Toughness by design: multi-scale interpenetrating lattices with size-enhanced fracture resistance

Architecture has long been used as a tool to enhance material toughness, but despite the extensive investigations into toughening mechanisms, the role of feature size remains poorly understood. In this work, we demonstrate the powerful connection between size and toughness by combining fracture experiments on macro-and nanolattices with an analytical framework that integrates fracture size-effects. We fabricate rhombic dodecahedron, octet-truss, and interpenetrating macrolattices from brittle (Vero) and less-brittle (Avero) polymer, along with nanolattices made using a single polymer (IP-Dip) processed to be brittle or ductile. Nearly all lattices failed in a ductile manner, even when made from nominally 'brittle' constituents. Notably, lattices undergoing this brittle-to-ductile transition achieved a substantial 3-6× higher work of fracture than their base materials despite having only 25% relative density. Our analytic framework reveals two key mechanisms for this enhancement: (1) size-affected brittle-to-ductile transitions at the material scale amplify lattice plastic energy dissipation, and (2) interpenetrating architectures enhance the size and thereby energy dissipation of the damage zone in brittle-strut lattices. This framework establishes a generalized method to design resilient metamaterials from virtually any brittle or ductile constituent.