Mechanical Characteristic Analysis of Special Hybrid Adhesive-Packaged FBG Strain Sensor at Liquid Helium Environment

Structural health monitoring (SHM) of aerospace equipment is the key to ensure the equipment’s on-orbit performance, operation safety, and service life. Aiming at the problem of SHM of the carrier rocket fuel tank under a cryogenic environment, a kind of fiber Bragg grating (FBG) strain sensor based on special hybrid adhesive-packaging technology was proposed, which realizes temperature response measurement and mechanical characteristics analysis at liquid helium condition. The FBG inscribed by the femtosecond laser line-by-line method was directly packaged on the metal substrate through a special hybrid adhesive, and the optical sensing and strain transfer mechanism of FBG strain sensor were analyzed theoretically. Experimental results show that the proposed sensor exhibits a temperature sensitivity of −17.19 pm/K, linearity ${R}^{{2}}$ is 99.957% within the temperature range of 291–5 K. From experiments of five load-unload cycles for the designed strain sensor at 5 K, its strain sensitivity is 1.62 pm/N. In addition, the elastic stage and plastic stage are also analyzed due to the importance of FBG strain sensor in low-temperature structure, and the strain sensitivity is 1.64, 1.39, 1.41, and 1.39 pm/N, respectively, within the strain range of 300–1500, 2000, 2500, and 3000 N. In our work, the reusability, reproducibility, hysteresis, and error analysis of the FBG strain sensor were also evaluated to demonstrate its potential for precise temperature–strain monitoring at cryogenic conditions.