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• We characterized the pressure wave produced by the Tonga volcano using cosmic rays. • We believe this is the first time that a Lamb wave is studied using cosmic rays. • Our high-altitude data provides additional information about this unique event. High-energy cosmic rays that hit the Earth can be used to study large-scale atmospheric perturbations. After a first interaction in the upper parts of the atmosphere, cosmic rays produce a shower of particles that sample it down to the detector level. The HAWC (High-Altitude Water Cherenkov) gamma-ray observatory in Central Mexico at 4,100 m elevation detects air shower particles continuously with 300 water Cherenkov detectors with an active area of 12,500 m2. On January 15th, 2022, HAWC detected the passage of the pressure wave created by the explosion of the Hunga volcano in the Tonga islands, 9,000 km away, as an anomaly in the measured rate of shower particles. The HAWC measurements are used to determine the propagation speed of four pressure wave passages, and correlate the variations of the shower particle rates with the barometric pressure changes. The profile of the shower particle rate and atmospheric pressure variations for the first transit of the pressure wave at HAWC is compared to the pressure measurements at the Tonga island, near the volcanic explosion. By using the cosmic-ray propagation in the atmosphere as a probe for the pressure, it is possible to achieve very high time-resolution measurements. Moreover, the high-altitude data from HAWC allows to observe the shape of the pressure anomaly with less perturbations compared to sea level detectors. Given the particular location and the detection method of HAWC, our high-altitude data provides valuable information that contributes to fully characterize this once-in-a-century phenomenon. [ABSTRACT FROM AUTHOR]

• A convolutional sparse coding-based UNet (CSCUNet) is proposed to analyze the ultrasonic Lamb wave signal, which enables high-resolution imaging of damage in composite laminates. • The CSCUNet incorporates the multi-layer convolutional sparse coding block in its encoder, enhancing both performance and interpretability of the model. • The effectiveness of the proposed method has been verified through testing on both numerical simulations and experimental data obtained from the surfaces of composite specimens. With the increasing utilization of composite materials due to their superior properties, the need for efficient structural health monitoring techniques rises rapidly to ensure the integrity and reliability of composite structures. Deep learning approaches have great potential applications for Lamb wave-based damage detection. However, it remains challenging to quantitatively detect and characterize damage such as delamination in multi-layered structures. These deep learning architectures still lack a certain degree of physical interpretability. In this study, a convolutional sparse coding-based UNet (CSCUNet) is proposed for ultrasonic Lamb wave-based damage assessment in composite laminates. A low-resolution image is generated using delay-and-sum algorithm based on Lamb waves acquired by transducer array. The encoder-decoder framework in the proposed CSCUNet enables the transformation of low-resolution input image to high-resolution damage image. In addition, the multi-layer convolutional sparse coding block is introduced into encoder of the CSCUNet to improve both performance and interpretability of the model. The proposed method is tested on both numerical and experimental data acquired on the surface of composite specimen. The results demonstrate its effectiveness in identifying the delamination location, size, and shape. The network has powerful feature extraction capability and enhanced interpretability, enabling high-resolution imaging and contour evaluation of composite material damage. [ABSTRACT FROM AUTHOR]

• An improved cased-hole ultrasonic RTM approach is proposed to image the annulus-formation interface in cased wells. • The missing ultrasonic waveforms between far and near receivers are reconstructed using phase-shift interpolation. • The hybrid illumination imaging condition is applied to mitigate the imaging noises around the true reflectors. • Field examples demonstrate that the proposed approach has the potential to detect the absence of cement. Cased-hole logging using ultrasonic pitch-catch modality is a proven technique for cement bond evaluation in cased holes. However, complex measuring environments such as tool and casing eccentering makes it difficult to precisely separate and pick the third interface echoes from full ultrasonic Lamb waves, leading to ambiguous identification of annulus-formation interface. To overcome this problem, we propose an improved cased-hole reverse time migration approach for ultrasonic pitch-catch measurements to image the annulus-formation interface. The missing ultrasonic Lamb waveforms between far and near receivers due to insufficient spatial sampling are reconstructed based on the optimized theoretical phase velocity of zero-order anti-symmetric Lamb mode waves. Additionally, we apply the hybrid illumination imaging condition to mitigate the imaging noises around the true reflectors and sources. Data examples from a physical experimental well and a field well demonstrate that the proposed approach is an effective method for characterizing the annulus-formation interface without requiring precise knowledge of the velocity distribution in the region behind the casing. Furthermore, test on an experimental well has indicated that the method has the potential to detect the absence of cement in the annulus medium. [ABSTRACT FROM AUTHOR]

• The improved VMD and DWT are combined to denoise and reconstruct the signal under strong noise without a reference. • The selection of wavelet function and transform level in DWT is optimized by Shannon entropy to denoise the signal without a reference. • The RAPID and the CNN are used to achieve spatial localization and visualization of the composite delamination. In this paper, a damage spatial imaging approach based on novel signal extraction is suggested to reconstruct the Lamb wave response signal under strong noise and realize the spatial localization of damage. First, the Variable Mode Decomposition (VMD) parameters are optimized by the improved Grey Wolf optimization method (IGWO) to decompose the response signal. To rebuild the response signal, the correlation coefficient is used to choose the optimal modal component and the residual. To give the best wavelet function and transform level for adaptive denoising of the reconstructed signal without a reference signal, an enhanced Discrete Wavelet Transform (DWT) based on Shannon entropy is proposed. To achieve damage localization imaging, a damage spatial localization model is built utilizing a reconstruction algorithm for probabilistic inspection of damage (RAPID) approach and a convolutional neural network (CNN). The suggested method may successfully increase the signal-to-noise ratio (SNR) of the reconstructed response signal and lower the error of spatial localization under strong noise through experiments. The spatial localization of composite damage using Lamb wave under strong noise is expanded in this paper. [ABSTRACT FROM AUTHOR]

Air-coupled ultrasonic Lamb wave damage imaging is a method for rapid, non-contact damage detection of plate specimens. However, the current technique based on Lamb waves has limitations regarding accuracy and quality of imaging. To address this issue, an improved bilateral boundary-enhanced reconstruction algorithm for the probabilistic inspection of defects (BBERAPID) is proposed. This algorithm optimizes the probability distribution near the damage boundary to enhance the accuracy of damage imaging. The skewed bilateral boundary distribution algorithm can improve the probability distribution of edges on both sides of the damage and alleviate the problem of abnormal boundary expansion. In addition, a new damage index (DI) calculation method is proposed to accelerate the DI trend near the damage. The new DI can improve the fineness of the damage boundary, thus better distinguish the healthy area from the damage area. Traditional DI has the problem of abnormal distribution at the damage boundary, which BBERAPID solves. The probability distribution near the damage is optimized to improve the quality and accuracy of damage imaging. Experimental studies are performed using composite plates to validate the proposed method further. The effectiveness and accuracy of the proposed BBERAPID algorithm for detecting damages in composite plates are verified by comparing it with existing imaging methods through experimental validation. The proposed non-contact air-coupled Lamb wave damage imaging method is suitable for rapidly and accurately characterizing defects in composite plates. [ABSTRACT FROM AUTHOR]

