Embedded fibre Bragg grating sensors for non-uniform strain sensing in composite structures

A methodology for evaluating the response of embedded fibre Bragg grating (FBG) sensors in composite structures based on the strain in a host material is introduced. In applications of embedded FBG sensors as strain sensing devices, it is generally assumed that the strain experienced in a fibre core is the same as the one measured in the host material. The FBG sensor is usually calibrated by a strain gauge through a tensile test, centred on obtaining the relationship between the surface strain in the host material and the corresponding Bragg wavelength shift obtained from the FBG sensor. However, such a calibration result can only be valid for uniform strain measurement. When the strain distribution along a grating is non-uniform, average strain measured by the strain gauge cannot truly reflect the in-fibre strain of the FBG sensor. Indeed, the peak in the reflection spectrum becomes broad, may even split into multiple peaks, in sharp contrast with a single sharp peak found in the case of the uniform strain measurement. In this paper, a strain transfer mechanism of optical fibre embedded composite structure is employed to estimate the non-uniform strain distribution in the fibre core. This in-fibre strain distribution is then utilized to simulate the response of the FBG sensor based on a transfer-matrix formulation. Validation of the proposed method is preceded by comparing the reflection spectra obtained from the simulations with those obtained from experiments.