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• A symplectic analytical wave propagation model is proposed for the forced vibration of ABH indentation plate. • ABH indentation can significantly reduce vibration of plates in the whole frequency range. • Embedding plates with multiple ABH indentations can increase stiffness without losing the ABH effect. • The ABH indentation plate with reinforcing stiffeners is proposed to alleviate the stiffness loss. • The quantity of stiffener has significant impact on the ABH effect. In recent years, acoustic black holes (ABHs) have been revealed as an effective bending wave control technology and widely used to regulate the vibration and noise reduction performance of plate structure. However, the isolation effect of most traditional ABH plate can only be aroused at frequencies above the cut-off frequency. In order to widen the frequency range of the ABH effect, this paper embedded single and multiple ABH indentations at the end of plate. For the forced vibration response of the ABH indentation plate, an analytical wave propagation model was proposed based on the symplectic method. The governing equations of bending vibration of thin plate was first introduced into the symplectic dual system, then the symplectic analytical solution of wave propagation parameters and waveforms were obtained. A Gaussian-discretization scheme was used to discretize the ABH indentation, whereas the proposed method was used to evaluate the wave amplitude. The vibration response was finally obtained using the superposition principle and wave propagation, reflection, and transmission relationships. The effectiveness of the proposed method was verified using the finite element results. The results showed that the ABH indentation plate, especially the plate with multiple ABH indentations, had significant vibration reduction performance. Considering that embedding ABH indentation can weaken the structural rigidity of the plate, the ABH indentation plate with reinforcing stiffeners was then proposed to alleviate this limitation, and the influence of the width and quantity of reinforcing stiffeners on the vibration response was studied. The results indicated that the increase of width will to some extent weaken the ABH effect, and the quantity of stiffeners had a significant impact on the ABH effect. [ABSTRACT FROM AUTHOR]

Abstract In this study the nonlocal bond-based peridynamic (PD) theory was applied to simulate Lamb wave transmission in 2D bimaterial plates. Plates of unlike materials were jointed end-to-end to make a bimaterial plate. The surface of the plate was then excited tangentially by single-frequency and multi-frequency force signals. The influence of changing material properties on travelling of the symmetric and antisymmetric Lamb waves were studied in detail. The phase and group velocities of travelling wave in bimaterial plates were calculated and it was found that dissimilar material properties of two joint plates can significantly alter the amplitude and arrival time of the wave. Moreover, Fast Fourier Transform (FFT) of the time history of symmetric Lamb wave velocity was calculated and verified. Accuracy and consistency of PD wave model was confirmed entirely using Spectral Finite Element Method (SFEM). The comparison between the results of two applied methods shows a good agreement. [ABSTRACT FROM AUTHOR]

Abstract Scanning laser Doppler vibrometry (SLDV) is a promising method for visualizing defects in plate-like structures. The Lamb wave-based SLDV method using continuous excitations has been recently developed in nondestructive evaluation (NDE). In addition, various signal processing techniques have been developed to enhance damage detection capability for the method. To visualize defects, the A 0 mode is commonly used because of its sensitivity to the thickness variations, i.e. the wave speed changes according to the thickness of a plate. The A 0 mode is easily generated by lead zirconate titanate (PZT) transducers. However, for thick plates, the A 0 mode is found to be less sensitive to thickness variations compared to thin structures. To address this issue, a mode-tuned inter-digital transducer (IDT) is adopted to efficiently generate and utilize the symmetric modes for damage detection and visualization of thick plates. Based on the wavelength of the S 0 mode using the dispersion curve of the plate, the IDT is designed and fabricated for efficient symmetric mode generation. To evaluate the capability of the mode-tuned IDT-based SLDV, a 6-mm-thick plate (40 mm × 40 mm) with defects of 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm depths is used. The S 0 mode is selected for its wavenumber sensitivity and the degree of separation between modes. It is demonstrated that the detectability of shallow defects in a thick plate is improved compared to that when using the A 0 mode. [ABSTRACT FROM AUTHOR]

The present study focuses on critical analysis of various aspects related to the evaluation of amplitude based material nonlinearity parameter γ amp through the numerical simulations and theoretical justifications. Two different materials with distinct nonlinearities are interrogated using Lamb waves in the numerical simulations. The effects of input force amplitude, number of cycles in the tone burst, tone burst windowing functions, material model parameters, excitation frequency, and wave propagation distance on γ amp are studied in detail. Also, γ amp estimated for the data obtained from the numerical simulations considering S 0 - S 0 mode pair at low frequencies and resonant S 1 - S 2 mode pair at higher frequencies are compared with the physics based material nonlinearity parameter γ phy . The difference between the results is marginal. Thus, the nonlinear Lamb wave technique can be effectively used even at low frequencies to estimate the actual material nonlinearity of the (metallic) plate materials with fair accuracy using the proposed nonlinearity models. [ABSTRACT FROM AUTHOR]

