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  • JOURNAL ARTICLE
    Carpenter G, Bozorgi S, Vladescu S, Forte AE, Myant C, Potineni RV, Reddyhoff T, Baier SKet al., 2019,

    A study of saliva lubrication using a compliant oral mimic

    , Food Hydrocolloids, Vol: 92, Pages: 10-18, ISSN: 0268-005X

    © 2019 Elsevier Ltd Due to ethical issues and the difficulty in obtaining biological tissues, it is important to find synthetic elastomers that can be used as replacement test media for research purposes. An important example of this is friction testing to understand the mechanisms behind mouthfeel attributes during food consumption (e.g. syrupy, body and clean finish), which requires an oral mimic. In order to assess the suitability of possible materials to mimic oral surfaces, a sliding contact is produced by loading and sliding a hemispherical silica pin against either a polydimethyl siloxane (PDMS), agarose, or porcine tongue sample. Friction is measured and elastohydrodynamic film thickness is calculated based on the elastic modulus of the samples, which is measured using an indentation method. Tests were performed with both saliva and pure water as the lubricating fluid and results compared to unlubricated conditions. PDMS mimics the tongue well in terms of protein adhesion, with both samples showing significant reductions in friction when lubricated with saliva versus water, whereas agarose showed no difference between saliva and water lubricated conditions. This is attributed to PDMS's –O–Si(CH3)2- group which provides excellent adhesion for the saliva protein molecules, in contrast with the hydrated agarose surface. The measured modulus of the PDMS (2.2 MPa) is however significantly greater than that of tongue (3.5 kPa) and agarose (66–174 kPa). This affects both the surface (boundary) friction, at low sliding speeds, and the entrained elastohydrodynamic film thickness, at high speeds. Utilising the transparent PDMS sample, we also use fluorescence microscopy to monitor the build-up and flow of dyed-tagged saliva proteins within the contact during sliding. Results confirm the lubricous boundary film forming nature of saliva proteins by showing a strong correlation between friction and average protein intensity signals (cross correlati

  • JOURNAL ARTICLE
    Peng B, Spikes H, Kadiric A, 2019,

    The Development and Application of a Scuffing Test Based on Contra-rotation

    , TRIBOLOGY LETTERS, Vol: 67, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Geng Z, Puhan D, Reddyhoff T, 2019,

    Using acoustic emission to characterize friction and wear in dry sliding steel contacts

    , Tribology International, Vol: 134, Pages: 394-407, ISSN: 0301-679X

    © 2019 Acoustic emission (AE) was recorded during tribological tests on 52,100 steel specimens under different loads. AE signals were transformed to the frequency domain using a Fast Fourier Transform and parameters such as power, RMS amplitude, mean frequency, and energy were analyzed and compared with friction coefficient and wear volume measurements. Results show that certain acoustic frequencies reflect friction while others reflect wear. If frequencies are chosen optimally, AE and friction signals are highly correlated (Pearson coefficients >0.8). SEM and Raman analysis reveal how plastic deformation and oxide formation affect friction, wear and AE simultaneously. AE recordings contains more information than conventional friction and wear volume measurements and are more sensitive to changes in mechanism. This all demonstrates AE's potential as a tool to monitor tribological behavior.

  • JOURNAL ARTICLE
    Rosenkranz A, Costa HL, Profito F, Gachot C, Medina S, Dini Det al., 2019,

    Influence of surface texturing on hydrodynamic friction in plane converging bearings - An experimental and numerical approach

    , Tribology International, Vol: 134, Pages: 190-204, ISSN: 0301-679X

    © 2019 Elsevier Ltd The frictional behaviour of plane converging bearings was experimentally and numerically studied for four texture geometries fabricated by ultra-short pulse laser texturing (single pocket, line-, cross- and dot-like texture) and convergence ratios under full-film lubrication in the presence of thick oil films (up to 100 μm). Regarding the experiments, small variations in the spread of results between different textures and a general improvement over the untextured reference can be observed. Numerical simulations help to clarify the expected variations and conditions under which these occur. For high convergences, the simulations demonstrated that textures are beneficial for friction reduction, regardless of load and relative texture's position. For low convergences, a significant friction reduction occurs for textures being located at the bearing's inlet.

