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The effect of grain shape, size distribution, intergranular friction, confinement, and initial compaction state on the high strain rate compressive mechanical response of sand is quantified using Long Split Hopkinson Pressure Bar (LSHPB) experiments, generating up to 1.1 ms long load pulses. This allowed the dynamic characterisation of different types of sand until full compaction (lowest initial void ratio) at different strain rates. The effect of the grain morphology and size on the dynamic compressive mechanical response of sand is assessed by conducting experiments on three types of sand: Ottawa Sand with quasi-spherical grains, Euroquartz Siligran with subangular grains and Q-Rok with polyhedral grain shape are considered in this study. The adoption of rigid (Ti64) and deformable (Latex) sand containers allowed for quasi-uniaxial strain and quasi-uniaxial stress conditions to be achieved respectively. Additionally, the effect of intergranular friction was studied, for the first time in literature, by employing polymer coated Euroquartz sand. Appropriate procedures for the preparation of samples at different representative initial consolidation states are utilized to achieve realistic range of naturally occurring formations of granular assembly from loose to dense state. The results identify material and confining sample state parameters which have

The ability to model impact and penetration behaviour of granular materials-such as sand-largely depends on the understanding of the stress-strain and volume change characteristics at high strain rates. Split Hopkinson Pressure Bar has been used extensively in the past to evaluate strain rate dependent constitutive response of a wide range of advanced and natural materials. However, the deformation behavior of geo-materials-including granular media-needs careful interpretation in order to provide data for calibration and validation of numerical models. In this paper, a new procedure for the determination of the smallest Representative Volume Element (RVE) of granular media is proposed. The procedure relies upon the simulations of consolidated granular assembly using Discrete Element Method (DEM). Results indicate the role of void ratio as an important state variable, which influences the dynamic mechanical performance of granular materials considerably.

SUMMARYThe morphology of many natural and man-made materials at different length scales can be simulated using particle-packing methods. This paper presents two novel 3D geometrical collective deposition algorithms for packed assemblies with prescribed distribution of radii: the 'planar deposition' and the '3D-clew' method. The 'planar deposition' method mimics an orderly granular flow through a funnel by stacking up spirally ordinated planar assemblies of spheres capable of achieving the theoretical maximum for monodisperse aggregates. The '3D-clew' method, instead, mimics the winding of a clew of yarn, thus yielding densely packed 3D polydispersed assemblies in terms of void ratio of the aggregate. The morphologies of such geometrically generated assemblies, achieved at several orders of magnitude reduced computational cost, are comparable with those consolidated uni-directionally by means of discrete element method. In addition, significantly faster simulations of mechanical consolidation of granular media have been performed when relying upon the proposed geometrically generated assemblies as starting configurations.

Quasi static and dynamic experiments were conducted to characterise the mechanical response of Etnean volcanic sand. Stress and strain histories were measured in near-uniaxial strain and near-uniaxial stress conditions at strain rates ranging between 5Á10-4 and 1.5Á10 3 s-1 using bespoke experimental setups. The effects of the lateral confinement and initial consolidation state were assessed. Etnean volcanic sand exhibited a noticeable strain rate dependent behaviour when characterised in its loose consolidation state but not when densely packed before loading. The effect of volcanic particles impingement on Ti-6Al-4 V alloy was assessed by conducting dynamic experiments at different incident angles using targets of different geometry. The texture of thus eroded surfaces was analyzed by means of non-contact 3D-profilometry. The surface analysis provided insights on the erosion mechanisms and quantitative data on the roughness increment caused by the collision and rubbing with volcanic sand.