Issue 51

G.S. Serovaev et alii, Frattura ed Integrità Strutturale, 51 (2020) 225-235; DOI: 10.3221/IGF-ESIS.51.18 226 insignificant change of stiffness of composite materials with embedded optical fibers is usually observed [1–3]. The decrease in strength is most pronounced under compressive loads compared to tensile loads and when optical fiber is oriented perpendicular to the load direction. A significantly larger negative effect of the embedded optical fibers on the strength of composite materials is observed under fatigue and impact tests in the literature [4–6]. Despite its small size, the diameter of the optical fiber is comparable to the size of a layer of composite material and, if embedded, can cause distortion of the layers. In this case during the manufacturing process of a composite material, this area around the optical fiber becomes filled with epoxy resin. Such technological defect in shape resembles the eye and in literature is called the resin pocket [7]. The size and shape of the resin pocket are determined by many factors, the main of which is the orientation of optical fiber to reinforcing fibers [8]. Among other factors are the mechanical properties of the composite material, the layer’s thickness, the number and stacking sequence of the layers, the size of the optical fiber, curing pressure and etc. In the study [9] an approach to determine the shape of a resin pocket, based on numerical finite element modeling of the pressing process in a plane-strain formulation of the theory of elasticity, is considered. Verification of the obtained numerical results with real cross-sectional images of polymer composite material (PCM) samples with embedded optical fiber showed the reliability of this method for determining the shape of the resin pocket. A numerical study of the effect of an embedded optical fiber on a local stress concentration is performed in [10]. The authors proposed a description of the fracture mechanism under tensile and compressive loads which starts in the region of the resin pocket. The study [11] investigated the effect of the resin pocket geometry, caused by the introduction of the microvascular canal into glass fiber reinforced plastic, on stress concentration and stress distribution during the failure of composites with two types of stacking [0/90] 4s and [90/0] 4s . Experimental studies, demonstrating a significant strain concentration in the vicinity of an embedded optical fiber are described in [12,13]. Aside from the issues related to the influence of embedded optical fibers on the host material, the problem of reliable strain measurements is of utmost importance. The study of the strain transfer from the host material to the embedded optical fiber with the use of strain transfer matrix is performed in [14]. Also it is known that the reflected optical signal of fiber-optic sensors based on Bragg gratings is distorted under transverse loads [15–18] and non-uniform strain distribution along the length of the grating [19–21]. The problem of reliable strain measurements by the embedded optical fibers, questions of how to choose the model linking the strain of the optical fiber in the area of the fiber Bragg grating with the measurement of its resonant wavelength and calibration problem as well as particular practical problems of measuring strains in composite materials with fiber-optic strain sensors are discussed in [22–25]. Most of the known studies focus on the problems that arise when optical fibers are embedded in layered composite materials with a unidirectional reinforcement structure. For such materials, the fact of dependence of appearance and dimensions of the resin pocket on the orientation of the embedded optical fiber with respect to the direction of reinforcing fibers is well known and experimentally verified. However, there is an insufficient number in studies concerning the incorporation of optical fibers into composite materials having other than unidirectional reinforcement structure. In this paper, layered composite materials with a woven reinforcement structure were studied. The introduction of such a foreign object as an optical fiber between the layers of this type of composite materials can have a significantly different effect on the distortion of adjacent layers in the region of the optical fiber due to a different internal structure. In the framework of the work, glass fiber reinforced plastic (GFRP) and carbon fiber reinforced plastic (CFRP) samples with embedded optical fibers were made. Using a digital microscope, cross-sectional images of the fabricated samples were obtained, which made it possible to analyze the structural distortions of the considered types of composite materials due to the embedded optical fiber and to evaluate whether a resin pocket technological defect occurs. Different kinds of sensors are based on the fiber-optic technology and are widely used in structural health monitoring [26]. In this study fiber-optic sensors (FOS) based on the fiber Bragg gratings (FBG) were used and the reflected optical signal from embedded sensors was analyzed. M ANUFACTURING OF COMPOSITE MATERIAL WITH EMBEDDED OPTICAL FIBERS he process of creating products from polymer composite materials (PCM) includes several basic steps: laying out a reinforcing material in the specialized form, package assembly, the polymerization process and removal of manufactured sample. These technological steps are common for every manufacturing technique with some additional features depending on the particular process. For the present study a woven fabric with weave style 2  2 twill was chosen as the reinforcement material. The weaving pattern is shown in the Fig. 1. The specimens were made with the help of compression molding manufacturing technique which allows to control the entire process and to achieve composite parts with uniform thickness. T