Alumina–Silica Hybrid Reinforcement in Jute/Epoxy Composites: Mechanical Characterization and Modified Rule of Mixtures for Strength–Toughness–Hardness Optimization

ABSTRACT Natural fiber composites offer sustainable alternatives to synthetic materials but need performance enhancement for structural applications. This study investigates the mechanical behavior of jute fiber–reinforced epoxy composites modified with alumina–silica hybrid fillers at varying compositions and loadings. Sixteen formulations containing 0–15 wt% filler with different alumina‐to‐silica ratios were fabricated using the hand layup technique with alkali‐treated jute fibers, followed by thermal post‐curing at 80°C. Mechanical characterization revealed complex interactions between filler composition and performance. Optimal tensile strength improvement of 20.8% was achieved with 2% alumina and 3% silica, while maximum flexural strength enhancement of 48.6% occurred at 1% alumina and 4% silica. Impact resistance similarly peaked at this lower alumina–higher silica combination, showing 27.9% improvement. In contrast, stiffness and hardness increased with both filler content and alumina proportion, with tensile modulus improving 38.8% and hardness 21.7% at 15% filler loading dominated by alumina. A modified Rule of Mixtures analysis with reinforcement efficiency coefficients quantified hybrid effects, revealing that 5% total filler maximized synergistic interactions. Higher concentrations showed diminishing returns, indicating agglomeration and disrupted matrix continuity. These findings are expected to guide the optimization of filler compositions to balance strength, toughness, and stiffness in sustainable composites.