Abstract
Optical fibers were integrated into graphite-epoxy composites in their manufacturing process. Two types of embedded fiber optic sensors were tested. One was an embedded fiber optic strain sensor, and the other was an embedded sensor for detection of cracking, utilizing optical fiber breaks. First, in order to evaluate the bonding property between optical fiber and epoxy resin, the optical fibers were integrated into epoxy resins, and the tensile test was conducted. As a result, by removing an acrylate coating of the optical fiber with H2SO4, the optical fiber showed good adhesion to the surrounding epoxy resin. The ultimate strain of the uncoated optical fiber was about 1.4 to 1.5%. Second, the embedded fiber optic strain sensor was evaluated. The fiber optic strain sensor consisted of Michelson interferometer using a single-mode coupler. The strains in graphite-epoxy composite laminates were measured by the embedded fiber optic strain sensor, and their strain sensitivities agreed well with the theoretical values. Third, the crack detection experiment using the embedded optical fiber was conducted. When the crack was induced by the tensile load, the embedded optical fiber was broken and the light through the optical fiber went out immediately. Such a relation between cracking and optical fiber break was confirmed by tension test with ultrasonic C-scan images. The cracking induced by the impact load in the composite laminates was also monitored. Internal cracking was induced at the early stage of impact load. As the results of these evaluations, it was proved that the embedded fiber optic sensors were effective for the measurement of strain and the detection of cracking in graphite-epoxy composite laminates.
