A novel method for quantitative evaluation of defects from thermographic images

This paper illustrates a novel method for the detection, localization and sizing of partial detachment defects affecting thermal barrier coatings, using the Long Pulsed Thermography (LPT). The scientific literature provides many applications, proving the LPT effectiveness in providing clear and intelligible thermal image contrasts. The proposed technique consists of a series of operational steps to be directly implemented on the raw thermograms acquired form the specimen surface. The authors of references [1] and [2] have shown that the processing techniques, commonly used for Dirac delta heating pulse, can be applied to finite duration heating pulse. The core of this paper is to introduce a series of developments concerning the technique of the linear fit of the logarithmic time history of the specimen’s surface temperature, during the cooling stage. The defect detection effectiveness of the fit slope (m) and the determination coefficient (R2) will be significantly enhanced adopting an optimization algorithm to better utilize the spatial information deriving from thermographic data. Additionally, the authors introduce the fit intercept (q) and fit standard deviation (Sρ) as defect detectors. In the presented technique, the image segmentation plays a vital role in specimen analysis and defect identification. It is the process of sorting and classifying possible defective pixels into a number of regions that are not overlapped according to the characteristics of grey scale, colour, texture or shape, [3]. The proposed approach not only improves the detection resolution, but also facilitates automated selection of the suspected regions without any a priori knowledge about defect location. The experimental results strongly indicate that the algorithm can reliably extract the inner defect information and improves the contrast between the defect area and the sound area, allowing also an accurate and precise sizing of the detected defects. The study has been performed with experimental data from a campaign of measurements carried out on thermal barrier coatings (TBCs). The defect on which the attention has been focused, is the debounding between the coating and the substrate, a very common structural defect of TBCs, [4].