3.1 Physical properties
The density, FVF, thickness, and void contents were measured as described in the "Materials and Methodology" section earlier in Table 2. The density of hybrid composites increased slightly (+ 4.9%) compared to pure kenaf/epoxy composite (K1), and maximum density (1.77 g/cm3) was measured for pure carbon/epoxy composite (C2) as expected due to a higher density of carbon fibers than kenaf fiber. The burn test method has a good accuracy compared to other methods discussed in the literature and was used for calculating the FVF and void contents in C2. The degradation of natural fiber starts after 200 oC because of its structure-thermal resistance [52, 53]. Therefore, this method cannot be used for natural fiber reinforced polymer composites due to high-temperature limitations (more than 400 oC) for burning the epoxy.
By alternating two carbon plies with kenaf fabrics in, KC1, FVF of carbon reached 9%, and kenaf FVF was decreased to 20% with 35% total hybrid FVF. The carbon FVF in other hybrid composites (KC2-KC5) was increased by approximately 16%, and kenaf FVF was decreased to 25% with 41% total hybrid FVF. As the number of kenaf and carbon plies was kept constant, the void contents were found between 2–5% in these hybrids. The variation in FVF may occur because of instabilities in the epoxy amount impregnation, which is essential to the VARI fabrication process employed for the composites. Non-uniform distributions of kenaf fiber in a fabric, or high vacuum pressure (80 KPa) in VARI used in the current study, or the trapping of mechanical air during resin flow [54]; during the curing, gas is formed as a result of chemical reactions [55] and dissolved gas nucleation in the resin [56] can be the reason for the variation of void contents and FVF. The primary source of air entrapment is inhomogeneous fiber architecture, which results in non-uniform permeability of the fiber preform, causing local variations in resin velocity. The capillary effect, dominant at the micro-scale, exacerbates this local velocity variance [57].
The number of kenaf-carbon interfaces in the interlayer hybrid composites is 4, 6, 4, 2, and 1 for KC1, KC2, KC3, KC4, and KC5, respectively. This distribution is also normalized in Fig. 2, along with the relative distribution of carbon and kenaf fibers by weight, calculated using Eq. 3.
The interface bonding between the fibers and epoxy is shown in Fig. 3a and 3f show good wetting of kenaf fibers and carbon fibers in hybrid composites as revealed by SEM micrographs. Fiber surfaces that contain epoxy debris (e.g., Fig. 3b) indicate good fiber-epoxy adhesiveness. Epoxy resin can penetrate fiber walls through the porous behavior of plant fibers because of their low viscosity. In addition, the surface irregularity of natural fibers generates mechanical interlocking that improves the strength of fiber-matrix interfaces.
3.2 Tensile Properties
The experimental tensile and flexural testing results for pure composites (kenaf/epoxy and carbon/epoxy) and kenaf/carbon/epoxy hybrid composites are given in Table 3. The tensile strength (σ) and modulus (E) of the hybrid composites are compared with pure kenaf/epoxy tensile strength (\({\sigma }_{K}\)), and modulus (\({E}_{K}\)). Pure kenaf/epoxy (K1) composites had an average tensile strength of 58.62 MPa. However, the highest tensile strength (294.13 MPa) and modulus (22.38 GPa) are achieved by locating kenaf fabric layers alternatively between carbon fabrics (KC2). Carbon fabrics are approximately 4 times more potent than kenaf fabrics, according to Table 1, so the tensile strength of K1 increased by 401.8 % b including 15.8 vol.% carbon fabrics (KC2). Thus, KC2 was selected to examine the impact of carbon fiber amount in hybrid, as shown in Fig. 4.Variations in tensile strength and modulus measured between KC2 and KC5 were minimal, except for KC2's modulus, since they essentially had identical FVF for kenaf/carbon, and they differed only in how they were stacked. The most ductile but least stiff hybrid laminate among KC2-KC5 was KC4, with the highest hybrid fiber FVF.
|
Tensile
|
|
|
Flexural
|
|
|
Table 3
Tensile and flexural experiments results.
