Reassessing anisotropy in 3D printed structures: The role of extrudate geometry vs interface bonding

Mechanical performance of 3D-printed parts produced through fused filament fabrication (FFF) and fused granular fabrication (FGF) is governed by interface bond quality and extrudate geometry. This study investigates these factors in Polyethylene Terephthalate Glycol (PETG) parts printed at small (FFF) and large (FGF) scales, subjected to tensile testing along extrudate and transverse directions. Anisotropy in 3D-printed parts arises from incomplete interface bonding and formation of periodic ridges (interface notches) between extrudates. A machining method is used to remove the notches, enabling independent evaluation of these mechanisms. Results show that interface notches reduce part strength, with tensile strength decreasing by 28 % in small-scale and 70 % in large-scale samples. Digital image correlation (DIC) quantifies strain fields induced by the notches during loading, showing that the strain concentration factor (K_ε) decreases with smaller layer heights. Mesoscale finite element analysis (FEA), validated by DIC, confirms the experimental findings and highlights the critical influence of notch root radius in K_ε. Once interface notches are eliminated, isotropic properties are achieved, demonstrating full bond strength. These findings emphasize the primary role of extrudate geometry in anisotropy and premature failure, guiding optimization of printing parameters and the design of next-generation 3D printing equipment aimed at mitigating these issues.