Force modulation with a scanning force microscope: an analysis

A magnetic force modulation microscope (FMM) has been employed to measure the dynamic behavior of a contact between a scanning force microscope (SFM) tip and a surface. Our experimental results show the inefficiency of the classical models (two Kelvin-Voigt elements in parallel). A new model which takes into account the normal and tangential stiffness of the contact, and also the geometrical and mechanical properties of the cantilever which hold the tip, is proposed. This model shows that the natural frequency is sensitive to the normal stiffness, only if the ratio of the normal contact stiffness to the cantilever stiffness is between 0.2 and 200. Above this domain, the natural frequency is sensitive to sliding (Mindlin theory).     

拉杆转子轮盘结合面切向接触刚度试验研究

运用分形接触理论,分析得到了轮盘结合面切向刚度模型,通过轮盘表面材料性能参数、拉杆转子法向载荷、切向载荷及轮盘表面的分形参数来得到轮盘结合面的切向接触刚度,对其变化规律进行数值仿真,并通过试验进行验证.研究结果表明:在定性理论模型上,分形接触理论是正确的,但分形理论有其适用范围,适用于粗糙度较小、拉杆转子法向载荷较大的...     

基于域扩展因子和微凸体相互作用的结合面接触刚度模型研究

结合面接触刚度直接影响了机械设备的整机动态特性,为了建立更为准确的接触刚度模型,以分形几何理论为基础,利用单一微凸体承受局部载荷时的弹性变形特性,并基于域扩展因子引入微接触截面积分布函数,推导了考虑表面微凸体相互作用影响的结合面接触刚度分形模型。为了验证所提出模型的准确性,通过三维非接触式测量,获得了试验试样的表面轮廓数据,并根据结构函数法,计算了各个试样的表面分形参数,进而将理论接触刚度与试验结果对比分析,结果表明：法向接触刚度的增长速率与粗糙面表面临界接触面积有关,临界接触面积决定了结合面内的弹性变形占比。考虑微凸体相互作用后,所提出模型的预测曲线更加符合试验中法向载荷与接触刚度的关系。     

轮轴弯曲刚度对轮轨垂向动态载荷影响分析

以国内某型地铁车辆为例,研究轮轴弯曲刚度对轮轨垂向动态载荷和轮对垂向振动的影响。在常规多刚体动力学模型的基础上,结合BM3000轮对和北京地铁轮对两种不同的弹性轮对模型,对比分别采用刚性轮对模型和弹性轮对模型时的轮对垂向振动加速度和轮轨垂向力。结果表明,对BM3000弹性轮对模型来说,由于其弯曲刚度相对较小,随着运行速度的增大,轮对垂向振动加速度和轮轨力与刚性轮对的差距不断加大,而对于轮轴弯曲刚度较大的北京地铁轮对来说,其弹性轮对模型和刚性轮对模型的结果比较接近,在计算的速度下轮对的振动峰值及频率均有明显的降低。因而,通过加大轮轴弯曲刚度可明显改善轮对的垂向振动和轮轨垂向力,实现改善轮轨动态接触状态的目的。     

Dynamic cutting force modeling and experimental study of industrial robotic boring

In this paper, a new approach based on industrial robotic boring is proposed to solve problems associated with intersection holes during aircraft assembly. A model is established to predict the dynamic cutting force of a robotic machining system. The robot stiffness coupling, chip deformation, and plowing interference affecting the cutting force are considered using the principles of cutting mechanics and the Oxley orthogonal cutting model. By solving a numerical solution of motion differential equation, the cutting force components in the radial, tangential, and feed directions are obtained by the model. In addition, an advanced curve intersection method is developed to identify the instantaneous uncut chip area and cutting edge contact length. Verification tests were performed on an ABB-IRB6600-175/2.55 robot for titanium alloy TC4 to determine the accuracy of the predictions. The results show that the simulated and measured cutting forces were in good agreement under different cutting conditions. By analyzing simulated and experimental results, we show that the model can be applied to predict the occurrence of vibration and has application value in terms of suppressing vibration during robotic boring.     

Dynamic response analysis of a cantilevered beam due to an elastic impact

The beam structure models with impact or contact parts under impact forces have been applied to the design of mechanical and electronic accessories. Switches, pick-ups and sensors are the typical examples of the structure to be designed to colliding with other parts. The damping characteristics and stiffness of an impacter have a great effect on dynamic responses of structures subjected to impact forces. Since the response characteristics of structures have an effect on the generation of impact forces, both theoretical and experimental investigations for the dynamic relationships between the impacter and impact mechanism are required for the structural design to reduce impact forces or eliminate unwanted vibration modes. In this paper, in order to examine the relationship between the changes of the stiffness and damping of an impacter and the variations of the dynamic characteristics of the impact model of a cantilevered beam with an impacter, the impact force of the impacter and response characteristics of the cantilevered beam were analyzed by both numerical simulation and experiments. Since the stiffness and damping of the impacter are highly nonlinear, the contact model using revised Hertz-model was established by experiments. Also, the results of numerical analyses for the dynamic response and impact force of a cantilevered beam with an impacter have a good agreement with experimental results. It is believed that the mathematical model and the analytical method presented here can be used for the design analysis and the improvement of performances of the beam-shape switches and pickup devices.     

