Abstract
We present results from two experiments that provide the first quantification of inter-particle force networks in stiff, 3D, opaque granular materials. Force vectors between all grains were determined using a mathematical optimization technique that seeks to satisfy grain equilibrium and strain measurements. Quantities needed in the optimization – the spatial location of the inter-particle contact network and tensor grain strains – were found using 3D X-ray diffraction and X-ray computed tomography. The statistics of the force networks are consistent with those found in past simulations and 2D experiments. In particular, we observe an exponential decay of normal forces above the mean and a partition of forces into strong and weak networks. In the first experiment, involving 77 single-crystal quartz grains, we also report on the temporal correlation of the force network across two sequential load cycles. In the second experiment, involving 1099 single-crystal ruby grains, we characterize force network statistics at low levels of compression.
Highlights
The stress, stability, and transport properties of granular materials are controlled by the network of contact forces between grains (e.g., [1, 2])
Researchers have studied these networks for decades in an effort to build multiscale models for granular media
To move beyond the spherical, frictionless, soft 3D grains explored in past experiments, X-ray computed tomography (XRCT) and 3D X-ray diffraction (3DXRD) have recently been combined to explore the contact network and grain strains in stiff, 3D, frictional grains [7, 8]
Summary
The stress, stability, and transport properties of granular materials are controlled by the network of contact forces between grains (e.g., [1, 2]). Researchers have studied these networks for decades in an effort to build multiscale models for granular media. Experimental capabilities for measuring forces in stiff, 3D, frictional materials remain limited. Such data are required to validate models and provide access to broader ranges of materials and environments than are currently available in computations. Recent work has enabled the calculation of the full force network in these materials from the XRCT and 3DXRD data [9]
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