PSI - Issue 5

N.A. Kosheleva et al. / Procedia Structural Integrity 5 (2017) 99–106 V.P. Matveenko et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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, d (2) Mathematical modeling of the strain interaction between the sensor elements and the sensor element itself with investigated surface makes it possible to evaluate effectively the dependence of the calibration factor on all the above factors. Modeling was carried out within the framework of elastic deformation of all sensor elements. For numerical implementation, finite element software (ANSYS) was used. Fig. 6a shows the general structural scheme of the FOS, where the substrate dimensions are 60×10×0.2 mm, the diameter of the quartz fiber is 125 μm, the thickness of the inner acrylic coating is 125 μm, the thickness of the outer polymer coating is 125 μm, the diameter and thickness of the adhesive point is 3 mm and 2.5 mm, the diameter of the technological holes is 1 mm. Materials of sensor design elements have the following mechanical characteristics: substrate E s = 2.1 · 10 11 Pa,  s = 0.3; the optical fiber E o = 71.4 · 10 9 Pa,  o = 0.17; the inner coating E ic = 1.56 · 10 9 Pa,  ic = 0.13; the outer coating E oc = 3.1·10 9 Pa,  oc = 0.15; epoxy adhesive E ea = 3 · 10 9 Pa,  ea = 0.32. а b K   

3

4

4

3

1

2

Fig. 6. а ) Structural scheme of FOS ( 1 − substrate, 2 − optical fiber, 3 − adhesive point, 4 − technological holes); b) ¼ sensor symmetric part.

4%

a b c d

25%

42%

29%

Fig. 7. The degree of influence of various design factors on the values of calibration factors K ε : a − optical fiber; b − mounting; c − adhesive; d − technological hole.

The analysis of numerical experiments results was done with a mathematical model that described the three dimensional spatial distribution of the strain of all FOS elements on a substrate. The sensitivity of the calibration coefficient to the type of adhesive joint of the fiber and the presence of technological holes in the metal substrate was established, as well as to the method of mounting the substrate to the study surface. In particular for determination the most rational way of adhering the FOS metal substrate to the investigated surface, the strain response of the fiber sensor to a homogeneous uniaxial strain (ε) was established with two fixing methods. The first method is to adhering only on the side faces of the substrate. The second method consists of adhering the entire contour of the substrate. It was assumed that from the two methods that was considered above, the one for which the calibration coefficient is closest to unity was chosen as the rational one. For the first method, the coefficient is K ε = 1.23, for the second K ε = 1.007. The choice of the mounting the FOS substrate to the investigated surface by adhering the substrate throughout the contour was indicated by this ratio of the values of the coefficients. From the analysis of the totality of the numerical experiments results, the influence degree of various design factors on the values of the calibration coefficients K ε was established. As such factors were considered: the physical and mechanical properties of optical fiber; the presence of a technological hole on a metal substrate; physical and mechanical properties of the adhesive; conditions for adhesive joint the metal substrate. The influence degree of these factors on the calibration factor as a percentage is shown in Fig. 7.