Interparticle Contact Research Articles

Investigation of consolidation mechanisms induced by applied electric/electromagnetic fields during the early stages of spark plasma sintering

This work aims at investigating the role of the electric/electromagnetic fields generated by the pulsed current applied during Spark Plasma Sintering (SPS) process on the sintering mechanisms. The selected model material was a metallic granular medium composed of microsized particles of pre-oxidized copper. Its electrical behavior and correlated microstructural modifications were studied in two complementary enclosures. First, a demonstrator equipped with a modulable pulsed electric current generator has allowed analyzing, at low temperature and without mechanical loading, the effects related to the application of an electric wave, by controlling and modulating its characteristics (i.e. shape, frequency, amplitude). An abrupt electrical transition, named “Branly effect” from an insulating to a conductive state, is observed in the granular copper medium. The increase of pulses frequency is shown to strongly reduce the critical time to generate the electrical transition. Moreover, a commercial SPS device, with specific electrical and thermal instrumentation, was implemented by varying applied stress and using conductive and insulating dies. The analysis of the electrical behavior coupled with post-mortem microstructural observations allowed to highlight that the coupling between pulsed current and mechanical stress promotes specific mechanisms in SPS. Under high stress, interparticle contact area increase and the insulating oxide layer is damaged by microcracking. The coupling with the flow of the pulsed current involves local overheating and dielectric breakdown, favoring ignition of Branly effect. Micro-welds are formed between particles, creating privileged paths of the pulsed current and initiating densification at low temperature (250 °C).

Nondestructive Shear Stiffness Evaluation of EPS-Sand Composites Using Quartz and Calcareous Aggregates

The influence of expanded polystyrene (EPS) beads inclusion on the shear stiffness of a quartz-based material and a calcareous-based material has been examined by performing a comprehensive series of non-destructive bender element tests. To this end, siliceous-based Firoozkooh and calcareous-based Hormuz sands are mixed with different percentages of expanded polystyrene beads and then tested with a bender element apparatus embedded in a triaxial cell. Accordingly, the influences of confining pressure, void ratio, grain size distribution, and different percentages of expanded polystyrene beads on the small-strain shear modulus of siliceous and calcareous sands are thoroughly studied and discussed. For all mixtures of both sands, the maximum shear modulus increases with increasing confining pressure, but decreases with increasing void ratio and expanded polystyrene beads content in the mixture. The contribution of expanded polystyrene beads to the reduction of small-strain shear modulus is observed to be more pronounced for the quartz-based aggregate as compared to the calcareous-based aggregate. The confining pressure is also observed to have no influence on the small-strain shear modulus of the composites at high expanded polystyrene bead contents due primarily to the notably high compressibility of soft beads diminishing the augmented interparticle contact forces caused by increasing isotropic confinement. Using the results of experiments, different small-strain shear modulus expressions are developed for quartz-based and calcareous-based compacted fill-expanded polystyrene beads composites, which can be utilized for design purposes in earthen structures.

Cross-scale study of the evolution of interparticle contact behavior using smart aggregate

Structural internal health monitoring is a critical step in the construction and maintenance of pavement, the current pavement health monitoring methods need to be improved in terms of accuracy, timeliness, and the understanding of the interaction mechanism between the sensors and surrounding media. In this paper, compaction and splitting tests of cement stabilized macadams (CSM) are conducted using the wireless embedded sensors-Smart aggregate (SA) and a new evolution indicator Sv is proposed to describe the generation and propagation of damage within the material. In addition, spherical and mixed particle assemblies are modeled using the discrete element method to investigate the variation of microscopic contact parameters between the SA and the surrounding granular medium under loading. The results show that SA sensor is an effective tool to monitor the inter-particle force and rotation changes within the mixture at meso-scale, and the fluctuation coefficient Sv can be used to more accurately characterize the evolution of inter-particle contact parameters. Moreover, there is a strong correlation between the SA readings and the microscopic contact parameters (e.g., coordination number, normal contact force), while the size and shape of particles in the medium can likewise significantly affect the monitoring results of SA sensors.

