Experimental validation of analytical chemistry methods for detecting contaminants on composite surfaces

There is a continuous need for the improvement of robust quality control methods for adhesive bonding processes that will lead to improved safety, durability, and both operational and material economics. This is because there are grave concerns of surface contaminants that can potentially denature the composite surface activity, thereby creating weaker bonds between an adhesive and adherent. Although conventional non-destructive inspections (NDIs) may mitigate the possibility of having adhesive bonds with detectable composite defects and cracks, they are incapable of providing full assurance of bond strength and long-term durability because the weak bonds due to the surface contamination are on a molecular level out of the sensitivity of the NDIs. This study analyzes two cost-effective and practical surface analysis techniques for detecting chemical contamination on the surface of carbon fiber reinforced composites: 1) a chemical force microscope with an epoxy modified probe and 2) an all solid-state electrochemical sensor. Sample composite coupons were manufactured using either polyester or nylon peel-plies and bonded together. Lap shear test results indicate that the shear strength of the polyester peel-ply samples were significantly higher than that of the nylon peel-ply samples. Correlations between shear strengths, adhesion forces and surface activity were investigated. Results from the atomic force microscopy (AFM) force curve analysis indicate that using a random approach for determining the specific locations for analysis is not sufficient and a more systematic technique is required to obtain reliable data. Although the modified AFM tip could differentiate between anomalies on the composite surface, additional analysis is required to draw correlations. Results obtained from the solid-state electrochemical sensor using electrochemical cyclic voltammetry show that there are differences in surface activity between the nylon and polyester peel-ply prepared coupons, demonstrating the potential for the sensor as an in-field service tool.