Mechanical, morphological and micromechanical characterization of multiscale walnut shell powder, E-glass fibre and graphene reinforced epoxy hybrid composites

1 Introduction

In a composite material, one constituent serves as the reinforcing phase, providing the primary load-bearing capability and stiffness, while the other acts as the bulk matrix phase, encapsulating the reinforcement, maintaining its geometric orientation, and facilitating stress transfer across the composite structure (Greenhalgh et al., 2025; Le et al., 2026). The concept of composite materials is inherently biomimetic. Nature provides an abundance of naturally occurring composites, such as tree leaves, bamboo, jute fibres, and various forms of wood, where high-strength cellulose fibres are embedded within a rigid lignin matrix (Amiandamhen et al., 2020; Devarshi et al., 2023; Peng et al., 2022). In the past few decades, the industrial adoption of composite materials has paved the way to synthetic composite materials substituting nearly 50% of traditional metals in aerospace, automotive, marine, and civil infrastructure fields, directed by their remarkable specific stiffness, high durability, and tailored properties (Phiri et al., 2024; Oulanti et al., 2026; Janeikaitė et al., 2025). The mechanical behaviour of a composite is not the sum of the properties of its matrix and reinforcements, but relies on multiple fabrication and geometric parameters (López-Santos et al., 2024; Cherouat and Borouchaki, 2009). The length of the reinforcement directs the efficiency of load transfer, wherein long fibres are favoured for high impact resistance, dimensional stability, and lower volumetric shrinkage, in spite of short fibres offering cost reduction and convenient processing (Elkafrawy et al., 2024; Wang et al., 2019). The orientation and alignment of the fibres determine the anisotropy of the composite. Unidirectional fibres provide maximum strength and stiffness along the longitudinal direction but possess very high transverse weakness (Zerbino et al., 2012). Conversely, multidirectional mats or woven cloths sacrifice some longitudinal peak strength to provide balanced, bi-directional structural rigidity (Faridul Hasan et al., 2021; Chowdhury and Summerscales, 2024). Wan et al. (Wan et al., 2013a) extensively investigated the dispersion mechanics and interfacial bonding of graphene/epoxy composites utilizing a facile surfactant-assisted process resulting in a remarkable 57% enhancement in tensile properties at a mere 0.1 wt% graphene loading. Shen et al. (Shen XJ. et al., 2012) explored the reinforcing effect of graphene nanosheets on the cryogenic mechanical properties of epoxy resins, varying the graphene content from 0.05 to 0.50 wt%, enhancing the tensile and impact strengths by 3.5% and 17%, respectively.

To bridge the gap between the ecological benefits of natural fibres and the high-performance requirements of synthetic materials, researchers have increasingly turned to hybrid composites. Ramesh et al. (Ramesh et al., 2013) evaluated the comparative properties of hybrid glass fibre, sisal, and jute reinforced epoxy architectures, indicating that sisal/glass hybridization optimized tensile strength (69 MPa), while jute/glass hybridization optimized flexural load-bearing capacity (1.03 kN). Furthermore, Arpitha et al. (SIC, 2026) conducted research on sisal/glass fibre and epoxy matrix with SiC as a filler phase. By titrating the amount of SiC up to 9 wt%, they concluded that a 3 wt% SiC in the hybrid composite showcased a significant tensile strength of 160 MPa and a flexural strength of 560 MPa, exhibiting that multiscale hybridization prepared using continuous fibres with rigid micro-particulates creates highly synergistic mechanical improvements. The fundamental objective of this research is the conception, fabrication, and comprehensive mechanical characterization of the multiscale hybrid composites that are prepared using natural reinforcement (Walnut Shell Powder), synthetic reinforcement (E-Glass), and nano-scale reinforcement (Graphene) with a Bisphenol epoxy matrix. Beyond experimental characterization, multiple analytical frameworks have been utilized to predict the response of composite specimens from constituent properties alone. The Rule of Mixtures remains the simplest first-order estimator for continuous fibre systems, while the semi-empirical Halpin–Tsai equations extend this treatment to discontinuous, particulate, and nano-scale reinforcements of arbitrary aspect ratio (Halpin and Tsai, 1967). For randomly oriented in-plane reinforcements, Tsai and Pagano (Tsai and Pagano, 1968) proposed an invariant, orientation-averaged formulation that accounts for the loss of directional efficiency inherent to non-unidirectional architectures. Separately, for particulate systems exhibiting weak interfacial adhesion, Nicolais and Narkis (Nicolais and Narkis, 1971) developed a debonding-based strength-reduction model that has since become a standard benchmark for evaluating particulate–matrix interfacial quality (Fibre Reinforcement, 2026). While these models are individually well established, relatively few studies apply them in combination to multiscale hybrid systems that simultaneously incorporate nano, micro, and macro-scale reinforcements in a single matrix. This gap motivates a coupled empirical–analytical approach in the present work, wherein sequential (two-step) homogenization is used to isolate the discrete mechanical contribution of nano-scale graphene from that of the co-reinforcing macro-scale E-glass or micro-scale WSp phases, allowing the empirical hybridization results to be benchmarked directly against established micromechanical theory.

