Application of the solution of the contact wear problem to modeling wear during fretting corrosion

Fretting corrosion wear was modeled using the deformation model of Winkler and the assigned elementary law of wear. The obtained theoretical results were compared with the available experimental data. It is demonstrated that the proposed model is adequate for the actual process of fretting corrosion.     

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.     

Dual-rotary fretting wear behavior of 7075 aluminum alloy

Friction and wear behavior of dual-rotary fretting (DRF) combined by torsional and rotational fretting modes have been investigated. Such fretting mode is essentially achieved by changing tilt angles of the rotary axis and varying rotary angular displacement amplitudes The DRF behavior has been characterized from the dynamic behavior, wear damage, third-body behavior, wear mechanisms. The running condition fretting map (RCFM) of DRF fretting wear was established by using the tilt angles and angular displacement amplitudes. The evolution of the wear volume vs the tilt angle under varied angular displacement amplitudes was quantificationally measured. In addition, the competition between the local wear and fatigue (cracking, wear) has been discussed in detail. The results indicated that the damage of 7075 aluminum alloy induced by DRF was strongly dependent upon the tilt angle and the angular displacement amplitude.     

Characteristics of wear debris in impact sliding

The characteristics of debris arising from repetitive compound (sliding) impact are presented. The debris was collected from a variety of tests. Microscopy and X-ray diffraction analysis establish (1) that debris may arise from specimen and counterface, (2) that metallic particles consist of very small and/or distorted primary crystals, (3) that oxide formation occurs and (4) that phase transformations of metallic constituents may occur. Further, particular debris characteristics are shown to be dependent on the nominal contact stress and the relative sliding velocity.     

Design effects on the fretting wear behaviour of ball bearings

A change in design of a ball bearing is described based on the results of numerical and experimental analysis to reduce fretting wear. Increasing the radii of curvature of the inner and outer races by a small amount reduces the product of the relative slip δ and the tangential traction τ at the contact region, both of which are caused by Heathcote slip. This results in the consequent reduction in fretting wear because there is a good correlation between the amount of fretting wear and τδ. This prediction is confirmed experimentally by increasing the groove radius of the inner race from 4.02 to 4.21 mm for a ball of radius 3.97 mm.     

三环减速器偏心套微动磨损分析

三环减速器在运转过程中产生磨损及发热的原因是作用于偏心套上的交变转矩在平键联接处产生的微动现象。本文提出了消除微动磨损的有效措施。     

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.     

The role of adhesion and delamination in the fretting wear of metallic materials

Fretting corrosion is a mild form of wear which is distinguished from unidirectional sliding wear by the small amplitude of movement and the low relative velocity of the surfaces. Macroscopic adhesion occurs in the early stages of the process, and there is evidence of the making and breaking of local welds and a roughening of the surface. Eventually adhesion falls off, the surfaces become smoother and removal of material from the surfaces occurs by delamination. The change-over from adhesion to delamination is a function of the particular material and the nature of the environment. The process is investigated in a number of materials of commercial interest such as mild steel, stainless steel and titanium alloys. The adhesion stage is more pronounced the nobler the metal and the more inert the environment. In systems exhibiting adhesion the number of fretting cycles to develop maximum adhesion corresponds to the number of cycles to initiate a propagating fatigue crack. However, it is thought that fall-off in adhesion is the significant factor.     

Frequency effects in fretting wear

The effect of frequency of vibration on fretting wear has been investigated in the 10 – 1000 Hz range with additional experiments at 20 000 Hz. Fretting tests were performed with two materials, a low carbon steel (AISI 1018) and an austenitic stainless steel (AISI 304). The experiments showed that two cases of fretting contact can be distinguished and related to the displacement amplitude. If the amplitude is low, the contact situation is characterized by partial stick at the interface. At these conditions the wear rate (measured as the volume of material removed per cycle) is little affected by frequency. However, in low amplitude fretting material damage by surface degradation and fatigue crack initiation is usually of more concern than the actual wear itself. Both of these parameters are found to be greatly accelerated by an increase in frequency. In high amplitude fretting, in contrast, gross slip occurs at the interface and wear becomes the dominant damage mode. At these conditions variations in frequency appear to have little effect on fretting wear and related mechanisms. Therefore, in the case of fretting at high displacement amplitudes, it may be possible to apply high frequency fretting to obtain accelerated testing conditions.     

