Effect of thermal residual stress on the reflection spectrum from FBG sensors embedded in CFRP composites

When fiber Bragg grating (FBG) sensors are embedded in carbon fiber reinforced plastic (CFRP) laminates, the reflection spectrum from the FBG sensors split into two peaks because of the non-axisymmetric thermal residual stress. This deformation of the spectrum will lead to misreading in strain measurements or crack detection in the laminates. In the present research, three types of FBG sensors: uncoated normal, polyimide-coated normal, and polyimide-coated small-diameter FBG sensors were embedded in CFRP laminates, and reflection spectra from the sensors were measured during the fabrication process of the laminates in order to evaluate the effect of thermal residual stress on the reflection spectra. Through the comparison of results obtained for the three FBG sensors, it was found that the effect of thermal residual stress on the reflection spectrum could be decreased when the optical fiber was coated with polyimide in the present laminate configuration and embedment position. Furthermore, these changes in the spectra during the curing process were simulated theoretically. Thermal residual stress components at the core were obtained by FEM analysis, and the spectra were calculated using the couple mode theory and the transfer matrix method. Considering the birefringence effect, the calculated spectra reproduced the measured spectra very well.