刚性弹体对素混凝土厚靶侵彻响应的LDPM数值模拟研究EI北大核心CSCD

基于一种新的细观离散元模型Lattice Discrete Particle Model(LDPM),该研究建立了刚性弹侵彻素混凝土厚靶的数值仿真模型。对LDPM基本假设和细观模型构建简单介绍,结合三轴压缩响应曲线,对23 MPa强度素混凝土进行LDPM参数标定。通过对比弹体减速度和侵彻深度试验值,验证数值模型对于混凝土厚靶侵彻问题的适用性。LDPM模拟弹体恒定速度侵彻混凝土厚靶,获得侵彻行程中侵彻阻力变化曲线,结合Forrestal阻力公式得到靶体静态阻应力。仿真结果表明,尖卵形弹头不同CRH值以及侵彻速度对靶体静态阻应力基本没有影响;弹径为最大骨料直径3倍、6倍和8倍的弹体受到靶体静态阻应力分别为260 MPa、175 MPa和163 MPa。该结果对混凝土侵彻缩比实验研究具有重要的实际工程意义。     

刚性弹体对素混凝土厚靶侵彻响应的LDPM数值模拟研究

基于一种新的细观离散元模型Lattice Discrete Particle Model(LDPM),该研究建立了刚性弹侵彻素混凝土厚靶的数值仿真模型。对LDPM基本假设和细观模型构建简单介绍,结合三轴压缩响应曲线,对23 MPa强度素混凝土进行LDPM参数标定。通过对比弹体减速度和侵彻深度试验值,验证数值模型对于混凝土厚靶侵彻问题的适用性。LDPM模拟弹体恒定速度侵彻混凝土厚靶,获得侵彻行程中侵彻阻力变化曲线,结合Forrestal阻力公式得到靶体静态阻应力。仿真结果表明,尖卵形弹头不同CRH值以及侵彻速度对靶体静态阻应力基本没有影响;弹径为最大骨料直径3倍、6倍和8倍的弹体受到靶体静态阻应力分别为260 MPa、175 MPa和163 MPa。该结果对混凝土侵彻缩比实验研究具有重要的实际工程意义。     

加载速率及其突变对混凝土压缩破坏影响的数值研究

混凝土动态力学行为具有明显的率相关性,探讨了加载速率及其突变对混凝土压缩破坏模式及宏观力学性能的影响。考虑混凝土细观结构非均质性的影响,从细观角度出发将混凝土看作由骨料、砂浆基质及过渡区界面组成的三相复合材料。采用耦合应变率效应的塑性损伤本构关系模型来描述砂浆基质及界面的动态力学行为;认为骨料不产生断裂破坏,为弹性体。采用Monte Carlo法建立了混凝土二维随机骨料模型,首先对Dilger等混凝土动态压缩试验进行数值模拟,数值与试验结果的良好吻合证明了方法的可行性及细观参数选取的准确性。进而探讨了细观组分应变率效应的影响,对比了混凝土非均质模型与宏观均匀模型的应变率效应,最后分析了软化阶段加载速率突变对混凝土破坏模式及宏观应力-应变关系的影响,并得到了一些有益结论。     

弹体侵彻混凝土靶板的数值模型

基于细观力学原理,采用离散元软件PFC3D对Hanchak侵彻的部分试验进行了数值建模,使用平行粘结模型来模拟混凝土颗粒之间的接触力和力矩,并通过模拟弹体以不同速度侵彻混凝土靶板,将弹体在不同速度下剩余速度与试验值进行对比分析,数值模拟得到的结果相对于试验值的偏差都在允许范围内,验证了离散元侵彻模型和程序编写及算法的有效性。     

基于对混凝土破损过程的模拟研究

从细观力学层次出发,把混凝土看作是由骨料、砂浆基质及两者之间的界面过渡区组成的三相复合材料,通过引入细观损伤变量及损伤变量的演化规律模拟了混凝土的破坏过程。首先生成骨料位置随机分布且颗粒级配符合Fuller曲线的数值试件,然后对每一相材料分别赋以服从Weibull分布的随机力学参数(弹模、强度和泊松比等)并规定相应的破坏准则,将通过上述途径获得的混凝土数值试件导入有限元软件Abaqus进行数值模拟。与实验结果的对比显示,提出的随机损伤数值模型能够较好地模拟混凝土板式试件在单拉、单压下的应力-应变关系曲线以及混凝土中裂纹的萌生、开裂过程。     

