Prediction of fretting wear using boundary element method

The simulation method of the fretting wear prediction using boundary element method is developed. The contact pressure and the contact width which is the first step to predict fretting wear are obtained from contact analysis of a semi-infinite solid based on the use of influence functions and patch solutions. The geometrical updating is based on nodal wear depths computed using Archard’s equation for sliding wear. The prediction of fretting wear for two cases of contact problems is performed; one is two-dimensional cylinder on flat contact which is for the comparison with a previous model by finite element method; the other is three-dimensional spherical contact. It is observed that for two-dimensional cylindrical contact the boundary element method developed in this study reduced the calculation time by 1/48 compared to FE method. We also showed the use of developed simulation technique is efficient to predict the fretting wear for three-dimensional spherical contact.     

电连接器微动磨损的超声识别与性能退化模型

针对振动环境下电连接器易产生微动磨损而接触性能降低这一问题，采用超声检测方法监测微动磨损过程中电连接器接触件间磨屑的特征值，研究了不同振动条件下接触件的磨损程度及接触性能的退化规律。结果表明，振动频率、振动加速度和振动次数对接触面磨屑的堆积和接触电阻的波动都有正向累积效应；电连接器轴向振动时，磨屑累积效应最为明显。接触电阻和磨屑特征值总量在高振频及加速度下呈现极高的相关性。以磨屑特征值构建的麻雀搜索算法优化BP神经网络性能退化模型的平均绝对误差小于5%。     

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.     

微动疲劳参数对钢丝微动疲劳磨损演化的影响*

微动疲劳易引起钢丝表面磨损和横截面积损失,进而造成钢丝断裂失效并缩短钢丝绳使用寿命。不同微动疲劳参数（接触载荷、疲劳载荷、钢丝直径和交叉角度）引起差异的钢丝微动疲劳磨损特性,故研究微动疲劳参数对钢丝微动疲劳磨损演化规律影响至关重要。基于摩擦学理论和Marc仿真软件构建钢丝微动疲劳磨损模型,探究接触载荷、疲劳载荷、交叉角度和钢丝直径对钢丝微动疲劳磨损演化的影响规律。结果表明：钢丝微动疲劳磨损体积主要与接触载荷和疲劳载荷有关;疲劳钢丝的磨损深度、磨损率及磨损体积随着接触载荷的增加而增大,且不同接触载荷下疲劳钢丝磨损体积均随着循环次数的增加而呈线性增加;随疲劳载荷幅值的增加,疲劳钢丝的磨损深度、磨损率及磨损体积均呈增加趋势;在不同疲劳载荷范围下疲劳钢丝的磨损体积均随着循环次数的增加而呈线性增加;当接触载荷、疲劳载荷及钢丝间摩擦因数相同时,不同交叉角度和不同加载钢丝直径下疲劳钢丝的磨损体积相同。     

Experimental investigation on sliding and fretting wear of steam generator tube materials

In nuclear power steam generators, high flow rates can induce vibration of the tubes resulting in fretting wear damage due to contacts between the tubes and their supports. In this paper, the sliding and fretting wear tests were performed using Inconel 600HTMA and 690TT against STS 304, which are the steam generator tube materials. The sliding wear tests with a pin-on-disk type tribometer were carried out under various applied loads and sliding speeds at air environment. The fretting wear tests were carried out under various vibrating amplitudes and applied normal loads.

The result of sliding and fretting wear tests show that the heat-treated Inconel 690TT has better wear resistance than Inconel 600HTMA in air. The fretting wear regimes were plotted using the test results and the wear coefficient was calculated also. From the results, it was observed that the wear and tear by stick-slip has very strong effect on the fretting wear behavior.     

