  Fatigue refers to the changes in material properties caused by repeatedly
applied loads. It has been widely studied for, e.g., construction materials,
but much less has been done on soft materials. Here, we characterize the
fatigue dynamics of a colloidal gel. Fatigue is induced by large amplitude
oscillatory stress (LAOStress), and the local displacements of the gel are
measured through high-frequency ultrasonic imaging. We show that fatigue
eventually leads to rupture and fluidization. We evidence four successive steps
associated with these dynamics: (i) the gel first remains solid, (ii) it then
slides against the walls, (iii) the bulk of the sample becomes heterogeneous
and displays solid-fluid coexistence, and (iv) it is finally fully fluidized.
It is possible to homogeneously scale the duration of each step with respect to
the stress oscillation amplitude $\sigma_0$. The data are compatible with both
exponential and power-law scalings with $\sigma_0$, which hints at two possible
interpretations in terms of delayed yielding in terms activated processes or of
the Basquin law. Surprisingly, we find that the model parameters behave
nonmonotonically as we change the oscillation frequency and/or the gel
concentration.
