Fracture Behavior of Natural Fiber‐Reinforced PLA/TPU Hybrid Composites Produced by Material Extrusion Additive Manufacturing

ABSTRACT Natural fibers provide sustainable, lightweight reinforcement for polymer composites, offering biodegradability and attractive mechanical performance. This study examines the fracture behavior of FDM 3D‐printed polylactic acid (PLA)/thermoplastic polyurethane (TPU) composites reinforced with short flax fibers. Composite filaments were fabricated using a customized single‐screw extruder with compounding capability at flax loadings of 4, 8, and 12 wt.%. To improve fiber dispersion and interfacial bonding, flax fibers were surface‐modified via alkali and silane treatments. Fracture performance was quantified using single‐edge notched bending (SENB) tests, reporting conditional fracture toughness and conditional strain energy release rate. PLA reinforced with 8 wt.% silane‐treated flax exhibited the highest values, with both parameters increasing by approximately 60% compared with neat PLA. Interfacial assessment indicated improved fiber‐matrix adhesion, as evidenced by approximately 26% fewer fiber pull‐outs for silane‐treated fibers relative to untreated fibers. The influence of TPU addition (8 and 16 wt.%) to PLA and PLA/flax systems was also evaluated. Incorporating TPU into the PLA matrix increased the conditional strain energy release rate but reduced the conditional fracture toughness compared with neat PLA. These results underscore the critical role of fiber surface modification and natural‐fiber reinforcement in tailoring the fracture response of 3D‐printable PLA‐based composites.