Page 177 - Carbon Nanotube Fibres and Yarns

168 Carbon Nanotube Fibers and Yarns

where the strain rate  ε represents the dimensionless strain rate based on
 ε 0 = 1/s, and 0.0186 is a strain rate sensitivity coefficient. A pure CNT yarn
tended to break at the weakest point, leading to unraveling of the yarn along
the twist direction, while a polymer-infiltrated composite CNT fiber tended
to break into several fragments. For pure CNT yarns, the strain rate effect
arises not only from the rate dependence of millions of CNTs, but also from
the interaction among CNTs. Irregular defects and voids in CNT yarns can
be regarded as a factor in elevating the strength as the strain rate increases.
The internal defect distributions could significantly influence the strain rate
sensitivity. At the quasistatic loading rates, the failure strength was dominated
by the strength of the weakest link, but the failure strength gradually got
closer to the average material strength as the strain rate was increased to high
levels.
The tensile behaviors at sonic strain rates can be used to elucidate vis-
coelastic characteristics, which have been used to reflect some aspects of the
interactions between molecules, fibrils, and fibers in polymer fiber materials
[69]. In a perfect linear elastic material, the sonic velocity (c) in the material
is related to its Young’s modulus (E) and mass density (ρ) by the well-known
wave propagation equation c =√(/ )ρ = E′ , where E′ = E/ρ (N/tex) is
E
the specific modulus of the material. The elastic modulus determined by the
acoustic method is known as dynamic modulus or sonic modulus. The ratio
between the sonic modulus and the quasistatic modulus (E s /E qs ) can be
used as an indication of the internal friction in textile fibers and fiber-fiber
friction in yarns [70, 71].
For a twist-spun CNT yarn, the degree of CNT orientation decreases
as the twist in the yarn increases, resulting in a decrease of both dynamic
and quasistatic moduli of the yarn, as shown in Fig. 7.21A [69]. The ratio
between the dynamic modulus and the quasistatic modulus (E s /E qs ) followed
a parabolic curve with a maximum at a twist angle of about 30–40 degrees,
as shown in Fig. 7.21B. This can be explained by frictional slippage between
CNTs in the yarn, which depends on both the intimacy of CNT-CNT
contact and the freedom of CNT-CNT relative movement under load. As
the twist increases, the contact force increases but the tendency of slippage
decreases. The combined effect of the two factors plateaued at the interme-
diate twist level.
A torque-free plied CNT yarn can be formed by twisting multiple sin-
gles yarns together in the opposite direction to the twist of the singles
yarns. The CNT tortuosity (misalignment) in the final plied yarn is greater
than that in the original singles yarns. This could result in decreases in yarn