A solution to the spatial tribocontact problem of the wear of a tooth gear using electric simulation

The problem is analyzed of the contact between the barrel-shaped teeth of a tooth gear with account of wear. Electric simulation is applied to solve the problem. It is analyzed how wear affects the shape and size of the contact area and the contact-pressure distribution. The epures of contact pressure are plotted from the characteristic cross sections of the contact area. It is demonstrated that the wear of contacting surfaces alters considerably the geometrical parameters of the contact area and the pattern of contact-pressure distribution.     

The experimental and numerical analysis of quasi-steady wear processes for a sliding spherical indenter

The wear process on a frictional interface in a relative sliding motion of a punch on a substrate induces shape evolution of contact interface. For fixed contact zone the process tends to a steady state with fixed contact stress and strain distribution. Such steady state wear regimes were analyzed in the previous papers [1], [2], [3], [4], [5] by applying the principle of minimum wear dissipation rate. However, when the contact zone evolves in time due to wear process, as in the case of sliding spherical indenter, a quasi-steady state is reached with stress intensity dependent on the contact size parameter. On the other hand, the contact pressure distribution satisfies the condition of steady state. The simplified numerical analysis of wear process is presented by applying the quasi-steady conditions. The numerical predictions are confronted with experimental test results for two specific cases of soft and hard substrates. The quantitative specification of wear parameters is provided, first assuming constant values throughout the whole wear process, next assuming linearly or exponentially varying during the initial period and tending to constant values.     

On the model for identification of wear law parameters of viscoelastic materials based on ball-on-disk wear tests

An analytical model of contact interaction between spherical indenter and rotating disk of viscoelastic material (ball-on-disk configuration) is considered. The numerical analysis for influence of load–speed regime on wear rate of disk is carried out. It is shown that adequate choice of contact interaction model which takes into account rheological behavior of the disk material is important in identification of wear law parameters based on the testing results.     

Finite element prediction of blanking tool cost caused by wear

The total cost of a blanked part is determined by a large number of factors, including the material cost, manufacturing costs. Predicting the manufacturing costs of a blanked part requires accurate estimation of tool cost caused by wear. The aim of this paper is to develop a finite element model allowing for the numerical prediction of the blanking tool life which allows for the evaluation of the cost rate of blanking tool caused by wear needed to assess the total cost of a blanked part. A wear prediction model has been implemented in the finite element code Abaqus in which the tool wear is a function of the normal pressure and some material parameters. In the present work, the tool is modeled as rigid body hypothesis, and the wear variables are computed in the contacting elements. The altered tool contact surface and contact pressure tool shapes are updated iteratively to simulate wear over a long period of time of about 100,000 cycles. A damage model is used in order to describe crack initiation and propagation into the sheet. The distribution of the tool wear on the tool profile is obtained and compared to industrial observations.     

A case of wear particle formation through shearing-off at contact spots interlocked through microroughness in adhesive wear

The shape and size distribution of wear particles and the interface morphology enables different wear modes to be distinguished, and hence allows comparison with corresponding wear models. The particles must be collected as soon as possible after their formation, to minimize deformation and chemical attack between the generation and analysis of the particles, and the concurrent damage to the interface.To this end, a correlated study of surface morphology and of wear particles was performed on bundles of copper wires of various diameters subject to unlubricated sliding on a copper substrate in adhesive wear. Different sliding conditions were used. In some tests the contact spot temperature was artificially increased through the passage of current to generate Joule heat; other tests were performed in argon rather than in the unregulated atmosphere.The results strongly support the “shearing-off” model of wear, operating at contact spots mechanically interlocked through microroughness, as indicated by the following.

• 1.(1) There was no evidence of any clinging of wear particles or fragments thereof to the harder side, as expected if adhesive forces had played a significant role in the primary wear event.
• 2.(2) Numerous partially formed but not yet fully detached wear particles were observed whose shape conformed exactly to that predicted by the shearing-off model.
• 3.(3) Evidence of microroughness was discovered on the surfaces of partly formed particles, the corresponding surfaces of wear particles and those of the contact spots on the other side.
• 4.(4) By contrast, none of these surfaces showed evidence of tearing or puckering that would indicate breaking of adhesive bonds.
• 5.(5) For all wear conditions, the size of the wear particles matched the size of the contact spots on the opposing side.
• 6.(6) The wear particles exhibited “similitude”, i.e. their ratio of length to width to thickness was virtually independent of the wear conditions, as predicted by the shearing-off model.
• 7.(7) The ratio of length to width to thickness of the particles was about 1:0.5:0.3, which is in close conformity with expectations from the shearingoff model without assistance from adhesion, whereas strong adhesive forces are expected to generate equiaxed wear particles.