Specimens
|
σ (MPa)
|
ε (%)
|
E(GPa)
|
\({\sigma }_{f}\)(MPa)
|
\({{\epsilon }}_{f}\) (%)
|
\({E}_{f}\) (GPa)
|
|
Av
|
σ/\({\sigma }_{K}\)b
|
Av
|
ε/\({\epsilon }_{K}\)b
|
Av
|
E/\({E}_{K}\)b
|
Av
|
\({\sigma }_{f}/{{\sigma }}_{{{f}}_{{K}}}\)b
|
Av
|
\({{\epsilon }}_{f}/{{\epsilon }}_{{{f}}_{{K}}}\)b
|
Av
|
\({E}_{f}/{E}_{{f}_{K}}\)b
|
|
K1
|
58.62
|
1.00
|
1.14
|
1.00
|
5.18
|
1.00
|
100.14
|
1.00
|
2.11
|
1.00
|
5.71
|
1.00
|
|
KC1
|
178.34
|
3.04
|
1.35
|
1.18
|
13.26
|
2.56
|
188.26
|
1.88
|
3.84
|
1.82
|
6.19
|
1.09
|
|
KC2
|
294.13
|
5.02
|
1.40
|
1.23
|
22.38
|
4.32
|
369.32
|
3.69
|
2.30
|
1.09
|
20.02
|
3.51
|
|
KC3
|
265.49
|
4.53
|
1.28
|
1.12
|
20.99
|
4.05
|
267.24
|
2.67
|
3.30
|
1.56
|
9.20
|
1.61
|
|
KC4
|
261.82
|
4.47
|
1.51
|
1.33
|
17.96
|
3.47
|
400.96
|
4.00
|
1.64
|
0.77
|
29.27
|
5.13
|
|
KC5
|
215.41
|
3.67
|
1.06
|
0.93
|
20.49
|
3.95
|
298.09
|
2.98
|
5.80
|
2.75
|
10.60
|
1.86
|
|
C2
|
680.42
|
1.00
|
1.33
|
1.00
|
51.39
|
1.00
|
683.99
|
1.00
|
2.45
|
1.00
|
32.88
|
1.00
|
|
σ: Tensile strength, ε: tensile strain, E: tensile modulus, \({\sigma }_{f}\): flexural strength, \({{\epsilon }}_{f}:\) flexural strain,\({E}_{f}\): flexural modulus, and Av: average value of three samples. K: Kenaf, b: increment ratio concerning pure kenaf/epoxy |
Carbon fibers influence the overall properties of a composite. The stress-strain characteristics of kenaf/epoxy are very different from those of carbon/epoxy composite (Fig. 5a). As a result of defects or dislocations in natural fibers, these fibers exhibit nonlinear tensile behavior [58, 59]. The volume fraction of carbon fibers has a significant effect on the degree of nonlinearity in composites formed by hybridizing with carbon fibers (Fig. 5b). KC5 hybrid composite with single carbon epoxy plies is noted to deviate from linear behavior more than hybrid composite with blocked carbon plies (KC2-KC4) for the similar carbon FVF.
Kenaf/carbon/epoxy hybrid composites had high overall tensile performance primarily due to their FVF of carbon fiber, but it is possible that hybrid fiber dispersion could also influence this performance. Compared to hybrid laminates KC3-KC5 with similar carbon content, the tensile modulus was increased (between 6.2–19.7%) when the hybrid plies were stacked alternately in KC2. KC2 has an average failure strain of 8.7% and 24.3% lower than KC3 and KC5, respectively, and 8.1% higher than KC4, while its tensile strength deviates by a maximum of 26.8%. A significant reason for the difference is the lack of carbon ply blocking in KC2 versus KC3-KC5. The first important point is that in woven fiber-reinforced composites, out-of-plane regular forces develop fundamentally [60]. Due to the nature of the surrounding kenaf plies, if one carbon ply is interspersed with single kenaf layers, the carbon plies will exhibit more deformation under the out-of-plane stresses. In the case of carbon layers bordered by like carbon layers, however, more significant stress and stiffness result because the carbon layers are constrained by like carbon. There are relatively large resin-rich zones in kenaf plies, which produces their high compliance and lower density because of their lumens. Aisyah, et al. [40] reported that plain woven kenaf/carbon hybrid, one-layer kenaf fabric at the center and carbon fibers at the upper and lower layers, had 122 MPa and 7.97 GPa tensile strength and modulus.
The morphology of failed composites after the tensile test is displayed in Fig. 6. Nevertheless, fiber pull-out was observed at fracture surfaces as well, although the lengths of pull-out were reportedly minimal, suggesting a uniform fracture. Figure 6a and Fig. 6d show micro-fractography analysis of brittle fiber failures and kenaf fibers (Fig. 6a-f) indicate that brittle fiber failures predominate. The matrix in Fig. 6b and Fig. 6d is clearly showing evidence of ductile failure. Pure kenaf/epoxy composite showed higher brittle failure with a lack of fiber pull in the form of fiber splitting compared to pure carbon/epoxy composite, which had higher fiber splitting (Fig. 3c). For hybrid composites, fiber pull (in the form of fiber splitting) was highest in hybrid KC2 among all hybrids (Fig. 4c). Figure 7 shows the specimens of the tensile test. Most specimens failed outside of gauge length. It is an expected behavior among the fiber-reinforced polymer composites using ASTM D3039 [60].
