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- Athanasopoulos, Stefanos, et al.
(författare)
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A novel multiscale neutron diffraction based experimental approach for granular media
- 2019
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Ingår i: Géotechnique Letters. - : Thomas Telford Ltd.. - 2045-2543. ; 9:4, s. 284-289
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Tidskriftsartikel (refereegranskat)abstract
- The complexity of granular (geo-)materials is associated with the mobility and interaction of their constituent particles. Under the effect of loading, these materials exhibit a highly inhomogeneous behaviour, which might vary significantly as the loading develops (i.e. certain grain assemblies take up the load whilst other, neighbouring assemblies fall into states of lower load). To that end, understanding the (micro-)mechanisms related to the distribution and evolution of forces/stresses through granular media requires appropriate, spatially resolved measurements. Neutron strain scanning (NSS) is an experimental technique based on diffraction measurements that has been successfully used in recent years to infer force/stress distribution in granular materials under load, by measuringthe crystallographic strains of grains. In this work, first results from a new experimental approach involving simultaneous NSS and digital image correlation (DIC) of quartz sand under load in a specially designed plane-strain apparatus are presented. The combined use of these techniques allows the investigation of deformation mechanisms in granular media, such as sand, at different scales. Therefore, a completely novel multiscale analysis of granular (geo-)materials can be made – that is, associating the traditional macroscale measurements with the mesoscale characterisation of the strain field (through DIC) and the inferred microscale stress distribution (through NSS).
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| 3. |
- Athanasopoulos, Stefanos D., et al.
(författare)
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Mapping Grain Strains in Sand Under Load Using Neutron Diffraction Scanning
- 2018
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Ingår i: Micro to MACRO Mathematical Modelling in Soil Mechanics. - Cham : Springer International Publishing. - 2297-0215 .- 2297-024X. - 9783319994734 - 9783319994741 ; , s. 23-33
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Bokkapitel (refereegranskat)abstract
- Towards the improvement of the understanding of force/stress distribution in granular media under load, a new experimental approach is suggested. Neutron diffraction, a non-conventional experimental technique, has been successfully used to map the evolution of intragranular strains in sand specimens loaded in a novel plane-strain apparatus. Representative preliminary results from recent experiments are presented, focusing on the correlation between the macro- and micro-scale response of the material, to highlight the potential of the experimental approach.
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| 8. |
- Athanasopoulos, Stefanos
(författare)
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Multiscale granular mechanics: A neutron diffraction based experimental approach
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Granular media (i.e., assemblies of grains, containing voids), such as sand, are highly complex materials, possessing inherently heterogeneous structure and properties that are manifested by the mobility and interaction of their constituent particles. Despite having been widely studied for centuries (i.e., the study of granular media in modern science essentially begins with the work of Charles-Augustin de Coulomb on sand, in 1776), there still exist key pieces of information regarding their (micro-)mechanical behaviour that have been largely eluding the scientific community. More specifically, under the effect of applied stress, these materials exhibit highly inhomogeneous mechanical behaviours, indicating varying and evolving local stress-strain relationships. As far as the strain is concerned, over the past decades, increasingly fine details have been revealed into the underlying grain-scale mechanisms, the origins of heterogeneous behaviour, including localisation, and how these lead to macroscopic material failure. However, details on the evolution of force/stress distribution are a key, missing piece of information and to be understood, requires appropriate, spatially-resolved local measurements.This thesis presents a novel, multiscale experimental approach, to characterise quantitatively the (micro-)structural evolution of granular media during loading, by associating traditional macroscale boundary measurements with microscale information acquired by neutron diffraction (ND), as well as digital image correlation (DIC), at a mesoscale in between. The ND method provides mapping of the distribution of stresses throughout the granular skeleton of the material, by inference from measured crystallographic strains of the grains, whilst DIC provides the complementary total strain field mapping, opening the possibility for local stress-strain analysis. A key component that enabled this novel approach is the development of a new, specially designed plane-strain loading apparatus. A series of experiments was realised with this apparatus, demonstrating the potential of the experimental approach through combined stress and strain mapping. These experimental developments also highlighted the need for appropriate, reference ND measurements for granular media, for the effective employment of the ND method, which, as of yet, do not exist. This need has been addressed and a newly assembled dataset of reference measurements for the material under study (i.e., Fontainebleau quartz sand) is presented in this thesis. With this reference dataset, a proper basis has been set for the better analysis of future experiments. Together, these experimental developments and results have laid the foundation for future, more detailed investigations of granular mechanics, where both stress and strain may be characterised locally in a specimen under load, in a full-field sense.
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| 11. |
- Tudisco, Erika, et al.
(författare)
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Localised strain in fissured clays : The combined effect of fissure orientation and confining pressure
- 2022
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Ingår i: Acta Geotechnica. - : Springer Science and Business Media LLC. - 1861-1125 .- 1861-1133. ; 17:5, s. 1585-1603
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports the main results of an experimental study on the mechanics of intensely fissured natural clays, extending our previous studies on scaly clay from Santa Croce di Magliano. While previous work focused on the influence of the orientation of fissures with respect to the loading direction, the present investigation specifically explores an additional, important variable: the stress level. The combined effect of fissure orientation and confining pressure was studied by setting up a large campaign of plane strain compression experiments, in which different combinations of these two variables were tested. Conventional global stress–strain measurements were complemented by measuring displacement and strain fields through two-dimensional digital image correlation. Such rich information provided a clear and consistent picture of the interplay between fissure orientation and stress level and revealed complex deformation patterns, which cannot be ignored for a proper interpretation of the material response.
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- Tudisco, Erika, et al.
(författare)
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Neutron imaging and digital volume correlation to analyse the coupled hydro-mechanics of geomaterials
- 2017
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Ingår i: Rivista Italiana di Geotecnica. - 0557-1405. ; 51:4, s. 60-68
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Tidskriftsartikel (refereegranskat)abstract
- A new approach to characterise the evolution and coupling of deformation and fluid flow in geomaterials is presented. The method exploits some key features of neutrons, namely penetration of dense materials used for triaxial pressure cells, sensitivity to hydrogen and the possibility to distinguish hydrogen from its isotope deuterium (in normal water, H2O, and heavy water, D2O, respectively). Illustration of the approach is provided with results from a combined fluid flow/triaxial compression test on a cemented sand specimen performed in-situ (i.e., acquiring images during loading) at a neutron imaging station. Quantitative analysis of neutron tomography images acquired at different stages of deformation is made by Digital Volume Correlation to provide full 3D strain fields that highlight the evolution of localised deformation features. Spatio-temporal tracking of the effect of the evolution of the permeability in the sample was possible by neutron radiographies acquired during pressure driven flow of H2O into the sample saturated with D2O. By exploiting the H2O/D2O neutron transmission contrast and similarities of their flow behaviour, the tracking of the H2O/D2O front can be considered as an indicator of the permeability of the sample that is correlated to the measured evolution of the deformation.
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