Study on the mechanical characteristics of a natural Fiber-based hybrid polymer composite

Abstract

In this work, the hybrid fiber-based epoxy composite samples have been prepared with the application of three unlike organic natural fibers viz., Sisal (S), Ramie (R), and Flax (F) and epoxy as the matrix, with a weight ratio of 70:30 for epoxy and fibers. Six different stacking sequences of fibers, pure fibers and pure matrix were used for specimen preparation, namely S/F/F/F/S, F/S/S/S/F, F/R/R/R/F, R/F/F/F/R, R/S/S/S/R, S/R/R/R/S, PS, PR, PF, and Pure matrix. The composite specimen was prepared by manually and uniform thickness was obtained by the compression moulding process. The mechanical trials such as, tensile, impact, hardness, and flexural tests had been carried out on the composite samples. The results revealed that among all the fiber-stacking combinations, the maximum tensile strength was observed in the Sisal and Flax fiber-based sequence F/S/S/S/F, showing a 366.7% enhancement compared to pure epoxy. The composite materials with the layering sequences R/S/S/S/R, F/R/R/R/F, and R/F/F/F/R demonstrate superior flexural strength, impact strength, and hardness values, respectively, compared to other combinations of composites. The maximum tensile, flexural, and impact strengths and hardness of F/S/S/S/F, R/S/S/S/R, F/R/R/R/F and R/F/F/F/R are 36.28 MPa, 77.8 MPa, 13.38 kJ/m², and 57 h, respectively, compared to all the remaining sequences of the specimens. Various defects in the composite specimens, including fiber breakage, porosity, voids due to fiber pullouts, and matrix regions, were identified through scanning electron microscope images.

Introduction

The automotive and aeronautical sectors demand materials with a high strength-to-weight (HSW) ratio to enhance system efficiency. Composites, predominantly fiber-reinforced polymeric systems, perfectly meet this requirement^1,2. They exhibit lower density compared to conventional materials, but they claim a remarkable HSW ratio^3,4. Over the past two decades, natural fiber-based composites have attained widespread use as a prodigious substitute to synthetic fibers, primarily owing to their numerous advantages and applications^5–7. The advantages of such fibers include availability, cost-effectiveness, ease of processing, and environmental friendliness. However, natural fiber-based composites have a few disadvantages, including lower mechanical properties. These drawbacks have been addressed through recent fabrication procedures, namely hybridization and surface treatment of fibers. A hybrid composite involves combining a double or multiple different fibers, resulting in enhanced mechanical properties. These hybrids are classified based on the fibers used, namely a combination of more synthetic, a combination of synthetic and natural, and a combination of more natural fibers^6–8. The mechanical characteristics of hybridized composites are primarily influenced by environmental conditions, the volume fraction of the fiber content, stacking sequences, and the surface treatment of the fibers. Consequently, scholars are increasingly concerned in replacing single fiber-based composites with hybridized composites⁹. Various researchers have explored different fiber combinations. Arthanarieswaran et al.,¹⁰ steered tensile strength assessments using combinations of banana and sisal fibers between layers of glass fibers. To achieve better strength, glass fibers were positioned on the exterior and middle layers of the hybrid system. The results indicated significantly improved flexural strength when banana/sisal was positioned between the glass fibers. Sathishkumar et al.,¹¹ analyzed the characterization of a hybrid composite system involving sisal and cotton fibers. They have piloted the tests on mechanical characteristics, and reported results displayed the great enhancement at 40% combinations of fibers in the composite. It was concluded that the highest load-bearing capacity was attained with the longitudinal orientation of sisal compared to that of cotton fibers. Zafeiropoulos et al.,¹² found the enhanced results from the mode I fracture toughness and interlaminar shear tests with hybrid systems consisting of flax and glass fibers compared to their pure forms. Alavudeen et al.,¹³ made a comparative analysis of the treated hybrid specimen consisting of banana and kenaf fibers with the untreated hybrid composite. In addition, they found that there was a 10% enhancement for the treated composite compared to the untreated composite.

