Mechanical properties of carbon fiber reinforced with carbon nanotubes and graphene filled epoxy composites: experimental and numerical investigations

The mechanical properties of carbon fiber-reinforced epoxy composites were identified by adding carbon-based nano-reinforcements, such as multi-wall carbon nanotubes (CNTs) and graphene platelets (GP), into the epoxy matrix by conducting suitable experiments. The main focus of this study is to compare the tensile modulus, tensile strength, flexural modulus, flexural strength, and thermal conductivity of carbon fiber-reinforced epoxy composites with nanoparticle reinforcement. The results revealed that adding CNTs and GP nanoparticles improved the mechanical properties compared to a pure carbon fiber-reinforced plastic composite. However, compared to CNTs, the GP’s addition has increased the mechanical properties of the CFRP composite. In addition, scanning electron microscopy (SEM) images were presented to explore the microstructural characterization of carbon fiber-reinforced nanoparticle-reinforced composites. Further, using numerical studies, the transverse modulus, major and minor Poisson’s ratio of the carbon fibre reinforced with CNT and GP particle reinforcement were estimated. The current study is applied to the efficient design of nanoparticle reinforced carbon fibre reinforced composites.


Introduction
Carbon fiber-reinforced composites were the most frequently recommended materials in the composite field due to their high specific strength and stiffness. These carbon fiber-reinforced composites apply to load-carrying structures such as fuselages and aircraft wing structures and frames in the automotive industry [1]. Many authors were drawn to carbon fibre reinforced composites' excellent properties, such as their good chemical resistance and outstanding mechanical properties, electrical properties [2,3]. Fillers were often put into matrix materials [4][5][6][7] in order to improve the performance of carbon fiber-reinforced composite structures.
Epoxy is a common thermoset group polymer that is ideal for making carbon fiber-reinforced composites due to its low shrinkage, low volatile release during curing, and good control in hot and humid environments [8,9]. At one atom thick, graphene is the thinnest material known to man. It is also tremendously stronger than Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. steel. Furthermore, graphene is a good conductor of heat and electricity, as well as having remarkable light absorption properties. It is a real material that has the potential to change the world because it can be used in almost every industry.
Research studies were performed on the addition of nano-sized filler in the epoxy matrix of carbon fiberreinforced laminate. Yung-Chuan Chiou presented the mechanical properties of carbon fibre reinforced with graphene nanoparticles, carbon aerogel, and epoxy composite. The authors found that an infusion of 1 wt% of graphene and carbon hybrids improved the tensile and flexural strengths [10]. Carbon Nanotube (CNT) is another carbon allotrope in the form of a hollow cylindrical shape that has been recommended for improving conventional composite materials. CNT addition will also improve the interface between the carbon fibre and the epoxy matrix [11].
Impact strength was studied by comparing graphene and carbon nanotubes (CNTs). CNTs were found to have the highest impact strength, while graphene reinforcement showed the least improvement [12]. The addition of CNT improved the mechanical properties of pure carbon fiber-reinforced composites [13].
On the other hand, micromechanics and the finite element method are used to figure out the elastic properties of the unidirectional carbon fibre reinforced composite with CNT, fullerene, and graphene-infused matrix composite [14]. Xuming Yao et al [15] conducted research studies on carbon fibres coated with CNTs and graphane oxide and discovered that both were significant when compared to one another, with CNT reinforcement exhibiting higher shear strength and graphane oxide reinforcement exhibiting superior humidity resistance. Due to its interesting features, including great tensile strength, the half-integer quantum Hall effect, and superior electrical and thermal conductivity, graphene, a ground-breaking discovery of the modern era, has emerged as one of the most investigated materials [16].
Another filler utilised to improve the produced composites' mechanical qualities is silicon. The mechanical, chemical, and geometrical characteristics are improved by combining silicon and hydroxyapatite [17]. The composite's brittleness and grain structure were both enhanced by the alloying with Silicon [18].
The combination of two monolithic and matrix materials is contrasted to monolithic material [19]. Combination of experimental and finite elemet simulation studies are performed to identify the tensile behaviour of bone-implant interface of bone-Implant models [20].
The percentage of elements and the quantity of alloying elements used in magnesium determine its mechanical and corrosion behavior [21]. The outcomes of the simulations can be very helpful in forecasting a variety of behaviours material under static and dynamic conditions [22]. The mechanical and wear properties influencing parameters are explored for Al based metal matrix composites [23].
By knowing the amazing potential and versatility of CNT and graphene, the mechanical properties of FRP composites will be substantially improved [24]. The natural frequency and damping ratio effects of MWCNTs and GNPs were investigated [25]. The aspect ratio of the MWCNT plays a critical role while deciding the Young's modulus and thermal response of resulting composite material [26].
The majority of the comparison work on carbon fibre and CNT mixed epoxy composites and carbon fibre and graphene mixed epoxy composites is done in terms of longitudinal modulus. However, because the characterization of composites is orthotropic, the transverse modulus and the major and minor poisson's ratio must be obtained. Many findings are devoid of this information. Efforts are being directed toward determining the longitudinal and transverse moduli, as well as the major and minor poisson's ratios, of plane carbon fibre reinforced epoxy composites, carbon fibre reinforced CNT mixed epoxy composites, and carbon fibre reinforced with graphene filler mixed epoxy composites, using both experimental and numerical methods. While validating the experimental and numerical results, the distribution of filler was considered to be uniform and random. In this work, CFRP composites were prepared using carbon-based nanoreinforcement. The tensile modulus, tensile strength, flexural modulus, flexural strength, and thermal conductivity of the carbon fiberreinforced epoxy composites were estimated by conducting suitable experiments. These properties are used to design effective carbon fiber-reinforced composites with carbon-based nanofillers, which are used in aerospace, sailboats, bicycle frames, many industirs etc [27][28][29].

