  Stress-induced fluidization of a simple yield stress fluid, namely a carbopol
microgel, is addressed through extensive rheological measurements coupled to
simultaneous temporally and spatially resolved velocimetry. These combined
measurements allow us to rule out any bulk fracture-like scenario during the
fluidization process such as that suggested in [Caton {\it et al., Rheol Acta},
2008, {\bf 47}, 601-607]. On the contrary, we observe that the transient regime
from solidlike to liquidlike behaviour under a constant shear stress $\sigma$
successively involves creep deformation, total wall slip, and shear banding
before a homogeneous steady state is reached. Interestingly, the total duration
$\tau_f$ of this fluidization process scales as $\tau_f \propto 1/(\sigma -
\sigma_c)^{\beta}$, where $\sigma_c$ stands for the yield stress of the
microgel, and $\beta$ is an exponent which only depends on the microgel
properties and not on the gap width or on the boundary conditions. Together
with recent experiments under imposed shear rate [Divoux {\it et al., Phys.
Rev. Lett.}, 2010, {\bf 104}, 208301], this scaling law suggests a route to
rationalize the phenomenological Herschel-Bulkley (HB) power-law classically
used to describe the steady-state rheology of simple yield stress fluids. In
particular, we show that the {\it steady-state} HB exponent appears as the
ratio of the two fluidization exponents extracted separately from the {\it
transient} fluidization processes respectively under controlled shear rate and
under controlled shear stress.
