can present various defects like inclusions, delaminations, cracks… In this work, we are interested in the detection of inclusions using guided waves propagation in such structures. These waves also called Lamb waves have the advantage of propagating over long distances while informing us about the totality of the inspected volume. According to the range of frequencies considered and the method used for their generation, it is possible to make profitable use of different propagation modes. To serve this purpose, laser-induced thermoelastic excitation of the first antisymmetric Lamb waves mode is studied. The results obtained are analysed using signal processing methods and then compared in order to clearly highlight the potentialities of these guided waves for the detection of inclusions in such samples. [Copyright &y& Elsevier]

Abstract: A nonlinear Lamb wave approach is presented for characterizing the surface properties of a solid plate. This characterization approach is useful for some practical situations where ultrasonic transducers cannot touch the surfaces to be inspected, e.g. the inside surfaces of sealed vessels. In this paper, the influences of changes in the surface properties of a solid plate on the effect of second-harmonic generation by Lamb wave propagation were analyzed. A surface coating with the different properties was used to simulate changes in the surface properties of a solid plate. When the areas and thicknesses of coatings on the surface of a given solid plate changed, the amplitude–frequency curves both of the fundamental waves and the second harmonics by Lamb wave propagation were measured under the condition that Lamb waves had a strong nonlinearity. It was found that changes in the surface properties might clearly affect the efficiency of second-harmonic generation by Lamb wave propagation. The Stress Wave Factors (SWFs) in acousto-ultrasonic technique were used for reference, and the definitions of the SWFs of Lamb waves were introduced. The preliminary experimental results showed that the second-harmonic SWF of Lamb wave propagation could effectively be used to characterize changes in the surface properties of the given solid plate. [Copyright &y& Elsevier]

Abstract: A novel combined spectral estimate (CSE) method for differentiation and estimation the phase velocities of multimode Lamb waves whose wave numbers are much close or overlap one another in multiplayer plates is presented in this paper, which based on auto-regressive (AR) model and 2-D FFT. Simulated signals in brass plate were processed by 2-D FFT and CSE. And experiments are performed by using two conventional angle probes as emitter and receiver on the same surface of three-layered aluminum/xpoxy/aluminum plates, which include symmetrical and unsymmetrical plates. The multimode Lamb waves are excited in these laminates, and the received signal is processed by 2-D FFT and CSE, respectively. The results showed that the phase velocities of multimode signals whose wave numbers are much closed cannot be differentiated by 2-D FFT, but CSE has strong spatial resolution. Compared the measured phase velocities with the theoretical values, the error is smaller than 2% on the whole. It promises to be a useful method in experimental signals processing of multimode Lamb waves. [Copyright &y& Elsevier]

Abstract: The objective of the research presented here is the investigation of the interaction of guided waves with welds, defects and other non-uniformities in steel plates loaded by liquid. The investigation has been performed using numerical simulation for 2D and 3D cases by the finite differences method, finite element method and measurement of 3D distributions of acoustic fields. Propagation of the S 0 mode in a steel plate and its interaction with non-uniformities was investigated. It was shown that using the measured leaky wave signals in the water loading of the steel plate and by application of signal processing, the 3D ultrasonic field structure inside and outside of the plate can be reconstructed. The presence of leaky wave signals over the defect caused by the mode conversion of Lamb waves has been proved using the numerical modelling and experimental investigations. The developed signal and data processing enables to visualise dynamics of ultrasonic fields over the plate, and also to estimate spatial positions of defects inside the steel plates. [Copyright &y& Elsevier]

