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Guided Lamb waves are highly valued in structural health monitoring for identifying critical damage. The essence of Lamb wave-based damage detection is finding and processing damage-sensitive features (DSFs) that can reliably reflect and interpret structural damage. The primary emphasis of this work is the first time a comprehensive evaluation and comparison of the fifteen most commonly used DSFs in the time, frequency and time–frequency domain for Lamb waves, along with their statistical and wave interactive features based on the proposed five feature evaluation criteria. Five feature evaluation criteria are proposed concerning diagnostic capability, stability against environmental variability, computational complexity, sensor framework and structural complexity. These criteria aim to facilitate the evaluation of the robustness and reliability of each feature category. These evaluations highlight the strengths and limitations of each feature and qualitatively quantify its performance at four levels. Additionally, the paper provides explicit guidance on selecting appropriate Lamb waves' features, including when to use specific features for damage description and determination. The Lamb wave-based data pre-processing and damage characterization approaches, as well as the influence of environmental and operational factors on damage features, are also discussed in this work. Conclusively, this work outlines four prospective research avenues concerning DSFs. • Proposed are 5 feature evaluation criteria for damage-sensitive features (DSFs) evaluation. • Evaluated and compared are the 15 commonly used Lamb waves' DSFs for the first time. • Offered are 7 recommendations on the selection of appropriate DSFs. • Presented are 5 methods of excitation and reception of lamb waves. • Discussed in detail are the effects of 6 different environmental and operating conditions. [ABSTRACT FROM AUTHOR]

This paper presents an approach exploiting the sensitivity of Lamb waves for characterizing the viscoelastic moduli and thickness of plates. The analytical sensitivity functions are first derived in the case of an isotropic plate and are integrated into an iterative inverse problem to optimize its viscoelastic moduli and thickness based on a zero-finding approach (Gauss–Newton algorithm for a multivariable problem). This method is validated numerically for a viscoelastic plate and shows high accuracy and low computational cost when compared to existing methods. Experimental validation demonstrates the ability of the algorithm to assess simultaneously the viscoelastic moduli and the thickness of isotropic plate-like structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel two-stage non-destructive procedure for determination of elastic constants was proposed. • Ultrasonic waves were successfully used to characterize elastic behaviour of AM samples with different raster angles. • Integrated use of bulk and guided waves was proved to be a simple but robust approach. • The method allowed characterization of samples represented by transversely isotropic and orthotropic material models. The subject of the current work was a simple but robust novel two-stage procedure for the non-destructive determination of effective elastic constants using ultrasonic wave propagation. First, ultrasonic bulk wave velocities measured on cubic samples were used to calculate most of the elements of the stiffness matrix. Secondly, the remaining elements were determined using the dispersion curves of elastic guided waves measured on plate samples. Based on the complete stiffness matrix it was possible to calculate the complete set of effective elastic constants. The algorithm was verified for AM elements produced from PLA filament satisfying the conditions of transversely isotropic and orthotropic material models. For the transversely isotropic samples, Young's moduli (E) varied from 2.6 to 2.9 GPa, shear moduli (G) equalled between 0.9 and 1.2 GPa, whereas Poisson's ratios (ν) ranged between 0.20 and 0.32. In orthotropic sample the corresponding values were: E = 1.3–2.8 GPa, G = 0.6–1.2 GPa, and ν = 0.07–0.59. The results of the current study have been compared with references from the literature, giving satisfactory agreement. [ABSTRACT FROM AUTHOR]

[Display omitted] • An advanced ultrasonic technique is employed to detect glass cracks in solar modules. • The Lamb waves scanning method produces map of the defects locations efficiently. • The investigated defects are due to mishandling, representing a real-world scenario. Ensuring the structural integrity of solar photovoltaic modules is crucial to maintain power production efficiency and fulfill the anticipated product lifespan. Hence, implementing quality control procedures and structural health monitoring is necessary throughout the stages of manufacture and operation. The ultrasonic examination has some benefits compared to electroluminescence and infrared approaches, namely in identifying mechanical flaws in the front glass. The Lamb waves (LW) method is an auspicious inspection approach among ultrasonic-based methods. This is primarily attributed to its quicker measurement speed than the standard ultrasonic c-scan. The LW method offers an advantage in terms of its ability to provide long-range coverage. The predominant approach in LW inquiry often centers on using fundamental modes within the low-frequency range. Nevertheless, the findings of this investigation demonstrate that the higher-order mode exhibits superior effectiveness for the specific objective of this research, as it displays a higher level of sensitivity towards cracks in the front glass of the module. The current study ultimately showcases the use of the LW scan. A damage indication threshold is determined by the fraction of energy spectral density (ESD) associated with the crack-sensitive mode. This technology effectively produces a comprehensive map of the designated area by comparing the ESD values at various measurement positions with a predetermined threshold. This map provides precise indications of the existence of areas that are affected by cracks. [ABSTRACT FROM AUTHOR]

• Contact surface boundary conditions between the two surfaces of the vertical crack were considered to study contact acoustic nonlinearity (CAN) from the breathing contact crack in conjunction with the modal decomposition method, Fourier transform, and variational principle-based algorithm. • The clapping effects when the crack closed due to the different ratios of incident wave amplitude to crack width with the contact boundary conditions between the crack surfaces were studied and analyzed to get the quantitative correlation between analytical calculations and FE simulation. • When the crack state was modulated at a low frequency (much lower than the one of the probing ultrasonic waves), the related mode conversion coefficients were modulated accordingly both in the fundamental frequency and harmonic frequency. To characterize fatigue crack, an analytical calculation and finite element (FE) simulation of Lamb wave propagating through the region of a breathing crack in a two-dimensional(2D) isotropic plate were studied. Contact surface boundary conditions between the two surfaces of the vertical crack were considered to study contact acoustic nonlinearity (CAN) from the breathing contact crack in conjunction with the modal decomposition method, Fourier transform, and variational principle-based algorithm. Reflection and transmission coefficients in the fundamental frequency and second harmonic frequency were calculated and analyzed quantitatively. Different ratios of incident wave amplitude to crack width were studied to calculate CAN results related to micro-crack width. In addition, a low-frequency (LF) vibration(10 Hz) excitation was introduced to perturb the free surface vertical crack to close, and an interrogating Lamb wave(1 MHz) was used to study crack-related CAN in different conditions for interpreting the modulation mechanism. The contact boundary conditions between two surfaces of vertical crack were set which were dynamically changed due to the low frequency modulation. The clapping effects when the crack closed due to the modulation of the contact boundary conditions between the crack surfaces were studied and analyzed to get the quantitative correlation between CAN and LF modulation. The results obtained from the analytical model were compared with those from the FE simulation, showing good consistency. Knowledge of these effects is essential to correctly gauge the severity of surface cracks in the plate, which can be spotlighted in its application to quantitative evaluation of micro fatigue cracks in structural health monitoring(SHM). [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]

• A passive sensor for ultrasound based on thermochromic liquid crystals (TLCs) is introduced. • Its use for characterising ultrasound sensors and for visualising wavefields in plates is presented and explained. • The sensor offers the potential for immediate visualisation without the need for scanning, and can be applied directly onto a sample or in air. • Ultrasound is visualised via local heating of the TLC and an absorbing backing layer. Acoustic field and vibration visualisation is important in a wide range of applications. Laser vibrometry is often used for such visualisation, however, the equipment has a high cost and requires significant user expertise, and the method can be slow, as it requires scanning point by point. Here we suggest a different approach to visualisation of acoustic fields in the kHz – MHz range, using paint-on or removable film sensors, which produce a direct visual map of ultrasound displacement. The sensors are based on a film containing thermochromic liquid crystals (TLC), along with a backing/underlay layer which improves absorption of ultrasound. The absorption generates heat, which can be seen by a change in colour of the TLC film. A removable sensor is used to visualise the resonant modes of an air-coupled flexural transducer operated from 410 kHz to 7.23 MHz, and to visualise 40 kHz standing waves in a Perspex plate. The thermal basis of the visualisation is confirmed using thermal imaging. The speed and cost of visualisation makes the new sensor attractive for use in condition monitoring, for fast assessment of transducer quality, or for analysis of acoustic field distribution in power ultrasonic systems. [ABSTRACT FROM AUTHOR]

