Cracking a granular column

To explain mechanical and bulk properties of the granular assemblies, a hierarchical multi-scale framework of constitutive stress-strain relations is generally considered. At the microscales, granulates are modelled by the elasto-plastic theories while at the representative length scales associated with its macroscopic behavior, they are described by the continuum elastic models. Our ability to use these constitutive expressions crucially depend on the following two aspects: (i) identification of suitable averaging lengths of the microstructures over which such a relation is scale invariant and (ii) successful transfer of information across different length scales. However, great challenges lie in bridging these length scales for the microstructural responses to the bulk phenomena of the granular assemblies. In this talk I, by studying the mechanical properties of a free-standing granular column, will speak about the scale-bridging relation between plastic events (at the sub-grain scale) and peristaltic modulations (at the system scale). The column, built inside water using hydrophobic sand grains, behaves effectively as a load-bearing shell on its exterior. Mechanical instabilities contributes to the microscopic plasticity whereas long wavelength-like peristaltic modulations are driven by minimization of the surface energies. While deformed under external stresses, these instabilities migrate to a region near the lowest Rayleigh-Plateau lobe, followed by a crack-opening (precursor of the column failure). The experimental findings incorporate the microstructural deformation in describing the macroscopic behavior of the submerged granular structure and hence, shed light into the granular dynamics from one length scale to another.