Interfacial adhesion and failure transition governed by fibre embedded length in 3D-printed continuous carbon fibre–reinforced PETG composites

The mechanical performance of additively manufactured composites is strongly governed by fibre–matrix interfacial adhesion, which can be compromised by process-induced imperfections. This study examines how fibre embedded length controls interfacial adhesion behaviour and failure mechanisms in 3D-printed continuous carbon fibre-reinforced polyethylene terephthalate glycol (PETG) composites, using single-fibre pull-out testing combined with scanning electron microscopy (SEM). Specimens with embedded lengths ranging from 1 mm to 50 mm were tested to capture the transition between fibre slippage and fibre fracture. A critical embedded length ( L c ) of 4.88 ± 0.54 mm was determined, beyond which fibre fracture dominates. The apparent interfacial shear strength (IFSS) was measured as 18.6 ± 1.2 MPa, while the interfacial fracture energy ( G c ) was calculated as 27.3 J/m 2 . A linear traction–separation cohesive law was fitted based on these experimental parameters. SEM imaging reveals distinct microstructural features associated with the two failure modes. These findings establish practical benchmarks for optimising fibre placement, orientation strategies, and load transfer efficiency in 3D-printed composites. The outcomes contribute to the advancement of lightweight composite structures for aerospace and automotive applications.