• Under stress-free and clamped boundary conditions, SH waves within a thin micropolar plate display both symmetric and skew-symmetric modes. However, these modes are notably absent under mixed boundary conditions. Similarly, P-SV waves can also generate both modes. Notably, both SH and P-SV waves exhibit dispersive characteristics. • The dispersion relationship governing SH-waves under mixed boundary conditions remains unique, regardless of which surface of the plate is stress-free or clamped. Notably, an intriguing transition point emerges in the phase velocity trend for SH waves under mixed boundary conditions. Preceding this point, the phase velocity decreases with an increasing wave number; however, beyond this point, the trend reverses. This phenomenon strongly suggests a non-linear relationship between phase velocity and plate properties under mixed boundary conditions. • The phase velocities of both SH and P-SV waves exhibit a direct correlation with both the plate thickness and the characteristic length parameter of micropolar plate. • The coupling number demonstrates a positive correlation with the phase velocity of SH waves under stress-free and clamped conditions, along with a similar relationship observed in the phase velocity of P-SV waves. The effects of the coupling number on the phase velocity of SH waves under mixed boundary conditions are particularly intriguing. In this scenario, the phase velocity initially rises and then declines with higher coupling numbers, with more noticeable impacts observed in the lower wave number range. This insight significantly contributes to a better comprehension of vibration behavior in thin plates under diverse conditions. • It is observed that number of modes increases with the increase in non-dimensional wavenumber. The observed phenomenon suggests that, across all modes, the phase velocity undergoes a reduction and eventually stabilizes at a constant value with an increasing wave number. Additionally, as the wavenumber increases, higher modes emerge with an augmented phase velocity. Studying wave propagation characteristics in plates is vital for comprehending the material dynamics to enhance the strength of structures and for optimizing the effectiveness of non-destructive testing devices. Micropolar elasticity displays microcontinuum behavior by integrating rotational and coupling effects through displacements and micro-rotations. The present study thoroughly investigates horizontally polarized shear waves (SH) within a thin micropolar plate by employing diverse boundary conditions namely stress-free, clamped, and mixed conditions. Shear horizontal waves propagate within a plate by shearing the material along the surface, excluding any motion in perpendicular direction. Moreover, for a more comprehensive grasp of wave behavior in a micropolar plate, the study investigates P-SV type waves under plane strain conditions. Analytical techniques are utilized to derive the dispersion relations for SH and P-SV type waves in a micropolar plate. Graphical representations are provided to showcase the impacts of various micropolar parameters such as coupling number, characteristic length, and plate thickness on the phase velocities of SH and P-SV type waves. The study also includes a dispersion analysis of the multimode aspect of SH and P-SV waves. This thorough investigation can significantly augment the comprehension of wave propagation phenomena within plate-like structures. [ABSTRACT FROM AUTHOR]

• Guided waves in nano-aluminum plates are simulated by molecular dynamics. • The polynomial method is developed to study the nonlocal strain gradient models. • The size parameters of the nonlocal strain gradient theory are calibrated. • Lamb waves and SH waves have different size parameters. The nonlocal strain gradient theory (NSGT) describes long-range interatomic interactions and higher-order strain gradients by introducing size parameters. However, the NSGT is constrained in studying the elastic wave propagation problems due to the difficulty in obtaining the precise size parameters. In this paper, the molecular dynamics (MD) simulations of the elastic wave propagation in the aluminum nano-plate are conducted, and the size parameters in the NSGT are calibrated based on the MD simulation results. The MD simulation process is divided into three stages: relaxation, excitation, and free vibration analysis. The embedded atom method is used to describe the interactions between the metal atoms. During the MD simulations, appropriate boundary conditions and excitation methods are proposed to obtain the group velocities of the elastic bulk, Lamb and SH waves. In addition, based on the three-dimensional (3D) elasticity theory and analytical integration Legendre polynomial method, the theoretical dispersion curves of the elastic wave propagation in the NSGT model are obtained. It is found that both the nonlocal theory and the NSGT can predict the guided elastic wave group velocity, and the NSGT prediction and MD simulation can be well matched when the size parameters are restricted to a certain calibrated regional band. The vibration of atoms in the Lamb wave propagation process is more complex than that in the SH wave propagation, leading to a more substantial size effect on the Lamb wave dispersions. [Display omitted] [ABSTRACT FROM AUTHOR]

• Cumulative second harmonic S0 Lamb waves are enhanced using a topologically optimized metamaterial structure. • The metamaterial-structure-aided monitoring system presents improved sensitivity to material microstructural changes. • The mode conversion efficiency is evaluated in simulations and experiments. Cumulative second harmonic Lamb waves in nonlinear media feature increasing amplitudes with propagation distance, conducive to the monitoring of material microstructural changes in structures. The phenomenon can be readily generated by zero-order symmetric (S0) mode waves in the low-frequency range. However, in a practical piezoelectric-transducer-activated system, both S0 and A0 (zero-order antisymmetric) mode Lamb waves are inevitably excited, while only the former is responsible for cumulative effects. The generation efficiency of the cumulative second harmonics is then affected by the presence of the A0 waves. To tackle the problem, this study develops a metamaterial structure, referred to as a meta-structure, to tactically enhance the cumulative second harmonic S0 Lamb waves by converting the A0 mode components into S0 mode waves. Topology optimization is conducted to design the meta-structure, which is surface-mounted onto the structure under inspection, to achieve high-efficiency A0-to-S0 wave mode conversion. Through tuning the parameters and constraints of the optimization, the designed single-sided meta-structure breaks the structural symmetry in the thickness direction, while facilitating its practical implementation. Typical scenarios with different meta-structure materials are discussed. Numerical simulations demonstrate that the strain amplitudes of the fundamental S0 mode waves can be increased by 60% with the deployment of the meta-structure, alongside an enhancement of the second harmonic S0 mode waves at different sensing distances. Finally, the designed meta-structure is fabricated via 3D printing technique and tested experimentally on an aluminum plate subjected to thermal aging treatment for monitoring the heating-induced microstructural changes inside the structure. Experimental results confirm an increase in the wave amplitudes of the linear S0 mode waves with the assistance of the meta-structure. The developed system improves the sensitivity of nonlinear Lamb wave-based monitoring methods in characterizing material microstructural changes, which shows great promise for detecting incipient damage in practical structural health monitoring applications. [ABSTRACT FROM AUTHOR]

• An energy focusing method of broadband Lamb wave by metamaterial is proposed. • All frequencies waves can reach the focus simultaneously by designing excitation. • The focusing energy is improved in proposed method compared to traditional method. • The method is effective for different transmitter/receiver position. In the field of structure health monitoring (SHM), the use of Lamb wave to locate damage is a common method. Energy focusing is beneficial for damage localization because of higher SNR and higher resolution. Optimization design of elastic metamaterials is promising for energy focusing based on speed modulation. However, current design scheme is effective only for narrowband Lamb waves. Compared to narrowband waves, broadband Lamb waves with a larger frequency range carry richer structural information. In this study, an energy focusing method based on broadband Lamb waves by simultaneous designing excitation waveforms and elastic metamaterials is proposed. Firstly, COMSOL finite element simulation software is used to calculate the relationship between the metamaterial structure and the excitation wave. Subsequently, the metamaterial structure and the excitation signal form are designed according to the relationship. Finally, the metamaterial structure is bonded with the aluminum plate using 3D printing PA2200 nylon to verify the effectiveness of the method. [ABSTRACT FROM AUTHOR]

