# Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure
## 1 Introduction
Some of the other methods require a variety of sensors to be mounted on the rotor blade and are therefore contact-based. Other measurement methods are time-consuming, only suitable for conductive material or have a complex measurement setup [10]. In the recent years, however, thermography has been proven to be a useful non-destructive measuring technique method for the inspection of rotor blades. Thermography works without contact, offers a simple measuring setup, has sufficient imaging capabilities (fast image acquisition of large areas) and the provided image results in the form of thermograms can be directly interpreted during inspection [11]. Furthermore, it enables the detection of sub-surface defects [10], which include production and material defects. In active thermography, the test specimen is first heated by an energy source and in the subsequent cooling phase, the differences in the temperature distribution on the surface of the test specimen are recorded with an infrared camera and visually displayed in a thermogram [12]. Different active thermography methods like pulse, step-heating and long-pulse thermography have been developed, which differ in their excitation and recording routine. In pulse thermography, the sample is excited for a few milliseconds (mostly 2 ms to 10 ms) with a flash lamp at high power. The thermograms are recorded during the cooling phase. In contrast, long-pulse and step-heating thermography principles excite the sample for several seconds (ranging from 5 s to 300 s and more) using halogen lamps at lower power. In long-pulse thermography, the recording takes place during the cooling phase while in step-heating thermography the thermograms are already recorded during the excitation in the warm-up phase [13]. These different active thermography methods have been used in various investigations to obtain defect detection for rotor blades of wind turbines. To evaluate the thermographic detectability of defects, the depth-to-diameter ratio is used [14]. It results from the depth at which the defect is located below the surface divided by the corresponding defect diameter. Due to the lateral heat transfer that takes place in the material between the surface and the defect, a defect with a depth-to-diameter ratio of \(\geq\)1 is considered to be barely detectable. In most experimental investigations, flat and uncoated reference objects (GFRP and CFRP) modified with flat bottom holes were studied [15; 16; 17; 18] to reduce unfavourable influences on the results due to a curved geometry or a coated surface. [PERSON] et al., for example, carried out investigations on a GFRP plate modified with flat bottom holes [19]. Both pulse thermography and step-heating thermography were applied. In the case of step-heating thermography, the cooling phase was also considered in addition to the warm-up phase, which corresponds to the principle of long-pulse thermography. By comparing the different types of excitation, it was found that pulse thermography is particularly well suited for the investigation of near-surface defects while step-heating thermography emphasizes deeper defects. The results of the raw data of both thermographic methods show that defects with a diameter of 4 mm in a depth of 2 mm could be detected, which equals a depth-to-diameter ratio of 0.5. As long-pulse thermography has a higher sensitivity for defects in low thermal conductivity and low thermal response rate materials (such as GFRP) than pulse thermography, long-pulse thermography is preferred for the detection of deeper-lying defects [20]. In addition to the investigations on flat plates, there are also approaches for the thermographic investigation of curved surfaces. [PERSON] et al. used pulse thermography to investigate impact damage on a curved CFRP specimen that was exposed to both thermal loads and ballistic impacts [21]. Both principal component analysis (PCA) and specially trained neural networks were able to detect the damage. [PERSON] et al.