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An experimental numerical study of the response of a long fibre Bragg grating sensor near a crack tip

The use of fibre Bragg grating (FBG) sensors and the associated signal interrogation and analysis methods for structural monitoring and micro-mechanics is an important area of research. Long FBG sensors are very well suited for relatively non-invasive detection and measurements of complex strain distributions typically arising when cracks or delaminations are present. However, when the fibre is embedded in a non-homogeneous strain field, the interpretation of its response as a function of the position along the sensor is complicated. Thus, accurate strain data rely on the methods used to decode the wavelength changes and deduce the associated strain distribution. In this paper, the response of a 13 mm long FBG sensor embedded normal to the crack plane in a compact tension specimen of epoxy material is analysed. To introduce highly non-uniform strains along the fibre, a natural crack is grown by cyclic loads and two configurations are investigated: (a) when the fibre is ahead of the crack front, (b) when the fibre is behind the crack front. In both cases, analysis of the signals is carried out using the conventional T-matrix and a method based on optical low-coherence reflectometry (OLCR) and inverse scattering which provides a direct reconstruction of the axial strains in the fibre without any assumptions. The results of the measurements and analysis demonstrated that the predicted spectra from the T-matrix method are not in good agreement with the recorded ones. These differences are attributed to the high strain gradients along the fibre due to its proximity to the crack front. The results from the OLCR-based method directly provide the non-uniform strains along the fibre. The experimental measurements with the latter method are also in good agreement with a three-dimensional numerical model of the actual experimental configuration.