Ultrasonic testing using shear polarised waves is widely applied in medical and engineering fields, commonly employed for hardness or stress measurement. The advantage of ultrasonic shear waves for wear measurement lies in their lower wave propagation speed and their sensitivity in measuring the wear scars formed through the shear motion. An in-situ wear measurement method of thin metallic coatings using ultrasonic shear waves is proposed in this study. A finite element analysis was used to investigate the interaction between an ultrasonic shear wave and various geometric wear scars. Compared with longitudinal waves, the result shows that 10 MHz shear waves and 22 MHz longitudinal waves have a similar performance in the measurement of undamaged metallic coatings. Whereas, for discontinuously distributed scars, the 10 MHz shear wave shows an amplitude decrease, where the energy has been reflected to both sides. Then, the in-situ tests were conducted, and the shear wave measurements of wear were compared with the microscope results. For 350 mu m aluminium-alloy coated samples, the maximum deviation between shear wave results and microscope results was 5.13 mu m, with a relative error of 1.5%. For 250 mu m bronze-lead coatings, the maximum deviation was 5.54 mu m, with a relative error of 2.51%. The practicality of using shear waves to determine continuous wear progression in bearings is briefly discussed, and their potential for monitoring the health of bearing coatings in service.

Track failures in the form of thermally induced buckling events or rail breakages on continuously welded rails (CWRs) have long posed challenges for the rail industry. These issues arise because CWRs cannot freely expand or contract as rail temperatures fluctuate during operation, resulting in compressive or tensile stresses. As temperature variations increase, these stresses can lead to rail buckling under compression or separation under tension. Buckling, in particular, poses a serious threat, as it is often associated with derailments, carrying substantial social (rail safety, public perception) and economic (track downtime, damage to freight, rolling stock, and infrastructure) consequences. For this reason, understanding the thermal stress state or neutral rail temperature (NRT) of installed track is critical for rail network owners and operators. This knowledge enables more precise planning of maintenance, targeted rail replacement, and monitoring of high-risk areas. Consequently, there is growing demand for in-situ, non-destructive methods to measure NRT. This paper reviews existing methods for measuring NRT, encompassing approaches developed from the 1960s to the present. Each method is critically evaluated, with the findings summarized in a table that outlines their operational principles, limitations, reported accuracy, and current technology readiness level.

Shear-polarized ultrasonic sensors have been instrumented onto the outer liner surface of an RTX-6 large marine diesel engine. The sensors were aligned with the first piston ring at top dead center and shear ultrasonic reflectometry (comparing the variation in the reflected ultrasonic waves) was used to infer metal–metal contact between the piston ring and cylinder liner. This is possible as shear waves are not supported by fluids and will only transmit across solid-to-solid interfaces. Therefore, a sharp change in the reflected wave is an indicator of oil film breakdown. Two lubricant injection systems have been evaluated—pulse jet and needle lift-type injectors. The needle lift type is a prototype injector design with a reduced rate of lubricant atomization relative to pulse jet injectors. This is manifested as a smaller reduction in the reflected ultrasonic wave, showing less metal–metal contact had occurred. During steady-state testing, the oil feed rate was varied; the high flow rate case was shown to reduce the amount of piston ring–liner contact, while no changes in the lubricant film thickness had previously been detected using traditional longitudinal ultrasonic sensors. This displays the increased sensitivity of shear sensors relative to longitudinal sensors in respect to the quantity of lubricant present. Piston ring oil film breakdown was also studied at a range of steady-state loading levels and engine slow down, showing the amount of contact decreased as engine load decreased, providing a real-time indication of the lubrication regime of the piston rings. When the load was further decreased, into total shutdown of the engine, the amount of contact increased until the engine had stopped rotating. The study has demonstrated the capability of shear ultrasonic sensors to detect changes in solid contact caused by injector design, oil feed rate, engine load, and engine shut down.

