Noncontact laser vibrometry-based fence-like arrays with wavefield filtering-assisted adaptive imaging algorithms for detecting multiple pits in a compact cluster

Pitting corrosion presents challenges for ultrasonic nondestructive evaluation due to the small pit dimension. Few Lamb wave-based techniques have achieved the identification of individual pits as subwavelength wave scatterers that are densely packed in a small cluster. In this article, noncontact laser vibrometry-based fence-like arrays with wavefield filteringassisted adaptive imaging algorithms are developed for detecting and identifying small pits in a cluster. Signals of back scattering waves induced by a cluster of subwavelength scatterers are acquired by noncontact laser Doppler vibrometry at sensing points and form a fence-like array surrounding the area of inspection. The signals are then processed by our array imaging algorithms to construct inspection images which take advantage of three techniques, including the wave mode and wave direction filters to extract single-mode back scattering Lamb waves induced by subwavelength scatterers, the pseudo-reversal propagation of back scattering waves to address the dispersion effect and improve the radial imaging resolution, and the adaptive weighting to improve the angular imaging resolution. Moreover, this work introduces the wave diffraction-related Rayleigh and Abbe limits that are conventionally used for characterizing optic lenses, for characterizing the resolution limit of Lamb wave-based arrays, and optimizing the array configuration. To validate our array imaging approach, a proof-of-concept experiment has been performed to detect a cluster of 3 3 3 pits with the pit diameter of 2 mm and the interval of 2 mm in a 3.2-mm thick aluminum plate; the experimental imaging result shows that our method can identify most pits except the one at the center of the pit cluster. We believe this study will benefit the design, characterization, and optimization of Lamb wave-based arrays for subwavelength resolution imaging and enable potential applications for the noncontact inspection of hidden pitting corrosion in civil, petrochemical, nuclear, and aerospace structures.