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This study is based on the principles of acoustoelasticity, analyzing the variation in sound wave time-of-flight(TOF) under different conditions using the critical refracted longitudinal(L CR) wave method. A simulation model was developed using the COMSOL finite element simulation software to mimic the propagation of L CR wave in a stress-free plate. The model was then subjected to uniaxial loading, and the change in wave TOF was observed by varying the angle between the stress direction and the sound wave propagation direction under constant load. The results were compared with theoretical calculations, showing a high degree of agreement, validating the accuracy of the designed model. Subsequently, during the tensile testing, the influence of different uniaxial loads and angle variations on the TOF of L CR wave was observed. Upon comparing the experiment data with theoretical calculations, a high degree of consistency in the trend of variation is observed, this demonstrates that the detection platform established in this study is feasible for the assessment of residual stresses in practice. [Display omitted] • The accuracy of the simulation results is verified by the comparison of theoretical calculations. • By analyzing the variation of TOF in different propagation distances, the optimum propagation distance is 45 mm. • The sound velocity of L CR wave increases with the increase of the propagation Angle, and the change is most obvious at 45°. • The feasibility of L CR method for residual stress detection is further proved by tensile test. [ABSTRACT FROM AUTHOR]

Nondestructive estimation of residual stresses has been a major challenge in engineering. Herein, the collinear Lamb wave mixing method is proposed for residual stress measurement in metal plates under immersion ultrasonic testing. Numerical simulations have been conducted to investigate the influences of excitation parameters on the performance of the wave mixing method. The results showed that the receiver position and cycle numbers of primary waves affect the amplitude of the method, and optimal excitation parameters recommended. Ultrasonic immersion experiments are conducted on two types of plate-like specimens using the collinear Lamb wave mixing method. Experimental results showed that the proposed method is effective for measuring the residual stress in metal plates. The immersion ultrasonic testing can significantly reduce the influence of inconsistent coupling conditions between the transmitters and the specimen on experimental results, therefore it is more reliable than conventional contact testing. [ABSTRACT FROM AUTHOR]

Ultrasound has been identified as a suitable method for inspecting complex structures (such as nuclear reactors), as it can propagate through both liquids and steel elements to be inspected. Leaky Lamb waves have been extensively researched to detect damages in plate-shaped structures, which are waves that reemit bulk waves towards the surrounding fluid. An imaging method based on the time reversal of Lamb waves has been applied in preliminary work to detect the edge of a single immersed plate and a machined notch. In this study, the frequency-domain topological energy method is applied to a set of two parallel immersed plates to succeed in accurately localizing defects represented by the edges of the plates. Firstly, the behaviour of leaky Lamb waves is briefly addressed. A 2D-STFT (two-dimensional Short Time Fourier Transform) is then used to separate the different Lamb modes, which is a powerful tool for tracking the modes involved in the propagation over time. The Fast Topological Imaging Method (FTIM), which allows for faster computations with less data storage, is applied to the case of Lamb waves in a set of parallel plates. Lamb modes can be identified then selected to improve the analysis of the obtained images. An original methodology is proposed based on spots (maxima) analysis for interpreting the results of this topological energy and identifying the edges of the two plates. This process could be applied, in the same manner, to other scattering defects in ultrasound non-destructive testing of plates. • A new concept of topological energy spots is defined in the topological energy method. • The analysis of the topological spots allows to distinguish the real image of the defects among all the topological maxima. • Lamb waves modes are tracked over the time with a 2D-Short Time Fourier Transform. [ABSTRACT FROM AUTHOR]

Medium-range guided wave testing is commonly employed for inspection of areas with restricted access. The technique usually works in pulse echo mode and at high frequency-thickness products, around 20 MHz ⋅ mm, offering good sensitivity and resolution. Defect sizing is based on the reflection amplitude of the received mode(s). However, the scattering of guided waves is complex, and the amplitude of the reflected modes does not provide sufficient information for defect sizing. This work aims to overcome this limitation using a focusing technique based on Lamb waves. Specifically, multiple Lamb wave modes are excited individually and superimposed to form a new mode with a desired through-thickness energy distribution. This way, energy is focused on a single point in the structure. Using weighting functions, the location of the focal point is swept across the thickness of the sample. The technique allows for accurate sizing of flaws, such as cracks and wall loss. [ABSTRACT FROM AUTHOR]

