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  • Journal article
    Campen S, Green JH, Lamb GD, Spikes HAet al., 2015,

    In situ study of model organic friction modifiers using liquid cell AFM: self-assembly of octadecylamine

    , Tribology Letters, Vol: 58, ISSN: 1573-2711

    Liquid cell AFM has been applied to study in situ the formation and properties of self-assembled films formed on mica surfaces by octadecylamine from alkane solution. Mica surfaces immersed in hexadecane or dodecane at room temperature show no identifiable surface films. However, when octadecylamine solution is injected into the cell, a boundary film forms almost immediately. This film takes the form of irregular islands of mean diameter approximately 1–3 µm and thickness typically 1.5 nm when measured in contact mode. These islands are believed to correspond to patches of vertically oriented but tilted octadecylamine or ammonium salt held together primarily by van der Waals forces between adjacent alkyl chains. These films are quite labile in that during scanning of the tip in both tapping mode and contact mode changes to the shape of the islands take place, including consolidation of the island density in the scanned region and depletion from around this area. In situ experiments in which the temperature of the cell is varied over time show that the initially formed islands disappear at a temperature of ca. 35 °C but are reformed when the cell is re-cooled. Similar tests on samples that remain immersed in solution for extended periods show more stable films, with islands being lost only above about 50–60 °C. The work shows that liquid cell AFM has great promise for studying the formation and properties of the boundary films formed by organic friction modifiers.

  • Journal article
    Reddyhoff T, Dobre O, Le Rouzic J, Gotzen N-A, Parton H, Dini Det al., 2015,

    Friction Induced Vibration in Windscreen Wiper Contacts

    , Journal of Vibration and Acoustics-Transactions of the ASME, Vol: 137, ISSN: 1048-9002

    This research is aimed at understanding the mechanisms that give rise to friction induced noise in automotive windscreen wipers, with a focus on frequencies between 500 and 3500 Hz. To study this phenomenon, experimental friction, sound, and high-speed video measurements are combined with finite element modeling of a rubber wiper/glass contact. In agreement with previous research, simultaneous sound and friction measurements showed that wiper noise in this frequency range results from the negative damping effect caused by the dependence of friction on speed in the mixed lubrication regime. Furthermore, during sliding, the friction induced noise recorded by the microphone occurred in one of two frequency ranges (close to 1000 Hz and between 2000 and 2500 Hz). These coincided closely with the eigen-frequencies of first two bending modes, predicted by finite element modeling. Experimental observations also showed the wiper to be oscillating backward and forward without any torsional motion and that the thickness of the glass had no effect on the emitted noise. These observations highlight how friction induced noise—although caused by conditions within contact—has characteristics that are determined by the structure of the excited component. A number of additional findings are made. Most importantly, both experiment and finite element modeling showed that the presence of water in contact with the wiper modulates the frequency and amplitude of the emitted noise by effectively adding mass to the vibrating system. While this is occurring, Faraday-like standing waves are observed in the water. In addition to this, friction induced vibration is shown only to occur for glass surfaces with intermediate surface energies, which is possibly due to high contact angles preventing water reaching the contact. Based on the understanding gained, a number of suggestions are made regarding means of reducing windscreen wiper noise.

  • Journal article
    van Arkel RJ, Amis AA, Cobb JP, Jeffers JRTet al., 2015,

    The capsular ligaments provide more hip rotational restraint than the acetabular labrum and the ligamentum teres

    , Bone & Joint Journal, Vol: 97B, Pages: 484-491, ISSN: 2049-4394

    In this in vitro study of the hip joint we examined which soft tissues act as primary and secondary passive rotational restraints when the hip joint is functionally loaded. A total of nine cadaveric left hips were mounted in a testing rig that allowed the application of forces, torques and rotations in all six degrees of freedom. The hip was rotated throughout a complete range of movement (ROM) and the contributions of the iliofemoral (medial and lateral arms), pubofemoral and ischiofemoral ligaments and the ligamentum teres to rotational restraint was determined by resecting a ligament and measuring the reduced torque required to achieve the same angular position as before resection. The contribution from the acetabular labrum was also measured. Each of the capsular ligaments acted as the primary hip rotation restraint somewhere within the complete ROM, and the ligamentum teres acted as a secondary restraint in high flexion, adduction and external rotation. The iliofemoral lateral arm and the ischiofemoral ligaments were primary restraints in two-thirds of the positions tested. Appreciation of the importance of these structures in preventing excessive hip rotation and subsequent impingement/instability may be relevant for surgeons undertaking both hip joint preserving surgery and hip arthroplasty.

