When a brittle material is loaded to the limit of its strength, it fails
by the nucleation and propagation of a crack1. The conditions for
crack propagation are created by stress concentration in the region
of the crack tip and depend on macroscopic parameters such as the
geometry and dimensions of the specimen2. The way the crack
propagates, however, is entirely determined by atomic-scale phenomena,
because brittle crack tips are atomically sharp and propagate
by breaking the variously oriented interatomic bonds, one at
a time, at each point of the moving crack front1,3. The physical
interplay of multiple length scales makes brittle fracture a complex
‘multi-scale’ phenomenon. Several intermediate scales may arise
in more complex situations, for example in the presence of microdefects
or grain boundaries. The occurrence of various instabilities
in crack propagation at very high speeds is well known1, and significant
advances have been made recently in understanding their
origin4,5. Here we investigate low-speed propagation instabilities
in silicon using quantum-mechanical hybrid, multi-scale modelling
and single-crystal fracture experiments. Our simulations predict
a crack-tip reconstruction that makes low-speed crack propagation
unstable on the (111) cleavage plane, which is conventionally
thought of as the most stable cleavage plane. We performexperiments
in which this instability is observed at a range of
low speeds, using an experimental technique designed for the
investigation of fracture under low tensile loads. Further simulations
explain why, conversely, at moderately high speeds crack
propagation on the (110) cleavage plane becomes unstable and
deflects onto (111) planes, as previously observed experimentally6,7.
