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Face-shear deformation was observed in various square piezoelectric transducers prepared using the anti-parallel co-poling method, which led to uniform polarization and the generation and receipt of pure fundamental shear horizontal (SH 0) waves without any Lamb wave. The amplitude of the SH 0 wave was found to be proportional to the piezoelectric charge constant (d 33) of the piezoceramic, suggesting that the d 33 value can be used as a figure of merit for piezoelectric transducers with strong SH 0 waves. The amplitude of the SH 0 wave associated with the square piezoelectric transducer is large at 0° and 90° but almost zero at 45°; therefore, a piezoelectric transducer with good omnidirectional properties needs to be developed. A ring-type omnidirectional piezoelectric transducer was produced using eight trapezoidal piezoceramics. The minimum amplitude of the SH 0 wave generated and received by the ring type-transducer exceeded 70% of the maximum amplitude. Hence, this ring-type transducer is highly omnidirectional and can be used for nondestructive testing and structural health-monitoring applications. [ABSTRACT FROM AUTHOR]

• Resonance verification ensures energy concentration within the feature region of the welded joint, preventing leakage. • The 3D time-step FE method allows for unambiguous identification of ZGV propagation modes. • The presence of ZGV-FGWs in the welded joint is confirmed. Extensive research has been conducted on zero-group-velocity (ZGV) Lamb waves in elastic plates, demonstrating significant progress in the field of nondestructive testing. However, there is a scarcity of studies focusing on ZGV modes in complex structures. In this paper, we present our research investigating the presence of ZGV feature guided waves (FGWs) in a welded joint. Our approach follows a similar methodology used to study ZGV Lamb waves in elastic plates. By employing two-dimensional (2D) finite element (FE) modeling, we analyze the response spectra of the welded joint when subjected to a force source, revealing the occurrence of resonance in the response spectra. To investigate resonance modes in the welded joint, we employ the three-dimensional (3D) time-step FE method. By applying spatial 2D and short-time Fourier transforms to the received time-domain signals, we analyze the frequency content and spatial distribution of the signals. This analysis allows us to verify the existence of non-propagation and propagation modes in the welded joint. The non-propagation mode refers to the presence of signals with a zero wavenumber, indicating that they do not propagate or travel along the welded joint. These signals are typically associated with local resonances or vibrations within the welded joint itself. On the other hand, the propagation mode corresponds to signals with nonzero wavenumbers, suggesting that they propagate or travel along the welded joint. Furthermore, by further analyzing the propagation mode in the welded joint, similar to the analysis of ZGV modes in solid plates, we have observed that it also exhibits ZGV characteristics based on the wavenumber-frequency spectra. To further analyze acoustic field distributions at resonance frequencies, we utilize the semi-analytical finite element method in conjunction with the perfectly matched layer method. The results obtained from this analysis are consistent with those obtained from the 2D FE method and 3D time-step FE method, thereby confirming that propagation modes with ZGV characteristics at resonance frequencies correspond to FGWs, which we refer to as ZGV-FGWs. Through this step-by-step analysis, we ultimately establish the existence of ZGV-FGWs in the welded joint. This study introduces fresh ideas and serves as a point of reference for future research on ZGV-FGWs in complex structures. [ABSTRACT FROM AUTHOR]

• A tunable bidirectional SH wave transducer was proposed. • The transducer was based on antiparallel d 15 piezoelectric strips (APS) • Simulations and experiments validated the proposed SH wave transducer. • The transducer can selectively suppress SH 0 wave and excited SH 1 wave. • The transducer can also excite circumferential SH wave in pipelines. Guided wave based defects inspection is very promising in the field of structural health monitoring (SHM) and nondestructive testing (NDT) due to its less dissipation and thus long distance coverage. In comparison with the widely used Lamb waves, shear horizontal (SH) waves are relatively simple but less investigated probably due to the traditional notion that SH waves were usually excited by electromagnetic acoustic transducers (EMAT). In this work, we proposed a tunable method to excite single-mode bidirectional SH waves in plates using antiparallel thickness-shear (d 15) piezoelectric strips (APS). The proposed SH wave driving mechanism here is similar to that by using the periodic permanent magnetics (PPM) based EMAT with the period of strips equal to half of the wavelength. Both finite element simulations and experiments were conducted to validate this transducer in excitation of bidirectional SH waves. Results show that the Lamb waves excited by single piezoelectric strip can be suppressed very well. The radiation angle of the excited bidirectional SH wave can be reduced by extending the strip length, increasing the driving frequency or using more strips. Moreover, the APS transducer can selectively excite SH 1 wave and suppress the SH 0 wave at 174 kHz and 273 kHz in a 10 mm-thick aluminum plate. Considering its simple structure, flexible design and low excitation energy, the APS SH wave transducer is expected to be widely used in near future. [ABSTRACT FROM AUTHOR]

Guided waves, especially Lamb waves, propagating in plate-like structures with constant thicknesses have been investigated intensively in the past decades in the context of structural health monitoring (SHM) and non-destructive testing (NDT). However, structures with variable thicknesses are also very common in practice, such as ship hulls whose thickness varies for satisfying various load conditions, and defect identification for such structures is also of importance. In this paper, we compare the propagation characteristics of both Lamb modes and SH 0 mode in variable thickness plates and propose to use the SH 0 mode for defect identification in such waveguides because of its non-dispersive property. First, the non-dispersive property of the SH 0 mode in waveguides with variable thickness is demonstrated in theory. Then, a wave prediction method is adopted to illustrate the differences between the Lamb modes and the SH 0 mode when propagating in waveguides with constant or variable thickness. Finally, experiments are conducted on a variable thickness plate to verify the non-dispersive property of the SH 0 mode and further compare the results with those of the Lamb modes. Recently developed thickness-poled omnidirectional shear horizontal piezoelectric transducers (OSH-PTs) are used to produce pure SH 0 waves. It is observed that the velocity of the Lamb modes may change significantly and the group velocity of the S 0 mode even decreases by 55.1% during propagation, which hampers Lamb modes from defect identification in variable thickness plates. By contrast, the SH 0 mode is non-dispersive thus promising for damage detection, and its effectiveness for defect localization in such structures is validated experimentally. [ABSTRACT FROM AUTHOR]

