Previous studies reported improved mechanical performance of conventional composites with the addition of nano fillers [119
]. Researchers also verified that the toughness of the composite improves because of nano fillers [125
]. A nanocomposite phase could be produced by using them on the composite interface as shown in Figure 6
a–d. This improves interlaminar fracture property and gives an advantage for nanocomposite over the pure matrix. The methodology to improve the toughness of composites using nanofillers is shown in Figure 6
e by a schematic diagram. The primary objective here is to explore the use of nano-fillers in the polymer (particularly because the nanocomposite has improved properties than pure polymers) as matrix material on the interlaminar site (or interleaved material) to form traditional carbon fiber based composites. The field of multiscale composite has been revolutionized by adding nanocomposites with traditional composites. Since it is a primarily emerging field, further work on framing of this multiscale/nano-filled/nano-interfaced fiber reinforced plastic is called for. Details of different types of nanofillers and nanocomposites are elaborated in Section 4.1
Moreover, the benefits of increasing delamination resistance in fiber reinforced composites have been proven using multiscale reinforcement, using fibers and CNTs together on the resin system or on the surface of the fibers [5
]. The ILFT of the multiscale composite is of further interest. As the name suggests, “multiscale” composites comprise reinforcements at varying scales, such as continuous or discontinuous fibers of the order of mm, along with microscale fibers and/or nanoscale fillers or tubes. They are manufactured by merging nanoscale-sized fillers (e.g., CNTs) with traditional fiber reinforcements like glass and carbon [150
]. The fiber resists in-plane load whereas the nanoscale reinforcement improves the performance of the through-thickness direction. Many techniques have been used by researchers to produce multiscale composites:
4.1. Nanofillers in Polymer
Many classes of resin systems such as including metals, ceramics, and polymers are used in fabricating CNTs-based composites (i.e., nanocomposites) and their mechanical response has been studied [134
]. The schematic diagram of nanofillers in polymers is shown in Figure 7
. Different categories of nanofillers are available which can to be used in conjunction with the resin to form nanocomposites like nano silica, nano-aluminum oxide (Al2
), nano-titanium oxide (TiO2
), polyhedral oligomeric silsesquioxane (POSS®
), layered silicate (nano clay), Halloysite nanotubes (HNTs), montmorillonite (MMT), graphite, carbon nanofibers (CNFs), and CNTs. Nanoparticles are further sub grouped into particulates, layered, and fibrous materials. So, multiple types of nanofillers are available today. Carbon nanotubes are mainly mentioned, according to the scope of this research study. CNTs are extensively used owing to high strength and low density. They also have superior electrical and thermal conductivity performance. CNTs have a tremendous impact on the overall material properties even if they are mixed in low quantities of weight fractions [125
]. CNTs are suitable for nanoscale reinforcement and provide versatility to fiber-reinforced composites. CNTs tend to agglomerate to form bundles. Dispersing the CNTs uniformly in the resin system is tedious owing to the presence of the van der Waals force effect. Here comes the role of functionalization of CNTs. The addition of functionalized groups onto the molecules by chemical methods is called functionalization and there are many approaches to the functionalization of CNTs [155
]. Some of them include defect, noncovalent, and covalent functionalization. This prevents the nanotubes from agglomerating and helps in achieving stabilization and good dispersion of CNT within the base matrix. This engagement between the particles and the resin system improves the final properties of the CNTs/polymer composites [156
]. The carboxyl (-COOH) or hydroxyl (-OH) groups present on the CNTs’ surface provide a conducive environment for the various chemical reactions to take place [166
]. Ma et al. [169
] reported an exhibition of better wettability and high surface energy with epoxy matrix by amino-functionalized CNTs in comparison to pristine CNTs. The CNTs with amino functional groups improved interfacial adhesion, thereby resulting in better flexural and thermo-mechanical properties. Traditional composite fibers are chemically modified to increase the adhesion between resin and fibers. The idea that the interaction between CNTs and polymer matrix can be a sum of Van der Waals bonds and first order chemical bonds strongly supports the fact that the CNTs/polymer results are comparable with those of strongly bonded composite system supports.
The characteristics of traditional polymer matrix composites are changed by introducing nano based particles such as clay nanoparticles, nanotubes, and CNFs, etc. Nanotubes improve many functional properties such as toughness, stiffness, and thermal performance of pure polymers and their composites, unlike macroscopic fillers that decrease the impact resistance and strength of the pure polymers and composites. The major benefit of using nanotubes is the capability to remarkably increase the delamination resistance properties of the composites. Gojny et al. [170
] studied the effect of the addition of nanoparticles on toughness properties of epoxy resins using double-wall CNTs and carbon black. They found that, in comparison to neat epoxy, nanocomposites had significantly higher fracture toughness. The dominating mechanism for dispersion of energy as reported by them was nanotube bridging cracks and deflection at small clusters. Enhancement of toughness is affected by the propagation of cracks and reduces the growth of nanopores. While CNTs and other nanoparticles can improve a few of the properties of composites, others investigations [171
] indicated that it also resulted in reduction of some of the mechanical properties.
