A joint work with colleagues from the University of Bologna (Italy) and Fraunhofer IKTS (Dresden) has been accepted for publication in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control.
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 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 artefacts. To tackle this aspect, literature offers a beamforming algorithm called Double Stage DMAS (DS–DMAS), firstly introduced in photoacoustic imaging and medical ultrasound imaging. In this paper, the DS–DMAS performance is analysed for Lamb waves inspection, in order 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 artefacts and enhance the quality of the imaging. The algorithms have been compared and characterized by exploiting an online free data set for guided waves inspection (http://openguidedwaves.de/) which collects piezo pitch–catch signals travelling through a quasi-isotropic Carbon Fiber Reinforced Plate (CFRP) at different actuated frequencies and damage positions.
Malatesta, M.; Neubeck, R.; Moll, J.; Tschoeke, K. & De Marchi, L., Double Stage DMAS with Fresnel Zone Filtering in Guided Waves Damage Imaging, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2022 (accepted in March 2022)