Page 225 - Carbon Nanotube Fibres and Yarns

216 Carbon Nanotube Fibers and Yarns

[18–35]. These methods utilize micro-strain sensors that can capture strain
variation due to piezoresistive effects [18], resonance monitoring [19, 20],
piezoelectric effects [21–23], capacitance variation [24–27], or changes in
optical properties [28–33, 36].
Among all the SHM techniques, CNT fibers are gaining more attention
as they possess low weight due to their hollow structure, which when com-
bined with their high aspect ratio and multifunctional properties, is ideal
for composite structures [5, 37–40]. In this method, a grid of CNT-based
sensors can be created over a large area of a structure and used to monitor
the strain field or damage in the structure. When a load is applied to the
structure and a strain reaches the CNT fiber sensor, it will trigger a change
in CNT’s resistance value depending on the magnitude of the damage.
The resistance value of the CNT fiber would be restored when the load
is removed. The concept is that CNT yarns are integrated in a laminated
composite material forming a continuous sensor circuit, and their inherent
piezoresistive sensitivity would capture even small amount of strain within
the host material [40]. In laminated composites, delamination occurs due
to the separation of their layers mostly due to interlaminar stress and matrix
failure. Delamination represents a considerable damage risk to their integ-
rity [40–43] and can occur almost at any place on the laminate, on the edge,
near the surface, or at the center of a laminated composite. While the range
in size of delamination damage can vary drastically, it may elude the de-
tection capabilities of most techniques that monitor change in material
geometry such as metallic foil strain gauges, which are more suitable for
surface strain detection, or optical fiber monitoring that requires complex
equipment and data analysis. The ability of the CNT yarn sensors to de-
tect mode II-dominated delamination in laminated composite materials has
been reported [39–42]. 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 Fig. 9.1,
which includes stitched yarn sensors and transverse, or longitudinal, yarn
sensors. The yarn sensors stitched through the thickness of the laminates al-
lows 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. It is worth mentioning that damage detection based on a
significant resistance change increase does not require highly precise resis-
tance measurements and thus two-point probe measurements are deemed
appropriate and sufficient.