Particle-scale Mechanisms Research Articles

Acoustic emission and energy dissipation in soils during triaxial shearing

Acoustic emission (AE) monitoring offers the potential to sense particle-scale interactions that lead to macro-scale responses of granular materials. However, there remains a gap in fundamental understanding of how particle-scale mechanisms and properties influence AE generation, which limits the application of AE monitoring and interpretation of AE measurements. Addressing this gap in knowledge was the principal focus of this study. A benchmarking study was conducted first whereby a programme of seven 3D DEM simulations of drained triaxial tests with energy tracking were performed and compared with experimental measurements, which ensured the adopted simulation approach captured realistic behaviour. Dissipated plastic energy was influenced in the same way as measured AE by imposed confining pressure, displacement rate, and load-unload-reload compression and shearing. A parametric analysis was subsequently conducted using a programme of 25 3D DEM simulations. The findings show that sliding and rolling friction were the dominant mechanisms in plastic energy dissipation and hence particle-scale properties that influence sliding and rolling friction have the most significant influence on AE generation (e.g., particle shape, surface roughness, hardness). Relationships have been established to quantify how changes in particle-scale properties in DEM (sliding and rolling friction coefficients, normal and shear stiffness and their ratio, and local damping coefficient) lead to changes in dissipated plastic energy (R2 of 0.99); however, it should be noted that the relations in the sensitivity analysis cannot be interpreted as representative of specific granular materials. This new knowledge enables improved interpretation of AE and underpins the development of theoretical and numerical approaches to model and predict AE behaviour in particulate materials.

The importance of initial particle position on filter performance in granular soils

This paper conducted an extensive set of gravity-driven filtration simulations using the discrete element method (DEM). The simulations were used to statistically analyse the inherent variability in filter performance due to variations in initial particle position using one-way ANOVA for repeated measures tests. The simulations accounted for group effects associated with the interaction of base particles within the voids of the filter particles, and considered a size ratio that captured the complete range of filtration behaviour. The simulations were performed in a highly controlled manner, such that the sole difference between the base-filter realisations was initial particle position. Four filtration characteristics, based on filter efficiency and filling degree, were considered to define filter performance. One-way ANOVA for repeated measures tests conclusively showed that initial filter particle position had a statistically significant effect on filter performance for all four characteristics. Significant variability was observed with normalised ranges up to 16.0%. Increasing the domain width did not eradicate the observed statistically significant differences in filter performance. These observations highlighted the importance of considering multiple stochastically generated base-filter realisations in DEM studies before drawing conclusions about the performance of filters due to the inherent variability of the particle-scale mechanisms underlying the filtration process.

A novel laboratory investigation to characterize the mechanical behaviors of polyurethane-ballast bonds under complex stress states

The polyurethane has been successfully applied to reinforce the railway ballasted track and the bonding properties of the polyurethane to ballast particles have significant effects on track performance. However, the mechanical behaviors of the polyurethane-ballast bonds(PBBs) haven’t been reported leaving the particle scale mechanism of the polyurethane reinforcement still unclear. A novel laboratory experimental method was developed in this paper to characterize the mechanical behaviors of the PBBs. Using standard test specimens with various specimen size and bond thickness, a series of laboratory tests were conducted to measure the stress–strain responses of the PBBs under tensile, compressive and shear loadings. The measured results show that the stress of a PBB increases nonlinearly with its strain when it subjects to uniaxial tensile or compressive loads. The test results also validate that the strength envelope of a PBB follows the linear Mohr-Coulomb criterion. A modified parallel bond model was proposed to predict the nonlinear stress–strain behaviors of the PBB with high accuracy. Furthermore, it is found that the bond thickness has distinct effects on the tensile and the cohesion strength of the PBB but negligible effect on its friction angle. The initial tangent elastic modulus of the PBB increases approximately linearly with the increase of the bond thickness. The experimental method, results and the modified bond model introduced in this paper are very helpful for the performance evaluation and the numerical simulations of polyurethane reinforced ballast.

