## Materials and Methods  
### Preparation of the Test Specimens  
For the investigations, a single base type of test specimen is manufactured, and different kinds of initial defects are introduced. The basic structure of the specimen is adapted from a real rotor blade leading edge with a laminated GFRP half tube as a supporting structure. On the GFRP, filler and coating are applied, as shown in Figure 1.  
To recreate realistic rotor-blade-leading-edge-like specimens, different defects such as voids and delaminations are introduced into different layers of the test specimen, labeled in the following with the corresponding letter from Table 1. The GFRP test specimens were manufactured using the vacuum infusion process. The laminate structure on the specimen surface is \(\pm 45^{\circ}\) with a structure of the scrim (surface) as biaxial, biaxial, unidirectional, and biaxial. The specific gravity of the fibers is biaxial \(830\) g/m\({}^{2}\) and unidirectional \(990\) g/m\({}^{2}\) with a fiber volume content between \(50\%\) and \(55\%\). The resin system used was RIM 135 with harden  
Figure 1: Schematic representation of the basic specimen structure with the modification of individual layers.  
The deviations in the GFRP composite material (**D**) were already introduced during the layer-by-layer buildup of the GFRP test specimen. For this purpose, round Teflon foil cutouts with a diameter of 7 mm-10 mm were positioned at defined points between the GFRP layers, see Figure 2b. The Teflon foil provides local debonding between the individual fiber layers. The styrofoam balls for imitating voids in the coating system (S) have a diameter of \(\sim 1\) mm \(\pm\) 0.4 mm and were introduced manually directly during the coating process of the filler, see Figure 3. Due to the electrostatic properties of the styrofoam balls, they had to be adhered to the specimen surface using a thin underlayer. This underlayer was applied as thinly as possible to the putty material using a flexible comb. On top of the underlayer with applied styrofoam balls, a covering layer of the putty material was applied. After a sufficiently long drying time, the putty was ground off mechanically to produce the geometrically correct shape of the specimens and to generate a sufficiently rough surface in preparation for the coating of the specimens afterward.  
The introduction of voids in the coating (**B**) was carried out with the same styrofoam balls, which were first positioned on an undercoat (on top of the putty) of the material applied as thinly as possible before, in the next step, the test specimens including the applied styrofoam balls were poured with coating. Note that the specimen with the applied styrofoam balls in the coating had to be overpoured with several layers to completely cover the applied styrofoam balls. Therefore, the specimens with this initial defect have a significantly higher coating thickness compared to the other specimens.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Sign** & **Modification** & **Introduction Method of the Modification** \\ \hline
**L** & Voids in GFRP & Introduction by air intake during GFRP lamination \\ \hline
**D** & Delamination in GFRP & Introduction by Teflon film between individual laminate layers \\ \hline
**B** & Voids in coating & Introduction of styrofoam balls withing the coating \\ \hline
**S** & Voids in filler & Introduction of styrofoam balls withing the filler \\ \hline
**R** & Reference specimen & No explicitly introduced initial defects \\ \hline \hline \end{tabular}
\end{table}
Table 1: Overview of the different test specimen modifications.  
Figure 2: Photograph of part of the GFRP specimen with initial defects as (**a**) introduced voids (within the red circle) and (**b**) introduced delamination, adapted with permission from [19].