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This study investigates the impact of particle plasticity on the mechanical behaviour of a model granular system under plane stress conditions simulated with the Discrete Element Method. A contact model transitioning from nonlinear elasticity to linear plastic deformation is integrated to analyse its effects on a 576-ball-bearing assembly subjected to varying loads. Simulations were conducted using Yade, comparing them with experimental results and traditional elastic models. The findings show that incorporating plastic deformation improves the accuracy of simulated force distributions and the material’s frictional response, particularly under high external loads. These results underscore the need for plasticity-inclusive models in realistic granular simulations, providing valuable insights for practical applications in industries handling high-stress granular systems. [ABSTRACT FROM AUTHOR]

Aggregates consisting of submicron-sized cohesive dust grains are ubiquitous, and understanding the collisional behavior of dust aggregates is essential. It is known that low-speed collisions of dust aggregates result in either sticking or bouncing, and local and permanent compaction occurs near the contact area upon collision. In this study, we perform numerical simulations of collisions between two aggregates and investigate their compressive behavior. We find that the maximum compression length is proportional to the radius of aggregates and increases with the collision velocity. We also reveal that a theoretical model of contact between two elastoplastic spheres successfully reproduces the size- and velocity-dependence of the maximum compression length observed in our numerical simulations. Our findings on the plastic deformation of aggregates during collisional compression provide a clue to understanding the collisional growth process of aggregates. [ABSTRACT FROM AUTHOR]

Granular materials are ubiquitous in nature and industry; their mechanical behavior has been a subject of academic and engineering interest for centuries. One of the reasons for their rather complex mechanical behavior is that stresses exerted on a granular material propagate only through contacts between the grains. These contacts can change as the packing evolves. This makes any deformation and mechanical response from a granular packing a function of the nature of contacts between the grains and the material response of the material the grains are made of. We present a study in which we isolate the role of the grain material in the contact forces acting between two particles sliding past each other. By using hydrogel particles, we find that a viscoelastic material model, in which the shear modulus decays with time, coupled with a simple Coulomb friction model, captures the experimental results. The results suggest that particle material evolution itself may play a role in the collective behavior of granular materials. [ABSTRACT FROM AUTHOR]

Since the seminal work of Cundall and Strack (1979), the Discrete Element Method (DEM) has now become accepted as a key tool amongst researchers exploring the fundamental behavior of granular materials. Along with a sustained increase in the number of publications documenting use of DEM in research, intensive development of new open-source and commercial DEM codes has taken place in the last decades. The credibility of these software packages depends on their capacity to replicate physical observations and to reproduce theoretical expressions. Researchers often calibrate DEM codes against laboratory data to gain confidence about their predictions, however, theoretical verifications at the macro and particle levels are often omitted or not explicitly documented or acknowledged. The validation of DEM codes against theoretical expressions is fundamental to guarantee reproducibility and generality of the software, and to avoid bias in more complex simulations.In this article, a dataset providing numerical simulation data along with input files is presented. The dataset relates to a series of theoretical validation approaches, previously documented in the literature, that were here applied to verify the open-source DEM code LAMMPS. The ability of LAMMPS to capture the macroscopic behaviour of granular packages is evaluated by shearing a face-center-cubic (FCC) array of monosized spheres. The calculation of particle translational/rotational motions and forces/torques is checked by considering a clump rolling down an inclined plane. Additionally, the stress-strain behavior of Toyoura sand under “drained” and “undrained” shearing is characterized by a series of LAMMPS outputs. The dataset collected from these simulations can be employed by users to benchmark new or existing DEM codes. Both the LAMMPS input scripts and the simulation results for all the cases are available in a public repository.

Granular materials are ubiquitous in nature and industry; their mechanical behavior has been a subject of academic and engineering interest for centuries. One of the reasons for their rather complex mechanical behavior is that stresses exerted on a granular material propagate only through contacts between the grains. These contacts can change as the packing evolves. This makes any deformation and mechanical response from a granular packing a function of the nature of contacts between the grains and the material response of the material the grains are made of. We present a study in which we isolate the role of the grain material in the contact forces acting between two particles sliding past each other. By using hydrogel particles, we find that a viscoelastic material model, in which the shear modulus decays with time, coupled with a simple Coulomb friction model, captures the experimental results. The results suggest that particle material evolution itself may play a role in the collective behavior of granular materials.