Microstructure response to shear strength deterioration in loess after freeze-thaw cycles

Shear strength deterioration owning to freeze-thaw (F-T) cycles affects loess slope stability in seasonal frozen region. The change in loess shear strength is closely related to particle microstructure. In this study, multiple F-t-tests (i.e., 0, 1, 3, 5, 10, 15, 20, and 30 cycles) were performed on natural loess from Jing'yang, China. Unconsolidated-undrained triaxial tests and scanning electron microscopy (SEM) were performed on the specimens after the predefined number of F-T cycles. Particle microstructure was quantitatively analyzed using fractal dimension theory. The relationship between shear strength index and particle microstructure characteristics parameters was evaluated using the grey correlation method. Changes in the loess microstructure involved two main processes: transformation and stabilization, and the particle shape, arrangement, and interparticle contact were transformed into different types. The particle shape changed from complex to round with debris detaching from the particle surface. The orderliness of particle arrangement increased and point-face intercontact was formed. The internal friction angle and cohesion decreased with an increasing number of F-T cycles, and the change pattern was similar to that of the particle microstructure characteristics, both of which changed significantly before 5 F-T cycles and stabilized after 15 F-T cycles. An evaluation of the correlation between shear strength index and particle microstructure characteristic parameters demonstrated that, the change in particle shape contributed the most to deterioration of shear strength during F-T cycles, followed by particle arrangement and particle distribution. These results provide insights into the mechanism of degradation of loess shear strength in seasonally frozen areas at the microstructural scale.

Synthesis and Evaluation of Engineering Properties of Polymer-Coated Glass Beads.

Modern construction projects are often challenging, which has increased the demand for innovative materials that ensure improved safety, durability, and functionality. To explore the potential of enhancing soil material functionality, this study synthesized polyurethane on the surface of glass beads and evaluated their mechanical properties. The synthesis of polymer proceeded according to a predetermined procedure, where the polymerization was confirmed through analysis of chemical structure by Fourier transform infrared spectroscopy (FT-IR) and microstructure observation by a scanning electron microscope (SEM) after complete synthesis. The constrained modulus (M) and the maximum shear modulus (Gmax) of mixtures with synthesized materials were examined by using an oedometer cell equipped with bender elements under a zero lateral strain condition. Both M and Gmax decreased with an increase in the contents of polymerized particles due to a decrease in the number of interparticle contacts and contact stiffness induced by the surface modification. The adhesion property of the polymer induced a stress-dependent change in M but was observed to have little effect on Gmax. Compared to the behavior of the rubber-sand mixtures, polymerized particles show the advantage of a smaller reduction of M.

Discrete element modelling of the effect of aspect ratio on compaction and shear behaviour of aggregates

Coarse aggregates are essential for constructing railways and highways, as they affect structural performance through their compacted state and shear strength. Particle geometry influences these properties, but little research has quantitatively investigated how shape indexes impact mechanical indicators. This study addresses this gap by exploring the effect of particle aspect ratio on coarse aggregate compaction and shear behavior using the discrete element method (DEM). The study's key contribution is the development of novel equations that describe the correlation between packing porosity, inter-particle contact state, and aspect ratio control thresholds in a practical format. To do so, first, a point cloud database is compiled from laser scans of 700 natural aggregate particles. Each particle is classified using three aspect-ratio parameters: elongation, flatness, and flatness-elongation index (FEI). They are then digitized as rigid block particle models and compared against actual point clouds. Next, nine aggregates of representative shapes are assessed for their macro and mesoscopic mechanical responses using DEM-based vibro-compaction and direct shear tests. The compaction simulations indicate an exponential relationship between porosity and inter-particle coefficient of friction and show that aggregates with FEI ≈ 0.5 are a good choice. The direct shear simulations consider the same initial porosity, and show that the relationship between peak shear strength and vertical stress follows a power function. Finally, the analysis of contact forces and fabric tensors implies that FEI ≈ 0.5 may also facilitate particle fragmentation control.

