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Granular materials belong to the class of complex materials within which rich properties can emerge on large scales despite a simple physics operating on the microscopic scale. Most notable is the dissipative behaviour of such materials mainly through non-linear frictional interactions between the grains which go out of equilibrium. A whole variety of intriguing features thus emerges in the form of bifurcation modes in either patterning or un-jamming. This complexity of granular materials is mainly due to the geometrical disorder that exists in the granular structure. Diverse configurations of grain collections confer to the assembly the capacity to deform and adapt itself against different loading conditions. Whereas the incidence of frictional properties in the macroscopic plastic behavior has been well described for long, the role of topological reorganizations that occur remains much more elusive. This paper attempts to shed a new light on this issue by developing ideas following the configurational entropy concept within a proper statistical framework. As such, it is shown that contact opening and closing mechanisms can give rise to a so-called configurational dissipation which can explain the irreversible topological evolutions that granular materials undergo in the absence of frictional interactions. [ABSTRACT FROM AUTHOR]

The focus of this study is the experimental characterization of cemented granular materials, with the aim of identifying the microscopic properties of the solid bonds and describing the extension to macroscopic mechanical strength of cemented samples. We chose to use artificially bonded granular materials, made of glass beads connected by solid paraffin bridges. The results of several sets of laboratory tests at different scales are presented and discussed. Micromechanical tests investigate the yield strength of single solid bonds between particles under traction, shearing, bending and torsion loading, as a function of variations in particle size, surface texture and binder content. Macro-scale tensile tests on cemented samples explore then the scale transition, including influence of confining walls through homothetic variations of the sample size. Despite the large statistical dispersion of the results, it was possible to derive and validate experimentally an analytical expression for micro tensile yield force as a function of the binder content, coordination number and grain diameter. In view of the data, an adhesive bond strength at the contact between bead and solid bond is deduced with very good accuracy and it is even reasonable to assume that the other threshold values (shear force, bending and torsion moments) are simply proportional to the tensile yield, thus providing a comprehensive 3D model of cemented bond. However, the considerable dispersion of the data at the sample scale prevents validation of the extended model for macroscopic yield stress. A final discussion examines the various factors that may explain intrinsic variability. By comparison with other more realistic systems studied in the literature in the context of bio-cementation, our artificial material nevertheless appears suitable for representing a cemented granular material. Being easy to implement, it could thus enable the calibration of discrete cohesion models for simulation of practical applications. [ABSTRACT FROM AUTHOR]

We introduce a novel numerical method for the simulation of soft granular materials, in which the particles can undergo large strains under load without rupture. The proposed approach combines an explicit total Lagrangian formulation of Material Point Method (TLMPM) with the Contact Dynamics (CD) method. The TLMPM resolves particle bulk deformations whereas the CD treats contact interactions between soft particles. The efficiency and accuracy of this approach are illustrated by analyzing diametral compression of a soft circular particle and the compaction of an assembly of soft particles up to very high levels of packing fraction. We show that although the assembly undergoes a jamming transition, the particles continue to rearrange as they get increasingly distorted under load. Interestingly, as the packing fraction increases, a transition occurs to a regime fully governed by particle shape change. The evolution of the global stress as well as the connectivity of the particles as a function of the packing fraction are discussed and a predictive model relating stress to packing fraction beyond jamming transition is proposed. [ABSTRACT FROM AUTHOR]

The softening effect has been widely accepted as the fundamental mechanical property of the granular materials, which underlies some specific phenomena such as fluidization during vibration. In this paper, a series of resonance column experiments are performed to observe the modulus softening of granular materials. A statistical softening model is subsequently proposed and its applicability is verified through a quantitative analysis of the variation of the normalized modulus by changing the external confining pressure. The average potential energy in grain contact has been found to be a power-law scaling with grain size. An evolution model is further implemented to account for the experimental findings on the evolution of modulus of the granular system subjected to different confining pressures. The modulus evolution, including softening and recovery, can be captured by the unified evolution model. [ABSTRACT FROM AUTHOR]

Granular materials may undergo static liquefaction under undrained shearing, which is related to many natural hazards, such as landslides. Despite great efforts, the overall process of static liquefaction remains largely unclear. Numerical undrained shear tests on granular assemblies are performed using the discrete element method, and network-based methods are introduced to investigate the evolution of the contact network. The occurrence of static liquefaction is attributed to the collapse of the contact network induced by contact loss. The weak subnetwork is broken before reaching the liquefaction point, while the strong contact subnetwork remains relatively unchanged. The failure of the strong subnetwork is further investigated by the mechanical features of two important mesoscopic structures, namely force chains and contact loops. The buckling events with buckling ratio exceeding the envelope line and the transition from small loops to large loops significantly destroy the stability of force chains, which causes the failure of force chains and eventually the occurrence of static liquefaction. The relationship of macroscopic stress with microscopic and mesoscopic structures is also identified. The evolution of node degree and global efficiency versus macroscopic stress presents a two-stage development mode, and the buckling events accelerates the transition of the development mode. Our analysis elucidates the occurrence of static liquefaction from microscopic and macroscopic perspectives, which are essential for better prediction and modeling of the catastrophic failures under undrained loading path of granular materials. [ABSTRACT FROM AUTHOR]

