双正交提升小波在非对称表面形貌分析中的应用

由平顶珩磨获得的表面形貌是非对称的、多尺度的,而传统滤波函数处理这类表面形貌时存在畸变效应.利用双正交小波的优点,使用提升方案,采用边界对称延拓策略,解决了传统滤波函数在突变处和边界处的畸变效应,并以缸套内表面为例,选用bior4.4双正交小波,证明了提升双正交小波在表面形貌分析中的改善作用.     

A statistical model of elasto-plastic asperity contact between rough surfaces

This work models statistically elasto-plastic contact between two rough surfaces using the results of a previous finite element analysis of an elasto-plastic sphere in contact with a rigid flat. The individual asperity contact model used accounts for a varying geometrical hardness effect that has recently been documented in previous works (where geometrical hardness is defined as the uniform pressure found during fully plastic contact). The contact between real surfaces with known material and surface properties, such as the elastic modulus, yield strength, and roughness are modeled. The asperity is modeled as an elastic-perfectly plastic material. The model produces predictions for contact area, contact force, and surface separation. The results of this model are compared to other existing models of asperity contact. Agreement exists in some cases and in other cases it corrects flaws, especially at large deformations. The model developed by Chang, Etsion and Bogy is also shown to have serious flaws when compared to the others. This work also identifies significant limitations of the statistical models (including that of Greenwood and Williamson).     

粗糙表面弹流润滑的摩擦系数的分析计算

本文在弹性流体动力润滑情况下对摩擦系数的计算方法进行了分析,考虑了流体的非牛顿流变特性和微凸体接触压力的影响。以平均型条纹粗糙表面为计算模型,在各种工况下对摩擦系数进行了计算对比。     

基于冗余提升形态小波的非对称表面特征提取研究

针对提升小波提取非对称表面形貌特征时存在平移变动性和较差的方向选择性,将具有平移不变性和良好的方向选择性的冗余提升形态Haar小波应用到非对称表面的特征提取中。开展了该形态小波对具有显著特征的非对称平顶珩磨的缸套内表面进行特征提取的分析;同时结合仿真的方法,研究了该形态Haar小波对一个模拟的具有一定长度沟槽特征的空间表面进行特征提取的效果。研究结果表明,冗余提升形态Haar小波对非对称表面形貌进行形态特征提取时,在各尺度重构的信号沿特征边缘几乎没有变动且无畸变,可实现准确提取。     

Frictionally-excited thermoelastic contact of rough surfaces

Frictional sliding contact between two elastically similar half-planes, one of which has a sinusoidally wavy surface, is studied in the full-contact regime. The steady-state regime is evaluated, within the limits imposed by the well-known phenomenon of thermo-elastic instability (TEI). TEI gives a critical speed whose value depends on the wavelength of the perturbation, and above which the perturbation itself grows arbitrarily with time. It is found that the TEI critical speed, V_cr, is clearly identified by the steady-state solution only in the special and limiting case when the flat half-plane is non-conductor; in that case, V_cr is the speed for which the steady-state predicts infinite amplification. In all other cases, V_cr (appropriate to the wavelength of the profile) does not correspond to infinite amplification, nor to the maximum one, V_M. In the limiting case of thermoelastically similar materials, not only the system is unconditionally stable (V_cr=∞) for f H₁<0.5, where f is the friction coefficient and H₁ a certain thermoelastic constant, but the regime at the maximum amplification is also always stable, and arbitrarily large amplification is obtained for f H₁ tending to infinity. However, it is found that in most practical cases of braking systems, V_crV_M, and so the limiting conditions are reached at V_cr. At this speed, the amplification is typically not extremely high.     

