Strain monitoring in FRP laminates and concrete beams using FBG sensors

The fibre-optic Bragg grating (FBG) sensor is broadly accepted as a structural health monitoring device for fibre reinforced plastic (FRP) materials by either embedding into or bonding onto the structures. The accuracy of the strain measured by using the FBG sensor is highly dependent on the bonding characteristics among the bare optical fibre, protective coating, adhesive layer and host material. In general, the signal extracted from the embedded FBG sensor should reflect the straining condition of the host structure. However, due to the existence of an adhesive layer and protective coating, part of the energy would convert into shear deformation. Therefore, the mechanical properties of these materials would affect the resultant strain measured by embedding a FBG sensor into the structure. This paper presents a theoretical model to evaluate the differential strains between the bare fibre and host material with different adhesive thickness and modulus of the protective coating of the embedded FBG sensor. The results are then compared with numerical analysis by using the finite element method (FEM). Experimental work was conducted for both glass fibre composites and FRP strengthened concrete beams with embedded FBG sensors. Externally bonded strain gauges were used to compare the results obtained from the FBG sensors. The theoretical predictions reveal that the axial strain measured at the fibre-core region is lower than the true strain of the host material with increasing thickness of adhesive layer. A thick adhesive layer and low modulus of coating material would enlarge the shear stress concentration area at the bonded end region. An experimental investigation also shows that the FBG sensor can be confidently used with sufficient bond length.