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The interaction of laser-generated Lamb waves propagating in a plate with a sharp-angle conical hole was studied experimentally and numerically. Part of the energy of the incident wave is trapped within the conic area in two ways: the antisymmetric Lamb wave orbiting the center of the hole and the wave localized at the acute edge. Parameters and conditions for optimal conversion of the incident wave into the trapped modes were studied in this work. Experiments were performed using the laser stroboscopic shearography technique, which delivers the time evolution of the acoustic field in the whole area of interest. The effect of trapping can be used for efficient damping, similar to the one-dimensional acoustical black hole effect. [ABSTRACT FROM AUTHOR]

The spatial-wavenumber filtering technique is an effective approach to distinguish the propagating direction and wave mode of Lamb wave in spatial-wavenumber domain. Therefore, it has been gradually studied for damage evaluation in recent years. But for on-line impact monitoring in practical application, the main problem is how to realize the spatial-wavenumber filtering of impact signal when the wavenumber of high spatial resolution cannot be measured or the accurate wavenumber curve cannot be modeled. In this paper, a new model-independent spatial-wavenumber filter based impact imaging method is proposed. In this method, a 2D cross-shaped array constructed by two linear piezoelectric (PZT) sensor arrays is used to acquire impact signal on-line. The continuous complex Shannon wavelet transform is adopted to extract the frequency narrowband signals from the frequency wideband impact response signals of the PZT sensors. A model-independent spatial-wavenumber filter is designed based on the spatial-wavenumber filtering technique. Based on the designed filter, a wavenumber searching and best match mechanism is proposed to implement the spatial-wavenumber filtering of the frequency narrowband signals without modeling, which can be used to obtain a wavenumber-time image of the impact relative to a linear PZT sensor array. By using the two wavenumber-time images of the 2D cross-shaped array, the impact direction can be estimated without blind angle. The impact distance relative to the 2D cross-shaped array can be calculated by using the difference of time-of-flight between the frequency narrowband signals of two different central frequencies and the corresponding group velocities. The validations performed on a carbon fiber composite laminate plate and an aircraft composite oil tank show a good impact localization accuracy of the model-independent spatial-wavenumber filter based impact imaging method. [ABSTRACT FROM AUTHOR]

Lamb wave technique has been widely used in non-destructive evaluation (NDE) and structural health monitoring (SHM). However, due to the multi-mode characteristics and dispersive nature, Lamb wave propagation behavior is much more complex than that of bulk waves. Numerous numerical simulations on Lamb wave propagation have been conducted to study its physical principles. However, few quantitative studies on evaluating the accuracy of these numerical simulations were reported. In this paper, a method based on cross correlation analysis for quantitatively evaluating the simulation accuracy of time-transient Lamb waves propagation is proposed. Two kinds of error, affecting the position and shape accuracies are firstly identified. Consequently, two quantitative indices, i.e., the GVE (group velocity error) and MACCC (maximum absolute value of cross correlation coefficient) derived from cross correlation analysis between a simulated signal and a reference waveform, are proposed to assess the position and shape errors of the simulated signal. In this way, the simulation accuracy on the position and shape is quantitatively evaluated. In order to apply this proposed method to select appropriate element size and time step, a specialized 2D-FEM program combined with the proposed method is developed. Then, the proper element size considering different element types and time step considering different time integration schemes are selected. These results proved that the proposed method is feasible and effective, and can be used as an efficient tool for quantitatively evaluating and verifying the simulation accuracy of time-transient Lamb wave propagation. [ABSTRACT FROM AUTHOR]

A fully non-contact single-sided air-coupled and laser ultrasonic non-destructive system based on the generation and detection of Lamb waves is implemented for the characterization of A 0 Lamb wave mode dispersion in a composite plate. An air-coupled transducer (ACT) radiates acoustic pressure on the surface of the composite and generates Lamb waves within the structure. The out-of-plane velocity of the propagating wave is measured using a laser Doppler vibrometer (LDV). In this study, the non-contact automated system focuses on measuring A 0 mode frequency–wavenumber, phase velocity dispersion curves using Snell’s law and group velocity dispersion curves using Morlet wavelet transform (MWT) based on time-of-flight along different wave propagation directions. It is theoretically demonstrated that Snell’s law represents a direct link between the phase velocity of the generated Lamb wave mode and the coincidence angle of the ACT. Using Snell’s law and MWT, the former three dispersion curves of the A 0 mode are easily and promptly generated from a set of measurements obtained from a rapid ACT angle scan experiment. In addition, the phase velocity and group velocity polar characteristic wave curves are also computed to analyze experimentally the angular dependency of Lamb wave propagation. In comparison with the results from the theory, it is confirmed that using the ACT/LDV system and implementing simple Snell’s law method is highly sensitive and effective in characterizing the dispersion curves of Lamb waves in composite structures as well as its angular dependency. [ABSTRACT FROM AUTHOR]

Interaction of ultrasonic guided waves with kissing bonds (closed delaminations and incipient surface breaking cracks) gives rise to nonlinear features at the defect location. This causes higher harmonic frequency ultrasonic radiation into the ambient air, often referred to as Nonlinear Air-Coupled Emission (NACE), which may serve as a nonlinear tag to detect the defects. This paper summarizes the results of a numerical implementation and simulation study of NACE. The developed model combines a 3D time domain model for the nonlinear Lamb wave propagation in delaminated samples with a spectral solution for the nonlinear air-coupled emission. A parametric study is conducted to illustrate the potential of detecting defect location, size and shape by studying the NACE acoustic radiation patterns in different orientation planes. The simulation results prove that there is a good determination potential for the defect parameters, especially when the radiated frequency matches one of the resonance frequencies of the delaminated layer, leading to a Local Defect Resonance (LDR). [ABSTRACT FROM AUTHOR]

The non-contact feature of the Laser/EMAT ultrasonic (LEU) technique is attractive for its NDT applications. However, it is challenging to apply it in thin structures because of the difficulties in the signal interpretations. In this work, the LEU technique is used to inspect the notch depths in thin steel plates. A Continuous Wavelet Transform (CWT)-based algorithm is developed to calculate the transmission coefficients of laser-generated Lamb waves. The effect of varying notch depths on Lamb waves’ transmission coefficients is investigated both numerically and experimentally. The transmission coefficients of laser-generated Lamb waves calculated using CWT have been used successfully to predict the notch depths in thin structures. [ABSTRACT FROM AUTHOR]