Chisholm et al. [173
] utilized 1.5 to 3.5 wt% SiC (silicon carbide) dispersed in the epoxy polymer and carbon fiber (satin weave) for manufacturing multiscale composites by a resin infusion process. The composites reinforced with 1.5 wt% SiC displayed an enhancement in tensile strength (up to 16%) and tensile modulus (up to 45%) of the composites.
Gojny et al. [174
] via resin transfer moulding manufactured nano-particle reinforced FRP composites, with CNTs and carbon black. The measured flexural and interlaminar shear strength of the matrix showed a significant increase of 20% with the addition of only 0.3 wt% CNTs (double wall) whereas the tensile properties did not show improvement and remained fiber-dominated. CNT influenced the fracture toughness behaviour also. The addition of carbon black in a similar proportion was found to be less effective in improving the mechanical properties of pure polymer material.
Tsantzalis et al. [175
] used CNF and lead zirconate titanate (PZT) particles to modify the CFRP laminates. They reported fracture toughness increase of 100% in GI
(Mode I) with an addition of 1% CNF in the matrix. Yokozeki et al. [176
] found similar improvement of tensile and shear ILFT modes (97% and 29% respectively) for 5 wt% cup stacked carbon nanotubes (CSCNTs)-laminates.
Kim et al. [177
] investigated the delamination resistance characteristics of CFRP composite by mixing multi-walled carbon nanotubes (MWCNTs) into the matrix. Composites with 0.2 and 0.7 wt% of MWNTs enhanced theMode I ILFT at the cryogenic temperature. It was also concluded that a lower improvement of critical energy release rate of the modified resin was observed at very low and cryogenic temperatures compared to room temperature. Chen et al. [178
] manufactured laminates using functionalized MWCNTs with varied surface synthesis as reinforcements. MWCNTs were dispersed efficiently on the epoxy by modifying the surfaces and subsequently resulted in better mechanical properties of the composite. The micrographs revealed that CNTs tend to realign during processing. They also noticed that the well oriented and positioned CNTs as reinforcements influenced those properties that were attributed to the fiber.
Green et al. [179
] produced fiber reinforced multiscale composites which consisted of dispersed CNFs in an epoxy resin. By adding 0.1 and 1 wt% CNF, the flexural strength increased by 16% and 20% while the modulus also improved by 23% and 26% respectively. The shear strength (ILSS) was also enhanced by 6% and 25% for the 0.1 and 1 wt% CNFs, respectively when compared to fiber reinforced composites without CNFs dispersion.
Several groups of researchers investigated the effect of toughening the brittle matrix system as an effective way to reduce delamination onset [180
]. Hunston et al. carried out a detailed investigation and showed that 3 J/m2
higher toughened resins in general increases the composite interlaminar fracture toughness by 1 J/m2
compared to neat resin composite laminates [183
]. Recently Ozdemir et al. also studied the toughening attributes of rubber nano particles in carbon fiber polymer composites and 250% increase in delamination resistance was reported with the addition of 20 parts per hundred of rubber particles in the epoxy matrix [184
]. Although certainly, the research in this direction has reached greater heights, there are still problems like particle dispersion during ex-situ processes using dry fabrics with liquid resin injection and the associated cost of the filler particles.
4.2. Dry CNTs Transfer to the Composite Interface
Research on dry CNT transfer on the composite interface has also gained significant attention and a schematic is depicted in Figure 8
. Techniques like spraying [185
], sprinkling [186
], buckypaper (dry CNTs) [124
], electrophoresis [139
] etc. are used to improve the properties of composite specimens with nanofillers as localized reinforcements.
Thakre et al. [188
] manufactured the nanotube composites with a spray of nanotube–ethanol solution sonicated (for an hour at 40 kHz) on the carbon fabric lamina using vacuum assisted resin transfer moulding (VARTM). Both the functionalized and non-functionalized single-walled carbon nanotubes (SWCNTs) were added and the mechanical performance was compared to laminates without any nanotubes. The ILSS showed an improvement of 4.4% with functionalized nanotubes whereas no improvement was seen for the case of the pristine or non-functionalized nanotubes.
The electrophoresis method was implemented by Bekyarova et al. [139
] for the deposition of the fillers (MWCNTs and SWCNTs) on woven carbon preform and composite manufactured by transfer moulding using epoxy resin. The carbon fabric/epoxy composites with CNTs incorporation exhibited 30% improvement of ILSS as opposed to baseline composites with no CNTs and demonstrated considerable improvement in electrical conductivity in the transverse direction.
