Optical backscatter reflectometer to study the response of cascaded FBGs embedded in fibre reinforced polymer composites during flexural tests

Due to their high mechanical and corrosion resistance and their small dimensions, optical fibre sensors and more particularly fibre Bragg gratings have demonstrated their high potential in the composite material field for monitoring purpose when there are placed under constrain, vibration or temperature variation. In this paper, we evaluate the capability of an Optical Backscatter Reflectometer (OBR) to address two configurations of sensors. The first one is with FBGs of different wavelengths distributed along a unique optical fibre. The second one consists in a high number of identical Fibre Bragg Grating (FBG) sensors cascaded in a single optical fibre and embedded in fibre reinforced polymer composites. In this last case, the optical fibre was placed in such a way that the set of FBGs yields a mapping of the flexural strain applied on the composite sample. For this, the Bragg wavelength evolution of the different FBGs subject to flexion (three and four-points bending) is computed from the OBR trace. This equipment being used in these two special FBGs sensors distribution, we compare them and we present advantages and drawbacks of each others.