CORDIS - Forschungsergebnisse der EU

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

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

Granular materials, such as sand, are complex systems in which relatively simple building blocks (i.e. contacting particles - sand grains) behave and interact collectively and in complicated ways. These interactions produce structural evolution over a range of different spatial and temporal scales from contacting-particle interactions to intermediate (meso-) scale communication and structure formation (e.g. localised deformation such as shear bands) to longer-range pattern formation. These kinematic effects are associated with the build-up of stress and possible relaxation via structural reorganisation and particle damage. This research tackles the experimental challenge of characterizing the mechanics of granular materials and, in particular, the quest for one of the holy grails of granular mechanics: the combined measurement of the kinematics and force transmission in real 3D materials during loading. The research has implications in terrestrial geomechanical/geotechnical engineering challenges, e.g. in optimising foundations or tunnels to mitigate risks of structural failure, and even extra-terrestrial challenges relating to mechanical behaviour of soils at lunar/martian landing sites. In a more general sense, the research will have impact on more diverse areas involving granulaer mechanics including industrial powder processing, pharmaceutical pill manufacturing and food science, amongst others. In the following the final outcomes of the project are outlines after first providing the general context and initial aims.

The overall aim of the project was to significantly advance the understanding of the role of microstructure and microstructural processes on the mechanical behaviour of granular materials such as natural sand. Such, new, quantified understanding aims towards facilitating development and calibration of advanced theoretical and numerical procedures for the modelling of the mechanics of granular materials. The project focussed on the development of novel experimental, data analysis and 3D image processing approaches to measure simultaneously particle kinematics, structural evolution, force transfer and elasticity in sand during mechanical loading.

The cornerstone to the research, and the key technological advance, is the development of novel methods to measure simultaneously the internal strains and kinematics of all the individual sand grains, within samples of many grains, during deformation. The key tools that employed to achieve this are three-dimensional x-ray diffraction (3DXRD), which was originally developed to study deformation in metals, and x-ray microtomography. In this project the 3DXRD technique has been adapted to measure crystallographic lattice strains in individual “sand” grains (quartz crystals); with such measurements, each grain essentially acts as a local 3D strain gauge or, for elastic deformations of the grains, a force gauge. Combined with tomographic x-ray imaging and 3D digital image analysis/correlation it has been possible to obtain, for the first time, measured grain kinematics and inferred force transmission within stiff, opaque, frictional materials. These measurements have been connected to macroscopic behaviours and properties such as the “macroscopic” stress-strain response and sample stiffness as well as the statistical configuration of the system. As a result, a major step forward has been made in the understanding of granular mechanics at scales from the crystal lattices of the individual grains, to the grain-grain interactions and up to the formation of force-transfer structures (“force chains”) that could control stiffness and failure at meso- and macro-scales. The combined techniques permit analysis of grain kinematics, grain strains, inter-granular force transmission, stiffness evolution and deformation mechanisms (including grain rotation and sliding, grain spalling and force-network reorganisation). Simultaneous measures have been allowing new understanding of different aspects of the mechanics.

Key collaborations have been established, and will continue, to realise the described outcomes of the project. In particular an important collaboration has been with J. Wright for the development of the combined 3DXRD and x-ray tomography with in-situ deformation experiments and the data analysis. A second important collaboration is with R. Hurley (previously at CalTech and now at Lawrence Livermore National Laboratory), which has led to the development of the 3D contact force inference technique that has provided the first 3D pictures of force transfer in opaque, stiff granular materials.
Outwith the primary scientific aims of the project, the fellow has become an established member of his host division and university (Division of Solid Mechanics at Lund University) and also an active member in both national and international communities in areas relating to granular mechanics, geomechanics, experimental mechanics and x-ray/neutron methods. The fellow has been able to develop a strong, independent scientific profile in these communities and has developed new competences and collaborations including concerning the mechanics of polymer and fibre materials, x-ray and neutron imaging and Li-ion batteries. Furthermore, the fellow has established experimental mechanics as a strong discipline at the host department and has set-up a new laboratory for 3D and 4D imaging (the 4D Imaging Lab @ LTH). The fellow has been awarded funding for a number of projects relating to the extended subject area of the project and in new areas opened up through collaborations within the host group. This includes national funding, faculty funding and funding from private foundations. In particular, the fellow was awarded a large grant by the engineering faculty at Lund University to establish a laboratory x-ray tomography facility (the aforementioned 4D Imaging Lab). The fellow is also strongly involved in the development of the MAXIV synchrotron and European Spallation Source (ESS) in Lund and is involved in a number of related national and international collaborations and programs. Through his work with x-ray imaging, the 4D Imaging Lab, the MAXIV synchrotron and ESS the fellow has also been involved in a number of outreach activities for academic and industrial audiences.

The fellow gained his “docent” title at Lund University during the project and has been granted an Honary Associate Professor at Heriot-Watt University in Edinburgh (UK) as well as a visiting professor at the University of Grenoble (France). He has supervised 2 postdoctoral researchers and a doctoral student plus he is / has been co-supervisor of 5 further doctoral students, 2 within the fellow’s group, one at the division of Biomechanics at Lund University, one in collaboration with the European Spallation Source and the Danish Technical University and another in a joint project with the University of Grenoble. The fellow has also supervised a number of MSc projects in his group and has established a new advanced level course on experimental mechanics. Other important aspects of the fellow’s integration are his roles: (i) as a consultant scientist for the European Spallation Source; (iii) as co-director of a new doctoral research school at Lund University on X-ray and neutron imaging at large-scale facilities; (iii) leading a proposal for an imaging beamline at the new MAXIV synchrotron; (iv) as a member of 2 Scientific and Technical Advisory Panels for the European Spallation Source (Engineering/Imaging and sample environment); (v) as chairperson for the scientific advisory panel of the NeXT neutron imaging station at the Insitut Laue Langevin in Grenoble.

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