Publications
225 results found
Morimoto T, O'Sullivan C, Taborda D, 2021, Exploiting DEM to Link Thermal Conduction and Elastic Stiffness in Granular Materials, Journal of Engineering Mechanics, Vol: 148, ISSN: 0733-9399
Estimating the effective thermal conductivity (ETC) of granular materials is important in various engineering disciplines. The ETC of a granular material is not unique, rather it depends upon the material's packing characteristics, i.e. porosity and coordination number. Directly measuring the ETC of granular materials with a particular packing density and subjected to specific stress conditions is experimentally challenging. There is a need to develop reliable, indirect experimental methods to measure the ETC of granular materials. Here we explore the possibility of linking the ETC of granular materials to their elastic moduli.This study used a thermal pipe network model implemented in a Discrete Element Method (DEM) code to generate ETC data for ideal, virtual two-phase granular samples with stagnant pore fluid. Parametric studies considered the sensitivity of the ETC to the sample packing. Data from small deformation probes were used to explore links between the samples' elastic moduli and their ETCs. The results provide a theoretical background for the development of an indirect experimental method to predict the ETC or trends in the variation in the ETC by considering stiffness data which are relatively straightforward to acquire. The study shows how DEM can be used as a sophisticated thought experiment to explore novel ideas for developing experimental procedures.
Yu M, Reddyhoff T, Dini D, et al., 2021, Using ultrasonic reflection resonance to probe stress wave velocity in assemblies of spherical particles, IEEE Sensors Journal, Vol: 21, Pages: 22489-22498, ISSN: 1530-437X
A high-sensitivity method to measure acousticwave speed in soils by analyzing the reflected ultrasonic signalfrom a resonating layered interface is proposed here.Specifically, an ultrasonic transducer which can be used to bothtransmit and receive signals is installed on a low-high acousticimpedance layered structure of hard PVC and steel, which in turnis placed in contact with the soil deposit of interest. The acousticimpedance of the soil (the product of density and wave velocity)is deduced from analysis of the waves reflected back to thetransducer. A system configuration design is enabled bydeveloping an analytical model that correlates the objectivewave speed with the measurable reflection coefficient spectrum.The physical viability of this testing approach is demonstratedby means of a one-dimensional compression device that probesthe stress-dependence of compression wave velocity of differentsizes of glass ballotini particles. Provided the ratio of thewavelength of the generated wave to the soil particle size issufficiently large the data generated are in agreement with dataobtained using conventional time-of-flight measurements. Inprinciple, this high-sensitivity approach avoids the need for thewave to travel a long distance between multiple transmitterreceiver sensors as is typically the case in geophysical testingof soil. Therefore it is particularly suited to in-situ observation ofsoil properties in a highly compact setup, where only a single transducer is required. Furthermore, high spatialresolution of local measurements can be achieved, and the data are unaffected by wave attenuation as transmitted insoil.
Bandera S, Angioletti-Uberti S, Tangney P, et al., 2021, Coarse-grained molecular dynamics simulations of clay compression, Computers and Geotechnics, Vol: 138, Pages: 1-18, ISSN: 0266-352X
This paper outlines a framework for using molecular dynamics to simulate the compression of kaolinite saturated at alkaline pH (=8) in a low (1 mM) concentration solution. The particles are modelled as flat (3D) ellipsoids and their interactions are described by a modified form of the Gay-Berne potential, calibrated against DLVO theory. The LAMMPS software was used to generate monodisperse and slightly polydisperse samples, and to simulate isotropic compression to 100 kPa. The influences of sample size and strain rate on the void ratio and the arrangement of particles within the samples were investigated via parametric studies. It is useful to consider the extent to which the system temperature (a measure of the average kinetic energy) is controlled when assessing whether the applied strain rate is appropriate. It is found that the number of particles that can be considered a representative element volume is orders of magnitude larger than the number simulated in earlier studies and that larger number of particles are required in polydisperse samples than in the monodisperse case. A comparison between the results obtained and those of published experimental studies show that the methodology proposed can deliver sensible results for the material considered.