Understanding the behaviour of engine and gear oils, especially the viscosity, under temperature and shear is important to improve machine operation. A novel viscometer using ultrasound is presented and used under a range of temperatures. A single cross-temperature (between 20°C and 60°C) ultrasonic calibration is shown to be sufficient. Next, the ultrasonic viscometer is compared to a conventional viscosity measurement technique for Newtonian and non-Newtonian lubricants. Newtonian viscosity standard fluids and shear-thinning engine oils are studied. Both viscosity measurement techniques match for Newtonian fluids but ultrasonic measurements are consistently lower for shear-thinning fluids. It suggests that the ultrasonic viscometer is similar to a high-shear viscometer with a shear rate of about 106s−1.

It has been demonstrated that acoustic-emission (AE), inspection of structures can offer advantages over other types of monitoring techniques in the detection of damage; namely, an increased sensitivity to damage, as well as an ability to localise its source. There are, however, numerous challenges associated with the analysis of AE data. One issue is the high sampling frequencies required to capture AE activity. In just a few seconds, a recording can generate very high volumes of data, of which a significant portion may be of little interest for analysis. Identifying the individual AE events in a recorded time-series is therefore a necessary procedure for reducing the size of the dataset and projecting out the influence of background noise from the signal. In this paper, a state-of-the-art technique is presented that can automatically identify cluster the AE events from a probabilistic perspective. A nonparametric Bayesian approach, based on the Dirichlet process (DP), is employed to overcome some of the challenges associated with this task. Additionally, the developed model is applied for damage detection using AE data collected from an experimental setup. Two main sets of AE data are considered in this work: (1) from a journal bearing in operation, and (2) from an Airbus A320 main landing gear subjected to fatigue testing.

This work explores experimentally the effects of DC electrical currents on lubricant film thickness alteration in lubricated sliding steel contacts in the boundary and mixed regime as measured by ultrasound. The experiments were performed in a two-electrode cell-based pin-on-disk tester instrumented with ultrasonic transducers. Unelectrified and electrified tribological tests were conducted on steel flat-on-flat contacts under various speeds and loads using both a mineral base oil and a gear oil. Film thickness, coefficient of friction (CoF), and electrical contact resistance (ECR) were measured during short experiments (30 s) in unelectrified and electrified (1.5 and 3 A) conditions. The results suggest that film thickness, CoF, and all ECR are altered by passing DC currents through the contact. In particular, film thickness increased and decreased, respectively, by applying electricity at the different speeds and loads tested. These alterations were majorly ascribed to oil viscosity decrease by local heat and surface oxidation caused by electrical discharge and break down at the interface.

Li-ion batteries (LIBs) are increasingly used in applications from personal electronics to electric vehicles (EVs) and grid scale storage. Research into LIB monitoring, such as state-of-charge (SOC) and state-of-health (SOH), and the effects of abuse on LIBs has received increased attention to allow for better battery performance and safety. To improve LIB safety better detection of thermal runaway (TR) is required for the mitigation of the associated consequences or to prevent it entirely. This paper reviews the growing field of ultrasound (US) sensing of LIBs for state monitoring and thermal runaway detection, with an additional perspective on of advancements made in thermal runaway testing. In this work, US is categorised by: hardware used in research; application for SOC and SOH monitoring. Further, TR is categorised by abuse scenario: overheating; penetration; overcharging; and gas generation. This review summarises the development of US to detect changes within a LIB. However, it is found that further developments are required to (1) isolate and characterise the various abuse/failure mechanisms using US and (2) decouple temperature and charge effects on the US signal. It is shown that decoupling the temperature-charge relationship within the US signal is necessary for accurate SOC and SOH monitoring.

This paper considers the problem of the dynamic modelling of macro slip in spherical roller bearings. By revisiting the fundamental physics which drive these systems, potential issues in existing models have been identified. Furthermore, in pure rolling conditions it was found that governing differential equations become “stiff”, requiring the use of implicit methods of time integration. The problem of individual roller macro slip in a wind turbine main bearing is then investigated using a simplified representation of system dynamics. Model results indicate clear links between slip/friction and the operational strategy of the wind turbine, as well as significantly higher frictional effects in the downwind main bearing row. Due to modelling simplifications, these results should not yet be considered conclusive, with further work required.

