A simultaneous dual-parameter optical fibre single sensor embedded in a glass fibre/epoxy composite

A simultaneous two-parameter single sensor based on weakly tilted Fibre Bragg Grating (TFBG), embedded in a 1 mm glass fibre/epoxy composite plate, is demonstrated to measure independently the temperature and strain variations induced in the material by the exposure to heating lamps. The spectrum of weakly TFBGs is composed of several peaks that can be used for different sensing purposes. Here, the shifting of the Bragg and the Ghost peaks are considered to calculate the strain and temperature variations through thermomechanical sensitivity coefficients of the selected peaks. To prove the reliability of the TFBG measurements, the resulting strain values were compared with the strain measurements obtained from the TFBG when compensated by a K-thermocouple embedded close to the optical fibre sensor. Furthermore, the numerical simulation of the full Finite Element Model (FEM) (composite + TFBG) and partial FEM (composite only) models were carried out by assuming a 3-D Gaussian temperature profile. This allowed the TFBG experimental measurements to be compared with the simulated results. A study focused on the strain deviation showed a good match between the full FEM and the TFBG measurements with an average error of ~5% in the case of the dual-parameter sensor and ~2% for the compensated TFBG.