  We study mode-I fracture in lattices with noisy bonds. In contrast to
previous attempts, by using a small parameter that perturbs the force-law
between the atoms in perfect lattices and using a 3-body force law, simulations
reproduce the qualitative behavior of the beyond steady-state cracks in the
high velocity regime, including reasonable micro-branching. As far as the
physical properties such as the structure factor $g(r)$, the radial or angular
distributions, these lattices share the physical properties of perfect lattices
rather than that of an amorphous material (e.g., the continuous random network
model). A clear transition can be seen between steady-state cracks, where a
single crack propagates in the midline of the sample and the regime of unstable
cracks, where micro-branches start to appear near the main crack, in line with
previous experimental results. This is seen both in a honeycomb lattice and a
fully hexagonal lattice. This model reproduces the main physical features of
propagating cracks in brittle materials, including the behavior of velocity as
a function of driving displacement and the increasing amplitude of oscillations
of the electrical resistance. In addition, preliminary indications of power-law
behavior of the micro-branch shapes can be seen, potentially reproducing one of
the most intriguing experimental results of brittle fracture.
