Vacuum-Bag Manufacturing of Honeycomb Structures: Opportunity for In-Situ Process Monitoring and Non-Destructive Voidage Estimates

A helicopter crew door was manufactured using out-of-autoclave prepregs and vacuum-bag-only processing techniques. Non-metallic honeycomb core was used to stiffen the structure. Miniature pressure sensors were embedded in the demonstrator to measure the honeycomb core pressure throughout the lay-up, including intermittent de-bulking, the pre-cure vacuum hold, and the elevated temperature cure. The sensors identified two insightful process phenomena: 1) gas evacuation increased with additional plies, implying that de-bulking may increase a skin's air permeability, thereby decreasing total process time, and 2) a non-uniform pressure response was observed in the part during cure, potentially leading to variations in part quality. Visible part quality was acceptable, excluding small radii geometrical features, a known source for defects in bag processing of woven materials. Internal part quality (voidage) was estimated in the honeycomb regions using pulsed infrared thermography non-destructive-evaluation. Calibration curves were generated by comparing thermography to micro-computed tomography images of the same samples. Macro-porosity around fibre bundles was identified as the major voidage source in these structures, and the thermography inspection identified significant local voidage variations over the part.