沥青混合料颗粒间接触力力链及位移场细观分析

为了从细观角度分析沥青混合料内部颗粒在荷载作用下所形成的接触力力链和位移场,利用离散元方法生成5种排列形式的沥青混合料细观模型,并对5种细观结构施加0.35kN荷载,追踪了颗粒间接触力力链的分布和大小,以及颗粒所产生的位移。结果表明,3种交叉排列的结构形成各向异性的拉、压力力链,以压力力链为主,最大接触力分别为5598N、17680N、9680N,产生的X向和Y向位移基本呈左右对称分布,随深度的增加位移减小;2种非交叉排列的结构仅产生压力力链,最大接触力分别为385.6N、334.5N,产生的X向和Y向位移变化趋势与交叉排列结构类似,但位移较小。     

集料骨架特性的分形分析和细观模型评价

为了研究组成沥青混合料的集料骨架特性,利用分形理论计算了集料级配分形维数,基于离散元方法从细观角度描述了集料颗粒间的接触状态,追踪了颗粒间的接触力和分布,并以车辙试验验证了5种沥青混合料的集料骨架抗变形能力。结果表明:对于相同最大公称粒径的沥青混合料,SMA-20的分形维数、接触力、动稳定度比AC-20分别增大2.93%、43.10%、9.28%,SMA-16的分形维数、接触力、动稳定度比AC-16分别增大3.03%、19.25%、25.64%,AC-16的分形维数和动稳定度比OGFC-16分别增大3.40%、4.88%,但SMA-16的接触力小于OGFC-16;分形维数、接触力和动稳定度越大,集料骨架越稳定,抗变形能力越强。     

三体摩擦界面剪切膨胀过程典型阶段的力链形态及承载特性

为研究研磨过程中颗粒体系的力链特性,该文运用离散元构建了一个双平行板剪切模型模拟无限剪切运动,并观察颗粒受剪切时相关参数变化。模拟将剪切膨胀过程分为塑性爬行阶段、脆性破坏阶段、颗粒重组阶段三部分,并运用力链在x轴与y轴的分量比值来观察力链方向性变化,用承载率-分布率曲线描述力链的分布及承载变化,用力链的形态图形来观察力链形态变化。结果显示:在剪切膨胀三阶段内,力链的形态、方向、分布及承载特性各不相同。其中,塑性爬行阶段力链的形态近似倾斜网格,分布比较集中,力链承载特性良好;而脆性破坏与颗粒重组阶段,强弱力链值相差较大,承载性差。并由此得出,在塑性爬行阶段研磨条件最好。     

考虑级配颗粒物质间接触力力链的分布概率分析

为了揭示颗粒物质间接触力力链的分布规律,选取10 mm、13 mm、16 mm、19 mm和20 mm五组粒径的刚性颗粒进行复写纸压痕试验,利用概率分析方法研究了单一粒径和考虑级配不同粒径颗粒体内部接触力力链的分布概率。研究结果表明:两种单一粒径的法向力与平均法向力比值f的分布概率呈先增大达到峰值后减小的趋势,分布概率达到峰值处所对应的f值接近1;10 mm颗粒的分布概率曲线所产生的误差较小,20 mm颗粒的分布概率曲线产生的误差相对较大;考虑级配的颗粒体,其分布概率均呈指数形式单调衰减,未出现分布概率峰值,并且强力链比例基本小于10 mm和20 mm的单一粒径颗粒体。     

周期边界条件下垂直螺旋输送机颗粒流数值模拟

采用离散单元法对垂直螺旋输送机中颗粒流动进行模拟计算,提出运用周期边界条件解决垂直螺旋输送机中计算效率问题,选用全尺寸模型和应用了周期边界条件的周期模型2种计算模型,计算得到2种模型中颗粒的流动特征,包括颗粒运动形态、正向接触力的力链网络和轴向速度的大小及其概率密度分布,讨论输送机在结构、颗粒流动形态、正向接触力的力链网络和轴向速度上的周期性。结果表明:2种模型在填充率较大且转速为300~1 700 r/min时模拟的颗粒流动特征一致;在一些工况中应用周期边界条件的仿真模型可以用来探寻垂直螺旋输送机中颗粒流动特征。     

