Morphology-dependent load transfer governs the strength and failure mechanism of carbon nanotube yarns

The outstanding properties of individual carbon nanotubes (CNTs) have motivated interest in CNT fibers and yarns for composite materials, high-strength conductors and multifunctional textiles. However, despite advances in manufacturing, the strength of CNT yarns remains 10-100 fold less than individual CNTs. In light of the complex, multi-scale load transfer in CNT yarns, a hierarchical model taking into consideration the morphology-dependent mechanics is necessary to understand this limitation. We present a coupled analytical and finite element model of three-dimensional morphology and the full tensile behavior of CNT yarns. By incorporating load-induced changes in morphology, simulations of yarns in tension show different load paths such as fracture-type or stick-slip failure depending on the waviness and number density of CNTs. The strength of untwisted pristine CNT yarns is shown to be limited to <10% of the intrinsic CNT strength, even at practical limits to CNT packing density and alignment. Load-induced changes in CNT morphology are verified by tensile testing of CNT yarns along with in-situ X-ray scattering. In addition, the nominal structure of the yarn is shown to strongly influence the improvement in strength achieved by densified and/or cross-linking, and a sublinear relationship between CNT contact enhancement and yarn strength is predicted. Thus, this work provides a means to investigate the complex load transfer mechanisms in CNT yarns and other assemblies, to further study their process-structure-property relationships, and to understand potential property limits.