This study investigates the enhancement of mechanical, fatigue, creep, and flame-retardant properties of epoxy composites reinforced with pistachio shell–derived lignin and thermally aged, surface-treated Sunn hemp fibers. The composites were fabricated with 40% fiber and varying lignin contents (3–5 vol.%) and subjected to thermal ageing to improve fiber–matrix interfacial bonding and matrix cross-linking. Results show that the hybrid reinforcement significantly improves mechanical performance, with tensile and flexural strengths reaching 167 MPa and 167 MPa, respectively, and impact energy of 4.7 J for the ESL1 composite. Higher lignin content (ESL2) further enhanced hardness (Shore D 99) and reduced creep deformation, demonstrating improved dimensional stability under prolonged stress. The ES composite exhibited the lowest flame propagation rate of 6.14 mm/min, indicating enhanced flame-retardant behavior. These findings demonstrate that the combination of pistachio lignin and thermally aged fibers provides a sustainable approach to producing high-performance epoxy composites with improved structural stability and fatigue resistance, suitable for advanced structural and automotive applications.
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