• The mechanism of the action of corrugated plates on solitary waves was studied. • A predicted model for the film thickness under turbulent conditions is proposed. • The prediction model considers wall structural parameters and Re l. The effect of wall distortion of the scrubbing-cooling tube on turbulent falling film is significant. The wall distortion was simplified by using corrugated plates; the effect of structural parameters (amplitude and wavelength) of corrugated plates on the action mechanism of solitary waves and the axial distribution of liquid film thickness were investigated. The results show that the Coanda effect decelerates and accelerates the flowing liquid film attached to the wall, which leads to the generation and extinction of solitary waves on the liquid film. The intensification of the local perturbation easily promotes the solitary waves to form slender water columns. Under the effect of Plateau-Rayleigh instability, the tails of the slender water columns break up to form tiny droplets. The mean liquid film thickness prediction model can accurately predict the mean liquid film thickness on corrugated plates (amplitude/wavelength = 0.02 ∼ 0.08) and the relative errors within ± 15%. [ABSTRACT FROM AUTHOR]

This paper describes a completely non-contact ultrasonic method of gas flow metering using air-coupled leaky Lamb waves. To show proof of principle, a simplified representation of gas flow in a duct, comprising two separated thin isotropic plates with a gas flowing between them, has been modelled and investigated experimentally. An airborne compression wave emitted from an air-coupled capacitive ultrasonic transducer excited a leaky Lamb wave in the first plate in a non-contact manner. The leakage of this Lamb wave crossed the gas flow at an angle between the two plates as a compression wave, and excited a leaky Lamb wave in the second plate. An air-coupled capacitive ultrasonic transducer on the opposite side of this second plate then detected the airborne compression wave leakage from the second Lamb wave. As the gas flow shifted the wave field between the two plates, the point of Lamb wave excitation in the second plate was displaced in proportion to the gas flow rate. Two such measurements, in opposite directions, formed a completely non-contact contra-propagating Lamb wave flow meter, allowing measurement of the flow velocity between the plates. A COMSOL Multiphysics® model was used to visualize the wave fields, and accurately predicted the time differences that were then measured experimentally. Experiments using different Lamb wave frequencies and plate materials were also similarly verified. This entirely non-contact airborne approach to Lamb wave flow metering could be applied in place of clamp-on techniques in thin-walled ducts or pipes. [ABSTRACT FROM AUTHOR]

This paper presents a multipath exploitation approach for defect imaging in thin-walled structures using Lamb waves under the sparse reconstruction framework. An image of the defects in the region of interest is obtained by inverting a multipath propagation model via group sparse reconstruction. This permits accurate imaging of regions close to the structure boundary without the introduction of ‘false’ defects or ghosts. Real-data measurements of defects close to the edge of an aluminum plate are used to demonstrate the effectiveness of the multipath exploitation scheme. [ABSTRACT FROM AUTHOR]

The propagation of plane-strain harmonic waves in multilayered plates is studied using a multiscale approach which couples an equivalent first-order single-layer theory and a discrete-layer cohesive-interface model using a homogenization technique to enforce continuity conditions on tractions at the layer interfaces. The model captures the effects of the inhomogeneous material structure and the presence of interfacial imperfections on local fields and global behavior through homogenized equilibrium equations which depend on the kinematic variables of the equivalent single-layer theory only. The equations are tractable and frequency equations and dispersion curves are derived in closed-form for plates with an arbitrary number of layers, interfacial imperfections and layup anisotropy. Among the advantages of the multiscale treatment over classical structural approaches: prediction of lowest modes is accurate over a larger range of frequencies and material inhomogeneities; the correction factors in the low-frequency regime are independent of the layup and accurate explicit expressions are derived for the first two cut-off frequencies; zig-zag displacement fields and jumps at the imperfect interfaces are reproduced; changes in modes of propagation and reduction of the cut-off frequencies due to softer layers, adhesives or delaminations are captured. [ABSTRACT FROM AUTHOR]

Since the identification of micro-cracks in engineering materials is very valuable in understanding the initial and slight changes in mechanical properties of materials under complex working environments, numerical simulations on the propagation of the low frequency S 0 Lamb wave in thin plates with randomly distributed micro-cracks were performed to study the behavior of nonlinear Lamb waves. The results showed that while the influence of the randomly distributed micro-cracks on the phase velocity of the low frequency S 0 fundamental waves could be neglected, significant ultrasonic nonlinear effects caused by the randomly distributed micro-cracks was discovered, which mainly presented as a second harmonic generation. By using a Monte Carlo simulation method, we found that the acoustic nonlinear parameter increased linearly with the micro-crack density and the size of micro-crack zone, and it was also related to the excitation frequency and friction coefficient of the micro-crack surfaces. In addition, it was found that the nonlinear effect of waves reflected by the micro-cracks was more noticeable than that of the transmitted waves. This study theoretically reveals that the low frequency S 0 mode of Lamb waves can be used as the fundamental waves to quantitatively identify micro-cracks in thin plates. [ABSTRACT FROM AUTHOR]

