  Using the global fiber bundle model as a tractable scheme of progressive
fracture in heterogeneous materials, we define the branching ratio in
avalanches as a suitable order parameter to clarify the order of the phase
transition occurring at the collapse of the system. The model is analyzed using
a probabilistic approach suited to smooth fluctuations. The branching ratio
shows a behavior analogous to the magnetization in known magnetic systems with
2nd-order phase transitions. We obtain a universal critical exponent
$\beta\approx 0.5$ independent of the probability distribution used to assign
the strengths of individual fibers.
