Long-gauge ultra-weak FBG strain sensor insensitive to structural defects: numerical simulation and experimental verification.

Strain monitoring of concrete structures with surface defects and material inhomogeneity is a challenge in the field of structural monitoring. In this paper, based on the characteristics of long-gauge structure and ultra-weak fiber Bragg grating (UWFBG) array, a strain sensor array with long distance and low requirement for structural surface flatness is proposed. The main measurement principle of the sensor is to convert the continuous strain of the measured structure into the average strain of each UWFBG, which can suppress the strain mutation phenomenon caused by structural defects. In the simulation, the long-gauge sensor can effectively reduce the interference of the defect on the strain data, and the sensitivity of the sensor to the structural defect gradually decreases with the increase of the gauge length. In the experimental tests, the sensor proposed in this paper can accurately reflect the surface strain field distribution and the overall deformation of the concrete structure compared with the traditional point-type fiber Bragg grating (FBG) sensor.