Low energy impact damage detection in CFRP using eddy current pulsed thermography

The carbon fiber reinforced polymer (CFRP) is widely used in aircraft and wind turbine blades because of their high strength and low weight. The CFRP is sensitive to impact damage and detecting damages in CFRP is important for ensuring the structural safety. This study is conducted to detect impact damages using eddy current pulsed thermography (ECPT). The impact damages were artificially produced by impact energies of 4 J, 6 J, 8 J and 10 J, respectively. The structural impact damages lead to non-uniform distribution of eddy current in CFRP excited by coil with high frequency alternating current. The eddy current can be represented by heat distribution according to Joule's law. And the damages also influence the transmission of heat in the CFRP. The change of heat is represented by surface temperature recorded in the form of thermal image sequence by IR camera. Then defects are evaluated by analyzing the heat distribution and patterns in thermal images. In addition, due to the 4 J energy is low, the damage size of the CFRP is small and cannot be observed directly from the original thermal images. In order to distinguish the small defects, a multi-resolution statistical analysis method, employing the wavelet transform combined with principal component analysis (PCA), is used to extract the defective characteristics of impact damage. Through this method, the information of thermal images has been improved to distinguish defects and the detectability of impact energy producing damage are also discussed.