Abstract: In Lamb waves inspection, an air-coupled transmitter transducer is oriented at a specific angle such that it generates a pure Lamb mode which propagates along the structure and interacts with any existing defects. For this inspection system, amplitude losses appears when small inclinations of the tested structure occurs. An important factor which affects directly these losses has been observed, it consists of the Lamb waves beam (LWB) deviation due to this bad alignment. In this work, a simple expression of LWB deviation has been deduced. This expression includes the test structure angle, phase velocity of generated Lamb mode, and the phase velocity of waves propagating in the coupled medium. A 0 Lamb mode is generated and detected in 1mm thick aluminium plate sample using 1MHz PZ27 piezoelectric transducers of 20mm of diameter. Experimental LWB deviation angles are measured for different inclination angles of the test sample. A comparative study is released with theoretical results. For 1° of misalignment in the aluminium plate inclination, and transducers separation distance of 35mm, LWB deviation angle is around 7° and the amplitude is reduced by around 11%. Then, for a large separation distance, we must move the receiver transducer to detect the deviated LWB. It is shown that, for both theoretical and experimental studies, the LWB deviation and its measured amplitude are very sensitive to the alignment of the tested structure with respect to the transmitter–receiver transducers plane. In metal plates it is most satisfactory to use A 0 mode compared with S 0 mode since it is easy to excite and has a large amplitude and small deviation beam angles. [Copyright &y& Elsevier]

Abstract: The dispersive characteristics of higher order mode Lamb waves (HOMLW) excited by interdigital transducers (IDT) are measured and analyzed, which are necessary for designing micro-sensor in ultrahigh frequency (UHF). A measurement system is set up, in which dispersive characteristics of HOMLW are obtained by the method of transform between frequency and time domains. The characteristics of amplitude–frequency and phase–frequency of Lamb wave are auto-measured by the system. By IFFT, the pulse response of the IDT device was obtained. Different modes were separated in time domain and dispersive curve of each mode is calculated by FFT. The best mode is chosen to design the micro-sensor in UHF. The phase velocity of HOMLW is greater than the surface wave (SAW) velocity and an oscillator in higher frequency can be made, so the absolute sensitivity of micro-sensor can be increased. In this paper, the dispersive characteristics of HOMLW excited by an IDT in a 127.86° rotated Y-cut, X propagating lithium niobate plate is analyzed. An oscillator using a 13 mode is made, the phase velocity of which is measured about 19652m/s when h/λ =0.94 (h =plate thickness, λ =wavelength). [Copyright &y& Elsevier]

Abstract: Two methods have been always used to calculate the electromechanical coupling coefficient of a Lamb wave in a multilayered plate: one is an approximate method using the acoustic velocity difference under different electric boundary conditions and the other is the Green’s function method. The Green’s function method is more accurate but more complicated, because an 8N-order matrix is used for calculating the electromechanical coupling coefficient of the Lamb wave in an N-layered plate, which induces great computation loads and some calculation deviations. In this paper, a transfer matrix method is used for calculating the electromechanical coupling coefficient of Lamb waves in a multilayered plate, in which only an 8-order matrix is needed regardless of the number of layers of the plate. The results show that the transfer matrix method can obtain the same accuracy as those by the Green’s function method, but the computation load and deviation are greatly decreased by avoiding the use of a high order matrix used in the Green’s function method. [Copyright &y& Elsevier]

Abstract: Anti-symmetrical Lamb wave mode A0 presents a large sensitivity to mass loading and can be used in contact with liquids with a small attenuation. The advantages of this system are the possibility to get a large mass sensitivity. The sensitivity increases when the thickness of membrane decreases. Therefore the problem is to obtain thin piezoelectric membranes. A membrane of AlN with a thickness of 2μm has been made. The measured mass sensitivity with a fluid is 200cm2 g−1. In a practical use point of view, the problem in this kind of sensor is its temperature sensitivity. In order to reduce effective temperature sensitivity, a device with thin metallic strips is presented. On the same membrane two different waves with perpendicular propagating directions are produced. Experimentally, temperature sensitivity is rather different depending on the propagation direction but mass sensitivity is almost the same, this allows distinguishing temperature effects from those due to mass loading on the frequency shift measurements. [Copyright &y& Elsevier]

