Influence of particle contact models on soil response of poorly graded sand during cavity expansion in discrete element simulation

The discrete element method(DEM) has been extensively adopted to investigate many complex geotechnical related problems due to its capability to incorporate the discontinuous nature of granular materials. In particular, when simulating large deformations or distortion of soil(e.g. cavity expansion),DEM can be very effective as other numerical solutions may experience convergence problems. Cavity expansion theory has widespread applications in geotechnical engineering, particularly to the problems concerning in situ testing, pile installation and so forth. In addition, the behaviour of geomaterials in a macro-level is utterly determined by microscopic properties, highlighting the importance of contact models. Despite the fact that there are numerous contact models proposed to mimic the realistic behaviour of granular materials, there are lack of studies on the effects of these contact models on the soil response.Hence, in this study, a series of three-dimensional numerical simulations with different contact constitutive models was conducted to simulate the response of sandy soils during cylindrical cavity expansion. In this numerical investigation, three contact models, i.e. linear contact model, rolling resistance contact model,and Hertz contact model, are considered. It should be noted that the former two models are linear based models, providing linearly elastic and frictional plasticity behaviours, whereas the latter one consists of nonlinear formulation based on an approximation of the theory of Mindlin and Deresiewicz. To examine the effects of these contact models, several cylindrical cavities were created and expanded gradually from an initial radius of 0.055 m to a final radius of 0.1 m. The numerical predictions confirm that the calibrated contact models produced similar results regarding the variations of cavity pressure, radial stress, deviatoric stress, volumetric strain, as well as the soil radial displacement. However, the linear contact model may result in inaccurate predictions when highly angular soil particles are involved. In addition, considering the excessive soil displacement induced by the pile installation(i.e. cavity expansion), a minimum distance of11 a(a is the cavity radius) is recommend for practicing engineers to avoid the potential damages to the existing piles and adjacent structures.

Wetting Behaviours of a Single Droplet on Biomimetic Micro Structured Surfaces

Natural surfaces with super hydrophobic properties often have micro or hierarchical structures.In this paper, the wettingbehaviours of a single droplet on biomimetic micro structured surfaces with different roughness parameters are investigated.Atheoretical model is proposed to study wetting transitions.The results of theoretical analysis are compared with those of experimentindicating that the proposed model can effectively predict the wetting transition.Furthermore, a numerical simulationbased on the meso scale Lattice Boltzmann Method （LBM） is performed to study dynamic contact angles, contact lines, andlocal velocity fields for the case that a droplet displays on the micro structured surface.A spherical water droplet with r= 15 μmfalls down to a biomimetic square-post patterned surface under the force of gravity with an initial velocity of 0.01 m·sand aninitial vertical distance of 20 μm from droplet centre to the top of pots.In spite of a higher initial velocity, the droplet can stillstay in a Cassie state; moreover, it reaches an equilibrium state at t≈17.5 ms, when contact angle is 153.16° which is slightlylower than the prediction of Cassie-Baxter’s equation which gives θ=154.40°.

面向脱轨后被动安全防护的转向架结构优化

为了验证和优化列车转向架部件的脱轨后被动安全防护效果,提出基于多边形接触模型的列车脱轨后接触建模方法,并建立列车脱轨后动力学模型,模拟不同结构参数组合下的列车脱轨后运行行为。将所得横移量和结构参数组合作为反向传播神经网络模型的训练样本,拟合两者之间的非线性关系,采用基于锦标赛选择的自适应遗传算法搜索横移量最小解及其对应的参数组合。以拖车制动盘优化问题为例,运用该优化模型进行结构参数优化。研究结果表明:在100 km/h脱轨速度下,制动盘的最优结构参数组合的半径为335 mm、厚度为80 mm、安装位置为320 mm;与原设计相比,车辆脱轨后横移量减少80.2%,大幅度提高了制动盘被动安全防护能力。本文所提出的建模方法和优化模型可推广应用于其他列车结构的脱轨后被动安全防护能力优化和验证,具有较大的工程实用价值。

