Fig. 1. Time spectrum diagram based on db4 wavelet transform. (a) Defect depth 0.5 mm, angle 20°; (b) defect depth 1.5 mm, angle 40°; (c) defect depth 3.5 mm, angle 140°; (d) defect depth 10 mm, angle 100°

Fig. 2. Network structure diagram

Fig. 3. Meshing of the finite element simulation model

Fig. 4. Time-domain signal and locally enlarged image of reflected wave with different depth defects. (a) Time-domain signal; (b) locally enlarged image

Fig. 5. Time-domain signal and locally enlarged image of reflected wave with different angle defects. (a) Time-domain signal; (b) locally enlarged image

Fig. 6. Schematic diagrams of experimental platform. (a) Principle diagram of laser non-destructive testing; (b) physical image of laser ultrasonic testing system; (c) top view of metal sample; (d) detailed diagram of laser parameter settings

Fig. 7. Laser ultrasonic signals obtained on specimens with different defect widths, depths and angles and comparison of experimental and simulation data. (a)‒(c) Laser ultrasonic signals; (d) comparison of experimental and simulation data

Fig. 8. Defect depth and angle prediction of validation set and test set. (a) VGG19 network loss function change diagram; (b) ResNet101 network loss function change diagram; (c) DenseNet169 network loss function change diagram; (d) VGG19 model validation set prediction result diagram; (e) ResNet101 model validation set prediction result diagram; (f) DenseNet169 model validation set prediction result diagram; (g) VGG19 model test set prediction result diagram; (h) ResNet101 model test set prediction result diagram; (i) DenseNet169 model test set prediction result diagram; (j) prediction results of VRD+SVR model on simulation data; (k) prediction results of VRD+SVR model on experimental + simulation data

Fig. 9. Prediction results of different methods