# Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure
## 5 Conclusions and Outlook  
For the investigation of the rotor blade leading edge of wind turbines, long-pulse thermography was presented as a measuring method, which is suitable for the detection of initial inhomogeneities below the surface. A GFRP test specimen with a curved surface, an applied coating system and modified with different drillings served as a reference object. The structural and material design as well as the geometrical dimensions correspond to a real rotor blade leading edge, which allows for a transferability of the results to reality. To reduce side effects like inhomogeneous heating or thermal reflections in the thermographic image as well as to increase the contrast, PCA was used as the image processing method. All output images were quantified and analysed by calculating the CNR value of each detected defect. The investigations have shown that  
* Sub-surface defects can also be detected on strongly curved and coated test specimens, which are similar in size and structural design to the leading edge of a rotor blade in the tip area. Figure 10: Results of test series 3 (\(\beta=45^{\circ}\), ROI A, \(t_{\text{exc}}=25\,s\), \(t_{\text{rec}}=15\,s\), \(k=89\,\text{mm}\), (**a**) surface temperature of the whole camera FOV, (**b**) result for the reference method without filtering at \(t_{\text{rec}}=0\,s\), (**c**) reference image with filtering, automatic defect detection and calculation of the CNR, (**d**) result for the PCA-based method without filtering, (**e**) PCA-based image with filtering, automatic defect detection and calculation of the CNR. Sub-surface defects with a depth-to-diameter ratio of up to 1.04 were detected, which exceeds the previously detected depth-to-diameter ratios of up to 0.825 on flat plates. * The determined CNR value depends significantly on the depth at which a defect is located rather than the diameter of the defect. * Sub-surface defects that have an angular offset from the optical axis of the camera and are therefore subject to the object's curvature can also be detected, but with a decreasing CNR value as the viewing angle increases. * There is an optimal excitation time depending on the depth of a defect: For a defect with a depth of 1.5 mm, an excitation time of 15 s to 25 s was found to be optimal. For defects at a depth of 2.5 mm, no optimal excitation duration could be determined based on the available experimental data. For a complete examination of the leading edge, the effects of different surface curvatures, eccentric inhomogeneities as well as typical defects like voids in the coating system or delaminations in the GFRP should additionally be examined regarding their detectability. In addition, modified test specimens will be exposed to rain loading to observe the damage evolution of the sub-surface defects and provide a prediction for the size as well as the timing of the occurrence of a surface defect. For further investigations concerning an optimal excitation duration as a function of the defect depth, it is therefore recommended to perform parameter studies by means of experiments and simulations. In addition, it is recommended to apply other evaluation methods in addition to PCA, such as pattern and edge detection, to provide defect detection suitable for on-site applications. Finally, investigations in the open field at the rotor blade leading edge of a real wind turbine are to be carried out to validate the in situ capability of the proposed measurement method. Conceptualization, F.J., M.S. and A.F. ; methodology, F.J.; software, F.J. and M.T. ; validation, F.J. and M.T. ; formal analysis, F.J.; investigation, F.J. and M.T. ; resources, F.J., M.S. and A.F. ; data curation, F.J. and M.T. ; writing--original draft preparation, F.J.; writing--review and editing, M.S. and A.F. ; visualization, F.J. and M.T. ; supervision, M.S. and A.F. ; project administration, F.J.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.