Fig. 1. Principle diagram of THz-TDS

Fig. 2. FRP internal structure

Fig. 3. Defect design

Fig. 4. Optical path of terahertz waves in typical three-layer structure

Fig. 5. 3D spectral data cube

Fig. 6. Local equivalent diagram of prototype

Fig. 7. Phase delay suppression graph

Fig. 8. THz time-domain signal power spectrum of FRP debonding simulation sample

Fig. 9. Long and wide B-Scan images of trapezoidal wedge groove positions. (a) 3# row B-Scan image; (b) 3# column B-Scan image; (c) 2# column B-Scan image; (d) 1# column B-Scan image

Fig. 10. Images obtained by THz reflection tomography for FRP at different flight time. (a) Flight time is 132.5 ps; (b) flight time is 166.3 ps; (c) flight time is 200.7 ps

Fig. 11. THz waveforms at three different depths at normal and defect regions. (a) Normal region; (b) wedged-groove defect region; (c) PTFE-film defect region

Fig. 12. FRP defect maps at different depths after image enhancement. (a) Upper layer; (b) middle layer; (c) lower layer

Fig. 13. THz time-domain waveforms after fringe suppression. (a) Waveform of upper defect after fringe suppression; (b) waveform of middle defect after fringe suppression;(c) waveform of lower defect after fringe suppression

Fig. 14. FRP defect maps at different depths after fringe suppression. (a) flight time is 134.2 ps; (b) flight time is 166.7 ps; (c) flight time is 200.7 ps