  The irregularity of particle motions during quasi-static deformation is
investigated using discrete element (DEM) simulations of sphere and
sphere-cluster assemblies. A total of three types of interparticle movements
are analyzed: relative motions of particle centers, relative motions of
material points of two particles at their contact, and the traversal of
contacts across the surfaces of particles. Motions are a complex combination of
rolling, sliding, and elastic distortion at the contacts, and all motions are
highly irregular and variant, qualities that increase with increasing strain.
The relative motions of particle centers diverge greatly from those of an
affine displacement of the particles. The motions of the nonconvex
sphere-cluster particles were more regular that those of the spheres. The paper
also investigates the effect of the distance between two remote particles and
their pair-wise relative displacements. Even for particle pairs separated by
more than six intermediate particles, the relative motions do not conform with
the mean deformation (affine) field. Force chains are shown to be transient
features, which survive only briefly across elapsed strains.
