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• The number of elements and pitch value of a conventional linear array probe were determined for single Lamb wave mode excitation. • A pitch condition for unidirectional Lamb wave propagation was derived. • At 20 MHz mm, using a conventional linear array probe, a single Lamb wave mode was excited. • Voltage amplitudes were optimized to further enhance the purity of the targeted mode. Lamb wave excitation at high-frequency-thickness products offers a potential solution for high-resolution guided wave testing. The method is attractive for crack imaging and corrosion mapping, especially in hidden locations where direct access is limited. However, multiple modes may propagate, complicating signal interpretation, which is undesirable. In this work, a systematic approach is presented, in an effort to determine the influence of the key parameters related to single higher order Lamb wave mode excitation using a conventional linear array transducer. Specifically, a linear time delay law is used to enhance the targeted mode, while the array's length, pitch and apodisation profile remain to be optimally selected. First, an analytical solution is derived based on modal analysis. This provides a natural decomposition of the amplitude of a guided wave mode into the product of the response of a single element and the excitation spectrum. Then, a key observation is made, associating the excitation spectrum to the directivity function for bulk wave phased array steering. This allows the application of well-established phased array analysis tools to guided wave phased array excitation. In light of this fact, minimisation of the spectrum's bandwidth, elimination of the grating lobes and derivation of an apodisation profile are performed, to enhance the purity of the targeted mode. Finally, experiments conducted on an aluminium plate verify the above theoretical results. The Full Matrix is acquired, and all signals are reconstructed synthetically. [ABSTRACT FROM AUTHOR]

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

In this paper, nonlinear Lamb waves with phase-velocity mismatching are proposed to detect accurately closed cracks by excluding the intrinsic material nonlinearity. Simulations and experimental studies were conducted to analyze double frequency Lamb waves (DFLWs) induced by the closed cracks and the intrinsic material nonlinearity. The results show that DFLWs induced by the material nonlinearity are negligible as compared with that caused by the contact acoustic nonlinearity (CAN) of the closed cracks. The closed cracks can be accurately detected using the proposed method, and the acoustic nonlinearity parameter increases monotonically with the crack evolution. The findings of this study provide a feasible method for detection and characterization of closed cracks. • Closed cracks are effectively detected using phase-velocity mismatching method. • Material nonlinearity is excluded in the detection of closed cracks. • Acoustic nonlinearity parameter increases monotonously with crack length. [ABSTRACT FROM AUTHOR]

With the improvement of the stable flow generation function of circulating water channels (CWCs), CWCs are gradually developing toward multi-functionality. The wave test capability of a CWC is limited by its short test section; so, an efficient wave absorption device is required. This paper proposes the design concept of an overtopping wave absorption device (OWAD). An optimization design of the device by combining CFD and simulation-based design (SBD) is presented. The SBD system included optimal Latin hypercube design (Opt LHD), SVR, and particle swarm optimization (PSO). The reflection coefficient was used to estimate the wave absorption device performance. Numerical analysis was performed using the Unsteady Reynolds Averaged Navier–Stokes (URANS) solver and the boundary wave generation method, and was verified and validated via a procedure recommended by International Towing Tank Conference (ITTC). Under two wave conditions, four different OWAD design variables (slope angle, porosity, extension depth, and extension height) were optimized using the SBD system to minimize the reflection coefficient. Based on optimization results, the OWAD was assembled and its wave dissipation performance was confirmed experimentally. Installing an OWAD caused the reflection coefficient to be reduced from 0.161 to 0.089 for short-wave design conditions and from 0.528 to 0.128 for long-wave design conditions. A wave absorption test also showed that the wave absorption performance of the OWAD is significantly better than that of honeycomb and multiple porous plate wave absorption devices. The OWAD produces not only lower reflection coefficients but also smoother wave surfaces due to its two-dimensional structures. • A concept of an overtopping wave absorption device (OWAD) is proposed for use in a wave-making circulating water channel. • The calculation results of numerical wave tank are verified and validated by an uncertainty analysis recommended by ITTC. • OWAD design variables are optimized by simulation-based design (SBD) during OWAD development. • In a wave period, the OWAD wave absorption process has two steps including wave breaking and wave overtopping. • The OWAD produces smaller reflection coefficients and smoother wave surfaces due to its two-dimensional structure. [ABSTRACT FROM AUTHOR]

Estimating the damage state of composites is essential since carbon fibre reinforced polymers (CFRP) have a complex damage and propagation behaviour. Accordingly, a comprehensive study of the effects of matrix cracks – e.g. their size and crack density – and the influence of different damage types and their interaction on the vibro-acoustic modulation (VAM) behaviour of CFRP specimens under fatigue loading was carried out. These effects were evaluated in two sets of commonly employed laminate layups. Firstly, in a cross-ply (CP) layup, the number of 90°layers was varied, with a constant areal weight (268 g/m 2). Secondly, quasi-isotropic (QI) specimens were tested, where the areal weight was reduced from thick-ply to thin-ply (30–268 g/m 2), changing the failure mode from a delamination-dominated to a brittle failure. After damage introduction under tension–tension loading (R=0.1), high-frequency Lamb waves are introduced via piezoceramic actuators and modulated by a low-frequency vibration using a servo-hydraulic testing machine. VAM correlates with the different damage behaviour and damage conditions in CFRP. For this purpose, the evolution of the sideband amplitudes, the modulation index, the signal in time domain and X-ray scans are used. Regardless of the layup, the degree of stiffness-reducing inter-fibre fractures and large-scale delaminations can be detected. • Reliable health monitoring of thin- and thickply CFRP with varied lay-ups via VAM measurements • Modulation envelope and sideband amplitudes provide damage progression during fatigue • Detection of delaminations and the degree of stiffness-reducing inter-fibre fractures, highly sensitive to crack size • Enabling a VAM threshold prior to delamination occurrence to predict failures [ABSTRACT FROM AUTHOR]

