## 4 Conclusions and Outlook  
Within this work, investigations were carried out to establish a correlation between initial defects in rotor-blade-leading-edge-like specimens and erosion damage after rain load to clarify the causes and mechanisms of premature damage. By modifying specimens that closely resemble the structure and the material of wind turbine rotor blade leading edges and impacting them in a rain erosion test facility, it was possible to generate realistic erosion damage in the laboratory. The investigations have shown that erosion damage occurs preferentially in those areas of the specimens that were already damaged before the rain load, for example, in the form of voids in the coating. Early erosion damage is  
Figure 22: Result presentation of a section of a specimen with voids in the coating as a comparison between the applied measurement methods, including visual images, height images, thermography (normalized raw image), and the Sensor 3D segmentation results of the CT data, in (**a**) unloaded and (**b**) loaded condition.  
thus due, among other things, to a non-ideal specimen condition or non-ideal condition of the rotor blade leading edge. The experiments with different measurement methods like CT and XRM show that the surface erosion damage below the surface is larger than what the surface damage would suggest. These results were obtained with the measurement method of active thermography as well. In addition, thermographic measurements can detect defects below the surface and on the surface within one image. Thus, in contrast to simple surface analysis like visual inspection, the entire damaged state of the specimen can be visualized and validated by the CT examinations. The feasibility study of thermography thus shows promising potential for future in-situ applications.  
In the further course of the investigations, the defect formation will be examined in more detail through cyclic erosion tests and the performance of intermediate examinations to make statements about the damage growth and to enhance our understanding of damage mechanisms. Furthermore, a quantitative evaluation of the damage condition in the different erosion stages is planned.  
Conceptualization, F.J., M.S. ([PERSON]), A.F., O.F., E.A.A., A.K., M.S. ([PERSON]) and C.T.; methodology, F.J., A.F., O.F., A.K. and C.T.; software, F.J., O.F. and E.A.A.; validation, F.J., O.F., E.A.A., A.K. and C.T.; formal analysis, F.J., O.F., E.A.A., A.K. and C.T.; investigation, F.J., O.F., E.A.A., A.K., C.T. and C.L.; resources, F.J., O.F., A.K. and C.T.; data curation, F.J., O.F., A.K. and C.T.; writing-original draft preparation, F.J., O.F., E.A.A., A.K. and C.L.; writing-review and editing, M.S. ([PERSON]), A.F., B.M., M.S. ([PERSON]) and A.H.; visualization, F.J., O.F., E.A.A., A.K., C.T. and C.L.; supervision, M.S. ([PERSON]), A.F., M.S. ([PERSON]), B.M. and A.H.; project administration, F.J., O.F., A.K. and C.T.; funding acquisition, M.S. ([PERSON]), O.F., C.T. and M.S. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Federal Ministry of Economic Affairs and Climate Action Germany (BMWK), grant number 20794 N/2, and the German Federation of Industrial Research Association (AiF).  
Not applicable.  
The authors would like to thank the 3M and AkzoNobel companies for the preparation of the test specimens used in this work. Furthermore, we would like to thank the company InfraTec for their advice and support during the thermographic measurements. In addition, thanks go to [PERSON] and [PERSON] for their surface measurement support.  
The authors declare no conflict of interest.  
The following nomenclature is used in this manuscript:  
\begin{tabular}{l l} AI & artificial intelligence \\ CT & X-Ray Computed Tomography \\ CNN & convolutional neural network \\ GFRP & glass fiber-reinforced plastic \\ ROI & region of interest \\ XRM & X-Ray Microscopy \\ \(t_{exc}\) & excitation time in [s] \\ \end{tabular}