Discrete element analysis of the mechanical properties of deep-sea methane hydrate-bearing soils considering interparticle bond thickness

Due to increasing global energy demands, research is being conducted on the mechanical properties of methane hydrate-bearing soils (MHBSs), from which methane hydrate (MH) will be explored. This paper presents a numerical approach to study the mechanical properties of MHBSs. The relationship between the level of MH saturation and the interparticle bond thickness is first obtained by analyzing the scanning electron microscope images of MHBS samples, in which is the bridge connecting the micromechanical behavior captured by the DEM with the macroscopic properties of MHBSs. A simplified thermal-hydromechanical (THM) bond model that considers the different bond thicknesses is then proposed to describe the contact behavior between the soil particles and those incorporated into the discrete element method (DEM). Finally, a series of biaxial compression tests are carried out with different MH saturations under different effective confining pressures to analyze the mechanical properties of deep-sea MHBSs. The results of the DEM numerical simulation are also compared with the findings from triaxial compression tests. The results show that the macromechanical properties of deep-sea MHBSs can be qualitatively captured by the proposed DEM. The shear strength, cohesion, and volumetric contraction of deep-sea MHBSs increase with increasing MH saturation, although its influence on the internal friction angle is obscure. The shear strength and volumetric contraction increase with increasing effective confining pressure. The peak shear strength and the dilation of MHBSs increase as the critical bond thickness increases, while the residual deviator stress largely remains the same at a larger axial strain. With increasing the axial strain, the percentage of broken bonds increases, along with the expansion of the shear band.     

Relationship between single and bulk mechanical properties for zeolite ZSM5 spray-dried particles

In this work typical mechanical properties for a catalyst support material, ZSM5 （a spray-dried granular zeolite）, have been measured in order to relate the bulk behaviour of the powder material to the single particle mechanical properties. Particle shape and size distribution of the powders, determined by laser diffraction and scanning electron microscopy （SEM）, confirmed the spherical shape of the spray-dried particles. The excellent flowability of the material was assessed by typical methods such as the Hausner ratio and the Cart index, This was confirmed by bulk measurements of the particle-particle internal friction parameter and flow function using a Schulze shear cell, which also illustrated the low compressibility of the material. Single particle compression was used to characterize single particle mechanical properties such as reduced elastic modulus and strength from Hertz contact mechanics theory. Comparison with surface properties obtained from nanoindentation suggests heterogeneity, the surface being harder than the core. In order to evaluate the relationship between single particle mechanical properties and bulk compression behaviour, uniaxial confined compression was carried out. It was determined that the Adams model was suitable for describing the bulk compression and furthermore that the Adams model parameter, apparent strength of single particles, was in good agreement with the single particle strength determined from single particle compression test.     

粗粒土离散元非球趋真颗粒模型

颗粒形状对粗粒土的物理力学特性有着显著的影响。离散元法广泛应用于粗粒土宏观物理力学特性的细观机理研究。为了考虑颗粒形状的影响,亟待发展计算高效的离散元非球趋真颗粒模型。本文基于X射线CT扫描技术并结合数字图像处理技术,对光滑和棱角性两类典型粗粒土（鹅卵石与碎石）进行三维重构,并提出了两类趋真颗粒模型,分别采用扩展超椭球模型和球多面体模型进行趋真逼近;开展了两类颗粒试样的3D打印和单轴压缩试验,分析了配位数和局部孔隙率分布等细观特性;基于离散元开源程序SudoDEM开展了两类试样的离散元模拟,并将模拟细观分析结果与物理试验进行了对比。结果表明,提出的两类趋真颗粒模型能够较好地对粗粒土颗粒进行离散元建模。     

Variation of granule mass fraction with coordination number in wet granulation process

In granulation, fine particles combine to form a coarse granule in the form of a particle matrix partially or fully saturated with a binder liquid. The final product of granulation possesses a wide variety of granule size distributions with surface mean diameters which differ with operating conditions. The final granule size depends on the operating conditions, e.g. operating gas velocity, inlet air temperature, initial feed particle size, and viscosity of the binder. The objective of this paper is to find out the uniformity in the relation between the granule mass fraction in the final granule size distribution and the number of feed particles present in the granules. The total number of granules obtained depends on the experimental conditions but the granule mass fraction and the number of feed particles forming a single granule are independent of operating variables, feed material and method of granulation. The paper purports further to compare the uniform nature of mass fraction of the granules in final granule size distribution and the primary particles required to form that particular granule size irrespective of experimental conditions of granulation.     