Ramnath et al.,¹⁴ analyzed the mechanical parameters, namely tensile, flexural, impact, and double shear. They utilized various fibers, including glass/kenaf/neem. Among the different specimen preparations, the combination of glass/twisted neem/twisted kenaf/twisted neem/glass demonstrated a remarkably high tensile ultimate strength of 70 MPa. Additionally, this configuration exhibited an absorbed impact strength of 12 J. Jose et al.¹⁵ found the impact and hardness characterization of the curaua/glass fibers; the higher contribution of glass fibers shows better results due to very good interaction of the fibers and the matrix. Devireddy and Biswas¹⁶ reported various mechanical behavior tests, including tensile, flexural, impact, and shear strength. Hybridization improved mechanical properties, and the combination of banana and jute fibers demonstrated exceptionally good interlaminar shear strength at a 30% fiber content. Prasanna et al.¹⁷ suggested the mechanical behavior of the hybridization of the sisal/bamboo is enhanced with a higher contribution of the bamboo fibers. Surface treatment, namely, the alkalinization process, has enhanced the mechanical characteristics, namely tensile strength and impact, by 30% and 40%, correspondingly. Epoxy resin-based composite with neem oil has reduced mechanical properties. In addition, the kenaf fibers and hybrid kenaf elements in the composite specimen would upsurge the mechanical characteristics as suggested by Arun and Viswanathan¹⁸. Arshad et al.,¹⁹ considered the mechanical characterization of basalt fiber composite in the presence of coir fiber and nano-titanium carbide. Results reported that the mechanical performance along with the fiber-matrix load transfer capability. The thermal stability test reports revealed that the hybrid fiber-based composites have advanced thermal resistance compared to the pure polymer specimen. Khan et al.,²⁰ studied the mechanical properties of hybrid fiber-based composites consisting of kenaf (K) and jute (J) fibers. They used two distinct stacking sequence for specimen preparation using hand layup process. The results revealed that the mechanical strength of the K/J/K fiber-based hybrid composites exhibit very good results compared to the J/K/J.

Sumesh et al.,²¹ revealed the characterization of composite material consisting of pineapple/sisal/banana fibers. They found that the 3% of filler material enhances characterization upto 20%. Dhakal et al.²² studied the characterization of flax/carbon-based hybrid fibers. Results revealed that the elongation of the hybrid composite is enhanced compared to pure carbon fiber-based composites. However, the flexural properties of the hybrid composite has reduced owing to the lower strength of Flax fiber. Cavalcanti et al.,²³ examined the characterization of the hybrid composites, viz., jute/sisal, jute/sisal, and jute/curaua. They reported that the mechanical characteristics of the hybrid composite was augmented compared to their pure form; specifically, the tensile strength of the jute/sisal and jute/curaua had been significantly upsurged by 68% and 77%, respectively. Several authors have explored the hybridization of fiber-based composite materials^24,25. Some studies focus on incorporating various fibers in different proportions, while others examine the effects of different stacking sequences^26,27. Certain researchers have experimented with stacking sequences involving combinations of natural-natural and natural-synthetic fibers, leading to notable improvements in the mechanical characterization of materials^28,29. Additionally, some authors have employed nanoparticles and surface treatments to enhance fiber bonding with matrix materials and interlaminar strength^30,31. Most of these investigations aimed to assess the mechanical behavior of fiber-based composites, specifically in terms of tensile, impact, and flexural behaviour.

While many studies have explored hybrid natural fiber composites^32,33, the available research is quite scattered. Most of the work reported so far revolves around only a few fiber combinations and often repeats similar stacking arrangements. Because of this, there is still no clear understanding of how different stacking orders of entirely natural fibers affect the overall mechanical behavior of the composite. Although some scholars have experimented with various natural fiber hybrids, no study has systematically investigated the mechanical performance of composites fabricated using sisal, flax, and ramie fibers in different stacking sequences, nor compared these hybrid configurations against their pure form. This lack of comparative stacking sequence evaluation limits the ability to optimize natural fiber composite designs for load-bearing applications in automotive and aircraft structures. This study, therefore, focuses on filling this gap by generating experimental data and correlating performance trends for sisal/flax/ramie-based hybrid natural fiber composites configured in multiple stacking patterns.

