Effect of fiber coating on crack detection in carbon fiber reinforced plasticcomposites using fiber Bragg grating sensors

Abstract

Fiber Bragg grating (FBG) sensors have been applied to the detection oftransverse cracks in carbon fiber reinforced plastic (CFRP) cross-ply laminatesusing the sensitivity of the sensors to non-uniform strain distribution. However,there is the possibility that the resin coatings of the FBG sensors willrelax the non-uniform strain distribution and deteriorate the sensitivity ofthe sensors. Hence, in the present research, the strain transfer from thelaminate to the core of an embedded optical fiber was calculated by atheory developed from the shear-lag model. As a result, it was found thatthe variation of the axial strain in the core of a polyimide coated FBGsensor was smaller than that of an uncoated FBG sensor. Nevertheless, thereflection spectra of the coated FBG sensor and the uncoated one, which werecalculated from the strain distributions in their cores, were almost thesame. This result shows that the coating of the optical fiber does notdeteriorate the sensitivity of the FBG sensor for crack detection. Furthermore,the effectiveness of the coated sensor was also confirmed experimentally.