Diffusion as a model of formation and development of surface topography

Changes of surface topography in tribological systems are due to inelastic processes as plastic deformation, detaching of wear particles and their reintegration into the surfaces. Due to these processes, the material particles are transported either along a surface or from one tribological partner to the other. Both processes are due to random interactions between surface asperities and are stochastic processes. The stochastic mass transfer between the surfaces is interpreted and described in the paper as a random deposition, the transport along a surface as a `diffusion' processes with some effective diffusion coefficient. We consider the development of the surface topography due to the described two kinds of random processes. There exist some stationary (in statistical sense) random surface topography with a power spectrum (spectral density) typical for many real frictional surfaces. The parameters of the model can either be obtained from comparison with measured topography or extracted from simulations at a lower space scale. The proposed mass transport model further allows to determine the wear rate in the system.     

曲面模型重构方法比较及误差分析

三维CAD模型重建是逆向工程中的关键环节,在逆向设计中起到至关重要的作用。在运用Geomagic Studio软件进行点云处理、错误修复和曲面生成的基础上,重点讨论了曲面阶段的精确曲面重构和参数化曲面重构两种不同的曲面重构方法,并对同一种模型进行重构和误差分析。精确曲面重构方法可保证模型数据的完整性与精确性,从而生成高质量的模型;参数化曲面重构方法通过参数转换功能将初始曲面导入Solid Works软件中进行编辑,最后得到参数化模型。实际应用中可结合这两种方法各自特点及优势,高效优质地构建CAD曲面模型。     

具有复杂型面机架的快速曲面重构技术

零件的曲面模型是获得其逆向设计数据等后续处理的关键环节。为了快速获得具有复杂型面机架的设计数据,研究了该机架点云曲面重构的获取方法。鉴于机架不同部位对逆向设计精度要求不同,采用Imageware和Geomagic Studio软件对机架的销孔和外形点云分别进行曲面重构之后,再实施整个模型的曲面整合策略。介绍了应用两种软件完成机架曲面模型的主要处理过程及关键环节。该方法能快速获得符合精度要求的机架曲面模型,可行且高效,为机架曲面模型的实体化及其后续处理提供了可靠依据。     

A simple model for stability of a long axial surface crack in a thin walled cylinder

Growth in the thickness direction of a long axial surface crack at the inner surface of a thin walled cylinder has been analyzed for loads generated by internal pressure and a thermal gradient through the wall thickness. Plane-strain deformations have been considered. It has been assumed that the cracked cross-section is fully plastic, but that the plastic zone width in the circumferential direction is very small. The cracked cross-section transmits a normal force and a bending moment, which have been considered as external forces on an equivalent cut ring element, to compute the deformation of the cracked cross-aection. An analytical expression has been derived for the crack-opening-displacement, as a function of the loads and the crack depth. Stable and unstable crack growth have been investigated on the basis of a critical crack-opening-displacement and a smoothly rising crack-opening R-curve. The condition for unstable crack growth depends primarily on the magnitude of the internal pressure. A thermal gradient by itself is less likely to cause unstable crack propagation.     

Monte carlo calculations for the mechanical relaxation of a self-assembled monolayer and for the structures of alkane/metal interfaces

Configurational-bias Monte Carlo calculations have been used to study the mechanical relaxation of a monolayer of CH₃(CH₂)₁₅S admolecules on a gold substrate when subjected to indentation by a force-microscope tip. Compression leads to substantial but reversible changes in molecular tilt angle and chain conformations. The same Monte Carlo technique has been used to probe the structural properties of liquid hexadecane films in contact with a flat metal substrate.     

Friction and slip of a simple liquid at a solid surface

We report a novel experimental technique using total internal reflection – fluorescence recovery after photobleaching (TIR‐FRAP) to probe the velocity of a liquid near a wall with a resolution of the order of 100 nm. As an example of use, we have investigated the boundary condition of the liquid velocity during lubricated friction and studied the influence of a classical additive (stearic acid) in a base oil (hexadecane), and demonstrate that simple Newtonian fluids can develop slip at the wall. This revised version was published online in September 2006 with corrections to the Cover Date.     

Minimization of the error of reconstruction of a continuous periodic signal with uniform sampling

Various approaches to solving the problem of filtration of a continuous periodic signal distorted by stationary additive non-correlated noise are considered. Particular attention is paid to the case where the known signal period is non-divisible by the signal sampling period.     

BJ202X越野车虚拟造型的逆向重构方法比较

将CAS与CAD两类软件相结合,以STEP格式把BJ202X越野车的虚拟造型导入工程软件进行逆向重构,并对四种逆向重构方法进行了比较.重构的模型以STL格式与快速成型软件MAGICS直接对接进行切片和数字化快速成型,从而使得研发周期缩短,成本降低.由此为工程人员提供了一种在CA-TIA软件环境中替代传统油泥模型制作与实物逆向的繁杂工序,直接对虚拟造型进行软件逆向的新方法.     

Three-dimensional reconstruction of a freeze-dried and metal-shadowed bacterial surface layer

Three-dimensional reconstructions were performed from tilt series of a rotary-shadowed regular bacterial surface layer. Two series, representing the two surfaces of the HPI layer, were processed. The metal film stands out very clearly from the surrounding matter (protein, carbon) in the reconstructions and can, in good approximation, be regarded as reproducing the surface geometry of the protein layer. Thus, by combining information from both surfaces, it is possible to reconstruct profiles of the protein structure. Shadowing is advantageous in that it provides excellent reproducibility in specimen contrast, and the metal film is stable in the electron beam. Moreover, unlike negative staining, information is obtained from surfaces that are not in contact with the specimen support, and the technique is also applicable to freeze-dried material.     

