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Article

Tensile and Dynamic Toughness of Kenaf Fiber-Reinforced Epoxy Composites

by
Thuane Teixeira da Silva
1,
Matheus Pereira Ribeiro
1,*,
Lucas de Mendonça Neuba
1,
Pedro Henrique Poubel Mendonça da Silveira
1,
Noan Tonini Simonassi
2,
Sergio Neves Monteiro
1 and
Lucio Fabio Cassiano Nascimento
1
1
Department of Materials Science, Military Institute of Engineering—IME, Praça General Tibúrcio 80, Rio de Janeiro 22290-270, Brazil
2
Advanced Materials Laboratory (LAMAV), Department of Materials Engineering, State University of the Northern Rio de Janeiro—UENF, Avenida Alberto Lamego, 2000, Campos dos Goytacazes 28013-602, Brazil
*
Author to whom correspondence should be addressed.
Fibers 2025, 13(9), 120; https://doi.org/10.3390/fib13090120
Submission received: 27 July 2025 / Revised: 25 August 2025 / Accepted: 1 September 2025 / Published: 5 September 2025
Browse Figures
Versions Notes

Abstract

Highlights

What are the main findings?
  • Single kenaf fibers exhibited an average absorbed energy of 28.3 kJ/m2, with thinner fibers showing superior tensile properties.
  • Epoxy composites with 20–30 vol% kenaf fibers achieved tensile toughness above 49 kJ/m2 and Izod impact absorption up to 38.8 kJ/m2, representing more than a twentyfold improvement over neat epoxy.
  • Tensile toughness values systematically exceeded impact toughness, indicating distinct fracture mechanisms: fiber pullout under tensile loading versus fiber/matrix decohesion under impact.
What is the implication of the main finding?
  • Kenaf fiber/epoxy composites present strong potential as sustainable alternatives to synthetic-fiber composites for structural applications requiring high energy absorption under quasi-static conditions.
  • Their superior tensile toughness suggests suitability for automotive, construction, and protective structures subjected to prolonged or controlled loading, while hybridization with synthetic fibers may be needed for impact-critical applications.

Abstract

The environmental impact of petroleum-based materials in driving climate change has stimulated growing interest in natural lignocellulosic fibers (NLFs) as reinforcements for polymeric matrices. NLFs exhibit specific mechanical properties that, in some cases, rival those of synthetic fibers such as aramid, carbon, and glass. Among the wide variety of NLFs, kenaf has been extensively investigated in applications including textiles, construction, and furniture, owing to its long-established global cultivation. Previous studies have also demonstrated its potential as a reinforcement in polymeric matrices for engineering applications, including ballistic protection. In this context, the present work reports, for the first time, on the tensile and dynamic impact toughness of epoxy matrix composites reinforced with 10, 20, and 30 vol% kenaf fibers. The tensile toughness, defined as the area under the stress–strain curve up to fracture, ranged from 9.36 kJ/m2 at 10 vol% to 52.30 kJ/m2 at 30 vol% fiber content—representing a three- to tenfold increase compared to the neat epoxy matrix. In Izod impact tests, the composites containing 30 vol% kenaf fibers absorbed 22 times more energy than the neat epoxy, rising from 1.8 to 38.8 kJ/m2. On average, the tensile toughness values exceeded those of the corresponding dynamic impact toughness. Scanning electron microscopy revealed the fracture morphology and highlighted the influence of the fibers under both toughness conditions.