• Ellipse of uncertainty (EOU) -based algorithm was developed for quantitative evaluation of defect localization using Lamb waves. • Parametric EOU formula was provided. • Correlated ToF data of each inspection pair were fused based on the acoustic reciprocity theorem and the fuzzy control parameter. • Accurate defect localization was realized by searching individuals at the intersection of multiple temporal-spatial mapping trajectories. • Quantitative evaluation of defects localization was realized by analyzing the distribution of individuals and their relative deviation of arrival times to every inspection pairs. Measurement deviation of time of flight (ToF) is inevitable in nondestructive testing based on the sparse array and ultrasonic Lamb waves. It affects the influence zone of temporal-spatial mapping trajectories (TSMTs) of signal parameters in the imaging zone, and further limits the quantitative evaluation of defect localization. In the paper, the ellipse of uncertainty (EOU) of TSMTs was derived from multiple parameters, including the group velocity, ToFs and their measurement deviations, distances between actuators and receivers. Then, an EOU-based algorithm was developed for quantitative evaluation of defect localization. The defects were localized by searching the individual scatterers at the intersection of multiple TSMTs. Based on the eccentricity of the uncertainty ellipse, a fuzzy scaling factor was introduced. It was combined with a fuzzy control parameter to tune the influence zone of TSMTs. Based on the acoustic reciprocity theorem and the fuzzy control parameter, the ToFs of scattering waves were fused to establish the one-to-one relation between individual scatterers and inspection pairs. Experimental results showed that the EOU-based algorithm can reduce the interferences of EOU in the detection; and the quantitative evaluation of defect localization was realized by analyzing the distribution of individuals and their ToF difference to inspection pairs. [ABSTRACT FROM AUTHOR]

• A water-immersed steel plate is insonified at normal beam incidence. • The axial pressure is studied in the frequency range of the lower leaky Lamb modes. • The near-field of the plate extends to very large ranges from the plate. • A piston angular spectrum model is developed and used to analyze the axial pressure. Axial radiation from leaky Lamb waves propagating in a 6.05 mm water-immersed steel plate being excited by a sound beam normally incident to the plate, is investigated as a function of axial distance, z , and frequency, f , over the 350–1000 kHz frequency band of the S 2 , A 2 , and A 3 Lamb modes in the plate. For certain leaky Lamb modes, prior literature has revealed complex characteristics in the transmitted pressure field close to the plate, caused by diffraction due to the finite angular spectrum of the incident beam. The present work extends earlier work by bringing insight into the changes of these field characteristics in the near- and far-field of the transmitted beam, over the frequency band of leaky Lamb modes, for normal beam incidence to the plate. A baffled piston source in a full-wave angular spectrum propagation model is used to analyze the phenomena involved. Maxima and minima that can not be described with plane wave theory are observed in the frequency spectrum of the axial pressure transfer function through the plate. At very long ranges the normalized transmitted sound beam tends to attain characteristics of the plate's plane-wave transmission coefficient, for two of the leaky Lamb modes. Near-field interference phenomena not described in prior literature are identified. For the leaky Lamb mode associated with a backward-wave branch close to the fundamental thickness-extensional resonance in the plate, TE1, the axial near-field is shown to extend to very far ranges. Supplementary measurements add confidence to the simulation results and findings. Besides of their fundamental significance in the study and understanding of sound beams transmitted through a fluid-immersed solid plate, the results are of importance e.g. in immersion applications where material characterization is made using fluid-coupled ultrasonic transducers in a through-thickness resonant transmission setup, such as plate thickness or material properties measurements. [ABSTRACT FROM AUTHOR]

The use of guided waves to identify damage has become a popular method due to its robustness and fast execution, as well as the advantage of being able to inspect large areas and detect minor structural defects. When a travelling wave on a plate interacts with a defect, it generates a scattered field that will depend on the defects geometry. By analysing the scattered field, one can thus characterize the type and size of the plate damage. A Bayesian framework based on a guided waves interaction model for damage identification of infinite plate for the first time is presented here. A semi-analytical approach based on the lowest order plate theories is adopted to obtain the scattering features for damage geometries with circular symmetry, resulting in an efficient inversion procedure. Subsequently, ultrasound experiments are performed on a large aluminium plate with a circular indentation to generate wave reflection and transmission coefficients. With the aid of signal processing techniques, the effectiveness and efficiency of the proposed approach are verified. A full finite element model is used to test the damage identification scheme. Finally, the scattering coefficients are reconstructed, reliably matching the experimental results. The framework supports digital twin technology of structural health monitoring. • A fast physics-informed damage identification Bayesian framework in 2D structures. • A semi-analytical approach for modelling wave damage interaction is presented. • Physical ultrasound experiments are performed to extract scattering coefficients. • Numerical, experimental case studies are conducted to verify the proposed framework. • The computational efficiency of the proposed framework can be greatly improved. [ABSTRACT FROM AUTHOR]

• Demonstration of wave mixing of two low-frequency S 0 waves with the incorporation of phase-reversal approach for fatigue damage evaluation. • Investigation of different effects of incident wave on wave mixing. • The sum combinational harmonic shows good sensitivity to the accumulation of fatigue damage and progressive fatigue damage. Fatigue damage is difficult to detect and evaluate non-destructively, specifically at its early stages (before the macro-crack formation). In this study, fatigue damage is evaluated based on the growth rate of the combinational harmonics generated by mixing of two fundamental symmetric mode (S 0) of Lamb waves in the low frequency range. The incorporation of the phase reversal approach to the wave mixing method could potentially improve the evaluation of the combinational and second harmonics and avoid the influence of other undesirable harmonics. A series of parametric case studies are carried out using the three-dimensional (3D) finite element (FE) method to investigate the effects of the excitation frequencies and time delay of the incident waves in wave mixing on the transient response of a weakly-nonlinear material. The numerical results and experimental results show that the sum combinational harmonic and second harmonics are sensitive to weak material nonlinearities. Further experiments on damaged samples by cyclic loading demonstrate that the sum combinational harmonic has much better sensitivity to the progressive fatigue damage than the the second harmonics. In general, the outcomes of this study indicate that the damage evaluation of early stage fatigue damage is feasible and effective with the wave mixing method using the S 0 waves generated at low frequency, and the phase-reversal approach improves considerably the quality of experimental results in the fatigue damage evaluation. [ABSTRACT FROM AUTHOR]

