Multiscale granular mechanics: A neutron diffraction based experimental approach

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.