• Wave-propagation characteristics are depicted between Lamb waves and various orientation crack. • A propagation distance calculation method considering dispersion effects was achieved using Sparse Decomposition. • A novel algorithm to quantify various orientation and length crack without baseline signals has been developed. Crack damage is one of the significant factors that may accumulate at the stress concentration area of engineering structures and cause catastrophic accidents. In this paper, we proposed a novel approach to identify the crack location and size by exploiting the reflections and diffractions of Lamb waves. The interaction mechanism between the crack and Lamb wave has been analyzed thoroughly, our analysis of the interaction between the crack and Lamb wave revealed that both reflections and diffractions carry valuable damage information that reflects the size and orientation of the crack. As the interaction coefficients between these two components and the crack are different, there are supposed to exist differences between them in the amplitude value. We implemented a threshold to classify the signals received from all paths into two groups: reflections and diffractions. Then we constructed an overcomplete dictionary of waveforms corresponding to different propagation distances to extract the damage information. Using sparse decomposition, the received signals were mapped to their corresponding propagation distances without the use of baseline signals. The diffractions allow us to determine the crack's tip points, while the reflections provide information about the edge points. The kinked crack's size and orientation were visualized based on the time-domain signals acquired in our experiment. We provided a comprehensive description of our algorithm and verified it through numerical simulation and experimental data. Our results show high agreement with actual cracks, demonstrating the efficacy of our proposed method. [ABSTRACT FROM AUTHOR]

• A WDR method optimized tomography algorithm is developed for robust localization using Lamb waves. • WDR excitation is designed to alleviate the frequency selection problem in defect localization. • Reconstruction independent component analysis is adopted for mode separation. • Time-frequency index is calculated as the damage index in the tomography algorithm. Tomography imaging of defects using ultrasonic Lamb waves has attracted much attention in nondestructive testing of plates. However, there are two challenges for robust localization of defects: frequency sensitivity to defects, and multi-mode interference on damage index (DI) extraction. Aiming for that, a wideband dispersion reversal (WDR) method optimized tomography is developed. Pre-dispersive wideband excitations of a certain Lamb wave mode are generated based on the configuration of transducers, and reconstruction independent component analysis is used for wave mode separation. According to the acoustic reciprocity principle, self-compensation phenomenon of mode signals can be recorded on the intact path, where the signal energy concentrates at a fix self-compensation point in the time–frequency domain, yet such compensation effect would be impaired encountering defects. Thus, a time–frequency index can be used as the DI, which is calculated based on the weighted Euclidean distance from the self-compensation point to the pixel points in the time–frequency domain. Experimental results show the WDR optimized tomography has a robust performance for magnet-simulated defects imaging without excitation frequency optimization. [ABSTRACT FROM AUTHOR]

Abstract Lamb wave is a promising tool for nondestructive testing and evaluation (NDT&E). Due to dispersion and multi-mode, wave components generally overlap with each other at Lamb wave signal, which makes the interpretation difficult. To solve the overlapping problem, Lamb wave sparse decomposition has been developed in the past decade. However, the performance is limited because original tone-burst or dispersive tone-burst is directly used as dictionary atom, without fully considering the waveform change during propagation. In this paper, to overcome this limitation, a systematic dictionary design scheme is established by taking excitation waveform, dispersion, multi-mode, amplitude modulation and environmental condition into comprehensive consideration. First, pulse compression technique is applied for signal-to-noise (SNR) improvement, and auto-correlations of various excitation waveforms are considered as the initial dictionary atoms. Second, dispersion and multi-mode are placed on excitation auto-correlations for dictionary modification. Third, amplitude modulation during Lamb wave propagation is considered for further dictionary modification. Finally, the effect of environmental condition on dictionary design is investigated to make it adaptive to different conditions. An experiment is given to validate the established dictionary design scheme. [ABSTRACT FROM AUTHOR]

• The increase of volumic fraction d of bonded Aluminum/Adhesive/Aluminum plate caused the occurring of double/simple ZGV-mode in S 0 /S 2 modes. • In viscous plate, and at the frequencies of ZGV-modes, the dispersion curves are connected. This phenomenon gives rise to a coupling mode. • The effects of d on these guided modes, in viscoelastic bonded plate, are investigated. These studies are of great importance for NDT applications since they play a critical role in the selection of the optimal inspection modes and frequencies. Before a nondestructive testing process gets set up, the sensitivity of the Lamb waves to the bonded plate's properties must be well understood. The dispersion and attenuation curves in thin bonded Aluminum/Adhesive/Aluminum plate are calculated by the Stiffness matrix method. The increase in the volumic fraction d caused the appearance of backward mode in the S 0 and S 2 dispersion curves. The effect of d on forward waves, and on backward waves and associated ZGV-modes, are studied. The frequencies of ZGV-modes decrease when d increases. The effects of d on dispersion and attenuation curves of the damped symmetric modes are shown. From certain values of d, and at the frequencies of the ZGV-modes, the dispersion curves of the symmetric modes are connected. This phenomenon gives rise to a C coupling mode. The attenuation levels of the ZGV-modes for different d values are calculated. The increase in d caused the shift of C-dispersion and C-attenuation curves toward the decreasing direction of the frequency axis, without significant change in the values of the phase velocity, and with slight decreases in the attenuation level of the C coupling mode. [ABSTRACT FROM AUTHOR]

