Bio-inspired multi-hierarchical structure for toughness enhancement in fused filament fabrication

• A bio-inspired multi-hierarchical structure was fabricated through Fused Filament Fabrication process. • The combination of core-shell structure and Bouligand strucuture turns brittle material into ductile failure. • Theoretical analysis and numerical simulation provide reliable estimates of effective elastic modulus. • The design principle has significant potential for application in composite material design. Fused Filament Fabrication (FFF) is a widely used material extrusion-based additive manufacturing (MEX) technique in commercial and industrial sectors. However, the FFF-finished parts are typically either stiff and brittle, or tough but soft, limiting their application in various circumstances. This study introduces a bio-inspired, multi-hierarchical structure combining a dual-phase core–shell structure, inspired by biomaterial silk, and a Bouligand structure, inspired by crustacean shells, organized across two hierarchical levels. Experimental results indicate that this two-level architecture turns the brittle material into ductile material, achieving a remarkable 3.8-fold increase in toughness, with a 37.3 % reduction in stiffness, while retaining 70 % stiff phase by volume. The silk-inspired core–shell structure enhances toughness along the material orientation, while the Bouligand architecture enhances in-plane damage tolerance. This synergy results in a prolonged failure process, providing early warning before catastrophic failure and improving energy dissipation. Theoretical analysis and numerical simulation provide good predictions of effective elastic modulus. Furthermore, the influence of pitch angle on failure mode is discussed. The findings of this study illustrate the potential of expanding mechanical properties in FFF applications through a cost-effective approach, while also offering design principles applicable to other material structure design methodologies for modern material development.