To address the proposed research gap, the present study involved fabricating composite laminates using pure sisal, flax, and ramie fibers, followed by hybrid configurations of these three fibers with varied stacking sequences. The tensile, flexural, impact, and hardness characteristics have been evaluated to determine the most effective fiber arrangement. The outcomes of this study are intended to offer practical guidance for optimizing stacking designs in natural fiber composites, moving closer to developing strong, lightweight, and sustainable materials suitable for real-world engineering use.

Materials and methods

This study utilized the purest forms of different fibers (sisal, flax, and ramie) along with an epoxy resin matrix to fabricate composite materials. The mechanical performance indicators, namely tensile, flexural, impact and hardness, of the natural fiber-based composites are comprehensively assessed to find their suitability in automotive and aircraft structural applications. The tensile testing evaluates the composite’s ability to endure axial forces and provides information about tensile strength and ductility, which are critical for load-bearing and structural applications. The flexural testing simulates bending conditions and displays the material’s reluctance to distortion and rupture under bending loads, especially relevant for panel, beam, and structural components²⁰. The impact testing displays the material’s capability to absorb energy under sudden or shock loading, which is crucial for evaluating toughness and suitability in dynamic environments (e.g., automotive interiors). The hardness testing gives insight into surface resistance to penetration and wear, which plays an important role for durability and surface performance in tribological applications³⁴. The above-mentioned fibers and their combinations were chosen since ramie is superior in tensile strength (3422 MPa)³⁵, sisal seems to have a good impact strength³⁵, and flax has a superior flexural strength of 89.6 MPa³⁶ compared to the other two fibers. The differences in mechanical properties among the sisal, ramie, and flax fiber composites can be attributed to their individual structural and chemical characteristics. Ramie fibers, with high cellulose content and crystallinity, contribute to higher tensile strength due to their stiffness and better bonding with epoxy³⁷. Flax fibers offer a balance of tensile and flexural properties because of their fine, uniform structure and good fiber–matrix interaction³⁸. Sisal fibers, while coarser and lower in crystallinity, contribute to higher impact resistance due to their higher elongation at break and energy absorption capacity³⁹. The nature of the fiber surface also influences the interfacial adhesion, which is critical for effective stress transfer. These factors collectively govern the overall mechanical response of each composite system. A comparison was made between the pure fiber-matrix combination and six different stacking sequences of composite materials: S/F/F/F/S, F/S/S/S/F, F/R/R/R/F, R/F/F/F/R, R/S/S/S/R, and S/R/R/R/S. Table 1 presents the comparative mechanical properties of the fibers.

Table 1.

The mechanical properties of the proposed fibers.

Fibers	Tensile strength (MPa)	Tensile modulus (GPa)	Flexural strength (MPa)	Flexural modulus (GPa)	Impact strength (kJ/m2)
Ramie35	3422	1.83	56.03	2.247	9.36
Sisal35	28.55	1.49	53.42	4.25	9.61
Flax36	34.67	1.03	89.60	1.59	9.36

The fibers and matrix were procured from Kovai Green Fibers in Coimbatore, Tamil Nadu, India, in bidirectional and mat forms measuring 1 m². The matrix material used was LY556 grade epoxy blended with amine-based HY951 hardener. The densities of the hardener and epoxy were recorded as 1.18 g cm^-2 and 0.98 g cm^-2, respectively. The tensile and flexural test have been accompanied using the universal testing machine (UTM), SDTRX model, made by Deepak Poly Plast Pvt, Ltd, Gujarat, India. The maximum load capacity that can be tested is up to 10 kN. Delta made a variable speed motor built into the machine with a speed range of 1 mm min⁻¹ to 200 mm min^-1, and the speed accuracy is in the range of + 1 to -1 mm min^-1. The impact test has been performed using the Izod/Charpy tester machine, made by Deepak Poly Plast Pvt, Ltd, Gujarat, India. The maximum impact speed is 3.46 m s^-1, and the scale range is between 5 J and 25 J. The hardness of the samples has been examined by using a Rockwell hardness testing machine, MTMS Engineers Pvt Ltd, Pune, India. The maximum testing loads in the machine are 60 kgf, 100 kgf and 150 kgf. The composite fabrication involved a ratio of 70:30 for mat-form fiber and resin. The hand lay-up technique was employed for fabrication, and a compression molding technique ensured uniform strength of the composite specimens^40,41. The entire specimen was subjected to a pressure of 5 bar for 7 h.