Abstract In the ultrasonic detection using nonlinear Lamb wave, only the cumulative second-harmonic Lamb wave is useful for the nondestructive evaluation of plate-like structures. One of the conditions for cumulative second-harmonic Lamb wave generation is phase velocity matching between the primary and the secondary wave. However, in most cases, the phase velocities are approximate matching. In this paper, we theoretically analyzed and discussed the cumulative effect of second-harmonic Lamb wave. The theoretical results show that, when the phase velocities of the primary and the secondary Lamb waves are approximate matching, there is still a cumulative effect in the secondary wave within a specific distance, which is referred as accumulation distance. Based on the accumulation distance, an excitation window is proposed as the basis for mode selection and excitation condition determination of nonlinear Lamb wave. It is considered that, in the case of approximate phase velocity matching, the mode pair with larger excitation window could generate second-harmonic Lamb wave easier than the other mode pairs. Then, the excitation windows of different Lamb wave mode pairs are calculated and compared theoretically. In order to verify the validity of the excitation window as the basis for mode selection, the ranges of two mode pairs' primary Lamb wave that could generate cumulative second-harmonic Lamb wave are measured and compared in a specific plate. In the experiment, the primary and the secondary Lamb waves around a longitudinal mode pair and a crossing mode pair are excited and measured using wedge piezoelectric transducers. In the measurement, the second-harmonic Lamb wave is generated by the primary wave in different frequency and phase velocity ranges, and then the ranges of the primary Lamb wave are confirmed in the phase velocity dispersion curves, using the time-frequency analysis. The experimental result shows that, the primary wave range of the longitudinal mode pair is larger than that of the crossing mode pair, which means that the selected longitudinal mode pair could generate second-harmonic Lamb wave easier than another one. The experimental result is in accordance with the comparing result of the excitation windows. The theoretical and experimental results show that, the excitation window could be an indicator for mode selection and excitation condition determination of cumulative second-harmonic Lamb wave generation, and it can be used in the field of ultrasonic detection using nonlinear Lamb wave. [ABSTRACT FROM AUTHOR]

Abstract In recent years, solar energy becomes one of the most popular renewable energy resources. Among the solar energy systems, solar cells are considered to be one of the most critical components, which are naturally fragile during either fabrication or service. Thus, nondestructive examination of solar cells is required practically. In this research, solar cells made from monocrystalline silicon are selected as the typical samples. The air-coupled ultrasonic nondestructive testing system, based on the generation and reception of the A0 mode Lamb waves, will be adopted for defects detection. Firstly, the dispersion curves of Lamb waves in anisotropic monocrystalline silicon are calculated theoretically, which provides a theoretical foundation for analyzing the behavior of Lamb waves propagating in silicon solar cells. Then, experiments on monocrystalline silicon solar cells are carried out by introducing the air-coupled ultrasonic Lamb waves. The amplitude distributions of Lamb waves propagating along various rotation angles in silicon solar cells are obtained. It shows a strong anisotropic properties of the tested samples. Finally, artificial cracks in solar cells are scanned circularly and laterally. The results of amplitude distributions give the scattering pattern of cracks, which can be used to determine whether the crack breaks through or not. And the extracted amplitude cross-correlation coefficients can help to estimate the length of them. [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 Mutual wave interactions provide a very promising technique for nondestructive testing and evaluation due to their exceptional sensitivity to micro-scale damage growth in metallic materials. This article describes detection of localized fatigue damage in aluminum plates using polyvinylidene difluorine (PVDF) sensors to simultaneously receive shear-horizontal waves and the secondary Lamb waves that they generate. Finite element simulations explore several aspects of wave interaction and mode identification. Then laboratory experiments confirm secondary wave generation at the sum frequency in an aluminum plate using the PVDF sensor by conducting frequency-wave number domain and supporting analyses. The PVDF sensor enables computation of amplitude ratios even though the primary and secondary waves have different polarities. Finally, a simple guided wave technique based on electrically scanning the wave mixing zone around a plate is shown to detect early localized fatigue damage in an aluminum plate. [ABSTRACT FROM AUTHOR]

• Besides dispersion, comprehensive amplitude modulation can also change the appearance of Lamb wave signals. • A dictionary reconstruction strategy is established with considerations of phase and amplitude information. • The performance of sparse imaging method relies on the denoising parameters and numbers of sources. Composite structure is increasingly used in civil and aerospace applications due to its high mechanical performance. Lamb wave based sparse reconstruction imaging for damage localization is promising for structural health monitoring (SHM) and nondestructive evaluation (NDE) by using few measurements. However, this dictionary based method requires accurate atoms to represent Lamb wave propagating features in structure very well. Besides dispersion, signal changes caused by amplitude modulation should be considered for waveform distortion when constructing the dictionary for sparse imaging method. In this paper, a non-contact laser is used for Lamb wave excitation which exhibits a strong amplitude modulation in low frequency. Additionally, the strong attenuation resulting from material damping would also presents a distance-dependent amplitude modulation. To reconstruct an amplitude model of Lamb wave, the decomposition method of system response and attenuation is proposed. Then, the influence of amplitude modulation on signal representation is analyzed, which shows the restriction of dictionary without considering amplitude modulation. On this basis, the amplitude considered dictionary is built together with the phase considered dictionary for sparse imaging in terms of damage detection. Furthermore, according to Lamb wave reflection model, the solution for sparse reconstruction imaging is given. Finally, the performance of sparse imaging method is discussed by experimental investigation with different parameters. The results show the efficiency of the proposed method with improved imaging performance and give comparisons for better parameter choice. [ABSTRACT FROM AUTHOR]