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

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

Air-coupled ultrasonic Lamb wave damage imaging is a method for rapid, non-contact damage detection of plate specimens. However, the current technique based on Lamb waves has limitations regarding accuracy and quality of imaging. To address this issue, an improved bilateral boundary-enhanced reconstruction algorithm for the probabilistic inspection of defects (BBERAPID) is proposed. This algorithm optimizes the probability distribution near the damage boundary to enhance the accuracy of damage imaging. The skewed bilateral boundary distribution algorithm can improve the probability distribution of edges on both sides of the damage and alleviate the problem of abnormal boundary expansion. In addition, a new damage index (DI) calculation method is proposed to accelerate the DI trend near the damage. The new DI can improve the fineness of the damage boundary, thus better distinguish the healthy area from the damage area. Traditional DI has the problem of abnormal distribution at the damage boundary, which BBERAPID solves. The probability distribution near the damage is optimized to improve the quality and accuracy of damage imaging. Experimental studies are performed using composite plates to validate the proposed method further. The effectiveness and accuracy of the proposed BBERAPID algorithm for detecting damages in composite plates are verified by comparing it with existing imaging methods through experimental validation. The proposed non-contact air-coupled Lamb wave damage imaging method is suitable for rapidly and accurately characterizing defects in composite plates. [ABSTRACT FROM AUTHOR]

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

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

Despite the demonstrated effectiveness of methods based on local defect resonance (LDR) in defect identification and sizing, the precision of LDR prediction is limited owing to the inconsistency between the fundamental assumption of existing methods and the reality. To tackle this deficiency, an analytical framework to interpret the formation of LDR induced by defects in multilayered structures is proposed. In this framework, reflections of Lamb waves at defect boundaries are investigated analytically, whereby the phase shift of reflected waves at defect boundaries is obtained. On this basis, the resonance formation is analyzed from a perspective of wave propagation and superposition, and the relation between the LDR frequency components and the defect size is ascertained. Experimental validations are performed in which LDR generated by defects in adhesively bonded multilayer structure are measured. This investigation provides an analytical insight into the defect-induced LDR and improves the precision of quantitative characterization of defects using LDR. • an interpretation of the formation of local defect resonance. • a new approach to analyze the interaction between GUWs and disbond in multilayered structures. • improved precision in prediction of local defect resonance frequencies. • demonstrated capability of accurately characterizing disbond defect. [ABSTRACT FROM AUTHOR]

Resistance spot welding can instantaneously join two or more plates by the resistance heating of the metal and is in high demand owing to its high productivity and high-work efficiency. For the quality control of resistance spot welding, the size and quality of the welded part, called the nugget, are important. Therefore, this study aims to establish a highly efficient and high-speed resistance spot-weld inspection method that can be applied to whole-lot inspection, which is currently a difficult process. The objective of this study is to measure the nugget diameter using laser ultrasonic technique that enable remote, non-contact ultrasonic inspection. An investigation of the available ultrasonic waves using simulated test specimens demonstrated the feasibility of estimating the distance from the generation/detection point to the joint using the diffraction of the Lamb wave, which can propagate long distances in a thin plate. By measuring the actual resistance spot welding specimens, it was determined that differences in the nugget diameter of approximately 0.5 mm could be clearly distinguished from the arrival time of the diffraction waves. It was also inferred that the nugget diameter could be calculated by determining the propagation velocity of the diffraction wave with a similar accuracy to that of the measurement using a contact-type probe. [ABSTRACT FROM AUTHOR]

