An alternative finite element model for simulation of frictional gap

This study is an attempt to modeling a frictional gap in a crack closure process under compressive loading conditions in which the crack surfaces are in touch and the effects of friction between them are significant. An iterative finite element (FE) solution is developed to model a finite crack in an interfacial layer with varying material properties. A mere application of a Lagrange multiplier formulation (node-to-node, NTN, or node-to-segment, NTS) in a developed FE framework to fulfill the contact constraints between contacting surfaces is discussed which improves the penalty formulation used in ANSYS. We then argue that the penalty formulation allows for a certain amount of crack surface interpenetration whereas the Lagrange multiplier formulation fulfils the contact constraints more accurately. This technique is easy to implement and offers higher accuracy than the equivalent FE solution, available in commercial FE software such as ANSYS 9.0, to the same system.     

Field electron and ion emission from charged surfaces: a strategic historical review of theoretical concepts

The field-electron (FE) and field-ion techniques directly observe and measure atomic-level surface processes that occur in very high electric fields. In theoretical terms, the high fields put large additional terms into Hamiltonians and free energies, and significantly modify many aspects of the surface physics and chemistry, as compared with the field-free situation.This paper presents a strategic review of the fundamental science of some of these high-field surface effects and processes, as developed in the context of the field electron and ion emission techniques. It outlines the main theoretical concepts developed, notes some twists of scientific history, and suggests useful contributions made to mainstream science. Topics covered are basic aspects of FE emission, surface field ionisation, localised field adsorption, charged surfaces theory, field-ion image contrast theory and associated imaging-gas kinetics, field evaporation, and aspects of the thermodynamics of charged surfaces. Despite many years of effort, important aspects of the theory remain incomplete. Some theoretical challenges are noted.     

A comparison of methods to evaluate the behavior of finite element models with rough surfaces

Finite element (FE) modeling of rough surfaces is becoming increasingly common. However, the quality of the assumptions being made in these models, and thus the quality of the models themselves, is often unclear. Decisions about the geometry of the surface to be modeled, including the size of the surface to be modeled, the lateral resolution of the measured surface data to be used, and the formulation of the probabilistic surface to be used, can have a significant effect on a model's behavior. Similarly, varying model parameters, including the FE mesh density, can change the results by a factor of three or more. This work examines some of the metrics that can be used to evaluate the influence of these assumptions and parameters on FE models with rough surfaces and discusses the relative merits of each option. In particular, qualitative comparison of result plots, quantitative comparison and convergence of results parameters, qualitative and quantitative comparison of distributions of result values over various model dimensions, and more sophisticated comparison techniques inspired by image and signal processing are discussed.     

Numerical and finite element contact temperature analysis of real composite-steel surfaces in sliding contact

Numerical techniques have been developed and used to evaluate the contact temperature distribution between real composite-steel surfaces in sliding contact. To characterise the contact temperature problem of composite materials new definitions for composite Peclet numbers have been introduced. In case of `slow sliding' problems a stationary numerical technique was applied, whereas for `intermediate and fast sliding' problems transient finite element (FE) solutions were preferred. At first sliding contacts of a single steel asperity over polyetheretherketone (PEEK) or carbon fibre (CF)/PEEK composite surfaces were modelled in order to study the contact temperature development on a microscopic level. It was followed by contact temperature results for real composite-steel sliding surfaces; the latter helped to provide information about the actual stress conditions, which are necessary to model the wear process of this pair of materials in future works.     

Prediction of contact pressure, slip distance and wear in cold rotary forging using finite element methods

This paper aims to utilize finite element (FE) methods to predict the wear of dies and workpiece in cold rotary forging through systematically investigating the contact pressure and the contact slip distance. For this purpose, a 3D elastic-plastic FE model of cold rotary forging is developed using the FE code ABAQUS. Using this reliable 3D FE model, the evolution and the distribution of contact pressure and contact slip distance are investigated in detail. Consequently, a new insight into the wear that occurs at the surfaces of the dies and the workpiece is gained based on Archard's wear law.     