We consider a dense aggregate of elastic, frictional particles isotropically compressed and next uniaxial strained at constant pressure. We show how failure can be predicted if fluctuations in the kinematics of contacting particles are introduced. We focus on the second order work and the possibility that at some stressed states it becomes negative under proper perturbations. Our analysis involves both a theoretical model and numerical simulations based upon the distinct element method (DEM). The theoretical model deals with contacting particles with incremental relative displacements that deviate from the average deformation in order to ensure their equilibrium. Because of this, the macroscopic stiffness tensor of the aggregate, that relates increments in stress with increments in strain, does not have the major symmetry. Consequently, in the hardening regime, we predict stressed states in which the second order work vanishes. The model seems transparent, and it makes clear and illustrative the role played by the fluctuations introduced in the kinematics of contacting particles in relation to the vanishing of second order work in an aggregate of compressed particles. The comparison with numerical simulations data supports the model. Statistical representation of the aggregate: conditional average. [ABSTRACT FROM AUTHOR]

To study the acoustic radiation characteristics of forced vibration of granular assemblies, the acoustic radiation model of a single particle and the acoustic radiation model of granular assemblies are established. Combined with experimental verification, the variation law of radiated sound power in the process of forced vibration of granular assemblies is studied. The effects of vibration characteristics, material characteristics, mass characteristics, and distance characteristics on the acoustic radiation of forced vibration of granular assemblies are explored. The radiation range of radiated ultrasound in the process of forced vibration of granular assemblies is analyzed. The research results show that under the condition of forced vibration, the radiated sound power of the granular assemblies is obvious. The internal vibration of the granular assemblies will cause the granular particles to radiate the sound power of multiple frequencies outward. The granular particles will produce obvious ultrasonic sound power in the violent vibration state. The frequency of the sound wave will generally have a significant attenuation effect at 10 cm. Considering the relevant experimental research and engineering application of granular materials, the ultrasonic radiation characteristics should be analyzed to avoid the influence of ultrasonic radiation caused by its severe vibration. [ABSTRACT FROM AUTHOR]

The standard rate‐and‐state friction (RSF) has extensively captured frictional behaviors, but it fails to explain the velocity dependence of frictional stability transition and widespread slow‐slip events (SSEs) in experiments and nature adequately. An alternative microphysical Chen‐Niemeijer‐Spiers (CNS) model can well describe the velocity dependence of frictional behaviors of granular gouges. Using the original CNS model, standard RSF parameters can be quantified microphysically. However, some micro‐parameters are not easy to estimate quantitatively, making it difficult to extrapolate to natural and experimental conditions. Here, we simplify the microphysically‐derived RSF parameters including direct effect a, evolution effect b, and critical slip distance Dc, as well as equivalent values (aeq, beq, and Deq). The simplified friction parameters directly illustrate their velocity dependence, namely the essentially constant a, aeq, and Dc, negatively velocity‐dependent b and beq, as well as varying Deq for different laws. They are roughly consistent with experimental results in various fault gouges. A modified CNS model is further derived from the original CNS model, establishing a direct link between the standard RSF and CNS models. The modified CNS model exhibits virtually identical frictional behaviors to the original CNS, but differs from the standard RSF at large velocity perturbations. Moreover, the linearized stability analysis indicates that the critical stiffness for the modified CNS model is velocity‐dependent. Compared with the standard RSF, the modified CNS model not only explains the velocity dependence of frictional stability transition, but also exhibits a more gradual transition for SSEs with a broader range of stiffness ratios. Plain Language Summary: Geological observations have illustrated a growing body of slow fault slip events that radiate little or no seismic energy, but the underlying mechanisms are not fully understood. This study examines two widely‐used friction models, namely, an empirical friction model and a microphysically‐based model. Both of them have been employed to capture experimental and natural observations, but the microphysical model shows significant velocity dependence of friction and is able to interpret friction parameters of the empirical friction model microphysically. We further examine the microphysically‐derived friction parameters, omit some micro‐parameters that contribute negligibly, directly describe their velocity dependence, and derive a simple friction model. The modified friction model is roughly consistent with the microphysically‐based model. More importantly, the modified model can not only describe the velocity dependence of frictional stability transition, but also explain the widespread slow fault slip events in experiments and nature. Key Points: Microphysically‐derived rate‐and‐state friction parameters are simplifiedA modified Chen, Niemeijer, and Spiers (CNS) model is derived from the original microphysical modelThe modified CNS model captures a more gradual transition for slow‐slip events [ABSTRACT FROM AUTHOR]

