R.S. Dwyer-Joyce

A Study of the Interaction between Ultrasound and a Partially Contacting Solid--Solid Interface

The measurement of the reflection of ultrasonic waves from a partially contacting solid-solid interface can be used to study the contact conditions at that interface. This paper describes measurements and predictions of the reflection of ultrasonic waves from partially contacting aluminium-aluminium interfaces, performed in the low frequency regime where the wavelength of the ultrasound is large compared to the size of the gaps. The proportion of the incident wave which is reflected at the interface (the reflection coefficient) was measured as a function of frequency with a single wideband ultrasonic transducer. When load was applied across the interface three regions of contact can be seen; no contact, partial contact and perfect contact. In the no contact region the measured reflection coefficient was unity at all frequencies. In the partial contact region the measured reflection coefficient increased with frequency. No measurements were taken in the perfect contact region in which the reflection coefficient is known to be zero at all frequencies as this state is the same as a continuous piece of aluminium. The reflection coefficient variation with frequency was modelled using a spring model, good agreement between experiments and predictions being achieved. Reflection coefficient measurements were then used to study the contact between two aluminium surfaces under repeated loading and unloading cycles. Plastic flow on first loading was evident while subsequent loading cycles revealed largely elastic behaviour. Both elastic and plastic statistical contact models, as well as a numerical contact model, were used to predict the variation of interfacial stiffness with pressure. These models agreed qualitatively with the experimentally determined stiffness variations and the predicted stiffness was within an order of magnitude of the measured value in all cases.

The measurement of lubricant-film thickness using ultrasound

Ultrasound is reflected from a liquid layer between two solid bodies. This reflection depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. If the wavelength is much greater than the liquid–layer thickness, then the response is governed by the stiffness of the layer. If the wavelength and layer thickness are similar, then the interaction of ultrasound with the layer is controlled by its resonant behaviour. This stiffness governed response and resonant response can be used to determine the thickness of the liquid layer, if the other parameters are known. In this paper, ultrasound has been developed as a method to determine the thickness of lubricating films in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant–film layer. The transducer is used to both emit and receive wide–band ultrasonic pulses. For a particular lubricant film, the reflected pulse is processed to give a reflection–coefficient spectrum. The lubricant–film thickness is then obtained from either the layer stiffness or the resonant frequency. The method has been validated using fluid wedges at ambient pressure between flat and curved surfaces. Experiments on the elastohydrodynamic film formed between a sliding ball and a flat surface were performed. Film–thickness values in the range 50–500 nm were recorded, which agreed well with theoretical film–formation predictions. Similar measurements have been made on the oil film between the balls and outer raceway of a deep–groove ball bearing.

An investigation into the mechanisms of closed three-body abrasive wear

Abstract Contacting components frequently fail by abrasion caused by solid contaminants in the lubricant. This process can be classified as a closed three-body abrasive wear process. The mechanisms by which trapped particles cause material removal are not fully understood. This paper describes tests using model elastohydrodynamic contacts to study these mechanisms. An optical elastohydrodynamic lubrication rig has been used to study the deformation and fracture of ductile and brittle lubricant-borne debris. A ball-on-disk machine was used to study the behaviour of the particles in partially sliding contacts. Small diamond particles were used as abrasives since these were thought not to break down in the contact; wear could then be directly related to particles of a known size. The particles were found to embed in the softer surface and to scratch the harder. The mass of material worn from the ball surface was approximately proportional to the particle sliding distance and abrasive concentration. Small particles tumbled through the contact, whilst larger particles ploughed. Mass loss was found to increase with abrasive particle size. Individual abrasion scratches have been measured and related to the abrading particle. A simple model of the abrasive process has been developed and compared with experimental data. The discrepancies are thought to be the result of the uncertainty about the entrainment of particles into the contact.

The Use of Ultrasound in the Investigation of Rough Surface Interfaces

The measurement of ultrasonic reflection has been used to study the contact between rough surfaces. An incomplete interface will reflect some proportion of an incident wave; this proportion is known as the reflection coefficient, If the wavelength is large compared with the width of the gaps in the plane of the interface then the reflection mechanism can be modeled by considering the interface as a spring. The proportion of the incident wave reflected (reflection coefficient) is then a function of the stiffness of the interface and the frequency of the ultrasonic wave. The sensitivity of the ultrasonic technique has been quantified using a simple model, from which the stiffness of individual gaps and contacts are calculated and their effect on the ultrasonically measured stiffness predicted. The reflection of ultrasound at a static interface between a rough, nominally flat aluminum plate and a rough, nominally flat hardened steel punch has been investigated. Plastic flow on first loading was evident, while repeated loading was largely elastic. However, subsequent cycles indicate a small amount of further plasticity and contact irreversibility. The effect of surface roughness on the resultant contact has also been investigated. A simple plastic contact model is described which allows prediction of the average size of the asperity contacts and their number. This model shows that the average size of the contacts remains constant over most of the loading whereas the number of contacts increases almost linearly. The contact stiffness has also been modeled with two well known elastic rough surface contact models. These models predicted a lower interface stiffness than was observed in the experiments. However they provide a useful way of interpreting the ultrasonically measured interface stiffness data.