Active Lamb wave detection is an effective and simple monitoring method for structural health monitoring (SHM). Piezoelectric lead zirconate titanate (PZT) ceramics are most commonly used as transducers to excite and receive Lamb wave. However, due to their hardness, thickness and brittleness, PZT ceramics are severely limited in detecting damage on curved surface structures. Herein, we report that flexible PZT film sensor is an excellent candidate for receiving Lamb wave used in Lamb wave-based SHM of aircraft. An improved piezoelectric coefficient d 33 (~130 pm V−1) is obtained in epitaxial PZT films grown on flexible inorganic mica substrates via van der Waals heteroepitaxy. Superior stable ferroelectricity and piezoelectricity retain in flexible PZT film after bending 104 cycles under a radius of 8 mm. As proven by experiment and simulation, flexible PZT film is demonstrated as an ultrasonic sensor with high sensitivity to be bonded on the curved aluminum plate for Lamb wave-based damage monitoring. Our work demonstrates that flexible PZT films have enormous potential for real-time light-weight and high-sensitivity receiver sensors for the SHM of aging aircraft. [ABSTRACT FROM AUTHOR]

• The Lamb wave dispersion curves have a significant rising stage before convergence. • Modal displacement in couple stress theory is different from classical theory. • Displacement components vanish for specific modes and frequencies. • The modal displacements are verified by mathematical derivations and calculations. In micro-scale structures, the influence of scale effects on the mechanical properties cannot be ignored, and the continuum mechanics theory cannot effectively describe guided wave propagation. This paper studies modal displacements in an isotropic micro/nano-scale plate — an essential factor for the mode selection of Lamb waves in non-destructive testing technology. To achieve this, the consistent couple stress theory, which additionally accounts for the impact of scale effects on wave propagation, has to be employed. A mathematical derivation method with singular value decomposition is proposed, and the excitation amplitude is computed by applying a normal stress source to evaluate the in-plane and out-of-plane displacements. The results indicate that the in-plane displacement vanishes on the plate surface for anti-symmetric modes when the phase velocity equals the velocity of shear waves. Moreover, the intersections of symmetric and anti-symmetric dispersion curves identify specific frequencies at which the out-of-plane displacement vanishes on the free surface. These findings highlight the difference in modal displacement characteristics between micro-scale and macro-scale structures. The results from this study may furnish theoretical guidance for ultrasonic non-destructive testing of microstructures. [ABSTRACT FROM AUTHOR]

• Damage imaging is achieved under an excitation with a small amplitude. • Long-duration excitation is used to improve transmitting energy. • Pulse compression and dispersion compensation is used to improve resolution. Considering a trade-off between temporal-spatial resolution and multi-mode nature of Lamb waves, tone bursts with short durations are usually used as excitations in Lamb wave based damage detection. A short-duration excitation usually requires a large amplitude to carry sufficient energy so as to obtain response signals with enough signal-to-noise ratio and cover a large inspection area. In this paper, an alternative Lamb wave damage imaging method using nonlinear chirp (nonlinear frequency modulation, NLFM) excitation with a long duration and a small amplitude is proposed. The signal processing techniques of pulse compression and dispersion compensation are adopted to compress the long-duration wave packets of response signals into short ones. Compared with conventional tone burst excitations with short durations and small amplitudes, due to the long duration of the nonlinear chirp excitation and the use of pulse compression, sufficient energy can be applied to transducers under small amplitude excitations so the image contrast in imaging will not degrade. Furthermore, as large amplitude excitations are no longer required, high voltage amplifiers are not necessary so the hardware of the Lamb wave testing system is simplified. Experiments on a carbon steel plate with an artificial crack are carried out and Lamb wave signals are collected using a linear array consisting of nine PZTs. Experimental results under the NLFM signal and conventional tone bursts are provided. Experimental results show that under the condition of the same excitation amplitude, the proposed method under the NLFM excitation can achieve better imaging quality compared with methods under conventional tone bursts. [ABSTRACT FROM AUTHOR]

Lamb wave-based structural health monitoring offers excellent potential for real-time delamination detection in laminated structures. It necessitates fast and accurate numerical simulation of guided wave interaction with delaminations. Towards this objective, we present an efficient layerwise zigzag theory (ZIGT) based time-domain spectral element for wave propagation analysis of composite beam- and panel-type structures (strips) containing delamination at arbitrary locations. The ZIGT delivers accuracy like layerwise theories by allowing slope discontinuities in the in-plane displacement field at layer interfaces while maintaining computational efficiency like equivalent single-layer (ESL) theories, making it ideal for such analyses. The high-order elemental nodes at Lobatto points have only four displacement variables, irrespective of the number of layers in the laminate. Delamination is modelled using the region approach, adapting the recently proposed hybrid point-least squares continuity method to satisfy the nonlinear displacement field continuity at the delamination fronts. The model's efficacy is examined vis-à-vis elasticity-based elements, ESL theory-based elements, and the ZIGT-based standard finite element for free vibration and guided wave propagation responses of composite strips featuring multiple delaminations. The present model shows superior overall efficiency, accuracy, and convergence. An application of the model is also explored for identifying delamination locations from Lamb wave velocity fields. [ABSTRACT FROM AUTHOR]

• The MC1-DCGRU was proposed to solve the problem that 1-DCNN cannot capture time-varying information in Multi-channel Lamb waves. • Proposed an "end-to-end" damage localization model combining wavelet threshold denoising and MC1-DCGRU. • High-precision and high-resolution damage localization were realized with only single-point Lamb wave excitation and seven sensors. • Multi-point cumulative damage localization was realized by the MC1-DCGRU generalization capabilities, which could also be applied to self-healing CFRP. Lamb wave has become one of the most promising methods for carbon fiber-reinforced plastics (CFRP) damage detection. However, the wave velocity difference caused by CFRP anisotropy makes the time information-based localization methods lack accuracy. Obtaining high resolution and accuracy with limited excitations and sensors is difficult, which leads to difficulties in localizing small-size impact damage. Therefore, this paper proposed multi-channel one-dimensional convolutional gated recurrent unit (MC1-DCGRU) for surface dent deformation and inter-laminated damage caused by low-velocity impact behavior localization combining the feature extraction capability of the multi-channel one-dimensional convolutional neural network (MC1-DCNN) and the temporal information capturing ability of gated recurrent unit (GRU). The localization mean Euclidean distance (MED) of single-point and two-point cumulative damage is 4.29 mm and 5.78 mm, respectively. The proposed method overcomes high-resolution requirements for damage localization. It achieves higher accuracy without increasing excitations and sensors, which also achieves multi-point cumulative damage localization. [ABSTRACT FROM AUTHOR]

