Flax-aramid fiber reinforced epoxy composites: mechanical and thermal evaluation for sustainable automotive componentsABSTRACT
The escalating demand for lightweight, high performance materials for the automotive sector has accelerated the development of hybrid composites. This study explores the mechanical and thermal behavior of epoxy based composites reinforced with a combination of flax and aramid fibers. Five hybrid configurations were developed by varying the fiber ratios and the experimental investigation includes tensile, flexural, compression, impact, water absorption, thermal stability, and hardness tests to assess the performance of the composites. Among all formulations, Combination 4 (67% epoxy, 9% flax, and 24% aramid) delivered the most balanced performance, exhibiting a tensile strength of 42.494 MPa and a flexural strength of 171.493 MPa. These values are substantially higher than representative properties reported for commonly used engineering thermoplastics such as Acrylonitrile Butadiene Styrene (ABS) in published literature. Microstructural evaluation via Scanning Electron Microscopy (SEM), thermal analysis through Thermogravimetric Analysis (TGA), and Shore D hardness tests confirmed the superior integrity and heat resistance of this configuration. The findings reveal combination No.4 as a strong, thermally stable, and weight efficient replacement for conventional polymer materials used in automotive structural components.
Keywords:
Hybrid Composites; Epoxy Resin; Flax Fiber; Aramid Fiber; Mechanical Properties.
1. INTRODUCTION
The growing demand for lightweight, sustainable, and high performance materials has positioned natural fiber reinforced polymer composites as promising alternatives to conventional synthetic reinforcements such as glass and carbon fibers. Natural fibers including flax, jute, hemp, sisal, coir, and bamboo, along with lignocellulosic waste materials, have attracted considerable attention due to their renewability, low environmental impact, and ease of processing. Their biodegradability and reduced carbon footprint further enhance their suitability for applications requiring environmentally responsible material solutions.
Although extensive research has been conducted on natural fiber composites, their relatively lower mechanical strength and variability in properties limit their direct replacement of synthetic fibers in structural applications. To address these limitations, hybridization strategies combining natural and synthetic reinforcements have been explored. This approach enables the integration of the environmental benefits of natural fibers with the superior strength, stiffness, and durability of synthetic fibers. In this context, flax fiber, known for its low density and favorable tensile properties, when combined with aramid fiber, which offers high tensile strength, impact resistance, and thermal stability, presents a promising hybrid system for structural applications.
Recent studies have primarily focused on individual reinforcement effects, fiber orientation, or bio-based resin modifications to enhance composite performance. However, systematic evaluation of hybrid flax-aramid composites across a defined composition range under consistent fabrication and testing conditions remains limited. In addition, variability in natural fiber availability and performance consistency continues to pose challenges in large scale structural applications, necessitating a more structured investigation into hybrid reinforcement strategies.
In this context, the present study investigates the mechanical, thermal, and moisture absorption behavior of flax-aramid reinforced epoxy composites by varying fiber weight fractions within a defined matrix composition. Five hybrid formulations were fabricated using a consistent manual lay-up and compression molding process to isolate the influence of reinforcement ratio on composite performance. The evaluation includes tensile, flexural, compressive, impact, hardness, water absorption, and thermal degradation characteristics.
The novelty of this work lies in providing a structured comparative assessment of hybrid composite formulations fabricated under identical processing conditions, enabling clearer interpretation of composition-property relationships. Unlike studies focusing on resin modification approaches such as epoxidized methyl ricinoleate or castor oil-based systems, the present work examines hybrid reinforcement as an alternative pathway for performance enhancement while retaining a conventional epoxy matrix. This approach offers practical insight into optimizing hybrid composite configurations for lightweight and sustainable structural applications. While similar compositional ranges have been reported in hybrid composites, the present work provides a consistent comparative evaluation of flax-aramid hybrid systems under identical processing conditions, enabling clearer interpretation of composition-property relationships.
1.1. Literature survey
RANAKOTIA et al. [1] examined the incorporation of Tasar silk waste in jute/Grewia optiva fiber reinforced epoxy laminates and reported noticeable improvements in mechanical response, indicating that alternative natural reinforcements can contribute to enhanced structural performance. POKHRIYAL and RAKESH [2] investigated the processing and characterization of himalayacalamus falconeri fiber based biodegradable composites, highlighting the feasibility of utilizing lesser explored natural fibers to achieve satisfactory strength and sustainability characteristics. Similarly, KUMAR et al. [3] analyzed the adhesive joining behaviour of banana/bagasse epoxy composites with different joint configurations and demonstrated that reinforcement selection and joint design significantly influence load transfer and bonding performance.
Recent studies have highlighted the potential of natural and synthetic fiber reinforced epoxy composites for lightweight and sustainable automotive applications. MOUDOOD and RAHMAN [4] investigated flax/bio-epoxy composites, reporting that environmental factors such as moisture and hydrothermal aging significantly affect tensile, flexural, and impact properties, emphasizing the need for durability considerations in structural applications. Hybridization with synthetic fibers has been shown to improve mechanical performance. SARWAR and BHUIYAN [5] demonstrated that Kevlar/flax/epoxy hybrid composites exhibit enhanced tensile strength, flexural modulus, and impact resistance compared to single fiber composites, suggesting that fiber synergy can overcome the limitations of natural fibers alone.