• How cointegration theory can be applied to SHM is clearly explained for the first time. • Common trend removal and anomaly detection using cointegration are clarified. • Two case studies using time series data and Lamb wave data are presented. • Providing a clear explanation of the technique for readers and potential users. Cointegration theory has been effectively applied to SHM. However, there is no work that has focused on explaining specifically how cointegration can be applied to SHM problems. Consequently, the technique is not fully clear for readers and one may experience technical challenges when approaching it. This work aims to mitigate the drawback through providing a clear explanation and illustration for three concerns: (1) how can common stochastic trends, induced by varying operational and environmental conditions, be removed from the analysed data? (2) how can a fault or damage be detected using cointegration residuals? (3) how are the relations between the number of cointegrated variables, common stochastic trends, and cointegration residuals? First, these concerns are explained using simulation data generated by four time series processes. Then, a case study using Lamb wave data in the form of time series is presented. Three different damage conditions are considered. Lamb wave series in the presence of an increasing temperature trend and a simulated nonlinear trend are analysed using cointegration. The results show that cointegration can remove both natural and artificial common trends from Lamb wave series, and the peak-to-peak and variance values can be used to detect and classify the damage. [ABSTRACT FROM AUTHOR]

This paper proposes a damage assessment approach for composites based on the Lamb wave factorization method. It can exploit both the forward and back-scattering waves and visualize the two-dimensional (2D) defect by solving the inverse scattering problem. When performing the method, a transducer array is firstly adopted, which would take turns to choose one element as the actuator and record the responses using the other transducer elements. On the basis, the scattered waves of defect are then extracted and transformed to the frequency domain to form a transmission-reception matrix. The detection area would be next sampled into numerous points whose pixel values can be determined by calculating the corresponding indicator function using the singular system of the matrix. By visualize the area using all the pixel values, the defect image can be finally obtained. Experiments on different 2D defects are carried out as validations, where both the defect shape and transducer quantity are discussed. Results demonstrates the effectiveness and accuracy of the method for damage assessment in composite laminates. [ABSTRACT FROM AUTHOR]

• The dispersive equation of SH guided waves propagating in a plate subject to the uniaxial stress state has been derived. • The experiment of the LCR wave probe and proposed SH 0 wave transducer has been carried out and compared. • The thermal modulation experiments have been performed to monitor the thermal stress in the aluminum and steel specimen. Thermal stress is one of the major causes of failure of engineering structures and its measurement has attracted more attention in recent years. The ultrasonic wave method is very promising in stress measurement due to its non-destructive nature and easy manipulation. The traditional ultrasonic wave transducers require a coupling medium which would introduce large repeatability errors in travel time measurement and thus in the measured stress. In this work, a methodology based on bonded shear horizontal (SH) guided wave piezoelectric transducers was developed to monitor thermal stress in metal plates. The adhesive bonding between the transducer and the specimen ensures the repeatability in travel time measurements, and the strain gauges are also employed to monitor the wave path length. The dispersive equation of acoustoelastic SH wave propagating in an isotropic medium under the uniaxial stress is derived. Both the uniaxial tension test and thermal modulation test have been performed in aluminum and steel plates. The results show that the acoustoelastic constants of the SH 0 wave are identical to that of the shear bulk wave as predicted by the acoustoelastic theory and the thermal stress measured from −60 °C to 100 °C by the proposed method has a very good repeatability (better than 2 MPa) in both the aluminum and steel plates. Considering the convenience and reliability of the bonded SH 0 wave piezoelectric transducer, the proposed method is very promising for monitoring of thermal stress in engineering structures, such as rails, etc. [ABSTRACT FROM AUTHOR]

• Efficient and accurate simulation of PZT-based Lamb wave health monitoring using three-dimensional SBFEM. • Utilizing SBFEM to model the electromechanical vibration of low-profile PZTs based on equivalent force concept. • Simple discretization of complex configurations by linking the SBFEM meshing module to standard finite element mesh generators. • Reduced computational cost for simulation of structural health monitoring techniques based on high-frequency transient wave motion. Exploiting scattering and reflection related data of ultrasonic Lamb wave interactions with damage is a common approach to health monitoring of thin-walled structures. Using thin PZT sensors, the method can be implemented in real-time. Simulation of Lamb wave propagation and its interaction with damage plays an important role in damage diagnosis and prognosis. It is, however, a time-consuming task due to the high-frequency waves that are commonly used to detect tiny damage. The current study employs the Scaled Boundary Finite Element Method (SBFEM) for effective modeling of Lamb wave health monitoring of homogenous thin plates. The electromechanical effects of piezoelectric sensors are included in the model to improve accuracy and make the results comparable to those of laboratory experiments. Simple meshing of complex topologies is possible by converting standard finite elements to scaled boundary elements. The 3D SBFEM wave motion equations are solved in the time domain to capture the sensor's PZT response to a high-frequency tone-burst actuation. The results are validated by pitch-catch and pulse-echo laboratory tests carried out on thin plates. SBFEM is used to study wave propagation in complex configurations, such as a stiffened plate, and the results are compared to their FEM counterparts. According to the findings, SBFEM significantly reduces the computational costs associated with simulation of Lamb wave health monitoring while also providing significant accuracy in comparison to the experimental results. [ABSTRACT FROM AUTHOR]

