Micromechanical Origin of Rate Independence of the Stress in Sheared Granular Materials

The stress in slowly sheared granular materials is independent of the shear rate, but individual particles transmit momentum at a rate proportional to the shear rate. How a rate-dependent microscopic momentum flux leads to a rate-independent macroscopic stress is a fundamental, unanswered question. Another prominent feature is the occurrence of large stress fluctuations. Here, we show that the two features are closely related. The fluctuations arise from recurrent jamming-yielding dynamics, with deformation being elastic in the jammed phases and yielding occurring in short bursts. These findings lead to an explanation for the rate independence of the stress.