Influence of Short Carbon and Glass Fibers on the Mechanical Performance, Thermal Stability, and Fracture Behavior of FDM-Printed Composites

Fused Deposition Modeling (FDM) is a widely used additive manufacturing technique due to its efficiency in producing complex structures at low cost with minimal material waste. However, weak interlayer adhesion and void formation limit the mechanical performance of printed components. This study evaluates the effect of short fiber reinforcement (carbon and glass) and printing orientation (0°/90° and 45°/-45°) on the mechanical properties, fracture resistance, and thermal stability of FDM-printed composites. PLA, PLA-CF, and PLA-GF, each reinforced with 20 wt% short carbon or glass fibers, were analyzed. Tensile, flexural, and fracture toughness (K _IC , G _IC , P _Q ) tests were performed according to ASTM standards, along with thermogravimetric analysis (TGA), porosity measurements, and fracture surface characterization using scanning electron microscopy (SEM). PLA exhibited the highest tensile strength in a 45°/-45° orientation (50.83 MPa), while PLA-GF in 0°/90° showed the highest flexural strength (17.79 MPa). Fracture resistance followed a similar trend, with PLA-GF achieving the highest K _IC (4.71 MPa·m¹/²) and P _Q (1186.66 N), while PLA exhibited the highest G _IC (10.31 kJ/m²). Fractographic analysis revealed fiber pull-out and interfacial debonding, indicating differences in fiber-matrix adhesion. TGA confirmed a higher thermal stability for PLA-CF, while porosity analysis demonstrated a direct correlation between void content and mechanical performance. These findings emphasize the role of fiber type, printing orientation, and microstructural integrity in the mechanical behavior of FDM-printed composites, particularly for applications requiring enhanced fracture resistance.