Dilatance (granular matter)
Dilatancy is the fundamental property of a granular, granular material , e.g. B. a bulk material or granules to increase its volume under the action of shear forces by loosening. Dilatancy is necessary for certain purposes.
For the dilatancy of gases and liquids, see Dilatance (Fluid) .
discovery
The dilatance was discovered by Osborne Reynolds in 1885 . He realized that compacted granular matter can only be deformed under shear forces if the grains push one another, which increases the volume and reduces the density .
Dilatance angle
The material parameter for the dilatation is the dilatation angle (dilatation angle), which describes the loosening and volume increase of the material associated with the shear deformation. It is defined as the ratio of volume expansion to shear expansion and is given in degrees. The angle of dilatation increases with the angle of friction and is also influenced by the shape of the grain : sharp-edged and flat grains have a larger angle of dilatation than round and compact ones.
Examples
- Vacuum packed coffee has a high density. The packaging prevents an increase in volume due to shear forces and is therefore very stable. If the opened packaging is deformed, the volume can increase due to dilatation and the grain structure is no longer stable.
- A footprint in wet sand is dry because the pressure of the foot shifts the grains of sand, thereby increasing the volume. The water penetrates into the enlarged pore space, which makes the surface appear dry.
- A rod that is stuck in a cylindrical body (e.g. bucket) with dry, compacted sand can only be pulled out with difficulty because the increase in volume of the sand is made more difficult by the bucket walls. The entire sand bucket can be pulled up on the rod because of the grains of sand that get caught in this way. However, the grains of sand relax after a while and can slide past each other. The rod comes loose and the container falls down.
- A balloon filled with dry sand feels soft when pressed on because only small shear forces arise. However, if the balloon is thrown to the ground with force, it will be severely deformed when it hits it, resulting in high shear forces. The balloon then keeps its deformation and feels very stable. The sand got stuck under the action of the shear forces and has become stable as a result. The balloon envelope ensures that the grains of sand cannot loosen on the surface, which would otherwise cause the lump of sand to disintegrate immediately.
Contractance and critical density
Are granular materials stored very loose, so increases their density with increasing shear displacement ( Kontraktanz ).
In the case of large shear paths, the density tends towards a certain critical density , which increases with increasing effective hydrostatic pressure .
This applies both to dilatant behavior, in which the current density of the material approaches the critical density from above, and to contractant behavior, in which the critical density is approached from below.