Birefringence Effect of Embedded FBGs Due to Peel Stress at the Overlap End of Single-Lap Joint

摘要

As advanced sensing technologies, fiber-optic distributed sensors which can be embedded into an adhesion layer are attractive tools for determine strain and stress distributions inside the adhesive and at the interfaces between adhesive and the adherents. We have developed a distributed strain measurement system using long gauge fiber Bragg gratings (FBGs) based on optical frequency domain reflectometry (OFDR) system. It can measure a strain distribution with the high spatial resolution of less than 1 mm at an arbitrary position along the FBGs. So it enables us to measure precisely strain distributions fluctuating locally in the adhesion layer. In this study, we embedded the long gauge FBGs whose gauge length was about 100 mm into the adhesion layer of a single-lap joint and we could successfully measure the longitudinal strain distribution at the interface between the adhesive and the adherent along the FBGs when the specimen was subjected to the tensile loads. In this test, we also found that the reflected spectrums around the overlap end of the single-lap joint were split, because there were large peel stresses which could cause the birefringence effect. In this paper, we propose the method to identify the magnitude of the peel stress by analyzing the reflected spectrums split due to the birefringence effect.