Processing and properties of composite-to-composite bonding using Scotch-Weld TM AF-555M structural adhesive were investigated. Bonding surfaces of T800H/3900-2 composite were prepared by co-curing the dry and wet peel-plies. Surface topologies of the peel-plies and the co-cured composite surfaces were examined by microscopy, contour mapping using a coordinate measuring machine equipped with a ruby sphere probe, and contact angle goniometry. Curing of the adhesive was conducted in an autoclave or vacuum press at 177°C (350°F) for 2 h under 310 KPa (45 psi). Common bagging practices for composite fabrication in an autoclave were followed. It was found that a prolonged vacuum application (i.e., overnight) prior to the application of temperature and pressure was a critical element to produce porosity-free, high-quality bonds with this adhesive system. Following this procedure, a strong bond line was consistently produced, which routinely provided a single-lap shear strength more than 10% higher than the nominal value of the adhesive (i.e., 35.9 MPa or 5200 psi) when tested at room temperature. An adhesive failure mode at the interface was noted on the fractured surfaces of specimens with strong bonds whereas a premature cohesive failure mode was more evident for the specimens with weaker bonds, probably due to porosities in the bond lines. Photomicrographs showed that the weak single-lap shear strengths occurred on specimens with significant porosity in the bond line, apparently caused by entrapped air from insufficient vacuum application prior to curing. The results of this study are discussed herein.

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