Third generation wavelet for the extraction of morphological features from micro and nano scalar surfaces

This paper investigates wavelet theory and techniques for the extraction of morphological structures from micro/nano scalar surfaces. It will significantly extend the authors’ previous accomplishments in the programme (EU: SMT CT98-2209). This previous work identified problems in the extraction of morphological structures that requires further study. Under a new EPSRC found project, the present work tries to overcome these problems and facilitates more practical and feasible methodologies for the identification and extraction of morphological structures on micro/nano scalar surface textures.In the current work, a complex wavelet model is introduced to solve the problems of nanometre roughness analysis of surface texture with linear features and curve-like features, in which small shifts of the input signal can cause large variations in the distribution of energy between wavelet coefficients at different scales. As a result, linear and curve-like features on a surface topography can be identified and extracted with refined accuracy. Case studies are conducted using automotive cylinders and femoral heads to demonstrate the application of using the new wavelet model in the assessment of surface topography.     

Stress history in rolling–sliding contact of rough surfaces

An investigation into the non-Hertzian, elastic stress history, due to the contact of two rough surfaces is presented. A complex evolution of stress is produced whose magnitude and rate depend strongly upon the roughnesses and speeds of the contacting bodies. The key features of the stress fields are illustrated by plots of stress versus time and horizontal distance, for a range of depths and for various contact conditions. The stresses near the surface are many times higher than in an equivalent smooth contact and the roughness on thecounterface generates a moving stress field which, when sliding is present, greatly increases the number of cycles of stress during each passage of the contact. This may account, in part, for the observation that the rolling fatigue life of hard steels declines more rapidly with sliding speed for rough, than for smooth surfaces and suggests that counterface roughness is especially important in determining the fatigue life.     

Simulation of rough surfaces with FFT

A numerical procedure for the simulation of three-dimensional surfaces has been developed. The method is based on Fast Fourier Transform (FFT). This method can simulate surfaces with given spectral density or auto-correlation function (ACF). Although this method cannot guarantee that each profile of the generated surface has the correct ACF, the average ACF of profiles for the generated surfaces is very close to the given one.     

Abrasive wear between rough surfaces in deep drawing

In tribology, many surface contact models are based on the assumption that surfaces are composed of a collection of small asperities of which the tips are equally sized and spherically shaped and have some kind of statistical height distribution. This approach was used in 1966 by Greenwood and Williamson and was successfully followed by many researchers during the following decades. The statistical representation of surface topography enables calculation of contact forces and asperity deformations with reasonable accuracy using well established equations. Although this approach has proven to be suitable for static contact situations, alternative representations of the surface topography are required when modelling abrasive wear. In the current work an elastoplastic contact model is developed in which a representation of the surface topography is obtained by best fit approximations of the micro-contacts, obtained from real, measured surface height data. In this deterministic surface representation, the tips of the contacting asperities are assumed to have an ellipsoidal shape. Given the material parameters and contact conditions, the load and deformation of a single asperity can be computed. Subsequently, the wear induced by each individual asperity is obtained by inserting its size and shape and the conditions into a “single asperity micro-abrasion model”. By summing the contributions of all individual asperities, the total abrasive wear volume is obtained. The results of the developed abrasive wear model are compared with results obtained using a statistical approach.     

随机型条纹粗糙表面的弹流润滑

本文在不同的工况及形貌参数下，用Ｎｅｗ－ｔｏｎ—Ｒａｐｈｓｏｎ完全数值解法，对任意条纹方向两维粗糙度的随机雷诺方程进行了数值求解，讨论了表面粗糙度及其纹路方向对机械零件润滑状态的影响。     

混粉电火花技术在粗加工中的应用研究

在对混粉电火花加工机理进行系统研究的基础上 ,对混粉电火花在粗加工中的加工效率和加工表面粗糙度进行实验研究。结果表明 ,通过合理选择放电参数 ,混粉电火花在相近加工表面粗糙度下 ,能够显著提高加工效率 ,从而为混粉电火花加工技术在粗加工中应用提供依据     