The aims of the research work summarised in this paper are twofold. The first goal is to make available a large number of new experimental results generated by testing specimens of grey cast iron under both constant and variable amplitude fretting fatigue loading. The second goal is to formulate an advanced fretting fatigue design approach based on the combined use of the Modified Wӧhler Curve Method, the Theory of Critical Distances and the Shear Stress-Maximum Variance Method. The validation exercise based on the experimental results being produced demonstrates that the proposed methodology is a powerful tool suitable for designing mechanical assemblies against fretting fatigue.

Wind turbine gearbox failures, many of which result from load-driven rolling element bearing failures, account for long downtimes and high maintenance costs. Non-invasive, in-situ measurement of bearing roller loads using ultrasound facilitates better understanding of the load history and failure mechanisms these gearbox bearings experience. In this study, ultrasonic load measurements from an instrumented high-speed shaft bearing were compared against results from a static multibody simulation. Measurements taken under steady operating conditions were found to agree well with the model. After quantification of the error in ultrasonic measurements the model enabled identification of load outliers which were attributed to transient events. Over- and under-loading instances may act to promote a variety of failure mechanisms, leading to premature bearing failure.

This paper describes a means to predict the internal structure of a lithium-ion battery from the response of an ultrasonic pulse, using a genetic algorithm. Lithium-ion batteries are sealed components and the internal states of the cell such as charge, health, and presence of structural defects are difficult to measure. Ultrasonic inspection of lithium-ion batteries is a recent and growing area of research. Reflected and transmitted ultrasound pulses are proposed as a non-invasive means of gaining insights into the internal structure and changes within the closed body of a cell. However, the multiple layers present in a lithium-ion cell are problematic when attempting to interpret waveforms as many internal reflections superimpose. Attributing specific features of a cell to wave characteristics is challenging.In this work a genetic algorithm has been developed as a means to reverse engineer a single ultrasound wave response to predict the internal layered structure of a lithium-ion battery cell. A first randomised guess at the layered structure is made. A numerical wave propagation model is used to predict the ultrasound waveform associated with that structure. This waveform is then compared with a measured or reference waveform to establish its fitness. The layered structure is generationally mutated until the predicted waveforms converge on the reference signal. As this occurs the predicted layered body reveals insights into the cell structure under inspection.Initially, the algorithm was tested against an idealised model battery and its predicted waveform, giving a model-model verification. Further, experimental ultrasonic reflection signals were captured from small capacity lithium-ion cells. Estimations of layer structure predicted by the model were compared with CT-scans of the cells to assess performance. The genetic algorithm was found to be effective in converging the predicted wave response to the reference signal and creating accurate battery structures. It was shown that only part of the waveform was required to generate accurate predictions, which is helpful in avoiding parts of the signal contaminated by near field transducer effects.It was demonstrated that the genetic algorithm can predict material wave speed to 40-1100 m/s (3-29 %) accuracy when battery layer geometry is provided; and thicknesses to within approximately 0.2-7.5 & mu;m (1-13 %) when material properties are provided. Providing the genetic algorithm with parameter constraints; either the layer topology and/or the material properties, substantially improved predictions to estimate the wave speeds on average to approximately & PLUSMN;50 m/s (3-4 %) and the layer thicknesses & PLUSMN;5 & mu;m (7-8 %).This raises the possibility of the use of this approach to predict state of charge when the battery construction is known, or the presence of internal defects and damage to a known battery material composition.

It has been shown that ultrasonic techniques work well for online measuring of circumferential oil film thickness profile in journal bearings; unfortunately, they can be limited by their measuring range and unable to capture details of the film all around the bearing circumference. Attempts to model the film thickness over the full range of the bearing rely on deterministic approaches, which assume the observations to be true with absolute certainty. Unaccounted uncertainties of the film thickness may lead to a cascade of inaccurate predictions for subsequent calculations of hydrodynamic parameters. In the present work, a probabilistic framework is proposed to model the film thickness with Gaussian Processes. The results are then used to estimate the location of the bearing shaft under various operational conditions. A further step in the process involves using the newly-constructed dataset to generate likelihood maps displaying the probable location of the shaft centre, given the bearing rotational speed and applied static load. The results offer the possibility to visualise the confidence of the predictions and allow the true location to be found within an area of high probability within the bearing’s bore.

