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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the reflected signal that occurs at the manufacturing stage due to the complex stress state in the region of the FOS (Serovaev and Kosheleva, 2019; Matveenko et al. , 2021). Nevertheless, the use of embedded FOSs based on Bragg gratings can be useful for recording the deformed state of many different materials. This study demonstrates the wide possibilities of embedding FBG in materials such as epoxy resin, cement, poured liquid plastic and thermoplastic materials used for 3D printing, as well as investigates their performance after implementation. The features associated with the embedment of FBG into these materials are noted, which must be taken into account to ensure the sensor integrity. 2. Sample manufacturing The sensitive element of the FOSs considered in this work is a fiber Bragg grating, which is a periodic change in the refractive index over a certain length of the optical fiber core. FBG reflects a narrow spectrum of an optical signal launched through an optical fiber by an interrogator. The reflected signal spectrum has a resonant wavelength, which depends on the effective refractive index of the optical fiber core in the grating region and the grating period. When the temperature or strain changes, the reflected spectrum and the resonant wavelength shift, which makes it possible to measure these physical quantities at the FBG location. For a single-mode optical fiber, the equations establishing the relationship between the change in the resonant wavelength of the reflected spectrum and the strains of the optical fiber in the Bragg grating region have the form presented in the work (Luyckx et al. , 2011). Epoxy resin is the main binder in polymer composite material. The hardening process of epoxy resin largely affects the subsequent properties of polymer composite materials. Therefore, when the optical fiber is embedded into an epoxy resin sample, an important task is to obtain information on process-induced strains in the process of sample creation. For the manufacturing of a cube-shaped sample, a special dismountable mold was created (Fig. 1a), which makes possible to extract the formed sample without chips and breaks. Holes are made on two opposite walls of the mold, with the help of which the positioning of the optical fiber is carried out before pouring the material in such a way, that after pouring and curing the material, the FOS is located in the central part of the sample. To ensure the integrity of the fiber-optic line, protective tubes were inserted into the side holes. A universal two-component epoxy resin was used to create the sample. Manufacturer's recommended resin to hardener ratio of 2:1 was applied. The epoxy cube-shaped sample obtained after 36 hours with embedded fiber is shown in Fig. 1b. 2.1. Embedment of FOS into an epoxy resin sample

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b Fig. 1. (a) Mold for creation the samples; (b) Epoxy sample with embedded FOS.

Fig. 2. Evolution of FOS readings and reflected spectra at different points in time in an epoxy sample.

Throughout the entire technological process of sample manufacturing, the readings of the FOS based on FBG were recorded. Fig. 2 shows the change in the shift of the resonant wavelength of the reflected spectrum and the shape of the reflected spectrum at different points in time during the material formation, where 1 − is the spectrum at the