基于离散元方法的沥青混合料抗裂性能分析

为了研究沥青结合料性质、集料表面特性和加载条件对沥青混合料抗裂性能的影响,利用离散元方法生成马歇尔数字试件,以伺服控制机理施加荷载,分析了粘结刚度比、粘结强度比、颗粒摩擦系数和加载速率4种细观参数对应力-应变曲线、劈裂强度、配位数和微裂缝的影响,给出了荷载作用后试件内部颗粒的细观响应。模拟结果表明:粘结强度比引起劈裂强度、配位数和微裂缝数的变异性最大,劈裂强度和配位数的变异系数分别为56.9%和5.9%,微裂缝数相差43条,加载速率引起劈裂强度、配位数和微裂缝数的变异性最小,劈裂强度和配位数的变异系数分别为2.3%和1.7%,微裂缝数仅相差1条,从细观角度验证了沥青结合料性质对沥青混合料抗裂性能的贡献率占主导地位。     

粘结颗粒材料双轴试验的数值模拟研究

利用离散元法数值模拟了5组试件的双轴试验,得到了粒径对弹性模量、泊松比、强度和裂缝初始应力的影响,分析了20MPa围压条件下粒径与弹性模量、泊松比、强度和裂缝初始应力的关系,以及6种围压与裂缝初始应力的关系,给出了5组试件荷载作用后颗粒间的接触力和颗粒位移矢量,从细观角度阐释了荷载作用下颗粒的运动模式。模拟结果表明,随着粒径的增大,弹性模量、强度和裂缝初始应力均呈减小的趋势;随着围压的增大,各粒径范围试件的裂缝初始应力呈增加的趋势,并且粒径越大的试件最大接触力和最大位移越大。     

颗粒流润滑剪切膨胀的力学机制研究

颗粒流润滑是可用于苛刻环境下的新型润滑方法,颗粒流的剪切膨胀特性是全面建立颗粒流润滑理论亟需解决的关键问题。该文基于离散单元法建立了颗粒流润滑的分析模型,研究了润滑启动阶段剪切膨胀现象的变化规律,剪切膨胀的速度适应机制和压力适应机制,并在总结剪切速度和压力载荷对剪切膨胀影响的基础上分析得出了剪切膨胀现象的力学机制。研究结果表明:润滑启动阶段,颗粒润滑系统的剪胀率随时步的变化服从二次多项式规律;颗粒润滑系统的剪切膨胀幅度随剪切速度,压力载荷的增大均呈现逐步递减的变化趋势;颗粒润滑系统的剪切膨胀特性与颗粒润滑介质间细观力链的波动密切相关,当细观力链的波动幅度变大时,颗粒润滑系统剪切膨胀的变化幅度亦增大,反之则减小。颗粒润滑介质间的接触力分布按幂函数规律变化,且颗粒润滑介质间力链的强度随着剪胀率绝对值的增大而变强。     

考虑颗粒形状的面板堆石坝振动台模型试验DEM模拟

提出了一种复杂形状颗粒的DEM模拟方法,实现了包括凹多边形在内的复杂形状颗粒的模拟,通过参数控制使得生成的颗粒形状更加真实,模拟了面板堆石坝振动台模型试验,从细观角度揭示了面板堆石坝动力特性。计算结果表明:在地震作用下面板与底板容易滑动,随着峰值加速度的增大,下游坝坡颗粒滑移面角度变大,坝顶沉降和坝体断面剪胀加剧;震前坝体强力链法向接触力主轴方向接近于竖直方向,震后向下游偏转;随着峰值加速度的增大,法向接触力各向异性程度逐渐增强;在相同初始组构条件下面板堆石坝激发的摩擦激励是一定的,与加载条件无关。     

块石含量和空间分布对土石混合体抗剪强度影响的离散元分析

土石混合体的抗剪强度在很大程度上取决于其块石的含量、空间分布、形态等参数。该文采用球体单元模拟土颗粒,通过组合颗粒单元构造非规则形态的块石,对不同含石量和块石空间分布下土石混合体的直剪过程进行了离散元数值模拟。计算结果表明,在低含石量下,土石混合体的抗剪强度随含石量的增加而增加;在中含石量下,受块石空间分布的影响,其抗剪强度呈现很强的波动性;在高含石量下,其抗剪强度显著增强,波动性相对较小。抗剪强度在中高含石量下波动现象的细观机理是块石空间分布影响下的力链结构特性。当块石的空间分布使其形成较强力链结构时,抗剪强度较高;反之,块石间的力链结构不稳定,抗剪强度相对较低。以上研究有助于从细观尺度揭示土石混合体变形和剪切强度特性的内在机理。     

