Distributed optical fibre sensor measurements on rods and bridge cable wires – Part II: Experimental

A new type of sensor for distributed strain or temperature readings on structures, based on the use of Optical Fibres (OF) utilising the Brillouin scattering effect, has been recently proposed. The use of this new sensor has been successfully carried out not only in laboratory, but also in field tests on bridges and dams. As a matter of fact, several authors have shown that, by means of distributed experimental strain readings, it is easy the safety monitoring and assessment of large structures as bridges, pipes, high rise buildings, dams and tunnels. Moreover, the greatest utility of this new sensors is demonstrated by the present Authors to detect the raising of defects in large structures, and accuracy and reliability of measurements are discussed. Some uncertainties of this kind of measure related both to mechanical aspects of the sensor response and to the optical signal processing are still present. In particular, the mechanical response of optical fibre, when bonded on a straining structure was deeply addressed by some of the present Authors. The sensors were modelled in the framework of elastic Functionally Graded Material Cylinders (FGMCs), under symmetrical load conditions. Analytical closed form solutions both for the decaying and for non-decaying cases were already found and also assessed from the experimental point of view. In particular, for the application of these sensors to the interesting case of rods and wires, which constitute elements widely present in many types of bridge structures, the theoretical mechanical response of the sensors connected to the elements to be monitored, was already obtained. The base theoretical results already carried out, have shown that thanks to the embedding technique of the bonding of the sensor to the rod element, a non-decaying response of the optical sensor should be expected. Indeed, in this case, the axial strain transmitted by the deforming rod to the central glass core of the optical fibre, should equate with the supporting one. With the aim to show consistency and accuracy of the proposed theory concerning the mechanical response of the couple rod-OF-sensor (OFS), in the present work laboratory tests are carried out on aluminium large rods in extension, equipped with embedded-type optic fibre sensors. Experimental strains read by means of embedded OFS, showed very good agreement with the forecasts obtained by means of the above cited theory. This result confirms, from the experimental point of view, the reliability of the theoretical provisions. In particular, the adopted measuring technique, which utilises embedded OFS, shows great consistency and accuracy for a class of important problems related to rod structures. Rods are commonly used in several structural systems, such as trusses, suspension or cable-stayed bridges. Currently, some experiments are under preparation for the detection of surface deficiencies on high strength steel bridge wire surface, using the new Optical Fibre Sensors. The work on the bridge wire application is a joint research effort between the University of Naples Federico II and Bridge Technology Consulting in the United States. Finally, the characterization of the mechanical response of optical fibres, joined with some new solutions obtained for FGMCs, leads to a very accurate calibration of the sensors when they are utilised for strain distributed readings.