1. Introduction

The escalating environmental impact of petroleum-based materials has intensified the search for sustainable alternatives in engineering applications. Natural lignocellulosic fibers (NLFs) have emerged as attractive reinforcements for polymer matrices owing to their renewability, biodegradability, low cost, and wide availability [1,2,3,4]. Their industrial relevance is evidenced by applications in the automotive [5,6,7], construction [8,9], and textile sectors [10], among others [11,12,13,14].
Several NLFs, including hemp [15,16,17,18], bamboo [19,20,21], jute [22,23,24], sisal [25,26,27], flax [28,29,30,31], coconut [32], and curaua [33,34], exhibit mechanical properties that rival synthetic fibers. For instance, curaua fibers present tensile strengths of up to 3000 MPa at low density, yielding a higher specific strength than carbon, aramid, or E-glass fibers [35]. Such performance underscores the potential of NLFs as eco-friendly reinforcements for structural composites.
Within this context, kenaf (Hibiscus cannabinus L.) has gained attention due to its global cultivation, favorable mechanical performance, and increasing use in composite materials [36]. Kenaf fibers have been incorporated into a variety of polymer matrices, including epoxy [37,38,39], polypropylene [40,41,42,43], HDPE [44,45,46,47], polystyrene [48,49,50], PLA [51,52,53,54], PVC [55,56,57], and polyester [58,59,60,61]. In epoxy systems, however, studies remain comparatively limited. While early reviews [36] reported no examples of kenaf/epoxy composites, subsequent research has primarily focused on thermal properties [62] and flexural or fatigue behavior [63,64,65,66,67], with fewer studies addressing tensile or impact performance [68,69,70].
Despite these efforts, important gaps remain. For example, Bakar et al. [63] reported Izod impact results for kenaf/epoxy composites that were comparable to neat epoxy, whereas Mutasher et al. [64] observed higher values. Rassmann et al. [65] and Mahjoub et al. [67] emphasized tensile strength, while more recent studies [68,69,70] explored alkali treatment effects and dynamic behavior. Nevertheless, comprehensive evaluation of toughness—particularly absorbed energy under tensile loading—remains scarce.
To address these gaps, the present study investigates, for the first time, the tensile and dynamic (Izod impact) toughness of epoxy composites reinforced with 10, 20, and 30 vol% of untreated kenaf fibers. The research scope included (i) the characterization of individual fiber properties, (ii) the fabrication and testing of composite specimens under standardized tensile and impact conditions, and (iii) the analysis of the relationship between fiber content, fracture mechanisms, and toughness. These objectives were defined to establish a comprehensive understanding of kenaf fiber as reinforcement in epoxy matrices and to provide a foundation for future structural applications.

2. Materials and Methods

Kenaf fibers were supplied by Tapete São Carlos Ltd.a. (São Paulo, Brazil). They were extracted from the herbaceous plant Hibiscus cannabinus (Malvaceae family) [71], then manually separated, washed by immersion in water for 24 h, cut into 150 mm lengths, and oven-dried at 70 °C for 24 h. No chemical treatment was applied to the fibers.
The polymeric matrix consisted of a commercial epoxy resin, diglycidyl ether of bisphenol A (DGEBA), cured with triethylenetetramine (TETA) at a stoichiometric ratio of 13 phr. Both resin and hardener were produced by Dow Chemical (São Paulo, Brazil) and distributed by Resinpoxy Ltd.a. (Rio de Janeiro, Brazil).

2.1. Kenaf Fiber Characterization

2.1.1. Diameter Measurement

The experiment included a detailed analysis of fiber dimensions, encompassing both cross-sectional diameter and length, performed using light microscopy. A total of 100 fibers were individually measured at three distinct positions and two orientations (0° and 90°). Each fiber was subsequently weighed on an electronic balance (AG-200, Sipcam Nichino Brasil, São Paulo, Brazil) with a precision of 0.0001 g. Dimensional measurements were obtained using an optical microscope (BX53M, Olympus, Center Valley, PA, USA).
The diameters of the 100 pre-selected fibers were measured and categorized into six classes, enabling the determination of the diameter distribution and the average diameter of the as-received kenaf fibers. Representative micrographs were also acquired to illustrate the measurement procedure. Figure 1 presents a typical example of an individual fiber measured at three points along its length, highlighting the dimensional heterogeneity characteristic of natural lignocellulosic fibers.

2.1.2. Fiber Tensile Tests

Tensile tests of kenaf fibers were carried out in accordance with ASTM D3822 [72] using a universal testing machine (Instron 3365, Instron, Norwood, MA, USA) equipped with a 10 kN load cell, providing ±0.5% accuracy for both load and strain measurements. The crosshead speed was set at 0.4 mm/min.
Based on material availability, an equal number of fibers from each dimensional group were tested. Specifically, five fibers from each of the six groups were selected, each with a gauge length of 40 mm. The results were statistically analyzed to obtain mean values and standard deviations, and further evaluated at a 95% confidence level.

