  We study the effect of strong heterogeneities on the fracture of disordered
materials using a fiber bundle model. The bundle is composed of two subsets of
fibers, i.e. a fraction 0<\alpha<1 of fibers is unbreakable, while the
remaining 1-\alpha fraction is characterized by a distribution of breaking
thresholds. Assuming global load sharing, we show analytically that there
exists a critical fraction of the components \alpha_c which separates two
qualitatively different regimes of the system: below \alpha_c the burst size
distribution is a power law with the usual exponent \tau=5/2, while above
\alpha_c the exponent switches to a lower value \tau=9/4 and a cutoff function
occurs with a diverging characteristic size. Analyzing the macroscopic response
of the system we demonstrate that the transition is conditioned to disorder
distributions where the constitutive curve has a single maximum and an
inflexion point defining a novel universality class of breakdown phenomena.
