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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application of fiber-optic sensors (FOSs) based on a fiber Bragg gratings (FBG) since they can be embedded in various materials, forming an intelligent structure. Compared to conventional electrical gauge configurations, FOSs combine many advantages, such as their small size, immunity to electromagnetic interference and radiation, and multiplexing capability. The authors of various works have paid great attention to the possibilities and problems arising from the embedding of optical fibers into materials. In particular, in (Novo et al. , 2002), a description of an intelligent plate made of a composite material with embedded FOSs based on Bragg gratings is given. Intelligent plates were installed in the hull of the ship in areas subjected to heavy loads. FOSs are used as a tool capable to detect structural failures on time. Among the works that describe practical applications of intelligent structures based on Bragg gratings, the following studies should be noted. In (Gebremichael et al. , 2005) FOSs are embedded into a polymer composite bridge. The main functions of the FOSs embedded into the bridge are: obtaining real-time data on temperature and strains. The authors of the study (Read and Foote, 2001) describe how FOSs are used for structural monitoring of sailing composite yachts and turboprop aircraft. Fiber-optic sensor data was obtained during a flight in real-time, including extreme loads and difficult environmental conditions. Articles (Lee et al. , 2003) and (Ghoshal et al. , 2015) describe the use of FOSs based on Bragg gratings as intelligent elements embedded into the wing and composite elements of a helicopter, respectively. Data on the mechanical state of controlled objects is obtained under various dynamic loads. However, it should be taken into account that when the FOS is embedded in composite materials, a problem arises with assessing the effect of the embedded fiber on the mechanical characteristics of the material. The works (Luyckx et al. , 2010, 2011) are devoted to this problem. In addition, the possibility of the appearance of technological defect in the form of "resin pocket", and, as a consequence, the need to assess the redistribution of the stress-strain state in the vicinity of the optical fiber embedded in the material cannot be excluded (Shivakumar and Emmanwori, 2004; Shivakumar and Bhargava, 2005; Lammens et al. , 2016). Fiber-optic sensors are widely used in the creation and monitoring of intelligent concrete structures, such as bridges (Tennyson et al. , 2001; Lin et al. , 2005; Rodrigues et al. , 2010), geotechnical objects (Hong et al. , 2016), engineering structures (Makul, 2020). In (Wong et al. , 2007), the use of FOSs based on Bragg gratings embedded in concrete for monitoring the temperature and evolution of strains, starting from the early stages of product manufacturing, is given. The possibility to measure process-induced strains and to find optimal modes of drying for samples, to control shrinkage and investigate the problems associated with early cracking due to shrinkage of young concrete with the help of the embedded FOSs is presented in (Slowik, Schlattner and Klink, 2004). The rapid development of additive materials obtained using 3D printing, as well as the relatively low cost, flexibility and easy customization of the production process, and wide application possibilities have led to the fact that these materials began to be used as a tool for the development of intelligent materials with sensors based on FBGs (Leal Junior et al. , 2019). Using the advantages of 3D printing, FBGs were embedded into materials at the stage of their manufacturing using a 3D printer. In (Fang et al. , 2016), the strain change measured by means of FBG sensors embedded into the structure during printing was investigated. The FBG data were compared with the theoretical results and a conclusion was made about the reliability of the readings obtained from the embedded FBG sensors. In (Hong, Zhang and Borana, 2019) a vertical pressure measurement sensor was developed and created using 3D printing, where a FOS based on a Bragg grating was used as a sensitive element. The FBGs were embedded inside a prototype pressure transducer made from PLA material. The authors note that the designed and printed sensor with embedded FBG has been tested under cyclic loading in laboratory tests. Worth noting that in most practical applications a fiber Bragg grating is “packed” (Torres et al. , 2011) into additional protective coatings and extra shells due to the fragility of FBGs written in a silica glass optical fiber and harsh conditions that can affect the FBG longevity and performance. In (Yan et al. , 2020) special protective “packaging s ” for FBG are created using a 3D pr inter. The influence of the packing scheme and material on the sensitive characteristics of the packed FBG is investigated. Special attention is paid to the temperature characteristics of the packaging material. The authors of the work have shown that the structure of the package, created using 3D printing, can not only effectively protect the FBG from breakage, but also be used to obtain a fiber-optic strain sensors based on FBGs, which have high stability and reproducibility. One of the most promising materials for the embedment of FOSs are layered polymer composite materials, since they are widely in demand in the creation of critical structures. This area of research has received considerable attention in the literature, including the authors of this work (Matveenko et al. , 2018). One of the important problems related to the embedment of FOSs based on Bragg gratings into a laminated composite material is the change in the spectrum of