胶结颗粒接触力学特性测试装置研制

为验证天然结构性砂土离散元模拟中接触模型及其参数的合理性,设计了一套用于理想胶结颗粒成型及实现不同加载条件下接触力学特性测试装置。通过胶结颗粒成型装置在两大小相同的铝棒间形成具有特定几何尺寸的胶结物,随后,采用一系列辅助加载装置实现简单及复杂加载条件下胶结颗粒接触力学特性的测试。试验结果表明:该装置可用于胶结颗粒在不同加载条件下接触力学特性的测试,实测胶结颗粒接触力学响应与天然砂土离散元中接触模型基本相符,且其抗剪和抗扭强度均随着法向压力的增大而增大,在三维应力空间中胶结颗粒强度包线呈椭圆抛物面状。

结构性砂土粒间胶结效应的二维数值分析

将理想胶结颗粒接触力学特性的测试结果引入到离散元胶结接触模型中,对结构性砂土粒间胶结效应进行离散元数值模拟。首先,胶结颗粒被理想化为两铝棒在指定部位形成胶结,通过一系列加载试验(拉伸、压缩、压剪)获得胶结铝棒在不同应力路径下的接触力学响应。随后,将测试结果提炼总结后引入到自行开发的二维离散元程序NS2D中,用以模拟不同初始密度和胶结强度的结构性砂土等向压缩试验。最后,通过与人工胶结砂土的试验数据进行比较,对文中的数值模拟结果进行验证。研究表明:离散元数值模拟能够有效的捕捉结构性砂土的主要力学特性,即屈服强度和体积模量均随初始密度和胶结强度的变化而变化,且胶结试样的屈服强度与试样内部颗粒间胶结点破坏率密切相关。

节理接触机理与闭合性质研究

介绍了节理闭合试验技术和节理初始开度的测试方法，探讨了节理闭合时非线性和不可恢复变形的机理，研究了节理在不同接触状态下组合形貌的特征和闭合变形性质，得到了能反映节理接触状态和受载历史的包含有节理无量纲初始开度等节理组合形貌参数的法向力－法向变形本构方程．

剪切载荷作用下岩体结构面动态接触特征数值模拟

准确认识岩体结构面的动态接触特征是评价和预测结构面剪切行为的基础。首先,通过FISH语言修正了FLAC^(3D)软件自带的INTERFACE本构模型,使其能够准确表达粗糙结构面在剪切过程中的力学行为;然后,采用数值模拟和室内试验相结合的方法对剪切过程中结构面接触和受力的分布及其演化规律进行了研究,分析了法向应力和结构面粗糙度系数(JRC)对接触特征的影响规律;最后,研究了结构面表面形貌-接触特征-剪切应力3者间的关系,揭示了结构面剪切应力演化机理。结果表明:FLAC^(3D)自带INTERFACE模型无法很好地模拟结构面峰后剪切行为,是由于结构面凸起未啃断导致黏聚力和法向应力偏大,通过对INTERFACE破坏单元的黏聚力和法向刚度进行修正可很好地模拟结构面的损伤破坏行为。随着剪切位移增大,结构面的接触面积逐渐减小,但最大渗透深度却不断增加;随着法向应力的增大,结构面的接触面积和渗透深度都增大;随着JRC的增大,结构面的接触面积减小,最大渗透深度增大。剪切过程中结构面接触面积逐渐减小,结构面剪切应力集中现象越来越显著,当INTERFACE单元达到最大剪切应力后,最大剪应力分布范围增大;随着法向应力增大,结构面剪切应力分布范围和高剪切应力的分布范围都增大;随着JRC增大,结构面接触面积减小,但高剪切应力的分布范围显著增大。临界视倾角是判断结构面接触面积的重要参数,其随着结构面剪切位移的增大而增大,但增加速度逐渐变缓,呈幂函数关系;临界视倾角随着法向应力的增大而减小,随着JRC的增大而增大。结构面的剪切应力与接触面积有较强的相关性,不同剪切位移阶段的结构面接触面积比下降速率不同,弹性阶段下降最快,位移-软化阶段次之,残余阶段下降最慢。

