Fig. 1. Schematics of laser induced ultrasound. (a) Thermoelastic mechanism; (b) ablation mechanism

Fig. 2. Principle diagram of laser ultrasound bilateral line scanning detection method

Fig. 3. Surface morphologies of rail steel specimen (including crack). (a) Macro morphology of rail steel specimen (including crack);

Fig. 4. Schematic of laser ultrasound experimental system

Fig. 5. Comparison of ultrasound waveforms before and after noise reduction

Fig. 6. Calibration diagram of standard values for reflected waves

Fig. 7. Left B-scan images of crack before and after noise reduction. (a) Before noise reduction; (b) after noise reduction

Fig. 8. Single-side line scan imaging of crack. (a) Left side of crack; (b) right side of crack; (c) enlarged image of scanning image on left side of crack; (d) enlarged image of scanning image on right side of crack

Fig. 9. Dual-side line scan imaging of crack

Fig. 10. Comparison of ultrasonic waveforms with and without cracks during left line scanning of crack

Fig. 11. Scanning position-maximum amplitude difference graphs obtained by line scan on both sides of crack. (a) Left side of crack;