Page 164 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn structures and properties 155

the alignment of CNTs in the final yarn (Fig. 7.15B and C), resulting in an
increase of yarn specific strength from 0.3 to 1 N/tex.

7.2 Tensile strength of CNT fibers and yarns

7.2.1 Tensile testing conditions
Many CNT researchers use engineering stress unit (MPa or GPa) to express
CNT yarn strength and modulus. All CNT yarns are porous and even a low
strain can cause the yarn diameter to decrease dramatically. We demonstrated
how the strength of the same yarn could be stated as either 648 or 1630 MPa
depending on whether the initial diameter or the instantaneous diameter of
the yarn was used in calculating the yarn stress [14]. This is why yarn-specific
tensile properties have always been used in the textile industry. Yarn-specific
stress is equal to the applied tensile force (in N or cN) divided by the yarn
−2
linear density in tex and expressed in N/tex or cN/tex (1 cN/tex = 10 N/
tex). The specific breaking strength of the yarn is known as tenacity. Dividing
engineering stress by the yarn density gives the yarn-specific stress in GPa/
3
(g/cm ), which is equal to the value in N/tex.
Tensile testing gauge length (distance between the two gripping points
on the specimen) can have an important influence on the tensile test results.
As a specimen always breaks at its weakest link, the longer the specimen,
the weaker its weakest link becomes [40]. The effect of gauge length on
yarn tensile strength is especially large when the gauge length is close to the
fiber length. In other words, yarn strength tested at a gauge length slightly
shorter than the fiber length would be significantly higher than that tested
at a gauge length slightly longer than the fiber length. Most commonly used
tensile gauge lengths for CNT yarn testing are 10 and 20 mm [10, 13, 14,
41], which are many times longer than the length of their constituent nano-
tubes. In one study, Zhang et al. [42] measured CNT yarn strength at gauge
length between 1 and 20 mm. Surprisingly, the tensile strength showed a
very weak dependence on the gauge length, as displayed in Fig. 7.16A. This
was because the shortest gauge length (1 mm) was much longer than the
length of the CNTs in the yarn.
−5
−1
On the other hand, the strain rate (2 × 10 –2 × 10 1/s) used in tensile
testing showed a much greater impact on the tensile properties of dry-spun
CNT yarns, as shown in Fig. 7.16B [42]. As the strain rate increased, both
the strength (fracture stress) and the Young’s modulus increased while the
elongation (fracture strain) displayed a decrease. This dependence of tensile
properties on strain rate may be explained by the viscoelasticity of CNT
yarns. At a low strain rate, the yarn is subjected to a gradual attenuation and