# Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure
## 3 Experimental Set-Up  
### Measurement Object  
For the sake of similarity to a real rotor blade leading edge of a wind turbine, the test specimen to be examined is accordingly adapted with respect to the geometry and the structural design. The left picture in Figure 2 shows the schematic structure of the test specimen. It consists of a hand-laminated GFRP half pipe with a length of 230 mm and an inner radius of \(r_{\text{i}}=11\) mm. The specimen has a GFRP wall thickness of \(s_{\text{GFRP}}=3.25\) mm. It consists of 4 layers of fiberglass laminate in biaxial \(\pm 45^{\circ}\) and unidirectional arrangement, with 815 g/m\({}^{2}\) per layer, and epoxy resin is used as matrix resin. While wind turbine rotor blades are often built with more layers of the composite material and thicker wall layers, the test specimen's materials are identical with those of real rotor blades. The coating system of the test specimen consists of a filler and a gel coating, both consisting of polyurethane. The filler and the coating are applied to the GFRP and have a combined thickness of approximately 0.5 mm. This coating system is applied in exactly the same way as in real rotor blades, so that the entire test specimen has a similar thermographic behaviour as the leading edge of a wind turbine rotor blade. The outer radius of the specimen including the coating system is 14.75 mm. For comparison, a typical blade profile of wind turbines, such as the NACA 4415 or the NACA 23015 [32], at a chord length of 1 m in the middle of the blade has a leading edge radius of about 25 mm. A chord length of 0.5 m at the tip region of the blade (where the load is the highest) the leading edge has a radius of about 12 mm [33]. Therefore, the present specimen radius is found in the outer third of the rotor blade towards the tip area.  
To imitate initial defects, the specimen is prepared with different drillings. In this case, voids are imitated by the application of standard drillings with a tapered tip, because it  
Figure 2: **(a)** schematic representation of the test specimen structure, **(b)** photograph of the rear side of the test specimen with seven drillings to imitate voids in the GFRP.  
closely matches the shape of trapped air bubbles near the surface. The drillings are placed at the rear side of the half pipe, so that when viewed from the front side, the drillings are at different depths \(h_{j}\) below the surface. This type of specimen preparation is easy to implement, allows reproducible tests and is used as standard method, in this case for the first time on curved and coated specimens. The right image in Figure 2 shows the rear side of the sample, divided into two regions of interest (ROI) A and B. Drillings 1 to 3 are approximately 1.5 mm below the surface and have a diameter of \(d_{1}=3\) mm, \(d_{2}=2.5\) mm and \(d_{3}=2\) mm, respectively. Drillings 5 to 7 are located approximately 2.5 mm below the surface and have diameters of \(d_{5}=3.5\) mm, \(d_{6}=3\) mm and \(d_{7}=2.5\) mm. This results in depth-to-diameter ratios between 0.5 and 1.04; see Table 1. Drilling 4 is a through-hole and serves as a marker for a better orientation in the thermograms.