A bulk material underlie various external forces. At storing a bulk material in a silo, the pressure to which the bulk material is exposed increases with increasing depth. With increasing depth, the pressure on the bulk material is larger and the packing density of the particles increases (the density of the bulk material increases).
The change in the packing density of the particles has a great influence on the behavior of the bulk material. With smaller particle spacing, the contact surfaces between the particles are larger and the influence of the forces between the individual particles is increased.
The bulk density (ρb) describes the ratio of the mass (m) of the bulk material to volume (V). The density of a bulk material at a pressure of P = 0 is called as bulk density (ρb0). Especially with cohesive materials, the resulting bulk density is not completely reversible and has a hysteresis.
At filling a container or mass flow in the shaft and if no shear movement has taken the bulk material, the bulk material is subject to uniaxial compression. By shearing a reorientation of the particles to each other is possible, so that sets a usually higher density.
During measuring of the yield locus, we measure the density simultaneously under the influence of pressure and shear stress. Even if the DIN only considers uniaxial compaction, both types of compaction can be used in the automatic evaluations.