Abstract
We are simulating the dynamical failure of three-dimensional notched solids under tension using molecular dynamics, simple interatomic force laws and system sizes from 10 to more than 100 millions atoms. The rare-gas solid will cleave brittlely at failure under mode I loading in the ⟨110⟩ direction. With sufficient loading, the crack velocity can reach a speed where a transition from brittle failure to ductile failure occurs. We show that this critical speed is independent of the loading history and is approximately 0.36 of the Rayleigh sound speed, consistent with our suggestion that this transition is initiated by an intrinsic instability in the crack dynamics.