Effect of coordination number of particle contact force on rutting resistance of asphalt mixture

Optimizing asphalt mix design at the indoor stage is of significant importance for enhancing the rutting resistance of asphalt mixture, which is affected by its structural characteristics. In this work, the coordination number of particle contact force (CNpcf) was proposed as an indicator to represent contact characteristics of skeleton structure aggregates in asphalt mixture. Nine asphalt mixtures with different gradations were designed, and the relationship of CNpcf with the number of aggregate contact zones (CZ) was established by combining rutting tests and digital image processing technique (DIP). The Mann-Whitney U test was implemented to analyze the distribution properties of inter-particle contacts before and after the rutting test. In addition, the resistance to the further expansion of rutting was analyzed. The results revealed a significant positive correlation (PCCs = 0.843, R2 = 0.711) between CNpcf and CZ. The content of coarse aggregates in the dominant structure did not exhibit monotonic related to anti-rutting performance of the asphalt mixture. Therefore, an optimum aggregate content of 57% was utilized. The Mann-Whitney U test revealed that the mesoscale skeleton structure of the asphalt mixes before and after rutting exhibited excellent stability. This study further indicated the applicability of combining CNpcf to adjust the mix design to enhance the rutting resistance of asphalt mixture and to prevent rutting expansion in flexible pavement.

Highly conductive polystyrene/carbon nanotube/PEDOT:PSS nanocomposite with segregated structure for electromagnetic interference shielding

Highly conductive polymer nanocomposites (CPNs) are promising alternatives to metals for electromagnetic interference (EMI) shielding applications. However, constructing a well-established conductive network within a polymer matrix using conventional processes is still challenging. This research aimed to improve the EMI shielding performance of CPNs by developing highly conductive segregated structures through a facile innovative dispersion mixing process. The nanocomposites were fabricated by dispersing polystyrene beads (PS), CNT, and PEDOT:PSS in deionized water, followed by vacuum filtration, solvent treatment, and hot press molding. The employed technique effectively constructed a highly conductive network in the PS/CNT nanocomposite, resulting in the lowest ever reported percolation threshold of 0.009 vol% among CNT-based segregated structures. Moreover, adding PEDOT:PSS to the nanocomposite as an additional constituent significantly promoted the conductive network by improving the dispersion of CNTs and the interparticle contact. The PS/CNT/PEDOT:PSS (100:2:4 w/w/w) exhibited a high electrical conductivity of 2.352 S/cm with notable specific EMI shielding effectiveness (SE) of 55.7 dB/mm (with dominant absorption mechanism), which is among the best performance reported for CNT-based conductive segregated structures, to the best of our knowledge. In brief, this work proposed a novel approach of using a facile, cost-effective, and eco-friendly method to fabricate highly CPNs for EMI shielding applications.

Enhanced rheological and tribological properties of nanoenhanced greases by tuning interparticle contacts

HypothesisDespite the wide spectrum of available nanoparticles, their utilization in lubricant and grease formulations remains challenging. To enhance their performance, an improved link between the interparticle contacts, brittleness of the resulting particle network, time-dependent rheology and tribology is required. ExperimentsWe systematically changed interparticle contacts and examined their effect on the colloidal stability, microstructure, rheological and tribological behavior of model greases by investigating four types of nanoclays: montmorillonite (Cloisite Na+), oleic-acid functionalized Cloisite Na+ (OA-Cloisite Na+), organomodified montmorillonite (C20A) and oleic-acid functionalized C20A (C20A-OA). FindingsWe observed a range of behaviors, starting from the lack of colloidal stability in greases derived with Cloisite Na+ and OA-Cloisite Na+ to semi-solid type systems with C20A and C20A-OA. Consistent with previous studies, the rheological and tribological properties of C20A systems scale with nanoclay loadings. Surprisingly, the functionalized C20A-OA system exhibited a delayed transition towards hydrodynamic lubrication, and enhanced lubrication properties, both of which were largely independent of nanoclay loadings. Coupled microstructural investigation and time-dependent rheology reveal that this behavior is governed by increasing repulsive forces, decreasing inter-particle friction between C20A-OA nanoparticles, and faster reorganization of the C20A-OA nanoparticle network under shear. Increased interparticle repulsion enables C20A-OA nanoclays to pass each other under shear and align in direction of shear, which reduces the overall viscosity, while the presence of OA on nanoclays decreases inter-particle friction and particle-steel surface friction.

