Numerical study of shear-history memory in slowly compressed granular media made of soft, frictional grains

Memory effects have been reported by several groups for sheared granular and matter and suspensions, often by applying cyclic or reversing shear. We use standard DEM simulation methods to study shear-history memory in granular media in a different situation, applying time-dependent shear as a perturbation to slow compression of the medium linear in time. Dense random packings of soft, frictional grains (spherical or non-spherical) are subject to increasing compression along with weak time-dependent shear. Due to the randomness of the system new contacts are formed as the system is compressed. When it is later decompressed, signatures of the applied shear can appear in macroscopic variables like the pressures exerted by the grains on the confining walls. These signatures can reflect plastic deformations of the grain pack, and possibly also changes to the transverse loading on contacts formed during the sample compression. Variations of these effects with system size, grain shape, and coefficient of friction are reported.