Modeling static friction behavior of elastic–plastic spherical adhesive microcontact in full-stick condition

The static friction behavior of an elastic–plastic spherical adhesive microcontact between a rigid flat and a deformable sphere under combined normal and tangential loading is studied by the finite element method(FEM).The contact between the sphere and the rigid flat is assumed to be full-stick,and the sliding inception is related to a loss of tangential stiffness.The intermolecular force between the rigid flat and the sphere is assessed by the Lennard–Jones(LJ)potential,which is applied to the sphere and the rigid flat by a user subroutine.The evolution of the adhesive force with tangential displacement in the full-stick condition is revealed.The results indicate that the increasing effect of adhesive energy on the static friction coefficient gradually diminishes with an increase in the adhesive energy and the external normal load.Finally,based on an extensive parametric study,an empirical dimensionless expression is obtained to predict the static friction coefficient of the spherical adhesive microcontact considering the intermolecular force.

Model for the static friction coefficient in a full stick elastic-plastic coated spherical contact

Finite element analysis is used to investigate an elastic-plastic coated spherical contact in full stick contact condition under combined normal and tangential loading. Sliding inception is associated with a loss of tangential stiffness. The effect of coating thickness on the static friction coefficient is intensively investigated for the case of hard coatings. For this case, with the increase in coating thickness, the static friction coefficient first increases to its maximum value at a certain coating thickness, thereafter decreases, and eventually levels off. The effect of the normal load and material properties on this behavior is discussed. Finally, a model for the static friction coefficient as a function of the coating thickness is provided for a wide range of material properties and normal loading.

Relationship between contact size and static friction:An approach for rigid crystalline surfaces

Relationship between contact size(A)and static friction(f)has been studied for rigid crystalline systems.We built a series of systems with two identical surfaces but different orientations and investigated the effects of the size and shape of the contact area on static friction.In these systems,there are numerous nontrivial commensurate contacts.Our results confirmed that the relationship between A and f was determined by both commensurability and shape of the contact.For commensurate contacts,f∝A independent of the shape.For incommensurate contacts,generally f∝A^(0) for regular shapes or f∝A^(1/4) for irregular shapes;however,in very few cases of regular shapes,f∝A^(1/2).Moreover,in above systems,commensurability of a contact can be easily changed by a perturbation of the misfit angle.Therefore,if the perturbation caused by the lateral force and the deformation of the surface are considered(as is the case in real systems),further research is necessary.

The Role of Friction in the Static Equilibrium of a Fixed Ladder: Theoretical Analysis and Experimental Test

In a recent publication the author derived and experimentally tested several theoretical models, distinguished by different boundary conditions at the contacts with horizontal and vertical supports, that predicted the forces of reaction on a fixed (i.e. inextensible) ladder. This problem is statically indeterminate since there are 4 forces of reaction and only 3 equations of static equilibrium. The model that predicted the empirical reactions correctly used a law of static friction to complement the equations of static equilibrium. The present paper examines in greater theoretical and experimental detail the role of friction in accounting for the forces of reaction on a fixed ladder. The reported measurements confirm that forces parallel and normal to the support at the top of the ladder are linearly proportional with a constant coefficient of friction irrespective of the magnitude or location of the load, as assumed in the theoretical model. However, measurements of forces parallel and normal to the support at the base of the ladder are linearly proportional with coefficients that depend sensitively on the location (although not the magnitude) of the load. This paper accounts quantitatively for the different effects of friction at the top and base of the ladder under conditions of usual use whereby friction at the vertical support alone is insufficient to keep the ladder from sliding. A theoretical model is also proposed for the unusual circumstance in which friction at the vertical support can keep the ladder from sliding.

Existence results for unilateral contact problem with friction of thermo-electro-elasticity

This work studies a mathematical model describing the static process of contact between a piezoelectric body and a thermally-electrically conductive foundation. The behavior of the material is modeled with a thermo-electro-elastic constitutive law. The contact is described by Signorini＇s conditions and Tresca＇s friction law including the electrical and thermal conductivity conditions. A variational formulation of the model in the form of a coupled system for displacements, electric potential, and temperature is de- rived. Existence and uniqueness of the solution are proved using the results of variational inequalities and a fixed point theorem.

Friction phenomena in a two-dimensional Frenkel-Kontorova model

By using the molecular dynamic simulation method with a fourth-order Runge--Kutta algorithm, a two-dimensional dc- and ac-driven Frenkel--Kontorova （FK） model with a square symmetry substrate potential for a square lattice layer has been investigated in this paper. For this system, the effects of many different parameters on the average velocity and the static friction force have been studied. It is found that not only the amplitude and frequency of ac-driven force, but also the direction of the external driving force and the misfit angle between two layers have some strong influences on the static friction force. It can be concluded that the superlubricity phenomenon appears easily with a larger ac amplitude and lower ac frequency for some special direction of the external force and misfit angle.