Relationship between acoustic emission and energy dissipation: a DEM study of soil–structure interaction

Acoustic emission (AE) monitoring offers the potential to sense particle-scale interactions that lead to macro-scale responses of granular materials; however, there remains a paucity of understanding of the fundamental links between particle-scale mechanisms and AE generation in particulate materials, which limits interpretation of the measured AE. The objective of this study was to establish links between particulate-scale energies and AE activity measured at the macro-scale in experiments. To achieve this, a programme of 3D DEM simulations was performed on granular soil/steel structure interfaces and the results were compared with experimental measurements. The findings show that the fundamental particulate-scale mechanisms that contribute to AE generation are friction and damping in particulate rearrangement, with friction being the dominant mechanism (i.e. > 95% of the total energy). Dissipated plastic energy was influenced in the same way as measured AE activity by unload–reload behaviour, imposed stress level, mobilised shearing resistance, and shearing velocity. Relationships have been established between AE and dissipated plastic energy (R2 from 0.96 to 0.99), which show AE generated per Joule of dissipated plastic energy is significantly greater in shearing than compression. A general expression has been proposed that links AE and plastic energy dissipation. This new knowledge enables improved interpretation of AE measurements and underpins the development of theoretical and numerical approaches to model and predict AE behaviour in particulate materials.

Calibration of the simulation parameters of jujubes in dwarfing and closer cultivation in Xinjiang during harvest period

In order to obtain accurate contact parameters of jujubes in dwarfing and closer cultivation in Xinjiang during the harvest period, based on the Hertz-Mindlin (no-slip) contact model in EDEM, the contact parameters, including the coefficient of restitution, static friction, and rolling friction, between jujube and machine components have been calibrated through physical and simulation test. The coefficient of restitution, static friction, and rolling friction between the jujubes and steel (jujube-steel) were calculated as 0.33, 0.46, and 0.04, respectively. The coefficient of restitution, static friction, and rolling friction between the jujubes and polyvinyl chloride (PVC) (jujube-PVC) were calculated as 0.39, 0.61, and 0.06, respectively. The optimum contact parameters among jujubes (inter-jujubes) were 0.23, 0.33, and 0.04. The accumulation test was applied for comparing the simulation and testing values of jujube's repose angle. In the results, the relative error was 1.27% which means the discrete element model and the contact parameters of jujube can be used for the discrete element simulation test. In addition, the results can be adequately used to simulate particle scale mechanics (grain commingling, flow/motion, forces, etc.) of jujube in harvest as well as the design and working mechanism study of post-harvest processing machinery and devices. Keywords: jujube, EDEM, contact parameter, experiment, calibration DOI: 10.25165/j.ijabe.20221502.6006 Citation: Yao S Q, Shi G K, Wang B S, Peng H J, Meng H W, Kan Z. Calibration of the simulation parameters of jujubes in dwarfing and closer cultivation in Xinjiang during harvest period. Int J Agric & Biol Eng, 2022; 15(2): 256–264.

Particle-scale observation of seepage flow in granular soils using PIV and CFD

Seepage-induced instabilities pose a challenge in many geotechnical applications. Particle-scale mechanisms govern the initiation of instability. However, current understanding is based on a macro-scale perspective that draws on continuum mechanics. Recent developments in imaging and numerical analysis can provide the particle-scale fundamental perspective needed to develop a comprehensive insight. This contribution demonstrates the value of combining particle-scale experimental and numerical studies. The experiments consider transparent soil samples created using refractive image matching and monitored by particle image velocimetry (PIV). Three-dimensional pore topology is extracted from a series of two-dimensional images and imported into computational fluid dynamics (CFD) simulations. Permeability is estimated by three distinct approaches: using flow rate, PIV- and CFD-generated data. The flow fields obtained from PIV and CFD are in good agreement considering both flow rate contour plots and flow rate distributions; this demonstrates the successful reconstruction of three-dimensional pore structure and flow-field analysis. The comparison also reveals that the side boundary effects in CFD simulations are constrained within a limited region. The multi-plane results characterise the variance of flow velocity with the three-dimensional pore topology. Finally, the fluid–particle interactions obtained from CFD results show a larger variance in the angular particle packings.

