Interparticle Bonds Research Articles

Effect of the clump size for bonded particle model on the uniaxial and tensile strength ratio of rock

The Discrete Element Method (DEM) has been increasingly used to study the behaviour of rock. Despite the advantage of classical DEM formulations in using simple interaction laws to model fracture initiation and propagation, they have limitations to properly simulate brittle rock behaviour. The code cannot predict the high values of unconfined compressive strength/tensile strength (UCS/TS) ratio associated with non-linear failure envelopes, as observed for hard rock, such as granite, unless significant modifications are made. This paper proposes and implements a clumped-particle model into a two-dimensional DEM code. The increase in the interaction between discrete elements, which locally increases the density of interparticle bonds, can improve the results to obtain the desired values of UCS/TS ratio. This approach seems able to significantly increase both the potential and the predictive capabilities of DEM for rock modelling purposes. The novelties introduced in this work are the presentation of a numerical procedure to determine the equivalent micro-mechanical properties of intact rocks and, aware of a gap in our knowledge, the characterisation of uncertainties that a clumped-particle model introduces in the numerical results. A series of numerical simulations, including uniaxial compression and direct tension tests, were carried out, by varying the relation between the clump and the minimum particle radius, using the Monte Carlo simulation technique. The results were compared with experimental data on Lac du Bonnet granite, from the bibliography, which allowed to determine the equivalent micro-mechanical properties. Results, from a simulation using a core sample of Lac du Bonnet granite, show that the variance in the values of the mechanical properties calculated decreases with the decrease in the particle radius, but the influence of the clump size on the behaviour requires the execution of a considerable amount of simulations to achieve results with an acceptable relative error of 5%. Nevertheless, the mechanical behaviour obtained from DEM simulations was in good agreement with the experimental data and the model captured both the tensile and the unconfined compression strength values.

Dynamic Weakening of Sandstone Subjected to Repetitive Impact Loading

Dynamic weakening is commonly observed when stone is subjected to a single or repetitive impact loading. In a series of impact loading experiments conducted using the split Hopkinson pressure bar system with external confinement pressure, we observe evident nonlinear dynamic response for each impact loading, an accompanying dynamic weakening effect, and significant plastic deformation of the specimen. The dynamic response can be predicted accurately by the nonlinear granular model (Johnson and Jia in Nature 437:871–874, 2005). In the framework of statistics, an analytical model for dynamic weakening is proposed and it suggests that the weakening effect is induced by the increasing number of broken inter-particle bonds after impact. This is related to a decrease in the dynamic modulus that can be described by introducing a confinement pressure-dependent energy portion parameter $$\mu$$ . A further nonlinear analysis of the experimental data provides detailed insights into the nonlinear dynamic response. The proposed weakening mechanism is based on inter-particle statistics and it comprises a wealth of dynamic regimes, including modulus softening and damage evolution, which can be extended to other granular materials but not limited to rocks.

Influence of structure on the compression behaviour of two very stiff fissured high plasticity Eocene clays

A series of one-dimensional compression tests have been performed on reconstituted specimens of Søvind Marl. A clear correlation is obtained between the location and the gradient of the intrinsic compression lines and the void ratio at the liquid limit. The found correlation has subsequently been used to normalise the compression curves of 12 one-dimensional compression tests on intact specimens of Søvind Marl and Little Belt Clay obtained from various depths at two sites at Aarhus Harbour. Søvind Marl and Little Belt Clay are both marine sedimentary clays of Eocene age, which can be characterised as very stiff fissured clays with a very high plasticity. From a comparison of the reconstituted and intact compression paths the influence of structure is analysed and discussed in the paper. The majority of intact specimens of Søvind Marl and Little Belt Clay from Aarhus Harbour show similar behaviour; a very stiff recompression path and yielding at high stress level after crossing the intrinsic compression line. The displayed behaviour indicates an insignificant influence of fissuring on the pre-yield behaviour. Post-yield the compression paths indicate a stable structure dominated by fabric rather than metastable interparticle bonds. The paper furthermore highlights the importance of considering the effect of pore water salinity when assessing the influence of structure on the compression behaviour of smectite rich natural marine clays.