3.3 Flexural Properties
The flexural strength (\({\sigma }_{f}\)) and modulus (\({E}_{f}\)) of the pure kenaf/epoxy and hybrid composites are compared based on Table 3. Pure carbon/epoxy composite, C2, showed 583 % ad 476 % hgher flexural strength and modulus, respectively, than pure kenaf/epoxy, K1. The flexural strength (100.14 MPa) and modulus (5.71 GPa) of K1 increased by 88 % ad 8.4 % b including 9 vol. % carbon fibers (KC1). The maximum flexural strength (400.96 MPa) and modulus (29.27 GPa) were achieved by incorporating 16.6 vol. % carbon fibers (KC4) showed 300 % ad 412.6 % iprovement in K1, respectively. The effect of carbon content by volume on the flexural properties of kenaf/epoxy composite is given in Fig. 8a and 8b.
The stacking sequences of hybrid composites (KC2 to KC5) showed the significant variations in the flexural properties. It is concluded that the poor properties of pure kenaf/epoxy can be improved by hybridization with carbon fiber in hybrid composites while keeping carbon fiber as skin layers (hybrid KC2 and KC4). However, hybrid KC4 had 8% and 46% higher flexural strength and modulus than hybrid KC2, respectively (Table 3). The flexural stress and strain response has been shown in Fig. 9a and Fig. 9b. Hybrid KC5 showed higher strain, indicating slow crack formulation during flexural loading compared to other tested hybrids (Fig. 9b). Aisyah, et al. [40] reported that plain woven kenaf/carbon hybrid had 224 MPa and 17.68 GPa flexural strength and modulus. They used single-ply kenaf fabric at the center and carbon fibers at the upper and lower layers in the hybrid. Bakar, et al. [61] found around 56.4 MPa and 3.5 GPa flexural strength and modulus for the ratio of 16:4 wt.% kenaf/carbon hybrid composite fabricated by compression molding. Singh, et al. [46] reported 335.6 MPa flexural strength for silanized kenaf/carbon hybrid (CC/KKK/CC) composite fabricated by hand layup. Our hybrid KC4 showed around 400.96 MPa flexural strength, 19.5% higher than the reported study on the exact configuration [46].
After flexural test, the failure response of pure and hybrid composites are fiber breakage and delamination, as shown in Fig. 8c and Fig. 9c, respectively. Pure composite K1 displayed pure brittle behavior in the form of fiber breakage and pulled out. There was crack formation too fast that cannot observe during flexural loading due to the brittle nature of kenaf compared to pure composite C2 (Fig. 8c and Fig. 9a). Hybrid composites showed improvement in a strain which indicates lower brittle behavior (Fig. 9b and Fig. 9c). Figure 10 shows SEM images of the fiber failure morphology and crack formation in the composites. Interface crack formation in the form of delamination and fiber breakage was observed, fiber-matrix interface crack (Fig. 10a), and fiber breakage (Fig. 10b – Fig. 10f) during flexural load. Hybrids showed slow crack formation due to the high stiffness of carbon, which makes them resistant to during flexural loading (Fig. 9c).
3.4 Interlaminar shear strength (ILSS)
Composite structures may be subjected to high stresses over the service life, resulting in cracking propagation across fiber-matrix interactions. As a result, increased fiber-matrix adhesion, greater strength, and a tougher matrix are desired. Short-beam shear tests were conducted on pure and hybrid composites to calculate their ILSS and the strength of fiber-matrix bonding. It is challenging to ensure pure shear failure during the experiment because shear failure might not even arise in the mid-plane [62]. Fiber rupture, micro-buckling, indentation, flexure, and interlaminar shear cracking of the composites are common failures during the shear experiment [63, 64]. Figure 11 shows the load vs displacement response during the short beam shear test. Pure carbon/epoxy showed a higher load than pure kenaf/epoxy composite, and although hybrid KC4 had lower displacement, KC4 demonstrated the highest load compared to other hybrids.
