Finite element simulation of nanostructuring burnishing

Dynamics simulation of burnishing of a thin steel layer beneath an indenter applied with a constant force and then moving with a constant velocity was performed by the finite element method in the plane strain approximation. The indenter was modeled by a perfectly rigid body, and the steel was modeled by an elastoplastic body with isotropic hardening according to an experimentally defined law. The regularities of changes in the stressstrain state of the material near the treated surface were studied and mechanisms of the formation of a nanostructured layer were disclosed. The effect of the friction coefficient and the burnishing force on the height of a bulge of edged material was analyzed. The results of studies agree well with experimental data.     

Tribological aspects in nanostructuring burnishing of structural steels

The paper studies tribological aspects of nanostructuring burnishing of steels. The efficiency of the process in improving the tribological properties of steels is assessed as regards the choice of an indenter material and lubricant-coolant reasoning from the friction coefficient at the “indenter-treated part” contact and from the absence of adhesive bond and fatigue microcracks. It is shown that synthetic diamond and dense boron nitride are promising indenter materials for nanostructuring burnishing of corrosion-resistant 20X13 steel and cement 20X steel. It is demonstrated that nanostructuring burnishing increases the wear resistance of structural steels under abrasive action and sliding friction in different media (lubricant, water, air, and argon) due to suppressed processes of microcutting, plastic edging, fatigue and oxidation wear, and adhesive bonding.     

Static friction of sinusoidal surfaces: a discrete dislocation plasticity analysis

Discrete dislocation plasticity simulations are carried out to investigate the static frictional response of sinusoidal asperities with (sub)-microscale wavelength. The surfaces are first flattened and then sheared by a perfectly adhesive platen. Both bodies are explicitly modelled, and the external loading is applied on the top surface of the platen. Plastic deformation by dislocation glide is the only dissipation mechanism active. The tangential force obtained at the contact when displacing the platen horizontally first increases with applied displacement, then reaches a constant value. This constant is here taken to be the friction force. In agreement with several experiments and continuum simulation studies, the friction coefficient is found to decrease with the applied normal load. However, at odds with continuum simulations, the friction force is also found to decrease with the normal load. The decrease is caused by an increased availability of dislocations to initiate and sustain plastic flow during shearing. Again in contrast to continuum studies, the friction coefficient is found to vary stochastically across the contact surface, and to reach locally values up to several times the average friction coefficient. Moreover, the friction force and the friction coefficient are found to be size-dependent.     

Existence and analytical predictions of periodic motions in a periodically forced,nonlinear friction oscillator

The grazing bifurcation, stick phenomena and periodic motions in a periodically forced, nonlinear friction oscillator are investigated. The nonlinear friction force is approximated by a piecewise linear, kinetic friction model with the static force. The total forces for the input and output flows to the separation boundary are introduced, and the force criteria for the onset and vanishing of stick motions are developed through such input and output flow forces. The periodic motions of such an oscillator are predicted analytically through the corresponding mapping structure. Illustrations of the periodic motions in such a piecewise friction model are given for a better understanding of the stick motion with the static friction. The force responses are presented, which agreed very well with the force criteria. If the fully nonlinear friction force is modeled by several portions of piecewise linear functions, the periodically forced, nonlinear friction oscillator can be predicted more accurately. However, for the fully nonlinear friction force model, only the numerical investigation can be carried out.     

纳米级随机粗糙表面微观滑动摩擦力的计算研究

表面形貌很大程度上决定了摩擦副的摩擦性能, 而所有的表面都不可能是绝对光滑的.由于摩擦表面形貌的随机性, 决定了实际的摩擦过程具有随机性的特点, 因此为了获得与随机形貌对应的摩擦特性, 建立合理的随机摩擦模型是必要的. 本文基于Lennard-Jones势能建立了纳米级随机粗糙表面和原子级光滑的刚性平面间的随机摩擦模型; 模型中, 界面势能由法向载荷和界面间平衡距离决定.通过数值计算的方法, 推导了微观滑动摩擦力的计算公式和摩擦力与法向载荷之间的关系. 研究结果表明摩擦力随着法向载荷的增加而增加, 但不是线性增长. 结果也说明界面间的表面势能可能是微观摩擦力的本质起源. 关键词： 随机粗糙表面 Lennard-Jones势能 微滑动摩擦力 微摩擦     

Influence of the normal force and contact geometry on the static force of friction of an oscillating sample

The paper is devoted to an experimental and theoretical investigation of the static friction force between a rapidly oscillating sample and a steel plate. The static frictional force is studied experimentally as function of the oscillating amplitude, the normal force and the contact geometry. A simplest model of tangent contact with a constant friction coefficient is proposed and shows a good agreement with experiment. The static friction force is proved to be a universal function of the ratio of the oscillation amplitude, the indentation depth and to the friction coefficient.     