As the solar photovoltaics power system sees rapid growth in installed capacity and plays a significant role in the future global energy mix, its reliability becomes a crucial factor in maintaining the stability of the electricity supply. Manufacturing imperfections and harsh operating environments may compromise the module's structural integrity, leading to performance deterioration and power loss. Therefore, nondestructive inspection becomes an indispensable part of the quality assurance or maintenance program to detect defects at any stage of the module's lifecycle. Ultrasound is an essential method for material inspection, and ultrasonic-guided waves have been long explored as a flaw detection technique on plate-like structures, taking advantage of its long-range detection that yields an efficient inspection process superior to the conventional pulse-echo technique. Inspired by the same idea, this work assesses the prospect of harnessing ultrasonic guided waves, particularly Lamb waves, to detect cracks, as they exist in an actual module. However, unlike the commonly investigated plates, solar photovoltaic modules contain stacks of a-few-microns-thick layers of different materials that add complexities to the structure. The investigated specimen is a thin film photovoltaic module with cracks caused during transportation and handling. It, therefore, represents a real-life research case that may occur in situ. Numerical and experimental methods are performed to reveal various Lamb modes that propagate in the structure, where the results of both methods are mutually confirmed. Unlike other works, this investigation is not confined to the utilization of the non-dispersive mode but attempts to find the defect-sensitive mode that can be used to detect cracks. An analysis of the experimental results reveals the mode most sensitive to cracks, while numerical simulations explain the corresponding phenomena. • This work assesses the prospect of harnessing UGW to detect cracks in the front glass of a thin film PV module. • The technique has potentials for QC process in continuous manufacturing lines or in-situ structural health monitoring. • The investigated specimen has actual defects caused by operational activities, hence representing a real-life research case. • This investigation attempts to discover the defect-sensitive mode that can be used to detect cracks. • By analyzing Lamb waves dispersion curves, the particular mode sensitive to cracks in the front glass panel can be identified. [ABSTRACT FROM AUTHOR]

• The quantitative classification of composite structures is challenging. • The sparse sensing method has become one of the application trends of structural health monitoring. • A sparse sensor arrays and guided waves based damage detection and evaluation method is proposed to achieve multi-area quantitative damage location, classification, and identification with a limited number of sensors. Damage detection techniques using Lamb waves have shown excellent capabilities in the diagnosis of composite structures. However, structural health monitoring of composite structures is challenging, especially for damage classification. This study proposes a machine learning-based method with a sparse sensor array to achieve quantitative classification of the damage location and severity on a composite plate. First, multi features extraction is used to construct a support vector machine (SVM) damage localization model. Second, optimal path extraction combined with principal component analysis (PCA) is used to construct an SVM model for classification. To reduce the operational burden of structures, the sparse array is employed. To improve the damage classification accuracy, Fisher clustering is proposed to extract the optimal detection path. Then, PCA is used to achieve data fusion. Experimental results on a glass fiber-reinforced epoxy composite laminate plate demonstrate that the proposed technique can accurately locate and classify the quantitative artificial damage. [ABSTRACT FROM AUTHOR]

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

In applications of spot welding, the nugget determines reliability of the material joining, which is characterized by nugget diameter. However, the conventional methods are complex and rough to measure the diameter of the nugget. This paper aims to propose a novel convenient method for measuring the diameter of spot welding nugget. The characteristic curve of laser ultrasonic Lamb waves in nugget area are obtained by laser ultrasonic detection technology under completely non-contact conditions. The variation characteristics of power spectral density in nugget area are analyzed by Fast Fourier Transform, to determine the geometric relationship between the scanning area and the nugget diameter, and the nugget diameter are obtained by further calculation. Experiments show that the accuracy of the proposed method is within 0.2 mm, and the relative errors of experimental data are within 3%. This paper provides an alternative method for the measurement of spot welding nugget diameter, and has great significance for the application of laser ultrasonic detection in spot welding. • A novel convenient method for measuring the diameter of spot welding nugget by laser ultrasonic detection (LUD) is proposed. • The variation characteristics of power spectral density in nugget area are analyzed, and the nugget diameter are obtained by further calculation. • Experiments show that the accuracy of the proposed method is within 0.2 mm, and the relative errors are within 3%. [ABSTRACT FROM AUTHOR]

• Third-harmonic phase velocity matching nonlinear Lamb waves are used for the early fatigue damage detection of aluminum alloys. • The third harmonic first increases and then decreases with the accumulation of fatigue life. When the fatigue life reaches about 80%, the amplitude of the third harmonic declines sharply. • The third-order nonlinear parameters increase continuously with the accumulation of fatigue life. When the fatigue life reaches about 80–90%, the third-order nonlinear parameters increase gradually and shows a downward trend. • The third harmonic and third-order nonlinear parameters are used as indicators of fatigue accumulation to realize detection of early fatigue damage. In this paper, third-harmonic phase velocity matching nonlinear Lamb waves are used for the early fatigue damage detection of aluminum alloys. An analysis is conducted regarding the relationship of third harmonic and third-order nonlinear parameters with fatigue accumulation. According to the analytical results, the amplitude of the third harmonic increases when the fatigue life falls below about 80%. However, when the fatigue life exceeds 80%, the third harmonic starts to decrease, and the third-order nonlinear parameters increase progressively before showing a downward trend. It can be found out that micro-cracks emerge with the accumulation of fatigue, thus reducing the amplitude of the third harmonic and causing the third-order nonlinear parameters to increase gradually and then decline. Furthermore, it is proposed to take the third harmonic amplitude and the third order nonlinear parameters as the indicators of early fatigue damage. [ABSTRACT FROM AUTHOR]

