PSI - Issue 54

V.P. Matveenko et al. / Procedia Structural Integrity 54 (2024) 218–224 Matveenko V.P., Serovaev G.S., Kosheleva N.A., Galkina E.B../ Structural Integrity Procedia 00 (2023) 000 – 000

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more pronounced change and shift in the spectrum for FBGs with a length of 5 and 10 mm. This result may be due to the inaccuracy of the FBG positioning during installation, since even a slight shift in the location of the sensor along the sample's length can lead to a considerable change in the level of the strain gradient that falls on the grating. It is worth noting that precise positioning of an FBG presents a challenging problem, given that the position of the sensor is not visually identifiable, and locating an FBG by specifying local strain along the optical fiber introduces a significant margin of error in determination. To more accurately determine the FBG's location after attaching to the surface of the sample, an optical backscatter reflectometer OBR4600 was used, which allows the measur ement of an optical fiber’s reflectogram with submillimeter resolution based on Optical Frequency Domain Reflectometry (OFDR). By conducting a distributed strain measurement using the OBR4600 and combining this distribution with a reflectogram, it is possible to accurately determine the location of the FBG under study and estimate the strain gradient that falls on the length of the grating. Fig. 4 shows a graph of the strain distribution along the sample obtained using a backscatter reflectometer and the strain values obtained using the studied point FBG sensors based on the relationship: 1     =  B k (1) Where   is the shift of the resonant wavelength relative to the reference state  B , k = 0.78 is strain sensitivity coefficient.

Fig. 4. Strain distribution along the sample obtained using distributed fiber-optic sensors (DFOS) and strain measurements based on studied FBG sensors.

It is worth noting that the presence of a strain gradient along the FBG results in a change in the shape of the reflected spectrum and its broadening that complicates the determination of the resonant wavelength. The results depicted in Fig. 4 correspond to the determination of the resonant wavelength based on the maximum peak value of the reflected spectrum. The results obtained showed that an FBG with a length of 5 mm is closer to the zone of maximum strain of the sample compared to FBGs with lengths of 10 and 15 mm. The strain magnitude obtained from the readings of a 5 mm long FBG was significantly higher than the strain in the area where the FBG is located, as determined using a distributed strain measurement. Table 1 presents the strain gradient values that occur on the studied FBGs at different load levels applied to the sample.