MADHU and KUMAR [6] carried out structural optimization of flax fiber reinforced epoxy composites and reported that fiber content, orientation, and distribution strongly influence tensile, flexural, and impact properties, highlighting the importance of design parameters in achieving high performance composites. BATI and KOC [7] studied matrix modified aramid fiber composites and found that enhancing out of plane properties through matrix modification significantly improves flexural and impact resistance, which is crucial for structural components subjected to multi directional loads. Similarly, UMAPATHI and KUMAR [8] explored flax fiber and Glass Fiber Reinforced Polymer (GFRP) hybrids, showing that the combination of natural and synthetic fibers improves stiffness, energy absorption, and load bearing capacity.
Surface modification and interfacial engineering play a critical role in achieving superior mechanical performance. CAI and ZHANG [9] demonstrated that unidirectional flax fiber/epoxy composites retain their tensile and flexural properties under hydrothermal aging, confirming the importance of fiber matrix bonding for durability. WANG et al. [10] reported that reinforcing aramid fiber/epoxy composites with strong hydrogen bonding interactions at the interface leads to significant improvements in tensile, flexural, and impact properties. XU et al. [11] further confirmed that surface functionalization of aramid fibers with multilayer coatings enhances fiber matrix adhesion, resulting in improved structural performance.
The architecture and hybrid design of composites are also crucial. PANICO and SANTULLI [12] and ALIPOUR and JAYARAMAN [13] found that hybridizing flax with basalt or varying fabric structures in flax/epoxy composites significantly influences mechanical properties, including stiffness, strength, and energy absorption. FEKIAČ and HRNČÁR [14] reported that optimization of epoxy composites and processing parameters can further enhance mechanical performance, while BACHMANN and LÖFFLER [15] highlighted that combining recycled carbon fiber with flax fiber improves flexural strength and stiffness. SACCONE and CICALA [16] investigated flexural performance in flax fiber reinforced composites and confirmed that fiber distribution and orientation play a key role in maximizing bending strength and modulus.
Studies on multi fiber hybrid composites have shown promising results for high performance applications. ÇAKIR and YILMAZ [17] reported that carbon-aramid hybrid fiber reinforced epoxy composites achieve superior tensile, flexural, and impact performance, while ISMAIL and ISMAIL [18] demonstrated that flax/carbon/aramid hybrids improve dimensional stability and thermal properties in addition to mechanical strength. These outcomes highlight the potential of hybrid composites in replacing conventional materials in lightweight automotive structures. SMEDER et al. [19] investigated hybrid composites incorporating glass, carbon, and aramid fibers and reported that multi fiber reinforcement significantly enhances tensile and flexural performance through synergistic load sharing among constituents. PANICO and SANTULLI [20] examined flax/aramid hybrid epoxy composites for lightweight applications and observed that hybridization improves the balance between stiffness, strength, and thermal stability, making such systems suitable for structural weight reduction strategies. Similarly, ALIPOUR and JAYARAMAN [21] studied the influence of combining aramid with natural fibers in epoxy based composites and demonstrated that optimized hybrid configurations lead to improved mechanical performance compared to single fiber systems. These findings collectively support the relevance of hybrid reinforcement approaches in tailoring composite properties for advanced engineering applications. [22] demonstrated that fiber orientation significantly influences the mechanical, thermo mechanical, and interfacial adhesion behaviour of epoxidized methyl ricinoleate modified flax/epoxy composites, indicating that proper orientation control enhances load transfer efficiency and overall composite sustainability for structural applications [23]. Reported that the incorporation of castor oil based bio resins alters the mechanical and visco elastic response of flax fiber epoxy composites while also affecting moisture diffusion characteristics, suggesting that bio-based resin modification can improve durability and environmental compatibility [24] showed that epoxidized castor oil modification contributes to improved thermal stability and mechanical performance of epoxy systems along with reduced water uptake, highlighting its suitability as a sustainable toughening agent in polymer composite matrices [25] observed that epoxy methyl ricinoleate enhances visco-elastic response and impact resistance in flax fiber reinforced epoxy composites, demonstrating its potential in improving energy absorption capability without compromising structural integrity.
Recent studies have reported various developments in natural and hybrid fiber reinforced epoxy composites, particularly focusing on flax based systems and bio modified resin matrices. For instance, investigations on flax fiber orientation and bio resin modification have demonstrated improvements in interfacial adhesion and thermo mechanical response, while other works have explored moisture diffusion behavior and viscoelastic performance of flax reinforced systems.
In contrast, the present work evaluates a defined range of flax-aramid hybrid laminate compositions fabricated under identical processing conditions, enabling direct comparison of mechanical, thermal, and environmental performance across multiple formulations. The study further examines the relationship between thermal degradation behavior and mechanical response, providing insight into composition-property interactions relevant to lightweight structural applications. This comparative approach offers incremental understanding of hybrid fiber ratio optimization while complementing existing studies that primarily focus on matrix modification strategies.
2. MATERIALS AND METHODS
2.1. Materials
Epoxy resin was selected as the matrix material due to its excellent mechanical strength, adhesion characteristics, and resistance to environmental degradation. Flax fiber was chosen as the natural reinforcement owing to its low density, good tensile properties, and sustainability advantages. Aramid fiber was incorporated as a synthetic reinforcement because of its high tensile strength, impact resistance, and thermal stability.
Figure 1 shows (a) flax fiber in yarn form and (b) chopped flax fiber. Flax fibers were procured in yarn form and mechanically processed into chopped fibers to facilitate uniform dispersion within the matrix.
