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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]

• 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]