# Condition Monitoring of Wind Turbine Blades Using Active and Passive Thermography
## 5 Results and Discussion
### Passive Thermography  
Passive thermal imaging of the damaged blade section was conducted at different times of the day. Typical results are shown in Figure 13. The results demonstrated that passive thermography was capable of capturing cracks, delamination, and internal features of the blade section. Early morning experiments provided a visible contrast of the defects on the defect plate attached to the damaged blade section primarily due to the considerable temperature change on the blade during this period [2; 18; 19]. All defects in the plate, except the smallest 4 mm ones, were detected during this period. Less useful information about the defects was obtained when the experiment was performed around noon. None of the defects were visible during the evening (around 6 pm), which was mainly due to the balanced temperature on the blade surface after several hours of heating. Thermal contrasts associated with dirt (glue on the surface), within the blue box in Figure 13, were most pronounced at noon. These contrasts faded during the afternoon. The blade's internal features such as shear webs were detected as cold regions during the morning and at noon but became hot signatures during the evening after several hours of heating. Cracks and delamination on the suction side were apparent near the blade's trailing edge. At noon, with peak sunlight, cracks and delamination were detected. Delamination in the upper area, identified \begin{table}
\begin{tabular}{l c c c c c} \hline  & **1** & **2** & **3** & **4** & **5** \\ \hline
**A** & 38.5 (183\%) & 30.9 (144\%) & 29.2 (127\%) & 27.6 (180\%) & 23.6 (774\%) \\
**B** & 36 (302\%) & 32 (160\%) & 29.2 (132\%) & 27.5 (208\%) & 23.2 (896\%) \\
**C** & 21.5 (1211\%) & 26.3 (707\%) & 27.8 (164\%) & 26.6 (580\%) & 22.9 (907\%) \\
**D** & 19 & 13.3 (1044\%) & 18.3 (827\%) & 17.7 (710\%) & 12.7 \\ \hline \end{tabular}
\end{table} 
Table 10: SNR (dB) of phase image captured during 75 s heating. \begin{table}
\begin{tabular}{l c c c c} \hline  & **1** & **2** & **3** & **4** & **5** \\ \hline
**A** & 33.6 (106\%) & 41.9 (127\%) & 40.8 (120\%) & 38.3 (179\%) & 31.6 (834\%) \\
**B** & 26.9 (163\%) & 38.5 (142\%) & 37.6 (121\%) & 34.7 (191\%) & 29.2 (915\%) \\
**C** & 20.3 (706\%) & 26.2 (527\%) & 30.1 (130\%) & 28.7 (484\%) & 24.2 (842\%) \\
**D** & 8 & 13.1 (537\%) & 12.7 (455\%) & 10.8 (416\%) & 9.4 \\ \hline \end{tabular}
\end{table} 
Table 11: SNR (dB) of amplitude image captured during 75 s heating. \begin{table}
\begin{tabular}{l c c c c} \hline  & **1** & **2** & **3** & **4** & **5** \\ \hline
**A** & 46.2 (80\%) & 45.1 (81\%) & 42.2 (85\%) & 38.4 (98\%) & 29.7 (141\%) \\
**B** & 41 (68\%) & 41.3 (106\%) & 39.6 (94\%) & 36.7 (109\%) & 28.7 (140\%) \\
**C** & 33.1 (349\%) & 33.7 (231\%) & 34.6 (128\%) & 33.4 (190\%) & 25.9 (338\%) \\
**D** & 14.9 & 23.5 (809\%) & 21.2 (763\%) & 17 & 13.6 \\ \hline \end{tabular}
\end{table} 
Table 9: SNR (dB) of amplitude image captured during cooling after 75 s heating. by the green box in Figure 13, were not detected clearly during the morning. The evening thermograms did not provide any information regarding cracks and delamination, so noon was the best time for crack and delamination monitoring. The transform-based technique, discussed in Section 5.2.2, SPAT, was employed for the first time to increase the quality of the passive thermography results. The phase images captured using this method were not sensitive to non-uniform heating. The FFT was applied to passive thermograms obtained at morning. The results are shown in Figure 14. Part (b) illustrates that the amplitude images considerably increased the quality and visibility of the visualized subsurface defects, as the visibility of cracks, delamination, and a large portion of the flat-bottomed hole defects was improved. Phase images have noticeably increased the contrast of shear webs signatures, marked by red arrows in Figure 14a. Cracks and delamination are marked by white boxes in Figure 14b.