Vacuum-bag-only prepreg processing of honeycomb structures: From lab-scale experiments to an aircraft demonstrator

An aircraft demonstrator was manufactured using out-of-autoclave prepreg and vacuum-bag-only processing techniques. The demonstrator featured flat, double-curvature, joggles, abrupt geometry changes, and honeycomb inserts. The scientific literature was used to assess and mitigate the processing risks for this demonstrator. Low through-thickness gas permeability and ply-bridging in small radii corners were thought to be the most significant risks. The through-thickness permeability of the target material was undetectable, therefore spiking was used to increase gas evacuation. Unfortunately, the available mould had small radii that could not be increased, as a result ply bridging was observed in these corners. Miniature pressure sensors were successfully embedded in the demonstrator to monitor 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, and 2) non-uniform pressure response was observed in the part during cure. Visible part quality was acceptable, excluding small radii geometrical features. Internal part quality (voidage) was estimated using pulsed infrared thermography non-destructive-evaluation. While void contents are not exact, the thermography revealed local voidage can vary significantly over the part.