  Predicting the time-dependent yielding of colloidal gels under constant
stress enables control of their mechanical stability and transport. Using
rotational rheometry, we show that the shear rate of colloidal gels during an
early stage of deformation known as primary creep can forecast an eventual
yielding. Irrespective of whether the gel strain-softens or strain-hardens, the
shear rate before failure exhibits a characteristic power-law decrease as a
function of time, distinct from the linear viscoelastic response. We model this
early-stage behavior as a series of uncorrelated local plastic events that are
thermally activated, which illuminates the exponential dependence of the yield
time on the applied stress. This precursor to yield in the macroscopic shear
rate provides a convenient tool to identify the fate of a gel well in advance
of the actual yielding.