We investigate the role of leaky guided waves in transcranial ultrasound transmission in temporal and parietal bones at large incidence angles. Our numerical and experimental results show that the dispersion characteristics of the fundamental leaky guided wave mode with longitudinal polarization can be leveraged to estimate the critical angle above which efficient shear mode conversion takes place, and below which major transmission drops can be expected. Simulations that employ a numerical propagator matrix and a Semi-Analytical approach establish the transcranial dispersion characteristics and transmission coefficients at different incident angles. Experimental transmission tests conducted at 500 kHz and radiation tests performed in the 200 – 800 kHz range confirm the numerical findings in terms of transmitted peak pressure and frequency-radiation angle spectra, based on which the connection between critical angles, dispersion and transmission is demonstrated. Our results support the identification of transcranial ultrasound strategies that leverage shear mode conversion, which is less sensitive to phase aberrations compared to normal incidence ultrasound. These findings can also enable higher transmission rates in cranial bones with low porosity by leveraging dispersion information extracted through signal processing, without requiring measurement of geometric and mechanical properties of the cranial bone. • Shear mode conversion leads to less phase aberration in transcranial ultrasound. • Optimal shear mode conversion is observed beyond the first critical angle. • The first critical angle can be found from dispersion data of leaky guided waves. • Dispersion data can be used when the properties of the cranial bone are unknown. • Transmission efficiency can improve in the temporal window at large incident angles. [ABSTRACT FROM AUTHOR]

Plate-like structures can be characterized by a variety of abrupt geometric changes affecting the Lamb wave propagation, similarly to damage occurring in service. Therefore, a deep knowledge of phenomena involved in the interaction between guide waves and discontinuities is required. For this purpose, an experimental investigation is carried out considering an isotropic plate where an abrupt thickness change is present. The fundamental modes excitation is operated by a piezoelectric transducer while the signal sensing in multiple locations, also across the discontinuity, is performed by a scanning laser Doppler vibrometer. The investigation reveals mode conversion and highlights how the effects on the wave propagation depend upon the discontinuity geometrical characteristics. A peridynamics-based model representing the examined problem is also defined and its effectiveness to simulate the observed phenomena is proven. • In-depth analysis of phenomena involved in the interaction between Lamb waves and geometrical discontinuities. • Wave propagation is affected according to the geometrical characteristics of the discontinuity altering the waveguide. • A peridynamics-based model representing the examined problem is defined and its effectiveness to simulate the observed phenomena is proven. • The results pave the way towards use of peridynamics in problems where the wave propagation takes place in structures with evolving fractures. [ABSTRACT FROM AUTHOR]

Tide gauge data indicated that ocean waves were generated by the eruption of the Hunga Tonga-Hunga Ha'apai volcano on January 15, 2022. These high-amplitude waves reached the Japanese coast earlier than expected for free ocean waves. These fluctuations in sea level were reasonably reproduced by an ocean model, with realistic bottom topography forced by sea-level pressure perturbations well reproduced by atmospheric general circulation models. While early arrival times are associated with atmospheric Lamb waves, the amplitude of the sea level changes cannot be fully explained by the resonance mechanism associated with sea-level pressure fluctuations of Lamb and Pekeris waves alone. Rather, resonance due to atmospheric waves in conjunction with the amplification effects caused by a steep continental slope is required in order to reproduce sea level fluctuations along the Japanese coast. In particular, the deep Northwest Pacific Basin to the east of Japan is important for strengthening the amplitude of the meteorological tsunami. • Sea level fluctuations due to the 2022 volcanic eruption in Tonga was reproduced. • Meteorological tsunamis were generated by Lamb and Pekeris waves. • Meteorological tsunamis by Pekeris waves are amplified due to Proudman resonance. • Meteorological tsunamis are amplified by entering the sharp continental slope. [ABSTRACT FROM AUTHOR]

• Photoelastic methods to achieve online or real-time measurements of both the photoelastic parameters simultaneously were introduced and reviewed briefly, especially the one with pixelated polarization CCD camera. • Three error factors of the new photoelasticity based on pixelated polarization CCD camera were introduced and their influences on measurement accuracy were experimentally evaluated. • Correction methods and algorithms of the three error factors were introduced in detail, and their effectiveness were validated with experiments. The utilization of pixelated polarization cameras has brought a technical breakthrough to traditional photoelasticity method. In this paper, the principles of the new method are introduced. Then a few of systematic error factors, especially, the non-ideal polarization state of background light (it mainly refers to elliptically polarized incidence, but the stress birefringence of imaging lens was also counted in) and the low extinction ratio of the pixelated polarization camera are analyzed and evaluated. Finally, the corresponding algorithms for error removal are proposed. For the non-ideal polarization state of background light, we first determine its Stokes vector field, and then we obtain the phase retardance and polarization azimuth of the specimen by solving the equations describing relationship between the measured Stokes vector and the theoretical ones of the emergent light field of specimen. For the low extinction ratio of internal polarizer in the camera, we analyze the mechanism of how extinction ratio affects the measurement accuracy, and remove the measurement error by applying the degree of linear polarization (DoLP) of a standard quarter wave plate as the correction coefficient to be divided by the measured DoLP of the specimen. The effectiveness of correction algorithm is validated by some typical experiments, while reasons for the phenomenon that the errors were not completely eliminated are also remarked. [ABSTRACT FROM AUTHOR]

