PSI - Issue 70

T. Ramya et al. / Procedia Structural Integrity 70 (2025) 469–476

471

Nomenclature λ B

Bragg wavelength grating period

Λ

n eff

effective refractive index of the fiber core

σ A F E λ ε

axial Stress

cross-sectional area of the pipe applied axial load (in Newton)

axial strain(μm/m) Young’s Modulus

Measured wavelength of the strain sensor (nm) Reference wavelength of the strain sensor (nm)

λ 0

k

Calibration factor

2. Fiber Bragg Grating (FBG) These sensors are capable of measuring strain and temperature in specific areas of interest (Saha et al.(2024).When integrated into pipelines, they provide continuous, real-time monitoring of structural health by detecting changes in the light spectrum reflected from the fiber (Zhou et al.(2025)).The basic principle involves transmitting light through an optical fiber, where the light interacts with the environment or a sensing element. The interaction causes changes in the light's properties (intensity, phase, wavelength, or polarization), which are then measured and analyzed (Torres

et al. (2011)). 2.1 Operation

As a resonant structure, a Fiber Bragg Grating (FBG) functions as a wavelength-selective mirror, acting as a narrowband optical filter (Rao (1998)). A fiber optic transmitter injects light into the optical fiber. As depicted in Figure 2, the light reflected from grating with a narrow spectral width centered on the Bragg wavelength, while the remaining light continues through the optical fiber to the next Bragg grating without any attenuation.

Fig.2. Fiber Bragg Grating (FBG) Principle

The Bragg wavelength (λ B ) is determined by both the period of gratings and the refractive index of the core (Chen and Zhang (2011)). The FBG has a symmetric design, hence it will consistently reflect light at the Bragg wavelength regardless of the direction from which the light approaches (Arora et al.(2011) and Kreuzer (2006) ).