PLATYMATCH – A particle-matching algorithm for the analysis of platy particle kinematics using X-ray Computed Tomography

Particle scale mechanisms influence the mechanical behaviour of geomaterials. A better understanding of soil micromechanics will help develop more accurate constitutive models and, by extension, safer and more economic geotechnical design. Sand-grained soils have been widely studied at the particle scale using X-ray Computed tomography and this has revealed important micromechanical behaviour of sands. Clays are least understood and studied at the particle scale. As technology rapidly progresses, to understand clay kinematics, a particle kinematic algorithm to match plate shaped (platy) particles will be required. Current algorithms developed for the matching of quartz and carbonate sand particles may not be suitable, since particle attributes that are strong discriminators of round particles may be less unique on platy particles, due to their extremely small c-axis and the reduced image resolution associated with their smaller size. This study presents, particle tracking of kaolin sample using mica markers and provides a platy soil particle matching algorithm for evaluating platy particle kinematics. After testing a range of alternative particle attributes, it was observed that a combination of particle major axis length, intermediate axis length, and perimeter was a good discriminant of the platy mica particles studied. On this basis, we developed PLATYMATCH, an optimised platy particle matching algorithm that considers a minimised combined normalised error of unique particle attributes within a defined search volume space. The algorithm was implemented such that particle attributes were compared in parallel, rather than sequentially, to avoid filtering out a potential particle match based on a single non-unique attribute. PLATYMATCH was then successfully validated using a compressed mica soil sample.

Micromechanics of slow granular flows

It has been contemplated for a long time that dense granular materials flow in a stick-slip manner, and large fluctuations in the stresses are associated with it. However, the particle scale mechanics for this type of macroscopic motion has not been understood so far. We have analyzed the time evolution of contact networks from particle dynamics simulations and found that the rate of change of elastic energy of the packing can distinguish the stick regimes and the slip events. The isostatic criterion (number of contacts for a minimally stable particle) has been used to construct a cascade failure mechanism which reveals that the effect of the random breaking of contacts due to applied shear can be system-spanning for some cases. The size of the cascade failures follows a power law that explains experimentally observed large fluctuations is stresses. We expect that this power law distribution can connect the microstructure of a granular packing to its mechanical response.

Normal stress differences in the consolidation of strong colloidal gels

Macroscopic models for the uniaxial consolidation of strong colloidal gels typically characterise the compressive strength of the particulate network in terms of the compressive yield stress Py(ϕ) or uniaxial elastic modulus K ′ (ϕ). Almost all industrial applications involve multi-dimensional (MD) configurations with arbitrary tensorial stress states, and it is unclear how to generalise these 1D constitutive models to MD consolidation. Several studies have attempted to extend these 1D constitutive models to MD by assuming either isotropic consolidation or zero Poisson’s ratio, but the validity of these assumptions is currently unknown. Lacking is a validated tensorial rheology for the consolidation of strong colloidal gels that is capable of predicting the consolidation of these materials. One step toward the development of such tensorial rheology is the consideration of normal stress differences (NSDs) during uniaxial consolidation. Thus, a tensorial constitutive model for the consolidation of colloidal gels cannot be developed without accounting for these NSDs. We address this problem by performing discrete element model (DEM) simulations of the uniaxial consolidation of a two-dimensional (2D) strong colloidal gel and investigate evolution of the tensorial stress state during consolidation. We show that during consolidation, the Poisson ratio increases from zero near the gel point to almost unity near close-packing and uncover the particle-scale mechanisms that underpin these observations. These results provide the first steps toward a complete tensorial rheology of colloidal gels that is capable of resolving the evolution of these complex materials under superposed differential compression and shear. Graphical abstract

Ultrasonic imaging of microscale processes in quartz gouge during compression and shearing