A comparative study of different wear measurement techniques applied to fretting wear of a spherical against a flat steel specimen

One of the main problems in fretting wear research is accurate measurement of the very small wear volumes (in microgams) produced. Different techniques, with their own advantages and disadvantages, can be used. Classic wear techniques lack sensitivity and often fail to measure the fretting wear volumes at all, or are not accurate enough. The authors have developed a Thin Layer Activation (TLA) method for quantification of the fretting wear of steel surfaces. The same method can also be used to determine material transfer between wear specimens. In this paper, the different methods are reviewed, and their accuracy in terms of the fretting wear measurement of steel surfaces is discussed. It is concluded that TLA has relatively good reproducibility and accuracy in comparison with classic fretting wear measurement techniques, such as normal approach measurement and spherical cap modelling. Modern threedimensional profilometry has better reproducibility than TLA, but is afflicted by systematic errors because of the oxidation of the metal surface. The different methods emphasise different features of the fretting process, and it is advisable to use them together for measuring fretting wear; their combined use provides a better understanding of the fundamentals of the fretting wear process.     

Fretting damage modeling of liner-bearing interaction by combined finite element – discrete element method

This paper presents a methodology to study the fretting damage behavior by combined finite element-discrete element method based on FFD method and cut boundary displacement method. A discrete element modeling technique is developed. An inter-element contact constitutive model and its microscopic parameters are determined and calibrated to reproduce continuous and discontinuous behaviors of material in DEM. A crack visualization technique is developed to display cracks according to their failure mode and time dependency. The effect of fretting condition on crack initiation and propagation as well as fretting damage is studied. The mechanism of fretting wear is investigated.     

Effect of contact pressure on fretting fatigue of austenitic stainless steel

The effect of contact pressure on fretting fatigue in solution-treated austenitic stainless steel was studied. With an increase in contact pressure, fretting fatigue life was almost unchanged at low contact pressures, however it decreased drastically at high contact pressures. At low contact pressures, stress concentration due to fretting damage occurred at the middle portion of the fretted area and the main crack responsible for failure was initiated there. At high contact pressures, concavity was formed at the fretted area without accompanying heavy wear. The main crack was initiated at the outer edge corner of the concavity which probably acted as a notch. Plain fatigue prior to the fretting fatigue test increased the fretting fatigue life at high contact pressures since the concavity formation was suppressed by the cyclic strain hardening.     

A fast and efficient contact algorithm for fretting problems applied to fretting modes I,II and III

A computational contact algorithm is presented to solve both the normal and tangential contact problems that describe fretting contacts between two elastic half-spaces. The coupling between the normal and tangential contact problems can or not be taken into account. Nevertheless the coupling should be introduced when materials are dissimilar. Fast and efficient methods are used. The contact solver is based on a conjugate gradient method and acceleration techniques based on the Fast Fourier transforms (FFT) are employed. Very good agreements are found with analytical solutions of three fretting examples representing each fretting mode. However it is shown that these analytical solutions are based on approximations that can be too strong when materials are dissimilar.     

关于微动磨损与微动疲劳的研究

微动磨损与微动疲劳是２种主要的微动模式,造成的损伤在工业中相当普遍,并可能引发灾难性的后果。主要研究了们移幅度、压力和疲劳应力３个基本微动参数,并以获得的微动区域、微动图为基础,分析了微动磨损与微动疲劳的运行机制和破坏规律。为更好地了解微动磨损与微动疲劳之间的内在联系,进一步探讨了接触磨损与局部疲劳、局部疲劳与整体疲劳之间的竞争机制。     

Finite Element Modeling and Experimental Validation of Fretting Wear Scars in a Press-Fitted Shaft with an Open Zone

A finite element (FE)-based method was developed for simulating the fretting wear scar in a press-fitted shaft with an open zone. The method is based on the energy wear approach and is implemented via the commercial FE code ABAQUS. The effects of open zone, mesh size, cycle jumps technique, and increments per fretting cycle were investigated for optimization of this methodology. The results show that when assuming that the surface profile can be changed only in the open zone, the FE wear model gives a good prediction of the scar width. The mesh size has a great influence on the dimensions and shape of the scar profile; when the mesh size is about 3% of the width of the wear scar for a press-fitted shaft, the best compromise between the wear scar shape and the computational time can be achieved. For the cycle jump ΔN, an optimum value of 3,000 is found; above this value, the depth of the fretting wear scar increases rapidly with increasing ΔN. The impact of increments per fretting cycle on the depth of the predicted wear scar is small by comparison with cycle jump ΔN. The results of the optimized model are validated with respect to the experimental data obtained in the interrupted fretting fatigue tests. The FE wear model can provide an accurate prediction of the maximum wear depth and the width of fretting wear scar. The predicted wear depth inside of the contact is slightly larger than that found experimentally.     

