Halogen Optical Referred Pulse-compression Thermography for Defect Detection of CFRP

Abstract In this paper, pulse-compression thermography nondestructive testing method is proposed for defect detection in carbon fiber reinforced composite in reflection mode, using halogen lamps as excitation source. It is known that care should be taken when pulse-compression algorithm is applied to data obtained using halogen lamps. This because standard commercial halogen lamps suffer from high on/off switching delay time, thus they are not well-following the time behavior of the coded waveform employed for the heating emission modulation. The hereby-proposed method, named “Referred Pulse-Compression Thermography”, relies on subtracting the step heating contribution from the recorded coded thermal sequence before applying the pulse-compression algorithm. In particular, the step heating contribution is recorded separately by mean of an additional measurement. It is shown that the proposed method is inherently immune to the halogen-lamp switching time delay as both the reference recorded step signal and the coded signal sequence are affected by the same setback value. The proposed method can effectively enhance the defect information and improving the thermal contrast between defect and non-defect areas when halogen lamps are used in combination with pulse-compression in reflection mode. The validity of the proposed method is verified by experimental results and a comparison study with step heating thermography on different composite samples, having complex shapes and embedded artificial defects at different depths.