Recent work has further clarified the loading-based performance of carbon-based nanofillers in epoxy systems. Azhagarsamy and Pannirselvam (Azhagarsamy and Pannirselvam, 2025a) investigated graphite nanoparticle-reinforced epoxy composites for a 0–7 wt% loading range, reporting highest tensile and flexural strength at 3 wt% graphite followed by a sharp decline at higher concentrations, and showcased that artificial neural network models could predict this loading-based performance with high accuracy (R > 0.96). In a related study (Azhagarsamy and Pannirselvam, 2025b) on carbon fibre-reinforced epoxy laminates showcased that graphene oxide loadings of 0.05–0.5 wt% produced a clear optimum at 0.3 wt%, achieving 24.5% and 21.3% improvements in tensile and flexural strength respectively, with a substantial improvement in glass transition temperature (102.3 °C–163.2 °C) confirmed via FTIR, TGA, and DSC. Sekar and Narayanan (Sekar and Narayanan, 2025) extended this loading-optimization approach to a graphene oxide–nanosilica hybrid nanofiller system in isophthalic polyester resin, exhibiting that a 0.3 wt% hybrid loading gained 59.5% and 82.2% improvements in tensile and flexural strength over the neat resin significantly exceeding fillers used individually and validated the hybrid dispersion via XRD. Furthermore, this resin-level optimization, Azhagarsamy and Pannirselvam (Azhagarsamy and Pannirselvam, 2025c) subsequently incorporated the optimized 0.3 wt% GO + 3 wt% nanosilica hybrids into basalt fibre-reinforced laminates, achieving tensile and flexural strengths of 671.5 MPa and 606.8 MPa with UL-94 V-0 flame-retardant performance. In overall, these studies establish that (i) an optimum, rather than monotonic, loading exists for carbon-based nanofillers in thermosetting matrices, and (ii) hybridizing a nano-scale filler with a second reinforcing phase can achieve mechanical gains better than either constituent alone. However, these studies are confined to nanofiller-only or nanofiller/single-macro- fibre systems within a single resin family (epoxy or polyester). The present work extends this body of literature by introducing a third, natural micro-scale particulate phase (WSp) alongside a macro-scale fibre (E-glass) and a nano-scale platelet filler (graphene) within a single epoxy matrix, and pairs the empirical characterization with established micromechanical models to analytically validate the observed multiscale synergy.

The study aims to isolate the tensile, flexural, and compressive strength responses of these individual reinforcements, contrast them against hybridized multiscale architectures, and validate the empirical mechanical data using micromechanical modelling and Scanning Electron Microscope (SEM) morphological failure analysis.

2 Materials and methods

2.1 Materials

The matrix utilized for all composite specimens in this investigation was Bisphenol-A (Grade 220), commercially referred to as diglycidyl ether of bisphenol A (DGEBA). Procured from Naptha Resins, Bangalore, India. The thermosetting epoxy was selected for its exceptional adhesive properties, robust mechanical baseline, low volumetric shrinkage during phase transition, and high thermal resistance. The neat resin possesses a standard density of 1.20 g/cm³. The curing of the composites was done at ambient temperature.

The reinforcements were critically chosen, one each from natural, synthetic, and nano-scale dimensions. The reinforcement quantity was decided after The 0.1 wt% graphene loading follows Wan et al. (Wan et al., 2013b)and Shen et al. (Shen X. et al., 2012), who reported peak or near-peak reinforcement efficiency for graphene/epoxy systems in the 0.05–0.1 wt% range, consistent with the loading-dependent optimum (typically 0.1–0.3 wt%) also reported by Azhagarsamy, Pannirselvam, Sekar and Narayanan (Azhagarsamy and Pannirselvam, 2025b; Sekar and Narayanan, 2025) for related carbon-nanofiller/thermoset systems. The 5 wt% WSp loading follows common practice in natural-particulate epoxy literature, where loadings above ∼5–10 wt% are frequently reported to increase agglomeration and void formation without proportional property gains (Pączkowski, 2023; Karagöz et al., 2024). The 15 wt% E-glass loading represents mat-based reinforcement fraction achievable via hand lay-up with the aerial weight (450 gsm) is used in this research (Nayak et al., 2018; Asi, 2009; Devendra et al., 2013; Muhammad and Ahmad, 2013).

• Natural Particulate - Walnut Shell Powder (WSp): Procured locally and powdered using mechanical grinding. Sieve analysis was carried out to obtain a uniform grit size of 65 with a typical particle size range of 170 microns to 300 microns. The granular powder possesses a density of 1.173 g/cm³ and a Young’s Modulus of 11.59 GPa. The WSp was used in pristine condition without any additional treatment and the powder was oven dried at 65 °C for 4 h to ensure removal of moisture content. WSp is utilized in industrial applications due to its lower density, higher abrasive qualities, and its biodegradable nature.
• Synthetic Fibre - E-Glass Mat: Obtained from Binani Glass Industries (Goa, India), the E-Glass reinforcement utilized is a randomly woven mat made of alumina-calcium borosilicate. The mat has an aereal weight of 450 g/m² (GSM), density 2.58 g/cm³, and a significant longitudinal Young’s Modulus of 70 GPa. Its random orientation in the composites ensured highly isotropic in-plane strength.
• Nano-Filler - Graphene Powder: Graphene was supplied by United Nanotech Innovations Pvt. Ltd., Bangalore, India. The graphene was synthesized with Chemical Vapor Deposition (CVD). The powder consisted of sp2-hybridized carbon atoms packed into a 2D honeycomb lattice. Other properties include its purity >97%, an average planar length of 1–2 microns, true density of 2.20 g/cm³ as per the specifications of the supplier. The bulk/tap density is not used as it does not represent the volume actually occupied by the filler within the cured matrix.

