基于颗粒物质力学的铁粉末压制中摩擦特性对力链演化影响

结合颗粒物质力学理论,通过离散元法实现铁粉末压制过程模拟并通过压制方程进行验证,针对粉末体系中的力链演化问题,提出力链特征定量分析方式,进一步通过分析不同颗粒间摩擦系数、侧壁摩擦系数与颗粒运动状态转变的方式,探讨摩擦特性对力链量化特征的影响,从而建立摩擦行为与力链演化间的联系. 研究结果表明:随颗粒间摩擦系数增大,整体力链数目变少,力链方向系数、承载不均匀度及单位屈曲度均变大,而随侧壁摩擦系数增大,力链特征差异较小,则颗粒间摩擦系数较侧壁摩擦系数对力链特征演化具有更显著影响. 同时发现,颗粒接触状态的改变与力链特征演化间具有对应性. 研究成果将进一步拓展粉末压制中考虑摩擦行为及力链演化过程在内的粉体致密化行为理论.      

双轴压缩下颗粒物质接触力与力链特性研究

为探究双轴压缩过程中颗粒物质接触力与力链特性,采用离散元法研究双轴压缩过程,得到双轴压缩过程中颗粒体系宏观力学特性,即偏应力、体积应变变化规律,并通过对接触力大小分布规律、接触力角度分布规律等细观层次分析,完成接触力大小、力链方向性量化描述,同时探究围压对颗粒物质力学特性的影响。研究表明,双轴压缩过程经历应力强化、应力软化、应力波动三个阶段。在各阶段,强接触与总体接触的各类型接触力大小分布规律可采用高斯函数或指数函数拟合,弱接触的法向与总接触力分布较为均匀;直径较大颗粒上具有接触力集中现象,集中程度随着围压升高而加强;力链网络形态经历"鱼鳞"状-"柱"状-"拱"状-"龟壳"状演变,由初始时基本各向同性,逐渐向最大主应力y向偏转,表现出强烈各向异性,最后又经历小幅度偏转偏离y向。不同围压下力链分量比变化规律证实了该发现,接触力角度分布规律也从力链承载方向验证了此结论。研究结果将为探究双轴压缩中颗粒物质力学特性提供理论基础。     

基于颗粒物质力学的粉末高速压制过程中应力传递分布分析

为探究粉末高速压制过程中应力变化规律,结合颗粒物质力学理论与粉末高速压制高速加载特点,采用离散元法研究粉末高速压制过程,获取了应力整体传递变化规律,应力大小和方向性变化规律及应力传递与力链间关系。研究结果表明:在粉末高速压制中应力逐层传递并在边界反射,其沿高度方向在不同时刻展现出峰值流动特性,随压制进行峰值分别处在0.014m、0.0075m、0.0025m、0.011m、0.014m高度位置,并随压制进行沿宽度方向在模壁处出现逐渐降低现象;应力大小分布概率密度基本呈现单峰特性,在0.5ms、0.8ms时刻峰值分别出现在归一化应力值为3和1.5附近,在1.3ms、1.7ms、4ms时刻峰值出现在归一化应力值为1附近;主应力方向经历无规律分布逐渐向各向异性即90°方向转变,最上层粉末主方向角变化最为剧烈,变化率峰值达到61.5?10(o)/s与6-2.5?10(o)/s,最下层粉末主方向角变化最为舒缓,变化率峰值仅为60.3?10(o)/s;同时,归一化力链强度值与应力值绝对差值保持在0.1下,1.3ms~3.3ms阶段平均增长斜率分别为0.31与0.32,应力传递与力链延展保持较为一致趋势。本研究有助于更深入地了解粉末颗粒物质在高速压制过程中受力变化规律,为进一步提高粉末压制件致密化程度与密度均匀性提供指导。     

回转窑多粒径颗粒流动与偏析的DEM模拟与实验研究

为明晰回转窑内颗粒的运动行为及偏析机理,以绿豆、黄豆和黑豆为颗粒介质,依次对3种装填顺序下的颗粒流动过程进行离散元模拟与实验研究,以颗粒质量分数和平均粒度为判据,对颗粒偏析进行评价。结果表明,回转窑内颗粒流动区可分为自由滚落区、渗流呆滞区以及窑壁携带区,自由滚落区颗粒流速最大,而渗流呆滞区流速最小。窑内颗粒沿轴向输运过程发生径向偏析,形成夹层结构,小颗粒受渗流作用在渗流呆滞区中心形成内核,大粒径和中等粒径颗粒集中在自由滚落区和窑壁携带区。窑内颗粒力链分布不均匀,强力链分布于近窑壁区,弱力链分布于自由滚落区和渗流呆滞区,且渗流呆滞区力链细而密集。当窑头附近不同粒径颗粒存在轴向速度差时,颗粒在轴向发生掺混,并产生径向偏析。     

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响. 探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础. 选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点. 以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method, DEM),通过对比相应实验结果,标定了该模型的参数. 对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律. 研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强. 研究结果可指导SLS中铺粉工艺的优化.      