电感式磨粒传感器中铁磁质磨粒特性仿真研究

针对机械装置的在线监测传感器,模拟了铁磁质磨粒通过传感器过程中传感器线圈的磁场和感应线圈的感应电压瞬态变化特性.考虑了线圈与铁磨粒的材料、线圈匝数和激励线圈的输入电压等因素,应用Jmag Designer I0.4软件建立了传感器的二维有限元模型.仿真结果揭示了磨粒运动过程中线圈磁场与感应线圈中感应电压的变化规律,获得了感应电压与球形磨粒的直径大小的立方成正比,与磨粒运行速度成正比.研究结果对于电感式磨粒传感器的开发具有重要的指导价值.     

On the fretting wear mechanism of Zr-alloys

Zirconium alloys are highly desirable in nuclear applications due to their transparency to thermal energy neutrons and for their high corrosion resistance. The main objective of this study is to investigate the fretting wear mechanism of Zr–2.5%Nb alloy. The experimental work was carried out in air at 265 °C, using a specially designed fretting wear tribometer. The transfer of material, the change in the wear volume and the maximum wear depth with the number of cycles were measured through 3D mapping of the topography of the fretted surface. SEM and Fourier Transform Infrared Interferometry methods were used to examine the microspall pits and to measure the distribution of the thickness of oxide layer in the fretting region. For relatively small slip amplitude, the results showed that the fretting wear mechanism is initially dominated by adhesion and abrasion actions and then by delamination and surface fatigue. The time variation of the wear losses was shown to be cyclic until a steady state value is reached. At high slip amplitudes, however, abrasion and delamination are the only dominant wear mechanisms. The volumetric wear losses were found to decrease monotonically with the number of cycles. A novel approach was introduced, whereby the thermal and electrical contact resistances of the fretting interface are simultaneously measured. The results demonstrated the potential use of this non-intrusive approach for real-time monitoring of the fretting wear mechanism.     

A generalized fretting wear theory

Combined impact-sliding fretting wear is a complex phenomenon due to the random nature of the excitation force and the self-induced tribological changes. Available models, which relate wear losses to the process variables, are empirical in nature and bear no physical similarity to the actual mathematical and physical attributes of the wear process. A generalized fretting wear theory is presented to mathematically describe the fretting wear process under various modes of motion; impact, sliding and oscillatory. This theory, which is based on the findings from the fracture mechanics analysis of the crack initiation and propagation processes, takes into consideration the simultaneous action of both the surface adhesion and subsurface fatigue mechanisms. The theory also accounts for the micro-, and macro-contact configuration of the fretting tribo-system. The closed form solution requires the calibration of a single parameter, using a limited number of experiments, to account for the effect of environment and the support material. The model was validated using experimental data that were reported for Inconel 600 and Incoloy 800 materials at room and high temperature environment, and for different types of motion. The results showed that model can accurately predict wear losses within a factor of ±3. This narrow range presents better than an order of magnitude improvement over the current state-of-the-art models.     

On the mechanisms of various fretting wear modes

According to relative motion directions for a ball-on-flat contact, there are four fundamental fretting wear modes, e.g., tangential, radial, torsional and rotational modes. In this paper, the mechanisms of these four fundamental fretting wear modes, particularly for the later three modes, have been reviewed from results obtained by the authors in combination with results from literature. Some general features have been reported. Differences both in running and degradation behavior have been discussed in detail. Results showed that some similar laws for three fretting regimes (partial slip regime, mixed regime and slip regime), fretting maps (running condition fretting map and material response fretting map), wear and cracking mechanisms obtained from the classic mode (i.e. tangential fretting) were also identified and useful to characterize the other modes. Nevertheless, the occurrence of relative slip for the radial fretting, the formation of mixed regime for the torsional fretting, the evolution of surface morphology for the rotational fretting were quite different compared to that of the classical fretting mode.     

The occurrence of spherical particles in fretting wear

The use of the scanning electron microscope to examine mild steel surfaces fretted in argon has revealed spherical debris as a characteristic feature of the wear process. They are found at all temperatures up to 500°C, usually with a smooth but layered surface. Sphere location is mainly in regions of high adhesion, on the scar surface itself or within surface cracks or tears. Metallographic examination has confirmed that the spherical particles have the same structure as the material from which they formed. Their formation and growth is a result of an alternating type of deformation occurring along a local interface.     