O'Sullivan C, Cheng H, Zhao J, 2021, Use of DEM in geomechanics: Special issue associated with the DEM 8 conference, Computers and Geotechnics, Vol: 137, Pages: 1-4, ISSN: 0266-352X
Schnaider Bortolotto M, Taborda DMG, O'Sullivan C, 2021, Thermal effects on the hydraulic conductivity of a granular geomaterial, 20th International Conference on Soil Mechanics and Geotechnical Engineering 2022, Publisher: 2022 Australian Geomechanics Society, Pages: 5017-5022
Geotechnical challenges arising from thermal loading are associated with many engineering applications such as ground source energy systems (5℃-40℃) and nuclear waste disposal (in excess of 100℃). The effects of temperature on soils have been the subject of limited research, particularly in terms of the fundamental characterisation of the non-isothermal behaviour of granular geomaterials. This study describes challenges associated with determining the hydraulic conductivity (k_ℎ) of such materials at different temperatures using a bespoke temperature-controlled triaxial apparatus. A methodology is proposed for interpreting thermo-hydro-mechanical (THM) tests on isotropically consolidated specimens and is applied to data obtained for a uniform sand. It is shown that the intrinsic head losses of the system need to be minimised in order to obtain reliable measurements; this requires a detailed calibration procedure. The developed approach is used to determine the hydraulic conductivity at ambient temperature and at 40℃, showing that the increase in k_ℎ with temperature is mostly due to the reduction in the viscosity of water. A detailed analysis of the volumetric response of the sample during heating is also carried out.
Morimoto T, O'Sullivan C, Taborda D, 2021, Analytical and DEM studies of thermal stress in granular materials, Powders and Grains 2021, Publisher: EDP Sciences, Pages: 1-4, ISSN: 2100-014X
The ability to predict thermal-induced stresses in granular materials is of practical importance across a range of disciplines ranging from process engineering to geotechnical engineering. This study presents an analytical formula to predict thermal-induced stress increments in mono-disperse granular materials subject to an initial isotropic stress state. A complementary series of DEM simulations were carried out to explore the applicability of the proposed analytical formula. The comparative analysis showed that the proposed expression can accurately predict stress changes in packings where there are negligible particle displacements as a consequence of the thermal loading (e.g. regular packings and medium/dense random packings); however large errors were observed in loose samples with a random packing.
Bernhardt-Barry M, Biscontin G, O'Sullivan C, 2021, Analysis of the stress distribution in a laminar direct simple shear device and implications for test data interpretation, Granular Matter, Vol: 23, ISSN: 1434-5021
Direct simple shear (DSS) testing allows observation of load-deformation response under rotation of the major principal stress plane, which is descriptive of many actual field problems. While the simplicity of the test configuration makes its use popular in research and industry, key uncertainties still remain regarding the interpretation of the laboratory data. This study uses laboratory validated discrete element method (DEM) models to examine the stress transmission in laminar-type direct simple shear devices under drained constant effective stress conditions. The DEM models (comprised of spheres) closely replicate physical specimens of precision chrome steel ball bearings for which the properties (e.g., shape, surface friction, and stiffness) were measured directly. The DEM models were also validated using experimental tests, so that conclusions regarding the system response can be derived with confidence from the available DEM data. The testing program included both loose and dense specimens, allowing for a comparison of the influence of density on stress state which has not been examined in previous simple shear DEM studies. Differences were observed between vertical effective stresses and shear stresses derived from boundary measurements (as commonly carried out in experimental programs) and those derived from force measurements within the DEM specimens. The failure state of the material in simple shear was also examined through Mohr’s circles of stress. The evolution of stresses on both the horizontally and vertically oriented planes were considered so that established methods of direct simple shear interpretation could be critically assessed. For the loose specimens, the angle of shearing resistance can be confidently estimated considering the maximum shear stress acting on the horizontal plane, which is easily inferred from measurements of the shear force during the physical test. This was true considering both internal and boundary calculated stress
Che H, O'Sullivan C, Sufian A, et al., 2021, A novel CFD-DEM coarse-graining method based on the Voronoi tessellation, Powder Technology, Vol: 384, Pages: 479-493, ISSN: 0032-5910
In unresolved flow CFD-DEM simulations, the porosity values for each CFD cell are determined using a coarse-graining algorithm. While this approach enables coupled simulations of representative numbers of particles, the influence of the porosity local to the particles on the fluid-particle interaction force is not captured. This contribution considers a two-grid coarse-graining method that determines a local porosity for each particle using a radical Voronoi tessellation of the system. A relatively fine, regular point cloud is used to map these local porosity data to the CFD cells. The method is evaluated using two different cases considering both disperse and tightly packed particles. The data show that the method conserves porosity data, is reasonably grid-independent and can generate a relatively smooth porosity field. The new method can more accurately predict the fluid-particle interactive force for polydisperse particle system than alternative methods that have been implemented in unresolved CFD-DEM codes.
