6. Elastic granular flows
Recently, Campbell [28,67,68] has been able to unify the
various granular flow theories and, in particular, fill in the gap
between the quasistatic and rapid flow regimes, and draw
complete flowmaps for shearing granular materials. The
missing link was to include the elastic properties of the particles
into the models – in effect to put the solid back into granular
solids. The principle quantity is the interparticle stiffness k, as it
governs how particles “see” one another mechanically and, as
mentioned previously, determines the bulk elastic properties of
the granular material. It requires the exercise of little
imagination to see the importance of the stiffness to the
rheology of dense granular flows. At the large concentrations of
common granular flows, particles are locked into force chains,
such as those seen in the shear cell in Fig. 6. Assuming that the
walls the cell are rigid, then the degree of compression of the
force chains and thus the deformation of each contact, is
determined only by the need for the material to shear at a given
concentration. Thus, if each particle in Fig. 6 was removed and
replaced with one with, say, twice the stiffness, the forces on
each contact and thus the contact stresses would double. As the
streaming stresses are insignificant at such large concentration,
this means that the stresses are proportional to the contact
stiffness.
Now imagine a high-concentration granular material, with
the forces distributed in force chains, undergoing shear at
constant volume. The shear will force particles together to form
the chain, cause the force chain to rotate until it becomes
unstable and collapses. As the chain rotates, it will want to dilate
the bulk material, but is prevented from doing so by the constant
volume constraint; instead, the rotation compresses the chain
and generates an elastic response. If Δ is the overall
deformation of the chain, then the deformation of each contact
is δ=Δ/N=Δd/L where N=L/d is the number of contacts in a
chain of length L composed of particles with characteristic size
d. Thus the force F=kδ=kΔd/L. The generated stress τ∼F/
d2∼k/d. Hence an appropriate dimensionless scaling for the
stress in the regime is τd/k. Note that following the above
argument, τd/k∼F/kd=δ/d; that is, τd/k can be interpreted as the
particle deformation δ represented as a fraction of the particle
diameter, d.
Campbell [28] divided the entire granular flow field into two
broad regimes the Elastic and the Inertial. The Elastic regimes
encompass all flows in which force is transmitted principally
through the deformation of force chains for which the natural
stress scaling is τd/k. Continuing from the above arguments,
follow a force chain through its life cycle. The chain will form
when particles are driven together by the shear rate γ and thus
the rate of chain production is proportional to γ. The chain then
rotates and is compressed. The degree of compression and thus
the force generated in the chain is determined by the necessity
of meeting the constant volume constraint; hence the magnitude