Influence of Stress Induced Birefringence on FBG Sensors Embedded in CFRP Laminates

Abstract

In previous research, the authors studied detection of transverse cracks in CFRP laminates using small-diameter fiber Bragg grating (FBG) sensors. FBG sensors were embedded in cross-ply or quasi-isotropic laminates, and tensile tests were carried out. As transverse cracks occurred, a reflection spectrum of FBG sensors was deformed due to a non-uniform axial strain distribution near transverse cracks. The spectrum width defined by FWQM (full width at quarter maximum) showed a good correlation with the crack density. However, calculated FWQM were slightly different from the measured one. One of the reasons is the presence of birefringence effect on embedded FBG sensors. When FBG sensors are embedded in the CFRP laminates, the sensors are subjected to an asymmetric residual stresses. The refractive index changes due to the photoelastic effects. Consequently, birefringence effect occurs and induces a split of the reflection spectrum. In this research, these effects were considered theoretically. Reflection spectrum was calculated by solving coupled-mode equations of x-polarization and y-polarization, respectively. Calculated results were compared with the previous analysis in which the birefringence effect was not considered. As a result, the theoretical calculation considering the birefringence effect showed a better corre-spondence with the experimental results.