As shown in Fig. 11c, the highest ILSS was found in pure carbon/epoxy composite compared to pure kenaf/epoxy because carbon fiber has excellent interlocking and load-carrying capacity in the matrix than kenaf. Similarly, the highest ILSS was observed in hybrid KC4 than in other types of hybrids. It showed around 281% higher ILSS compared to pure kenaf/epoxy. The amount of carbon fiber in the hybrid composites played a crucial role in ILSS. Hybrid KC2 showed around 108% higher ILSS than hybrid KC1 due to twice the plies of carbon layers. The stacking sequence of fiber plies in hybrid composites affected the ILSS of hybrid composites. The lowest ILSS was found for KC3 than hybrid KC2, KC4, and KC5. The carbon plies as skin layers in hybrid (KC4) showed excellent interlaminar shear performance; even this hybrid showed comparable shear strength to pure carbon/epoxy composite. It may be because of better interlocking between the kenaf and carbon layers in the hybrid. The ILSS is expected to improve because of hybridizing higher mechanical strength fibers (carbon) with low kenaf strength in the hybrid composites. In Fig. 11d, pure K1 showed a brittle response during shear loading compared to pure C2, which had a shear failure. Hybrid composites (KC1 – KC5) showed a slow shear crack formulation compared to K1, according to Fig. 11b and 11e.
3.5 Fracture toughness
Mode-II fracture testing of pure and hybrid composites is done as described earlier, and the load-displacement response of tested specimens is compared in Fig. 12a. A higher peak load is required to fail the hybrid composites than the pure kenaf/epoxy composite. For all composites, the load increased linearly until peak load, and unstable crack formation happened towards the center roller after peak load. Crack initiation in the form of consecutive rounding tops and bottom in the curves can be seen for hybrid composites. The crack formation in hybrid composites was not uniform compared to pure carbon/epoxy composite. Pure K1 (Ft) showed a complete linear trend until failure at a mid-plane and did not produce any crack formation after pre-crack length during testing. On the other hand, pure C2 (Ft) displaced uniform growth of crack from pre-crack position to midplane of specimens. The hybrid KC1 (Ft) had a higher load than other hybrid composites during the testing.
The Mode-II fractures toughness is given in Fig. 12b. Pure carbon/epoxy showed the highest toughness among all tested composites. Pure C2 (Ft) had approximately 130% higher fracture toughness than K1 (Ft) composite. Although hybridization of carbon at FVF is the same in all hybrid composites, KC1 (Ft) improved the fracture toughness by approximately 79%, and KC2 (Ft) improved only 35.8%. It highlighted the impact of stacking sequence in hybrid on fracture toughness performance of the composites. Hybrid KC1 (Ft) has two carbon layers at the center, and the crack was initiated in comparison to the other hybrid, which has kenaf layers in the center. Natural fibers have poor consistency in crack formation during fracture toughness (Mode II) loading. Even the poor performance (up to 51% decrement) was found for hybrid KC3 (Ft) - KC5 (Ft) among all composites, which was lower than pure kenaf/epoxy composite. According to Eq. 6, load and displacement should be high to achieve higher fracture toughness in the composites. In the case of these hybrids, load and displacement were lower than pure kenaf/epoxy (Fig. 12a). The variation in fracture toughness among hybrid composites indicates that the stacking sequence is crucial for the specific fracture toughness depending on the structural application.
The failure response after the fracture toughness (Mode II) test is shown in Fig. 13. The crack formation and delamination are crucial to analyzing the fracture performance of the composites. Pure C2 (Ft) displayed uniformed crack growth from pre-cracked point to mid-plane. However, pure K1 (Ft) showed any crack formation until pre-cracked and broke from midplane after reaching maximum load and displacement. The main reason may be that the composites have lower mechanical properties than carbon fibers. In the case of hybrid composites, Hybrid KC2 (Ft), KC3 (Ft), and KC4 (Ft) had uniform crack formation to hybrid KC1 (Ft) and KC5 (Ft). The maximum load was increased in hybrid composites by adding the carbon layers with kenaf. As shown in Fig. 13, cracks were found along the path of pre-cracked. Actual delamination may be difficult to detect during fracture loading. The local buckling and reduction of in-plane compressive strength occur due to cracks and delamination which are developed in the composites during the subsequently in-plane compressive loading [49]. Pure K1 (Ft) may behave like brittle materials during the testing than other tested ones. The fracture toughness performance of pure kenaf/epoxy can be improved by hybridizing carbon fiber as skin or center layers with two kenaf layers at the center, such as hybrid KC1 (Ft) and KC2 (Ft). The more than two layers of kenaf at the center in hybrid composites (KC3 (Ft), KC4 (Ft), and KC5 (Ft)) do not resist the crack formulation; therefore, the low load is developed, which is responsible for poor fracture toughness performance.
