  Unstable growth of cracks (rough crack surface and crack branching) in
dynamic fracture has long been observed in various materials. Until now, there
was no universally agreed upon explanation for these instabilities. Here, we
demonstrate that: 1) Due to the non-uniform stress distribution in the cracked
body and the expansion of high stress region as the crack velocity increases, a
force-bearing cracked body can be treated as a temporary layer-like material
(TLLM) with a "brittle layer" that the area of its automatically increases. 2)
For this TLLM, it takes only one crack for the stress in its "brittle layer" to
fall below a certain value at small velocities. Coupled with the asymmetry of
the whole system, this results in a rough crack surface. 3) The "brittle layer"
is large enough when the crack velocity reaches a certain value. Two cracks
must be formed, or else the stress cannot be reduced below the certain value,
resulting in crack branching. Crack propagation can be compared to cars
traveling on a road: the high stress region in the cracked body provides a
"road" to let cracks "pass ."