  • Journal article
    Medina S, Fowell MT, Vladescu S-C, Reddyhoff T, Pegg I, Olver AV, Dini Det al., 2015,

    Transient effects in lubricated textured bearings

    , Proceedings of the Institution of Mechanical Engineers Part J - Journal of Engineering Tribology, Vol: 229, Pages: 523-537, ISSN: 1350-6501
  • Journal article
    Forte AE, D'Amico F, Charalambides MN, Dini D, Williams JGet al., 2015,

    Modelling and experimental characterisation of the rate dependent fracture properties of gelatine gels

    , FOOD HYDROCOLLOIDS, Vol: 46, Pages: 180-190, ISSN: 0268-005X
  • Journal article
    Ma G, Wang L, Gao H, Zhang J, Reddyhoff Tet al., 2015,

    The friction coefficient evolution of a TiN coated contact during sliding wear

    , Applied Surface Science, Vol: 345, Pages: 109-115, ISSN: 0169-4332
  • Journal article
    Spikes H, Zhang J, 2015,

    Reply to the Comment by Scott Bair, Philippe Vergne, Punit Kumar, Gerhard Poll, Ivan Krupka, Martin Hartl, Wassim Habchi, Roland Larson on "History, Origins and Prediction of Elastohydrodynamic Friction" by Spikes and Jie in Tribology Letters

    , Tribology Letters, Vol: 58, ISSN: 1573-2711

    Bair, accompanied by some of his past co-authors, has commented on our paper “History, Origins and Prediction of Elastohydrodynamic Friction”. We believe that our paper presents a balanced summary of current understanding of EHD friction, its origins and prediction. However, Bair et al. appear to have chosen to interpret our paper as an assault on the high-stress viscometric approach that Bair has championed for some years and also as an espousal of the Eyring model of rheology which he has consistently denigrated over the same period. To combat this perceived assault, they suggest that our paper somehow misrepresents the literature and indeed contains numerous misstatements of fact and misrepresentations by omission. Our reply refutes this.

  • Journal article
    Liang H, Guo D, Reddyhoff T, Spikes H, Luo Jet al., 2015,

    Influence of thermal effects on elastohydrodynamic (EHD) lubrication behavior at high speeds

    , SCIENCE CHINA-TECHNOLOGICAL SCIENCES, Vol: 58, Pages: 551-558, ISSN: 1674-7321
  • Journal article
    van Kuilenburg J, Masen MA, van der Heide E, 2015,

    A review of fingerpad contact mechanics and friction and how this affects tactile perception

  • Journal article
    Flicek RC, Hills DA, Dini D, 2015,

    Sharp edged contacts subject to fretting: A description of corner behaviour

    , International Journal of Fatigue, Vol: 71, Pages: 26-34, ISSN: 1879-3452

    In this paper, we use the singular terms in Williams’ solution to quantify the behaviour at the edge of a complete (i.e. sharp edged) contact. To do this, we define two alternative parameters from the generalised stress intensity factors to bring out an internal length dimension from the solution. We then obtain an order of magnitude estimate of the extent of slip and/or separation when these remain near to the contact edge. When larger slip or separation lengths are implied, we derive only qualitative implications. Finally, we apply this analysis to an example problem.

  • Journal article
    Smith ER, Heyes DM, Dini D, Zaki TAet al., 2015,

    A localized momentum constraint for non-equilibrium molecular dynamics simulations

    , JOURNAL OF CHEMICAL PHYSICS, Vol: 142, ISSN: 0021-9606
  • Journal article
    Ingram M, Hamer C, Spikes H, 2015,

    A new scuffing test using contra-rotation

    , Wear, Vol: 328-329, Pages: 229-240, ISSN: 1873-2577

    The mode of lubricant failure known as scuffing provides a significant design constraint in high sliding gears, cams and metal cutting and forming processes. It is therefore important to have an effective test method to measure the scuffing resistance of lubricant formulations. In most existing scuffing bench tests, a moving surface is rubbed against a stationary one at a fixed sliding speed and the load at which scuffing occurs is determined. This approach has two disadvantages. One is that wear of the stationary surface can lead to a large decrease in effective contact pressure during a test. The second is that viscous lubricants often generate significant elastohydrodynamic films at the sliding speeds employed. This means that the scuffing tests measure a complex combination of the influence of the fluid and boundary film-forming properties of the lubricant on scuffing rather than reflecting solely the influence of lubricant formulation.This paper describes a new scuffing test method in which the two metal surfaces are rubbed together in mixed rolling–sliding with the two surfaces moving in opposite directions with respect to the contact, i.e. in contra-rotation. This enables the sliding speed to be decoupled from the entrainment speed so that the scuffing properties of a lubricant can be determined in boundary lubrication conditions over a wide range of sliding speeds. Also, because both surfaces move relative to the contact, wear is distributed and this minimises changes in contact pressure during a test.

