Mixed-mode fracture performance of 3D-printed carbon fiber onyx composites

Purpose This study aims to examine the fracture characteristics of composites produced using a MarkForged Mark Two 3D printer, utilizing Onyx filament, reinforced with different layers of continuous carbon fiber using a concentric fiber filling type. The purpose of this investigation was to gain a deeper understanding of how the addition of carbon fiber layers in Onyx can enhance its fracture response and potentially improve additive manufacturing applications. Design/methodology/approach Four different types of samples were created: Onyx without any reinforcement and Onyx with 2, 4 and 6 layers of carbon fiber, respectively. All samples had the same dimensions and the fracture behavior of these composites was assessed experimentally and numerically using a specialized loading fixture with a pair of grips. The tests were conducted by varying the loading angle, α, 0° 45° and 90° and the results were compared to evaluate the impact of carbon fiber reinforcement on the fracture strength and toughness of the Onyx composite material. Findings The results indicated that when subjected to loads at 0°, 45° and 90° orientations, double-layer carbon fiber-reinforced Onyx demonstrated a higher total strain energy release rate (measured at 2.114, 2.152 and 0.882 J/m², respectively) compared to its unreinforced counterpart (which reported 0.913, 0.963 and 0.344 J/m2 values). This increase confirms directly to enhanced fracture resistance in the reinforced samples. Furthermore, specimens utilizing a six-layer carbon fiber configuration demonstrated superior fracture toughness (7.682, 5.259 and 5.121 J/m² across the 0°, 45° and 90° angles) when compared to the four-layer composites (4.684, 4.568 and 3.022 J/m²), reflecting a corresponding increase in the fracture resistance for the six-layer Onyx composites. Originality/value The findings of this study can help researchers in this field to develop and propose a standard test method to determine the failure behavior, because there is no standard fracture test method for these materials. The fracture toughness properties under tensile, shear and mixed tensile-shear modes of 3D printed materials, especially the effect of various types of carbon fiber reinforcement, were investigated. As the main innovation, this paper used a specially designed combined-mode fracture device coupled with finite element analysis to investigate the effect of carbon fiber reinforcement on the failure behavior of Onyx 3D-printed composites.