シミュレーション 粉体成形プロセスのシミュレーション

Void ratio based representative volume element for modelling the high strain rate behaviour of granular materials

DEM simulation of the shear behaviour of breakable granular materials with various angularities

A non-invasive experimental method and a clustering DEM model were developed in this study to investigate micro-macro behaviors of real granular materials with irregular particle shape configurations. The investigated behaviors include global deformations, failure strengths and residual strengths, stress and strain distributions, local coordination number, local void ratio, particle kinematics, and fabric orientation distributions. The experimental method includes an approach to automatically identify and recognize multiple particles using x-ray computed tomography imaging (XCT) and an enhanced approach to digitally represent microstructures of granular materials. The digitally represented microstructure can be directly employed for numerical simulation setup. A compression test and a direct shear test on coarse aggregates were conducted and analyzed using this method. The experimental measurements were applied for the evaluation of DEM simulations. The clustering DEM model provided in this study extends the conventional DEM model by incorporating actual microstructure of materials into simulations. Real irregular particles were represented by clusters of balls in the clustering DEM model while spherical particles were employed in the conventional DEM model. The PFC3D commercial software was applied for 3D DEM simulations of the compression test and the direct shear test. Compared to the conventional DEM model, the clustering DEM model demonstrated a better capability of predicting both the micro and macro behaviors of granular materials, including dilation, strength, particle kinematics, and fabric evolution. Fabric distribution was investigated for both the conventional DEM model and the clustering DEM model. The clustering DEM model described the fabric distribution of actual materials more precisely. This feature enabled it to simulate micro-macro behaviors of materials more accurately. A theoretic stress-fabric tensor relationship was also evaluated using the simulated stress and fabric distributions based on the actual microstructure of a real material. This relationship incorporated the anisotropic microstructure characteristics of the material. Whether it can better describe behaviors of granular materials was evaluated in this study. Generally, this research provides a more inherent understanding of granular materials through both DEM simulations and experimental validations.

Electric discharge aided surface post-treatment of laser powder bed fused non-planar metallic components for enhanced form accuracy

As a critical factor governing the strength and deformation behaviours of granular materials, dilatancy characteristics inherently exhibit intricate complexity arising from stochastic particle arrangements and dynamic motion patterns. This article introduces a stress-dilatancy relationship considering the variation of contact number within granular assemblies from the perspective of micromechanics. At microscopic scale, average contact force and displacement expressions are derived using the true stress tensor. And then the variation of contact number is introduced to further optimise the description of the energy dissipation behaviours of granular materials. Following that, the stress dilatancy relationship for granular materials considering of the evolution of fabric anisotropy are established through macro-micro energy conservation. The stress-dilatancy relationship can well describe the phenomenon that the stress ratio for dense sand at the phase transition point is inconsistent with the critical stress ratio. Compared with the reults of experiments obtained from Ottawa sand, the established stress-dilatancy relationship can better describe macroscopic deformation response than the classical flow law such as the Cam-Clay model and Rowe dilatancy model. Moreover, under true triaxial stress paths and different initial anisotropies, the mechanical responses of the proposed stress-dilatancy relationship are consistent with the results of discrete element method simulation experiments.

The notion of hypo-elasticity originates from the work of Truesdell and involves a constitutive law expressing the stress rate as a properly invariant isotropic tensorial function of the stress- and strain-rate tensors. On the other hand hypoplasticity involves the same basic constitutive model as hypo-elasticity, except that the isotropic tensorial function is not necessarily a differentiable function of the strain-rate tensor. This non-differentiable dependence accomodates the known different behaviour of a granular material in compression and tension. Such constitutive models have, over the past decade, been successfully employed to account for the behaviour of various granular materials, including sand, soil and certain powders. The hypoplasticity model is inherently nonlinear, with the consequence that much of the progress to date has been predominantly of a numerical nature arising from its use as an incremental law. Here we examine certain symmetric dynamical cylindrical and spherical cavity problems, with a view to the determination of simple exact results. In the first part of the paper, assuming an infinite granular medium, we show that for any prescribed time-dependent pressure applied at the cavity wall, an exact stress profile may be determined, which satisfies the appropriate conditions at the cavity and at infinity. It is an exact solution in the sense that the underlying equations are properly satisfied, but the initial data for the stress and the void ratio, and the boundary data for the void ratio cannot be arbitrarily prescribed, and must take on those values generated by the stress profile. In the second part of the paper, we examine similarity solutions of cylindrical and spherical cavity problems and show that the governing equations admit a large number of such solutions, including some particularly simple power-law cases. Numerical results are given for the problem of an initially 'infinitesimally small' spherical cavity subjected to constant internal pressure.

