Damage detection in bending beams through Brillouin distributed optic-fibre sensor

The use of distributed optical fibre sensors for strain measurements in beams, by means of Brillouin scattering effect, has been proposed recently. Several researchers have stressed the theoretical and practical difficulties related to this kind of measurement. These include the mechanical characterization of optical fibres, the decay of strains in the protective coatings, the spatial resolution of the Brillouin scattering, the brittleness of the glass core, the elastic - plastic response of the coatings, the end effects, and the different effects of strain readings in dilatation or in contraction. A solution to each of the above-cited problems would entail further research effort. However, all the works pointed out that the qualitative strain response of bending beams is clearly accounted for by distributed optical fibre sensors. In spite of the above-mentioned uncertainties, the distributed nature of the sensor makes it very attractive when safety assessment of large structures, such as bridges, tunnels, dams or pipes, is involved. In the present paper, the detection of defects or damage in bending beams by means of distributed optic-fibre sensors is proposed. In particular, the fibre-optics distributed sensor has been used for measurement of the deformation of a steel beam in experimental laboratory tests. Comparison of the experimentally measured strains, carried out on both damaged and undamaged beams, revealed the presence and position of defects in the beam. Quality and accuracy of the measurements carried out with distributed optical-fibre sensors are discussed, focusing on the applicability of the identification method.