Programming fracture resistance in metamaterials via elastic instabilities

The design of fracture-resistant materials has long been hindered by the complexity of toughening mechanisms across multiple length scales^1,2. Mechanical metamaterials offer a promising platform to address this challenge, yet existing research has largely focused on passively characterizing fracture in conventional lattice architectures^3-8. Recent studies have demonstrated the potential of elastic instabilities to enhance functionalities in architected materials^9-18; however, their connection to fracture resistance remains unexplored. Here we demonstrate that fracture behaviours in mechanical metamaterials can be actively programmed by exploiting elastic instabilities, thereby bridging the two traditionally disconnected failure modes. Through a combination of experiments and simulations, we show that controlled manipulation of the inelastic zone size in pseudoplastic metamaterials enables a transition from intrinsic to extrinsic fracture behaviour, accompanied by up to a one-order-of-magnitude increase in fracture energy. This work represents a shift from passive observation to active control of fracture mechanics, establishing a new framework for designing metamaterials with tailored fracture resistance. Our findings not only advance the fundamental understanding of instability-fracture interactions in metamaterials but also suggest a broadly applicable route for programming fracture behaviours through instability design.