2.2. Composites Processing

Composite plates containing 10, 20, and 30 vol% kenaf fibers were fabricated using a steel mold with internal dimensions of 150 × 120 × 12 mm. Fibers were manually aligned along the mold length to promote unidirectional orientation before resin impregnation, after which the epoxy–hardener mixture was poured at the previously calculated fiber/resin ratio. Although no quantitative orientation distribution analysis was performed, care was taken to minimize misalignment during fabrication. The reproducibility of mechanical results across specimens indicates consistent fiber orientation. Fiber density was determined using the geometric linear density method, by weighing approximately 100 fibers and measuring their diameters and lengths under an optical microscope.
The average fiber density was calculated as 1.53 g/cm3, while the epoxy resin density was reported as 1.11 g/cm3 [73,74]. The curing process was carried out under a constant pressure of 5 tons at room temperature (≈25 °C) for 24 h, without additional thermal post-curing, in line with standard practices for DGEBA/TETA systems, which achieved complete curing at ambient conditions through stoichiometric hardener addition. The void content, measured by the Archimedes method, was below 5%. Table 1 summarizes the nomenclature adopted for the analyzed samples.
Two types of plates were produced to prepare specimens for mechanical testing: thin plates (150 × 120 × 2 mm) for tensile tests in accordance with ASTM D3039 [75], and thick plates (150 × 120 × 10 mm) for Izod impact tests in accordance with ASTM D256 [76]. This procedure ensured that the final specimen dimensions complied with the respective standard requirements.
No chemical treatment was applied to the fibers, as the purpose of this study was to assess the intrinsic contribution of raw kenaf fibers to the tensile and impact toughness of epoxy composites. Although surface treatments were often employed to enhance fiber–matrix interfacial adhesion, they may also introduce additional variables that complicate direct comparison with untreated fibers. Therefore, the use of raw fibers was intentionally chosen to isolate their natural reinforcing effect.

2.2.1. Composites Characterization

Composite Tensile Tests
Tensile tests of the kenaf fiber-reinforced epoxy composites were conducted in accordance with ASTM D3039 [75]. Specimens were extracted from composite plates measuring 150 × 120 × 2 mm and containing 0, 10, 20, or 30 vol% fibers. Seven specimens were prepared for each group. Testing was performed on an Instron 5582 universal machine at a crosshead speed of 2 mm/min. The measured properties included the tensile strength, elastic modulus, elongation at break, and toughness, with the latter determined from the area under the stress–strain curve.
Stress–strain curves were recorded directly from the testing machine without compliance correction. This approach was justified by the relatively large specimen dimensions compared with the machine stiffness, which minimized compliance effects. Furthermore, since all groups were tested under identical conditions, any residual compliance equally influenced the specimens, thereby preserved the validity of comparisons between neat epoxy and kenaf fiber composites.
Izod Impact Tests
Izod impact tests were conducted in accordance with ASTM D256 [76]. Specimens were cut from the composite plates to dimensions of 63.5 × 12.7 × 10 mm, yielding 10 specimens per group. Notches with a depth of 2.54 mm and an angle of 45 ± 1° were introduced using a manual broaching machine (IME, Rio de Janeiro, Brazil). The tests were performed on a PANTEC XC-50 pendulum impact tester (PANTEC, Campos do Goytacazes, Brazil) equipped with a 22 J hammer.
Scanning Electron Microscopy (SEM)
The fracture surfaces of the kenaf fiber-reinforced epoxy composites were examined using a scanning electron microscope (SEM, Quanta FEG 250, FEI, Hillsboro, OR, USA). Observations were carried out with a secondary electron detector at an accelerating voltage of 10 kV and magnifications ranging from 240× to 1600×. To improve conductivity and image quality, the samples were gold-coated using a Leica ACE 600 sputter coater (Leica Microsystems, Wetzlar, Germany).
Statistical Analysis
The mechanical results were statistically analyzed using analysis of variance (ANOVA) to evaluate significant differences among the mean values at a 95% confidence level. Post hoc comparisons were performed using Tukey’s test, also known as the honestly significant difference (HSD), calculated as follows:
H S D   =   q ⋅ E M S r
where q is the tabulated HSD constant for a 5% significance level, EMS is the error mean square obtained from ANOVA, and r is the number of replicates per treatment [77].

