  A force balance condition to predict quasistatic crack paths in anisotropic
brittle materials is derived from an analysis of diffuse interface continuum
models that describe both short-scale failure inside a microscopic process zone
and macroscopic linear elasticity. The derivation exploits the gradient
dynamics and translation symmetry properties of this class of models to define
a generalized energy-momentum tensor whose integral around an arbitrary closed
path enclosing the crack tip yields all forces acting on this tip, including
Eshelby's configurational forces, cohesive forces, and dissipative forces. This
condition is validated quantitatively by numerical simulations.
