Issue 78

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

localized stress concentrations and lower stress-transfer efficiency, is responsible for the little decrease for S3 and S4 (1.5 and 2 wt. %). However, their fracture toughness continued to be higher than that of S0ut and S0t, indicating that Si ₃ N ₄ has a long-lasting reinforcing effect. Processing parameters, including nanoparticle dispersion and ultrasonication conditions, may affect the ideal loading. The observed toughening mechanisms are consistent with earlier research on epoxy systems reinforced with nanoparticles. While Fe 3 O 4 –graphene hybrid nanofillers enhanced fracture toughness by crack bifurcation, twisting, and stress redistribution [35], nano-alumina has been shown to increase crack-path tortuosity and fracture-process-zone size [36]. In a similar vein, ceramic nanoparticles improved stress transmission and slowed the spread of cracks in basalt-fiber multiscale composites [37]. Through increased interfacial bonding and inhibition of microcrack coalescence, surface-treated natural fiber/epoxy nanocomposites incorporating nanoclay have also shown improved resistance to crack growth [38]. These results corroborate the current findings and show how well treated H/B fibers work as a multiscale toughening method when combined with optimally dispersed Si 3 N ₄ . The fracture toughness trend as obtained in the present study is in good agreement with these well-known mechanisms. As a result of alkali treatment, the strength of the H/B F-Ep bond increases from S0ut to S0t, indicating the beneficial effect that alkali treatment has on strengthening the hemp/basalt fiber–epoxy bond, while the further improvement following the addition of Si ₃ N ₄ suggests that multiscale reinforcement is effective in inhibiting crack propagation via crack-tip shielding and stress redistribution. It was found that the maximum fracture toughness was obtained for S2 (1 wt.% Si ₃ N ₄ ), indicating an optimum nanoparticle concentration at which a balance between nanoparticle dispersion and particle–matrix interaction is achieved. The slight decrease seen for S3 and S4 is believed to be due to the agglomeration of the nanoparticles which leads to the presence of localized stress concentrations, reducing the effectiveness of the toughening mechanisms. The agglomeration of Si ₃ N ₄ largely depends on the concentration of the Si ₃ N ₄ filler and dispersion conditions, including the ultrasonication time. The concentration of Si ₃ N ₄ hence, the optimum fracture toughness level may be influenced by the ultrasonication conditions. The optimal balance under the chosen processing settings is the one found in this investigation. At 1.5 and 2 wt. % Si ₃ N ₄ (S3 and S4), the expected densities rose to 1.3530 and 1.3570 g/cm³, respectively. The corresponding experimental densities were 1.3380 ± 0.013 and 1.3392 ± 0.014 g/cm³, with void contents of 1.11% and 1.31%, respectively, according to our earlier investigation [39]. During processing, the increased Si ₃ N ₄ loading restricted resin flow and encouraged local agglomeration and air entrapment by increasing resin viscosity and particle–particle interactions. Despite the higher theoretical density, the decrease in ILSS, flexural strength, and fracture toughness was probably caused by the increased void content with higher Si ₃ N ₄ loading. Overall, the good match of the experimental results with previous results supports the use of the hybrid combination of alkali-treated H/B fibers and Si ₃ N ₄ nanoparticles as an effective multiscale toughening strategy for epoxy composites. The fracture toughness enhancement is comparable with some recently reported Si ₃ N ₄ /nanoparticle – Ep composite systems, highlighting the huge potential of Si ₃ N ₄ as a high-performance ceramic nanofiller for the preparation of damage-tolerant hybrid polymer composites. Fractographic analysis of fracture toughness-tested Si ₃ N ₄ -modified H/B F-Ep hybrid nanocomposites Representative failed fracture toughness specimens for S0ut, S0t, and Si 3 N ₄ -modified composites (S1–S4) are shown in Fig. 9. Every specimen showed successful Mode-I fracture, with cracks starting at the machined notch and spreading throughout the width of the specimen. Improved resistance to crack propagation was demonstrated by the more rougher fracture pathways of the treated and Si 3 N ₄ -modified composites.

Figure 9: Representative photographs of K IC test specimens after failure for Si ₃ N ₄ -modified H/B F-Ep hybrid nanocomposites.

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