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The multiple signal classification (MUSIC) algorithm is originally used for direction of arrival estimation and frequency identification. MUSIC-like algorithms have gained their popularity in structural health monitoring (SHM) owing to their super-resolution characteristic and the virtue of baseline independence. MUSIC essentially is a searching algorithm to find the best phase difference matching between the eigenvector from signal subspace and the presupposed steering vector. However, in the guided-wave-based SHM (in plate-like structure, the guided wave is Lamb wave), the dispersion effect will definitely lead to phase distortion. Therefore, Xu et al. presented a focusing MUSIC algorithm where the time reversal technique is adopted to eliminate the dispersion in the way of back propagation. Seeing that the phase difference information, which is necessary for MUSIC, disappears after back propagation, the damage scattered wave packet, obtained by window truncation, is forward propagated again at the constant group velocity to recover the time of flight information. Nevertheless, the forward propagation process is superfluous from the view of phase matching. In the presented forward-propagation-free focusing MUSIC (FPF-F MUSIC) algorithm (the simplified version of Xu's work), by revising the steering vector in the form of phase coincidence, the matching relation for MUSIC is modified from phase difference matching to phase alignment matching, so that the forward propagation and the steering vector traversal setting for every inspection point are all omitted. The damage localization performance of FPF-F MUSIC is verified on the aluminum plate with double through-hole and surface defects under uniform linear and circular arrays. [ABSTRACT FROM AUTHOR]

The imaging method based on the conventional phased array (PA) can only realize angle focusing. To break through the above limitations, frequency diverse array (FDA) is first introduced from radar to Lamb wave, which opens up a possibility to realize angle–range focusing in damage localization. The energy pattern of FDA without considering the features of radar signals is re-derived, and a corresponding FDA focusing imaging method for Lamb wave is developed. In the presented method, by appending various frequency offsets to the redesigned anti-dispersive excitations, the desired FDA focusing and the dispersion elimination for Lamb wave are achieved simultaneously. Furthermore, based on full matrix capture being the data acquisition approach, a novel total focusing is realized with comprehensive consideration of positive and negative frequency offsets. Benefiting from FDA focusing, compared with the PA methods, the proposed method exhibits superior adjacent defects’ identification ability and preferable interference suppression capability with excellent location accuracy and strong parameter robustness. Its superiority is validated by the experiment on aluminum plate with multiple adjoining surface and inner defects. [ABSTRACT FROM AUTHOR]

Amplitude and phase characteristics are widely utilized in Lamb wave imaging. Nevertheless, the aforementioned methods are mostly dependent on the baseline (i.e. the health response from an undamaged specimen) and are more or less affected by artifacts from the undesired mode. Except the amplitude and phase features, the instantaneous frequency is also a crucial damage information source. However, it is often neglected. If using it, owing to the negligible change in the instantaneous frequency of the damage wave packets before and after baseline subtraction, the baseline would no longer be necessary. Owing to the distinction in frequency between the compensated A0 and S0 mode responses, suppression of the undesired mode is possible. Therefore, to diminish the influence of the undesired S0 mode and eliminate the baseline dependence, a dispersive instantaneous frequency imaging algorithm is proposed for Lamb waves. Profiting from the dispersive pre-compensated focusing strategy, the damage echo is compensated to be consistent with the excitation, which implies that its instantaneous frequency is identical to the center frequency of the excitation. By quantitatively comparing their consistency, an imaging index (i.e. the pixel value of an image) is assigned for every scanning point to generate C-scan images. As a simple extension of the instantaneous phase, the instantaneous frequency is obtained by taking its derivative. Therefore, frequency-based images not only preserve the merits of phase images without significantly increasing the calculation cost, but also incorporate baseline independence and undesired S0 mode suppression. Moreover, because the amplitude, phase and frequency features of the defects can be acquired successively for one path, a compound imaging method is presented. Experiments were conducted on an isotropic aluminum plate with diverse defects and array forms. In comparison with the amplitude-based and phase-based images, the S0 mode artifact in the frequency-based images is alleviated and the correctly identified defects are more conspicuous without using baseline. • The frequency information is first introduced into Lamb wave imaging. • The frequency-based images are independent of the baseline. • In frequency-based images, the undesired mode is suppressed. • A holographic imaging method is presented. • The presented methods are validated on an aluminum plate. [ABSTRACT FROM AUTHOR]

In the purpose of achieving composite structure damage identification and localization of structural health monitoring, a denoising algorithm of redundant second-generation wavelet transform considering neighboring coefficients is selected as the best solution from 18 denoising schemes performed in this paper. Through introducing fractal dimension as a damage-sensitive feature and adopting the probabilistic reconstruction algorithm, the damage status on the composite panel could be identified and located as tomography maps. The practicability of the presented approach is validated in the experiment operating in the composite damage monitoring system. [ABSTRACT FROM AUTHOR]

