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In conventional laser ultrasonic (LU) single-mode frequency domain synthetic aperture focusing technique (F-SAFT) imaging, the precision of the imaged defect profile is limited due to the influence of the directionality of laser-induced ultrasound and the directionality of detection. To solve this problem, LU-based F-SAFT imaging with the shear wave, longitudinal wave, and mode-converted wave is proposed in this article. In the experiment, multimode bulk waves are excited by a pulsed laser, and a laser Doppler vibrometer is employed to detect ultrasound waves. The F-SAFT imaging is performed with single-mode and multimode ultrasound wave signals, respectively. The results show that F-SAFT imaging fusing three mode signals (shear, longitudinal, and mode-converted waves) improves the range of the upper surface of the imaged defect by nearly two times. In addition, the profile of the imaged defect is more complete than that of using a combination of both longitudinal and shear waves. By analyzing the influence of the directionality of laser-induced ultrasound and the detection directionality caused by the out-of-plane ultrasound component on the imaging results, a theoretical approach is used to select a reasonable excitation–detection distance for the use of multimode-fused F-SAFT algorithm. Furthermore, the feasibility of the theoretical approach is verified by the experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]