Fast, faster, and fastest cracks

Abstract Cracks pass through solids with a wide range of velocities; from 0 to about - 1 103 m s . Within this range at least five critical velocities can be defined. - 1 Approximate values for them are as follows: 0 m s, first satisfaction of the - 1 2- 1 Griffith condition; 10 m s, dislocation emission from the tip stops; 10 m s, - 1 crack tips begin to flutter; 103 m s, bifurcation of crack tips begins; maximum metres per second, maximum velocity at high applied stresses. In principle, these critical velocities can be determined by solving coupled time-dependent differential equations. However, they can be estimated much more simply by taking advantage of the fact that the atomic configurations at moving crack tips are nearly invariant because they must satisfy critical conditions. The results are in good agreement with experimental observations.