Fig. 1. Schematic diagram of heat transfer near cracks under laser line of long pulse

Fig. 2. Schematic diagram of heat conduction in cracked air gaps

Fig. 3. Analog specimen design of crack width. (a) Schematic diagram of the structure; (b) Plug gauges; (c) Aluminum alloy slider; (d) Com-bined specimen

Fig. 4. Test platform for laser long pulse thermography. (a) Schematic diagram; (b) Test setup

Fig. 5. Thermal map of analog crack width under laser excitation

Fig. 6. Temperature rise distribution near the laser hot spot with different crack widths

Fig. 7. Variation in temperature rise of specimen surface. (a) Temperature rise over time; (b) Temperature rise amplitude ratio over time

Fig. 8. Distribution of specimen surface temperature rise. (a) Sampling line; (b) Distribution of temperature rise along the sampling line

Fig. 9. Centralized processing of the temperature rise distribution curve

Fig. 10. Absolute value of the first 3 principal components of centralized contour distribution

Fig. 11. Location distribution of the weight vector \begin{document}${\boldsymbol{ \nu}} $\end{document}

Fig. 12. Variation of \begin{document}$ \gamma $\end{document}with crack width

Fig. 13. Temperature rise curves at the location of the crack in the specimen

Fig. 14. Variation of \begin{document}$ \delta $\end{document} with crack width

Fig. 15. Variation law of index \begin{document}$\varLambda$\end{document} with crack width ( \begin{document}$ \varepsilon $\end{document}=25)

Fig. 16. (a) Thermal diagram of random crack width 1; (b) Thermal diagram of random crack width 2; (c) Temperature rise curve at random crack width 1 and 2