# Thermographic Non-Destructive Evaluation for Natural Fiber-Reinforced Composite Laminates
## 5 Experimental Results and Analysis  
Figure 8 shows the PT results of BFRP. PPT was performed and then the depth information was obtained by Equations (1) and (2) using the physical properties in Table 5. The images from the depths of 0.28 mm (0.6 Hz), 0.85 mm (0.065 Hz) and 1 mm (0.048 Hz) were obtained in flashes and halogen lamps modalities, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Specimen** & **Experimental Method** & **Image Processing Modality** \\ \hline \multirow{4}{*}{BFRP} & PT & RMF\(\rightarrow\)CIS\(\rightarrow\)PCT/PPT \\  & VT & RMF\(\rightarrow\)PCT \\  & UT & Amplitude image \\ \cline{1-1}  & CW THz & Amplitude image\(\rightarrow\)Median filter \\ \hline \multirow{2}{*}{Jute/hemp hybrid laminate} & PT & RMF\(\rightarrow\)CIS\(\rightarrow\)PCT/PPT \\  & VT & RMF\(\rightarrow\)PCT \\ \hline \multirow{2}{*}{SCB} & PT & RMF\(\rightarrow\)CIS\(\rightarrow\)PCT/PPT \\  & VT & RMF\(\rightarrow\)PCT \\ \cline{1-1}  & LT & RMF\(\rightarrow\)Phase transform \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Applied experimental and image processing methods. Figure 8: PT for the BFRP laminates after PPT and PCT processing. In Figure 8a,b, the resin-rich areas A and C were detected at the depth of 0.28 mm, and were barely detected at the depths of 0.85 mm (Figure 8d,e), then were not detected at all at the depth of 1 mm (Figure 8g,h). The resin-rich areas B and F were detected at the depth of 0.85 mm (Figure 8d,f) in the clearest view; they were not detected at the depth of 1 mm (Figure 8g,i). The delamination in Figure 8h (marked in purple) was detected at the depth of 1 mm, which suggests its depth. The impact damage (marked in yellow) grew along with the depth increase. Overall, flashes modality provided clearer results than halogen lamps modality, and PPT can provide the additional information. In particular, it can provide the depth information via frequencies for quantitative analysis, which is advantageous. Taking into account the UT results in Figure 9, the PT results evidently provided more information, especially for resin-rich areas (marked in red). The VT results in Figure 10 only showed clear identification for delamination, but not for resin-rich and impact damage areas. The delamination was shown more clearly in the front-side inspection in Figure 11. PPT provided additional information (Figure 11e-g, in which the delamination shrinked along with the depth increase. CW sub-THz can detect the delamination, but showed a rough result (Figure 11c). This may be caused by the low resolution (\(\sim\)3 mm) linked to its low radiation frequency (\(\sim\)0.1 THz). The depth information in CW inspection is inaccessible. The intensity curve of the CW sub-THz result in Figure 11d showed the delamination feature, which is deepest in the central zone. The fiber information was not detected in BFRP. Figure 10: VT for the BFRP laminates after PCT processing. Figure 9: UT amplitude images of the BFRP laminates. Figure 12 shows the thermographic results of jute/hemp hybrid laminates. The results are linked to the flashes modality because it provides clearer results, and the full thickness can be detected due to the higher thermal diffusivity \(\alpha\). The images from the depths of 0.44 mm (1 Hz) and 1.72 mm (0.066 Hz) were obtained by PPT. Impact damage areas (marked in yellow) other than the visible zones were detected, but not as clear as that in BFRP. The resin-rich areas (marked in red) were clearly detected. The fibers can be identified very clearly, which provides the possibility for the evaluation of fiber orientation. PPT (Figure 12a-f) provided more information than PCT (Figure 12g-i) and VT (Figure 12j-l), especially for the identification of the flaws by means of defect depth retrieval via frequencies. PCT showed the clear identification for both resin-rich and impact damage areas, but the impossibility to perform a quantitative analysis is the disadvantage.