We consider a random array of identical spheres that interact through
noncentral contact forces. We assume that the displacement of a contac
t relative to a center may be calculated from the average strain of th
e aggregate. The normal component of the contact force is assumed to b
e Hertzian and the tangential component is assumed to be linearly elas
tic until frictional sliding occurs. We consider the response of the m
aterial in triaxial compression. For monotone deformations, we calcula
te the evolution of the contact distribution, the volume change, the s
tress-strain response, the plastic string, and the strain hardening.