3.6 Water absorption and thickness swelling
The water absorption of pure and hybrid composites demonstrated a speedy rise in absorption over the initial few days that approximately arrived at steady-state saturation after 148 days; the same is illustrated in Fig. 14a. For all composites, the water absorption characteristics are distinctly different. The two stages were presented in the water absorption curves of the composites, as shown in Fig. 14a. The first stage was seen between 0 to 148 hours (Fig. 14a Section a), percentage of water absorption was found to rapidly increment in a quasi-linear behavior along with rising the immersion duration. After this, as described in Fig. 14a section b, the second stage followed a nonlinear response between water absorption and immersion time, and the water absorption rose steadily while waiting for the saturation condition. The water absorption of pure kenaf/epoxy K1 and pure carbon/epoxy C2 were the highest and lowest among the other tested hybrids composites.
Figure 14b shows two stages for thickness swelling variation concerning immersion time. In the first stage (Section a), the thickness swelling increased, exhibiting quasi-linear behavior as immersion time increased, and then gradually increased in the second stage b, until the saturation point was reached. Thickness swelling of hybrid KC3 and pure C2 was maximum and minimum value at the entire immersion duration, respectively, compared to other pure and hybrid composites. Due to the hydrophilic characteristic of kenaf fiber in composites, the highest water absorption and thickness swelling for pure K1 is expected. Another theory is that kenaf fiber comprises cellulose, hemicellulose, lignin, and cellulose is a hydrophilic polymer with hydroxyl groups. These hydroxyl groups react with the hydrogen bond of water molecules, causing the composite to absorb a lot of moisture [118]. The fibers are closely packed in the hybrid composite, with the water-impermeable carbon and glass fibers acting as barriers, preventing contact between the water and the kenaf fiber. These results show that the water absorption and thickness sensitivity of pure kenaf composites are reduced by adding carbon as a blockage layer to the water molecules. The water absorption in the kenaf composite is due to the hydrophilic nature of kenaf fiber. Furthermore, the lumen, a hollow region in the middle of the fibers that aid in the water absorption process, gives the fibers a tubular form [118].
Table 4
Water absorption properties of non-hybrid and hybrid composites.
Composite
|
QS (%)
|
S
|
D (mm2/s)
|
P (mm2/s)
|
\({{T}}_{{s}{w}{e}{l}{l}}\)(%)
|
K1
|
8.0
|
2.1
|
1.32 × 10− 11
|
2.72× 10− 11
|
5.6
|
KC1
|
6.5
|
1.6
|
1.29 × 10− 11
|
2.07× 10− 11
|
6.9
|
KC2
|
4.2
|
3.3
|
4.11 × 10− 12
|
1.34× 10− 11
|
6.6
|
KC3
|
5.7
|
1.6
|
1.99 × 10− 11
|
3.14× 10− 11
|
7.9
|
KC4
|
4.3
|
3.4
|
3.48 × 10− 12
|
1.18× 10− 11
|
3.9
|
KC5
|
6.3
|
2.1
|
1.07 × 10− 11
|
2.21× 10− 11
|
7.2
|
C2
|
0.9
|
2.4
|
5.88 × 10− 12
|
1.38× 10− 11
|
0.9
|
QS: percentages of water absorption at saturation time, \({T}_{swell}\): Maximum percentages of thickness swelling, S: Sorption coefficient, D: Diffusion coefficient, and P: Permeability coefficient
Table 4 shows that the hybrid KC4 composites have a lower diffusion coefficient and permeability coefficient than the other hybrids, indicating that the hybrid composites are water-resistant. However, hybrid KC3 showed a higher diffusion coefficient and permeability coefficient value than other hybrid composites, which show lower water resistance. Pure carbon/epoxy has the lowest water absorption and highest diffusion coefficient, which means water cannot penetrate the layer much more quickly compared to all composites; however, it cannot be absorbed by carbon due to the hydrophobic structure. Pure kenaf/epoxy composite has the highest water absorption due to its hydrophilic structure; however, low diffusion coefficient due to blocking the water passing through the epoxy layer by absorbing the water by each kenaf fiber in the fiber mat. In conclusion, although pure kenaf/epoxy composite has the highest water absorption and lowest dimensional stability, its water resistance can be improved by hybridizing kenaf with carbon. It can be highlighted that stacking sequence indicates a critical impact on water absorption and dimensional stability. Hybrid KC5 and KC3 performance were lowest compared to other hybrid composites. Bakar, Ahmad [119] found around 9% water absorption (for 120 days) for the ratio of 16:4 wt.% kenaf/carbon hybrid composite fabricated by compression.