7 Pages

Force distribution in a granular medium

Granular materials have a rich set of unusual behavior which prevents them from being simply categorized as either solids or fluids [1]. Even the most simple granular system, a static assembly of noncohesive, spherical particles in contact, holds a number of surprises. Particles within this system are under stress, supporting the weight of the material above them in addition to any applied load. The interparticle con­ tact forces crucially determine the bulk properties of the as­ sembly, from its load bearing capability [2,3] to sound trans­ mission [4-6] or shock propagation [7,8]. Only in a ciystal of identical, perfect spheres is there uniform load sharing between particles. In any real material the slightest amount of disorder, due to variations in the particle sizes as well as imperfections in their packing arrangement, is amplified by the inherently nonlinear nature of interparticle friction forces and the particles’ nearly hard-sphere interaction. As a result, stresses are transmitted through the material along “force chains’’ that make up a ramified network of particle contacts and involve only a fraction of all particles [9-11 ].