Altuhafi FN, O'Sullivan C, Sammonds P, et al., 2021, Triaxial compression on semi-solid alloys, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 52, Pages: 2010-2023, ISSN: 1073-5623
Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed.
Sufian A, Artigaut M, Shire T, et al., 2021, Influence of Fabric on Stress Distribution in Gap-Graded Soil, JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING, Vol: 147, ISSN: 1090-0241
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Liu D, O'Sullivan C, Harb Carraro JA, 2021, Influence of particle size distribution on the proportion of stress-transmitting particles and implications for measures of soil state, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 147, Pages: 04020182-1-04020182-14, ISSN: 0733-9410
It is generally accepted that the use of void ratio and bulk density as measures of soil8state have limitations in the case of gap-graded soils as the finer grains may not 9transmit stress. However, hitherto no one has systematically explored whether this 10issue also emerges for soils with continuous gradings. Building on a number of experimental and discrete element method (DEM) studies that have considered the idea of an effective void ratio for gap-graded or bi-modal soils, this contribution extends consideration of this concept to a broader range of particle size distributions. By exploiting high performance computers, this study considers a range of ideal isotropically compressed samples of spherical particles with linear, fractal and gap-graded (bimodal and trimodal) particle size distributions. The materials’ initial packing densities are controlled by varying the inter-particle coefficient of friction. The results show that even for soils with continuous particle size distributions, a significant proportion of the finer particles may not transmit stress and be inactive. Drawing on ideas put forward in relation to gap-graded soils, both a mechanical void ratio and mechanical bulk density that consider the inactive grains as part of the void space are determined. Even for the linear and fractal gradings considered here, the difference between the conventional measures and the mechanical measures is finite and density dependent. The difference is measurably larger in the looser samples considered. These data highlight a conceptual/fundamental limitation of using the global void ratio26as a measure of state in expressions to predict granular material behaviour
Dutta TT, Otsubo M, Kuwano R, et al., 2020, Evolution of shear wave velocity during triaxial compression, Soils and Foundations, Vol: 60, Pages: 1357-1370, ISSN: 0038-0806
Accurate design of geotechnical structures requires precise estimation of the shear wave velocity (Vs) and the small-strain shear modulus. However, the interpretation of Vs data measured in deformed/sheared soil has not been extensively considered. This study used a triaxial apparatus equipped with planar piezoelectric transducers to monitor the evolution of Vs during triaxial compression of cohesionless soils. Recognizing that the grain shape and surface characteristics affect the overall mechanical response of granular materials, various natural sands and glass bead samples were considered. Discrete element method (DEM) simulations using spherical particles were carried out to compute particle-scale responses that cannot be measured in the laboratory. The experimental results revealed that the Vs values for samples with different initial densities tend to approach one another and have similar values (merge) at large axial strains. This merging occurs at a lower strain level for spherical particles in comparison with non-spherical particles. The linear Vs-void ratio relationship, which is often developed and used for homogeneous and isotropic stress states, is no longer applicable during shearing. It is the mean coordination number that dictates the evolution of Vs during triaxial compression. Furthermore, the axial strain at which the peak Vs is achieved is found to be comparable to the axial strain at which specimen dilation takes place.
Otsubo M, OSullivan C, Ackerley S, et al., 2020, Selecting an appropriate shear plate configuration to measure elastic wave velocities, Geotechnical Testing Journal, Vol: 43, ISSN: 0149-6115
The (small-strain) elastic moduli of soil can be determined from stress wave velocity measurements. Bender/extender elements are widely used in laboratory experiments; however, discussion on how to accurately determine wave velocities using this method continues. Planar piezoelectric transducers (sometimes called shear plates) are a relatively new technology, whose use is not yet widely established, that appear to offer some advantages in comparison with bender/extender elements for laboratory geophysics tests. This contribution critically assesses the use of planar piezoelectric elements experimentally and using discrete element method (DEM) simulations. Planar piezoelectric elements capable of generating and receiving either shear or compression waves were placed in the top and base caps of a triaxial apparatus. Samples of glass ballotini were used so that stress wave propagation simulations could be performed on equivalent virtual samples using DEM. The appropriate shear plate configuration to effectively measure the shear wave velocity is explored. Considering both time- and frequency-domain responses, it is revealed that shear plate signals are sensitive to the surface area and thickness of the piezoelectric elements and to the lateral boundary conditions. Using a shear plate with the widest possible surface area exposed to the soil specimen is recommended to increase the signal-to-noise ratio and to produce more planar shear waves, resulting in a more accurate measurement of shear wave velocity.
