Defect imaging based on laser ultrasonic frequency domain synthetic aperture focusing technology with separated generation–detection and 2-D equivalent velocity mapping

An accurate frequency-domain synthetic aperture focusing technology (F-SAFT) based on 2-D equivalent velocity mapping is proposed for laser ultrasonic testing (LUT) to detect and image internal defects in specimens. The equivalent velocity mapping varies with depth and spatial frequency to match the characteristics of the separated excitation and detection commonly used in LUT. Bulk acoustic waves are non-destructively excited by a line-shaped laser source and are detected by a laser Doppler Vibrometer. By compensating for the distance shift arising from the separation of the laser spots, the imaging defect location is corrected. Several optimisations of the algorithm are carried out: the calculation time of the equivalent velocity is reduced by simplifying its expression, and the imaging quality, especially for defects with a shallow depth, is improved by the 2-D equivalent velocity mapping. The results show that compared with traditional F-SAFT, the optimised F-SAFT algorithm can not only improve the imaging quality but also locate the defect more accurately with separated excitation and detection. • F-SAFT with various velocity mappings for separated excitation and detection. • 2-D velocity mapping with high calculation efficiency. • Improved imaging quality of shallow defects by optimising equivalent velocity. • Corrected defect’s horizontal location by compensating imaging shift.