Comparative study of mechanical behaviors of additively manufactured continuous fiber-reinforced thermoplastic composites undergoing flexural and mode I fracture

Fiber-reinforced plastic (FRP) composite structures produced via additive manufacturing (AM) offer excellent specific strength for applications such as aeronautics and automobiles. However, the AM-made FRP composites have experienced limited deployment as their structural integrity can be influenced by printing parameters and process-induced defects. These defects can cause nonlinear mechanical responses when under bending-type deformation, causing premature failure due to accumulated microfracture. In this study, flexural and mode I fracture responses of 3D-printed fiber-reinforced plastic (FRP) composites were analyzed using three-point bending (3PB) and double cantilever beam (DCB) tests on samples reinforced with continuous carbon fiber (CCF) and continuous glass fiber (HSHTG) samples. The highest flexural modulus and strength are reported as 29.5 GPa and 457.1 MPa for CCF samples and 12.7 GPa and 268.8 MPa for HSHTG samples. The mode I fracture toughness (G IC ) of the 3D-printed composites reinforced with continuous carbon and glass fiber were characterized with samples of varying laminae sequences. The crack initiation G IC s for CCF and HSHTG samples are reported as 1015 and 1100 J/m 2 . It was observed that the crack propagation G IC fluctuated throughout the crack propagation processes, and the damping induced by nonuniform interfacial discontinuities was evident. G IC s were also observed to vary with the laminae sequences. Fracture responses in 3PB and DCB tests were compared, and failure modes associated with reinforcing strategies were identified using microscopic and fracture analysis. It is concluded that the combined effects of complex flexural loading and the AM-specific characteristic anisotropy result in self-arrested crack propagation and fluctuating G IC . The results of this study give insights into determining key processing parameters in laminated FRP structures made of layer-by-layer AM processes, considering the effects of their characteristic geometrical discontinuities on the respective failure mechanisms.