PSI - Issue 18

Valerii Matveenko et al. / Procedia Structural Integrity 18 (2019) 12–19 Author name / Structural Integrity Procedia 00 (2019) 000–000

15

4

three sensors (s4, s5, s6) to register longitudinal strains. The scheme of fiber-optic lines mounted on the sample surface is shown in the Fig. 1.

Table 2: Mechanical characteristics of optical fiber and epoxy.

Material silica glass polyimide epoxy E, GPa 71.4 2.5 2.9  0.17 0.35 0.36

Fig. 1. Scheme of optical fibers mounted on the sample surface.

To assess the effect of adhesive bonding on sensor data, numerical and experimental studies were performed. The physical quantity that is recorded by FOSS, based on the Bragg grating, is the change in the wavelength of the reflected spectrum. Relationships that establish the link in the wavelength change of the reflected spectrum with the strains of the optical fiber in the Bragg grating zone for a single-mode optical fiber have the following form (Geert Luyckx et al. 2010):

2 1 ( 2 1 (

*   

12 2      (

))

11 1 n p p

  

1

3

3

(3)

2 n p p 11 2

12 1 3      (

))

  

2

3

*

2



where 3  – strain along the fiber, 1 2 ,   – principal strains in the plane perpendicular to the optical fiber, * * 1 1 2 2 ,           – the difference of the resonant wavelengths of the reflected spectrum in the current 1 2 ,   and initial *  times, 11 12 , p p – Pockels coefficients, n - effective refractive index of optical fiber. The simplest way to use these relations to calculate the strains with the help of experimental information obtained from the sensor is based on the assumption of a uniaxial stress state in the area of the optical fiber and the Bragg grating. At these case: 1 2      and



  

 2 n p p p 2 12 11 (





1   

)

   

(4)

12

3

*  

1 k



(5)

  

3

*



Here  - Poisson ratio of optical fiber. For used silica glass fibers k = 0.78.