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

Wave energy converters together with flexible breakwaters not only provide shelter to the energy devices against wave attacks but also offer improved performance, longer survivability and lower costs. Thus, in the present work we are interested to investigate the interaction of linear water waves with an array of three thin vertical plates, comprising of two plate wave energy converters and one flexible breakwater, submerged in water of finite depth in a two dimensional space. The plate in the middle of the setup is a flexible plate of variable thickness, and on either side of it are piezoelectric plate wave energy converters with constant thickness. The problem is solved by converting the associated boundary value problem into a system of coupled integral equations. A modified form of the energy balance equation is derived, which results due to the presence of the two piezoelectric plates. We investigate how the structural properties of the piezoelectric plates like their submergence depths, plate lengths, distance from the breakwater and different edge conditions can affect the wave attenuation and wave power absorption efficiency. Further, it is also examined how the presence of the non-uniform flexible plate with different flexibility and lengths can improve the performance of the system. It is analysed that shorter sized piezoelectric plate wave energy converters in the presence of non-flat flexible plate offer higher wave power absorption efficiency at lower frequencies. • Wave power absorption by two vertically submerged piezoelectric plates is studied. • A non-uniform flexible plate is also included in the setup. • A modified form of the energy balance equation is derived. • Various physical quantities are investigated for efficient performance of the system. • Elastic plate helps reduce wave transmission and increase power absorption efficiency. [ABSTRACT FROM AUTHOR]

• Bending edge wave is studied in a thin transversely isotropic viscoelastic plate supported by a viscoelastic foundation. • The nonlocal elasticity theory is used to extract the micro-scale effect on the travelling edge wave. • The Kirchhoff plate theory and the sinusoidal waveform are considered to obtain the closed-form solution. • It is observed that due to the present of viscosity in the foundation the speed of the wave is reduced. The analysis consists of the dynamic behaviour of a thin semi-infinite functionally graded plate under bending edge wave propagation. The plate is made of transversely isotropic materials supported by an elastic foundation. A quadratic variation of the properties of the materials is used to examine the behaviour of the FGM (Functionally Graded Materials) plate in the bending edge wave propagation. The Eringen differential form is employed to transition from non-local to local elasticity. The parameter entailed in the differential form is used to extrapolate the microstructure's impact on the bending edge wave propagation in the functionally graded plate. Additionally, the surface elasticity theory is included to investigate the surface effects on the dispersion of edge wave. The viscosity effect of the plate and the foundation are taken into consideration while formulating the problem. To develop the kinematics of FGM plates, the Kirchhoff plate theory is considered. The motion of the plate is taken along the transverse direction of the plate. The viscoelastic theory proposed by Kelvin-Voigt is used to formulate the mathematical theory of the proposed model. A coupled equation corresponding to the moments and shear forces has been obtained while formulating the plate equation of motion. The rotatory inertia effect is neglected due to the small deflection of the plate. The numerical simulation presented in the paper shows the influence of the viscosity of the elastic foundation, surface effects, non-local elasticity, and the density of the materials on the natural frequency of bending edge wave propagation. [ABSTRACT FROM AUTHOR]

• Novel mathematical model for wave-particle drag effects in semiconductor plates. • Extended Mindlin method for coupled flexoelectric semiconductor plates. • Explicit coupling mechanisms in different waves (extensional, thickness-stretch, bending, and thickness-shear). • New 3D dispersion relations for Lamb waves in flexoelectric semiconductor plates. • New analytical and simple solutions with high accuracy at small wavenumbers. In this paper, the infinite power series of two-dimensional (2D) differential equations for flexoelectric semiconductor plates are derived by extending the Mindlin method. The zeroth-order and first-order theories are systematically presented and applied to the plate made of cubic crystals of class m3m, identifying explicitly the coupling among wave modes and charge distributions. More specifically, it is observed that the extensional and thickness-stretch waves are coupled with axial motions of free carriers; bending and fundamental shear are coupled with the thickness motions of free carriers, showing the dynamic phenomena of the flexoelectric electronic (flexoelectronic) effect; and the shear-horizontal waves, such as face-shear and thickness-twist, are decoupled. Furthermore, when comparing with the three-dimensional (3D) dispersion relations in the flexoelectric semiconductor plate which are also derived for the first time in this paper, the simple 2D theory is shown to be amazingly accurate. For instance, for an infinite plate of thickness 2 h = 0.2 µm, the extension, bending, and shear modes predicted from the present 2D theory agree well with the 3D benchmarks when the actual wavelength is larger than 1.26 µm, i.e., wavelength is at least more than six times of the thickness. To illustrate the flexoelectricity-based wave-particle drag effects, the wave modes and charge distributions are plotted. Due to its simplicity and computational efficiency, the present 2D theory provides us a powerful tool in understanding the physical mechanism of wave-particle drag in flexoelectric semiconductor plates and analyzing acoustoelectric devices when the flexoelectric effect is involved. [ABSTRACT FROM AUTHOR]