尼龙粉末在SLS预热温度下的离散元模型参数确定及其流动特性分析

尼龙粉末是增材制造中常用的粉体材料,温度对其流动性有重要影响.探索尼龙粉末增材制造预热温度下的流动性是研究选择性激光烧结(selective laser sintering, SLS)工艺中粉体铺展成形的基础.选取SLS技术中的尼龙粉末为原材料,采用离散元数值方法,研究尼龙粉末的流动行为,是增材制造工艺数值模拟和铺粉工艺优化的研究热点.以Hertz-Mindlin模型为基础,基于Hamaker理论模型和库伦定律,在尼龙粉末的接触动力学模型中引入范德华力和静电力,建立预热温度下尼龙粉末流动的离散元模型(discrete element method,DEM),通过对比相应实验结果,标定了该模型的参数.对加热旋转圆筒中尼龙粉末流动过程进行了DEM数值模拟,校核了所建模型的正确性,并研究了粉体粒径分布对尼龙粉末流动特性的影响规律.研究表明,尼龙粉末黏附力是静电力与范德华力的共同作用结果;随着粉体粒径的增大,尼龙粉末崩塌角增大,流动性增强;相对于高斯粒径分布,粒径均匀分布的尼龙粉末颗粒流动性更强.研究结果可指导SLS中铺粉工艺的优化.     

颗粒的堵塞行为对单向压制过程的影响

为了研究粉末冶金压制成形过程中粉体的堵塞状态并探索堵塞对粉体致密态的影响,利用离散元方法模拟了由多分散颗粒组成的体系在单向压制过程中的堵塞现象;分析了系统压力参数随密度参数的变化规律以及颗粒间偶关联函数的第一个峰值随体积分数的变化形式,并统计得出了临近堵塞状态下颗粒间量纲为一的接触力和颗粒运动方向的概率分布规律。结果表明:系统压力参数随密度参数的变化基本符合黄培云双对数压制方程;在临近堵塞状态下,偶关联函数的第一个峰值随着体积分数的增大迅速增大到一个稳定值,轴向运动颗粒减少而横向运动颗粒增加,且颗粒之间量纲为一的接触力达到一个相对稳定的分布状态;单向压制过程颗粒主要承受法向载荷,而切向载荷相对较小;颗粒的运动状况决定了系统的堵塞状态及其相对稳定的接触力分布,影响了系统致密化过程的密度分布。     

考虑粒子分布特征的复合材料细观力学方法

研究了颗粒增强复合材料中颗粒增强体粒径分布对复合材料力学性能的影响,利用分形思想将增强粒子的概率分布特征考虑进来,对已有的复合材料细观力学等效夹杂方法进行修正,建立了一个考虑粒子统计分布的细观等效力方法.以混凝土为例,分析了颗粒增强体体积含量、夹杂与基体的模量比和分形结构的分辨率对复合材料力学性能的影响.结果表明,这种新方法能够适用于分析颗粒增强复合材料的细观结构对力学性能的影响.     

基于PFC~(3D)的砂土直剪模拟及宏细观分析

为能更好地分析砂土在剪切过程中的体积变化及力学行为,通过PFC~(3D)颗粒流程序,按照室内实际级配建立数值试样,进行了不同垂直压力下的砂土直剪试验模拟,并将模拟结果与室内试验结果进行了对比;研究了砂土在剪切过程中的体积变化,并用两种方法分析了砂土剪切带的演变过程;从细观角度对试样颗粒的速度场及力链网络的发展变化进行了分析研究。结果表明,数值模拟结果与室内试验结果基本一致,砂土的体积变化表现为先剪缩后剪胀,剪胀量与垂直压力成反比;剪切带厚度约为11～12倍d_(50),在剪切带内颗粒的位移和欧拉角变化较大;试样内部强力链的演变较为明显,力链网络对外荷载变化的敏感性很高;剪切过程中上盒颗粒速度场方向的变化能够较好地解释砂土产生的剪胀现象。     

