Epifluorescent microscopy of edge-trimmed carbon fibre-reinforced polymers : an alternative to computed tomography scanning

X-Ray computed tomography (XCT) can be used to detect edge-milled carbon fibre-reinforced polymer (CFRP) defects. Significantly this method is able to show subsurface defects that cannot be captured by traditional methods such as stylus-based or more novel areal methods of surface quality measurement. While useful, this method can be prohibitive due to high equipment cost, scanning time and image resolution. XCT can often produce artefacts which falsely predict damage or obscure damage and depending on machine X-ray power often cannot resolve damage to fibre diameter which is critical when observing milled quality of the surface/subsurface. This study utilises epifluorescent (EF) optical microscopy to provide high-quality optical images as an alternative to XCT to observe through-depth damage of CFRP materials. The method of computing the novel damage criteria is presented, as well as the validation of the method which compares EF to XCT. Subsurface damage of fabric and unidirectional (UD) materials in 0°, 45°, 90° and −45° orientations to the cutting edge is observed to demonstrate typical defects. A novel metric resulting from the EF method provides a total area of damage when compared to a theoretically straight cut across the face of the edge-milled CFRP. The method shows that different subsurface damage exists for different fibre orientations to the cutting edge, highlighting the clear need for through-depth analysis of machined edges. In addition, the method is shown to be a suitable alternative to XCT with scope for further development of industrial aerospace and automotive quality control of machined CFRP parts.