2.2 Specimen preparation

Composite fabrication was carried out using the hand lay-up process that is highly beneficial for manufacturing large-dimension products with low internal residual stress accumulation. Two distinct high-precision aluminium molds were fabricated from 25-gauge aluminium sheets; a primary slab mold measuring 280 × 140 × 4 mm designed to yield specimens for tensile and flexural testing, and a secondary mold measuring 65 × 45 × 30 mm was prepared specifically for compression testing specimens.

Prior to resin infusion, the internal surfaces of the aluminium molds were thoroughly coated with a specialized silicon releasing spray to prevent matrix adhesion and ensure pristine, damage-free demolding. The necessary weight fractions of the Bisphenol-A matrix and the specified reinforcements were meticulously measured using a high-precision digital analytical balance featuring a least count of 0.01g available at KLS Gogte Institute of Technology, India.

The measured reinforcement was introduced into the liquid Bisphenol-A resin. Graphene was first pre-dispersed in a small amount of the liquid epoxy resin/acetone as a carrier solvent, followed by solvent removal using ultrasonication at 750 W, 20 kHz for 30 min, before mixing with the remaining resin and other reinforcements during hybrid samples preparation. The full mixture was then subjected to high-shear mechanical mixing at 2000 rpm for 15 min using an electric blender. The homogeneous dispersion, the promoter, accelerator, and catalyst were sequentially added in resin-to-hardener ratio of 100:1 by weight. The catalysed liquid composite was allowed to settle in the mixing container at ambient laboratory temperature of 24 °C and relative humidity 46% for 20 min, a critical degassing step that allows entrained air bubbles to rise and rupture at the surface, thereby minimizing the formation of internal voids that act as catastrophic stress concentrators (Nedbailo et al., 2023; Bertling et al., 2026). The degassed mixture was carefully poured into the prepared aluminium molds. The curing was conducted isothermally at ambient laboratory temperature of 24 °C and relative humidity 46% for a duration of 24 h so that the controlled dissipation of the exothermic heat of the curing reaction could be achieved through highly conductive aluminium mold (Nedbailo et al., 2023; Palmieri et al., 2021; Yang et al., 2019). This slow kinetic approach drastically minimizes volumetric shrinkage, mitigates residual thermal stresses, and prevents the formation of micro-cracks within the matrix prior to testing (Ding et al., 2024; Li et al., 2025). Following the 24-h cure cycle, the solid composite slabs were demoulded and precisely machined into standardized ASTM testing geometries utilizing a high-speed abrasive cutting apparatus equipped with a 1 mm cutting wheel.

Using the materials as specified in Section 2.1, the following composition samples were prepared as presented in Table 1.

TABLE 1. Specimen compositions.

2.3 Testing

• Tensile testing: Tensile characterization was carried out in accordance with the ASTM D3039 standard. The composite slabs were machined into precise rectangular geometries measuring 250 × 25 × 4 mm. To ensure proper stress transfer from the UTM jaw grips to the composite, protective tabbing was securely bonded to the specimen ends using industrial Araldite adhesive. Directly clamping the composite test specimens in the jaws often induces severe localized crushing and premature failure. Optimal groove angles were scoured on the tabs to avoid slippage. The UTM applied a monotonic longitudinal tensile load at a precisely controlled constant crosshead displacement rate of 2.0 mm/min until failure. The fabricated and prepared samples for tensile testing are showcased in Figure 1.
• Flexural testing: The flexural resilience of the composite specimens was assessed utilizing a three-point bending setup, following ASTM D 790–02 standard. Specimens were precisely cut into a rectangular slab shape of 127 × 12.7 × 4mm, maintaining a span-to-depth ratio of 16:1, establishing an active functional span length of 64 mm between the supporting cylindrical rollers during testing. A highly concentrated perpendicular load was applied dynamically at the exact mid-span of the specimen at a constant crosshead displacement rate of 1.7 mm/min until failure. Figure 2 exhibits the image of flexural testing samples.
• Compression testing: Compressive properties were characterized following the ASTM D695-02a standard protocols. Compressive test specimens require a highly specific, short geometric profile to ensure the material undergoes true compressive crushing failure rather than structural macroscopic buckling. Consequently, specimens were machined to 12.7 × 12.7 × 25 mm as shown in Figure 3. The specimens were positioned vertically between two perfectly parallel, flat compression platens within the UTM. A uniform axial compressive load was applied at a constant rate of 1.3 mm/min, and real-time observations were recorded, correlating the applied load to continuous volumetric deformation.
• The theoretical density of each composite sample was calculated utilizing the rule of mixtures with the constituent densities (epoxy: 1.20 g/cm³; WSp: 1.173 g/cm³; E-glass: 2.58 g/cm³; graphene: 2.20 g/cm³) and the respective weight fractions. The experimental densities of the cured laminates were determined via the water-displacement method in accordance with ASTM D792, using three specimens per formulation. The void content was then computed following ASTM D2734 as:

ρ theoretical = 1 ∑ W i ρ i (1)

V v % = ρ theoretical − ρ experimental ρ theoretical x 100 (2)

Wᵢ and ρᵢ are the weight fraction and density of constituent “i” in Equations 1, 2. Table 2 shows the density (ρ theoretical and ρ experimental) and the void content

TABLE 2. The ρ theoretical, ρ experimental, and void content.

