Fluctuations of local plastic strain in granular media
We experimentally study the heterogeneity of strain in a granular medium subjected to oscillatory shear in a rotating drum. Two complementary methods are used. The first method relies on optical imaging and grain tracking, allowing us to compute some components of the strain tensor and their variance. The second method, Diffusive Acoustic Wave Spectroscopy (DAWS), provides the quadratic strain within the bulk. Our results show that strain is spatially heterogeneous, with fluctuations about ten times larger than the mean, primarily dominated by variability at the grain scale. We then analyze in detail the strain fluctuations occurring during the forward and backward branches of the shear stress cycles, along with the intra-cycle plastic strain resulting from of each cycle. Both methods reveal that each shear cycle consists of two consecutive diffusive-like branches, and that the resulting plastic strain fluctuations scales with the mean plastic shear strain. We propose that plastic strain fluctuations result from irreversible strain heterogeneity that increases with applied shear -- reflected in forward-backward strain anticorrelations -- but is constrained by load-controlled induced memory.