结合面法向和切向接触刚度的MPSO-BP神经网络算法的建模

提出一种基于改进的粒子群和BP神经网络相结合的算法模型,用改进粒子群优化算法优化BP神经网络参数。以机械结合面法向接触刚度和切向接触刚度作为算例。考虑结合面配对副材料、接触载荷、表面加工方法、表面粗糙度和结合面间的介质作为主要因素,对8组结合面法向和切向接触刚度进行预测建模,并对仿真与实验结果进行比较与误差分析。结果表明,该方法实现了多种影响因素组合下的机械结合面法向和切向接触刚度较高精度的建模和预测。     

机械结合面法向动态接触刚度理论模型与试验研究

机械结合面经常可能在动态条件下工作,使得结合面的接触状态偏离静态工作状态,导致接触刚度发生改变。为了揭示动态接触刚度的变化规律,考虑两个粗糙结合表面上单个微凸体由弹性变形向弹塑性变形以至最终向完全塑性变形转化的接触过程,建立一个振动周期内各变形阶段微凸体的平均接触刚度模型;在此基础上,基于高斯分布假设,建立整个粗糙结合表面的动态接触刚度解析模型。该模型揭示了接触面压、振动频率和相对位移振幅对动态接触刚度的影响规律,并与试验结果和静态接触刚度计算结果进行了比较。研究表明:法向动态接触刚度和静态接触刚度与接触面压之间的关系基本一致,但有一定偏离。这种偏离程度随动态位移幅值和振动频率的增加而分别呈线性和非线性增大。     

基于绝对节点坐标法的钢索-滑轮系统动力学模型与响应迟滞特性

钢索-滑轮系统间的非线性接触特性严重影响了长距离软式传动系统变形及动态响应迟滞效应的准确计算。基于绝对节点坐标法建立了三维高阶钢索单元模型。在此基础上,建立了钢索-滑轮系统的动态接触模型,应用Hunt-Crossley接触模型计算法向接触力,采用LuGre微变模型计算较低相对运动速度下的切向接触力,准确描述了钢索与滑轮接触由动摩擦向静摩擦的转变过程。仿真结果与准静态实验结果的对比验证了模型的准确性,分析了钢索材料参数和末端载荷对钢索-滑轮系统动态响应以及钢索末端位移的影响。结果表明,钢索-滑轮系统的动态响应延迟时间由钢索材料特性及末端载荷决定;钢索末端位移主要受钢索刚度及末端载荷影响。相同条件下采用小比重钢索材料,适当增加末端载荷以增大钢索结构刚度,可减小系统传动中钢索的末端位移,有效提高钢索-滑轮传动系统的传动性能。     

基于加速度变化率的轮胎等效侧偏刚度估计方法

等效侧偏刚度精确地反映了轮胎的侧偏特性,是汽车操纵稳定性研究中的重要参数。为此提出了基于加速度变化率的汽车轮胎等效侧偏刚度估计方法。采用三自由度汽车动力学模型,把前后轮侧向力的时间变化率表示为加速度变化率的函数。利用轮胎力学特性理论推导出瞬时等效侧偏刚度的估计公式,在汽车纵向加速度为稳态的条件下,利用轮胎侧向力的时间变化率和轮胎侧偏角的时间变化率来估算轮胎瞬时等效侧偏刚度。通过ADAMS软件的整车常半径转向虚拟试验对估计方法进行了验证。虚拟试验结果表明提出的方法具有较好的估计精度。     

Atomistic study of lateral contact stiffness in friction force microscopy

The effective stiffness of a friction force microscope tip–substrate system is an important parameter that describes the relationship between lateral force and elastic deformation. In this study, we use a multi-spring model to simplify the system, where two contributions, the tip apex stiffness and the lateral contact stiffness, are discussed in detail. Molecular dynamics simulations are used to characterize stiffness by simulating a tip apex subject to shear or sliding over a substrate surface. The results show that, although the height of the tip apex and tip–substrate orientation affect the various stiffness contributions, the contact itself dominates the overall compliance.     