Prediction of contact pressure,slip distance and wear in cold rotary forging using finite element methods

This paper aims to utilize finite element (FE) methods to predict the wear of dies and workpiece in cold rotary forging through systematically investigating the contact pressure and the contact slip distance. For this purpose, a 3D elastic-plastic FE model of cold rotary forging is developed using the FE code ABAQUS. Using this reliable 3D FE model, the evolution and the distribution of contact pressure and contact slip distance are investigated in detail. Consequently, a new insight into the wear that occurs at the surfaces of the dies and the workpiece is gained based on Archard's wear law.     

Prediction of plastic deformation under contact condition by quasi-static and dynamic simulations using explicit finite element analysis

We compared the quasi-static and dynamic simulation responses on elastic-plastic deformation of advanced alloys using Finite element (FE) method with an explicit numerical algorithm. A geometrical model consisting of a cylinder-on-flat surface contact under a normal load and sliding motion was examined. Two aeroengine materials, Ti-6Al-4V and Super CMV (Cr-Mo-V) alloy, were employed in the FE analysis. The FE model was validated by comparative magnitudes of the FE-predicted maximum contact pressure variation along the contact half-width length with the theoretical Hertzian contact solution. Results show that the (compressive) displacement of the initial contact surface steadily increases for the quasi-static load case, but accumulates at an increasing rate to the maximum level for the dynamic loading. However, the relatively higher stiffness and yield strength of the Super CMV alloy resulted in limited deformation and low plastic strain when compared to the Ti-6Al-4V alloy. The accumulated equivalent plastic strain of the material point at the initial contact position was nearly a thousand times higher for the dynamic load case (for example, 6.592 for Ti-6Al-4V, 1.0 kN) when compared to the quasi-static loading (only 0.0072). During the loading step, the von Mises stress increased with a decreasing and increasing rate for the quasi-static and dynamic load case, respectively. A sudden increase in the stress magnitude to the respective peak value was registered due to the additional constraint to overcome the static friction of the mating surfaces during the sliding step.

     

Investigation on contact parameters in cold rotary forging using a 3D FE method

Cold rotary forging is an advanced but very complicated metal plastic forming technology with continuous local plastic deformation. Investigating the contact parameters between the contact interactions has a great importance to improve the contact performance. This paper is aimed to utilize the finite element (FE) method to study the contact parameters in a complex continuous local forming technology of cold rotary forging. For this purpose, a reasonable 3D elastic–plastic dynamic explicit FE model of cold rotary forging of 20CrMnTi alloy is developed using the FE software ABAQUS/Explicit. Based on this valid 3D FE model, some important contact parameters, such as the contact pressure, contact slip rate, contact slip distance, and contact force, are numerically examined in detail. The results of this research help to better understand the complicated contact mechanisms in cold rotary forging.     

Three-dimensional modeling of elasto-plastic sinusoidal contact under time dependent deformation due to stress relaxation

In the current work, the effect of stress relaxation in contact between sinusoidal surfaces is studied using FE simulations. There are a few works on the elastic and elasto-plastic contact between sinusoidal surfaces, but the transient effects such as creep and stress relaxation are not considered in these works. Stress relaxation causes significant change in the contact area and pressure between the contacting surfaces. The Garofalo formula is used to model the transient behavior of stress relaxation. The results for the contact area and contact pressure are presented and discussed. Empirical equations are developed to predict contact area and pressure by fitting to the FEM results. The equations are dependent on the initial surface separation, aspect ratio, and the Garofalo constants.     