Simulation on mechanical behavior of cohesive soil by Distinct Element Method

Analysis in the dynamic mechanical behavior of cohesive soils subjected to external forces is very important in designing and optimizing terrain machines. Distinct Element Method (DEM) is an ideal method to analyze large discontinuous deformations of soil, but the conventional DEM model is difficult in simulating the complex behavior of cohesive soil. In order to simulate and analyze the behavior of cohesive soil accurately, the DEM mechanical model of cohesive soil with parallel bonds between particles was established by considering the capillary and the dynamic viscous forces induced by the presence of water between soil particles. During the excavation process by a bulldozing plate, the dynamic behavior of cohesive soil was simulated by DEM software PFC2D. The phenomena that the discrete particles were bonded into clusters initially, and the clusters were broken into smaller clusters or discrete particles during the excavation process, are consistent with the ruptures and separations of the actual cohesive soils subjected to external forces.     

A study of influence of gravity on bulk behavior of particulate solid

This paper examines the influence of gravity on the bulk responses of a granular solid. The loading scenarios in this study include confined compression, rod penetration into a granular medium and discharging through an orifice. Similar loading and flow conditions are likely to be encountered in the stress and deformation regimes that regoliths are subjected to in extraterrestrial exploration activities including in situ resource utilisation processes. Both spherical and non-spherical particles were studied using the discrete element method (DEM). Whilst DEM is increasingly used to model granular solids, careful validations of the simulation outcomes are rather rare. Thus in addition to exploring the effect of gravity, this paper also compares DEM simulations with experiments under terrestrial condition to verify whether DEM can produce satisfactory predictions.The terrestrial experiments were conducted with great care and simulated closely using DEM. The key mechanical and geometrical properties for the particles were measured in laboratory tests for use in the DEM simulations. A series of DEM computations were then performed under reduced gravity to simulate these experiments under extraterrestrial environment. It was found that gravity has no noticeable effect on the force transmission in the confined compression case; the loading gradient in the rod penetration is linearly proportional to the gravity; the mass flow rate in silo discharge is proportional to square root of the gravity and the angle of repose increases with reducing gravity. These findings are in agreement with expectation and existing scientific evidence.     

A study of influence of gravity on bulk behaviour of particulate solid

This paper examines the influence of gravity on the bulk responses of a granular solid. The loading scenarios in this study include confined compression, rod penetration into a granular medium and discharging through an orifice. Similar loading and flow conditions are likely to be encountered in the stress and deformation regimes that regoliths are subjected to in extraterrestrial exploration activities including in situ resource utilisation processes. Both spherical and non-spherical particles were studied using the discrete element method （DEM）. Whilst DEM is increasingly used to model granular solids, careful validations of the simulation outcomes are rather rare. Thus in addition to exploring the effect of gravity, this paper also compares DEM simulations with experiments under terrestrial condition to verify whether DEM can produce satisfactory predictions. The terrestrial experiments were conducted with great care and simulated closely using DEM. The key mechanical and geometrical properties for the particles were measured in laboratory tests for use in the DEM simulations. A series of DEM computations were then performed under reduced gravity to simulate these experiments under extraterrestrial environment. It was found that gravity has no noticeable effect on the force transmission in the confined compression case; the loading gradient in the rod penetration is linearly proportional to the gravity; the mass flow rate in silo discharge is proportional to square root of the gravity and the angle of repose increases with reducing gravity. These findings are in agreement with expectation and existing scientific evidence.     

3-D DEM simulation of cohesive soil-pushing behavior by bulldozer blade

A numerical simulation based on discrete element method (DEM) was conducted on the excavation and pushing processes of soil by a bulldozer blade. Soil contains water and the resistance acting on the bulldozer blade is largely influenced by the cohesive force due to liquid bridges formed among soil particles. In the present study, a cohesive bond force model proposed by Utili and Nova [5] was introduced in which the microscopic behavior of cohesive force was modeled analogously with macroscopic shear failure characteristics. The dependency on the magnitude of microscopic cohesive force was verified. The behavior of particles changed greatly by taking into account the cohesive bond force. The characteristic behavior of excavated soil aggregates, such as rolling motion and intermittent collapsing, were observed in front of the blade surface.     