This paper is the first in a two-part study on lubrication in wind turbine main bearings. Elastohy-drodynamic lubrication is a complex field, the formulas and results from which should not be applied blindly, but with proper awareness and consideration of their context, validity and limitations in any given case. The current paper, "Part 1", therefore presents an introductory review of elastohydrodynamic lubrication theory in order to provide this necessary background and context in an accessible form, promoting cross-disciplinary understanding. Fundamental concepts, derivations and formulas are presented, followed by the more advanced topics of starvation, non-steady effects, surface roughness interactions and grease lubrication. "Part 2" applies the presented material in order to analyse wind turbine main-bearing lubrication in the context of available film thickness formulas and related results from lubrication theory. Aside from the main-bearing, the material presented here is also applicable to other lubricated non-conformal contacts in wind turbines, including pitch and yaw bearings and gear teeth.

Acoustic impedance is an important property used to interpret acoustic reflection measurements in tests to determine oil film thickness, a critical parameter dictating efficiency and wear rates of lubricated components. A new method to measure acoustic impedance of solid media, based on the well-established spring model, is described. The advantage of this method over existing techniques is that it can be applied to thin, multi-layered materials where individual reflections cannot be distinguished, common in many tribological systems such as bearings, piston rings and piston liners. The method is demonstrated experimentally for a range of materials. Results compare well with values calculated independently from acoustic velocity and density. The method has been applied to a bearing test rig to determine acoustic impedance of a thin-walled bearing. This study demonstrates that the technique is capable of measurements in dynamic systems and where traditional methods of calculating acoustic impedance are not feasible.

The lubricating effectiveness of an oil film in a journal bearing depends on the dynamic viscosity of the oil. The viscosity in turn depends on the local operating temperature, pressure, and shear rate. Reproducing these conditions in a laboratory viscometer to investigate the lubricant behaviour is a challenging task. As a result, methods that allow oil viscosity measurement in-situ in a film, would be preferred. Ultrasound technology utilising shear bulk acoustic waves (BAW) has been used to measure liquid viscosity in the bulk, as well as in-situ in a film; the reflection of a shear BAW from a solid-liquid interface depends on the liquid viscosity. Surface acoustic waves (SAW) have been also used for measuring bulk liquid viscosity. In this paper, shear-horizontal surface acoustic waves (SH-SAWs) were explored for measuring oil film viscosity, as they present good coupling with liquids and sensitivity to surface changes. The main objectives of this work were to generate SH-SAWs on metallic media, investigate the wave response at the metal-oil interface, relate the wave response to viscosity with the aim to apply this knowledge to a journal bearing application for measuring viscosity in-situ the lubricant film. Initially, the SH-SAW response was investigated at a solid-liquid interface. SH-SAWs attenuate at the solid-liquid interface, due to the liquid viscosity. This was modelled as a function of the liquid properties, material and geometry of the medium, and wave frequency. The SH-SAW attenuation-viscosity model was used to calculate the viscosity (in the range of ∼3 to 4600 cP) of different oils at a free surface, which agreed with the viscosity values from datasheets and bench-top viscosity measurements. This approach was then implemented in-situ in a journal bearing application. A bearing sleeve was instrumented with a pair of SH-SAW transducers and a shear BAW transducer installed inside the rotating journal. These two approaches were used to measure the film viscosity of 4 lubricants blended with different additives in two ways; the former by the leakage of the surface wave, and the latter by the reflection of the bulk wave. Both approaches were found to be in good agreement. They successfully distinguished the chemistry of the oil test samples according to their viscosities under various loading conditions and constant speed, and were able to monitor changes in the oil film viscosity in the loaded region. The SH-SAW sensors used were low cost and small sized and so can be fitted relatively conveniently into a bearing sleeve, requiring nothing but a function generator and digitiser to operate. This approach could then be used to evaluate lubricant formulations for their performance actually inside a bearing, rather than through the extrapolation of data from a conventional bench top viscometer.

