Damage detection in bending beams through Brillouin distributed optic-fiber sensors

In recent years the use of distributed optical fibre sensors for measurements of strains in beams, by means of Brillouin scattering effect, has been proposed. Several works have stressed the actual theoretical and practical difficulties connected to this kind of measurements such as mechanical characterization of optical-fibre, decay of strains in the protective coatings, spatial resolution of the Brillouin scattering, brittleness of the glass core, elastic-plastic response of the coatings, end effects, 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 damages in bending beams by means of distributed optic-fibre sensors is proposed. In particular, the fibre-optics distributed sensor has been used for the measurement of deformation of a steel beam in experimental laboratory tests. Comparison of the experimentally measured strains, carried out on a damaged and an undamaged beam, revealed the presence and position of the defects present in the beam. Quality and accuracy of the measurements carried out with distributed optical fibre sensors are discussed, focusing on the applicability of identification method.