  • JOURNAL ARTICLE
    Vlădescu SC, Fowell M, Mattsson L, Reddyhoff Tet al., 2019,

    The effects of laser surface texture applied to internal combustion engine journal bearing shells – An experimental study

    , Tribology International, Pages: 317-327, ISSN: 0301-679X

    © 2019 Elsevier Ltd We present an experimental study into the impact of surface texturing on crank shaft bearing friction. This was achieved by applying a series of laser-etched patterns to the surface of shell components and comparing friction measurements - obtained using a journal bearing test rig - to those of a non-texture reference. Tests were performed under a range of applied loads, speeds and lubricant viscosities in order to simulate the operating conditions encountered in an engine. Three texture configurations were assessed with varying dimple density and area coverage. The lubricant film temperature was also measured in order to remove shear heating effects and deduce accurate friction values. The bearing shell with a texture pattern applied to the whole of its circumference showed substantial reduction in friction (approximately 18%) when compared with the non-textured reference. This reduction in friction increases as the film thickness increases (i.e.: moving right on the Stribeck curve) and is most pronounced under full film hydrodynamic conditions. Friction reductions of up to 13% were observed using bearing shells that were only textured outside the loaded region. This is attributed to a reduction in shear rate due to an increased gap without alteration of the pressure generation and is important since this texture configuration maintains its effectiveness, even after the heavily loaded region becomes worn. These reported hydrodynamic friction reductions have practical implications for automotive bearings.

  • JOURNAL ARTICLE
    Sufian A, Knight C, O'Sullivan C, Van Wachem B, Dini Det al., 2019,

    Ability of a pore network model to predict fluid flow and drag in saturated granular materials

    , Computers and Geotechnics, Vol: 110, Pages: 344-366, ISSN: 0266-352X

    The local flow field and seepage induced drag obtained from Pore Network Models (PNM) is compared to Immersed Boundary Method (IBM) simulations, for a range of linear graded and bimodal samples. PNM were generated using a weighted Delaunay Tessellation (DT), along with the Modified Delaunay Tessellation (MDT) which considers the merging of tetrahedral Delaunay cells. Two local conductivity models are compared in simulating fluid flow in the PNM. The local pressure field was very accurately captured, while the local flux (flow rate) exhibited more scatter and sensitivity to the choice of the local conductance model. PNM based on the MDT clearly provided a better correlation with the IBM. There was close similarity in the network shortest paths, indicating that the PNM captures dominant flow channels. Comparison of streamline profiles demonstrated that local pressure drops coincided with the pore constrictions. A rigorous validation was undertaken for the drag force calculated from the PNM by comparing with analytical solutions for ordered array of spheres. This method was subsequently applied to all samples, and the calculated force was compared with the IBM data. Linear graded samples were able to calculate the force with reasonable accuracy, while the bimodal samples exhibited slightly more scatter.

  • JOURNAL ARTICLE
    Ewen JP, Gao H, Müser MH, Dini Det al., 2019,

    Shear heating, flow, and friction of confined molecular fluids at high pressure.

    , Phys Chem Chem Phys, Vol: 21, Pages: 5813-5823

    Understanding the molecular-scale behavior of fluids confined and sheared between solid surfaces is important for many applications, particularly tribology where this often governs the macroscopic frictional response. In this study, nonequilibrium molecular dynamics simulations are performed to investigate the effects of fluid and surface properties on the spatially resolved temperature and flow profiles, as well as friction. The severe pressure and shear rate conditions studied are representative of the elastohydrodynamic lubrication regime. In agreement with tribology experiments, flexible lubricant molecules give low friction, which increases linearly with logarithmic shear rate, while bulky traction fluids show higher friction, but a weaker shear rate dependence. Compared to lubricants, traction fluids show more significant shear heating and stronger shear localization. Models developed for macroscopic systems can be used to describe both the spatially resolved temperature profile shape and the mean film temperature rise. The thermal conductivity of the fluids increases with pressure and is significantly higher for lubricants compared to traction fluids, in agreement with experimental results. In a subset of simulations, the efficiency of the thermostat in one of the surfaces is reduced to represent surfaces with lower thermal conductivity. For these unsymmetrical systems, the flow and the temperature profiles become strongly asymmetric and some thermal slip can occur at the solid-fluid interface, despite the absence of velocity slip. The larger temperature rises and steeper velocity gradients in these cases lead to large reductions in friction, particularly at high pressure and shear rate.