Fault zones are usually composed of a granular gouge, coming from the wear material of previous slips, which contributes to friction stability. When considering a mature enough fault zone that has already been sheared, different types of infill materials can be observed, from mineral cementation to matrix particles that can fill the remaining pore spaces between clasts and change the rheological and frictional behaviors of the gouge. We aim to understand and reproduce the influence of grain‐scale characteristics on slip mechanisms and gouge rheology (Riedel bands) by employing the discrete element method. A 2D‐direct shear model is considered with a dense assembly of small polygonal cells of matrix particles. A variation of gouge characteristics such as interparticle friction, gouge shear modulus or the number of particles within the gouge thickness leads to different Riedel shear band formation and orientation that has been identified as an indicator of a change in slip stability. Interpreting results with slip weakening theory, our simulated gouge materials with high interparticle friction or a high bulk shear modulus, increase the possible occurrence of dynamic slip instabilities (small nucleation length and high breakdown energy). They may give rise to faster earthquake ruptures. Plain Language Summary: The center of a seismic fault zone is usually composed of a material with granular particles highly contributing to the way the fault moves. This zone may be composed of various infill materials from mineral cementation to smaller particles that can fill the remaining pore spaces between larger particles and change the properties of the fault zone. We aim to understand and reproduce the influence of grain‐scale characteristics on slip mechanisms by employing numerical simulations. A variation of granular particle characteristics leads to different deformation patterns that can be identified as an indicator of a change in slip stability. The obtained results suggest that some granular materials with high interparticle friction or a high bulk shear modulus, increase the possible occurrence of dynamic slip instabilities which may lead to faster earthquake ruptures. This work investigates in more depth the link between the characteristics of the granular material, the deformations of the fault core, and the possible occurrence of an earthquake. Key Points: 2D‐discrete element method simulations performed on numerical fault gouges composed of a very dense assembly of polygonal‐shaped particlesA small change in grain‐scale gouge properties impacts Riedel shear band formation, their orientation angle and the type of Riedel structure formedHigh interparticle friction and high bulk shear modulus increase the breakdown energy and the occurrence of dynamic slip instabilities [ABSTRACT FROM AUTHOR]

• PSA excels in sensing stresses in discrete and continuum materials. • PSA reveals force networks in granular and powder beds. • PSA uncovers size and wall effects in granular systems. • PSA aids DEM in understanding particle interactions in granular materials. • PSA research is applied to rocks, composite soil, snow, and the human eye. Granular materials, such as various grains and granulated powders, are essential in numerous industrial applications and natural processes, including the chemical, pharmaceutical, food, and materials processing industries. They also play a significant role in geotechnical phenomena like the shear-driven collapses of embankments and landslides, which can lead to extensive damage and disruptions. Despite a significant surge in research activities and advancements in discrete element modelling (DEM) tools since the late 1970s, understanding the mechanical characteristics of granular materials remains complex. Experimental techniques such as photo stress analysis (PSA), Nuclear Magnetic Resonance (NMR), and X-ray tomography (XRT) offer valuable insights, yet there is still a lack of detailed information on how granular materials respond mechanically at the microscale under various loading conditions. This knowledge gap affects industrial product quality, equipment safety, production efficiency, operational costs, environmental compliance, and overall safety. In this paper, we initially focus on our scientific contributions to the development of photo stress analysis (PSA) in unravelling the load-transmission characteristics of granular materials of varying sizes and structures (two-dimensional and quasi-three-dimensional) under different mechanical loading environments and applications. Where relevant, discrete element modelling (DEM) complements PSA to enhance our understanding of granular mechanics. We then present the application of PSA to selected key interdisciplinary areas from our recent work, demonstrating the power of PSA. Despite current challenges in applying PSA to three-dimensional granular systems, the results reported here significantly advance our fundamental understanding of the particle-scale and micromechanical characteristics in granular mechanics and particulate engineering. This work highlights the potential of PSA in addressing various inter- and multidisciplinary research problems in the future. [ABSTRACT FROM AUTHOR]

A method for modeling cubically anisotropic elasticity within the discrete element method is presented. The discrete element method (DEM) is an approach originally intended for modeling granular materials (sand, soil, and powders); however, recent developments have usefully extended it to model stochastic mechanical processes in monolithic solids which, to date, have been assumed to be elastically isotropic. The method presented here for efficiently capturing cubic elasticity in DEM is an important prerequisite for further extending DEM to capture the influence of elastic anisotropy on the mechanical response of polycrystals, composites, etc. The system demonstrated here uses a directionally assigned stiffness in the bonds between adjacent elements and includes separate schemes for achieving anisotropy with Zener ratios greater and smaller than one. The model framework is presented along with an analysis of the accessible space of elastic properties that can be modeled and an artificial neural network interpolation scheme for mapping input parameters to model elastic behavior.