Determining the shear‐velocity dependence of dry granular friction can provide insight into the controlling variables in a dry granular friction law. Some laboratories believe that the quality of this study is at the forefront of the discipline for the following reasons. Results suggest that granular friction is greatly affected by shear‐velocity (v), but shear experiments over the large range of naturally occurring shear‐velocities are lacking. Herein we examined the shear velocity dependence of dry friction for three granular materials, quartz sand, glass beads and fluorspar, across nine orders of magnitude of shear velocity (10−8–2 m/s). Within this range, granular friction exhibited four regimes, following a broad approximate "m" shape including two velocity‐strengthening and two velocity‐weakening regimes. We discuss the possible physical mechanisms of each regime. This shear velocity dependence appeared to be universal for all particle types, shapes, sizes, and for all normal stresses over the tested range. We also found that ultra‐high frequency vibration as grain surfaces were scoured by micro‐chips were formed by spalling at high shear velocities, creating ∼20 μm diameter impact pits on particle surfaces. This study provides laboratory laws of a friction‐velocity (μ‐v) model for granular materials. Plain Language Summary: Numerous phenomena of earth science can be simplified to shearing granular systems, such as landslide gouge and rock avalanches. It is believed that friction varies with shear velocity, and this is no exception for dry granular materials. Studying the law of friction variation over as large a range of velocity changes as possible aids in establishing a full life‐cycle hypothesis for landslides from initiation, through movement to cessation. We sheared granular materials across nine orders of magnitude, from 10−8 to 2 m/s and obtained an "M" shape law for the variation of friction with velocity. By analyzing the acoustic frequency of shear experiments at different velocities, we discovered that the acoustic frequency increased with shear velocity. By comparing the microscopic morphology of the particle surface before and after high‐velocity shearing, we hypothesized that the impact pits may be left by particle collisions during high‐velocity shearing. We proposed that the role of ultra‐high frequency vibration on high‐velocity shear weakening should be considered, which provides a basis for accurately revealing the physical mechanism of granular friction variation with velocity. Key Points: This paper examines the shear velocity dependence of friction for dry granular materials, across nine orders of magnitude in shear velocity (10−8–2 m/s)An "M‐shaped" law for the variation of friction with velocity is obtained including two velocity‐strengthening and two velocity‐weakening lawsThere is microscopic evidence that ultra‐high frequency vibration may cause high velocity shear weakening [ABSTRACT FROM AUTHOR]

Sustainable road infrastructure is largely dependent on the resilience of the road foundation to withstand dynamic stresses. From Empirical to Mechanistic Empirical Pavement Design Guidelines (MEPDG), the use of unbound granular materials (UGMs) has been specified for road foundation bases for flexible roads. Rutting is a common road failure from these designs. MEPDG includes climate change factors that essentially invalidate the empirical approach. However, both design approaches still rely on the anisotropic particle interlock of UGM upon compaction, notwithstanding traditional or intelligent compaction technology. The anisotropic character of compacted UGM only exhibits vertical stiffness with the highest shear stress located in the mid layer. This makes the interface between the surface of the compacted subgrade and the bottom of the compacted subbase incompatible. Consequently, the stiffness of the UGM is satisfactory for static loads, but when these become dynamic, the inter-granular transmission of these stresses creates a new challenge, non-linearity of stress and the ensuing propensity for stress rotation. A model subbase of a road foundation was modified acknowledging the natural angle of repose of UGM. The ribbed saw tooth structure with an inherent composite anisotropic character seemed to offer significant stiffness. A threepoint shear stress localisation (3-PSSL) was experimentally created with three stress levels identified within the subbase and a fourth stress level in the base. This geomorphic lock is possible by the use of an angular geo-lock compactor (AGL). Added stiffness in the road foundation should avert rutting, thereby increasing resilience and sustainability of road infrastructure, and ultimately stimulating socio-economic development. World Intellectual Property Organisation (WIPO) has documented this ongoing research as patent No. WO 2021/048590 A1 awaiting entry into The National Phase. This is Work in Progress, where particle intra-lock may invalidate particle interlock. [ABSTRACT FROM AUTHOR]

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In the last two decades, bottom ash has been widely used as an alternative to conventional granular fill in various construction activities, either by replacing the whole or a portion of the fill material. This study investigates the dynamic behaviour of bottom ash and compares its response with sand through strain-controlled constant volume cyclic simple shear tests. The effects of relative density, shear strain, vertical stress, and number of loading cycles on the dynamic behaviour of bottom ash and sand are discussed. The results of the study indicate that bottom ash is highly compressible than sand under one-dimensional anisotropic compression, attributed to the angular and vesicular texture of bottom ash particles. This texture results in associated particle breakage in dense bottom ash specimens. Further, the damping of bottom ash is lower than that of sand in most cases, and the shear modulus is found to be smaller or larger than that of sand depending on shear strain magnitude and the number of loading cycles. At medium shear strain magnitudes, bottom ash and sand exhibit almost the same initial shear modulus. However, the degradation of the shear modulus with the number of loading cycles is significant in sand compared to bottom ash, thereby indicating that bottom ash exhibits a better dynamic resistance than sand. The study highlights that specimen dimensions critically influence the results obtained from cyclic simple shear tests and that careful interpretation of results is necessary, especially when comparing materials with different particle sizes or admixtures. Furthermore, the influence of cyclic loading frequency on the overall dynamic response of tested materials is also discussed. The findings of the study contribute to the understanding of bottom ash behaviour and its potential use as a granular fill material in construction applications. [ABSTRACT FROM AUTHOR]

The study aims to investigate the normal and oblique impact of an elastic sphere (tennis ball) on a granular surface (clay) and two different plastic tape lines. In this research, we model the impact force with a mathematical elastoplastic force model, and a differential approach is used. The model is applied for an impact with granular material (green clay) and plastic surfaces (line tapes). We investigated the normal and oblique impact dynamics of a sphere (tennis ball). The impact duration is divided into two phases: compression with an elastoplastic force and restitution with an elastic force. The laboratory experiments in various configurations are recorded with a high frame-per-second camera and analyzed using image processing methods. The mathematical model for the impact with rebounds is verified with the experimental set-up for the considered surfaces. The viscoelastic and elastic forces agree well with the experimental data. The impact parameters of the granular surface and plastic tapes are compared. The ANOVA test suggests robust statistical significance in the coefficient of restitution between granular surfaces and plastic tapes. Our force model for impact performs well, and the impact responses of the sphere on the granular surface and the plastic line tapes are significantly different. [ABSTRACT FROM AUTHOR]