Tensile and flexural characteristics were assessed using an UTM with load cells of 2 kN and 5 kN, respectively. The sizes of the tensile and flexural specimens had been chosen according to ASTM D 638 standards, measuring 165 × 13 × 3 mm² and 127 × 12.7 × 3 mm², respectively. The crosshead velocities for the tensile and flexural tests were fixed at 1 mm min^-1 and 2.5 mm min^-1, respectively⁴². Figure 1 shows the fractured specimen after tensile test for a variety of tested combinations. Figure 2 shows the fractured specimen after the flexural test. Figure 3 displays the experimental setup for the tensile, flexural, impact and Rockwell hardness tests. The obtained results after the tensile and flexural tests are load in kg and elongation in mm. These load and elongation values need to be converted into stress and strain values, respectively, to draw the graph. The following Eqs. (1) and (2) have been used for converting load into stress and elongation into strain.

1

2

Fig. 1.

The fractured specimen after the tensile test (a) S/F/F/F/S, (b) F/S/S/S/F, (c) S/R/R/R/S, (d) R/F/F/F/R, (e) R/S/S/S/R, (f) F/R/R/R/F, (g) Pure epoxy, (h) Pure R, (i) Pure S and (j) Pure F.

Fig. 2.

The fractured specimens after the flexural test (a) R/S/S/S/R, (b) F/S/S/S/F, (c) S/R/R/R/S, (d) R/F/F/F/R, (e) S/F/F/F/S, and (f) F/R/R/R/F.

Fig. 3.

Experimental arrangement for (a) Tensile, (b) Flexural, (c) Impact and (d) Hardness tests.

Where ‘w’ denotes specimen’s width (13 mm), ‘t’ is its thickness (3 mm), and ‘l’ is its narrow length (57 mm).

A scanning electron microscope of model JSM 6390 has been used for the analysis of failures in the specimen with microstructural scale after the tensile test. The resolution, acceleration voltage and working distance are 3.0 nm, 30 kV, and 8 mm, respectively, and were utilized for the scan. Figure 4 shows the schematic diagram of fiber orientation and ply order of F/R/R/R/F. Similarly, all the combinations of fiber-based composite specimens have been prepared for this study.

Fig. 4.

Schematic diagram of fiber orientation and ply order of F/R/R/R/F.

Result and discussion

Tensile test

The hybrid composite specimens had been fabricated in the ASTM D638 standard. The obtained tensile strength values of the specimens consist of natural fibers namely, flax and sisal, along with pure fiber and matrix, with different stacking sequences are shown in Fig. 5. The stacking sequences of the composite specimen are as follows: S/F/F/F/S, pure matrix and fibers, and similarly, F/S/S/S/F, pure matrix, and fibers are shown in Fig. 5a and b, respectively. The tensile strength in the sisal and flax fiber-based composite during the tensile test of S/F/F/F/S, PF, and PS are 291.1%, 283.3% and 263.3% higher compared to the PE, as shown in Fig. 5a. Similarly, the tensile strength of Sisal and Flax fibers of sequence F/S/S/S/F-based composites is 366.7% higher compared to PE, as shown in Fig. 5b. The tensile strength of F/S/S/S/F is 19.15% higher compared to S/F/F/F/S, as shown in Fig. 5c. The strain of the PF is higher compared to PS, S/F/F/F/S and PE, which are 110%, 52.72% and 52.72%, respectively, as shown in Fig. 5a. Likewise, the strain of F/S/S/S/F is 52.72% higher compared to S/F/F/F/S, as shown in Fig. 5c. The highest tensile strength for the hybrid F/S/S/S/F (36.28 MPa) is slightly higher than the 34.27 MPa reported for flax-only epoxy composites with 40% flax content³⁶. This improvement can be attributed to the hybrid stacking sequence, which enhances better load distribution and reduces crack propagation. The R/F/F/F/R (29.58 MPa) and F/R/R/R/F (25.20 MPa) combinations also showed superior strength compared to the milled ramie root polyester composites reported in a previous study³⁵, where peak tensile performance was limited by the shorter fiber length and less effective stress transfer.

