Investigation of high temperature compaction on fracture toughness of 3D printed carbon fiber polyamide composites

One flaw associated with fused deposition 3D printing is that the process inherently produces intermittent voids between and inside of deposited layers. Post printing compaction at elevated temperatures of printed parts was previously proposed for reducing the void content within the part and resulted in increased modulus and strength. However, compaction can also lead to embrittlement as the level of crystallinity increases. This work examined the fracture toughness of short and long carbon fiber reinforced polyamide composites using double cantilever beam testing. The critical strain energy release rate behavior was investigated under different compaction temperature levels and for pristine composites. Lower Mode I fracture toughness was found in compacted samples than in unconditioned test samples. The significant variation in Mode I fracture toughness of FDM composites, treated and untreated, was observed and higher values corresponded with the increased intralayer fracture. Both intralayer fractures and interlayer fractures were observed during testing. Interlayer cracks proceeded between two deposited layers of material, while intralayer cracks propagated within a single bead. It was observed that interlayer cracks progressed in a more unstable way than intralayer cracks. Analysis of fracture toughness behavior showed that intralayer failure was more energy-consuming than interlayer fracture due to plasticity of the polymer. Fracture toughness of pristine sample was dominated by nylon plasticity, while increased degree in crystallinity and reduced void content contributed to a more brittle behavior after compaction.