In the last decade, a new multi‐scale FEM×DEM approach has been developed using Finite Element Method (FEM) coupled with Discrete Element Method (DEM) as a constitutive law to account for the specificities of the mechanical behavior of granular materials. In FEM×DEM model, a DEM calculation is performed on a particle assembly (volume element—VE) at each Gauss point. Recent publications have demonstrated that FEM×DEM approach naturally captures the discrete and anisotropic nature of granular materials. Despite its advantages, FEM×DEM with classical FEM, suffers from mesh dependency, especially when material enters softening phase and exhibits strain localization. To overcome this limitation, FEM×DEM model has been enriched by incorporating a local second gradient model. Nevertheless, the existence of multiple possible solutions is observed. In this paper, we study the variability and loss of uniqueness of numerical solutions to a boundary value problem. Different VEs with equivalent mechanical properties are generated and used to model the pressuremeter tests by means of FEM×DEM. The modeling results show a great variability of the numerical results, both in shape of borehole and in different modes of shear bands. For the same VE, the loss of uniqueness of numerical solutions is evidenced by a slight modification of loading history at the level of the internal pressure applied to the borehole. Finally, we show that when a certain heterogeneity is introduced by using different VEs within the same BVP, even if the uniqueness of the solution is not guaranteed, the set of possible solutions seems more restrained. [ABSTRACT FROM AUTHOR]

The clogging phenomenon finds extensive application in both industrial processes and daily life events. While this broad spectrum of application motivated extensive research to identify the general factors underlying the clogging mechanism, it results in a fragmented and system-specific understanding of the entire clogging process. Therefore, it is essential to establish a holistic understanding of all contributing factors of clogging based on the microscopic physical mechanisms. In this paper, we experimentally investigate clogging of granular materials in a two-dimensional hopper flow and present a self-consistent physical mechanism of clogging based on precursory chain structures. These chain structures follow a specific modified restricted random walk, and clogging occurs when they are mechanically stable enough to withstand the flow fluctuations. We introduce a single-particle model that can explain the arch-forming probability. Our results provide insight into the microscopic mechanism behind clogging and a broader understanding of the dynamics of dense granular flow. The study of the general factors underlying clogging are fragmented and application-driven due to its broad spectrum of applications. The authors propose a holistic understanding of the clogging process by investigating the clogging of a granular hopper flow using high-speed imaging, finding a signature formation of precursory chain structures. [ABSTRACT FROM AUTHOR]

Particle segregation in dense flowing size-disperse granular mixtures is driven by gravity and shear, but predicting the associated segregation force due to both effects has remained an unresolved challenge. Here, a model of the combined gravity- and kinematics-induced segregation force on a single intruder particle is integrated with a model of the concentration dependence of the gravity-induced segregation force. The result is a general model of the net particle segregation force in flowing size-bidisperse granular mixtures. Using discrete element method simulations for comparison, the model correctly predicts the segregation force for a variety of mixture concentrations and flow conditions in both idealized and natural shear flows. [ABSTRACT FROM AUTHOR]

The wave propagation in dilatant granular materials is described by a nonlinear evolution equation of the fifth order deduced by Giovine–Oliveri in (Meccanica 30(4) (1995) 341–357). In this paper, we study the well-posedness of the classical solutions for the Cauchy problem, associated with this equation. [ABSTRACT FROM AUTHOR]

We present a framework to study regolith segregation on rubble-pile asteroids—self-gravitating granular aggregates—due to seismic shaking induced by impacts sustained during their lifetimes. We first relate the amplitude and frequency of surface vibrations to the location and severity of an impact, and the rubble body's geometry and bulk properties. For illustration, the body is taken to be an ellipsoid with size and spin close to that of Itokawa, although more complex asteroid shapes may be incorporated. We then model the body's collisional history stochastically given the variability in the impact activity on an asteroid. Finally, we use discrete element simulations to investigate the regolith's response to impacts. In these simulations, in any sample collisional history, every time an impact occurs, a bin filled with a grain mixture and located at the region of interest on the asteroid is vibrated at that impact's associated amplitude and frequency. Using this framework, we find that impact-driven seismicity is sufficient to drive size segregation on small rubble-piles, but the segregation quality depends on several aspects, e.g. total impact energy supplied, placement of the region of interest, bulk wave speed and seismic diffusivity. [ABSTRACT FROM AUTHOR]

A simple continuum model is proposed for discrete granular avalanches, as observed in slowly rotating drums. Subject to granular transfer across the basal interface, the model evolves the transient surface inclination and mid-slope velocity profile of the avalanches, from failure to arrest. This is done using new boundary conditions that allow entrainment or detrainment contingent on the basal shear rate. For entrainment to occur, the basal shear stress must overcome the erosion resistance of the jammed deposit, while for detrainment to take place the basal shear rate must vanish. The resulting avalanche dynamics is controlled by a single dimensionless number, the ratio of excess slope at failure to excess resistance to erosion, that can be determined from inclination history data. This number provides a measure of slope brittleness, from which the peak flow rate and arrest inclination can be predicted. Analytical techniques are used to derive model solutions, and clarify the resulting behaviour. Finally, the model is tested by comparing solutions with laboratory experiments and discrete particle simulations. [ABSTRACT FROM AUTHOR]

