  We present a unified theory of fracture in disordered brittle media that
reconciles apparently conflicting results reported in the literature. Our
renormalization group based approach yields a phase diagram in which the
percolation fixed point, expected for infinite disorder, is unstable for finite
disorder and flows to a zero-disorder nucleation-type fixed point, thus showing
that fracture has mixed first order and continuous character. In a region of
intermediate disorder and finite system sizes, we predict a crossover with
mean-field avalanche scaling. We discuss intriguing connections to other
phenomena where critical scaling is only observed in finite size systems and
disappears in the thermodynamic limit.