A method for modeling cubically anisotropic elasticity within the discrete element method is presented. The discrete element method (DEM) is an approach originally intended for modeling granular materials (sand, soil, and powders); however, recent developments have usefully extended it to model stochastic mechanical processes in monolithic solids which, to date, have been assumed to be elastically isotropic. The method presented here for efficiently capturing cubic elasticity in DEM is an important prerequisite for further extending DEM to capture the influence of elastic anisotropy on the mechanical response of polycrystals, composites, etc. The system demonstrated here uses a directionally assigned stiffness in the bonds between adjacent elements and includes separate schemes for achieving anisotropy with Zener ratios greater and smaller than one. The model framework is presented along with an analysis of the accessible space of elastic properties that can be modeled and an artificial neural network interpolation scheme for mapping input parameters to model elastic behavior.

Abstract Crystallization of granular assemblies has broad implications for rapid and scalable creation of architected materials with applications ranging from structural materials to microarchitected battery electrodes. While significant advances have been made in understanding colloidal self‐assembly at nano to micro scale, the governing mechanisms for organization of dry assemblies of hard spheres remain unclear. Here, we investigate crystallization of mono‐size hard spheres with and without imposed vibration. Using X‐ray computed tomographic analysis coupled with discrete‐element simulations, we unravel the roles of gravity and imposed vibration on the three‐dimensional self‐assembly of the dry spheres. We use these insights to introduce gravity‐mediated epitaxial crystal growth with slow pouring of balls on seeding templates. Contrary to vibration‐induced crystallization, this method can form large single crystals with both close‐packed and rather surprisingly, nonclose‐packed metastable particle arrangements. Our results provide insight for the scalable manufacture of defect‐free granular assemblies that can be used as space‐holding templates to manufacture cellular materials, such as inverse opals and other related topologies. Key points Self‐assembly of hard spheres is a critical step for the scalable manufacture of micro‐architected solids. Via a combination of vibration experiments, 3D X‐ray tomographic observations, and simulations, we elucidate the critical role of gravity in the self‐assembly of hard spheres. We design seeding templates that can not only induce the self‐assembly into stable close‐packed crystal structures but also rather counterintuitively into metastable single crystal structures.

Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a varying amount of mineral cementation between particles, we aim to understand the influence of the strength of interparticle bonds on slip mechanisms by employing the discrete element method. We consider a direct shear model without fluid in 2D, based on a granular sample with angular and faceted grain shapes. Focusing on the physics of shear accommodation inside the granular gouge, we explore the effect of an increase of cementation on effective friction (i.e. stress ratio) within the fault. We find that brittleness and the overall shear strength are enhanced with cementation, especially for dense materials. For the investigated data range, three types of cemented material are highlighted: a poorly cemented material (Couette flow profile, no cohesion), a cemented material with aggregates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R), and a highly-cemented material behaving as a brittle material (with several Riedel bands followed by fault-parallel shear-localization Y). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that the effective friction of a cemented fault cannot be directly predicted from Mohr–Coulomb criteria because of the heterogeneity of the stress state and kinematic constraints of the fault zone. [ABSTRACT FROM AUTHOR]