Particle fabric imaging for understanding the monotonic and cyclic shear response of silts

The findings from geotechnical laboratory testing has shown that, in addition to the well understood influence of void ratio and effective confining stress, the particle fabric has a significant effect on the monotonic and cyclic shear behaviour of silts. There is a need to advance our understanding of these effects through systematic quantification of the particle fabric in a given silt matrix in terms of individual particle parameters (e.g., dimensional sizes, volumes, shapes, orientations) as well as inter-particle contact arrangements. In addressing this need, new methodologies were developed for X-ray micro-computed tomography (μ-CT) imaging of silts with specific attention paid to: sampling and preparing of silt specimens, scanning parameters to obtain the needed image resolutions, and digital processing of images to capture individual particle data. The ability of μ-CT imaging to effectively image and capture individual particle parameters and three-dimensional fabric of silt is demonstrated. The particle fabric in terms of orientation in reconstituted silt specimens is illustrated using the μ-CT images produced using standard-size silica particles. The fabric(s) quantified from the imaging of a natural low plastic silt is also presented, and the findings are in accord with those inferred from the mechanical laboratory element testing of the same silt. The work contributes to systematic accounting for fabric in understanding the macroscopic shear behaviour of natural silts.

Study of Strain-rate Effect in Two-dimensional Biaxial Test on Granular Material using Discrete Element Method

Numerical study to investigate the effect of various strain rates on global friction angle in the sand has been performed. Granular material behavior is influenced by several factors, among others: pack configuration, grain macro and micro roughness, confinement pressure, loading rate, etc. Sand is a granular material composed of discrete particles that the most refined microscopic techniques are needed to study its mechanical properties. In Indonesia, research related to the Discrete Element Method is still very limited. The two-dimensional discrete element method is capable to calculate the motion and interparticle contacts of large number of small particles, and each particle is modeled as a rigid circular element. The study started with the validation of the DEM model using YADE. Particles with a local friction angle of 35° are arranged in a closed rectangular box (frictionless wall). The number of particles used in this model validation simulation is 1000 sphere-type particles with monodisperse particle gradations. The simulation was done by a drained biaxial test with confinement stress of 100 kPa, thereafter varying strain rate are applied. Based on the deviatoric stress–axial strain curve from YADE, the result can be depicted on the Mohr circle to obtain the value of the global friction angle. It is found that the different value of strain rate affects its global friction angle. Increasing the value of the strain rate can increase the material global friction angle, which increases the strain rate from 1% to 5%, 10%, 25%, and 50% will increase its global friction angle by 5%, 5%, 14%, and 18%, respectively.

Characterization of Microscopic Damage Evolution of Functional Groove in Cageless Bearings

Bearings with partial function grooves as cageless bearings related to magnetic floating bearing protection have characteristics, such as fast offsetting of the impact force caused by rotor fall, and they gradually become important parts of ultra clean and cryogenic transportation systems. However, the functional failure mechanism of the bearing, which is caused by functional groove wear, is not clear. Therefore, this paper establishes a force–motion intrinsic model of particles inside the functional groove, which is based on the discrete element method, which it itself combined with the functional groove damage evolution trend analysis. Then, a hyper-quadratic surface model of inter-particle contact is established to simulate the time-varying friction coefficient of the functional groove by combining particles of different sizes to form particle clusters. Additionally, as the boundary condition, EDEM is used to solve the contact motion state of rolling element rolling through the functional groove for one week to obtain the overlap between particles and contact force change law. The results show that the wide side of the functional groove wears more seriously than the narrow side, and the rolling element leaves the functional groove with more impact than when it enters the functional groove, and the functional groove wears more seriously. In this paper, through the study of local functional groove wear of cageless ball bearing, we propose to characterize the damage extension of functional groove by using the number of particle fracture and motion trend in discrete element method, and this study provides theoretical guidance for the design of cageless bearings.

Estimating Earth Pressure during Tunneling in Gravel-cobble Stratum by Considering Sensor-particle Size Effect

When tunneling in gravel-cobble stratum with earth balanced shield, the ratio between the diameters of the pressure sensor embodied in muck chamber and cut gravels is generally small. The small sensor-particle size ratio leads to considerable difference between the measured and actual earth pressure. This makes the configuration of earth pressure a difficult task and furtherly affects tunneling efficiency and safety. This work characterizes the variation of stress measurement error in a granular packing with consideration of the sensor-particle size ratio, the particle size distribution as well as the contact force distribution. The probability density function of stress measurement error is built according to a general probability distribution of inter-particle contact force. After then, a probability-based regressive procedure is proposed to estimate the applied stress with the measured stress. For stress samples with a same target stress but different biases, the stress estimated by the probability-based procedure has a minor variation and thus is more reliable than the arithmetic mean stress. Finally, the probability-based procedure is adopted to estimate the actual earth pressure of a well-tunneled section in Chengdu gravel-cobble stratum, according to which the lateral pressure coefficient of Chengdu gravel-cobble stratum is suggested as 0.20 – 0.28.

