Monitoring strain gradients in adhesive composite joints by embedded fiber Bragg grating sensors

The adhesive bonding of composite materials can be applied to assemble light-weight structures for aeronautical applications. However, the application of adhesive bonds in primary load-bearing structures is limited by the lack of adequate techniques to monitor the quality of the joint during service. This work evaluates the feasibility of using Fiber Bragg Grating (FBG) arrays to monitor the distribution of strains developed along the bonded region in single lap shear specimens. Three different positions for the FBGs were both experimentally and numerically studied namely in the first ply, at the composite/adhesive interface, and in the adhesive. The mechanical performance of the joint was not modified due to the embedding of the optical fiber at the different positions. The simulations and the measurements showed good agreement for the three positions. The optical sensor placed at the composite/adhesive interface captured the strongest strain gradient.