On a direct determination of non-uniform internal strain fields using fibre Bragg gratings

A new method for measuring non-uniform strain fields using fibre Bragg grating sensors is proposed and demonstrated. The method is based on the measurement of the complex impulse response of the grating by optical low coherence reflectometry and the retrieval of the local Bragg wavelength by the layer-peeling technique. This method is applied to measurements of non-uniform strain distributions generated in a double-edge-notched thin plate made of epoxy, where a 24.1 mm long grating is embedded at the specimen centre along the loading direction and orthogonally to the notches. The axial strain distribution along the grating is determined with an absolute error of 30 με. The results are compared with finite element simulations and a good agreement is observed.