95 research outputs found
The Geometrical Structure of Disordered Sphere Packings
The three dimensional structure of large packings of monosized spheres with
volume fractions ranging between 0.58 and 0.64 has been studied with X-ray
Computed Tomography. We search for signatures of organization, we classify
local arrangements and we explore the effects of local geometrical constrains
on the global packing. This study is the largest and the most accurate
empirical analysis of disordered packings at the grain-scale to date with over
140,000 sphere coordinates mapped. We discuss topological and geometrical ways
to characterize and classify these systems, and discuss implications that local
geometry can have on the mechanisms of formation of these amorphous structures.Comment: 15 pages; 16 figure
Crystallisation in a granular material
The athermal and dissipative nature of packings of grains is still challenging our understanding of their compaction
as well as their crystallisation. For instance, some beads poured in a container get jammed in random disordered con gurations,
which cannot be denser than 64%, the random closed packing (RCP) limit. Remarkably it has been suggested that the
RCP bound is saturated with dense patterns of beads aggregated into polytetrahedral structures. Yet when a suitable vibration
is applied, a packing of beads might start to order and some regular patterns appear.
We present new experiments on the crystallisation of the packing of beads. By extending tapping techniques, we have
obtained packings with volume fractions φ ranging from the RCP to the crystal (φ = 0.74). Computing tomography has been
used to scan the internal structure of large packings (≈200,000 beads). Voronoi and Delaunay space partitions on the grain
centres were performed to characterise the structural rearrangements during the crystallisation. This allows us to describe
statistical properties of the local volume uctuations and the evolution of the densest patterns of beads.
In terms of statistical description, a parameter based on the volume uctuations discloses different regimes during the
transition. In terms of geometry, we con rm that polytetrahedral dense clusters are ubiquitous at the RCP. We describe some
intrinsic features of these clusters such as rings of tetrahedra and show how they disappear as the crystal grows.
This experiment enlightens how an athermal system jammed in a complex frustrated con guration is gradually converted
into a periodic crystal
Local and Global relations between the number of contacts and density in monodisperse sphere packs
The topological structure resulting from the network of contacts between
grains (\emph{contact network}) is studied for large samples of monosized
spheres with densities (fraction of volume occupied by the spheres) ranging
from 0.59 to 0.64. We retrieve the coordinates of each bead in the pack and we
calculate the average coordination number by using three different methods. We
show that, in the range of density investigated, the coordination number is
larger than 4 and it increases with the packing fraction. At local level we
also observe a positive correlation between local packing fraction and number
of neighbors. We discover a dependence between the local densities of
configurations with few neighbors in contact and the global sample-denities.
This might indicate that local configurations with small number of neighbors
are able to deform plastically when the sample is compactifying.
PACS: 45.70.-n, Granular Systems; 45.70.Cc, Static sandpiles; Granular
Compaction.Comment: 10 pages, 6 figure
Experimental investigation of the mechanical stiffness of periodic framework-patterned elastomers
Recent advances in the cataloguing of three-dimensional nets mean a systematic search for framework structures with specific properties is now feasible. Theoretical arguments about the elastic deformation of frameworks suggest characteristics of mechanically isotropic networks. We explore these concepts on both isotropic and anisotropic networks by manufacturing porous elastomers with three different periodic net geometries. The blocks of patterned elastomers are subjected to a range of mechanical tests to determine the dependence of elastic moduli on geometric and topological parameters. We report results from axial compression experiments, three-dimensional X-ray computed tomography imaging and image-based finite-element simulations of elastic properties of framework-patterned elastomers
Modelling and characterization of cell collapse in aluminium foams during dynamic loading
Plate-impact experiments have been conducted to investigate the elastic–plastic behaviour of shock wave propagation and pore collapse mechanisms of closed-cell aluminium foams. FE modelling using a meso-scale approach has been carried out with the FE software ABAQUS/Explicit. A micro-computed tomography-based foam geometry has been developed and microstructural changes with time have been investigated to explore the effects of wave propagation. Special attention has been given to the pore collapse mechanism. The effect of velocity variations on deformation has been elucidated with three different impact conditions using the plate-impact method. Free surface velocity (ufs) was measured on the rear of the sample to understand the evolution of the compaction. At low impact velocities, the free-surface velocity increased gradually, whereas an abrupt rise of free-surface velocity was found at an impact velocity of 845 m/s with a copper flyer-plate which correlates with the appearance of shock. A good correlation was found between experimental results and FE predictions
In-Situ Quasistatic Compression and Microstructural Characterization of Aluminium Foams of Different Cell Topology
Quasistatic compression and micro structural characterization of closed cell aluminium foams of different pore size and cell distributions has been carried out. Metallic foams have good potential for lightweight structures for impact and blast mitigation and therefore it is important to find out the optimized foam structure (i.e. cell size, shape, relative density, and distribution) to maximize energy absorption. In this paper, we present results for two different aluminium metal foams of density 0.5 g/cc and 0.7 g/cc respectively that have been tested in quasi-static compression. The influence of cell geometry and cell topology on quasistatic compression behavior has been investigated using computed tomography (micro-CT) analysis. The compression behavior and micro structural characterization will be presented
Revisiting stigmergy in light of multi-functional, biogenic, termite structures as communication channel
Termite mounds are fascinating because of their intriguing composition of nu- merous geometric shapes and materials. However, little is known about these structures, or of their functionalities. Most research has been on the basic com- position of mounds compared with surrounding soils. There has been some targeted research on the thermoregulation and ventilation of the mounds of a few species of fungi-growing termites, which has generated considerable inter- est from human architecture. Otherwise, research on termite mounds has been scattered, with little work on their explicit properties.
This review is focused on how termites design and build functional structures as nest, nursery and food storage; for thermoregulation and climatisation; as defence, shelter and refuge; as a foraging tool or building material; and for colony communication, either as in indirect communication (stigmergy) or as an information channel essential for direct communication through vibrations (biotremology).
Our analysis shows that systematic research is required to study the prop- erties of these structures such as porosity and material composition. High res- olution computer tomography in combination with nonlinear dynamics and methods from computational intelligence may provide breakthroughs in un- veiling the secrets of termite behaviour and their mounds. In particular, the ex- amination of dynamic and wave propagation properties of termite-built struc- tures in combination with a detailed signal analysis of termite activities is re- quired to better understand the interplay between termites and their nest as superorganism. How termite structures serve as defence in the form of disguis- ing acoustic and vibration signals from detection by predators, and what role local and global vibration synchronisation plays for building are open ques- tions that need to be addressed to provide insights into how termites utilise materials to thrive in a world of predators and competitors
Tomographic analysis of jammed ellipsoid packings
Disordered packings of ellipsoidal particles are an important model for disordered granular matter. Here we report a way to determine the average contact number of ellipsoid packings from tomographic analysis. Tomographic images of jammed ellipsoid packings prepared by vertical shaking of loose configurations are recorded and the positions and orientations of the ellipsoids are reconstructed. The average contact number can be extracted from a contact number scaling (CNS) function. The size of the particles, that may vary due to production inaccuracies, can also be determined by this method
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