In engineering practice, failures due to fatigue cracks in metallic structures have always been difficult to predict. In our study, nonlinear Lamb wave-mixing was applied in the detection of micro-cracks in plates. The analysis of the nonlinear interaction of these waves with cracks of various lengths and widths was performed using finite-element simulations. The simulation results showed that the sideband at the sum frequency provide a sensitive means for micro-crack detection. Moreover, the sideband amplitudes show a monotonic increase with micro-crack length, but a decrease with micro-crack width. Experiments using Lamb wave-mixing were conducted on plates with fatigue cracks. The experimental results show that a proposed acoustic nonlinearity parameter related to the sideband at the sum frequency is sensitive to micro-cracks in plate, and is well correlated with the damage degree. [ABSTRACT FROM AUTHOR]

The transmission behavior of Lamb waves and the possible occurrence of resonance at an adhesive butt joint of plates are studied experimentally. To this purpose, two 2.5-mm thick aluminum alloy plates are bonded at their edges using cyanoacrylate-based adhesive. Bonded plate specimens with different joint conditions are prepared by changing the bonding procedure. The measurements are performed for the transmission characteristics of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb modes for the frequency range of 0.4–0.6 MHz below the cut-off frequency of the higher-order modes. The experimental results show that the transmission coefficients of the S0 and A0 modes exhibit different frequency-dependent characteristics depending on the joint condition. Furthermore, for the incidence of the S0 mode at the center frequency of 1 MHz, the transmitted S0 mode in weakly bonded specimens shows a long oscillation tail due to the resonance effect. The experimental results are discussed in the light of the theoretical results based on the spring-type interface model. The interfacial stiffnesses identified from the transmission coefficients are shown to be correlated with the bonding condition of the joint and give reasonable estimates of the resonance frequencies of weakly bonded specimens. [ABSTRACT FROM AUTHOR]

Effective application of the Lamb waves for structural health monitoring and damage identification intensively relies on the accurate damage-related feature extraction in the received signals. Most of existing signal processing methods extract the damage-related features from the time–frequency joint spectrum which requires a quite amount of effort. In this paper, the soft-thresholding process, based on different signal decomposition methods, is introduced to damage identification so that the damage-related signal features can be manifested more distinctively. By applying two popular signal decomposition methods (i.e., the discrete wavelet transform (DWT) and the empirical mode decomposition (EMD)), the signal of interest can be represented by a series of components with different frequencies. Since most noises exist in the high frequency range, it is feasible to alleviate noise by restricting the energy of high-frequency components. Finally, a denoised signal is synthesized using the corresponding reconstruction method. As an application, the soft-thresholding process is performed to detect a small crack on an isotropic aluminum plate under the white Gaussian noise contamination. The results, from both the numerical finite element simulation and experimental test, indicate that the soft-thresholding process is capable of effectively reducing the effect of noise, convincingly improving the sensitivity of damage identification, and discriminating relatively small damage. [ABSTRACT FROM AUTHOR]

Antisymmetric and symmetric Lamb-type feature guided waves (FGW) have recently been shown to exist in small angle plate bends. This paper reports Semi-Analytical Finite Element (SAFE) method simulations revealing the existence of a new family of Shear Horizontal ( SH B ) type of FGW mode in 90° bends in plate structures. Mode shapes and velocity dispersion curves are extracted, demonstrating the SH -like nature of a bend-confined mode identified in studies of power flow across the bend. The SH B mode is shown to have reduced attenuation in the higher frequency range, making it an ideal choice for high-resolution inspection of such bends. Further modal studies examine the physical basis for mode confinement, and argue that this is strongly related to FGW phenomena reported earlier, and also linked to the curvature at the bend region. Wedge acoustic waves discussed widely in literature are shown as arising from surface-limiting of the SH B mode at higher frequencies. The results are validated by experiments and supported by 3D Finite Element (FE) simulations. [ABSTRACT FROM AUTHOR]

Air-coupled ultrasonic inspection using leaky Lamb waves offers attractive possibilities for non-contact testing of plate materials and structures. A common method uses an air-coupled pitch–catch configuration, which comprises a transmitter and a receiver positioned at oblique angles to a thin plate. It is well known that the angle of incidence of the ultrasonic bulk wave in the air can be used to preferentially generate specific Lamb wave modes in the plate in a non-contact manner, depending on the plate dimensions and material properties. Multiple reflections of the ultrasonic waves in the air gap between the transmitter and the plate can produce additional delayed waves entering the plate at angles of incidence that are different to those of the original bulk wave source. Similarly, multiple reflections of the leaky Lamb waves in the air gap between the plate and an inclined receiver may then have different angles of incidence and propagation delays when arriving at the receiver and hence the signal analysis may become complex, potentially leading to confusion in the identification of the wave modes. To obtain a better understanding of the generation, propagation and detection of leaky Lamb waves and the effects of reflected waves within the air gaps, a multiphysics model using finite element methods was established. This model facilitated the visualisation of the propagation of the reflected waves between the transducers and the plate, the subsequent generation of additional Lamb wave signals within the plate itself, their leakage into the adjacent air, and the reflections of the leaky waves in the air gap between the plate and receiver. Multiple simulations were performed to evaluate the propagation and reflection of signals produced at different transducer incidence angles. Experimental measurements in air were in good agreement with simulation, which verified that the multiphysics model can provide a convenient and accurate way to interpret the signals in air-coupled ultrasonic inspection using leaky Lamb waves. [ABSTRACT FROM AUTHOR]