Acoustic microscopy is widely used for the material characterization of thin plates having thicknesses < 100 μ m. For thin plate material characterization, the acoustic microscopy-based approach utilizes the guided Lamb wave modes generated in the plates by defocusing a single high-frequency focused transducer. Identification of the guided wave modes is vital in material characterization. Frequency-wavenumber analysis using 2D-FFT is commonly used to identify guided wave modes. In PVDF-based focused transducers used in acoustic microscopy, edge waves originating from the transducer rim influence the generation of guided waves, making the conventional frequency-wavenumber analysis erroneous. This study proposes a new water path corrected frequency-wavenumber (f-k) analysis using 2D-FFT to overcome the challenges imparted by the edge waves. In this approach, the water path of the Lamb wave modes is calculated and removed from the waveforms before performing 2D-FFT. The proposed approach has been demonstrated on thin metal plates of Aluminum, Copper and Brass having thicknesses of 45 μ m , 52 μ m and 52 μ m , respectively. A good correlation is obtained between theoretical and experimental results. [ABSTRACT FROM AUTHOR]

Dominant flexural mode excitation of helical waves in hollow cylinders by applying helical surface tractions, equivalent to exciting obliquely propagating plate waves in the unwrapped plate, is reported. However, this approach cannot be generalized with theoretical underpinnings. The response of hollow cylinders under helical surface tractions is investigated using normal mode expansion method. Purity of the target flexural mode depends on the mode wave structures at excitation frequencies and the helix angle of the helical excitation. With helical surface tractions, only flexural modes with large circumferential and longitudinal displacements, usually torsional flexural modes, can be effectively generated. The target flexural mode and excitation frequency should be selected carefully for better purity. The helix angle for the target mode should diverge considerably from other modes, especially those with large circumferential and longitudinal displacements. The apparently oblique propagation of the flexural mode can also be explained theoretically. The helically excited flexural mode comprises two orthogonal wave structures with identical flexural modes that are out of phase in the propagation direction, causing periodically changing circumferential orientation of the combined wave structure and oblique wave front. Finite element simulations and experiments agree well with theoretical predictions. [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]

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]

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 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 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 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 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]

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 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]

Abstract Lamb waves offer an efficient way for monitoring the structural integrity of large carbon fiber reinforced polymer (CFRP) structures. This paper presents a new method that is able to detect and assess delamination's length in anisotropic CFRP plates. It uses a chirp signal, which contains a wide range of frequencies, as excitation. Thus the material is interrogated by Lamb waves with various wavelengths. This method is based on that, for a specific delamination length, only some particular frequency components are disturbed. Simulation and experimental results show that the lowest frequency that is perturbed by a delamination decreases as the delamination length increases. Therefore, by observing which frequency components of the chirp-exited Lamb wave are disturbed, delamination length can be estimated. Wavelet analysis is implemented for better comparison of signals at multiple frequencies. [ABSTRACT FROM AUTHOR]

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

In this paper, a phase unwrapping-based local wavenumber imaging method is proposed for delamination detection in the composite plate. Local wavenumber imaging has shown significant advantages in damage localization and quantification. However, a dense of spatial measurements and a high requirement of sampling intervals are crucial for conventional spatial Fourier transform methods. To address these challenges, a phase unwrapping method adopting low-sampled wavefields for local wavenumber estimation is proposed for damage imaging. Specifically, the wavefield sampling process can divide the detection area into various grids while making each grid consist of four neighbor measurements. Under the far-field assumption, the wave path distances of grid measurements can be approximately obtained by projecting their real distances along the wave vector direction. Meanwhile, by revealing the phase differences of grid measurement pairs with the low-sampled wavefield data, the frequency-related wavenumbers are estimated subsequently. Hence, the local wavenumber can be recovered robustly by averaging the estimated wavenumbers from all measurement pairs in the grid. On this basis, the structural damage can be geometrically imaged and quantitatively evaluated. Both the simulation study and the experimental investigation have shown the efficiency of the phase unwrapping-based method for local wavenumber imaging, which demonstrates the strong capability of damage localization and quantification in composite structures with low-sampled wavefield. • An improved phase unwrapping method is proposed for local wavenumber estimation with less measurements. • The proposed method can provide a high-quality wavenumber image with low-sampled wavefields. • The location, size, and depth can be accurately evaluated for the composite delamination. [ABSTRACT FROM AUTHOR]