非均质混凝土材料破坏的三维细观数值模拟

在随机骨料模型的基础上,采用特征单元尺度对网格进行剖分,建立了非均质混凝土材料损伤破坏及宏观力学特性研究的三维细观单元等效化模型。对单轴拉伸、单轴压缩条件下湿筛混凝土试件的破坏过程及宏观力学性能进行了模拟分析;研究了混凝土梁的三分点弯拉力学特性,并与平面模型结果作了对比。研究表明:1) 与平面计算模型相比,三维模型更真实地模拟混凝土材料在外荷载作用下的损伤破坏过程,更准确的描述非均质混凝土材料的宏观力学性能,且与实验结果吻合;2) 骨料空间分布形式基本不影响混凝土材料的宏观弹性模量及强度,但影响其破坏过程和破损路径;3) 与随机骨料模型等细观力学方法相比,该方法具有高效性。     

小麦PFC模型细观参数标定方法的验证与分析

为了验证PFC模型细观参数标定方法的可行性,以小麦试样为例,根据PFC软件提供的模型细观参数标定方法的要求,在实验室进行了三轴压缩试验,并利用PFC软件对实验室三轴压缩试验进行了数值模拟。结果表明:通过反复调整PFC模型细观参数能够使数值模拟结果与试验结果基本一致,但很难使二者完全吻合。同等条件下,数值模型的颗粒摩擦系数对偏应力峰值的影响显著,颗粒法向刚度控制偏应力-应变关系曲线的初始斜率,颗粒孔隙率影响偏应力-应变关系曲线达到峰值后的平缓趋势。     

考虑孔隙及微裂纹影响的混凝土宏观力学特性研究

混凝土是一种典型的多孔介质材料,孔隙分布错综复杂,孔径尺寸跨越微观尺度和宏观尺度,对混凝土弹性模量及强度等力学参数产生巨大影响.认为混凝土是由骨料、孔隙及砂浆基质组成的三相复合材料,采用Monte Carlo 法将孔隙、微裂纹及微缺陷与骨料颗粒随机投放在砂浆基质中.根据三相球模型及中空圆柱形杆件模型得到含孔材料的有效力学性质,并推导得到含孔材料的等效本构模型.建立含孔隙混凝土试件的细观单元等效化力学模型,对二级配含孔隙混凝土试件在单轴拉伸及压缩条件下的反应进行了非线性分析.结果表明:孔隙、微裂纹的存在对混凝土宏观弹性模量、强度及残余强度等力学性质都有很大影响,在对混凝土宏观力学特性分析及研究混凝土损伤断裂时不应忽略其影响.     

基于混凝土细观骨料模型的SHPB仿真模拟研究

将混凝土视为由粗骨料和砂浆基体组成的双相复合材料,建立了圆柱状三维混凝土细观骨料模型,并将其应用于分离式Hopkinson压杆仿真实验中。通过五种不同梯形载荷的加载计算,得到了试件不同应变率下的应力—应变曲线,并将模拟结果与实验结果进行对比分析。结果表明,使用细观骨料模型可以有效模拟混凝土SHPB实验并反映材料的动态力学性能,而且其宏观破坏形式也与传统的均匀材料模型有所不同,所建立的模型同时为多种非均匀材料的细观数值模拟提供了方便。最后通过几种不同骨料含量和骨料尺寸混凝土模型的计算研究了这两种因素对混凝土动态强度和应力均匀性的影响。     

基于细观单元等效化方法的混凝土动态破坏行为分析

混凝土材料具有明显的应变率效应,对其力学性质增强机理的认识还不统一。在细观随机骨料模型基础上,采用特征单元尺度划分试件网格,推导了考虑材料拉/压强度应变率效应的细观单元等效本构关系,建立了非均质混凝土材料的细观单元等效化数值模型。基于二维模型对Dilger等混凝土动态压缩试验进行了数值模拟,获得的数值结果与试验数据及随机骨料模型结果吻合良好,证明了细观单元等效化方法的准确性;进而对三维混凝土试件动态单轴拉伸和压缩破坏模式及宏观力学性质的加载速率效应进行了研究。数值结果表明:随着加载速率的增加,混凝土裂纹(损伤)数量增大,混凝土破坏将耗散更多的能量,是混凝土动态强度提高的主要原因。     

