Development of a Rapid Drop Test for Surface Quality Inspection

In material bonding, the surface energy of the adherend can be an excellent predictor of adhesive bond strength. In this research, the use of the sessile drop test as a pre-bond inspection tool for bonded composite repairs was investigated. A portable goniometer suitable for a maintenance workshop in the aviation industry was designed and built. Dimethyl sulfoxide and water were used as testing fluids, dispensed in 3 ul quantities. Contact angles were measured using the spherical approximation for a range of surfaces which included samples that had been peel-plied, hand sanded, grit blasted and contaminated with silicone release agent. The surface energies of the samples were then evaluated using the Owens-Wendt and Girifalco-Good-Fowkes-Young (GGFY) methods. Although the GGFY method successfully distinguished contamination, difficulties arose when evaluating surface roughness. Peel-ply surfaces also had varying contact angles due to irregular droplet shapes. The Owens-Wendt method was also found to be inaccurate with only two test fluids although this is regarded as industry standard, It is for these reasons that traditional surface energy methods were considered impractical for a workshop environment. An alternate measurement parameter considered was the droplet diameter where the same testing fluids were imaged from the top instead. This measurement successfully differentiated contamination and also allowed the abrasion process to be determined. In addition, the degree of abrasion could be determined between grit blasted and untreated peel-ply surfaces. This test was found to be more accurate than the traditional surface energy techniques and has the potential to form part of a pre-bond surface certification process for bonded composite repairs.