Abstract This paper deals with the introduction of a spectral-element method, in the time domain, to address problems of ultrasonic wave propagation in the field of the nondestructive ultrasonic testing and evaluation. Two kinds of problems are addressed. The first focuses on guided waves that are often used to control immersed layered defective structures. The second focuses on the use of bulk waves to inspect layered anisotropic media that may contain a defect. This full-wave technique allows for the use of significantly coarser meshes compared to other standard finite-element methods that can be used for non-flat defective or damaged models, as using one element per shortest wavelength is sufficient. It is thus well suited to the simulation of wave propagation phenomena in complex structures at high frequencies. With the goal of first validating our approach for simple cases, we begin by presenting results of simulations for a homogeneous plate. We compare the dispersion curves obtained with the analytical ones. The results are in excellent agreement, and the spectral-element method is fast enough to allow for simulations having a high level of accuracy. We then reproduce and analyze the transmission losses of a quasi plane wave across the immersed plate, and discuss the influence of the finite size of the ultrasonic beam in real physical experiments. We subsequently study the transmission of an ultrasonic wave through an immersed tri-layer composed of two aluminum plates glued together and with a defect, a model that is already not accessible to quasi-analytical calculation techniques. We again determine the dispersion curves, and then study transmission through the structure having a delamination defect compared to transmission through a healthy structure. We then compute the dispersion curves of the same tri-layer structure but with a varying glue thickness. These dispersion curves are compared to those of the healthy structure. We then perform a three-dimensional simulation of a pulse-echo experiment in an immersed medium composed of layers of transversely-isotropic austenitic steel, the axis of symmetry of each layer being tilted differently. This medium represents a very simple model of a weld, in which we include two kinds of defects: a gas bubble resulting from the welding process, and then a branching and crossing Y-shaped crack resulting e.g. from aging. We illustrate their effect on ultrasonic waves by computing the scattered field. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic is known to be a promising technique to characterize the microstructural degradation in engineering materials. This work demonstrates the use of nonlinear Lamb wave mixing technique to assess the localized deformation in modified 9Cr 1Mo steel during creep. Two Lamb wave modes of different frequencies (ω 1 & ω 2 ) are allowed to mix within the material under certain resonant condition to generate third type of harmonic waves of frequencies (ω 1 ±ω 2 ). This new generated wave carries the information of material nonlinearity from the mixing site and independent of the other extraneous nonlinear factors. Amplitude of the generated third wave depends on the third order elastic constants of the material. This study reveals that nonlinear Lamb wave mixing technique could be used to assess the localized deformation much prior to its failure. [ABSTRACT FROM AUTHOR]

Numerous Lamb wave dispersion curve estimation methods have been developed to support damage detection and localization strategies in non-destructive evaluation/structural health monitoring (NDE/SHM) applications. In this paper, the covariance matrix is used to extract features from an ultrasonic wavefield imaging (UWI) scan in order to estimate the phase and group velocities of S0 and A0 modes. A laser ultrasonic interrogation method based on a Q-switched laser scanning system was used to interrogate full-field ultrasonic signals in a 2-mm aluminum plate at five different frequencies. These full-field ultrasonic signals were processed in three-dimensional space-time domain. Then, the time-dependent covariance matrices of the UWI were obtained based on the vector variables in Cartesian and polar coordinate spaces for all time samples. A spatial covariance map was constructed to show spatial correlations within the full wavefield. It was observed that the variances may be used as a feature for S0 and A0 mode properties. The phase velocity and the group velocity were found using a variance map and an enveloped variance map, respectively, at five different frequencies. This facilitated the estimation of Lamb wave dispersion curves. The estimated dispersion curves of the S0 and A0 modes showed good agreement with the theoretical dispersion curves. [ABSTRACT FROM AUTHOR]

Lamb waves are being investigated extensively for structural health monitoring (SHM) because of their characteristics of traveling long distances with little attenuation and sensitivity to minor local damage in structures. However, Lamb waves are dispersive, which results in the complex overlap of waveforms in the damage detection applications of the SHM community. This paper proposes a sparse representation strategy based on an l 1 -norm optimization algorithm for guided-Lamb-wave-based inspections. A comprehensive dictionary is designed containing various waveforms under diverse conditions so that the received waveform can be decomposed into a spatial domain for the identification of damage location. Furthermore, the l 1 -norm optimization algorithm is employed to pursue the sparse solution related to the physical damage location. The functionality of the created dictionary is validated by both metal beam and composite wind turbine experiments. The results indicate a great potential for the proposed sparse representation using a dictionary algorithm, which provides an effective alternative approach for damage detection. [ABSTRACT FROM AUTHOR]

In this study, the hardening process of ultra high performance concrete (UHPC) was monitored non-destructively using a single embedded sensor system and the characteristics of guided waves, especially the Lamb wave. Lamb wave propagation depends on the material properties of the medium and boundary conditions. Since the boundary conditions of the embedded sensor system continuously change during the hardening process of concrete materials, the measured characteristics of the propagating waves also vary. To understand the variations in wave propagation, the Lamb modes were decomposed using the polarization characteristics of piezoelectric sensors, which were used to measure wave responses. Additionally, a traditional penetration resistance method was adopted to estimate the time for phase transition of UHPC. The decomposed Lamb modes were compared to measurements of penetration resistance. The strength development of UHPC, with and without short-fiber reinforcement, was estimated using the variation of patterns of the decomposed Lamb modes after the phase transition. Based on the proposed methodology, which measures the propagation and variation of the Lamb waves, it is possible to estimate the time of phase transition and the strength development of UHPC. [ABSTRACT FROM AUTHOR]

• A genetic algorithms-based waveform inversion method is developed for wavenumber extraction of one Lamb wave signal. • The plate properties are identified with the chromosome of searched individual. • The elastic formula of Lamb waves is solved via the bisection method to derive the wavenumber curves. Wavenumber dispersion of ultrasonic Lamb waves associating with the thickness and material property of plates plays an important role in waveguide characterization and non-destructive testing. Classical waveguide characterization with wavenumber estimation methods is performed based on a large amount of Lamb wave signals or sensitive to wave mode overlapping and noise. A genetic algorithms-based waveform inversion method is developed for wavenumber extraction of one Lamb wave signal and characterization of plate in this work. The individual chromosome is encoded in real number by the thickness and material properties of the structure. The elastic formula of Lamb waves is solved by the bisection method to further derive the wavenumber curves; then the wave mode signals are generated based on the propagation function, and are linearly combined to form the individual signal. The signal with the minimum residual ratio to the analysis signal is iteratively searched by the genetic algorithm. Finally, the plate properties and wavenumber of Lamb waves are identified with the chromosome of searched individual. Numerical and experimental studies prove the developed waveform inversion method can realize accurate wavenumber extraction of a Lamb wave signal, even though its time–frequency representation has low sparsity, and characterization of plate. The residual ratio between the inversion wavenumber and theory curves is around 1%; and the estimation deviation of material properties of plate is around 6%. [ABSTRACT FROM AUTHOR]

