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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into the mold, in a ratio of 1:5. The scheme of the experimental sample with an embedded optical fiber is shown in Fig. 1b, where the sections of the embedded optical fiber which will experience the tension strain under the three point bending tests are represented in green, and compression in orange color respectively.
Fig. 1. (a) The view of the mold filled with cement mixture with placed optical fibers; (b) The scheme of the sample with embedded optical fiber.
The process of hardening and strengthening of the cement in the mold was carried out for 21 days, after which the sample was removed from the mold. At this stage special attention must be paid to the integrity of the optical fiber in the input and output areas as the most fragile zones. A single-mode silica glass optical fiber with diameter of a cladding and protective acrylate coating of 0.125 mm and 0.245 mm respectively was used for embedding in the sample. It should be noted that, in order to ensure the best strain transfer from the sample material to the optical fiber and to avoid the need to introduce additional calibration coefficients, the embedded fiber-optic line did not contain any additional protective coatings, except for the standard acrylate. Due to the fact that the used measuring equipment has one measuring channel, after removing the sample from the mold, the respective ends of the optical fiber were fused so that all the embedded portions of the optical fiber form one continuous optical fiber line. A method based on measuring the spectral shift in Rayleigh backscattering by optical frequency domain reflectometry (OFDR), which allows distributed measurements of strain and temperature using a standard single mode optical fiber was used for strain registration in the sample by embedded optical fiber. In distributed strain and temperature measurements, the entire optical fiber under test (FUT) is considered as a sensitive element, in contrast to single point fiber-optic sensors, for which only pretreated regions of the optical fiber are sensitive 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 fluctuations in the material structure (Güemes, Fernández López and Soller, 2010). For different optical fibers, the change in refractive index along the length will be different, but will remain 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 serving as a sensitive element. When an optical fiber is subjected to external influence (strain, temperature), the signal in the frequency domain shifts relative to the reference one. To calculate the strain or temperature, the signal shift is estimated at a certain window relative to the reference and this shift value is multiplied by the coefficient of strain or temperature sensitivity of the optical fiber. This method allows to obtain the distribution of the measured value with a very high spatial resolution. The measurements were taken with an OBR 4600 backscatter reflectometer from Luna Inc. The maximum optical fiber length for measurement is 70 m. The manufactured sample was loaded by three-point bending scheme for which it was placed on cylindrical supports with the distance between the supports of 260 mm. The transfer of force to the sample was carried out using an octagonal prismatic element (see Fig. 2). In the course of the experiment, a stepwise increase in the load was carried out with time delays and unloadings at each step.
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