Fig. 1. (a) Excitation controller module;(b) excitation driver module; (c) resonant coil for eddy current thermal excitation; (d) eddy current excitation device in service

Fig. 2. Physical maps of test specimens.(a) Groove-like defects with different depths;(b) circular defects with different depths

Fig. 3. Variation of thermal excitation efficiency factor with resonant frequency

Fig. 4. Variation of thermal excitation efficiency factor with lift-off distance

Fig. 5. Variation of thermal excitation efficiency factor with input power

Fig. 6. Relationship between groove-like defect depth and average grayscale difference

Fig. 7. Relationship between circular defect depth and average grayscale difference

Fig. 8. Relationship between circular defect depth with average grayscale difference when the depth is less than 7 mm

Fig. 9. Infrared thermal images of verifying specimens.(a) Groove-like defect with 5 mm depth;(b) circular defect with 6 mm depth