Type of materials
The materials used for the present study were carbon fibre mat, epoxy resin, multi-wall carbon nanotubes, and graphene. Fabrics made of carbon fibre were used in automobile and aerospace structures due to their high specific strength and modulus. Carbon nanotubes (CNTs) are long, hollow cylindrical tubuler structures made of graphite sheets. The multi-wall nanotubes and graphene nanosheets of 99% purity were purchased from Vriksha Composites, Andhra Pradesh. The epoxy resin (LY556) and compatible hardener (HY951) were used as matrix materials. Using the hand layup method, the composite specimens were prepared. The carbon mat is placed in the mold, and pure epoxy with hardener in the measured quantity is poured over the carbon fabric. The epoxy resin is spread uniformly in the mold, and the entire mould is left for curing for 24 h. Whereas in the carbon-based nano composites, the carbon mat is placed inside the mold, and the epoxy and the nano filler (CNT or graphene) mixed after sonication are poured over the carbon mat and cured for 24 h. After curing, the specimens are removed from the mould, cut to ASTM standards, and tested for the required properties.
The weight fraction of the carbon mat is 40%, the nanoparticle reinforcement is 0.5%, and the remaining is epoxy resin [25,30]. According to previous research, 0.5% addition of CNT or graphite has a significant effect on tensile strength. To cure the specimens, the hardener is used in a 1:10 ratio. The mechanical properties that are provided by the (the vruksha composites, Tenali, Andhra Pradesh, India) supplier which were given in table 1.

Composite specimen preparation process
Using the hand layup technique, the carbon fibre reinforced epoxy (C/E), carbon fiber/multi-wall carbon nanotubes mixed epoxy (C/CNT + E), and carbon fiber/graphene mixed epoxy composite (C/G + E) were prepared. First, the carbon nanotubes were incorporated into the epoxy matrix using an ultrasonicator. The epoxy and carbon nanotube mixture was sonicated for 3 h to ensure a uniform distribution because the dispersion duration have a significant depends on shape, and aspect ratio of the CNTs considered for study [31]. Later, these mixtures were used to prepare the composite specimen, as shown in figure 1. Using the hand layup method, test samples of carbon fiber/carbon particle-mixed epoxy composites were made.
The mould used for the sample is cleaned and coated with a release agent. A carbon mat layer was placed in the mould, and a nanoparticle-mixed epoxy matrix was poured over the carbon mat. Using the rollers, the matrix was uniformly distributed over the layer. Next, another layer was placed on the mould, and matrix material was poured over the carbon layer. This procedure was repeated for three carbon layers until the mould was filled. The prepared mould was cured for 24 h. This curing process is used to release the voids or porosity created during the hand lay-up process [32]. Following ASTM standards, the samples were taken out of the mould and made ready for tensile and flexural tests.

Tensile testing
Using the digital universal tensile testing machine, the tensile testing was performed at room temperature by following ASTM D3039 standards, as shown in figure 1. The composite tensile modulus was obtained from the tensile testing machine. Each testing result is obtained by testing four specimens and taking the average value as the final result.Four tests were performed at each concentration of the constituents. A carbon fiber-reinforced pure epoxy composite was also prepared and tested to compare the enhancement with nano-reinforcement.  With a crosshead speed of 10 mm min −1 , the load is put on the specimen until the results are seen or the specimen fails.