In this paper, a modified fisher discriminant criterion is used to identify and rank damage sensitive features for debonding classification. For this purpose, the analysis of the correlation between features (ACF) based on Pearson correlation coefficient and the Fisher discriminant ratio (FDR) criterion were combined to select the most relevant features. The debondings classification was evaluated in a multiclass classification by considering three different classes of damage and one class of undamaged. The nondestructive inspection was performed in a composite-adhesive-composite lap-joint using guided Lamb wave testing with angle beam transducers mounted in a pitch-catch configuration. From the collected data, twelve features were extracted by using statistical functions and signal processing techniques. From the extracted features, the most damage sensitive features were selected based on the proposed method (ACF and FDR) and the results were compared using the conventional FDR method. For three different classifiers, the feature selection strategies were tested and their performance evaluated using the metrics: accuracy, precision, recall and F1 score. For the dataset collected in this study, the results support that the proposed method yields an improvement of the performance of the classifiers in comparison with the use of the conventional FDR-based feature selection. [ABSTRACT FROM AUTHOR]

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

In this paper, an in situ technique for the localisation and the sizing of cracks in plate-like structures is presented. The excitation and the reception of the ultrasonic diagnostic signal – Lamb waves – are achieved by permanently installed piezoelectric ceramic disks. The technique simplifies the underdetermined problem of the full characterisation of cracks to an over-determined problem of the localisation of crack tips. Also, it overcomes the difficulties associated with using the pitch-catch excitation scheme for crack characterisation. The development of the technique was done systematically using analytic models and the finite element method. The technique has been validated by an experiment and assessed by extensive parametric studies. While it was designed to be a convenient approach, highly accurate crack characterisation has been attained as evident through the results of both numerical studies and physical experiments. [ABSTRACT FROM AUTHOR]

This work presents an omnidirectional electromagnetic acoustic transducer (EMAT) for generating and receiving A 0 mode of Lamb waves in plate structures. The proposed EMAT is composed of a spiral meander coil (SMC) and a new concentric permanent magnet pairs with opposite polarity (CPMP-OP) for enhancing the generation and detection of A 0 mode. A finite element model was established to simulate the directions of the static magnetic field produced by the permanent magnets and the incentive process of guided waves. To verify the performance of the EMAT, several experiments were performed. Firstly, we experimentally verified that the developed EMAT could generate and receive a pure single A 0 mode omnidirectionally. Then, the relationship between the frequency response characteristic of the developed EMAT and the interval l (the distance between the two adjacent annular wires of the SMC) were explored. Furthermore, the circumferential consistency of the transducer was well demonstrated. Finally, the sensing performance of the developed omnidirectional A 0 mode EMAT employing a CPMP-OP was compared with that of EMAT with different structures of coils and permanent magnets. The experimental results indicated that the developed omnidirectional A 0 mode EMAT employing a CPMP-OP could enhance the signals of the A 0 mode. [ABSTRACT FROM AUTHOR]

The use of waveguides for ultrasonic transduction is advantageous in numerous applications such as measurement of flow, temperature and material properties under harsh environments. However, typically waveguide transducers do not use any mode control. This paper describes aspects of mode selection for waveguide based transduction of bulk longitudinal ultrasonic waves in a target metallic specimen. It is proposed that specific waveguide modes with suitable modal structure and matched acoustical impedance can significantly enhance the transduction of bulk ultrasonic waves in the specimen sample. Using circular cylindrical waveguides excited by magnetostriction, experimental results guided by finite element simulations and analysis are presented, demonstrating the feasibility of the approach. An example application in the form of a bulk longitudinal wave ultrasonic transducer is also provided illustrating the improvement in the signal quality using the proposed concept. [ABSTRACT FROM AUTHOR]