• High-bandwidth and multiband polarization-controlled waveplates in the near-infrared band are realized using an all-dielectric metasurface. • By breaking the correlation between phase delay and frequency, independent polarization control over two wavelengths (2 μm and 4 μm) can be achieved. • We utilize micro-spheres structure on the back side as an anti-reflection layer, and the highest transmittance can reach about 87%. In this study, we demonstrate an all-dielectric metasurface that can efficiently manipulate polarization for two broadband polarization control in the infrared band. By breaking the correlation between phase delay and frequency, independent polarization control over two wavelengths (2 μm and 4 μm) can be achieved. The phase delay can be freely controlled by controlling the number of waveguide modes in the grating. When operating in single mode, the phase delay increases in a linear fashion with the thickness of the grating. While at two modes, the phase delay is periodically perturbed with increasing thickness, which is caused by the mutual interference between the two modes. Therefore, the phase delay of two independent bands can be freely controlled. Here, a free combination of a dual-band polarized half-wave plate and quarter-wave plate are designed. Half-quarter wave plate and quarter-quarter wave plate, are fabricated and experimentally demonstrated to have efficient broadband polarization conversion efficiency with relative bandwidths exceeding 42.8 %. Since only is the device made of pure silicon, we utilize micro-spheres structure on the back side as an anti-reflection layer, and the highest transmittance can reach about 87 %. [ABSTRACT FROM AUTHOR]

This study investigates nonlocal and surface effects on the dispersion behaviors of Shear horizontal (SH) waves in piezoelectric(PE)-piezomagnetic(PM) bilayer systems. The interface between these two layers is imperfectly bonded. The general governing equations are derived from the nonlocal magnetoelectroelastic (MEE) theory by adding an inherent length. The G-M model and generalized Young-Laplace equations have been used to incorporate surface effects into the boundary conditions of the bilayer systems. The closed-form dispersion equation is obtained analytically for electrically open and magnetically short conditions. Numerical solutions are utilized to investigate the effects of nonlocal scale parameters and surface parameters on SH surface wave propagation. Contrary to the results of classical theory, the coupling effects of nonlocal small-scale and surface piezoelectricity are more significant than individual effects. Also, it has been observed that the imperfectness parameter across the interface and the thickness ratio of the bilayer significantly affect the phase velocity. Moreover, 2D and 3D plots of the mode shapes of field variables for the propagation of SH waves are presented graphically. These results are validated by conducting analyses excluding nonlocal effects. This allows us to isolate the specific impact of surface effects in the piezoelectric–piezomagnetic bilayer system, drawing connections to existing results and enhancing the robustness of our findings. This study provides valuable insights into complex wave dynamics, helping to optimize the performance and functionality of such smart composites in various engineering applications. • Theoretical study of the propagation of SH wave in bilayer plate is investigated. • The coupling influence of surface piezoelectricity and nonlocality are discussed. • Thickness ratio of bilayer plates and imperfect interface have significant impacts. • Mode-shape plots of field variables for the SH wave are presented graphically. • This study provides valuable insights into complex wave dynamics in nanostructures. [ABSTRACT FROM AUTHOR]

One-dimensional shell chain structures can cause stress wave dispersion, and the dispersion ability is mainly affected by the apparent elastic Poisson ratio of the unit cell composing the chain. In this study, a novel plate–shell chain structure with a variable Poisson ratio is proposed by combining bent plates and arc shells (BPAS). The dispersive wave propagation features in the BPAS chains under striker impact are studied theoretically, numerically, and experimentally. An equivalent continuum model of the unit cell under compression in a linear-elastic deformation regime is established, which contains two material parameters, i.e., the apparent elastic modulus and the apparent elastic Poisson ratio. It is found that the unit cell can reach a relatively large Poisson ratio by adjusting its structural parameters. A continuum-based wave propagation model considering transverse inertia is developed, and an explicit analytical solution is obtained to represent the dispersion wave propagation behavior within a linear-elastic deformation regime. The model can reasonably predict the force and velocity evolution features of the chains and is verified by the numerical/experimental tests. The results show that the BPAS chain with a large Poisson ratio exhibits superior wave dispersion performances. The underlying mechanism is that more impact energy is dispersed by the transverse movement of the cells in the BPAS chain. Increasing the aspect ratio of the unit cell can improve the wave dispersion ability of the BPAS chains, achieving a high attenuation of the stress wave and is helpful for impact mitigation. • A plate–shell chain with a variable Poisson ratio is proposed for tuning wave propagation. • An equivalent continuum model of the unit cell composing the chain is established. • An analytical continuum-based wave model considering transverse inertia is developed. • The plate–shell chain can achieve strong wave dispersion due to its transverse inertia. • The wave dispersion ability of the plate–shell chain is superior to the hollow elliptical cylinder chain. [ABSTRACT FROM AUTHOR]

The thin layer method (TLM) is an effective semi-discrete numerical tool for analyzing wave motion in stratified media. The TLM can calculate the eigenvalues and eigenvectors for both propagating and decaying waves, which is essential to simulate waves near the source. The eigenvectors are particularly useful in various applications, such as estimating modal contributions, calculating synthetic seismograms, determining Green's function, analyzing site response, and solving wave amplification problems in earthquake engineering, to name a few. In practice, most engineering applications use linear element-based TLM. However, the overall performance of TLM depends upon the type of element and the number of thin layers. The linear element TLM requires relatively thin sub-layers to obtain an accurate solution, making it computationally expensive and time-consuming. This study explores the potential of using non-linear higher-order TLM (HTLM) for forward modeling. The mathematical framework of TLM is used to formulate the generalized HTLM in determining the multimodal dispersion curve of Rayleigh, Love, and Lamb waves. The convergence and computational efficiency of different orders of HTLM have been investigated on a diverse set of published soil profiles and plate-like structures. Since non-linear interpolation functions better capture the change in displacement and stress between the layers, the findings reveal that the HTLM consistently outperforms the linear element TLM in terms of accuracy. Furthermore, the computational efficiency of different order HTLM is analyzed by comparing the total degrees of freedom and CPU time. Remarkably, higher-order HTLMs exhibit improved accuracy without increasing the computational burden, making them an attractive option for practical applications such as global search-based inversion and full spectrum inversions where fast and economical forward modeling is essential. Therefore, the HTLM will become helpful in seismic wavefield modeling, dynamic soil characterization, non-destructive evaluation methods, wave propagation analysis through waveguides, seismic site response analysis, etc. • Higher order thin layer method for theoretical dispersion calculation of surface waves. • The higher order converges much faster than the linear thin layer method. • Accurately captures the changes in element's stress and displacement field. • Exhibit improved accuracy without increasing the computational burden. [ABSTRACT FROM AUTHOR]

• Broadband signal is used for excitation and the full wavefield is measured using SLDV. • A multi-stage vibrometric NDT approach is introduced for defect detection in plates. • The proposed broadband entropy-based two-stage damage imaging algorithm provides high contrast damage map, revealing both shallow and deep defects. • The Method is successfully tested on the experimental data of different test specimens containing various defect types. This paper presents a new algorithm called broadband entropy-based two-stage damage imaging (BE2DI) for the detection and imaging of defects in multi-layer plates. The method differentiates defects from either local scattering phenomena of propagating Lamb waves or locally increased vibrational activity for stimulated global plate resonances by exploiting the higher vibrational activity of full-field broadband vibration signals at defective regions and the distinct spatial behavior of defects and artifacts. Using these features, first, a candidate damage map is constructed by means of iterative thresholding based on descriptive statistics of broadband data. Afterwards, an adaptive regional damage index based on local entropy is introduced, which utilizes operational deflection shapes to process previously identified candidate regions and subsequently reveal both shallow and deep defects. BE2DI can be seen as an automated local defect resonance identification approach in the case of shallow defects. Multiple plates having various defect types with different sizes and depths are investigated. Broadband chirp signals are used to excite the plates using low-power piezoelectric actuators, and vibrational data is registered by 3D scanning laser Doppler vibrometry. The obtained results quantitatively and qualitatively demonstrate that the proposed method can effectively detect different kinds of damage, regardless of being shallow or deep. [ABSTRACT FROM AUTHOR]