Many subglacial environments consist of a fine‐grained, deformable sediment bed, known as till, hosting an active hydrological system that routes meltwater. Observations show that the till undergoes substantial shear deformation as a result of the motion of the overlying ice. The deformation of the till, coupled with the dynamics of the hydrological system, is further affected by the substantial strain rate variability in subglacial conditions resulting from spatial heterogeneity at the bed. However, it is not clear if the relatively low magnitudes of strain rates affect the bed structure or its hydrology. We study how laterally varying shear along the ice‐bed interface alters sediment porosity and affects the flux of meltwater through the pore spaces. We use a discrete element model consisting of a collection of spherical, elasto‐frictional grains with water‐saturated pore spaces to simulate the deformation of the granular bed. Our results show that a deforming granular layer exhibits substantial spatial variability in porosity in the pseudo‐static shear regime, where shear strain rates are relatively low. In particular, laterally varying shear at the shearing interface creates a narrow zone of elevated porosity which has increased susceptibility to plastic failure. Despite the changes in porosity, our analysis suggests that the pore pressure equilibrates near‐instantaneously relative to the deformation at critical state, inhibiting potential strain rate dependence of the deformation caused by bed hardening or weakening resulting from pore pressure changes. We relate shear variation to porosity evolution and drainage element formation in actively deforming subglacial tills. Plain Language Summary: The ice at the base of certain glaciers moves over soft sediments that route meltwater through the pore spaces in between the sediment grains. The ice shears the sediment, but it is not clear if this slow shearing is capable of changing the structure or volume of the pore space, or the path of the meltwater that flows through the sediment. To study the relations between the shearing of the sediment and the changes in its pore space, we use computer simulations that portray the sediment as a collection of closely packed spherical grains, where the pores are filled with meltwater. To shear the simulated sediment, the grains at the top are pushed with fixed speeds in the horizontal direction. Despite the slow shear, which is generally thought of as having no effect on pore space, our results show that shearing changes the sizes of the pores in between the grains, where large pores are formed near the top of the sediment layer. If the grains at the top are pushed with uneven speeds, then the largest pores are formed in the areas where grain speeds vary the most. We show that the exchange of meltwater between neighboring pores is faster than the movement of the grains, indicating that the meltwater can adjust quickly to changing pore space. Key Points: Large shear gradients at the ice‐till interface create a narrow zone of elevated porosity in tillThe porosity of granular beds increases with shear strain rate even for subglacial strain ratesPore pressure equilibrates rapidly at the grain scale during critical state shear [ABSTRACT FROM AUTHOR]

Granular excavation is the removal of solid, discrete particles from a structure composed of these objects. Efficiently predicting the stability of an excavation during particle removal is an unsolved and highly nonlinear problem, as the movement of each grain is coupled to its neighbors. Despite this, insects such as ants have evolved to be astonishingly proficient excavators, successfully removing grains such that their tunnels are stable. Currently, it is unclear how ants use their limited information about the environment to construct lasting tunnels. We attempt to unearth the ants' tunneling algorithm by taking three-dimensional (3D) X-ray computed tomographic imaging (XRCT), in real time, of Pogonomyrmex ant tunnel construction. By capturing the location and shape of each grain in the domain, we characterize the relationship between particle properties and ant decision-making within an accurate, virtual recreation of the experiment. We discover that intergranular forces decrease significantly around ant tunnels due to arches forming within the soil. Due to this force relaxation, any grain the ants pick from the tunnel surface will likely be under low stress. Thus, ants avoid removing grains compressed under high forces without needing to be aware of the force network in the surrounding material. Even more, such arches shield tunnels from high forces, providing tunnel robustness. Finally, we observe that ants tend to dig piecewise linearly downward. These results are a step toward understanding granular tunnel stability in heterogeneous 3D systems. We expect that such findings may be leveraged for robotic excavation. [ABSTRACT FROM AUTHOR]

Results from an experiment involving the measurement of individual particle stresses in a two-dimensional mono-disperse assembly of 579 ball bearings are presented. Using a combination of neutron radiography and strain scanning techniques, the full bi-axial stress state was obtained for each particle from which the full contact force network could be established. The evolution of this network was examined over a series of five monotonically increasing loads. Significant levels of inhomogeneity were observed in the form of prominent force chains that showed complex interaction with regions of order and disorder within the assembly. A reduction in the level of inhomogeneity with increasing load was also observed. [ABSTRACT FROM AUTHOR]

