Issue 78

R. Shivashankar et alii, Fracture and Structural Integrity, 78 (2026) 454-473; DOI: 10.3221/IGF-ESIS.78.26 Good fiber–matrix adhesion is still visible for S3 (1.5 wt. % Si ₃ N ₄ ; Fig. 11e, f), but matrix fissures, microvoids, and localized agglomerates become more noticeable. The minor decrease in fracture toughness can be explained by these characteristics, which can serve as stress-concentration sites and lower stress-transfer efficiency. Excessive loading of ceramic nanoparticles has been linked to similar consequences [41, 42]. Increased fiber pull-out, matrix breaking, resin-rich areas, and more noticeable nanoparticle aggregation are all present in S4 (2 wt. % Si ₃ N ₄ ; Fig. 11g, h). These flaws limit the efficiency of crack-pinning and bridging mechanisms and offer preferred crack-initiation sites, which further lowers K IC . However, compared to the unfilled composites, the rough fracture surface and residual fiber-matrix adhesion show higher fracture resistance. Overall, the SEM data confirm that the optimal loading is 1 wt.% Si ₃ N ₄ (S2), which offers the best compromise between fracture resistance, interfacial bonding, and nanoparticle dispersion. Fracture toughening mechanism From S0ut to S2, fracture toughness gradually increased as a result of the combined impacts of Si 3 N ₄ reinforcement and fiber surface alteration. While well-dispersed Si ₃ N ₄ encouraged fracture deflection, pinning, crack-tip shielding, and fiber bridging, alkali- and silane-treated fibers enhanced interfacial adhesion and stress transfer [41, 42]. As a result, S2 (1 wt. % Si ₃ N ₄ ) showed the highest fracture toughness. Fracture toughness slightly decreased with higher loadings due to stress concentrations brought about by nanoparticle agglomeration and microvoids, which also decreased the efficiency of crack arrest mechanisms. Therefore, the ideal loading under the current processing conditions was 1 wt. % Si ₃ N ₄ . CORRELATION OF ILSS , FLEXURAL , AND FRACTURE PROPERTIES he synergistic effects of fiber surface modification and Si ₃ N ₄ nanoparticle reinforcement consistently improve the ILSS, flexural characteristics, and fracture toughness of the H/B F-Ep hybrid nanocomposites. While silane treatment of basalt fibers improved their chemical compatibility with the epoxy matrix, alkali treatment of hemp fibers eliminated surface contaminants and increased surface roughness. These changes increased ILSS, flexural strength, modulus, and resistance to crack initiation by improving fiber–matrix adhesion and stress transmission. By creating a multiscale reinforcement network, the addition of Si 3 N ₄ improved the mechanical performance even further. The nanoparticles were efficiently distributed and reinforced the fiber–matrix interphase at the ideal loading of 1 weight percent (S2). Cohesive matrix fracture, fiber fracture, crack deflection, crack pinning, crack-tip shielding, matrix tearing, and fiber bridging were all shown by SEM studies to promote energy dissipation and resistance to flexural and interlaminar failure. As a result, S2 demonstrated the best overall fracture and mechanical performance. There were minor decreases in ILSS, flexural characteristics, and fracture toughness at increasing Si ₃ N ₄ concentrations (S3 and S4). Nanoparticle agglomeration and related microvoids, which produce localized stress concentrations, lower stress transfer efficiency, and offer preferred locations for crack initiation, are responsible for this behavior. However, compared to the untreated and nanoparticle-free systems, these composites maintained superior properties. Overall, the concordance between the mechanical results and SEM observations verifies that interfacial integrity, stress transfer, and damage tolerance are enhanced by efficient fiber surface modification in conjunction with optimal Si ₃ N ₄ dispersion. The S2 composite provides an efficient multiscale strengthening and toughening method for H/B F-Ep hybrid nanocomposites by demonstrating that 1 wt.% Si ₃ N ₄ achieves the optimal balance between nanoparticle dispersion and interfacial reinforcement. C ONCLUSIONS n the present study, it was shown that the mechanical properties of hybrid hemp/basalt fiber-reinforced epoxy composites can be greatly improved by the synergistic effect of fiber surface modification and Si ₃ N ₄ nanoparticle reinforcement. The key findings are outlined below: • Improved mechanical performance and fiber–matrix adhesion was associated with silane treatment of basalt fibers and alkali treatment of hemp fibers, most likely because of improved interfacial interactions and surface modification. • The ILSS, flexural characteristics, and fracture toughness were all increased by Si 3 N ₄ inclusion; these improvements were probably caused by improved interfacial contacts, crack-deflection, and energy-dissipation mechanisms. T I

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