Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Another approach for determining the elastic modulus of discontinuously aligned
fiber-reinforced composite is to use the Halpin–Tsai equation [10]:
ECL
Em
ECT
Em
¼ 1 þ 2ðl=dÞh LVf
1 h LVf
¼ 1 þ 2h TVf
1 h TVf
ð4:7Þ
ð4:8Þ
where ECL and ECT are longitudinal and transverse composite modulus, respectively,
h Land h T are given by the following equations:
h L ¼ ðEf =EmÞ 1
ðEf =EmÞþ2ðl=dÞ
4.1 Strengthening Mechanismj105
ð4:9Þ
hT ¼ ðEf =EmÞ 1
: ð4:10Þ
ðEf =EmÞþ2
For the randomly oriented short-fiber-reinforced composite, the elastic modulus of
the composite can be calculated using the following equation [11]:
Erandom ¼ 3
8 ECL þ 5
8 ECT: ð4:11Þ
The macroscale tensile test is one of the primary mechanical measurements
commonly used to characterize the deformation of CNT–metal nanocomposites
subjected to axial stress. Enhanced yield strength and elastic modulus have been
observed in CNT–metal and CNT–ceramic systems during macroscale tensile
deformation. The experimental data are compared with theoretical results predicted
from micromechanical models of composites [12, 13]. For CNT–metal nanocomposites,
effective load transfer from the matrix to the nanotubes is essential to achieve
enhanced mechanical strength. From a microscopic viewpoint, strong interfacial
bonding between the matrix and nanotube is needed to promote the load transfer.
Fundamental understanding of the nature and mechanics of load transfer is crucial
for making CNT–metal nanocomposites with tailored mechanical properties.
At present, direct tensile testing of CNT–metal nanocomposites in the nanoscale
region is nonexistent in the literature. Only deformation of pristine CNTs at
the nanoscale has been reported in the literature. Such nanoscale deformation was
performed experimentally by means of atomic force microscopy (AFM) [Chap. 1,
Ref. 144, 14, 15]. In AFM, individual nanotubes can be manipulated at the nanoscale
using a cantilever with a sharp probe. When the tip comes close to the specimen
surface, forces between the tip and the nanotube produce a deflection of the
cantilever. The tip deflection can easily be detected by a photodetector that records
the reflection of a laser beam focused on the top of the cantilever. Mechanical
deformation can be evaluated from the force/tip-to-sample distance curves. Several
approaches have been adopted to measure the nanoscale deformation of materials in
AFM including nanoindentation, three-point bending and tension.
The resistance of materials to deformation (hardness) in very low volumes can be
determined experimentally by nanoindentation. In this context, a small indentation is