     

Abrasive wear between rough surfaces in deep drawing

In tribology, many surface contact models are based on the assumption that surfaces are composed of a collection of small asperities of which the tips are equally sized and spherically shaped and have some kind of statistical height distribution. This approach was used in 1966 by Greenwood and Williamson and was successfully followed by many researchers during the following decades. The statistical representation of surface topography enables calculation of contact forces and asperity deformations with reasonable accuracy using well established equations. Although this approach has proven to be suitable for static contact situations, alternative representations of the surface topography are required when modelling abrasive wear. In the current work an elastoplastic contact model is developed in which a representation of the surface topography is obtained by best fit approximations of the micro-contacts, obtained from real, measured surface height data. In this deterministic surface representation, the tips of the contacting asperities are assumed to have an ellipsoidal shape. Given the material parameters and contact conditions, the load and deformation of a single asperity can be computed. Subsequently, the wear induced by each individual asperity is obtained by inserting its size and shape and the conditions into a “single asperity micro-abrasion model”. By summing the contributions of all individual asperities, the total abrasive wear volume is obtained. The results of the developed abrasive wear model are compared with results obtained using a statistical approach.     

Fretting wear of laterally supported tube

The fretting wear of a tube, which is in contact with a lateral support, is examined experimentally. A fretting wear tester is specifically designed. Elastic springs are used as the support, which can simulate the contact between a spacer grid and a fuel rod in pressurized water reactor fuel. The tubes and the springs are made of Zircaloy-4. The experiments are conducted in air at room temperature. The experimental conditions, i.e. the normal and shear forces on the contact, the slip range and the number of cycles, are set to be the same. To investigate the influence of the contact geometry on the wear, the spring supports have a concave, a flat or a convex contour. The influence on the axial and transverse slip directions is investigated to incorporate the actual tube motion caused by such a flow-induced vibration in the reactor. The wear on the tube is examined by the surface roughness tester, which measures the depth, and the contour of the worn surface of the tube. Since the shape and the distribution of wear are found arbitrary, a method for evaluating the wear volume is proposed using the signal processing technique. It is found that wear can be restrained when the slip direction is transverse, and if the support has a concave contour.     

A study of tool wear using statistical analysis of metal-cutting acoustic emission

Many aspects of the interactions between cutting tools, workpiece material and the chips formed during machining that affect the wear and failure of the tool are not fully understood. The analysis of acoustic emission signals generated during machining has been proposed as a technique for studying both the fundamentals of the cutting process and tool wear and as a methodology for detecting tool wear and failure on line. A brief review of the theory of acoustic emission is presented. Acoustic emission data from reduced contact length machining experiments and tool flank wear tests are analyzed using distribution moments. The analysis shows that the skew and kurtosis of an assumed β distribution for the r.m.s. acoustic emission signal are sensitive to both the stick-slip transition for chip contact along the tool rake face and progressive tool wear on the flank of the cutting tool.     

Development of a wear equation for polymer-metal sliding in terms of the fatigue and topography of the sliding surfaces

A wear equation has been derived using the concept of fatigue failure due to asperity interactions in the contact region between sliding bodies. One of the three principal stresses that arise in the contact zone under the effect of a normal as well as a tangential load is of tensile nature. It is this principal stress that has been considered to be responsible for the initiation and propagation of fatigue cracks. It is assumed that the deformation in the contact zone is of elastic nature and that both the contacting surfaces are covered with asperities that have spherical tips. The wear equation involves the asperity height distribution φ(z). The particular distribution for a sliding situation is determined from experimental studies of the topography of sliding surfaces. The wear equation indicates that the wear rate depends upon the fatigue properties of the weaker material, normal load, sliding speed, coefficient of friction, moduli of elasticity of the contacting materials, asperity density, asperity radius of curvature and the distribution and standard deviation of asperity heights. The variation of wear with these parameters as indicated by the wear equation is in agreement with the experimental studies already reported in the literature.     

Rolling—sliding wear damage in rail and tyre steels

The wear behaviour of several rail steels in rolling-sliding contact with a tyre steel is investigated in the laboratory using an Amsler wear testing machine. Three wear regimes are identified, and a metallurgical examination to determine the characteristic wear modes within these regimes is described. Relations between the wear rates and test contact parameters are presented for two of these regimes. A comparison of laboratory test wear modes with those found to occur in side-worn rails shows sufficient correlation for the laboratory test to be used to offer a prediction of the relative wear behaviour of pearlitic rail steels in curved track.     

