Surface porosity development in tool-side facesheets of honeycomb core sandwich structures during co-cure

Co-cure of honeycomb core sandwich structures combines the consolidation of composite prepreg facesheets with bonding of facesheets to a low-density core in a single thermal cycle for efficient manufacturing. The coupling of multiple process phenomena and the complex geometry of honeycomb cores, however, can lead to defects in cured parts. Effects of co-cure on void formation at the tool-side facesheet surface remain unclear. We employ autoclave processing and an in situ visualization technique to elucidate physical mechanisms by which surface porosity forms, and the effects of pressure on this formation. Results displayed a multi-stage development, including evacuation of entrapped air, followed by evolution and subsequent entrapment of dissolved volatiles in the prepreg resin, and demonstrated the utility of elevated pressure in reducing porosity. These findings describe the physics underlying porosity formation at the tool-facesheet interface, and provide insight into potential mitigation strategies to produce sandwich panels with defect-free facesheet surfaces.