基于有限元与光滑粒子耦合的弹丸挤进过程分析

基于有限元（FEM）与光滑粒子（SPH）耦合算法,建立弹丸身管耦合系统动力学模型。通过对某大口径火炮弹丸挤进过程进行数值模拟,弥补弹丸挤进过程有限元分析方法无法有效模拟弹带大变形缺陷,成功模拟出弹丸挤进阶段弹带塑性流动过程,在此基础上深入研究弹带的应力应变变化规律。分析弹丸初始装填角、弹炮间隙、弹丸装填不到位等因素对挤进过程中弹丸动力响应影响。结果表明,仿真与实弹射击的弹带变形一致性较好。     

An Improved Concrete Damage Model for Impact Analysis of Concrete Structural Components by using Finite Element Method

This paper presents the development of an improved concrete damage model for projectile impact on concrete structural components. The improvement is in terms of reduction of input material parameters for nonlinear transient dynamic impact analysis by employing concrete damage model. The experimental data such as pressure vs volumetric strain, triaxial compression failure and pressure vs stress difference have been used for evaluation of the important parameters of concrete damage model. Various contact algorithms have been outlined briefly to model the interface between the projectile and target. The nonlinear explicit transient dynamic analysis has been carried out by using finite element method to compute the responses. It is observed that the computed penetration depth obtained in the present study is in good agreement with those values of corresponding experimental studies and LS-DYNA.     

Modelling of concrete behaviour in uniaxial compression and tension with DEM

The paper focuses on the DEM modelling of the behaviour of plain concrete during uniaxial compression and uniaxial tension using the discrete element method. The model takes into account the concrete heterogeneity at the meso-scale level. The effects of concrete density, size of aggregate grains and specimen size on the stress–strain curve, volume changes and fracture process are studied. In addition, the evolution of contact forces, grain rotations, displacement fluctuations and strain localization during deformation is investigated. The elastic, kinetic, plastic and numerical dissipated energy is calculated and analysed at a different stress–strain stage. Concrete is described as a 1-phase or 3-phase material. The macroscopic 2D and 3D results are compared with the corresponding experiments. A satisfactory agreement between experiments and calculations is achieved.

     

Analysis of reinforced concrete plates subjected to impact employing the truss‐like discrete element method

The prediction of the response of reinforced concrete structures subjected to projectiles impact still presents open questions. These include the rate dependence of material properties, the interaction between concrete and steel reinforcement and the simulation of fracture and fragmentation. Because the appearance of discontinuities in the target structure is difficult to account using a continuum approach, the application of discrete models was developed as an appealing alternative. A version of the discrete model in which nodal masses are linked by an array of uniaxial elements, herein called discrete element method, is used in this study. This method was implemented in the system Abaqus to take advantage of its numerical and post‐processing capabilities. A reinforced concrete rectangular plate subjected to impact of a projectile is examined in detail. Comparisons between experimental and numerical results are shown with the aim of validating the proposed method.     

Micromechanical analysis of cohesive granular materials using the discrete element method with an adhesive elasto-plastic contact model

An adhesive elasto-plastic contact model for the discrete element method with three dimensional non-spherical particles is proposed and investigated to achieve quantitative prediction of cohesive powder flowability. Simulations have been performed for uniaxial consolidation followed by unconfined compression to failure using this model. The model has been shown to be capable of predicting the experimental flow function (unconfined compressive strength vs. the prior consolidation stress) for a limestone powder which has been selected as a reference solid in the Europe wide PARDEM research network. Contact plasticity in the model is shown to affect the flowability significantly and is thus essential for producing satisfactory computations of the behaviour of a cohesive granular material. The model predicts a linear relationship between a normalized unconfined compressive strength and the product of coordination number and solid fraction. This linear relationship is in line with the Rumpf model for the tensile strength of particulate agglomerate. Even when the contact adhesion is forced to remain constant, the increasing unconfined strength arising from stress consolidation is still predicted, which has its origin in the contact plasticity leading to microstructural evolution of the coordination number. The filled porosity is predicted to increase as the contact adhesion increases. Under confined compression, the porosity reduces more gradually for the load-dependent adhesion compared to constant adhesion. It was found that the contribution of adhesive force to the limiting friction has a significant effect on the bulk unconfined strength. The results provide new insights and propose a micromechanical based measure for characterising the strength and flowability of cohesive granular materials.     