  • JOURNAL ARTICLE
    Reddyhoff T, Schmidt A, Spikes H, 2019,

    Thermal Conductivity and Flash Temperature

    , TRIBOLOGY LETTERS, Vol: 67, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Yu M, Arana C, Evangelou SA, Dini Det al., 2019,

    Quarter-Car Experimental Study for Series Active Variable Geometry Suspension

    , IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, Vol: 27, Pages: 743-759, ISSN: 1063-6536
  • JOURNAL ARTICLE
    Manieri F, Stadler K, Morales-Espejel GE, Kadiric Aet al., 2019,

    The origins of white etching cracks and their significance to rolling bearing failures

    , INTERNATIONAL JOURNAL OF FATIGUE, Vol: 120, Pages: 107-133, ISSN: 0142-1123
  • JOURNAL ARTICLE
    Vladescu S-C, Putignano C, Marx N, Keppens T, Reddyhoff T, Dini Det al., 2019,

    The Percolation of Liquid Through a Compliant Seal-An Experimental and Theoretical Study

    , JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 141, ISSN: 0098-2202
  • JOURNAL ARTICLE
    Stevenson H, Jaggard M, Akhbari P, Vaghela U, Gupte C, Cann Pet al., 2019,

    The role of denatured synovial fluid proteins in the lubrication of artificial joints

    , Biotribology, ISSN: 2352-5738
  • JOURNAL ARTICLE
    Hu S, Reddyhoff T, Wen J, Huang W, Shi X, Dini D, Peng Zet al., 2019,

    Characterization and simulation of bi-Gaussian surfaces induced by material transfer and additive processes

    , Tribology International, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Puhan D, Wong J, 2019,

    Properties of Polyetheretherketone (PEEK) transferred materials in a PEEK-steel contact

    , Tribology International, ISSN: 0301-679X

    Polyetheretherketone (PEEK) is a high performance polymer that can be an alternative to metal for some moving components in unlubricated conditions. During rubbing, PEEK is transferred to the counterface. The formation and properties of PEEK transfer films on steel and sapphire are studied by in-situ observations of PEEK wear process, contact temperatures and triboemission, as well as FTIR and Raman spectroscopies ex situ. Our results suggest that frictional heating alone may not be sufficient to generate PEEK degradation observed in the transfer materials. Triboplasma observed during rubbing, together with the mechanical shear, may promote generations of radicals and degradation of PEEK, which subsequently influence the properties of PEEK transfer film and performance of polymer-metal tribopair.

  • JOURNAL ARTICLE
    Fatti G, Righi MC, Dini D, Ciniero Aet al.,

    First-Principles Insights into the Structural and Electronic Properties of PTFE in its High-Pressure Phase (Form III)

    , Journal of Physical Chemistry C, ISSN: 1932-7447
  • JOURNAL ARTICLE
    Vidotto M, Botnariuc D, De Momi E, Dini Det al., 2019,

    A computational fluid dynamics approach to determine white matter permeability.