Ultrathin wave plates based on metallic and dielectric metasurfaces have attracted much attention for independently controlling the phase, amplitude, and polarization of incident light. However, the issues of narrow bandwidth and low polarization conversion need to be improved for their optical applications. In this study, we show that an array of paired metal nanowire grids (PMNG) covered with a dielectric layer controls the polarization state in a broadband of visible to mid-infrared wavelengths. By analyzing the phase difference and amplitude ratio of orthogonal electric-field components passing through the SiO 2 -covered PMNGs, it is observed that the wavelengths operating as a circular polarizer appear regularly in the range from 450 nm to 4.0 μm with a stable phase difference of π/2. Also, the array of PMNG continuously converts linear polarization into circular or elliptical polarization by rotating the polarization angle of the incident light. Then, we demonstrate experimentally accessible polarization control by investigating the change in polarization ellipse between input and output light for this PMNG structure. Moreover, the PMNG can play an effective role as a wire grid polarizer in the IR region. Therefore, the suggested PMNG platform can be variously applied to emerging fields requiring ultrathin, multifunctional, and high-performance planar polarization components. • We resolve the issue of narrowband phase difference by using an array of paired metal nanowire grids (PMNG). • The wavelengths operating as a circular polarizer appear regularly in a broad wavelength of 450 nm to 4.0 μm. • The PMNG regularly converts linear polarization into circular polarization by rotating the polarization angle of incident light. • This new type of QWP, PMNG, also serves as an efficient wire grid polarizer in the IR region. [ABSTRACT FROM AUTHOR]

Monitoring material degradation and in-service damage in fibrous composites at an early stage is challenging, yet critical for many industrial applications. Second harmonic Lamb waves, which result from the interaction of fundamental waves with material microstructural defects, have great potential for Structural Health Monitoring (SHM) applications. However, understanding of the second harmonic generation in composite laminates remains largely insufficient, which hinders practical SHM applications. This issue is investigated in this study to highlight the quasi-cumulative second harmonic B 2 mode Lamb wave in the low frequency range. Theoretical analyses are conducted to ascertain how anisotropy affects the quasi-cumulative effect, followed and validated by numerical simulations and analyses considering different wave propagation angles. The characteristics of the second harmonic B 2 mode Lamb waves are further explored in terms of the cumulative feature, robustness to wave beam divergence, and excitability. Experiments are finally carried out to monitor material degradation during the thermal aging of a carbon fiber reinforced panel. The results confirm the omnidirectional cumulative effect of quasi-phase-matched second harmonic B 2 mode Lamb waves. It is revealed that the 0° propagating Lamb waves exhibit superior cumulative effect, high robustness to wave beam divergence, and good excitability, rendering them an ideal tool for SHM applications. Furthermore, the high sensitivity of the second harmonic B 2 mode waves to material degradation is demonstrated, confirming their promise for material degradation monitoring applications in composite structures. • The influence of the anisotropy of composite materials on the second harmonic generation is theoretically analyzed. • The cumulative feature, robustness to wave beam divergence, and excitability of the second harmonic B 2 mode Lamb waves are numerically explored. • The second harmonic B 2 mode Lamb waves are experimentally applied to monitor material degradation during the thermal aging of a carbon fiber reinforced panel. [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]

The air-coupled ultrasonic technique is a reliable method for measuring the stress state of composite plates. This paper aims to address the requirement for efficient and accurate stress measurement of large component composite plates by proposing an air-coupled ultrasonic in-situ measurement method. Ultrasonic Lamb waves can propagate over long distances while maintaining sensitivity to slight changes and are suitable for in-situ stress measurement of CFRP plates. However, due to the serious mismatch in acoustic impedance, the air-coupled ultrasonic Lamb waves have a low signal-to-noise ratio, which can affect the accuracy of stress measurement. Focusing on this problem, the method employs the Lyapunov exponent with a moving window to accurately extract the acoustic-time characteristic instead of just judging the presence or absence of a weak signal compared with the traditional method of using the Lyapunov exponent. To evaluate the effectiveness of the proposed method in different fiber directions of CFRP plates, seven uniaxial tensile specimens are obtained along 0°, 15°, 30°, 45°, 60°, 75°, and 90° fiber directions, respectively. Utilize stress coefficient calibration results K L to realize the uniaxial stress measurement with a spatial resolution of 150 mm. The experimental results demonstrate that the stress measurement errors are within ±7.29 MPa in the range of 0–150 MPa, and the standard deviations of the repeatability measurement experiment are below 4.57 MPa. The proposed method exhibits significant advantages in terms of measurement accuracy and repeatability compared with traditional stress methods based on peak extraction or cross-correlation. It is a suitable technique for health monitoring and evaluation of composite plates. [ABSTRACT FROM AUTHOR]

• Mode-entangled resonance (MER) enables mode-preserving full transmission of a Lamb wave. • Symmetric (S) or asymmetric (A) Lamb wave can pass through an asymmetric waveguide using the MER. • MER develops when S- and A-modes individually satisfy specific phase-matching conditions. • Super-enhancement of transduction efficiency for A-mode is achievable using the MER. We report a novel resonance phenomenon called the mode-entangled resonance. It can occur at a specific frequency in a plate carrying both symmetric and antisymmetric Lamb wave modes. We show that at this resonance, a symmetric or antisymmetric Lamb wave can be fully transmitted with its mode preserved from one plate to another through a matching plate geometrically misaligned with the two base plates. This phenomenon appears to be counter-intuitive because both symmetric and antisymmetric waves are generally formed in the reflected and transmitted wave fields when the misaligned plate is used as a matching plate. Our analysis shows that if both symmetric and antisymmetric Lamb wave modes at the resonance are entangled in the misaligned matching plate to satisfy the established phase matching condition, the matching plate fully transmits any wave mode across the two base plates. The discovered phenomenon is related to the classical Fabry-Perot resonance (FPR), but FPR is only applicable for single-mode transmission through a matching plate aligned with two base plates. Numerical and experimental results were presented to validate the discovered mode-entangled resonance phenomenon. Furthermore, we show that the discovered phenomenon can be used for super-amplification of the amplitude of the antisymmetric Lamb wave when it is excited by plate-surface-mounted patch transducers. [Display omitted] [ABSTRACT FROM AUTHOR]

Solar chimney technology has received considerable attention in recent years as it offers suitable thermal comfort at the desired space and involves no running cost and harmful emissions. The heat transfer and ventilation performance of the solar chimney greatly depend on the configuration of the absorber plate. Thus, the present paper numerically investigates the effect of variations in geometrical configurations of the wavy absorber plate such as the number of waves, wave amplitude, inclination angle of the chimney, inclination angle of the glass plate, and air gap. The numerical analysis suggested optimum number of waves to be 10 (Nu = 210), optimum inclination of the chimney to be 55° (Nu = 220), and optimum wave amplitude to be 6.25 cm (Nu = 232). However, no optimum values were obtained for variations in glass plate inclination and the air gap. The inverted T plate surface geometry (Nu = 252–257) delivered the highest enhancement in heat transfer performance relative to other surface geometries. Correlations have been developed for the performance parameters by performing several numerical experiments and the proposed mass flow rate correlation predicts 91.3% of the data within ±7%, whereas, the Nusselt number correlation predicts 93.47% of the data within ±7%. [ABSTRACT FROM AUTHOR]