  • Journal article
    Heyes DM, Dini D, Brańka AC, 2015,

    Scaling of Lennard-Jones liquid elastic moduli, viscoelasticity and other properties along fluid-solid coexistence

    , Physica Status Solidi (B), Vol: 252, Pages: 1514-1525, ISSN: 0370-1972

    Static and dynamical properties of the Lennard-Jones (LJ) fluid along the fluid-solid coexistence line are determined by molecular dynamics simulation. A number of properties, such as the radial distribution function, Einstein frequency, mean force, root mean square force, and normalised time correlation functions are shown to be essentially invariant or structurally isomorphic along this line when scaled by so-called macroscopic variables (MRU). Other quantities subject to MRU such as the potential energy, pressure and infinite frequency compressional modulus are not constant along this line of states but can be reproduced using simple formulae of the form for Roskilde fluids. The elastic moduli fall within the domain of isomorphism theory. A generalised Cauchy relationship in which the infinite frequency longitudinal modulus is proportional to the longitudinal modulus of the fluid was found to be obeyed very well for the LJ fluid phase along this coexistence line.

  • Journal article
    Rathbone D, Dini D, Marigo M, van Wachem BGMet al., 2015,

    An accurate force–displacement law for the modelling of elastic–plastic contacts in discrete element simulations

    , Powder Technology, Vol: 282, Pages: 2-9, ISSN: 1873-328X

    This paper presents an accurate model for the normal force–displacement relationship between elastic–plastic spheres in contact for use in discrete element method (DEM) simulations. The model has been developed by analysing the normal force–displacement relationship between an elastic–perfectly plastic sphere and a rigid surface using the finite element method (FEM). Empirical relationships are found that relate the parameters of the new model to material properties. This allows the model to be used in the DEM for direct simulation of well characterised elastic–plastic materials without fitting parameters to experimental results. This gives the model an advantage over models in the literature for which fitting to experimental results is required. The implementation of the model into an existing DEM code is discussed and validated against the results from FEM simulations. The new model shows a good match to the FEM results and the DEM implementation correctly distinguishes between the loading, unloading and re-loading phases of contact between two spheres.

  • Journal article
    Vladescu S-C, Olver AV, Pegg IG, Reddyhoff Tet al., 2015,

    The effects of surface texture in reciprocating contacts - An experimental study

    , TRIBOLOGY INTERNATIONAL, Vol: 82, Pages: 28-42, ISSN: 0301-679X
  • Journal article
    Arana C, Evangelou SA, Dini D, 2015,

    Series Active Variable Geometry Suspension for Road Vehicles

    , IEEE-ASME TRANSACTIONS ON MECHATRONICS, Vol: 20, Pages: 361-372, ISSN: 1083-4435
  • Journal article
    Campen S, Green JH, Lamb GD, Spikes HAet al., 2015,

    In situ study of model organic friction modifiers using liquid cell AFM; saturated and mono-unsaturated carboxylic acids

    , Tribology Letters, Vol: 57, ISSN: 1573-2711
  • Journal article
    Nyqvist J, Kadiric A, Ioannides S, Sayles Ret al., 2015,

    Semi-analytical model for rough multilayered contacts

    , Tribology International, Vol: 87, Pages: 98-112, ISSN: 1879-2464

    This paper presents a new model for analysis of non-conformal rough surface contacts where one or both of the contacting bodies are coated with a multilayered coating. The model considers elastic contact of arbitrary geometry with real measured roughnesses and both normal and tangential surface loads. It predicts contact pressure distribution, surface deformations and full subsurface stress field. As such, the model offers an optimisation tool for analysis and development of multilayered coatings. Influence coefficients approach is utilised to obtain contact pressures and subsurface stresses while the contact solver is based on a standard conjugate gradient method. To improve model efficiency, a semi-analytical approach is devised, where the influence coefficients for displacements and stresses are expressed explicitly by solving the fundamental equations in the frequency domain. Fast Fourier Transforms in conjunction with discrete convolution are then utilised to provide the solution in spatial domain. Selected results are presented to first validate the model and then illustrate the potential improvements that can be achieved in the design of multilayered coatings through application of the model.