Impact monitoring is an important research topic within aircraft structural health monitoring. Piezoelectric lead zirconate titanate (PZT) ceramics are commonly used as sensors for detecting impact stress waves. However, due to their brittleness, high density, small ultimate strain, and poor impact resistance, PZT ceramics are severely limited in impact monitoring on curved surface structures. In this study, a novel 0–3 PZT/Silicone resin flexible piezoelectric composite sensor (PZT/SR-FPCS) was developed. It serves as an excellent candidate for an aircraft structural impact monitoring sensor. The developed PZT/SR-FPCS has an elastic modulus of 34.2 MPa lower than that of PVDF. Its yield failure strain of 7.2 % and fatigue failure strain of 9200 με are significantly higher than those observed in typical aircraft structures. The strain sensitivity ranges from about 8.51 mV/με to 9.81 mV/με, and the Lamb wave signal can still be correctly detected in a high-temperature environment of 75 ℃. To investigate the potential of PZT/SR-FPCS for impact monitoring, the stress wave sensing theory of PZT/SR-FPC is derived. Both FEA and experimental studies demonstrate that the dominant energy narrow-band of the impact stress wave in the plate structure is the A 0 mode Lamb wave. The impact source localization method based on the arrival time difference and wave velocity of the dominant energy narrow-band is proposed, and the impact source localization verification experiments are carried out on a curved structure using PZT/SR-FPCS. As proven by the experiment, impact source localization results coincide with the actual impact location. The study shows that the developed PZT/SR-FPCS and the proposed impact source localization method are both feasible and effective. [Display omitted] • Developed a novel 0–3 PZT/Silicone resin flexible piezoelectric composite sensor (PZT/SR-FPCS). • Performance tests confirm that the PZT/SR-FPCS is an excellent candidate for an aircraft structural impact monitoring sensor. • An impact source localization method based on the dominant energy narrow band of the impact stress wave is proposed. • As proven by FEA and experiments, PZT/SR-FPCS and the proposed impact source localization method are feasible and effective. [ABSTRACT FROM AUTHOR]

In addressing the construction of beam solutions for elastic guided waves in plates, we propose expressions of the amplitude function for the Lamb wave and the shear horizontal plate wave. The amplitude function is governed by an in-plane amplitude equation describing plate waves that has a same form as the two-dimensional Helmholtz equation. Therefore, beam solutions can be constructed within the same framework as this Helmholtz equation. The paraxial approximation is then applied to this in-plane amplitude equation. On the basis of this approximation, Gaussian beam solutions for both the Lamb and shear horizontal plate waves are derived. The effects of beam width, wavenumber, and guided-wave mode upon the wavefields are examined through several numerical examples. [Display omitted] • We defined an amplitude function governed by an in-plane amplitude equation. • The in-plane amplitude equation has a similar form to the 2D Helmholtz equation. • We derived Gaussian beam solutions for Lamb and shear horizontal plate waves. • Each guided-wave mode in the beam retains its original characteristics. • Diffraction diminishes with higher wavenumbers and wider beam widths. [ABSTRACT FROM AUTHOR]

The damage status of Ni-based single crystal alloy determines its remaining service life. However, the study of the damage status of Ni-based single crystal alloy in the early service stage has not been reported. Therefore, this paper will establish a theoretical relationship that reflects the interaction between the microstructure evolution of Ni-based single crystal alloy and the Lamb wave. Micro-defects of different specifications and sizes are actively produced by laser cratering on the surface of the Ni-based single crystal alloy. Then the prepared samples with different damage levels are experimentally measured, and the damage statuses of these samples are quantitatively characterized by nonlinear parameter. Finally, the molecular dynamics method is used to quantitatively calculate the dislocation density, dislocation chord length and stress of Ni-based single crystal alloy. Through experimental measurements, it can be observed that in the early damage stage of the Ni-based single crystal alloy, the second harmonic amplitude and nonlinear characteristic parameter enlarge with the increase of the micropit diameter. Molecular dynamics simulation can reveal the variation and cause of the second harmonic amplitude and nonlinear characteristic parameter. Moreover, it is found that with the increase of micropit depth, the corresponding second harmonic amplitude and nonlinear characteristic parameter become larger. And under the same micropit depth, the corresponding second harmonic amplitude and nonlinear characteristic parameter become larger with the increase of micropit diameter, which is consistent with the experimental results. The above research results fully indicate that the second harmonic amplitude and nonlinear characteristic parameter change with the degree of damage intensification, which can provide a theoretical basis for the prediction of remaining service life of Ni-based single crystal alloy in the early stage. • The relationship between the microstructure evolution of Ni-based single crystal alloy and the Lamb wave is established. • The damage statuses of these samples can be quantitatively characterized by nonlinear parameter. • The dislocation density and dislocation chord length of Ni-based single crystal alloy by molecular dynamics method. [ABSTRACT FROM AUTHOR]

Abstract The application prospect of a permanently installed structural health monitoring (SHM) system for composite structural prognosis may be determined by the reliability, cost and the added weight of the system. In this paper, we aim at exploiting information encoded in multipath scattering Lamb wave signals, so that damage imaging could be achieved with as few as possible sensors. Firstly, dispersion compensation is applied to the residual signal between two measurement sequences, and thus the propagation distance of each wave packet could be identified. Assuming that acoustic rays propagate according to Fermat's principle, the multipath scattering between the known features (e.g. edges, stiffeners, lap joints, and rivets) and the damage is equivalent to adding a virtual transducer distributed in a symmetrical position. Thus enlarges the element-number and the aperture of the sensor array. On this basis, a modified elliptical method is established to accommodate for these multipath scattering signals for damage imaging. An experimental example is introduced, where the specimen is a carbon fiber-reinforced polymer (CFRP) laminate and the damage is simulated by an added mass. The results show that the damage could be correctly detected and accurately localized even with a single transmitter-receiver pair. [ABSTRACT FROM AUTHOR]

Highlights • Dynamic fundamental solutions are derived for the first time for the Kane–Mindlin plate theory. • First BEM formulation based on the higher-order plate theory for extensional motion of plates. • The formulation is efficient for modelling high-frequency S0 Lamb wave mode. • The formulation is able to handle thickness-stretch displacement of the S0 mode and out-of-plane loads. Abstract This paper presents a new boundary element formulation for modelling the fundamental symmetric Lamb wave (S0 mode) propagating in the high frequency range. At such high frequencies, the S0 mode reveals significant dispersive character. Conventional BEM formulations governed by the generalised plane stress theory fail to accurately represent the dispersive properties of the S0 mode and to handle out-of-plane loads because the effects of thickness-stretch are not considered. Therefore, a new BEM formulation is proposed based on the dynamic fundamental solutions which are derived for the first time for a higher-order plate theory (Kane–Mindlin) taking into account coupling between extensional motion and the first mode of thickness vibration. Only plate edges are required to be discretized using simple line elements in the proposed BEM formulation. Three benchmark examples are presented where the solutions from the new BEM formulation are shown to be in excellent agreement with analytical and three-dimensional finite element results. Furthermore, the advantage of the proposed formulation is demonstrated through comparisons with the BEM results based on the generalised plane stress theory. [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]