3. Results and Discussions

3.1. Kenaf Fiber Properties

The six diameter intervals obtained for the kenaf fibers and their respective frequencies are summarized in Table 2. The first four intervals exhibited the highest frequencies, with the most common diameter being 0.058 mm.
From the measurements of 100 fibers, the average diameter corresponding to the most frequent interval (0.058 mm) was consistent with the value reported by Ramesh [72], approximately 0.067 mm. Owing to their heterogeneous structure, NLFs naturally exhibit diameter variations along the length of individual fibers [34]. Figure 2 presents representative stress–strain curves for single kenaf fibers corresponding to each of the six diameter intervals listed in Table 2. These curves reveal an initial linear elastic response followed by abrupt rupture, a characteristic feature of brittle fracture. As observed in Figure 2, an increase in fiber diameter generally correlates with reduced tensile strength and decreased elongation at break.
Table 3 presents the average and standard deviation of tensile strength and absorbed energy for single kenaf fibers across the diameter intervals defined in Table 2.
To quantify the relationship between fiber diameter and mechanical performance, statistical analyses were conducted. Pearson’s correlation revealed strong and significant negative relationships between fiber diameter and tensile strength (r = −0.90, p = 0.014), as well as between fiber diameter and absorbed energy (r = −0.96, p = 0.003). Linear regression further confirmed these results, with coefficients of determination of R2 = 0.82 for tensile strength and R2 = 0.91 for absorbed energy. These findings, illustrated in Figure 3 and Figure 4, demonstrate that thinner kenaf fibers consistently exhibit superior tensile properties, supporting the hypothesis that defect density increases with fiber thickness.
Both tensile strength and absorbed energy were higher in the fibers of the smallest diameter range compared to those of larger diameters, as shown in Table 3. Increasing fiber diameter resulted in a substantial reduction in both properties. This behavior is attributed to the greater defect density present in thicker fibers, which impairs mechanical resistance [34]. Notably, the absorbed energy values in Table 3 indicate that thinner kenaf fibers exhibit relatively high tensile toughness, a result not previously reported for brittle NLFs.
Based on the absorbed energy data in Table 3, ANOVA was performed to verify the statistical reliability of the mean values, particularly in light of the relatively high standard deviations observed for the smaller diameter intervals. The corresponding analysis of variance is presented in Table 4.
The absorbed energy results of single kenaf fibers were found to be statistically different, as the calculated F value exceeded the tabulated critical F. To further identify differences among the groups, Tukey’s test was applied. With an HSD value of 21.29, the results presented in Table 5 revealed significant differences among the diameter intervals.

3.2. Composite Tensile Properties

Figure 5 presents representative tensile stress–strain curves for neat epoxy and epoxy composites reinforced with 10, 20, and 30 vol% kenaf fibers.
The average mechanical properties obtained from the seven tensile specimens in each group are summarized in Table 6.
A slight increase in tensile properties was observed between neat epoxy and the 10 vol% kenaf composite. By contrast, a significant improvement was found between 10 and 20 vol% fiber content, even considering the standard deviations. This suggests the existence of a threshold fiber content (between 10 and 20 vol%) required to achieve more effective reinforcement in epoxy composites. Such behavior is not commonly observed in epoxy composites reinforced with other NLFs, which often exhibit reduced tensile properties at a low fiber content, as reported for hemp [18] and caranan fibers [78].
Table 6 also indicates that the tensile properties of composites with 20 and 30 vol% kenaf fibers are statistically similar. This can be attributed to the reduced fiber–matrix adhesion at higher fiber contents [79], which hinders effective stress transfer. For example, Krishna and Kanny [69] reported a tensile strength of 16 ± 1 MPa for an epoxy composite with 41 wt% kenaf fibers, which is significantly lower than the present result of 60.23 ± 7.33 MPa for 37 wt% (30 vol%).
Comparisons with other studies further highlight the variability of results. Mahjoub et al. [67], without statistical analysis, reported higher values of tensile strength, the modulus, and elongation at break than those in Table 6. Conversely, Mutasher et al. [64] reported a tensile strength of only 27.72 MPa for an epoxy composite with 24 wt% kenaf fibers, which is markedly lower than the present results. These discrepancies underscore the inherent variability of NLFs [34], even within the same fiber type.
Within the scope of this work, the tensile toughness results in Table 6 warrant particular attention. Since no fiber selection was performed during composite fabrication, the mean absorbed energy of single kenaf fibers (Table 3) must be considered. By accounting for the frequency distribution of fiber diameters (Table 2), the overall contribution was estimated as follows:
27.79 ± 0.23 + 26.95 ± 0.38 + 17.73 ± 0.24 + 7.66 ± 0.10 + 5.84 ± 0.04 + 4.73 ± 0.01 = 28.33 kJ/m2.
When this average absorbed energy of single fibers is added to the neat epoxy tensile toughness of 4.23 ± 1.57 kJ/m2, the resulting values closely match those of EK20 and EK30 composites in Table 6. This quantitatively demonstrates the contribution of kenaf fibers to the tensile toughness of epoxy composites.

