Manufacture of Composite Tube Spar Wing Assembly Manufacturing Demonstration Units with Matrix Reflow Joints

The manufacturing process for a tubular spar wing assembly is presented with focus on a novel spar-to-skin bonding technique. Two manufacturing demonstration units (MDU) are described. Each MDU consisted of two tubular spars bonded to a skin panel. Both composite spars had the same laminate configuration but were manufactured by different methods. In this study, three of the four spars were created by using steel clamshell tooling. Plies were laid down onto two halves of the steel clamshell tools and then spliced together when the tools were closed together; next the plies were envelope-bagged to the steel tools and cured in the autoclave. The last spar was manufactured by wrapping plies around a wash-out mandrel, which was removed after cure. The wash-out process involves removing the tool by spraying pressurized water. The spars were bonded to a wing-skin panel in a secondary cure process. During the initial cures of the spar and skin panel, each of the surfaces that were to be bonded during the secondary process had an outer prepreg layer containing a higher epoxy volume fraction than typical prepreg plies. This epoxy was un-reacted during the initial cure. During the secondary cure, prepreg with additional hardener was added at the joint interface, along with a layer of adhesive. This bonding process resulted in a joint comparable to a co-cured assembly rather than a traditional adhesive bond. The two different MDUs had subtle variations in the secondary bonding process. Coordinate measurement machine (CMM) three-dimensional (3D) scans of the two MDUs revealed that the MDU that used an envelope bag during secondary cure was more warped than the MDU that had been bagged to a flat tool surface. Hardener rich ply orientation may have also played a role in level of warp observed in the MDUs.