In the presented work, the characterization of plates using zero group velocity Lamb modes is discussed. First, analytical expressions are shown for the determination of the k – ω location of the zero group velocity Lamb modes as a function of the Poisson’s ratio. The analytical expressions are solved numerically and an inverse problem is formulated to determine the unknown wave velocities in plates of known thickness. The analysis is applied to determine the elastic properties of tungsten and aluminum plates based on the experimentally measured frequency spectra. [ABSTRACT FROM AUTHOR]

In the industry, many metal structures are constituted by thin plates which undergo, during their use, fatigue, corrosion, defects etc., these effects often result in a thickness variation. It is so, important to be able to measure, accurately, the thickness of these plates. Furthermore, only one side of the plate is often available, this disadvantage increases the difficulty of measuring the thickness variations. Generally, ultrasonic waves are used to test these structures, without dismantling or modifying them. This paper presents a method to measure the thickness of thin metal sheet by using the propagation of the first antisymmetric A 0 Lamb mode generated by a piezoelectric transducer. Principle is based on the proportionality relation which exists between the group velocity and the thickness of the thin sheet, at low frequencies. We put in evidence this relation which we test on thin metal sheets (150 mm long and 70 mm (58 mm for the sheet of 25 μm thickness) width) of which we know the thicknesses (25; 75; 100; 160 and 200 μm), measured with a palmer (micrometer). This work is a feasibility study which shows that it is possible to use piezoelectric transducers to measure small thicknesses with the A 0 Lamb mode method. [ABSTRACT FROM AUTHOR]

We previously formulated a semi-analytical finite element technique for Lamb waves in a plate surrounded by fluids and investigated the dispersion curves and wave structures for leaky Lamb waves. Herein, this technique is extended to the calculation of transient responses both in a plate and in fluids for dynamic loading on the plate surface. To gain fundamental insights into guided wave inspection for a water-filled pipe or tank, guided waves generated upon transient loading of a flat plate water-loaded on one side were analyzed. The results show that a quasi-Scholte mode propagating at the plate-water interface is useful for the long-range inspection of a water-loaded plate because of its non-attenuation and minimal dispersion; moreover, this mode has superior generation efficiency in the low-frequency range, while it is localized near the plate-water interface at higher frequencies. [ABSTRACT FROM AUTHOR]

Structures with composite materials in daily use are subjected to impact. Determining if the material is damaged is an important aspect of structural health monitoring of composites. In this paper, a method to determine structural damage as a result of impact and localize the position of the damage based on lamb wave mode is presented. First, the paper analyses the characteristics of lamb wave mode generated by force loading on the plate. These results are validated based on the analysis of experimental results. From the results, it can be concluded that S 0 mode lamb waves were generated by damage of the plate structure and it can be used as the characteristic signal of damage. Then, empirical mode decomposition (EMD) is utilized to decompose to the Intrinsic Mode Functions (IMF) of S 0 mode inside the emission signals of complex acoustics to realize if damage occurs and localize the position of impact damage on the composite material plate. [ABSTRACT FROM AUTHOR]

• We proposed a method to measure dispersion curves base on clustering algorithms. • The method could realize more accurate and reliable measurement results with less measured data. • This method was experimentally verified with an isotropic aluminum plate and an anisotropic CFRP plate. • We obtain A 0 mode phase-velocity dispersion curves for CFRP plate with various braiding angles. Due to the large detection range and high sensitivity to damages, Lamb waves are widely used in the localization and quantification of structural damages in a plate structure. The dispersion curve measurement is significant in the applications of Lamb waves, especially in a material with unknown properties. In the study, a method was proposed to measure dispersion curves based on clustering. Compared with traditional methods, the proposed method could realize more accurate and reliable measurement results with less measured data. This method was experimentally verified with an isotropic aluminum plate and an anisotropic CFRP plate. The relative error between measured and real values in an aluminum plate was less than 1%. With this method, A 0 mode phase-velocity dispersion curves in CFRP plate with various braiding angles were experimentally obtained. This method facilitated Lamb wave defect detection and material parameter inversion. [ABSTRACT FROM AUTHOR]

Recent improvements in tomographic reconstruction techniques generated a renewed interest in short-range ultrasonic guided wave inspection for real-time monitoring of internal corrosion and erosion in pipes and other plate-like structures. Emerging evidence suggests that in most cases the fundamental asymmetric A0 mode holds a distinct advantage over the earlier market leader fundamental symmetric S0 mode. Most existing A0 mode inspections operate at relatively low inspection frequencies where the mode is highly dispersive therefore very sensitive to variations in wall thickness. This paper examines the potential advantages of increasing the inspection frequency to the so-called constant group velocity (CGV) point where the group velocity remains essentially constant over a wide range of wall thickness variation, but the phase velocity is still dispersive enough to allow accurate wall thickness assessment from phase angle measurements. This paper shows that in the CGV region the crucial issue of temperature correction becomes especially simple, which is particularly beneficial when higher-order helical modes are also exploited for tomography. One disadvantage of working at such relatively high inspection frequency is that, as the slower A0 mode becomes faster and less dispersive, the competing faster S0 mode becomes slower and more dispersive. At higher inspection frequencies these modes cannot be separated any longer based on their vibration polarization only, which is mostly tangential for the S0 mode while mostly normal for the A0 at low frequencies, as the two modes become more similar as the frequency increases. Therefore, we propose a novel method for suppressing the unwanted S0 mode based on the Poisson effect of the material by optimizing the angle of inclination of the equivalent transduction force of the Electromagnetic Acoustic Transducers (EMATs) used for generation and detection purposes. [ABSTRACT FROM AUTHOR]