Delayed feedback for controlling the nature of bifurcations in friction-induced vibrations

We analyse the control of friction-induced vibrations using time-delayed displacement feedback. We have used the exponential model for the drooping characteristics of the friction force for which the bifurcation is subcritical in nature. With an appropriate choice of the control parameters we have managed to change the nature of the bifurcation to supercritical along with increasing the stability boundaries. A nonlinear controller is required when the control force is applied in a direction parallel to the friction force. In contrast, a linear time-delayed displacement feedback applied in a direction normal to the friction force achieves our dual objective of controlling the nature of the bifurcation as well as quenching the vibrations. We also consider a dynamic friction model (the LuGre model) and observe that the qualitative change in the nature of the bifurcation is independent of the complexity considered in modeling the friction force.     

Towards a statistical theory of solid dry friction

Wearless dry friction of an elastic block of weight N, driven by an external force F over a rigid substrate, is investigated. The slider and substrate surfaces are both microscopically rough, interacting via a repulsive potential that depends on the local overlap. The model reproduces Amontons’s laws which state that the friction force is proportional to the normal loading force N and independent of the nominal surface area. In this model, the dynamic friction force decays for large velocities and approaches a finite static friction for small velocities if the surface profiles are self-affine on small length scales.     

Correlation mechanism between force chains and friction mechanism during powder compaction

The relation between friction mechanism and force chains characteristics has not yet been fully studied in the powder metallurgy research area.In this work,a uniaxial compression discrete element model is established based on the compaction process of ferrous powder.Furthermore,the correlation mechanism between force chains and the friction mechanism during powder compaction is investigated.The simulation results reveal a strong correlation between the variation of the friction coefficient and the evolution of force chains.During the powder compaction,the friction coefficient would eventually tend to be stable,a feature which is also closely related to the slip ratio between particles.The side wall friction and the friction between particles would have an important effect on the direction of force chain growth in about one-third of the area near the side wall.The research results provide theoretical guidance for improving the densification process of the powder according to the force chain and friction.     

Velocity dependence of atomic friction

Sliding friction between the tip of a friction force microscope and NaCl(100) was studied to deduce the velocity dependence of friction forces on the atomic scale. A logarithmic dependence of the mean friction force is revealed at low velocities. The experimental data are interpreted in terms of a modified Tomlinson model which is based on reaction rate theory.     

基于耦合振子模型的摩擦力计算研究

以界面摩擦为研究对象,探讨了基于耦合振子模型(coupled-oscillator model)的滑动摩擦微观机理,分析了滑动过程中的能量耗散问题. 采用Maugis-Dugdal接触模型替代界面摩擦中的Lennard-Jones势能,并将该模型融入耦合振子模型之中,通过计算振子在一个周期内的能量增加值,推导出了界面摩擦力的理论计算公式. 理论分析表明,对于探针-试样接触系统,滑动摩擦力近似随着法向载荷的2/3次方增加,这与纳米摩擦学经典理论是相符的.理论计算结果与超高真空原子力显微镜镀铜探针在Cu(111)晶面扫描实验结果符合良好,表明本文提出的理论和方法可行. 关键词： 耦合振子模型 界面摩擦 摩擦力 法向载荷     

Frenkel-Kontorova模型中垫底势对最大静摩擦力的影响

用一维Frenkel-Kontorova模型,对相互接触的两个单原子分子链具有相对运动趋势时所产生的最大静摩擦力进行了研究.分别在相邻原子的距离与周期势场的周期比b/a为可公度(commensurate)、黄金分割(golden mean)、螺旋分割(spiral mean)三种情况下,描述了特殊垫底势力的振幅A与分子链静摩擦力的关系,在特殊垫底势力的作用下上层原子链弹性系数K对静摩擦力的影响.研究表明,垫底势力的形式对静摩擦力的大小有很重要的影响.     

