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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At a constant temperature, the measured longitudinal strain can be expressed by the relation:







    

cK

K

0 





where   is the difference between the resonant wavelength of the FBG in the loaded and unloaded states,   - is the spectral shift,  is the mean optical frequency, K  and T K , the coefficients of strain and temperature sensitivity, c is the speed of light in vacuum,  is the central scanning wavelength. The purpose of the considered in the work FOSs is to measure strain in various sections of the controlled structure, which allows collecting information about the current mechanical state and signaling the possible appearance and development of damage. As a rule, the damage of the material begins in areas with an inhomogeneous strain distribution. Therefore, FOSs must provide reliable measurements in both uniform and gradient strain fields. In particular, under conditions of static loading, it is possible to implement a damage detection method based on the ratios of FOS readings in zones of gradient strain distribution caused by the appearance of damage and in zones located at a distance from the damaged areas with a uniform strain distribution (Matveenko, Kosheleva and Serovaev, 2021). In this paper, the measurements of point FOSs based on FBGs and DFOSs based on Rayleigh backscattering are compared when measuring uniform and gradient strain fields. To implement a close to uniform strain distribution, a uniform-strength beam was used, which geometric and loading schemes are shown in Fig.1a and b. The beam was 3D printed from PLA (Polylactic acid) plastic. Two lines of silica glass optical fiber SM1250(9/125)P w ith a polyimide coating with an outer diameter of 150 µm were attached to the surface of the beam by cyanoacrylate glue, as shown in Fig. 1c. One of the glued optical fibers was used for distributed measurements of strain. On the second optical fiber, a 5 mm long FBG was inscribed at a distance of 17 mm from the clamping area.

Fig. 1. (a) The scheme of the sample; (b) loading scheme; (c) the sample with glued FOSs.