Page 224 - Carbon Nanotube Fibres and Yarns

Sensors based on CNT yarns 215

contribution of intrinsic resistance in the fiber and reduce the effect of
inter-tube resistance due to contact. When a free or neat CNT fiber is
stretched, the deformation mechanism is expected to be dominated by:
(1) breaking of contact due to fiber unraveling and bond breaking; and (2)
slippage [15]. The first phenomenon leads to an increase in contact resis-
tance. In the presence of a matrix, tunneling seems to drive the piezore-
sistivity due to matrix infiltration of the porous fibers creating barriers for
electron tunneling to occur.
One important factor to consider when characterizing a sensor is the
sensitivity, an output change for a given change in an input parameter. For
a strain gauge, this is represented by the ratio of relative change in electrical
resistance ∆R/Ro, to the mechanical strain.
The sensitivity of the CNT yarn represented by the gauge factor is
given as:
∆ / ∆ RR
RR
/
GF = 0 = 0 (9.3)
ε ∆ / 0
LL
where R 0 is the initial resistance, ∆R is the change in resistance, Ɛ is the strain,
which is defined as the ratio of the change in length ∆L over the original
length L 0 .
Although the GF values for SWCNT-based piezoresistive strain sensors
have been shown to be greater than 2900 [16], CNT-fiber-based sensors
have shown values lower than that. The reported gauge factor of neat CNT
fibers was around 0.5 [11, 15]. The dominance of contact resistance between
CNT bundles in the fiber means that the contribution of intrinsic resistance
in the individual CNTs is minimal in a CNT fiber. In conventional terms
for textile yarns, the term yarn represents an aggregation of fiber or fiber
bundles. However, both terminologies have been used interchangeably to
describe CNT fibers. In the context of this article, the term yarn and fiber
are used interchangeably.

9.2 Damage sensors

One of the proposed applications for the piezoresistivity of CNT fibers is
as strain sensors for real-time structural health monitoring (SHM) of com-
posite parts and structures. SHM methods provide constant and immediate
feedback of the state of health of a structure including potential damage
[17]. SHM methods may include vibration analysis, strain gauges, fiber optic
sensors, stress wave propagation techniques as well as several other methods