Bowles AJ, Fowler GD, O'Sullivan C, et al., 2020, Sustainable rubber recycling from waste tyres by waterjet: A novel mechanistic and practical analysis, Sustainable Materials and Technologies, Vol: 25, Pages: 1-15, ISSN: 2214-9937
Production and disposal of car tyres are major contributors to environmental damage. The first stage in tyre rubber recycling is granulation to smaller particle sizes. The sub-optimal physical, mechanical and chemical properties of mechanically ground tyre rubber (GTR) when incorporated into recycled blends are major obstacles to wider use of this potentially sustainable, recovered resource. Consequently, newly manufactured tyres contain less than 5% recycled material. This study compares two types of GTR product: mechanically ground crumb (MGC) and ultrahigh pressure waterjet-produced rubber crumb (WJC). A novel image analysis method showed that when the two particle types were compared, MGC was associated with both greater convexity and sphericity: the geometric mean ratio of MGC/WJC sphericity was 1.67. When part-recycled rubber blends comprising 30% crumb of particle size < 300 μm were compared to virgin polymer, the WJC blend exhibited superior mechanical properties to the MGC blend. These results can be explained by the higher surface area to volume ratio of WJC when compared to MGC which results in strong bonding in new blends using WJC. Further analysis by scanning electron microscopy (SEM) elucidated significant shape and textural variation within the WJC sample, allowing grouping into two sub-categories: “W1” which comprises particles with complex geometries, and “W2” particles which have a relatively simple topology that is similar to MGC. Maximising the W1:W2 particle ratio is likely to be crucial to the optimisation of output quality in the WJC process, and so a composite model is proposed that unifies three well-established fluid effects: brittle fracturing, impact cratering and cavitation. Impact cratering and cavitation effects should be maximised by altering process parameters with the aim of producing a higher proportion of crumb with a more irregular surface morphology to achieve better bonding properties in recycled
Su TC, O'Sullivan C, Yasuda H, et al., 2020, Rheological transitions in semi-solid alloys: in-situ imaging and LBM-DEM simulations, Acta Materialia, Vol: 191, Pages: 24-42, ISSN: 1359-6454
Rheological transitions from suspension flow to granular deformation and shear cracking are investigated in equiaxed-globular semi-solid alloys by combining synchrotron radiography experiments with coupled lattice Boltzmann method, discrete element method (LBM-DEM) simulations. The experiments enabled a deformation mechanism map to be plotted as a function of solid fraction and shear rate, including a rate dependence for the transition from net-contraction to net-dilation, and for the initiation of shear cracking. The LBM-DEM simulations are in quantitative agreement with the experiments, both in terms of the strain fields in individual experiments and the deformation mechanism map from all experiments. The simulations are used to explore the factors affecting the shear rate dependence of the volumetric strain and transitions. The simulations further show that shear cracking is caused by a local liquid pressure drop due to unfed dilatancy, and the cracking location and its solid fraction and shear rate dependence were reproduced in the simulations using a criterion that cracking occurs when the local liquid pressure drops below a critical value.
Smith E, Trevelyan D, Ramos-Fernandez E, et al., 2020, CPL library - a minimal framework for coupled particle and continuum simulation, Computer Physics Communications, Vol: 250, Pages: 1-11, ISSN: 0010-4655
We present an open-source library for coupling particle codes, such as molecular dynamics (MD) or the discrete element method (DEM), and grid based computational fluid dynamics (CFD). The application is focused on domain decomposition coupling, where a particle and continuum software model different parts of a single simulation domain with information exchange. This focus allows a simple library to be developed, with core mapping and communication handled by just four functions. Emphasis is on scaling on supercomputers, a tested cross-language library, deployment with containers and well-documented simple examples. Building on this core, a template is provided to facilitate the user development of common features for coupling, such as averaging routines and functions to apply constraint forces. The interface code for LAMMPS and OpenFOAM is provided to both include molecular detail in a continuum solver and model fluids flowing through a granular system. Two novel development features are highlighted which will be useful in the development of the next generation of multi-scale software: (i) The division of coupled code into a smaller blocks with testing over a range of processor topologies. (ii) The use of coupled mocking to facilitate coverage of various parts of the code and allow rapid prototyping. These two features aim to help users develop coupled models in a test-driven manner and focus on the physics of the problem instead of just software development. All presented code is open-source with detailed documentation on the dedicated website (cpl-library.org) permitting useful aspects to be evaluated and adopted in other projects.
