Spatial repartition of local plastic processes in different creep regimes in a granular material

Granular packings under constant shear stress display below the Coulomb limit a logarithmic creep dynamics. However, the addition of small stress modulations induces a linear creep regime characterized by an effective viscous response. Using Diffusing Wave Spectroscopy, we investigate the relation between creep and local plastic events spatial distribution (hot spots) contributing to the plastic yield. The study is done in the two regimes, i.e. with and without mechanical activation. The hot-spot dynamics is related to the material effective fluidity. We show that far from the threshold, a local visco-elastic rheology coupled to an ageing of the fluidity parameter, is able to render the essential spatio-temporal features of the observed creep dynamics. Copyright (C) EPLA, 2016