3.3. Statistical Analysis of Tensile Toughness

ANOVA was performed on the tensile toughness values (area under the stress–strain curve) presented in Table 6 to verify the presence of statistically significant differences. The results are summarized in Table 7.
The analysis indicated that the tensile toughness values of the kenaf fiber composites were significantly different, as the calculated F exceeded the critical F value. To further identify differences among the groups, Tukey’s test was applied. With an HSD value of 16.96, the results shown in Table 8 confirm significant differences among the composites.
Specifically, no statistical difference was observed between EPOXY and EK10, as their toughness values were below the HSD threshold. In contrast, EK20 and EK30 displayed statistically similar performance but with significantly higher toughness than both EPOXY and EK10.

3.4. Tensile Fracture Mechanism

Figure 6 presents representative SEM micrographs of the fracture surfaces of neat epoxy and kenaf fiber/epoxy composites.
In Figure 6a, the neat epoxy exhibits characteristic river markings that are typical of brittle fracture, consistent with its low toughness and elongation values reported in Table 6. For the EK10 composite (Figure 6b), limited fiber pullout is observed together with interlaminar cracks. These defects, attributed to reduced fiber–matrix adhesion and the presence of occasional voids or impurities, account for the only modest improvement in mechanical properties relative to neat epoxy.
At higher fiber fractions, EK20 (Figure 6c) displays a greater density of pulled-out fibers, whereas EK30 (Figure 6d) reveals a combination of fiber fracture and pullout. The reduced prevalence of interfacial cracks in EK20 and EK30 correlates with the higher tensile strength, elastic modulus, and toughness values in Table 6. Notably, the increased proportion of fiber fracture relative to pullout indicates more efficient stress transfer across the fiber–matrix interface, consistent with the improved load-bearing capacity of these composites.

3.5. Dynamic Izod Impact Toughness

Figure 7 presents the macroscopic appearance of typical impact-tested kenaf fiber/epoxy composite specimens. Notably, the 30 vol% specimens (EK30) shown in Figure 7c did not fracture into two separate parts.
According to ASTM D256 [76], such specimens cannot be statistically considered; however, their partial failure strongly indicates a higher energy absorption capacity. This observation is consistent with the fact that incomplete fracture requires greater impact energy for failure. Table 9 summarizes the absorbed energy values obtained in the Izod impact tests.
For validation, it is relevant to note that the DGEBA/TETA epoxy manufacturer reports a typical Izod impact strength of approximately 2.0 kJ/m2 for neat epoxy, which agrees with the 1.8 kJ/m2 measured in the present study, confirming the reliability of the experimental procedure.
A clear trend of increasing absorbed energy with higher fiber content is evident in Table 9, consistent with reports on other, less widely investigated NLF composites [80,81,82]. The effect of incorporating 30 vol% kenaf fibers was particularly remarkable, resulting in an energy absorption that was more than 20 times greater than that of neat epoxy.
Figure 8 graphically compares the tensile and Izod impact toughness values obtained in this study. It is worth emphasizing that, on average, the tensile toughness of kenaf fiber/epoxy composites exceeded their corresponding Izod impact toughness. Such results, reported here for the first time in NLF-reinforced polymer composites, highlight the relevance of these materials for applications where mechanical energy absorption is critical.