During an earthquake, fault slip weakening is often explained by frictional heating phenomena, generally promoting melt production on the fault surface. Here, we investigate the influence of melt production at the scale of the grains composing a fault gouge. We use a modern version of Discrete Element Modeling able to deal with realistic grain shapes in 2D, and couple it with a Multibody Meshfree Approach able to provide a satisfactory proxy for the mechanical behavior of molten grains. Frictional sliding of solid grains and viscous shearing of molten grains are monitored during simulations. Our results confirm the natural tendency of granular gouge to localize deformation in a thin layer, and thus to trigger local melt production. We also show that the appearance of melt is likely to enhance this localization, and might create a positive feedback to its own production. We propose guidelines for the future writing of a friction model including melting, inspired by these simulation results. Key Points: Simulations confirm that fault gouges naturally tend to localize shearing in a narrow area, corresponding to a few tens of grain diameterMolten grains in the fault gouge lead to a reduction in friction, which follows a sigmoid‐like weakening phenomenologyMelt presence leads to a more intense localization of the shearing, leading to a positive feedback to its own generation [ABSTRACT FROM AUTHOR]

Fragmentation of granular clusters may be studied by experiments and by granular mechanics simulation. When comparing results, it is often assumed that results can be compared when scaled to the same value of E / E sep , where E denotes the collision energy and E sep is the energy needed to break every contact in the granular clusters. The ratio E / E sep ∝ v 2 depends on the collision velocity v but not on the number of grains per cluster, N. We test this hypothesis using granular-mechanics simulations on silica clusters containing a few thousand grains in the velocity range where fragmentation starts. We find that a good parameter to compare different systems is given by E / (N α E sep) , where α ∼ 2 / 3 . The occurrence of the extra factor N α is caused by energy dissipation during the collision such that large clusters request a higher impact energy for reaching the same level of fragmentation than small clusters. Energy is dissipated during the collision mainly by normal and tangential (sliding) forces between grains. For large values of the viscoelastic friction parameter, we find smaller cluster fragmentation, since fragment velocities are smaller and allow for fragment recombination. [ABSTRACT FROM AUTHOR]

Colloidal glasses (CGs) made of polymer (polymethylmethacrylate) nanoparticles are promising metamaterials for light and sound manipulation, but fabrication imperfections and fragility can limit their functionality and applications. Here, the vibrational mechanical modes of nanoparticles are probed to evaluate the nanomechanical and morphological properties of various CGs architectures. Utilizing the scanning micro-Brillouin light scattering (µ-BLS), the effective elastic constants and nanoparticles' sizes is determined as a function of position in a remote and non-destructive manner. This method is applied to CG mesostructures with different spatial distributions of their particle size and degree of order. These include CGs with single-sized systems, binary mixtures, bilayer structures, continuous gradient structures, and gradient mixtures. The microenvironments govern the local mechanical properties and highlight how the granular mesostructure can be used to develop durable functional polymer colloids. A size effect is revealed on the effective elastic constant, with the smallest particles and ordered assemblies forming robust structures, and classify the various types of mesoscale order in terms of their mechanical stiffness. The work establishes scanning µ-BLS as a tool for mapping elasticity, particle size, and local structure in complex nanostructures., (© 2024 The Author(s). Small Methods published by Wiley‐VCH GmbH.)

Summary: Capturing the interaction between objects that have an extreme difference in Young's modulus or geometrical scale is a highly challenging topic for numerical simulation. One of the fundamental questions is how to build an accurate multiscale method with optimal computational efficiency. In this work, we develop a material‐point‐spheropolygon discrete element method (MPM‐SDEM). Our approach fully couples the material point method (MPM) and the spheropolygon discrete element method (SDEM) through the exchange of contact force information. It combines the advantage of MPM for accurately simulating elastoplastic continuum materials and the high efficiency of DEM for calculating the Newtonian dynamics of discrete near‐rigid objects. The MPM‐SDEM framework is demonstrated with an explicit time integration scheme. Its accuracy and efficiency are further analyzed against the analytical and experimental data. Results demonstrate this method could accurately capture the contact force and momentum exchange between materials while maintaining favorable computational stability and efficiency. Our framework exhibits great potential in the analysis of multi‐scale, multi‐physics phenomena. [ABSTRACT FROM AUTHOR]

