Issue 51

G.S. Serovaev et alii, Frattura ed Integrità Strutturale, 51 (2020) 225-235; DOI: 10.3221/IGF-ESIS.51.18 228 After the manufacturing process described above, the resulting plates were cut into individual specimens and the performance of embedded Bragg grating sensors was checked. FBG SPECTRA AFTER MANUFACTURING PROCESS iber-optic sensors based on fiber Bragg gratings (FBG) embedded into material can give a lot of useful information about the mechanical state of the controlled object during its life cycle. This means that it is possible to measure such parameters as strain and temperature not only during operation but also during the manufacturing stage, which provides addition control measure for such complex materials as layered reinforced composites. Also, a high- density mesh of controlled points can be established in the object of study due to the simplicity of multiplexing of FBGs in optical fiber, which means that a large number of gratings can be written and simultaneously interrogated in the single optical fiber. The principle of FBG operation, illustrated in the Fig. 4, is based on sending the broadband optical signal through the core of the optical fiber and measuring the reflected from the grating narrowband part of the initial optical signal which is centered on the so-called Bragg wavelength. The Bragg wavelength in the initial unloaded state depends on the effective refractive index of the core of the optical fiber and grating period. Under load, such as strain or temperature change, the reflected spectrum shifts and therefore the Bragg wavelength changes, and this change can be used to calculate the strain or temperature in the FBG area. Figure 4 : The principle of FBG operation. The reflected spectrum in case of FBG written in low birefringence single mode optical fiber and operating in close to uniaxial stress state has only one peak value which can be easily detected. But it is known that when optical fiber in the Bragg grating area experience high transverse strains or strain gradient along the grating, the spectrum becomes distorted due to induced birefringence. In this case several peaks can occur for the single FBG and under loading the maximum value can jump from one peak to another. So, it becomes problematic to measure the Bragg wavelength shift and such situation can lead to significant measurement errors. Manufacturing of composite materials is a complex technological process during which the constituents are exposed to severe external loads such as pressure and temperature. And the optical fiber, which is confined between the layers of composite material, experience these external loadings too. For the woven GFRP material, manufactured by the technological process described in the previous section, the specimen before the cutting contained an embedded optical fiber with 5 FBGs (Fig. 5). F