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 the average rotation of the spheres about their centers be distinguished from the average rotation of the aggregate. Without such a distinction, the stress in an anisotropic aggregate, subjected to a deformation that involves an average rotation, would not be symmetric. We illustrate this by calculating the incremental stress±strain relation for an orthotropic aggregate