Propellant tanks typically operate under low-temperature and high-pressure conditions. However, existing fiber optic sensors often fail to meet the comprehensive and reliable assessment requirements for large-range strain measurements and mechanical performance in such extreme environments. To address this challenge, two types of metallized-packaged fiber Bragg gratings (FBGs) strain sensors were designed and fabricated, demonstrating exceptional suitability for high-precision localized and long-term strain monitoring at ultra-low temperatures. The FBGs were inscribed using a femtosecond laser line-by-line method and securely bonded to a metallic substrate with tin (Sn) solder, ensuring robust strain transfer and enhanced mechanical stability during cryogenic testing. Experimental results demonstrated that the FBG temperature sensors effectively performed temperature sensing in the ultra-low temperature liquid helium (LH) environment (5 K). To evaluate the mechanical performance of the FBG strain sensors in the LH environment, a cyclic load test of 1000 N was first conducted, followed by four overload tests with maximum loads reaching 3000 N. Experimental results demonstrated that the metallized FBG strain sensors exhibited a well-defined elastic region under loads up to 2500 N, characterized by a strain sensitivity of over 1.26 pm/N, and a hysteresis error less than 0.1 nm. Beyond 2500 N, the sensors transitioned into a plastic deformation regime, with the hysteresis error increasing to more than 1.47 nm.
He et al. (2026) studied this question.