Abstract: The characteristics of the three lowest order plate waves (A0, S0, and SH0) propagating in piezoelectric plates whose thickness h is much less than the acoustic wavelength λ are theoretically analyzed. It is found that these waves can provide much higher values of electromechanical coupling coefficient K 2 and lower values of temperature coefficient of delay (TCD) than is possible with surface acoustic waves (SAWs). For example, in 30Y − X lithium niobate, the SH0 mode has K 2 =0.46 and TCD=55ppm/°C. The corresponding values for SAW in the widely used, strong coupling material of 128Y − X lithium niobate are K 2 =0.053 and TCD=75ppm/°C. Another important advantage of plate waves is that, unlike the case of SAWs, they can operate satisfactorily in contact with a liquid medium, thus making possible their use in liquid phase sensors. [Copyright &y& Elsevier]

Abstract: This paper deals with a study of the conversion of a Lamb wave at the loaded edge of a plate. An experimental study is performed to know the interaction of surface waves with a viscoelastic material. A Lamb wave is generated by a wedge. At the end of the plate, this wave gives rise to several reflected waves. A first study makes it possible qualitatively to know the Lamb modes present in reflection. Secondly, conversion phenomena are studied to know if surface waves are sensitive to evolution in the time of the viscoelastic material structure. The selected material is a sol–gel containing silica. [Copyright &y& Elsevier]

Abstract: Air-coupled ultrasonics is fine alternative for the immersion testing technique. Usually a through transmission and a pitch–catch arrangement of ultrasonic transducers are used. The pitch–catch arrangement is very attractive for non-destructive testing and evaluation of materials, because it allows one-side access to the object. However, this technique has several disadvantages. It is sensitive to specularly reflected and edge waves. A spatial resolution depends on a distance between the transducers. A new method for detection and visualisation of inhomogeneities in composite materials using one-side access air-coupled ultrasonic measurement technique is described. Numerical predictions of Lamb wave interaction with a defect in a composite material are carried out and the interaction mechanism is explained. Experimental measurements are carried out with different arrangements of the transducers. The proposed method enables detect delamination and impact type defects in honeycomb materials. [Copyright &y& Elsevier]

Abstract: This research is focused in the measurement and modeling for the dispersion relations of Lamb waves propagating in LiNbO3 and LiTaO3 plates. A theoretical model based on a stiffness matrix method with recursion computation algorism is used to provide numerical calculations for the dispersion relations. Also, a dry, noncontact laser ultrasound technique is used to measured dispersion multi-mode dispersion relations. For all the experiments, the measured dispersion curves show good agreement with the theoretical calculation, indicating the reliabilities in the measurement and modeling. [Copyright &y& Elsevier]

Abstract: We present work on the development of an ultrasonic texture measurement system for sheet metals using non-contact transducers, suitable for use both in the laboratory and on the production line. Variation of the velocity of the zero-order symmetric (S 0) Lamb wave is used to determine the crystallographic texture of polycrystalline metal sheets ranging in thickness from 0.1 to 3mm. This system features improvements on previous state-of-the-art ultrasonic technology in that it probes velocity over a continuous range of angles using only two electromagnetic acoustic transducers (EMATs). This is demonstrated to offer a significant improvement in accuracy and allows the detection and investigation of asymmetric anisotropies in the sheets. Another advantage of the system is its potential for combining several different measurements using a single pair of transducers. The capability is demonstrated for through-thickness shear wave measurements as well as the zero-order symmetric Lamb wave measurements which are the primary means of determining the texture. The change between generating Lamb and through-thickness bulk waves can be made entirely by changing the electrical circuit connected to the EMATs without modifying the transducer assembly in any way. Measurement of all of the above waves can provide information on the sheet thickness and other physical properties of the sheet in addition to texture. Certain texture parameters can be calculated from both Lamb and shear wave velocities, allowing self-calibration of the system. [Copyright &y& Elsevier]