Quantitative assessment of alkali and carbon nanotube reinforcement effects on the tensile reliability of sustainable sisal fiber bio-based epoxy composites

The present study investigates a two-stage reinforcement strategy to enhance the tensile performance and reliability of sisal fiber–reinforced bio-based epoxy composites, aligning material development with sustainability-driven design principles. In the first stage, sisal fiber mats were treated with 4 wt% and 5 wt% NaOH to improve fiber–matrix interfacial efficiency, while in the second stage, multi-walled carbon nanotubes (MWCNTs) were incorporated into the epoxy matrix at low weight fractions of 0.15, 0.25, and 0.35 wt% using a combined mechanical stirring and ultrasonication approach. Tensile testing conducted in accordance with ASTM D3039 revealed a systematic increase in ultimate tensile strength (UTS) from 71.24 MPa for untreated composites to 103.32 MPa for 5 wt% NaOH-treated composites, corresponding to an improvement of approximately 45% due to enhanced interfacial bonding. Subsequent CNT modification further improved tensile performance, with an optimum response observed at 0.25 wt% MWCNT, achieving a maximum UTS of 129.36 MPa and an elastic modulus of 8.1 GPa. Regression-based mathematical modelling captured the near-linear strengthening behavior induced by alkali treatment and the non-linear saturation-dominated response associated with CNT addition, with model predictions remaining within experimental scatter. Statistical reliability assessment using Weibull analysis demonstrated reduced strength variability for alkali-treated and optimally CNT-modified composites. Fracture surface analysis using scanning electron microscopy revealed a clear transition from interfacial debonding and fiber pull-out to cohesive fracture, crack bridging, and crack deflection mechanisms at optimized reinforcement levels. This study quantifies the combined effect of alkali treatment and low-loading CNTs on sisal bio-epoxy tensile behavior, achieving ~ 82% strength improvement with an optimum at 0.25 wt% CNT, while enhancing stiffness and maintaining controlled variability within the tested range. By integrating renewable natural fibers, low nanofiller content, and data-driven modelling, this study contributes to sustainable materials innovation (SDG 9), responsible material utilization (SDG 12), and reduced environmental impact through lightweight composite design (SDG 13).