盐浴硫氮碳共渗45钢的接触疲劳寿命

本文对盐浴硫氯碳共渗45钢的接触疲劳性能进行了研究。基于试验数据,根据韦布尔分布函数等,分别计算出了不同失效概率下共渗试样的接触疲劳极限及绘制了P-S-N 曲线。并对其失效机理进行了分析。

点接触弹流动压性能的多网格法分析

点接触问题主要研究在一定载荷作用下，运动球体间的接触力学性能，涉及物体的弹性变形和流体动压的相互作用。本文应用多网格重积分方法，在给定的物体几何结构、载荷、速度、流体性质条件下，对运动球体的弹性变形、流体膜的厚度分布、流体膜的压力分布等参数的性能及其关系做了研究。分析结果反映出这类问题的基本特征和参数之间相互作用的规律。这一分析技术为相关结构和参数的设计及进一步分析提供了一个强有力的手段。

无限维线性系统的有限维接触元

本文通过简洁而又直观的方法讨论了线性情形时的有限维接触无的渐近行为，特别地，给出了渐近解的表达式。

论大学生的行为控制

行为控制、强调人的内在因素、大学生是具有一定文化修养的青年人．如何运用行为科学的规律去分析、研究当代青年大学生的思想状态和行为，是高等院校政治思想工作中的关键环节．

A New Method for Modelling and Simulation of the Dynamic Behaviour of the Wheel-rail contact

This paper presents a new method for modelling and simulation of the dynamic behaviour of the wheel-rail contact. The proposed dynamic wheel-rail contact model comprises wheel-rail contact geometry, normal contact problem, tangential contact problem and wheelset dynamic behaviour on the track. This two-degree of freedom model takes into account the lateral displacement of the wheelset and the yaw angle. Single wheel tread rail contact is considered for all simulations and Kalker’s linear theory and heuristic non-linear creep models are employed. The second order differential equations are reduced to first order and the forward velocity of the wheelset is increased until the wheelset critical velocity is reached. This approach does not require solving mathematical equations in order to estimate the critical velocity of the dynamic wheel-rail contact model. The mathematical model is implemented in Matlab using numerical differentiation method. The simulated results compare well with the estimated results based on classical theory related to the dynamic behaviour of rail-wheel contact so the model is validated.

超疏水结构表面液滴撞击行为研究进展

固体表面的液滴撞击是实际应用中遇到的许多现象的重要部分,如微流控设备、冷却喷雾、喷墨打印、农药喷洒等。此外,通过超疏水结构表面修饰应用设备的功能表面能够显著提高设备的防污、抗粘附等功能,但是这种基于微纳结构的功能性表面抵抗液滴钉扎效应的能力很大程度限制了其应用。因此,研究液滴撞击超疏水表面行为对提高设备的抗腐蚀、自清洁等性能具有重要意义。首先介绍了非均匀表面的润湿理论,然后总结了近年来针对超疏水表面静态和动态液滴润湿行为的研究成果,着重阐述了结构润湿性、物理形貌及冲击速度对液滴接触时间和最大铺展系数的重要影响,介绍了液滴最大铺展系数预测模型的研究进展,进而指出了目前研究的不足之处。最后,对液滴回弹机制和最大铺展系数的研究进展进行了展望。

落生水电站压力钢管进人孔应力分析

利用ANSYS有限元分析程序,分别进行了考虑人孔补强圈与主管接触或不接触两种情况下的应力计算,以此来分析接触行为对钢管开口区及补强圈应力的影响,提供落生水电站压力钢管3号人孔参考。