In this paper we consider analytical properties of a class of general nonlinear systems known as Quasi-Polynomial systems. We analyze sufficient conditions for permanence and boundedness of solutions, and illustrate our approach with an application to Lamb equations describing the evolution of electromagnetic modes in an optical maser. [ABSTRACT FROM AUTHOR]

We propose an ultrathin plasmonic metasurface-enabled quarter waveplate using a subwavelength broken rectangular annulus (BRA) arrays. The BRA structure is formed by two pairs of slits with perpendicular orientation embedded in a silver thin film. The elimination of the overlapping parts at four corners of a regular rectangular annulus increases significantly the phase anisotropy induced by localized surface plasmons, and thus decreases significantly the thickness of the metal film in order to achieve the required phase difference for a designed waveplate. Simulations show that an ultrathin quarter waveplate can be obtained with a as thin as 10 nm thickness of a silver BRA arrays at wavelength band of 1.55 µm which provides great potential in advancing manufacture of nanophotonic devices. [ABSTRACT FROM AUTHOR]

This paper presents a new acoustic emission (AE) source localization for isotropic plates with reflecting boundaries. This approach that has no blind spot leverages multimodal edge reflections to identify AE sources with only a single sensor. The implementation of the proposed approach involves three main steps. First, the continuous wavelet transform (CWT) and the dispersion curves of the fundamental Lamb wave modes are utilized to estimate the distance between an AE source and a sensor. This step uses a modal acoustic emission approach. Then, an analytical model is proposed that uses the estimated distances to simulate the edge-reflected waves. Finally, the correlation between the experimental and the simulated waveforms is used to estimate the location of AE sources. Hsu-Nielsen pencil lead break (PLB) tests were performed on an aluminum plate to validate this algorithm and promising results were achieved. Based on these results, the paper reports the statistics of the localization errors. [ABSTRACT FROM AUTHOR]

The Laser/EMAT ultrasonic (LEU) technique has shown the capability to measure weld penetration depths in thick structures based on ray-tracing of laser-generated bulk and surface waves. The ray-tracing method is not applicable to laser-generated Lamb waves when the LEU technique is used to measure weld penetration depths in thin structures. In this work, transmission coefficients of Lamb waves present in the LEU signals are investigated against varying weld penetration depths. An artificial neural network is developed to use transmission coefficients of sensitive Lamb waves and LEU signal energy to predict weld penetration depths accurately. The developed method is very attractive because it allows a quick inspection of weld penetration depths in thin structures. [ABSTRACT FROM AUTHOR]

All-fiber polarization transformer (AFPT) is a length of variably spun birefringence fiber which can transform the state of polarization as a wave plate. A set of complete analytic solutions for the coupled-mode equation of AFPT with a specific function of spun rate is deduced by a modified method through deliberate matrix transformation. And the analytical expressions of the transmitting eigen-modes from the zero spun-rate end to the maximum spun-rate end are derived. Moreover, when the linear polarized light is input as single eigen-mode, the ellipticity of quasi-circular polarization light at the output end is calculated as function of the fiber structure parameters. The equivalent retardation of AFPT is evaluated and the critical constraints between the fiber length and the maximum spun rate are illustrated according to practical requirements. [ABSTRACT FROM AUTHOR]

In this work, a structural health monitoring system has been implemented with damage identification purpose on a winglet of a general aviation aircraft. Using a pitch-catch approach, guided waves (Lamb waves) have been measured by means of an array of piezoelectric sensors able to excite and record the dynamic response of the structure. The undamaged configuration has been used as reference state (baseline) for the structural health monitoring (SHM) purpose while the damaged configuration (current state) has been obtained by low velocity impact test. The effectiveness of the damage has been verified using a Non-Destructive Inspection (NDI) by means of a C-SCAN Olympus Omni SX. The calculation of the damage index obtained comparing the measured wave propagation data in a reference state and the current state is introduced as a determinant of structural damage. Its calculation in different paths associated with the Probability Ellipse (PE) method has been used to identify the position of the damage. Additional radar graphs have been developed for the measurement of the directionality of the Lamb waves and further considerations have been introduced in order to evaluate the sensitivity of the Lamb waves with the aerodynamic load. [ABSTRACT FROM AUTHOR]

We report a unidirectional single-longitudinal-mode Ho:YLF ring laser. An acousto-optic modulator and two half-wave plates were used to enforce the Ho:YLF ring laser in a unidirectional operation. The single-longitudinal-mode output power could reach 3.73 W successfully when the incident pump power was 16.4 W. The corresponding slope efficiency was 27.1%. The wavelength of the single-longitudinal-mode Ho:YLF ring laser was 2063.8 nm. The M 2 factor was 1.12. The results illustrated that the single-longitudinal-mode output power could be further enhanced by increasing the radio frequency power of the acousto-optic modulator. [ABSTRACT FROM AUTHOR]

In the present study, experiments are carried out on an Aluminium (Al) plate with and without damage, using Lamb waves and an array of four Piezoelectric Wafer (PW) transducers bonded onto it. The PW transducers are used to actuate and sense Lamb waves. These transducers are very compact, cheap, and can be easily bonded onto the structure. The responses recorded from the experiments, are further analyzed using a new algorithm developed by the authors, in order to localize the damage. The damage location is obtained as a small enclosed area formed by the intersection of various curves generated by an algorithm proposed by the authors. The damage location is further refined within this enclosed area using the optimization methods. Three different types of damages are considered in the present work. The case of multiple damage localization is also reported here. The results obtained by the algorithm are in excellent agreement with the actual location of damage. Thus, the present method has the capability of locating single as well as multiple damages in plate structures using minimal number of transducers. [ABSTRACT FROM AUTHOR]