Cold‐box sand (CBS) belongs to the granular materials and consists of sand and a binder. The behavior of CBS is simulated with a micropolar model, whereby the additional degree of freedom of the model describes the rotation of the sand grains. The model is used to generate a shear band under pressure for three different meshes, where the force‐displacement curves of the three meshes converge so that no mesh dependence occurs. Another requirement of the model is the consideration of asymmetric behavior for compression and tension. Due to the additional degree of freedom the implicit implementation of the micropolar continuum is very time‐consuming. Therefore, an explicit implementation is considered as an alternative possibility. This paper compares the advantages and disadvantages of both methods and the results for both calculations. [ABSTRACT FROM AUTHOR]

The purpose is to study seismic response and dynamic lateral pressure of the silo wall with different storage conditions under the earthquake. FEM was used to conduct dynamic time history analysis of the silo. The seismic response of the empty silo was smaller than that of the filled silos, and for the latter, under seismic action, the bulk sand particles and the silo wall had a more complex dynamic interaction, and nodes displacements on the wall and columns mainly fluctuated rising. For the deformation of the bulk sand, it was similar to a parabolic pattern in section, the closer to the silo center the greater the deformation was, and the farther away from the silo center the smaller the deformation was. The maximum acceleration was near the top of the wall in the empty silo, near the middle of the wall in contact with bulk sand in the filled silos. Nodes maximum overpressure coefficient was 1.6 and 2.9 in half fully and fully filled silos, respectively, but the coefficient 2.9 was greater than the standardized value of 2.0, therefore, the suggested overpressure coefficient was proposed. Meanwhile, Mises stress was investigated, and it was smaller in the empty silo, but larger in the filled silos, particularly in and around the ring beam due to the dynamic interaction between the sand and the silo wall, where the stiffness had a sudden change, and the maximum stress at the ring beam increased gradually with the seismic action. [ABSTRACT FROM AUTHOR]

Particle crushing takes place in a wide range of engineering applications and affects fluid flow through granular materials. This study examines volumetric changes and the evolution of hydraulic conductivity during particle crushing using a combination of high-stress experiments, data compilation from published studies and particle-scale numerical simulations. An asymptotically correct hyperbolic model fits void ratio versus stress data and provides reliable estimates of the yield stress. Alternatively, hydraulic conductivity follows a power law with void ratio and the trend extends into the grain crushing regime. Hydraulic conductivity diminishes rapidly as sand grains start crushing; the reduction in hydraulic conductivity can reach 1-to-4 orders of magnitude depending on mineral composition, particle shape and initial packing density. Crushing involves the progressive detachment of small fragments; these small particles move into the larger pores, increase the drag resistance, clog pore constrictions, and effectively hinder fluid flow after the yield stress. [ABSTRACT FROM AUTHOR]

A 46‐year‐old male was treated with corticosteroids for nonspecific interstitial pneumonia (NSIP). He was referred to our hospital and admitted for worsening dyspnea and diffuse ground‐glass opacity on chest computed tomography (CT) during corticosteroid treatment. Gottron's sign was observed, and the patient was diagnosed with clinically amyopathic dermatomyositis on skin biopsy. We increased the corticosteroid dose and added immunosuppressive agents; however, the opacity on the chest CT worsened. Based on periodic‐acid‐Schiff‐positive granular material in the bronchoalveolar lavage fluid and the presence of anti‐GM‐CSF antibodies, the patient was diagnosed with autoimmune pulmonary alveolar proteinosis (APAP). The concentration of anti‐GM‐CSF antibodies in preserved serum was also elevated when the patient was diagnosed with NSIP. Thus, we assumed that NSIP and APAP coexisted, and that APAP manifested during immunosuppressive therapy. When exacerbation is observed during the treatment of interstitial pneumonia with immunosuppressive agents, it is necessary to consider APAP. [ABSTRACT FROM AUTHOR]

Robot grippers based on the jamming of granular material have been studied widely in previous years. Recently, also other benefits and challenges of granular material have emerged for robotics. We discuss various functions of granular matter in robotic actuation, sensory processing, locomotion, and manipulation. We also provide a review of the design and methods of robots for moving in or on challenging granular environments. Drawing on the properties of granular material and their potential applications, we propose our unique perspectives and innovative ideas for future research and development in this field. [ABSTRACT FROM AUTHOR]

Moving packed-bed heat exchangers in concentrated solar power (CSP) plants involves heat transfer between heated falling particles and supercritical carbon dioxide. The overall effective thermal conductivity of the moving packed bed and particle-side channel contact resistances are still the bottlenecks in achieving the desirable thermal transport levels. To this end, a novel moving packed bed heat exchanger consisting of an Octet lattice packed between the walls of the particle-side channel is proposed in this study. Granular flow analysis in Octet lattice moving packed bed heat exchanger (OLHX) was conducted through experiments and discrete element method (DEM)-based numerical simulations. The experimental images clearly demonstrated stagnation regions upstream of lattice fibers, void regions downstream of the fiber junctions, and wavy-type unobstructed flow on the lateral sides of the fibers. DEM simulations were successful in capturing all these critical flow phenomena. Larger flow velocities were observed on the lateral sides of the fibers in the simulations. Also, when the particles in the silo were emptied, the final images showed an accumulation of particles on the inter-fiber as well as fiber-channel wall junctions. Moreover, the fiber connections resulted in some regions devoid of particle contact on the channel endwall, which means that these regions would suffer from poor thermal exchange. The overall mass flowrate increased with increasing porosity for a fixed particle diameter. [ABSTRACT FROM AUTHOR]

