Ulf Olofsson

Wear, plastic deformation and friction of two rail steels: a full-scale test and a laboratory study

Form change due to wear and plastic deformation on a rail can reduce the service life of a track. The purpose of this investigation was to study the development of these damage mechanisms on new and 3-year-old rails in a commuter track over a period of 2 years. The experimental results from the form measurements show that there was a significant change in rail profile due to wear as well as to plastic deformation. Plastic deformation and wear was a continuing process even for rail that had been in service for 5 years. The plastic deformation mechanism was plastic ratchetting. Compared with the UIC 900A grade rail, the form change was less for the UIC 1100 grade rail. However, the contact situation in terms of sliding velocity and contact pressure had more influence on form change than the change of material. The results from 3D surface measurement showed that there were different wear mechanisms involved at different parts of the rail. Mild wear dominated at the rail head, but at the rail edge severe wear clearly influenced the amount of wear. The severe wear showed traces of seizure. Material tests were performed on two different testing machines: a two-roller and a pin-on-disk machine. On the basis of results from the material testing, a simple wear map was constructed. In the wear map, the wear coefficient is presented as a function of sliding velocity and contact pressure. The results from laboratory tests showed that wear coefficient depended strongly on sliding velocity. The increase in the wear coefficient when increasing sliding velocity was due to a change of wear mechanism from mild wear to severe wear.

Contact mechanics analysis of measured wheel-rail profiles using the finite element method

Abstract A tool has been developed for contact mechanics analysis of the wheel-rail contact. Using measurements of wheel and rail profiles as input, the tool is based on the finite element (FE) code ANSYS. Traditionally, two methods have been used to investigate the rail-wheel contact, namely Hertzs analytical method and Kalkers software program Contact. Both are based on the half-space assumption as well as on a linear-elastic material model. The half-space assumption puts geometrical limitations on the contact. This means that the significant dimensions of the contact area must be small compared with the relative radii of the curvature of each body. Especially in the gauge corner of the rail profile, the half-space assumption is questionable since the contact radius here can be as small as 10 mm. By using the FE method (FEM) the user is not limited by these two assumptions. The profile measurement system Miniprof was used to measure the wheel and rail profiles that were used as input when generating the FE mesh. As a test case, a sharp curve (303 m radius) in a unidirectional commuter train track used by X1 and X10 trains was chosen. The results of two contact cases were compared with the results of the Hertz analytical method and the program Contact. In the first contact case the wheel was in contact with the rail gauge corner. In the second case the wheel was in contact with the rail head. In both contact cases Hertz and Contact presented very similar results for the maximum contact pressure. For the first contact case, a significant difference was found between the FE method and the Hertz method and the program Contact in all of output data. The Hertz and Contact methods both presented a maximum contact pressure that was three times larger (around 3 GPa) than the FE solution. Here, the difference was probably due to the combination of both the half-space assumption and the elastic-plastic material model. For the second contact case, there was no significant difference between the maximum contact pressure results of the three different contact mechanics methods employed.

Wheel-Rail Interface Handbook

Many of the engineering problems of particular importance to railways arise at interfaces and the safety-critical role of the wheel-rail interface is widely acknowledged. Thus better understanding ...Out-of-round railway wheels can have a detrimental influence ontrack and vehicle components, contributing to increased risks of rail breaks, sleeper cracking, high-cycle fatigue of wheels and axles and bearing damage. Environmental consequences are rolling noise and impact noise, ground vibrations and passenger discomfort. Causes of different types of out-of-round wheels are surveyed. Experimental equipment and assessment procedures for quantification of out-of-roundness are explained. Results from field tests illustrating consequences of out-of-round wheels are presented. Wheel impact load detectors and examples of alarm limits used in different countries for detection of detrimental wheel defects are listed. Mathematical models and computer programs for simulation of the influence of out-of-round wheels on wheel-rail contact forces and vehicle/track responses are described.

Influence of leaf, humidity and applied lubrication on friction in the wheel-rail contact: Pin-on-disc experiments

Abstract A novel test method has been used to study how applied and natural lubrication (leaf and humidity) influences the coefficient of friction in the wheel-rail contact. A pin-on-disc tribometer placed in a climate chamber was used as the test equipment. The pin-on-disc contact simulates the wheel-rail contact caused by commuter train traffic on straight track. The results show that the coefficient of friction decreases when the relative humidity increases and decreases even more when a leaf is used as a lubricant. By using an elm leaf as the lubricant, the coefficient of friction is reduced by a factor of four compared with the unlubricated case. However, the coefficient of friction decreases even more when a rail lubricant is used.

Simulation of mild wear in boundary lubricated spherical roller thrust bearings

Owing to the curved contact surfaces in a spherical roller thrust bearing, the rollers will undergo sliding. For an unskewed roller there will be two points along each contact where the sliding velocity is zero. At all other points along the contact, sliding is present. Under boundary lubricated conditions the sliding can give rise to mild wear. Experimental results show that this wear can cause a significant change in the surface profile outside the zero sliding points. The mild wear in the contact was simulated using Archards wear law. An iterative wear model is described in which the normal load distribution, the tangential tractions and the sliding distances are repeatedly calculated to simulate the changes in surface geometry due to wear. Good qualitative agreement was achieved between the simulation results and the previously presented experimental results.