Arai et al. [189
] fabricated a CFRP/CNF hybrid laminates by inserting CNFs with a small quantity of ethanol (solvent) between prepregs. Then the solvent with an areal density between 10 g/m2
and 30 g/m2
was allowed to volatilize from the prepreg sheets. For hybrid laminates, they found the Mode I ILFT shows improvement of half a factor compared to unmodified composites. The Mode II ILFT was also improved about three times compared to unmodified laminates but the in-plane rigidity decreases by 12% relative to the unmodified CFRP laminate.
4.3. Nanofillers in Interleave
The approach of using nanofillers in interleave with an aim to toughen the composite system is widely used by researchers and a simplified schematic is shown in Figure 9
. Well dispersed CNTs are formed as thin interleave film [5
]. The nanocomposites thin film can also be manufactured using electrospinning [194
] and electrospun [198
] processes. These methods have gained popularity and are found to be an effective technique to also produce the electrospinning/electrospun nano-interlayers [201
] which toughen the composite laminate. Klein et al. [206
] reported the ILFT of specimens (satin woven CFRPs) with carbon nanotube/epoxy films. They noted that the interleaving films with the inclusion of carbon nanotubes was diminished. They observed this consequence as a thick epoxy layer was created in the mid-plane due to the film.
Warren et al. [207
] also prepared and examined B-staged epoxy/SWCNT nano-composite thin films toughened laminate by nylon particles. The epoxy nanocomposites had well-dispersed SWCNTs and nylon particles that were made into films by controlling the curing and viscosity of the epoxy resin. They noticed that the surface functionalization of SWCNT and the inclusion of preformed nylon particles enhanced the mechanical performance of the nano-composites. The thin films seamlessly integrate into a laminated composite system upon heating and serve as interleaves for improving the mechanical properties and conductivity.
Davis et al. [208
] fabricated nano composite (carbon fiber/epoxy) laminates by spraying fluorine functionalized carbon nano tubes (0.2, 0.3 and 0.5 wt%) on either side of 4 hardness satin carbon fabric and through a resin infusion process. The fabricated nano composites showed improved mechanical properties and durability under both tension-compression (with R ratio of −0.1) and tension-tension (with R ratio of +0.1) cyclic loadings. The CNTs incorporation toughened the fiber matrix interfaces and deflected the interfacial crack development and delamination both under cyclic and static loading.
Yokozeki et al. [186
] utilized the combination of three different techniques to improve the ILFT of unidirectional CFRP laminates; namely, (1) Sprinkling of CSCNTs between the prepreg layers during stacking of plies, (2) Incorporation of resin films with CSCNT dispersed in matrix films, and (3) Dispersion of CSCNTs into the epoxy resin using the epoxy to produce prepregs. Two types of CSCNTs were used; aspect ratio of about 10 and 100 (designated as AR 10 and AR 100 respectively). They found that the incorporation of CSCNT (AR 10) films with CSCNT-dispersed resin was the most effective in enhancing Mode I (up to 300%) and Mode II (up to 160%) fracture resistance.
4.4. Multiscale Reinforcement
One of the techniques used to produce multiscale composites is by growing carbon nanotubes on the fiber surface and is termed as multiscale fiber approach [209
]. The schematic of multiscale reinforcement approach is shown in Figure 10
CNTs were grown on the fibre surface by the chemical vapour deposition method in a study by Thostenson et al. [128
] which resulted in a multi scale reinforced material. The localized effect of adding nano-reinforcements on transferring the load at the fiber matrix interface was studied by manufacturing the single-fiber composites. It demonstrated that the interlaminar shear (ILS) of the composite was increased with the addition of nanotubes at the interfacial region.
Woven carbon fiber laminate was prepared in situ with CNTs by Kepple et al. [210
] to investigate the influence of growing nanotubes on carbon fiber. They noticed that the carbon fibers with grown nanotubes improve the delamination resistance of the laminates by up to 50%. Also, it was reported that the structural integrity of the final part was unaffected. Even 5% increase in the flexural modulus was reported. This work also demonstrated that CNTs on the flexible substrate even after functionalization remain as such, and most CNTs are rigid enough to tolerate the high temperatures higher than 800 °C undergone during the synthesis [210
Mathur et al. [211
] also grew CNTs on different carbon fiber substrates by chemical vapour deposition to produce hybrid/phenolic composites. They manufactured uni-directional cafron fibers tows, bi-directional (2D) CF cloth, and 3D CF felt. For unidirectional, 2D and 3D composites, the flexural strength was enhanced by 20%, 75%, and 66% respectively as opposed to the one without nanotubes under similar test scenarios.