A novel multiscale neutron-diffraction-based experimental approach for granular media

S D Athanasopoulos,J F Kelleher,S A Hall

doi:10.1680/jgele.18.00234

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 measuring the 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).

Highlights

The vast growth of full-field measurement techniques over the past few decades
For granularmaterials, in particular, various techniques have been employed towards the understanding of their complexity (e.g. Charalampidou et al, 2011; Desrues & Andò, 2015), which is manifested by the mobility and interaction of their constituent particles
The corresponding σa2x1i1al mappings exhibit significant spatial and temporal variations, implying overall grain reorganisation, σa2x1i1al increases with an increasing σmacro. These successive shifts suggest that the micro-stress distribution evolves through unloading in one region of the specimen leading to another area taking up the load

Summary

INTRODUCTION

The vast growth of full-field measurement techniques over the past few decades When Bragg’s law is fulfilled, a reflection (i.e. a Bragg peak), corresponding to a specific hkl orientation, can be identified in a diffraction pattern and shifts in its position are directly related to elastic changes in d-spacing (i.e. elastic crystallographic – or grain – strains) of the respective planes. Other characteristics of a Bragg peak can provide further information on a material’s microstructure (e.g. subgrain plastic deformations, such as dislocations, can be attributed to peak width variations), but in the present work only peak position shifts are considered, which are associated with the elastic component of strain For polycrystalline materials, such as sand, it is assumed that within a sub-volume of a specimen illuminated by neutrons (i.e. the gauge volume – GV), typically of the order of a few mm, there exists a substantial subset of scattering grains with a specific hkl orientation that fulfils Bragg’s law (i.e. the scatterers).

RESULTS AND DISCUSSION

Section III

CONCLUSIONS