Fig. 1. Laser line source model based on thermoelastic effect. (a) Main view; (b) top view

Fig. 2. Heat source function of excitation laser

Fig. 3. Finite element model of laser ultrasound

Fig. 4. Meshing of titanium alloy plate. (a) Meshing of internal cracks; (b) meshing of surface cracks

Fig. 5. Distributions of ultrasonic sound fields at different moments in 3D model. (a) 160 ns; (b) 320 ns; (c) 660 ns; (d) 1660 ns; (e) 820 ns; (f) 1000 ns; (g) 1160 ns; (h) 1500 ns

Fig. 6. Section diagrams of interaction between ultrasonic wave and crack at different moments. (a) 1660 ns; (b) 1500 ns

Fig. 7. Differential B-scan images. (a) x-z section of internal cracks; (b) x-z section of surface cracks; (c) y-z section of internal cracks; (d) y-z section of surface cracks

Fig. 8. Reconstructed images of laser ultrasonic synthetic aperture. (a1)(a2) Internal cracks; (b1)(b2) surface cracks

Fig. 9. Multi-mode combination reconstruction of images

Fig. 10. H-shaped TC4 raw material

Fig. 11. Experimental process of laser ultrasonic positioning. (a) Overall diagram; (b) local diagram

Fig. 12. Time and frequency domain components of laser ultrasound signal after multivariate mode decomposition. (a) Time domain; (b) frequency domain

Fig. 13. Laser ultrasonic displacement curves at excitation points with different distances. (a) Comparison; (b) 2 mm; (c) 3 mm; (d) 4 mm; (e) 5 mm

Fig. 14. Laser ultrasonic displacement curves under different crack depths

Fig. 15. Laser ultrasonic displacement curves under different crack widths

Fig. 16. Fitting relationship between crack width and reflected wave peak value. (a) Raw data; (b) exponential fitting; (c) polynomial fitting; (d) linear fitting