Frenkel-Kontorova模型中垫底势对最大静摩擦力的影响

用一维Frenkel-Kontorova模型,对相互接触的两个单原子分子链具有相对运动趋势时所产生的最大静摩擦力进行了研究.分别在相邻原子的距离与周期势场的周期比b/a为可公度(commensurate)、黄金分割(golden mean)、螺旋分割(spiral mean)三种情况下,描述了特殊垫底势力的振幅A与分子链静摩擦力的关系,在特殊垫底势力的作用下上层原子链弹性系数K对静摩擦力的影响.研究表明,垫底势力的形式对静摩擦力的大小有很重要的影响. 关键词： Frenkel-Kontorova模型 静摩擦力     

摩擦微观能量耗散机理的复合振子模型研究

提出无磨损界面摩擦微观能量耗散机理的复合振子模型，指出滑动摩擦过程同时存在整体做低频弹性振动的宏观振子和界面原子受激励产生热振动的微观振子，并在此基础上分析了宏观振子和微观振子对摩擦能量耗散的不同影响. 通过对界面原子的动力学分析，指出摩擦过程界面激励力的频率是能量转换的关键:在平衡力作用阶段，界面作用力的频率趋于零，因而可以直接作用到每个原子，力的作用效果是整体和均匀的；在失稳跳跃阶段,由于界面激励力的频率极高，造成摩擦界面原子获得的能量分布很不均匀，从而产生不可逆的能量耗散过程. 与目前通用的独立振子模型比较，复合振子模型能够更准确描述摩擦能量耗散过程，可为摩擦控制提供理论指导. 关键词： 摩擦 能量耗散机理 复合振子模型 独立振子模型     

Instability criteria for a motion of a vortex subject to a non-linear friction force

Here we study the instability of the vortex motion caused by non-monotonous dependence of the friction force on the vortex velocity in the presence of a gyroforce. The analysis is done within the frameworks of “minimal” model for the elastic string subject to friction force and gyroforce. We demonstrate that even a weak gyroforce renders the condition for onset of instability more strict than for a vortex not subject to gyroforce: either the friction force must exceed a threshold, or its non-monotonic velocity dependence must be a rather steep function.     

颗粒物质中圆棒受到的静摩擦力

通过实验研究了圆棒在颗粒物质中受到的摩擦力与颗粒填充高度及棒径的关系.观测到摩擦力随颗粒高度和棒径而增大.用连续介质模型推导了摩擦力公式，与实验观测基本一致.结果表明，摩擦力和棒径成正比；当颗粒高度很小时，摩擦力与颗粒高度平方成正比；而当颗粒堆积很高时，摩擦力与高度成正比，即静摩擦力与接触面积成正比. 关键词： 颗粒物质 摩擦力     

Application of Two-Dimensional Frenkel-Kontorova Model to Nanotribology

The dry friction force between two contacting surface layers is studied. The upper layer is arranged on a two-dimensional square lattice and driven by an external driving force. The lower layer is approximated by a two-dimensional periodic substrate potential. This model, usually called two-dimensional Frenkel-Kontorova model, is applied to study the friction forces in this paper. The behaviors of different substrate potential strongly affect the static frictionforce. It is found that the system has strong anisotropic character. The possibility to obtain superlubricity is suggested.     

扩展位错低频内耗的模型

本文提出面心立方合金中扩展位错引起低频内耗峰的一个模型,用来解释高浓度Cu-Al和Cu-Zn合金经过冷加工后在210K附近出现的弛豫型内耗峰(测量频率约为1Hz)。文中指出,扩展位错在外力作用下的运动,可以分解为相对运动(两个部分位错之间的相对位移)和整体运动(扩展位错中心的位移)。除了考虑两个部分位错之间交互作用引起的回复力和位错线张力引起的回复力以外,本文引入了邻近位错之间长程交互作用引起的回复力,并且论证了这种回复力只影响扩展位错的整体运动,但不影响其相对运动。扩展位错的相对运动和整体运动,分别导致相对运动内耗峰和整体运动内耗峰。二者峰温相近。实验上观察到的内耗峰是这两个内耗峰的叠加。本文模型的推论与实验结果相符。 关键词：     

多晶体压剪试样静态加载有限元计算

基于晶体塑性理论研究了晶体织构对数值计算结果的影响,建立了带有织构的多晶体压剪试样(SCS)模型。从材料和试样结构两方面研究了静态加载条件下微观晶粒在有限变形过程中对试样宏观力学性能的影响。由于模型几何结构的特殊性,重点对模型斜槽部分的应力、应变及变形特点进行了分析。考虑到试样在压缩过程中受摩擦的影响,数值分析了不同摩擦系数对变形过程的影响,在此基础上计算了相同摩擦系数下不同晶粒数目、不同单元数目以及单元类型对多晶体压剪模型力学性能的影响,并对试件关键部位不同取向晶粒的应力状态进行了分析。