PSI - Issue 17

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

5

375

Fig. 2. Geometric parameters of the capillary tube.

The capillary tube must meet the following conditions: the realization of a uniaxial stress state in the Bragg grating area and the coincidence of the strain values along the fiber directly in the fiber and in the material adjacent to the fiber. Fulfillment of these conditions will provide a reliable measurement of the longitudinal strain with the help of the FOSS. On the other hand, the dimensions of the additional external coating and cavity should ensure a uniform distribution of the strain along the entire length of the Bragg grating and exclude the possibility of touching of the outer coating and the optical fiber in a wide load range. With the help of numerical simulations, the parameters of a capillary tube were estimated on a model problem for a parallelepiped with embedded optical fiber and a capillary in the Bragg grating area (figure 3a). The optical fiber has a diameter of 0.124 mm and a protective coating made of polyimide with a thickness of 0.012 mm. The Bragg grating length is 33 optical fiber diameters (5 mm). The length of the parallelepiped is 8 times longer than the Bragg grating length and in the cross section has the shape of a square with a side equal to 40 diameters of the optical fiber. As an external load, pressure P distributed along the parallelepiped edge in the direction perpendicular to the optical fiber is considered. In this loading case, the greatest difference from the uniaxial stress state takes place in the optical fiber. The calculation of the stress-strain state was carried out by the finite element method in the framework of the linear elastic theory. Perfect contact of all adjacent parts of the model is assumed. Due to symmetry, a quarter of the model with the corresponding boundary conditions was considered. In the finite element method implementation, a mesh with a refinement in the inclusion region was used (figure 3b).