Adaptive finite element simulation of wear evolution in radial sliding bearings

This article employs an adaptive wear modeling method to study the wear progress in radial sliding bearings contacting with a rotary shaft. Mixed Lagrangian–Eulerian formulation has been used to simulate the contact condition between the bearing and the shaft, and the local wear evolution is modeled using the Archard equation. In the developed wear processor algorithm, not only remeshing is performed on the contact elements, but also is executed for their proximity elements. In this way the wear simulation becomes independent of the size of the contact elements. Validation was done for a laminated polymeric composite bearing. The composite has been modeled as a linear orthotropic material. The wear coefficients were obtained from flat-on-flat experiments and were applied as pressure and velocity dependent parameters in the wear processor. Finally, the effect of the clearance on the wear of the radial bearings has been studied numerically. The simulations also demonstrate how the contact pressure evolves during the wear process, and how the clearance influences this evolution.     

Residual stresses and sliding wear

The residual stresses that develop during the wear of AISI-SAE 1018 and 4340 steels have been examined. The entire three-dimensional stress tensor was obtained. A normal stress perpendicular to the surface, predicted by theory, has been found, but its magnitude is too small to affect the wear rate. There are also significant shear stresses. The wear process rapidly alters any initial stress distribution produced by heat treatment or peening to such a degree that the wear rate is not affected by these stresses, unless they are initially larger than those that can be produced by the wear parameters.     

Tread wear and footprint geometrical characters of truck bus radial tires

Wear and mileage performance are the foremost performances for truck bus radial (TBR) tires. There are a lot of researches about the tire wear performance as well as the contact patch phenomenon by using finite element analysis (FEA) method or testing. But there is little published data on the correlations between the footprint geometry and the tread wear performance of tires. In this paper, an experiment on tire-ground performance of TBR tires is carried out by using Tekscan. The real-time changes of contact-area pressure distribution that occurred during the process of continuous load and unload are recorded. Three types of tires that act differently in behavior under normal usage are analyzed. A new method of researching in tire tread wear, which focuses on the geometrical characters of the footprint, is put forward. The experimental results of the three tires are described by using footprint geometrical characters. On the basis of studying the changing laws of footprint geometrical characters during the loading process and considering consumer survey and factory feedback information, the correlations between the geometrical character of footprints and tread destruction form are built. The analyzed results show that a greater contact area coefficient and a steady coefficient of contact result in a better wear performance for TBR tires. The footprint-shape coefficient changing laws in the process of loading are found to have a very good coincidence with the tread wear of the three types of tires. Tires with a smaller footprint-shape coefficient are likely to have an average tread wear while avoiding the shoulder wear first. The proposed research provides a new solution to predict tire-ground performance at the point of footprint and several useful references for improving tire design.     

Journal bearing housing design—A statistical study with FEM

When designing journal bearing housings for construction equipment, one goal is to get an even pressure distribution over the length of the bushing. This is to avoid excessive wear due to contact pressure peaks.A housing that is considered good, does not allow the pin to deflect and keeps the stresses low in the weld. To do this it must be stiff and this will lead to high contact pressures on the edges of the bushing, which is not preferable since wear is highly dependent on the contact pressure level.If the distribution of the contact pressure could be smoothed out over the bushing, the material might be used more efficiently. The normal way to do this is to crown the bushing to allow for pin deflection. However this leads to reduced area in contact. Another method to avoid high pressures in the bearing is to optimize the bearing housing for optimum stiffness.This paper describes one way to optimize journal bearing housings in regard to the contact pressure in the tribo-contact. A statistical approach was applied to a parameterized finite element model with contact elements. Three parameters were analyzed at different loads; set ring thickness, set ring width and fillet weld size.The contact pressure distributions were evaluated in two different ways to a single value to meet the statistical demand of measurable result. The results show that the set ring thickness and width are the parameters that influence most the contact pressure distribution. To reduce the maximum contact pressure the set ring thickness should be kept small.     

Evolution of contact pressure during wear of the coating in a thrust sliding bearing

The evolution of contact pressure is analyzed during the wear of a thrust sliding bearing following the law of nonlinear wear. The coating’s deformation properties are described using the nonlinear Winkler model. It is demonstrated that steady wear can exist with a definite contact pressure distribution.     