Nucleation of Condensed Phase in Water Vapor on the Nanostructured Surface of a β-AgI Crystal. 1. Spatial Organization

The Monte Carlo method has been employed to carry out computer simulation of condensed water phase nuclei on a basal face of a silver iodide crystal at temperatures of 260 and 400 K. Comparison between the calculated spatial correlation functions of molecules located on nanostructured and smooth substrates has indicated a destructive action of the electric field of the nanostructure on molecular clusters. At the initial stage of absorption, vapor molecules are retained on the nanostructured surface without the formation of interparticle bonds; therewith, initially absorbed molecules are bonded to the structured substrate substantially more strongly than to the smooth surface, on which intermolecular interactions play a significant role in their retention. The electric field of the nanostructured surface has a destructive effect on intermolecular hydrogen bonds. The temperature dependence of the number of hydrogen bonds exhibits an inverse pattern, at which an increase in the temperature within some region of conditions is accompanied by the recovery of some bonds rather than their rupture. A simplified model has been proposed to explain this effect.

Brittle solid collapse to simple liquid for a waxy suspension.

We show that thanks to the existence of a continuous (percolating) network of weak interparticle bonds in a liquid, wax suspensions behave as "soft breakable (brittle) solids". It appears that, under the action of either a large stress over a short time or oscillating low stress (fatigue test), the initially solid network of these materials is broken and dispersed in the liquid, which makes them turn abruptly ("collapse") and irreversibly into a low viscous fluid. This collapse is more dramatic as the concentration increases. These phenomena are related to the evolution of the microstructure directly observed after different flow histories. The interpretation of these results provides new perspectives for understanding the physical origin of the brittleness or plasticity of solid or pasty materials, and suggests such materials might be used as model systems to simulate and explain natural catastrophic events such as landslides and avalanches.

Strength and ductility of powder consolidated ultrafine-grain tantalum

Bulk ultra-fine grain tantalum was successfully fabricated from pure tantalum powder particles with an average particle size of 50 μm by equal channel angular extrusion at 900° C and 1200° C using three different processing routes. The effects of extrusion route and temperature on the consolidation performance are evaluated through microstructural analysis and room temperature mechanical testing. The consolidated tantalum has a “wood grain” like structure with strong interparticle bonds and a grain size <100 nm. The lamellar structure is helpful in resisting crack formation and fracture. The bulk tantalum consolidates are much stronger than annealed wrought tantalum and have significant ductility. Tensile tests for consolidated tantalum at room temperature show a yield strength of 340–376 MPa, an ultimate strength of up to 618 MPa, and strains to failure of ~19–25%. These properties are comparable to wrought Ta. The present study demonstrates that equal channel angular extrusion is a feasible method for fabricating bulk nanostructured refractory alloys from precursor powder, with a combination of high strength and good ductility.

Liquid Structure of Shock-Compressed Hydrocarbons at Megabar Pressures.

We present results for the ionic structure in hydrocarbons (polystyrene, polyethylene) that were shock compressed to pressures of up to 190GPa, inducing rapid melting of the samples. The structure of the resulting liquid is then probed using insitu diffraction by an x-ray free electron laser beam, demonstrating the capability to obtain reliable diffraction data in a single shot, even for low-Z samples without long range order. The data agree well with abinitio simulations, validating the ability of such approaches to model mixed samples in states where complex interparticle bonds remain, and showing that simpler models are not necessarily valid. While the results clearly exclude the possibility of complete carbon-hydrogen demixing at the conditions probed, they also, in contrast to previous predictions, indicate that diffraction is not always a sufficient diagnostic for this phenomenon.

The rheological approach in an assessment of interparticle interactions in soils

The rheological parameters were determined by the amplitude sweep test on the MCR-302 rheometer and have shown a behavior specificity of soils of different genesis depending on a texture and organic matter content. Light loamy gley podzolic soils of the northern taiga have a dilatant hardening of interparticle bonds, but their stability is extremely low. Soddy-podzolic soils of the Moscow region have somewhat greater stability, and it is similar to a gley podzolic soils’ stability. The typical chernozem has the greatest stability to stresses.

Experimental study of the deformation and breakage of 3D printed agglomerates: Effects of packing density and inter-particle bond strength

Characterization of the mechanical properties of agglomerates is important in order to understand their deformation and breakage. However, research progress has been hampered by limitations in our ability to manufacture reproducible agglomerates with well-controlled and fully characterised mechanical properties. In this paper, we report on the preparation and testing of agglomerates with tuneable properties using 3D printing technology. Two typical agglomerate structures with different packing densities were designed and printed using a PolyJet 3D printer. Each agglomerate consisted of rigid primary particles connected by either rigid or rubber-like inter-particle cylindrical bonds. Compression tests (using speeds in the range 0.02–0.5 mm/s) and drop weight impact tests were carried out to investigate the effect of bond material and strain rate on mechanical properties of the agglomerates. The results show that strain rate affects their deformation and breakage significantly, and breakage patterns of the two structures are different under uniaxial compression and impact test conditions. These results demonstrate the broad utility of 3D printed agglomerates as ideal “test” agglomerates for a range of breakage studies, including validating computer simulations of DEM breakage.

