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A laser ultrasonics (LU) based frequency domain synthetic aperture focusing technology (LUB-F-SAFT), which uses 2D equivalent velocity (SEV) and fuses multi-mode ultrasound, is developed to image the internal horizontal hole defects with a diameter of ∼ 1. 0 mm in bimetallic composites. Bulk waves non-destructively excited by a line-shaped laser are detected by a vibrometer away from the excitation. The results indicate that the LUB-F-SAFT with SEV can improve the imaging quality and locating capability, and the testing error is about 0. 02 mm. The improvement in the imaging quality using SEV for different ratios of the first layer thickness to defect-to-interface distances is discussed. In addition, a theoretical approach based on the directionality of the detected ultrasound is applied to determine a reasonable excitation-detection distance for better defect imaging. A LUB-F-SAFT imaging combining multi-mode waves is employed to improve the range of defect profiles. The results show that the range of the imaged defect profile becomes larger using the multi-mode fused LUB-F-SAFT imaging, which is an improvement of about 49.2% compared to the shear wave imaging result. • LUB-F-SAFT imaging with SEV is developed for bilayer media. • The SEV corrects distortion and position bias of defect. • A optimized excitation-detection distance for better defect imaging. • A multi-mode fused LUB-F-SAFT improve the profile of the defect. [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]