Granular Matter最新文献

Laser powder bed fusion (LPBF) is an additive manufacturing technique that utilizes laser-induced melting of specific regions within a powder layer to create complex parts. Achieving high-quality products in LPBF requires the optimization of process parameters based on the unique characteristics of the powder material. Since experimental optimisation can be both time-consuming and costly, we propose a computational model capable of simulating the particle micro-mechanics in LPBF, offering a more cost-effective solution.

We have developed a novel thermal discrete particle and contact model that accurately captures the essential phenomena of melting, coalescence, and consolidation within LPBF. Our model assumes that solid particles partially melt under the influence of heat, subsequently coalesce, and form solid bonds during the cooling phase. The rate of coalescence is determined by the material’s surface tension and viscosity as it undergoes melting. To account for phase transitions, we employ an apparent heat capacity method. We first introduce our contact model and provide verification against analytical solutions for a two-particle system. We then demonstrate the efficacy of our model by applying it to a multi-particle example, successfully capturing the coalescence and consolidation behaviour observed in LPBF. The model has been implemented in the open-source code MercuryDPM. The current model is developed for polymer material, but it can be extended to metal and ceramic.

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Thermal Discrete Particle Model of Particle melting and Coalescence

Driven by the “dual carbon” strategy for low-carbon steel transformation, raising the pellet ratio in the blast furnace (BF) burden offers a core technical path to optimize burden structure and cut carbon emissions. The distribution of BF burden critically influences operation by affecting internal gas flow, heat and mass transfer, and chemical reactions. This study employs the discrete element method (DEM) to examine how burden structure affects bed porosity at high pellet ratios, emphasizing mixed-layer formation. Key findings include: (1) Porosity evolution patterns in mixed burden layers under varying furnace charge configurations were characterized. (2) Porosity is minimized at the ore-coke interface because of particle penetration. Additionally, this effect intensifies as the coke-ore size difference increases. (3) Increasing the pellet ratio enhances lump-zone bed porosity and permeability. As the pellet ratio increased from 30 to 90%, bed porosity rose from 33.75 to 36.19%, a 2.43% increase.

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In this study, a contact-scale approach is developed in order to measure the strength of the bio-cemented bond under shear loading. Fifteen pairs of bio-cemented sand grains, coming from two different bulk samples with different calcite content, were first observed using high-resolution synchrotron X-ray tomography in order to compute the contact surface area, before being subjected to the shear loading. After failure, these samples were also observed using scanning electron microscopy and energy-dispersive X-ray spectroscopy in order to determine the failure mode. The results have shown that the shear strength is 2.11 times higher that the tensile strength for this material, and has an average value of 5.81 ± 1.99 MPa. Digital image correlation was used in this study in order to distinguish the samples that broke due to shear from those that broke due to rolling. Similarly to the case under tensile loading, failure was also observed to occur at the interface between the sand and the calcite crystals. The percentage of active calcite crystals (hbox {f}_textrm{c}) was also found to be around 25%, independently of the amount of calcite present in the initial bulk sample.

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Main steps followed in the study that allow to find the shear strength of a bond between bio-cemented sand grains.

This paper investigates the effects of recent stress history (RSH), specifically sudden changes in the stress path direction, on the small-strain stiffness and stiffness degradation characteristics of granular materials, using three-dimensional discrete element method (DEM) simulations. Specimens with varying RSHs are prepared via a series of approach paths from diverse directions, involving deviations from and returns to a common stress state. The norm of the induced plastic strain, obtained through a stress probing technique, is proposed as a criterion to determine the extent of the approach path. Subsequently, constant-p triaxial compression tests are conducted to analyze the effects of different RSHs on stress–strain response, small-strain stiffness, and stiffness degradation. Results reveal that specimens experiencing full stress reversal exhibit significantly higher small-strain shear stiffness than those under sustained load directions. Specimens loaded with counterclockwise stress rotation angles exhibited slightly greater stiffness than those with equivalent clockwise angles. Furthermore, the configuration of stiffness degradation curves, defined by onset and reference shear strain, is strongly influenced by the stress rotation angle. Stress probing reveals that RSH mainly affects total shear stiffness degradation by altering the extent of plasticity, with the elastic stiffness remaining largely unaffected. Finally, a modification to an existing stiffness model is proposed by introducing an RSH-dependent factor, complementing the effects of void ratio, stress state, and shear strain, to enhance predictive accuracy.

