A comparative study on submillimeter flaws in stitched T-joint CFRP by infrared thermography , micro-CT , ultrasonic c-scan and microscopic inspection

Stitching is used to reduce dry-core (incomplete infusion of T-joint core) and reinforce T-joint structure. However, it may cause new types of flaws, especially submillimeter flaws. In this paper, microscopic inspection, ultrasonic c-scan, pulsed thermography, vibrothermography and laser spot thermography are used to investigate the internal flaws in a stitched T-joint CFRP. Then, a new micro-laser line thermography is proposed. Micro-CT is used to validate the infrared results. A comparison between micro-laser line thermography and micro-CT is performed. As a conclusion, micro-laser line thermography can detect the internal submillimeter defects. However, the depth and the size of defects can affect the detection results. The micro-porosities with a diameter of less than 54 μm are not detected in the micro-laser line thermography results. Micro-laser line thermography can detect the micro-porosity (a diameter of 0.162 mm) from the depth of 90 μm. However, it cannot detect the internal micro-porosity (a diameter of 0.216 mm) from the depth of 0.18 mm. The potential causes are given. Finally a comparative study is conducted.