It is found that leaky backward Lamb waves, i.e. waves with negative energy-flux velocity, propagating in a plate submerged in a liquid possess extraordinary energy properties distinguishing them from any other type of waves in isotropic media. Namely, the total time-averaged energy flux along the waveguide axis is equal to zero for these waves due to opposite directions of the longitudinal energy fluxes in the adjacent media. This property gives rise to the fundamental question of how to define and calculate correctly the energy velocity in such an unusual case. The procedure of calculation based on incomplete integration of the energy flux density over the plate thickness alone is applied. The derivative of the angular frequency with respect to the wave vector, usually referred to as the group velocity, happens to be close to the energy velocity defined by this mean in that part of the frequency range where the backward mode exists in the free plate. The existence region of the backward mode is formally increased for the submerged plate in comparison to the free plate as a result of the liquid-induced hybridization of propagating and nonpropagating (evanescent) Lamb modes. It is shown that the Rayleigh's principle (i.e. equipartition of total time-averaged kinetic and potential energies for time-harmonic acoustic fields) is violated due to the leakage of Lamb waves, in spite of considering nondissipative media. [ABSTRACT FROM AUTHOR]

In this paper, the S 0 -mode Lamb wave phase velocity at low frequency range is chosen to characterize fatigue damages accumulated in composite laminates under cyclic loadings. A stiffness/velocity degradation model is proposed based on three distinctive damage mechanisms: fiber breaks, matrix cracks and delamination, all of which are always involved in fatigue damage process. In order to reduce the complexity of the damage model, fiber damage is treated as an independent damage and delamination is coupled with matrix cracks. An approximation of shear-lag model is then proposed to avoid any direct measurements of crack density for it is highly impractical in real applications. Following this, energy release rate for formation of matrix cracks is extracted based on measured phase velocity and damage evolution is realized by incorporating a modified Paris law. Controlled fatigue experiments were conducted where in-situ phase velocity was measured by a laser ultrasonic system. The fatigue damages were then characterized using the proposed damage model with the measured velocity. Finally, the ability of proposed model to predict residual lives was also validated and good accuracy has been obtained. [ABSTRACT FROM AUTHOR]

The present study aims to investigate the feasibility of using the time-reversed Lamb wave as a new method for noninvasive characterization of long cortical bones. The group velocity of the time-reversed Lamb wave launched by using the modified time reversal method was measured in 15 bovine tibiae, and their correlations with the bone properties of the tibia were examined. The group velocity of the time-reversed Lamb wave showed significant positive correlations with the bone properties ( r = 0.55–0.81). The best univariate predictor of the group velocity of the time-reversed Lamb wave was the cortical thickness, yielding an adjusted squared correlation coefficient ( r 2 ) of 0.64. These results imply that the group velocity of the time-reversed Lamb wave, in addition to the velocities of the first arriving signal and the slow guided wave, could potentially be used as a discriminator for osteoporosis. [ABSTRACT FROM AUTHOR]

Lamb wave propagating behavior in woven glass fiber reinforced epoxy (WGF/epoxy) composite laminates was studied by simulation and experiment. A 3D finite element (FE) model was built in Abaqus/Explicit software with the help of user material subroutine to capture the wave propagating features in WGF/epoxy composite plate. The Lamb wave tests were carried out by a non-circular array with 8 piezoelectric (PZT) wafers. A defect imaging algorithm was developed using MATLAB based on the ellipse theory in time-dependent domains. Two defect imaging cases were carried out in the WGF/epoxy composite plate to verify the feasibility of the proposed algorithm. A good agreement between the simulated waveforms and experimental data was achieved. This indicates a great potential of the proposed algorithm in the field of damage localization and identification for WGF/epoxy thin-walled laminates. [ABSTRACT FROM AUTHOR]

This paper presents a study of the detectability of the level of adhesion at the Aluminum/Epoxy interface using the propagation of ultrasonic guided waves. The topic is relevant to the long-range ultrasonic inspection of structural adhesive bonding. The samples considered are epoxy-coated aluminum: samples were fabricated with partially or totally crosslinked epoxy groups and with different surface treatments of aluminum substrates. Experimentally measured dispersion curves and attenuation coefficients of a selected mode are used to quantitatively track changes at the epoxy-aluminum interface. The results show that modes could be very sensitive to the nature of the interface, that is to say to the level of adhesion, if properly selected. Numerical predictions via the Jones rheological model and their comparison with experimental results highlight three adhesion levels: weak, medium and high one. [ABSTRACT FROM AUTHOR]

A plasmonic quarter-wave plate which is based on U-shaped silver nanopatches array in the near-infrared range is designed and presented by us. It utilizes localized surface plasmon resonances with an abrupt phase shift to achieve the desired phase modulation in the transmitted field. The optimized quarter-wave plate can perfectly convert a linearly polarized light to circularly polarized light, whose ellipticity reaches to 1 at the wavelength of 1.55 μm. The bandwidth, in which the ellipticity is larger than 0.83, is 120 nm. This structure offers an enormous potential applications in optical sensor systems, advanced nanophotonic devices and integrated optical circuits. [ABSTRACT FROM AUTHOR]