3 Results and discussions

3.1 Mechanical properties

The prepared samples were tested for tensile, flexural, and compressive strength. Three specimens were tested, and the average result is tabulated in Table 3. The response of the composite samples to uniaxial tensile stress reveals the efficiency of the Interfacial Shear Strength (IFSS) between the polymer matrix and the reinforcing fillers. Under pure tension, structural failure is typically initiated by interfacial debonding or matrix micro-cracking. The empirical data for tensile loading and corresponding deformation are presented in Table 3, while the improvement in the properties in comparison with neat epoxy samples is presented in Table 4.

TABLE 3. Empirical tensile load and deflection parameters.

TABLE 4. Percentage improvement in Tensile Strength, Flexural Strength and Compressive Strength.

Mechanical properties are reported as mean ± standard deviation across three specimens per formulation. The coefficient of variation was generally higher for the WSP-containing and E-Gr compressive results (e.g., E-Gr compressive strength: 104.78 ± 22.53 MPa, CV ≈ 21.5%) than for the neat resin or E-glass tensile results, consistent with the particulate agglomeration and localized void formation discussed in Section 3.4. The hand lay-up technique does not guarantee uniform particulate or nano-filler distribution, and this variability is expected to propagate into greater scatter.

The improvement in the test results of the prepared samples compared to the test results of the neat epoxy samples is listed in Table 4.

The neat Bisphenol-A epoxy established a standard baseline tensile strength of 79.00 MPa and a relatively low elastic modulus of 2.46 GPa, typical of unreinforced thermosets. The results of tensile strengths exhibited by various materials is showcased in Figure 4.

The incorporation of a virtually negligible 0.1 wt% weight fraction of Graphene induced a massive 24.47% increase in ultimate tensile strength (98.33 MPa) and nearly doubled the structural stiffness (4.80 GPa). This exponential enhancement at an ultra-low concentration is governed by the extraordinary specific surface area of graphene (186 m²/g) and its extremely high inherent aspect ratio (Tetlow et al., 2014; Harito et al., 2019; Wu et al., 2021). The 2D-sp² carbon lattice interacts largely with the encompassing epoxy polymer chains, acting as a cross-linking bridge. The tensile stress lodges a microscopic crack in the brittle epoxy during loading; the propagating crack collides with the rigid graphene. The graphene nanoparticle forces the crack to change its path and absorbs a huge amount of kinetic energy, acting like a toughening mechanism and crack deflector (Khalid et al., 2023; Li et al., 2016; Shokrieh et al., 2014; Irez et al., 2020).

The addition of 5 wt% Walnut Shell Powder, in contrast, exhibits the lowest tensile enhancement, culminating at a moderate 88.22 MPa (a 11.67% improvement). This underperformance is an outcome of the fundamental incompatibility between highly hydrophilic natural Walnut Shell particles and highly hydrophobic synthetic epoxy (Mohammed et al., 2025; McKay et al., 2024; Zwawi, 2021). The varying surface energies inhibit optimal resin wetting during the composite fabrication (Shejkar et al., 2021). The WSp particles do not disperse uniformly in the epoxy matrix, forming localized, high-density agglomerations that are also noticed from the SEM image showcased in Figure 5. When uniaxial tension is applied, these agglomerates act as large macro-defects and stress concentrators (Zhang et al., 2018; Joffre et al., 2014). The weak interfacial bonds fracture at high rates, completely converting the particulate inclusions into huge internal voids, paving the way to rapid, catastrophic crack propagation and nullifying any achieved tensile gains (Yao, 2014; Bain et al., 2017; Hou et al., 2021). The 15 wt% E-Glass reinforced composite exhibited an enhancement of 53.58% (121.33 MPa) in the tensile strength, acknowledging that continuous macro-fibres substantially bear and distribute the longitudinal loads better than dispersed particulates (Ding et al., 2020; Fong et al., 2025). The hybrid E-WSpG yielded a notable enhancement in tensile properties compared to both neat epoxy (NE) and single-reinforced systems (E-WSp and E-Gr). The tensile strength of E-WSpG reached 113.61 MPa, representing a 43.81% increase over neat epoxy (79.00 MPa), and a 28.8% improvement over the binary E-WSp composite (88.22 MPa). This improvement indicates that high-aspect-ratio graphene nanosheets facilitate efficient load transfer across the matrix, while the bio-particulate WSp acts as a secondary structural filler, restricting polymer chain mobility under axial tensile loading (Palaniyappan et al., 2024; Shejkar et al., 2023). However, a superior performance was showcased by the multiscale hybrid E-EgG, exhibiting significant improvement in tensile performance. This E-EgG hybrid achieved an ultimate tensile strength of 135.61 MPa and an exceptional Young’s Modulus of 9.22 GPa; a substantial 71.66% enhancement over the neat resin. While the macroscopic E-glass fibres absorb the primary tensile strain, the nano-dispersed graphene particles impregnate the highly localized interstitial microscopic spaces of the epoxy matrix (Uppin et al., 2022; Garg et al., 2025; Shanmugam and Meenakshisundaram, 2023; Turaka and Bandaru, 2023). Graphene significantly enhances the Interfacial properties between the E-glass surface and the surrounding epoxy, creating a highly cohesive, chemically locked structural network (Park et al., 2015). By arresting early-stage matrix micro-cracking and preventing interfacial delamination, the graphene allows the E-glass fibres to be pushed to their absolute theoretical failure limits (Hamidul Islam et al., 2024; Varghese et al., 2025; Megahed et al., 2024).

