Fatigue Behavior and Multiscale Structural Evolution of Polyamide 66 Industrial Fiber Under Cyclic Loading

ABSTRACT This study systematically examines the room‐temperature fatigue behavior and microstructural evolution of high‐tenacity (HT) and super‐high‐tenacity (SHT) polyamide 66 (PA 66) industrial fibers under cyclic loading at various stress levels. By integrating multiple characterization techniques, such as wide‐angle x‐ray diffraction (WAXD), small‐angle x‐ray scattering (SAXS), birefringence measurements, and Fourier transform infrared spectroscopy (FTIR), the underlying fatigue mechanisms were elucidated. The SHT fiber demonstrated significantly lower initial and total fatigue strain than the HT fiber under identical stress conditions. The crystalline structure showed no significant changes, including crystallite size, crystalline orientation, and crystallinity in both fibers before and after fatigue testing. In contrast, the amorphous regions underwent substantial reorganization. The amorphous orientation and amorphous thickness of the SHT fiber increased slightly with increasing fatigue stress, which is attributed to the alignment of coil chains under tensile fatigue stress. The molecular chains in the amorphous regions of the HT fiber were more easily extended and oriented, leading to more significant structural changes in its amorphous regions. The fatigue mechanisms of both fibers involved a transition from gauche to trans conformation, and this conformational change was more evident in the HT fiber. Additionally, the HT fiber sustained a lower maximum cyclic stress, which was associated with its lower degree of amorphous orientation. It is demonstrated that developing industrial fibers with a higher degree of amorphous orientation is crucial for enhancing their fatigue resistance.