Traditionally dispersion characteristics for any plate structure are evaluated using the Rayleigh-Lamb solution and an assumption that the material properties are known. In cases where the material properties may be difficult to obtain the opposite is proposed. Specifically, the dispersive characteristics of Lamb waves may be used to extract the mechanical properties of a given test coupon. This paper presents a particle swarm optimisation (PSO) methodology to achieve this. The results from a series of tests on an aluminium coupon were used to validate the approach. The PSO was found to successfully match dispersion characteristics identified in a 2D FFT. [ABSTRACT FROM AUTHOR]

The developed method is based on acoustic underwater scattering measurements and can be applied to characterize a defect in a blade of a marine current turbine. To simplify the study, the blade is replaced by a rectangular plate immersed in water having a groove opening out. The measurements are made on horizontal plane perpendicular to the long axis of the stainless steel plate. The frequency range of signal is between 50 and 400kHz. The transducer is remote from the plate. The measurements are recorded on 360° with a 1° step. The experimental trajectories of the scattering signals are compared to the trajectories calculated from the group velocities of Lamb waves A and S 0. Spectra give also an information on groove position. The present study is the first step toward remote monitoring of blades on a marine current turbine. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mode conversion of Lamb waves can be used to size notch-type defects in plates. [•] Spectral leakage can hide small amplitude modes in the frequency–wavenumber domain. [•] Baseline subtraction technique is applied to 2DFFT to detect low amplitude modes. [•] Lower limits of detection method are tested via the FEM. [Copyright &y& Elsevier]

Highlights: [•] Our aim is to analyze the mode transmission in the notched plate within the relatively short propagation distance. [•] The originally-transmitted modes and newly-converted modes were separated. [•] The velocities and energy of converted modes were extracted to locate and evaluate the notches, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Several Lamb wave modes can be coupled to a particular structure, depending on its geometry and transducer used to generate the guided waves. Each Lamb mode interacts in a particular form with different types of defects, like notches, delamination, surface defects, resulting in different information which can be used to improve damage detection and characterization. An image compounding technique that uses the information obtained from different propagation modes of Lamb waves for non-destructive testing of plate-like structures is proposed. A linear array consisting of 16 piezoelectric elements is attached to a 1mm thickness aluminum plate, coupling the fundamental A0 and S0 modes at the frequencies of 100kHz and 360kHz, respectively. For each mode two images are obtained from amplitude and phase information: one image using the Total Focusing Method (TFM) and one phase image obtained from the Sign Coherence Factor (SCF). Each TFM image is multiplied by the SCF image of the respective mode to improve contrast and reduce side and grating lobes effects. The high dispersive characteristic of the A0 mode is compensated for adequate defect detection. The information in the SCF images is used to select one of the TFM mode images, at each pixel, to obtain the compounded image. As a result, dead zone is reduced, resolution and contrast are improved, enhancing damage detection when compared to the use of only one mode. [Copyright &y& Elsevier]

Abstract: In this paper, the group velocities of the zero order antisymmetric (A0) Lamb modes in a phononic crystal plate with single layer cylindrical holes parallel to the surface of the plate were investigated theoretically. The results show that by increasing the filling fraction, the A0 mode can be efficiently slowed down and the group velocity of the A0 modes can be tuned from the positive to the negative referring to the phase velocity. Moreover, the zero group velocity of the A0 modes can be obtained with a given filling fraction. These results may be useful in designing acoustic devices. [Copyright &y& Elsevier]

Abstract: Numerical simulations were carried out to quantify the reflection and transmission characteristics of the fundamental Lamb modes propagating in aluminium sub-plates, which are formed due to a semi-infinite horizontal crack. It was observed that, a Lamb mode propagating in a sub-plate when incident at the edge of a crack, undergoes reflection and transmits through the main plate, as well as the other sub-plate. The mode transmitted through the sub-plate has been termed the ‘Turning Lamb Mode’ (TLM). Furthermore, a mode converted mode also propagates along with the TLM. This mode has been termed the ‘Mode Converted Turning Lamb Mode’ (MCTLM). Reflection and transmission characteristics of the fundamental Lamb modes in aluminium sub-plates were studied at frequencies 150kHz, 175kHz, and 200kHz. Experiments conducted to validate the observations made in numerical simulations, confirmed that the transmission and reflection characteristics depend on the thickness ratio. From this study it is surmised that when a Lamb mode propagates through a plate containing horizontal crack, the TLM and the MCTLM start propagating from one sub-plate to the other at the rear edge of the crack and amplitude of these modes depends on the location of the crack across the plate thickness. [Copyright &y& Elsevier]