健康教育影响洲滩型血吸虫病流行区居民接触疫水行为的研究

研究结果表明,健康教育能够显著降低血吸虫病流行区居民接触疫水率,人均接触疫水率下降了42.02%;居民接触疫水频数、指数 A 和指数 B 分别下降了63.71%、33.3%和13.48%。不同职业人群学生指数 A、指数 B 有所上升,其他人群均有所下降;20岁以下青少年指数 B 有所上升;不同接触疫水方式改变情况不一致,游泳、打棕叶、放牧上述三项指标均有所上升。进入疫区的外来人群接触疫水频数、指数 A、指数 B 分别上升了64.28%、88.39%、90.93%。

Numerical investigation on wheel-turnout rail dynamic interaction excited by wheel diameter difference in high-speed railway

The wheel-rail relationship in turnout is more complicated than that in ordinary track. Profile wear and machining errors of the wheelset cause deviations Of the rolling radius on different wheels. Therefore, wheelsets move to the direction of smaller diameter wheels in search of a new stable state and to change the condition before entering the turnout. Thc main aim of the present work is to examine the wheel-turnout rail dynamic interaction combined with the static contact behaviour. Calculations are performed on a high-speed vehicle CRH2 and the No. 12 turnout of the passenger dedicated line. The wheel-turnout contac! geometric relationship and normal contact behaviour under wheel diameter difference are assessed by the trace principle and finite element method. A high-speed vehicle-turnout coupling dynamic model is established based on SIMPACK software to analyse the wheel-rail dynamic interaction, riding comfort, and wear. Both the wheel diameter amplitudes and distribution patterns are accounted for. The simulation shows that wheel diameter difference can greatly disturb the positions＇ variation of wheel-rail contact points and affect the normal contact behaviour on switch rails by changing the load transition position. The effect of wheel diameter diffierence on wheel-turnout rail dynamic interaction can be divided into three according to its amplitude： when the wheel diameter difference is within 0-1.5 mm, the wheel flange comes into contact with the switch rail in advance, causing a rapidly increased lateral wheel-rail force; when it is within 1.5 2.5 mm, trains are subject to instability under equivalent in-phase wheel diameter difference; when it is larger than 2.5 mm, the continuous flange-switch rail contact helps strengthen the vehicle stability, but increases the wheel-rail wear. It is recommended to control the wheel diameter difference to within 2.5 mm but limit it to 2 mm if it is distributed in-phase.

The influence of substrate and atmosphere on the properties of FeSiB(Cu,Nb) alloy melts

The surface tensions and contact angles of Fe_(78)Si_9B_(13) and Fe_(73.5)Cu_1Nb_3Si_(13.5)B_9 alloy melts were studied as a function of temperature in various atmospheres(vacuum, Ar and N_2 gas) and on different substrates(Si C, Al_2O_3 and BN). It is indicated that Si_3N_4, NbN, Fe_2 Al B and B_(13)C_2 are generated as new phases at the interface between the melt and substrate, and reactive wetting behaviour exists during the heating process. The surface tensions of two alloy melts on BN substrate both firstly decrease and then increase along with increasing temperature, leading to V-shaped surface tension versus temperature, which results from atomic diffusion effects in the surface layer during the oxidation of BN and formation of C-rich layer. Comparably, the surface tensions on Al_2O_3 and Si C substrates decrease with increasing temperature throughout the entire temperature range. Among three substrates, BN exhibits the mildest wetting behaviour. The vacuum environment has the strongest protective effect on melt stability among the tested atmospheres. These findings enrich our knowledge about the effects of the substrate and atmosphere on Fe-based alloy melts at a high temperature, and provide theoretical reference for designing jet nozzles in melt-spinning techniques.

Parameter studies on impact in a lap joint

To represent a loose lap joint, a beam impacting four springs with gaps is modeled. Modal analysis with base excitation is solved, and time histories of contact points are closely monitored. Using the impulse during steady state response, six influential parameters are studied： damping ratio, contact stiffness, intermediate contact position, gap, excitation amplitude and beam height. For all parameters, the system response is highly controlled by modes with two contacting springs. Each parameter＇s effect on system response is presented including unstable regions, unique trend behaviours result. Recommendations for structural designers are also noted.