PSI - Issue 33

V.P. Matveenko et al. / Procedia Structural Integrity 33 (2021) 925–932 Author name / Structural Integrity Procedia 00 (2019) 000–000

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reliability. Due to the rapid development of monitoring systems, there is a constant search for new types and variants of sensitive elements that would make it possible to overcome the known disadvantages of traditionally used sensors. The development of photonics technologies has led to the active use of fiber-optic sensors (FOS) for structural health monitoring of various engineering structures: buildings, bridges, dams and power plants. FOS have a number of important features, due to which this type of sensors is widely demanded. These features include high sensitivity and accuracy of FOS, corrosion resistance, resistance to electromagnetic interference, and a long length of optical fiber, on which it is possible to measure strain and temperature. In addition, the embedment of sensors from the initial phase of the construction of structures, in order to register the entire history, including the stages of manufacturing and preliminary maintenance, opens up new prospects for monitoring the structures integrity. The obtained information about the engineering structures’ behavior, received from the early stages, gives a more complete picture of their specific problems (Wong et al. , 2007; Yun, Jang and Wang, 2012), on the basis of which recommendations can be made to experts and manufacturers to optimize their building procedures (Slowik, Schlattner and Klink, 2004). Among fiber-optic sensors, distributed FOS (DFOS) have great prospects, since they have all the main advantages of single point fiber-optic sensors and allow to make measurements with over long distances of optical fiber without preliminary recording the sensitive areas in the fiber structure (Drake, Sullivan and Wilson, 2018). Currently, fiber-optic systems for distributed strain and temperature measurements are based on the registration of three types of scattering in an optical fiber – Raman, Brillouin and Rayleigh scattering (Barrias, Casas and Villalba, 2016). Raman scattering is sensitive only to temperature changes. Brillouin scattering is capable for distributed strain and temperatures measurements over a long distance of an optical fiber, but has limitations associated with large distances between measurement points (Bao and Chen, 2012). The approach considered in this work, based on measuring the spectral shift in Rayleigh backscattering by optical frequency domain reflectometry (OFDR), makes it possible to carry out distributed measurements of temperature and strains with a high spatial resolution (Ding et al. , 2018). Distributed fiber-optic sensors are a convenient tool for monitoring the mechanical state and occurrence of damage, for example, for detecting cracks in concrete structures (Barrias, Casas and Villalba, 2019). Thus, the authors (Fischer, Thoma and Crepaz, 2019) have shown experimental confirmation that distributed FOSs are suitable not only for detecting cracks and their further observation along a certain measurement line in concrete structures, but also for early detection of incipient shear cracks. The paper (Barrias, Casas and Villalba, 2017) describes an experimental study associated with testing of two concrete beams under three-point bending. Distributed FOS are embedded in concrete beams in such a way that part of the sensor is embedded without a protective coating into the reinforcing bar inside the concrete, and the remaining fiber is glued to the surface of the element after the concrete has hardened. This makes it possible to directly compare the resulting strains on the surface of concrete and reinforcement using a single optical fiber. The data obtained during the beams testing using distributed FOS were compared with traditional electrical strain gauges and showed a good correlation. The present study aims to measure strains in a cement sample under loading according to the three-point bending scheme using embedded distributed fiber-optic sensors. The comparison between obtained measurements, data received from embedded single point fiber-optic sensors based on fiber Bragg gratings (FBG) and numerical simulation results by the finite element method was carried out. For this purpose, single mode silica glass optical fiber with thin acrylate coating was used. The operability of DFOS has been demonstrated as well as sensitivity to the appearance and development of the defect. 2. Application of embedded distributed FOS for strain measurement in cement sample under three-point bending The fiber-optic line was embedded into the cement sample at the stage of sample manufacturing. Cement mixture was poured into a prismatic mold made of moisture-resistant plywood with a size of internal cavity of 400×70×70 mm 3 . The inputs and outputs of the optical fiber from the mold were made through the technological holes located at a distance of 20 mm from the bottom and the upper surface of the mold (see Fig. 1a). The tension was applied to the optical fiber during its positioning which is necessary to maintain its straight-line arrangement. After placing and fixing the optical fiber, a cement mixture made of Portland cement 400 and river sand was poured