  • Journal article
    Spikes H, 2015,

    Basics of EHL for practical application

    , LUBRICATION SCIENCE, Vol: 27, Pages: 45-67, ISSN: 0954-0075
  • Conference paper
    Mastrandrea LN, Giacopini M, Dini D, Bertocchi Eet al., 2015,

    Elastohydrodynamic analysis of the conrod small-end of a high performance motorbike engine via a mass conserving cavitation algorithm

    Copyright © 2015 by ASME. In this contribution a complementarity formulation for the solution of EHL problem in presence of cavitation is employed in order to investigate the tribological behavior of the conrod small-end of a high performance motorbike engine. The influence of different physical and geometrical parameters is discussed. In particular, the clearance between the conrod small-end and the piston pin, the lubricant physical properties, the surface roughness and the stiffness of the piston pin are investigated, thus providing preliminary guidelines for the correct design of the coupling. Due to the negligible influence of the transversal forces acting on the conrod small-end and of the relative sliding speed between the mating surfaces, a two symmetrical model of the assembly is prepared and results are compared with those obtained adopting a simply symmetrical model.

  • Conference paper
    Dini D, Giacopini M, Giuseppe MA, Bertocchi Eet al., 2015,

    The influence of textured surfaces on the tribological behaviour of hip replacements employng a mass conserving complementarity algorithm

    , Pages: 4-6
  • Journal article
    Flicek RC, Hills DA, Barber JR, Dini Det al., 2015,

    Determination of the shakedown limit for large, discrete frictional systems

    , EUROPEAN JOURNAL OF MECHANICS A-SOLIDS, Vol: 49, Pages: 242-250, ISSN: 0997-7538
  • Journal article
    Costa HL, Spikes H, 2015,

    Effects of Ethanol Contamination on Friction and Elastohydrodynamic Film Thickness of Engine Oils

    , TRIBOLOGY TRANSACTIONS, Vol: 58, Pages: 158-168, ISSN: 1040-2004
  • Conference paper
    Profito FJ, Dini D, Zachariadis DC, 2015,

    A general finite volume method solution for the reynolds lubrication equation with Mass-Conserving cavitation model

    , Pages: 303-305
  • Journal article
    Wong JSP, Hu M, Shi D, Li RKY, Wong JSSet al., 2014,

    In-situ Monitoring on Dynamics of Solute Transport in Polymer Films

    , Polymer, Vol: 58, Pages: 67-75, ISSN: 0032-3861

    A new and non-invasive technique based on confocal laser scanning microscopy (CLSM) that allows the visualization of penetrant diffusion in-situ has been developed and was applied to quantify local solute dynamics in polymeric films. The effectiveness of the proposed technique was demonstrated using a model penetrant, rhodamine-6G (Rh-6G), and a system of polyvinyl alcohol (PVA) films with different degree of cross-linking, and different content of montmorillonite (MMT) clay. The penetrant's transport across PVA films were monitored by measuring the time evolutions of through thickness fluorescence intensity profiles. These profiles were then converted to concentration profiles, which allow local diffusion coefficients of the model solute (i.e. Rh-6G) to be determined. The developed methodology was applied to both single layer and bilayers films and local diffusion heterogeneity was detected. Hence the technique developed can be applied to multi-layer films, and can be beneficial to film developments for packaging and filtration technology.

  • Journal article
    Tavasci A, Arizzi F, Dini D, Rizzi Eet al., 2014,

    Heat flux evaluation in high temperature ring-on-ring contacts

    , Wear, Vol: 330-331, Pages: 320-326, ISSN: 1873-2577

    A comprehensive methodology to investigate heat flux in a ring-on-ring tribometer is presented. Thermal fluxes under high contact pressures and temperature differences were evaluated through an experimental campaign and by a numerical procedure of inverse analysis applied to surface temperature measurements. An approximation of a two-dimensional time-dependent analytical solution for the temperature distribution was first developed and subsequently adapted to mimic the specific testing configuration characteristics; the problem was finally simplified to enable further inverse analysis. Experiments were performed using an innovative high temperature ring-on-ring tribometer. The evaluated contact heat-transfer rates were reported as a function of normal load and temperature difference between the discs under steady-state conditions; the results reported here show that, in the present test configuration, the temperature difference has stronger influence than the applied load in terms of heat transfer induced by contact.

