Depth detection of spar cap defects in large-scale wind turbine blades based on a 3D heat conduction model using step heating infrared thermography

The defects dispersed in a spar cap often lead to the failure of large-scale wind turbine blades. To predict the residual service life of the blade and make the repair, it is necessary to detect the depth of spar cap defects. Step-heating thermography (SHT) is a common infrared technique in this domain. However, the existing methods of SHT on defect depth detection are generally based on 1D models, which are unable to accurately detect the depth of spar cap defects due to ignoring material anisotropy and in-plane heat flow. To improve the depth detection accuracy of spar cap defects, a 3D model based on the theory of heat transfer is established by using the equivalent source method (ESM), and a defect depth criterion is proposed based on the analytical solution of the heat conduction equation. The modeling process is as follows. The heat conduction model of SHT was established by ESM. Then, coordinate transformation, variables separation, and Laplace transformation were utilized to solve the 3D heat conduction equation. A defect depth criterion was proposed based on emerging contrast Cr. A glass fiber reinforced plastic composite plate containing 12 square flat-bottom holes with different sizes and depths was manufactured to represent a spar cap with large thermal resistance defects, such as delamination and cracks. The experimental results demonstrate the validity of the 3D model. Then, the model was applied to an on-site SHT test of a 1.5 MW wind turbine blade. The test results prove that the depth detection accuracy of spar cap defects can be significantly improved by using the 3D model. In addition, by using an improved principle component analysis (PCA) method containing a contrast enhancement factor, artifacts can be reduced and the recognition time of defects can be shortened. The 3D model provides a tool for detecting the depth of deep-lying defects in a thick composite structure, and the SHT technology is optimized by improved PCA.