This preparation ensures enhanced fiber matrix interlocking, efficient load transfer, and isotropic mechanical response in the final composite product. Aramid fibers were obtained in chopped form and used without additional surface treatment. The combination of flax and aramid fibers enables the development of a hybrid system that integrates lightweight characteristics with enhanced mechanical and thermal performance. Aramid fibers are advanced synthetic materials made up of long chain polyamides, with over 85% of their amide bonds directly connected to aromatic rings, contributing to their exceptional performance characteristics. Their tightly packed and highly crystalline polymer structure, stabilized by strong interchain hydrogen bonding, contributes to their exceptional tensile strength, dimensional stability, and thermal resistance. These properties make aramid fibers particularly well suited for structural applications requiring durability under mechanical and thermal stress. The aramid fibers were incorporated without additional surface treatment, leveraging their inherent bonding characteristics with the epoxy resin. Figure 2 shows the chopped aramid fiber used in the composite fabrication.
2.2. Selection and design of composite formulations
The composition ratios of epoxy, flax, and aramid fibers were determined based on insights from recent literature and the mechanical performance trends reported for hybrid natural synthetic composites. The epoxy content was maintained between 65% and 68% to ensure adequate wetting, uniform fiber dispersion, and optimal load transfer between the matrix and the reinforcements. Several studies have indicated that epoxy matrices below 65% lead to poor interfacial adhesion and void formation, while exceeding 70% tends to reduce the reinforcing effect of the fibers and increase brittleness [4, 9].
The proportion of flax fiber was varied from 9% to 15%, as natural fibers at moderate volume fractions have been shown to enhance tensile and flexural strength without compromising the structural integrity of the matrix. Beyond 15%, flax fibers tend to absorb more moisture and reduce interfacial adhesion, leading to degradation in mechanical and thermal performance [20, 21]. Similarly, the aramid fiber content was adjusted between 20% and 25%, as prior investigations demonstrated that hybrid composites with aramid reinforcement above 25% exhibit increased stiffness but reduced impact strength and elongation at break [10, 17].
The selection of the five hybrid composite formulations was not empirical, but was guided by a structured composition screening strategy derived from published literature and preliminary fabrication feasibility considerations. Previous studies on natural-synthetic hybrid epoxy composites consistently report that effective mechanical performance is achieved when the epoxy matrix content is maintained within a narrow optimal range, ensuring adequate fiber wetting, reduced void formation, and improved load transfer between the matrix and reinforcement phases. Based on these findings, the epoxy content in the present study was intentionally restricted to a range of 65–68 wt.% across all combinations.
Within this fixed matrix range, flax and aramid fiber contents were systematically varied to examine their complementary roles. Flax fiber was limited to a maximum of 15 wt.% to avoid excessive moisture absorption and interfacial degradation, while still contributing to weight reduction and sustainability. Aramid fiber content was correspondingly adjusted between 20–25 wt.% to enhance load bearing capability, impact resistance, and thermal stability without introducing excessive stiffness or processing difficulties.
The five compositions were designed to represent a systematic variation of natural and synthetic fiber proportions rather than isolated trial points. This approach enables comparative evaluation of the interaction between flax and aramid fibers under consistent processing conditions. Therefore, the chosen combinations (1–5) were formulated to study the effect of gradual variation in flax and aramid fiber ratios while maintaining the matrix content within an optimum range. The percentage compositions for each combination are presented in the Table 1.
These formulations were chosen to analyze how different proportions of flax and aramid fibers influence the mechanical properties, thermal resistance, and failure characteristics of the hybrid composites.
2.3. Composite fabrication
Hybrid composite laminates were fabricated using a manual lay-up process followed by compression molding. Prior to fabrication, flax and aramid fibers were dried at 80°C to remove moisture and improve interfacial bonding.
The epoxy resin and hardener were mixed in a 90:10 ratio as per manufacturer specifications and stirred thoroughly to ensure uniform consistency. The chopped fibers were then uniformly distributed in molds according to the designed compositions. The laminates were subjected to compression molding at a pressure of 5 MPa for one hour at room temperature, followed by ambient curing for 24 hours.
After curing, the laminates were demolded and machined into test specimens as per relevant ASTM standards. Each specimen was visually inspected for defects such as voids or delamination before proceeding with mechanical and thermal characterization. Figure 3 illustrates the fabricated composite laminates for all five combinations.
2.4. Mechanical and thermal characterization
To evaluate the functionality of the developed hybrid composites for potential use in automotive structural applications, a series of standardized mechanical and thermal tests were conducted. These tests were designed to assess strength, stiffness, impact resistance, and thermal stability. Figure 4 shows the Universal Testing Machine (UTM) used for performing tensile and flexural tests on the fabricated composite specimens. Mechanical and thermal properties of the fabricated composites were evaluated using standardized testing procedures.
Universal Testing Machine (UTM) used for conducting tensile and flexural tests on the fabricated composite specimens.
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Tensile test was conducted according to ASTM D3039 using a Universal Testing Machine at a crosshead speed of 2 mm/min [26].
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Flexural test was performed following ASTM D790 under three-point bending configuration [27].
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Impact strength was measured using a Charpy impact tester as per ASTM D256 [28].
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Thermogravimetric analysis (TGA) was carried out under a nitrogen atmosphere from room temperature to 800°C to assess thermal stability and degradation behavior.
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Scanning Electron Microscopy (SEM) was used to examine fractured surfaces to evaluate fiber-matrix interaction, failure mechanisms, and microstructural features.
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Hardness was measured using a Shore D hardness tester.
All specimens were inspected prior to testing to ensure the absence of visible defects such as voids and delamination.
