  In their letter, Andersen, Sornette, and Leung [Phys. Rev. Lett. 78, 2140
(1997)] describe possible behaviors for rupture in disordered media, based on
the mean field-like democratic fiber bundle model. In this model, fibers are
pulled with a force which is distributed uniformly. A fiber breaks if the
stress on it exceeds a threshold chosen from a probability distribution, and
the force is then redistributed over the intact fibers. Andersen et al. claim
the existence of a tricritical point, separating a "first-order" regime,
characterized by a sudden global failure, from a "second-order" regime,
characterized by a divergence in the breaking rate. We show that a first-order
transition is an artifact of a (large enough) discontinuity put by hand in the
disorder distribution. Thus, in generic physical cases, a first-order regime is
not present. This result is obtained from a graphical method, which, unlike
Andersen at al.'s analytical solution, enables us to distinguish the various
classes of qualitatively different behaviors of the model.
