On the observation with polarised light of a granular medium under stress

Paper in proceeding, 2013

Whereas a two dimensional assembly of photoelastic discs shows clear coloured fringes when viewed with polarised light, a three dimensional granular medium composed of glass beads or grains shows structured patterns in the form of stripes of light of varying intensity which appear to have orientation related to the overall direction of major principal stress. Any light ray passes through many individual grains, each of which contributes to the retardation of the polarised light which emerges from the sample. The retardation within a grain is the result of nonuniform stress states within that grain which themselves result from the distribution of the contact forces between neighbouring grains in the assembly. In this paper we combine the framework of Jones calculus and the stress optic law in order to calculate and predict the effect that the randomly distributed contact forces on randomly distributed grains should have on the intensity of the light emerging from such a complex three dimensional assembly.