Research on torsional fretting wear behaviors and damage mechanisms of stranded-wire helical spring

Wear on the local area of steel wires’ surface is attributed to torsional fretting on the working process of stranded-wire helical spring. A mathematical model to calculate normal contact force and angular displacement amplitude among the wires is established first when the spring is impacted. With the experimental parameters obtained from the model, the torsional fretting test, which stimulates torsional fretting among the wires in the working process of the spring, is realized successfully on a newly developed fretting tester. Torsional fretting behaviors are strongly dependent upon normal contact force, angular displacement amplitude, and number of cycles. There are three basic types of T-θ curves (short for torque), angular displacement curves during the process of torsional fretting, namely, parallelogram, elliptic, and linear T-θ curves. To analyze the damage mechanisms, distribution maps of oxygen in the wear scar of spring wires under different working conditions are revealed. The damage gets slight in the partial slip region, mainly with the abrasive wear and the slight oxidative wear, whereas the wear mechanisms are mainly the abrasive wear, the oxidative wear, and the delamination, accompanied with obvious plastic deformation in the mixed fretting and slip regions.

     

Evaluation of fretting wear based on the frictional work and cyclic saturation concepts

It is time consuming or even impossible to simulate the whole process of fretting wear, since it always involves millions of cyclic loadings. This paper focuses on the modeling and evaluation method of fretting wear for the typical bridge type fretting test with a flat pad. The frictional work on the contact interface is chosen as the parameter to evaluate the fretting wear. To verify the fretting wear model, the predicted wear profile is compared with that obtained by the experimental results. Fretting wear always includes plastic deformations due to the edge stress singularity. The effect of cumulative plastic deformation is also taken into account in the wear model. The role of the coefficient of friction at the contact interface on the fretting wear has also been discussed.     

The role of plastic anisotropy deformation in fretting wear predictions

The deformation occurring under fretting conditions occurs over length scales of the same order as the grain size, so the plastic anisotropy plays a significant role in the very local region near the contact edge during fretting process. The present study first describes plastic anisotropy by unified anisotropy plastic model coupling with Archard's wear law on the fretting behavior incorporating the effect of wear debris into such a quantitative model. The finite element method, utilizing this model, is used to analyze gross slip fretting conditions. The implementation of the wear simulation tool together with anisotropy cyclic plasticity analysis during fretting process is applied to the wear depth simulation. The present study validates the experiment phenomena from numerical simulation that failure location of the specimens under the flat-on-flat configuration is very close to the trailing edge. The scar at the trailing edge is much deeper than any other locations and the larger relative slip range resulted in considerably deeper surface damage. Another interesting discovery is that when material with different orientations the degree of wear also develops differently and the quantitative prediction is given.     

Three dimensional fretting wear analysis by finite element substructure method

In steam generator of nuclear power plant, flow induced vibration in a U-tube bundle could cause wear and fatigue failure. This vibration causes fretting-wear between the supporting plates and tubes by generating infinitesimal friction. In this study, a substructure method is developed for three dimensional finite element models of fretting wear problems and its feasibility is also verified. The substructure method can reduce large amount of computation time required by conventional finite element analysis.     

