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Contact force measurements and stress-induced anisotropy in granular materials


Interparticle forces in granular media form an inhomogeneous distribution of filamentary force chains. Understanding such forces and their spatial correlations, specifically in response to forces at the system boundaries1,2, represents a fundamental goal of granular mechanics. The problem is of relevance to civil engineering, geophysics and physics3,4,5, being important for the understanding of jamming, shear-induced yielding and mechanical response. Here we report measurements of the normal and tangential grain-scale forces inside a two-dimensional system of photoelastic disks that are subject to pure shear and isotropic compression. Various statistical measures show the underlying differences between these two stress states. These differences appear in the distributions of normal forces (which are more rounded for compression than shear), although not in the distributions of tangential forces (which are exponential in both cases). Sheared systems show anisotropy in the distributions of both the contact network and the contact forces. Anisotropy also occurs in the spatial correlations of forces, which provide a quantitative replacement for the idea of force chains. Sheared systems have long-range correlations in the direction of force chains, whereas isotropically compressed systems have short-range correlations regardless of the direction.

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Figure 1: Experimental set-up and representative data.
Figure 2: Comparison of experimental images (a, c) and computed images (b, d).
Figure 3: Probability distributions of the normal forces, the tangential forces, and the mobilized friction, for the sheared and the isotropically compressed systems.
Figure 4: Contact orientation, variation of the mean force and spatial correlations for the sheared and the isotropically compressed systems.

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This work was supported by NSF DMR, NSF DMS, and NASA. We thank J. Snoeijer and collaborators, J. Socolar and B. Tighe for providing their data and for discussions. We thank H. Phatnani for critical reading of the manuscript.

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Correspondence to R. P. Behringer.

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Majmudar, T., Behringer, R. Contact force measurements and stress-induced anisotropy in granular materials. Nature 435, 1079–1082 (2005).

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