基于DIC的颗粒间接触力计算及力链分析

首次利用数字图像相关方法(Digital Image Correlation method,DIC),依据牛顿力学理论和颗粒线性动量平衡,分析了集中力作用下的二维钢珠颗粒体系,定量地获得了颗粒间接触力。采用CCD相机进行观测并采集了集中力作用下钢珠颗粒体系的变形图像;利用数字图像相关方法对采集的序列图像进行处理,获得了全场位移及应变,并根据胡克定律求得全场应力。对每个颗粒面积区域内各点应力值求和并取平均,获得各个颗粒的平均应力。对采集的数字图像进行图像分析,得到各个颗粒及接触点的排列结构。以颗粒平均应力和颗粒的排列结构作为初始条件,根据牛顿力学理论和颗粒线性动量平衡,最终获取了颗粒间接触力的大小和方向,实现了真实颗粒体系接触力的定量计算。另外从受力方面对集中力作用下颗粒体系内部力链的发展与演变进行了分析。研究结果表明:力链起源于加载位置,随着外荷载的部分耗散,颗粒间接触力逐渐发生变化,力链向四周扩展而形成力链网络。通过对不同压入深度颗粒间接触力变化和分布的分析,定量地描述了加载过程中颗粒间的滚动或滑动引起的力链断裂和重构的现象。     

Automatic extraction method of force chain information and its application in the flow photoelastic experiment of granular matter

The force chain is the core of the multi-scale analysis of granular matter. Accurately extracting the force chain information among particles is of great significance to the study of particle mechanics and geological hazards caused by particle flow. However, in the photoelastic experiment, the precise identification of the branching points of force chains has not been effectively realized. Therefore, this study proposes an automatic extraction method of force chain key information. First, based on the Hough transform and the Euclidean distance, a particle geometric information identification model is established and geometric information such as particle circle center coordinates, radius, contact point location, and contact angle is extracted. Then, a particle contact force information identification model is established following the color gradient mean square method. The model realizes the rapid calibration and extraction of a large number of particle media contact force information. Next, combined with the force chain composition criterion and its quasilinear feature, an automatic extraction method of force chain information is established, which solves the problem of the accurate identification of the force chain branch points. Finally, in the photoelastic experiment of ore drawing from a single drawpoint, the automatic extraction method of force chain information is verified. The results show that the macroscopic distribution of force chains during ore drawing from a single drawpoint is left–right symmetrical. Strong force chains are mostly located on the two sides of the model but in small numbers and they mainly develop vertically. Additionally, the ends are mostly in a combination of Y and inverted Y shapes, while the middle is mostly quasilinear. Weak force chains are abundant and mostly distributed in the middle of the model, and develop in different directions. The proposed extraction method accurately extracts the force chain network from the photoelastic experiment images and dynamically characterizes the force chains of granular matter, which has significant advantages in particle geometry information extraction, force chain branch point discrimination, force chain retrieval, and force chain distribution and its azimuthal characterization. The results provide a scientific basis for studying the macroscopic and microscopic mechanical parameters of granular matter.     

密度梯度柱壳链的弹性波传播特性研究

均匀圆柱壳链可以调控弹性波传播, 引入密度梯度有望进一步提高波形调控能力. 通过建立密度梯度柱壳链的细观有限元模型和连续介质模型, 研究了密度梯度柱壳链的弹性波传播特性. 通过将密度梯度柱壳链等效为变密度连续介质弹性杆, 建立了其在应力脉冲作用下的控制方程. 运用拉普拉斯积分变换方法, 考虑杆中密度遵循线性分布, 获得了方程的解析解. 以三角形应力脉冲作用为例, 通过与细观有限元模拟结果比较, 发现解析解可以较好地预测梯度柱壳链中载荷的演化趋势. 正梯度链中载荷峰值随着波传播逐渐增大, 负梯度链中载荷峰值随着波传播逐渐减小. 正梯度链支撑端峰值载荷高于均匀链, 负梯度链支撑端峰值载荷低于均匀链, 这表明相较于均匀柱壳链, 密度梯度柱壳链可以在更大范围内对波形进行调控. 线性密度梯度参数对梯度柱壳链的波形调控能力影响较大, 梯度参数越小, 传递到支撑端的峰值载荷越小; 相反, 梯度参数越大, 支撑端的峰值载荷越大. 建立的理论模型及其解析解为研究梯度柱壳链中应力波传播规律及揭示载荷调控机理提供了理论基础.      

Estimation of structural element sizes in sand and compacted blocks of ground calcium carbonate using a void network model

An algorithm has been developed for the calculation of the size of the effective structural or skeletal elements which make up the solid phase of an unconsolidated or consolidated porous block. It is based on a previously presented algorithm, but it has now been validated on unconsolidated samples and tested on consolidated samples. It also includes a virtual reality representation of the structures. First, a network model named Pore-Cor is made to reproduce the percolation behaviour of the experimental sample, by matching its simulated percolation characteristics to an experimental mercury intrusion curve. The algorithm then grows skeletal elements between the cubic pores and cylindrical throats of the void network model until they touch up to four of the adjacent void features. The size distributions of the simulated solid elements are compared with each other and with experimentally determined particle size distributions, using a Mann–Whitney test. The algorithm was shown to simulate skeletal elements with the correct trends in size distribution for two different sand samples, provided the sand packed itself optimally under the applied mercury pressure. It was also applied to two samples of variously compressed calcium carbonate powder, having fine and coarse particle size distributions respectively. The simulation demonstrated that on compressing the powder at the minimum force, the skeletal elements differed from the constituent particle sizes, as expected. The average size of the skeletal elements increased as the compression force was increased on the calcium carbonate powders. The results suggest that the method could be useful as a tool for probing the effect of compaction on aggregation or sintering, and for studying other effects such as cementation in geological samples, where other more direct techniques cannot be applied.     

