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

The scope of this study is the monitoring of the adhesion integrity of composite Single Lap Joints (SLJs) through the assessment of the M U L 2 CODE, software developed by the M U L 2 Research Group - Department of Mechanical and Aerospace Engineering of Politecnico di Torino. The M U L 2 CODE is implemented through the Carrera Unified Formulation (CUF) for 2D structures based on Hierarchical Legendre Expansion (HLE) polynomials. An efficient method for the Structural Health Monitoring (SHM) of bonded composite joints is experimentally verified and simulated by commercial FE software and by the CUF approach, in order to reduce the computational cost of analyses: using transient excitations via toneburst signals, the structural health of damaged SLJ is analytically, experimentally and numerically evaluated. The interaction mechanism between the waves traveling through the investigated specimens is numerically simulated with a simple Finite Elements (FE) model and it is solved via M U L 2 CODE and commercial software Ansys Workbench (with 2D and 3D models), respectively. Experimental campaigns data are compared with analytical, CUF and Ansys results demonstrating the consistence of the M U L 2 formulation that is computationally simpler, but very efficient for the joint analysis. The presented and discussed CUF application is able to quantify with a high accuracy the debonding extension in the damaged SLJ, simply tuning the excitation frequency of the SHM technique. • Elastic waves interference is evaluated by using analytical approach, experimental data, CUF and Ansys simulations. • The method is tested on a composite single lap joint, simple experimental set-up representative of an actual scenario. • A good agreement between experiments and simulations confirmed the validity of the proposed method. • The CUF approach provides finer results with lower elapsed computational time respect to the Ansys FE model. [ABSTRACT FROM AUTHOR]

The effects of laser-induced heat on the performance degradation of liquid crystal (LC) devices are critically important in applications involving laser systems. In this study, we investigated the influences of 1064-nm continuous-wave (CW) laser loading on an LC variable retarder (LCVR). The results show that thermal deposition initially causes an overall downward shift in the electronically controlled phase retardation curve. This phase retardation reduction is restored under a certain voltage and laser power by reducing the driving voltage. For common nematic LC wave plates that do not require voltage application, the decrease in phase retardation can be pre-compensated by increasing the corresponding LC cell thickness. For electrically controlled LC devices, the phase retardation under a high-power CW laser can be preconfigured by plotting phase response curves versus voltages at different laser powers. Based on these calibration curves, the LC devices can still service high-power CW lasers as long as the resulting temperature does not reach the clear point of the LC material. These results clarify the fundamental mechanisms of phase modulation variation induced by thermal deposition and could be useful for the fabrication optimization and effective implementation of LC devices in high-power laser systems. • Thermal effect makes the LCVR electronically controlled phase retardation curve shifts down overall. • Without voltage, the degradation of LCVR performance is attributed to the change in the refractive index of LC material. • Applying a range of voltage, thermal effect leads to an additional increase in the deflection angle of LC molecules. • For nematic passive LC waveplates, phase retardation decrease can be pre-compensated by increasing LC cell thickness. • For active LC devices, phase retardation under CW lasers can be preconfigured by calibrated phase-voltage response curve. [ABSTRACT FROM AUTHOR]

Transmissive metasurfaces have provided an efficient platform to manipulate electromagnetic (EM) waves, but previously adopted multilayer meta-atoms are too thick and/or the design approach fully relies on brute-force simulations without physical understandings. Here, based on coupled-mode theory (CMT) analyses on multilayer meta-atoms of distinct types, it is found that meta-atoms of a specific type only allows the phase coverage over a particular range, thus suitable for polarization-control applications. However, combinations of meta-atoms with distinct types are necessary for building ultra-thin wavefront-control meta-devices requiring 360° phase coverage. Based on these physical understandings, high-efficiency meta-atoms are designed/fabricated, and used to construct three typical meta-devices, including quarter- and half-wave plates and a beam deflector. Our results elucidate the physics underlying the interplay between thicknesses and performances of transmissive metasurfaces, which can guide the realizations of miniaturized transmissive meta-devices in different frequency domains. [ABSTRACT FROM AUTHOR]

