P. N. B. Reis, L. Távara, M. Muñoz‐Reja, M. T. Aranda, F. Nur Ainin, M. D. Azaman, Luís Ferreira
ABSTRACT To address plastic waste and agricultural residue underutilization, this study evaluates the thermal (DSC and FTIR), microstructural (SEM), and mechanical behavior of polylactic acid (PLA) reinforced with flax fibers and olive pit particles, processed by fused filament fabrication (FFF). The results show the FFF reduces neat PLA's tensile strength by 10.8% and its tensile modulus by 31.6%, primarily due to process‐induced voids, poor interlayer bonding, and polymer degradation that affect load transfer. The incorporation of flax fibers and olive pit particles leads to a further decrease in mechanical performance, reducing tensile and bending strength by up to 37% and 39.6%, respectively. In flax fiber composites, this is mainly caused by fiber degradation, poor interfacial adhesion, and random dispersion, which limit load transfer efficiency. Similarly, while olive pit particles exhibit greater thermal stability, their hydrophobic nature restricts adhesion to the PLA matrix, resulting in insufficient stress transfer. Moreover, the composite parts present complex failure mechanisms, including fiber pull‐out and crack deflection. In conclusion, the natural reinforcements reduce the PLA's mechanical performance and modify its fracture behavior. In its current form, they show potential for sustainable composites in non‐structural applications where aesthetics and environmental factors are important. To expand their use into structural applications, future work should focus on improving filler‐matrix compatibility, optimizing particle size, fiber content, and processing conditions.