Discrete element modeling of asphalt concrete cracking using a user-defined three-dimensional micromechanical approach

We established a user-defined micromechanical model using discrete element method (DEM) to investigate the cracking behavior of asphalt concrete (AC). Using the “Fish” language provided in the particle flow code in 3-Demensions (PFC3D), the air voids and mastics in asphalt concrete were realistically built as two distinct phases. With the irregular shape of individual aggregate particles modeled using a clump of spheres of different sizes, the three-dimensional (3D) discrete element model was able to account for aggregate gradation and fraction. Laboratory uniaxial complex modulus test and indirect tensile strength test were performed to obtain input material parameters for the numerical simulation. A set of the indirect tensile test were simulated to study the cracking behavior of AC at two levels of temperature, i e, −10 °C and 15 °C. The predicted results of the numerical simulation were compared with laboratory experimental measurements. Results show that the 3D DEM model is able to predict accurately the fracture pattern of different asphalt mixtures. Based on the DEM model, the effects of air void content and aggregate volumetric fraction on the cracking behavior of asphalt concrete were evaluated.     

Discrete Element Modeling of Asphalt Concrete Cracking Using a User-defined Three-dimensional Micromechanical Approach

We established a user-defined micromechanical model using discrete element method (DEM) to investigate the cracking behavior of asphalt concrete (AC). Using the Fish language provided in the particle flow code in 3-Demensions (PFC3D), the air voids and mastics in asphalt concrete were realistically built as two distinct phases. With the irregular shape of individual aggregate particles modeled using a clump of spheres of different sizes, the three-dimensional (3D) discrete element model was able to account ...     

Effects of Aggregate Size and Specimen Scale on Asphalt Mixture Cracking Using a Micromechanical Simulation Approach

A micromechanical model based on discrete element method(DEM) was employed to investigate the effects of aggregate size and specimen scale on the cracking behavior of asphalt mixture. An algorithm for generating three-dimensional aggregates that can reflect the realistic geometry such as shape, size and fracture surface of aggregate particles was developed using a user-defined procedure coded with FISH language in particle flow code in three-dimensions(PFC3 D). The parallel-bond model(PBM), linear contact model(LCM), and slip model(SM), whose sets of micro parameters were obtained by comparing experimental tests with numerical simulation results, were used to characterize the internal contact behavior of asphalt mixture. Digital asphalt mixture specimens were used to simulate the effects of aggregate size and specimen scale on the cracking behavior by the indirect tensile(IDT) test. Some conclusions can be drawn as follows: Both cracks and IDT strength decrease with increasing aggregate size. However, the heterogeneity of contact-force distribution augments with increasing aggregate size, especially with 13.2-16 mm aggregate. The aggregate size of 4.75-9.5 mm dominates in forming skeleton structure for asphalt mixture. The IDT strength decreases and cracks augment with increasing sample scale. The crack growth can be well interpreted from the perspective of energy analysis. The conclusions show that the proposed micromechanical model is suitable for the simulation of crack propagation. This study provides an assistant tool to further study the cracking behavior of particle-reinforced composites material such as asphalt mixture and Portland cement concrete.     

Numerical Investigation into the Effect of Air Voids on the Anisotropy of Asphalt Mixtures

The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson's ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digital samples of asphalt mixture with different shapes of single air void were built using discrete element software PFC~(3D). The aggregate gradation, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the three cubic samples loaded in three different directions. Dynamic modulus in three directions and Poisson's ratio in six directions were calculated from the compression stress-strain responses. Results show that both the modulus and Poisson's ratio are dependent on the preferential orientation of air voids. The anisotropy of the modulus and Poisson's ratio increases as the pressure loading on the asphalt mixture increases. Compared to the modulus, Poisson's ratio due to air voids has been shown to be more anisotropic. The maximum of Poisson's ratio and modulus is shown to be up to 80% and 11% higher than the minimum, respectively.     

Viscoelastic micromechanical model for dynamic modulus prediction of asphalt concrete with interface effects

A viscoelastic micromechanical model is presented to predict the dynamic modulus of asphalt concrete(AC) and investigate the effect of imperfect interface between asphalt mastic and aggregates on the overall viscoelastic characteristics of AC. The linear spring layer model is introduced to simulate the interface imperfection. Based on the effective medium theory, the viscoelastic micromechanical model is developed by two equivalence processes. The present prediction is compared with available experimental data to verify the developed framework. It is found that the proposed model has the capability to predict the dynamic modulus of AC. Interface effect on the dynamic modulus of AC is discussed using the developed model. It is shown that the interfacial bonding strength has a significant influence on the global mechanical performance of AC, and that continued improvement in surface functionalization is necessary to realize the full potential of aggregates reinforcement.     