A novel structure consisting of a Si plate sandwiched between two ZnO layers is suggested as propagation medium for Quasi Longitudinal (QL) acoustic Lamb waves. Considering a low-dispersive quasi-longitudinal mode as an example, the phase velocity v , electromechanical coupling constant k 2 and mechanical displacements U 1 , U 3 versus plate and ZnO layers thicknesses have been calculated starting from uncoated plate through the same plate with one layer to the plate with two layers onto opposite faces. A remarkable increase in the electromechanical coupling, together with an essential decrease in the vertical displacement U 3 , at the ZnO surface, have been theoretically demonstrated and experimentally verified for definite combinations of the films/plate thicknesses. This property of the structure is attractive for applications to microwave liquid sensors. [ABSTRACT FROM AUTHOR]

The scattering of high frequency guided ultrasonic waves by a fatigue crack at the side of a fastener hole has been studied. The guided wave pulse consists of the superposition of the two fundamental Lamb modes A 0 and S 0 above the cut-off frequencies of the higher modes. The scattered field was simulated using a three-dimensional finite difference algorithm with a staggered, Cartesian grid for the limited area of interest around the hole and an analytical phase angle correction for the additional, variable propagation distance. Experimentally, the modes were selectively excited using a standard ultrasonic wedge transducer and measured using a laser interferometer, resulting in good spatial resolution. The scattered field was measured and simulated for an undamaged hole, a small, part-thickness quarter-elliptical fatigue crack, and a through-thickness fatigue crack. Good agreement was found and a significant influence of the fatigue cracks on the scattered field was observed. The complex difference of the scattered field due to additional scattered waves at the fatigue cracks of variable depth and length was evaluated. This allows for the prediction of high frequency guided wave sensitivity for fatigue crack detection at fastener holes, a significant maintenance problem for ageing aircraft. [ABSTRACT FROM AUTHOR]

A dynamic solution is presented for the propagation of harmonic waves in magneto-electro-elastic plates composed of piezoelectric BaTiO 3 (B) and magnetostrictive CoFe 2 O 4 (F) material. The state-vector approach is employed to derive the propagator matrix which connects the field variables at the upper interface to those at the lower interface of each layer. The ordinary differential approach is employed to determine the wave propagating characteristics in the plate by imposing the traction-free boundary condition on the top and bottom surfaces of the layered plate. The dispersion curves of the piezoelectric–piezomagnetic plate are shown for different thickness ratios. The numerical results show clearly the influence of different stacking sequences as well as thickness ratio on dispersion curves and on magneto-electromechanical coupling factor. These findings could be relevant to the analysis and design of high-performance surface acoustic wave (SAW) devices constructed from piezoelectric and piezomagnetic materials. [ABSTRACT FROM AUTHOR]

Quarter-wave plates are primarily used to change the polarization state of light. Their retardation usually varies depending on the wavelength of the incident light. In this paper, the design and characteristics of an achromatic quarter-wave plate, which is formed by a cascaded system of birefringent plates, are studied. For the analysis of the combination, we use Jones matrix method to derivate the general expressions of the equivalent retardation and the equivalent azimuth. The infrared achromatic quarter-wave plate is designed based on the simulated annealing (SA) algorithm. The maximum retardation variation and the maximum azimuth variation of this achromatic waveplate are only about 1.8 ° and 0.5 ° , respectively, over the entire wavelength range of 1250–1650 nm. This waveplate can change the linear polarized light into circular polarized light with a less than 3.2% degree of linear polarization (DOLP) over that wide wavelength range. [ABSTRACT FROM AUTHOR]

Interdigital transducers (IDTs), thanks to their multiple capabilities have the potential of increasing versatility of SHM systems. Migration of the IDT technology in SHM systems and devices is reviewed in this paper. A summary review of different types of IDTs is presented and their salient features are presented in terms of their applicability in the Lamb wave based SHM systems. Comprehensive review is provided concerning the implementation of the IDT capabilities towards the development of SHM systems. Experimental results obtained with prototype IDTs are provided for illustration. Finally, future development directions of the IDTs dedicated to SHM systems are outlined. [ABSTRACT FROM AUTHOR]

Arterial stiffness is highly correlated with the functions of the artery and may serve as an important diagnostic criterion for some cardiovascular diseases. To date, it remains a challenge to quantitatively assess local arterial stiffness in a non-invasive manner. To address this challenge, we investigated the possibility of determining arterial stiffness using the guided circumferential wave (GCW) induced in the arterial wall by a focused acoustic radiation force. The theoretical model for the dispersion analysis of the GCW is presented, and a finite element model has been established to calculate the dispersion curve. Our results show that under described conditions, the dispersion relations of the GCW are basically independent of the curvature of the arterial wall and can be well-described using the Lamb wave (LW) model. Based on this conclusion, an inverse method is proposed to characterize the elastic modulus of artery. Both numerical experiments and phantom experiments had been performed to validate the proposed method. We show that our method can be applied to the cases in which the artery has local stenosis and/or the geometry of the artery cross-section is irregular; therefore, this method holds great potential for clinical use. [ABSTRACT FROM AUTHOR]

In this study, a Lamb wave based nonlinear method is used to detect delamination in a composite laminate. For this purpose, an artificial delamination is created in a composite laminate using a thin Teflon sheet. The research includes an experimental study as well as finite element simulation. Two higher harmonics are observed in the Lamb wave response as a result of contact nonlinearity at the delamination. A spectral damage index is extracted from the spectral information of the fundamental and higher harmonics. It is found to be invariant to the sensor location and its values show decreasing trend with increase in the delamination width. Further, a new hybrid method is introduced, wherein the spectral and temporal data are used together in order to locate a delamination. [ABSTRACT FROM AUTHOR]

