Comparative study on submillimeter flaws in stitched T-joint carbon fiber reinforced polymer by infrared thermography, microcomputed tomography, ultrasonic c-scan and microscopic inspection

Abstract

Abstract. 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.

Pulsed micro-laser line thermography on submillimeter porosity in carbon fiber reinforced polymer composites: experimental and numerical analyses for the capability of detection

“…However, these techniques cannot detect submillimeter porosity because the distance between the specimen and the camera is usually small (approximately 4.5 cm) when using a microlens, which is difficult for the experimental configurations. Flying spot laser thermography used to detect the subsurface submillimeter flaws in previous composites work was reported [1,2]. However, the results revealed that the technique cannot work on micro-sized flaws.…”

“…The authors proposed micro-laser line thermography (pulsed micro-LLT) [1], which was verified to work for subsurface submillimeter flaws in composites [2]. In this article, pulsed micro-LLT was used to detect the subsurface submillimeter porosities in a 3D preformed CFRP specimen for the evaluation of detection capabilities.…”

“…Since these artifacts are more or less constant during the whole acquisition, including before heating when the image is cold, this image or the average of several images can be subtracted before heating, so their effect can be reduced [2]. Figure 3 shows an example with an academic aluminum plate.…”

“…Large-sized porosities in composites can be detected using classical IRT techniques including pulsed thermography and lock-in thermography [1,2]. However, these techniques cannot detect submillimeter porosity because the distance between the specimen and the camera is usually small (approximately 4.5 cm) when using a microlens, which is difficult for the experimental configurations.…”

“…However, the results revealed that the technique cannot work on micro-sized flaws. The authors proposed a new micro-laser line thermography (micro-LLT) technique [1], which was verified to be suitable for subface submillimeter flaws in composites [2].…”