Fig. 1. Ultrasonic imaging models. (a) Traditional laser ultrasound SAFT method; (b) improved laser ultrasound SAFT method

Fig. 2. Schematic of equivalent synthetic aperture

Fig. 3. Comparison of rectangular and circular defect waveforms

Fig. 4. Schematic of single defect numerical calculation model

Fig. 5. Schematic of grid division

Fig. 6. Differential pre and post ultrasound signals detected by defect model and defect free model at N=75. (a) Differential pre ultrasound echo signal; (b) differential ultrasound signal

Fig. 7. B-scan before and after signal amplitude difference calculation. (a) B-scan of defective model; (b) differential signal B-scan

Fig. 8. Comparison before and after Hilbert transform

Fig. 9. Comparison before and after maximum linear interpolation processing

Fig. 10. SAFT imaging before and after averaging processing. (a) Before processing; (b) after processing

Fig. 11. Imaging results of single defect model. (a) B-scan imaging; (b) unprocessed SAFT imaging; (c) pre-processed SAFT imaging

Fig. 12. Normalized amplitude at defect imaging location. (a) Longitudinal depth; (b) lateral width

Fig. 13. Schematic of multi defect model

Fig. 14. Imaging results of the multiple defect model. (a) B-scan imaging; (b) unprocessed SAFT imaging; (c) pre-processed SAFT imaging

Fig. 15. Normalized amplitude at defect imaging location. (a) Longitudinal depth; (b) lateral width