Application of influence function method on the fretting wear of tube-to-plate contact

The structural integrity of steam generators in nuclear power plants is much dependent on the fretting wear characteristics of Inconel 690 U-tubes. The influence function method for the tube-to-plate contact model is demonstrated in this study to investigate the fretting wear problems on the secondary side of the steam generator, which are caused by flow induced vibrations between the U-tubes and supports. Two-dimensional numerical contact model is developed and formulated in terms of the Cauchy singular integral equation. The distributions of normal pressures, shear stresses and displacement fields are derived between two contact bodies which have similar elastic properties. Based on the algorithms for normal pressures and relative slip, a numerical approach is developed to simulate the fretting wear of tube-to-plate contacts. The work rate model is adopted in this study to find the wear amounts between two materials. The results are compared with the solutions by finite element analyses to validate the application of the present method to fretting wear problems.     

The effect of gross sliding fretting wear on stress distributions in thin W-DLC coating systems

The effect of fretting wear on stress evolution in a thin, hard diamond-like carbon coating deposited on a high strength steel is simulated using a finite element (FE) based method under gross sliding conditions. The effects of coating stiffness, thickness, and coefficient of friction are studied. The trailing edge tensile stress and the leading edge compressive stress are generally predicted to reduce with wear, while the shear stress at the interface is predicted to increase. An extrapolation procedure for predicting coating wear life is shown to provide accurate and slightly conservative estimates compared with explicit modelling to complete coating wear.     

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.     

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.     

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.     

Friction behavior of quenched and tempered steel in partial and gross slip conditions in fretting point contact

Tangential traction caused by friction in contacting surfaces is a major factor in fretting fatigue that increases stress levels and leads to a reduction in fatigue life. Friction in fretting contact was studied in partial, mixed and gross slip conditions on quenched and tempered steel. Measurements were made with sphere-on-plane contact geometry for polished and ground surfaces. Friction was evaluated from on-line energy ratio and, after the tests, from wear marks. A maximum friction coefficient of over 1.0 was measured at mixed slip zone with polished surfaces, whereas ground surfaces promote lower values in similar operating conditions. The friction coefficient dependence on load cycles and loading frequency is also presented and briefly discussed. The friction data and understanding thus gained is to be used for evaluation of crack initiation with the numerical fretting fatigue model.     

Friction and wear of Inconel 690 for steam generator tube in elevated temperature water under fretting condition

Recently, material of Inconel 690TT (thermal treatment) for the steam generator tubes in a nuclear power plant was substituted for the existing material of Inconel 600HTMA (high temperature mill-annealing). Inconel 690TT has more chromium than Inconel 600HTMA in order to improve the corrosion resistance. In this study, to evaluate the friction and wear characteristics of Inconel 690TT under fretting condition, the fretting tests as well as sliding tests were carried out in air and in elevated temperature water environment, respectively. Fretting tests of the cross-cylinder type were done under various applied normal loads, and sliding tests of pin-on-disk type were also carried out to compare with the results of the fretting test. In summary, the results of the fretting tests correlated with the results of the sliding tests. The wear mechanism of Inconel 690TT in air was delamination wear and the mechanism in water was affected by micro-pitting. Also, it was found that the fretting wear coefficients in water were increased with increase in the temperature of water.     

Fretting wear behavior of AZ91D and AM60B magnesium alloys

The fretting wear behavior of the AZ91D and AM60B magnesium alloys are investigated using a reciprocating fretting wear machine under dry conditions with different numbers of cycles, different normal loads, slip amplitudes and frequencies. The worn surfaces and wear debris were examined using scanning electron microscopy and optical microscopy in order to understand the predominant wear mechanisms of two magnesium alloys. The results indicate that the AZ91D alloy displays a lower friction coefficient and lower wear quantity than the AM60B alloy. The AZ91D shows a higher capability than AM60B in resisting crack nucleation and propagation. Both AZ91D and AM60B show similar friction and wear characteristics. The wear quantity increases with increasing normal load, but decreases with increasing frequency. The friction coefficient also decreases as the normal load is increased. Fretting frequency had little effect on the friction coefficient. In a long term, the fatigue wear and abrasive wear were the predominant wear mechanisms for AM60B and delamination wear, adhesive wear and abrasive wear for AZ91D.     