Flexural testing subjects complex combined stress states on the specimens while testing, inducing compression on the concave upper surface, tension on the convex lower surface, and interlaminar shear forces along the neutral axis (Burgani et al., 2022; Chowdhury et al., 2020; Kanığ and Kutlu, 2025). The results of Flexural strength exhibited by various materials is showcased in Figure 5.

The E-Eg composite yielded a significant rise of 78.58% in flexural strength. The randomly distributed, multidirectional woven nature of the E-glass mat provides immense resistance to out-of-plane deformation, completely distributing the localized point load throughout the composite (Heckadka et al., 2015; Moreira Arouche et al., 2021; Gunarti et al., 2024). The E-EgG composite exhibited significant gain in ultimate flexural strength of 201.23 MPa and a bending modulus of 9.63 GPa, a substantial 123.81% enhancement compared to the neat epoxy. The governing mechanism depends on the crack-bridging and pinning ability of the nano-graphene (Riccio et al., 2020). In the three-point bending flexural test, the structural failure originates from the formation of micro-cracks on the highly stressed lower face of the composite (Nurazzi et al., 2021; Mehdikhani et al., 2019). The dispersed 2D graphene nano-platelets adequately bridge these initiating crevices, maintaining structural integrity (Parente et al., 2025). Concurrently, the macroscopic E-glass mat adds to the overall beam rigidity, effectively delaying the catastrophic flexural yield (Zhu et al., 2024). A key finding is the significant flexural performance exhibited by the E-WSpG hybrid composite specimens. E-WSpG showcased a flexural strength of 173.40 MPa and a flexural modulus of 6.82 GPa, a rise of 92.86% and 147.1% over neat epoxy, respectively. Notably, the flexural strength of the Walnut Shell powder/Graphene hybrid system outperformed the synthetic E-Glass fiber-reinforced composite without graphene. This demonstrates that the synergistic coaction amongst 2D graphene sheets and fine bio-particulates adequately resists bending-induced tensile and compressive stresses, proving that E-WSpG is a highly admissible eco-friendly substitute to the traditional glass fiber composites in flexural performance requirement applications (Matykiewicz, 2020; Riccio et al., 2020). The E-WSpG exhibited immense synergistic potential, possessing a flexural strength of 173.40 MPa. The graphene networks effectively disperse the localized stress concentrations evolving from the rigid WSp agglomerates and create a uniform strain field, further delaying the commencement of flexural fracture (Karagöz et al., 2024; Barman et al., 2025; Zeinedini and Shokrieh, 2024; Saleh et al., 2025; Singh et al., 2022).

Compressive testing assesses the matrix’s structural yield stability and the reinforcement’s resistance to localized buckling and crushing (Riccio et al., 2020; Rozylo and Wysmulski, 2021; Yerramalli and Waas, 2004). The results of Compressive strength exhibited by various materials is showcased in Figure 6.

The Walnut Shell Powder (WSp) provided a significant 75.30% improvement in compressive strength compared to the neat epoxy. This performance features the typical governing physics of particulate-reinforced composites. In tensile loading, the poorly bonded WSp particulates act as stress-concentrating voids that rapidly initiate fracture points (Koyunucu and Ulay, 2023). However, under pure axial compression, the highly rigid, highly packed cellular structure of the walnut shell effectively absorbs and disperses the crushing energy (Wang et al., 2023; Gharibzahedi et al., 2012; Xiao et al., 2021). The physical volume occupied by the WSp particulates immensely restricts the kinetic mobility of the epoxy polymeric chains (Pączkowski, 2023). The E-Eg composite showcased a substantial 311.23% compressive strength enhancement due to the lateral strength of the interwoven glass fibres that resists buckling. The E-WSpG composite demonstrated significant stress resistance, exhibiting a compressive strength rise from 47.12 MPa (NE) to 121.31 MPa (+157.45%). While the incorporation of particulate WSp individually improved compressive resistance, the addition of graphene in E-WSpG relieved matrix crushing and localized shear band movement. The rigid planar structure of graphene prevents micro-buckling of the epoxy matrix, yielding a more robust structure under heavy compressive loads (Yang and Meng, 2026). However, ultimate compressive strength was again achieved by E-EgG, culminating in an extraordinary compressive strength of 214.73 MPa (355.71% improvement) and modulus of 3.55 GPa. The continuous network of the E-glass mat suppresses macroscopic geometric buckling, while the nano-level dispersion of graphene actively anchors the epoxy matrix tightly around individual glass fibres (Turaka and Bandaru, 2023). The interaction amongst the reinforcements prevents fibres from buckling sideways and crushing the surrounding resin, making the composite much tougher in compression (Riccio et al., 2020; Salman et al., 2015; Abu Bakar et al., 2020).

