  The growth dynamics of a single crack in a heterogeneous material under
subcritical loading is an intermittent process; and many features of this
dynamics have been shown to agree with simple models of thermally activated
rupture. In order to better understand the role of material heterogeneities in
this process, we study the subcritical propagation of a crack in a sheet of
paper in the presence of a distribution of small defects such as holes. The
experimental data obtained for two different distributions of holes are
discussed in the light of models that predict the slowing down of crack growth
when the disorder in the material is increased; however, in contradiction with
these theoretical predictions, the experiments result in longer lasting cracks
in a more ordered scenario. We argue that this effect is specific to
subcritical crack dynamics and that the weakest zones between holes at close
distance to each other are responsible both for the acceleration of the crack
dynamics and the slightly different roughness of the crack path.
