Page 238 - Carbon Nanotube Fibres and Yarns

228 Carbon Nanotube Fibers and Yarns

model was used to obtain the mechanical properties in the piezoresistive
layer considering the fact that the CNT yarns do not form a continuous
phase. The effective properties of the strain gauge sensor were then cal-
culated by homogenizing the properties of the CNT yarns and those of
the polymer substrate. The polymer could be an isotropic material with
elastic modulus, E m , Poisson’s ratio, ν m , and electrical resistivity, ρ m (m and
f subscripts denote the polymer matrix and the CNT fiber, respectively).
The CNT fiber was considered a transversely isotropic material with elastic
moduli, E 1f and E 2f , Poisson’s ratio, ν f , shear modulus, G 12f , and electri-
cal resistivities, ρ 11f and ρ 22f . The sensitivity of the foil strain gauge sensors
was calculated by changing several geometrical and material parameters
including the shape and dimensions of the strain gauge sensors and the
exerted load. The strain gauge sensor was shown to be sensitive to all trac-
tions although the highest sensitivity is achieved when a normal traction is
relatively aligned with the CNT yarn direction [64]. It was reported that
size, geometry and the relative arrangement of the piezoresistive membrane
within the sensor significantly affect the performance of strain gauge sen-
sors [64]. The higher the Poisson’s ratio and the lower the spacing factor, the
higher the sensitivity of the strain gauge. The spacing factor is represented
by the normalized separation between the CNT yarns, that is, the ratio of
distance between CNT yarns and the diameter of the CNT yarn.
The strain gauge design objective is to obtain the location and config-
uration of the material that maximizes its sensitivity to external loading.
From the relative resistance and strain history curves (Fig. 9.8) and the GF
obtained (Fig. 9.8), it is shown that these foil strain gauge sensors compris-
ing of carbon nanotube yarn are sensitive enough to capture strain and can
replicate the loading and unloading cycles.
Based on these findings, there is an on-going effort to design flexible
and stretchable foil strain gauges using CNT yarn. The goal is to have a
foil strain gauge that can measure one order of magnitude higher strain
and with also a sensitivity at least one order of magnitude higher than that
of conventional foil strain gauges using constantan wires. To achieve that,
the first consideration is the substrate of the sensor. The substrate used in
the design of the prototype CNT yarn foil strain gauge is Kapton HN™
polyimide films. Polyimide films could be very thin films and are easily
machinable, that is, they can sustain micro-channels (grooves) that are cre-
ated by laser prototype machine drills. Most importantly, their coefficient of
thermal expansion (CTE) is close to that of CNT yarns. The use of elasto-
mers as substrate could also make it possible to achieve all the requirements
needed from the substrate. A study is still underway and could be very sem-
inal in the production of large-strain surface foil strain gauges.