An understanding of the hydroelastic response of a flexible circular plate to water waves is relevant to many problems in ocean engineering ranging from offshore wave energy converters and solar wind devices to very large floating structures such as floating airports and ice sheets. This paper describes results from physical model tests undertaken in the COAST laboratory at the University of Plymouth. Response amplitude operators (RAOs) of a floating flexible circular disk are determined for incident monochromatic and irregular wave trains, the latter defined by JONSWAP spectra. Free-surface displacements are measured using wave gauges, and the plate motion recorded using a QUALISYS® motion tracking system. Different basin depths and plate thicknesses are considered in order to quantify the effects of water depth and flexural plate rigidity on the overall dynamic behaviour of the circular disk. We present synchronous and subharmonic nonlinear responses for monochromatic waves, and displacement spectra for irregular waves. The measured wave hydrodynamics and disk hydroelastic responses match theoretical predictions based on linear potential flow theory. [Display omitted] • Experimental data on floating flexible disk dynamics in water waves. • Potential flow theory provides close match to experimental data. • 2nd & 3rd harmonic responses in regular waves are identified. [ABSTRACT FROM AUTHOR]

• Thermoelastic Lamb waves in functionally graded nanoplates are studied based on integral form of modified nonlocal theory. • The Legendre polynomial approach is improved based on the proposed iterative solution integration method. • The escape frequency, which appears in differential model of modified nonlocal theory, does not exist in the integral one. • Nonlocal effect on attenuation is more notable than that on phase velocity. • Nonlocal effect strengthens attenuations before their peak frequency, but then weakens them. Based on the integral form of the modified nonlocal theory, the dispersion and attenuation characteristics of thermoelastic Lamb waves in functionally graded material nanoplates are investigated. The improved Legendre polynomial series approach is presented to conquer the difficulty of the very time-consuming integration calculation, which contains the nonlocal factor, the Legendre polynomials, rectangular window function and their derivative. An iteration solution integration method is derived by using the properties of Legendre polynomials and the integration by parts. The proposed approach possesses the advantages of a small scale of the characteristic matrix and can directly obtain the complex wave number solutions without iteration. Comparisons with the available data indicate the validity of the proposed approach. Numerical examples show that the nonlocal effect on attenuation is more notable than that on phase velocity, especially at the frequency of the maximal attenuation. Besides, the nonlocal effect is very weak at the low frequency. Importantly, the escape frequency, which appears in the results from the differential form of modified nonlocal theory, does not exist in the integral form of modified nonlocal theory. In fact, the escape frequency originates from the approximate transformation from integral model to differential model. [ABSTRACT FROM AUTHOR]

Turbines, as power sources for power generation, ships, etc. are capable of absorbing energy from high-temperature and high-pressure gas, converting thermal energy into mechanical energy. We propose a built-in variable wavelength traveling wave turbine that establishes a traveling wave mechanism in the rectangular flow channel. This device has the same function as a conventional turbine, extracting energy from incoming high-pressure gas, but it operates in a completely different way from axial flow and centripetal types. Numerical simulation is adopted to investigate the energy absorption characteristics and flow mechanism of the variable wavelength traveling wave turbine of two wave numbers. The traveling wave turbine is simplified to a two-dimensional traveling wave plate with analytical functions to control the motion parameters. The results show that influenced by variable wavelength, the motion waveform forms expansion type, equal wavelength type and contraction type. The curve of energy absorption rate of traveling wave turbine shows first increase and then decrease with the highest efficiency reaching 89.71%. The mass flow coefficients under two wave numbers correspond to the efficiency curves with approximately the same trend. There is the obvious distinction between pressure regions divided by the moving traveling wave plate, indicating that fluid energy is mainly transferred to traveling wave plate. The velocity varies prominently in X direction near the leading edge and trailing edge of the plate. Interestingly, as the variable wavelength changes, the vortex direction alters where the first formed vortex sheet shed the trailing part of the traveling wave plate. The area of vortex sheet varies significantly. [ABSTRACT FROM AUTHOR]