Inconsistent effect of dynamic load waveform on macro- and micro-scale responses of ballast bed characterized in individual cycle: a numerical study

Cyclic load is widely adopted in laboratory to simulate the effect of train load on ballast bed. The effectiveness of such load equivalence is usually testified by having similar results of key concerns of ballast bed, such as deformation or stiffness, while the consistency of particle scale characteristics under two loading patterns is rarely examined, which is insufficient to well-understand and use the load simplification. In this study, a previous laboratory model test of ballast bed under cyclic load is rebuilt using 3D discrete element method (DEM), which is validated by dynamic responses monitored by high-resolution sensors. Then, train load having the same magnitude and amplitude as the cyclic load is applied in the numerical model to obtain the statistical characteristics of inter-particle contact force and particle movements in ballast bed. The results show that particle scale responses under two loading patterns could have quite deviation, even when macro-scale responses of ballast bed under two loading patterns are very close. This inconsistency indicates that the application scale of the DEM model should not exceed the validation scale. Moreover, it is important to examine multiscale responses to validate the effectiveness of load simplification.

Pressure‐controlled secondary flows and mixing in sheared Platonic solid‐shaped particles

AbstractGranular materials exhibit unique secondary flow behaviors upon shearing. We demonstrate, using particle dynamics simulations, that the secondary flow patterns are controlled by a pressure exerted on particle bed. A threshold pressure, at which vortex flow transitions to disturbed or chaotic flow, depends on particle shape, that influences interparticle contacts and rheological performance. Our results show that the flow patterns are essentially determined by a dimensionless term combining pressure and granular temperature for all the spherical and Platonic solid‐shaped particles explored. Particle mixing is promoted by the vortex flow or disturbed flow with strong diffusion. The highest mixing rate under a specified pressure is obtained for cubic particles, due to the significant microstructural ordering near the boundaries causing a high gradient of packing density. These findings shed light on how applied pressure and particle shape affect secondary flows which is critical to the understanding and control of granular mixing.

Numerical simulation of the force output of a pressurized sand damper under cyclic loading

This study implements the discrete element method (DEM) to capture the mechanical behavior of a pressurized sand damper (PSD) and characterizes the dissipated energy under cyclic loading. The idea of a PSD is to exploit the increase in shearing resistance of sand under external pressure and the associated ability to dissipate energy through interparticle contact sliding. DEM employs a simple linear contact model for the entire assembly and utilizes irregularly shaped particles to mimic the behavior of sand grains. The series of DEM simulations reported herein examine the effects of multiple parameters on the magnitude of dissipated energy. These parameters include stroke amplitude, grain size distribution, the magnitude of pressure imposed on the sand, and different geometric configurations of the PSD. The results reveal that the main energy dissipation mechanism is interparticle frictional sliding in the sand. Moreover, when multiple spheres are attached to the piston rod significant increase in the magnitude of dissipated energy is achieved. The numerical results compare favorably with experimental data recorded from a PSD that was subjected to cyclic testing.

Evolution of the force chain structure of partially saturated granular material under triaxial compression conditions

Saturation conditions play a significant role in the mechanical behavior of granular material and the evolution of an interparticle contact system during the loading process. Using data derived from discrete element simulations of granular assemblies subject to triaxial compression loading with different suctions, we examine the properties of an interparticle contact network and force chain structure to provide a comprehensive picture of the evolution of a particle-scale contact system. The study focuses on the evolution of the topological statistic features (including degree and triangles) and the force chain structure characteristics influenced by suction conditions. To comprehensively evaluate the characteristics of the force chain system, combined statistical parameters are proposed for describing the linearization and intensity of the force chain structure, which account for multiple features of force chains. Accordingly, a novel expression combined with multiscale properties is proposed for strength prediction that convincingly explains data results in the studied suction range. This study illustrates the consistency of the evolving force chain structure features with mechanical behavior that provides new insights for understanding the particle-scale force chain structure evolution from which these properties dominate the macroscopic response.

