  We examine the effect of small, spatially localized, excitations applied
periodically in different manners, on the crackling dynamics of a brittle crack
driven slowly in a heterogeneous solid. When properly adjusted, these
excitations are observed to radically modify avalanche statistics and
considerably limit the magnitude of the largest events. Surprisingly, this does
not require information on the front loading state at the time of excitation;
applying it either at a random location or at the most loaded point gives the
same results. Subsequently, we unravel how the excitation amplitude, spatial
extent and frequency govern the effect. We find that the excitation efficiency
is ruled by a single reduced parameter, namely the injected power per unit
front length; the suppression of extreme avalanches is maximum at a
well-defined optimal value of this control parameter. This analysis opens a new
way to control largest events in crackling dynamics. Beyond fracture problems,
it may be relevant for crackling systems described by models of the same
universality class, such as the wetting of heterogeneous substrates or magnetic
walls in amorphous magnets.