• Cross-shaped sensor cluster is proposed to be used together with nonlinear Lamb waves for microcrack localization. • The determination criteria of appropriate Lamb wave mode pair for microcrack localization are revealed. • Extraction of TDOAs of nonlinear microcrack characteristic signals is investigated. • The investigations verify that the proposed method can realize microcrack localization without prior group velocity. Using the nonlinear interaction effect between ultrasonic Lamb waves and microcracks to detect and locate microcracks has the advantages of fast detection speed and high sensitivity. In this paper, a method for microcrack localization based on cross-shaped sensor clusters in a plate is proposed by combining nonlinear ultrasonic Lamb wave technology and time difference of arrival (TDOA) technology. The antisymmetric (A0) mode at low frequency is chosen as the primary Lamb wave to simplify the complication of the dispersion and multi-mode properties of Lamb waves. The selected mode pair (A0-s0) weakens the influence of the cumulative growth effect of higher harmonics induced by the inherent material nonlinearity on the microcrack characteristic signals. Pulse inversion technique and cross correlation function are used to extract the TDOAs of the nonlinear characteristic signals including microcrack information. The cross-shaped sensor clusters approach proposed for the first time can achieve reliable and fast microcrack localization without being affected by the duration of the excitation signal, and a priori knowledge of group velocities of primary wave modes or generated harmonics. Experimental and numerical results validate the proposed method in isotropic and anisotropic plates. This paper provides a new idea for nonlinear ultrasonic nondestructive evaluation and structural health monitoring of microcracks in plates. [ABSTRACT FROM AUTHOR]

This study proposes a modified virtual time-reversal (VTR) algorithm for baseline signal-free damage detection in plate-like structures. The physical actuation and sensing of Lamb waves are performed using a broadband Gaussian excitation instead of the conventional narrowband modulated tone burst excitations. The forward response and the reconstructed signal due to the time-reversal process for a narrowband input signal are then constructed virtually using the broadband transfer function. The method eliminates the possibility of numerical errors encountered in the conventional VTR method based on narrowband excitations. It is also more efficient than the traditional VTR algorithm because it can probe at multiple excitation frequencies using a single measurement for each sensing path. This modified VTR algorithm is employed in the recently developed refined time-reversal method (RTRM), which uses an extended signal length of the reconstructed signal for computing damage index (DI) and probes the structure at the best reconstruction frequency. The new method is termed the virtual refined time-reversal method (VRTRM). The DIs based on the VRTRM are used in the reconstruction algorithm for probabilistic inspection of defects to achieve baseline signal-free localization of damages. The efficacy of the proposed VRTRM for damage localization is experimentally verified with the established technique RTRM. Experiments are performed in an aluminium plate equipped with a network of surface-bonded piezoelectric patch transducers to illustrate the conventional VTR's pitfalls and the modified VTR's accuracy for a single mass damage scenario. The results show that the proposed VRTRM is as accurate as the established technique RTRM in estimating the reconstructed signals and localizing a block mass damage. Finally, the VRTRM is shown to localize in a dual damage scenario with excellent accuracy. In contrast, the conventional main mode-based VTR method fails to localize the damages with or without single-mode tuning. • A modified virtual time-reversal (VTR) method is proposed using broadband measurements. • The method eliminates possible numerical errors in conventional VTR method. • It can compute reconstructed signals for multiple frequencies using a single measurement. • The new VTR is combined with refined time-reversal method for baseline-free damage localization. • The method is shown to very efficient and accurate in single and dual-damage scenarios. [ABSTRACT FROM AUTHOR]

• A resonant SH 0 wave can be generated by a pair of S 0 -mode Lamb waves and SH 0 waves when the resonance condition is satisfied. • The critical limit of frequency deviation is considered as ± 10% under imperfect resonance condition. • The damage region can be located by the time-domain signal of the resonant wave. • One-way S 0 - SH 0 mixing method has distinguishable package of the resonant wave and long detection range in experiment. This paper numerically and experimentally investigates the resonant behavior of one-way Lamb and SH (shear horizontal) mixing method in thin plates with quadratic nonlinearity. When the primary S 0 -mode Lamb waves and SH 0 waves mix in the region with quadratic nonlinearity, both numerical and experimental results verify the generation of the resonant SH 0 waves if the resonance condition ω S 0 / ω S H 0 = 2 κ / (κ + 1) is satisfied. Meanwhile, we find that the acoustic nonlinear parameter (ANP) increases monotonously with material quadratic nonlinearity, the length of the damage region and the frequency of the resonant wave. Furthermore, the damage region can be located by the time-domain signal of the resonant wave based on one-way S 0 - SH 0 mixing method. This study numerically and experimentally reveals that one-way Lamb and SH mixing method is feasible to quantitatively evaluate and locate the damage region of quadratic nonlinearity in thin plates. [ABSTRACT FROM AUTHOR]

• The beat-effect of the second-harmonic generation (SHG) of low-frequency Lamb wave in a layered plate was experimentally observed. • Influence of the interfacial property on the SHG effect of low-frequency Lamb wave was analytically and experimentally examined. • The proposed nonlinear low-frequency Lamb wave approach is competent for accurately characterizing a minor change in the interfacial property. This work investigates the feasibility of using a nonlinear low-frequency Lamb wave approach for characterizing the interfacial property of a two-layered plate. Compared with the case of the exact phase-velocity matching, the approximate phase-velocity matching in the low-frequency region can still guarantee the cumulative second-harmonic generation (SHG) of primary Lamb wave propagation, which overcomes the drawbacks arising from the inherent dispersion and multimode features of Lamb wave propagation. For a given two-layered plate, the appropriate mode pair at low frequency consisting of primary Lamb wave and double-frequency Lamb wave (DFLW), which satisfies the approximate phase-velocity matching and nonzero energy flux, is selected to ensure that the amplitude of the generated second harmonic grows within the maximum cumulative distance (MCD). Meanwhile, the numerical analyses indicate that the variation of the SHG efficiency is maximized at the MCD during the interfacial degradation. Using the nonlinear ultrasonic measurement-based experimental setup, the time-domain signal of the second harmonic generated at different propagation distances is conveniently extracted, and then the relative nonlinear acoustic parameter curve consistent with the theoretical prediction is obtained. For examining the influence of interfacial property on the SHG effect of low-frequency Lamb wave propagation, the different annealing cycles of the thin adhesive layer (acrylics) are used to simulate minor changes in the interfacial property of the given two-layered plate. It is found that the relative nonlinear acoustic parameter at the MCD decreases monotonically and sensitively with the increment of annealing cycle number, which verifies the quantitative correlation between the SHG efficiency of low-frequency Lamb wave propagation and the degree of the interfacial degradation. The consistency between the numerical analysis and the experimental measurement shows the potential of using the SHG effect of low-frequency Lamb wave propagation to characterize a minor change in the interfacial properties of a layered composite plate. [ABSTRACT FROM AUTHOR]

