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.     

Simulation of a soil loosening process by means of the modified distinct element method

We apply the Distinct Element Method (DEM) to analyze the dynamic behavior of soil. However, the conventional DEM model for calculation of contact forces between elements has some problems; for example, the movement of elements is too discrete to simulate real soil particle movement. Therefore, we modify the model to solve the difficulties. To investigate the validity of the modified model, we conduct an experiment in which soil is cut with a pendulum-typeblade, and simulate the soil loosening process with the modified DEM model. This paper presents details of the experimental apparatus and the comparison of soil behavior and energy absorption between the simulation and the experiment. Some characteristic phenomena of the experiment are reproduced in the simulation giving us confidence that the modified model is better than the conventional model for the simulation of soil behavior.     

Numerical simulation and testing verification of the interaction between track and sandy ground based on discrete element method

Tractive effort of tracked vehicles plays an important role in military and agricultural fields. In order to solve the problem of low precision in numerical simulation of the interaction between track and sandy ground, a systematic and accurate discrete element modeling method for sandy road was proposed. The sandy ground was modeled according to the mechanical parameters measured by soil mechanics tests. The interaction coefficients of sandy soil were measured by the repose angle test and triaxial compression test combined with the corresponding simulation. On this basis, a discrete element interaction model of track-sandy ground was established, which can be used to test the tractive effort of track. Numerical simulation calculation of track model at different speeds was carried out, and the simulation results were compared with the results of indoor soil bin test for verification. The verification results show that the interaction between track and sandy ground based on DEM simulation is consistent with the actual soil bin test. The discrete element modeling method in this paper can be used to model the track and sandy ground accurately, and the simulation model can be used to test the tractive effort of tracked vehicle.     

基于离散元法的刚性履带低含水软地面附着特性数值分析

针对低含水软地面低黏性、高摩擦的力学特性,以刚性履带在干燥壤土行驶这一典型工况为试验条件,基于离散单元法选取合理的颗粒细观参数,在三轴剪切试验的基础上建立并验证了土壤数值仿真模型。通过数值牵引试验,对刚性履带低含水软地面的附着特性进行了离散元细观分析。分析表明,土壤附着力随履刺的向后排列而递减;垂直于履带行驶方向的附着作用不可忽略;离散元法能更好地还原低含水软地面的力学特性,揭示了土壤颗粒在动态荷载下非连续的本质,适合于干燥、松散的低含水软地面数值研究。     

A 2D DEM–LBM study on soil behaviour due to locally injected fluid

Leakage from underground pipes could result in foundations being undermined and cause damage to adjacent infrastructure. Soil particles surrounding the leaking area could be mobilised, displaced, and even washed out of the soil matrix by the leaking fluid, generating a void or cavity. A two-dimensional simulation using a coupled discrete element method and lattice Boltzmann method (DEM–LBM) has been used to investigate the behaviour of a soil bed subject to a locally injected fluid, which represents a leak in a pipe. Various values of inter-particle surface energy were also adopted to model the mechanical effects of cohesive particles. The results suggest that the inter-particle surface energy greatly influences the bed response with respect to the leaking fluid, including the excess pressure initiating the cavity, the cavity shape and its evolution rate.     

基于离散元的杆件断裂全过程数值模拟

杆件的断裂会涉及到大变形、非线性以及不连续等问题,通常的数值计算方法模拟这种复杂力学行为具有局限性。本文基于颗粒离散元法DEM,将接触粘结处的分布式弹簧用梁纤维进行等效,提出了一种适于结构弹塑性分析的DEM纤维梁模型,然后在此基础上构建了构件断裂模拟算法以及纤维破环准则。将该模型应用于悬臂梁结构,模拟了悬臂梁从弹性到弹塑性阶段,再到断裂破坏的全过程,数值模拟得到的结构响应和截面开裂破坏形态均较合理。最后将该方法应用于单层网壳倒塌破坏模拟,并与网壳振动台倒塌试验进行对比,结果表明,数值模拟得到的杆件断裂过程及结构倒塌模式与试验现象一致,验证了该模型的正确性和适用性。     

2D FE–DEM analysis of tractive performance of an elastic wheel for planetary rovers