Glacier surges are quasi-periodic episodes of rapid ice flow that arise from increases in slip rate at the ice-bed interface. The mechanisms that trigger and sustain surges are not well understood. Here, we develop a new model of incipient surge motion for glaciers underlain by sediments to explore how surges may arise from slip instabilities within a thin layer of saturated, deforming subglacial till. Our model represents the evolution of internal friction, porosity and pore water pressure within the till as functions of the rate and history of shear deformation, and couples the till mechanics to a simple ice-flow model. Changes in pore water pressure govern incipient surge motion, with less permeable till facilitating surging because dilation-driven reductions in pore water pressure slow the rate at which till tends towards a new steady state, thereby allowing time for the glacier to thin dynamically. The reduction of overburden (and thus effective) pressure at the bed caused by dynamic thinning of the glacier sustains surge acceleration in our model. The need for changes in both the hydromechanical properties of the till and the thickness of the glacier creates restrictive conditions for surge motion that are consistent with the rarity of surge-type glaciers and their geographical clustering. [ABSTRACT FROM AUTHOR]

A novel theoretical framework for the interpretation and modeling of the critical-state behavior of granular soils is proposed in this paper. The theory is built upon the hypothesis that the fabric state of granular assemblies is attracted by a critical fabric surface under continuous shearing. A fabric ratio-stress ratio relation and a fabric-porosity relation are proposed based on recent discrete element modeling (DEM) experimental results. It follows that the existence of a critical-state line in the conventional e-p-q space can be derived by combining the aforementioned elements, offering a new perspective on the critical-state phenomenon. Simple fabric evolution laws are proposed to capture the first-order features of the stress-strain-fabric behaviors of granular soils. The basic evolution laws are then enhanced to capture the behavior of natural sand. The computed results agree well with the data from Toyoura sand under a wide range of pressure and density conditions. This exploratory study reveals a promising pathway to integrate various microstructure information in the continuum modeling of granular soils. The generality and limitations of the model is also discussed at the end of this paper. [ABSTRACT FROM AUTHOR]

Pavement compaction is a process of structure evolution of materials accompanied by energy dissipation, which is closely related to the inherent granular properties of asphalt mixture. This research achieved dynamic characterization of particle migration energy and contact force development through particle tracking technique and discrete element simulation. Analytical theories for multi-stage structure evolution and compaction mechanisms were proposed based on granular properties. Results indicate that kinetic, dissipative, and potential energies dynamically represent stages of structure evolution, determined by particle collisions, self-organization into arches, and interlocking reinforcement. The initial density state is established after interparticle collisions, while self-organization occurs as particles adaptively rearrange spatially by rotation in response to unbalanced forces, accompanied by a cyclical strengthening trend of arch formation and collapse. Arches facilitate stress redistribution toward isotropy, with strong contact forces exhibiting a top-down and side-center trend, progressively supported by particles larger than 4.75 mm. Theoretical measures for compaction control were proposed, including pre-calculation of I C , selection of compaction temperature corresponding to the optimum energy efficiency, and control of principal stress deflection within the friction cone. This research aims to provide insight into granular properties and compaction dynamics, thereby contributing to intelligent compaction. [Display omitted] • Multi-stage structure evolution and compaction mechanisms of asphalt mixtures were illustrated through granular properties. • Particle migration energy was proposed to reflect the inherent dynamic response of the asphalt mixture. • Frictional dissipation, arching effect, and interlocking govern collision, self-organization, and stress deflection. • Optimizing the temperature and I C based on the Stribeck curve and particle characteristics affects compaction quality. [ABSTRACT FROM AUTHOR]

Abstract Granular collisions are characterized by a threshold velocity, separating the low-velocity regime of grain sticking from the high-velocity regime of grain bouncing: the bouncing velocity, v b . This parameter is particularly important for nanograins and has applications for instance in astrophysics where it enters the description of collisional dust aggregation. Analytic estimates are based on the macroscopic Johnson-Kendall-Roberts (JKR) theory, which predicts the dependence of v b on the radius, elastic stiffness, and surface adhesion of grains. Here, we perform atomistic simulations with model potentials that allow us to test these dependencies for nanograin collisions. Our results not only show that JKR describes the dependence on materials parameters qualitatively well, but also point at considerable quantitative deviations. These are the most pronounced for small adhesion, where elastic stiffness does not influence the value of the bouncing velocity.

