Topological mechanical metamaterial for robust and ductile one-way fracturing

Fracturing is an unavoidable fact that threatens the reliability and functionality of various materials. Therefore, being able to regulate the propagation of cracks in a predictable and ductile manner is of paramount importance. In this study, we exploit the properties of topological mechanical metamaterials (TMMs) to realize a versatile mechanism to guide cracks in a unidirectional and sequential way and turn fracturing of lattices made of brittle materials into ductile events. Inspired by quantum topological states, recent discoveries of TMMs have uncovered a variety of unconventional mechanical phenomena, ranging from one-way wave propagation to polar elasticity. Here, we go beyond conventional linear theories of TMMs to reveal remarkable potentials of using topological mechanics to realize robust control of fracturing. We show that polarized floppy modes occurring in TMMs lead to strongly asymmetric stress fields localizing around the crack tips, giving rise to ductile one-way fracturing, in sharp contrast to classical theories of fractures in brittle materials. Using theories, simulations and experiments conducted on a variety of lattice configurations, we demonstrate the universality of this fracture unidirectionality feature, which we ascribe to the TMM's bulk topology, providing robust solutions in fracture control in engineering.