Multi-directional strain measurement under thermomechanical loading using embedded phase-shifted fiber Bragg grating

This study presents a multi-directional strain measurement technique under thermomechanical loading, utilizing the sensitive birefringence of an embedded phase-shifted fiber Bragg grating. The spectral response of the phase-shifted fiber Bragg grating to temperature and multi-directional strains is mathematically elucidated and simulated using a modified transfer matrix method considering the polarization state. Strain decoupling and reconstruction algorithms are newly proposed based on thermomechanical analysis, which relate the spectral characteristics, strains in the sensor, and strains in the host material. After calibration of the temperature and multi-directional strain responses, a phase-shifted fiber Bragg grating is embedded into a polyetherimide specimen and subjected to thermomechanical loading, achieving the measurement of pressure as well as three-directional strains in the sensor and polyetherimide. The presented technique is demonstrated to be reliable for accurate multi-directional strain measurement under thermomechanical loading with minimal errors and shows potential for in-situ measurement of advanced structures.