Lamb waves have the characteristics of long propagation distance, and they can be used to realize rapid and wide-scale nondestructive testing of plates. The detection sensitivity can be improved by using a high-order Lamb wave mode with a short wavelength. However, Lamb waves have multimode characteristics. The number of Lamb wave modes greatly increases at high frequencies, and it is difficult to obtain a pure single high-order Lamb wave mode with electromagnetic acoustic transducers (EMATs); this increases the difficulty of signal processing and analysis. In this study, an obliquely incident EMAT based on the inclined static magnetic field was designed to solve this problem. The mode selectivity of the proposed obliquely incident EMAT is superior to that of the traditional EMAT. A new parameter is proposed to improve the mode selectivity of EMATs: the polarization angle of the permanent magnet. The incident angle of incident ultrasonic waves is controlled by selecting the polarization angle of permanent magnet. A rectangular permanent magnet is obliquely polarized to provide a biased inclined static magnetic field. The obliquely incident Lorentz force is formed through the coupling between the eddy current and the inclined static magnetic field by an obliquely polarized permanent magnet. The obliquely incident Lorentz force is the source of the obliquely incident ultrasonic wave. Simulation and experimental results show that the developed obliquely incident EMAT can reduce the amplitude of the unwanted modes and enhance the mode selectivity. It was also verified that the developed obliquely incident EMAT had better frequency-response characteristics and sound-field directivity. • The polarization angle of permanent magnet was added to select Lamb wave mode generated by proposed EMAT. • Obliquely incident Lorentz force was formed by coupling between eddy current and inclined static magnetic field. • The amplitude of unwanted modes was reduced and mode selectivity was enhanced by obliquely incident EMAT. • Obliquely incident EMAT has better frequency-response characteristics and sound-field directivity. [ABSTRACT FROM AUTHOR]

• A hybrid numerical-analytical model is presented for elastic-plastic plate impact. • The model has been verified by the experiment of sphere-plate impact. • The impact-induced waves have been detected by the experiments and the model. • The accuracy and efficiency are verified by three-dimensional finite element simulations. The aim of this study is to present an efficient model for the analysis of complicated nonlinear transient dynamics of an elastic-plastic plate subjected to a transversely eccentric low-velocity impact. A mixed numerical–analytical model is presented to predict the transient dynamic behaviours consisting of either plate impact responses or wave propagations induced by the impact in a plate with an arbitrary shape and support. This hybrid approach has been validated by comparison with results of laboratory tests performed on an elastic-perfectly plastic narrow plate eccentrically struck by an elastic sphere, and results of a three-dimensional finite element (FE) analysis for an elastic-perfectly plastic simply-supported rectangular plate eccentrically struck by an elastic sphere. The advantages of this hybrid approach are in the simplification of local contact force formulation, computational efficiency over the FE model, and convenient application to parametric study for eccentric impact behaviour. The hybrid approach can provide accurate predictions of the plate impact responses and plate wave propagations. [ABSTRACT FROM AUTHOR]

Abstract This work reports a study aimed at quantifying damage severity in composite materials by an in-field inspection technique known as Acousto-Ultrasonics (AU) where Lamb waves are transmitted through a damage zone by exciting a pulse onto the composite surface and capturing the received waves after their interactions with the damage. It builds on a previous study that proposed as quantitative descriptors of the received signal a set of stress wave factors (SWFs) extracted from the power spectral density (PSD) distribution of the AU signal. The SWFs were previously shown to detect the spatial extent of impact damage, but the severity of the damage within the impact damage zone was not quantified. The present study takes a step in the direction to achieve that goal. Lamb waves are transmitted through a cross ply laminate in which transverse ply cracks of increasing density (number of cracks per unit axial length) are induced. The PSD distribution is first analyzed for the case of an undamaged laminate and then the SWFs are correlated with the crack density. The implications of the correlations obtained and the future prospects of quantifying the damage severity by such correlations are discussed. [ABSTRACT FROM AUTHOR]