Improved charge transfer through the minimal addition of Pb as a sintering aid to TiO2-based low-temperature dye sensitised solar cell

The poor interparticle connectivity between the nanoparticles architecture of photoanode due to insufficient sintering temperature has been an issue for developing flexible dye sensitised solar cell (DSSC). This issue has led flexible DSSC to yield low conversion efficiency. This research aims to implement lead (Pb) as sintering aid to improve the interparticle connection of the photoanode by using the concept of liquid phase sintering. With low melting point of Pb (327.5 °C), necks were formed at the titanium dioxide (TiO2)–Pb interface that improved the connection and lowered the electronic resistance even at low sintering temperature of 150–250 °C. Morphological studies showed the formation of these necks, while phase analysis indicated the more desirable TiO2 anatase phase was present. Specimens containing 5 wt% Pb in the TiO2 matrix showed the highest efficiency value of 8.73% at 250 °C, which is even higher compared to their high-temperature (450 °C) counterpart by 12.21%. This is due to surface fusion of Pb at a lower temperature, leading to enhanced interparticle contact and reduction in recombination reactions. Further increase in Pb did not improve the conversion efficiency which can be due to high charge trapping sites and layer cracking due to high amounts of Pb in TiO2 matrix.

Pinning and Depinning Dynamics of an Evaporating Sessile Droplet Containing Mono- and Bidispersed Colloidal Particles on a Nonheated/Heated Hydrophobic Substrate

The present study is primarily focused on the coupled effects of substrate heating, colloidal dispersion, and particle size variation on the contact line (CL) pinning-depinning dynamics of evaporating droplets containing mono- (3/4.5 μm) and bidispersed (3 and 4.5 μm) polystyrene colloidal particles on poly(dimethylsiloxane) (PDMS) substrates. Experimental techniques such as high-speed visualization, optical microscopy, infrared thermography, and scanning electron microscopy are implemented to discover the plausible causes dictating the underlying physics. In the case of the nonheated substrate, there exists a significant delay in the CL depinning for the evaporating droplets containing bidispersed particles, as opposed to the monodispersed cases. A first-order model is illustrated for the determination of the net horizontal force acting on the particles near the CL. Interestingly, the model's findings revealed that due to the difference of particle size in the case of the bidispersed suspension, the interparticle contact force gets modified, thus enhancing the CL pinning. For the heated substrate cases, droplets with monodispersed particles (3 μm) exhibit a substantial delay in the CL depinning, whereas a nearly complete pinning of the CL is witnessed for the case of bidispersed colloidal suspension droplets. It is mainly due to the augmentation of particle deposition near the CL because of the circulatory thermal Marangoni and outward capillary flows. Thus, the mobility of the CL is inhibited, which is further reinforced by the presence of different-sized particles. Eventually, a ring-like deposition is observed, as opposed to an inner deposit commonly observed from the evaporation of colloidal droplets on hydrophobic substrates.

A machine learning-based strategy for experimentally estimating force chains of granular materials using X-ray micro-tomography

A machine learning-based strategy is presented to estimate the contact force chains of uniformly sized spherical granular materials under triaxial compression, using particle kinematics and inter-particle contact evolution data measured by X-ray micro-tomography (μCT). To this end, a graph neural network (GNN) is introduced to predict the contact force chains of granular materials at the end of a shear increment based on the evolution of contact network and grain displacement during that shear increment. Meanwhile, discrete-element modelling (DEM) is performed for a glass bead specimen under triaxial compression with the use of in situ μCT scanning. The DEM model has the same initial conditions as the glass bead specimen and is validated by comparing the calculated stress–strain curves, particle kinematics and inter-particle contact fabric evolution of the numerical specimen with the experimental results. The DEM model is used to generate sufficient virtual data to train the GNN model, which is applied to the glass bead specimen to predict the evolution of contact force chains. The model-predicted results yield a power-law relationship between the above mean normalised particle maximum normal contact forces and the probability density function, which is consistent with the findings reported in previous numerical studies.