Microstructure modeling and virtual test of asphalt mixture based on three-dimensional discrete element method

The objective of this work is to model the microstructure of asphalt mixture and build virtual test for asphalt mixture by using Particle Flow Code in three dimensions(PFC~(3D))based on three-dimensional discrete element method.A randomly generating algorithm was proposed to capture the three-dimensional irregular shape of coarse aggregate.And then,modeling algorithm and method for graded aggregates were built.Based on the combination of modeling of coarse aggregates,asphalt mastic and air voids,three-dimensional virtual sample of asphalt mixture was modeled by using PFC~(3D).Virtual tests for penetration test of aggregate and uniaxial creep test of asphalt mixture were built and conducted by using PFC~(3D).By comparison of the testing results between virtual tests and actual laboratory tests,the validity of the microstructure modeling and virtual test built in this study was verified.Additionally,compared with laboratory test,the virtual test is easier to conduct and has less variability.It is proved that microstructure modeling and virtual test based on three-dimensional discrete element method is a promising way to conduct research of asphalt mixture.     

基于离散元法的沥青混合料虚拟疲劳试验方法

为了从细观角度深入分析沥青混合料疲劳机理,运用离散元方法实现了沥青混合料小梁试件的虚拟疲劳试验。首先,根据概率理论,建立了由混合料三维体积级配至沥青混合料二维数字试件数量级配推导公式;接着,根据蒙特卡洛方法,编制了考虑集料不规则形状的沥青混合料二维数字试件生成程序,以此生成了沥青混合料二维数字试件;然后,结合离散元基本原理,提出了包括沥青混合料离散元模型的建立、离散单元的接触模式选择和疲劳失效判断标准在内的一整套沥青混合料虚拟疲劳试验方法;最后,进行了沥青混合料小梁疲劳试验,并与基于离散元方法的虚拟试验结果进行了比较分析。研究结果表明,虚拟试验结果与真实试验结果相差无几,所建立的虚拟疲劳试验方法可以作为沥青混合料疲劳性能分析的辅助手段。     

Dynamic elastic modulus of cement paste at early age based on nondestructive test and multiscale prediction model

This paper introduced a nondestructive testing method to evaluate the dynamic elastic modulus of cement paste. Moreover, the effect of water-cement ratio and conventional admixtures on the dynamic elastic modulus of cement paste was investigated, in which three kinds of admixtures were taken into account including viscosity modifying admixture （VMA）, silica.fume （SF）, and shrinkage-reducing admixture （SRA）. The experimental results indicate that the dynamic elastic modulus of cement paste increases with decreasing water-cement ratio. The addition of SF increases the dynamic elastic modulus, however, the overdosage of VMA causes its reduction. SRA reduces the dynamic elastic modulus at early age without affecting it in later period. Finally, a multiscale micromechanics approach coupled with a hydration model CEMHYD3D and percolation theory is utilized to predict the elastic modulus of cement paste, and the predictive results by the model are in accordance with the experimental data.     

轻集料混凝土局部受压试验及动力本构分析

为探究轻集料混凝土单轴局部受压动力特性,采用试验机对轻集料混凝土单轴局部受压本构关系进行相关试验研究.考虑了5种不同加载应变率和4种不同加载面积,在位移控制下,得到轻集料混凝土单轴局部受压应力-应变曲线.根据应力-应变曲线特征点的峰值应力、峰值应变和弹性模量,分析不同加载应变率与加载面积对特征点的影响.同时根据混凝土黏弹塑性损伤本构模型,考虑不同加载应变率和加载面积的影响修正模型参数,并通过试验数据验证理论模型的合理性.研究结果表明:加载应变率和加载面积对轻集料混凝土应力-应变曲线影响明显;不同加载面积对轻集料混凝土弹性模量、峰值应力及峰值应变有较大影响;所提出的理论模型能够准确描述轻集料混凝土材料局部动力荷载作用下的应变率效应和非线性行为.     