Flexural testing
Flexural testing was performed on the same digital universal tensile testing machine by changing the loading setup. Using the standard ASTM D7264, the specimens were tested for flexural strength. The cross-head speed of 10 mm mm −1 is maintained while applying the bending load.

Scanning electron microscope
After testing the specimens, the specimens were studied using scanning electron microscope images. A SEM study was conducted using the VEGA3 Tescan SEM at an enhanced voltage of 10 kV at Vignan's University, Andhra Pradesh.

Results and discussion
3.1. Tensile property Carbon fibre with carbon nanotube mixed epoxy composite, carbon fibre reinforced with carbon nanographene mixed epoxy composite, tensile modulus, and tensile strength were presented in figures 2(a) and 3. The maximum tensile strength of 81.75 MPa is exhibited with the carbon graphene reinforcement in the carbon/ epoxy composite. The tensile strength for carbon nanotube-reinforced carbon/epoxy composites is 51.66 MPa. Differently shaped carbon allotropes mixed in the carbon/epoxy composites showed 23.83% and 95.96% improvement compared to pure carbon/epoxy composites with graphene and carbon nanotube reinforcement [33,34]. Carbon nanotubes were in hollow cylindrical structures, whereas graphene is in sheet form. Although the CNT and GN were allotropes of carbon, the infusion of GN has given more benefits than the CNT ( figure 2(b)). The sheet structure of the GNs is aligned parallel to the load, as is visible in the SEM images. As a result, the composite failed along the plane of the carbon layer (figures 1 and 7(e) and (f)). The same analogy of the contact nature between the epoxy with carbon nanotubes and graphene oxide is illustrated in figure 7 of Xuming Yao et al work [15].
Fiber pull-outs were observed in the SEM images of the CNT mixed carbon fibre reinforced composite (figures 6(a), and(b), and the CNTs show more strength in the longitudinal direction. And because aligning carbon nanotubes to the loading direction is difficult [35], the full benefits of CNTs were not realised in carbon fibre and CNT-mixed epoxy composites due to deviation of the uniform dispersion [36]. Past literature reveals that random-oriented CNTs in the polymer composite show only a moderate or no strength enhancement [37]. Carbon fiber with out the pull out is presented in figure 6(c) the carbon fibers in the failre region is presented in figures 6(a) and (d). This clearly shows that the alignment of graphene in the direction of the carbon mat gave more strength to the applied load, while the alignment of the carbon nanotubes was random. Because of this, the full tensile strength potential of CNT was not reached in this study ( figure 2(b)).
Notable improvements in tensile modulus and tensile strength are observed with carbon graphene filler. This combination, i.e., carbon fibre and graphene filler in an integrated epoxy composite, withstands most of the load better than carbon fibre and carbon nanotube filler in a mixed epoxy composite under tensile loading. Figure 4(a) shows the bending strength of carbon-based composites. It was seen that carbon fibre reinforced with graphene had a higher bending strength than carbon fibre reinforced with nanocarbon tubes mixed with epoxy composites. The increase in flexural strength of composites is due to the bonding between the carbon graphene and the epoxy matrix. The carbon fibre and carbon graphene combination showed the maximum flexural strength at around 366.816 MPa, and the flexural strength of the carbon fibre and carbon nanotube  mixed epoxy composite was 284.97 MPa. Compared to pure carbon/epoxy composites, carbon/carbon graphene showed a 95.93% improvement and a 23.83% improvement in carbon fiber/carbon nanotube mixed epoxy, respectively. Under bending load, the CNTs may also randomly align in the epoxy matrix; however, the void portion in the hollow CNT is responsible for less bending strength and modulus enhancement than GN's reinforcement. In figure 4(b) the same idea is depicted. The spaces inside the CNT were shown in figure 4 in the form of voids, but due to the sheet structure of the GN, these voids were not present [38,39]. Figure 5 shows the flexural modulus of the carbon fibre reinforced carbon filler-based composites. The C/G + E combination showed the greatest improvement in flexural modulus [40]. Under bending load, the alignment of the graphene to the carbon mat supported and resisted the load better. Because of this, the flexural modulus is higher for C/G + E than for C/CNT + E.

Flexural test
The flexural modulus is a very important factor that avoids the bending of the material under the same type of load. In this case, the sheet shape of the graphene has given full potential to the applied bending load, whereas in the CNT-reinforced composite, the differences in the alignment did not create a chance to explore the full potential of the CNTs. As a result, the C/G + E composite showed higher performance in terms of flexural strength than C/CNT + E.

