Water-filled capillary tubes are a kind of standard component in both life science (e.g., blood vessels, interstitial pores, and plant vessels) and engineering (e.g., MEMS microchannel resonators, heat pipe wicks, and watersaturated soils). Under sufficiently low temperatures, water in capillary tubes undergoes phase transition and exhibits frost heave, which can cause deformation, damage, and even fracture of tube wall. However, the thermomechanical analysis of freezing water-filled capillary tubes remains obscure, particularly regarding the rapid change in water temperature due to thermal transient effects. We develop a thermal model of freezing in a waterfilled capillary tube that is suddenly exposed to cold air flow, with the time domain divided into two regimes, separated by the thermal penetration time tp. The effect of thermal penetration on temperature distribution is solved. Then, a distinction is made between freezing occurring before thermal penetration and those occurring after thermal penetration. We next analyze transient mechanical stresses acting at tube wall, with interfacial tension and frost heave effect accounted for. Results obtained are not only useful for preventing frost heave failure but also provide theoretical guidance for tailoring the freezing resistance of microfluidic devices used in MEMS.

The deformation of foundation soil caused by freeze-thaw cycles is a typical geological disaster in engineering construction in permafrost areas. Fiber optic sensing technology provides an important technical means for accurate and distributed real-time monitoring of frozen soil deformation. To explore the feasibility of distributed fiber optic strain sensing in monitoring frozen soil deformation, this study utilized a self-developed optical cable-frozen soil interface mechanical characteristics tester to investigate the failure mechanism of the cable-soil interface in frozen soil samples with different dry densities and initial water contents. The experimental results indicate that the fiber optic strain monitoring results accurately reflect the progressive failure characteristics of the cable-soil interface, and the strain softening model can better describe the mechanical properties of the interface. During the freezing process, the liquid water in the soil becomes ice, causing the movement of the freezing front and water migration, and resulting in significant differences in the mechanical properties of the interface. The evolution process of the shear stress at the cable-soil interface at different depths reflects the deformation coordination state with the frozen soil during the cable pullout process, indicating that the measurement range of the cable and the coupling of the interface are closely related to the dry density and initial water content of the soil. This study provides a reference for the application of optical fiber sensing technology in deformation monitoring of frozen soil foundation in cold regions.