Fracture of 3D-printed micro-tensile specimens: filament-scale geometry-induced anisotropy

The interlayer interface was widely considered as the reason for anisotropic mechanical properties in 3D-printed parts produced by material extrusion additive manufacturing (MEAM). Still, the cause has remained widely debated. Utilising a specially developed micro-tensile specimen formed by single filaments, this study examines the roles of their orientation and filament-scale geometric features on mechanical performance. The specimens were loaded in two directions: (i) longitudinal (F), coinciding with the main axis of extruded filaments, and (ii) transverse (Z), normal to the interface between layers. To replicate the geometrical groove features found at the interlayer interfaces in Z specimens, some of the F specimens were scored manually perpendicular to the load prior to tensile testing to produce similar filament-scale features. Tensile testing of all specimens with microscopic characterisation showed that both F specimens (with and without manual grooves) and Z specimens shared very similar strength characteristics, close to those of bulk polylactide (PLA). Manually grooved F specimens demonstrated significantly reduced plasticity, strain-at-fracture and load-bearing area - very close to the Z specimen's characteristics indicating that the presence of natural grooves in Z specimens is the predominant cause of mechanical anisotropy in MEAM as opposed to commonly assumed poor interlayer molecular diffusion.