Electron-acoustic and surface electron beam induced voltage signal formation in scanning electron microscopy analysis of semiconducting samples

The conditions for the detection of electron-acoustic (EA) and surface electron beam induced voltage (SEBIV) signals using a common sample mount and bottomside detection scheme are hereby discussed. It is shown that while the intrinsic properties of the sample under electron-beam irradiation would chiefly determine the presence of these contrast mechanisms, the manner in which the sample is mechanically and electrically configured in relation to the signal detection is crucial in determining the actual signal coupling mechanisms at work and hence the assumptions by which a robust and consistent interpretation of experimental results can be made. EA signals are detectable only if electrical coupling between the sample and the detector is defeated, a necessary pre-requisite as the signal magnitude of carrier-generated SEBIV coupling is 2-3 orders larger in most cases. With regards to SEBIV detection, bottomside SEBIV detection may be preferable to topside detection owing to minimization of topographic signal contribution, higher signal coupling efficiency and a less complex sample-detector mounting procedure.     

Chip formation in microscale milling and correlation with acoustic emission signal

This work investigated the effects of different workpiece materials on chip formation and associated mechanisms in microcutting. The wavelet transformation technique was used to decompose acoustic emission (AE) signals generated from orthogonal micromilling of different workpiece materials. This allowed studying energy levels corresponding to deformation mechanisms. Resulting chip forms were characterised. The results indicated that the computed energies of decomposed frequency bands can be positivity correlated with chip morphology. The work provides significant and new knowledge on the utility and importance of AE signals in characterising chip formation in micromachining. Understanding chip formation mechanisms is important in managing the size effect in micromachining.     

逆向工程与曲面重构技术

重点分析逆向工程中常用的三种曲面重构方法,即B样条曲面、NURBS曲面及三角Bézier曲面的构造理论及方法,并介绍获得高质量重构曲面所必须的几项关键技术,即曲面点云数据的获取技术、测量数据的分割技术,曲面拟合及光顺技术。     

Optimization of manufacturing cell formation with a multi-functional mathematical programming model

Cellular manufacturing (CM) has received considerable interest owing to its promising application as an appropriate manufacturing philosophy for just-in-time (JIT) and flexible manufacturing systems (FMS). Cell formation (CF), i.e. the grouping of parts with similar design features or processing requirements into part families and the corresponding machines into machine cells, is the first stage of designing an effective CM system.Numerous analytical approaches have been proposed to solve the CF problem. Among which, mathematical programming (MP) represents the large percentage of literature as heuristic procedures. However, the related MP models have not been synthesized and classified in a systematic manner for the novice. Furthermore, most MP models attempting to adopt large number essential features are extremely complicated. Therefore, in this work, we present a multi-functional MP model that incorporates the merits of related CF models based on the systematic study of MP models. The proposed model can offer the suitable modules that include the different objective functions and constraints for user to solve the related problem. In addition, analysis results demonstrate that the proposed model’s performance to outperform the other related models.     

A model of liquid-mediated adhesion with a 2D rough surface

A computational analysis is conducted on a solid–liquid–solid interface: a rough, elastic surface is brought into contact with rigid flat while being influenced by an intervening liquid film of fixed volume. The problem is solved (iteratively) by satisfying the equations of elasticity and capillarity, while conserving liquid volume and honoring the constraints of no surface interpenetration and no negative pressures at dry contact points. As the liquid volume is increased from low values, a critical condition is reached whereby there are sudden increases in the values of certain output parameters, such as tensile force and wetted length. For the smaller values of surface roughness, the critical point is associated with a near complete collapse of the surface topography.     

NURBS曲线曲面重构的方法

CAD/CAM中用有理多项式函数表示曲面越来越广泛。由于非均匀有理B样条(NURBS)可以精确表示解析形状和自由曲线曲面,国际标准组织(ISO)与1991年把NURBS作为表示工业产品几何形状的工业标准。这里主要讨论了NURBS曲线曲面重构的方法。     

Comparison of buoyancy and surface tension in a square cavity

Laminar natural convection induced by free surface temperature gradient has been investigated numerically. Buoyancy effect and surface tension effect were made to oppose against each other. For fluid of unit Prandtl number and cavity of unit aspect ratio, the flow field was examined to determine the relative importance of surface tension force and buoyancy force. The flow patterns of each regime are: free surface-concentrated streamlines for surface tension flows, opposite direction circulation for buoyancy driven flows, and surface tension cell above with buoyancy driven cell below for mixed flows. The borderlines of each flow regime were obtained by interpolation and the flow regime map was obtained.     

小波技术在自由曲面重构和光顺处理的应用研究

详细分析小波技术在自由曲面重构和光顺处理等方面的应用情况,指出小波技术在该领域应用的优势和不足。同时展望小波技术在三角域的应用。     

Microstructure alterations at the surface of a heavily corrugated rail with strong ripple formation

The topography, morphology, structure and residual stress state at the surface of a rail with severe longitudinal ripples was characterized using a combination of spectrometry, microscopy, scattering and diffraction methods. The investigations reveal that white etching layers develop at the ripple hills, while the ripple valleys contain a severely plastic deformed layer. While the structure in the ripple valleys remains pearlite, the structure of the white etching layers on top of the ripple hills is nanocrystalline martensite containing cementite particles of a few nanometer size. The microhardness of the white etching layers on top of the ripple hills is about three times higher than the microhardness in the ripple valleys. The microhardness throughout the white etching layer is not homogeneous due to the non-uniform amount of carbon in solute solution and also due to differences in the amount and the size of the fine carbide particles. The whole running surface is under biaxial compressive residual stresses. In the ripple valleys the direction of the principal residual stresses nearly coincides with the longitudinal and the transverse direction on the rail. On the ripple hills the compressive residual stresses due to the martensite formation are significantly higher and the stress state is nearly homogeneous.