# Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure
## 1 Introduction  
The rotor blade leading edge of a wind turbine is heavily loaded by the high rotational speed (over 300 km/h at the blade tip) and the environmental mechanical influences such as rain, wind or dust. The resulting damage due to erosion leads to an increase in surface roughness at the leading edge, which changes the aerodynamic properties of the rotor blade negatively [1]. Such partial surface damages lead to premature transitions of the boundary layer flow from laminar to turbulent [2], which has a significant influence on the degree of efficiency of the wind turbine [3] and thus leads to losses of the annual energy production [4]. The repair of a rotor blade leading edge with severe damage is expensive and complicated and the quality of the repair is strongly dependent on the experience of the technicians. Furthermore, poorly executed repairs bring new susceptibility to erosion during operation. Therefore, various solutions to protect the leading edge have been developed, such as coatings, leading edge tapes and erosion shields (erosion shields are currently not used because an attachment solution is missing [5]). Nevertheless, erosion damage still occurs. Regular monitoring and early maintenance of the rotor blade is therefore recommended. The rotor blade itself consists of two half-shells glued together and supported by central thrust webs. The half-shells are made of a glass fiber-reinforced plastic (GFRP) strengthened by a sandwich structure of foam or balsa. Carbon fiber-reinforced plastic (CFRP) chords in the center of the rotor blade and ribs in the rear blade area provide increased stiffness. The rotor blade is coated with a polyurethane-based coating system. The first few millimeters of the leading edge material thus consist of the coating system andGFRP laminate. Studies indicate that initial sub-surface defects, such as voids, delaminations and low resin content in the GFRP, voids or foreign particles in the coating system and at the boundary layer between coating (or tape) and GFRP are responsible for premature erosion [6; 7]. Voids occur, for example, as a result of air infiltration during the production of GFRP laminates, when the coating or the leading edge tape is applied. Foreign particles, e.g., dust, that become trapped in the coating during its application are also production defects [8]. Since material defects are stress raisers that can provoke rotor blade damages, they need to be investigated in detail. The very first step is to enable the detection of sub-surface material defects, which then allows to study the defects' potential to result in premature erosion damage as well as to study the damage progression under rain loading. Therefore, a non-destructive measuring technique is required, which enables the contactless detection of near-surface defects in coated and curved GFRP and is applicable on rotor blades of wind turbines even in the open field. To examine a rotor blade leading edge regarding production and material defects, a non-destructive and contactless measurement technique is required that can detect blade defects in the near-surface area within or underneath a coating. These defects include voids, delaminations, inclusion of foreign particles or low resin content. They range in lateral size from around 1 m for delaminations to a few millimeters or even micrometers for voids or trapped dust particles, although the effect of the defect size on the later damage development is not yet clear.. Furthermore, the measuring technique should have imaging capabilities to enable the fast investigation of the entire leading edge in as few sections as possible. Several different non-destructive measuring techniques for the investigation and monitoring of wind turbine blades exist, such as acoustic emission measurements, guided waves, ultrasonic testing, strain measurements, thermographic measurements, radiography testing or visual inspection [9]. Indeed, visual inspection is the standard maintenance method, but sub-surface defects remain hidden.