Programmable fracture toughness in architected mechanical metamaterials

• A metamaterial with tunable orthogonal stiffness ratio was designed. • Fracture tests were performed on pre-cracked metamaterials. • Fracture toughness was evaluated based on the concept of discrete fracture mechanics. • The fracture toughness can be programmed by tuning the stiffness ratio. Architected mechanical metamaterials—engineered materials whose mechanical behavior primarily arises from the structure—have recently enabled on-demand tuning of stiffness, Poisson’s ratio, and mechanical wave propagation properties. However, the programmable control of fracture toughness—a key material property governing fracture resistance—has been investigated, but its potential has yet to be fully explored. In particular, no existing metamaterial has achieved this control without altering its structural topology and deformation stiffness along the loading direction. Here, we present a two-dimensional spring-lattice metamaterial that enables programming of fracture toughness decoupling it from deformation stiffness through orthogonal stiffness modulation. By varying the stiffness ratio between vertical and lateral springs ( α ), the load transfer near the crack tip was independently tuned while maintaining the vertical deformation stiffness identical. Pre-cracked specimens with different α values were subjected to tensile test experiments and finite-element analysis. Despite identical deformation stiffness, the energy release rate at crack propagation (fracture toughness) considerably varied with α across specimens. These results demonstrated that fracture toughness could be programmed purely via structural stiffness design. Furthermore, this concept was shown to be extendable to three-dimensional architectures. This finding introduces a new design axis in architected materials and suggests a route toward mechanically robust, tunable systems for structural reliability.