The detection of the nonlinear Lamb wave using a PVDF comb transducer is described in this research. The construction features of the PVDF comb transducer is firstly described. Then, the harmonic-frequency characteristic of PVDF comb transducer as the receiver in the nonlinear Lamb wave inspection is qualitatively investigated by the output-vector addition method. The theoretical result shows that, when the wavelength of the fundamental Lamb wave is equal to the PVDF comb transducer's finger spacing, the transducer can receive the fundamental wave. When the frequency of the higher-harmonic Lamb wave is integral multiple of the frequency of the fundamental one, the PVDF comb transducer can also receive the higher-harmonic Lamb wave. In order to verify the harmonic-frequency characteristic of the PVDF comb transducer, we conducted an experiment based on the cumulative effect of the nonlinear Lamb wave. That is, the value of the relative nonlinearity parameter will grow higher when the propagation distance increases. In order to receive the fundamental and second-harmonic Lamb waves, a PVDF comb transducer with special finger spacing was made and pasted on an aluminum plate surface. The desired Lamb-wave mode was generated by a wedge transducer. It had been observed that the value of relative nonlinearity parameter increased with the wave propagation distance. That means that the PVDF comb transducer had received both the fundamental and the second-harmonic Lamb waves. Both the theoretical and experimental results show that, because of the PVDF comb transducer's simultaneous response to the fundamental and second-harmonic signals, the nonlinear Lamb wave can be detected using a PVDF comb transducer. [ABSTRACT FROM AUTHOR]

This research reports a selective detecting and focusing approach for fatigue cracks through use of decomposition of the time reversal operator (DORT) in combination with nonlinear Lamb waves. In the proposed approach, the time reversal operator (TRO) is only decomposed at the second-harmonic frequency, which has been widely utilized in recent years to evaluate micro-damage, rather than the fundamental frequency in the traditional DORT method. Further, an imaging algorithm based on the total focus method is introduced to locate nonlinear scatterers. The proposed approach is experimentally investigated to locate two fatigue cracks with different fatigue cycles in an aluminum plate with a set of surface bounded piezoelectric (PZT) transducers. The results show that the significant eigenvalues are related to the fatigue cycles of fatigue cracks when the TRO is decomposed at the second-harmonic frequency, and the number of significant eigenvalues is exactly equal to the number of fatigue cracks. The two fatigue cracks are selectively located accurately and are also distinguished from linear scatterer by use of the proposed method. [ABSTRACT FROM AUTHOR]

Propeller inspection is mandatory for safe operation of aircraft. Damage evaluation on such rotating structures requires dedicated measurement techniques. In this study efforts to create a stroboscopic technique are reported. Lamb waves were excited on a rotating blade with a Q-switched Nd:YAG laser synchronized to the sample rotation, whereas the wave amplitude was obtained by a laser Doppler vibrometer. A surface breaking notch on an aluminum sample rotating at 415 rpm was detected and sized with millimeter accuracy. The technique has potential for automatic non-contacting damage detection on rotating structures such as helicopter blades and turbines. [ABSTRACT FROM AUTHOR]

This paper presents a multipath exploitation approach for defect imaging in thin-walled structures using Lamb waves under the sparse reconstruction framework. An image of the defects in the region of interest is obtained by inverting a multipath propagation model via group sparse reconstruction. This permits accurate imaging of regions close to the structure boundary without the introduction of ‘false’ defects or ghosts. Real-data measurements of defects close to the edge of an aluminum plate are used to demonstrate the effectiveness of the multipath exploitation scheme. [ABSTRACT FROM AUTHOR]

Wall thickness reduction, which typically results from corrosion, erosion, or operating conditions, is a significant challenge in many industries. In order to generate a broadband pure shear horizontal wave mode in plate structures, this paper proposes a magnetostrictive patch transducer called the Staircase Magnetostrictive Patch (ScaMP) Transducer. For ScaMP transduction, an optimised patch width offers a broadband frequency response is employed. The lowest remnant thickness of the sample was estimated using the wideband SH1 dispersive guided wave mode's cut-off property. The broad frequency excitation method enabled measurement over a range of wall thinning up to 80% of the nominal thickness with a single data acquisition. Experiments were carried out on an 8 mm nominal wall thickness aluminium plate with gouge defects to evaluate the proposed sensor's broad frequency response. The measurement of remnant thickness was estimated with a maximum error of 5.5%. • Proposes a novel magnetostrictive patch transducer for broadband pure SH wave mode generation in plate structures. • The proposed method enabled measurement over a range of wall thinning up to 80% of the nominal thickness. • Experimentally estimated lowest remnant thickness up to 25% using the proposed technique with a maximum error of 5.5%. [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]