Lost circulation is a pervasive problem in geothermal wells that can create prohibitive costs during drilling. The main issue with treatment is that the mechanism of plug formation is poorly understood. Here we applied two experimental approaches to characterize the clogging effectiveness of different materials. Fracture flow tests with different geometries were conducted with various individual materials and mixtures at relevant conditions. A high-temperature flow loop system was also developed to inject single- and mixed-material plugs into a gravel pack with a non-uniform geometry to compare with the fracture tests. The fracture tests revealed that single materials tended to form no plug or an unstable plug, while mixtures of materials were uniformly better at sealing fractures. Gravel pack tests at high temperatures show most of the materials are intact but degraded. The fibrous materials can create partial or unstable plugs in the gravel pack, but mixed-material plugs are far more effective at clogging. Both test types suggest that (1) mixed materials are more effective at blocking fluid flow and (2) fibrous materials seal fracture openings better, while granular materials seal inside fractures or pore throats better. Further research is needed to study the long-term stability of different plug configurations. [ABSTRACT FROM AUTHOR]

The uniform mixing of particles is crucial in many industrial applications. In industries, most granular materials are nonspherical, with ellipsoids being a common deviation from the sphere. The study employs the DEM model to investigate the mixing of a binary mixture in a vibrated packed bed with fixed amplitude and frequency. The study explores the behavior of the binary mixture of coarse and fine particles, with one particle being ellipsoid of aspect ratios (0.25–3.0) and the other being a sphere. The subdomain mixing index quantifies the mixing. The mixing of binary mixtures is determined by the aspect ratio of coarse and fine particles. The mixing efficiency is indicated by the average velocity of reference spherical particles, reflecting the convection strength within the bed. The results show higher average velocity in an ellipsoidal‐sphere mixture than in a sphere‐sphere mixture. The DEM results are validated with an identical experimental system. [ABSTRACT FROM AUTHOR]

Featured Application: The failure analysis of soil in the field of geotechnical engineering. The localization of deformation in shear bands is a fundamental phenomenon in granular materials like soil. In this study, we focus on the characteristics of shear bands, particularly the size effect, by implementing biaxial discrete element method (DEM) modeling. Firstly, we describe the establishment of the biaxial experimental model with dense sands. Then, we implement analyses of specimens with different sizes and find that there is a clear size effect in the stress–strain curve after the peak strength point, and there is less of a size effect in the angle of the shear band; the angle is consistent with Arthur's theory. Finally, the reason for the size effect is analyzed using the width of the shear band and the porosity inside the shear band. As the specimen size increases, the ratio between the shear band area and the whole specimen decreases. This effect reduces as the isotropic confining stress increases. The difference in the proportion of the shear band area mainly causes the size effect that affects the specimen deformation characteristics. We also find that with the increase in isotropic confining stress, the type of shear band gradually changes from cross-type to single-type. Our study provides valuable insights into understanding the behavior of granular materials. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study investigating the effect of particle shape of granular soils on the shear strength characteristics via direct shear tests. Thirty direct shear tests were conducted on spherical and crushed sand mixtures with different proportions (i.e. 0%, 25%, 50%, 75% and 100%). The overall regularity (OR) parameter of sand particles varies between 0.788 and 0.909, fairly reflecting the particle shape of sand mixture. A particular relation between OR and maximum shear strength was found, namely that maximum shear strength increases with a decrease in OR. Furthermore, a gradual increase was observed in the improvement factor (If) due to geotextile reinforcement with an increase in OR except for S25C75, representing 25% spherical and 75% crushed sand, at high and low normal stress levels (i.e. 29 kPa and 116 kPa). The interfacial friction angle (ϕ) of sand mixtures is improved by geotextile reinforcement. Additionally, geotextile reinforcement caused an apparent increase in ϕ with decreasing OR values. [ABSTRACT FROM AUTHOR]

