Molecular dynamics of cracks

Brittle objects fail because of cracks. But how and why do they move? The answers to these questions hide down at the atomic scale. Simple analytical models point to numerical simulations of brittle fracture that can be compared directly with laboratory experiments. These simulations do not yet agree with experimental results because the atomic force laws on which the computations rest are not yet known well enough. The author discusses how atomic discreteness affects crack motion, and explains three qualitative phenomena: lattice trapping, a velocity gap, and crack-tip instabilities. The mathematics that originally predicted lattice trapping and the velocity gap is elaborate, but the author has found explanations that rely on simple intuition about atomic motions. He then shows how scaling ideas make it possible to extrapolate with confidence from nanometers to centimeters, and picoseconds to microseconds, so as to compare theory and experiment for brittle fracture