The complex design of composite structures makes it very difficult for conventional visual inspection techniques to detect defects in these materials. Therefore, numerous types of nondestructive testing and structural health monitoring techniques have been developed for damage identification in composite structures, among which ultrasonic guided waves, particularly Lamb waves, have gained much popularity. In recent years, apart from piezoelectric pointwise measurements, laser Doppler vibrometry has been used for full wavefield measurements of propagating Lamb waves. Damage imaging can be done with animations of Lamb waves interacting with defects. In this work, two end-to-end deep learning-based models of many-to-one sequence prediction were developed to perform pixel-wise image segmentation. A large dataset of full wavefield images resulting from the interaction with delamination of random size, shape, and location was utilised to train the proposed deep learning models. The main goal of this work is to investigate the feasibility of implementing deep learning methods for delamination identification in composite laminates by only utilising the animations of Lamb waves. The elaborated models worked well on the numerically generated test data and have also shown good performance on the real-world experimental data. These methods can enable the automation of delamination identification and to create damage maps without the intervention of the user. [ABSTRACT FROM AUTHOR]

Ultrasonic guided wave is a promising technique for structural health monitoring and nondestructive testing. However, due to the anisotropy and complexity of composite materials, the imaging performance of numerous signal processing methods deteriorates with significant artifacts and unsatisfactory accuracy. To obtain a better damage imaging performance of Lamb waves in noisy and noise-free conditions, a weighted delay-and-sum (DAS) imaging method based on denoising autoencoder (DAE) learning is developed for complex composite structures. The traditional DAS formulation is modified to be more compatible with anisotropic materials. The DAE with feature learning capability is then employed to extract potentially efficient features and remove noise from ultrasonic signals. Several verification experiments conducted on flat or curved and stiffened composite structures have confirmed the ability of the DAE-DAS method to suppress artificial noise and to intensify singularities induced by the anomalies. By comparing with the unweighted DAS methods and the weighted DAS method without feature extraction, the proposed algorithm has satisfactory robustness to achieve higher localization accuracy and fewer artifacts. [ABSTRACT FROM AUTHOR]

Zero group velocity (ZGV) Lamb modes have already shown their potential in nondestructive testing applications as they are sensitive to the sample structural characteristics. In this paper, we first consider an aluminum sample to validate a method based on the beam shaping of the generation laser. This method is proven to enhance ZGV Lamb modes in aluminum, and then advantageously applied to a composite material plate. Finally, based on the proposed method, scanning the sample over healthy and flawed zones demonstrates the ability to detect subsurface flaws. [ABSTRACT FROM AUTHOR]

The multi-mode and dispersion natures of elastic guided waves bring big challenges for wave manipulation, and also provide more possibilities for smart device design. In this study, a piezoelectric bimorph plate connected to a periodic arrangement of electric circuits is proposed, and both propagating and evanescent guided Lamb waves are investigated through the complex band structure resulting form using different shunting capacitor or inductor distributions. Results show that keeping and breaking the symmetry of the electric boundary condition can, respectively, lead to decoupling or hybridizing of multi-mode Lamb waves. Accordingly, the asymmetric transmission at a desired frequency can be realized by combining the symmetric and asymmetric electric boundary conditions. Harmonic analysis shows that numerical results are in good agreement with the prediction of complex band structure. Actively changing the electric boundary conditions can be used for guided wave propagation in piezoelectric bimorph plates to easily control the system without changing the structure's geometry. The framework presented herein offers enhanced capabilities for controlling guided wave propagation for engineering applications and to develop nondestructive testing techniques. [ABSTRACT FROM AUTHOR]

To control the propagation of multi-mode guided waves in plate is very significant in nondestructive testing and structural health monitoring for identifying damage and detecting structural integrity. In this paper, a novel metasurface is proposed to simultaneously manipulate multi-mode guided waves in plate, including shear horizontal wave, symmetric mode and anti-symmetric mode Lamb waves. For controlling the guided wave propagation in plate, the functional unit is designed by a staggered arrangement of in-plane polarized and out-of-plane polarized piezoelectric patches attached on both surfaces of host plate to form a metasurface, and each piezoelectric patch is individually connected with negative capacitance circuit. The transmitted phase of each mode guided wave through the functional unit can be shifted arbitrarily in 0 ∼ 2 π range by adjusting the shunting negative capacitance circuits. Furthermore, the transmitted phase shifts profile along the metasurface can be coded to control the wave front of multi-mode guided waves in plate, and tunable functions including anomalous refraction and wave focusing can be realized. Results indicate that the proposed metasurface can simultaneously modulate and control the wave front of each mode guided wave to achieve wave mode splitting and purifying from multi-mode incident waves. In addition, a wave source shifter is realized by an annularly arranged metasurface to achieve equivalent wave source control. Therefore, the proposed piezoelectric metasurface has great potential in the fields of tunable manipulation of guided waves and design of smart devices. • First elastic metasurface to achieve tunable manipulation of multi-mode guided waves. • The working frequency of the proposed metasurface can be tuned by the shunting circuit in a wide frequency range. • Multiple functions including anomalous refraction and wave focusing are realized for each mode guided wave individually. • Metasurface-based wave source shifter is further designed. [ABSTRACT FROM AUTHOR]