钢筋混凝土桥墩截面能力的概率分析

采用随机模拟方法研究钢筋混凝土桥墩截面的屈服曲率、极限曲率、屈服抗弯能力、极限抗弯能力、曲率延性、塑性转动能力以及抗剪能力的概率特性,得到:屈服曲率、屈服抗弯、极限抗弯和抗剪能力服从正态分布;极限曲率服从极值I型分布;曲率延性、塑性转角服从对数正态分布。分析了桥墩轴力(轴压比)变化对上述截面能力的概率分布及变异性的影响,表明:轴力变化基本上不影响它们的概率分布模型,但对其变异性影响较大;对截面的抗弯能力及截面的屈服曲率,其变异系数随轴力的增加而减小,对截面的极限曲率、曲率延性和塑性转角,其变异系数随轴力的增加而增大。     

石灰岩沥青混合料抗滑性能衰变特性研究

为了了解石灰岩沥青混合料抗滑性能衰变特性的影响因素,使用自行开发的室内加速磨耗模拟装置,针对AC-13及SMA-13混合料,研究了混合料类型和集料特性等因素对混合料抗滑衰变特性的影响。试验结果表明,随着磨耗圈数的增加,AC-13混合料的摆值持续降低,SMA-13混合料则先升后降;增大级配中粒径大于2.36mm集料的含量,有利于改善AC-13及SMA-13混合料的抗滑衰变特性,AC-13混合料的抗滑衰变特性与贝雷法级配参数之间有着良好的线性关系;集料的磨光值、洛杉矶磨耗损失和压碎值对两种混合料的抗滑衰变特性均有显著影响,且影响程度依次降低;粗集料针片状含量与混合料的抗滑衰变特性之间为明显的负相关关系,当集料粒形接近球体或立方体时混合料的抗滑衰减速率减缓。     

基于原子力显微镜技术对全气候老化沥青的老化非线性预测

采用全气候加速老化箱对马歇尔成型的SMA-13、AC-13沥青混合料分别进行0、1000、2000及3000h的老化,对混合料中的老化沥青进行抽提。沥青老化后的形貌高度(Height)、粘附力(Adhesion)以及模量(Modulus)用原子力显微镜(AFM)进行量化。选用Verhulst模型,采用1stOpt的LM+通用全局优化法对试验数据进行非线性拟合,分别建立AFM试验指标与老化时间(t)之间的非线性微分模型。结果表明:在全气候老化条件下,非线性方程可以精确地从微观角度对沥青混合料中沥青的老化规律进行描述,模型参数L和r对老化状态和老化速率进行量化。随着老化时间的增长,形貌粗糙度值降低,粘附力和模量的粗糙度值增加。     

Study on the void reduction behaviour of porous asphalt pavement based on discrete element method

The objective of this study was to analyse the void reduction behaviour of porous asphalt mixture under load. A three-dimensional discrete element model of porous asphalt mixture based on aggregate gradation and void gradation was built in PFC3D software. The parameter of the model was obtained from creep test. The rutting test was simulated using this discrete element model. And a new method was developed to obtain and analyse the void structure in discrete element model. The simulation results were compared with one of the laboratory test. The comparative analysis indicates that, the discrete element method can be used to simulate the creep response and void reduction behaviour of porous asphalt mixture. Further research shows that porosity, effective porosity, number of connected components and section pores have a good correlation with strain of porous asphalt mixture. With the increase in strain, the proportion of section pores with diameter less than 2 mm increases. In the initial stage of loading, the void reduction is the main reason for rut increment of porous asphalt mixture. In the later stage, the void structure is almost incompressible; the lateral deformation of mixture becomes the domination factor.     