• Simultaneous excitation of multiple transmitters is efficient for data acquisition. • Sparse reconstruction imaging for Lamb wave simultaneous excitation is proposed. • The proposed method is applied for high-resolution damage imaging in CFRP. Most Lamb wave methods utilize transducer array for damage detection and localization. Simultaneous excitation of multiple transmitters is a strategy for efficient data acquisition, in which matched filtering is generally applied for signal separation from different sources. However, the signal-to-noise ratio (SNR) and subsequent damage imaging performance is limited due to transmitter interference. To obtain both efficient data acquisition and high imaging performance, sparse reconstruction imaging for Lamb wave simultaneous excitation system is proposed in this paper that exploits the sparsity of structural damage. The key of success is to construct accurate damage-reflection Lamb wave model based on the linear characteristic of simultaneous excitation system. In addition, the model performance is further improved by excitation waveform design for each transmitter. The effectiveness of the proposed method is experimentally demonstrated with array imaging of simulated delamination in composite laminates. [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]

Abstract Ultrasonic, non-contact defect detection for polymer pipes can provide an easy and reliable method to establish on site quality control without destructive mechanical testing and finally may help to prevent damage from leaking pipes. The target is to inspect thin walled polymer pipes made of PVDF and PP with small defects in the pipe wall. Air coupled transducers with a nominal frequency of 200 kHz are used to excite Lamb waves in the pipe wall that interact with potential defects. A reference measurement on a flawless part of the pipe helps to mitigate the effects of geometrical and material inhomogeneities. The possibility to excite only the A 0 Lamb wave mode in thin plates and pipes using air coupled transducers is shown. Defects attenuate the propagating Lamb waves, resulting in a characteristic pattern in the receiver signal. Simulations on the Lamb wave propagation help to identify the dispersion parameters and allow to understand the effect of defects better. The detection of defects is challenging because of other imperfections in the specimen. With a reference measurement the detection of defects down to a size of 1 mm is possible. [ABSTRACT FROM AUTHOR]

Highlights • Transmission ratios of the S0 and A0 Lamb waves across a closed crack are analyzed. • Spring-type interface model is applied to model a partially closed crack. • S0 and A0 modes show remarkably different interaction with a closed crack. • Experimental results are presented for a fatigue crack under variable tensile loading. Abstract The transmission characteristics of Lamb waves across a partially closed through-thickness crack in a plate are investigated numerically and experimentally. In the numerical analysis, the spectral element method is used to simulate the transmission of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb modes across a crack in a low-frequency range. The analysis is carried out for an open crack with traction-free surfaces as well as for a partially closed crack modeled as a spring-type interface characterized by normal and tangential stiffnesses. The transmission ratios of both modes are obtained from the spectral amplitude of the simulated transmission waveforms for different crack lengths and interfacial stiffnesses. The numerical results show that the transmission ratio of the S0 mode increases monotonically with the interfacial stiffness, but that of the A0 mode depends on the interfacial stiffness in a non-monotonic manner depending on the frequency. The Lamb wave transmission measurements are carried out for aluminum alloy plates with artificial slits or a fatigue crack. The experimental results for the plates with slits show reasonable agreement with the numerical results for open cracks. The measured transmission ratio of the S0 mode is shown to decrease with the tensile load applied to the plate, but that of the A0 mode shows different load dependence for different frequencies. The qualitative features of the experimental results for the fatigue crack are discussed based on the numerical simulation for closed cracks. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a theoretical, numerical and experimental analysis of Lamb wave propagation and joint-debond identification in an advanced composite structure with core-junction. In the process, the wave modes in the recorded sensor-signals are effectively identified based on the theoretically obtained time-history response and dispersion curves for the sample structure. In order to study the core-junction and joint-debond effects, the finite element based numerical simulations were carried out in ABAQUS and the results verified with laboratory experiments. It was observed that the presence of core-junction in the structure reduces the propagating Lamb wave mode amplitudes, whereas the presence of localized joint-debond significantly increases the modal amplitudes. The study was further extended for the analysis of variable core-junction thickness and joint-debond size effects on the amplitude difference between the baseline and affected Lamb wave signals. Finally, a baseline-free debond detection strategy is proposed for the detection of hidden joint-debond locations in the target structure. Highlights • Lamb wave based analysis of core-core joint and joint-debond effects in an advanced composite structure are presented. • Presence of core-junction in the sandwich composite structures substantially reduces the wave mode amplitudes. • The increase in adhesive bond thickness further decreases the A0 mode amplitudes. • Presence of joint-debond at the core-junction significantly amplifies the propagating Lamb wave modes. • The increase in joint-debond size leads to a further increase in A0 mode amplitude. • A SHM strategy is proposed to effectively locate such joint-debond locations in the advanced composite structure. [ABSTRACT FROM AUTHOR]

Highlights • Damage localization method by the time-reversed Lamb waves is proposed for plates. • Mode conversion behavior of Lamb waves is considered in the time reversal process. • Localization of a notch-type defect in plates is numerically simulated. • Focused spots appear at the damage location by the proposed time reversal method. Abstract A damage localization method based on the time reversal focusing of the mode-converted scattered Lamb wave is proposed for plate structures with a non-symmetric defect in the thickness direction. Dual transducers are attached symmetrically on the upper and lower surfaces of the plate to selectively emit and receive the lowest-order symmetric (S0) and antisymmetric (A0) modes. The localization of damage is achieved by the numerical time-reversed (TR) simulation of the mode-converted Lamb wave generated at the defect. To investigate the validity of the proposed method, the signals of the Lamb waves in a plate with a partial-thickness notch are numerically simulated by the three-dimensional elastodynamic finite integration technique (EFIT). When the S0 mode is emitted in the damaged plate, not only the S0 mode is scattered but also the A0 mode is generated due to mode conversion at the notch. Similar mode conversion behavior is confirmed when the A0 mode is emitted. The time reversal of the mode-converted scattered Lamb waves creates focused spots at the damage location without using baseline data for the undamaged plate. The proposed method reduces the magnitude of the artifacts compared to the time reversal of the non-mode-converted Lamb wave, and yields the focused spot whose size is associated with the size of the notch and the half wavelength of the time-reversed wave mode. Furthermore, the proposed method is applied to a plate with a notch-type defect adjacent to an a priori known through-thickness hole, demonstrating the damage localization in a relatively complicated structure. [ABSTRACT FROM AUTHOR]

