The effect of controlled pre-twisting on the tensile properties of compact carbon nanotube fibers.

Tensile experiments on individual solution-spun carbon NanoTube (CNT) fibers after controlled pre-twisting reveal a significant trade-off: a marked enhancement in ductility alongside a reduction in stiffness and strength. Although the baseline fiber exhibits a densely-packed macroscopic appearance, scanning electron microscopy indicates that twisting separates the fiber into detached mesoscale fibrils with a helical architecture. A mechanical model is proposed based on two hypotheses. (1) Since detached fibrils begin with identical lengths, twisting them generates a helical path mismatch. When the fiber is pulled, this mismatch forces the fibrils to engage sequentially from the outer edge to the core, driving a non-linear drop in overall stiffness. We demonstrate this mechanism of sequential engagement using a physical analogue made of nylon filaments. This behavior contrasts sharply with standard twisted yarns, where filaments are arranged along their helical paths without length discrepancies, resulting in simultaneous engagement under tension. (2) The twisted fibrils exhibit ductility exceeding the ultimate strain of the untwisted configuration. Consistent with prior studies, we propose that helical confinement enhances the inter-CNT sliding capacity within the fibrils at the nanoscale. A parametric analysis evaluating various shapes of "constitutive continuation" for the fibrils demonstrates that plastic-like responses yield excellent agreement with the experimental data. Since our approach is derived from purely geometric considerations and can be combined with different constitutive laws, it is readily extendable to different fiber configurations and other filamentous materials, providing a physically-grounded framework for designing hierarchical fibers through controlled pre-twist.