# Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure
## 1 Introduction
also carried out investigations on a curved CFRP specimen modified with flat bottom holes using step-heating thermography [22]. In the cooling phase, inserted defects with a depth-to-diameter ratio of 0.25 could be detected. Curved GFRP specimens with an inner radius of 92 mm were investigated by [PERSON] et al. [23]. Using long-pulse thermography, delaminations in the form of teflon strips inserted into the laminate as well as the effect of static loads on the specimen were mapped experimentally and in simulations. Although the tests were performed on curved specimens, the used specimens did not possess a coating nor a strong enough curvature to be able to make comparisons with a real rotor blade leading edge in the tip section. To extract the information that is contained in the raw thermograms efficiently, signal processing algorithms are applied. The most common advanced post-processing methods are thermographic signal reconstruction, PCA, absolute thermal contrast and phase Fourier analysis [24; 25]. These data processing methods reduce thermal image artefacts, which are caused by non-uniform heating or thermal reflections from the environment. In this way, the detectability of defects is increased and a quantification is possible [26]. [PERSON] et al., for example, performed a comparison of pulse and step-heating thermography on a coated GFRP sample. They concluded that the collected data during both phases (warm-up and cooling) of step-heating thermography with a subsequent evaluation of the phase images (generated by Fourier transformation) gave the best results [27]. Defects larger than 4 mm could be detected in a depth of up to 2 mm, which is a depth-to-diameter ratio of 0.5. [PERSON] et al. made a quantitative comparison of phase, step-heating, and long-pulse thermography [18]. For this purpose, they likewise modified a GFRP plate with flat drilled holes of 10 mm to 2.5 mm diameter, which were located between 0.5 mm and 3 mm below the surface to be excited. Phase image analysis as well as a PCA for all excitation methods were used for the data evaluation. The results of long-pulse thermography in combination with a PCA revealed most defects. Thus, defects with a diameter of 5 mm at a depth of 2 mm and defects with a diameter of 2.5 mm at a depth of 1.5 mm were detected, which results in a depth-to-diameter ratio of 0.4 and 0.6, respectively. [PERSON] et al. have achieved similar results by the combination of long-pulse thermography and PCA on a GFRP plate modified with flat bottom holes [16]. [PERSON] et al. performed a case study on a GFRP plate modified with various flat bottom holes by using long-pulse thermography with halogen lamps as excitation [15]. Different heating times between 5 s and 40 s were applied to the test specimen. With the use of PCA, defects with a depth-to-diameter ratio of 0.825 (depth = 6.6 mm, diameter = 8 mm) could be detected. Phase Fourier analysis as well as PCA are widely used and are established image processing methods that extract the desired information from thermograms and have therefore been used in various research papers. While PCA uses general orthogonal functions, which provide a sparse representation of the thermographic data, phase Fourier analysis works on the basis of harmonic functions. Since in long-pulse thermography the sample is excited with only one light pulse, oscillatory signals in the thermographic data are less pronounced, which is why the application of PCA is more obvious here. To assess the signal quality, the signal-to-noise ratio (SNR) is a typically used figure of merit. The higher the SNR, the better the signal is distinguished from background noise. For the quantification of the defect detectability in thermograms, a modified version of the SNR is often used, in which the contrast between two regions of the image (e.g., defect-affected area and defect-free/surrounding area) is defined as a signal and related to the surrounding noise [15; 17].