Reversibility and diffusion in a meso-scale model for amorphous under cyclic shear.

We present results on a meso-scale model of amorphous plasticity subject to cyclic shear. We show that, after a transient, depending on the amplitude of cycling, the steady state behavior falls into one of three cases, in order of increasing strain amplitude: i) pure elastic behavior with cessation of all plastic activity, ii) reversible periodic plasticity with the period being an integer number of strain cycles (not necessarily a single cycle), and iii) irreversible plasticity with long-time diffusion. This behavior is consistent with what is seen in experiments on 2D amorphous particle rafts and confocal microscopy of emulsions and in athermal quasi-static particle-based simulations. We further show that, in the large amplitude regime, the steady state single-cycle plastic strain field is localized along line-like structures similar to the ones seen during avalanches during steady plastic shear, but there is little persistence in the localization from one cycle to the next.