3.2 Void content

The void content of the fabricated composites was quantified using the theoretical-versus-experimental density method (ASTM D2734), with results summarized in Table 2. All the composite samples exhibited relatively low void fractions, ranging from 1%–5% for the multiscale E-EgG hybrid. A distinct trend was observed in the void content that increased with the compositional complexity of each sample, increasing from the neat resin through the single reinforcement samples (E-Gr: 1.58%; E-WSp: 1.85%; E-Eg: 2.20%) to the hybrid samples (E-WSpG: 2.21%; E-EgG: 2.74%).

This trend quantitatively substantiates the morphological observations of SEM morphology discussed in Section 3.4. The increased void content of the samples containing WSp (E-WSp, E-WSpG) justifies the poor interfacial wetting and particulate agglomeration visible in Figure 5, wherein hydrophilic WSp particulates are unable to achieve complete matrix encapsulation, leading to the localized micro-voids at the filler-matrix boundary. Similarly, the higher void fractions in the E-glass reinforced composite samples (E-Eg, E-EgG) showcase the inherent difficulty of achieving complete resin impregnation within the interstitial spaces of the woven mat under manual hand lay-up. Notably, despite possessing the highest void content of all six composite samples, the E-EgG hybrid delivered a higher mechanical performance across all three loading modes, proving that the reinforcing efficiency and interfacial synergy of the E-glass/graphene combination is sufficiently dominant to compensate for the marginal strength loss due to the elevated void fraction. The measured void content of all the composite samples ranged in 1%–5%.

3.3 Micromechanical modelling

The weight fraction was converted to the volume fraction using Equation 3, and the values obtained are used for the micromechanical models and their calculations.

Vi = Wi ρ i ∑ j W j ρ j (3)

• Rule of Mixtures (upper-bound/Voigt) for the E-glass modulus (Frankland et al., 2003) (Equation 4)

E c = V f E f + V m E m (4)

• b. Random in-plane orientation correction (Tsai–Pagano) for the E-glass reinforcement (Pan, 1996). The E-glass reinforcement is a randomly oriented mat and not unidirectional; therefore, a defensible model for a random-in-plane mat is presented in Equation 5. For the E-glass mat, the random-in-plane orientation assumption underlying the Tsai–Pagano treatment is reasonably consistent with the manufacturer-reported “randomly woven mat” architecture, though the mat’s woven (rather than chopped-random) structure likely explains why the simple Rule-of-Mixtures upper bound matched experimental data more closely than the random-orientation model as a woven mat retains more directional continuity than an idealized randomly oriented discontinuous-fibre assembly.

E r a n d o m = 3 8 E 11 + 5 8 E 22 (5)

where E _*11* is the longitudinal modulus (from ROM above), and E _*22* is the transverse modulus, obtained from Halpin–Tsai (Raju et al., 2018; Zhao et al., 2020; Loos and Schulte, 2015; Budarapu et al., 2019) as shown in Equation 6.

E 22 = Em 1 + ξ η V f 1 − η V f , η = E f / E m − 1 E f / E m + ξ (6)

using ξ = 2 for circular/roughly cylindrical transverse fiber reinforcement.

• c. Nicolais–Narkis model—for the WSp particulate system (Zare, 2016a; Zare, 2016b). This model is specifically designed for poorly-bonded, non-adhering particulate fillers as shown in Equation 7. The spherical/equiaxed particle assumption (ξ = 2) underlying the Halpin–Tsai treatment is a simplification as WSp was produced by mechanical grinding to a grit-65 size, which typically yields angular, irregular particle morphology rather than true spheres. The systematic underprediction of modulus for both WSp-containing formulations (E-WSp: −16%; E-WSpG: −13%) is consistent with this simplification, since angular/irregular particles generally provide greater mechanical interlocking and a higher effective aspect ratio than spheres, which ξ = 2 does not capture.

σ c = σ m 1 − 1.21 V f 2 3 (7)

• d. Halpin–Tsai model for the graphene nanoplatelet system (E-Gr)

Graphene platelets possess high aspect ratios and are discontinuous for a simple rule of mixtures. Halpin–Tsai is utilized as presented in Equation 6 with an assumption of ξ as shown in Equation 8. The platelet-geometry assumption (ξ = 2a/t) is structurally appropriate given graphene’s inherent 2D morphology, but its numerical accuracy depends entirely on the assumed aspect ratio. a is the platelet planar length and t is the platelet thickness.

ξ = 2 a t (8)

The Halpin–Tsai prediction for the E-Gr system is sensitive to the assumed platelet aspect ratio, which was set to ξ ≈ 600 (implying a ∼5 nm, few-layer platelet thickness for the reported 1–2 µm planar length). Table 5 shows varying this assumption across a physically plausible range (ξ = 100–1,000, corresponding to platelet thicknesses of 3–30 nm) shifts the predicted modulus from 3.29 to 4.52 GPa, a ∼35% spread, before even accounting for the volume-fraction uncertainty.

