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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Table 1. Strain gradient for studied FBGs under different load levels.

FBG

Strain gradient (µε/mm), F = 0.98 N

Strain gradient (µε/mm), F = 1.96 N

Strain gradient (µε/mm), F = 2.94 N

Strain gradient (µε/mm), F = 3.92 N

FBG 5 mm FBG 10 mm FBG 15 mm

64 87 76

130 174 153

195 262 229

259 348 305

It is known that the strain limit for FBGs inscribed using phase mask technology is lower compared to FBGs inscribed point-by-point with a femtosecond laser without removal of the protective coating. This feature was observed during the experiments: the optical fiber in the 5 and 10 mm FBG region was damaged under a load of 4.9 N, in contrast to a 15 mm FBG, which maintained its integrity up to a load of 8.44 N. Fig. 5 shows the reflected spectra of a 15 mm FBG under loads up to 8.44 N for both tension (blue curve) and compression (red curve) cases.

Fig. 5. Reflected spectra of 15 mm FBG inscribed by femtosecond laser under various load levels for tension (blue curve) and compression (red curve) cases.

The reflected spectra of FBG under tension and compression are almost symmetrical. 4. Conclusions

The study presents results on the influence of the strain gradient along the length of FBG on the shape of the reflected spectrum. In order to achieve sufficient strain gradient different sample designs were investigated and geometric configuration of the sample with non-linear variation of the cross-sectional area was chosen for experiments. FBGs of different lengths and fabricated using two types of inscription techniques: phase mask (5 and 10 mm) and point-by-point inscription using femtosecond laser (15 mm) were studied. To precisely locate the studied FBGs on the sample it was proposed to employ distributed measurements based on Rayleigh scattering. It was shown that FBG inscribed with femtosecond laser without removing protective coating withstands much higher load and show symmetrical spectral change in cases of tension and compression.