SEM analysis
After the failure, scanning electron microscope images of the present tensile and flexural specimens were presented in figures 6-7. Along with the properties, the interaction between the nanoparticle and the epoxy matrix is presented, and the reason for the enhancement in the properties is observed. The bonding between the filler and matrix, and the carbon fibre is good in both cases. The main failure mechanism associated with the both composite materials is fiber pullout. Figures 6(a), (b) shows th efiber pullout of C/CNT + E composite. The nanofiller supported the carbon fibre. Carbon fibre pullout is observed in the C/CNT + E composite, whereas in the C/G + E composite, layer separation is observed under tensile failure. At the same failure location, the SEM images were captured and presented in figures 7(a)-(d). In figure 7(a), fiber separation is marked. At the failure region, the damage caused to the fibre is less, and these graphene particles have supported the fibre; as a result, a better enhancement is attained in graphene filler than in CNT filler (figures 7(c)-(d)).

Thermal conductivity Test
Carbon fiber-reinforced composites with graphene and CNT-mixed composites were further tested for thermal conductivity. The thermal conductivity of the composite specimens is measured using the thermal conductivity testing apparatus available at the Prasad V. Potluri Siddhartha Institute of Technology. The prepared specimens are placed in the thermal conductivity apparatus, heat energy is supplied, and the difference in the temperatures are identified from the sensors attached to the machine.The samples were prepared according to ASTM 1530. The diameter of the prepared specimen is 50 mm, and the thickness is 10 mm. In figure 8, the thermal conductivity apparatus is presented along with the variation of thermal conductivity. The specimen testing is repeated four times by supplying the same input parameters to the apparatus. The average value is taken as the final result. Because CNTs have a larger surface area than graphene sheets, CNT reinforcement improves thermal conductivity [41]. When compared to pure epoxy, thermal conductivity capacity is increased with carbon fibre reinforcement, and the percentage of improvement is recorded as 80.5%. Furthermore, the addition of CNT and GP increases conductivity. The percentage of improvement is 35.3% and 20% with CNT and GP additions, respectively, compared to pure carbon fiber-reinforced composites. The CNT alignment may not be a problem for the flow of heat energy. As a result, the conductivity is high for CNT reinforcement.

Elastic properties of carbon fiber reinforced filler mixed composite using micromechanics
In this section, the micromechanics approach with finite element method support is used to show the transverse modulus, major and minor poissons ratio of the carbon fibre reinforced with filler mixed epoxy matrix. The current simulation studies are solved using the finite element method software ANSYS. The main assumption for this work is that there are perfect bonds between all the parts of the composites. The relation between the continuum models of CNT and graphene are presented [41,42]. One of the assumptions that will be considered in the micromechanics is that the reinforcement distribution is uniform. However, this condition will determine the percentage error between the experimental and numerical simulations. To decrease the deviation between the experimental numerical simulations, the radom distribution will be taken into consideration. The space between the reinforcements is constant in the uniform distribution, but it is assumed to vary in the random distribution. Solid 95 elements are selected to perform the finite element simulations. This element is defined by 20 nodes, and each node is provided with three degrees of freedom in the x, y, and z directions. The continuum model approach is considered for the CNT and graphene [43].
When comparing Young's moduli from experiments and simulations, the differences are caused by the assumptions used in the FE models. Perfect bonding between the reinforcement and the matrix, the lamina is initially stress-free and linearly elastic, and the matrix is void-free. These assumptions may not be true while performing experimental studies. In this work, the nano-CNT or graphene particles were assumed to be distributed uniformly in the epoxy matrix. But when the above assumption was used to compare the experimental results to the simulation results, a lot of differences were seen. The maximum and minimum limits of the Young's modulus were found by changing the RVE while keeping the nano-reinforcement the same. This brought the results from experiments and simulations closer together. From figure 10(a), it is observed that the  CNT were equally spaced in the X-Y plane, whereas in figure 10(b), the CNT were close with respect to the X axis (zone A). Because the CNTs were not equally spaced along the Y axis (zone B), the equally spaced assumption may not yield a closed form of solution. Considering this, the maximum limit and minimum limit of the Young's modulus were predicted. In our previous work [14], we showed how to figure out the RVE for nano-CNTs and graphite. References [46][47][48][49] highlights some recent advancements in numerical methods regarding different mesh points.

