Effect of thermal residual stress on the reflection spectrum from fiber Bragg grating sensors embedded in CFRP laminates

When FBG sensors are embedded in CFRP laminates, the reflection spectrum from the FBG sensors splits 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 cross-ply laminates, and reflection spectra from the sensors were measured during the fabrication process of the laminates. 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 and its diameter was small in the present laminate configuration and embedment position. Furthermore, these changes of the spectra during the curing process could be simulated by theoretical calculation considering the birefringence effect.