Understanding the mechanisms of friction stir welding based on computer simulation using particles

Abstract Friction stir welding (FSW) is a novel technique for joining different materials without melting. In FSW the welded components are joined by stirring the plasticized material of the welded edges with a special rotating pin plunged into the material and moving along the joint line. From the scientific point of view, the key role of the FSW processes belongs to formation of the special plasticized conditions and activation of physical mechanisms of mixing the materials in such conditions to produce the strong homogeneous weld. But it is still a lack of complete understanding of what are these conditions and mechanisms. This paper is devoted to understanding the mechanisms of material mixing in conditions of FSW based on a computer simulation using particles. The movable cellular automaton method (MCA), which is a representative of the particle methods in mechanics of materials, was used to perform all computations. Usually, material flow including material stirring in FSW is simulated using computational fluid mechanics or smoothed particle hydrodynamics, which assume that the material is a continuum and does not take into account the material structure. MCA considers a material as an ensemble of bonded particles. Breaking of inter-particle bonds and formation of new bonds enables simulation of crack nucleation and healing, as well as mass mixing and micro-welding. The paper consists of two main parts. In the first part, the simulations in 2D statements are performed to study the dynamics of friction stir welding of duralumin plates and influence of different welding regimes on the features of the material stirring and temperature distribution in the forming welded joints. It is shown that the ratio of the rotational speed to the advancing velocity of the tool has a dramatic effect on the joint quality. A suitable choice of these parameters combined with additional ultrasonic impact could considerably reduce the number of pores and microcracks in the weld without significant overheating of the welded materials. The second part of the paper considers simulation in the 3D statement. These simulations showed that using tool pins of different shape like a cylinder, cone, or pyramid without a shoulder results in negligible motion of the plasticized material in the direction of workpiece thickness. However, the optimal ratio of the advancing velocity to the rotational speed allows transporting of the stirred material around the tool pin several times and hence producing the joint of good quality.

Probing the early stages of tablet disintegration by stress relaxation measurement

Rapid tablet disintegration is a requirement for the efficient dissolution of the active pharmaceutical ingredient (API) from immediate release tablets. From the mechanistic viewpoint, tablet disintegration begins by the wetting of the tablet surface and the ingress of dissolution medium into the tablet pore structure, followed by the loosening of inter-particle bonds. The present work introduces a new methodology for probing and quantifying the early stages of tablet disintegration by stress relaxation measurements using texture analysis (TA). The method is based on applying a pre-defined load on the tablet by means of a needle-shaped probe and measuring the tablet resistance in time after the addition of the dissolution medium. This measurement provides information about the extent and rate of stress relaxation within the tablet upon hydration. Using a tablet formulation containing ibuprofen as the API and lactose as excipient, the effect of the API content, compaction pressure, and pH of the dissolution medium on the stress relaxation rate was systematically investigated. It is shown that using a dissolution medium pre-saturated by the formulation components has only a minor effect on the tablet disintegration rate compared to a pure phosphate buffer, meaning that the surface dissolution of particles within the tablet is not the main pre-requisite of disintegration in this case. On the other hand, pH of the dissolution medium was found to have a very strong effect on the stress relaxation rate in the tablet after wetting, suggesting that van der Waals interactions rather than solid bridges are the predominant particle bonding mechanism in the investigated formulations.

Magnetite Nanocontainers: Toward Injectable Highly Magnetic Materials for Targeted Drug Delivery.

Nanocontainers based solely on magnetite NPs have been synthesized by indirect gelation of stable magnetite hydrosol at ambient temperature using the microemulsion-assisted sol-gel method. Containers synthesized have adjustable size and consist of ∼10 nm magnetite nanoparticles linked by Fe-O-Fe interparticle bonds. The material demonstrates high magnetization values up to 60 emu/g and low cytotoxicity against both HeLa and postnatal human fibroblast (up to 260 μg/mL). The systems developed are perspective as a drug depot, particularly for magnetically controlled thrombolysis.

