Identification of defects and strain error estimation for bending steel beams using time domain Brillouin distributed optical fiber sensors

In recent years the use of distributed optical fiber sensors for measurements of strain in beams, by means of the Brillouin scattering effect, has been proposed. Several works pointed out the practical difficulty of this kind of measurement, connected both to theoretical and to experimental problems, e.g. mechanical characterization of optical fibers, decaying of strains in the protective coatings, spatial resolution of the Brillouin scattering, brittleness of the glass core, elastic-plastic response of the polymeric jackets, end effects and the different responses of the fiber for dilatation and contraction. Dealing with each of the above problems still requires a great research effort. However, recent literature shows that distributed optical fiber measurement techniques are extremely useful for finding qualitative responses in terms of strains. Indeed, in spite of the above-mentioned uncertainties, the great advantage of the proposed distributed measurement of strains remains evident for the safety assessment of large structures, such as bridges, tunnels, dams and pipelines, over their whole lifetimes. In view of this, in the present paper the detection of defects or damage in bending beams-by using distributed optical fiber sensors in a method based on time domain stimulated Brillouin scattering-is proposed. In particular, laboratory tests were carried out to measure the strain profile along a steel beam. Two tests were performed: the first one involves an integral steel beam, while the second experiment is performed on a damaged beam. Comparison between these two tests allows the detection of the position and the establishing of bounds on the size of the defect. At the end, the quality and accuracy of the measurements are discussed and a sensitivity analysis of the strain readings taking into account the bonding conditions at the interface between the structure and the fiber is also carried out by means a parametric numerical simulation.