PSI - Issue 37

V.P. Matveenko et al. / Procedia Structural Integrity 37 (2022) 508–516 Matveenko V.P., Kosheleva N.A., Serovaev G.S./ Structural Integrity Procedia 00 (2021) 000 – 000

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Fig.10. Samples from various materials with embedded FOS.

The loading of the epoxy resin specimen was carried out with a constantly increasing load from 0 to 8 kN. Fig. 11 shows a sample loading scheme: (a) along the direction of the optical fiber, (c) perpendicular to the direction of the optical fiber. The main purpose of these tests was to check the performance of the sensors under various loading conditions. So, for example, under the load presented in Fig. 11a, it is assumed that a stress state of the optical fiber in the region of the FBG is close to uniaxial. As a result, the reflected spectra shown in Fig. 11b contains one peak for all load levels. When a load is applied perpendicular to the optical fiber (Fig. 11c), the FBG region is in a complex stressed state. As a result, two resonant peaks are observed in the reflected spectrum of the FBG sensor (Fig. 11d) experiencing the same strain level (the similar strain levels are noted in Fig. 11b, c as ε 1 , ε 2 , ε 3 , ε 4 ) in the direction of the optical fiber. Additional research on the strain values obtained on the basis of the recorded physical quantities is required due to the induced birefringence that occurs in a single-mode optical fiber. a b

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Fig.11. (a, c) Epoxy resin sample compressive loading scheme; (b, d) Reflected spectra during loading for epoxy resin sample.

The compression tests at a load increasing speed of 2 mm/min and a loading step of ~ 900 N with an increase in the load from 0 to 10 kN, with a one-minute holdings at each stage were carried out with a cement sample. The load was applied perpendicular to the direction of the optical fiber (Fig. 12a). The reflected spectra taken at different load levels: 1 − without load, and 6 − at a maximum compressive load of 10 kN are shown in Fig. 12b. It can be noted that despite the fact that the optical fiber experiences a complex-stressed state, with an increase in the load, only a broadening of the spectrum is observed, with the preservation of a single peak.