Aggregate distribution influence on the indirect tensile test of asphalt mixtures using the discrete element method

The purpose of this study is to investigate the effect of horizontal aggregate distribution, i.e. aggregate distribution in horizontal cross sections, on the indirect tensile (IDT) test of asphalt mixtures. An index of aggregate homogeneity, used to evaluate the aggregate distribution in a two-dimensional (2D) cross section, was comprehensively described; the horizontal aggregate distribution was evaluated by the index. A microstructure-based discrete element model for predicting the IDT test results was established by a discrete element program called particle flow code in two dimensions (PFC2D). Based on this model and by loading horizontal cross sections of asphalt mixtures along different directions, the effects of horizontal aggregate distribution on the splitting strength and maximum horizontal stress with regard to an IDT test were numerically simulated by means of the discrete element method; the obtained results were verified by performing an actual IDT test. Results reveal that the splitting strengths and maximum horizontal stresses in the IDT test exhibit anisotropy. Furthermore, it is revealed that there is an insignificant correlation between the horizontal aggregate distributions and the average splitting strengths and average maximum horizontal stresses, as well as a significant correlation between the horizontal aggregate distributions and the variations in the splitting strengths and maximum horizontal stresses.     

Intensification of vibration mixing of particulate solids by means of multi-layer loading of components

The objective of the study is to show how initial distribution of dissimilar particulate components influences the mixing time and mixture quality. The dissimilar components have a tendency to segregate in one another, and it is impossible to achieve the perfect mixture of them in industrial settings. Nevertheless, the situation can be improved if the components are loaded as a sequence of several sandwiches, each of these sandwiches containing layers of components that are proportional to their share in the mixture. In this case, a sort of pre-mixing occurs while still at the loading stage – which allows reducing the optimum mixing time and increasing the homogeneity of the mixture. The theory of Markov chains was used to simulate the mixing kinetics. It is shown that the number of loaded sandwiches has a very strong influence on the process efficiency. A loading device that can effectively realize multi-layer loading is proposed. The mixing kinetics for ternary mixture of glass beads was investigated experimentally at a lab scale vibration mixer. A one-time loading and a two-sandwich loading were compared. It was shown that the optimum mixing time and non-homogeneity of the mixture were reduced by half in the latter case.     

Dynamic modulus simulation of the asphalt concrete using the X-ray computed tomography images

The objective of this study is to predict the dynamic modulus of asphalt mixture using both two-dimensional (2D) and three-dimensional (3D) Distinct Element Method (DEM) generated from the X-ray computed tomography (X-ray CT) images. The 3D internal microstructure of the asphalt mixtures (i.e., spatial distribution of aggregate, sand mastic and air voids) was obtained using the X-ray CT. The X-ray CT images provided exact locations of aggregate, sand mastic and air voids to develop 2D and 3D models. An experimental program was developed with a uniaxial compression test to measure the dynamic modulus of sand mastic and asphalt mixtures at different temperatures and loading frequencies. In the DEM simulation, the mastic dynamic modulus and aggregate elastic modulus were used as input parameters to predict the asphalt mixture dynamic modulus. Three replicates of a 3D DEM and six replicates of a 2D DEM were used in the simulation. The strain response of the asphalt concrete under a compressive load was monitored, and the dynamic modulus was computed. The moduli of the 3D DEM and 2D DEM were then compared with both the experimental measurements results. It was revealed that the 3D discrete element models successfully predicted the asphalt mixture dynamic modulus over a range of temperatures and loading frequencies. It was found that 2D discrete element models under predicted the asphalt mixture dynamic modulus.     

集沙铲集沙过程的受力状况仿真分析与试验研究

集沙铲是某款小型铁路轨道除沙车的重要工作部件。集沙铲在实际使用中受到的冲击力大、铲板刃缘变形严重,为改善集沙铲受力状况,对集沙铲集沙过程的受力状况进行研究。应用离散单元法（discrete element method,DEM）与多体动力学,分别对相同工况下直板、15°板和30°板集沙铲的总受力和X、Y、Z三个方向的受力进行比较。同时,利用离散单元法对不同时刻集沙铲切分沙土瞬间的沙土颗粒的速度分布状况进行分析。最后设计并制作了集沙铲试验台,模拟了集沙铲集沙过程,并对直板和30°板集沙铲的工作转矩进行了测量。研究结果表明：增大集沙铲铲板刃缘倾角可以减小集沙铲的冲击受力;随着集沙铲铲板刃缘倾角变化,集沙铲触沙瞬间沙土颗粒的速度分布发生明显变化。研究结果对改善集沙铲冲击受力具有一定指导意义,同时也对利用离散元法研究铲板-沙土相互作用过程具有一定指导意义。