• e. Two-step homogenization for the hybrid systems (E-WSpG, E-EgG) (Castricum et al., 2021). Hybrid systems that combine a nano-filler with a macro-filler are effectively modelled in two sequential steps.

TABLE 5. Effect of multiple ξ assumptions.

(1) Homogenize the graphene into the epoxy to obtain an effective matrix modulus (graphene-toughened epoxy), using the local graphene volume fraction within the epoxy + graphene sub-volume only, as shown in Table 6.

TABLE 6. Effective matrix modulus.

(2) Treat the macro-filler (WSp or E-glass) as reinforcing that effective matrix, first for the E-WSpG: WSp (ξ = 2, spherical/equiaxed assumption) into E_m, eff, and then for the EgG: E-glass (ROM, continuous fiber) into E_m, eff.

The results obtained using the models listed in Section 3.3 are presented in Table 7.

TABLE 7. Micromechanical models result.

The Halpin–Tsai and the two-step framework systematically underpredict all three graphene-containing moduli by 15%–46%. At Vf as low as 0.05%–0.06%, Halpin–Tsai (a dilute, non-interacting-particle theory) does not capture percolation, interphase, or network-bridging effects that become proportionally more important at ultra-low true volume fractions which is a noteworthy limitation of the model and not an error in the experimental data.

3.4 Scanning electron microscopy

In order to correlate the macroscopic experimental mechanical results with microscopic failure mechanics, in-depth morphological evaluation of the post-fracture composite samples was conducted. Imaging was executed with a high-resolution Scanning Electron Microscope (JEOL JSM-6360LV) available at Shivaji University, India. Prior to observation in the high-vacuum chamber, the non-conductive polymeric fracture surfaces were subjected to a thin layer of gold-sputter coating to avoid severe electron charging artifacts and ensure better imaging clarity.

The incorporation of 5 wt% WSp completely altered the fracture topography. Figures 7A,B exhibited a higher surface roughness compared to the neat resin, indicating that the propagating crack front is physically forced to deviate around the rigid particulate reinforcement, thereby dissipating kinetic energy. At high magnification, a critical flaw was observed that shows massive, distinct areas of particulate agglomeration. These high-density clusters of WSp lacked homogeneous encapsulation by the epoxy matrix, as a result of poor interfacial wetting dynamics and thermodynamic incompatibility during the mixing of the individual components (Koyunucu and Ulay, 2023). These unbonded agglomerations act as severe stress concentrators, and under applied tension, the matrix gets debonded from the WSp clusters, initiating rapid interfacial delamination.

SEM images of the 15 wt% E-Glass composites (Figures 8A,B) demonstrated an intensely disorganised, highly twisted fracture surface, indicating immense energy dissipation prior to failure. The micrographs showcased extensive instances of fibre pull-out and fibre fracture, and the matrix was visibly seen tightly adhering to the fibres, suggesting effective load transfer between the epoxy and glass fibres directly to the high-strength glass reinforcement until the ultimate tensile capacity of the fibres was substantially exceeded. This fracture morphology is consistent with, and provides qualitative support for, the improved load transfer inferred from the 53.58% tensile and 78.58% flexural strength gains.

At 0.1 wt% graphene loading, the fracture surface topography (Figures 9A,B) exhibited moderate, highly uniform topological roughening. Crucially, the high-magnification images exhibited the presence of distinct, narrow topological bands running parallel to the direction of crack growth that serve as distinct evidence of a crack deflection mechanism. As the micro-crack advances through the polymer matrix, it repeatedly collides with the highly rigid, 2D graphene nano-platelets. Unable to fracture the graphene (E = 500 GPa), the crack is physically forced to constantly tilt, twist, and shift its propagation plane (Wazalwar et al., 2021; Dong et al., 2024). This continuous, rapid deflection exponentially increases the total surface area of the fracture, consuming huge amounts of kinetic energy and fundamentally elevating the composite’s overall fracture toughness and rigidity.

The SEM images of hybrid E-WSpG composite showcased in Figures 10a,b exhibited a complex, multi-scale fracture morphology characterized by a highly convoluted fracture surface and pronounced plastic deformation. High-magnification fractography revealed a synergistic failure behaviour driven by the interaction between the micro-scale bio-filler WSp and nano-scale graphene. The micro-scale WSp agglomerates served as primary crack initiation sites due to localized stress concentration under axial tensile loading. As micro-cracks propagated outward from the WSp-matrix boundary, they encountered high-aspect-ratio 2D graphene nanoplatelets dispersed in the epoxy resin. The thin graphene sheets pinned crack fronts, forcing them to bifurcate and follow the twisted, out-of-plane paths (crack deflection). Nano-scale graphene sheets bridged micro-voids around the WSp bio-particles, enhancing strength and slowing catastrophic crack growth. The multi-scale toughening mechanism induced by the hybrid reinforcements explains the elevated tensile strength (113.61 MPa) and tensile modulus (5.37 GPa) for the E-WSpG hybrid composite relative to both neat epoxy and binary E-WSp systems.

