  The rolling resistance between a pair of contacting particles can be modeled
with two mechanisms. The first mechanism, already widely addressed in the DEM
literature, involves a contact moment between the particles. The second
mechanism involves a reduction of the tangential contact force, but without a
contact moment. This type of rotational resistance, termed creep-friction, is
the subject of the paper. Within the creep-friction literature, the term
"creep" does not mean a viscous mechanism, but rather connotes a slight slip
that accompanies rolling. Two extremes of particle motions bound the range of
creep-friction behaviors: a pure tangential translation is modeled as a
Cattaneo-Mindlin interaction, whereas prolonged steady-state rolling
corresponds to the traditional wheel-rail problem described by Carter,
Poritsky, and others. DEM simulations, however, are dominated by the transient
creep-friction rolling conditions that lie between these two extremes. A
simplified model is proposed for the three-dimensional transient creep-friction
rolling of two spheres. The model is an extension of the work of Dahlberg and
Alfredsson, who studied the two-dimensional interactions of disks. The proposed
model is applied to two different systems: a pair of spheres and a large dense
assembly of spheres. Although creep-friction can reduce the tangential contact
force that would otherwise be predicted with Cattaneo-Mindlin theory, a
significant force reduction occurs only when the rate of rolling is much
greater than the rate of translational sliding and only after a sustained
period of rolling. When applied to the deviatoric loading of an assembly of
spheres, the proposed creep-friction model has minimal effect on macroscopic
strength or stiffness. At the micro-scale of individual contacts,
creep-friction does have a modest influence on the incremental contact
behavior.