Fig. 5.

The tensile strength of hybrid composite (Sisal and Flax) with different stacking sequence, (a) S/F/F/F/S, pure fibers and matrix, (b) F/S/S/S/F pure fibers and matrix and (c) composites S/F/F/F/S and F/S/S/S/F.

In contrast, the S/R/R/R/S (7.01 MPa) and R/S/S/S/R (7.51 MPa) sequences yielded much lower strengths, likely due to poor interfacial connection between the surface fiber and the epoxy, leading to early failure. The differences between studies highlight the critical influence of fiber type, fiber orientation, and processing parameters on tensile properties. The current results confirm that proper hybridization and stacking order can deliver improvements beyond those observed in single-fiber systems reported in earlier literature.

The natural fibers, namely Flax and Ramie of different stacking sequences are used to fabricate composite specimens as shown in Fig. 6. Figure 6 revealed outcomes of the tensile test of the experimented composites, pure fibers and matrix. The layer-by-layer sequences of the composite specimen are as follows: F/R/R/R/F and R/F/F/F/R, shown in Fig. 6a and b, respectively. The tensile strength produced during the tensile test of F/R/R/R/F, PF, and PR are 240%, 283.3% and 6.67% higher compared to the PE, as shown in Fig. 6a. Similarly, the tensile strength produced by fibers of sequence F/S/S/S/F is 206.7% higher compared to PE, as displayed in Fig. 6b. The PR strain is up to 5.5 mm, which is higher compared to PF, EP, and F/R/R/R/F by 144%, 243%, and 243%, respectively, as shown in Fig. 6a. Figure 6b displays that the PR strain is 450% higher compared to R/F/F/F/R. Figure 6c shows the tensile strength and strain of F/R/R/R/F are 10.92% and 26%, respectively, which are higher compared to R/F/F/F/R.

Fig. 6.

The tensile strength of hybrid composites (Ramie and Flax) with different stacking sequence, (a) F/R/R/R/F pure fibers and matrix, (b) R/F/F/F/R pure fibers and matrix and (c) composites F/R/R/R/F and R/F/F/F/R.

Figure 7 demonstrates the tensile strength of pure matrix, fibers and different stacking sequences of hybrid composites namely, Ramie and Sisal. The load of PS is higher compared to PR, PE, and R/S/S/S/R by 259.3%, 283.3% and 342.3%, respectively, as exposed in Fig. 7a. Similarly, the tensile strength of PS is 296.5% higher compared to S/R/R/R/S, as publicized in Fig. 7b. The tensile strength of S/R/R/R/S is 11.5% higher compared to R/S/S/S/R, as shown in Fig. 7c. The strain of the PR is higher compared to R/S/S/S/R, PE, and PS by 10%, 358.33% and 450%, respectively, as shown in Fig. 7a. Similarly, PR is 5.77% higher compared to S/R/R/R/S, as shown in Fig. 7b. The strain of the R/S/S/S/R and S/R/R/R/S does not show much difference in Fig. 7c. Table 2 shows the one-way ANOVA was performed to evaluate differences among three trial conditions for the six composites, namely FRRRF, RFFFR, FSSSF, SFFFS, SRRRS, and RSSSR. Among the six different composites, For FRRRF and RFFFR, the F-values (47.24 and 25.42, respectively) with p < 0.001 notifies statistically noteworthy changes among groups. In contrast, FSSSF, SFFFS, SRRRS, and RSSSR showed p > 0.05, indicating no significant variation between trials.

Fig. 7.

The tensile strength of hybrid composite (Ramie and Sisal) with different stacking sequence, (a) R/S/S/S/R, pure fibers and matrix, (b) S/R/R/R/S pure fibers and matrix and (c) composites R/S/S/S/R and S/R/R/R/S.

Table 2.

One-way ANOVA for six composites namely FRRRF, RFFFR, FSSSF, SFFFS, SRRRS, and RSSSR.