基于数字图像处理和有限元建模方法的沥青混合料劈裂试验数值模拟

应用数字图像处理技术及工业CT扫描获得试件图像,结合有限元建模方法,建立了包含集料、空隙和胶浆在内的沥青混合料有限元模型,并模拟研究了沥青混合料劈裂试验。结果表明:通过定义应力集中因子,可用来描述沥青混合料内部应力的不均匀分布,级配类型、模量比、有无空隙及加载位置都对模拟结果有较大影响。采用DIP-FEM方法能够很好地将沥青混合料的微观结构和宏观力学性能结合起来。     

鼓泡流化床流动特性的直接颗粒模拟

为了考察流化床中颗粒和气泡运动的基本规律，对两维鼓泡流化床进行了数值模拟.在假设气相为无黏不可压的条件下，求解了体积平均后的欧拉方程，并利用离散单元法（DEM）模拟了颗粒间的碰撞及颗粒与壁面间的碰撞过程.模拟中跟踪了约90 000个颗粒，统计了不同床高截面上颗粒及气体纵向平均速度的分布，并对不同表观空气风速和床高下床内压降以及压力波动进行了研究.结果表明，DEM方法能很好地模拟鼓泡床内的流动特性.在床内较低截面上，颗粒及气体的平均速度沿水平方向呈现两个峰，而在较高截面上合并为一个峰.且表观风速越大，气泡运动速度越高.     

部件复杂表面影响土壤扰动行为的离散元宏细观分析

通过分析毛管水形成液桥对土壤微观力学结构的影响,引入并行约束来表征土壤颗粒间液桥的黏性作用,建立了土壤颗粒接触非线性力学模型,并对波纹形复杂表面推土板前端土壤的动态扰动行为进行了离散元宏细观模拟。结果表明,离散元模拟不仅准确再现了实验分析中复杂表面结构对土壤动态扰动行为影响的研究结果,而且从土壤颗粒运动以及土块破碎等细观角度准确分析了触土部件复杂表面对土壤动态扰动行为的影响因素和机理。     

基于数字图像的沥青混合料离散元几何建模方法

沥青混合料的性能与其细观结构密切相关,离散元方法在解决沥青混合料这种复杂、多相、不连续介质材料的细观问题时,具有很大优势。针对以往沥青混合料离散元建模不能较好模拟集料颗粒形状的问题,结合数字图像处理技术,提出了一种新的沥青混合料离散元几何建模方法,主要有3个步骤：1）使用特征聚类方法,将沥青混合料切片图像中的集料与沥青砂浆分离;2）并利用链码边界跟踪和线段表算法将分离的集料区域填充为离散元中的＂聚粒＂;3）用离散元的圆盘单元填充沥青胶砂区域,实现沥青混合料切片图像的二维离散元几何建模。利用该模型,较好地模拟了沥青混合料在贯入荷载作用下变形的动态过程。     

Finite element analysis for radiative heat transfer in multidimensional participating media

A finite element model is developed to simulate the radiative transfer in 2D and 3D complex-geome-tric enclosure filled with absorbing and scattering media. This model is based on the discrete ordinates method and finite element theory. The finite element formulations and detailed steps of numerical calculation are given. The discrepancy of the results produced by different space and solid angle discretization is also investigated and compared. The effect of the six-node quadric element on the accuracy is analyzed by a 2D rectangular enclosure. These results indicate that the present model can simulate radiative transfer in multidimensional complex-geometric enclosure with participating media effectively and accurately.     

Lateral Pressure of RC Silos with Static and Dynamic Granular Materials

This paper aims at analyzing material-induced lateral pressure of RC cylinder silo in both static and dynamic condition using the finite element method (FEM).In the finite element software ABAQUS, concrete material is modeled by concrete damaged plasticity model, and stored materials in silo is modeled by the hypoplastic theory.In terms of numerical model, shell elements (S4R) and solid elements (C3D8) are applied for model silo wall and stored materials respectively. The interaction between silo wall and stored materials is simulated by Coulomb friction model and penalty contact constrain provided by ABAQUS.The numerical results are verified with the existing experimental data that are designed to ensure the validation of such numerical model using FEM and it obtains good agreements between numerical results and experimental data.Then the material parameters are analyzed in both static and dynamic condition.According to the analysis, it is clear that critical friction angle, initial void ratio and minimum void ratio have an obvious effect on static lateral pressure while all the material parameters affect dynamic lateral pressure at different levels. In addition, differences of silo wall between elastic and plastic state are analyzed in dynamic condition.The numerical results show that it contributes to increasing dynamic pressure when silo wall enters into the plastic state.Finally, this paper discusses the time-history lateral pressure at different heights along silo wall, and analytical results indicate that larger acceleration values play main roles in producing the maximum lateral pressure at higher part of the silo wall.     