Fluid injection can induce mechanical deformation in granular porous media due to the elevation of internal pore fluid pressure. This gains more significance when more than two immiscible fluids are involved, attributable to capillary and viscous drag forces. Such a coupled hydromechanical behavior associated with immiscible fluid flows plays an important role in injection, storage, and recovery of fluids in deformable porous media. This study presents a dimensionless map with newly proposed dimensionless parameters to predict deformation occurrence due to an immiscible fluid flow in deformable porous media. A series of hydromechanically coupled pore network simulations are first performed while varying the capillary number, mobility ratio, medium stiffness, and effective confining stress over orders of magnitudes. The compilation of simulation results with previously published Hele–Shaw experiment results is analyzed with the dimensionless parameters, such as the capillary number, mobility ratio, particle-level force ratios, and particle-level pressure ratios. Particularly, the particle-level pressure ratios include the capillary pressure ratio, defined as the ratio of capillary pressure to fracture pressure, and the viscous drag pressure, defined as the ratio of viscous drag pressure to fracture pressure. The dimensionless map based on the particle-level pressure ratios, where the capillary pressure ratio and viscous drag pressure ratio are defined as the ratios of capillary pressure and viscous drag pressure to fracture pressure, effectively delineates four deformation regimes—no deformation, capillary-induced deformation, drag-driven deformation, and mixed-mode deformation. The results demonstrate that capillary-induced deformation occurs when the capillary pressure ratio is greater than 10−1, while drag-driven deformation is observed when the viscous drag pressure ratio exceeds 10−2. The presented dimensionless map and dimensionless parameters are expected to be applicable for geological subsurface processes, including geological storage of carbon dioxide and hydrogen, and enhanced oil recovery. Article Highlights: Hydromechanically coupled PNM examines hydromechanical responses of granular porous media subjected to immiscible fluid flows. A dimensionless map with capillary and viscous pressure ratios effectively captures flow-driven deformation in deformable porous media. The dimensionless map is applicable for practices involving injection, storage, and recovery of fluids in deformable porous media. [ABSTRACT FROM AUTHOR]

Although the internal stress state of soils can be affected by repetitive loading, there are few studies evaluating the lateral stress (or K0) of soils under repetitive loading. This study investigates the changes in K0 and directional shear wave velocity (Vs) in samples of two granular materials with different particle shapes during repetitive loading. A modified oedometer cell equipped with bender elements and a diaphragm transducer was developed to measure the variations in the lateral stress and the shear wave velocity, under repetitive loading on the loading and unloading paths. The study produced the following results: (1) Repetitive loading on the loading path resulted in an increase in the K0 of test samples as a function of cyclic loading number (i), and (2) Repetitive loading on the unloading path resulted in a decrease in K0 according to i. The shear wave velocity ratio (i.e. Vs(HH)/Vs(VH), where the first and second letters in parentheses corresponds to the directions of wave propagation and particle motion, respectively, and V and H corresponds to the vertical and horizontal directions, respectively) according to i supports the experimental observations of this study. However, when the tested material was in lightly over-consolidated state, there was an increase in K0 during repetitive loading, indicating that it was the initial K0, rather than the loading path, which is responsible for the change in K0. The power model can capture the variation in the K0 of samples according to i. Notably, the K0 = 1 line acts as the boundary between the increase and decrease in K0 under repetitive loading.

The mechanical properties of granular materials at a low shear rate are frequently viewed as rate-independent, and a rate-independent constitutive relation is used to describe the mechanical behaviors of the granular material. However, time-dependent behaviors (or creeping behaviors) become significant in the long run. In this paper, we conduct triaxial experiments to observe the creep of granular materials under both dynamic and static loading conditions. Three typical creeping behaviors, aging, transitional, and fluidic, have been observed. To describe the time-dependent behaviors, an internal state variable characterizing the fluidity of the granular system is introduced to propose a rate-dependent constitutive relation which we call in this paper, a state evolution model. A characteristic strain is also introduced into the model to account for the influences of historic strain on the current state. The experiments are analyzed using the proposed model and it has been found that our model can well discern the observed three creeping behaviors. A criterion to identify the stability of a granular system is also given out based on the proposed model. Two key factors dominating the stability of the granular system are recognized – shear wave velocity and critical velocity. Our laboratory experiments in combination with the proposed criterion offer a physical explanation of the intrinsic triggering mechanism of system evolution from a stable to fluidic state: decrease in wave velocity or/and critical velocity. [ABSTRACT FROM AUTHOR]

Repeated/cyclic shearing can drive amorphous solids to a steady state encoding a memory of the applied strain amplitude. However, recent experiments find that the effect of such memory formation on the mechanical properties of the bulk material is rather weak. Here, we study the memory effect in a yield stress solid formed by a dense suspension of cornstarch particles in paraffin oil. Under cyclic shear, the system evolves toward a steady state showing training-induced strain stiffening and plasticity. A readout reveals that the system encodes a strong memory of the training amplitude (γT) as indicated by a large change in the differential shear modulus. We observe that memory can be encoded for a wide range of γT values both above and below the yielding albeit the strength of the memory decreases with increasing γT. In situ boundary imaging shows strain localization close to the shearing boundaries, while the bulk of the sample moves like a solid plug. In the steady state, the average particle velocity v inside the solid-like region slows down with respect to the moving plate as γ approaches γT; however, as the readout strain crosses γT, v suddenly increases. We demonstrate that inter-particle adhesive interaction is crucial for such a strong memory effect. Interestingly, our system can also remember more than one input only if the training strain with smaller amplitude is applied last. [ABSTRACT FROM AUTHOR]

