Fig. 1. Schematic of experimental system

Fig. 2. Schematic of position of excitation laser, probe laser and crack on surface of sample

Fig. 3. b-scan image of ultrasonic signal based on TOFD

Fig. 4. Time domain waveforms obtained by TOFD. (a) Excitation laser scanning to step 9; (b) excitation laser scanning to step 40

Fig. 5. Schematic of position of excitation laser, heating laser, probe laser and crack on surface of sample during real-time monitoring

Fig. 6. Schematic of experimental process

Fig. 7. First type of experimental results. (a) Real-time monitoring results of tR signals for cracks under different power heating and natural cooling conditions; (b) real-time monitoring results of tL-R signals for cracks under different power heating and natural cooling conditions

Fig. 8. Second type of experimental results. (a) Real-time monitoring results of tR signals for cracks under different power heating and natural cooling conditions; (b) real-time monitoring results of tL-R signals for cracks under different power heating and natural cooling conditions

Fig. 9. Third type of experimental results. (a) Real-time monitoring results of tR signals for cracks under different power heating and natural cooling conditions; (b) real-time monitoring results of tL-R signals for cracks under different power heating and natural cooling conditions; (c) real-time monitoring results of tR signals for cracks under 200 mW power heating and natural cooling conditions

Fig. 10. Three-dimensional AFM image of crack at 1.5 mm from starting point of crack (a) and variation of asperity height on crack wall along crack direction (b)

Fig. 11. Crack morphology characteristics corresponding to first type of experimental results

Fig. 12. Crack morphology characteristics corresponding second type of experimental results

Fig. 13. Crack morphology characteristics corresponding to third type of experimental results