Discrete element modeling of inherently anisotropic granular assemblies with polygonal particles

In the present article, we study the effect of inherent anisotropy, i.e., initial bedding angle of particles and associated voids on macroscopic mechanical behavior of granular materials, by numerical simulation of several biaxial compression tests using the discrete element method (DEM). Particle shape is considered to be irregular convex-polygonal. The effect of inherent anisotropy is investigated by following the evolution of mobilized shear strength and volume change during loading. As experimental tests have already shown, numerical simulations also indicate that initial anisotropic condition has a great influence on the strength and deformational behavior of granular assemblies. Comparison of simulations with tests using oval particles, shows that angularity influences both the mobilized shear strength and the volume change regime, which originates from the interlocking resistance between particles.     

FAE炸药跌落撞击安全性数值分析

提出了冲击载荷作用下燃料空气炸药(FAE)的三相混合物材料模型. 在模型中, FAE炸药被模型化为具有一定的微观结构, 其液体组分及气体组分填充于金属固相颗粒互相搭结构成的骨架孔洞内. 另外, 材料模型中引入了气体的绝热压缩模型, 使得整个模型可以给出在冲击加载条件下FAE炸药中各组分的应力状态及温度场, 从而为合理地判定FAE装药的安全性提供了条件. FAE三相混合物材料模型作为用户自定义子程序嵌入到动力学程序AUTODYN中. 最后, 应用FAE炸药材料模型对某FAE战斗部从20\,m高处垂直跌落至钢质地板的撞击过程进行了数值模拟. 基于FAE装药在这种跌落撞击过程中的应力和温度状态,对其安全性进行了评价.      

Size-dependent coalescence kernel in fertilizer granulation-A comparative study

Granulation is a key process in several industries like pharmaceutical, food, fertilizer, agrochemicals, etc. Population balance modeling has been used extensively for modeling agglomeration in many systems such as crystallization, aerosols, pelletisation, etc. The key parameter is the coalescence kernel, β（ij） which dictates the overall rate of coalescence as well as the effect of granule size on coalescence rate. Adetayo, Litster, Pratsinis, and Ennis （1995） studied fertilizer granulation with a broad size distribution and modeled it with a two-stage kernel. A constant kernel can be applied to those granules which coalesce successfully. The coalescence model gives conditions for two types of coalescence, Type I and II. A twostage kernel, which is necessary to model granule size distribution over a wide size distribution, is applied in the present fluidized bed spray granulation process. The first stage is size-independent and non-inertial regime, and is followed by a size-dependent stage in which collisions between particles are non-random, i.e. inertial regime. The present work is focused on the second stage kernel where the feed particles of volume i and j collide and form final granule ij instead of i ＋j （Adetayo et al., 1995） which gives a wider particle size distribution of granules than proposed earlier.     

WET-GRANULATION RESEARCH WITH APPLICATION TO SCALE-UP

Summary Granulation is a unit operation by which larger granules are produced from fine, powdery particles to improve appearance, flow properties and mixedness, reduce dustiness and, in general, produce engineered particles with superior attributes. Agglomeration in wet granulation is achieved by introducing a “binder” fluid onto a shearing mass of fine powders. This paper gives a general overview of the process with emphasis on a simplified granulation model based on a dimensionless parameter containing inertia and viscous dissipation energies between colliding particles: the so-celled Stokes number. The model incorporates most common features of all granulation devices (mixers) used in the pharmaceutical industry.Also described in the paper is a computer simulation that captures the movement of flowing powder in an ideal mixer-granu/ator with constant shear rate. A fraction of the total number of particles is wet (covered by binder and there-fore “sticky”) while the rest of the particles are dry. The numerical simulation depicts two distinct regimes of agglomeration found in a typical granulator: granule growth and subsequent breakup. During granule growth-simulations, final granule size and shape distributions are obtained by analyzing the size and shape of formed granules using a pattern-recognition technique. A second kind of simulation, also using rapid granular flow modeling, follows the rotation and deformation of an “agglomerate” held together by a liquid binder. Results from these simulations yield critical values of the Stokes number. Below the cdticel value, the agglomerates are stable and only rotate in response to shear while above the critical value they break into two or more pieces. At the critical value, they attain a steady elongated shape. Using values of the critical Stokes number, the model predicts the size of formed granules.The existence of the critical state in which granules attain a characteristic elongated shape is used to measure shear forces in a granulator by employing calibrated “test” particles of known strength. This knowledge is employed in granulation scale-up to determine a kinematic rule that conserves stresses in the small and the large-scale machines. It was found that conserving the magnitude of internal stresses in the moving powder yields granules with similar attributes in granulators of different size.     

