Issue 78

composite having 1.5 wt.% Si ₃ N ₄ showed the highest interlaminar shear strength of 27.77 MPa (70.9% higher) and flexural strength of 258.91 MPa (42% higher) than the H/B F-Ep composite without Si ₃ N ₄ . The maximum fracture toughness (2.6828 MPa. √ m) was obtained at 1 wt.% Si ₃ N ₄ , which is a 57.3% increase because of crack pinning, crack deflection and crack bridging. However, 2 wt.% Si ₃ N ₄ led to the agglomeration of the nanoparticles and to localized stress concentrations, which slightly lowered the mechanical performance. The results show that the optimized incorporation of Si ₃ N ₄ , together with the chemically modified H/B hybrid reinforcements, is a promising approach for the enhancement of damage tolerance of the epoxy nanocomposites for lightweight structural applications in the automotive and aerospace sectors. K EYWORDS . Hybrid composites, Silicon nitride, Interfacial adhesion. Flexural properties, Fracture toughness, Fractographic analysis

R. Shivashankar et alii, Fracture and Structural Integrity, 78 (2026) 454-473; DOI: 10.3221/IGF-ESIS.78.26

I NTRODUCTION

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iber-reinforced polymer composites (FRPCs) are widely used in aerospace, automotive, marine, defense, and civil engineering due to its low weight, high specific strength and stiffness, corrosion resistance, and design flexibility. Natural fiber-reinforced polymer composites, which offer low density, renewability, biodegradability, low cost, and less environmental effect, are among the sustainable alternatives that are gaining popularity due to growing environmental concerns [1]. Because of its high cellulose content, favorable mechanical qualities, quick growth, and accessibility, hemp fiber is very appealing. However, fiber–matrix adhesion and stress transfer may be weakened by its hydrophilicity, moisture absorption, dimensional instability, and limited durability [2]. Surface modification is one of the effective methods to enhance the inter-material properties of natural fibers. The alkali treatment is a simple and economic method for the removal of hemicellulose, lignin, waxes and other surface impurities. It can enhance surface roughness, fibrillation, wettability and mechanical interlock with the polymer matrix, which helps to enhance load transfer. It has been reported that the mechanical performance of hemp fiber polymer composites is significantly improved by optimized alkali treatment [3,4]. The drawbacks of natural fiber composites can be addressed by hybridization with synthetic fibers. Because of its great strength, resistance to heat and chemicals, resistance to corrosion, and advantageous environmental properties, basalt fiber is especially appealing. Thus, hemp/basalt hybrid composites present a promising compromise between sustainability and structural performance [5]. With an elastic modulus of 85–95 GPa and a tensile strength of roughly 3000 MPa, basalt fibers— which are generated directly from volcanic rock—are appropriate for structural applications [6]. Tensile, flexural, impact, and environmental resistance can all be enhanced by their integration [7–9]. Thus, basalt fiber contributes to improved load bearing capacity, while hemp offers lightweight and sustainable reinforcement. Despite these advantages, interfacial failure remains a major limitation in hybrid composites. Damage may initiate through matrix cracking, fiber–matrix debonding, fiber pull-out, and interlaminar delamination. Therefore, interlaminar shear strength (ILSS), flexural strength, and fracture toughness are important indicators of load transfer, structural integrity, and damage tolerance [10]. Improving fiber–matrix adhesion and resistance to crack initiation and propagation remains essential for achieving reliable hybrid composites. By enhancing matrix stiffness, strength, fracture toughness, and damage tolerance, nanoparticle reinforcement provides an efficient way to get around these restrictions. To prevent the spread of cracks, nanoparticles might encourage energy dissipation processes like matrix deformation, debonding, bridging, deflection, and pinning [11]. The mechanical and fracture properties of epoxy composites have been reported to be improved by a variety of nanofillers, such as carbon nanotubes, graphene, nanosilica, nanoclay, alumina, TiO ₂ , and SiC [12–14]. Among these, silicon nitride (Si ₃ N ₄ ) is appealing due to its low density, wear resistance, chemical and thermal stability, high hardness, and fracture toughness [15]. In addition to promoting crack deflection and improving mechanical and fracture performance, well-dispersed Si 3 N ₄ nanoparticles can also increase particle–matrix interaction. While Mohammed et al. [17] noted increased fiber–matrix adhesion, decreased stress concentration, and delamination, Suresha et al. [16] found improved flexural and fracture behavior. Therefore, SiO 2 has the ability to strengthen the fiber-reinforced epoxy composites at several scales.

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