PSI - Issue 50

Valerii Matveenko et al. / Procedia Structural Integrity 50 (2023) 184–191 Valerii Matveenko, Natalia Kosheleva, Grigorii Serovaev / Structural Integrity Procedia 00 (2022) 000 – 000

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fiber-optic technology are one of the fastest growing fields thanks to advantages such as: light weight, small size, reliability and stability, resistance to external electromagnetic disturbances, low power consumption, high sensitivity (Wu et al. , 2020). Given the many advantages of fiber-optic sensors (FOSs), it is not surprising that they have already replaced traditional sensors in various applications. Sensors for strain, vibration, acceleration, rotation, pressure, temperature, linear and angular position, humidity, viscosity, etc. already exist successfully using fiber optic technology (Jayawickrema et al. , 2022). In addition, FOSs have real-time remote monitoring and data transmission capabilities. This is why sensors based on fiber-optic technology are increasingly being chosen for Structural Health Monitoring systems (SHM). Today the most common types of FOSs are point based fiber Bragg gratings (FBGs), distributed fiber-optic sensors (DFOSs) based on Rayleigh, Brillouin or Raman scattering and interferometric FOS based on Fabry-Perot or Mach-Zehnder Interferometer (Leung et al. , 2015). These FOSs embedded in the volume or mounted on the surface of the structure are capable of integrated, quasi-distributed and fully distributed measurements even at long lengths. The scope of FOSs applications for SHM grows every year and in various fields, e.g. civil engineering structures (Bado and Casas, 2021; Wijaya, Rajeev and Gad, 2021; He et al. , 2022), aerospace (Rocha, Semprimoschnig and Nunes, 2021) and medical applications (Tosi et al. , 2018) and also in the manufacture of special sensors (Sun et al. , 2019). The simultaneous use of point and distributed FOSs provides a more complete picture of the mechanical state of an object. Point-type FOSs can measure strain at specific points, while DFOSs are able to provide strain measurements along the entire length of the fiber-optic cable. At the same time these sensors can complement each other with the necessary information for temperature compensation and to separate the effects of temperature induced strain response from other influences that affect sensor readings. However, there are a number of problems that have received attention in the literature. One of the problems is related to the differences in the readings of these two types of sensors. For example, in (Ye et al. , 2020) DFOSs based on Brillouin scattering and point FOSs based on FBGs were used to measure strain and temperature for two and a half years of the reinforced concrete beams on top of a constructed railway bridge. Two types of prestressed reinforced concrete beams were involved in the study: two inner beams and two edge beams. The mechanisms of prestressing loss, including immediate prestressing losses due to elastic shortening of concrete and time-dependent losses due to steel relaxation, concrete shrinkage and creep, were investigated using two types of FOSs. The authors point out considerable discrepancies in the data obtained with point and distributed FOSs. The authors suggest the following as possible reasons for such discrepancies in the readings of the two types of FOSs: frequent failure of the analyzer during the data collection stage; incorrect temperature compensation for DFOSs due to inaccurate temperature coefficient values, which was assumed constant along the entire length of the DFOSs; lower level of accuracy of DFOSs compared to point FBG sensors. Also, effects such as the resistance of the sensors to their installation environment, differences in the bonding conditions between the FOSs and the concrete, differences in the method of fixing the FOSs and differences in the cross sectional deformation (FBG sensors and DFOS were fixed to different prestressing points of the beam) may have affected the FOSs readings. In (Drusová et al. , 2021) a comparison of three types of FOSs for temperature control in a groundwater flow simulator is given. Particular attention is paid to the measurement errors of the sensors from the attachment method, their packaging and their cross-sensitivities to temperature and strain. The paper compared FBG sensors, DFOS based on Raman scattering and Rayleigh scattering. The authors investigated the factors that were important in selecting the type of FOS designed for temperature measurement. The accuracy and temperature resolution of the three types of FOSs were investigated and a comparison with PT100 reference sensors was given. It was shown that in terms of spatial resolution, the FBG sensors were the most suitable for the experiment, as the distance between the sensors was chosen based on the dimensions of the installation. However, Mach-Zehnder interferometer-based DFOS are not suitable for this experiment, because the spatial resolution of 1 m proved insufficient for such a small scale setup. Raman scattering-based DFOS are shown to have the best temperature resolution, but these DFOS need more temporal averaging to achieve this resolution. This accuracy and resolution can be achieved if the Raman scattering-based DFOS are continuously calibrated during the experiment. FBG sensors and DFOSs based on Rayleigh scattering can be used for temperature measurement if the effects caused by strain acting on the sensor are