  The variational principle of safe equilibrium for inhomogeneous elastic
cracked bodies is formulated. Using the standard calculus of variations, we
show that the crack remains in safe equilibrium as long as the maximum energy
reduction rate of the virtually growing crack is negative. The crack starts to
grow in the direction of the maximum energy reduction rate when the latter
becomes zero. This energetic criterion implies the criteria proposed by He and
Hutchinson (1989). As an application we use this criterion to predict the
growth direction of an interface crack in a bimaterial.