Due to their advantages—longer internal force delay compared to bulk materials, resistance to harsh conditions, damping of a wide frequency spectrum, insensitivity to ambient temperature, high reliability and low cost—granular materials are seen as an opportunity for the development of high-performance, lightweight vibration-damping elements (particle dampers). The performance of particle dampers is affected by numerous parameters, such as the base material, the size of the granules, the flowability, the initial prestress, etc. In this work, a series of experiments were performed on specimens with different combinations of influencing parameters. Energy-based design parameters were used to describe the overall vibration-damping performance. The results provided information for a deeper understanding of the dissipation mechanisms and their mutual correlation, as well as the influence of different parameters (base material, granule size and flowability) on the overall damping performance. A comparison of the performance of particle dampers with carbon steel and polyoxymethylene granules and conventional rubber dampers is given. The results show that the damping performance of particle dampers can be up to 4 times higher compared to conventional bulk material-based rubber dampers, even though rubber as a material has better vibration-damping properties than the two granular materials in particle dampers. However, when additional design features such as mass and stiffness are introduced, the results show that the overall performance of particle dampers with polyoxymethylene granules can be up to 3 times higher compared to particle dampers with carbon steel granules and conventional bulk material-based rubber dampers. [ABSTRACT FROM AUTHOR]

This study focused on the seismic performance of an intermediate unit of scrap tire–granular material composite columns under cyclic loading. A Laboratory test study regarding the dynamic deformation characteristics of the intermediate unit of a scrap tire–granular material composite column was conducted. The hysteresis loops, skeleton curves, stiffness degradation curves, and energy–dissipation capacities of the three specimens with different initial vertical loads were experimentally evaluated. Based on the test results, a restoring force model of the tire–granular material unit is established. The results demonstrated that the external force had a significant influence on the strength of the intermediate unit of the scrap tire–granular material composite column. The area of the hysteresis loop of the tire–granular material unit was large, indicating a good seismic performance. As the initial vertical load increased, the horizontal bearing capacity and lateral stiffness of the intermediate unit of the scrap tire–granular material composite column increased. The calculation results of the skeleton curve and hysteresis curve were in good agreement with the test results. As a base damping material, the scrap tire–granular material composite column is feasible, and provides a new concept for the resource utilization of the original scrap tire. [ABSTRACT FROM AUTHOR]

Soil arching is significantly influenced by relative density, while its mechanisms have barely been analyzed. A series of DEM numerical simulations of the classical trapdoor test were carried out to investigate the multi-scale mechanisms of arching development and degradation in granular materials with different relative density. For analysis, the granular assembly was divided into three zones according to the particle vertical displacement normalized by the trapdoor displacement δ. The results show that before the maximum arching state (corresponding to the minimum arching ratio), contact forces between particles in a specific zone (where the vertical displacement of particles is larger than 0.1δ but less than 0.9δ) increase rapidly and robust arched force chains with large particle contact forces are generated. The variation in contact forces and force chains becomes more obvious as the sample porosity decreases. As a result, soil arching generated in a denser particle assembly is stronger, and the minimum value of the arching ratio is increased with the sample porosity. After the maximum arching state, the force chains in this zone are degenerated gradually, leading to a decrease in particle contact forces in microscale and an increase in the arching ratio in macroscale. The recovery of the arching ratio after the minimum value is also more significant in simulations with a larger relative density, as the degeneration of contact force chains is more obvious in denser samples. These results indicate the importance of contact force chain stabilities in specific zones for improving soil arching in engineering practice. [ABSTRACT FROM AUTHOR]

The critical-state theory (CST) serves as the foundation of modern soil mechanics and has gained widespread acceptance for explaining the behavior of granular materials. The classical CST does not address the fabric anisotropy of granular materials, leading to questions about the uniqueness of the critical state when considering such anisotropy. Recently, the development of anisotropic critical-state theory (ACST) highlights that fabric anisotropy of granular materials should reach a unique normalized value at the critical state. However, ACST does not specify which fabric tensor is used to characterize the microstructure of soils. In this study, we present a comprehensive characterization of various soil fabrics, including contact-based fabric tensor, void-based fabric tensor, and particle–void fabric, respectively. We employ three-dimensional clumped particles to approximate different shapes of Toyoura sand particles, which makes the characterization of soil fabric more accurate. Discrete element method simulations of conventional triaxial compression tests under both drained and undrained conditions are conducted on soil samples with different initial confining pressures and void ratios. The evolution of various fabrics toward the critical state is compared and discussed. Moreover, we establish relationships among contact-based fabric, void-based fabric, particle–void fabric, mean effective stress, and void ratio at the critical state. The study provides new insights into understanding the relations between the critical state and the microstructure of granular soils. [ABSTRACT FROM AUTHOR]

Although some efforts have been made separately to investigate the effects of the partial drainage condition and the intermediate principal stress ratio on the mechanical behaviors of sand and the instability that ensues, the interplay and combined influence of these two important factors have not been adequately investigated. In this study, a series of proportional strain loading tests considering a variety of combinations of different intermediate principal stress ratios (b = 0, 0.5, and 1.0) and generalized drainage conditions, including excessive, full, partial, zero (undrained), and expansive drainage conditions are conducted on transversely isotropic specimens through an advanced discrete element servomechanism. The second-order work criterion is used to detect the occurrence of instability. The evolution of the load-bearing structure of granular specimens is quantified through a contact-normal-based fabric tensor. The fabric evolutions and their interplay with the external loadings are illustrated. The simulation results show that even dense specimens can fully liquefy under expansive drainage conditions. Under otherwise identical conditions, the specimens under extension loading (b = 1) exhibit more significant non-coaxiality between the loading direction and the major principal direction of the fabric, which induces smaller shear strength and weaker resistance to instability than that of the compressive loading condition (b = 0 or 0.5). [ABSTRACT FROM AUTHOR]