• An omnidirectional high-order SH wave piezoelectric transducer (OSH n -PT) was proposed. • The excitation function of the OSH n -PT was derived explicitly. • Simulations and experiments validate the significant mode selectivity of the OSH n -PT. • The SNR of the OSH 1 -PT was over 19 dB in pitch-catch tests (30.7 dB at 490 kHz). • An OSH 2 -PT was theoretically designed and validated by FE simulations. Guided wave tomography, as an advanced structural health monitoring (SHM) method, has offered a feasible solution to wall thickness quantification which is essential in petrochemical industries. However, previously used low-frequency Lamb waves (A 0 and S 0) limit the resolution of tomography. Recently, the first-order shear horizontal guided wave (SH 1) was found very promising in tomography for its capability in resolution improvement. However, the SHM-required omnidirectional piezoelectric transducers for selectively generating and receiving the SH 1 wave, namely OSH 1 -PT, are not available yet. In this work, a general method was developed to design an OSH 1 -PT based on the thickness-poled PZT half-ring configuration. Firstly, the excitation function of the OSH 1 -PT was explicitly derived and validated through finite element simulations. Secondly, a design formula was obtained and used to determine the size of the OSH 1 -PT. Then, the designed OSH 1 -PT was fabricated and tested by using a 2D laser Doppler vibrometer. Significant mode selectivity was observed in all directions (0 ∼ 90°) with the excited SH 1 to SH 0 ratio higher than 15 dB. Pitch-catch tests were conducted from 400 ∼ 520 kHz and the received SH 1 to SH 0 ratio was found higher than 19 dB at all frequencies and reached its maxima of 30.7 dB at 490 kHz, which is very close to the designed working frequency of 500 kHz. Finally, an OSH 2 -PT was theoretically designed and validated by FE simulations. Due to its simplicity and effectiveness in designing the OSH n -PT, the proposed method is expected to pave the road to wide applications of high-order SH wave tomography. [ABSTRACT FROM AUTHOR]

• Investigating an innovative approach to determine the damping property of a viscoelastic coating using ultrasonic Lamb wave in a metal-polymer waveguide. • Dropping amplitude signals as well as changing the phase of Lamb wave when travels through the coated waveguide. • Exploiting the obtained damping property in FEM software, ABAQUS. • Reducing computational cost and time by taking advantage of the experiment-based fitting method. • Verification of amplitude ratio as well as group velocity dispersion curves related to the FEM simulation by experimental results with an error of less than 1%. • Confirming the excitability of piezo discs in a range of frequency with tangible precision. This work presents an innovative approach according to an experiment-based fitting method to determine the damping property of a viscoelastic coating layer, in a simple, low-cost, and time-effective manner. In this experiment, symmetric and asymmetric ultrasonic Lamb waves were applied to two coated plates with different thicknesses, and the waves were generated using piezo discs. A viscoelastic coating influences the signal amplitude as well as the wave phase. By comparing the amplitude ratio (AR) of the transmitting signals between the coated and bare plates, the damping property of the viscoelastic coating was experimentally determined. Similar to the experiments employing the finite element method (FEM) software, in this experiment, ABAQUS, was employed to verify the conformity between numerical and experimental AR. By selecting a non-dispersive Rayleigh damping β for the coating layers at all frequencies, the computational cost reduced significantly to one-tenth the original cost. Apart from corroboration by AR matching, the numerical dispersion curves of the group velocity were also validated by experimental curves. The FEM dispersion curves in the frequency range of the tests were found to be highly reliable, with an average error of less than 1% for the first experimental setup and 10% for the second setup. Furthermore, in coated waveguides, the proposed technique could precisely estimate the damping property of the viscoelastic coating layers, where excitability in a wide range of frequencies is required. However, this precision strongly relies on the selected mode, frequency range, PZT quality, and waveguide thickness. [ABSTRACT FROM AUTHOR]

The inspection of sizeable plate-based metal structures such as storage tanks or marine vessel hulls is a significant stake in the industry, which necessitates reliable and time-efficient solutions. Although Lamb waves have been identified as a promising solution for long-range non-destructive testing, and despite the substantial progress made in autonomous navigation and environment sensing, a Lamb-wave-based robotic system for extensive structure monitoring is still lacking. Following previous work on ultrasonic Simultaneous Localization and Mapping (SLAM), we introduce a method to achieve plate geometry inference without prior knowledge of the material propagation properties, which may be lacking during a practical inspection task in challenging and outdoor environments. Our approach combines focalization to adjust the propagation model parameters and beamforming to infer the plate boundaries location by relying directly on acoustic measurements acquired along the mobile unit trajectory. For each candidate model, the focusing ability of the corresponding beamformer is assessed over high-pass filtered beamforming maps to further improve the robustness of the plate geometry estimates. We then recover the optimal space-domain beamformer through a simulated annealing optimization process. We evaluate our method on three sets of experimental data acquired in different conditions and show that accurate plate geometry inference can be achieved without any prior propagation model. Finally, the results show that the optimal beamformer outperforms the beamformer resulting from the predetermined propagation model in non-nominal acquisition conditions. • Ultrasonic measurements provide useful information for the robotic inspection of large metal structures. • Lamb waves reflections can be used to recover a metal structure geometry with sufficient precision. • The wave propagation model can be recovered simultaneously with the structure geometry through optimal beamforming. • Optimal beamforming yields accurate mapping results even when the acquisition conditions deteriorate. [ABSTRACT FROM AUTHOR]

