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

Abstract

The use of fibre Bragg grating (FBG) sensors and the associated signal interrogation andanalysis methods for structural monitoring and micro-mechanics is an important area ofresearch. Long FBG sensors are very well suited for relatively non-invasive detection andmeasurements of complex strain distributions typically arising when cracks ordelaminations are present. However, when the fibre is embedded in a non-homogeneousstrain field, the interpretation of its response as a function of the position along the sensoris complicated. Thus, accurate strain data rely on the methods used to decode thewavelength changes and deduce the associated strain distribution. In this paper, theresponse of a 13 mm long FBG sensor embedded normal to the crack plane in a compacttension specimen of epoxy material is analysed. To introduce highly non-uniform strainsalong the fibre, a natural crack is grown by cyclic loads and two configurations areinvestigated: (a) when the fibre is ahead of the crack front, (b) when the fibre is behind thecrack front. In both cases, analysis of the signals is carried out using the conventionalT-matrix and a method based on optical low-coherence reflectometry (OLCR) and inversescattering which provides a direct reconstruction of the axial strains in the fibre withoutany assumptions.The results of the measurements and analysis demonstrated that the predicted spectra from theT-matrix method are not in good agreement with the recorded ones. These differences areattributed to the high strain gradients along the fibre due to its proximity to the crackfront. The results from the OLCR-based method directly provide the non-uniform strainsalong the fibre. The experimental measurements with the latter method are also in goodagreement with a three-dimensional numerical model of the actual experimentalconfiguration.