Double-Stage DMAS With Fresnel Zone Filtering in Guided Waves Damage Imaging

: Digital beamforming methods in plate-like structures are widely exploited for Lamb waves-based damage imaging. Among them, the delay and sum (DAS) imaging technique is the most popular thanks to its low-computational cost and ease of implementation. However, the imaging outputs are low quality due to the high levels of side lobes and limited off-axis signal rejection, which leads to limited image resolution and contrast. Recently, the delay multiply and sum (DMAS) beamforming has been applied to nondestructive testing (NDT) field as a promising DAS alternative able to enhance the imaging reconstruction in terms of contrast and damage detectability. However, DMAS is still affected by high levels of artifacts. To tackle this aspect, literature offers a beamforming algorithm called double-stage DMAS (DS-DMAS), first introduced in photoacoustic imaging and medical ultrasound imaging. In this article, the DS-DMAS performance is analyzed for Lamb waves inspection, to provide an exhaustive comparison between DAS, DMAS, and DS-DMAS. As a further step, a filtering process addressed as Fresnel zone filtering (FZF) is used to restrict the beamforming partial sums in a physical way to the area around the scattering point. The proposed approach is an adaptation of a well-established technique in seismic data processing called Fresnel migration, able to suppress artifacts and enhance the quality of the imaging. The algorithms have been compared and characterized by exploiting an online free dataset for guided waves inspection (ht.tp://openguidedwaves.de/) which collects piezo pitch-catch signals traveling through a quasi-isotropic carbon fiber-reinforced plate (CFRP) at different actuated frequencies and damage positions.