叶片机械过盈配合的接触分析

叶片机械中广泛采用过盈配合技术将叶轮、轴套和轴联成一体,这是典型的三维多体接触问题。文中以某柴油机涡轮增压器的压气机叶轮为例,采用有限元参数二次规划法,并结合多重子结构技术分析求解叶轮与轴套、轴套与轴的过盈接触问题,利用JIFEX程序,针对不同的过盈量、摩擦因数和转速进行研究,获得叶轮、轴套与轴之间接触应力的相应分布规律,以为设计、制造参考。     

A Numerical Contact Model Based on Real Surface Topography

A numerical finite element contact model is developed to make use of the high precision surface topography data obtained at the nanoscale by atomic force microscopy or other imaging techniques while minimizing computational complexity. The model uses degrees of freedom that are normal to the surface, and uses the Boussinesq solution to relate the normal load to the long-range surface displacement response. The model for contact between two rough surfaces is developed in a step-by-step manner, taking into account the far-field effects of the loads developed at asperities that have come to contact in previous steps. Method accuracy is verified by comparison to simple test cases with well-defined analytical solutions. Agreement was found to be within 1 % for a wide range of practical loads for the high precision models. Applicability of extrapolation from lower precision models is presented. The real contact area estimates for micrometer-size tribology test machine surfaces are calculated and convergence behavior with mesh refinement is investigated.     

A new roughness parameter to evaluate the near-surface deformation in dry rolling/sliding contact

Within this work, a two-dimensional finite element model for rolling contact of a wheel on a rail is presented that accounts for the roughness of the contact surfaces. The rail material is modeled with elastic–plastic behavior. The maximum of the plastic shear strain is concentrated close to the surface of the rail and is mainly influenced by the surface roughness. A concept is proposed that demonstrates one crucial parameter of the roughness determines surface deformation (based on results of a sinusoidal roughness model). This roughness parameter depicts the ratio between asperity height and width. Numerical validation is achieved for predicting plastic shear strains in rough surfaces. The plastic shear strain is associated with surface damage, such as cracks and wear.     

Numerical Investigation on the Effects of Machining-Induced White Layer during Rolling Contact

It is well known that a thin phase-transformed white layer can be formed on component surfaces produced by hard machining. However, it is not clear as to how the white layer affects component performance, for example, in rolling contact fatigue. This study aims to determine the effects of white layer and associated residual stress on rolling contact stresses and strains. It is nearly impossible for an experimental study to identify the effects of white layer alone on rolling contact. Furthermore, small-scale contact stresses and strains (less than 30 μm) of the phase-transformed region are difficult to measure using the current experimental techniques. Therefore, a finite element analysis simulation model of rolling contact incorporating machining-induced surface integrity has been developed in this study. Three cases were investigated to decouple the effects of surface integrity factors: surface with white layer only, surface with residual stress only, and surface with white layer and residual stress. The simulation results show that distinct material properties of the white layer significantly influence the magnitudes and distributions of near-surface stresses and strains instead of those in the subsurface. Furthermore, it can be inferred that the white layer would affect near-surface fatigue damage instead of subsurface fatigue damage. The simulated near-surface fatigue damage mechanisms have been substantiated by the fatigue test data.     

Surface separation and contact resistance considering sinusoidal elastic–plastic multi-scale rough surface contact

The current work considers the multi-scale nature of surface roughness in a new model that predicts the real area of contact and surface separation as functions of load. This work is based upon a previous rough surface multi-scale contact model which used stacked elastic–plastic spheres to model the multiple scales of roughness. Instead, this work uses stacked 3D sinusoids to represent the asperities in contact at each scale of the surface. By summing the distance between the two surfaces at all scales, a model of surface separation as a function of dimensionless load is obtained. Since the model makes predictions for the real area of contact, it is also able to make predictions for thermal and electrical contact resistance. In accordance with concerns in previous works that the iterative calculation of the real contact area in multi-scale methods does not converge, this work not only tests but also gives conditions required for convergence in these techniques. The results are also compared to other existing rough surface contact models.     

