  We sandwich a colloidal gel between two parallel plates and induce a radial
flow by lifting the upper plate at a constant velocity. Two distinct scenarios
result from such a tensile test: ($i$) stable flows during which the gel
undergoes a tensile deformation without yielding, and ($ii$) unstable flows
characterized by the radial growth of air fingers into the gel. We show that
the unstable regime occurs beyond a critical energy input, independent of the
gel's macroscopic yield stress. This implies a local fluidization of the gel at
the tip of the growing fingers and results in the most unstable wavelength of
the patterns exhibiting the characteristic scalings of the classical viscous
fingering instability. Our work provides a quantitative criterion for the onset
of fingering in colloidal gels based on a local shear-induced yielding, in
agreement with the delayed failure framework.
