PSI - Issue 50

6

Valerii Matveenko et al. / Procedia Structural Integrity 50 (2023) 184–191 Valerii Matveenko, Natalia Kosheleva, Grigorii Serovaev / Structural Integrity Procedia 00 (2022) 000 – 000

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The results of the measurements of the FBG sensors under uniaxial tension of the samples were taken from the work (Matveenko et al. , 2018). To carry out a distributed measurements of strain under uniaxial tension, samples of a similar shape from the same batch of material with optical fibers glued on the surface of the samples were used. The test modes were similar to that described in (Matveenko et al. , 2018). The distribution of strain along the length of the sample at different load levels obtained with the help of FBG sensors (horizontal red lines, the length of which corresponds to the length of the FBGs) and DFOS (blue solid line) is shown in Fig. 4a for a sample with circular cutouts and in Fig. 4b for a sample with triangular cutouts. The graphs also show the results of numerical simulation by the finite element method (black dashed line) for the mechanical characteristics of the material: E 11 = 22 GPa, E 22 = 22 GPa, E 33 = 6 GPa, G 12 = 4 GPa, G 13 = 3 GPa, G 23 = 3 GPa,  12 = 0.14,  13 = 0.13,  23 = 0.18.

Fig. 4. Strain distribution along the length of the sample with (a) round cutouts; (b) triangular cutouts.

It is worth noting the differences in the shape of the strain distribution for the samples under consideration. For a sample with circular cutouts, the shape of the strain distribution is close to the normal distribution graph with a single peak in the central part. Triangular cutouts form a section with two peaks in the central distribution area, which creates additional difficulties when conducting distributed measurements. Satisfactory qualitative and quantitative correspondence of numerical simulation results, FBG and DFOS measurements with a spatial resolution of 1 mm and gage length of 5 mm was obtained for the sample with circular cutouts. For a sample with triangular cutouts, there is a satisfactory correspondence of the measurements in the areas of the largest strain gradient, however, there is a discrepancy in the results in the region of two peaks in the central part of the strain distribution. The distributed measurement of strain is carried out by measuring Rayleigh scattering profiles at the initial, unloaded moment of time and in the loaded state. Then the scattering profiles are divided into fragments of a given length along the measured section of the optical fiber, which are translated into the frequency domain using the Fourier transform, where a cross-correlation procedure is performed for each section to determine the spectral shift (Kreger et al. , 2009). The length of the fragments into which the scattering profile is divided (gage length) affects the readings during measurement. As the sensor length increases, the number of points on the scattering profile increases, which are used to transfer data from the time domain to the frequency domain, and consequently, the spectral resolution and measurement accuracy increase. On the other hand, the presence of a significant strain gradient along the length of the fragment (gage length) makes it difficult to calculate the cross-correlation peak (Luna Technologies Inc., 2013). In addition, with an increase in the length of the sensor, it becomes difficult to determine the local strain gradient, due to averaging of measurement data.