In this work we develop an experimental method to study interparticle force transmission in 3D. An initial 2D study of volumetric data for commercially available polyethylene fluorescent microspheres shows that three-dimensional effects play a significant role in deformation of real granular assemblies. This highlights the need for more experimental work to validate existing numerical models and motivate further development of theory and models describing real 3D granular media. A full 3D analysis method is described where volumetric images are captured via confocal microscopy and the displacement fields for each particle are determined using digital volume correlation (DVC). This data is then used to determine the average strain in each particle as well as the assembly's fabric (geometric descriptors of particles and contacts) which are in turn used for the Granular Element Method (GEM) to determine interparticle forces. Additionally, we perform a DVC analysis for an in-house produced polyacrylamide copolymer grain with a volumetric fluorescent speckle pattern. This demonstrates that with sufficiently small particles, it is possible to use our imaging and analysis methodology to determine intergranular force transmission in 3D experiments. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Layered soil materials are characterised using different techniques. • Failure patterns of layered soil systems are analysed using DPIV. • A correlation between the slip surface angle β and H/B is presented. • A good level of comparisons between the DPIV and FEM results. • A new methodology is proposed to estimate UBC of layered soil. Abstract In the geotechnical and terramechanical engineering applications, precise understandings are yet to be established on the off-road structures interacting with complex soil profiles. Several theoretical and experimental approaches have been used to measure the ultimate bearing capacity of the layered soil, but with a significant level of differences depending on the failure mechanisms assumed. Furthermore, local displacement fields in layered soils are not yet studied well. Here, the bearing capacity of a dense sand layer overlying loose sand beneath a rigid beam is studied under the plain-strain condition. The study employs using digital particle image velocimetry (DPIV) and finite element method (FEM) simulations. In the FEM, an experimentally characterised constitutive relation of the sand grains is fed as an input. The results of the displacement fields of the layered soil based DPIV and FEM simulations agreed well. From the DPIV experiments, a correlation between the slip surface angle and the thickness of the dense sand layer has been determined. Using this, a new and simple approach is proposed to predict theoretically the ultimate bearing capacity of the layered sand. The approach presented here could be extended more easily for analysing other complex soil profiles in the ground-structure interactions in future. [ABSTRACT FROM AUTHOR]

Crystallization of dry particle assemblies via imposed vibrations is a scalable route to assemble micro/macro crystals. It is well understood that there exists an optimal frequency to maximize crystallization with broad acceptance that this optimal frequency emerges because high-frequency vibration results in overexcitation of the assembly. Using measurements that include interrupted X-ray computed tomography and high-speed photography combined with discrete-element simulations we show that, rather counterintuitively, high-frequency vibration underexcites the assembly. The large accelerations imposed by high-frequency vibrations create a fluidized boundary layer that prevents momentum transfer into the bulk of the granular assembly. This results in particle underexcitation which inhibits the rearrangements required for crystallization. This clear understanding of the mechanisms has allowed the development of a simple concept to inhibit fluidization which thereby allows crystallization under high-frequency vibrations.

According to classical critical state theory (CST) of granular mechanics, two conditions on the stress ratio and void ratio are satisfied when reaching and maintaining a critical state (CS). Therefore, CST dictates the necessity of these two conditions although their sufficiency has not been demonstrated, but only assumed. The present work challenges this assumption based on the results of a virtual twodimensional (2D) discrete element method (DEM) experiment. The virtual sample is first brought to CS and then rotation of the principal axes (PA) of stress is imposed while keeping stress principal values fixed. The rotation induces a void ratio reduction and thus, abandonment of CS, despite the fact the two CST conditions are satisfied at the initiation of the rotation process, since the stress principal values remain fixed and the void ratio is at its critical state value for the given fixed pressure. The recently proposed anisotropic critical state theory (ACST) remedies this incompleteness of CST by enhancing its two conditions by a third, related to the critical state value of a fabric anisotropy variable, defined as the trace of the product of the fabric anisotropy tensor and the loading direction tensor. This third condition is violated by the stress PA rotation and can explain the aforementioned void ratio reduction. ACST can also explain various other response characteristics that cannot be addressed by classical CST with no fabric anisotropy consideration. In conclusion, the three conditions of ACST are shown to be both necessary and sufficient for reaching and maintaining CS. [ABSTRACT FROM AUTHOR]

