  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.