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]

The article presents the results of a study of perforated plates effectiveness to ensure safety in case of emergencies on spacecraft and launch pads. The degree of shock wave attenuation by perforated plates with different permeability coefficients has been studied experimentally and numerically. Experiments were carried out with conical shock tube, which was used to produce spherical shock waves of varying intensity, and perforated plate with a permeability coefficient of 41% and. Shock wave reflection and attenuation coefficients were obtained. To study the qualitative picture of a shock wave interaction with perforated plate, a numerical model using Ansys Fluent was created and verified. The numerical model made it possible to qualitatively evaluate the breakup and coupling of a shock wave front after passing perforated plate. The degree of a shock wave attenuation by plates with permeability coefficients of 29% and 20.6% was numerically studied. It was shown that perforated plates can effectively reduce a shock wave overpressure. • Perforated plates effectively reduce shock wave overpressure. • Shock wave attenuation nonlinearly depends on permeability coefficient. • Perforated plates do not change shock wave positive phase duration. • Shock wave breakup and coupling does not depend on characteristic size of hole. [ABSTRACT FROM AUTHOR]

Fish swim by flexing their bodies and energy is transferred from the fish to the water. And the plate which imitates traveling wave motion of fish can absorb the energy of fluid under certain motion parameters. By arranging the traveling wave plate in a vertical rectangular flow channel, the flow mode is changed from external flow mode to internal flow mode. This internal flow device is called traveling wave pump-turbine, and it can operate in both pumping mode and turbine mode. The traveling wave plate is simplified into a two-dimensional plate, and the model of the traveling wave pump-turbine is established. The numerical simulation method is used to study its performance characteristics at different wave speeds when it is in pumping mode. The results show that: although it operates in pumping mode by changing the volume of the working chamber, it can output stable mass flow rate. With the increase of wave speed, the volumetric efficiency increases gradually, the time-averaged efficiency first increases and then decreases, and the maximum time-averaged efficiency can reach 0.9. The instantaneous pressure and power are relatively stable when the wave speed is low, while they fluctuate more sharply with the increase of wave speed. [ABSTRACT FROM AUTHOR]

• When a plate is submerged in a fluid, no wave blocking occurs in the plate mode irrespective of compressive force. • Multiple propagating modes occur due to wave blocking when a submerged plate rest on a Winker foundation. • In wave scattering by articulated submerged plate, energy relation depends on wave amplitude and the energy transfer rate. • Discontinuities in the reflection and transmission coefficients occur at blocking and saddle points in wave scattering. • Irregularities in plate deflection occur due to the superposition of multiple propagating waves within blocking frequencies. Surface gravity wave interaction with an articulated flexible submerged plate in the presence of lateral compressive force is investigated within the framework of blocking dynamics, assuming that the submerged plate is resting on a uniform Winkler foundation. The problem is studied considering small-amplitude plate response and linearised water wave theory in finite water depth. The loci of the dispersion relation reveal the occurrence of wave blocking in the submerged plate mode for a range of wave frequency and compressive force when the submerged plate is resting on a Winker foundation. However, in the absence of the Winkler foundation, no wave blocking occurs in the plate mode irrespective of the values of compressive force, and plane wave propagation in plate mode does not happen for particular values of compressive force. Four propagating wavenumbers exist within the blocking frequencies, of which one is in the surface mode, and others are in the submerged plate mode. Thus, to account for the four propagating wave modes, the existing solution approach for a pair of propagating wave modes is modified in an appropriate manner within the limit of blocking frequencies. Consequently, the energy relation is obtained using Green's identity, in which the reflection and transmission coefficients are a combination of the amplitudes of different reflected and transmitted wave modes and the associated energy transfer rates. The reflection coefficient in the submerged plate mode jumps from one to zero for very small values of frequency due to the introduction of the Winkler foundation in the presence of compressive force. In addition, zero wave reflection occurs for a higher frequency range with an increase in the values of the Winkler constant. Removable discontinuities are observed at the blocking and saddle points, whilst jump discontinuities occur in the reflection and transmission coefficients due to the change of incident wave mode within the blocking frequencies. Linear time-dependent displacements in surface and submerged plate modes are presented prior to the blocking limit. [ABSTRACT FROM AUTHOR]

