Particle spin in anisotropic granular materials

We adopt the simplest theory for the description of the mechanical behavior of a random array of elastic spheres and show that it is necessary that th e average rotation of the spheres about their centers be distinguished from the average rotation of the aggregate. Without such a distinction, the str ess in an anisotropic aggregate, subjected to a deformation that involves a n average rotation, would not be symmetric. We illustrate this by calculati ng the incremental stress-strain relation for an orthotropic aggregate. (C) 2001 Elsevier Science Ltd. All rights reserved.