Composite		DF	Sum of Squares	Mean Square	F Value	Prob > F
FRRRF	Model	2	36237.88902	18118.94451	47.23831	4.44089E-16
	Error	128	49096.27303	383.56463
	Total	130	85334.16205
RFFFR	Model	2	16539.90136	8269.95068	25.42004	1.62605E-9
	Error	92	29930.532	325.33187
	Total	94	46470.43336
FSSSF	Model	2	77.1498	38.5749	0.31005	0.73418
	Error	92	11446.29331	124.41623
	Total	94	11523.44311
SFFFS	Model	2	61.27582	30.63791	0.30056	0.74114
	Error	91	9276.21624	101.93644
	Total	93	9337.49207
SRRRS	Model	2	0.69922	0.34961	0.14003	0.86949
	Error	111	277.13032	2.49667
	Total	113	277.82953
RSSSR	Model	2	0.68575	0.34287	0.23115	0.79403
	Error	103	152.78076	1.48331
	Total	105	153.46651

The load-withstanding capacity of the addition of flax fiber on the uppermost and lowermost layers of the composite is higher compared to other stacking combinations, as shown in Fig. 8a. Similarly, it displays that the addition of Sisal fiber on the uppermost and lowermost layers of the composite exhibits higher strain compared to other fibers and lower load tensile strength. Figure 8b shows the addition of flax fibers in the inner core of the composite produces tensile strength and similarly, the addition of the sisal fibers in the inner core of the composite produces more strain and tensile strength. Addition of the flax fibers on the uppermost and lowermost layers of the composite produces more tensile strength compared to the innermost part of the composite, as shown in Fig. 8. For sisal fiber-based composites, both core and outer layers produce approximately the same tensile load and strain, as shown in Fig. 8. Venkatesh et al.¹⁷ investigated the tensile test of the specimen, which consists of sisal and bamboo fiber-based composites and it exhibits 23.4 MPa tensile strength for untreated fibers and 38.4 MPa tensile strength of treated fibers.

Fig. 8.

The tensile strength of three different fibers with altered orientation: (a) Flax and Sisal fibers on the outer layer of fiber stacking combinations, (b) Ramie and Sisal on the outer layer of fiber stacking combinations.

Flexural test

Figure 9 displays the flexural strength of the composite specimen having different fiber orientations. The flexural test has been repeated three times for each composite specimen of different orientation and the variations have been noted as error bars in Fig. 9. R/S/S/S/R fiber-based composites produce the highest flexural strength, 23.91% higher compared to the flexural strength produced by R/F/F/F/R fiber-based composites. Similarly, F/R/R/R/F fiber-based composites produce 9.79% higher flexural strength compared to the R/F/F/F/R composites. The remaining composites have been shown in Fig. 9 produce smaller variations of flexural strength. Sapiai et al.⁴³ exposed that the flexural strength of Kenaf/glass fiber-based composites is in the range of 77.61 MPa to 115.71 MPa. They fabricated the composite specimen with the proportions of fiber and matrix at 70:30, which is the same as the proportions of the present experiment. The obtained value of the flexural strength is slightly lesser than the flexural strength of kenaf and glass fiber composites owing to the substitution of biodegradable natural fibers in place of glass fibers.

Fig. 9.

Flexural strength of the composite specimen having different fiber orientation.

Impact test

Figure 10 shows the impact strength of the specimens having different fiber orientations. Each experiment has been repeated three times in the composite specimen and the range of readings has been used for the error graph. F/R/R/R/F fiber-based composites produce 67.74% higher impact strength compared to the S/R/R/R/S fiber-based composites. Similarly, F/S/S/S/F fiber-based composites produce 45.16% higher impact strength compared to the S/R/R/R/S fiber-based composites. The remaining sequences of fibers, namely, S/F/F/F/S, R/S/S/S/R and R/F/F/F/R, produce impact strengths of 35.5%, 23.2% and 9.67%, respectively, compared to the S/R/R/R/S fiber-based composites. Venkatesh et al.¹⁷ inspected the impact strength of the sisal and bamboo-based composites; the range of impact strength varies from 14.8 kJ m⁻² to 19.7 kJ m^-2. The obtained impact strength of the present study varies from 7.82 kJ m^-2 to 13.52 kJ m^{− 2}. this difference in impact strength is due to the property difference of the natural fibers and the bonding strength of the composite specimen.

