Multi-frequency Fused Lock-in Thermography in Detecting Defects at Different Depths

Lock-in thermography (LT) is a non-destructive evaluation method capable of inspecting defects in composites by calculating differences of the thermal amplitude and phase between sound and defective regions. This method greatly reduces influences of non-uniform heating and emissivity in detection, while a single excitation frequency limits the depth resolution of the subsurface defects, as defects at a specific depth can only be detected with a specific excitation frequency. In this paper, a multi-frequency fused LT is proposed to improve the depth resolution and obtain better inspection results. Sequential infrared images are acquired under multiple excitation frequencies and then the phase information is extracted along with a specified enhancement scheme. Finally, a fusion image containing defects with multiple depths is constructed by using principal component analysis method. The proposed method has been applied to characterization of the flat-bottomed holes in a carbon fiber reinforced polymer (CFRP) panel and compared with other active thermography techniques. To improve the detection efficiency, an optimal selection method of excitation frequency is also proposed. The experimental results show that the use of the proposed method is able to identify all defects at multiple depths with clear edges and high contrast.