航空活塞发动机机匣微振磨损可用度分析与预测

针对航空活塞发动机机匣微振磨损问题,经统计分析后,发现各机匣微振磨损程度近似呈正态分布,其均值和标准差随工作时间增加而增大。基于正态分布密度函数,建立了机匣微振磨损可用度分析模型。通过建立灰色GM(1,1)模型,对机匣微振磨损程度的均值进行预测,并利用最小二乘拟合多项式拟合均值与方差的非性线关系,实现了可用度的有效预测。预测实例表明,可用度预测结果与实际较吻合,具有较高的预测精度。机匣微振磨可用度的分析与预测,可准确地获得可用度的变化规律,对发动机科学维修具有一定指导意义。     

Influence of the normal force,abrasive slurry concentration and abrasive wear modes on the coefficient of friction in ball-cratering wear tests

The purpose of this work is to study the influence of the normal force (N), abrasive slurry concentration (C) and abrasive wear modes on the coefficient of friction in ball-cratering wear tests. Experiments were conducted with balls of AISI 52100 steel, an AISI H10 tool-steel specimen and abrasive slurries prepared with black silicon carbide (SiC) particles+distilled water. The tangential (T) and normal loads were monitored throughout the tests and the results have shown that: (i) the coefficient of friction behavior was independent of the normal force and (ii) both the concentrations of abrasive slurries and the subsequent action of the abrasive wear modes, generally, did not affect the behavior or magnitude of the coefficient of friction.     

Surface morphology in engineering applications: Influence of roughness on sliding and wear in dry fretting

Influence of initial surface roughness on friction and wear processes under fretting conditions was investigated experimentally. Rough surfaces (Ra=0.15-2.52 μm) were prepared on two materials: carbon alloy (AISI 1034) and titanium alloy (Ti-6Al-4V). Strong influence of initial surface roughness on friction and wear processes is reported for both tested materials. Lower coefficient of friction and increase in wear rate was observed for rough surfaces. Wear activation energy is increasing for smoother surfaces. Lower initial roughness of surface subjected to gross slip fretting can delay activation of wear process and reduce wear rate; however, it can slightly increase the coefficient of friction.     

Fretting fatigue and wear damage of structural components in nuclear power stations—Fitness for service and life management perspective

Fretting fatigue and wear problems have major economical and safety impact on the nuclear industry. This keynote paper provides examples of the fretting problems encountered in nuclear power stations and an overview of the methodologies used to assess their root cause, their potential effect on the integrity of structural components and the future damage projection for risk management. The limitations of existing models that are commonly used to predict fretting wear rate are discussed. A system approach to the fretting wear/fatigue problem allowed us to significantly improve the capability of predicting fretting damage through the recognition of the problem nonlinearity, and the effect of self-induced changes. The application of linear elastic fracture mechanics principles for predicting the fretting wear and fretting fatigue strength is demonstrated. The paper underlines the critical roles of the following two factors. First, the validation of the above mentioned methodologies, through experimental investigation of the long-term fretting wear and fatigue behavior of structural components under realistic operating conditions. Second, the qualification of in -situ measurements of fretting wear damage using nondestructive evaluation NDE and inspection methods.     

Proposition of a finite element-based approach to compute the size effect in fretting fatigue

The aim of this paper is to propose a finite element (FE)-based methodology capable to predict the pad size effect on specimen life. This phenomenom has been observed in a number of available fretting fatigue tests, which will be considered to validate the analysis. The proposed methodology differs from conventional FE analysis as (i) it provides a direct evaluation of the fatigue strength at nodal points, besides the traditional stress/strain results and (ii) it proposes that, for problems involving the presence of severe stress gradients, as it is the case in many mechanical assemblies subjected to fretting, the size of the FE should be defined so that it characterizes the stress state in a process zone rather in a point. A discussion of how to define a priori the size of such FE is also conducted. Multiaxial stress-based criteria incorporated into the code are considered to carry out the fatigue analysis. The results obtained show that there is not a single range of FE sizes capable to correctly predict the fretting fatigue limit for all tests considered.     