2.5. Experimental replication and data consistency
For each test, a minimum of three specimens were evaluated under identical conditions, and the reported values represent the mean of these measurements. Error bars in the graphical results indicate standard deviation (n = 3). Minor variability observed in the results is attributed to inherent inconsistencies associated with natural fiber distribution and manual fabrication processes. However, consistent trends across repeated trials indicate acceptable experimental reproducibility.
3. RESULTS AND DISCUSSION
3.1. Tensile behaviour
To investigate the tensile behavior of the flax-aramid fiber reinforced epoxy composites, tensile testing was performed on all five hybrid combinations using standardized procedures.
Test Methodology
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Standard Followed: ASTM D3039/D3039M
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Specimen Dimensions: As prescribed in ASTM D3039
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Testing Equipment: Universal Testing Machine (UTM) equipped with an extensometer
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Test Parameters: Conducted at ambient room temperature with a constant crosshead speed of 2 mm/min
Each specimen was tested in triplicate, and average values of ultimate tensile strength and elongation at break were calculated. Figure 5 shows the comparison of tensile strength for the five composite combinations.
Tensile strength comparison of composite combinations [Values represent mean ± standard deviation (n = 3)].
Combination 5 (68% epoxy, 11% flax, 21% aramid) achieved the second optimized performance, with a tensile strength of 29.881 MPa and an elongation of 0.883%, suggesting its potential use in semi structural applications that require moderate flexibility with considerable strength. Combination 3 (65% epoxy, 15% flax, 20% aramid) showed balanced tensile properties with a strength of 14.064 MPa, making it suitable for components under moderate mechanical stress. On the other hand, Combinations 1 and 2, which incorporated 25% and 23% aramid fiber respectively but less favorable flax proportions, showed comparatively lower tensile strengths of 15.601 MPa and 11.148 MPa respectively. These results indicate that aramid fiber content alone does not govern tensile strength; optimal interaction between both fibers and the matrix is essential. Although Combination 4 exhibited the highest average tensile strength, the relatively higher standard deviation indicates variability in mechanical response, which may be attributed to non-uniform fiber distribution and inherent inconsistencies associated with manual fabrication processes. Among the five fabricated formulations, Combination 4 (67% epoxy, 9% flax fiber, 24% aramid fiber) recorded the highest tensile strength of 42.494 MPa and an elongation of 1.450%, demonstrating superior load bearing capacity and ductility. This performance can be attributed to the higher aramid fiber content, which contributes to improved load-bearing capability, along with sufficient epoxy content that promotes effective fiber wetting and reduces void formation. It stands out as the most promising formulation for applications requiring high tensile strength and flexibility. These findings reinforce the importance of precise fiber ratio optimization in hybrid composites and their direct influence on tensile performance, especially for load bearing structural automotive applications.
3.2. Compressive behaviour
The compression test is essential for assessing a material’s ability to withstand axial compressive forces, offering insights into deformation resistance, buckling behavior, and structural integrity under load. This evaluation is particularly critical for automotive structural components, where materials frequently encounter compressive stresses. The test also reveals the efficacy of matrix-fiber interaction and the contribution of each reinforcement phase to compressive strength.
Test methodology
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Standard Followed: ASTM D695
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Specimen Preparation: As per ASTM standards
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Testing Equipment: Universal Testing Machine (UTM)
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Test Conditions: Conducted at room temperature with uniform axial load application
Each of the five composite combinations was subjected to compression testing to determine peak load capacity and ultimate compressive strength. Figure 6 shows the comparison of compressive strength for the five composite combinations.
Compressive strength values of hybrid composites [Values represent mean ± standard deviation (n = 3)].
The compressive evaluation highlights how the internal architecture of the composite resists axial deformation and micro buckling. Interestingly, Combination 3 (15% flax) recorded the highest compressive strength at 20.908 MPa. This suggests that at higher weight fractions, the flax fibers which possess a more complex cell wall structure act as effective bulk reinforcements that improve the composite’s resistance to out of plane buckling under compression. While Combination 4 followed closely with 20.388 MPa, the slight reduction compared to Combination 3 may be attributed to its lower flax percentage, reducing the fiber packing density required for compressive stability. Conversely, the significant drop in Combination 5 (14.097 MPa) points to a threshold where the matrix becomes too resin rich (68%), leading to localized shear failure before the fibers can fully engage the load.
Although Combination 3 and Combination 4 exhibited comparatively higher compressive strength values, the presence of noticeable standard deviation in certain combinations indicates variation in mechanical response among the tested specimens. The slight variations indicated by the error bars are primarily attributed to the inherent morphological variability of the natural flax fibers and potential micro-voids formed during the hand-layup process, which is common in natural-fiber reinforced polymers. Combination 3 emerges as the most suitable formulation for high compressive load applications, with Combination 4 offering comparable performance. These findings indicate that optimization of the fiber matrix ratio is critical for achieving the desired compressive strength in hybrid composites intended for load bearing automotive structural elements.
3.3. Flexural test
The flexural test, also known as the bending test, is a critical method for evaluating a material’s resistance to deformation under applied bending loads. This test provides key indicators such as flexural strength, modulus, and peak load capacity, which are essential in assessing the stiffness and structural integrity of materials used in load bearing applications. The test is especially relevant for fiber reinforced polymer composites, where the interplay between fiber type, orientation, and matrix material significantly influences bending performance.
Test methodology
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Standard Followed: ASTM D790
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Specimen Dimensions: As per ASTM D790 guidelines
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Testing Apparatus: Universal Testing Machine (UTM)
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Support Span: 100 mm
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Crosshead Speed: 2 mm/min
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Test Environment: Room temperature
Each composite specimen was subjected to three points bending, with the load applied centrally until failure or maximum deflection was reached. The results were recorded in terms of average peak load (N) and flexural strength (MPa).