As a key part of elliptical vibration cutting (EVC) technology, a two-dimensional ultrasonic elliptical vibration cutting (UEVC) device has become a prevalent area of importance and research interest. This paper dissects through the conventional structural design and creatively proposes the UEVC device with flexible guided wave propagation. The device resolves the problems of poor coupling efficiency of two-way excitation signal, complex structure design, non-orthogonal, and the difficulty in controlling the ellipse trajectories of the tool tip. It has been confirmed that a flexible guided wave exhibits an effective unidirectional propagation and lateral load decoupling ability via establishing its system dynamic response and frequency impedance characteristics. Here, we explain the coupling mechanism of the two-way flexible guided wave in the UEVC device. Results suggested that the experimental measurement of the impedance for the cutting device and the individual transducer shows that the cutting device with the transducer and the tool bar connected by the flexible guided wave has a lower impedance increase ratio compared to the individual transducer, and hardly influence the tool bar mode of the cutting device. At the same time, the tool tip trajectory of the cutting device having a phase difference of 90° showed a satisfactory "elliptical" shape, while the microstructure processed by the tool tip ellipse trajectory with a phase difference of 60° reached a burr-free state. Theoretical and experimental findings reveal that the flexible guided waves exhibit excellent weak coupling characteristics in the non-transmitting direction. This is convenient to the independent, orthogonal and stable propagation of signals in the two directions. Based on flexible guided wave propagation technology, our findings provide theoretical foundation and application value for the design of a two-dimensional UEVC device. • Proposed an innovative method to realize ultrasonic elliptical vibration cutting driven by two-way flexible guided wave. • The frequency impedance model of flexible guided wave ultrasonic elliptical vibration cutting device is established. • The two excitation signals independently drive the tool bar, and the device design becomes simple. • The ultra-precision burr-free machining of microstructures is completed by using the controllable ellipse trajectory. [ABSTRACT FROM AUTHOR]

• Beams of 1 H 1+, 7Li3+, 16O6+ and 58Ni12+ ions were chopped successfully in a wide range beam energy. • The measurements carried out to test the performance of the chopping system confirm the validity of the expressions used in the design. • The analytical expression should be used for interpretation of BPM profile of the beam modified by chopper action to reconstruct the correct maximum beam displacement. • If a parallel plate sine wave chopper is used, there is an evidence of significant loss of transmission through the 90° non-achromatic analyzing magnet, due to the increase of beam emittance, especially for heavy ions. A chopping system of a tandem accelerator based on two pairs of parallel plates is investigated. The first chopper operating at 37.5 MHz cuts off the tails of the bunched beam and the second, at a lower frequency of 4.6875 MHz, eliminates the satellite low intensity pulses. The experimental procedures for measurement of the performance of a chopping system including a Beam Profile Monitor (BPM) technique and a pulsed beam diagnostic monitor are described. The results of simulations and experimental observations are presented. An analytical technique to reconstruct the real profile of deflected beams from the profile measured with a BPM is discussed in detail. Theoretical and experimental data on the chopper scanning window are presented. An analyses of the factors limiting the chopping system performance is conducted and technical solutions to overcome it are discussed. [ABSTRACT FROM AUTHOR]

Introduction: In the pathogenesis of femoral pseudoarthrosis, the arched shape of the bone is important when determining traction force on the anterolateral plane and posteromedial compression. In plate osteosynthesis, flexion forces affect the implant and can determine its breakdown and osteosynthesis failure. The wave plate seeks to solve the problem by transferring the support to the cortex under the plate or creating the resistance of a leaf spring.Objectives: To evaluate the bone healing pattern of femoral pseudoarthrosis treated with wave plate and its effectiveness in consolidation.Materials and Methods: We retrospectively evaluated 18 patients with femoral pseudoarthrosis treated with wave plate.Results: Three months after surgery, one patient presented healing of pseudoarthrosis, six months after surgery, 16 patients presented healing of pseudoarthrosis and at 12 months, all patients had complete bone healing. Regarding the type of pseudoarthrosis, nine patients had avascular and nine vascular pseudoarthrosis. Fifteen patients had medial cortical failure and three, lateral cortical failure. In all cases, consolidation started where there was bone failure, be it medial or lateral.Discussion: The wave plate technique is well indicated for the existence of cortical bone failure, both in fractures and in pseudoarthrosis. Objective is to stabilize the fragments by transferring and allowing contact between them. The formation of the bone callus occurred in our sample from the region of the bone defect to the region of bone contact, using either a tricortical or cancellous bone graft. The elastic force of the wave and the eccentric distribution of loads in the femur favored the formation of a callus where it was distracted (bone failure) and the location with concentration of movement (proximity between the fragments) was the end of bone healing.Conclusion: The wave plate technique was effective in bone healing of femoral pseudoarthrosis, promoting consolidation from the cortical failure to the bone contact zone. [ABSTRACT FROM AUTHOR]

Porous materials contain a network structure composed of distributed pores. Due to the distribution of pores, the volume of the material will be less than the actual dense volume. The material coefficients such as density and elastic modulus can change due to the influence of porosity. Therefore, the characteristics of porosity can be used as a measurement standard in the properties detection. This study explores the dispersion relationship of the wave propagation behaviour of the Lamb wave of porous material plates such as 316 L stainless steel and permeable alumina ceramic with different porosities in water fluid environment. We used laser to excite the ultrasonic wave and used probe receiving as a method to measure the guided wave dispersion relationship. The results of the study show that the effect of porosity of the porous plate on the density and elastic modulus can be shown through the dispersion relationship. The theoretical model and the experimentally measured dispersion relationship results were in good agreement. In addition, by altering the density, Poisson's ratio, shear modulus and independent porosity of the porous plate, the dispersion curve can be shifted, which can be used for the development of ultrasonic measurement in the future. [ABSTRACT FROM AUTHOR]

• The method of multiple scales is applied to analysis of internally resonant guided waves. • The amplitudes of the internally resonant guided waves are nonlinearly dependent to propagation distance. • The propagation-distance dependence of the amplitudes is affected by detuning parameters. Taking into account damping effects, the propagation-distance dependence of the amplitude of nonlinear guided waves arising from an internal resonance was investigated. In the analysis of Lamb-wave propagation, we introduced nonlinear quadratic effects derived from both geometric and material nonlinearities, referred to as classical nonlinearities, and damping effects by expressing the Lamé constants as complex numbers. Assuming a small detuning from the phase matching condition with respect to wavenumber and frequency, and using the method of multiple scales and self-adjointness of the governing equations with boundary conditions, the amplitude equations describing the nonlinear coupling of the two wave modes are derived as solvability conditions. This derivation also provides both physical and mathematical insights into both the phase and group velocity matching conditions. Numerical results obtained from these amplitude equations show that because of the nonlinear coupling an internal resonance between the two modes can occur under the above matching conditions. Without detuning, one mode increases monotonically in amplitude while the other decreases monotonically. Nevertheless, their changes gradually decrease with propagation distance, and each amplitude converges to a finite value. However, with wavenumber and frequency detuning, both amplitudes increase and decrease cyclically with propagation distance, resulting in a weakening of this resonance. Furthermore, results with damping effects show an attenuation of both amplitudes with propagation distance, which limits the distance resonance waves can propagate. [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 electrostatic precipitator has been widely utilized to control particulate pollutants in industry. It is important to optimize the configurations of the discharge and collecting electrode in order to increase the efficiency of electrostatic precipitators. The purpose of this paper is to investigate the influence of wavy collecting plates in a single-stage electrostatic precipitator. The effects of the applied voltage, volumetric flow rate per unit height, wavelength. h, and amplitude on the particle charging and behavior are extensively analyzed. The numerical model is validated by comparison with previous experimental and numerical data. The wavy collecting plate enhances the charging and transport behavior of the particles. Furthermore, changing the amplitude of the wavy collecting plate leads to a much greater effect on the particle charging and particle behavior in the wavy electrostatic precipitator, resulting in the highest collection efficiency. Unlabelled Image • The wave amplitude of plates is a crucial factor to increase the corona current. • Vortex in the convex section affects the gas flow field and makes flow faster. • The wavy collecting plate enhances the particle charging process. • The wave amplitude of plates significantly affects the collection efficiency. [ABSTRACT FROM AUTHOR]

