Yield stress of granular material

Debris flow material is commonly accepted to exhibit yield stress. However, the concept of yield stress is often not very clear since many different experimental and theoretical approaches to the yielding process of fluid granular materials have been proposed and this can give rise to misleading conclusions. This paper attempts to clarify the physical meaning of yield stress for granular materials and to examine its significance as a factor not only directly affecting the initiation of motion but also indirectly related to the shear resistance causing motion to cease. Experimental data from a 60 cm diameter cone-and-plate rheometer using plastic granular material suggests the existence of hysteresis between shear stress in initiation (yield stress) and cessation of motion of a granular material.