Active ultrasonic monitoring in rock joints and gouge materials has the potential to detect the signatures of shear failure for a wide range of sliding modes, from slow and stable movements to fast and unstable sliding. While these collected measurements currently are being used to identify the seismic precursors to shear failure in rock joints and gouge materials, the underlying physical processes and contact scale mechanisms that control the changes in ultrasonic wave attributes are still poorly understood. To address this knowledge gap, this paper aims to investigate the relationship between the variations in ultrasonic wave attributes and the underlying particle scale mechanisms during both compression and shearing. Our double direct shear experiments were coupled with ultrasonic wave propagation measurements on granular quartz gouges, in which the gouge layers were sheared under different sliding velocities and constant normal stress conditions. Simultaneously, ultrasonic waveforms were continuously recorded during shearing with a fast data acquisition system and three pairs of ultrasonic wave transducers embedded at the two sides of the gouge layers. Different particle comminution mechanisms were observed from the non-uniform distribution of normal and shear stresses through the changes in ultrasonic transmissivity and scanning electron microscope (SEM) images. Our results show that the signatures of the geometry- and time-dependent variations of the inter-particle contact quality and pore volume changes with sliding velocity and slip accumulation were clearly captured from the variations in the transmitted wave amplitude and the dominant frequency, respectively. In addition, we found that variations in dominant frequency corresponded to dilation and compaction of the granular gouge layer during compression as well as stable and unstable sliding. Our results therefore confirmed that non-destructive acoustic techniques are capable of capturing a variety of micromechanical processes during fault gouge deformation and may prove useful in natural settings. • Identifying particle abrasion and splitting at different locations using SEM images. • Nonlinear evolution of amplitude with normal stress and particles comminution. • Reverse relation between amplitude and slip velocity in stable and unstable sliding. • Opposite correlation between dominant frequency and porosity variation.

DEM simulation of grain-coating type methane hydrate bearing sediments along various stress paths

This study explores the mechanical behaviors of grain-coating type methane hydrate bearing sediments (MHBS) using the discrete element method (DEM) under four typical compression conditions, i.e. triaxial, isotropic, constant stress ratio, and true triaxial. The particle-scale mechanism of grain-coating type MHBS is taken into account in the DEM contact model. The numerical results show that MH saturation and bond result in increased structural yield stress. The mean stress and volumetric change are larger under lower stress ratio in the constant stress ratio compression tests. The yield envelop obtained from the true triaxial compression test agrees well with the Lade-Duncan criterion. The shear strain contours in deviatoric plane can be described by changing the size of Lade-Duncan yield surface. Grain-coating type MHBS behaves like weakly-bonded sand, as cohesive MH bonds break at the very early stage of loading. Upon loading, the frictional properties of MH host sands evolve according to the change in grain morphology (i.e., particle enlargement and angulation). The simulation results and revealed particle-scale mechanism are useful in future development of constitutive models for MHBS.

Effect of particle morphology and contacts on particle breakage in a granular assembly studied using X-ray tomography

The macroscopic response of a geomaterial is entirely determined by changes at the particle scale. It has been established that particle crushing is affected by particle size, shape and mineral composition and initial density; and the initiation of breakage has often been related to the onset of yielding. Encouraged by the success of X-ray tomography in revealing particle-scale mechanisms of deformation, we present our findings regarding the onset of particle breakage, deriving from our study of 3D images of a dry granular assembly undergoing crushing. We propose two bespoke image analysis algorithms, which allow us to track breakage and identify contacts prior to breakage. The combination of the two algorithms, along with the high resolution of the 3D images enables us for the first time to track breakage of individual particles, identify different breakage modes for each particle and simultaneously study the effect of particle morphology and coordination number on breakage. Three different breakage types are identified: chipping, splitting and fragmentation. We have found that particle heterogeneity and sphericity mainly contribute to fragmentation, whereas the coordination number also affects chipping. The confining stress state within the particles with high coordination number made them more resistive to fragmentation, whereas particles with low coordination number mainly undergo fragmentation. The shearing of the particles at their contact points, leads to local stress concentrations resulting into surface chipping. Finally, we discuss the relation between the initiation of breakage and yielding, showing that some breakage occurs before the point where yielding is traditionally defined.

The smoothing of landscapes during snowfall with no wind

ABSTRACTEvery winter, snowy landscapes are smoothed by snow deposition in calm conditions (no wind). In this study, we investigated how vertically falling snow attenuates topographic relief at horizontal scales less than or approximately equal to snow depth (e.g., 0.1–10 m). In a set of three experiments under natural snowfall, we observed the particle-scale mechanisms by which smoothing is achieved, and we examined the cumulative effect at the snowpack scale. The experiments consisted of (a) a strobe-light box for tracking the trajectories of snowflakes at deposition, (b) allowing snow to fall through a narrow gap (40 mm) and examining snow accumulation above and below the gap, and (c) allowing snow to accumulate over a set of artificial surfaces. At the particle scale, we observed mechanisms enhancing (bouncing, rolling, ejection, breakage, creep, metamorphism) and retarding (interlocking, cohesion, adhesion, sintering) the rate of smoothing. The cumulative effect of these mechanisms is found to be driven by snowpack surface curvature, introducing a directional bias in the lateral transport of snow particles. Our findings suggest that better quantification of the mechanisms behind smoothing by snow could provide insights into the evolution of snow depth variability, and snow-vegetation interactions.

