Page 212 - Carbon Nanotube Fibres and Yarns

202 Carbon Nanotube Fibers and Yarns

A simulation of a typical yarn consisting of CNTs with 10 nm diam-
eter and four walls showed a yarn strength of 1.59 N/tex, see Fig. 8.11E.
The stress-strain behavior is initially linear, followed by stick-slip behavior
with decreasing stress after the peak load. To understand the mechanism of
load transfer, the normalized number of contacts remaining at each load
step relative to the initial number, N c /N 0 , is also plotted (Fig. 8.11E). In
general, as the stress in the yarn increases, the contacts are progressively
loaded. As weaker contacts break, the CNTs in the vicinity of the broken
contacts detach and relax simultaneously. As a result, these CNT segments
do not carry any load, although they remain intact. The simulations revealed
that progressive stick-slip and failure of CNT-CNT contacts caused isolated
load paths to emerge, leading to yarn failure at less than 10% of the intrinsic
CNT strength.

8.5 Summary and outlook

The assembly parameters of CNT yarns, such as twist angle, packing density,
alignment, entanglement and straightness of the constituent CNTs, affect
the yarn mechanical properties in many ways, making yarn mechanics mod-
eling very complicated. Simple analytical modeling can only predict the
yarn performance qualitatively while atomistic simulation works only at
the nanoscale. The coarse-grained treatment based on mesoscopic models
has been very successful in investigating the densification, twisting, aligning,
and breaking processes of the yarn. At a higher size scale, the atomistic and
coarse-grained tools show obvious limitations. Therefore multi-scale mod-
eling techniques have become useful. Very recently, Gao et al. used a three-
level hierarchical model to deal with such problem [121]. The strength loss
from an individual CNT to a CNT yarn was divided into three levels of
weakening mechanisms, i.e., stress localization due to the presence of de-
fects in individual CNTs, insufficient load transfer in closely packed bundle,
and porosity and misalignment of CNT bundles in the yarn.
From this review of the various modeling methods and simulation re-
sults, we conclude that the key to produce strong CNT yarns is to maximize
the intertube shear properties, which was also concluded very recently by
Jung et al. [122]. For this purpose, bio-inspired structural design of nano-
composites [123] provides some very useful guidelines for the improvement
of CNT yarn mechanical properties: (1) aligning and densifying the CNTs,
(2) polymer infiltration and cross-linking, and (3) surface modification and
interfacial covalent bonding.