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

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

Abstract In this study the nonlocal bond-based peridynamic (PD) theory was applied to simulate Lamb wave transmission in 2D bimaterial plates. Plates of unlike materials were jointed end-to-end to make a bimaterial plate. The surface of the plate was then excited tangentially by single-frequency and multi-frequency force signals. The influence of changing material properties on travelling of the symmetric and antisymmetric Lamb waves were studied in detail. The phase and group velocities of travelling wave in bimaterial plates were calculated and it was found that dissimilar material properties of two joint plates can significantly alter the amplitude and arrival time of the wave. Moreover, Fast Fourier Transform (FFT) of the time history of symmetric Lamb wave velocity was calculated and verified. Accuracy and consistency of PD wave model was confirmed entirely using Spectral Finite Element Method (SFEM). The comparison between the results of two applied methods shows a good agreement. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The Parylene-C as waveguide layer which enhanced the performance of Lamb wave sensor. • The evanescent filed of P-Lamb wave sensor was used for activated partial thromboplastic time (aPTT) measurement. • The relative standards deviation (RSD) of measured activated partial thromboplastic time was 2.11%, with P-Lamb. • A comparative measurement was done between with SYSMEX CS 5100, clinical coefficient index (R2) was 0.99467. Abstract Blood clotting time monitoring is of great importance in predicting the risk of haemorrhage and cardiovascular diseases. In this study, a new P-Lamb wave sensor was developed to measure activated partial thromboplastic time (aPTT) with enhanced sensitivity. The sensor utilizes a piezoelectric and hydrophobic material, Parylene-C, as a waveguide for the P-Lamb wave sensor in order to monitor the aPTT in whole blood samples. With a smooth surface, the P-Lamb wave sensor reduced energy loss and prevented leaky modes, with a decrease of 33% ± 1.25% in the surface roughness and an increase of 2.75% ± 0.15% in the signal. The experimental results demonstrated that, compared to a quartz crystal microbalance (QCM), a P-Lamb sensor with high sensitivity (the frequency shift of a P-Lamb sensor is approximately 200 times greater than that of a QCM) improved stability and enhanced repeatability. Parylene-C worked as a phonon coupling layer, which improved the stability and sensitivity of the Lamb sensor. The relative standards deviation (RSD) of the aPTT measurement was 2.11%, which was 1.22 times lower than reported values. The P-Lamb wave sensor was also tested for its sensitivity to different concentrations of heparin and was found to exhibit an increase in coagulation time with an increasing heparin concentration. Compared to the SYSMEX CS 5100 haematology analyser, the clinical coefficient index (R2) was 0.99467. Considering its compact size, low cost, and mass production of the chip unit, the developed P-Lamb wave sensor is a promising device for point-of-care diagnosis of haemostasis and personal health monitoring. [ABSTRACT FROM AUTHOR]

We present a theoretical method for single frame Mueller matrix polarimetry. The proposed technique combines the concept of channeled spectrum for spectral measurements and the property of achromatic axially symmetric quarter wave plate (AASQWP) for spatial measurements. A circular grating at the output ensures that the spectral information is spread out in the radial direction and the spatial birefringence information for a particular wavelength is spread out along the circumference with radius corresponding to the particular wavelength. The diffraction pattern from the circular grating therefore contains a wealth of information from which it is possible to evaluate all the elements of the Mueller matrix and for any desired wavelength contained in the broadband source illuminating the sample. [ABSTRACT FROM AUTHOR]

Abstract The present paper is aimed to test the idea of using scattering resonance frequencies, which are sensitive to the crack's size and depth variation, for the express non-destructive estimation of its severity. The width and depth of artificial strip-like delaminations are estimated by matching theoretical eigenfrequencies, calculated in the framework of a corresponding 2D elastodynamic boundary value problem, with the experimentally obtained resonance frequencies. The high accuracy of the delamination characteristics obtained by this way indicates its potential utility for practice. Graphical abstract Image 1 Highlights • Delamination eigenfrequencies depend strongly on its width and depth. • Experimental scattering resonance frequencies are evaluated from LDV B-scans. • Matching theoretical and experimental resonances reliably estimates damage severity. [ABSTRACT FROM AUTHOR]

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

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

• A theoretical model of nonlinear lamb wave in a linearly varying plate was proposed. • Its properties in linearly varying plate are discussed and verified. • The application of nonlinear Lamb wave technique may be extended from the uniform-thickness plate to the varying-thickness plate. The aim of this paper is to investigate propagation characteristics and the generation mechanism of the nonlinear lowest-order symmetric Lamb mode (S 0) which propagates downslope in free elastic plates with slowly linearly varying-thickness. From theoretical analyses, in a low frequency-thickness product (fd) range, the S 0 mode is slightly dispersive, it is easy to generate, and it approximately satisfies the principle of the phase velocity matching. Therefore, if a S 0 mode is excited at a proper frequency in the low fd range, the amplitude of the second harmonic wave is linearly increasing in a certain propagating-distance, which is valuable for the practical NDE application of the second harmonic wave. Moreover, numerical simulations and experiments have been carried out to validate theoretical results. Our investigation of properties of the second harmonic wave can be applied to characterize and evaluate micro-structural damages in varying-thickness waveguides. [ABSTRACT FROM AUTHOR]

