Strain Distributions in Bonded Composites Using Optical Fibers and Digital Image Correlation

Abstract

Adhesive bonding is often utilized to assemble lightweight composite structures in the aeronautical industry. However, bonded composite joints are seldom exclusively used in primary load bearing structures due to the lack of adequate testing techniques to monitor the quality of the bonds during service. Continuous optical fibers can be used to monitor adhesive bonds since they are small (diameter~155 microns) and have minimal effect on the structural integrity of the bond. This work investigates the feasibility of using embedded optical fibers to monitor strain distributions within double-lap shear joints. Optical sensors were embedded in one ply from the adhesive/composite interface, and in the adhesive. Surface strain fields were recorded using a digital image correlation system. The specimens were tested in tension and the internal strain distributions were obtained from optical fibers. Optical sensors placed in one ply from the interface captured the greatest strain gradient. Experimental data was compared to a 3-D finite element model that includes the geometry of the specimens and the optical fibers. The simulations and the optical fiber measurements are in good agreement.