Grain crushing underpins key mechanical behaviours of granular materials. A variety of factors, including grading, particle shapes and loading conditions, have been recognised to affect the crushability of grains and the overall behaviour of a granular material. Among them, the role of intermediate principal stress in a general stress condition on the shear behaviour of crushable granular sand remains less understood, owing to the scarcity of experimental data and analytical tools available. In this paper, a multi-scale computational approach is employed to investigate the shear behaviour of crushable granular sand under general stress conditions with varying intermediate principal stresses and confining pressures. The computational approach features multi-scale coupling between non-smooth contact dynamics and peridynamics, and offers a rigorous way to consider the intertwined evolution of particle size and shape during the process of grain crushing. The numerical study helps to quantify comprehensively and analyse the grain crushing-induced changes of macro- and micro-scale material behaviours including strength, deformability, particle size and shape evolution, particle-scale forces and contact conditions, and the development of anisotropy. The competition between a void-filling mechanism due to grain size change and enhanced friction and interlocking due to grain shape change in dictating the deformation of crushable sand is further discussed. The findings offer insights into the complex behaviours of crushable granular materials under general stress conditions and facilitate future development of physics-based constitutive theories on crushable sand.

Effect of the grain elongation on the behaviour of granular materials in biaxial compression

A DEM study on microstructural behaviour of soluble granular materials subjected to chemo-mechanical loading

The behavior of granular materials mainly depends on the mechanical and engineering properties of particles in its structural matrix. Crushing or breakage of granular materials under compression or shear occurs when the energy available is sufficient to overcome the resistance of the material. Relatively little systematic research has been conducted regarding how to evaluate or quantify particle crushing and how it effects the engineering properties of the granular materials. The aim of this study is to investigate the effect of crushing on the bulk behavior of granular materials by using manufactured granular materials (MGM) rather than using a naturally occurring cohesionless granular material. MGM allow changing only one particle parameter, namely the “crushing strength”. Four different categories of MGM (with different crushing strength) are used to study the effect on the bulk shear strength, stiffness modulus, friction and dilatancy angle “engineering properties”. A substantial influence on the stress–strain behavior and engineering properties of granular materials is observed. Higher confining stress causes some non-uniformity (strong variations/jumps) in volumetric strain and a constant volumetric strain is not always observed under large shear deformations due to crushing, i.e. there is no critical state with flow regime (with constant volumetric strain).

The valorisation of waste glass is part of a sustainable development plan. It presents several challenges, starting with the elimination of the various types of waste sent to landfill and the related costs, the reduction of the inconveniences associated with glass (shards, landfill fires), the development of new economic sectors and raw material saving (aggregate quarries). Several research works have previously been carried out to highlight the interest of using waste glass as a partial replacement for unbound granular aggregate in pavement structure. Nevertheless, industrialists remain reluctant to apply such a technique, which encourages researchers to extend their studies in order to properly control the behaviour of granular materials containing glass grains and consequently give more confidence to companies to enforce this technical solution. The present work consists in studying the influence of the shape of glass grains less than 5 mm (rounded and sub-rounded) on the mechanical behaviour of granular pavement materials. The study consists in making a comparison, from a behaviour point of view, between the two forms of glass grains obtained from two different grinding processes. As a result, the Proctor test clearly shows that there is no significant difference between rounded and sub-rounded shape in terms of compactabiliy. It was noted that the addition of particles of recycled glass makes the mixture more insensitive to water. It has been also found that the limiting percentage of addition of glass particles is situated between 20% and 30% reported to the total mass of the mixture.

Granular materials are often used in pavement structures. The influence of anisotropy on the mechanical behaviour of granular materials is very important. The coupled effects of water content and fine content usually lead to more complex anisotropic behaviour. With a repeated load triaxial test (RLTT), it is possible to measure the anisotropic deformation behaviour of granular materials. This article initially presents an experimental study of the resilient repeated load response of a compacted clayey natural sand with three fine contents and different water contents. Based on anisotropic behaviour, the non-linear resilient model (Boyce model) is improved by the radial anisotropy coefficient γ3 instead of the axial anisotropy coefficient γ1. The results from both approaches (γ1 and γ3) are compared with the measured volumetric and deviatoric responses. These results confirm the capacity of the improved model to capture the general trend of the experiments. Finally, finite element calculations are performed with CAST3M in order to validate the improvement of the modified Boyce model (from γ1 to γ3). The modelling results indicate that the modified Boyce model with γ3 is more widely available in different water contents and different fine contents for this granular material. Besides, based on the results, the coupled effects of water content and fine content on the deflection of the structures can also be observed.

Modeling of textural anisotropy in granular materials with stochastic micro-polar hypoplasticity

This paper uses the discrete element method (DEM) in three dimensions to simulate cone penetration testing (CPT) of granular materials in a calibration chamber. Several researchers have used different numerical techniques such as strain path methods and finite element methods to study CPT problems. The DEM is a useful alternative tool for studying cone penetration problems because of its ability to provide micro mechanical insight into the behaviour of granular materials and cone penetration resistance. A 30° chamber segment and a particle refinement method were used for the simulations. Giving constant mass to each particle in the sample was found to reduce computational time significantly, without significantly affecting tip resistance. The effects of initial sample conditions and particle friction coefficient on tip resistance are investigated and found to have an important effect on the tip resistance. Biaxial test simulations using DEM are conducted to obtain the basic granular material properties for obtaining CPT analytical solutions based on continuum mechanics. Macro properties of the samples for different input micro parameters are presented and used to obtain the analytical CPT results. Comparison between the numerical simulations and analytical solutions show good agreement.

Modelling phase transition in granular materials: From discontinuum to continuum