基于球形颗粒几何排列的离散元试样高效生成方法

在球体离散元数值模拟中,颗粒的初始排列状态是影响计算效率和计算结果的重要环节。本文采用前进面几何构造算法,提出了一种基于网格搜索的球形颗粒随机排列高效算法。通过求解空间三边方程,满足了粒径设置的任意大小的颗粒依次置入前进面的外侧,并与构成前进面的三个颗粒相互接触。为获得高体积分数的颗粒簇,该算法允许颗粒改变其粒径大小。采用颗粒网格化方法可以简化前进面的搜索,并由此提高排列效率。通过计算平均配位数、体积分数和二阶结构张量的特征值,对不同粒径比下得到的立方体试样进行了分析,得到试样配位数及体积分数均随着粒径比的增大而增大,且得到的试样为各向同性。此外,空间网格的大小和初始颗粒的生成点对随机排列的效率均会产生显著的影响。最后,对非规则铁路道砟进行了精细构造及压碎模拟,发现DEM模拟得到的应力-应变曲线与试验结果基本吻合,验证了该算法得到的颗粒试样在模拟道砟裂纹起裂、扩展等过程的有效性。     

橡胶粘结颗粒材料粘弹性性能的试验研究与离散元模拟

制备了颗粒规则四方排列和六方排列的橡胶粘接颗粒材料试样,实验测试了所制备试样在单向拉伸载荷下的应力松弛曲线和不同应变率时的应力应变曲线。基于所测试的应力松弛曲线,采用曲线拟合方法得到了所测试材料的宏观Burger’s粘弹性本构模型参数。采用离散元模型中单元间连结模型代表颗粒间橡胶粘接剂的作用,并基于试样的宏观Burger’s模型参数与离散元模型中细观Burger’s连结模型参数间的关系,建立了橡胶粘接颗粒材料的无厚度胶结离散元分析模型。最后采用所建立的离散元模型计算了所测试试样的松弛和拉伸力学性能。离散元预测结果与实验结果的对比表明,采用无厚度胶结离散元模型能较好的计算颗粒规则排列的橡胶粘接颗粒材料松弛和拉伸力学性能,但基于应力松弛实验拟合而来参数不能准确反应橡胶粘接剂在高应变率条件下其力学性能的应变率相关性。     

Investigation on the granulation behavior of iron ore fine in a horizontal high-shear granulator

High-shear granulation is widely used in many particulate industries for its good capability to improve the size, strength and composition uniformity of powder substances. This work conducted an experimental study to investigate the granulation behavior of iron ore fine in a horizontal high-shear granulator, such as granules size distribution, granules growth rate, and permeability of the granules bed. The results show that the granule size and permeability of packed granules bed increase gradually with increasing the granulation time, and the growth of granules can be divided into three stages: the rapid growth stage, the slow growth stage and the relatively stable stage. Both the higher rotational speed and larger number of impellers increase the kinetic energy and collision frequency of the particles, which causes the increase of average granule size, growth rate and permeability of granules packed bed. On the other hand, the shear damage effect of the impellers on the granules is also enhanced with the increase of rotational speed and impeller number, resulting in significant granule size segregation.     

Simulation of soil deformation and resistance at bar penetration by the Distinct Element Method

A mechanical model of soil is constructed using the Distinct Element Method (DEM) which makes it possible to analyze the discontinuous property of soil. To discuss the applicability of the soil model by the DEM, a bar penetration test was conducted and the result was compared with the simulation results. From the results of the behavior of elements, it could be said that the mechanical model by the DEM could well simulate the discontinuous behavior of soil and the parameters used in the simulation play important roles to make the soil model useful. As for the penetrating resistance, some problems which lie in the present DEM model are discussed and the key to solving these problems is indicated. Moreover, the method to determine the time interval used in the DEM simulations is mentioned in terms of the stability of the solution in the calculation.     

