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.