Stress-induced fluidization of a simple yield stress fluid, namely a carbopolmicrogel, is addressed through extensive rheological measurements coupled tosimultaneous temporally and spatially resolved velocimetry. These combinedmeasurements allow us to rule out any bulk fracture-like scenario during thefluidization 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 regimefrom solidlike to liquidlike behaviour under a constant shear stress $\sigma$successively involves creep deformation, total wall slip, and shear bandingbefore 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 themicrogel, and $\beta$ is an exponent which only depends on the microgelproperties and not on the gap width or on the boundary conditions. Togetherwith recent experiments under imposed shear rate [Divoux {\it et al., Phys.Rev. Lett.}, 2010, {\bf 104}, 208301], this scaling law suggests a route torationalize the phenomenological Herschel-Bulkley (HB) power-law classicallyused to describe the steady-state rheology of simple yield stress fluids. Inparticular, we show that the {\it steady-state} HB exponent appears as theratio of the two fluidization exponents extracted separately from the {\ittransient} fluidization processes respectively under controlled shear rate andunder controlled shear stress.
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