Capacitive Imaging Technique for the Inspection of Composite Sucker Rod

Composite sucker rod has been extensively used due to its high strength, light weight and corrosion resistive nature. However, such composite sucker rod is diffcult for conventional non-destructive evaluation(NDE) techniques to inspect because of its complex material and/or structure. It is thus useful to embark research on developing novel NDE technique to comply the inspection requirement. This work demonstrates the feasibility of using the capacitive imaging(CI) technique for the inspection of composite sucker rod. Finite element(FE) models were constructed in COMSOL to simulate the detection of defects in the glass-fiber layer and on the carbon core surface. An FE Model based inversion method is proposed to obtain the profile of the carbon core. Preliminary CI experimental results are then presented, including the detection of surface wearing defect in the glass-fiber layer, and obtaining the profile of the carbon core. A set of accelerated aging experiments were also carried out and the results indicate that the CI technique is potentially useful in evaluating the ageing status of such composite sucker rod. The CI technique described in this work shows great potential to target some challenging tasks faced in the non-destructive evaluation of composite sucker rod, including quality control, defect detection and ageing assessment.     

基于接触单元的磨床螺栓连接面有限元建模与模型修正

以接触单元和弹簧－－阻尼单元建立了磨床螺栓连接件的结合面动力学有限元模型。并以前3阶试验模态及有限元模型理论计算的固有频率均方差最小为目标函数,对包含接触单元和弹簧－－阻尼单元的结合面特征参数进行优化,在此基础上修正并取得更为符合实际的结合面动力学模型,这对机床结构的动态优化设计具有重要意义。     

二维多粗糙峰涂层表面的弹塑性接触力学分析

应用有限元方法对二维多粗糙峰涂层表面的弹塑性接触力学行为进行了分析。对不同涂层材料弹性模量、不同屈服极限、不同涂层厚度及不同表面形貌的粗糙表面与刚性平面的弹塑性接触问题进行了模拟,分析了这几种因素对接触压力、接触面积、表面轮廓变形及应力场分布的影响。     

Parametric study on the fatigue life of railways under rolling contact fatigue by three-dimensional numerical analysis

The current paper presents the results of parametric analyses on the stress intensity factor (SIF) of railways with inclined cracks under rolling contact fatigue (RCF). A 3D finite element (FE) model was proposed to demonstrate the shear mechanism in RCF. The feasibility of the suggested numerical model was verified through the SIF (K) obtained from advanced 3D FE analysis compared with existing 2D FE results. Based on the series of FE analyses, the sensitivity analysis on the cracked depth, surface/crack friction coefficients, and inclined angle, which mainly affected SIF history at the cracked tip, was examined. SIF distributions for various locations of the wheel along the cracked tip were also presented.     

Contact and thermal analysis of transfer film covered real composite-steel surfaces in sliding contact

For composite-steel surfaces in sliding contact an anisotropic numerical contact algorithm has been developed to study the ‘layer type’ problems. An FE contact analysis was applied to evaluate the contact parameters (real contact area, contact pressure distribution and normal approach). The contact temperature rise was determined by using both a numerical thermal algorithm for stationary and a FE transient thermal technique for ‘fast sliding’ problems.The effect of a continuous transfer film layer (TFL), that had built up during wear of the PEEK matrix material on the steel counterpart, was considered. Its thickness was assumed to be t=1 μm, and its material properties were that of PEEK at room temperature or, in the case of frictional heating, at a temperature of 150°C (i.e. above the glass transition temperature of the polymer matrix).Results are presented for a spherical steel asperity, with/without TFL, sliding over composite surfaces of different fibre orientation, and in addition, for real composite-steel surfaces (based on measured surface roughness data) in sliding contact. The TFL has an effect on the contact parameters especially at higher operating temperatures (i.e. 150°C); it results in the production of a larger contact area and a lower contact pressure distribution. The contact temperature rise is clearly higher if a TFL is present. Due to the low thermal conductivity of PEEK, the TFL is close to the melting state or it even gets molten within a small vicinity of the contact area.     

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).