In recent years, several fitting techniques have been presented to reconstruct the parameters of a plate from its Lamb wave dispersion curves. Published studies show that these techniques can yield high accuracy results and have the potential of reconstructing several parameters at once. The precision with which parameters can be reconstructed by inverting Lamb wave dispersion curves, however, remains an open question of fundamental importance to many applications. In this work, we introduce a method of analyzing dispersion curves that yields quantitative information on the precision with which the parameters can be extracted. In our method, rather than employing error minimization algorithms, we compare a target dispersion curve to a database of theoretical ones that covers a given parameter space. By calculating a measure of dissimilarity (error) for every point in the parameter space, we reconstruct the distribution of the error in that space, beside the location of its minimum. We then introduce dimensionless quantities that describe the distribution of this error, thus yielding information about the spread of similar curves in the parameter space. We demonstrate our approach by considering both idealized and realistic scenarios, analyzing the dispersion curves obtained numerically for a plate and experimentally for a pipe. Our results show that the precision with which each parameter is reconstructed depends on the mode used, as well as the frequency range in which it is considered. • Database approach yields fast multiparameter material characterization. • Lamb mode inversion is sensitive to random noise. • Different combinations of material parameters can yield similar dispersion curves. • New values to describe reliability of multiparameter Lamb mode inversion. [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]

A rapid, fully non-contact, hybrid system which encompasses an air-coupled transducer (ACT) and a laser Doppler vibrometer (LDV) is presented for profiling A 0 Lamb wave dispersion of an isotropic aluminum plate. The ACT generates ultrasonic pressure incident upon the surface of the plate. The pressure waves are partially refracted into the plate. The LDV is employed to measure the out-of-plane velocity of the excited Lamb wave mode at some distances where the Lamb waves are formed in the plate. The influence of the ACT angle of incidence on Lamb wave excitation is investigated and Snell’s law is used to directly compute Lamb wave dispersion curves including phase and group velocity dispersion curves in aluminum plates from incident angles found to generate optimal A 0 Lamb wave mode. The measured curves are compared to results obtained from a two-dimensional (2-D) Fast Fourier transform (FFT), Morlet wavelet transform (MWT) and theoretical predictions. It was concluded that the experimental results obtained using Snell’s law concept are well in accordance with the theoretical solutions. The high degree of accuracy in the measured data with the theoretical results proved a high sensitivity of the air-coupled and laser ultrasound in characterizing Lamb wave dispersion in plate-like structures. The proposed non-contact hybrid system can effectively characterize the dispersive relation without knowledge of neither the materials characteristics nor the mathematical model. [ABSTRACT FROM AUTHOR]

This research investigates a Lamb-wave based structural health monitoring approach matching an out-of-phase actuation of a pair of piezoceramic transducers at low frequency. The target is a typical quasi-isotropic carbon fibre reinforced polymer aeronautical laminate subjected to artificial, via Teflon patches, and natural, via suitable low velocity drop weight impact tests, delaminations. The performance and main influencing factors of such an approach are studied through a Design of Experiment statistical method, considering both Pulse Echo and Pitch Catch configurations of PZT sensors. Results show that some factors and their interactions can effectively influence the detection of a delamination-like damage. [ABSTRACT FROM AUTHOR]

Influence of attenuation on acoustic emission (AE) signals in Carbon Fiber Reinforced Polymer (CFRP) crossply and quasi-isotropic panels is examined in this paper. Attenuation coefficients of the fundamental antisymmetric (A 0 ) and symmetric (S 0 ) wave modes were determined experimentally along different directions for the two types of CFRP panels. In the frequency range from 100 kHz to 500 kHz, the A 0 mode undergoes significantly greater changes due to material related attenuation compared to the S 0 mode. Moderate to strong changes in the attenuation levels were noted with propagation directions. Such mode and frequency dependent attenuation introduces major changes in the characteristics of AE signals depending on the position of the AE sensor relative to the source. Results from finite element simulations of a microscopic damage event in the composite laminates are used to illustrate attenuation related changes in modal and frequency components of AE signals. [ABSTRACT FROM AUTHOR]

Defects that propagate from the inside of a structure can be difficult to detect by traditional non-destructive inspection methods. A non-contact inspection method is presented here that uses the near-field interactions of ultrasonic Lamb waves to detect defects propagating into a 1.5 mm thick aluminium sheet from the opposite side to that which is inspected. Near-field interactions of the S0 Lamb waves with the defects are shown to give rise to a characteristic increase in the wave magnitude, which is used to position and characterise these hidden defects. It is shown that such defects are difficult to detect from a study of their influence on ultrasonic transmission alone. Single defects of different depths, and systems of multiple defects with varying separations and relative depths, are successfully detected in both experimental trials and FEM simulations. Reliable single defect detection is achieved for defects with a minimum depth of 30 % of the plate thickness, and resolution of multiple defects is achieved for defect separations of 5 mm. [ABSTRACT FROM AUTHOR]

In this study the orthogonality relation-based method for post-processing finite element (FE) predictions and experimental measurements is applied in order to separate Lamb modes at a plate edge at normal incidence. The scattered wave field from the free edge is assumed to be a superposition of all the eigenmodes of an infinite plate. The eigenmode amplitudes of the reflected wave field are determined by implementing the orthogonality-based method on the measured plate edge displacements. Overlapping wavepackets of Lamb modes at a plate edge are simulated by using the FE model and the experiment in the case of an incident S 0 mode in a plate with a notch. In the experiment a 3D Scanning Laser Doppler Vibrometer (3D SLDV) (Johansmann and Sauer, 2005) is used to measure 3 dimensional vibrations and thus the edge two-dimensional displacement components simultaneously. It is demonstrated that it is possible to extract signals of various propagating and non-propagating modes in time-domain. The influences of the errors in practical measurements on the extraction procedure have also been studied. [ABSTRACT FROM AUTHOR]

