Characterization of residual strains in an epoxy block using an embedded FBG sensor and the OLCR technique

Cylindrical specimens made of epoxy and centrally located glass fibre are used to characterize residual stresses due to curing and post-curing of the epoxy. The fibre contains a Bragg grating which is used as a strain sensor along the fibre axis. A novel optical low-coherence reflectometry technique combined with a layer-peeling algorithm allows the direct reconstruction of the optical period of the grating and provides the strain distribution due to polymerisation of the epoxy along the fibre without any assumption. For the purpose of analysis, the polymerisation process is considered as an equivalent thermo-elastic problem. Using the experimental data as an input to a finite element model of the fibre-epoxy specimen, a non-homogeneous strain field is predicted in the whole specimen. Particularly, the experimental data are in good agreement with finite element simulations and a theoretical model [8] along the embedded fibre.