Experimental Study of the Fretting Wear Behavior of Incoloy 800 Alloy at High Temperature

The fretting wear behavior of the nuclear power material Incoloy 800 was investigated in this study. A PLINT high-temperature fretting tester was used on an Incoloy 800 cylinder against a 304SS cylinder at vertical cross contact under different temperatures (25, 300, and 400°C). During testing, a normal load of 80 N was applied, and the displacement amplitudes ranged from 2 to 40 µm. The fretting wear mechanism at high temperatures and the kinetic character of the materials of the Incoloy 800 steam generator tube were analyzed. Results showed that the fretting running regimes varied little with ncreasing temperature, and some microcracks were observed in both the mixed fretting regime (MFR) and the partial slip regime (PSR) at high temperatures. Slight abrasive wear and microcracks were the main wear mechanisms of the Incoloy 800 alloy in PSR, whereas those in the MFR and the gross slip regime were oxidative wear, abrasive wear, and delamination.     

690合金材料的微动磨损特性研究

利用微动磨损试验机,在载荷50N以及位移幅值为60μm、100μm、150μm的工况下,研究了690合金材料在常温下的微动磨损行为及其动力学特性,采用激光共焦扫描显微镜(LCSM)和扫描电子显微镜(SEM)观察磨痕微观形貌。结果表明,载荷和位移幅值对微动特征有很大的影响,微动运行完全处于滑移状态。在滑移区,滑移磨损严重、磨痕面积大。690合金材料的磨损机制主要表现为磨粒磨损与剥层的共同作用。     

蒸汽发生器传热管湍流激励功率谱密度计算

湍流激励是核蒸汽发生器换热管发生流致振动的重要机理之一,也是微振磨损的重要诱因。为计算换热管的微振磨损速率,需要首先获得湍流激励功率谱密度（PSD）,其此前只能通过试验的方法获取。为利用CFD方法获得换热管上的湍流激励功率谱密度,建立了蒸汽发生器换热管束流场的数值模型,基于LES方法计算得到了管束内部的流场分布,提取了传热管上所受流体力并计算得到了其功率谱密度,计算结果与试验结果吻合良好。结果显示,管束内部传热管所受流体力为具有尖峰特征的宽频信号,其能量集中在一定带宽内。其结果为利用CFD方法计算传热管微振磨损建立了基础。     

金属橡胶内螺旋金属丝微动磨损预测模型研究*

目前金属丝微动磨损预测模型多适用于垂直接触，而锐角交叉下的模型存在计算过程繁琐、表征不全面等不足，难以便捷有效地预测金属橡胶内部复杂无序的金属丝磨损情况。基于有限元分析，确定螺旋曲率对磨损结果的影响极小，因此将金属橡胶内部螺旋金属丝接触对微元理想化为直金属丝接触对，探究无序接触下金属丝磨损特征的演化规律。结果表明，无序接触状态下的磨损特征演化规律与金属丝接触夹角大小密切相关。依据几何学分析，得到任意锐角接触下磨损磨痕位于金属丝1/2接触夹角处的特殊位置关系，据此建立任意接触形态下的微动磨损演化预测模型，并利用已有文献中的金属丝微动磨损试验结果对预测模型进行验证。结果显示，建立的任意锐角下的磨损演化模型能够较准确地预测金属丝的磨损结果，误差均在15%以内。研究结果为预测金属橡胶内部金属丝微动磨损和使用寿命提供一定理论基础。     

Investigation of nuclear-reactor friction units operating in heavy high-temperature liquid-metal heat carriers. Part 1. Experimental equipment

Experimental equipment designed for solving tribological problems arising in friction units operating in heavy liquid-metal heat carriers for nuclear-reactor circuits is discussed. In order to study fretting corrosion, a setup has been produced that simulates the wear of a fuel element in contact with a spacer grid cell. Thus, the vibration strength of the steam generator of a fast-neutron reactor with a lead heat carrier is substantiated. An experimental setup for studying and carrying out endurance testing of cylindrical gears with standard circuit and plain bearings operating in fast-neutron reactors in the medium of a lead heat carrier is described. Experimental specimens of circulation pumps for reactor equipment are developed for investigation of the wear of steel and cast-iron plain bearings operating in lead medium and the wear of eutectic lead-bismuth and lead-lithium alloys.