The flexural characteristics, illustrated in Figure 7, provide insights into the composite’s stiffness and resistance to multi axial bending stresses typical in automotive floor panels. Combination 4 demonstrated a flexural strength of 171.493 MPa, indicating effective load transfer and strong interfacial interaction within the hybrid laminate under bending conditions. The high aramid content (24%) in this configuration acts as a robust tensile reinforcement on the bottom surface of the bend, while the epoxy matrix effectively manages the compressive stresses on the top surface. The lower performance of Combination 5 (79.048 MPa) suggests that without sufficient reinforcement density, the matrix dominated regions become vulnerable to crack initiation during bending, highlighting the necessity of a high fiber to matrix ratio for structural rigidity.
Flexural strength comparison of composite combinations [Values represent mean ± standard deviation (n = 3)].
The flexural strength results show noticeable variation among the tested specimens, as reflected by the standard deviation values. In particular, Combination 4 exhibited the highest average flexural strength; however, it also showed a relatively higher standard deviation compared to other combinations. This variation is mainly influenced by the significantly higher value recorded for one specimen, which may be attributed to localized fiber alignment and improved stress transfer in that specific sample. In general, the observed variability in flexural response across different combinations can be associated with non-uniform fiber distribution, minor differences in resin-rich regions, and inherent inconsistencies arising from the manual lay-up fabrication process. The results of the flexural test indicate that Combination 4 offers the optimized balance of stiffness and strength, making it suitable for automotive structural components where bending resistance and lightweight properties are critical. The findings highlight the importance of optimizing fiber ratios particularly favoring higher aramid content and moderate flax integration for improved mechanical performance under flexural loads.
3.4. Impact test
The impact test is a critical evaluation method used to determine a material’s toughness its ability to absorb and dissipate energy under sudden or dynamic loading conditions. For fiber reinforced composites, this property is particularly important in automotive structural applications, where components are frequently subjected to impact forces.
The impact resistance of flax and aramid fiber reinforced epoxy composites was assessed using the Izod test in compliance with ASTM D256. This method measures the energy absorbed by a notched specimen upon fracture, providing valuable information on the material’s toughness and its ability to resist crack initiation and propagation under dynamic loading conditions.
Methodology
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Test Standard: ASTM D256 (Izod Method)
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Testing Machine: Izod Impact Testing Machine
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Specimen Dimensions: 63.5 mm × 13 mm × 3 mm
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Notch Type: Single V-notch
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Number of Combinations: Five (Combination 1 to 5)
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Data Recorded: Energy absorbed (in Joules)
Each specimen was impacted with a pendulum hammer, and the energy engrossed during fracture was recorded. Figure 8 shows the comparison of impact strength for the five composite combinations.
Impact energy absorption of all combinations (Izod test) [Values represent mean ± standard deviation (n = 3)].
Combination 5 exhibited the highest impact energy absorption (1.05 J), indicating excellent toughness. This is attributed to its higher epoxy content (68%), which contributes to improved crack propagation resistance, and a well balanced fiber composition. Combination 4, closely following with an impact value of 1.02 J, also demonstrated superior resistance to dynamic loading, making it a strong contender for impact critical automotive applications. Combinations 1, 2, and 3 recorded significantly lower impact values (0.37 J, 0.50 J, and 0.48 J respectively), suggesting brittle behavior due to relatively higher flax content and less favorable fiber matrix interaction. The impact test results exhibit variation among the tested specimens, as reflected by the standard deviation values. Combination 4 and Combination 5 showed the highest average impact strength, indicating better energy absorption capability under sudden loading conditions. However, a comparatively higher standard deviation was observed in Combination 5, which may be attributed to greater variation in specimen response. This variability can be associated with differences in fiber distribution, local fiber orientation, and the presence of microstructural inconsistencies such as voids or resin-rich regions introduced during the manual fabrication process. In contrast, Combination 4 exhibited relatively lower standard deviation, suggesting more consistent impact behavior among the specimens. The impact test results suggest that Combination 5 and Combination 4 exhibit superior toughness, making them suitable for dynamic structural applications in the automotive sector. The results also highlight the importance of optimizing the epoxy fiber ratio to enhance energy absorption while maintaining structural integrity. Among the investigated formulations, Combination 5 demonstrates a favorable balance between toughness and composition.
3.5. Water absorption test
The moisture absorption properties of composite materials are a critical factor in determining their durability and performance in humid or aqueous environments. In this study, water absorption tests were carried out on epoxy based hybrid composites reinforced with flax and aramid fibers to evaluate their moisture uptake characteristics. The test was conducted in accord with ASTM D570 standards, where specimens were immersed in water for a fixed duration, and the percentage increase in weight was calculated to quantify water absorption. Figure 9 shows the comparison of water absorption behavior for all composite combinations.
Water absorption comparison for all combinations [Values represent mean ± standard deviation (n = 3)].
Methodology
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The water absorption behavior of the composite specimens was evaluated as per ASTM D570. Each specimen was first oven dried and weighed (W1) to record its initial mass.
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The samples were then immersed in distilled water at room temperature for 24 hours. After immersion, the specimens were surface dried and reweighed (W2).
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The percentage of water absorption was calculated using the relation:
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Water Absorption (%) = (W2−W1)/W1 × 100
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Combination 4 demonstrated the lowest water absorption (3.43%), indicating excellent resistance to moisture ingress. This performance is attributed to its higher epoxy content, which acts as a moisture barrier, and lower flax fiber content, which minimizes hydrophilic pathways.