考虑结合面法向刚度的拉杆转子轴向振动特性

针对接触刚度解析模型参数确定困难、精度难以保证等问题,提出了根据弹塑性粗糙表面微体单元受力变形的有限元分析结果确定轮盘结合面法向刚度的方法;为准确获取拉杆转子的轴向振动特性,建立了考虑轮盘结合面法向刚度的集中质量动力学模型;运用上述方法和模型计算了某型实验转子轴向振动的固有频率,并将结果与实测数据进行对比,误差低于5%,证明了该方法的有效性;改变拉杆预紧力,进一步研究预紧力对拉杆转子动力学行为的影响,结果表明：拉杆预紧力对转子的作用效果存在一个饱和区域,可为拉杆预紧力数值的确定提供重要的设计依据。     

具有微织构形貌的结合面法向接触刚度分形模型的研究

研究微织构结合面上的表面形貌参数对结合面法向接触刚度的影响。根据微织构表面的形貌特征,将微织构表面分为织构前表面和织构区域两部分,由分形接触理论计算出织构前表面上微凸体的基本参数,忽略织构区域底部未接触部分,将微凸体在接触载荷作用下的变形分为三个阶段:弹性阶段、弹塑性阶段、塑性阶段。由接触力学理论,首先建立织构前表面上单个微凸体的法向接触刚度模型。然后由微观到宏观,结合微织构表面的织构形貌特征,构造整个宏观微织构结合面的法向接触刚度计算模型,研究不同的表面形貌参数对于微织构界面上法向接触载荷、微凸体因载荷产生的变形以及法向接触刚度的影响。经过仿真分析之后,结果表明,当微织构结合面的法向接触载荷不断增大时,结合面的法向接触刚度总体呈单调上升趋势;并且随着织构密度的增加,结合面上由于织构形貌的存在,使得一部分微凸体未产生变形,从而减小结合面的法向接触刚度,并且当法向载荷增加时,这种效果会更加明显;在相同的接触载荷下,塑性指数越高,处于塑性变形状态的微凸体就越多,从而使具有微织构形貌的结合面的法向接触刚度变大。     

多平面锥形针接触建模与影响因素研究

针对平面锥形针在组织接触变形阶段受力变化难以分析的现状,根据弹性组织与针接触时的变形特点,建立了多平面锥形针的接触模型,并重点探讨了该阶段针的受力与穿刺性能问题。接触模型揭示了在穿刺深度相同的情况下,锥形针的接触受力受组织剪切模量、针形半顶角和针的平面数三个因素共同影响。通过实验对接触模型进行验证,发现实验曲线和理论曲线能够较好地吻合,表明该模型是有效可行的。由研究可得：为减小平面锥形针的接触受力、提高针的穿刺性能,可选择较小剪切模量的组织、减小针形半顶角或减少针的平面数。     

Modeling of the centerless infeed (plunge) grinding process

A computer simulation method for investigating the form generation mechanism in the centerless infeed (plunge) grinding process is described. For a 3-D simulation model of form generation, contact points are assumed to be on least squares contact lines at the grinding wheel, regulating wheel, and work-rest blade. Using force and deflection analyses, the validity of this assumption is shown. Based on the 2-D simulation model developed in the previous work and the least squares contact line assumption, a 3-D model is presented. To validate this model, simulation results were compared with the experimental works. The experiments and computer simulations were carried out using three types of cylindrical workpiece shapes with varying flat length. The experimental results agree well with the simulation. It can be seen that the effect of flat end propagated to the opposite end through workpiece reorientation.     

Investigation into the Normal Contact Stiffness of Rough Surface in Line Contact Mixed Elastohydrodynamic Lubrication

In this work, the statistical asperity microcontact models in combination with the acoustic spring model and the load sharing concept are utilized to study the interfacial normal contact stiffness for a rough surface in line contact elastohydrodynamic lubrication (EHL). Two different statistical microcontact models of Greenwood and Williamson (GW) and Kogut and Etsion (KE) are employed to derive the normal contact stiffness expressions for a dry rough line contact considering the purely elastic contact and the multiple regimes elastic–elastoplastic–fully plastic contact, respectively. The liquid film stiffness is calculated based on the relationship between film thickness and bulk modulus of the lubricant. The lubricant film thickness equations are employed in conjunction with the load sharing concept and the empirical formulas for the maximum contact pressure in a dry rough contact are fitted for the GW model and the KE model, to evaluate the relationship between film thickness and motion velocity for the purely elastic GW microcontact model and the multiregime KE microcontact model, respectively. The comparison with experimental results shows that the KE model predicts closer total contact stiffness results than the GW model. The stiffness contributions from the solid asperity contact and lubricant film are obtained and effects of surface roughness, applied load, motion velocity, and type of lubricant on the normal contact stiffness are analyzed.     

