Page 289 - Carbon Nanotube Fibres and Yarns

278 Carbon Nanotube Fibers and Yarns

Fig. 11.6 Mechanics of CNT-twisted yarn coil actuators [22]. (Source: X. Yang, W. Wang,
M. Miao, Moisture-responsive natural fiber coil-structured artificial muscles, ACS Appl.
Mater. Interfaces 10 (38) (2018) 32256–32264.)
A mechanic explanation of the spring-like yarn coils is provided in
Fig. 11.6. The free body diagram shows that the lifting force F is balanced
by the torque Q developed in the twisted yarn, as expressed in Eq. (11.4).
2 Q
F = (11.4)
D
Combining Eqs. (11.3) and (11.4), we can see that the lifting force of
the twisted yarn coil (F) increases with the cubic power of the yarn radius
(r) and with the increases of yarn tension and twist angle.
More complex architectures formed from twisted CNT yarns can
be obtained by changing the number of yarn plies, the yarn orientation,
and assembly structure. For instance, Shang et al. [45] twisted two straight
yarns, one straight yarn and one coiled yarn, and two coiled yarns into
a double-helix structure, respectively. The two-stage loading behavior of
the double-helix CNT yarn with helical structure of individual yarns was
observed during tensile testing. While one of the yarn components breaks
early under tension due to the highly twisted state, the second yarn pro-
duces much larger tensile strain and significantly prolongs the process until
ultimate fracture. Chen et al. [42] tuned the chirality of CNT fibers by vary-
ing the spinning direction. The CNT fibers with left- or right- handedness
were bundled together in parallel and over twisted into a spring-like fiber
by stabilizing one end while rotating the other end. No entwisting or un-
twisting was observed in a relaxing state of these spring-like fibers.

11.3.4 Fabric actuators

Weaving and knitting are the two most common textile processing methods.
A woven fabric has two perpendicular and individual thread systems, warp
and weft threads, that come in close contact and result in a rigid fabric. Chen