Distributed Optical Fibre Sensor Measurements on Rods and Bridge Wires - Part I: Theory

Recently, the use of Optical Fiber Sensors (OFS) utilizing Brillouin scattering effect for reading distributed measurements of temperature and strains has been addressed. Several authors have stressed the actual theoretical and practical difficulties connected to this new kind of measurement such as 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 coatings, end effects, and different effects of strain readings in dilatation or in contraction. Moreover for strain readings, a few technical devices have been proposed to connect the optical fiber sensor to the structure, in order to obtain better results in terms of accuracy of measurements. Recently, several authors have shown that, by means of distributed experimental strain readings, the safety monitoring and assessment of large structures as bridges, pipes, high rise buildings, dams, and tunnels is made easier. In spite of the above mentioned uncertainties, the greatest utility of these new sensors is demonstrated by the authors for detection of defects rising in large structures, and accuracy and reliability of measurements are discussed. A recent paper by some of these authors has provided for the mechanical response of optical fiber, when it is embedded in a bearing supported beam-like element, where OFSs are treated in the framework of elastic Functionally Graded Material Cylinders, under symmetrical load conditions. The achieved theoretical results characterize the complete set of the so-called no-decaying solutions, which present axial strain in the system core-jacket not varying with the radius, and hence equal to the value assumed in the supporting element. Those analytical solutions are particularly efficient for strain readings utilizing embedded sensors, because in this case the fiber core should exhibit an axial strain equal to the one on the structure to which the sensor is linked. Moreover, another set of so-called decaying solutions is obtained which was evaluated for the rate of decaying of the axial strain between the core and the support. In Part II of the current work laboratory tests will be shown, carried out on aluminium large rods in extension, equipped with embedded-type optic fiber sensors. Rods are commonly used in several structural systems, such as trusses, suspension, or cable-stayed bridges.