1. Introduction
Carbon
nanotubes can be classified by the number of walls in the tube,
single-wall, double wall and multiwall. Each wall of a carbon nanotube
can be further classified into chiral or non-chiral forms. Carbon
nanotubes are currently manufactured as agglomerated nanotube balls or
bundles [1, 2]. Use of carbon nanotubes and graphene as enhanced
performance additives in batteries is predicted to have significant
utility for electric vehicles, and electrical storage in general.
However, utilization of carbon nanotubes in these applications is
hampered due to the general inability to reliably produce individualized
carbon nanotubes [3, 4]. Single-walled carbon nanotubes are a novel
form of carbon. They are closed-caged, cylindrical molecules,
approximately 0.5 to 3 nanometers in diameter and a few hundred
nanometers long [5, 6]. They are known for their excellent
electrical and thermal conductivity and high tensile strength [7,
8]. Since their discovery in 1993, there has been substantial research
to describe their properties and develop applications using them [9,
10]. From unique electronic properties and a thermal conductivity
higher than diamond to mechanical properties where the stiffness,
strength and resilience exceeds any current material, carbon nanotubes
offer tremendous opportunities for the development of fundamentally new
material systems.
Utilization of carbon nanotubes in conductors has been attempted [11,
12]. However, the incorporation of carbon nanotubes into polymers at
high enough concentrations to achieve the desired conductivity typically
increases viscosities of the compound containing the carbon nanotubes to
very high levels [13]. The result of such a high viscosity is that
conductor fabrication is difficult [14]. A typical example of a high
concentration is one percent, by weight, of carbon nanotubes mixed with
a polymer [15, 16]. Currently, there are no fully developed
processes for fabricating wires based on carbon nanotubes [17, 18],
but co-extrusion of carbon nanotubes within thermoplastics is being
contemplated, either by pre-mixing the carbon nanotubes into the
thermoplastic or by coating thermoplastic particles with carbon
nanotubes prior to extrusion [19, 20]. Application of carbon
nanotubes to films has been used extensively, but not to wires [21,
22]. Utilization of carbon nanotubes with thermosets has also been
widely studied in recent years [23, 24]. However, thermosets are
crosslinked and cannot be melted at an elevated temperature [25,
26]. Finally, previous methods for dispersion of carbon nanotubes onto
films have not focused on metallic carbon nanotubes in order to maximize
current-carrying capability or high conductivity [27, 28]. The
above-mentioned proposed methods for fabricating wires that incorporate
carbon nanotubes will encounter large viscosities, due to the large
volume of carbon nanotubes compared to the overall volume of carbon
nanotubes and the polymer into which the carbon nanotubes are dispersed.
Another issue with such a method is insufficient alignment of the carbon
nanotubes. Finally, the proposed methods will not produce the desired
high concentration of carbon nanotubes.
The use of high-performance, fiber-reinforced composites has expanded
substantially in recent years [29, 30], as improvements in these
composites have allowed them to meet the final performance requirements
of advanced material systems [31, 32]. For example, extensive
research and development in carbon fiber-reinforced composites has led
to significant improvements in the properties of these composites [31,
32], such as in-plane mechanical properties [33, 34]. Furthermore,
composites formed using two-dimensional and three-dimensional woven
fiber reinforcements can be formed into the final net shapes [35,
36]. However, the out-of-plane properties of fiber-reinforced
composites remain problematically low [37, 38]. Out-of-plane
properties are dominated by the matrix surrounding the reinforcing
fibers, which is relatively weak compared to the fibers [37, 38].
Additionally, fiber-reinforced composites generally possess matrix-rich
regions within the interlaminar region between the fibers [39, 40],
and these regions have proven difficult to reinforce with fiber
reinforcements [41, 42]. As a result, cracks may easily initiate and
propagate under load within these regions, leading to composite failure
[43, 44]. Therefore, there exists a continued need for improved
reinforcements for composite materials so as to form hybrid carbon
nanotube fiber reinforcements by depositing of carbon nanotubes on fiber
substrates.
Introducing a uniform distribution of carbon nanotubes into a polymer
matrix can yield property enhancements that go beyond that of a simple
rule of mixtures. The challenge is to take full advantage of the
exceptional properties of carbon nanotubes in the composite material.
Carbon nanotubes are ideal reinforcing material for polymer matrices
because of their high aspect ratio, low density, remarkable mechanical
properties, and good electrical and thermal conductivity. However,
property improvements are not significant to date, apparently due to
poor interfacial carbon nanotube-polymer bonding and severe carbon
nanotube agglomeration. The present study is focused primarily upon the
mechanical properties of fiber-reinforced polymer composites containing
graphene-carbon nanotube hybrid materials. The graphene-carbon nanotube
fiber-reinforced polymer composites utilize nanotechnology enhancements
to provide advantageous durability and structural stability improvements
over conventional fiber-reinforced polymer composites not containing
graphene or carbon nanotubes. The effect of the hybrid material weight
fraction on the modulus of elasticity and hardness is evaluated for the
fiber-reinforced polymer composite. Stress-strain responses of the
composite tensile deformation are illustrated and the effect of strain
on the bond order parameters of the tensile deformation is investigated
for the fiber-reinforced polymer composite. The present study aims to
explore how to effectively improve the mechanical properties of polymers
by utilizing graphene-carbon nanotube hybrid materials. Particular
emphasis is placed upon the effect of weight fraction on the mechanical
properties of polymer composites reinforced with graphene and carbon
nanotubes.