FAST, FASTER, AND FASTEST CRACKS

Cracks pass through solids with a wide range of velocities; from 0 to about 10(3) m s(-1). Within this range at least five critical velociti es can be defined. ms Approximate values for them are as follows: 0 m s(-1), first satisfaction of the Griffith condition; 10 m s(-1), dislo cation emission from the tip stops; 10(2) m s(-1), crack tips begin to flutter; 10(3) m s(-1), bifurcation of crack lips begins; maximum met res per second, maximum velocity at high applied stresses. In principl e, 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 configura tions at moving crack tips are nearly invariant because they must sati sfy critical conditions. The results are in good agreement with experi mental observations.