Particle-Scale Mechanisms in Undrained Triaxial Compression of Biocemented Sands: Insights from 3D DEM Simulations with Flexible Boundary

Particle-Scale Mechanisms in Undrained Triaxial Compression of Biocemented Sands: Insights from 3D DEM Simulations with Flexible Boundary

A peek into the origin of creep in sand

This paper presents the results of an experimental study of the particle scale mechanisms that underpin creep, on-going deformations under constant external load, in dry non-cemented sand under 1D oedometric compression loading at 2500 kPa. Traditional observations on the boundary of the sample are complemented with simultaneous measurements of the 3D kinematics of both the entire grain assembly and details of grain-scale mechanisms using synchrotron based X-ray tomography at two different spatial resolutions. Both the continuum response and the local grain scale response are captured using two spatial resolutions, i.e. {6.5},{upmu }hbox {m} and {0.65},{upmu }hbox {m} respectively. The results, for the first time, illustrate that small displacements measured at the boundary can be the result of rather pronounced fracturing at the individual grain scale.

Soil microstructural changes induced by suffusion: x-ray computed tomography characterization

Suffusion is one mechanism of internal erosion, which occurs in gap-graded or broadly graded soils when the fine particles are detached and transported by the seepage flow through the void space formed by the granular soil skeleton. Suffusion is therefore a particle scale mechanism. During this microscale, the initial soil fabric may change due to both fines migration and coarse grains rearrangement, leading to an increase/decrease of global/local porosity and hydraulic conductivity, besides of a probable appearance of heterogeneity, which can, in turn, impact the mechanical behaviour of the eroded soil. In the literature, suffusion test results give only a macroscopic point of view and fail to quantify the effect of suffusion at the scale of the soil's induced heterogeneities. In this paper, x-ray tomography is used to get microscopic observations of soil sample microstructure evolution during a suffusion test. The results reveal that suffusion is not a homogeneous process; the removal of fine particles takes place mainly around the soil sample circumference leading to a higher void ratio at the periphery. Besides, the inter-granular void ratio decreases significantly but almost uniformly throughout the sample owing to the progressive collapse and reorganization of the coarse grains induced by the loss in fines.

Investigating the mechanical behavior of grain-coating type methane hydrate bearing sediment in true triaxial compression tests by distinct element method

Methane hydrate is viewed as one of the potential energy resources to alleviate the current and future energy needs. Sediments containing methane hydrate are referred to as methane hydrate bearing sediments. In laboratory tests, the methane hydrate bearing sediments are commonly synthesized by the gas saturated method, and their mechanical properties are tested under simple stress path. When the gas saturated method is used to form the methane hydrate bearing sediments, methane hydrate tends to generate at the surface and meniscus of particles, namely the grain-coating type methane hydrate bearing sediments. In this study, to obtain the mechanical behavior of grain-coating type methane hydrate bearing sediment subjected to complex stress paths, a discrete element method simulation scheme accounting for three particle-scale mechanisms of grain-coating type methane hydrate bearing sediments, i.e. cementation, particle enlargement and particle angulation, along with the experimental data of pure methane hydrate that indicate the strong correlation between the mechanical property of pure methane hydrate and the pore pressure/ambient temperature, is adopted to explore the mechanical behavior of grain-coating type methane hydrate bearing sediments under true triaxial compression tests. The results show that: (1) the discrete element method simulation scheme accounting for microscopic mechanism is effective to simulate the mechanical property of grain-coating type methane hydrate bearing sediments; (2) the cementation effect of methane hydrate in the grain-coating type methane hydrate bearing sediments is weak and the mechanical behavior of grain-coating type methane hydrate bearing sediments in true triaxial compression tests is similar to that of cohesionless granular materials; (3) in true triaxial compression tests, the peak and residual deviator stresses of methane hydrate bearing sediments are larger under a lower value of intermediate principal stress ratio. The Lade-Duncan criterion can be used to describe the strength of grain-coating methane hydrate bearing sediments.