• Semi-analytical method is developed for dispersion of composite plates periodically attached with ABH resonators. • Equivalent dynamic stiffness of the ABH resonator is obtained analytically. • Dynamic coupling between host plate and resonator is described innovatively in terms of waves. • Symplectic method and WKB method are combined to establish the eigenproblem. A semi-analytical method is developed to calculate the wave propagation properties of thin orthotropic composite plate structures periodically attached to acoustic black hole (ABH) resonators. First, the free and forced vibration of the host orthotropic plate is described in terms of the analytical wave shapes obtained by the symplectic method. Second, a wave-based description is provided for the ABH resonators using the higher-order WKB method. Third, the equivalent dynamic stiffness of the resonator is obtained by considering the deformation-force relationship at the connections. Finally, an eigenvalue problem is established for the dispersion of the periodic plate based on wave propagation analysis, where the dynamic coupling between the host plate and resonator is innovatively described in terms of directly excited waves. Because the wave-based derivations in the proposed method are analytical, the calculations highly efficient and accurate, and were validated through several numerical examples. [ABSTRACT FROM AUTHOR]

The plate with traveling wave motion can obtain energy from fluid under certain motion parameters. This energy absorption method, which imitates motion of fish, is called traveling wave energy absorption method. In order to improve the energy absorption efficiency, the traveling wave plate is arranged in the rectangular flow channel and is driven by the high pressure gas coming through the inlet. This internal flow mode is called traveling wave turbine. The traveling wave plate is simplified as a two-dimensional plate. The aerodynamic characteristics of traveling wave turbine are studied by numerical simulation. The results show that: Benefiting from the confinement effects of the rectangular flow channel, the incoming flow can maintain high pressure. Therefore, the time-averaged energy absorption efficiency of traveling wave turbine can reach 62.84%. The pressure of the fluid is high before the traveling wave plate, but it becomes low when the fluid flows through the plate. This means that the energy of the fluid is absorbed by the traveling wave plate. There are vortexes in the traveling wave turbine and the vortexes with opposite rotating directions will accelerate the fluid between them. [ABSTRACT FROM AUTHOR]

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]

Zero-Group-Velocity (ZGV) guided mode has recently received particular attention because of its highly sensitive to structural changes, such as defects. Parallel to ZGV, in the present work, we introduce a new guided mode so-called Minimum-Group-Velocity (MGV) that is a remarkable phenomenon in defect detection applications. Strictly speaking, when any two phase-velocity dispersion curves of Lamb modes approach each other without overlapping, we observe this new mode in group-velocity dispersion curves. It is worth noting that this MGV mode has never been addressed in literature. We compute the Lamb-like modes in asymmetric viscoelastic sandwich plates using the Stiffness Matrix Method (SMM). We show that the change in properties of this sandwich can dramatically affect frequency ranges associated with ZGV and MGV modes. However, a linear correlation between phase velocities and this change on viscoelastic properties is discussed, where we observed an interconnection phenomenon in the dispersion curves. This interconnection happens between specific points on the dispersion curves, each corresponding to a ZGV- or MGV-mode. On the other hand, we verify that the detection of changes in level of viscoelastic properties or variations in the thickness of the buffer viscoelastic layer is possible within a limited segment of the damped mode curve. [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]

• 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]

• 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]

• 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]

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]

• 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]

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]

• 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]

• 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]

• 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]

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]

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]

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]

A jet-jet interaction technique using multiple supersonic fluidic injectors can reduce a shock cell length generated from a supersonic jet. In this study, the effects of the supersonic jet-jet interaction technique on an underexpanded jet impinging on a perpendicular plate was experimentally investigated under an atmospheric pressure. Four supersonic sub jets of Mach 1.5 interacted perpendicularly with a Mach 2.0 main flow at nozzle pressure ratio of 8.5, and the main flow impinged on several plate positions. Jet-jet interaction prevents the development of a central shock wave above the plate surface, which results in a maximum stagnation pressure decrease of 48% on the plate. Instead of the wall pressure decrease, the axial thrust of the main jet is decreased by jet-jet interaction; however, it is less than 6.1%. Jet-jet interaction causes a radial expanded main jet that prevents the flow fluctuation downstream of the jet-jet interaction area as well as in the vicinity of the plate surface because jet-jet interaction negates the oscillating shock cells. Hence, the jet-jet interaction technique is useful for various engineering applications that have issues due to the surface pressure increase and flow fluctuation. • A supersonic jet interaction on the impingement characteristics were investigated. • Jet-jet interaction prevents the surface pressure increase and flow fluctuation. • The axial thrust of a main jet is decreased by jet-jet interaction. [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]

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]

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]

• 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]

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 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]

• 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]

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]

• 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]

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]

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]