High-performance carbon nanotube (CNT) fibers inevitably encounter complicated thermal environments while used in aerospace structures and intelligent actuators. Catastrophic damage is unavoidably induced due to the loss of their mechanical properties. Herein, a modified Hopkinson bar and an in-situ SEM tensile equipment were introduced to study the high-temperature time-dependent mechanical behaviors and microstructure evolution of individual CNT fibers. It was found that the interaction between CNTs was weakened by thermal expansion and defects were introduced into CNTs by thermal oxidation. In addition, the effects of temperature on the viscoplastic strain transition and durability of CNT fibers were studied through a series of relaxation experiments. A shear-lag unit cell and a three-level mathematical model were developed for describing the hierarchical structure evolution of CNT fibers. These models clarified the mechanism of strength loss of CNT fibers at high temperatures and characterized the effect of thermal expansion on their microstructure and mechanical properties. The overlap length and the distance between tubes as a function of temperature were discussed. According to the modeling, an effective strategy was proposed to improve the mechanical properties of CNT fibers at high temperatures by restraining thermal expansion and thermal oxidation. This work would provide crucial theoretical and experimental guidance for the improvement of the thermal dynamic stability of CNT fibers and their composites in extremely high-temperature operating conditions.

高性能碳纳米管（CNT）纤维在用于航空航天结构和智能执行器时不可避免地会遇到复杂的热环境。由于其机械性能的丧失，不可避免地会引起灾难性损坏。在此，引入改进的霍普金森杆和原位 SEM 拉伸设备来研究单根 CNT 纤维的高温时间依赖性机械行为和微观结构演变。结果发现，CNT 之间的相互作用因热膨胀而减弱，并且通过热氧化将缺陷引入 CNT。此外，通过一系列松弛实验研究了温度对 CNT 纤维粘塑性应变转变和耐久性的影响。开发了剪切滞后晶胞和三级数学模型来描述 CNT 纤维的层次结构演化。这些模型阐明了 CNT 纤维在高温下强度损失的机制，并表征了热膨胀对其微观结构和机械性能的影响。讨论了重叠长度和管之间的距离作为温度的函数。根据该模型，提出了一种通过抑制热膨胀和热氧化来提高 CNT 纤维在高温下的机械性能的有效策略。这项工作将为提高碳纳米管纤维及其复合材料在极端高温操作条件下的热动态稳定性提供重要的理论和实验指导。