Highlights • The effect of ablation on the shape of laser-induced elastic waves is studied. • The elastic waves were generated and observed by projecting laser pulses. • Two numerical simulations were performed using heat flux and normal stress input. • The effects of thermal expansion and radiation pressure depend on energy intensity. Abstract In this paper, the effect of ablation on the shape of elastic waves generated by laser excitation is studied numerically and experimentally. Laser-induced ultrasound has been widely used in the nondestructive testing (NDT) field because it has the advantage that the sensor does not have to be directly attached to the target structure. In the safety assessment process, low energy excitation is used, and thus the structure is not damaged. Most studies related to laser ultrasound have focused on the method of detecting cracks within the elastic range, and there have been few studies on the effect of ablation. This research consists of experiments and numerical analyses. In experiments, elastic waves were generated in an aluminum plate by projecting laser pulses with different energy intensities. The velocities in the thickness direction were measured using a Laser Doppler Velocimeter (LDV) at a point 135 mm away from the excitation point. In the numerical study, two numerical simulations were carried out using heat flux and normal stress input to mimic laser pulse excitation. A thermo-mechanical simulation by heat flux was conducted to simulate thermal expansion by the laser pulse, and the normal stress was applied to reflect the effect of radiation pressure by ablation, respectively. Waveforms were synthesized by using different magnitude ratios of the obtained numerical responses and were compared with the experiment results. It is found that the effect of radiation pressure should not be neglected if the energy intensity is large although the effect of radiation pressure decreases as the energy intensity decreases. At the energy intensity with which ablation occurs, the effects of thermal expansion and radiation pressure exist simultaneously, and the contribution to the response depends on the energy intensity. [ABSTRACT FROM AUTHOR]

Highlights • Incremental Lamb wave scattering from a notch emanating from a through-hole is analyzed. • Multiple frequencies and hole diameters are considered as well as a notch with no hole. • Numerical results are validated with experimentally measured scattering patterns. • The presence of even a small hole significantly affects notch incremental scattering. • Incremental scattering from off-angle incident waves is a strong function of frequency. Abstract Lamb wave scattering from a crack originating at a through-hole is of practical importance because of the abundance of fastener holes used in engineering structures. Notches are often used to simulate cracks so that Lamb wave methods can be more conveniently investigated in the laboratory. A linear, three-dimensional finite element model is employed in this paper to study incremental scattering of the fundamental anti-symmetric (A 0 ) Lamb wave mode from notches emanating from through-holes. The term “incremental scattering” refers to the change in scattering caused by introduction of the notch and is motivated by structural health monitoring for which transducers are fixed and signal changes are interpreted to detect damage. Far-field angular scattering patterns are generated for multiple incident angles and frequencies, and such patterns are experimentally validated at one frequency by laser vibrometry measurements. Comparisons are made between a vertical notch alone (no hole) and notches located above and below the through-hole. Additionally, holes of different sizes are considered to investigate the effect of hole diameter on incremental scattering patterns. Results show that the presence, location and size of the through-hole affect both the shape and strength of notch incremental scattering patterns. [ABSTRACT FROM AUTHOR]

Highlights • The new series of differential equations of free vibration of thick functionally graded plates, including the lateral extension, have been derived. • Based on the dispersion relation and Lamb wave velocity, the validation of the differential equations has been investigated. • Legendre spectral element method is utilized to analyze the thick functionally graded plates subjected to impulsive loading. • The accuracy of the numerical simulation is examined by comparing the group wave velocity of first symmetrical and asymmetrical modes of Lamb waves derived from the dispersion relations and numerical models. Abstract In this paper, a new series of differential equations which can predict the propagation and dispersive behavior of first symmetric and antisymmetric Lamb wave modes in functionally graded material (FGM) thick plates are developed. Then, given the accuracy and cost effectiveness of Legendre spectral element method in combination with central difference method, these methods are utilized to set up the numerical models and solving the dynamic discretized differential equations. Finally, based on the wave front velocity, the percentage of error is estimated and it is shown that considering some limitations which are inherently included in the utilized method of error estimation, the proposed equations and the numerical procedure can predict the behavior of first symmetric and antisymmetric Lamb wave modes in FGM thick plates precisely. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract We investigate experimentally Lamb wave propagation in coupled-resonator elastic waveguides (CREWs) formed by a chain of cavities in a two-dimensional phononic crystal slab with cross holes. Wide complete bandgaps, extending from 53 to 88 kHz, are first measured in a finite phononic crystal slab sample. A straight waveguide and a wave splitting circuit with 90° bends are then designed, fabricated and measured. Elastic Lamb waves are excited by a piezoelectric patch attached to one side of the phononic slab and detected using a scanning vibrometer. Strongly confined guiding and splitting at waveguide junctions are clearly observed for several guided waves. Numerical simulations are found to be in excellent agreement with experimental results and allow for the identification of the involved resonant cavity modes. The influence on the dispersion of guided waves of the slab thickness and of the hole length is also investigated. The results have implications for the design of innovative phononic devices with strong confinement and tailorable dispersion. [ABSTRACT FROM AUTHOR]

Abstract Lamb waves offer an efficient way for monitoring the structural integrity of large carbon fiber reinforced polymer (CFRP) structures. This paper presents a new method that is able to detect and assess delamination's length in anisotropic CFRP plates. It uses a chirp signal, which contains a wide range of frequencies, as excitation. Thus the material is interrogated by Lamb waves with various wavelengths. This method is based on that, for a specific delamination length, only some particular frequency components are disturbed. Simulation and experimental results show that the lowest frequency that is perturbed by a delamination decreases as the delamination length increases. Therefore, by observing which frequency components of the chirp-exited Lamb wave are disturbed, delamination length can be estimated. Wavelet analysis is implemented for better comparison of signals at multiple frequencies. [ABSTRACT FROM AUTHOR]

Highlights • We propose a modified design of the omnidirectional electromagnetic acoustic transducer (EMAT) for antisymmetric (A0) Lamb wave generation. • Numerical simulation based on FEM is performed to characterize the magnetic configurations and determine the coil parameter of the EMAT. • The double-turn coil is used instead of a single spiral coil. • The proposed EMAT can generate and receive pure single A0 mode Lamb wave signal with high signal to ratio. Abstract This work presents a modified design of electromagnetic acoustic transducer (EMAT) that generates only antisymmetric (A0) omnidirectional Lamb wave. The new magnetic configuration includes a solid cylindrical magnet and a ring-shaped magnet surrounding the outside of the solid cylindrical magnet. The diameter of the solid cylindrical magnet is slightly small than the inner diameter of the ring-shaped magnet. The double-turn coil is used instead of a single spiral coil. Numerical simulation is applied to characterize the magnets and determine the coil parameters. Experimental results on a 3 mm-thick aluminum plate show that the modified EMAT we proposed can generate and receive pure single A0 mode Lamb wave with high signal to noise ratio and can effectively meet the imaging requirements. [ABSTRACT FROM AUTHOR]