Effect of wear debris on wear in rolling-sliding motion

The effect of wear particles on wear was investigated using a four-ball extreme pressure lubricant test apparatus. The wear particles present in both filtered and unfiltered oil samples were examined using a duplex Ferrograph analyser and a bichromatic microscope. The wear track widths were measured for various wear modes and were found to increase if the wear debris was recirculated. The size of the wear particles increased with test duration.     

Effect of oil supply on fretting wear

Fretting wear of lubricated flat steel surfaces (100 mm × 100 mm) was studied using a specially prepared test rig. The amplitudes of reciprocating sliding and the apparent contact pressure were 0.1–0.75 mm and 10 MPa respectively. The specimen surfaces were given various heat treatments and were mesh grooved for oil supply. Gear oil was fed to the sliding surfaces. Ungrooved surfaces gave severe fretting wear under the experimental sliding conditions used. However, significant reduction in fretting wear was obtained by grooving the surface. The effect could be obtained only when the spacing between the grooves was small, presumably because the oil was fed to the whole contact region owing to the smaller spacing.     

A novel modular fretting wear test rig

A novel modular experimental apparatus was designed and developed to measure and visualize fretting wear and friction for Hertzian circular and elliptical contacts and flat on flat contacts. The experimental apparatus utilizes a magnetostrictive actuator to reciprocate a flat, ball, or cylinder between two fixed specimens. Two stationary flat or cylindrical specimens mounted on a rotary table clamp the reciprocating specimen from the top and bottom to generate the fretting contact. The two stationary test specimens installed on the rotary table perpendicular to the moving specimen form a crossed cylinder geometry which creates a well-defined circular contact. An elliptical contact with different aspect ratios can be obtained by varying the angle between the fixed and the moving specimens. Dead weights placed on top of the upper stationary specimen provide the normal load. A force sensor located in line between the actuator output shaft and the specimen is used to measure friction. The test rig's modular design allows it to be configured for Hertzian circular (ball-on-flat, crossed cylinder), elliptical (crossed cylinder), and conformal (flat-on-flat) contacts. In the ball on flat configuration a steel flat or sapphire window is used in contact with the reciprocating ball. When the sapphire window is used a microscope and high speed camera is employed for in situ visualization and recording of the contact.     

A tentative explanation for the tribochemical effects in fretting wear

In many fretting investigations, tribochemical reactions have been reported to critically determine the wear and friction behavior, however, different and contradictory assessments of the importance of mechanical and thermal effects on these reactions have been suggested. Since fretting is characterized by relatively slow sliding speeds, high temperatures are not generated over the entire nominal contact area. However, evidence for phase transformations, which are typical of high temperatures, have been observed many times in fretting experiments. In other words, there exists a discrepancy between the macro- and micro-scale observations. In our previous experimental and theoretical work, the tribochemical transformations of steel and ceramics were extensively investigated and the presence of very high flash contact temperatures under gross slip fretting was confirmed. In this paper we present a tentative explanation of the mechanism for the observed tribochemical changes under selected fretting conditions, which can also explain the discrepancy in the results from macro- and micro-scale studies. The proposed wear mechanism considers the tribochemical transformations at the asperity spot-to-spot contacts due to high flash temperatures, while the heat generation and dissipation at apparent contact area remain significantly lower. The observed overall wear transition occurs due to gradual accumulation of the transformed material, which in “closed” fretting contacts remains in great part within the contact.     

Quantitative methods in wear debris analysis

Wear debris has been recovered from several test systems and analyzed using different methods. These methods produced specific information concerning the particulate size and composition. A magnetic debris recovery method was quantitatively evaluated using debris samples and also using collections of manufactured particulates having known sizes and compositions. Small 5 μm diameter SiO₂ spheres, some containing nickel, were used to simulate debris. Other particulates of iron and nickel in different size ranges were also used in order to investigate such matters as size resolution, lubricant dilution techniques, particle overlap difficulties and the general problem of calibration of debris recovery systems. A comparison between chemical analysis and particulate analysis findings is presented. The application of optical and electron microscope methods and X-ray microanalysis in characterizing the wear particulates was carried out directly on the recovery substrate; these techniques are described.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,