3.6. Statistical Analysis of Izod Impact Toughness

ANOVA was performed on the dynamic toughness (absorbed energy) values obtained from the Izod impact test (Table 9) to assess the presence of statistically significant differences. The ANOVA results are presented in Table 10.
The analysis confirmed that the impact toughness values of the kenaf fiber/epoxy composites were statistically significant, as the calculated F exceeded the critical F value. To further discriminate among groups, Tukey’s test was applied. With an HSD value of 2.30, the results in Table 11 indicate significant differences among all composites.
Examination of the intervals revealed that all groups differed significantly, as the variation in mean toughness values exceeded the HSD threshold. Notably, EK30 exhibited the most pronounced difference, confirming its superior energy absorption capacity.

3.7. Izod Impact Fracture Mechanism

Figure 9 shows SEM fractographs of kenaf fiber/epoxy composites after Izod impact testing. All samples, including neat epoxy, exhibit river markings that are characteristic of brittle thermosets.
For EK10 (Figure 9a), relatively few fibers are fractured, with most being cleanly detached from the matrix, consistent with the limited energy absorption reported in Table 9. With increasing fiber content, EK20 and EK30 (Figure 9b,c) display larger fracture surface areas, more extensive fiber breakage, and pronounced interfacial decohesion. This morphology accounts for the higher absorbed impact energy in these composites, as crack propagation along the fiber/matrix interface generates additional fracture surfaces that dissipate energy.
In comparison with tensile loading, distinct fracture mechanisms are evident. Under tensile conditions (Figure 6), fiber pullout played a major role in energy dissipation, enhancing toughness. In contrast, under impact loading (Figure 9), interfacial decohesion dominates, leading to matrix cracking that limits efficient stress transfer. This explains why the relative increase in impact resistance is less pronounced than the gain in tensile toughness.
Overall, the SEM observations provide a consistent microstructural rationale for the mechanical behavior: higher fiber content promotes fiber fracture and the formation of larger crack surfaces, correlating with the improvements in strength, modulus, and energy absorption reported in Table 6.

4. Final Remarks

This study provides the first direct comparison between the tensile and dynamic (Izod impact) toughness of kenaf fiber/epoxy composites with fiber contents up to 30 vol%. The results demonstrate a consistent increase in absorbed energy with increasing fiber fraction, particularly from 10 to 20 vol%, within statistical significance. This behavior highlights the reinforcing effect of kenaf fibers on energy dissipation. Furthermore, tensile toughness values were systematically higher than impact toughness, indicating distinct dominant fracture mechanisms: fiber pullout under quasi-static loading versus interfacial cracking under dynamic loading.
These findings have important implications for the design of structural components. The superior tensile toughness compared with impact toughness suggests that kenaf/epoxy composites may perform more efficiently under quasi-static loads than under high strain-rate conditions. For instance, panels or components subjected to long-term tensile stresses or controlled deformation could benefit from the enhanced tensile toughness, whereas applications exposed to sudden impacts may require hybridization with other fibers (e.g., aramid or glass) to compensate for the relatively lower impact toughness. Understanding this balance is therefore crucial for optimizing the use of natural fiber composites in automotive, construction, and protective structures.

5. Conclusions

  • Single kenaf fibers exhibited an average absorbed energy of 28.33 kJ/m2, with thinner fibers demonstrating superior toughness.
  • Epoxy composites reinforced with 10, 20, and 30 vol% kenaf fibers showed significantly higher tensile and impact toughness than neat epoxy.
  • Toughness increased with fiber content, with the best performance achieved at 20 and 30 vol%.
  • The higher tensile toughness compared with impact toughness is explained by distinct fracture mechanisms: fiber pullout under tensile loading and fiber/matrix interfacial cracking under impact loading.

