Non-Destructive Inspection of Adhesively Bonded Joints using Amplitude Modulated Thermography

The goal of this work is to design thermal waves specifically for pulsed-phase infrared thermography (PPT) of bonded joints and to test their effectiveness on adhesive lap joints in carbon-fiber reinforced polymers structures. The method was applied to both single and double lap joint specimens to test different defect depths and multiple lap joint interfaces. An optimal thermal wave was designed for each specimen type based on the blind frequency information obtained from square pulse thermal wave excitation of the lap joint specimens. Modifications to the optimal thermal wave were then implemented based on the physical limitations of the lamp systems and different threshold settings for the blind frequency calculation. Thermal input waves based on the linear combination of discrete frequency waves were also implemented. The discrete frequency waves improved the defect image contrast in both the single and double lap joint specimens, while the optimal thermal input wave had the opposite effect and instead highlighted surface features. The difference in performance between the two thermal input waves is presumed to be due to the ability to separate frequency components in the transformation of data and not due to a critical thermal energy input threshold.