The need for fast and effective Non-Destructive Testing (NDT) techniques is ever present. Existing techniques such as ultrasonic testing, whilst established and reliable, face many limitations when considering large structures such as those found in the aerospace and green energy sectors. Wave mode, as well as other wavenumber based filtering techniques have been presented to address many of these limitations. This work describes a novel application of Wave Mode Spectroscopy (WMS) along with feature detection for complex geometric shapes. The specimen's geometry is found during the wavefields measurement through the use of a 3D Scanning Laser Doppler Vibrometer (SLDV) allowing the wavefield to be mapped to a 2D plane with limited distortion of the wavelength and without any prior knowledge of the part's geometry. This was shown to allow WMS to be applied to continuous, multi-frequency wavefields and generate accurate thickness maps. Monogenic signal analysis has been applied to the same measurement data to generate amplitude maps that allow the automatic detection of edge features through the use of a Canny edge detection algorithm. [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]

As a good information carrier, solitary waves have broad application prospects in the field of non-destructive testing. In this paper, we studied the coupling effect between highly nonlinear solitary waves in the one-dimensional particle chain and composite plate with matrix cracking. We based on the tensor internal variable damage model and the residual stiffness model, obtained the couple differential equation of the one-dimensional particle chain and the composite plate with matrix cracking. The fourth-order Runge-Kutta method is used to solve the differential equations, and the change law of the particle velocity, internal force, and displacement curve is obtained. This paper analyzed the influence of the damage amount and the geometric parameters on the reflected solitary waves of composite plates with different material properties. It is found that the reflected waves are sensitive to the thickness of the plate, the average distance between cracks, and the thickness of the damage layer. The research results of this paper have enriched and perfected the theory of nondestructive testing of solitary waves in the field of composite materials and provided broad application prospects for the practical application of one-dimensional particle chains. [ABSTRACT FROM AUTHOR]

Dispersion of guided waves propagating in layered media containing layers with equal acoustic impedances is analyzed by Cauchy sextic formalism coupled with the exponential fundamental matrix method. The cases of layers with equal acoustic impedances apparently have not been studied yet. However, dispersion in such a medium exhibits several peculiarities that may be essential for developing the acoustic non-destructive testing methods. • Dispersion of guided waves propagating in layered media containing layers with equal acoustic impedances is analyzed. • Cauchy sextic formalism coupled with the exponential fundamental matrix method is used. • The cases of layers with equal acoustic impedances are studied. • Dispersion exhibits the anomalous behavior. • The outer layers become invisible. [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]

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]

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]

This work assesses the suitability of the two zero-order Lamb wave modes to detect multiple barely-visible impact damage in composite material. Three specimens were subjected to damage at three different low-energy levels and one was left as an undamaged reference sample. Ultrasonic Lamb wave modes were selectively generated by surface-bonded piezoceramic wafer transducers in two tuned configurations. By using an algorithm based on the Akaike Information Criterion the time-of-flight of the Lamb modes was determined, allowing their threshold detection capabilities for the studied application to be successfully benchmarked. The results were consistently validated by digital shearography, ultrasonic C-scan and optical microscopy. A study of the effects on structural integrity was completed with an assessment of the damping ratio and residual bending strength proving to be sensitive parameters to the induced damage. [ABSTRACT FROM AUTHOR]

Lamb wave is widely studied in the non-destructive testing of composite materials due to its wide detection range and high sensitivity to defects. In order to solve the problem of extracting the effective signal of damage in a strong noise environment to improve the accuracy of damage location determination. A damage location imaging method based on power spectral density and Lamb wave tomography is proposed in this paper. The damage location imaging of composite materials under strong noise environment is realized through simulation and experiment respectively. Firstly, the simulation analysis the propagation characteristics of Lamb waves in composite materials. The circular sensor array is evenly arranged on the composite material. Each sensor is used as an actuator to generate Lamb wave in turn clockwise. Other sensors are responsible for collecting signals. Strong noise is added to the collected signal to simulate the signal collected in the strong noise environment. Finally, the damage information is characterized by power spectral density to determine the damage factor, and by using the probability imaging algorithm, the damage location imaging is totally realized. The experimental results show that the maximum error of imaging positioning for single damage and multiple damage under strong noise environment is 5.10 mm and 8.04 mm, respectively. This method does not need to pretreatment the signal with strong noise, and can directly image the original signal. At the same time, the extraction process of complex reflected signals is avoided, and it has a great potential in the location and identification of composite materials damage in a strong noise environment. [ABSTRACT FROM AUTHOR]

• The Lamb wave can be only efficient excited and received when the transmitter and receiver are arranged along with the warp or weft direction of the CFRP laminates. • The BVIDs (Barely Visible Impact Damages) in 3 J and 7 J impact areas can be intuitively located, and the BVIDs around the macro impact damage can be also clearly displayed. • The method shows high application significance in improving the capability of nondestructive testing, and can be used for ensuring the safety operation of CFRP composites. The traditional linear ultrasonic technique may not provide a clear indication of the underlying damage in some cases of invisible or barely-visible impact damage. Considering nonlinear ultrasonic methods have been used for the detection of defects such as closed crack or delamination, a new nonlinear Lamb wave imaging method for detecting the Barely Visible Impact Damage (BVID) of Carbon Fiber Reinforced Polymer (CFRP) laminates was proposed in this work. First, the testing platform of nonlinear Lamb wave was built, and the test specimen with different impact damages was prepared. Second, the Lamb wave modes that meet the requirements of nonlinear testing were excited, and then, the correlation between the relative non-linear coefficient and status of low-energy impact damage were investigated. Lastly, the scanning imaging methods were developed, and the Damage Index (DI) for imaging impact damage was defined. The research results indicated that the DI based on the relative non-linear coefficient has higher detection sensitivity for low-energy impact damage, and the BVID and the extension of macro damage can be displayed intuitively by use of the new nonlinear Lamb wave imaging method. [ABSTRACT FROM AUTHOR]