Abstract: The reflection of obliquely incident symmetric and anti-symmetric Lamb wave modes at the edge of a plate is studied. Both in-plane and Shear-Horizontal (SH) reflected wave modes are spawned by an obliquely incident in-plane Lamb wave mode. Energy reflection coefficients are calculated for the reflected wave modes as a function of frequency and angle of incidence. This is done by using the method of orthogonal mode decomposition and by enforcing traction free conditions at the plate edge using the method of collocation. A PZT sensor network, affixed to an Aluminum plate, is used to experimentally verify the predictions of the analysis. Experimental results provide support for the analytically determined results. [Copyright &y& Elsevier]

Abstract: Defect imaging using scanning laser source technique has been investigated for a plate with rounded defects and notch-type defects in our previous studies. This paper examines frequency dependence of the defect images with both calculations and experiments in order to acquire clearer images. Both calculation and experimental results for a straight notch revealed that clearer images of notch-type defects can be obtained in the range of low frequency-thickness product below about 200kHzmm. Moreover, images of the defects of various shapes were obtained by synthesizing images from eight receiving transducers, and similarly to the case of the straight notches, they became more clearly in the low frequency range. [Copyright &y& Elsevier]

Abstract: Acoustic emission (AE) is an inspection technique for monitoring of metals. AE parameters depend on the source mechanism and proper study enlightens the material’s condition. Classification criteria based on AE parameters are established concerning the material’s status. However, the shape of the AE waves depends on the propagation path. Thin plates exhibit strong dispersion that influences waveform parameters. This paper numerically studies wave propagation through a homogeneous plate. The study does not concern classical parameters like velocity and attenuation, but AE waveform parameters. It is shown that any classification, should incorporate the source location in order to exclude the effect of dispersion. [Copyright &y& Elsevier]

Abstract: This paper is intended to examine the effect of inclusion shapes, inclusion contents, inclusion elastic constants, and plate thickness on the dispersion relations and modes of wave propagation in inclusion-reinforced composite plates. The shape of inclusion is modeled as spheroid that enables the composite reinforcement geometrical configurations ranging from sphere to short and continuous fiber. Mori–Tanaka mean-field theory is used to predict the effective elastic moduli of the composite plate explicitly. The effective elastic moduli are able to elucidate the effect of inclusion’s shape, stiffness, and volume fraction on the composite’s anisotropic elastic behavior. The resulting moduli are then used to determine the dispersion relations and the modal patterns of Lamb waves using the dynamic stiffness matrix method. The types (symmetric or antisymmetric) of Lamb waves in an isotropic plate can be classified according to the wave motions are symmetrical or antisymmetric about the midplane of the plate. Classifying the wave type in an anisotropic plate is not as simple as that in an isotropic plate, and has not received proper attention in the literature. The wave types and orders are identified by analyzing the dispersion curves and inspecting the calculated modal patterns, and the results indicate that the Lamb waves in an orthotropic composite plate can also be classified as either symmetric or antisymmetric waves. It is also found that the inclusion contents, aspect ratios and plate thickness affect propagation velocities, higher-order mode cutoff frequencies, and modal patterns. Propagation speed is generally increased with the aspect ratio, e.g., using longer fibers generally results in a higher propagation speed. [Copyright &y& Elsevier]

Abstract: A new type of hydrogen sensor using Lamb waves is proposed and fabricated. The sensor consists of an aluminum thin plate-based hydrogen sensing medium that can be easily replaced, a medium grip component, and a piezoelectric sensor. In addition to structural uniqueness, the proposed hydrogen sensor provides two wave parameters, peak amplitude and phase shift, to detect changes in hydrogen concentration. The proposed sensor can provide reliable sensitivity, decreased fabrication and operational costs, and significantly increased lifetime because its hydrogen sensing medium can be replaced when it shows low sensitivity. [Copyright &y& Elsevier]

Abstract: We have implemented three-dimensional (3D) elastodynamic finite integration technique (EFIT) simulations to model Lamb wave scattering for two flaw-types in an aircraft-grade aluminum plate, a rounded rectangle flat-bottom hole and a disbond of the same shape. The plate thickness and flaws explored in this work include frequency–thickness regions where several Lamb wave modes exist and sometimes overlap in phase and/or group velocity. For the case of the flat-bottom hole the depth was incrementally increased to explore progressive changes in multiple-mode Lamb wave scattering due to the damage. The flat-bottom hole simulation results have been compared to experimental data and are shown to provide key insight for this well-defined experimental case by explaining unexpected results in experimental waveforms. For the rounded rectangle disbond flaw, which would be difficult to implement experimentally, we found that Lamb wave behavior differed significantly from the flat-bottom hole flaw. Most of the literature in this field is restricted to low frequency–thickness regions due to difficulties in interpreting data when multiple modes exist. We found that benchmarked 3D EFIT simulations can yield an understanding of scattering behavior for these higher frequency–thickness regions and in cases that would be difficult to set up experimentally. Additionally, our results show that 2D simulations would not have been sufficient for modeling the complicated scattering that occurred. [Copyright &y& Elsevier]

