Issue 51

G.S. Serovaev et alii, Frattura ed Integrità Strutturale, 51 (2020) 225-235; DOI: 10.3221/IGF-ESIS.51.18 233 Despite the fact that optical fiber lies in resin rich region, only an insignificant distortion of the internal structure of the material in the area of the embedded optical fiber without the formation of a resin pocket, for all studied samples of GFRP, was found. The microscopic images of CFRP samples for polished and unpolished surfaces are presented in the Fig. 13. Due to a number of differences between carbon fibers and glass fibers (physico-mechanical properties, fiber size and density, etc.), a composite with carbon fiber reinforcement has a more ordered structure, in which, using an optical microscope, it was not possible to isolate individual warp fibers, only the weft fibers. In this case, the advantage of polishing the surface is clearly expressed, since in the absence of surface treatment the optical fiber is weakly or not at all distinguishable. In the obtained images, it was not possible to trace the distortions of the internal structure of the woven laminated composite material in the region of the embedded optical fiber. The presence of a resin pocket in the area of embedded optical fiber is common for unidirectional composite materials and is a potential source of cracking and delamination. The absence of signs of a resin pocket formation for the considered composite materials with a woven reinforcement structure, obtained in the present study points at differences in the interaction of the internal structure of the composite materials and the embedded optical fiber. A possible explanation is that unlike unidirectional reinforced layers, which are bent around embedded optical fiber in case when the reinforcement direction is perpendicular to the optical fiber, the individual weft and warp fibers of woven structure provide more space for the optical fiber to be more efficiently integrated into the structure of the composite material. C ONCLUSIONS he present work is devoted to the study of the influence of embedded optical fibers on the internal structure of composite materials. One of the main goals was to expand knowledge on this problem for various types of composite materials. Unlike known studies, devoted to the materials with a unidirectional reinforcement structure, two types of composite material (GFRP and CFRP) with woven reinforcement structure and with embedded fiber-optic sensors based on Bragg gratings were studied. The evaluation of the readings of Bragg grating sensors embedded into composite material revealed that the reflected spectrum has distortions due to manufacturing process that can lead to significant errors in the measurements of strain. This result indicates the need for additional mechanical protection of FBG when embedded in a composite material, which will allow avoiding strong distortions of the reflected signal during the manufacturing stage, or to use the more advanced methods for determining the Bragg wavelength shift, taking into account the shape of the spectrum. Cross-sectional images of studied polymer composite materials with different types of surface treatments were analysed for structural changes due to the embedded optical fibers. Polishing of the studied surface makes it possible to more clearly distinguish the location of the optical fiber in the structure of the composite material, however, for the case of glass fiber, the visibility of the structural components of the composite deteriorates. For carbon fiber composite samples without polishing, it is almost impossible to detect optical fiber. However, for studied materials, analysing both polished and unpolished surfaces provides more complete information about the internal structure of the material. An analysis of the structure did not reveal any significant distortion of the layers in the region of the embedded optical fiber and the formation of a resin pocket for the considered composite materials with a woven reinforcement structure. The obtained results indicate that the use of embedded optical fibers is promising and feasible for assessing the mechanical state of structures made from composite materials. However, in order to build effective and reliable monitoring systems based on this technology, it is necessary to take into account such factors as the type and structure of the reinforcement of the composite material into which the optical fiber is embedded, the manufacturing process, possible distortions of the reflected optical signal, on the basis of which the measured physical quantities (strain, temperature) are determined. A CKNOWLEDGEMENTS he research was supported by RFBR (project No. 17-41-590684 r_ural_a). T T