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Combination 3 followed with a relatively low water absorption (4.57%), likely due to its balanced fiber composition and effective fiber matrix bonding.
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Combinations 1 and 2 showed moderate absorption values (5.70% and 7.00%, respectively), suggesting acceptable moisture resistance, though not optimal for prolonged exposure to moisture.
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Combination 5 exhibited relatively low water absorption of 3.54%, indicating improved resistance to moisture ingress compared to Combinations 1 and 2.
The water absorption behavior of the hybrid composites is influenced by both material composition and microstructural characteristics. While flax fibers are inherently hydrophilic due to the presence of hydroxyl groups in their cellulose structure, moisture uptake is not governed by fiber content alone. The absorption process is also affected by microvoids, fiber distribution, and local fiber-matrix interaction, which can create pathways for moisture ingress and diffusion within the composite. The experimental results show relatively low variation among the tested specimens, as indicated by the small standard deviation values for most combinations. Combination 4 exhibited the lowest average moisture uptake along with minimal standard deviation, suggesting consistent resistance to water absorption across all specimens. Slight variations observed in other combinations, particularly in Combination 2 and Combination 3, may be attributed to differences in fiber dispersion, void content, and continuity of the matrix phase. Composites with higher flax content tend to exhibit increased moisture uptake due to the hygroscopic nature of lignocellulosic constituents, whereas higher epoxy content reduces water absorption by forming a relatively impermeable matrix that limits moisture penetration.
The results confirm that epoxy content and fiber distribution critically influence the moisture resistance of flax-aramid reinforced epoxy composites. Among the five configurations, Combination 4 stands out as the most moisture-resistant formulation. The observed mechanical and environmental response trends are consistent with recent findings reported for flax-based composite systems where matrix modification and reinforcement architecture were identified as dominant factors governing stiffness, viscoelastic behavior, and moisture transport. While earlier studies primarily examined resin chemistry effects, the present investigation emphasizes the combined influence of flax and aramid fibers as a complementary strategy for improving load transfer and thermal resistance. The comparative behavior obtained across the tested formulations reinforces the broader understanding that both matrix tailoring and reinforcement hybridization play interconnected roles in determining composite performance.
3.6. Shore D hardness test
The Shore Hardness Test is a widely used method to measure the resistance of polymer based materials and composites to surface indentation. Shore D hardness is specifically used for medium hard materials, including fiber reinforced composites, and provides insights into the material’s wear resistance, durability, and surface integrity. This test is particularly significant for automotive applications, where door pads and other structural components must withstand continuous mechanical interactions and environmental exposure.
The Shore D Hardness test is performed to measure the surface hardness of polymer composites, providing insights into their resistance to indentation and wear. Higher hardness values indicate better scratch resistance, structural durability, and load bearing capability, which are essential for automotive structural applications.
Shore D Hardness testing was conducted on five hybrid epoxy composite combinations, reinforced with flax and aramid fibers, to evaluate their mechanical performance. Table 2, shows the Shore D hardness measurements for all five composite combinations.
Key observations:
Figure 10 shows the comparison of Shore D hardness for the five composite combinations. Combination 4 exhibits the highest average Shore D hardness (80.0), indicating superior surface resistance to deformation. The Shore D hardness results exhibit relatively low variation among the tested specimens, as reflected by the small standard deviation values for all combinations. Combination 4 demonstrated the highest average hardness along with a comparatively lower standard deviation, indicating consistent surface resistance and uniform material response across specimens. Minor variations observed in other combinations may be attributed to slight differences in local fiber distribution, resin-rich regions, and surface irregularities introduced during specimen preparation. However, the overall low standard deviation values suggest good repeatability of the measurements and confirm that the hardness behaviour is not significantly affected by random variability. Combination 1 follows with an average hardness of 73.8, showing good mechanical strength and indentation resistance. Combination 5 has the lowest hardness (69.8), suggesting a relatively softer surface compared to other combinations. The higher aramid fiber content in Combination 4 (24%) contributes to increased hardness, as aramid fibers have high mechanical strength and toughness. The natural flax fiber in lower proportions improves matrix bonding, but its softer nature slightly reduces hardness in some combinations. Combination 5, with lower aramid content (21%), shows the least hardness, indicating that aramid plays a crucial role in enhancing the composite’s hardness properties. The Shore D hardness evaluation indicates that Combination 4 exhibits the highest surface hardness among the tested composites, underscoring its suitability for applications demanding enhanced wear resistance and long term durability. This superior hardness is primarily attributed to the elevated aramid fiber content (24%), which contributes to improved structural reinforcement and matrix rigidity. These results highlight the potential of Combination 4 for use in automotive structural components such as door panels, underbody shields, and other mechanically stressed regions, where high surface integrity and mechanical robustness are essential.
3.7. Summary of mechanical and physical properties
A consolidated summary of the mechanical and physical properties obtained for all hybrid composite formulations is presented in Table 3. The values represent the mean and standard deviation calculated from three specimens tested under identical conditions.
Summary of mechanical and physical properties of flax-aramid hybrid composites (Mean ± Standard Deviation).
3.8. Scanning electron microscopy (SEM)
Scanning Electron Microscopy (SEM) is a widely used technique for characterizing the surface morphology and micro structural features of composite materials. Unlike optical microscopy, SEM employs a focused electron beam to scan the sample surface, generating high resolution images that provide valuable insights into fiber matrix interactions, void formations, crack propagation, and failure mechanisms. The analysis is crucial for understanding how the reinforcement phases (flax and aramid fibers) interact with the epoxy matrix and how these micro structural characteristics influence the composite’s overall mechanical behavior.