Abstract: Using finite element method, we have made a theoretically study of the band structure of Lamb wave in a locally resonant phononic crystal strip waveguide with periodic soft rubber attached on the two sides of epoxy main plate. The numerical results show that the Lamb wave band gap based on local resonant mechanism can be opened up in the stub strip waveguides, and the width of the local resonant band gap is narrower than that based on the Bragg scattering mechanism. The results also show that the stub shape and width have influence on the frequency and width of the Lamb wave band gap. [Copyright &y& Elsevier]

Abstract: The thermal degradation in ferritic Cr–Ni alloy steel plates is measured using the nonlinear effect of Lamb wave propagation. Experiments were carried out to introduce controlled levels of thermal damage to determine the nonlinear response of Lamb waves. A “mountain-shape” change in the normalized acoustic nonlinearity of Lamb wave versus the level of thermal degradation in the specimens has been observed. The variation in the measured acoustic nonlinearity reveals, based on metallographic studies, that the normalized acoustic nonlinearity increases due to the second phase precipitates in the early stage and it decreases as a combined result of dislocation change and micro-void initiation in the material. The results show a potential application of the nonlinear Lamb waves for the quantitative assessment of thermal damage in metallic plates or pipes. [Copyright &y& Elsevier]

Abstract: In this paper we propose a concept and report experimental results based on a circular array of Piezoelectric Wafer Active Sensors (PWASs) for rapid localization and parametric identification of corrosion type damage in metallic plates. Implementation of this circular array of PWASs combines the use of ultrasonic Lamb wave propagation technique and an algorithm based on symmetry breaking in the signal pattern to locate and monitor the growth of a corrosion pit on a metallic plate. Wavelet time-frequency maps of the sensor signals are employed to obtain an insight regarding the effect of corrosion growth on the Lamb wave transmission in time-frequency scale. We present here a method to eliminate the time scale, which helps in identifying easily the signature of damage in the measured signals. The proposed method becomes useful in determining the approximate location of the damage with respect to the location of three neighboring sensors in the circular array. A cumulative damage index is computed from the wavelet coefficients for varying damage sizes and the results appear promising. Damage index is plotted against the damage parameters for frequency sweep of the excitation signal (a windowed sine signal). Results of corrosion damage are compared with circular holes of various sizes to demonstrate the applicability of present method to different types of damage. [Copyright &y& Elsevier]

Abstract: This paper deals with developing a parametric 2D finite-element technique, which can size strip-like defects in elastic or viscoelastic, isotropic or anisotropic material plates. A pure incident Lamb wave mode is sent towards the defect and reflection and transmission coefficients produced by mode conversion phenomenon are used as input data for the inversion process, which consists in quantifying two unknown parameters representing the geometry of the defect. A finite-element-based model is used to simulate the Lamb wave scattering for various values of the aimed parameters, and specific post-processing based on Shkerdin’s orthogonality relation is applied to predict the needed reflection and transmission coefficients. The target is known reflection and transmission coefficients initially obtained from an experiment, which can be either a numerical experiment for validation purposes, or proper measurements on a sample for NDE application purposes. The inversion process is stopped when the predicted reflection and transmission coefficients fit at best those obtained from initial experiment. In this paper, we have quantified two geometrical parameters for three types of defects in different waveguides: the through-thickness depth (or position) and width (or radius) of (1) a circular hole inside an aluminium plate, (2) a rectangular notch at the surface of a Perspex plate, (3) a cracked zone inside a composite plate, representing an impact damage. [Copyright &y& Elsevier]

Abstract: An analysis is given to the band structure of the two-dimensional phononic crystal plate constituted of a square array of elastic anisotropic, circular Pb cylinders embedded in elastic isotropic epoxy. The numerical results show that the band gap can be tuned by rotating the anisotropic material orientation. It is found that the influence of anisotropy on band gap of Lamb wave is clearly different from that on the band gap of bulk waves. The thickness of the system under study is a sensitive parameter to affect the influence of anisotropic materials on the normalized gap width. [Copyright &y& Elsevier]

Abstract: Propagation of the anti-symmetric fundamental Lamb mode (A 0) in a composite laminate containing a semi-infinite delamination was studied through numerical simulations and experimental studies. Here, the wave was generated on a sub-laminate above the delaminated region, and the reflection and transmission of the Lamb modes at the edge of the discontinuity were quantitatively studied. The wave group (named as ‘turning mode’), which was made to propagate from one side of the delamination to the other side (through delaminations) was captured and its transmission factor was estimated. Reflection and transmission factors of wave groups were estimated for three different laminates – quasi-isotropic, unidirectional and cross-ply, containing delamination at various interfaces across thickness. Propagation of ‘turning modes’ were also captured experimentally, employing air-coupled transducers, in quasi-isotropic laminates containing delaminations at different interfaces. Attenuation compensation was incorporated in the experimental signals to improve the comparison of transmission and reflection factors between numerical simulations and experiments. [Copyright &y& Elsevier]