Posture Maintenance Control of 2-Link Object By Nonprehensile Two-Cooperative-Arm Robot Without Compensating Friction

In this paper, a method to posture maintenance control of 2-link object by nonprehensile two-cooperative-arm robot without compensating friction is proposed. In details, a mathematical model of the 2-link object is firstly built. Based on the model, stable regions for holding motion of nonprehensile two-cooperative-arm robot are obtained while the 2-link object is kept stable on the robot arms with static friction. Among the obtained stable regions, the robust pairs of orientation angles of the 2-link object are found. Under the robust orientation angles, a feedback control system is designed to control the arms to maintain the 2-link object's posture while it is being held or lifted up. Finally, experimental results are shown to verify the effectiveness of the proposed method.

Approximation of thermoelasticity contact problem with nonmonotone friction

The paper presents the formulation and approximation of a static thermoelasticity problem that describes bilateral frictional contact between a deformable body and a rigid foundation. The friction is in the form of a nonmonotone and multivalued law. The coupling effect of the problem is neglected. Therefore, the thermic part of the problem is considered independently on the elasticity problem. For the displacement vector, we formulate one substationary problem for a non-convex, locally Lipschitz continuous functional representing the total potential energy of the body. All problems formulated in the paper are approximated with the finite element method.

考虑负摩阻力的桩基设计研究

以广东省广州市南沙区的房屋建筑工程为例,对复杂地质情况下桩周负摩阻力的桩基设计进行研究,详细分析了桩侧阻力的计算方法、桩基静载试验检测要点,探讨了桩侧阻力成因和影响因素,并提出负摩阻力防治措施。经过在复杂地质条件下的工程应用,表明本工程所提出的方案具有良好的实践效果,证明未能正确考虑桩周负摩阻力的桩基设计存在较大的安全风险,必须全面掌握桩周负摩阻力的成因、计算方法,有效进行桩基静载试验检测,采取有效的防治措施,才能确保桩基设计的安全和良好的经济效益。

基于分形理论的磨削粗糙表面静摩擦系数模型

为更准确地计算磨削粗糙表面的静摩擦系数,本文综合考虑了微凸体相互作用和区域扩展系数的影响,基于分形几何理论和Hertz接触理论,并利用切向接触载荷的基本理论,推导了磨削粗糙表面的真实接触面积、法向总载荷和切向总载荷,建立磨削粗糙表面的静摩擦系数分形模型,通过数值仿真研究了不同法向总载荷、分形维数、高度尺度参数以及材料参数对粗糙表面静摩擦系数的影响规律。仿真结果表明,磨削粗糙表面的静摩擦系数随着法向总载荷的增加而增加,随着高度尺度参数或材料参数的增大而减小;静摩擦系数和分形维数存在非线性关系,当分形维数小于2.65时,静摩擦系数随着分形维数的增大而增大,当分形维数大于2.65时,静摩擦系数随分形维数的增大而减小。最后通过试验和现有模型验证了本文模型的有效性。

海上试桩不同桩型承载力特性研究

天津南港某高桩码头拟建场地有深厚密实砂层,通过试桩测定设计桩型对砂层的穿透及贯入能力,研究不同基桩承载力特性。试验结果表明:混凝土预制桩及钢管桩均有较好的穿透及贯入能力;基桩轴向极限承载力高应变检测结果小于静载试验,误差均在5%以内;混凝土预制桩桩端承载力占总承载力不小于44.6%,钢管桩桩端承载力占总承载力不小于21.9%;相同荷载作用下,混凝土预制方桩静载与动载沉降量之比为1.40,钢管桩为0.90,两者差异明显;基桩承载力达到极值时桩侧摩阻力要明显大于规范极限侧摩阻力,规范取值偏保守。

一种摩擦力演示装置设计

摩擦力是一种常见的力,同时也是一种多因素共同决定的力。很多人认为摩擦力的概念抽象而不容易理解,尤其是摩擦力的大小与接触面的关系问题。为了辅助摩擦力教学,设计了一台摩擦力演示装置。同时介绍了该演示装置的设计原理、装置构成、制作方法以及仪器的详细使用方法。它适用于学生做基础的摩擦力验证实验。

SEBIM安全阀活塞组件最大静摩擦力测量工艺研究

核电厂SEBIM安全阀先导控制柜探测单元活塞组件的最大静摩擦力对其整定值的精度与稳定性具有重要影响。为避免阀门开启定值和关闭定值的差值超标,设计了一套测量工具并给出其测量方法和相关建议,对测量曲线和具体算法进行分析说明。结果表明,该工艺能够准确测量探测单元活塞组件的最大静摩擦力,提高SEBIM安全阀设备的可靠性,确保其对系统安全的保护功能。

