Thermoviscoelastic freezing in glass‐to‐metal seals: In situ capturing of the set point and strain entrapment

Abstract Sealing glasses retain frozen‐in features formed under nonequilibrium conditions, yet the interplay between structural relaxation and strain evolution remains insufficiently understood. Glass‐to‐metal (GTM) seals provide a representative system wherein such strain memory is both measurable and performance‐critical. Here, femtosecond‐laser‐inscribed fiber Bragg grating (FBG) sensors were embedded into sealing glass to enable in situ strain monitoring under three controlled cooling rates (1, 5, and 10°C/min). The set point ( T Q ), marking the onset of strain freezing, was experimentally identified and theoretically predicted based on relaxation dynamics. T Q consistently increased with faster cooling, as did the strain‐freezing offset, indicating suppressed relaxation capacity of the glass network. Real‐time strain profiles further revealed a five‐stage evolution: prefreezing relaxation, glass transition‐induced freezing, viscoelastic compaction, rebound dynamics, and residual entrapment. A free‐volume‐based framework is proposed to explain the interaction among atomic mobility, constrained relaxation, and rebound phenomena. Quantitative analysis indicates that faster cooling amplifies the deviation of residual strain from theory due to restricted structural kinetics. These findings elucidate the role of thermoviscoelastic freezing in relaxation–strain coupling and validate embedded FBG sensing as a real‐time structural diagnostic approach for sealing glasses.