The optimum topology of bimaterial phononic crystal (PhCr) plates with one-dimensional (1D) periodicity to attain maximum relative bandgap width of low order Lamb waves is computationally investigated. The evolution of optimized topology with respect to filling fraction of constituents, alternatively stiff scattering inclusion, is explored. The underlying idea is to develop PhCr plate structures with high specific bandgap efficiency at particular filling fraction, or further with multiscale functionality through gradient of optimized PhCr unitcell all over the lattice array. Multiobjective genetic algorithm (GA) is employed in this research in conjunction with finite element method (FEM) for topology optimization of silicon–tungsten PhCr plate unitcells. A specialized FEM model is developed and verified for dispersion analysis of plate waves and calculation of modal response. Modal band structure of regular PhCr plate unitcells with centric scattering layer is studied as a function of aspect ratio and filling fraction. Topology optimization is then carried out for a few aspect ratios, with and without prescribed symmetry, over various filling fractions. The efficiency of obtained solutions is verified as compared to corresponding regular centric PhCr plate unitcells. Moreover, being inspired by the obtained optimum topologies, definite and easy to produce topologies are proposed with enhanced bandgap efficiency as compared to centric unitcells. Finally a few cases are introduced to evaluate the frequency response of finite PhCr plate structures produced by achieved topologies and also to confirm the reliability of calculated modal band structures. Cases made by consecutive unitcells of different filling fraction are examined in order to attest the bandgap efficiency and multiscale functionality of such graded PhCr plate structures. [ABSTRACT FROM AUTHOR]

Composite materials are increasingly being used in advanced aircraft and aerospace structures. Despite their many advantages, composites are often susceptible to hidden damages that may occur during manufacturing and/or service of the structure. Therefore, safe operation of composite structures requires careful monitoring of the initiation and growth of such defects. Ultrasonic methods using guided waves offer a reliable and cost effective method for defects monitoring in advanced structures due to their long propagation range and their sensitivity to defects in their propagation path. In this paper, some of the useful properties of guided Lamb type waves are investigated, using analytical, numerical and experimental methods, in an effort to provide the knowledge base required for the development of viable structural health monitoring systems for composite structures. The laboratory experiments involve a pitch-catch method in which a pair of movable transducers is placed on the outside surface of the structure for generating and recording the wave signals. The specific cases considered include an aluminum plate, a woven composite laminate and an aluminum honeycomb sandwich panel. The agreement between experimental, numerical and theoretical results are shown to be excellent in certain frequency ranges, providing a guidance for the design of effective inspection systems. [ABSTRACT FROM AUTHOR]

As a type of broadband source of ultrasonic guided waves, laser pulses can be used to launch all modes of interests. In this paper, Lamb waves are excited by imposing heat flux mimicking the supply of the heat from laser pulses, and effects by defects on the received Lamb waves in a plate are investigated by means of the finite element method. In order to alleviate the heavy computational cost in solving the coupled finite element equations, a sub-regioning scheme is employed, and it reduces the computational cost significantly. A comparison of Lamb waves generated by unfocused and line-focused laser sources is conducted. To validate numerical simulations, the group velocity of A 0 mode is calculated based on the received signal by using the wavelet transform. The result of A 0 mode group velocity is compared with the solution of Rayleigh–Lamb equations, and close agreement is observed. Lamb waves in a plate with defects of different lengths are examined next. The out-of-plane displacement in the plate with a defect is compared with the displacement in the plate without defects, and the wavelet transform is used to determine the arrival times of Lamb waves traveling at the A 0 mode group velocity. A strong correlation is observed between the extent of defects and the magnitude of wavelet coefficients. [ABSTRACT FROM AUTHOR]

The Peano-series expansion is used to investigate the propagation of the lowest-order symmetric ( S 0 ) and antisymmetric ( A 0 ) Lamb wave modes in a functionally graded piezoelectric material (FGPM) plate. Aluminum nitride has been retained for illustration, it is polarized along the thickness axis, and at the same time the material properties change gradually perpendicularly to the plate with an exponential variation. The effects of the gradient variation on the phase velocity and the coupling electromechanical factor are obtained. Appropriate curves are given to reflect their behavior with respect to frequency. The highest value of the electromechanical coupling factor has been observed for S 0 mode, it is close to six percent, conversely for A 0 mode it does not exceed 1.5%. The coupling factor maxima undergo a shift toward the high frequency area when the corresponding gradient coefficient increases. The Peano-series method computed under Matlab software, gives rapid convergence and accurate phase velocity when analysing Lamb waves in FGPM plate. The obtained numerical results can be used to design different sensors with high performance working at different frequency ranges by adjusting the extent of the gradient property. [ABSTRACT FROM AUTHOR]

We propose hybrid phononic-crystal plates which are composed of periodic stepped pillars and periodic holes to lower and widen acoustic band gaps. The acoustic waves scattered simultaneously by the pillars and holes in a relevant frequency range can generate low and wide acoustic forbidden bands. We introduce an alternative double-sided arrangement of the periodic stepped pillars for an enlarged pillars' head diameter in the hybrid structure and optimize the hole diameter to further lower and widen the acoustic band gaps. The lowering and widening effects are simultaneously achieved by reducing the frequencies of locally resonant pillar modes and prohibiting suitable frequency bands of propagating plate modes. [ABSTRACT FROM AUTHOR]

• Developed a wave mixing technique to detect early material degradation. • Analyzed combined harmonics theoretically and experimentally. • Cubic nonlinearity parameter proportional to propagation distance. • Existing nonlinear ultrasonic techniques are compared with the proposed technique. Early detection of accumulated damage or material degradation in structures is important to ensure their structural safety. Nonlinear ultrasonic techniques are widely used to measure the quadratic nonlinearity parameter that represents the third-order elastic constants of materials for material degradation detection. In addition, there are ongoing efforts to exploit both the third- and fourth-order elastic constants that describe the cubic nonlinearity parameter to detect material degradation. This study develops a nonlinear ultrasonic three-wave mixing technique to generate and measure third-order combined harmonics (TOCH) in plate-like structures and to measure cubic nonlinearity parameter. The proposed three-wave mixing technique generates three primary Lamb waves in the target structure and measures the TOCH produced by nonlinear cross-interaction of the primary Lamb waves. A theoretical model is developed to describe the generation of TOCH in a nonlinear elastic and homogeneous plate, and the effectiveness of the theoretical model is validated experimentally. Measurements of the TOCH are conducted for intact and degraded aluminum specimens with different degradation levels. Because inherent material nonlinearity and material degradation alter the third- and fourth-order elastic constants of a structure, the three-wave mixing technique for measuring the TOCH can be used to identify the inherent material nonlinearity and material degradation. In particular, the experimental results indicate that the proposed technique is more sensitive to early-stage material degradation than existing nonlinear ultrasonic techniques such as two-wave mixing, and third harmonic generation techniques. [ABSTRACT FROM AUTHOR]