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]

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]

This paper investigates a method to generate only symmetric omnidirectional Lamb waves and proposes specially-tuned double-coil EMATs. While methods to generate omnidirectional Lamb waves have been reported, the symmetric mode is typically accompanied by unwanted antisymmetric modes. The double spiral coil works as a wavelength filter but the relation between the coil geometries and the wavelength of the symmetric mode that minimizes the unwanted antisymmetric mode has not been established. Axisymmetric analysis was performed to find the relation. Excellent rejection of antisymmetric modes with successful symmetric mode generation was confirmed by experimental results that favorably agree with the theoretical results. [ABSTRACT FROM AUTHOR]

Visualizing the propagation of Lamb waves generated by scanning laser injection is a promising and effective method for the nondestructive inspection of carbon fiber reinforced plastic (CFRP) structures. We evaluate the use of two-dimensional time-slowness maps of the acquired data for defect detection in CFRP laminates. The slowness maps can be constructed from the differences in wave propagation characteristics between two adjacent laser injection points. Using a CFRP plate, in which an artificial delamination is located at the center, we show that the delamination can be clearly observed as a hotspot in the two-dimensional slowness map. The visible hotspot is due to the dispersion of the lowest antisymmetric (A 0) mode of the Lamb wave: the A 0 mode slows down at the delaminated region. We also demonstrate that our method can be applied to an L-shaped structure. Two-dimensional slowness images can be useful for developing and applying practical inspection methods based on laser ultrasonic waves. • ・Defects in CFRP laminates are inspected using laser ultrasonic guided waves. • ・A two-dimensional slowness map is made from the data obtained by scanning laser. • ・The slowness map is simple and useful for defect detection. [ABSTRACT FROM AUTHOR]

To improve the resolution of Lamb wave inspection, a waveform separation and image fusion strategy is established. The integration of the adaptive Chirplet transform and the time-varying band-pass filtering provides a methodology for extracting interest waveforms from the overall Lamb wave signals. On this basis, image generation and image fusion is employed to establish a visual description of the overall health status of structures. The strategy could be still available even under the conditions where the dispersion characteristics of the structure cannot be correctly estimated and the reference signals are not accessible. The merits of the proposed strategy are demonstrated by an experimental example. [ABSTRACT FROM AUTHOR]

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

Aimed at improving the imaging performance of the classical total focusing method (TFM), which is an effective post-processing technique for Lamb wave phased array based damage imaging, a waveform correlation factor (WCF) weighted TFM imaging algorithm is proposed. In the proposed algorithm, besides the signal amplitude used in the classical TFM, the waveform correlation coefficients between any two waveforms after dispersion compensation are also utilized for damage imaging. Waveform correlation coefficients will be large if the wave packets are caused by the same imaging point, or they will be small due to the change in waveform shape arising from dispersion. For each imaging point, dispersion compensation for all collected signals is respectively implemented so that the scattering waveforms can be extracted. Then the waveform correlation coefficients between any two scattering waveforms are calculated and weighted on the corresponding amplitude used in the TFM. By comparing the imaging results of the proposed algorithm with that of the classical TFM algorithm, it is found that the proposed algorithm can significantly reduce the background noise and thus improve the imaging performance. • A new damage imaging algorithm to improve imaging performance of the classical TFM. • Fusion of signal amplitudes and waveform correlation coefficients for imaging. • Remarkable reduction in background noise of imaging results. • Experimental validations using linear and circular arrays. [ABSTRACT FROM AUTHOR]

