Orthotropic mechanical properties of PLA materials fabricated by fused deposition modeling.

• FDM printed PLA material exhibits orthotropic anisotropy in stiffness and strength. • Anisotropy introduced by interlayer bonds is more pronounced. • A -0.05 mm air gap reduced voids and improved mechanical performance. • A -0.05 mm air gap cannot eliminate mechanical anisotropy. • Strength of FDM printed PLA was accurately predicted by Hill48 yield criterion. Polylactic Acid (PLA) sample manufactured by fused deposition modeling (FDM) technique is typically assumed to be transversely isotropic without a thorough examination of its orthotropy in three-dimensional space. This study investigated the mechanical orthotropy of PLA samples with two air gap levels (0 mm and -0.05 mm) and various loading directions. Tensile strength and elastic modulus were experimentally measured and the Hill48 yield model and orthotropic elastic model were calibrated. Results revealed that the anisotropy induced by interlayer bond was more pronounced than that within layer. Introducing a -0.05 mm air gap remarkably reduced voids in printed samples, enhancing the stiffness and strength of tensile samples. It also delayed the transition of fracture modes in intra-layer samples as the filament angle increased from 0° to 90°, which shifted from filament breakage to combined fracture mode and subsequently to interface failure. Despite these improvements, the inherent anisotropy of FDM printed PLA materials remained due to the oriented molecular chains and insufficient chain diffusion. The study emphasizes the importance of orthotropic mechanical models, demonstrating their reliability through calibration with acceptable accuracy. [ABSTRACT FROM AUTHOR]