Lamb wave is a promising means for active structural health monitoring and nondestructive evaluation of laminated composite plates. The inevitable reflections from structural geometric boundaries and other interferences limits the imaging performance of many existing imaging methods. The image generated through the well-known delay-and-sum method holds a wide mainlobe width and high level of sidelobes. Although the sparse reconstruction method is free from those deficiencies, it is sensitive to the regularization parameter. To overcome those limitations, a Lamb wave imaging method to locate anomalies or damage in laminated composite plates is proposed. The scattering signals are sparsely decomposed one by one with a prior weights penalized on the undetermined sparse coefficients, and the minimum variance distortionless response algorithm is adopted to process those sparse coefficients so as to generate the image. Experimental results on a quasi-isotropic laminated composite plate demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Lamb wave-based structural health monitoring techniques have the ability to scan a large area with relatively few sensors. Lamb wave imaging is a signal processing strategy that generates an image for locating scatterers according to the received Lamb waves. This paper presents a Lamb wave imaging method, which is formulated as a weighted structured sparse reconstruction problem. A dictionary is constructed by an analytical Lamb wave scattering model and an edge reflection prediction technique, which is used to decompose the experimental scattering signals under the constraint of weighted structured sparsity. The weights are generated from the correlation coefficients between the scattering signals and the predicted ones. Simulation and experimental results from an aluminum plate verify the effectiveness of the present method, which can generate images with sparse pixel values even with very limited number of sensors. [ABSTRACT FROM AUTHOR]

A narrowband tone burst excitation is usually required in most Lamb wave inspection techniques in order to reduce the dispersion effect and maintain mode purity, especially in the case that dispersion compensation is hard to implement, such as the scenarios prior knowledge of accurate dispersion curves is not available. The duration of the excitation is relatively large because of its narrow bandwidth, which degrades the resolution for inspection. Following the emerging sparse representation theory, a combined method of sparse reconstruction and delay-and-sum (DAS) for high resolution Lamb wave inspection is proposed in this paper. The scattering signals are compressed in time domain without losing the information of the arrival time of each echo. Unlike most methods that signal decomposition and reconstruction use the same dictionary, the proposed approach utilizes two different dictionaries: one for signal decomposition and another for feature extraction. The DAS imaging method combined with the feature signals is applied to damage imaging for carbon fiber reinforced plastic (CFRP) laminates. The effectiveness of the proposed approach is validated on a quasi-isotropic laminated CFRP plate. The imaging results show that the proposed approach can achieve a higher localization resolution and lower noise floor without degrading the localization accuracy. [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]

Piezoelectric transducer arrays are utilized in Structural Health Monitoring systems as a means for excitation and sensing of elastic waves. Anomalies of propagating waves have enabled to develop damage detection algorithms. Depending on actuation-sensing strategies these algorithms can be classified as pitch-catch and pulse-echo. Despite many signal processing methods such as delay-sum, time-reversal, probability-based diagnostic imaging, etc. the spatial damage information provided by the actuator-sensor paths to reconstruct the damage image is limited. A novel strategy based on Lamb wave focusing is proposed in order to increase damage imaging resolution. In the proposed method all actuators are used at the same time exciting specially designed waveforms so that inspect one specific point of the structure. Damage map is created by applying appropriate signal processing. It uses dispersion curve of selected Lamb wave mode for dispersion compensation. The dispersion curve is acquired by using laser scanning Doppler vibrometer. The damage indicator is calculated based on the energy of compensated signals registered by sensors. It is shown that apart from high energy level at excitation point, energy is concentrated exactly in the damaged region. An example of crack detection and visualization in an aluminum plate is shown confirming the accuracy of the proposed method. Also the proposed method is compared to well-established delay-and-sum algorithm. [ABSTRACT FROM AUTHOR]

• A WDR method optimized tomography algorithm is developed for robust localization using Lamb waves. • WDR excitation is designed to alleviate the frequency selection problem in defect localization. • Reconstruction independent component analysis is adopted for mode separation. • Time-frequency index is calculated as the damage index in the tomography algorithm. Tomography imaging of defects using ultrasonic Lamb waves has attracted much attention in nondestructive testing of plates. However, there are two challenges for robust localization of defects: frequency sensitivity to defects, and multi-mode interference on damage index (DI) extraction. Aiming for that, a wideband dispersion reversal (WDR) method optimized tomography is developed. Pre-dispersive wideband excitations of a certain Lamb wave mode are generated based on the configuration of transducers, and reconstruction independent component analysis is used for wave mode separation. According to the acoustic reciprocity principle, self-compensation phenomenon of mode signals can be recorded on the intact path, where the signal energy concentrates at a fix self-compensation point in the time–frequency domain, yet such compensation effect would be impaired encountering defects. Thus, a time–frequency index can be used as the DI, which is calculated based on the weighted Euclidean distance from the self-compensation point to the pixel points in the time–frequency domain. Experimental results show the WDR optimized tomography has a robust performance for magnet-simulated defects imaging without excitation frequency optimization. [ABSTRACT FROM AUTHOR]