Bonding quality monitoring of Carbon Fiber Reinforced Plastics bonded structures by fiber Bragg gratings

This paper deals with a new method for the continuous and remote monitoring of the bonding quality of Carbon Fiber Reinforced Plastics (CFRP) bonded structures. The proposed approach is based on the use of Fiber Bragg Gratings (FBGs) sensors and relies on the dependence of their temperature-induced strain sensitivity, generated by local inductions of (even small) thermal changes, on the quality of the bonding. Bonding composite laminates is a structural solution often adopted in aeronautics. The presence of disbonded areas during production is normally investigated with nondestructive tests (NDT), generally based on ultrasonic techniques (UT). However, these techniques do not always allow to detect areas with poor bonding quality (insufficient adhesion forces). To simulate such situations, some specimens were used for the experimental tests consisting of CFRP flat skins partially overlapped in a bonded area. To reproduce various bonding conditions, the specimens were bonded with different adhesive laying techniques. Experimental results here reported demonstrate that the FBGs thermal sensitivity can change significantly depending on the quality of the bonding in proximity of their position and can increase up to similar to 50 % if the FBG is positioned in proximity of areas of the composite laminates presenting a disbonding or defects. The proposed monitoring approach is useful for in-service structural health monitoring of aircraft, supporting the NDT methods usually included in maintenance manuals.