Study on Lock-in Thermography Defect Detectability for Carbon-Fiber-Reinforced Polymer (CFRP) Sheet with Subsurface Defects

A simple theoretical expression is described to demonstrate the thermal response characteristics of the thermal wave field in the frequency domain by means of the Green's function method. The defect detectivity using lock-in thermography (LIT) for a carbon-fiber-reinforced polymer (CFRP) with a subsurface defect is investigated, and theoretical analysis results of the defect depth resolution as well as the detection range determined by the thermal wave phase information have shown good agreement with experimental results. The detectable size of a lamination defect in a CFRP has also been studied using lock-in thermographic imaging. The experimental results indicate that LIT is reliably available for evaluating subsurface defects of a CFRP panel at the appropriate modulation frequency, and the defect diameter-to-depth ratio can be as low as 3.0.