Experimental and mesoscopic investigation on the dynamic properties of coral aggregate concrete in compression

To investigate the dynamic responses and comprehending the damage mechanism of coral aggregate concrete(CAC) in compression, both the experimental and numerical investigations were implemented in the present work. Firstly, the dynamic mechanical properties of CAC at different strain rates were tested through the Split-Hopkinson pressure bar(SHPB) tests.Moreover, the effects of concrete strength grade and strain rate on CAC were discussed and analyzed. Subsequently, we developed the three-dimensional(3D) random mesoscale model considering the randomness of aggregate shape, size and distribution at meso-level, which was validated and employed in the numerical simulation of CAC in compression. The results indicate that the splitting failure passing through the coral aggregate is CAC'S primary failure mode. It has been found that the failure pattern, deformation process, and dynamic increasing factor of CAC are associated with both the strain rate and concrete strength grade. Furthermore, by comparing the experimental and mesoscopic results, it has been proven to be reliable to employ the developed 3D mesoscale modelling method to simulate CAC's dynamic performances, which has enormous potential in future research of CAC under intense dynamic loadings.     

月壤静力学特性的离散元模拟

以典型月壤的物理力学性质为参照标准,采用颗粒流程序PFC3D对月壤静力学特性进行了离散元模拟研究。通过离散元模拟三轴试验反复调整颗粒细观参数,建立了月壤离散元接触力学模型;描述了月面探测车辆行驶下的月壤承压特性模型与剪切特性模型;从月面探测车辆的行驶观点出发,模拟压板试验,得出了载荷-沉降关系;通过模拟履带板试验得出了剪切应力-位移关系。最后,计算出了月壤的变形模量K为1635 kN/mn+2、变形指数n为1.22,剪切变形模量k为1.35 cm。结果表明,模拟试验值与模型预测趋势一致。     

基于细观结构特征的沥青混合料空隙三维分布特征研究

细观结构特征对沥青混合料宏观性能有着重要影响,其中细观结构的空隙率特征是影响沥青混合料车辙、水损害的关键因素之一.通过工业CT分别对最大公称粒径分别为13 mm与16 mm的AC、SAC、SMA的6种沥青混合料试样进行无损扫描,获取其内部细观结构特征信息,并对其试样进行三维重构.提取试样的内部空隙体积参数并计算其三维空隙率;选取部分断面图像计算其二维空隙率,并对两种方法计算出的空隙率进行对比分析.研究结果表明:试样内部空隙数量随着空隙体积增大呈递减趋势,且0～50 mm3体积区间内空隙数量最多;相同级配下,SAC的空隙率小于AC和SMA;最大公称粒径为13 mm的空隙率小于16 mm的空隙率;空隙率由试样两端向中间呈逐渐减小的趋势,且SMA较SAC表现得明显,最大公称粒径为16 mm的试样比最大公称粒径13 mm的表现得更明显;3D计算空隙率比2D计算空隙率更接近实测值.     

基于宏微观纹理特征融合的路面摩擦性能预测

通过非接触式三维激光表面测试、机器学习,开展基于宏微观纹理特征融合的路面摩擦性能智能预测模型研究. 来自俄克拉荷马州的45个测试站点被选取作现场测试平台. 利用三维激光检测车和GripTester,分别获取行车道轮迹带路面摩擦数据、宏观纹理;利用LS-40三维激光表面分析仪获取集料表面三维微观纹理数据,测算4类微观纹理参数. 利用机器学习算法,将路面摩擦与宏微观纹理特征建立联系. 综合模型训练与测试,评价路面摩擦性能预测模型的准确率. 模型的测试标准差为0.047,测试集R²为0.865. 研究结果表明,86.5%的测试数据适用于所建立的机器学习预测模型,开发的评价指标及预测模型能够较好地预测路面摩擦性能.     

基于离散单元方法的多孔沥青混合料级配比选

应用离散单元方法结合PFC2D软件,对多孔沥青混合料的级配进行了评价,建立了细观尺度上的直观离散元数值模型,构建了粗细两种级配的PAC-13多孔沥青混合料骨架结构二维模型,进行了结构稳定性虚拟试验.仿真结果表明,相同空隙率条件下,PAC-13粗型级配的骨架结构比细型级配更稳定,结构承载力和抵抗局部变形的能力更佳.同时进...