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Combined 4D experimental grain-scale characterisation of grain-strains, force transfer and kinematics in natural granular material (sand) under load

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Role of microstructures in granular materials

Granular materials, such as sand, are complex systems within which individual sand grains behave and interact collectively and in complicated ways. An EU-funded initiative studied their mechanical behaviour through advanced experiments involving 3D x-ray tomography and diffraction.

Fundamental Research

The project GRANULAR MECHANICS (Combined 4D experimental grain-scale characterisation of grain-strains, force transfer and kinematics in natural granular material (sand) under load) investigated the role of microstructure and microstructural processes involved in the mechanical behaviour of granular materials such as sand. A better quantified understanding of granular materials will help in the development and calibration of advanced theoretical and numerical techniques for modelling their mechanics. Scientists therefore used novel experiments, data analysis and 3D image processing to simultaneously measure particle kinematics, structural evolution, force transfer and elasticity in sand during mechanical loading. Research focused on developing new methods for simultaneously quantifying the kinematics and the internal strains of all the individual sand grains, within samples containing many grains during deformation. X-ray tomography and image analysis provided the granular kinematics, whilst 3D X-ray diffraction (3DXRD)), developed for studies of metals, was adapted to measure crystallographic lattice strains in all the individual sand grains (quartz crystals). By combining 3DXRD with tomographic X-ray imaging and 3D digital image analysis researchers obtained, for the first time, measured grain kinematics and inferred force transmissions within stiff, opaque materials. These measurements have been linked to macroscopic behaviour and properties such as stress-strain response, sample stiffness and statistical configuration of the system. The project was a major step forward towards a better understanding of granular mechanics from the scale of the crystal lattice of individual grains of sand, to grain on grain interactions and up to the formation of force transfer structures. These structures, known as force chains, may control stiffness and failures at meso- and macro-scales. GRANULAR MECHANICS outcomes will help scientists to develop better models of granular material mechanics. Its innovative work will benefit sectors as diverse as industrial powder processing, pharmaceutical pill manufacturing and food science. It can even help to predict the behaviour of soils at lunar and Martian landing sites.

Keywords

Granular materials, sand, GRANULAR MECHANICS, force transfer, 3DXRD

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