Page 209 - Carbon Nanotube Fibres and Yarns

Mechanics modeling of carbon nanotube yarns 199

strength of CNT yarns with random CNT stacking is found to be higher.
This study is interesting as it revealed three microstructural evolution
mechanisms, namely CNT stretching, fiber untwisting, and intertube slid-
ing, and correctly described the effect of twist and intertube interactions.
Another similar CGMD study on the effect of twisting (spinning) on the
assembly of CNT yarn was performed by Mirzaeifar et al. [115]. The CGMD
method bridges two different length scales and incorporates CNT deforma-
tions at the atomistic level into a larger scale model. The simulations revealed
how twisting a bundle of CNTs improves the shear interaction between the
nanotubes up to a certain level due to the increase in the interaction surface,
in agreement with the result predicted by the analytic model. Furthermore,
over-twisting the bundle weakens the intertube interactions due to excessive
deformation in the cross sections of individual CNTs in the bundle.
Bratzel et al. [114] studied the mechanics of CNTs with a binding polymer.
Nanotube pull-out test and bundle tensile test were performed on nonco-
valently cross-linked CNT/polymer bundles. The cross-link length and con-
centration are found to influence the tensile strength and toughness. With the
inclusion of 1.5-nm-long cross-linking polymer at 17 wt% concentration, the
bundle’s strength and toughness can be increased four- and fivefold, respectively.
Due to the generality of the microstructural evolution under mechan-
ical loads, the mechanics of CNT networks can provide a solid foundation
for the tensile mechanics of CNT yarns. Based on CGMD simulations,
Xie et al. [116] reported that the mechanical behaviors of CNTs under
uniaxial tension are closely correlated to their microstructural evolution, as
illustrated in Fig. 8.10. In the beginning stage of tensioning, local stretching
of vdW binding sites leads to an affine deformation and the in-plane stress
is uniform over the whole material (Fig. 8.10A and B). With increasing
tension, transverse contraction due to the Poisson’s effect results in a com-
pression of the porous network structures (Fig. 8.10C). Additionally, some
distinct changes take place as some CNTs bundle together to form several
main threads (Fig. 8.10D and E). Finally, the deformation field becomes
non-affine and the network eventually breaks at some of these threads by
losing intertube binding (Fig. 8.10F).
Besides static tensile behaviors, CNT assemblies, including yarn, film,
and aerogel, exhibit interesting damping behaviors [36,38,117,118]. A good
understanding of this dynamic mechanical property is also very helpful to
the analysis of other yarn properties. Yang et al. [113] studied the mechani-
cal response of a randomly entangled network of long CNTs to an applied
cyclic shear strain with different amplitudes and frequencies and at different