Fracture toughness of additively manufactured carbon fiber reinforced composites

Abstract

The fracture properties (stress intensity factor and energy release rate) of additively manufactured (AM) polylactic acid (PLA) and its short carbon fiber (CF) reinforced composites have been studied. The effects of CF reinforcement, nozzle geometry and bead lay-up orientations in fracture properties, void contents, and interfacial bonding were investigated. The fused filament fabrication (FFF)-based AM specimens using both circular and square shaped nozzle were printed and compared with the conventional compression molded (CM) samples. Compact tension (CT) specimens with different CF concentrations (0 wt.%, 3 wt. %, 5 wt.%, 7 wt.% and 10 wt.%) were printed with two bead lay-up orientations (450/-450 and 00/900) using PLA and CF/PLA composite filaments. The results show significant improvement in fracture toughness and fracture energy for CF/PLA composites in comparison to neat PLA. The fracture toughness was increased by 42% for 00/900 and 38% for 450/-450 bead orientations, respectively with 5% CF loading. The increase in fracture energy was observed to be about 77% for 00/900 and 88% for 450/-450 bead orientations, respectively for the same fiber reinforcement (5 wt. %). Such improvement in fracture properties is expected to be higher for all 900 bead orientations. The samples printed by square-shaped nozzle showed enhanced fracture toughness with less inter-bead voids and larger bonded areas in comparison to the circular-shaped nozzle. Although the fracture toughness showed very negligible differences between 00/900 and 450/-450 specimens, distinguishable variation may be seen in the case of 00 and 900 bead orientations. The crack propagation path and fracture mechanisms were studied using optical microscopy (OM) and scanning electron microscopy (SEM) examinations. Fractography revealed different modes of failure with a very high fiber orientation along the printing direction and a relatively higher void contents for 7 and 10 wt. % fiber reinforcement.