A numerical model of friction between rough surfaces

This paper describes a computational method to calculate the friction force between two rough surfaces. In the model used, friction results from forces developed during elastic deformation and shear resistance of adhesive junctions at the contact areas. Contacts occur between asperities and have arbitrary orientations with respect to the surfaces. The size and slope of each contact area depend on external loads, mechanical properties and topographies of surfaces. Contact force distribution is computed by iterating the relationship between contact parameters, external loads, and surface topographies until the sum of normal components of contact forces equals the normal load. The corresponding sum of tangential components of contact forces constitutes the friction force. To calculate elastic deformation in three dimensions, we use the method of influence coefficients and its adaptation to shear forces to account for sliding friction. Analysis presented in Appendix A gives approximate limits within which influence coefficients developed for flat elastic half-space can apply to rough surfaces. Use of the method of residual correction and a successive grid refinement helped rectify the periodicity error introduced by the FFT technique that was used to solve for asperity pressures. The proposed method, when applied to the classical problem of a sphere on a half-space as a benchmark, showed good agreement with previous results. Calculations show how friction changes with surface roughness and also demonstrate the method's efficiency.     

Modeling and simulation of AFM cantilever with two piezoelectric layers submerged in liquid over rough surfaces

In this paper, the vibration of an atomic force microscope (AFM) cantilever in tapping mode with two whole piezoelectric layers submerged in liquid medium is investigated. In the performed modeling, the sample surface has been considered as rough, and to show these surface asperities, two models of Rumpf and Rabinovich have been employed for analyzing the attractive Van der Waals force. This paper has been organized in two sections. The first section deals with the functioning of cantilever over rough surfaces, which accompanies the changes of the attractive Van der Waals force, and the second section involves the changes in the Van der Waals force which lead to a change in the liquid medium. The cantilever is totally submerged in the liquid. To show the effect of liquid on cantilever, first, only the cantilever tip is immersed in the liquid and it is dynamically analyzed. Then, the cantilever is totally submerged and then taken out of the liquid, so that the additional mass and damping of the cantilever could be calculated. In these two manners of cantilever immersion in liquid, the effects of the added mass and damping on the cantilever can be measured. When a cantilever vibrates totally in liquid, since the mass and damping of the liquid that is present on the cantilever cannot be determined, first, the cantilever's natural frequency in liquid is estimated in the laboratory and then by using this frequency and the cantilever stiffness (which is not medium-dependent and is always considered as constant), the additional mass and damping of the cantilever are determined.     

Modeling of fretting wear evolution in rough circular contacts in partial slip

This paper presents a numerical model that maps the evolution of contact pressure and surface profile of Hertzian rough contacting bodies in fretting wear under partial slip conditions. The model was used to determine the sliding distance of the contacting surface asperities for one cycle of tangential load. The contact pressure and sliding distance were used with Archard's wear law to determine local wear at each surface asperity. Subsequently, the contact surface profile was updated due to wear. The approach developed in this study allows for implementation of simulated and/or measured real rough surfaces and study the effects of various statistical surface properties on fretting wear. The results from this investigation indicate that an elastic–perfectly plastic material model is superior to a completely elastic material model. Surface roughness of even small magnitudes is a major factor in wear calculations and cannot be neglected.     

Analysis of plane and rough contacts, subject to a shearing force

We analyse the surface traction conditions induced in plane contact between two bodies whose surfaces are rough. It is assumed that the roughness may be idealised by a surface of regularly spaced cylindrical “bumps”, and the overall geometry may be in the form of a cylinder, flat ended punch or wedge. The stick-slip regime experienced by each individual asperity contact is found, and hence it is shown how the applied shearing force produces concentrated regions of surface damage. Conditions for crack initiation are then discussed, and compared with equivalent results found for nominally smooth contacts.     