Standard approaches to source localization usually rely on trilateration, using signals detected at multiple receiving points, with further research focused on improving localization robustness and reducing the number of sensors required. Time Reversal (TR) is a valid alternative, however it is often difficult to define a reliable hybrid (computational/experimental) approach, particularly in the case of plates where dispersive properties of the propagating waves play a crucial role to obtain focusing. In this paper, we present an analytical method that describes wave propagation in thin plates, accounting for edge reflections and dispersive properties, and validate it by comparison to experimental data. The Image Source Method (ISM), employed and extended in this study, provides an analytical means of Green's function computation through multiple edge-reflected propagation paths and proves to be reliable and fast to study propagation of Lamb waves in thin plates. A one-channel hybrid TR approach based on ISM is also proposed, utilizing experimental signals transferred to the model for backpropagation computation. In particular, the determination of the optimal signal duration to be time-reversed is discussed. • We propose an analytical method for simulating the dispersive propagation of elastic waves in plates. • Multiple edge-reflected propagation paths are included. • We perform an experimental/computational one-channel Time Reversal with backpropagation simulated using the proposed method. • We define measures of the simulation efficiency to determine the optimal signal duration to be time-reversed. • The optimal signal duration avoids focus deterioration due to accumulation of phase delays. [ABSTRACT FROM AUTHOR]

Based on the guided wave, a crack detection method for U-shaped flexural plate (UFP) butt welds is established to realize damage localization and quantitative identification. Firstly, the semi-analytical finite element (SAFE) method was used to obtain the dispersion curve in a plate with weld by discretizing the cross-section and then solving the characteristic equation. Subsequently, numerical studies were performed to investigate the mode conversion and energy transfer efficiency in weld bending. Then the method was determined after the wave packet analysis of voltage responses under different damage conditions. Finally, a validation experiment was conducted in an actual UFP butt weld. The result indicates that there exist 10 guided wave modes in a plate with weld at 150 kHz, while only 2 modes in a flat plate; For curvature radius of weld bending substantially greater than the weld thickness, the guided wave energy mainly passes through the bending in the form of transmitted waves and the reflected wave formed in the bending can be ignored; The damage can be located with the defect echo of the specific guided wave and the error distance only accounting for 1.5% of the total weld length. It is effective to take wavelet energy as an indicator to characterize the damage degree. The method significantly improves crack detection efficiency and provides a scientific basis for monitoring similar structures. • Detect weld with bending in the U-shaped flexural plate using the guided waves. • The method proposed can realize the full line scanning of the weld with bending with just one excitation. • The damage location and severity can be further identified by the guided wave echo and the selected damage indicator. [ABSTRACT FROM AUTHOR]

The utilization of corrugated steel plates in concrete filled steel tube (CFST) structures has shown advantageous performance and considerable economy due to their high out-of-plane stiffness and local buckling resistance. A novel composite shear wall composed of multiple CFST components using double horizontally-corrugated steel plates and connected using pure corrugated steel plates with large wave heights was proposed. Fifteen CFST components with double horizontally-corrugated steel plates were designed and statically tested to evaluate the eccentric compression performance. Parameters including nominal eccentricity ratios, nominal sectional aspect ratios, steel plate thicknesses, and diameters of reinforced longitudinal reinforcements were considered. The bearing capacity, stiffness, and ductility of the CFST component with double horizontally-corrugated steel plates were investigated. In addition, the non-uniformities of confinement along with cross-sectional depth was studied through the analysis of measured strains and finite element simulations. Moreover, two capacity models of the eccentrically loaded components were proposed based on eccentricity reduction coefficient and stress balance of the cross-section, respectively. Both models effectively predicted the experimental and simulated results. • The eccentrically loaded behavior of a novel component utilizing double horizontally-corrugated steel plates was studied. • The corrugated steel plates effectively mitigated longitudinal buckling and effectively confined the core concrete. • The novel component showed good performance even though its sectional width-thickness ratio exceeding the specified limit. • Two capacity models were proposed to predict the strength of the novel CFST components. [ABSTRACT FROM AUTHOR]

Energy transformation is crucial for sustainable development, with pumped storage playing a key role. Central to this system is the hydraulic turbine, necessitating innovation to overcome traditional issues like cavitation and vibration. This study proposes the traveling wave turbine, integrating a mechanism of traveling wave motion into a vertical rectangular flow channel. Gravitational potential energy of water flow propels the traveling wave plate and the traveling wave mechanism is elegantly modeled as a two-dimensional wave plate. Through numerical simulation, we explore the influence of peak of wave-channel clearance on traveling wave turbine performance. Results show that decreasing peak of wave-channel clearance coefficient enhances time-averaged efficiency, reduces volume loss coefficient, and weakens fluctuations in instantaneous mass flow and power coefficients. High sensitivity to peak of wave-channel clearance changes is observed under a high head condition. Reduction in peak of wave-channel clearance decreases pressure difference between front and rear of the traveling wave plate at t/T = 0 and increases it at t/T = 0.25. Simultaneously, decreasing clearance coefficient leads to reduced fluid velocity at the contraction point, causing a noticeable decrease in vortex scale and intensity. [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]

• A new non-destructive testing technique is proposed for Directed Energy Deposition. • A wave model describes wave propagation in both the substrate and the thin wall. • A laser vibrometer scans the thin wall fabricated by additive manufacturing. • A 0 mode Lamb wave in the thin wall is enhanced by frequency tuning. • A small defect caused by inconsistent powder delivery is detected and localized. Thin-walled metallic structures produced by the Directed Energy Deposition (DED) Additive Manufacturing (AM) process are prone to various fabrication defects, which hinder the wider applications of the technique in practice. In-situ inspection and monitoring methodologies are in high demand for improved quality control of printed parts. This paper presents an ultrasonic guided-wave-based method and a prototype that can potentially be used for in-situ inspection of thin-walled structures produced by DED. Lamb waves are excited by a Lead zirconate titanate (PZT) piezoelectric transducer bonded on the DED substrate remotely from the thin wall. The substrate works as a waveguide to transmit the waves which then propagate along the thin wall. A non-contact laser vibrometer is applied to measure the guide wave signals by scanning the surface of the thin wall. The mechanisms of guided wave generation and propagation along the substrate and printed part are theoretically studied. It allows for choosing proper inspection parameters to enhance the measurement sensitivity of guided waves and help interpret the signals for defect detection. Experiments were conducted with DED-produced stainless steel (316L) thin-walled structure. The new method is demonstrated in one example to detect and localize a small defect caused by inconsistent powder delivery of a fabricated thin wall sample, via analysing the B-scan ultrasonic guided wave signals. The new technique provides strong potential for in-situ online monitoring of the DED process. [ABSTRACT FROM AUTHOR]

• A multimodal Lamb wave baseline-free method for corrosion detection. • It applies the diversity of mode types and mode sensitivities to replace the baseline signal. • It demonstrates a great robustness and effectiveness to damage detection. The multi-mode and dispersion nature of Lamb waves means that a variety of modes with individual mode structures and distinct dispersion behaviors would propagate in the structures simultaneously. The existence of a corrosion would result in thickness reduction, which means the frequency-thickness product under a specific excitation would also decrease. Due to dispersion diversity, the interaction of each individual Lamb mode at the corrosion may be distinct, i.e., the velocity varies in different extent and even in opposite trends. In this paper, the combination of multiple modes, rather than a single sensitive mode, is used for structure diagnosis. Specifically, two Lamb modes both sensitive to corrosion but with opposite variation trends are taken and the corrosion index is defined on the ratio of their time-of-flight, so as to enhance the sensitivity to corrosion and eliminate the influence of difference path lengths in the sensor network. On this basis, a probabilistic reconstruction algorithm is introduced for corrosion detection and localization. Since the two modes are extracted from the same wideband Lamb wave response, the proposed method is baseline-free. The influence of mode conversion on the effectiveness of the proposed method is discussed. Ultimately, the performance of the proposed method is demonstrated by an experimental example. The results show that the defect could be correctly identified and accurately localized. [ABSTRACT FROM AUTHOR]