Calibration of discrete element model parameters: soybeans

Discrete element method (DEM) simulations are broadly used to get an insight of flow characteristics of granular materials in complex particulate systems. DEM input parameters for a model are the critical prerequisite for an efficient simulation. Thus, the present investigation aims to determine DEM input parameters for Hertz–Mindlin model using soybeans as a granular material. To achieve this aim, widely acceptable calibration approach was used having standard box-type apparatus. Further, qualitative and quantitative findings such as particle profile, height of kernels retaining the acrylic wall, and angle of repose of experiments and numerical simulations were compared to get the parameters. The calibrated set of DEM input parameters includes the following (a) material properties: particle geometric mean diameter (6.24 mm); spherical shape; particle density ( $$1220~\hbox {kg m}^{-3}$$ ), and (b) interaction parameters such as particle–particle: coefficient of restitution (0.17); coefficient of static friction (0.26); coefficient of rolling friction (0.08), and particle–wall: coefficient of restitution (0.35); coefficient of static friction (0.30); coefficient of rolling friction (0.08). The results may adequately be used to simulate particle scale mechanics (grain commingling, flow/motion, forces, etc) of soybeans in post-harvest machinery and devices.

Discrete Element Modeling of Micro-scratch Tests: Investigation of Mechanisms of CO2 Alteration in Reservoir Rocks

The injection of CO2 into geological formations leads to geochemical re-equilibrium between the pore fluid and rock minerals. Mineral–brine–CO2 reactions can induce alteration of mechanical properties and affect the structural integrity of the storage formation. The location of alterable mineral phases within the rock skeleton is important to assess the potential effects of mineral dissolution on bulk geomechanical properties. Hence, although often disregarded, the understanding of particle-scale mechanisms responsible for alterations is necessary to predict the extent of geomechanical alteration as a function of dissolved mineral amounts. This study investigates the CO2-related rock chemo-mechanical alteration through numerical modeling and matching of naturally altered rocks probed with micro-scratch tests. We use a model that couples the discrete element method (DEM) and the bonded particle model (BPM) to perform simulations of micro-scratch tests on synthetic rocks that mimic Entrada sandstone. Experimental results serve to calibrate numerical scratch tests with DEM–BPM parameters. Sensitivity analyses indicate that the cement size and bond shear strength are the most sensitive microscopic parameters that govern the CO2-induced alteration in Entrada sandstone. Reductions in cement size lead to decrease in scratch toughness and an increase in ductility in the rock samples. This work demonstrates how small variations of microscopic bond properties in cemented sandstone can lead to significant changes in macroscopic large-strain mechanical properties.

DEM simulation of bonded granular material. Part II: Extension to grain-coating type methane hydrate bearing sand

In free gas environment, methane hydrate (MH) tends to grow on sand grain surfaces and forms cementations (bonds) at contacts where the MH coating layers overlap. This grain-coating type growth habit of hydrate endows methane hydrate bearing sediment (MHBS) with special mechanical behavior. This study first examines the mechanics of grain-coating type MHBS based on previously reported experimental results. Then, three particle-scale mechanisms are proposed, i.e. contact cementation (MH acting as bonds at contacts), particle enlargement (MH coating grain surface and thus increasing effective particle size) and particle angulation (the presence of rough fracture surface when a MH bond is broken). The three mechanisms are implemented into a bonded contact model for the discrete element simulations of MHBS. Grain angularity, MH growth habit and mechanical properties of MH are incorporated in the bonded contact model in a consistent way. The simulation results show that the experimentally observed effects of MH saturation and test conditions (temperature and pore pressure) on MHBS behavior can be well reproduced, thus validating the three proposed mechanisms. Cementation effect can be fully seen under very low stress state, whereas under in-situ stress conditions for MHBS, cementation only influences the small-strain behavior. Particle enlargement and angulation are dominant to control strength and dilation of MHBS.