Distantly spaced in X axis
The studies were performed by adopting two-stage homogenization methods. In the first stage of homogenization, the filler properties (CNT or graphene) of the mixed epoxy matrix were identified. Later, the properties of the carbon fibre reinforced with filler and epoxy composite were identified. The complete methodology used for these two homogenization procedures is taken from the previous research published by the first and second authors of the present work [14,45,50]. The finite element model at 0.5% of CNT and graphene in the epoxy matrix is presented in figures 9(a), (b). The Fe model of the carbon mat-reinforced filler mixed matrix is presented in figure 9(c). All these studies were performed by considering the symmetry of the geometrical model, loading applied to the Fe model, and boundary conditions. Uniaxial pressure is applied in the thickness direction (Z-direction), and longitudinal modulus is determined by using Hook's law and major and minor Poisson's ratio, which were obtained by dividing the lateral strain with the longitudinal strains of FE models. The major poisson's ratio is associated with the longitudinal directional loading, whereas the minor poisson's ratio is associated with the transverse loading of the FE model. Along with the strength of the material, the relation between the one-directional load (Z) and opposite-direction material deformation (X or Y) is necessary while designing any material because the strength alone is not sufficient to recommend the material for a particular application. These relationships will be given by the major and minor poisson's ratios.  Applying the pressure load in the through-thickness direction gives the longitudinal modulus (E 1 ) and major Poisson's ration (ν 12 ). Applying a pressure load in the transverse direction (X or Y) predicts the transverse modulus (E 2 ) and minor poisson's ratio (ν 21 ) The tensile modulus obtained from experimental findings is compared with FE results. For that, the maximum and minimum limits of the Young's modulus were obtained at a fixed percentage of CNT and graphene (0.05). The experimental results were in the middle between the maximum and minimum limits of the tensile modulus ( figure 12).
For C/CNT + E, the maximum longitudinal modulus is 13119 MPa, the minimum longitudinal modulus is 10200 MPa, whereas the longitudinal modulus from the experimental studies is 11441.27 MPa. For C/G + E composite, the minimum modulus is 11423 MPa, the maximum longitudinal modulus is 31456 MPa, and from the experimental results, the longitudinal modulus for the same composite is 28331 MPa ( figure 12). The experimental results are in the range between the maximum and minimum magnitude bands. It was demonstrated in earlier research using micromechanics techniques that the addition of appropriately shaped and scattered graphenes enhanced the elastic performance [44].
The transverse modulus of carbon fibre reinforced with CNT reinforcement is found to be high as shown in figure 13. Compared to pure carbon fiber-reinforced epoxy composite, the infusion of CNT and GP is beneficial in terms of transverse modulus. About 292% and 200% improvements were observed with CNT and GP filler reinforcement, respectively. Because transverse direction has a lower modulus than longitudinal direction, it is referred to as 'negative direction.' With the CNT and filler reinforcement, the transverse modulus was greatly increased compared to a pure carbon fiber-reinforced composite. In a similar vein, it has been discovered in the past literature that the formation of an uneven distribution of CNTs increased the transverse modulus [45,50].  The major and minor Poisson's ratios of the carbon fiber-reinforced composites were presented in figure 14. These two properties were significant for forecasting the deformation of the material at different loading conditions [46]. Significant differences were not noticed due to the nanofiller. However, a lot of changes were noticed in the minor Poissons ratio.

Conclusions
The present work addresses the enhancement of carbon fiber-reinforced composites with the addition of carbon nanotubes and carbon graphene filler without a change in the weight of the carbon fiber-reinforced epoxy composite. The stiffness and strength of the same composite were improved with carbon nanotube and graphene additions. Two different-shaped nanofillers, such as C + CNT + Epoxy and C + G + Epoxy, were used. The results show that the nanofiller reinforcement significantly enhances the tensile and flexural strength  of the materials. The addition of 40% carbon fibre and 0.5 wt% graphene filler in epoxy resin significantly enhanced tensile and flexural strength.
• The maximum tensile strength of 81.75 MPa is exhibited with the carbon graphene reinforcement in the carbon/epoxy composite. The tensile strength for carbon nanotubes reinforced carbon/epoxy composite is 51.66 MPa.
• Carbon/carbon graphene showed a 95.93% improvement in tensile strength and a 23.83% improvement in carbon fiber/carbon nanotube mixed epoxy compared to pure carbon/epoxy composites.
• When compared to pure epoxy, thermal conductivity capacity is increased with carbon fiber reinforcement and the percentage of improvement is recorded as 80.5%. Furthermore, the addition of CNT and GP increases conductivity.The percentage of improvement is 35.3% and 20% with CNT and GP additions, respectively, compared to pure carbon fiber reinforced composites.
• Compared to pure carbon fiber reinforced epoxy composite, the infusion of CNT and GP is beneficial in terms of transverse modulus. About 292% and 200% improvement are observed with CNT and GP filler reinforcement, respectively.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).