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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transverse strain, very strict adherence to the orientation of the optical fiber is required during embedding process, which is a complex technological problem. Another factor influencing the shape of the reflected Bragg grating spectrum and introducing complexity to the definition of strain is the non-uniform strain distribution along the Bragg grating length (Peters et al. (2001)). The Bragg grating length can vary over a wide range of values. When optical fiber is embedded in the structure of such materials as laminated polymer composites, concretes, etc. consisted of several structural constituents there is a high probability of the occurrence of process-induced strains nonuniformly distributed along the length of the grating at the manufacturing stage. Thus, it is necessary to carry out studies that suggest ways to solve the problems described above for the use of FOSS based on the Bragg gratings embedded in the material. In this paper, we study the possibility of using a capillary in the Bragg grating area, which will provide the uniaxial stress state of the grating and uniform strain distribution along its length.

3. Application of the capillary tube

One of the directions for obtaining reliable information about the strains measured by the embedded Bragg grating sensors is associated with a change in the conditions of interaction with the host material in the Bragg grating area. In a constructive implementation, this can be achieved by using a capillary tube around the fiber in the section of the Bragg grating (figure 1).

Fig. 1. The scheme of a capillary tube in the Bragg grating area.

The essence of this scheme is that the capillary tube, which represents the additional external coating (capillary coating) and the cavity between the coating and the optical fiber, protects the Bragg grating from the effects of transverse strain. Thus, the sensor region will be under conditions of a uniaxial stress state, under which there is a direct correspondence between the measured value of the Bragg wavelength shift and the longitudinal strain using the relation (4). The interaction of the capillary tube and the optical fiber is provided through the connecting elements (binders) on opposite sides of the tube. In this case, the Bragg grating region can be, as covered by a protective coating or remain without any protective coating. To measure compressive strain, it is necessary to provide the preliminary tension of the optical fiber. The degree of which will determine the maximum possible value of the recorded compressive strain. The geometrical dimensions of the capillary will be determined by the thickness t , the length l , and the size of the cavity t c (figure 2). In addition, it is important to choose the material of the capillary coating, and therefore, to choose the elastic modulus c E and the Poisson’s ratio c  (assuming that the coating is made of isotropic material).