Role of contact frequency on the wear rate of steel in discontinuous sliding contact conditions

This work presents a novel approach of sliding ball-on-disk wear tests where the disc material is investigated. Each part of the wear track on the disc is in discontinuous contact with the counterbody. The contact frequency at each part of the wear track on the disc with the counterbody is defined by the rotation frequency of the disc. The sliding speed is however a function of both the rotation frequency and wear track diameter. In this work, the effect of the contact frequency on friction and wear was investigated on carbon steel in discontinuous sliding contact with corundum balls. Various sliding speeds were used while maintaining the contact frequency at a fixed value, and various contact frequencies were applied at constant sliding speeds.The wear rate of the disk material is shown to depend not only on the usual wear test parameters, namely sliding speed and contact load, but also on contact frequency. Moreover, contact frequency is shown to be a key factor determining the wear mode even at constant sliding speed and load. At contact frequencies above 9 Hz, the dominant wear mechanism is oxidational wear, while at frequencies below 4 Hz the dominant wear mechanism is adhesive wear. This transition from adhesive to oxidational wear takes place together with a change in the type of debris generated and in the value of the coefficient of friction.The validity of the Garcia-Ramil-Celis model proposed earlier for discontinous sliding contact conditions, is demonstrated for the case of carbon steel disks sliding against a chemically inert counterbody.     

An explanation of the wear of metals

Mathematical analysis establishes that the well-known empirical linear wear law for the adhesive wear of metals is the consequence of the statistics of surface roughness and is almost independent of the assumed contact model. A strain ratio fatigue failure criterion (i.e. the Manson-Coffin low cycle fatigue law) coupled with a probabilistic treatment of surface asperity height distribution and surface contact provides, for the first time, a fundamental explanation for the formation of wear particles.     

What role for contact spots and dislocations in friction and wear?

Sixteen years of research on the properties of 'contact spots' of ductile materials in 'adhesive' wear are surveyed. They are the locations (typically occupying less than 1% of the macroscopic area of contact) at which solids ale in load-bearing contact during wear. Their properties determine the coefficient of friction, and friction and Joule heat are generated at them. It was found that the mechanical behavior of contact spots is governed by ordinary dislocation plasticity. They remain elastic if the local pressure falls short of the indentation hardness of the softer side. In that case (almost) no wear occurs. Elastic contact spots are promoted by light loads and fine polishing. Ordinarily, wear particles are detached through tangential shearing-off where, statistically, the two sides at contact spots momentarily interlock. The Holm-Archard wear 'law' is a simple direct result of this mechanism. However, quite typically the bulk of sliding takes place within adsorbed moisture films which are ubiquitous in our daily surroundings. Outside of contact spots the adsorbed moisture behaves much like ordinary water. At the typical contact spot the moisture is squeezed down to but two monomolecular layers; and the relative motion between sliding solids overwhelmingly takes place between these. This causes the prevalence of friction coefficients about μ 0.3 in our surroundings. The complex behavior of the adsorbed water can cause stick-slip. As water is desorbed at 170 °C, the determination of flash temperatures at contact spots is critical. A corresponding theory was developed and verified in connection with experiments on graphite lubrication which depends on the presence of adsorbed moisture. Further, plastic contact spots and solid lubrication have been simulated by means of a Bridgman anvil apparatus. The results show that the same work-hardening behavior applies at contact spots as known from the bulk. They also proved that at contact spots intimate mixing of the materials of the two sides gives rise to a finely mixed layer which can amorphize by much the same dislocation mechanism as is believed to cause melting. The amorphous material promptly recrystallizes at least in the Cu-Ag system. It is proposed to try and inhibit such recrystallization by selected lubricants, so as to promote beneficial wear-resistant tribo-films.     

Relationship of surface slope distributions to the wear mechanisms of ceramics

Many ceramics wear by more than a single wear mechanism. To understand how different wear mechanisms contribute to the wear properties of ceramics, quantitative measures of those mechanisms are required. Surface slope distribution analysis was investigated as a method of quantitatively analysing wear mechanism signatures, based on a simple model of the variation of slope with wear mechanism. Slope distribution was shown to be similar for materials within the same class of ceramics, e.g. silicon carbides. However, the range of slope parameters for a mechanism overlapped with that of other mechanisms, so that a mechanism was not found to yield a unique combination of slope parameters. A comparison of wear rate and slope distribution showed that wear could be correlated with specific sections of the distribution, and that this correlation was dependent upon the ‘dominant’ wear mechanism. Secondary effects on the slope distribution, e.g. porosity, invalidated the data for that material for use in slope analysis.     

Thermomechanical wear modelling

It is assumed that the contact between bodies in sliding motion produces a stress field and frictional heat source that may induce severe wear from material yielding or fracture. For this situation, a thermomechanical wear model is used to develop a wear transition equation for identifying the dominant factors that will reduce or control such wear, by employing thermoelasticity analyses and contact mechanics. This equation is used to construct wear maps for ease of analysis. Studies are used to substantiate the thermomechanical wear model with experimental results that emphasise the transition from mild to severe wear for dry and lubricated metallic and ceramic sliding contacts under load.