Wear mechanisms and transitions in railway wheel steels

Abstract The need to improve safety and reduce costs means that new specifications are being imposed on railway wheel wear. These mean that more durable wheel steels are required. In order to develop such materials, a greater understanding is needed of the wear mechanisms and transitions occurring in wheel steels. In this work, twin-disc wear testing has been carried out to study the wear characteristics of R8T railway wheel steel. The results have indicated that, compared with previous wheel steels, R8T offers greater wear resistance. Three wear regimes were identified; mild, severe, and catastrophic. Wear rates were seen to increase steadily initially and then to level off, before increasing rapidly as the severity of the contact conditions increased. This paper is concerned with the form of the data and the reasons for the transitions. Analysis of the contact conditions indicated that the first transition in the wear rate was 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. This improved understanding will help in progressing towards the aim of eventually attaining a wear modelling methodology reliant on material properties rather than wear constants derived from testing.

Wear of human teeth: A tribological perspective

Abstract The four main types of wear in teeth are attrition (enamel-on-enamel contact), abrasion (wear due to abrasive particles in food or toothpaste), abfraction (cracking in enamel and subsequent material loss), and erosion (chemical decomposition of the tooth). They occur as a result of a number of mechanisms including thegosis (sliding of teeth into their lateral position), bruxism (tooth grinding), mastication (chewing), toothbrushing, tooth flexure, and chemical effects. In this paper the current understanding of wear of enamel and dentine in teeth is reviewed in terms of these mechanisms and the major influencing factors are examined. In vitro tooth wear simulation and in vivo wear measurement and ranking are also discussed.

Wear debris and associated wear phenomena - fundamental research and practice

Abstract The effect and consequences of wear on the present-day operation and maintenance of industrial machinery are considerable and far reaching in terms of the performance, cost and underlying business environment. One of the ways open to maintenance engineers to keep track of the wear occurring is to monitor the condition, or ‘health’, of critical items of machinery by either measuring or capturing representative samples of wear products, primarily the debris that is generated in the contact which is subsequently released and transported away from the source by the lubricant behaving as a carrier fluid. Commencing with a brief description of some wear-related failures of critical components, such as bearings and gears, the different manifestations of wear debris are reviewed with particular regard to the methods employed to measure or capture the debris and how their characteristics are determined. Of particular importance is the need to establish and single out those morphological features which relate to the underlying wear mechanisms involved which led to their generation. The manifestation of wear phenomena and the causal effects are to be addressed by examining the mechanisms in the light of current fundamental research in which the links between wear and the generation of the associated debris are reviewed and discussed.

Wear at the wheel/rail interface when sanding is used to increase adhesion

Abstract Sanding is used in train operation to improve adhesion at the wheel/rail interface during both braking and traction. An experimental study has been carried out to determine the effect of sanding on the wear of wheel and rail materials. Static tests were performed using actual wheel and rail sections. Dynamic tests were carried out with and without sand on a twin disc machine, where wheel and rail steel discs are loaded together and driven under controlled conditions of rolling and sliding. Sand was fed into the disc contact through a standard compressed air sanding valve. In both static and dynamic tests, sand caused severe surface damage. During the dynamic tests, because of the application of sand, wear increased by factors between 2 and 10. The wheel steel wear rates showed the largest increases. Wear in wet conditions was higher than that in dry conditions because the wet discs entrained a larger amount of sand through the contact that otherwise was ejected when the discs were run dry. The mechanisms of sanding wear have been investigated. Severe plastic flow as well as a high material removal rate and surface corrugation takes place. The rail discs are scored by sand particles that embed and stick in the softer wheel disc. As well as wear by abrasion, the discs were subject to a rapid fatigue process and large chunks of material fractured from the surface. A simple abrasive wear model has been developed to predict wear of rail material caused by sand in the wheel/rail contact, which shows good correlation with test results. There are a number of idealizations inherent in the test simulations that lead to increased severity over the actual wheel/rail contact. These include the amount of sand entering the contacts in both types of test and the disc geometry and motion in the dynamic tests. While the twin disc specimens have been scaled down, the sand is as used on the railway network, which may lead to surface damage appearing more severe than it would be in an actual wheel/rail contact. Results are therefore only to be taken as a guide to what happens in the full size wheel/rail interface.

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 the transmission of ultrasound across solid–rubber interfaces

The reflection coefficient at the solid–rubber interface of a rubber coupled ultrasonic transducer may be used as a measure of its coupling performance; the lower the solid–rubber reflection coefficient, the better the coupling. An experimental and theoretical study of the transmission of ultrasound across dry coupled solid–rubber interfaces, and in particular the effect of surface roughness and particulate contaminants on solid coupling, is described. The effect of surface roughness is modeled using a numerical contact model of the solid–rubber interface from which the static stiffness is calculated. The calculated interfacial stiffness is then used in a spring model of the interface to predict the solid–rubber reflection coefficient behavior. By repeating this process for different loads applied to the solid–rubber interface the variation of reflection coefficient with interfacial pressure is found. The agreement between these predictions and the experiments is shown to be reasonable in general and good...