  • Journal article
    Parkes M, Myant C, Dini D, Cann Pet al., 2014,

    Tribology-optimised silk protein hydrogels for articular cartilage repair

    , Tribology International, Pages: ---, ISSN: 0301-679X
  • Journal article
    Puthumanapully PK, Harris SJ, Leong A, Cobb JP, Amis AA, Jeffers Jet al., 2014,

    A morphometric study of normal and varus knees

    , KNEE SURGERY SPORTS TRAUMATOLOGY ARTHROSCOPY, Vol: 22, Pages: 2891-2899, ISSN: 0942-2056
  • Journal article
    Gurrutxaga-Lerma B, Balint DS, Dini D, Eakins DE, Sutton Aet al., 2014,

    Dynamic Discrete Dislocation Plasticity

    , Advances in Applied Mechanics, Vol: 47, ISSN: 0065-2156

    This chapter concerns with dynamic discrete dislocation plasticity (D3P), a two- dimensional method of discrete dislocation dynamics aimed at the study of plastic relaxation processes in crystalline materials subjected to weak shock loading. Traditionally, the study of plasticity under weak shock loading and high strain rate has been based on direct experimental measurement of the macroscopic response of the material. Using these data, well-known macroscopic constitutive laws and equations of state have been formulated. However, direct simulation of dislocations as the dynamic agents of plastic relaxation in those circumstances remains a challenge. In discrete dislocation dynamics (DDD) methods, in particular the two-dimensional discrete dislocation plasticity (DDP), the dislocations are modeled as discrete discontinuities in an elastic continuum. However, current DDP and DDD methods are unable to adequately simulate plastic relaxation because they treat dislocation motion quasi- statically, thus neglecting the time-dependent nature of the elastic elds and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. This chapter reproduces the ndings by Gurrutxaga-Lerma, Balint, Dini, Eakins, and Sutton (2013), who proved that under shock loading, this assumption leads to models that invariably break causality, introducing numerous artifacts that invalidate quasi- static simulation techniques. This chapter posits that these limitations can only be overcome with a fully time-dependent formulation of the elastic elds of dislocations. In this chapter, following the works of Markensco and Clifton (1981) and Gurrutxaga- Lerma et al. (2013), a truly dynamic formulation for the creation, annihilation, and nonuniform motion of straight edge dislocations is derived. These solutions extend the DDP framework to a fully elastodynamic formulation that has been called dynamic discrete dislocation plasticity (D3P). This chapter describes the s

  • Conference paper
    Forte AE, Dini D, 2014,

    Modelling and Characterisation of Soft Tissue Deformation and Indentation with Particular Application to Articular Cartilage and Brain

    , STLE Frontiers 2014
  • Journal article
    Gattinoni C, Mackowiak S, Heyes DM, Branka AC, Dini Det al., 2014,

    Boundary-controlled barostats for slab geometries in molecular dynamics simulations

    , PHYSICAL REVIEW E, Vol: 90, ISSN: 1539-3755
  • Journal article
    Putignano C, Le Rouzic J, Reddyhoff T, Carbone G, Dini Det al., 2014,

    A theoretical and experimental study of viscoelastic rolling contacts incorporating thermal effects

  • Journal article
    Strozzi A, Giacopini M, Bertocchi E, Dini Det al., 2014,

    Formulation of the tangential velocity slip problem in terms of variational inequalities

    , Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol: 228, Pages: 1122-1135, ISSN: 2041-305X
  • Journal article
    Borges PDN, Forte AE, Vincent TL, Dini D, Marenzana Met al., 2014,

    Rapid, automated imaging of mouse articular cartilage by microCT for early detection of osteoarthritis and finite element modelling of joint mechanics

    , OSTEOARTHRITIS AND CARTILAGE, Vol: 22, Pages: 1419-1428, ISSN: 1063-4584
  • Journal article
    Spikes H, Jie Z, 2014,

    History, Origins and Prediction of Elastohydrodynamic Friction

    , TRIBOLOGY LETTERS, Vol: 56, Pages: 1-25, ISSN: 1023-8883
  • Journal article
    Liu X, Spikes H, Wong JSS, 2014,

    In-situ pH responsive fluorescent probing of localized iron corrosion

    , Corrosion Science, ISSN: 0010-938X
  • Conference paper
    Geraldes D, Hansen U, Jeffers J, Amis Aet al., 2014,

    A framework for parametric analysis of glenoid implant design

    , International Society for Technology in Arthroplasty 2014
  • Conference paper
    Forte AE, galvan S, manieri F, Dini Det al., 2014,