Lamb wave tomography (LWT) is a potential and efficient technique for non-destructive tomographic reconstruction of damage images in structural components or materials. A two-stage inverse algorithm proposed by the authors for quickly reconstructing the damage images was applied to hollow cylinders. An aluminum hollow cylinder with an internal surface pit and a Carbon Fiber Reinforced Plastic (CFRP) laminated hollow cylinder with an artificial internal surface damage were used to validate the proposed method. The results show that the present method is capable of successfully reconstructing the images of the above damages in a larger inspection area with much less experimental data compared to some conventional ultrasonic tomography techniques. [ABSTRACT FROM AUTHOR]

This paper proposes a new multichannel time reversal focusing (MTRF) method for circumferential Lamb waves which is based on modified time reversal algorithm and applies this method for detecting different kinds of defects in thick-walled pipe with large-diameter. The principle of time reversal of circumferential Lamb waves in pipe is presented along with the influence from multiple guided wave modes and propagation paths. Experimental study is carried out in a thick-walled and large-diameter pipe with three artificial defects, namely two axial notches on its inner and outer surface respectively, and a corrosion-like defect on its outer surface. By using the proposed MTRF method, the multichannel signals focus at the defects, leading to the amplitude improvement of the defect scattered signal. Besides, another energy focus arises in the direct signal due to the partial compensation of dispersion and multimode of circumferential Lamb waves, alongside the multichannel focusing, during MTRF process. By taking the direct focus as a time base, accurate defect localization is implemented. Secondly, a new phenomenon is exhibited in this paper that defect scattered wave packet appears just before the right boundary of truncation window after time reversal, and to which two feasible explanations are given. Moreover, this phenomenon can be used as the theoretical basis in the determination of defect scattered waves in time reversal response signal. At last, in order to detect defects without prior knowing their exact position, a large-range truncation window is used in the proposed method. As a result, the experimental operation of MTRF method is simplified and defect detection and localization are well accomplished. [ABSTRACT FROM AUTHOR]

Surface wave motions generated by a time-harmonic point load applied at the surface of an isotropic linearly elastic half-space are conventionally solved by the use of integral transform techniques. The inverse transforms, are often complicated and will not always yield closed-form solutions. In this paper expressions for the displacements for surface wave motions radiated from point-load excitation are determined in a simple manner by the use of the elastodynamic reciprocity theorem. It is shown that the radiated amplitudes of the surface displacements obtained by the reciprocity approach are identical to the corresponding results obtained by the use of Hankel transform and by Lamb in his classical paper. [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]

This article presents a review of air-coupled ultrasonics employed in the characterization or nondestructive inspection of industrial materials. Developments in air-coupled transduction and electronics are briefly treated, although the emphasis here is on methods of characterization and inspection, and in overcoming limitations inherent in the use of such a tenuous sound coupling medium as air. The role of Lamb waves in plate characterization is covered, including the use of air-coupled acoustic beams to measure the elastic and/or viscoelastic properties of a material. Air-coupled acoustic detection, when other methods are employed to generate high-amplitude sound beams is also reviewed. Applications to civil engineering, acoustic tomography, and the characterization of both paper and wood are dealt with here. A brief summary of developments in air-coupled acoustic arrays and the application of air-coupled methods in nonlinear ultrasonics complete the review. In particular, the work of Professor Bernard Hosten and his collaborators at Bordeaux is carefully examined. [ABSTRACT FROM AUTHOR]

Highlights: [•] We developed a semi-analytical finite element method for calculating leaky Lamb waves. [•] The calculation results agreed well with the previous theoretical studies. [•] We discussed dispersion curves for total transmission mode and leaky plate modes. [•] Attenuation curves and displacement distributions were also analyzed. [Copyright &y& Elsevier]

Highlights: [•] Three different misalignments – linear, orientation and synchronised orientation were introduced in air-coupled probes. [•] Experiments were carried out on a metallic plate employing the air-coupled probes. [•] A o mode was captured in various configurations and its amplitude was normalised with respect to the reference value. [•] Studied variation in normalised amplitude with the misalignment variable. [•] Gaussian curves were found to be representing appropriately the variation in amplitude with the misalignment variable. [ABSTRACT FROM AUTHOR]

Highlights: [•] We experienced the use of a PA system with two probes to study guided waves. [•] We used brass plate and bone plate as samples. [•] We studied the effect of steering angle on the excitation of guided waves in plates. [•] We found the change of the phase velocity spectrum with steering angles in agreement with the source influence theory. [•] It is feasible to use the PA system with two probes to perform in vivo study. [ABSTRACT FROM AUTHOR]

Highlights: [•] Experimental study is made on an immersed rough plate which is investigated with Lamb waves. [•] Numerical simulations are made in order to help experiments. [•] S 0 and A 0 Lamb waves are excited at different and low frequencies. [•] The retro-converted waves are detected even if their energy is weak. [ABSTRACT FROM AUTHOR]

Highlights: [•] Interaction of guided Lamb waves with structural discontinuity studied with FEM. [•] Generation of “turning” and “direct” modes in a helicopter rotor blade section. [•] Experimental correlation of measured A-Scan, velocity data with numerical data. [•] Use of turning and direct modes to inspect damage on spar of rotor blade. [ABSTRACT FROM AUTHOR]

Highlights: [•] A dedicated modeling technique for comprehending nonlinearity of ultrasonic waves. [•] An theoretical study on influence of fatigue damage on nonlinearity of ultrasonic waves. [•] Use of embeddable active sensing network to acquire nonlinearity of ultrasonic waves. [•] A strategy for evaluating initial fatigue damage and monitoring its growth. [ABSTRACT FROM AUTHOR]

Highlights: [•] Maximum and minimum of the surface displacements of Lamb waves is discussed. [•] Location of the vanishing displacements is predicted by the crossing of the bounds. [•] Experimental influence is demonstrated by a laser-ultrasonic technique. [•] Periodic variation of the maximum and minimum is predicted. [ABSTRACT FROM AUTHOR]

