  We argue that nucleation of brittle cracks in initially flawless soft elastic
solids is preceded by a nonlinear elastic instability, which cannot be captured
without accounting for geometrical precise description of finite elastic
deformation. As a prototypical problem we consider a homogeneous elastic body
subjected to tension and assume that it is weakened by the presence of a free
surface which then serves as a site of crack nucleation. We show that in this
maximally simplified setting, brittle fracture emerges from a symmetry breaking
elastic instability activated by softening and involving large elastic
rotations. The implied bifurcation of the homogeneous elastic equilibrium is
highly unconventional for nonlinear elasticity as it exhibits an extraordinary
sensitivity to geometry, reminiscent of the transition to turbulence in fluids.
We trace the post-bifurcational development of this instability beyond the
limits of applicability of scale free continuum elasticity and use a
phase-field approach to capture the scale dependent sub-continuum strain
localization, signaling the formation of actual cracks.