The early nucleation of a eutectic alloy during high-energy milling

The nucleation of a eutectic alloy –at an early stage– and the factors that determine whether rod or hexagonal growth occurs were evaluated as a function of the process control agent (PCA)—or the lack of it. The study is subdivided into three separated experiments; in all cases, the samples (elemental powders of Pb and Sn) were processed by high-energy milling under composition, pressure and temperature (c-P-T) vial conditions. In the first case, it was detected an excessive mechanical kneading when the powders were processed without PCA and a monolith –as long as 5 mm in length– was obtained. However, this route should not be underestimated because the presence of α−, β− and eutectic-phases –at the surface of the monolith– were traced as nanoparticles (<100 nm). In the second and third cases, to prevent the excessive mechanical kneading and interparticle bond of the powders during milling, two different mechanochemical routes with PCA were considered: (i) tributyl phosphate (TBP) and (ii) ethanol (ETOH). Both of them were proposed to modify the viscoelastic behaviour of the powders. Nanorods resulted when TBP was used. Although structural disorder –at nanometric scale– was not traced, other effects associated with TBP like organic debris and the presence of by-products were detected. Based on the results with the ETOH option, it was shown that the precursors store energy as structural defects during milling. This suggests that the early eutectic nucleation (hexagonal crystal structure) occurred due to structural defects, crystals interfaces and phase boundaries. These acted as preferential sites for nucleation. Moreover, a typical eutectic phase distribution was detected. Finally, it was demonstrated that even after prolonged milling time and nonetheless the kind of PCA –or the lack of it, only the eutectic formation was scarcely traced; that is to say, a partial eutectic formation took place.

Multiscale Insights Into Borehole Instabilities in High‐Porosity Sandstones

AbstractThis paper presents a multiscale analysis of the classical borehole instability problem in high‐porosity sandstones using a hierarchical multiscale approach. A rigorous, two‐way message‐passing coupling of finite element method and discrete element method is employed, where the finite element method is employed to solve a boundary value problem and the constitutive material responses required at each of its integration points are derived by the discrete element method solution to an embedded representative volume element instead of by using an assumed phenomenological model. We employ this multiscale approach to examine the successive failure of a borehole subjected to gradually decreased support stresses at the inner borehole wall or increased far‐field stresses. To reproduce the material behavior of high‐porosity sandstone, representative volume elements with a high‐porosity structure and interparticle bonds are generated. It is found that stress concentration triggers the initial failure at the borehole wall, and the subsequent failure in form of deformation bands is driven by further stress concentration ahead of the band tips. The failure pattern around the borehole varies with initial stress state and global loading path. The comparison of the local stress paths of the initial failure points in various cases reveals that the change of failure pattern from shear failure to mixed‐mode failure and to compaction failure is dominated by the increased mean stress. Cross‐scale parametric studies show that the failure mode changes from clear compaction bands to multiple arrays of shear bands by changing the high‐porosity specimen to a low‐porosity one. The increase in cohesion strength expands the yield locus and enlarges the critical mean stress between different failure patterns. Hence, the failure mode may change from compaction failure to mixed‐mode failure or from mixed‐mode failure to shear failure due purely to the increased cohesion strength. The qualitative comparisons indicate greater length, larger area, and more severe damage of the diametrically opposite failure with the increase in minor principal stress σ0 and principal stress ratio. The diametrically opposite failure mode is found a good indicator for σ0 direction, but it may also occur under hydrostatic far‐field stress due to material anisotropy.

Crack nucleation in rod-type nuclear fuel pellet

A plane problem of fracture mechanics on crack nucleation in a rod-type nuclear fuel pellet is considered. Nuclear reactor fuel pellets in operation may be damaged in various ways; in particular, crack nucleation. We consider a problem for the case of a heat-releasing fuel pellet with cladding: as the heat release intensity increases, zones of heightened stress are formed in the nuclear fuel pellet. The heightened stress will promote the appearance of prefracture bands that are simulated as zones of weakened interparticle bonds of the material. Interaction of prefracture zone faces is simulated by placing bonds between faces that have a specified deformation pattern. The problem of equilibrium of a fuel pellet with prefracture zones is reduced to the solution of a system of singular integral equations. An analysis of the ultimate state of the zone of weakened interparticle bonds of the material is realized on the basis of the criterion of critical opening of prefracture zone faces.