This study aims to measure non-propagating Lamb wave modes (NPLs) for detection of a surface-breaking fatigue crack. Crack-induced NPLs are the attractive feature to identify and localize an invisible crack, because they are not spatially propagated but significantly contribute to local Lamb mode amplitudes at a crack interface. By extracting only the crack-induced NPLs from Lamb wavefields measured at multi-spatial points, the crack is able to be identified and localized without relying on baseline data obtained from the pristine condition of a target structure. First, the existence of the crack-induced NPLs are analytically proved using a modal expansion method. Then, a NPL filter based on a frequency–wavenumber analysis is proposed so that only NPLs are isolated from Lamb wavefields. Subsequently, the theoretical proof and the proposed NPL filter are numerically and experimentally validated through fatigue crack detection tests. In particular, the quantities of NPL generation at the real fatigue crack interface are investigated under changing driving frequencies as well as crack widths. The numerical and experimental results confirm that NPLs remarkably appear at open crack interfaces, while they are rarely induced at closed crack interfaces. [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]

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]

• Carbon steel plates coated with a composite coating were fabricated and tested. • The inverse problem of Lamb wave propagation in coated structures was solved. • The elastic modulus and thicknesses of the substrate were estimated accurately. • The coating thickness is evaluated more accurately at low frequencies. • The material properties decay is assessed more efficiently at high frequencies. Nondestructive evaluation (NDE) of the mechanical properties and thickness loss of coated structures is of great importance for health monitoring purposes. This study aimed to propose an NDE method based on the Lamb wave propagation for the inspection of coated structures. The inverse problem of Lamb wave propagation in coated structures was developed and solved. The dispersion curves for the fundamental antisymmetric Lamb wave mode were obtained analytically, considering different thicknesses and a range of material properties decay for the substrate and coating. To assess the proposed inverse wave propagation NDE method, four carbon steel specimens coated by composite materials were fabricated and underwent the Lamb wave propagation tests. The Lamb wave phase velocity was measured for the specimens. The elastic modulus and thickness of the carbon steel plate were estimated for each specimen using the measured phase velocities through the inverse solution of the developed Lamb wave propagation model. The estimated elastic modulus and thickness of the specimens were in good agreement with the actual ones. Therefore, the validity of the proposed nondestructive Lamb wave propagation method for the evaluation of coated structures was confirmed. Prospective applications of this method for the in-situ inspection of coated structures to assess the thickness loss and material decay are envisioned. [ABSTRACT FROM AUTHOR]

• Current plate thickness evaluation methods require knowledge of Lamb waves. • Our model-based plate defect visualization method estimates wall-thinning. • Wavenumber information determines optimal mode to detect wall-thinning in plates. • Autonomous 2D digital filtering method is proposed using 2D FFT data. The visualization of the wall thinning of plate-like structures using scanning laser Doppler vibrometry (SLDV) is a promising method in nondestructive evaluation using laser ultrasonics. In particular, the Lamb-wave-based SLDV method that uses continuous excitation exhibits excellent performance for the estimation of the wall thinning of plates. Currently, plate thickness is quantitatively evaluated based on wavenumber analysis using measured signals. However, it is difficult to estimate plate thickness automatically below the product of frequency and thickness of 6 MHz·mm without knowing the wavenumber sensitivity and minimum wavenumber distance from reference mode owing to the lack of the physical understanding of Lamb waves. In this study, a model-based autonomous plate defects visualization method is proposed for the quantitative imaging of the wall thinning of plates so that inspectors can use scanning laser Doppler vibrometry (SLDV) without any knowledge of Lamb waves and its signal processing. Interdigital-transducer-based SLDV is utilized to validate the proposed method, and a 6-mm-thick carbon steel plate with 1–8% wall thinning, and a 2-mm-thick aluminum plate with Y-shaped wall thinning are used. Experiments demonstrate that the capability of the proposed method for detecting wall thinning in plates is equivalent to that of manual plate defects visualization method. [ABSTRACT FROM AUTHOR]

Lamb wave is a promising means for active structural health monitoring and nondestructive evaluation of laminated composite plates. The inevitable reflections from structural geometric boundaries and other interferences limits the imaging performance of many existing imaging methods. The image generated through the well-known delay-and-sum method holds a wide mainlobe width and high level of sidelobes. Although the sparse reconstruction method is free from those deficiencies, it is sensitive to the regularization parameter. To overcome those limitations, a Lamb wave imaging method to locate anomalies or damage in laminated composite plates is proposed. The scattering signals are sparsely decomposed one by one with a prior weights penalized on the undetermined sparse coefficients, and the minimum variance distortionless response algorithm is adopted to process those sparse coefficients so as to generate the image. Experimental results on a quasi-isotropic laminated composite plate demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

• The interaction of solitary waves with pristine and corroded plates is investigated. • Analytical and coupled discrete/finite element models are presented to predict the reflected solitary waves. • Numerical results are in good agreement with the experimental results. • The sensitivity of reflected solitary waves to corrosion is evaluated in plates of different thicknesses. • The sensitivity of the method in granular chains of different material and geometric properties is studied. Over the last decade, a novel nondestructive evaluation (NDE) method based on the application of highly nonlinear solitary waves has emerged. The method is based on the actuation and detection of solitary waves propagating along a medium made of uniform spherical particles, the last of which is in contact with the material to be assessed noninvasively. The hypothesis is that the dynamic interaction between the wave and the material/structure to be inspected is dependent upon the condition of the material/structure. The study presented in this paper aims at designing and developing the NDE framework for the detection of localized corrosion in metals. In particular, this paper presents a numerical investigation of the interaction of highly nonlinear solitary waves with plates in pristine and corroded conditions. A coupled discrete/finite element model is used to predict the time of flight and the relative amplitudes of the reflected solitary waves as localized corrosion progresses in the plate. The sensitivity of the method is quantified in terms of different parameters such as plate thickness, chain length, and particles' mechanical and geometric properties. It is found that the sensitivity, i.e. the ability to detect corrosion at earlier stages, of this novel NDE technique is inversely proportional to the plate thickness and is proportional to the diameter and the stiffness of the particles. In the future, the findings of this study can be used to optimize the design of solitary wave transducers, which are devices that can be used to trigger, sustain, and detect the propagation of the solitary waves and their interaction with the structure of interest. [Display omitted] [ABSTRACT FROM AUTHOR]