WC/Al2O3颗粒增强Cu基复合材料爆炸粉末烧结实验研究

利用爆炸粉末烧结工艺,探索WC /Al2O3同时作为增强基颗粒制取多种颗粒增强Cu基复合材料的可行性,同时分析了工艺参数对压实坯致密度的影响。研究了复合材料的微观组织和致密度、韧性和硬度等性能,爆炸粉末烧结法可以成功制出WC/ Al2O3/Cu多种颗粒增强金属基复合材料。     

Static and dynamic compaction of ceramic powders

The static and dynamic compaction of ceramic powders was investigated experimentally using a high-pressure friction-compensated press to achieve static stresses of 1.6 GPa and with a novel gas gun setup to stresses of 5.9 GPa for a tungsten carbide powder. Experiments were performed in the partial compaction region to nearly full compaction. The effects of variables including initial density, particle size distribution, particle morphology, and loading path were investigated in the static experiments. Only particle morphology was found to significantly affect the compaction response. Post-test examination of the powder reveals fracture of the grains as well as breaking at particle edges. In dynamic experiments, steady structured compaction waves traveling at very low velocities were observed. The strain rate within the compaction waves was found to scale nearly linearly with the shock stress, in contrast with many fully dense materials where strain rate scales with stress to the fourth power. Similar scaling is found for data from the literature on TiO₂ powder. The dynamic response of WC powder is found to be significantly stiffer than the static response, probably because deformation in the dynamic case is confined to the relatively narrow compaction wave front. Comparison of new static powder compaction results with shock data from the literature for SiO₂ also reveals a stiffer dynamic response.     

Study of particle rearrangement during powder compaction by the Discrete Element Method

This paper presents simulations of cold isostatic and closed die compaction of powders based on the Discrete Element Method. Due to the particulate nature of powders, densification of the compact proceeds both through the plastic deformation at the particle contact and the mutual rearrangement of particles. The relative weight of each mechanism on the macroscopic deformation process depends on the contact law, the relative density, and the type of stress exerted on the particles (shear or pressure). 3D computer simulations have been carried out to investigate the role of these parameters on the deformation mechanisms of powder compacts. The effect of rearrangement is studied by comparing simulations that use a homogeneous strain field solution for which local rearrangement is omitted and simulations that include local rearrangement. It is shown that local rearrangement has some effect on average quantities such as the average coordination number, the average contact area and the macroscopic stress. The effect on averaged quantities is much stronger for closed die compaction than for isostatic compaction. However the main effect of local rearrangement is to widen the distribution of the parameters that define the contact (contact area in particular). The results of these simulations are compared to available experimental data and to statistical models that use a homogeneous strain field assumption.     

The flow behavior of powder stearin-based methyl ester sulfonates and the effect of glidant

The preparation and behavior of powder stearin-based methyl ester sulfonates (MES) with the addition of builder as a glidant agent was studied. MES is an anionic surfactant and its performance is equivalent to petroleum-based linear alkylbenzene sulfonates (LAS), the workhorses of the detergent industry. At the same time, zeolites or soda ash were the alternative builders for non-phosphate-based detergent. The behavior of powder MES properties was measured in terms of morphology, particle size, cohesion, caking, and compaction. Before that, these palm stearin-based MES of carbon chain 16–18 with ratios of 98:2, 80:20, and 60:40 in flakes were ground using a high shear mixer by 23,000 rpm to produce powder particles. Decreasing the particle size of MES powder can reduce the powder's cohesion, caking, and compaction. The addition of zeolite or soda ash as a glidant in powdered MES can improve the caking behavior. The high stearic in MES greatly influenced getting fine particle powders and required more glidant agents. This powder MES with excellent powder characteristics can perform well as a bio-based cleaning product.     

粉末热压扩散与应力场耦合的力学模型

以弹性接触应力场为初始条件,建立了热压条件下球形颗粒表面扩散与应力场耦合的力学模型. 引入包含表面能项级数形式的应力函数,以描述随时间演化的表面扩散过程及扩散对应力场演化的影响. 而应力场通过改变化学势梯度,又会促进(或阻止)表面扩散结合的进程.利用该模型分析了压力、温度和界面区应力场演化对致密化参数的影响. 比较了满足粘着或非粘着对结合宽度和应力分布的影响,将考虑粘着的弹性接触应力场作为初始条件,分析了弹性变形和表面扩散共同驱动的粉末冶金热压烧结致密化规律.