Residual strain-induced birefringent FBGs for multi-axial strain monitoring of CFRP composite laminates

Several solutions to measure multi-axial strain in composite materials using fibre Bragg gratings have already been presented in the literature. Most of them use Bragg sensors written in highly birefringent fibres. Unless they exhibit a higher transverse strain sensitivity, a major drawback of those highly birefringent Bragg sensors is the necessity to orient them before embedding them into the composite material, because otherwise an underestimation of the present strain field can occur. Therefore, in this paper we describe how the residual strain, which is built up during the cure cycle of a composite laminate, can in-situ create a highly birefringent fibre Bragg sensor making the need of extra manipulations before embedding redundant. Consequently, in the paper this Bragg sensor is used to quantitatively measure multi-axial strain fields of cross-ply laminates loaded along their three principal mechanical axes. Good similarities were found for the measured strains and the reference strains.