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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Comparing the results of stress distribution along the fiber boundary with the polymer coating and without it, it can be stated that the local stress concentration, which is caused by singular points (point A ) on the outer surface where the polymer coating and composite material have a contact, decreases along the thickness of the polymer coating. Based on this result, an additional element in the input-output zone can be introduced in the form of coating with a length of 2 l. In this example it is a fluoroplastic coating with 0.2 mm thickness and the following mechanical characteristics: E fc = 0.545 · 10 9 Pa, ν fc = 0.466. Fig. 5c shows the stress intensity distribution over the surface of the quartz fiber guarded by the fluoroplastic coating without and with a modified geometry nearby the singular point (see Fig. 4b). The results show that the fluoroplastic coating with the corresponding thickness eliminates the local stress concentration in the optical fiber. In this case, the change in the geometry in the vicinity of the singular point A practically does not affect on the stress state picture in the optical fiber.

P

P

a

b

z

l

l

r

A

A

O

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Fig. 4. Options for the input-output of the optical fiber from the material.

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c

b

i P 

i P 

i P 

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1 2

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0 .5

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- 0 .1 - 0 .0 5

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Fig. 5. The distribution of stress intensity over the surface of a quartz fiber: a) without a polymer coating, b) with a polymer coating, c) with a polymer and ftoroplastic coatings; 1 — without rounding; 2 — with rounding.

4. Evaluation of the influence of various structural factors of the fiber-optic sensor on the substrate on the values of the calibration factor To register axial strain on the surface of deformable bodies, fiber optical sensors (FOS) with Bragg gratings on substrates in various designs are used. The common thing that unites them is that they represent the structure that consists of heterogeneous interacting elements. At least there are three elements in them: an optical fiber with a Bragg grating, a substrate and an adhesive that connects the fiber to the substrate. All components of the sensor and their interaction determine how the strain values on the surface of the deformed body are correlated with the strain values of the optical fiber which are recorded with the help of the Bragg grating. Also, the method of the sensor mounting to the surface, which is under investigation, is an important factor affecting to this correspondence. The correspondence between the strain values on the surface of the deformable body (  ) and the values of the optical fiber strain (  d ) are determined by the calibration factor: