Characterization of the mixed-mode interlaminar fracture toughness of an additive manufacturing continuous carbon fiber reinforced-thermoplastic composite

There is a lack of knowledge concerning the interlaminar fracture toughness under mixed-mode ratios of 3D-printed composites. In this work, several additive manufacturing (AM) continuous Fiber Reinforced Thermoplastic (cFRT) specimens have been tested to characterize the initiation and propagation of interlaminar fracture toughness under three different mixed-mode G_II/(G_I + G_II) ratios: 25, 50, and 75%. The results obtained do not exhibit the common tendency seen in traditional laminated composite materials, in which the fracture toughness increases with the mixed-mode ratio. While the fracture toughness for the 50% mixed-mode ratio falls between the corresponding mode I and mode II values, the fracture toughness for the 25% and 75% ratios falls outside this range. To provide a reasonable explanation, fractography and microstructure analyses were conducted to quantify fiber, matrix, and void contents. It was concluded that this uncommon behavior is probably related to the intrinsic variability of the material and manufacturing process. Highlights: Continuous fibers improve mechanical properties of 3D-printed composite parts. Delamination is a critical failure mode for laminated composite materials. Characterization of mixed-mode delamination is key for simulation and design. First complete study of mixed-mode delamination for 3D-printed composites.