Experimental study on mechanical performance and microstructural characterization of optimized sisal fiber reinforced polyester composites

This study investigates the mechanical, dynamic, and thermal behavior of sisal fiber–reinforced polyester composites. Specimens were fabricated using manual hand layup with randomly distributed 20 mm sisal fibers at weight fractions of 5%, 10%, 15%, and 20%. Mechanical tests (tensile, flexural, impact, double shear, and compressive), free vibration analysis, damping evaluation, thermal characterization, and SEM imaging were performed. Results show that increasing fiber content enhanced most mechanical properties, natural frequency, and storage modulus, although peak damping values (tan δ) decreased with higher fiber loading. The highest impact strength (16 J) and compressive strength (52.4 MPa) were obtained at 20% fiber, while the most balanced overall performance was achieved at 15%, yielding maximum tensile strength of 17.44 MPa, flexural strength of 52.65 MPa, and shear strength of 77.97 MPa. Free vibration tests recorded a frequency of 67.38 Hz with a damping ratio of 0.059, and dynamic mechanical analysis reported a storage modulus up to 7.5 GPa. Thermal stability was maintained up to 300 °C, with rapid degradation above 400 °C and heat flux peaks at 600 °C. SEM analysis confirmed that fiber distribution and orientation strongly influenced composite performance. Overall, 15% sisal fiber content was identified as optimal for structural applications requiring balanced strength, dynamic behavior, and thermal stability.