  We study how the loading rate, specimen geometry and microstructural texture
select the dynamics of a crack moving through an heterogeneous elastic material
in the quasi-static approximation. We find a transition, fully controlled by
two dimensionless variables, between dynamics ruled by continuum fracture
mechanics and crackling dynamics. Selection of the latter by the loading,
microstructure and specimen parameters is formulated in terms of scaling laws
on the power spectrum of crack velocity. This analysis defines the experimental
conditions required to observe crackling in fracture. Beyond failure problems,
the results extend to a variety of situations described by models of the same
universality class, e.g. the dynamics in wetting or of domain walls in
amorphous ferromagnets.