    , Biomech Model Mechanobiol

    Glioblastomas represent a challenging problem with an extremely poor survival rate. Since these tumour cells have a highly invasive character, an effective surgical resection as well as chemotherapy and radiotherapy is very difficult. Convection-enhanced delivery (CED), a technique that consists in the injection of a therapeutic agent directly into the parenchyma, has shown encouraging results. Its efficacy depends on the ability to predict, in the pre-operative phase, the distribution of the drug inside the tumour. This paper proposes a method to compute a fundamental parameter for CED modelling outcomes, the hydraulic permeability, in three brain structures. Therefore, a bidimensional brain-like structure was built out of the main geometrical features of the white matter: axon diameter distribution extrapolated from electron microscopy images, extracellular space (ECS) volume fraction and ECS width. The axons were randomly allocated inside a defined border, and the ECS volume fraction as well as the ECS width maintained in a physiological range. To achieve this result, an outward packing method coupled with a disc shrinking technique was implemented. The fluid flow through the axons was computed by solving Navier-Stokes equations within the computational fluid dynamics solver ANSYS. From the fluid and pressure fields, an homogenisation technique allowed establishing the optimal representative volume element (RVE) size. The hydraulic permeability computed on the RVE was found in good agreement with experimental data from the literature.

  • JOURNAL ARTICLE
    Puhan D, Nevshupa R, Wong JSS, Reddyhoff Tet al., 2019,

    Transient aspects of plasma luminescence induced by triboelectrification of polymers

    , TRIBOLOGY INTERNATIONAL, Vol: 130, Pages: 366-377, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Porte E, Cann P, Masen M, 2019,

    Fluid load support does not explain tribological performance of PVA hydrogels

    , JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS, Vol: 90, Pages: 284-294, ISSN: 1751-6161
  • JOURNAL ARTICLE
    Dzepina B, Balint D, Dini D, 2019,

    A phase field model of pressure-assisted sintering

    , JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 39, Pages: 173-182, ISSN: 0955-2219
  • JOURNAL ARTICLE
    Jeffreys S, di Mare L, Liu X, Morgan N, Wong JSSet al., 2019,

    Elastohydrodynamic lubricant flow with nanoparticle tracking

    , RSC ADVANCES, Vol: 9, Pages: 1441-1450, ISSN: 2046-2069
  • JOURNAL ARTICLE
    Ma S, Scaraggi M, Yan C, Wang X, Gorb SN, Dini D, Zhou Fet al., 2019,

    Bioinspired 3D Printed Locomotion Devices Based on Anisotropic Friction

    , SMALL, Vol: 15, ISSN: 1613-6810
  • JOURNAL ARTICLE
    Jean-Fulcrand A, Maser MA, Bremner T, Wong JSSet al., 2019,

    Effect of temperature on tribological performance of polyetheretherketone-polybenzimidazole blend

    , TRIBOLOGY INTERNATIONAL, Vol: 129, Pages: 5-15, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Wen J, Dini D, Reddyhoff T, 2019,

    Design and optimization of a liquid ring thrust bearing

    , Tribology International, ISSN: 0301-679X

    © 2019 Liquid menisci at millimeter length scales and smaller exhibit large Laplace pressures. To utilise these effects, liquid ring bearings have recently been developed, which consist of liquid rings confined between alternate superhydrophobic and hydrophilic patterns. We present a detailed experimental and theoretical performance analysis of such bearings. For a single, 100 μm thickness, liquid ring, the maximum supporting force is 0.13 N, which decreases with increasing the ring misalignment. The frictional torque increases linearly with rotational speed until a critical Reynolds number is reached. Above this, an instability occurs due the concave liquid ring meniscus, which further increases friction. These results show how liquid ring bearings can be optimised.

  • JOURNAL ARTICLE
    Dench J, di Mare L, Morgan N, Wong JSSet al., 2018,

    Comparing the molecular and global rheology of a fluid under high pressures

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 20, Pages: 30267-30280, ISSN: 1463-9076
  • JOURNAL ARTICLE
    Tan Z, Dini D, Rodriguez y Baena F, Forte AEet al., 2018,

    Composite hydrogel: A high fidelity soft tissue mimic for surgery

    , MATERIALS & DESIGN, Vol: 160, Pages: 886-894, ISSN: 0264-1275
  • JOURNAL ARTICLE
    Dawczyk J, Ware E, Ardakani M, Russo J, Spikes Het al., 2018,

    Use of FIB to Study ZDDP Tribofilms

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Kontou A, Southby M, Morgan N, Spikes HAet al., 2018,

    Influence of Dispersant and ZDDP on Soot Wear

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Ewen JP, Heyes DM, Dini D, 2018,

    Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

    , FRICTION, Vol: 6, Pages: 349-386, ISSN: 2223-7690
  • JOURNAL ARTICLE
    Vladescu S-C, Marx N, Fernandez L, Barcelo F, Spikes Het al., 2018,

    Hydrodynamic Friction of Viscosity-Modified Oils in a Journal Bearing Machine

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Ebrahimi MT, Dini D, Balint DS, Sutton AP, Ozbayraktar Set al., 2018,

    Discrete crack dynamics: A planar model of crack propagation and crack-inclusion interactions in brittle materials

    , INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 152, Pages: 12-27, ISSN: 0020-7683
  • JOURNAL ARTICLE
    Verschueren J, Gurrutxaga-Lerma B, Balint DS, Sutton AP, Dini Det al., 2018,

    Instabilities of High Speed Dislocations

    , PHYSICAL REVIEW LETTERS, Vol: 121, ISSN: 0031-9007
  • JOURNAL ARTICLE
    Hartinger M, Reddyhoff T, 2018,

    CFD modeling compared to temperature and friction measurements of an EHL line contact

    , TRIBOLOGY INTERNATIONAL, Vol: 126, Pages: 144-152, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Yu M, Arana C, Evangelou SA, Dini D, Cleaver GDet al., 2018,

    Parallel Active Link Suspension: A Quarter-Car Experimental Study

    , IEEE-ASME TRANSACTIONS ON MECHATRONICS, Vol: 23, Pages: 2066-2077, ISSN: 1083-4435
  • JOURNAL ARTICLE
    Gattinoni C, Ewen JP, Dini D, 2018,

    Adsorption of Surfactants on alpha-Fe2O3(0001): A Density Functional Theory Study

    , JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 20817-20826, ISSN: 1932-7447
  • JOURNAL ARTICLE
    Marx N, Fernandez L, Barcelo F, Spikes Het al., 2018,

    Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part I: Shear Thinning Behaviour

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Marx N, Fernández L, Barceló F, Spikes Het al., 2018,

    Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part II: Impact of Shear Thinning on Journal Bearing Friction

    , Tribology Letters, Vol: 66, ISSN: 1023-8883

    © 2018, The Author(s). In a companion paper, the temporary shear thinning behaviour of a series of viscosity-modifier (VM)-containing blends was studied over a wide shear rate and temperature range [Marx et al. in Tribol Lett, https://doi.org/10.1007/s11249-018-1039-5]. It was found that for almost all VMs the resulting data could be collapsed on a single viscosity versus reduced strain rate curve using time–temperature superposition. This made it possible to derive a single equation to describe the viscosity–shear rate behaviour for each VM blend. In the current paper, these shear thinning equations are used in a Reynolds-based hydrodynamic lubrication model to explore and compare the impact of different VMs on the film thickness and friction of a lubricated, isothermal journal bearing. It is found that VMs reduce friction and especially power loss markedly at high shaft speeds, while still contributing to increased hydrodynamic film thickness at low speeds. The model indicates that VMs can contribute to reducing friction in two separate ways. One is via shear thinning. This occurs especially at high bearing speeds when shear rates are large and can result in a 50% friction reduction compared to the equivalent isoviscous oil at low temperatures for the blends studied. The second is via their impact on viscosity index, which means that for a set viscosity at high temperature the low-shear-rate (and thus the high shear rate) viscosity of a high-VI oil, and consequently its hydrodynamic friction, will be lower at low temperatures than that of a low-VI oil. The identification and quantification of these two alternative ways to reduce friction should assist in the design of new, fuel-efficient VMs.