The lubricant inlet meniscus in a rolling element bearing acts as a reservoir that feeds the elastohydrodynamic contact, resulting in a sufficiently thick film to avoid wear. A shortening and/or thinning of the inlet meniscus towards the contact centre is an indicator of bearing starvation and impaired lubricant performance. This work introduces an ultrasonic method to measure meniscus dimensions. Using in situ ultrasonic sensors on a full-scale cylindrical roller bearing test rig, we show that ultrasonic waves can cause oil films to resonate, and that the resonant frequency is directly related to the film thickness. Using a benchtop rig we validate the relationship between resonant frequency and film thickness. This allows for the measurement of meniscus thickness and length in situ during bearing operation. Menisci were measured at both the inlet and outlet of rolling bearing line contacts while lubricated with different viscosity oils and grease. All lubricants used showed they could be monitored using this approach. The implication of this paper is that it is possible to measure a critically important lubrication mechanism during operation without major component modifications.

In this work, the feasibility of using low-sampled vibration signals for bolt joint tightness detection was investigated. Testing was carried out on multiple bolt joint configurations using a bench top electrodynamic shaker rig. Two data-processing methods were successfully used to deduce bolt joint loosening from the accelerometer measurements, namely the resonant frequency and regression methods (ARX and AR-ARX). Both methods were able to detect loosening of bolt joints, however, the latter possesses higher sensitivity in detecting the position of the loosened bolt among an array of bolts. As the resonant frequency of wind turbines is low (0.35-2 Hz), the minimum sampling rate for bolt joint tightness detection is consequently also low (twice the resonant frequency). This facilitates potential use of existing accelerometer instrumentation on wind turbines, typically sampled at low rates.

Lubrication is essential in metal cold rolling operation to regulate friction at the metal-roll interface, reduce energy loss and improve the product surface finish. A novel non-intrusive pitch-catch technique, based on the reflection of ultrasound, was employed on a pilot mill to evaluate oil-film thickness at the metal-roll interface during the metal cold rolling operation. During the metal rolling process, oil-film thickness was measured under varying rolling load and rolling speed. The oil-film thickness increases as the roll speed increases and reduces as the rolling load increases. The values of oil-film thickness obtained from this non-intrusive ultrasonic technique agree with theoretical values. This study is a proof of concept and has shown promising results. If further developed, the technique could be employed for in situ monitoring of lubricant during rolling operation in metal rolling industries. [GRAPHICS]

Abstract This article describes the design of a journal bearing test platform capable of high-accuracy film thickness measurements via ultrasonic transducers permanently embedded in both the bearing and rotating shaft. A bespoke hydraulic loading system with programmable valves allowed the application of dynamic loads with defined loading patterns, including the simulation of loading patterns found in real components such as automotive connecting rod bearings. Tests under a range of rotation speeds, temperatures, and loading profiles allowed the detailed analysis of film thickness response to rapidly changing loads. Unlike conventional methods, the ultrasonic technique offers a noninvasive direct measurement of the shaft–bearing interface, thus enabling the study of phenomena such as squeeze film effects. Measurements were achieved by applying a novel referencing method, referred to as the “snapshot technique”. Via this method, squeeze time was shown to reduce with increasing shaft rotation speed, applied load, and bearing temperature. Results were compared against inductive sensors and numerical techniques and good agreement was observed.

This paper is the second in a two-part study on lubrication in wind turbine main bearings. Where Part 1 provided an introductory review of elastohydrodynamic lubrication theory, this paper will apply those ideas to investigate lubrication in the double-row spherical roller main bearing of a 1.5 MW wind turbine. Lubrication is investigated across a “contact conditions dataset” generated by inputting main-bearing applied loads, estimated from hub loads generated using aeroelastic simulation software, into a Hertzian contact model of the main bearing. From the Hertzian model is extracted values of roller load and contact patch dimensions, along with the time rate of change of contact patch dimensions. Also included in the dataset are additional environmental and operational variable values (e.g. wind speeds and shaft rotational speeds). A suitable formula for estimating film thickness within this particular bearing is then identified. Using lubricant properties of a commercially available wind turbine grease, specifically marketed for use in main bearings, an analysis of film thickness across the generated dataset is undertaken. The analysis includes consideration of effects relating to temperature, starvation, grease thickener interactions and possible non-steady effects. Results show that the studied main bearing is at risk of operating under mixed lubrication conditions for a non-negligible proportion of its operational life, indicating that further work is required to better understand lubrication in this context and implications for main-bearing damage and operational lifetimes. Key sensitivities and uncertainties within the analysis are discussed, along with recommendations for future work.