The microscopic evaluation of the E-EgG composite shown in Figures 11a,b exposed the peak of composite toughening mechanics. The analysis showcased a highly compact, dense bundle-like structure of E-glass fibres seamlessly integrated into a toughened matrix environment. Unlike the standard E-glass composite, high-magnification views demonstrated ultra-thin layers of graphene rigidly adhered simultaneously to both the glass fibre surfaces and the surrounding bulk epoxy matrix. The observed morphology is consistent with graphene altering the local interfacial region, which is proposed to affect interfacial shear strength (IFSS), though IFSS itself was not directly measured in this study (Park et al., 2015). By exponentially increasing the adhesive friction and chemical forces bridging the macro-fibres and the matrix, the graphene actively resists fibre pull-out and delays catastrophic interfacial delamination (Mahmood et al., 2016). The interconnected, multi-tier energy dissipation network combining macro-scale fibre tension with nano-scale crack deflection and enhanced IFSS physically describes the cause for E-EgG composite systematically outperforming all the mechanical test results.

4 Conclusion

• The mechanical, analytical, and morphological characterization conducted in this research clarifies how multiscale reinforced polymer composites behave under load. The selection, dimensional scale, and combination of reinforcements together dictate the load-bearing capacity and failure mode of a thermosetting Bisphenol epoxy matrix.
• Walnut Shell Powder (WSp) produces uneven property gains. Poor interfacial wetting and particulate agglomeration limit its contribution to tensile strength (a modest 11.67% improvement), but its rigid, dense cellular structure restricts polymer chain mobility under compressive loading, yielding a 75.30% improvement in compressive strength. This makes WSp a viable, low-cost, eco-friendly filler for components subjected to compressive rather than tensile loads.
• E-glass continuous fibre mats outperform natural particulates in tension and flexure through efficient load transfer and structural interlocking. The larger gain, though, comes from multiscale hybridization: adding a volumetrically negligible 0.1 wt% of nanoscale graphene alongside the macroscopic E-glass fibres produces mechanical enhancements that exceed what either reinforcement achieves alone.
• In the E-WSpG hybrid, the micro-scale bio-particulates and 2D graphene sheets reinforce the resin matrix together rather than independently. The composite reached a flexural strength of 173.40 MPa and flexural modulus of 6.82 GPa, surpassing the conventional E-glass composite’s 160.56 MPa and pointing to a lightweight, eco-friendly alternative for bending-dominated structural applications.
• The E-EgG composite dominated the test results, elevating ultimate tensile strength by 71.66%, flexural strength by an immense 123.81%, and compressive strength by an extraordinary 355.71% relative to the neat baseline resin. High-resolution morphological images confirm that the exceptional performance is the result of a fundamentally novel mechanical interaction. The graphene nano-platelets alter the properties, multiplying the Interfacial Shear Strength (IFSS) between the continuous glass filaments and the epoxy matrix, while simultaneously serving as a strong, 2D physical barrier against micro-crack propagation within the bulk resin.
• The analytical validation utilizing micromechanical models endorsed the empirical findings across all compositions of the composite specimens tested in this research. Rule-of-Mixtures and Halpin–Tsai predictions matched the E-Eg within 2% and E-Gr moduli within 44.5% of experimental values, affirming efficient interfacial load transfer. A two-step homogenization framework, considering graphene as a matrix-toughening phase before superposing the macro-scale reinforcement, predicted the E-EgG modulus within 15.3% of the experimental result, while the E-WSpG hybrid modulus exceeded its corresponding prediction by approximately 46.1%. This deviation complements direct, model-based quantitative evidence for the synergistic WSp–graphene interaction proposed based on SEM observation, and establishes a transferable analytical framework for the design of future multiscale hybrid composites.

While the current methodology and the results achieved yield highly promising outcomes for industrial implementation, the prospect of composite materials engineering presents multiple avenues for further optimization. The study progressing from ambient room-temperature curing to highly controlled, elevated-temperature isothermal cycles could further enhance the degree of covalent cross-linking within the Bisphenol resin network. The application of silane coupling agents or typical chemical surface treatments to the Walnut Shell Powder could substantially alleviate its natural hydrophilicity. Composite fabrication methods such as Vacuum Assisted Resin Transfer Molding (VARTM) or automated pultrusion would further reduce the internal micromechanical void fraction, yielding high-performing composite materials.

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Footnotes

Edited by: Iacopo Bianchi, Marche Polytechnic University, Italy

Reviewed by: Azhagarsamy S., SRM Institute of Science and Technology (Deemed to be University) Research Kattankulathur, India

Mohammed Asif Kattimani, Lords institute of Engineering and Technology, India

Footnotes

SiC, Silicon Carbide; SEM, Scanning Electron Microscopy; WSp, Walnut Shell Powder; XRD, X-Ray Diffraction; CVD, Chemical Vapor Deposition; DGEBA, Diglycidyl Ether of Bisphenol A; IFSS, Interfacial Shear Strength; GSM, Grams Per Square Meter; ROM, Rule of Mixtures; FTIR, Fourier Transform Infrared Spectroscopy; TGA, Thermogravimetric Analysis; DSC, Differential Scanning Calorimetry.