In engineering practice, failures due to fatigue cracks in metallic structures have always been difficult to predict. In our study, nonlinear Lamb wave-mixing was applied in the detection of micro-cracks in plates. The analysis of the nonlinear interaction of these waves with cracks of various lengths and widths was performed using finite-element simulations. The simulation results showed that the sideband at the sum frequency provide a sensitive means for micro-crack detection. Moreover, the sideband amplitudes show a monotonic increase with micro-crack length, but a decrease with micro-crack width. Experiments using Lamb wave-mixing were conducted on plates with fatigue cracks. The experimental results show that a proposed acoustic nonlinearity parameter related to the sideband at the sum frequency is sensitive to micro-cracks in plate, and is well correlated with the damage degree. [ABSTRACT FROM AUTHOR]

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

[Display omitted] • A meter-scale ultra-low frequency wide-bandgap seismic metamaterial for Lamb waves is proposed for the first time, and the bandgap can cover the 2 Hz seismic peak spectrum. • Band structures, vibration modes and transmission spectrum of the seismic metamaterial are calculated and analyzed. • The influences of geometrical parameters, material parameters value on the bandgap width and location are discussed. • The structure is optimized, and the effects of the different forms of matrix on the attenuation degree of the transmission spectrum is discussed. Seismic metamaterials (SMs) have attracted the attention of many researchers in the field of damping and elastic wave isolation because of their bandgap properties, i.e., attenuating elastic waves in the range of bandgaps. However, the dimension of these SMs often reaches the 10-meter scale, which impedes practical engineering application. To break through this bottleneck and achieve a smaller scale, a meter-scale novel SM composed of steel and rubber is proposed for attenuating ultra-low frequency seismic Lamb waves. Firstly, the finite element method is used to analyze the band structure of SMs and calculate the bandgaps. To explain the mechanism of bandgap, the vibration modes of the waves at the bandgap boundary frequency are further analyzed. Subsequently, the transmission spectrum of Lamb waves incidents on the finite SMs system is analyzed to prove the authenticity of the bandgaps. Finally, parameter analyses including the geometric variables, material properties, equivalent mass density, and structural matrix forms with identical equivalent mass density are investigated numerically. The results show that Lamb waves in the range of 0–20 Hz are significantly attenuated by SMs, and the aforementioned parameters are significant factors affecting the bandgap properties and transmission properties. The proposed SM has a smaller size while maintaining some wide bandgaps at 0–20 Hz for ultra-low frequencies. It is worth noting that the 2 Hz seismic peak spectrum causing the destruction of building structures is covered effectively. [ABSTRACT FROM AUTHOR]

• A forward model for guided waves propagating along the ground silicon wafer is established. • Acoustic mode conversion caused by wafer thinning is studied. • A Bayesian approach is used for quantifying subsurface damage (SSD) and its uncertainty. • Effect of acoustic mode conversion on the quantization results of SSD is analyzed. A multimode photoacoustic method has been applied for evaluating the degree of subsurface damage (SSD) in the ground silicon wafer. Mode conversion of laser-generated broadband guided waves is caused by wafer thinning. Bayesian inversion is applied to quantify the effective elastic constants of the SSD layer together with the corresponding uncertainty estimates from the experimental dispersion data of multimode guided waves. Laser ultrasonic experiments have been performed on the polished and ground silicon wafers with different thicknesses to determine the ability of the method to characterize SSD and the effect of acoustic mode conversion on the quantization results. The experimental results show that the mode of the laser-generated broadband guided waves will gradually change from Rayleigh wave to Lamb wave as the wafer is thinned. Both the Rayleigh mode and the A 0 Lamb mode can accurately estimate the effective elastic constants of SSD, but the A 0 Lamb mode leads to longer confidence intervals, which has been theoretically explained. [Display omitted] [ABSTRACT FROM AUTHOR]

Lamb waves propagating in a layered structure consisting of layers with equal acoustic impedances, is studied by a variant of Cauchy sextic formalism and the exponential fundamental matrix method. It is shown that the dispersion portraits provide enough information to distinguish individual layers, even if their acoustic impedances are identical, making such layers indistinguishable by the pulse – echo method. It is revealed that the developed methodology is adequate for determining geometrical and physical properties of the individual layers with equal acoustic impedances. [ABSTRACT FROM AUTHOR]

In this paper, we propose a systematic and time-efficient approach to calibrate 2D VARANS-VOF models for simulation of wave interaction with porous plate in numerical wave tank. To reach this goal, we develop a data-driven approach in combination with numerical and experimental data to figure out the optimal empirical coefficient associated with linear and non-linear drag force coefficients (α , β). Advanced gradient boosting decision trees algorithms (GBDT) are adopted to capture the accuracy of the prediction model. Although only limited samples, for instance 72 samples are generated and chosen to train and test the prediction model, the GBDT model is able to identify the trend in non-linear influence of model parameters. The advantage of GBDT algorithm can obviously be seen through the regression analysis with very high regression coefficient (R 2 > 0.99). The developed model is subsequently employed for a large range of considered parameters as input features of the models to predict the possible hydrodynamics characteristics. The smallest mean squared error between predicted and experimental results, which is 0.00204, is found and used to determine the best performing combination of these model parameters. The numerical model together with selected values of empirical coefficients is then validated using available experimental data in literature. The free surface elevations in front of and behind the structure from the numerical model are nearly identical with the experimental data. • A time-efficient approach to calibrate the 2D VARANS-VOF models for simulation of wave interaction. • Advanced gradient boosting decision trees algorithms (GBDT) to capture the accuracy. • The developed model is used to predict the possible hydrodynamics characteristics. • The numerical model is validated using available experimental data in literature. [ABSTRACT FROM AUTHOR]