Deformation mechanism of FFF printed materials — inspiration for new printing strategies

Fused Filament Fabrication (FFF) offers a cost-effective and versatile method for fabricating complex geometries; however, its layer-by-layer deposition process inherently introduces microscopic voids, which degrade the mechanical integrity of printed parts. This study investigates the link between macroscopic anisotropic behavior and microscopic deformation mechanisms in FFF-printed specimens produced using two representative printing strategies: unidirectional and interlaced. Standardized specimens were subjected to off-axial tensile loading (i.e., the off-axial angle refers the angle between the deposited filaments and the loading axis in tensile tests), and their responses were analyzed through fracture morphology examination and digital image correlation (DIC) to capture both global and localized deformation modes. Results reveal that in unidirectionally printed specimens, the dominant failure mechanism transitions from breakage of deposited filaments to interfacial debonding as the off-axial angle increases. This shift correlates with notable reductions in stiffness, yield stress, and fracture strain, underscoring the strong anisotropy of this strategy. Conversely, specimens fabricated using an interlaced pattern consistently exhibited a combination of rupture of deposited filaments and interfacial debonding across all loading orientations. This led to more uniform mechanical performance, as the interplay of failure modes mitigated direction-dependent behavior. To further enhance interfacial bonding and mechanical resilience, a novel S-shaped interlaced printing strategy was proposed. Experimental validation showed substantial improvements in elastic modulus and yield strength, demonstrating the strategy's potential to reduce stress concentrations and improve structural reliability.