  The pattern development of multiple cracks in extremely anisotropic solids
such as bilayer or multilayer two-dimensional (2D) crystals contains rich
physics, which, however, remains largely unexplored. We studied crack
interaction across neighboring 2D layers by transmission electron microscopy
and molecular dynamics simulations. Parallel and anti-parallel ('En-Passant')
cracks attract and repel each other in bilayer 2D crystals, respectively, in
stark contrast to the behaviors of co-planar cracks. We show that the misfit
between in-plane displacement fields around the crack tips results in
non-uniform interlayer shear, which modifies the crack driving forces by
creating an antisymmetric component of the stress intensity factor. The
cross-layer interaction between cracks directly leads to material toughening,
the strength of which increases with the shear stiffness and decreases with the
crack spacings. Backed by the experimental findings and simulation results, a
theory that marries the theory of linear elastic fracture mechanics and the
shear-lag model is presented, which guides the unconventional approach to
engineer fracture patterns and enhance material resistance to cracking.