Surface cracks and wear of rail : a full-scale test on a commuter train track

Abstract Damage mechanisms such as surface cracks and wear on a rail can reduce the service life of a railway track. The purpose of this investigation was to study the development of these two damage mechanisms on new and 3-year-old rails in a commuter railway track over a period of 2 years. Four curves were studied with radius between 303 and 616m. In two of the curves, two different kinds of rail steel grade (UIC 900A grade with ultimate strength 900N/mm2 and UIC 1100 grade with ultimate strength 1100N/mm2) were used in each curve. In the other two curves, only the lower-strength rail was used. Four pieces of new rail, each 20m long, were inserted in the two curves with both UIC 900A and UIC 1100 grade rail. Lubrication was applied on the high rail of one of the curves with both UIC 900A and UIC 1100 grade rail and on one of the curves with only UIC 900A grade rail. The two remaining curves were not lubricated. Surface cracks in the form of headchecks could be noted on the surface of the new 1100 grade rails after 1 month of traffic. By contrast, the surface of the UIC 900A grade rails showed visible surface cracks in only two of four curves and that after approximately 2 years of traffic. Both materials seemed to be similarly sensitive to crack initiation but the 1100 grade rail was more sensitive to crack propagation and also more sensitive to the formation of headcheck cracks. Lubrication, as expected, reduced the profile change. A less expected outcome was that lubrication also reduced the rate of crack propagation; however, the lubricated UIC 1100 grade rail was as sensitive to crack initiation as the unlubricated UIC 1100 grade rail. By comparing the wear depth in the headcheck zone with the crack length, equilibrium between these two damage mechanisms was found for the lubricated UIC 1100 grade rail. Both the crack length and the wear depth showed low values. By using a lubricant with friction modifiers the stresses was low enough to prevent crack propagation; at the same time, the rail was hard enough to reduce the wear rate. This is probably the most favourable state in terms of rail maintenance cost.

Experimental Characterization of Wheel-Rail Contact Patch Evolution

The contact area and pressure distribution in a wheel/rail contact is essential information required in any fatigue or wear calculations to determine design life, re-grinding, and maintenance schedules. As wheel or rail wear or surface damage takes place the contact patch size and shape will change. This leads to a redistribution of the contact stresses. The aim of this work was to use ultrasound to nondestructively quantify the stress distribution in new, worn, and damaged wheel-rail contacts. The response of a wheel/rail interface to an ultrasonic wave can be modeled as a spring. If the contact pressure is high the interface is very stiff, with few air gaps, and allows the transmission of an ultrasonic sound wave. If the pressure is low, interfacial stiffness is lower and almost all the ultrasound is reflected. A quasistatic spring model was used to determine maps of contact stiffness from wheel/rail ultrasonic reflection data. Pressure was then determined using a parallel calibration experiment. Three different contacts were investigated; those resulting from unused, worn, and sand damaged wheel and rail specimens. Measured contact pressure distributions are compared to those determined using elastic analytical and numerical elastic-plastic solutions. Unused as-machined contact surfaces had similar contact areas to predicted elastic Hertzian solutions. However, within the contact patch, the numerical models better reproduced the stress distribution, as they incorporated real surface roughness effects. The worn surfaces were smoother and more conformal, resulting in a larger contact patch and lower contact stress. Sand damaged surfaces were extremely rough and resulted in highly fragmented contact regions and high local contact stress.

A Study of Airborne Wear Particles Generated From a Sliding Contact

Recently, much attention has been paid to the influence of airborne particles in the atmosphere on human health. Sliding contacts are a significant source of airborne particles in urban environment ...

Mapping Railway Wheel Material Wear Mechanisms and Transitions

In order to develop more durable wheel materials to cope with the new specifications being imposed on wheel wear, a greater understanding of the wear mechanisms and transitions occurring in wheel steels is needed, particularly at higher load and slip conditions. The aim of this work was to draw together current understanding of the wear mechanisms, regimes, and transitions (particularly with R8T wheel material) and new tests on R7T wheel material; to identify gaps in the knowledge; and to develop new tools for assessing wear of wheel materials, such as wear maps, that can be used to improve wear prediction. Wear assessment of wheel materials, as well as wear rates, regimes, and transitions, is discussed. Twin disc wear testing, used extensively for studying wear of wheel and rail materials, has indicated that three wear regimes exist for wheel materials: mild, severe, and catastrophic. These have been classified in terms of wear rate and features. Wear rates are seen to increase steadily initially and then level off, before increasing rapidly as the severity of the contact conditions is increased. Analysis of the contact conditions in terms of friction and slip has indicated that the levelling off of the wear rate observed at the first wear transition is caused by the change from partial slip to full slip conditions at the disc interface. Temperature calculations for the contact showed that the large increase in wear rates seen at the second wear transition may result from a thermally induced reduction in yield strength and other material properties. Comparisons made between discs and actual wheels have provided some support for the theories relating to the transitions observed. Wear maps have been produced using the test results to study how individual contact parameters such as load and sliding speed influence wear rates and transitions. The maps are also correlated to expected wheel—rail contact conditions. This improved understanding of wheel wear mechanisms and transitions will help in the aim of eventually attaining a wear modelling methodology reliant on material properties rather than on wear constants derived from testing.

A model for micro-slip between flat surfaces based on deformation of ellipsoidal elastic bodies

Micro-slip is a phenomenon that occurs between contacting surfaces when a frictional load, less than that necessary to produce macro-slip, is applied to the contacting surfaces. A model is presented for micro-slip between a flat smooth surface and a flat rough surface. The rough surface is covered with uniformly distributed ellipsoidal elastic bodies. The results from two test cases show that anisotropy of the contacting surfaces influences the tangential stiffness at zero displacement, the length of the micro-slip zone and the energy dissipated in the contact.