Structural health monitoring (SHM) requires efficient online crack detection and characterization to prevent structural failures, which mainly arise from fatigue cracks. Existing solutions for crack characterization involve analyzing sensed wave signals directly, but these approaches usually require onerous steps or many sensors to obtain sufficient and clear wave packets. An alternative strategy is a model-based inversion, which takes the full waveform into consideration and does not require analysis on a single wave packet. This approach can achieve accurate characterization with fewer sensors and simpler implementation. We propose an efficient model based on the Huygens' principle and the no-mode-conversion property of the A 0 mode Lamb waves to meet the requirements of online monitoring. We then verify the proposed model-based crack imaging method through simulation and experiments on smooth and rough cracks. The proposed method is easy, cheap, and efficient, making it a desirable option for SHM tasks. • Enabled in-situ crack imaging with a sparse array of Lamb wave transducers. • Capable of characterizing through-thickness, linear, and smooth cracks in plates. • Employed waveform inversion to exploit adequate data despite sensor shortages. • Adapted the method for in-situ imaging with an efficient forward model. • Leveraged the non-mode-conversion property of the A0 mode in the model. [ABSTRACT FROM AUTHOR]

• A method for predicting the cumulative characteristics of second harmonic Lamb waves in damped plates is proposed. • The influence of material damping and wave beam divergence on the cumulative effect of the second harmonic Lamb waves is analyzed. • The proposed method is validated numerically and experimentally. The second harmonic Lamb waves have high sensitivity to microstructural defects in materials and are therefore promising for incipient damage detection and monitoring of thin-walled structures. Existing studies have shown that the second harmonic Lamb waves can be cumulative with increasing propagation distance under the internal resonance conditions, which is conducive to nonlinear wave measurements in view of structural health monitoring. However, when propagating in a lossy structure with damping, the cumulative properties of the second harmonic Lamb waves are affected by energy dissipation and thus need to be re-examined. In this paper, a method for predicting the cumulative characteristics of second harmonic Lamb waves in damped plates is proposed. Instead of using material damping parameters which are difficult to obtain in practice, the proposed method relies on the attenuation patterns of Lamb waves at fundamental and double frequencies while taking into account the influence of the wave beam divergence. The proposed methodology is validated by finite element simulations and experiments. The results show that the cumulative second harmonic Lamb waves in the damped plate tend to increase and then decrease, and a "sweet" zone of relatively large amplitude can be predicted using the proposed method. The elucidation of the cumulative characteristics of the second harmonic Lamb waves provides guidance for effective system design for structural damage detection and monitoring applications. [ABSTRACT FROM AUTHOR]

• Nonlinear Lamb wave imaging of micro-damage is achieved based on wavefield data. • A new approach is proposed for weak nonlinear feature enhancement. • Demonstrated capability of accurately imaging the fatigue crack tip. • Visualize the wave field of the harmonics. The traditional nonlinear ultrasonic technique, as typified by the second-harmonic generation and the frequency mixing response, can be employed to identify and characterize the micro-damage. However, the research on micro-damage characterization using nonlinear Lamb wave imaging technique remains an ongoing challenge and is rarely reported. A method called standardized amplitude difference is proposed for nonlinear feature enhancement, and further for fatigue crack imaging based on the wavefield data. Wavefield data contain abundant information on the spatial and temporal variation of propagating waves in the damaged structure. The nonlinearity index β ′ of the signal difference under the high and low incident wave amplitudes is calculated for fatigue crack imaging. Two scanning methods, including local scanning and global scanning, are introduced to image the fatigue crack tip and visualize the wave field of the harmonics respectively. The experimental validation, based on the imaging results of an aluminum alloy plate specimen with a barely visible fatigue crack and a steel plate with a blind hole, manifests that the proposed method can be used to enhance and extract the nonlinear features and suppress the fundamental frequency, so as to improve the signal-to-noise ratio (SNR) of the micro-damage imaging results. [ABSTRACT FROM AUTHOR]

We present an ultrasonic method of detecting cracks in pipelines based on using normally incident transducers in a pitch-catch setup, which can only excite Lamb modes of higher order than the fundamental modes A 0 and S 0 commonly used in crack detection applications. By excitation and measurements of the Lamb modes S 1 , S 2 , and A 3 , in a steel plate immersed in fluid with and without a notch (to emulate a crack), the performance of the modes towards crack detection is quantified by assessing whether it returns a high leaky component and whether the notch has a large impact on the leaky component. In order to narrow the scope of measurements necessary to investigate notch sensitivity for different system parameters, and to potentially optimize the system setup, we present a computationally efficient theoretical model based on angular spectrum method (ASM) and the theoretical sensitivity kernel formulation from the field of seismology that accounts for a scatterer in the wave path between the transmitter and receiver. The model is compared against measurements, which show that the frequency components of the S 2 mode has both the largest leaky frequency component in the given setup and the largest sensitivity at a frequency close to the maximum leaky frequency such that a difference caused by the notch is easily measured. By using the measurements and the validation calculation as baseline reference, we calculate the expected S 2 mode sensitivity and leaky components for larger plate thicknesses and larger standoffs, which exemplifies how the model can be applied in measurement system design and optimization. • Crack detection using normally incident transducers to excite Lamb waves is proposed. • Measurements show that frequencies near the S2 mode are highly sensitive to a notch. • A model of notch sensitivity is compared against measurements. • Examples of theoretical notch sensitivity with parameter changes are shown. [ABSTRACT FROM AUTHOR]

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

Monitoring damage in advanced composite structures proves challenging due to insufficient transfer and reuse of diagnostic models. Such limitations fail to meet practical monitoring requirements. This paper proposes an innovative method to monitoring damage via Lamb waves based on transfer learning. Firstly, the proposed method utilizes algorithm-centric transfer learning to extract transferable features from the source domain and a limited amount of data from the target domain, using the domain adaptive feature mapping. The aim of this method is to achieve feature adaptation in both the source and target domains by obtaining approximate distribution patterns in the feature space. Subsequently, a fine-tuning method is presented based on data-centric transfer learning for adaptive damage identification. A screening mechanism utilizing a coarse-to-fine strategy is employed to select suitable samples for diagnosing and evaluating damage in the target domain. The proposed method is verified by targeted monitoring cases of advanced composite structures under four monitoring stages, namely, detection of the damage existence, identification of damage types, damage localization and quantification. The results show that the proposed method can present higher monitoring accuracy in general damages for advanced composite structures than nine state-of-the-art methods. [ABSTRACT FROM AUTHOR]

This paper aims at experimental study of delamination process in multidirectional composite laminate exhibiting elastic couplings by using the acoustic emission (AE) technique. The mode I double cantilever beam (DCB) test was conducted on carbon/epoxy laminate with the bending-twisting elastic couplings according to the ASTM D5528 Standard. Experimental research was supported with visual high resolution registration of propagated crack tip, as well as with measurements of elastic waves generated by developing internal damages by using the acoustic emission set Vallen AMSY-5 equipped with two piezoelectric sensors. Prior to test, values of fundamental Lamb wave modes velocities were estimated and the dispersion curves were calculated. Delamination initiation point was determined by using two different AE criteria. The mode I critical strain energy release rates were calculated by using three different data reduction schemes. Different damage mechanisms during crack propagation process were quantitatively evaluated by fundamental waveform features divided into three characteristic frequency bands groups. In addition, signal energy concentration regions were assessed by using the wavelet transforms. The outcomes revealed various damage mechanisms related with delamination process, even though the matrix cracking found to be the dominating mode of failure. • Application of the acoustic emission technique to examine of delamination process in elastically coupled laminates. • Determination of total fracture toughness by using two different AE criterion. • Classification of damage mechanisms by using waveform characteristic features. • Analysis of failure mechanisms by using wavelet transform. [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]