O'Sullivan C, Ciantia M, 2020, Calculating the state parameter in crushable sands, International Journal of Geomechanics, Vol: 20, Pages: 04020095-1-04020095-10, ISSN: 1532-3641
The state parameter (y) measures the distance from the current state to the critical state line (CSL) in thecompression plane. The existence of a correlation between both the peak angle of shearing resistance (�#$ )and peak dilatancy and y is central to many constitutive models used to predict granular soil behaviour. Thesecorrelations do not explicitly consider particle crushing. Crushing induced evolution of the particle sizedistribution influences the CSL position and recent research supports used of a critical state plane (CSP) toaccount for changes in grading. This contribution evaluates the whether the CSP can be used to calculate yand thus enable prediction of the peak angle of �#$ and peak dilatancy where crushing takes place. The dataconsidered were generated from a validated DEM model of Fontainebleau sand that considers particlecrushing. It is shown that where y is calculated by considering the CSL of the original uncrushed material therecan be in a significant error in predicting the material response. Where the CSP is used there is a significantimprovement in our ability to predict behaviour whether the CSP is accurately determined using a largenumber of tests or approximated using crushing yield envelopes. It is shown that the state parametercalculated using the previously available definition can give a false sense of security when assessingliquefaction potential of potentially crushable soils. The contribution also highlights the stress-pathdependency of the relationship between �#$ and y whichever approach is used to determine y
Sufian A, Knight C, O'Sullivan C, et al., 2020, Capturing Local Fluid Field Characteristics in Porous Media using Pore Network Models
Pore Network Models (PNMs) provides a computationally efficient approach to model porous media. A PNM employs a network representation of the pore space, where individual pores are taken as nodes of the network and are connected by constrictions which are edges of the network. In assemblies of spheres, this network representation is often achieved using Delaunay-based tessellation methods. Fluid flow is simulated in the network model using a Stokes flow algorithm which solves for continuity at each node in the network using a local conductivity model. The seepage induced drag forces can be obtained from the resulting flow field and geometric properties of the pores and constrictions. While PNMs are computationally efficient, there is a simplification of the pore space geometry and governing fluid equations. To better understand these assumptions, the local flow field and seepage induced drag obtained from a PNM is compared to fully-resolved Immersed Boundary Method (IBM) simulations. The local pressure field was very accurately captured, while the local flow rate exhibited slightly more scatter and sensitivity to the tessellation method. There was a close similarity in the network shortest paths, indicating that PNM captures dominant flow channels. Comparison of streamline profiles demonstrated that local pressure drops coincided with pore constrictions. In addition, the fluid-particle interaction force was determined reasonably accurately using PNMs. This suggests that a PNM is a viable option to investigate the local behaviour in porous media.
Knight C, O'Sullivan C, Dini D, et al., 2020, Computing drag and interactions between fluid and polydisperse particles in saturated granular materials, Computers and Geotechnics, Vol: 117, Pages: 1-16, ISSN: 0266-352X
Fundamental numerical studies of seepage induced geotechnical instabilities and filtration processes depends on accurate prediction of the forces imparted on the soil grains by the permeating fluid. Hitherto coupled Discrete Element Method (DEM) simulations documented in geomechanics have most often simulated the fluid flow using computational fluid dynamics (CFD) models employing fluid cells that contain a number of particles. Empirical drag models are used to predict the fluid-particle interaction forces using the flow Reynolds number and fluid cell porosity. Experimental verification of the forces predicted by these models at the particle-scale is non-trivial. This contribution uses a high resolution immersed boundary method to model the fluid flow within individual voids in polydisperse samples of spheres to accurately determine the fluid-particle interaction forces. The existing drag models are shown to poorly capture the forces on individual particles in the samples for flow with low Reynolds number values. An alternative approach is proposed in which a radical Voronoi tesselation is applied to estimate a local solids volume fraction for each particle; this local solids fraction can be adopted in combination with existing expressions to estimate the drag force. This tessellation-based approach gives a more accurate prediction of the fluid particle interaction forces.
