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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eliminated. It is concluded that at present it is not possible to avoid calibrating FOS using thermometers or thermistors. Nevertheless, fiber-optic technologies can also be combined and calibrated with each other during measurements. Special attention must be paid to spatial sampling intervals and sampling frequency of FOSs when selecting their type for an experiment. A review paper (Fan and Bao, 2021) discusses different types of FOSs used for monitoring corrosion in reinforced concrete. The authors focus on the measurement principle and key features: sensor design, sensor installation, sensitivity, resolution and sensor lifetime. The authors divide the considered FOSs according to the spatial measurement characteristic into point and distributed ones. DFOSs are shown to be better suited for monitoring corrosion propagation, as point-based FOSs detect corrosion condition only at locations where sensors are installed. It is concluded that a balance must be found between sensor lifetime and its sensitivity to corrosion. The problem of interference between different types of sensors with different sensing mechanisms, the nature of which is not yet clear, is described. It is concluded that further research is needed to clarify possible interactions between different types of FOSs. The present work is devoted to the comparison of DFOS based on Rayleigh scattering and point FOSs based on FBGs when measuring strain in homogeneous and gradient strain fields. 2. Strain registration in the homogeneous and gradient zones by two types of fiber-optic sensors Among the FOSs used to measure strain and temperature, point sensors based on the Bragg gratings and DFOSs based on Raman, Brillouin and Rayleigh scattering are the most common. A FBG is a periodic change in the refractive index at a certain length of the optical fiber core. FBG reflects a narrow spectrum of an optical signal with a central wavelength depending on the effective refractive index of the optical fiber core in the grating region and the grating period. When the temperature and strain change, a shift of the reflected spectrum and the resonant wavelength occurs, which makes it possible to measure these physical quantities at the location of the FBG. It is possible to create a quasi-distributed measuring system by inscribing several FBGs on one optical fiber and providing wavelength division multiplexing (WDM). Despite the possibility of a sufficiently dense recording of the FBGs along the length of the optical fiber, there are restrictions on the number of FBGs associated with the light source bandwidth and a decrease in the measurement range with an increase in the number of sensors. Among DFOSs, the method based on the measurement of the spectral shift in Rayleigh backscattering has the best spatial resolution, which allows distributed measurements of strain and temperature using a standard single mode optical fiber. For measurements, the OBR 4600 backscattering reflectometer of Luna inc. was used. With distributed measurement of strain and temperature, the entire optical fiber under test (FUT) acts a sensitive element, unlike point FOSs, for which only pre-processed areas of the optical fiber have sensitivity to changes in strain and temperature. This approach is based on the fact that the refractive index of any optical fiber along its length undergoes minor changes due to the presence of inhomogeneities in the structure of the material. For different optical fibers, the change in the refractive index along the length will be different, but it will be preserved from measurement to measurement (in the absence of external influences) for a single optical fiber. Thus, a reference profile of the reflected signal is formed for an optical fiber acting as a sensing element. When an external influence on the optical fiber (strain, temperature) occurs, the signal shifts in the frequency domain relative to the reference one. To calculate the strain or temperature, the signal shift is estimated at a certain window relative to the reference one and this shift value is multiplied by the coefficient of strain or temperature sensitivity of the optical fiber (Kreger et al. , 2006). This relation is similar to that used to calculate strain and temperature by the shift of the resonant wavelength of the FBG for cases close to the uniaxial stress state.

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