avril 21, 2016

Jenike or Peschl Shear Testing

After storage of powder, the intention is usually to make the powder flow or yield again. With free flowing material the shearing of the material is easily performed. With more cohesive materials, the initiation of flow is more difficult.

Jenike’s method is the most common method to measure powder flow properties after steady state flowing. The methods apply the history to the powder by consolidation after which the resulting strength is measured. In this way the situation in a silo or hopper is simulated, in which the powder undergoes a certain history of shear and consolidation in the top giving it strength when it reaches the bottom of the silo or hopper. The Jenike shear cell can analyze internal friction and wall friction on various wall surfaces.

A more modern version of Jenike's approach is the ring shear cell tester, which enables a much larger displacement than the Jenike cell.

Typically, a sample is placed in a shear cell of specific dimensions. The specimen is pre-consolidated by twisting or turning the cell cover while applying a compressive load normal to the cover. Subsequently, using a normal load to the cover the specimen is pre-sheared until a steady state. Finally, shearing in horizontal direction under a reduced load results in a shear force that goes through a maximum value. This maximum value is representative of a powder under certain conditions. The rotational split level shear tester designed by Peschl rotates the powder over itself producing a torque from which the shear stress can be calculated.

The data points, (maximum shear vs load) result in a yield locus with two Mohr circles. Several sets of Mohr circles provide a flow function. Combination of internal friction measurements with wall friction and bulk density measurements enables silo/hopper design resulting in a rational estimation of the hopper half angle and outlet opening of the hopper enabling mass flow.