Page 228 - Carbon Nanotube Fibres and Yarns

Sensors based on CNT yarns 219

even before the load history response of the laminated composite sample
indicates it (event A). In addition, the yarn sensor was able to withstand
loading, more than the maximum load, as well as capture the delamination
without its circuit failing. The yarn sensor failed (event C1) 94 s after event
A as shown in Fig. 9.2. The determination of the exact location of delam-
ination and its progression can be achieved with a configuration consisting
of a combination of different yarn sensors like the one shown in the inset
of Fig. 9.2, which includes stitched yarn sensors and transverse, or longi-
tudinal, yarn sensors. The yarn sensors stitched through the thickness of
the laminates allows for the determination of delamination only; additional
transverse yarn sensors parallel to the composite laminate layers and along
the beam’s width direction are required to establish the precise location of
the delamination or the damage. Abot et al., observed that the yarn sensor
closest to the delamination fails first and subsequently the other transverse
sensors as the delamination propagates and reaches their locations [42]. The
results obtained with the CNT fiber sensors have been validated with other
methods including using in situ optical fibers and x-ray tomography of the
entire laminated composite samples posttesting.

9.3 Torque sensors

Most applications that require rotational positioning and high torque gen-
eration for mechanical performance are bulky with a complex design that is
not ideal for nanotechnological applications. Twist-spun CNT yarn can serve
as an actuator for high-performance motion systems like artificial muscles
that require torsional rotation in addition to bending and contraction, and
micromechanical devices. In addition to their relatively high strength [44],
their nanoscale dimensions and high aspect ratio are attractive for torsional
sensing. Torsional acceleration in CNT yarns can be driven in both direc-
tions for conversion of mechanical energy to electrical energy. This can find
application in sensors that generate electrical signals through applied tor-
sional rotation. The change in electrical resistance upon the twist loading of
the CNT yarn is shown in Fig. 9.3 [45, 46]. The decrease in electrical resis-
tance upon application of torsional loading to the CNT yarns demonstrates
that applied twist increases fiber compaction, resulting in increased electrical
contact between nanotubes and a negative piezoresistance. The changes in
electrical resistance due to twist are mostly reversible but irreversible resis-
tance changes at higher shear strains exceeding 12.9% have been reported for
composite CNT fibers due to matrix failure (Fig. 9.3B) [46].
Untwisted CNT yarn shows a very large resistance increase in tor-
sional displacement compared to CNT yarns with twist. Fig. 9.4 shows