We propose two-dimensional reconfigurable phononic crystal slabs that support topologically protected edge states for Lamb waves. It consists of a triangular lattice of triangular-like air holes perforated in a slab and two sandwich cylinders are placed in the matrix of each unit cell. The pseudospin–orbit coupling is achieved by shifting the sandwich cylinders up or down. The robust transport of Lamb waves along reconfigurable interfaces is further demonstrated. The dynamically reconfigurable design provides an excellent platform for tuning the frequency range of topological edge states and steering efficient transport of Lamb waves along arbitrary pathways, which can be employed for elastic-wave communications, signal processing, and sensing. [ABSTRACT FROM AUTHOR]

• The time–frequency diagrams are obtained based on the Normal Time-Frequency Transform, and the characterization effect on Lamb waves is discussed. • A method to characterize signal differences by Singular Value Decomposition and Dynamic Time Warping is presented. • A new damage index is developed to quantify the damage in composite structures, and the matching performance of damage trends is investigated using experimental tests. Damage index (DI) is of vital importance for the health monitoring of composite structures. However, most of the existing DI construction methods cannot reveal the rich information hidden in Lamb waves, such as the time-varying characteristics of the signal amplitude and phase, and thus cannot well characterize the evolutionary trend of damage. To address this limitation, this study proposes a novel DI construction method for damage identification in composite structures. The normal time–frequency transform is first applied to obtain the unbiased time–frequency distribution of the received Lamb wave signal, and then the singular value decomposition method is used to obtain the singular value sequence of the two-dimensional time–frequency matrix to characterize its matrix sparsity. Next, the dynamic time warping method is used to measure the distance between singular value sequences of the current and baseline signals. A quantitative DI is finally defined using these distances. Two experimental studies are utilized to validate the effectiveness of the proposed method. Results show that the proposed method can reveal the rich information hidden in the Lamb wave signals and has better trend-matching performance in fitting the actual damage propagation trend than traditional methods. [ABSTRACT FROM AUTHOR]

• A real time-reversal operation based on the Lamb wave to detect the damage rather than virtual. • A multi-modal superposition finite difference time domain method is proposed to simulate the Lamb waves propagation with little calculation cost, which facilitates damage imaging output quickly. • A maximum energy frame method is presented to automatically determine the focusing time from wavefield animation, and enabling the detection of multiple damage points. • A detailed comparison between the proposed method and other four Lamb wave-based damage detection methods is evaluated. This study proposes a wavefield reconstruction method with a time-reversal operation (W R -TR) based on the Lamb wave to detect the damage in the plate. Currently, it is challenging to implement the wavefield reconstruction method for damage detection because of two issues. One is how to quickly simulate the Lamb wavefield. The other is how to determine the focusing time for searching the desired frame from wavefield animation, which can show the location and size of damage. In response, this study introduces a multi-modal superposition finite difference time domain method (MS-FDTD) to simulate the Lamb waves propagation with little calculation cost, which facilitates damage imaging output quickly. In addition, a maximum energy frame method (MEF) is presented to automatically determine the focusing time from wavefield animation, enabling the detection of multiple damage points. The simulations and experiments have demonstrated good noise robustness, anti-distortion ability, and broad applicability with dense or sparse array layouts. Moreover, a detailed comparison between the proposed method and other four Lamb wave-based damage detection methods is evaluated in this paper. [ABSTRACT FROM AUTHOR]

• A method for liquid level measurement based on leaky lamb waves was presented to improve the sensitivity and accuracy of liquid level detection. • Liquid level detection with lamb waves was simulated using the finite element method. • The field experiment of liquid level detection was performed, and the comparison tests of the detection of lamb waves and longitudinal waves were conducted. The accurate detection of liquid-level plays an important role in industrial process. This paper presents a method for liquid-level measurement based on Lamb waves. The dispersion equation of Lamb waves under unilateral liquid load is deduced to obtain the dispersion curves. The structure and attenuation curves are analyzed under a certain frequency and thickness, and the modes with more energy loss are selected. The ratio of the signal energy of the liquid-level at the upper and lower positions is defined as the discrimination to indicate the change of the liquid-level. The models of the liquid storage tank are built and the Lamb waves of different modes are simulated using the finite element method. The experimental results reveal that it is easy to detect the difference in ultrasonic signals when the liquid-level changes. Compared with the longitudinal wave detection method, the proposed method improves the discrimination of liquid-level detection. The experimental results are consistent with the theoretical calculations. [ABSTRACT FROM AUTHOR]

• Legendre Orthogonal Polynomial Expansion (LOPE) method mapping the relationship between the wave velocity and initial stresses was developed to generate the input characteristics of linear and non-linear variation for deep neural network. • A Deep Neural Network (DNN) was combined with LOPE for the inverse of initial stress based on Lamb wave dispersion date rather than objective function obtained from wave equations. • The performances of ReLU activation function and Sigmoid activation function were illustrated in detail for current inverse work. The prediction of the initial stress in composites is essential for the non-destructive testing (NDT) and structural health monitoring (SHM) of carbon fibre reinforced polymer (CFRP). This paper examines the potential of Lamb waves in the inverse of initial stress by calculating the influence of initial stress on the dispersion characteristics of Lamb waves propagating in multilayered CFRP laminates. By introducing the mechanics of incremental deformation into the linear three-dimensional elasticity theory, the Legendre orthogonal polynomial expansion (LOPE) method is used to mathematically model the Lamb wave propagating in multilayered CFRP laminates subjected to horizontal and vertical homogeneous initial stresses. Then, a three-hidden-layers Feed Forward Deep Neural Network (DNN) with Back Propagation (BP) algorithm is constructed to invert the magnitude and direction of the initial stresses. The input features are the phase velocities of fundamental Lamb wave A 0 mode at five different frequencies. Both training and testing samples are obtained by LOPE forward calculation. An ablation experiment is presented to compare the two different activation functions. Finally, the accuracy of the inverse is verified by comparing with the available outcomes of LOPE forward calculation. [ABSTRACT FROM AUTHOR]

• Measurement of the reflective function of highly attenuative material by using a pair of ultrasound transducers. • Transducer-pair method is proposed to acquire V(z) by using conventional ultrasound transducers with a high f-number and small aperture. • By comparing the results obtained from both the single transducer and the transducer-pair approach, the feasibility is demonstrated. Ultrasonic reflectivity using a V(z) technique is a powerful characterization method in acoustic microscopy to measure the elastic properties of materials. Conventional techniques generally use a low f-number with high frequency; however, to measure the reflectance function of the highly attenuative material, a low frequency is essential. In this study, the transducer-pair method based on Lamb waves is used to measure the reflectance function of a highly attenuative material. The results demonstrate the feasibility of the proposed method using a commercial ultrasound transducer with high f-number. [ABSTRACT FROM AUTHOR]