Micromechanical modelling of oval particulates subjected to bi-axial compression

One of the questions that still remain unanswered among researchers dealing with granular materials is how far the particle shape affects the micro-macroscopic features of granular assemblies under mechanical loading. The latest advances made with particle instrumentation allow us to capture realistic particle shapes and size distribution of powders to a fair degree of accuracy at different length scales. Industrial applications often require information on the micromechanical behaviour of granular assemblies having different particle shapes and varying surface characteristics, which still remains largely unanswered. Traditionally, simulations based on discrete element method (DEM) idealise the shape of individual particles as either circular or spherical. In the present investigation, we analyse the influence of particle shape on the shear deformation characteristics of two dimensional granular assemblies using DEM. We prepared the assemblies having nearly an identical initial packing fraction (dense), but with different basic shapes of the individual particles: (a) oval and (b) circular for comparison purposes. The granular assemblies were subjected to bi-axial compression test. We present the evolution of macroscopic strength parameters and microscopic structural/topological parameters during mechanical loading. We show that the micromechanical properties of granular systems are significantly influenced by the shape of the individual particles constituting the granular assemblies.     

Numerical analysis of compression mechanics of highly polydisperse granular mixtures with different PSD-s

The deformation of three-dimensional highly polydisperse packings of frictional spheres with continuous unimodal and discrete uniform particle size distributions (PSDs) under uniaxial compression was investigated, using the discrete element method. The stress transmission and elasticity parameters, i.e., the effective elastic modulus and the Poisson’s ratio of the granular assemblies were examined; these parameters are important in many engineering disciplines. The influence of the shape of the PSD on the porosity and the transmission of pressures in samples was observed; however, the shape of the PSD did not affect the stiffness and the Poisson’s ratio of polydisperse granular packings. The results revealed that, in some cases, knowledge of the grain size distribution is not a critical issue for modeling granular packings composed of non-uniformly sized spheres.     

Computational applications of a coupled plasticity-damage constitutive model for simulating plain concrete fracture

A coupled plasticity-damage model for plain concrete is presented in this paper. Based on continuum damage mechanics (CDM), an isotropic and anisotropic damage model coupled with a plasticity model is proposed in order to effectively predict and simulate plain concrete fracture. Two different damage evolution laws for both tension and compression are formulated for a more accurate prediction of the plain concrete behavior. In order to derive the constitutive equations and for the easiness in the numerical implementation, in the CDM framework the strain equivalence hypothesis is adopted such that the strain in the effective (undamaged) configuration is equivalent to the strain in the nominal (damaged) configuration. The proposed constitutive model has been shown to satisfy the thermodynamics requirements. Detailed numerical algorithms are developed for the finite element implementation of the proposed coupled plasticity-damage model. The numerical algorithm is coded using the user subroutine UMAT and then implemented in the commercial finite element analysis program Abaqus. Special emphasis is placed on identifying the plasticity and damage model material parameters from loading-unloading uniaxial test results. The overall performance of the proposed model is verified by comparing the model predictions to various experimental data, such as monotonic uniaxial tension and compression tests, monotonic biaxial compression test, loading-unloading uniaxial tensile and compressive tests, and mixed-mode fracture tests.     

Introduction of imperfections in the cubic mesh of the truss‐like discrete element method

In the present version of the truss‐like discrete element method (DEM), masses are considered lumped at nodal points and interconnected by means of unidimensional elements with arbitrary constitutive relations. In previous studies of non‐homogeneous concrete cubic samples subjected to nominally uniaxial tension, it was verified that numerical predictions of fracture using DEM models are feasible and yield results that are consistent with the experimental evidence so far available, including the prediction of size and strain rate effects. In the DEM formulation, material failure under compression is assumed to occur by indirect tension. In previous simulations, it was verified that the response is satisfactorily modelled up to the peak load, when a sudden collapse usually occurs, characteristic of fragile behaviour. On the other hand, experimental stress versus displacement curves observed in small specimens subjected to compression typically present a softening branch, in part due to sliding with friction of the fractured parts of the specimens. A second deficiency of DEM models with a perfectly cubic mesh is that the best correlations with experimental results are obtained with material parameters that differ in tension and compression. This paper examines another cause of the excessively fragile behaviour of DEM predictions of the response of concrete elements subjected to nominally uniaxial compression, which is due to the regularity of the perfect cubic mesh, unable to capture nonlinear stability effects in the material. It is shown herein that the introduction of small perturbations of the DEM regular mesh significantly improves the predicting capability of the model and in addition allows adopting a unique set of material properties, which are independent of the nature of the loading.