Fracture of 3D-printed polymers: Crucial role of filament-scale geometric features

Is mechanical anisotropy in extrusion-based 3D-printed parts caused by weak inter-filament bonding, as is widely accepted? This study demonstrates that filament-scale geometric features may be a more important factor than filament bonding. Specially designed 3D-printed compact tension specimens were tested normal to, and along, the direction of extruded filaments. Higher strength and toughness were found in the filament direction. These differences disappeared when small grooves, comparable to micro-features, were introduced in specimens tested along the former direction to replicate grooves that naturally occur between filaments/layers. Mechanical testing and fractography demonstrate that filament-scale geometric stress raisers are critically important and cause anisotropy in 3D-printed materials.