Elastic contact between rough surfaces: Effect of roughness at large and small wavelengths

The area and pressure distribution in elastic contacts between frictionless, nonadhesive surfaces is studied as a function of load and surface geometry using finite element calculations. Surfaces that follow self-affine scaling on all resolved scales are compared to surfaces with cutoffs at small and large length scales, and experimental surfaces that are not self-affine. In all cases the true area of contact is proportional to load and inversely proportional to elastic modulus and the mean slope of the surface. The constant of proportionality κ is nearly constant and lies between analytic predictions. Large wavelength cutoffs lead to a small increase in κ, a homogeneous distribution of contacts at large scales, and limit the size of the largest connected regions. Small wavelength cutoffs lead to local redistributions in pressure that decrease the probability of low and high local pressures.     

基于提升小波和PNN的三容水箱故障诊断

针对三容水箱是典型的非线性多变量耦合系统,提出将提升小波和概率神经网络相集合(LWPNN)方法用于三容水箱液位故障诊断.首先,建立三容水箱数学模型以获取实验数据,之后利用提升小渡对数据快速降噪,最后通过概率神经网络进行故障分类.实验结果表明,LWPNN集合方法较传统PNN故障诊断方法有较高的故障诊断率,是一种有效的故障...     

Contact analysis of three-dimensional rough surfaces under frictionless and frictional contact

A methodology for surface and sub-surface stress calculation of nominally flat on flat rough surface contact has been developed. This methodology is applicable for both large area contact (Hertzian contact) and small area of asperity contact (point load contact) with and without surface friction. A total of nine rough surfaces are generated by the computer with specified standard deviation of surface heights, σ, of 0.3, 1.0 and 3.0 nm, and correlation length, β^*, of 0.1, 0.5 and 0.9 μm. Under the typical applied load at the magnetic head slider-disk interface, small numbers of contact points are obtained and the deformation is purely elastic. Since these contact points are scattered and isolated, asperity contact behaves like point load contact. As β^* becomes larger, more adjacent points will be in contact at a certain contact spot and this is especially true at small σ. All the cases of flat on flat rough surface contact yield maximum von Mises stress on and near the surface at both frictionless and frictional contacts; no local maximum occurs in the sub-surface. In general, the friction effect in the vicinity of contact point is to increase the stress magnitude, while outside this region it also alters the stress distribution. For a surface of small β^* and large σ at high load of 1000 times of the nominal pressure at the head-disk interface, the contact pressure reaches the hardness at a few contact points and plastic deformation takes place in the near surface.     

A model for the liquid-mediated collapse of 2-D rough surfaces

Small-scale devices are particularly vulnerable to adverse effects of adhesion because of large surface area to volume ratios. Additionally, small gaps can be easily bridged at high humidity or when there are other contaminant liquids present. The bridging of a portion of an interface by a liquid droplet of given volume tends to pull surfaces in closer proximity due to the sub-ambient pressures that arise. In turn, the area spanned by the bridge will increase and lead to a greater adhesive force. A previous study considering a liquid bridge between two elastic half-spaces initially separated by a uniform gap demonstrated that, under certain conditions, an instability will arise whereby the surfaces come into full contact. Owing to the regularity of the geometry considered, the problem was amenable to a semi-analytical treatment and the stability condition was expressible analytically in terms of a single dimensionless group. In the present work, we develop a model to include the effects of surface roughness. The influence of asperities on the surface is treated by means of a recently developed multi-scale model that considers the full range of wavelengths comprising the surface profile. In the simulations, two nominally flat rough surfaces are brought together under a prescribed load and a liquid bridge of given volume is introduced in the contact, the initial areal coverage being determined by the initial mean surface separation. The interface configuration is then iterated until a configuration is found that satisfies the equations of elasticity and capillarity for the given liquid volume. As a result of the simulation, critical values are found for combinations of parameters that delineate stable and unstable conditions.     

基于提升小波的颤振图像边缘检测方法研究

这里基于提升小波理论重点研究了图像边缘检测问题，提出了一种新的图像多尺度边缘检测方法，该检测方法适用于多种线性或非线性双正交小波，具有运算速度快、检测精度高等特点；并基于此边缘检测技术，研究了飞机结构颤振的边界预测问题，给出了一种新的稳定性参数估计方法。