In composite material research, SEM is particularly valuable for:
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Investigating fiber matrix interactions, which influence mechanical properties such as tensile and flexural strength.
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Identifying micro structural features like voids, cracks, and delamination that affect composite performance.
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Analyzing fracture surfaces to understand failure mechanisms under mechanical loading, such as fiber pull out, matrix cracking, and brittle or ductile fracture behavior.
Experimental procedure
To examine the fracture behavior and fiber matrix interfacial characteristics, SEM analysis was performed on Combination 4 specimen, which exhibited the optimized mechanical performance. The imaging revealed fiber pull out, matrix cracking, voids, and the quality of interfacial bonding, allowing correlation of micro structural features with macroscopic mechanical properties. The SEM examination was carried out using a JSM-6390 model operating at an accelerating voltage of 18 kV. Samples were gold coated prior to imaging to enhance conductivity and ensure image clarity.
3.9. Sample preparation
Fractured surfaces of tensile and impact tested specimens were selected to evaluate fiber pull out, matrix cracking, and fiber matrix adhesion. Prior to imaging, the specimens were dried to remove residual moisture and sputter coated with a thin layer of gold to enhance electrical conductivity and prevent electron charging during scanning.
3.10. Imaging and analysis
The micro structural characteristics of the hybrid composites were examined using a JEOL JSM-6390 scanning electron microscope operated at an accelerating voltage of 18 kV. Images were captured at multiple magnifications to investigate morphological features, fiber matrix interactions, and failure mechanisms of combination 4, which exhibited the highest mechanical performance, allowing a clear correlation between micro structural attributes and observed mechanical behavior.
The analysis focused on identifying the following aspects:
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The bonding interface between the flax and aramid fibers and the surrounding epoxy matrix.
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Evidence of fiber pull out and interfacial debonding, which reflects the effectiveness of fiber matrix adhesion.
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The presence of micro voids, cracks, and resin rich zones, which may compromise the composites’ structural integrity.
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Key fracture modes and failure patterns, providing insight into the material’s mechanical response under stress.
This detailed micro structural evaluation helped correlate the surface morphology with the mechanical behavior observed during tensile and impact testing. The analysis further validated the superior performance of Combination 4, reinforcing its suitability for high strength and lightweight automotive applications.
3.11. Fiber-matrix adhesion
Figure 11 SEM micrograph of the fractured surface showing fiber-matrix interaction in the hybrid composite. The image reveals limited fiber pull-out and relatively uniform matrix coverage over the fibers, indicating effective fiber wetting and reduced interfacial gaps.
3.12. Fiber pull-out and fracture surface
Figure 12 SEM image illustrating fiber pull-out and matrix fracture, highlighting failure mechanisms in the hybrid composite. Evidence of fiber pull out is visible, which suggests partial debonding at certain points. This is expected in hybrid composites, where differences in fiber matrix adhesion can lead to localized failure zones. The fracture pattern indicates a combination of ductile matrix deformation and brittle fiber breakage, contributing to the composites’ high impact strength.
3.13. Crack propagation and matrix cracking
Figure 13 SEM image showing crack propagation and matrix cracking, revealing the composite’s fracture behavior under stress. Evidence of matrix cracking was found, particularly around flax fibers. This suggests that flax fibers absorb stress up to a limit before matrix crack initiates. However, the high aspect ratio of aramid fibers appears to arrest crack growth, improving the composites’ damage resistance.
3.14. Voids and porosity analysis
Figure 14 SEM image highlighting voids and porosity, indicating potential sites for stress concentration and mechanical property reduction. A few micro voids are detected in the matrix, likely due to air entrapment during fabrication. However, the overall void content is low, which indicates that the fabrication process was effective in minimizing porosity, ensuring consistent mechanical performance.
3.15. Fracture mechanism under load
This SEM image illustrates the composite’s fracture behavior under tensile and impact loading. Figure 15 SEM image illustrating the fracture mechanism under load, showing fiber breakage and matrix deformation. The failure mode is a combination of fiber breakage, matrix shearing, and interfacial debonding, confirming that Combination 4 effectively distributes mechanical stress and exhibits balanced toughness and strength.
In the present investigation, Scanning Electron Microscopy (SEM) analysis was primarily conducted on Combination 4, as it exhibited superior mechanical and thermal performance among all formulations. The objective of the SEM examination was to identify the dominant fracture features and microstructural characteristics contributing to its observed behavior.
SEM analysis was performed on fractured specimens to examine the microstructural features governing the mechanical response of the hybrid composites. The micrographs reveal features such as fiber distribution, matrix coverage on fiber surfaces, and the presence of microvoids. The fracture surfaces of the optimized composition show relatively uniform fiber dispersion and limited fiber pull-out, indicating effective interaction between the matrix and reinforcement phases. The presence of matrix material adhering to fiber surfaces suggests improved load sharing during mechanical loading.
In contrast, localized regions exhibiting fiber pull-out, interfacial gaps, and microvoids can act as stress concentration sites and contribute to variations in mechanical response. Microvoids and matrix cracking observed in certain regions may originate from entrapped air during fabrication and can serve as initiation points for crack propagation under loading.
Although manual lay-up followed by compression molding is generally susceptible to void formation and localized fiber agglomeration, careful control of resin viscosity, degassing during mixing, and uniform pressure application during molding were employed in the present work to minimize such defects. The SEM micrographs indicate relatively good fiber wetting and limited void presence, particularly in the optimized composition, suggesting effective impregnation under the adopted processing conditions.
