The lack of practical quality assurance techniques for qualifying the surface of an advanced composite for adhesive bonding is a major impediment to more widespread use of composite materials in aircraft design.  Surface energy is potentially the most critical predictor of performance for composite/composite adhesive bonds, but surface energy measurements on surfaces that have been roughened by sanding or grit blasting are very difficult when using multi liquid contact angle techniques. We have shown that the wetting behavior of a single probe liquid chosen to have surface tension characteristics very similar to the uncured adhesive can be an excellent predictor of subsequent adhesive bond performance.  This obviates the need for precise surface energy calculation.  When the wetting behavior of the probe liquid is quantified by measuring the diameter of a small drop of known volume, the technique becomes practical for application in a manufacturing environment using simple apparatus.  This drop diameter technique provides fundamental information about the relationship between contamination, surface energy and performance for adhesively bonded composites.

Graphite-reinforced composites with varying surface energy were created by grit blasting followed by controlled contamination with silicone based mold release.  The wetting behavior of small drops of DMSO (with surface energy characteristics very similar to epoxies) was studied as a function of contamination level.  The diameter of a drop of DMSO of known volume proved to correlate very well with the amount of contaminant as well as the fracture toughness and failure mode of adhesive joints prepared from these surfaces.  The ability of the adhesive to imbibe small amounts of contamination resulted in a threshold amount of contaminant, below which the fracture toughness was insensitive to contaminant.  However, these levels of contamination are readily detected using the drop diameter technique.  This work provides the basis for a QA tool currently under development.