• The ABH is introduced to attenuate AE reflection waves. • The parameter optimization method of the ABH is presented. • Simulations and tests show the ABH has a good prospect on reducing the AE boundary reflection. • The proposed method could improve the accuracy of the acoustic emission tests. Due to the difference between the characteristic impedance of air and plate structure, the acoustic emission (AE) wave propagating in a plate structure is reflected when reaching the boundary. It leads to the fact that the obtained signal contains not only direct AE components but also reflected components. The existence of the reflected AE wave brings error and difficulty to accurate AE source analysis. This paper attempts to introduce the acoustic black hole (ABH) into an experimental plate structure (a disc structure in this paper) to attenuate the reflected AE wave from the structure boundary and obtain the high Signal-Noise Ratio (SNR) direct AE wave. Firstly, the finite element model of a disc with an ABH is established to verify the attenuation of the reflected AE wave. Then, the orthogonal test is conducted to reveal the influence of geometric parameters of the ABH on reflection attenuation. And a method based on the optimal parameter combination of the ABH is proposed to attenuate boundary reflection. Finally, the broken-lead and friction experiments are performed on the processed disc with the ABH to verify the ability of the ABH to attenuate the boundary reflection of AE wave. Both analytical and experimental results show that the ABH is a candidate tool to reduce the reflected AE wave on the boundary. [ABSTRACT FROM AUTHOR]

• The aim of the present work is to develop a multi-mode imaging technique that will allow for identification of damage size and location using ultrasonic guided waves. • The proposed technique combines a reverse-time migration (RTM) imaging algorithm with a 3D wave propagation simulator using different wave modes. • This combination enables the separation of multiple modes using the wavefield filtering techniques, potentially providing more information to damage types. • Without the limitation of generating single dominant mode waves, wide frequency ranges are accessible, enabling optimal frequencies for a variety of ultrasonic data acquisition systems. The sensitivity of Lamb wave modes to a particular defect or instance of damage is dependent on various factors (e.g., the local strain energy density due to that wave mode). As a result, different modes will be more useful than others for damage detection and quantification, dependent on damage type and location. For example, prior work in the field has shown that out-of-plane modes may have a higher sensitivity than in-plane modes to surface defects in plates. The excitability of a certain data acquisition system and the corresponding resolution for damage imaging also varies with frequency. The aim of the present work was to develop a multi-mode damage imaging technique that enables characterization of damage type and size, general sensitivity to unknown damage types, higher resolution imaging, and detectability regardless of the data acquisition system used. A reverse-time migration (RTM) imaging algorithm was combined with a numerical simulator—the three-dimensional (3D) elastodynamic finite integration technique (EFIT)—to provide multi-mode damage imaging. The approach was applied to two simulated case studies featuring damaged isotropic plates. Sensitivities of damage type to wave mode were investigated by separating the A 0 and S 0 Lamb wave modes obtained from the resultant RTM wavefields. [ABSTRACT FROM AUTHOR]

This work aims to demonstrate the ability to evaluate bulk thermal aging of aeronautical composite materials using Lamb waves. These composite materials are bi-dimensional woven composite structures with polymers matrix. More than 20 different thermal aging profiles are obtained by modulating the temperature and curing duration. The impact on material properties is evaluated using normalized destructive characterization and x-ray computed tomography. Lamb waves are generated and detected using a single 3 MHz phased array transducer in contact with the composite structure. In that way, a spatio-temporal image of elastic wave propagation in the composite material is obtained. The dispersion curves are calculated by using a bi-dimensional discrete Fourier transform or singular value decomposition to project the experimental data into the wavenumber-frequency domain. Both methods allow for accurate estimation of the Lamb wave dispersion curves. Thermal aging is then evaluated by comparing experimental data with a set of dispersion curves that correspond to different controlled thermal aging profiles. Our promising results open the door toward quantitative evaluation of composite material thermal aging on the runway. [ABSTRACT FROM AUTHOR]

• Lamb waves in piezoelectric two layered structures show a wide range of properties. • Wave properties depend on mode order, layers thickness and transducer configuration. • Further improvements are expected for piezoelectric three layered structures. Using Bi 12 GeO 20 plate coated by AlN film, the main characteristics of the A 0 , S 0 , A 1 , and S 1 Lamb waves are numerically calculated and compared with those of SAW in the same structure. It is shown that (i) there exist only one SAW mode for this material combination, (ii) the mode exist in a narrow range of AlN thickness, (iii) it can be excited using four different transducer configurations, (iv) the velocity and coupling constants of the mode are, respectively, higher and lower as compared with the values for the uncoated Bi 12 GeO 20 substrate. On the other hand, for Lamb waves (i) there are several modes of different orders, (ii) the modes exist in a wide range of the AlN thickness, (iii) they may be excited using twelve IDT configurations, (iv) the velocities and coupling constants of the waves may be larger and smaller as compared with the values for the uncoated plate, (v) both characteristics of the modes depend on the mode order, film thickness, plate thickness, and transducer configuration, and (vi) the depth profiles of the modes are neither symmetrical nor anti-symmetrical. These properties make Lamb waves more attractive than SAW. Application of the waves and the structure for some applications are suggested. [ABSTRACT FROM AUTHOR]

