Fig. 1. Excitation mechanisms of laser ultrasound. (a) Thermoelastic mechanism; (b) ablative mechanism; SAW (c) generation and (d) detection mechanisms of laser ultrasound based on wavefront regulation

Fig. 2. Numerical simulation model. (a) Schematic diagram of simulation model; (b) spatial configuration diagram of original coordinate system (x, y, z) and rotating coordinate system (X, Y, Z); surface displacements due to SAW propagation when α is (c) 0° and (d) 90°, respectively

Fig. 3. Time-domain and frequency-domain spectra of SAW. Time-domain spectra of SAW obtained at detection points in (100) crystal planes when (a) α=90° and (b) α=0°, respectively; (c) frequency domain spectra of SAW for α from 0° to 90°

Fig. 4. SAW velocity curve varying with angle in (100) plane

Fig. 5. Time-domain and frequency domain spectra of SAW in (001) crystal plane. Time-domain spectra of SAW obtained at detection points when (a) α=90° and (b) α=0°, respectively; (c) frequency domain spectra of SAW for α from 0° to 90°

Fig. 6. SAW velocity varying with angle in (001) plane

Fig. 7. Optical path of laser ultrasound microimaging system based on wavefront regulation and photo of laser spots on sample surface. (a) Optical path of laser ultrasound microimaging system based on wavefront regulation; (b) photo of laser spots on sample surface

Fig. 8. SAW signals collected by system. (a) Time domain spectrum and (b) frequency domain spectrum of SAW when θ is 0°; (c) time domain spectrum and (d) frequency domain spectrum of SAW when θ is 90°

Fig. 9. SAW velocity curve varying with angle θ

Fig. 10. SAW velocity distributions on surface of monocrystalline and polycrystalline nickel-based superalloys. (a) Monocrystalline; (b) polycrystalline