Mechanical simulation and load test.

The mechanical properties of fiber Bragg Grating (FBG) strain sensors in ultra-low temperature environments are very important for structural health monitoring (SHM) in the aerospace field. In this paper, two kinds of FBG temperature sensors and strain sensors were fabricated by femtosecond laser technology and ultrasonic metallization packaging process. The strain transfer mechanism, microstructure behavior, and macroscopic mechanical properties of the FBG strain sensors were discussed in detail and the corresponding theoretical models were constructed. The stress-strain curve and the elastic-plastic stage of the FBG strain sensor in liquid helium environment (5K) were simulated. The experimental results showed that in the range of 280 K~5 K, the sensitivity of double-end fixed and full-coverage metal-packaged FBG temperature sensors was -33.9 pm/K and -39.8 pm/K, respectively. Then, the stability and reliability of the designed FBG strain sensor were verified by three cyclic load tests at 5 K. In the range of 300 N-1000 N, the sensitivity of the double-end fixed and the full-coverage metal-packaged FBG strain sensor were 1.46, 1.42, 1.47 pm/N, and 1.29, 1.25, 1.24 pm/N, respectively. Further, the mechanical properties of the strain sensor were characterized by overload experiments, and it was verified that the elastic deformation occurred within 2500 N of the designed two FBG strain sensors and the plastic deformation occurred when the load was greater than or equal to 3000 N. The experimental results agree with the theoretical simulation, and the results of this work pave the way for the analysis of mechanical properties and structural health monitoring system of FBG strain sensors under an ultra-low temperature environment.