Compaction characteristics and maximum dry density control constitute critical considerations for over coarse-grained soil filler. In this study, DEM simulations of surface vibration tests were conducted and parametric analyses were carried out to investigate the effect of maximum particle size and gradation on compaction characteristics and maximum dry density behavior of over coarse-grained soil. The results show that the maximum dry density was positively correlated with the maximum particle size. Pores could be sufficiently filled when the content of small particles reached 25%, and soil skeleton was loosened when the content of small particles exceeded 35%. Giant and medium particles at a low content increased the maximum dry density by replacing small particles and void between them, and forming the soil skeleton when the content exceeded 40%. The increase in maximum dry density caused by small particles filling the pores and the decrease caused by the loosening of soil skeleton result in a peak value of maximum dry density when the content of small particles is between 25 and 35%. The effects of particles in soil can be concluded into four effects: framework effect, filling effect, substitution effect, and loosening effect. This study provides a scientific basis for predicting models and compaction methods of over coarse-grained soil.

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We explored experimentally the static and dynamic behavior of magnetic repelling particles confined in a two-dimensional cell using two particle geometries, namely, disks and rectangular bars. Despite the contactless interaction, typical static features of granular materials are observed for both particle shapes when the material rearranges under the action of gravity: pile formation with an angle of repose, and pressure saturation (Janssen-like effect), which can be explained by considering the magnetically-induced torques that generate friction between particles and confining walls. When the material is forced to be rearranged by compression, particle shape effects become notorious: while disks rearrange increasing the hexagonal ordering, bars augment their orientational ordering forming larger non-contact force chains mediated by the magnetic field; however, in both cases, the resistance to compression rises continuously, in contrast with the fluctuating compression dynamics (stick–slip motion or periodic oscillations) that characterizes granular systems with inter-particle contacts. Our results indicate that continuum approaches of granular materials can be used to characterize the system, despite the contactless interaction and specific shape of the constitutive particles.

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Angle of repose, Janssen effect and other features of conventional granular matter are also observed in a system of contactless repelling particles.

Discrete element method and theoretical analyses were performed to investigate the evolution characteristics of dynamic normal stress on a silo wall with hyperbolic hopper. Results show that the hyperbolic hopper promotes the development of stagnant zone, which avoids the concentration of large dynamic normal stress on a silo wall effectively. Compared with the conical hopper, the hyperbolic hopper reduces the peak normal stress by 14.42%, and increases the range of large normal stress from 0.2 to 0.5 m. The large stagnant zone causes a flow zone with large velocity gradient to develop, which drives the granular materials to flow out from the hyperbolic hopper in an orderly manner, thereby weakening the oscillation strength of the granular materials against silo wall. The hyperbolic hopper weakens the inertial force generated by the instantaneous arch during its formation and breaking, resulting in reducing the dynamic normal stress on silo wall with a hyperbolic hopper.

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The influence of the raised-bottom on the dissipation behavior of three-dimensional (3D) oscillating closed granular system is studied by discrete element simulation in this work. Firstly, the damping effect and motion phase states of granular balls in a flat-bottom closed container are investigated in the interested range of excitation parameters, which reveals two different high damping granular dissipation behaviors. Then, the damping characteristics of the same quantity of granular balls in flat, concave and raised bottom containers are compared, which indicates that the granular system with a 2 mm raised-bottom exhibits relatively better damping effect. Moreover, the difference between flat-bottom and 2 mm raised-bottom granular system in the dissipation behavior of optimal damping granules is further analyzed. Finally, the essence of enhanced damping effect of the 3D granular system by the 2 mm raised-bottom is revealed, i.e., the fact that the change of injection mode of vibration energy into granular system caused by the raised-bottom makes it easier for ideal dense granular cluster to appear in the oscillating granular bed.

In order to accurately and efficiently predict the landslide hazard and post-failure behavior of soil-rock mixtures (SRM), this study adopts the smoothed particle hydrodynamics (SPH) method. Rocks with arbitrary shapes are generated by employing the Monte Carlo random sampling principle. Subsequently, a lattice-based particle generator is proposed to interpret the geometrical model of SRM slopes and to construct the SPH numerical model. Furthermore, this study examines the effects of varying rock contents, sizes and shapes on the failure characteristics of SRM slopes. The findings reveal that the shear zone exhibits non-circular form during SRM slopes failure, presenting four distinct plastic expansion modes: Bypass, diversion, penetration, and inclusion. For identical rock content, an increase in large-sized rocks enhances the interlocking effect, thereby improving SRM slope stability. Conversely, the roundness of rocks significantly affects their failure behavior within SRM slopes, with higher roundness contributing to easier instability. The results demonstrate that the SPH method provides an innovative approach for investigating the failure behavior of heterogeneous materials, such as geotechnical bodies. Moreover, this method exhibits substantial potential for broader applications across various geotechnical engineering domains.

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