Author Contributions

Conceptualization, methodology and investigation, T.T.d.S.; formal analysis and data curation, T.T.d.S., P.H.P.M.d.S., M.P.R., N.T.S., L.d.M.N.; software, P.H.P.M.d.S. and M.P.R.; writing—original draft preparation, T.T.d.S., M.P.R., P.H.P.M.d.S. and L.d.M.N.; writing—review and editing, L.F.C.N. and S.N.M.; supervision, L.F.C.N.; project administration, S.N.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research work is supported by the Brazilian Agency CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), Protocol #001. The author Pedro Henrique Poubel Mendonça da Silveira thanks to the Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro (FAPERJ) (Grant number E-26/202.081/2025). And, the APC was funded by Fibers.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Fibers 13 00120 g001
Figure 1. Optical microscopy image of a kenaf fiber with diameter measurements at different positions along its length.
Figure 1. Optical microscopy image of a kenaf fiber with diameter measurements at different positions along its length.
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Fibers 13 00120 g002
Figure 2. Relationship between kenaf fiber diameter and tensile strength.
Figure 2. Relationship between kenaf fiber diameter and tensile strength.
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Figure 3. Correlation between fiber diameter and tensile strength.
Figure 3. Correlation between fiber diameter and tensile strength.
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Figure 4. Correlation between fiber diameter and absorbed energy.
Figure 4. Correlation between fiber diameter and absorbed energy.
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Figure 5. Examples of stress–strain curves of (a) neat epoxy; (b) 10 vol% kenaf fiber composite; (c) 20 vol% kenaf fiber composite; and (d) 30 vol% kenaf fiber composite.
Figure 5. Examples of stress–strain curves of (a) neat epoxy; (b) 10 vol% kenaf fiber composite; (c) 20 vol% kenaf fiber composite; and (d) 30 vol% kenaf fiber composite.
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Figure 6. SEM images of the fractured surfaces of tensile test specimens: (a) EPOXY; (b) EK10; (c) EK20; (d) EK30.
Figure 6. SEM images of the fractured surfaces of tensile test specimens: (a) EPOXY; (b) EK10; (c) EK20; (d) EK30.
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Figure 7. Typical macroscopic aspect of the fractured specimens: (a) EK10; (b) EK20; (c) EK30.
Figure 7. Typical macroscopic aspect of the fractured specimens: (a) EK10; (b) EK20; (c) EK30.
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Fibers 13 00120 g008
Figure 8. Tensile and Izod impact absorbed energies as a function of the fiber fraction for the neat epoxy resin and kenaf fiber-reinforced composites.
Figure 8. Tensile and Izod impact absorbed energies as a function of the fiber fraction for the neat epoxy resin and kenaf fiber-reinforced composites.
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Fibers 13 00120 g009
Figure 9. SEM fractographs of typical Izod impact specimens: (a) EK10; (b) EK20; (c) EK30.
Figure 9. SEM fractographs of typical Izod impact specimens: (a) EK10; (b) EK20; (c) EK30.
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Table 1. Nomenclature of the composites used in this study.
Table 1. Nomenclature of the composites used in this study.
NomenclatureComposition
EPOXYNeat Epoxy
EK1010 vol% kenaf fiber
EK2020 vol% kenaf fiber
EK3030 vol% kenaf fiber
Table 2. Different kenaf fiber dimension ranges.
Table 2. Different kenaf fiber dimension ranges.
IntervalDiameter Interval (mm)Mean Diameter (mm)Frequency (%)
10.024–0.0470.03523
20.047–0.0690.05838
30.069–0.0920.08124
40.092–0.1150.10410
50.115–0.1380.1274
60.138–0.1610.1501
Table 3. Tensile strength of different diameter intervals of kenaf fibers.
Table 3. Tensile strength of different diameter intervals of kenaf fibers.
IntervalDiameter Interval (mm)Tensile Strength (MPa)Absorbed Energy (kJ/m2)
10.024–0.0471170.00 ± 332.1027.79 ± 12.57
