Thermally Assisted Consolidation Of Out Of Autoclave Prepreg For Concave Parts

"Vacuum Bag Only, Out-of-Autoclave prepreg processing offers a sustainable, cost-efficient alternative to conventional autoclave manufacturing of high strength composite parts. Flat or low curvature parts made with Out-of-Autoclave prepregs have demonstrated comparable mechanical properties to their autoclave counterparts, yet challenges still remain with the fabrication of parts with more complex features. One area of concern is corner thickening in concave corners. Best practices for reducing this defect exist; however, a more robust processing technique is desirable. In this work, a manufacturing process is proposed that involves heating the laminate during the vacuum bag compaction at regular intervals in the layup process. The aim of these heated debulks is to reduce corner thickening early on in the manufacturing process by encouraging inter-ply slipping, precipitating air removal, and increasing the degree of impregnation.Before the process could be designed, its potential was assessed through material characterization and demonstrator studies. Inter-ply friction was characterized for debulked samples using pull-out tests, allowing for a better understanding of the forces required to encourage prepreg slip. Dynamic mechanical analysis was used to assess changes in the degree of cure caused by debulking, and found a slight increase in curing with increased debulk temperature and duration. Investigating the removal of air from a tool-part interface showed no impact of heating on air removal rate. It was observed that air returned to the plies if vacuum was released when the resin viscosity was low.Initial trials were run on flat plates. There was a decrease in short beam shear strength as the debulk dwell temperature increased. Meanwhile, low temperature hot debulks led to an increase in voids. L-brackets were manufactured on a concave tool. The hot debulk led to a 25% reduction in corner thickening and a 80% reduction in void content compared to the part debulked at room temperature. Such improvements in quality motivated the development of a more robust design for a hot debulk process. This procedure consisted of using an infrared lamp to heat the prepreg, dry carbon fibre fabric as breather to absorb the heat while evacuating gases, and a bath of dry ice used to cool the tool before the vacuum bag was vented.Parts were then made using this procedure with different debulk durations, temperatures, and frequencies. Longer hot debulks led to the greatest reduction in corner thickening, although improvements were observed for short, high temperature debulks. Long, high temperature debulks led to an increase in voids. Fewer debulks led to thicker corners and higher void contents, regardless of debulk temperature. Short beam shear strength was not impacted by the debulk procedure.A better understanding of the relationship between debulk time and temperature was necessary so an existing model was modified to simulate the trends observed. There are several barriers, both theoretical and empirical, that need to be overcome before this model can be used for process refinement. The work presented here contributes to the field in being among the first publicly available literature to methodically develop a detailed hot debulk procedure and to establish processing conditions to lead to improvements in part quality." --