• A new method for computing RIs from Lamb waves with extended data length to accommodate extra indirect waves. • The damage missed by the direct wave may be exposed by the extra indirect waves. • The modified RIs demonstrate excellent sensitivity to damages and ensure a low threshold for the healthy condition. The Lamb wave reciprocity-based method is an effective candidate for the health monitoring of composite plates. However, if the damage is located symmetrically between the transmitter–receiver pair, the reciprocity still holds true and the method misjudges its presence. In the present work, a new method for computing the reciprocity index (RI) from Lamb wave signals with extended data length is proposed. This method exploits extra indirect waves that bounce one or more times between the damage and other reflectors. These waves probe the damage through different paths and from different directions. Thus, the damage missed by the direct wave may be exposed by the indirect waves. Benefiting from that, two modified RIs are defined and their performance is validated by two experimental examples. As expected, both indices demonstrate excellent sensitivity to damage even in the middle of the transmitter–receiver pair and ensure a low threshold for the pristine condition, illustrating an excellent ability to distinguish between health and illness. [ABSTRACT FROM AUTHOR]

Delamination is one of the most common types of damage in carbon fiber reinforced polymer composites. This paper analyzes the interaction of fundamental symmetric (S0) and anti-symmetric (A0) mode Lamb waves with the start and the end of a delamination using finite element simulations. Propagations of Lamb waves in a 4-layer [0/90]s laminate were compared with propagations obtained in a 1-layer [0] sub-laminate and in a 3-layer [90/90/0] sub-laminate, which are the two sub-laminates separated by the delamination. Results show that the Lamb waves travel at different velocities in the delaminated region and in the healthy region. Comparing the signals obtained in the delaminated region with the excitation, it was found that mode conversions happen at the edges of a delamination for both S0 and A0 waves. The mode conversion of S0 wave at the end of a delamination generates large A0 waves and causes most significant changes to the detection signal. Based on this feature, a reference-free localization method is proposed in this paper. By measuring the difference between the arrival times of two modes, the distance between pick-up sensor and the end of delamination can be estimated. This method was verified by signals obtained in finite element simulation. [ABSTRACT FROM AUTHOR]

Blood coagulation tests are a critical part in diagnosis of patients with bleeding disorders and cardiovascular diseases. However, most conventional blood coagulation tests require large and expensive instruments and skilled personnel. Here, we present a simple and low-cost approach to evaluate blood coagulation using Lamb wave-based device. To assess coagulation, we monitored the acoustic streaming of 5 μm fluorescent particles suspended in citrated plasma. Using a Lamb wave, citrated blood plasma and coagulation reagents can be mixed in a few seconds and the viscosity of the droplet increases due to blood coagulation. Then, the acoustic streaming of particles eventually stops, at which the “coagulation time” is defined. As a preliminary test, coagulation time was reduced with increasing calcium concentration from 0 to 25 mM. Prothrombin time results from the Lamb wave-based device were validated by comparing with those of commercial instruments with the correlation coefficient (R 2 ) as 0.9432. Using the Lamb wave for blood coagulation test, the temperature can be controlled as 37 °C simultaneously by modulating experimental conditions. Our device enables lithography-free, simple and low-cost measurement of blood coagulation, which can be applied to clinical settings and developing countries. [ABSTRACT FROM AUTHOR]

Time of flight (ToF) based method for damage detection using Lamb waves is widely used. However, due to the energy dissipation of Lamb waves and the non-ignorable size of damage in composite structure, the performance of damage detection is restricted. The objective of this research is to establish an improved method to locate and assess damages in composite structure. To choose appropriate excitation parameters, the propagation characters of Lamb waves in quasi-isotropic composite laminates are firstly studied and the broadband excitation is designed. Subsequently, the pulse compression technique is adopted for energy concentration and high-accuracy distance estimation. On this basis, the gravity center of intersections of path loci is employed for damage localization and the convex envelop of identified damage edge points is taken for damage contour estimation. As a result, both damage location and size can be evaluated, thereby providing the information for quantitative damage detection. The experiment consisting of five different sizes of damage is carried for method verification and the identified results show the efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

Silicon steel laminations are introduced as the back-plate to an electromagnetic acoustic transducer (EMAT) to increase the efficiency of the EMAT by increasing the magnitude of the EMAT coil's dynamic magnetic field and the eddy current in the sample surface. A two-dimensional, non-linear finite element model is developed to quantify the effectiveness of the back-plate’s different maximum permeability and saturation flux density, on increasing the eddy current density and the dynamic magnetic flux density in the specimen. A three-dimensional FE model is also developed, and confirms the expected result that the laminated structure of silicon steel (SiFe) markedly reduces the eddy current induced in the back-plate, when compared to a continuous slab of the steel. Experimental results show that silicon steel lamination can increase the efficiency of the EMAT in the cases both with and without a biasing magnetic field. [ABSTRACT FROM AUTHOR]

The time reversal method has been widely used to detect damage in composite plates. Based on the time reversibility of Lamb waves, the waveform of the original input signal will be reconstructed after the time reversal process. However, the time reversal operator of Lamb waves in composite plates is a function of frequency. Thus the shape of the reconstructed signal will not be identical to that of the original input signal. To alleviate the effects of the time reversal operator, an improved time reversal method (ITRM) is proposed. In this method, the response signal in the forward path and the reconstructed signal in the time reversed path are both modulated by a weight vector which is obtained as the product of a window function and the reciprocal of amplitude dispersion. The window function varies with the excitation signal adaptively, and its shape is also determined by a threshold. An experimental example is then introduced to illustrate the effectiveness of the ITRM. The results show that the time reversibility can be greatly enhanced and the impact damage in the composite plate can be exactly located by the ITRM. [ABSTRACT FROM AUTHOR]

This paper presents a study on model assessment for predicting structural fatigue life using Lamb waves. Lamb wave coupon testing is performed for model development. Three damage sensitive features, namely normalized energy, phase change, and correlation coefficient are extracted from Lamb wave data and are used to quantify the crack size. Four data-driven models are proposed. The average relative error and the probability of detection (POD) are proposed as two measures to evaluate the performance of the four models. To study the influence of model choice on the probabilistic fatigue life prediction, probability density functions of the actual crack size are obtained from the POD models given the Lamb wave data. Crack growth model parameters are statistically identified using Bayesian parameter estimation with Markov chain Monte Carlo simulations. The model assessment and the influence of model choice on fatigue life prediction are made using both coupon testing data with artificial cracks and realistic lap joint testing data with naturally developed cracks. [ABSTRACT FROM AUTHOR]

