Topological mechanical metamaterial for robust and ductile one-way fracturing

Fracturing is unavoidable and threatens the reliability and functionality of materials. Therefore, regulating the propagation of cracks in a predictable and ductile manner is of paramount importance. Herein, we exploit the properties of topological mechanical metamaterials (TMMs) as a versatile mechanism to guide cracks unidirectionally and turn fracturing of lattices made of brittle materials into ductile events. Inspired by quantum topological states, recent discoveries of TMMs have uncovered varieties of unconventional mechanical phenomena, ranging from one-way wave propagation to polar elasticity. We show that polarized floppy modes occurring in TMMs lead to strongly asymmetric stress fields localizing around the notch tips, leading to ductile one-way fracturing, in sharp contrast to classical theories of fractures in brittle materials. Our work demonstrates the universality of this fracture unidirectionality feature, which is protected by TMM's bulk topology, and provides robust solutions in programmable fracturing for a broad class of materials and structures.