Highlights: [•] We develop a spatial transversal filter model for meander-line coil EMATs. [•] The generated wavelengths have to belong to the wavelength spectrum of the EMAT. [•] The characteristics of multiple wavelengths will cause the multi-modes phenomenon. [•] Developing a new meander-line coil with special geometric pattern. [•] Proposing a method for removing the effect of multiple wavelengths. [Copyright &y& Elsevier]

Highlights: [•] Lamb waves are generated by the center of a bounded beam. [•] Rayleigh waves are generated by the borders of a bounded beam. [•] Experimental evidence is shown based on acousto-optic imaging. [Copyright &y& Elsevier]

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]

Highlights: [•] A new method of flaw severity characterization using pattern classification is presented. [•] We explore advanced analysis techniques for studying multi-mode Lamb wave signals. [•] Pattern classification provides a formal method of characterizing flaw interaction. [•] Several feature selection techniques are explored that are unique for damage detection. [•] Experimental data shows agreement between predicted and known class labels. [ABSTRACT FROM AUTHOR]

• A spectrogram decomposition strategy combining nonnegative matrix factorization and adaptive basis learning is developed for automatic mode extraction under severe overlapping and low signal-to-noise ratio (SNR). • The proposed method provides an accurate individual mode extraction method, and the extraction of each mode does not modify the energy distribution of other modes. • The proposed method facilitates ultrasonic guided waves based long cortical thickness assessment. Due to its multimode and dispersive nature, ultrasonic guided waves (UGWs) usually consist of overlapped wave packets, which challenge accurate bone characterization. To overcome this obstacle, a classic idea is to separate individual modes and to extract the corresponding dispersion curves. Reported single-channel mode separation algorithms mainly focused on offering a time–frequency representation (TFR) where the energy distributions of individual modes were apart from each other. However, such approaches are still limited to identifying the modes without significant overlapping in time–frequency domain. In this study, a spectrogram decomposition technique was developed based on a combination strategy of generalized separable nonnegative matrix factorization (GS-NMF) and adaptive basis learning, towards the automatic mode extraction under severe overlapping and low signal-to-noise ratio (SNR). The extracted modes were further used for cortical thickness estimation. The method was verified using broadband simulated and experimental datasets. Experiments were conducted on a bone-mimicking plate and bovine cortical bone plates. For simulated data, the relative errors between extracted and theoretical dispersion curves are 1.33% (SNR = ∞), 1.43% (SNR = 10 dB) and 0.88% (SNR = 5 dB). The root-mean-square errors of the estimated thickness for 3.10 mm-thick bone-mimicking plate, 3.83 mm- and 4.00 mm-thick bovine cortical bone plates are 0.039 mm, 0.049 mm, and 0.052 mm, respectively. It is demonstrated that the proposed method is capable of separating multimodal UGWs even under significantly overlapping and low SNR conditions, further facilitating the UGW-based cortical thickness assessment. [ABSTRACT FROM AUTHOR]

In this study, the effect of friction on the generation of dry-coupled Lamb waves is experimentally investigated. Recurrence analysis is performed to analyze the complex behavior of friction based dry-coupled Lamb waves. In particular, the effect of the normal force, which is necessary for a stronger dry-coupled Lamb wave generation and the friction, on the transmission of mechanical energy and determinism characteristics of Lamb waves are investigated. The results verify that larger friction coefficient and friction force are crucial for generation and propagation of strong Lamb waves supporting the fact that the main mechanism to transfer mechanical energy using dry-couplant is friction. The sensitivity of Lamb waves to the friction coefficient, highlights the importance of designing specific pads with respect to condition of the surface. Besides, the results show that the normal force and friction coefficient can change the determinism characteristics behavior of multimode Lamb waves. Furthermore, it is shown that the determinism value is sensitive to the friction coefficient and normal force. A similar trend is observed in the determinism values and friction coefficient. In general, a smaller friction coefficient indicates smaller determinism value. Additionally, it is shown that a normal load can change the behavior of a system, as observed from recurrence plots, owing to changes in the Lamb waves trajectories in the phase-space domain. In addition, it is shown that recurrence plots enable the detection of mode transitions in multimode Lamb waves. Recurrence analysis is a complementary tool to frequency domain methods for accurate analysis of multimode Lamb waves behavior. • The effect of friction and normal force in exciting dry-coupled Lamb waves is studied. • To increase the SNR the friction between pads and surface has to be maximized. • Recurrence analysis is applied to analyze dry-coupled ultrasonic Lamb waves. • It is shown that the determinism of ultrasonic Lamb waves and friction are correlated. • Recurrence plot is successfully used for Lamb wave modes arrival detection. [ABSTRACT FROM AUTHOR]

• A metamaterial ultrasound mode convertor is proposed and investigated to achieve complete mode conversion from Lamb waves into shear horizontal waves. • The mechanism behind the mode conversion phenomenon is revealed via finite element analyses. • The complete mode conversion behavior is demonstrated using both numerical simulations and experimental observations. • The mode convertor allows the switching of interrogative wave mode for SHM/NDE applications. This article reports a new mechanism involving a non-perforated resonant elastic metamaterial to achieve the complete conversion of Lamb waves (A 0 and S 0) into the fundamental shear horizontal (SH 0) wave. The proposed metamaterial ultrasound mode convertor is studied via the observation of the special resonant shear motion of its unit cells, initiating with a conventional additive stub design. Thereafter, such a stubbed structure is further modified to fully couple the Lamb modes with the shear horizontal stub motion. By investigating the band structure of the metamaterial unit cell through modal analysis and tuning the shear resonant motions, a complete SH 0 mode generation band within the simultaneous Lamb modes bandgap can be established in a wide frequency range. Such a special bandgap situation enables the complete mode conversion from Lamb waves into shear horizontal waves. The transformation capability of the proposed ultrasound mode convertor is further substantiated via the harmonic analysis of metamaterial chain model, showcasing the frequency spectrum of the transmitted wave modes. The optimal configuration is determined by conducting a parametric study to identify the most effective mode conversion performance. Finally, a coupled-field transient finite element simulation is carried out to acquire the dynamic response of the structure. The frequency-wavenumber analysis of the transmitted wave field illuminates the successful realization of the mode conversion behavior. Experimental demonstrations are presented to validate the numerical predictions. The proposed complete mode conversion capability may possess great potential for wave control and manipulation. [ABSTRACT FROM AUTHOR]