Wave propagation signals are commonly used as information carrier in structural health monitoring. To facilitate decision making, wave signals are often decomposed into multiple components to reveal its frequency or time-frequency content. In this research we investigate the use of time-frequency decomposition techniques for feature extraction. The method based on the adaptive harmonic wavelet transform (AHWT) possesses high computational efficiency and at the same time inherently avoids certain issues in some other time-frequency feature extraction methods, e.g., the empirical mode decomposition (EMD). EMD entertains the adaptivity with respect to signal features, but that adaptive nature affects the comparability of the resulting intrinsic mode functions (IMFs). In contrast, the AHWT based method has similar feature extraction capabilities in the time-frequency domain while using a deterministic basis. Therefore, wavelet features can become cross-comparable when common wavelet basis is used. Our case study shows that the AHWT based approach can identify the critical features in Lamb wave signals to realize effective and robust decision making in structural damage detection. [ABSTRACT FROM AUTHOR]

In order to gain basic insight into the interaction between ultrasonic guided waves and structural discontinuities with contacting surfaces, the transmission characteristics of Lamb waves at contacting edges of two plates are studied experimentally. The edges of two 2.5-mm thick aluminum alloy plates are mated together to constitute a contacting interface of plates and subjected to different levels of compressive loading. The transmission measurements of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb modes across the contacting interface are performed in a frequency range below the cut-off frequencies of the higher-order modes. As a result, it is found that the transmission coefficient of the S0 mode increases monotonically with the applied contact pressure, but the transmission coefficient of the A0 mode exhibits non-monotonic dependence on the contact pressure and the frequency showing a local minimum. For the incidence of the S0 mode, the resonance at the contacting interface is observed as a long-time oscillation tail in the transmission waveform. The resonance frequency is found to increase with the contact pressure. The experimental results are discussed in the light of the theoretical results based on the spring-type interface model. The normal and tangential stiffnesses of the contacting interface are identified from the transmission coefficients as well as from the resonance frequency. The estimated interfacial stiffnesses increase monotonically with the contact pressure, and indicate their dependence on the frequency. Implications of the present results to the Lamb-wave based characterization of the plate contact condition are discussed briefly. [ABSTRACT FROM AUTHOR]

• A solid-liquid-solid coupling interface model was designed. • Mechanism of AE signal propagating in the model was expounded. • The effect of oil film thickness and center frequency on AE signal was analyzed. • Passing coefficient was proposed to characterize the propagation law of AE signal. The propagation path of acoustic emission (AE) signal contains several structures with solid-liquid-solid coupling interfaces. In this paper, the effect of the coupling interface on the AE signal propagation is investigated. Based on the theory of Lamb wave, a solid-liquid-solid coupling interface model is designed. The propagation characteristics of the AE signals with different frequencies are predicted by simulation in the coupling models with different oil film thicknesses and contact areas. Furthermore, experimental verification is carried out. The results of the experiments are consistent with the numerical simulations. The results indicate that the oil film thickness and the center frequency will have an effect on the propagation of AE signal. Two passing coefficients are proposed to quantify the propagation characteristics. This paper provides guidance for the study of the AE signal propagation characteristics in the complex structure with multi-media interfaces. [ABSTRACT FROM AUTHOR]

This paper presents a delamination damage detection technique based on the nonlinear response of Lamb waves in composite laminates. In the approach, a network of transducers is employed to sequentially scan the structure by actuating and receiving dual guided waves. Using combined frequency waves originated due to contact nonlinearity when the incident dual frequency wave interacts with the damage, a damage localization imaging algorithm is proposed to locate the damage and determine the damage location. The proposed damage localization imaging algorithm is based on analysing the cross-correlation of the signal envelope at combined frequency wave with the excitation pulse envelope at incident waves for each transducer pair, the damage localization image is constructed. A numerical study using an experimentally validated finite element model is used to verify the proposed damage detection technique. The results of the numerical study show that, despite the small amplitude of the nonlinear components, the proposed method can effectively predict the damage location, and it does not require baseline measurements, which makes it potential to be a baseline-free damage detection technique. [ABSTRACT FROM AUTHOR]

A novel technique based on inverted Lamb wave MEMS resonator has been exploited for the realization of a DNA biosensor. Zinc oxide based Lamb wave MEMS resonator in the inverted configuration of ZnO/SiO 2 /Si/ZnO is fabricated for label free and efficient detection of Neisseria meningitidis , responsible for bacterial meningitis. Meningitis remains a devastating endemic in sub-Saharan Africa. Its early detection can prevent the spread and its lethal complications. The developed biosensor shows a very high sensitivity of 310 Hz(ngμl−1)−1 and very low detection limit of 82 pgμl−1 for symmetric mode of the Lamb wave device while the antisymmetric mode shows a sensitivity of 202 Hz(ngμl−1)−1 and the limit of detection of 84 pgμl−1. This very high sensitivity and very low detection limit of the Lamb wave resonator can be attributed to very high mass loading effect on the membranous structure of Lamb wave device, unlike the bulk substrate based devices. The indigenously developed MEMS based inverted Lamb wave biosensor shows high selectivity, long shelf life and good reproducibility. The ease of operation, low processing time and possibility of wireless integration of the of the Lamb wave DNA sensor paves a path towards the promising application in the field of meningitidis detection. The use of fabricated biosensor can be extended to other viral and bacterial detection applications as well. [Display omitted] • Novel biosensor based on Inverted Lamb wave MEMS resonator is developed for the detection of Neisseria meningitidis. • Inverted configuration of ZnO/SiO 2 /Si/ZnO has been exploited to avoid damping of waves. • RF magnetron sputtered ZnO thin film is used as piezoelectric and sensing layer both. • The fabricated Lamb wave based DNA sensor shows a very high sensitivity of 310 Hz(ngμl−1)−1 and low detection limit of 82 pgμl−1. • Various mechanism involved in the biosensing are discussed which can be exploited further to increase the selectivity and sensitivity of the sensor. [ABSTRACT FROM AUTHOR]

• The study presents a method to detect defects at interfaces in a double-casing well. • The relationship between ultrasonic Lamb waves and channels was investigated. • The method of assessing the channel thickness was applicable to all interfaces. In cement-bonding evaluation, channel detection by ultrasonic Lamb waves has been widely studied and applied in single-casing wells. However, its feasibility in double-casing wells remains unknown. The relationship between ultrasonic Lamb waves and channels based on finite-difference time-domain simulations was investigated. According to the Lamb wave propagation, channels near interfaces were determined by the Lamb wave amplitude changes. A channel detection approach in double-casing wells was proposed. The ability of the proposed strategy to accurately detect the spatial position and geometric shape of the channels in double-casing wells was seen. This study provided guidance in double-casing cement-bonding evaluation. [ABSTRACT FROM AUTHOR]