20.047–0.069568.90 ± 281.9026.95 ± 11.59
30.069–0.092488.10 ± 268.1017.73 ± 4.95
40.092–0.115236.00 ± 75.087.66 ± 1.30
50.115–0.138249.40 ± 159.205.84 ± 4.62
60.138–0.161130.90 ± 69.464.73 ± 1.69
Table 4. ANOVA for the absorbed energy results in Table 4 for single kenaf fibers.
Table 4. ANOVA for the absorbed energy results in Table 4 for single kenaf fibers.
Source of VariationDFSSqAQFcalculatedF
Treatment310,215.073405.0227.113.24
Residue162009.85125.62
Total1912,224.92
Table 5. Tukey’s test for the absorbed energy results in Table 3 for single kenaf fibers.
Table 5. Tukey’s test for the absorbed energy results in Table 3 for single kenaf fibers.
Interval123456
10.006.5913.7419.7818.7823.07
26.590.007.1513.1912.1816.47
313.747.150.006.045.049.33
419.7813.196.040.001.003.28
518.7812.185.041.000.004.29
623.0716.479.333.284.290.00
Table 6. Values for the tensile strength, modulus of elasticity, elongation at break, and tensile toughness of the composites.
Table 6. Values for the tensile strength, modulus of elasticity, elongation at break, and tensile toughness of the composites.
SampleTensile Strength (MPa)Elastic Modulus (GPa)Elongation at Break (%)Tensile Toughness (kJ/m2)
EPOXY7.92 ± 2.50.28 ± 0.320.32 ± 0.074.23 ± 1.57
EK1011.86 ± 9.420.33 ± 0.210.36 ± 0.169.36 ± 7.69
EK2061.13 ± 7.963.21 ± 0.532.00 ± 0.2249.36 ± 12.12
EK3060.23 ± 7.733.17 ± 0.951.77 ± 0.3252.30 ± 16.47
Table 7. ANOVA for the tensile toughness results of the kenaf fibers/epoxy composite.
Table 7. ANOVA for the tensile toughness results of the kenaf fibers/epoxy composite.
Source of VariationDFSSqAQFcalculatedF
Treatment311,739.323913.1032.623.10
Residue202399.08119.95
Total2314,138.40
Table 8. Tukey’s test for tensile toughness results of the kenaf fibers/epoxy composites.
Table 8. Tukey’s test for tensile toughness results of the kenaf fibers/epoxy composites.
SamplesEPOXYEK10EK20EK30
EPOXY0.005.1345.1348.07
EK105.130.0040.0042.94
EK2045.1340.000.002.94
EK3048.0742.942.940.00
Table 9. Energy absorbed by Izod impact test samples.
Table 9. Energy absorbed by Izod impact test samples.
Dynamic Izod Absorbed Energy (kJ/m2)
EPOXYEK10EK20EK30
1.8 ± 0.025.6 ± 0.723.9 ± 2.738.8 ± 3.8
Table 10. ANOVA for the impact toughness results of the kenaf fibers/epoxy composite.
Table 10. ANOVA for the impact toughness results of the kenaf fibers/epoxy composite.
Source of VariationDFSSqAQFcalculatedF
Treatment33857.791285.93949.733.01
Residue2432.491.35
Total273890.29
Table 11. Tukey’s test for impact toughness results of the kenaf fibers/epoxy composites.
Table 11. Tukey’s test for impact toughness results of the kenaf fibers/epoxy composites.
SamplesEPOXYEK10EK20EK30
EPOXY0.003.0917.5129.27
EK103.090.0014.4226.18
EK2017.5114.420.0011.76
EK3029.2726.1811.760.00
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Silva, T.T.d.; Ribeiro, M.P.; Neuba, L.d.M.; da Silveira, P.H.P.M.; Simonassi, N.T.; Monteiro, S.N.; Nascimento, L.F.C. Tensile and Dynamic Toughness of Kenaf Fiber-Reinforced Epoxy Composites. Fibers 2025, 13, 120. https://doi.org/10.3390/fib13090120

AMA Style

Silva TTd, Ribeiro MP, Neuba LdM, da Silveira PHPM, Simonassi NT, Monteiro SN, Nascimento LFC. Tensile and Dynamic Toughness of Kenaf Fiber-Reinforced Epoxy Composites. Fibers. 2025; 13(9):120. https://doi.org/10.3390/fib13090120

Chicago/Turabian Style

Silva, Thuane Teixeira da, Matheus Pereira Ribeiro, Lucas de Mendonça Neuba, Pedro Henrique Poubel Mendonça da Silveira, Noan Tonini Simonassi, Sergio Neves Monteiro, and Lucio Fabio Cassiano Nascimento. 2025. "Tensile and Dynamic Toughness of Kenaf Fiber-Reinforced Epoxy Composites" Fibers 13, no. 9: 120. https://doi.org/10.3390/fib13090120

APA Style

Silva, T. T. d., Ribeiro, M. P., Neuba, L. d. M., da Silveira, P. H. P. M., Simonassi, N. T., Monteiro, S. N., & Nascimento, L. F. C. (2025). Tensile and Dynamic Toughness of Kenaf Fiber-Reinforced Epoxy Composites. Fibers, 13(9), 120. https://doi.org/10.3390/fib13090120

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