An extension of the Legendre polynomial method is proposed, which allows to model coupled Lamb wave propagation in anisotropic multilayers with Kelvin-Voigt viscoelastic properties in a carbon-epoxy-layer sequence [0/ϕ/0]. In spite of complicated nonlinear dependences of the constituent equations on the viscoelastic properties, a linear relationship is found between the attenuation curves calculated by Hysteretic model, and the attenuation curves calculated by the Kelvin-Voigt model. In view of verifying the feasibility of using wave propagation analysis for non-destructive testing of composite materials, predictions are made concerning the dependence of the experimentally accessible phase velocities and attenuation coefficients of the modes on (virtual, defect induced, changes of) the viscoelastic constants in general and the fiber orientation in particular. In order to get a deeper insight in the material property sensitivity of the waves and in how to excite and detect them efficiently, also the through-thickness of displacements and energies profiles of different wave modes and frequencies are investigated, and how these depend on the viscoelastic moduli C ij. [ABSTRACT FROM AUTHOR]

• The state of the art of SH wave piezoelectric transducers and EMATs is reviewed. • The principles, merits and drawbacks of various SH wave transducers are discussed. • Applications for SH wave-based SHM techniques in plates and pipes are presented. • Piezoelectric, magnetostrictive principles and excitation mechanisms are covered. Shear horizontal (SH) waves are of great importance in structural health monitoring (SHM) and nondestructive testing (NDT), since the lowest order SH wave in isotropic plates is non-dispersive. The SH waves in plates, circumferential SH waves and torsional waves in pipes have remarkable resemblances in dispersion characteristics and wave structures, so the latter two can also be called as SH waves in pipes. This paper reviews the state-of-the-art research on SH wave transducers for SHM and NDT. These transducers are grouped into the following categories: Lorentz-force-based electromagnetic acoustic transducers (EMATs), magnetostrictive EMATs, shear wave piezoelectric wedge transducers, thickness-shear piezoelectric transducers and face-shear piezoelectric transducers. The working principles, applications, merits and limitations of different kinds of SH wave transducers are summarized, with a focus on discussing the various configurations for exciting and receiving directional, omnidirectional SH waves in plates and torsional waves in pipes. This paper is expected to greatly promote the applications of SH waves in SHM, NDT and the related areas such as elastic metamaterials. [ABSTRACT FROM AUTHOR]

Broadband control of polarization state is essential for terahertz applications ranging from non-destructive testing to imaging in medicine and wireless communications. We present a single-layered terahertz anisotropic metasurface quarter-wave plate consisting of hybrid metallic wire grating arrays, which enables broadband linear to pure circular polarization conversion with an ellipticity over 0.99 and 43% bandwidth of the central wavelength. The broadband effect is robust across a wide range of incident angles. Interestingly, angle-tunable arbitrary polarization states are implemented by changing the angle of incidence at other frequencies. Such single-layered broadband quarter-wave plate provides a route to develop broadband polarization conversion components, facilitating compact applications of terahertz technologies. • Broadband quarter-wave plate is realized based on single-layered metasurface. • Ellipticity over 0.99 and 43% bandwidth of the central wavelength is achieved. • Broadband effect of wave plate is applied to a wide range of incident angles up to 80°. [ABSTRACT FROM AUTHOR]

Wavefield imaging is a powerful visualization tool in nondestructive evaluation for studying ultrasonic wave propagation and its interactions with damage. To isolate and study damage scattering, damage-free baseline data is often subtracted from a wavefield. This is often necessary because the damage wavefield can be orders of magnitude weaker than the incident waves. Yet, baselines are not always accessible. When the baselines are accessible, the experimental conditions for the baseline and test data must be extremely similar. Researchers have created several baseline-free approaches for isolating damage wavefields, but these often rely on specific experimental setups. In this paper, we discuss a flexible approach based on ultrasonic guided wave digital surrogates (i.e., numerical simulations of incident waves) and transfer learning. We demonstrate this approach with two setups. We first isolate reflections from a circular, 2 mm diameter half-thickness hole on a 10 × 10 cm steel plate. We then isolate 8 circular, half-thickness holes of various diameters from 1 mm to 40 mm on a 60 × 60 cm steel plate. The second plate has a non-square geometry and the data has multi-path reflections. With both data sets, we isolate damage reflections without explicit experimental baselines. We also briefly illustrate the comparison of our dictionary learning method with wavenumber filtering technique which is often used to enhance the defect wavefields. • Damage wavefield reflections are isolated without explicit experimental baselines. • Numerical simulations are used to learn and transfer knowledge to experiment. • We use a non-square plate in the presence of multipath reflections. • Reflections from 8 half-thickness holes are isolated from incident waves. [ABSTRACT FROM AUTHOR]

Lamb waves are used in various non-destructive testing (NDT) and structural health monitoring (SHM) applications due to their velocity sensitivity to the properties of the material and their propagation over the entire thickness of the object over a long distance with low attenuation. However, the interpretation of the received signals is complex due to the Lamb waves dispersion nature, multiple mode generation, reflections and other. This work presents a novel method for the Lamb waves fundamental modes phase velocity dispersion evaluation and reconstruction of these curves segments using only two signals measured at two close points. The carried out investigation using simulated signals has shown that the proposed method works with an extremely high accuracy - the mean relative error from the true values is less than 0.01%. The proposed method has been experimentally verified in two different mediums: isotropic – aluminium plate and anisotropic GFRP plate. The obtained results have indicated have that the mean relative error from the true values is less than 1% in the aluminium case. The comparison of the results obtained in the GFRP plate case has indicated that the proposed method allows to reconstruct the phase velocity dispersion curves segments in the object of complex structure. [ABSTRACT FROM AUTHOR]