The effect of slip amplitude and test time on fretting wear in metal-metal contact

A study was conducted to determine the effect of amplitude and test time on surface damage in metal-to-metal contact under lubricated conditions. The test set up consisted of a ball loaded against a flat disc, with an external drive imparting a linear oscillatory motion to the ball on the flat. The materials were steel AISI 52100/AISI 52100, and the lubricant was ISO VG 220. Damage characteristics were defined for amplitudes in the range of 5 to 50 microns and for test times of 10 to 360 min.     

Simulation of wear and contact pressure distribution at the pad-to-rotor interface in a disc brake using general purpose finite element analysis software

Passenger car disc brakes are safety-critical components whose performance depends strongly on contact conditions at the pad-to-rotor interface. The interface can be classified as a conformal dry sliding contact. During braking both brake pad and rotor surfaces are worn, affecting the useful life of the brake as well as its behavior. This paper discusses how wear of the pad-to-rotor interface can be predicted using general purpose finite element analysis software. A three-dimensional finite element model of the brake pad and the rotor is developed to calculate the pressure distribution in the pad-to-rotor contact. A wear simulation procedure based on a generalized form of Archard's wear law and explicit Euler integration is used to simulate the wear of the brake pad under steady-state drag conditions.     

Friction and dry sliding wear behavior of carbon and glass fabric reinforced vinyl ester composites

Friction and dry sliding wear behavior of glass and carbon fabric reinforced vinyl ester composites have been presented. The results show that the coefficient of friction and wear rate increased with increase in load/sliding velocity and depends on type of fabric reinforcement and temperature at the interphase. The excellent tribological characteristics were obtained with carbon fiber in vinyl ester. It is believed that a thin film formed on counterface was seems to be effective in improving the tribological characteristics. The worn surfaces examined through SEM, showed higher levels of broken glass fiber in vinyl ester compared to carbon-vinyl ester composites.     

Suitability of CAE neural networks and FEM for prediction of wear on die radii in hot forging

Prediction of tool wear in hot die forging along the entire arbor radius by wear models known so far is a very difficult task. On these parts of tools significant changes of contact pressure and sliding lengths occur along the die curvature during the plastic flow of material formed. A new approach presented in the paper combines the use of a conditional average estimator neural network (CAE NN) with the exploitation of results obtained by the finite element method (FEM) and also data from other sources. Consequently new parameters as well as the results of experimental work can be taken into account. In this paper a brief overview of models for prediction of tool (die) wear are discussed. The theoretical background of CAE NN, as well as its application to the modeling of the tool wear phenomenon, is presented. Some results of FEM analysis of the hot forging process that serve as input parameters in the CAE NN model are also briefly discussed. Two relevant practical applications are shown. In the first example, tool wear was modeled at a higher number of strokes (blows), by knowing wear at a lower number of strokes. In the second example, the number of strokes was the output parameter—the number of strokes causing predetermined wear at any point of the tool engraving curvature (arbor radius) was predicted. A comparison between the measured and predicted values of wear demonstrated good agreement that was assessed by a corresponding coefficient of determination.     

Characterization of the contact between a punch and a half-infinite substrate in a fretting situation

This paper analyzes the behavior of a mechanical system formed by a flat punch with square edges at 90 over a half-infinite substrate made of the same material under normal reversible constant and tangential loads, typical of a fretting situation. Based on previous studies of the behavior of such a system, both static and cyclic behavior are analyzed. The analysis is carried out using both analytical and numerical solutions, the latter being obtained with finite and/or boundary element codes. The kinematic behavior of the contact zone is studied (slipping, separation, or adherence), as well as the stress state of the substrate in the area surrounding the edges of the punch. The very same analyses have been made of a system equal to the latter but replacing the contact by continuity. The comparison of the solutions of the two systems gives the validity range of different continuous models, easier to use in representing the behavior of the punch–substrate system.     

Analytic and FE based estimations of the coefficient of friction of composite surfaces

In this work the coefficient of friction (COF) of a body with a composite surface layer (CSL) in sliding contact with a homogeneous counterbody is studied. With an analytic model the upper and lower bounds of the COF are estimated, whereas the material and surface properties of the CSL components are known and no information about their geometrical arrangement is at hand. An analogy with the Voigt and Reuss bounds is invoked. The analytic results are compared to FE computations. Additionally the influence of plastic deformation and geometric properties of the CSL are treated.