  • JOURNAL ARTICLE
    Reddyhoff T, Underwood RJ, Sayles RS, Spikes HAet al., 2018,

    Temperature measurement of debris particles in EHL contacts

    , SURFACE TOPOGRAPHY-METROLOGY AND PROPERTIES, Vol: 6, ISSN: 2051-672X
  • JOURNAL ARTICLE
    Campen S, Smith B, Wong J, 2018,

    Deposition of Asphaltene from Destabilized Dispersions in Heptane-Toluene

    , ENERGY & FUELS, Vol: 32, Pages: 9159-9171, ISSN: 0887-0624
  • JOURNAL ARTICLE
    Vakis AI, Yastrebov VA, Scheibert J, Nicola L, Dini D, Minfray C, Almqvist A, Paggi M, Lee S, Limbert G, Molinari JF, Anciaux G, Aghababaei R, Restrepo SE, Papangelo A, Cammarata A, Nicolini P, Putignano C, Carbone G, Stupkiewicz S, Lengiewicz J, Costagliola G, Bosia F, Guarino R, Pugno NM, Mueser MH, Ciavarella Met al., 2018,

    Modeling and simulation in tribology across scales: An overview

    , TRIBOLOGY INTERNATIONAL, Vol: 125, Pages: 169-199, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Shen L, Denner F, Morgan N, van Wachem B, Dini Det al., 2018,

    Capillary waves with surface viscosity

    , JOURNAL OF FLUID MECHANICS, Vol: 847, Pages: 644-663, ISSN: 0022-1120
  • JOURNAL ARTICLE
    Kanca Y, Milner P, Dini D, Amis AAet al., 2018,

    Tribological evaluation of biomedical polycarbonate urethanes against articular cartilage

    , JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS, Vol: 82, Pages: 394-402, ISSN: 1751-6161
  • JOURNAL ARTICLE
    Stevenson H, Parkes M, Austin L, Jaggard M, Akhbari P, Vaghela U, Williams HRT, Gupte C, Cann Pet al., 2018,

    The development of a small-scale wear test for CoCrMo specimens with human synovial fluid

    , Biotribology, Vol: 14, Pages: 1-10

    © 2018 The Authors A new test was developed to measure friction and wear of hip implant materials under reciprocating sliding conditions. The method requires a very small amount of lubricant (<3 ml) which allows testing of human synovial fluid. Friction and wear of Cobalt Chromium Molybdenum (CoCrMo) material pairs were measured for a range of model and human synovial fluid samples. The initial development of the test assessed the effect of fluid volume and bovine calf serum (BCS) concentration on friction and wear. In a second series of tests human synovial fluid (HSF) was used. The wear scar size (depth and volume) on the disc was dependent on protein content and reduced significantly for increasing BCS concentration. The results showed that fluid volumes of <1.5 ml were affected by evaporative loss effectively increasing the protein concentration resulting in anomalously lower wear. At the end of the test thick deposits were observed in and around the wear scars on the disc and ball; these were analysed by Infrared Reflection-Absorption Spectroscopy. The deposits were composed primarily of denatured proteins and similar IR spectra were obtained from the BCS and HSF tests. The analysis confirmed the importance of SF proteins in determining wear of CoCrMo couples.

  • JOURNAL ARTICLE
    Heyes DM, Dini D, Smith ER, 2018,

    Incremental viscosity by non-equilibrium molecular dynamics and the Eyring model

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 148, ISSN: 0021-9606
  • JOURNAL ARTICLE
    Ewen JP, Kannam SK, Todd BD, Dini Det al., 2018,

    Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces

    , LANGMUIR, Vol: 34, Pages: 3864-3873, ISSN: 0743-7463
  • JOURNAL ARTICLE
    Morales-Espejel GE, Rycerz P, Kadiric A, 2018,

    Prediction of micropitting damage in gear teeth contacts considering the concurrent effects of surface fatigue and mild wear

