Unconfined yield strength (bulk strength) is the major principle stress that will cause material in an unconfined state to fail in shear. It is the primary flow property that governs the development of hang-ups in process equipment. It is used to compute critical arching and rathole dimensions for a given material in a hopper or bin. All hangups in process equipment result in the formation of a free surface. By definition, the stress acting normal to any free surface is zero. However, stresses acting along the free surface may not be zero. In a hang-up condition the material on a free surface is supported by stresses that act along the free surface and are equal to the unconfined yield strength of the material.