PARTICLE PROPERTIES—PROMISE AND NEGLECT

We see two major trends in Particle Technology. First, the focus is shifted from unit operations towards functional products, i.e. towards product engineering. Second, modeling will become more and more important. Processes cannot yet be designed from basic molecular understanding. Nanotechnology, however, begins to bridge this gap between molecules and particles and may thus open new ways not only for the production and handling of particulate matter but also for the engineered design of advanced material properties. Starting from the concept of product engineering we investigate the basic preconditions for tailoring nanoparticulate properties, i.e. the control of the particle interactions. Nanotechnology can only be transferred to industrial production if the interactions are effectively controlled. Material and particle properties are essential for predictive models. Although strong tools like MD, DEM or population balance models are available, these models are only predictive if realistic material and particle properties are available which is often not the case. We show for selected examples how particle properties can be obtained by studying the physically relevant elementary processes. The impact breakage behavior of many different materials is described by a master curve. Particle adhesion can be modeled if the roughness of particle and substrate and the Hamaker constant are known. The latter is obtained from adsorption studies.     

New evaluations for multiaxial-stress properties of ceramic materials

This paper suggests some new evaluations for multiaxial-stress properties of ceramic materials. These evaluations include some that have been used for other kinds of materials, as well as others which have not been previously employed. In some cases, these methods represent modifications of existing evaluations. The paper is confined to macroscopic behavior based upon bulk laboratory specimens. The influences of volume, stress gradients and localized behavior are not considered here since considerable attention has recently been devoted to these questions. The important problem of fracture strength will not be considered since this property appears to be considerably influenced by localized microscopic behavior. However, new evaluations of remaining mechanical properties for states of combined stresses will be presented. These include elastic and plastic strength, stiffness, ductility, resilience and toughness. Emphasis on combined-stress properties was selected since recent critical reviews indicate the need for for such an evaluation. Part A of this paper outlines new experiments that are needed to evaluate the mechanical properties and to confirm theories proposed in Part B. In Part B of this paper, new macroscopic engineering-type theories for combined-stress behavior are presented for the first time. These theories attempt to predict combined-stress behavior from uniaxial tension and compression (or pure bending and compression) behavior. These theories provide for materials such as ceramics with different properties in tension and compression. A final section of this presentation is devoted to improvements in the evaluations of other mechanical properties of materials as related to high-temperature creep and fatigue properties.     

冲击载荷下脆性空心颗粒破碎机理

为考察脆性空心颗粒在冲击载荷作用下的应变率效应和破碎行为的细观机理,以粉煤灰漂珠为研究对象,基于低速冲击实验和有限元数值模拟,对比了典型空心颗粒材料在不同加载速率下的力学响应特性和细观压溃行为,阐释了材料宏观应变率效应产生的细观机理,获得以下结果。（1）在0.001～300 s?1应变率范围,漂珠颗粒的破碎率和Hardin破碎势平均提升了约21%和10%～30%,材料比吸能提升了50%～125%,比吸能的额外增加主要与动态颗粒滑移产生的摩擦耗能相关。颗粒平均尺寸较大的试样体现出更强的应变率效应。（2）初始压溃阶段的应力应变响应特征的数值模拟结果与实验结果较吻合,低速冲击下动态二次压溃现象产生的细观机理为动态颗粒滑移和压紧行为对加载速率的依赖性。(3) 数值模拟表明,冲击加载下产生相同应变时颗粒的损伤程度和范围大于准静态加载,这与实验所得破碎势随应变率增加的结果一致。对比低速冲击实验的相对破碎势分析和细观数值模拟结果可知,脆性颗粒堆积材料在动态冲击下表现出的宏观应变率效应主要归因于颗粒压溃行为的率敏感性和动态加载下颗粒破碎能量利用率的降低。     

单轴压缩条件下混凝土细观损伤演化机理的CT试验研究

混凝土是一种非均质的材料,在细观层次上将混凝土看作由骨料、砂浆和两者之间的界面组成的三相复合材料.本文在对混凝土进行单轴压缩试验的基础上,根据CT扫描图像反映的细观破损过程,分析了基于CT数平均值变化规律的特点;将混凝土材料的损伤过程进行了分段,提出了损伤变量的提取方法.随后结合试验得到的宏观应力应变曲线,经过拟合得到...