## 3 Results and Discussion
### Analysis of the Erosion Evolution Using Active Thermography  
A feasibility study of thermographic measurements is carried out to show the potential for later in-situ monitoring. Figure 21 shows a specimen section's thermograms before and after rain exposure as an example.  
Before rain loading, the introduced defects appear as bright or hot spots on the specimen, see Figure 21a. Since the thermal conductivity of air with 0.0262 W/(m\(\cdot\)K) is significantly lower than the thermal conductivity of the coating with 0.2 W/(m\(\cdot\)K) (estimated value based on polyurethane as base material, since no data is available for the coating in this respect), the heat introduced during excitation accumulates in front of the defects. Here, the coating heats up more strongly than in areas without defects. The thermogram after rain loading shows evolved surface damages (A) in addition to sub-surface defects (B). The surface damages are indicated by lower temperatures or darker areas; see Figure 21b. Special attention should be paid to the pinhole in the center of the specimen at pixel position \(y=600\) px (C). Here, according to the bright border area of the defect, the damage below the surface is larger than it is visible at the surface in a visual image.  
For the validation of the thermographic measurement method, a comparison of the different measurement methods is shown in Figure 22.  
Figure 21: Thermogram of a specimen section with the environment at an excitation time of \(t_{exc}=10\) s; (**a**) unloaded condition; (**b**) loaded condition. The red boxes A–C show different defects; A: large surface damage, B: subsurface defects, C: pinhole.  
Figure 22a shows the image section in the unloaded state. While no irregularities can be seen in the visual image and the 3D topography, the thermogram shows irregularities in the form of hot spots. These irregularities can also be seen in the CT. Since the specimen has inserted voids, it can be assumed that exactly these initial defects were detected. Therefore, the specimen's condition is not ideal, even if it appears to be so from the outside.  
Figure 22b shows the same specimen section after a 4 h exposure to rain. In the visual image and the 3D profilometer, defects are now clearly visible on the surface of the specimen. In the thermogram, these defects can also be declared as surface defects in the form of dark spots. It is noticeable that the surface defects are surrounded by a bright border, which would mean that the defect under the surface is significantly larger than visible at the surface. A look at the CT image confirms that the defects have grown in size compared to before rain loading. Here, the defect size in the CT differs from the defect sizes in the visual images, indicating that the defects have also grown below the surface, which validates the thermographic results.  
The investigations show that it is possible to generate realistic erosion damage in the laboratory. Furthermore, the experiments show that premature erosion starts from initial defects in the test specimen and then enlarges to different sizes after rain loading. The CT and the XRM measurements show that surface defects are often bigger than visible at the surface. The thermographic feasibility study shows the same results. Therefore, only visual inspection of the rotor blade is not sufficient for integrated condition assessment of rotor blade leading edges. Furthermore, the thermographic investigations show that not only sub-surface defects but also surface damage can be visualized in a single thermographic image, and an additional visual camera is unnecessary.  
To conclude, the condition of a rotor blade's leading edge should therefore be inspected before it is used on a wind turbine. Active thermography, in particular, offers great potential as a measurement method since it is highly flexible compared to CT and can be used at various locations, including in the open field.