  The purpose of this paper is to fill the gap between the classical treatment
of brittle fracture mechanics and the new idea of considering the crack
evolution as a free discontinuity problem. Griffith and Irwin criterions of
crack propagation are studied and transformed in order to be no longer
dependent on any prescription of the geometry of the crack during its
evolution. The inequality contained in theorem 6.1. represents the link between
generalized Irwin and Griffith criterions of brittle crack propagation. The
physical meaning of this inequality is explained in the last section.
  Further information available at http://irmi.epfl.ch/cag/buliga_bfrac.html .