The comparatively uniform microstructure observed in Combination 4 supports its improved mechanical performance, indicating that reduced defect concentration and consistent fiber distribution contribute to enhanced structural response.
It should be noted that the present SEM analysis is qualitative in nature and focuses on comparative interpretation of fracture features. Quantitative evaluation of void fraction and interfacial parameters using standardized methods (e.g., ASTM D2734) was beyond the scope of this study; however, such analysis could provide additional insight into structure property relationships in future investigations.
3.16. Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) was conducted to evaluate the thermal stability of epoxy composites reinforced with flax and aramid fibers. All samples were heated from ambient temperature to 850°C at a rate of 10°C/min under a nitrogen atmosphere. The corresponding degradation parameters, including onset temperature, peak decomposition temperature, and residual mass, are summarized in Table 4, while the thermogravimetric curves are presented in Figure 16. The TGA curves exhibit a multi-stage degradation behavior associated with the composite constituents. The initial weight loss below approximately 150°C is attributed to the removal of absorbed moisture and low molecular weight volatiles. The primary degradation region, observed between approximately 280°C and 400°C, corresponds predominantly to the decomposition of lignocellulosic components of flax fibers, including hemicellulose and cellulose. At higher temperatures, further mass loss is associated with degradation of the epoxy matrix and partial decomposition of aramid fibers. Beyond 600°C, the curves tend to stabilize, indicating the presence of thermally stable char residue.
The influence of fiber composition on thermal behavior is evident across the formulations. Composites with higher flax content exhibit earlier onset of degradation due to the thermal sensitivity of lignocellulosic constituents. In contrast, increased aramid fiber content contributes to improved thermal resistance, owing to its aromatic molecular structure and higher thermal stability. The residual mass at elevated temperatures is governed by char formation, which enhances resistance to further thermal degradation.
Among the investigated formulations, Combination 5 shows the highest onset and peak decomposition temperatures, indicating improved initial thermal stability; however, its lower residual mass suggests comparatively reduced char formation. Combination 1 exhibits the highest residual mass, reflecting better char yield and potential resistance to thermal decomposition at higher temperatures. Combination 4 demonstrates a balanced thermal profile, combining relatively high degradation resistance with moderate char retention, indicating stable behavior across the degradation range.
The comparatively stable thermal response observed for Combination 4 is consistent with its balanced mechanical performance, suggesting that optimized hybrid composition contributes to both structural integrity and resistance to thermal degradation. The onset degradation temperatures of all formulations exceed typical automotive service temperatures (generally below 150–200°C), indicating that the developed composites possess adequate thermal tolerance for such applications. It is noted that the assessment of thermal suitability in this study is based on comparative behavior within the hybrid system and does not represent validation for high temperature structural deployment.
3.17. Derivative thermogravimetric (DTG) analysis
To further understand the thermal degradation behavior of the developed hybrid composites, derivative thermogravimetric (DTG) analysis was performed. The DTG curves represent the rate of mass loss with respect to temperature (dW/dT) and clearly indicate the temperatures corresponding to maximum degradation rate (Tmax).
As shown in Figure 17, the DTG curves of all hybrid composite formulations exhibit two distinct degradation stages. The first major degradation peak appears in the temperature range of approximately 350–450°C, which is primarily associated with the decomposition of the epoxy matrix along with partial degradation of flax fibers. The second degradation stage, observed between 550°C and 650°C, corresponds to the thermal breakdown of aramid fibers and the oxidation of residual char.
Among the five formulations, Combination 3 exhibited a comparatively higher secondary peak temperature, indicating improved char stability at elevated temperatures. Combination 4 displayed well defined and stable degradation peaks, consistent with its superior overall mechanical performance. The derivative thermogravimetric (DTG) results complement the Thermogravimetric Analysis (TGA) findings and provide clearer insight into the degradation behavior and thermal stability of the developed hybrid composites, supporting their potential suitability for moderate high temperature automotive applications.
4. CONCLUSION
This study evaluates the mechanical and thermal performance of flax-aramid reinforced epoxy hybrid composites for potential structural applications in the automotive sector. Five formulations with varying fiber proportions were fabricated and characterized through tensile, compressive, flexural, and impact testing, along with water absorption, thermogravimetric, and hardness analyses.
Among the investigated configurations, Combination 4 (67% epoxy, 9% flax, and 24% aramid) demonstrated the most balanced and consistent performance across the evaluated properties. It exhibited a tensile strength of 42.494 MPa, flexural strength of 171.493 MPa, impact strength of 1.02 J, and Shore D hardness of 80, along with the lowest moisture absorption (3.43%) and stable thermal behavior with degradation onset around 290°C. While the highest compressive strength was observed for Combination 3, Combination 4 showed comparable performance suitable for load bearing conditions. Comparisons with Acrylonitrile Butadiene Styrene (ABS) were included only for contextual reference based on literature reported values and should not be interpreted as direct equivalence, as no experimental benchmarking under identical conditions was performed.
The results indicate that the hybridization of flax and aramid fibers within an epoxy matrix can provide a favorable balance between mechanical performance, thermal stability, and environmental resistance. The present investigation focuses on baseline characterization, and further studies involving fatigue behavior, long term environmental exposure, and application-specific testing would provide additional validation for structural applications.
Overall, the findings demonstrate that optimized hybrid compositions can offer a viable pathway toward developing lightweight and partially sustainable composite materials for engineering applications.
5. DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available as they form part of an ongoing doctoral research work.
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