Abstract: In recent years, much attention has been paid to the use of Lamb waves for structural health monitoring. This choice is prompted by the high speed of Lamb wave inspection, although their dispersive nature can complicate the interpretation of results, especially when dealing with closely-spaced reflectors. In this paper, the objective is to improve the time resolution and signal-to-noise ratio of signals obtained from inspection of plates by the S 0 Lamb mode. The signal processing scheme used is based on deconvolution of the measured signal by Wiener filtering, followed by autoregressive spectral extrapolation. The deconvolution technique is applied to signals obtained from finite element models and also to experimentally measured signals; both sets of data are based on plates with various types of notch discontinuities. Using this technique, the separation distance between adjacent notches was estimated with high accuracy in both simulated and experimental ultrasonic signals. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the propagation behavior of Lamb waves in a thermal stress relaxation type functionally graded material (FGM) plate with material parameters that vary continuously along the thickness, the power series technique, which has been proved to have good convergence and high precision, is employed for theoretical derivations. The influence of the gradient coefficients of FGM on the dispersion curves is illustrated. The numerical results also reveal differences between the properties of Lamb wave propagation in the FGM plate and the corresponding properties in a homogenous plate. In terms of results, we find that both the normal and anomalous dispersions exist in the first and the second modes of the Lamb wave that propagates in the FGM plate, while only the anomalous dispersion is in the first mode and only the normal dispersion is in the second mode for the homogenous plate. The wave structure is asymmetric due to the asymmetric properties of the material. The dominance of in-plane and out-plane displacements is different between the metal-rich and ceramic-rich surfaces. All these results give theoretical guidance not only for experimental measurement of material properties but also for nondestructive evaluation using an ultrasonic wave generation device. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports simulations of the generation of Lamb wave modes in thin plates using a meander coil (meander line) EMAT, which works under the principle of Lorentz force mechanism in non-magnetic materials. The numerical simulations have been compared with measurements. The numerical work presented in this paper is divided into two parts. First, a 2D electromagnetic model is developed for calculating the Lorentz force density, which is the driving force for elastic wave generation within the plate. Second, the calculated force at each point in the metal is used as the driving force for generating elastic wave modes in the plate. These calculated Lamb wave modes are compared with those generated experimentally in a thin plate. Additionally, the measured wave modes are analyzed with the help of dispersion curves and a time–frequency analysis. Our numerical work is also extended to analyze the interaction of Lamb wave modes with defects. The simulations compare favorably with the measurements presented here. [Copyright &y& Elsevier]

Abstract: This paper proposes an acoustic multi-touch tactile sensing method. The proposed method is based on an amplitude disturbed ultrasonic wave diffraction pattern. An A 0 Lamb wave transmitted in a thin finite copper plate is processed to provide tactile information, for one or two fingers. A touch event is localized by identifying the diffraction signals among a database of diffracted Lamb wave references. Statistic models are used to improve the localization reliability. An artificial silicone finger is used in the calibration procedure. This touch interface is evaluated as a 2-touch interface. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the A0 Lamb wave propagating in a thin finite copper plate is investigated to design a new multi-touch tactile instrument, which could localize one or several fingers. The position of index is localized by identifying the absorption signals of Lamb waves when it is in contact with the plate. Statistic models are used to improve the robustness of this interface. An artificial silicone finger is fabricated in the calibration procedure. The multi-touch (two fingers or more) applications using this plate is also presented. [Copyright &y& Elsevier]

Abstract: A model for quantifying the performance of acoustic emission (AE) systems on plate-like structures is presented. Employing a linear transfer function approach the model is applicable to both isotropic and anisotropic materials. The model requires several inputs including source waveforms, phase velocity and attenuation. It is recognised that these variables may not be readily available, thus efficient measurement techniques are presented for obtaining phase velocity and attenuation in a form that can be exploited directly in the model. Inspired by previously documented methods, the application of these techniques is examined and some important implications for propagation characterisation in plates are discussed. Example measurements are made on isotropic and anisotropic plates and, where possible, comparisons with numerical solutions are made. By inputting experimentally obtained data into the model, quantitative system metrics are examined for different threshold values and sensor locations. By producing plots describing areas of hit success and source location error, the ability to measure the performance of different AE system configurations is demonstrated. This quantitative approach will help to place AE testing on a more solid foundation, underpinning its use in industrial AE applications. [Copyright &y& Elsevier]

Abstract: The aim of this work is to study the fundamental Lamb modes interaction with defects in isotropic plates. For these experimental investigations, symmetrical notches with various depths milled in aluminum plates are considered. Moreover, the incident Lamb wave of a specific mode is generated by means of two identical thin piezoceramic transducers placed at the opposite sides of the plate. The waves scattered by the notch are recorded with conventional transducers located on the plate surface in front and behind the defect. The selection of the A 0 or the S 0 modes is obtained by exciting the transducers with anti-phased or in-phased signals, respectively. Furthermore, a calibration process is investigated to correct errors caused by the presence of the receiver between the emitters and the defects. The power reflection and transmission coefficients are then obtained and the power balance is verified. Finally, these measurements are compared successfully with those obtained by a numerical method using the finite-element modeling described in a previous work. [Copyright &y& Elsevier]