Fig. 1. Acquisition and DFT of infrared image sequence by infrared detector after long pulse thermal wave excitation

Fig. 2. Schematic of panel with internal defects at different depths

Fig. 3. Simulation test results. (a) Phase data and reference phase data at corresponding points; (b) curve of phase difference at each point and corresponding integrated regions

Fig. 4. Flow chart of image sequence processing method for long pulse thermography

Fig. 5. GFRP sample. (a) Schematic of defect distribution in GFRP panel; (b) front and rear surfaces of sample

Fig. 6. Self-developed infrared non-destructive testing system

Fig. 7. Raw thermal images captured at different times. (a) 5 s; (b) 15 s; (c) 25 s

Fig. 8. Test results of GFRP sample. (a) Phase curves; (b) phase difference curves

Fig. 9. Phase images at different frequencies. (a) 0.019 Hz; (b) 0.039 Hz; (c) 0.078 Hz; (d) phase integrated image

Fig. 10. Images for size measurement of defects. (a) Image segmentation; (b) edge extraction

Fig. 11. Detection results with different algorithms. (a) ATC; (b) TSR; (c) PCA; (d) proposed method

Fig. 12. Quantitative evaluation of different algorithms. (a) Absolute grayscale difference profiles in horizontal line of defects; (b) SNR

Fig. 13. Test results of defective and sound regions. (a) Variation of temperature difference with time; (b) variation of R_SN with sampling frequency

Fig. 14. Detection results at different sampling frequencies. (a) 1 Hz; (b) 5 Hz; (c) 10 Hz; (d) 30 Hz