Liu D, O'Sullivan C, Carraro JAH, 2020, Stress inhomogeneity in gap-graded cohesionless soils - A contact based perspective, Geo-Congress 2020, Pages: 341-348, ISSN: 0895-0563
Gap-graded cohesionless soils, comprising mixtures of fine and coarse grains, pose a particular challenge in soil mechanics. Reasoning and experimental data indicate that some of the finer grains may exist in the void space without transmitting any stress. A number of authors have proposed considering at least some of the volume of these particles along with the void space when calculating the void ratio in the case of low fines contents. The concept of a transitional fines content has been proposed, i.e., a fines content delineating materials whose behavior is dominated by the coarser grains and materials whose behavior is determined by the finer grains. This contribution uses discrete element method (DEM) simulations to explore the nature of stress transmission in gap-graded materials comprised of spherical particles. Partitioning the stress tensor by considering the contributions of the contacts between coarse particles, the contacts between coarse and fine particles, and the contacts between fine particles is shown to provide useful insight into the contribution of each type of particle to the overall stress transmission. In general, for the mixtures considered here, the coarse-coarse contacts transmit a greater range of forces and a greater average force. For the mixture with size ratio of 3.7, the range of contact force magnitudes transmitted by each contact type reduces with increasing fines content and increasing sample density. This sensitivity is more evident for the lower fines contents studied.
Nadimi S, Otsubo M, Fonseca J, et al., 2019, Numerical modelling of rough particle contacts subject to normal and tangential loading, Soils and Foundations, Vol: 21, ISSN: 0038-0806
Our understanding of the mechanics of contact behaviour for interacting particles has been developed mostly assumingthat surfaces are smooth. However,real particlesof interest inengineering science are generally rough. While recent studies have considered the influence of roughness on the normal force-displacement relationship, surface roughness was quantified using only a single scalar measure, disregardingthe topology of the surface. There are some conflicting arguments concerning the effect of roughness on the tangential or shear force-displacement relationship. In this study,optical interferometry data are used to generate the surface topology for input into a 3D finite element model. This model is used to investigate the sensitivity of the normal force-displacement response to thesurfacetopology by considering differentsurfaces with similar overall roughness values. The effect of surface roughness on the tangential force-displacement relationshipand the influence of loading history are also explored. The results indicate that quantifying roughness using a single value, such as the root mean square height of roughness, Sq, is insufficient to predict the effect of roughness upon stiffness. It is also shown that in the absenceof interlocking,rough particle surfaces exhibit a lower frictional resistance in comparison with equivalent smooth surfaces.
Ciantia M, Arroyo M, O'Sullivan C, et al., 2019, Micromechanical inspection of incremental behaviour of crushable soils, Acta Geotechnica, Vol: 14, Pages: 1337-1356, ISSN: 1861-1125
In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood.
Dutta TT, Otsubo M, Kuwano R, et al., 2019, Estimating stress wave velocity in granular materials: Apparent particle size dependency and appropriate excitation frequency range, Geotechnique Letters, Vol: 9, Pages: 1-26, ISSN: 2045-2543
There is a lack of consensus in the literature on the influence of the median particle size on stress wave velocityin cohesionless soils. For assemblies of spherical particles with Hertzian contacts, the stress wave velocitiesshould not depend on particle size. However, a link between particle size and stress wave velocity has beenreported in laboratory experiments. In this study, to identify the reasons for the discrepancies, wave velocitymeasurements were performed using planar piezoelectric transducers on four different sizes of alkaline glassbeads and natural silica sands. The experimental results indicate that shear and compression wave velocities areindependent of the median particle size. In accordance with dispersion theory, both the experiments and discreteelement simulations demonstrate that the maximum frequency that can propagate through a granular assembly(i.e., the lowpass frequency) reduces with increasing median particle size. The relationship between the lowpassfrequency and the input signal frequency determines the quality of the received signal and hence the accuracy ofthe interpreted stress wave velocity data. To accurately estimate shear wave velocities, the selected inputfrequencies should match those frequencies which exhibit the largest gain factors and input frequencies shouldnot exceed the half of lowpass frequency. To determine the compression wave velocity, it is suggested to adoptthe start to start method and to choose an input frequency which is slightly lower than the lowpass frequency.
