Résumé:

We study the fatigue of a soft glass submitted to repeated oscillatory shear deformation using a unique instrument that simultaneously probes the mechanical response of the sample and its microscopic dynamics. The soft glass, a dense packing of microgel particles, exhibits at rest a spontaneous ballistic dynamics, characterized by compressed exponential relaxations, in line with rearrangements being due to the relaxation of internal stresses in a soft solid. Oscillatory shear measurements show a characteristic single step yielding process. To better understand the microscopic origin of yielding, we couple dynamic light scattering to shear rheology and track both the reversible non affine dynamics and the decay of higher order correlation echoes, up to 500 cycles. Several regimes are found by increasing the shear amplitude. Unperturbed spontaneous solid-like relaxation is measured in the linear regime. In this regime, the non-affinities are fullyreversible and originate presumably from spatial fluctuations of the sample elastic modulus. By contrast, the non-affinities become partially irreversible in the non-linear regime and stem from plastic rearrangements. At the onset of non-linearity (γ~6%), we find that dynamics accelerates sharply but still exhibit a solid-like compressed exponential relaxation. In the fully fluidized regime (γ>30%), the microscopic dynamics is however qualitatively different and exhibits a stretched exponential decay, characteristic of a supercooled liquid. Interestingly, we find over a relatively broad range of strain amplitude, which macroscopically corresponds to the regime of a prominent loss peak in the rheology data, the coexistence of a fast liquid-like mode and a slower solid-like mode. Overall, our experimental results that combine macroscopic and microscopic data provide a rational scenario for the fatigue yielding of a heterogeneous soft solid.