    , Wear, Vol: 398-399, Pages: 99-115, ISSN: 0043-1648

    © 2017 The Authors The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts for mixed lubrication conditions, stress history, and fatigue damage accumulation, is adapted here to deal with transient contact conditions that exist during meshing of gear teeth. The model considers the concurrent effects of surface fatigue and mild wear on the evolution of tooth surface roughness and therefore captures the complexities of damage accumulation on tooth flanks in a more realistic manner than hitherto possible. Applicability of the model to gear contact conditions is first confirmed by comparing its predictions to relevant experiments carried out on a triple-disc contact fatigue rig. Application of the model to a pair of meshing spur gears shows that under low specific oil film thickness conditions, the continuous competition between surface fatigue and mild wear determines the overall level as well as the distribution of micropitting damage along the tooth flanks. The outcome of this competition in terms of the final damage level is dependent on contact sliding speed, pressure and specific film thickness. In general, with no surface wear, micropitting damage increases with decreasing film thickness as may be expected, but when some wear is present micropitting damage may reduce as film thickness is lowered to the point where wear takes over and removes the asperity peaks and hence reduces asperity interactions. Similarly, when wear is negligible, increased sliding can increase the level of micropitting by increasing the number of asperity stress cycles, but when wear is present, an increase in sliding may lead to a reduction in micropitting due to faster removal of asperity peaks. The results suggest that an ideal situation in terms of surface damage prevention is that in which some mild wear at the start of gear pair operation adequately wears-in the tooth surfaces, thus reducing sub

  • JOURNAL ARTICLE
    Hili J, Pelletier C, Jacobs L, Olver A, Reddyhoff Tet al., 2018,

    High-Speed Elastohydrodynamic Lubrication by a Dilute Oil-in-Water Emulsion

    , Tribology Transactions, Vol: 61, Pages: 287-294, ISSN: 1040-2004

    © 2018 Society of Tribologists and Lubrication Engineers. When a concentrated contact is lubricated at low speed by an oil-in-water emulsion, a film of pure oil typically separates the surfaces (stage 1). At higher speeds, starvation occurs (stage 2) and the film is thinner than would be expected if lubricated by neat oil. However, at the very highest speeds, film thickness increases again (stage 3), though little is known for certain about either the film composition or the mechanism of lubrication, despite some theoretical speculation. In this article, we report the film thickness in a ball-on-flat contact, lubricated by an oil-in-water emulsion, at speeds of up to 20 m/s, measured using a new high-speed test rig. We also investigated the sliding traction and the phase composition of the film, using fluorescent and infrared microscopy techniques. Results show that, as the speed is increased, starvation is followed by a progressive change in film composition, from pure oil to mostly water. At the highest speeds, a film builds up that has a phase composition similar to the bulk emulsion. This tends to support the “microemulsion” view rather than the “dynamic concentration” theory.

  • JOURNAL ARTICLE
    Masen M, Cann PME, 2018,

    Friction measurements with molten chocolate

    , Tribology Letters, Vol: 66, ISSN: 1023-8883

    A novel test is reported which allows the measurement of the friction of molten chocolate in a model tongue–palate rubbing contact. Friction was measured over a rubbing period of 150 s for a range of commercial samples with different cocoa content (85–5% w/w). Most of the friction curves had a characteristic pattern: initially a rapid increase occurs as the high-viscosity chocolate melt is sheared in the contact region followed by friction drop as the film breaks down. The exceptions were the very high (85%) and very low (~ 5%) cocoa content samples which gave fairly constant friction traces over the test time. Differences were observed in the initial maximum and final friction coefficients depending on chocolate composition. Generally, the initial maximum friction increased with increasing cocoa content. At the end of the test, the rubbed films on the lower slide were examined by optical microscopy and infrared micro-reflection spectroscopy. In the rubbed track, the chocolate structure was severely degraded and predominately composed of lipid droplets, which was confirmed by the IR spectra. The new test provides a method to distinguish between the friction behaviour of different chocolate formulations in a rubbing low-pressure contact. It also allows us to identify changes in the degraded chocolate film that can be linked to the friction profile. Further development of the test method is required to improve simulation of the tongue–palate contact including the effect of saliva and this will be the next stage of the research.

  • JOURNAL ARTICLE
    Spikes H, 2018,

    Stress-augmented thermal activation: Tribology feels the force

    , FRICTION, Vol: 6, Pages: 1-31, ISSN: 2223-7690
  • JOURNAL ARTICLE
    Lu J, Reddyhoff T, Dini D, 2018,

    3D Measurements of Lubricant and Surface Temperatures Within an Elastohydrodynamic Contact

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Guo Y, di Mare L, Li RKY, Wong JSSet al., 2018,

    Cargo Release from Polymeric Vesicles under Shear

    , POLYMERS, Vol: 10, ISSN: 2073-4360

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