This study investigates mixed-mode fracture of fused deposition modeled (FDM) polycarbonate (PC) reinforced with low carbon nanofiber (CNF) contents. PC/CNF filaments containing 0, 0.05, 0.08, 0.10 and 0.20 wt% CNFs were prepared by melt blending and filament extrusion and used to fabricate ASTM D638 dog-bone tensile specimens and centrally slotted U-notched plates on a high-temperature CoreXY FDM printer. The U-notched specimens were tested in tension at three slot orientations, \(\theta={0}^{\circ}\) , \({20}^{\circ}\) and \({40}^{\circ}\) (measured from the axis perpendicular to the loading direction), providing opening and mixed-mode I/II loading conditions. CNF incorporation was verified by FTIR, Raman spectroscopy and thermogravimetric analysis (TGA), while fracture mechanisms were examined using SEM fractography and DIC-based stress mapping ahead of the slot root. Tensile tests showed modest stiffness gains, with the elastic modulus increasing from 1136 MPa for neat PC to 1315 MPa at 0.05 wt% CNF, accompanied by only small changes in strength and ductility. For the notched specimens, a global energy-based approach was used to determine the elastic ( \({G}_{el}\) ), plastic/non-linear ( \({G}_{pl}\) ) and total fracture energy ( \({G}_{tot}\) ), and the corresponding energy-based toughness ( \({K}_{c,tot}\) ). Plastic work dominated the fracture process, typically accounting for ~ 70–94% of \({G}_{tot}\) , confirming the elastic–plastic nature of fracture in these FDM structures. For neat PC, \({K}_{c,tot}\) increased from 9.16 MPa \(\sqrt{\text{m}}\) at \(\theta={0}^{\circ}\) to 11.87 MPa \(\sqrt{\text{m}}\) at \(\theta={40}^{\circ}\) . A pronounced synergistic optimum occurred at 0.05 wt% CNF, where \({K}_{c,tot}\) rose from 11.35 MPa \(\sqrt{\text{m}}\) ( \(\theta={0}^{\circ}\) ) to 20.57 MPa \(\sqrt{\text{m}}\) ( \(\theta={40}^{\circ}\) ) with \({G}_{tot}=321.7\) N/mm, whereas higher CNF contents (≥ 0.10 wt%) provided limited or reduced gains. Fractography revealed extensive fibrillation, voiding and fiber pull-out at 0.05 wt% CNF, while 0.10 wt% specimens showed features consistent with CNF agglomeration and more localized damage.
