Non-destructive testing of metal plates by guided wave propagation image using scanning sound source technique and airborne ultrasound phased array with suppressed grating lobe generation

We have been investigating non-destructive testing using a scanning airborne ultrasound source and a nonlinear harmonic method, and we have examined using an airborne ultrasound phased array (AUPA) to achieve high-speed non-destructive testing by electronically controlled scanning of the sound source. However, AUPAs can generate unnecessary foci, such as grating lobes (GLs), in addition to the main lobe (ML) generated at the focal point. In non-destructive testing, artifacts occur in the results because the ML and GLs excite the measurement object simultaneously. The GL generation can be suppressed by adjusting the spacing of the AUPA ultrasound emitters, which can decrease the generation of artifacts. In this paper, suppression of GL generation by using an AUPA and the reduction of artifacts in non-destructive testing results were verified by an experiment examining. We imaged a pseudo-defect on a metal plate by visualizing guided wave propagation without using an acoustic window.