In this paper slant stack ( SL ) transform is presented and its application for processing of multi-modal dispersive Lamb waves snapshots is proposed. The SL transform can facilitate the evaluation of dispersion curves based on a set of signals captured at the structure׳s surface. The presented technique leads explicitly to the frequency-phase-velocity representation of the processed signals. Theory behind the technique is presented and the SL results are compared to those obtained using the 2D discrete Fourier transform. The SL is used to process data acquired from an aluminum plate and to investigate anisotropic properties of a composite plate. [ABSTRACT FROM AUTHOR]

The increasing mechanical and economical demands in modern systems and structures are forcing an inevitable need for joining dissimilar materials, thus creating the challenge of establishing a process to inspect and monitor dissimilar joints. Condition monitoring is a necessity to ensure that the structures are being safely used and to extend their lifetime. Lamb waves (LWs) are ultrasonic guided waves that are widely used for structural health monitoring of mechanical, aerospace, and civil structures. This paper proposes a novel structural health monitoring approach for damage detection, localization, and assessment using a minimal LW sensor-actuator set-up. More specifically, the proposed framework provides damage detection, localization and assessment within a dissimilar-material joint by Bayesian inference of six parameters of damage extent and location. Finite element simulations are used to simulate the measured LWs and generate a dataset required to train artificial neural networks (ANN), acting as surrogate models for LW simulation with reduced computational cost. The ANN-based LW simulations are further used as forward model within an Approximate Bayesian Computation (ABC) framework to provide probabilistic inference of the damage size and position. The results show that damage of different sizes and locations can be successfully identified with a high level of resolution and with quantified uncertainty. The results also show that data fusion for ABC inference using multiple sensor measurements can be possible with improved inference results. However, a precise and robust damage inference can be achieved using a minimal sensing set-up based on one actuator and two sensing points, with consideration of certain levels of measurement noise. These findings imply a considerable reduction of complexity of LW actuator-sensor networks, and overall, they imply a significant reduction of computational resources and cost for damage detection and assessment in structures, thus providing a step forward towards online/onboard monitoring applications. • A novel framework to detect, localize, and assess damage using Lamb-waves, is proposed. • A reliable damage identification was achieved using only one actuator and two sensors. • Surrogate models for Lamb-wave propagation have been developed based on Artificial Neural Networks. • Probabilistic damage inference was performed using Approximate Bayesian Computation. • Data fusion was employed for ABC inference using multiple sensor measurements. [ABSTRACT FROM AUTHOR]

This work explores the generation of Lamb waves and their higher harmonics using a pattern source. An array of laser sources, when used with a short-pulsed laser, generate multiple Lamb modes in a plate-like component. The variation in duty ratio of the pattern source is found to influence the generation of fundamental modes and their harmonics. A two-dimensional finite element model is developed to simulate the narrowband Lamb wave generation process, and validated against targeted experiments. For a 'square-like' spatial profile of energy distribution, the second harmonic of the fundamental wave modes is fully suppressed at the duty ratio of 0.5, and the third harmonic of the fundamental wave modes is fully suppressed at duty ratios of 1/3 and 2/3. As the duty ratio increased from 0.2 to 0.8, the variation in amplitude of the fundamental modes is greater than that of the higher order modes. For duty ratio values less than 0.25 and greater than 0.75, the second harmonic component of the fundamental waves had approximately the same amplitude as the fundamental wave modes. Similarly, the third harmonic of the fundamental wave modes had the same amplitude or slightly higher amplitude than the higher order Lamb modes for duty ratio values less than 0.25 and greater than 0.75. [ABSTRACT FROM AUTHOR]

The developed method is based on acoustic underwater scattering measurements and can be applied to characterize a defect in a blade of a marine current turbine. To simplify the study, the blade is replaced by a rectangular plate immersed in water having a groove opening out. The measurements are made on horizontal plane perpendicular to the long axis of the stainless steel plate. The frequency range of signal is between 50 and 400kHz. The transducer is remote from the plate. The measurements are recorded on 360° with a 1° step. The experimental trajectories of the scattering signals are compared to the trajectories calculated from the group velocities of Lamb waves A and S 0. Spectra give also an information on groove position. The present study is the first step toward remote monitoring of blades on a marine current turbine. [ABSTRACT FROM AUTHOR]