Dynamic analysis of planar mechanical systems with clearance joints using a new nonlinear contact force model

We investigated the dynamic behavior of planar mechanical systems with clearance joints. First, the contact effect in clearance joint was studied using a new nonlinear contact force model, and the rationality of this model was verified by the results of numerical simulations, which are based on a journal and bearing contact model. Then, the dynamic characteristics of a planar slider-crank mechanism with clearance were analyzed based on the new nonlinear contact force model, and the friction effect of clearance joint was also considered using modified Coulomb friction model. Finally, the numerical results of the influence of clearance size on the acceleration of slider are presented, and compared with the published experimental results. The numerical and experimental results show that the new nonlinear contact force model presented in this paper is an effective method to predict the dynamic behavior of planar mechanical system with clearance joints, and appears to be suitable for a wide range of impact situations, especially with low coefficient of restitution.     

考虑微凸体水平距离分布及相互作用的结合面法向接触刚度建模

在涉及微凸体侧接触的粗糙表面接触建模过程中,通常需要假定微凸体之间侧接触的角度分布规律。提出一种考虑微凸体水平距离分布及相互作用的结合面法向接触刚度建模方法,该方法不再需要假定角度分布规律,而是基于首次发现的单个粗糙表面微凸体水平距离正态分布规律,根据统计学理论进行考虑微凸体相互作用的结合面法向接触刚度建模。对模型进行数字仿真发现：结合面法向接触刚度与接触载荷均随着微凸体水平距离标准差的减小而增大,并且考虑微凸体相互作用会使得结合面的法向接触刚度减小。结合面法向接触刚度随弹性模量的增大而减小,随材料硬度的增大而增大。通过有限元仿真结果与模态试验结果对比可知,基于模型的有限元仿真前三阶固有频率与试验所得结果基本吻合,并且误差相对GZQ模型更小。旨在通过研究单个粗糙表面微凸体水平距离分布,突破侧接触建模时接触角度分布函数仍需假设的理论瓶颈,为更加准确地预测结合面接触特性奠定基础。     

Evaluating local elasticity of the metal nano-films quantitatively based on referencing approach of atomic force acoustic microscopy

Traditional technique such nanoindenter(NI) can’t measure the local elastic modulus at nano-scale(lateral). Atomic force acoustic microscopy (AFAM) is a dynamic method, which can quantitatively determine indentation modulus by measuring the contact resonance spectra for high order modes of the cantilever. But there are few reports on the effect of experimental factors, such length of cantilever, contact stiffness on measured value. For three different samples, including copper(Cu) film with 110 nm thickness, zinc(Zn) film of 90 nm thickness and glass slides, are prepared and tested, using referencing approach in which measurements are performed on the test and reference samples (it’s elastic modulus is known), and their contact resonance spectra are measured used the AFAM system experimentally. According to the vibration theory, from the lowest two contact resonance frequencies, the tip-sample contact stiffness is calculated, and then the values for the elastic properties of test sample, such as the indentation modulus, are determined. Using AFAM system, the measured indentation modulus of copper nano-film, zinc nano-film and glass slides are 113.53 GPa, 87.92 GPa and 57.04 GPa, which are agreement with literature values MCu=105-130 GPa, MZn=88.44 GPa and MGlass=50-90 GPa. Furthermore, the sensitivity of contact resonance frequency to contact stiffness is analyzed theoretically. The results show that for the cantilevers with the length 160μm, 225μm and 520μm respectively, when contact stiffness increases from 400 N/m to 600 N/m, the increments of first contact resonance frequency are 126 kHz, 93 kHz and 0.6 kHz, which show that the sensitivity of the contact resonance frequency to the contact stiffness reduces with the length of cantilever increasing. The novel method presented can characterize elastic modulus of near surface for nano-film and bulk material, and local elasticity of near surface can be evaluated by optimizing the experimental parameters using the AFAM system.     

Quantitative elastic modulus measurement by magnetic force modulation microscopy

It is experimentally demonstrated that magnetic force modulation microscopy (MFMM) is a technique allowing quantitative elastic modulus measurements. A model of the cantilever–tip–sample interaction taking into account the lateral contact stiffness (i.e., the friction effects at the level of the tip–sample contact), the position of the magnetic force applied to the cantilever with respect to the tip position, as well as the inclination of the cantilever arm with respect to the sample surface is presented. The model shows that MFMM is much less sensitive to lateral force than the other modulation techniques and thus, in contrast to the latter, that the contrast of the stiffness images can be interpreted as a true elasticity contrast and not as a mixture of friction and elasticity. Thanks to the study of the normal contact stiffness versus normal load that allows the characterization of contact between tip and sample, it is possible to perform quantitative elastic modulus measurements with a dynamic modulation method.