We have been developing a simulation program for use with soil–wheel interaction problems by coupling Finite Element Method (FEM) and Discrete Element Method (DEM) for which a wheel is modeled by FEM and soil is expressed by DEM. Previous two-dimensional FE–DEM was updated to analyze the tractive performance of a flexible elastic wheel by introducing a new algorithm learned from the PID-controller model. In an elastic wheel model, four structural parts were defined using FEM: the wheel rim, intermediate part, surface layer, and wheel lugs. The wheel rigidity was controlled by varying the Young’s Modulus of the intermediate part. The tractive performance of two elastic wheels with lugs for planetary rovers of the European Space Agency was analyzed. Numerical results were compared with experimentally obtained results collected at DLR Bremen, Germany. The FE–DEM result was confirmed to depict similar behaviors of tractive performance such as gross tractive effort, net traction, running resistance, and wheel sinkage, as in the results of experiments. Moreover, the tractive performance of elastic wheels on Mars was predicted using FE–DEM. Results clarified that no significant difference of net traction exists between the two wheels.     

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.     

Experimental validation of distinct element simulation for dynamic wheel–soil interaction

The deformation behaviour of the soil during dynamic wheel–soil interaction was studied by using the discontinuum modelling technique, distinct or discrete element method (DEM). The simulation model was developed using DEM for two types of soil, soil-A (coarse sand) and soil-B (medium sand). A transparent sided soil bin was used to observe the soil deformation. Three CCD video camera photographic images of the validation experiments were analyzed and compared with the simulation program results.This paper presents the simulation and validation results for two types of soil at three different vertical loadings of 4.9, 9.8 and 14.7 N. Wheel sinkage, vertical and horizontal draft force acting on the rigid wheel and the soil deformation images from the validation experiments were some of the data used to compare the simulation program results with the validation experiments. The simulation program was helpful to understand the complex deformation behaviour of the soils. The simulated results for the deformation behaviour of soil-B showed better correlation with the validation experiments than soil-A. The results obtained have also been compared with the previous work on DEM to explain phenomena such as the high simulated sinkage of the rigid wheel.     

Equivalent boundary model of lunar soil drilling simulation by DEM

Aiming to solve the computational cost problem in the discrete element simulation for lunar soil drilling sampling, an equivalent boundary method was proposed. A high-accuracy DEM model of lunar soil was established firstly. As the novel alterable constitutive law, the accuracy of the model was verified to meet the performance of real lunar soil very much both in shear strength indices and elastic–plastic behavior. A common drill bit in the geological exploration field for sampling soil was chosen as the simulation object. In preanalysis, it was known that with the increase of drilling depth, the stress concentration area was always near the drill bit, while the affected area of the lunar soil was a cylindrical area around the drill pipe, which extended towards the drilling direction instead of extending around it. Then a big boundary drilling simulation scene was established to investigate the flow direction of lunar soil particles. The motion law of particles and the velocity field information were obtained, and a U-shape chain was described around the drill bit. Finally an equivalent boundary was set near the U-shaped chain, and the size was determined by comparing the soil stress in the fierce collision zone and around the reference boundary. This method could be a reference for other lunar soil drilling researches with other drills of different sizes.     

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。     

基于土体中结构面的岩土工程问题探讨

The structural plane exists also in soil mass and is concerned with geotechnical engineering problems. This paper studies geotechnical engineering problem's in consideration of structural plane of soil mass, in order to attract attention to the structural plane of soil mass. These problems are that , the structural plane has control over the failure of soil mass and affects the mechanical properties of soil mass,soil slope stability and mechanical parameter ets, etc., as along the structural plane the water and pollutants can transport and percolate in the soil mass. It can be analysed by using DEM model for soil mass.     

Evaluation of link-track performances using DEM

A two-dimensional discrete-element model for the interaction between link-track and soil is presented. The model was developed using commercial PFC2D code. Two different particles, sphere and clump of two spheres, were used to represent the soil. The soil parameters of the model were determined using Hertzian contact theory. Based on the model and soil parameters, simulations of biaxial tests and calculations of the internal angle of friction and cohesion were preformed. The simulation results showed that the internal angle of friction should not exceed the value of 0.65 when using the spherical particles. Based on the clumped particles model, simulations of shear tests with two grouser plates (lengths 100 and 150 mm) were performed under different soil conditions, normal pressures, and cleat heights. A curve fitting of the simulation results was performed using three semi-empirical models from Bekker, Janosi, and Wong for representing the shear stress–displacement relationship. The best fitting was achieved using Wong’s approach. The simulation results of the cleat effects were compared with Bekker’s grouser approach and McKyes’s formulation for soil–blade interaction. In most of the cases, the results of Bekker’s model were the lower bound and McKyes’s model, the upper bound of the DEM simulation results. The properties of the soil model for the DEM were determined using simulation results of shear tests by grouser plate. In the range investigated, the size of the shearing grouser plate is not significant in determining the soil model properties.     