• An efficient approach to the investigation of nonlinear scattering of guided waves. • The GPU implementation and the small-size model profile make the model highly efficient. • The wave damage interaction coefficients concept is extended for nonlinear scattering problems. • Super harmonic generation and selective mode conversion phenomena are observed. • The rough crack surfaces are considered. This article presents an efficient numerical approach to the investigation of nonlinear scattering and mode conversion phenomena of Lamb waves as they interact with breathing cracks. A Local Interaction Simulation Approach (LISA) is adopted, which possesses the versatility to capture arbitrary damage profiles. The stick-slip contact dynamics is implemented in the LISA model via the penalty method, which captures the nonlinear interactions between Lamb waves and breathing cracks. The LISA framework achieves remarkable computational efficiency with its parallel implementation using Compute Unified Device Architecture (CUDA) executed on powerful GPUs. A small-size LISA model with absorbing boundaries is tailored for the purpose of extracting the Lamb wave scattering and mode conversion features. Due to the explicit parallel CUDA implementation and the small-size model setup, the computation is highly efficient. Numerical case studies on nonlinear scattering of Lamb waves from breathing cracks are given. Distinctive higher harmonic generation and selective mode conversion phenomena are presented using the complex-valued Wave Damage Interaction Coefficients (WDICs) containing both amplitude and phase information of the scattered wave field. The effect of oblique incident angle on nonlinear scattering phenomenon is investigated. The rough crack surface feature with initial openings and closures is also considered to better approximate fatigue cracks in practical engineering scenarios. In addition, the wave amplitude effect on the nonlinear scattering and mode conversion is studied. This research may provide guidelines for the effective design of sensor arrays utilizing nonlinear Lamb waves for fatigue crack detection. [ABSTRACT FROM AUTHOR]

• Representations for waves of all types generated in the coupled structure. • Insight into the resonance transmission and backward mode phenomena. • Wave energy streamlines and guided wave profiles. • Source energy partition among the excited waves. Explicit integral representations for the wave fields generated by air-coupled or underwater transducers in an acoustic medium with a submerged elastic (metallic or laminate composite) plate are obtained in the form of path Fourier integrals. Together with the far-field asymptotics for the incident, reflected, transmitted and guided waves that were derived from those integrals, they proved to be a convenient tool for the in-depth study of wave phenomena inherent in such acoustic-elastic structures. The present paper is focused on the non-conventional properties of backward leaky waves such as the increased wave energy transfer through the plate due to the increasing occurrence of energy vortices impeding lateral outflow of wave energy. The transformation of classical Lamb waves into leaky guided waves due to a fluid loading, especially in the backward wave range, the energy properties of those waves and the appearing traveling Scholter-Stoneley waves as well as source energy partition among the reflected, transmitted and guided waves are also numerically illustrated and discussed. [ABSTRACT FROM AUTHOR]

Abstract This study is focused on the development of a finite difference model to simulate Lamb wave propagation through Single Lap Joints (SLJs). The main advantage of this model is the mathematical ability to easily reproduce the presence of a damage (debonding) as a discontinuity in velocity values. This makes our model suitable for continuous and embedded Structural Health Monitoring (SHM) of a complex structure. Numerical simulations and experimental campaigns are presented in order to validate the proposed model. [ABSTRACT FROM AUTHOR]

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

Abstract Favorable isolation performance in optical transmitting-receiving (T/R) channels is key to establishing full-duplex bi-directional laser communication links when designing mono-wavelength laser communication terminals. This study proposes a novel design scheme for T/R channels that use a single laser wavelength. The system includes measures such as the deflection at λ/4 wave plate angles, light trap installation at a polarization beam splitter, and removal of polarization analyzers in optimized systems. A mathematical model describing isolation in the optical T/R channels is presented. Three types of factors that affect the isolation of optical T/R channels were analyzed and simulated in detail. The present simulation results show that the roughness of the λ/4 wave plate and the solid angle of the detector field of view are exponentially related to T/R isolation, which can thus be regarded as the most significant factors affecting isolation. According to the present design scheme, the prototype T/R channel was developed for measuring isolation. The measured value (77.8 dB) agrees well with the result determined using the established mathmatical model (77.3 dB). Conclusively, the present theoretical model can provideguidance for optimizing isolation in a mono-wavelength optical channel. Thus, it can be applied extensively in laser communication systems, including inter-satellite networks. [ABSTRACT FROM AUTHOR]