Rodent models of behavior used in the fields of neuroscience and psychology generate a wealth of multimodal data. For instance, as a rodent moves and behaves in its environment, muscle contractions apply subtle forces to any surface the animal contacts. These forces generate acoustic waves that propagate through the waveguide as Lamb and shear horizontal (SH) waves and contain information about the rodent's physiology, behavior, and underlying psychological state. If the information in these waves were to be tapped, it would provide a novel, non-invasive way to study rodent behavior. This article lays the foundations for using guided ultrasonic waves generated by a mouse's movement on an aluminum plate for detecting behavior and drawing inferences about acoustic startle responses. The experimental setup involves piezoelectric sensors capturing the waves generated by the rodent's movement, which are then stored as discrete acoustic emission (AE) hits using an amplitude threshold-based data acquisition system. This method of data acquisition ensures that data collection occurs only when the animal moves or behaves, and each movement/behavior is represented by values of features within the generated wavepackets (AE hits). Through open field tests with C57BL/6J mice, utilizing piezoelectric sensors and the DAQ system, it was observed that every movement/behavior of the animal generated Lamb wavepackets within the frequency range of 20 kHz to 100 kHz. Furthermore, rearing behavior in the animals also led to the generation of SH wavepackets in the frequency range of 75 kHz to 230 kHz. This criterion was subsequently employed to detect rearing behavior. In the acoustic startle response test, where the animals' responses to intense sound pulse were recorded, AE hits' features proved useful in quantifying the response. These experimental findings validate the proposed technology's practicality and demonstrate its capability to enhance studies of rodent behavior. • An open-field arena is designed to capture the guided ultrasonic waves generated by the movement/behavior of mice. The arena features an aluminum base plate equipped with ultrasonic sensors. • In this setup, we treat the mouse as an acoustic source, and the resulting guided waves are regarded as acoustic emissions (AE). • As the mice freely explore the open-field arena, every movement/behavior gives rise to Lamb waves with frequency content spanning from 20 kHz to 100 kHz. • Notably, specific motions, such as rearing and occasionally sniffing, correlate with the generation of shear horizontal waves, with frequencies ranging from 75 kHz to 230 kHz. • Another novel aspect of this research lies in conducting the acoustic startle response (ASR) in an unconstrained environment, facilitating the quantification of the "intensity" of the response through the absolute energy of the AE hits. Additionally, this study marks the first observation of variations in the physical motions associated with ASR. [ABSTRACT FROM AUTHOR]

The interactions between ultrasonic waves and damage provide crucial information for developing effective damage detection techniques in various fields, including non-destructive testing and structural health monitoring, and they can be investigated through wave propagation. Then, this article presents an approach to investigate the longitudinal wave in thin plates with symmetric damage considering oblique incidence. The approach includes a strategy to determine the plate thickness reduction due to the damage presence by considering an oblique incidence of waves generated by the actuator. Modeling is developed to compute each wave packet of reflected and transmitted waves separately, which allows one to describe the wave scattering. Numerical simulations are carried out and the results show that the sensor size can be adjusted to improve the existence of damage detection and the determination of the damage depth. Results from experimental tests are presented to demonstrate the approach considering a circular actuator. The findings contribute to improving the comprehension of the interaction between guided waves and discontinuities, potentially encouraging the use of waves for damage sizing purposes rather than only for detection and improve the current state of the art in wave propagation by investigating the effects of different excitation frequencies and the influence of the damage parameters and sensor sizing on the resulting waves. • Analysis of the scattering of longitudinal waves in plates with symmetric damage. • A model that describes each wave packet is presented. • Optimal frequencies to detect symmetric damage are presented. • The optimal frequencies depend on the incidence angle. • The sensor size can be adjusted to improve the damage detection process. [ABSTRACT FROM AUTHOR]

Orthotropic steel bridge decks (OSDs) find extensive use in bridge engineering worldwide but suffer from fatigue cracking due to vehicle loads. The technology of Lamb wave longitudinal transmission has potential to monitor fatigue cracks. Experimental and numerical investigations were conducted to examine the impact of wave longitudinal transmission on fatigue crack monitoring, utilizing the lightweight arrangements of Lamb wave sensors. Three full scale specimens of OSDs were manufactured and subjected to testing. The sensors utilizing Lamb waves were continuously monitored two types of cracks that were initiated in tests. Standardized wave features were extracted by a wavelet transform algorithm from the received waves in order to evaluate crack growth. Finite element models was also established and verified by the experimental results. Parametric studies were conducted to investigate the effects of crack types, wave transmission distances, and excitation frequencies on wave features. The experimental results show that the wave features exhibit clear variation trends as both the fatigue life and the fatigue crack lengths increase. The findings of the parametric study reveal the proposed sensor arrangements are proved to be suitable for general application. Meanwhile, the different variations in wave features can be observed in the various crack types and lengths, but they are less noticeable when considering different wave transmission distances, as long as the excitation frequencies remain constant. The approach to improve the signal-to-noise ratio in engineering applications arises from the fact that the monitoring frequency of wave features should be chosen as the same as the excitation frequency. • Experimental and numerical investigations were conducted to examine the impact of wave longitudinal transmission on fatigue crack monitoring. • The proposed sensor arrangements are proved to be suitable for general application, even with OSD structures having changeable floorbeam spacing. • The different variations of standardized wave features in response to the various types and lengths of cracks are apparent. • Despite changes in wave transmission distances, the variations of standardized wave features are less noticeable when excitation frequencies remain constant. • The monitoring frequency should choose the same as the excitation frequency to improve the signal-to-noise ratio in engineering applications. [ABSTRACT FROM AUTHOR]

• Diffuse Lamb-wave field is utilised for fatigue-crack detection in an aircraft panel. • Extensive experimental programme of fatigue testing is undertaken. • Ultrasonic Lamb-wave responses are captured using a network of piezoceramic transducers. • Phase/frequency characteristics of Lamb waves are used to detect and monitor fatigue cracks. • Outlier statistical analysis is performed to support the analysis. A diffuse Lamb-wave field is utilised for fatigue-crack detection in a multi-riveted strap-joint aircraft panel. The work involves an extensive experimental programme of fatigue testing. Ultrasonic Lamb-wave responses are captured using a network of low-profile, surface-bonded piezoceramic transducers. Various phase/frequency characteristics of Lamb waves are used to detect and monitor fatigue cracks. An outlier statistical analysis – based on these characteristics - is also performed for damage detection. The results show that phase analysis can be used successfully for damage detection in such structures. In contrast, frequency analysis does not offer reliable damage detection. The results demonstrate the potential and limitations of the method for reliable fatigue-crack detection in complex aircraft structures. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the feasibility of evaluating the degree of early fatigue in solid plates by utilizing quasi-static components (QSC) generated by primary Lamb wave (PLW) propagation. The investigation is carried out through theoretical considerations, numerical simulation, and experimental examination. The paper begins by examining the generation of QSCs by PLW propagation under group velocity matching. Subsequently, a suitable mode pair comprising primary and zero-frequency Lamb waves, which satisfy group velocity matching, is chosen for a given aluminum plate to enable the QSC to accumulate and grow with propagation distance. The aluminum plates are then subjected to tension-tension cyclic loading, and experimental measurements are performed. By measuring the amplitudes of the PLW and the generated QSC at different numbers of loading cycles (N), a quantitative relationship between the relative nonlinear acoustic parameter and N is established. The study demonstrates that the relative nonlinear acoustic parameter increases sensitively and monotonically with the increase in fatigue loading cycles. These experimental results confirm the feasibility of utilizing the group-velocity-matching QSC-based approach to evaluate the degree of early fatigue in solid plates. • Feasibility of using QSC generated by PLW under group velocity matching for early fatigue assessment in solid plates. • Quantitative relationship between relative nonlinear acoustic parameter and loading cycles established through experiments. • Consistent findings validate the practicality of using group-velocity-matching QSC-based approach for early fatigue evaluation. [ABSTRACT FROM AUTHOR]