Bandera S, O'Sullivan C, Angioletti-Uberti S, et al., 2019, An evaluation of contact models for particle-scale simulation of clay, 7th International Symposium on Deformation Characteristics of Geomaterials (IS-Glasgow 2019), Publisher: EDP Sciences, Pages: 1-6
Geotechnical engineers are well aware that the particle surface chemistry and the pore fluid composition can significantly influence the mechanical behaviour of clay. Reference is often made to the Derjaguin-Landau-Vervey-Overbeek (DLVO) theory, which enables the electrochemical interactions between charged particles to be estimated. Hitherto, the absence of an effective framework for particle-scale simulation of clay has inhibited a direct link between these electrochemical interactions and clay behaviour (e.g. load:deformation response) or fabric (i.e. the development of a disperse or flocculated fabric). Ebrahimi [1] demonstrated the viability of using molecular dynamics simulations where the clay grains are simulated as ellipsoidal particles whose interactions are described by an analytical expression called the Gay-Berne (GB) potential. While promising when compared to other approaches documented in the literature, Ebrahimi's work considered only a single clay mineralogy and did not explicitly account for the pore fluid composition. This paper considers the use of the Gay-Berne potential in particle-scale modelling of clay from a more general perspective. Calibration of the GB model parameters to predict kaolinite particle interactions reveals a lack of generality in Ebrahimi's approach. The Gay-Berne potential cannot simulate situations in which attractive and repulsive interactions co-exist, which lead to the classical “cardhouse” fabric, as is the case of kaolinite particles interacting via an acidic pore fluid.
Artigaut M, Sufian A, Ding X, et al., 2019, Influence of stress anisotropy on stress distributions in gap-graded soils, ISSN: 2555-0403
The behaviour of gap graded soils comprising non-plastic fines (sand or silt) mixed with a coarser sand or gravel fraction has received attention from researchers interested in internal instability under seepage loading (a form of internal erosion) as well as researchers interested in load:deformation responses. Skempton and Brogan [1] postulated that resistance to seepage induced instability depends upon the proportion of the overall applied stress that is transmitted by the finer fraction. Shire et al. [2] explored Skempton and Brogan’s hypothesis using DEM simulations to look at the proportion of the applied stress transmitted by the finer fractions (α) in ideal isotropic samples. They showed that at low fines contents (FC< FC*) the average stress transmitted by the finer grains is less than the applied stress (α<1), while for FC>FC+ the fines play a key role in stress transmission (α>1); for FC*<FC< FC+, α depends on the sample density. The current contribution describes a series of constant p’ DEM triaxial test simulations carried out to assess the evolution of stress heterogeneity with shearing. The simulation data generated indicate that a sample can transition from being fines dominated (with the fines transmitting a significant proportion of the applied stress and α ≥ 1) to coarse or sand- dominated (with α < 1) as the material dilates during shear deformation. While α reduces as the samples dilate, the relationship between the α and the sample void ratio is non-trivial. The anisotropy of the coarse-coarse contact network exceeds the overall contact force anisotropy; this indicates that the deviator stress is transmitted through a strong force network passing through the coarse-coarse contacts supported by the fine-coarse contacts.
Nguyen H, O’Sullivan C, 2019, Linking macro-scale yielding and micro-scale response, ISSN: 2555-0403
The nature of the link between macro-scale yield and particle scale interactions is poorly understood. This contribution addresses this knowledge gap using discrete element method (DEM) triaxial test simulations performed with a modified version of the granular LAMMPS code, employing a simplified Hertz-Mindlin contact model. Each sample comprised monodisperse spheres and these samples were sheared from different initial density states. The positions of the yield surfaces were identified from the overall load:deformation behaviour, just as in a laboratory test. The overall sample responses observed in these DEM simulations confirmed that the size and positions of the sub-yield surfaces largely depend upon the sample density. The particle-scale parameters monitored included the coordination number, the second order fabric tensor and the proportion of sliding contacts (the sliding fraction). A correlation was established between the variation in coordination number with strain and stiffness degradation with strain. The sliding fraction data information showed that, in contrast to conventional understanding, the particles start to slide relative to each other even within the linear elastic zone.