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

Abstract In recent years, solar energy becomes one of the most popular renewable energy resources. Among the solar energy systems, solar cells are considered to be one of the most critical components, which are naturally fragile during either fabrication or service. Thus, nondestructive examination of solar cells is required practically. In this research, solar cells made from monocrystalline silicon are selected as the typical samples. The air-coupled ultrasonic nondestructive testing system, based on the generation and reception of the A0 mode Lamb waves, will be adopted for defects detection. Firstly, the dispersion curves of Lamb waves in anisotropic monocrystalline silicon are calculated theoretically, which provides a theoretical foundation for analyzing the behavior of Lamb waves propagating in silicon solar cells. Then, experiments on monocrystalline silicon solar cells are carried out by introducing the air-coupled ultrasonic Lamb waves. The amplitude distributions of Lamb waves propagating along various rotation angles in silicon solar cells are obtained. It shows a strong anisotropic properties of the tested samples. Finally, artificial cracks in solar cells are scanned circularly and laterally. The results of amplitude distributions give the scattering pattern of cracks, which can be used to determine whether the crack breaks through or not. And the extracted amplitude cross-correlation coefficients can help to estimate the length of them. [ABSTRACT FROM AUTHOR]

Abstract Scanning laser Doppler vibrometry (SLDV) is a promising method for visualizing defects in plate-like structures. The Lamb wave-based SLDV method using continuous excitations has been recently developed in nondestructive evaluation (NDE). In addition, various signal processing techniques have been developed to enhance damage detection capability for the method. To visualize defects, the A 0 mode is commonly used because of its sensitivity to the thickness variations, i.e. the wave speed changes according to the thickness of a plate. The A 0 mode is easily generated by lead zirconate titanate (PZT) transducers. However, for thick plates, the A 0 mode is found to be less sensitive to thickness variations compared to thin structures. To address this issue, a mode-tuned inter-digital transducer (IDT) is adopted to efficiently generate and utilize the symmetric modes for damage detection and visualization of thick plates. Based on the wavelength of the S 0 mode using the dispersion curve of the plate, the IDT is designed and fabricated for efficient symmetric mode generation. To evaluate the capability of the mode-tuned IDT-based SLDV, a 6-mm-thick plate (40 mm × 40 mm) with defects of 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm depths is used. The S 0 mode is selected for its wavenumber sensitivity and the degree of separation between modes. It is demonstrated that the detectability of shallow defects in a thick plate is improved compared to that when using the A 0 mode. [ABSTRACT FROM AUTHOR]

Abstract Lamination defect is one of the common defects in the manufacturing process of seamless pipes. In this paper, the quantitative detection of a lamination defect in thin-walled metallic pipe using circumferential Lamb waves is studied. To interpret the received time-domain signals and extract useful information about the lamination defect, wavenumber analysis is performed on these signals. A three-dimensional finite-element model is established using Matlab and ABAQUS commercial software. Owing to the processing technique, an aluminum ring structure with a three-quarters circumference is considered to represent the metallic pipe. The lamination defect constructed in the model is a "zero-volume" crack, which stretches from θ = 180° to 270° and locates in the mid-plane of the wall. A five-cycle 0.41-MHz sinusoidal tone-burst signal modulated by a Hanning window is carefully chosen to generate the appropriate excitation wave, in CL 0 mode. According to the received signals, the conclusion that the incident CL 0 mode interacts with the lamination defect for numbers of times can be obtained. The space-amplitude curve of incident waves is also depicted to reveal the amplitude distribution of incident waves. A fully non-contact experimental platform that adopts an electromagnetic acoustic transducer as a transmitter and a laser ultrasonic inspection system as a receiver is set up to verify the finite-element model. Three different wavenumber analysis methods are performed on the wavefield signals to explain the detectability of the lamination defect separately through both numerical and experimental studies. It can be concluded that, from the variation of wavenumber, the continuity of structure can be deduced. Not only can the location be calculated with an error of less than 10%, but the profile of the lamination defect can also be imaged. It is also found that very good consistency exists between numerical and experimental results. Highlights • Lamination defect in thin-walled metallic pipe is detected using C-Lamb waves. • Wave-propagation characteristics are depicted between C-Lamb waves and lamination. • Wavenumber analysis methods are used to quantitatively identify lamination defect. • A fully non-contact EMAT-laser platform is set up to perform wavenumber analysis. [ABSTRACT FROM AUTHOR]