Fig. 10.

Impact strength of the composite specimen having different fiber orientation.

Hardness test

Figure 11 displays the hardness of the specimens having different fiber orientations. The hardness test has been repeated three times and the range of values is used as error bars in the graph shown in Fig. 11. R/F/F/F/R fiber-based composite having the highest hardness values among the composite specimens, which is 26.67% higher hardness compared to the hardness value of the S/F/F/F/S fiber-based composite specimen. Likewise, the remaining four composite specimens, namely, S/R/R/R/S, R/S/S/S/R, F/R/R/R/F and F/S/S/S/F, display hardness values of 6.67%, 15.56%, 20% and 20% higher compared to the S/F/F/F/S composite specimen. Parandaman and Jayaraman⁴⁴ investigated the hardness property of the Coir and bagasse fiber-based composites. they used the Rockwell hardness test, C scale, perimeter 16 − 1 inch, and load 100 kg and they reported the hardness values varied from 33 to 51. The same experimental conditions have been used for this study and the obtained hardness values range from 45 to 57. Table 3 shows the maximum tensile modulus, strain to failure, flexural, impact, and hardness of various sequences of composite specimens. This table displays the comparison among the various properties of the sequence of the specimen, which were consider for the investigation.

Fig. 11.

Hardness of the composite specimens having different fiber orientations.

Table 3.

Maximum tensile, modulus, strain to failure, flexural, impact, and hardness of various sequence of composite specimens.

Different sequence of composite materials	Maximum Tensile strength (MPa)	Tensile modulus (MPa)	Strain to failure (%)	Maximum Flexural strength (MPa)	Maximum Impact energy (kJ m− 2)	Maximum Hardness (HR)
S/F/F/F/S	23.58	906.92	1.026	68.80	11.20	45
F/S/S/S/F	36.28	907	1.04	66.70	11.85	53
S/R/R/R/S	07.01	87.625	1.08	66.80	07.90	48
R/F/F/F/R	29.58	1643.33	1.02	62.10	08.20	57
R/S/S/S/R	07.51	70.84	1.106	77.80	10.21	52
F/R/R/R/F	25.20	1145.45	1.022	66.85	13.38	54

SEM image

Figure 12 displays SEM photographs of the tensile fractured specimen having different fiber orientations, namely, (a) S/F/F/F/S, (b) F/S/S/S/F, (c) S/R/R/R/S, (d) R/F/F/F/R, (e) R/S/S/S/R, and (f) F/R/R/R/F. More fiber pullouts and voids are present in the R/S/S/S/R and F/R/R/R/F composite specimens as shown in Fig. 12; this may be due to deprived interfacial bonding between the fibers and matrix, which can be minimized by surface modification of the fibers. The fiber breakdowns and matrix regions in the S/F/F/F/S- and F/S/S/S/F-based composite are publicized in Fig. 12; this may be attributed to the highest bonding strength between the fibers and matrix; hence, more load has been transferred to the fibers, and it results in fiber breakdown. While accumulation of the fibers causes formation of the matrix region in the composite. Similarly, more fiber pullouts and matrix regions are shown in the S/R/R/R/S and R/F/F/F/R-based composites as displayed in Fig. 12; this can be accredited to a lack of bonding strength between the corresponding fibers and their matrices; hence, surface modification has been emphasized to increase the interfacial bonding strength. In general, fiber breakdown occurs more at fiber pullout, which is majorly happening because of the deprived bonding between the hydrophilic characteristics of the utilized fiber and the hydrophobic characteristics of the corresponding resin material. Further, voids in the matrix regions act as crack initiation points and propagate into complete failure, also contributing to the poor blending quality^45,46. In the past studies, the researchers proposed that the composite specimen with surface treatment and incorporation of wood powder in the specimen will reduce the defects and increase the surface adhesion between the layers of the specimen. However, in this work, the fibers have been utilized in their untreated form to first understand how they naturally interact with the matrix without any chemical enhancement. The prime intention was to establish a baseline performance and identify the inherent failure mechanisms under simple, low-cost processing conditions. Although the chemical treatments may improve fiber–matrix bonding, it is highly required to observe the raw interfacial behavior before moving on to any modifications.