The relative equilibria for the spherical, finite density three-body problem are identified. Specifically, there are 28 distinct relative equilibria in this problem which include the classical five relative equilibria for the point-mass three-body problem. None of the identified relative equilibria exist or are stable over all values of angular momentum. The stability and bifurcation pathways of these relative equilibria are mapped out as the angular momentum of the system is increased. This is done under the assumption that they have equal and constant densities and that the entire system rotates about its maximum moment of inertia. The transition to finite density greatly increases the number of relative equilibria in the three-body problem and ensures that minimum energy configurations exist for all values of angular momentum. [ABSTRACT FROM AUTHOR]

Here we investigate three long-standing principles of granular mechanics and avalanche science: dilatancy, effective stress and dispersive pressure. We first show how the three principles are mechanically interrelated: Shearing of a particle ensemble creates a mechanical energy flux associated with random particle movements (scattering). Because the particle scattering is inhibited at the basal boundary, there is a spontaneous rise in the center of mass of the particle ensemble (dilatancy). This rise is connected to a change in potential energy. When the center of mass rises, there is a corresponding reaction at the base of the flow that is coupled to the vertical acceleration of the ensemble. This inertial stress is the dispersive pressure. Dilatancy is therefore not well connected to effective-stress-type relations, rather the energy fluxes describing the configurational changes of the particle ensemble. The strict application of energy principles has far-reaching implications for the modeling of avalanches and debris flows and other dangerous geophysical hazards. [ABSTRACT FROM AUTHOR]

RoboClam is a biomimetic burrowing robot that imitates the valve expansion/contraction digging pattern of the Atlantic razor clam, Ensis directus , to dig into submerged soil using an order of magnitude less energy than would be required to push into the soil with brute force. This paper examines whether it would theoretically be possible to use the same method to dig into dry soil. The stress state of the soil around the contracting robot was analyzed, and a target zero-stress state for dry soil digging was found. Then, the two possible modes of soil collapse were investigated and used to determine how quickly the robot would have to contract to achieve the target zero-stress state. It was found that for most dry soils, a RoboClam-like device would have to contract in 0.02 s, a speed slightly faster than the current robot is capable of, but still within the realm of possibility for a similar machine. These results suggest that the biomimetic approach successfully used by RoboClam to dig into submerged soil could feasibly be used to dig into dry soil as well. [ABSTRACT FROM AUTHOR]

A novel general constitutive formulation framework of rate independent plasticity is presented where the stress rate tensor is split in two components, and plastic loading is affected separately by each one of these two components in relation to a common loading direction normal to a single smooth yield surface in stress space. This split-stress rate separate effect provides greater flexibility for simulating the material response under combined stress rate components loading. The difficulty arises when loading and unloading events may occur interchangeably between the two stress rate components and special attention is required to avoid irrational results and guarantee continuity of response upon rotation of the total stress rate direction at a loading stress point. The formulation can acquire two different analytical schemes, one more complicated but incrementally linear, while the other is simpler but incrementally nonlinear. The key constitutive ingredient of split stress rate definition is a multifaceted choice, and three options are presented. One option which is of particular interest to granular mechanics, splits the stress rate into a component that changes the stress principal values at fixed principal axes, while the other component does exactly the opposite, i.e., rotates the stress principal axes at fixed principal values. For this option various constitutive ingredients and new concepts are introduced and extensively elaborated, with emphasis on dilatancy, understanding that these elaborations are of an exploratory nature intending to prompt further research along the proposed guidelines. [ABSTRACT FROM AUTHOR]

Pyroclastic density currents are generated in explosive volcanic eruptions when gas and particle mixtures remain denser than the surrounding atmosphere. These mobile currents have a diversity of flow regimes, from energetic granular flows to turbulent suspensions. Given their hazardous nature, much of our understanding of the internal dynamics of these currents has been explored through mathematical and computational models. This review discusses the anatomy of these currents and their phenomenology and places these observations in the context of forces driving the currents. All aspects of the current dynamics are influenced by multiphase interactions, and the study of these currents offers insight into a high-energy end-member of multiphase flow. At low concentration, momentum transfer is dominated by particle-gas drag. At higher concentration, particle collisions, friction, and gas pore pressure act to redistribute momentum. This review examines end-member theoretical models for dilute and concentrated flow and then considers insight gained from multiphase simulations of pyroclastic density currents. [ABSTRACT FROM AUTHOR]