Detection of Surface Contaminants on Aerospace Structural Composites Prior to Adhesive Bonding

Aerospace structural composites, owing to their light weight and advantageous combination of mechanical properties, have enabled a substantial advancement in the fabrication of commercial aircraft. These composite materials have allowed optimization in aerospace design by reducing weight, thus enabling the construction of aircraft that have excellent fuel efficiency and release fewer emissions. Costs in aerospace design and manufacturing can also be decreased by the use of composite materials. Further advancement of composite materials depends on the advancement of adhesive bonding and related processes. Adhesive bonding provides for a large stress-bearing area, fatigue resistance, and high strength-to-weight ratio. By adhesively bonding aerospace composite structures, riveting and mechanical fasteners can be minimized or eliminated, and the necessity of structural alteration to fit the mechanical fasteners can be reduced, thereby taking maximum advantage of the inherent mechanical capabilities of the composites.
In this dissertation, laser induced breakdown spectroscopy (LIBS) and optically stimulated electron emission (OSEE) were studied as surface characterization techniques for the detection of trace contamination levels of silicone on carbon fiber reinforced polymer (CFRP) composites. An OSEE instrument developed by NASA was used to characterize the composite surfaces. Prior to adhesive bonding, the CFRP surfaces were coated with different silicone thicknesses to determine the effect of silicone on the fracture characteristics and failure modes of the bonded structures. The OSEE experiments were performed before and after laser surface treatment under various conditions. The failure modes from the double cantilever beam (DCB) tests were correlated with the OSEE characterization results.
This research advanced the field of LIBS by investigating the mechanistic interaction of laser energies below 100 μJ, referred to as μLIBS. A novel LIBS system was designed, assembled, and used to experimentally characterize CFRP surfaces before and after laser surface treatment. The ability to detect and resolve levels of silicone contaminants on CFRP surfaces has been achieved in both bulk and thin films. The surface sensitivity of LIBS to detect ultralow level contaminants has now been proven. Time-resolved analysis was conducted to determine the optimal emission conditions of plasma plumes for LIBS. An ultraviolet (UV) picosecond laser source was used to determine the single pulse ablation threshold of CFRP substrates comprised of an epoxy matrix and carbon fibers.
LIBS and X-ray photoelectron spectroscopy (XPS) results were correlated to study the sensitivity and limit of detection (LOD) of LIBS to detect silicone contamination on CFRP surfaces before and after laser surface treatment. Different surface conditions, and thus different residual silicone concentrations, were produced by varying the laser powers. The CFRP surfaces were subsequently characterized by LIBS and XPS. It was found that LIBS demonstrated comparable surface sensitivity to that of XPS. Thus, LIBS could readily detect silicone concentrations below those known to be a threat to adhesive bonding. Overall, it was demonstrated that LIBS is an extremely sensitive, rapid, and practical technique to ensure that the CFRP surfaces are adequately prepared for adhesive bonding.