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

Abstract Mutual wave interactions provide a very promising technique for nondestructive testing and evaluation due to their exceptional sensitivity to micro-scale damage growth in metallic materials. This article describes detection of localized fatigue damage in aluminum plates using polyvinylidene difluorine (PVDF) sensors to simultaneously receive shear-horizontal waves and the secondary Lamb waves that they generate. Finite element simulations explore several aspects of wave interaction and mode identification. Then laboratory experiments confirm secondary wave generation at the sum frequency in an aluminum plate using the PVDF sensor by conducting frequency-wave number domain and supporting analyses. The PVDF sensor enables computation of amplitude ratios even though the primary and secondary waves have different polarities. Finally, a simple guided wave technique based on electrically scanning the wave mixing zone around a plate is shown to detect early localized fatigue damage in an aluminum plate. [ABSTRACT FROM AUTHOR]

Abstract The study provides a technique to quantitatively and quickly assess the debonding range between steel plate and substrates for concrete-filled steel tubular member (CFST member) or reinforced concrete (RC) structure strengthened by steel plate with epoxy as the bonding agent. The technique involves single test with a transducer 400 mm away from a mechanical impact source. The spectrogram of the signal is obtained by Short Time Fourier Transformation (STFT) and reassigning technique. Numerical and experimental studies were performed for two layered (steel + concrete) and three layered (steel + epoxy + concrete) composite plates containing debonding crack with different lateral sizes. The interfacial debonding is evaluated by a parameter derived from the group velocity profile of zero-order antisymmetric mode (A 0) of the steel plate with substrates. Quantitative relationships between the parameter and defect ratio are established for two-layered and three layered composite plates containing partially delaminated crack. The effects of impact-sources, and the defect position related to the test line were also discussed. [ABSTRACT FROM AUTHOR]

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

• Coupled models of EMATs are built via a mid-level method. • Approaches to solve the amplitudes of the S0 and A0 modes are compared. • The single frequency approach is used in characterizing excitability of Lamb waves. Electromagnetic acoustic transducers (EMATs) are versatile non-contact ultrasonic transducers with relatively low transduction efficiency. A fundamental problem not answered completely yet is how the structure of an EMAT influences the generation of various Lamb wave modes. We tackle this problem with fully coupled frequency domain EMAT models where the underlying integro-differential equations are solved directly and the magnitudes of the phasors of the displacement components at a point in the middle plane of a plate are used to represent the strength of the S0 and A0 mode waves. Idealized single-wire, two-wire and wire bundle EMATs with uniform bias magnetic field and practical meander line and tightly wound EMATs with distributed bias field are studied. Polar plots of the idealized EMATs with swept angle of the bias field show that the vertical component of the bias field decides the strength of the S0 mode waves. Sweeping of the width of the magnet for the practical EMATs generates S0 mode curves that could be explained from the distribution of the vertical component of the bias field. This work represents the first attempt to solve the proposed problem of characterizing excitability of Lamb waves with EMATs quantitatively, via the model-based approach. [ABSTRACT FROM AUTHOR]

• Fast phase shift technique for rapid inspection of samples is presented. • Results were compared with those extracted by three-step phase shift. • In term of accuracy, the obtained results are not bad. • This technique can be applied for fast inspection of optoelectronic samples. This paper presents a two-step phase-shifting interferometry technique for fast inspection of uniformity of optoelectronic samples. The in-line configuration was applied to make full use of the space-bandwidth product (SBP) of the camera and hence can resolve finer spatial details of the sample. Two-step phase-shifting (TSPS) algorithm was used to reconstruct the object with only two in-line interferograms generated when a quarter wave plate (QWP) is placed in the light path of one arm of the Mach-Zehnder interferometer. Experimental results show that the extracted phases calculated by the TSPS technique match with the phases extracted by the three-step phase-shifting (THSPS) technique. [ABSTRACT FROM AUTHOR]

