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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From the obtained results, it can be concluded that even with a small value of the cavity size and with the load, which significantly exceeds the material strength, the reduction of the gap between the external coating and the optical fiber does not exceed 30%. The change in the elasticity modulus of the external coating with the constant cavity size and thickness of the external coating (0.012 mm) also slightly affects the gap size between the coating and the optical fiber under external load. It should be noted that with the considered parameters of the capillary tube and the material properties, in which the optical fiber is embedded, it is possible to achieve a uniaxial stress state in the Bragg grating area. At this case, the relative difference between the longitudinal strain values in the fiber and in the material zone adjacent to the fiber does not exceed 3%. However, for the anisotropic properties of the host material with a circular section of the additional coating, these conditions are not met. Studies have shown that achieving an acceptable coincidence of strain along the fiber and strain in the material, made of carbon fiber-reinforced plastic with effective properties 84.1 x y E E = = GPa, 10.7 z E = GPa, 0.035 xy  = , 0.45 yz xz  =  = , 4.3 xy G = GPa, 3.5 yz xz G G = = GPa, is possible with an elliptical cross section of the capillary with a ratio of ellipse semi-axes equal to a/b = 0.6, where the semi-axis a coincides with the y axis. Embedding into the material such a foreign object as an optical fiber causes a redistribution of the stress-strain state in a certain vicinity around the embedded object and may cause a high stress concentration. As part of the study, analysis of the value y y K P =  was made that determines the stress concentration coefficient for the corresponding component of the stress tensor. The stress concentration field K y for the central along the length cross section of the model for the cases with embedded optical fiber without a capillary tube and with the presence of a capillary tube is presented in figure 6.

Fig. 6. The stress concentration field K y for material with embedded optical fiber: а ) without a capillary tube b) with a capillary tube.

The optical fiber embedding causes a stress concentration in the vicinity of the fiber, and taking into account in the calculation scheme of the cavity between the optical fiber and the external coating increases the stress concentration to a value of 3.03. A change in the cavity size by 2 times does not significantly affect the stress concentration value. These results are obtained for the studied isotropic material. However in case of composite material with effective mechanical properties described previously, the embedding of the optical fiber leads to a stress concentration 3.87 y K = and introducing the capillary into the model, increases the stress concentration up to the value of 6.31. This means that for anisotropic materials, such as composites, the risk of material failure in the vicinity of the embedded optical fiber and capillary tube is higher compared to the isotropic materials. These results should be taken into account during the design stage of product development. 1.81 y K =