Evaluation of Sandwich Structure Bonding In Out-of-Autoclave Processing

The out-of-autoclave-vacuum-bag-only (OOA-VBO) process is low in capital expenditures compared to the traditional autoclave, however, the material challenges for OOA-VBO workable material systems are high. Presently there are few such aerospace grade prepreg materials available commercially. In this study, we evaluated processing and properties of honeycomb sandwich structure (HC/SS) panels fabricated by co-curing composite face sheet with adhesives by the OOA-VBO process in an oven. The prepreg materials were IM7/MTM 45-1 and T40-800B/5320. Adhesives studied were AF-555M, XMTA-241/PM15, FM-309-1M and FM-300K. Aluminum H/C cores with and without perforations were included. It was found that adhesives in IM7/MTM 45-1/AF-555M, T40-800B/5320/FM 309-1M and T40-800B/5320/FM-300K panels all foamed but yielded high flatwise tensile (FWT) strength values above 8,275 kPA (1,200 psi). IM7/MTM 45-1/XMTA-241/PM15 did not foam, yet yielded a low FWT strength. SEM photomicrographs revealed that the origin of this low strength was poor adhesion in the interfaces between the adhesive and face sheet composite due to poor wetting associated with the high initial viscosity of the XMTA-241/PM15 adhesive.