Abstract The paper deals with numerical and experimental investigations aimed to develop a Finite Element (FE) model for predicting wave propagation in a blended composite winglet. Material anisotropy, inhomogeneity, multi-layered configuration and complex geometries tend to increase the complexity of the wave propagation phenomena and consequently the development of established FE models. Moreover, even if 2D finite elements seem to be not appropriate for modelling guided waves propagation, especially for complex anisotropic structural components, they are more attractive than 3D ones, because of the computational cost saving. For this reason, part of the presented research activity is addressed to investigate the efficiency of shell finite elements in modelling guided waves propagation in a such complex structure as a winglet. The development of an efficient model depends also on the numerical characterization of the medium within which guided waves propagate through. As a consequence, preliminary simple experimental bending and modal tests have been carried out to support the material properties modelling. Subsequently, guided wave propagation FE analyses were performed and the results compared with provided experimental data. A good agreement between numerical and experimental results of the different analyses has been achieved in terms of both signal time of flight and amplitudes. [ABSTRACT FROM AUTHOR]

Abstract The quarter wave plate made of spun birefringence fiber with complex-tangent spun rate profile has the exact analytical solution to the coupled-mode equation with variable coefficients. Based on the solution for wave plates with this profile, critical constraint between the normalized fiber length and the normalized maximum spun rate is analyzed according to typical specification. For given actual fiber length and maximum spun rate, the optional range and optimal value of beat length of prototypical birefringence fiber are deduced and figured out. [ABSTRACT FROM AUTHOR]

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

Highlights • A novel-joint damage detection algorithm based on Lamb waves and electro-mechanical impedance methods is developed. • A mobile transducer set contains PZT patches is designed. • A baseline-free damage characterizing algorithm is presented. Abstract Lamb waves and electro-mechanical impedance (EMI) based methods are increasingly used in damage detection owing to their high sensitivity to small structural defects. Lamb wave based methods are effective in detecting damages in a large area and electro-impedance based methods are suitable for characterizing the identified damage. Based on these two methods, a novel combined damage detection method is presented in this research. To achieve this, first, a mobile transducer set is developed, which can be used for both the Lamb waves and EMI based methods. Then, a baseline-free damage detection strategy that combines the Lamb waves and EMI methods is presented. Finally, a laboratory-sized test piece is used to validate the effectiveness of the proposed approach. The results achieved with the application of the presented combined method for characterizing an L-shape crack in an aluminum plate show better location accuracy and detection efficiency than those obtained by applying only one method. [ABSTRACT FROM AUTHOR]

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

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

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

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

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

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

Highlights • SnTe as capping layer coatings in the vacuum and extreme ultraviolet spectral regions. • Design and fabrication of VUV quarter wave plates (QWP) based on SnTe/Al bilayer. • Extreme ultraviolet (EUV) improved facility for ellipsometric measurements. • Studying the polarimetric properties of SnTe/Al sample at the wavelength of (121.6 nm). • Angle-dependence phase retarder properties of SnTe/Al also reported in this study. Abstract Vacuum ultraviolet and extreme ultraviolet polarized light plays a significant role in many applications from materials science and solar physics investigations to optical components development. In this paper, we study the optical performances and the quarter wave retarder properties of aluminum coating capped with SnTe/Al thin film. The measurements have been performed by using a reflectometer properly improved for ellipsometric characterizations. The specimen has been fully characterized at hydrogen Lyman–alpha line (121.6 nm) by deriving the reflectance and the ellipsometric parameters, i.e. the ratio r and the phase difference δ. We found that the SnTe/Al bilayer behaves as phase retarder, although the performances differ from the ideal case. In fact, the polarimetric analysis combined with the optical test at different wavelengths allowed to identify carbon contamination on the top layer and a mild oxidation of the aluminum coating that affect the optical throughput. [ABSTRACT FROM AUTHOR]

Abstract Elastic guided wave based evaluation is useful to detect delamination in composites. This paper reports a detailed account of guided wave interaction with delamination in laminated composites modeled using time domain spectral finite element (TSFE) method. Wave scattering due to different delamination positions is studied. Wave field near the delamination in different layer-wise positions is investigated, which is useful to further characterize and correlate the far field wave packet carrying the parametric signature of delamination. Off-axis delamination in composite laminate creates an asymmetry in the elastodynamic stress transfer. It leads to wave mode conversions. Sensitivity of the delamination to the wavelengths is studied. The outcome of this study indicates potential possibilities to use frequency-wavelength information to discriminate various sizes of delamination. Energy of the scattered waves and dissipation/conversion of the wave energy due to the defects depend on the resonance characteristics of the sub-laminates. Wave scattering effect due to length-wise multiple delaminations and edge delamination is also analyzed. The resonance patterns in the signals are analyzed with reference to the defect quantification problem. [ABSTRACT FROM AUTHOR]