A regime map for the normal surface impact of wet and dry agglomerates

The normal surface impacts of wet and dry agglomerates are simulated in a discrete element modeling framework. While the impact behavior of dry agglomerates has been addressed previously, similar studies on wet agglomerate impact are missing. By adding a small amount of liquid to a dry agglomerate, the impact behavior changes significantly. The impact behavior of the agglomerates at different moisture contents and impact energies are analyzed through postimpact parameters and coupled to their microscopic and macroscopic properties. While increasing the impact energy breaks more interparticle bonds and intensifies damage and fragmentation, increasing the moisture content is found to provide the agglomerates with higher deformability and resistance against breakage. It is shown that the interplay of the two latter parameters together with the agglomerate structural strength creates various impact scenarios, which are classified into different regimes and addressed with a regime map. © 2018 American Institute of Chemical Engineers AIChE J, 64: 1975–1985, 2018

MODELING OF CRACKS NUCLEATION IN FIBER COMPOSITE UNDER BENDING

Design of fiber-reinforced composite of minimum material consumption at guaranteed reliability and durability requires consideration of cases when cracks may appear in the binder. To know the limit bending loads at which cracks will occur in the binder, it is necessary to carry out the limit analysis of the composite. Proposed design model takes into account the presence of damages (zones of weakened inter-particle bonds of the material) in the fiber composite. Based on this design model a calculation method has been developed for composite parameters at which cracks appear. A thin plate of elastic isotropic medium (matrix) and inclusions (fibers) of another elastic material distributed in the matrix is considered. The plate is bending. It is assumed that at the loading of composite, the cracks initiation and fracture of the composite occur. A closed system of nonlinear algebraic equations is constructed. Solution of the obtained system allows to predict the cracking in composite under bending, depending on geometric and mechanical characteristics of the binder and fiber. A criterion of the cracks nucleation in the composite under the action of bending loads is formulated. Size of limit minimal zones of weakened inter-particle bonds of the material at which the cracks nucleation occurs is recommended to be considered as a design characteristic of the binder material.

A discrete element model of concrete for cyclic loading

This paper takes advantage of the discrete element method to develop a model of concrete for cyclic simulations. For this purpose, a micro-mechanical damage model that also allows stress-reversals is formulated for inter-particle bonds. Moreover, a multi-phase implementation of the discrete element method is developed and used for two distinct reasons. First, to characterize aggregate and mortar particles separately. Second, to allow the effect of the interfacial transition zone to be taken into account. A strict validation approach is taken in this work, whereby the developed model is only calibrated against monotonic stress-strain curves and then evaluated for its performance under cyclic loading. Simulation results are constantly compared against experimental values. These comparisons illustrate the capability of the model to predict cyclic properties of concrete. Progression of damage is discussed in terms of numerical variables and also through the visualization of force chains and crack propagation.

Annealing cycles and the self-organization of functionalized colloids

The self-assembly of functionalized (patchy) particles with directional interactions into target structures is still a challenge, despite the significant experimental advances in their synthesis. Self-assembly pathways are typically characterized by high energy barriers that hinder access to stable (equilibrium) structures. A possible strategy to tackle this challenge is to perform annealing cycles. By periodically switching on and off the inter-particle bonds, one expects to smooth-out the kinetic pathways and favor the assembly of targeted structures. Preliminary results have shown that the efficiency of annealing cycles depends strongly on their frequency. Here, we study numerically how this frequency-dependence scales with the strength of the directional interactions (size of the patch σ). We use analytical arguments to show that the scaling results from the statistics of a random walk in configurational space.

Effect of discretization at laboratory and large scales during discrete element modelling of brittle failure

The paper investigates the mode I fracture propagation in brittle rocks using numerical simulations based on the discrete element method. The rock material is represented as an assembly of bonded particles interacting according to an elastic-brittle contact law. The formulation of the method includes the ability for near neighbour interactions to occur so as to control the density of interparticle bonds in the medium, and thus ensures the non-dependence of the model's predictions with respect to its discretization. The adopted model was calibrated to reproduce the behaviour of a granitic rock with the aim to focus more specifically on brittle failure. Numerical samples with different discretizations were generated and subjected to different loading paths. Firstly, the response of laboratory scale pre-cracked specimens subjected to direct tension and three points bending tests was studied. Then, the fracture process zone was characterized by evaluating the volume of the damaged zone as well as the energy dissipated by cracks for different discretizations. It is shown that the proposed approach ensures the fracture process zone characteristics to be independent on the discretization of the model. In addition, large scale simulations of a well-documented underground excavation were performed in order to assess the model's capability to describe brittle failure mechanisms at the structure scale. The results confirm the independence of the model with respect to its discretization and highlight the pertinence of the proposed discrete approach for the study of large scale boundary value problems.