Fig. 12.

SEM image of tensile fractured specimen (a) S/F/F/F/S, (b) F/S/S/S/F, (c) S/R/R/R/S, (d) R/F/F/F/R, (e) R/S/S/S/R, and (f) F/R/R/R/F, Magnification factor – 1000X and Scale bar − 10 μm.

Conclusion

A hybrid composite consisting of fibers reinforced with epoxy has been fabricated using three different natural fibers: Sisal, Ramie, and Flax. The specimens were made-up through the hand lay-up procedure and compression molding, employing various stacking sequences: S/F/F/F/S, F/S/S/S/F, F/R/R/R/F, R/F/F/F/R, R/S/S/S/R, and S/R/R/R/S. Tensile, flexural, impact and hardness tests were conducted on the composite specimens, and the corresponding results have been reported.

The one-way ANOVA has been executed to evaluate the statistical significance between the six composites. Among the six composites, FRRRF and RFFFR show statistical significance. The sisal and flax fiber-based epoxy composites with stacking sequences of F/S/S/S/F and S/F/F/F/S displayed very high load-bearing capacity compared to other hybrid configurations, pure fibers, and the matrix. The Ramie and Sisal fiber-based hybrid composite with the stacking sequence of R/S/S/S/R and S/R/R/R/S exhibited significantly lower stress produced during the tensile test. However, it demonstrated very high strain value compared to other hybrids, pure fibers, and the matrix. The addition of flax fibers to the hybrid composites enhanced stress generation, while the incorporation of Ramie fibers increased strain. Nevertheless, the latter resulted in a significant reduction in the generation of stress. For the Sisal-based hybrid composite, no significant characteristics were observed. It is evident that the addition of Sisal to Flax and Ramie fibers-based composites enhances the stress and strain characteristics. For the flexural test, the R/S/S/S/R-layers-based composite produces 23.91% higher flexural strength compared to the R/F/F/F/R-layers-based composites. Likewise, the impact strength produced by the F/R/R/R/F-layer-based composites produces 67.74% higher results compared to the S/R/R/R/S-layer-based composites. For hardness tests, the R/F/F/F/R-layer-based composite produces 26.67% enhanced results compared to the S/F/F/F/S-layer-based composite. Scanning electron microscope images revealed tensile fractured defects namely, fiber breakage, porosity, voids owing to fiber pullouts in the matrix regions. These defects can be linked to the limitations of the hand lay-up process, which often results in uneven resin distribution and insufficient compaction. As a result, achieving uniform fiber wetting and completely eliminating voids becomes quite challenging with this manual technique. However, the study on the proposed hybrid composites shows that they could be the potential materials for fabricating internal structures of the automotives and the lightweight structural components.

In the future, the investigations may be conducted to gain a clearer understanding of the physio-mechanical behavior of these new natural fiber combinations with the help of water absorption tests and interlaminar shear strength measurements. Interlaminar shear strength would help to evaluate how effectively the fibers bond and hold together under external loading, while the water absorption test can show the extent to which the natural fibers absorb moisture when exposed to environmental conditions. Additionally, future work may also involve a comparative analysis of untreated and chemically treated fibers to highlight improvements in interfacial bonding and the reduction of void formation.

Author contributions

Mohan Kumar Anand Raj: *Conceptualization, Investigation, Writing – original draft.* P. Manoj Kumar: *Conceptualization, Formal analysis, Supervision.* Praveensabari Palanisamy: *Formal analysis, Validation, Visualization.* Sanjay Dharmaraju: *Validation, Visualization.* Sureshkumar Periyasamy: *Validation, writing-review and editing.* Murugesan Palaniappan: *writing-review and editing.* Dawit Tafesse Gebreyohannes: *Methodology, project administration, Writing – original draft.*.

Data availability

All data is provided within the article.

Declarations

Competing interests

The authors declare no competing interests.