Detection of very thin defects in CFRP by the lock-in thermography method

Quick development of constructional composite materials application is caused by their excellent mechanical and strength-related properties, combined with a low specific weight. One of the basic groups of reinforcement materials in composites are carbon fibres discovered back in 19th century. The main reason of defects in structures of composite materials is the variability of working charges in constructions during the process of using. Existed defects are complicated because of the effects like loss of continuity of reinfused fibres, binder cracks and loss of fibres adhesiveness to binder. Diagnostic methods, which are effective with relation to metals became little effective when used in detection of defects in composite materials. This caused greater interest of diagnostic techniques with using infrared thermography. Lock-in thermography is one of NDT methods providing phase images of thermal waves in a sample leading to receiving a distribution of internal defects and allowing for thermal properties evaluation. We used lock-in thermography in connection with modulated thermal source synchronized with the IR image acquisition camera. It was prepared sample of multilayer structure carbon composite with deliberately introduced defects for comparative purposes. Very thin defects of different sizes and shapes made of Teflon or copper having a thickness of 0.1 mm were searches. The results are reported in the paper.