Numerical modeling of the capillary in the Bragg grating area, ensuring uniaxial stress state of embedded fiber-optic strain sensor

In this paper, the possibility of providing a uniaxial stress state of FOSS based on the use of a capillary tube in the area of the Bragg grating is studied. The capillary tube, which consists of the additional external coating and a cavity between the coating and the optical fiber, protects the Bragg grating from the effects of transverse strain. This allows to use a direct relation between the measured value of the Bragg wavelength shift and the longitudinal strain. With the help of numerical simulation, the geometric and mechanical parameters of the capillary tube have been studied. Numerical calculations showed a slight effect of the elastic modulus of the capillary coating on the cavity size under load. However, the presence of a cavity in the capillary leads to an increase in stress concentration, which must be considered when designing a capillary tube for a particular application. The obtained results confirmed the effectiveness of the structural scheme of the capillary in the area of FOSS embedded into the controlled material. (C) 2019 The Authors. Published by Elsevier B.V.