Dispersive instantaneous frequency imaging

Abstract

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.