小吨位转体T构临时锁定称重配重技术

常规称重配重工作量大、程序繁琐且不易保证转体结构的精度。以某立交段工程转体桥项目为依托,提出并应用了小吨位转体桥T构临时锁定称重配重技术,即在施工过程前按照单侧悬臂梁施工重量偏差2%计算转体结构偏载,设计临时锁定型钢代替砂箱,将临时锁定型钢预埋转体结构中,转体前割断型钢测定型钢应变变化值,推算出转体桥不平衡弯矩并完成配重。推导了临时锁定称重配重计算公式,并与常规称重方案对比。结果证明,对于小吨位转体桥T构临时锁定称重配重技术可准确测量T构的配重力矩,给出合理配重方案,具有技术可行、安全可靠、方便快捷的优点。

减震器静摩擦力检测系统开发

针对减震器静摩擦力测试精度差、测试角度不可设的问题,开发了减震器静摩擦力检测系统。通过SolidWorks软件完成了检测系统的机械设计,采用PLC作下位机控制伺服电机完成测试的机械动作,使用LabVIEW虚拟仪器作为上位机完成控制数据输出、传感器参数采集、静摩擦力曲线绘制。最后对检测系统进行实际运行试验,试验结果表明:该检测系统有效完成了减震器静摩擦力测试,具有检测精度高、测试角度可设定、测试曲线绘制等特点。

大直径嵌岩灌注桩竖向承载特性试验研究

为了探讨泥岩地基大直径灌注桩竖向承载性能,依托青岛某机场桩基工程,对嵌入泥岩的9根大直径灌注桩进行了竖向抗压静载试验和桩身应力监测,分析大直径嵌岩灌注桩的竖向抗压性能,明确大直径灌注桩的荷载传递规律。结果表明,试桩桩身完整性较好,桩身质量可靠;试桩荷载沉降(Q-s)曲线均为缓变型,加载初期存在明显的压密段,桩顶沉降量为10.31 mm,最低回弹率大于50%;最大荷载作用下,各试桩的竖向抗压极限承载力特征值均不小于6480 kN,试桩竖向抗压承载能力仍有较大潜力;试桩均表现出端承摩擦桩的特性,轴力自桩顶到桩端逐渐衰减,桩侧摩阻力峰值发生在中风化泥岩段,桩端阻力占桩顶荷载的10.55%~34.48%。

海洋静力触探仪标准装置研究及不确定度评定

为解决海洋静力触探仪量值溯源问题,结合设备测量过程中受温度影响的分析,根据量值溯源技术路线,设计构建了可在模拟海洋温度条件下开展海洋静力触探仪校准的标准装置。在对该标准装置的稳定性和开展试验的重复性考核的基础上,对校准试验结果进行了不确定度评定分析,分析表明海洋静力触探仪标准装置所复现的孔隙水压力标准值扩展不确定度为0.017 MPa(k=2),锥尖阻力标准值扩展不确定度为0.031~0.270 kN(k=2),侧壁摩阻力标准值扩展不确定度为0.031~0.036 kN(k=2),构建的标准装置性能指标满足海洋静力触探仪的量值溯源需求。

静摩擦力数字化探究实验的再设计

静摩擦力是高中物理教学中的重要内容.受滑动摩擦力的影响,学生也会错误地认为接触面的粗糙程度以及正压力会影响静摩擦力的大小.但人教版必修第1册“摩擦力”一节设计的静摩擦力演示实验,这个实验效果不能解决学生的疑惑.利用“滴水装置”并结合数字化技术,创新设计静摩擦力的探究实验,帮助学生正确认识静摩擦力,培养和发展学生的科学探究能力和科学思维习惯.

高速铁路跨营业线T构转体称重试验关键技术

为减少施工过程对营业线的影响,现在跨铁路营业线桥梁施工中越来越普遍地采用转体法施工。而称重试验是转体施工中重要的一步,通过称重试验获得相关力学参数,为后续转体指挥决策提供依据。通过跨既有焦柳线铁路(56 m+56 m)T构桥梁转体施工称重试验施工实践,介绍了转体桥梁施工中称重试验的一般方法及原理,提出了相应配重方案的一般方法。

纺织品静电的产生及消除方法

静电会吸附灰尘而影响服装服用性及穿着舒适性,严重时会引起电子元器件的损坏及产生电火花,引起安全生产事故。探讨静电产生原理,包含摩擦起电、感应起电、电荷迁移和破断起电,通过对产生静电原理的深入探讨,从3个方面提出如何减少或消除静电产生的措施。一方面提高环境湿度及选择合适材料来减少静电的产生,另一方面采取措施使已产生的静电快速泄露,再一方面用性质相反的电荷去中和已经产生的静电。该研究为抗静电面料的生产开发及工业生产中消除静电提供参考借鉴。