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.     

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.     

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。     

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.     

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.     

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.     

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.     

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.     

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.     

滑块式万向联轴器和轧辊的装配体有限元分析

滑块式万向接轴因其能在倾角较大情况下传递较大的扭矩而被广泛应用,但是延用至今的强度计算方法,或因假设条件多,使计算误差大,或因经验公式范围有限而不能满足进一步研究的需要。利用有限元技术,对轧机滑块式万向联轴器和轧辊的装配体进行强度计算,计算出关键零件的危险位置。因假设条件少,更符合实际情况,因此计算结果更为准确、可靠。     

Effect of the specimen geometry on wear - combination of polyacetal (POM) and carbon steel for machine structures

Engineering plastics which have been shown to have good mechanical properties are now frequently used as materials for various machine elements. Engineering plastics are combined with other engineering plastics and metallic materials for machine construction. These machine elements are fabricated with contact surface forms, such as convex, concave, and plane surfaces. Therefore, when designing machines with a combination of materials containing engineering plastics, it is useful to know the wear and friction characteristics for various contact surface forms. In the present research, polyacetal (POM), an engineering plastic, and carbon steel, a metal often used for machine structures, were chosen as materials to study wear and friction. Wear tests were performed in the combination of a convex surface and a plane, and in the combination of a plane and a plane. As a result, some features of the wear and friction characteristic are clarified. (1) The worn mass when the flat specimen made of POM is rubbed by the POM pin specimen is larger than when with the pin specimen made of carbon steel. (2) When the flat specimen made of POM is rubbed by the POM or the carbon steel pin specimen, the same grade of wear is observed regardless of the pin specimen material. (3) The worn length of the steel spherical pin specimen on the steel flat specimen becomes close to the initial radius of the curvature of the pin specimen when the sliding distance is large. The initial condition of the spherical tip pin specimen on the flat specimen evolves toward a condition of the flat tip pin specimen on the flat specimen. So, the comparison between the two geometries is non-relevant. Such problem did not occur in POM pin specimen.     

Synergy between corrosion and wear of electrodeposited Ni-P coating in NaCl solution

An immersion block-on-ring tester was employed in this study to investigate the interaction between wear and corrosion of electroplated Ni-P coating in 5% NaCl solution. The weight loss and the coefficient of friction of the specimen under different bath temperatures and overpotentials were measured. The results showed that at temperature of 50 °C and overpotential of +600 mV_SCE, which was within the transpassive region of its polarization curve, a severe synergy between corrosion and wear was observed. However, at lower temperatures and overpotentials, the interaction between wear and corrosion was hardly noticed.     

Effect of molybdenum disulphide upon the friction and wear in ceramic–steel pair

Friction and wear tests between a stationary block and a rotating ring under lubrication with molybdenum disulphide (MoS₂) were carried out at room temperature at a sliding distance of 500 m. Silicon nitride and cemented carbide blocks were pressed against a bearing steel ring, silicon nitride-bearing steel and cemented carbide-bearing steel pairs, by a load of 1600 N. The effect of molybdenum disulphide upon the coefficient of friction and the wear of the steel ring was discussed for both pairs in comparison with mineral oil lubricants. Molybdenum disulphide was more effective in reducing the coefficient of friction and the wear of the ring than the oil lubricants. Various mechanical pretreatment for forming MoS₂ film on the ring surface prior to the sliding tests were also considered. The mechanical pretreatment enabled the sliding test with the low friction coefficient even without lubrication over the sliding distance of 500 m. In general, the coefficient of friction and wear loss of the steel ring were smaller in the silicon nitride-bearing steel pair than in the cemented carbide-bearing steel pair.     

On the use of the asymptotic scaled modal analysis for time-harmonic structural analysis and for the prediction of coupling loss factors for similar systems

The paper presents the results of an ongoing research on a scaling procedure aimed at the reduction of the computational cost associated, in a deterministic approach, to the wavelength simulation. The adoption of the energy distribution approach allowed to completely define the way in which a scaled model can represent the mean response of a given original model. This goal is achieved through a reduction in the dimensions not involved in the energy propagation and accordingly in an increase of the original damping loss factors. Here, the coupled systems under observation are those in which the same wave is allowed to travel, exchange and reflect. Specifically the results coming from the classical modal response and the statistical energy analysis are compared with those in output from the scaling procedure. Some highlights with experimental test-case results and comparisons for a plate assembly are also given. The work presents also an investigation about the possibility of estimating the energy influence coefficients through scaled finite element models.