PSI - Issue 17

Grigorii Serovaev et al. / Procedia Structural Integrity 17 (2019) 371–378 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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opportunities for assessing the state of objects as well as detecting defects at an early stage of their development. Due to the rapid growth of the telecommunications industry based on optical fibers and a lot of researches carried out in this field, the quality of fiber-optic components has significantly increased, while decreasing the production cost. This was one of the reasons for the study of the possibility of using optical fibers as sensitive elements. Fiber-optic sensors (FOSs) have several advantages compared to other sensing elements: they are not sensitive to electromagnetic effects, can operate in a wide range of temperatures, it is possible to place many FOSs on one optical fiber and simultaneously register measurements from all sensors on the line (Udd, (2011)). In this paper, Bragg grating sensors written in a standard single-mode optical fiber are considered. This type of FOSs is widely used to monitor the mechanical state of various objects (Gebremichael et al. (2005); Ghoshal et al. (2015); Hong et al. (2016); Lee et al. (2003); Sierra Pérez et al. (2016); Wymore et al. (2015)). Due to the small size of the optical fiber, the sensor can be embedded into the structure of the controlled object at the manufacturing stage. Thus, FOSs allow to assess the state of the product not only during its operation, but also to control the technological process of production (Matveenko et al. (2018)). The use of embedded fiber-optic strain sensors (FOSS) based on Bragg gratings, leads to a number of problems. One of which, is the redistribution of stresses in the vicinity of the embedded optical fiber, which can lead to dangerous stress concentrations. When an optical fiber is embedded in the structure of layered composite materials, there is a high probability of forming a technological defect (a resin pocket) in the inclusion region (Shivakumar and Emmanwori (2004)). In addition, an important task is to assess the reliability of the strain values, calculated based on the physical quantity measured by the sensor. Embedded optical fiber operates in a complex stress state, which prevents the direct determination of strain, since the direct correspondence between the measured Bragg wavelength shift and the longitudinal component of the strain tensor exists only for the case of uniaxial stress state of the optical fiber in the Bragg grating area. It should also be noted that in order to make reasonable use of fiber-optic strain sensor, it is necessary to evaluate the efficiency of the strain transfer from the host material to the embedded sensor. One approach to solving this problem is to use numerical simulation methods to calculate a strain transfer matrix (Luyckx et al. (2010)). In order to eliminate the effect of transverse strains on the FOSS measurements, the Bragg grating area may be surrounded by a capillary tube. (Voet et al. (2010)) have shown that the encapsulation of the Bragg grating in the capillary ensures the preservation of a single peak in the reflected spectrum after the implementation of the technological process of the composite material manufacturing. Another example of encapsulation of the Bragg grating area into the capillary is presented by (Li et al. (2014)), in which the use of a capillary is aimed at increasing the temperature sensitivity of a fiber-optic sensor. In this paper, the use of a capillary tube in the Bragg grating area to ensure uniaxial stress state of the sensor is studied. This enables the use of a direct relation between the measured value of the Bragg wavelength shift and the longitudinal strain. With the help of numerical simulation methods, geometrical and mechanical parameters of the structural scheme of a capillary tube were investigated. The stress concentration in the vicinity of the embedded optical fiber with a capillary was analyzed. The fiber Bragg grating (FBG) is a periodic change in the refractive index on a specific region of the core of a single-mode optical fiber. Such grating works as a narrowband reflecting optical filter. The light source sends a broadband optical signal through a fiber-optic core. Most of the light passes through the grating without reflection, and only light in a certain narrow wavelength range is reflected from the grating. The central wavelength (Bragg wavelength) λ * of the reflected signal is proportional to the effective refractive index n of the optical fiber core in the region of the grating and the geometric length of the grating period  (Othonos (1997)). 2. The principle of FBG operation

* 2 n  = 

(1)