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.      

Investigation of elemental shape for 3D DEM modeling of interaction between soil and a narrow cutting tool

Discrete Element Method (DEM) has been applied in recent studies of soil cutting tool interactions in terramechanics. Actual soil behavior is well known to be inexpressible by simple elemental shapes in DEM, such as circles for 2D or spheres for 3D because of the excessive rotation of elements. To develop a more effective model for approximating real soil behavior by DEM, either the introduction of a rolling resistance moment for simple elemental shape or the combination of simple elements to form a complex model soil particle shape cannot be avoided. This study was conducted to investigate the effects of elemental shape on the cutting resistance of soil by a narrow blade using 3D DEM. Six elemental shapes were prepared by combining unit spheres of equal elemental radius. Moreover, cutting resistance was measured in a soil bin filled with air-dried sand to collect comparative data. The elemental shape, with an axial configuration of three equal spheres overlapped with each radius, showed similar results of soil cutting resistance to those obtained experimentally for the six elemental shapes investigated.     

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.     

转子系统非线性动力学DEM建模研究

将DEM方法引入转子系统非线性动力学的分析计算,对其建模过程和有关本构关系,作了比较详尽的介绍。该模型可用于碰摩、弯曲振动和扭转振动等转子系统的大部分动力学和运动学问题分析计算,不仅适用于单跨转子系统,也适用于多跨转子系统,尤其是在碰摩计算时,可考虑其摩擦特性随时间历程而变的特点,从而使分析计算更加接近实际情况。     

基于离散元模型的土石混合体直剪试验分析

土石混合体是由高强度块石和低强度土体组成的一类特殊工程地质材料,其力学性质可通过直剪试验进行确定。本文针对土石混合体的细观材料特性,分别采用球形颗粒单元和非规则组合颗粒单元模拟土体和块石材料,对其在不同含石量和颗粒粘结强度下的直剪试验过程进行离散元分析。计算结果表明,土石混合体的抗剪强度随着含石量和粘结强度的增加而增加;通过不同法向应力下直剪试验的离散元分析,确定了不同含石量下土石混合体材料的内摩擦角和粘聚力。本文研究结果有助于进一步揭示土石混合体的抗剪强度特性。     

Discrete simulation and micromechanical analysis of two-dimensional saturated granular media

In this study, a novel approach to incorporate the pore water pressure in the discrete element method （DEM） to comprehensively model saturated granular media was developed. A numerical model was constructed based on the DEM by implanting additional routines in the basic DEM code; pore water pressure calculations were used with a two-dimensional （2D） model to simulate the undrained behavior of satu- rated granular media. This model coupled the interaction of solid particles and the pore fluid in saturated granular media. Finally, several 2D undrained shear tests were simulated. The test results showed that the model could predict the response of the saturated granular soil to shear loading. The effect of initial compaction was investigated. Biaxial tests on dense and loose specimens were conducted, and the effect of the initial density on the change in shear strength and the volume change of the system was inves- tigated. The overall behavior of loose and dense specimens was phenomenologically similar to the real granular material. Constant volume tests were simulated, and the results were compared to those from the coupled model. Induced anisotropy was micromechanically investigated by studying the contact force orientation. The change in anisotropy depended on the modeling scheme. However, the overall responses of the media obtained usinz the couoled and constant volume methods were similar.     

Discrete simulation and micromechanical analysis of two-dimensional saturated granular media

In this study, a novel approach to incorporate the pore water pressure in the discrete element method (DEM) to comprehensively model saturated granular media was developed. A numerical model was constructed based on the DEM by implanting additional routines in the basic DEM code; pore water pressure calculations were used with a two-dimensional (2D) model to simulate the undrained behavior of saturated granular media. This model coupled the interaction of solid particles and the pore fluid in saturated granular media. Finally, several 2D undrained shear tests were simulated. The test results showed that the model could predict the response of the saturated granular soil to shear loading. The effect of initial compaction was investigated. Biaxial tests on dense and loose specimens were conducted, and the effect of the initial density on the change in shear strength and the volume change of the system was investigated. The overall behavior of loose and dense specimens was phenomenologically similar to the real granular material. Constant volume tests were simulated, and the results were compared to those from the coupled model. Induced anisotropy was micromechanically investigated by studying the contact force orientation. The change in anisotropy depended on the modeling scheme. However, the overall responses of the media obtained using the coupled and constant volume methods were similar.