A frequency diverse array (FDA) of dispersive lamb waves for grating lobes suppression

Abstract

Governed by the spatial sampling theory, the element spacing of a phased array for damage imaging shall preferably be smaller than half the wavelength of the excitation signal. Otherwise, the grating lobes emerge, resulting in pseudo damage in the constructed damage images. The prevailing methods for grating lobe suppression often exhibit downgraded effectiveness when applied to dispersive Lamb waves. Drawing inspiration from the Frequency Diverse Array (FDA) technique, which originates from radar target detection, an FDA-based damage detection approach has been developed, which utilizes a specially designed zigzag frequency vector to address the challenges. Previous studies have demonstrated that multiple adjacent damage, which cannot be separated by conventional phased array, can be effectively identified by the FDA using Lamb waves. This is attributed to the ability of FDA to adjust the size of the focusing area (i.e., the in-phase gain area) by incorporating array signals of diverse frequencies. Similarly, the grating lobes, which result from the accidental in-phase gain of phased array signals, can be suppressed by the FDA. This is achieved by disrupting the in-phase gain through the introduction of signals with varying frequencies. By redesigning the anti-dispersive excitation with different frequencies, the dispersion compensation for Lamb waves and the frequency control of FDA can be integrated into a single step for damage imaging. The focusing capabilities of ordinary phased array and FDA are compared under various excitations and parameters. The ability of multiple damage identification and grating lobe suppression of FDA has been experimentally validated by characterizing damage under seven kinds of cases.