Otsubo M, Tanu Dutta T, Durgalian M, et al., 2019, Particle-scale insight into transitional behaviour of gap-graded materials – small-strain stiffness and frequency response, 7th International Symposium on Deformation Characteristics of Geomaterials, Publisher: EDP Sciences, Pages: 1-5
This study aims to develop a fundamental understanding of the role of fine particles on the small-strain stiffness of gap-graded granular soils. Stiffness was measured using cyclic triaxial probes, which give a measure of static stiffness, and dynamic wave propagation data, from which the dynamic stiffness can be measured. Assemblies of loosely packed spherical particles were considered. In the laboratory, local deformation transducers were used to measure the static stiffness, while the dynamic stiffness was calculated from stress wave velocities, measured using planar piezoelectric elements. To relate the particle-scale responses to the overall soil stiffness, complementary discrete element method (DEM) simulations were performed in which both static and dynamic stiffnesses were measured. Both the laboratory and the DEM data indicate that at low fines contents (< 30%) the stiffness decreases with increasing fines content. When the fines content increases from 30% to 35% there is a sharp increase in stiffness with increasing fines content; this is understood to mark the transition point at which the fines start to contribute significantly to the overall behaviour. Analyses of the frequency domain response of shear wave signals revealed that the lowpass frequency increases significantly at this transition point. This observation can be used to develop experimental interpretation protocols to assess to what extent fines are contributing to the overall soil stiffness.
Sufian A, Knight C, O'Sullivan C, et al., 2019, Ability of a pore network model to predict fluid flow and drag in saturated granular materials, Computers and Geotechnics, Vol: 110, Pages: 344-366, ISSN: 0266-352X
The local flow field and seepage induced drag obtained from Pore Network Models (PNM) is compared to Immersed Boundary Method (IBM) simulations, for a range of linear graded and bimodal samples. PNM were generated using a weighted Delaunay Tessellation (DT), along with the Modified Delaunay Tessellation (MDT) which considers the merging of tetrahedral Delaunay cells. Two local conductivity models are compared in simulating fluid flow in the PNM. The local pressure field was very accurately captured, while the local flux (flow rate) exhibited more scatter and sensitivity to the choice of the local conductance model. PNM based on the MDT clearly provided a better correlation with the IBM. There was close similarity in the network shortest paths, indicating that the PNM captures dominant flow channels. Comparison of streamline profiles demonstrated that local pressure drops coincided with the pore constrictions. A rigorous validation was undertaken for the drag force calculated from the PNM by comparing with analytical solutions for ordered array of spheres. This method was subsequently applied to all samples, and the calculated force was compared with the IBM data. Linear graded samples were able to calculate the force with reasonable accuracy, while the bimodal samples exhibited slightly more scatter.
Taylor HF, O'Sullivan C, Shire T, et al., 2019, Influence of the coefficient of uniformity on the size and frequency of constrictions in sand filters, Géotechnique, Vol: 69, Pages: 274-282, ISSN: 0016-8505
Constrictions between voids control the filtration and permeability properties of granular materials. This study uses high-resolution microcomputed tomography images and discrete-element modelling to analyse two important characteristics of constrictions in granular filters: (a) the constriction size distribution (CSD) and (b) the constriction density per unit volume. The results demonstrate the importance of the particle size distribution (PSD) and void ratio of the granular material in determining the constriction density, with more widely graded materials having more densely spaced constrictions. The PSD is shown to be the main determinant of the CSD, in agreement with previous studies. The data are used to examine proposed approaches to estimate constriction spacing or void size.
Su T-C, O'Sullivan C, Nagira T, et al., 2019, Semi-solid deformation of Al-Cu alloys: a quantitative comparison between real-time imaging and coupled LBM-DEM simulations, Acta Materialia, Vol: 163, Pages: 208-225, ISSN: 1359-6454
Semi-solid alloys are deformed in a wide range of casting processes; an improved understanding and modelling capability is required to minimise defect formation and optimise productivity. Here we combine thin-sample in-situ X-ray radiography of semisolid Al-Cu alloy deformation at 40–70% solid with 2D coupled lattice Boltzmann method - discrete element method (LBM-DEM) simulations. The simulations quantitatively capture the key features of the in-situ experiments, including (i) the local contraction and dilation of the grain assembly during shear deformation; (ii) the heterogeneous strain fields and localisation features; (iii) increases in local liquid pressure in regions where liquid was expelled from the free surface in the experiment; and (iv) decreases in liquid pressure in regions where surface menisci are sucked-in in experiments. The verified DEM simulations provide new insights into the role of initial solid fraction on the stress-deformation response and support the hypothesis that the behaviour of semi-solid alloys can be described using critical state soil mechanics.
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