Fracture metamaterials with on-demand crack paths enabled by bending

In many scenarios -- when we bite food or during a crash -- fracture is inevitable. Finding solutions to steer fracture to mitigate its impact or turn it into a purposeful functionality, is therefore crucial. Strategies using composites, changes in chemical composition or crystal orientation, have proven to be very efficient, but the crack path control remains limited and has not been achieved in load-bearing structures. Here, we introduce fracture metamaterials consisting of slender elements whose bending enables large elastic deformation as fracture propagates. This interplay between bending and fracture enables tunable energy dissipation and the design of on-demand crack paths of arbitrary complexity. To this end, we use topology optimisation to create unit cells with anisotropic fracture energy, which we then tile up to realize fracture metamaterials with uniform density that we 3D-print. The thin ligaments that constitute the unit cells confer them a strikingly distinct response in tension and shear, and we show that by controlling the orientation and layout of the unit cells the sequential progress of the crack can be controlled, making the fracture path arbitrarily tortuous. This tortuosity increases the energy dissipation of the metamaterial without changing its stiffness. Using bespoke arrangements of unit cells, metamaterials can have on-demand fracture paths of arbitrary complexity. Our findings bring a new perspective on inelastic deformations in mechanical metamaterials, with potential applications in areas as diverse as the food industry, structural design, and for shock and impact damping.