    A Novel Composite Phantom for Brain Tissue

    , EMBC 2014
  • Journal article
    Bodnarchuk MS, Dini D, Heyes DM, Chahine S, Edwards Set al., 2014,

    Self-assembly of calcium carbonate nanoparticles in water and hydrophobic solvents

    , Journal of Physical Chemistry C, Vol: 118, Pages: 21092-21103, ISSN: 1932-7455

    The self-assembly of Ca2+ and CO32– ions into nanoparticles in water and hydrophobic solvents is investigated using molecular dynamics (MD) computer simulation. A new three-stage particle assembly procedure is used which relaxes the structure of the nanoparticle toward a lower energy state. In hydrophobic solvent the bare particle is essentially spherical whereas in water it is ellipsoidally shaped. With surfactant stabilizer the nanoparticles typically exhibit nonspherical cores in model hydrophobic solvents. Binary surfactant systems exhibit synergistic effects where for example a salicylate-sulfonate combination forms a cage which promotes a compact core. Synergistic effects on the shape of the particle were also observed in a hydrophobic solvent for surfactant-stabilized systems with trace water as a third component. The simulations show that rather than being a rigid structure the carbonate core shape and stabilizing shell coverage are sensitive to solvent, surfactant, and small polar molecules which act as cosurfactants.

  • Journal article
    Flicek RC, Hills DA, Dini D, 2014,

    Refinements in the characterisation of mode-mixity and small scale yielding at sharp notch roots

    , ENGINEERING FRACTURE MECHANICS, Vol: 126, Pages: 73-86, ISSN: 0013-7944
  • Journal article
    Le Rouzic J, Reddyhoff T, 2014,

    Spatially Resolved Triboemission Measurements

    , Tribology Letters, Vol: 55, Pages: 245-252, ISSN: 1573-2711
  • Conference paper
    Cattilino M, Secoli R, Galvan S, Forte AE, Dini D, rodriguez y Baena Fet al., 2014,

    Development of a Dynamic Soft Tissue Phantom for Cooperative Control Testing in Robotic Surgery

    , Hamlyn Symposium
  • Conference paper
    Rasin I, Pekar Z, Sadowsky O, Forte AE, Galvan S, Dini D, Shoham M, Joskowicz Let al., 2014,

    Real-Time Modelling of Intra-operative Brain Shift Based on Video Tracking

    , The Hamlyn Symposium on Medical Robotics 2014
  • Journal article
    Medina S, Dini D, 2014,

    A numerical model for the deterministic analysis of adhesive rough contacts down to the nano-scale

    , INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 51, Pages: 2620-2632, ISSN: 0020-7683
  • Journal article
    Myant CW, fowell M, spikes H, kadiric Aet al., 2014,

    A Study of Lubricant Film Thickness in Compliant Contacts of Elastomeric Seal Materials Using a Laser Induced Fluorescence Technique

    , Tribology International, Vol: 80, Pages: 76-89, ISSN: 1879-2464

    A laser induced fluorescence technique was used to investigate the build-up of lubricant films in compliant contacts operating in the isoviscous elasto-hydrodynamic regime (I-EHL). The described technique utilises an optimised optical set-up with a relatively high signal-to-noise ratio and was shown to be able to produce film thickness maps of the complete contact area and measure a very wide span of thicknesses, from 50 nm to 100 μm. Maps of film thickness were obtained over a range of entrainment speeds and loads for three different contact configurations and two elastomer materials, polydimethylsiloxane (PDMS) and a fluorocarbon rubber (FKM) which is typically used in rotary seal applications. In a model contact of a nominally smooth PDMS ball sliding on a glass flat, a crescent shaped area of reduced film thickness was observed towards the contact exit. In contrast to typical elasto-hydrodynamic contacts, no side-lobes of reduced film thickness were recorded, while the central film region exhibited a converging wedge shape. The elliptical contact of an FKM O-ring rolling on a flat glass showed a central region of flat film while areas of minimum film thickness were located near the contact edges either side of the centre. The highly conformal contact of relatively rough FKM O-ring sliding against a concave glass lens, a geometry more representative of that found in elastomeric seals, showed discrete regions of reduced film, corresponding to surface roughness asperities. With rising entrainment speed, some lift-off was observed, with surface roughness asperities appearing to be increasingly compressed. Measured films thicknesses were compared to existing theoretical predictions for I-EHL contacts and the level of agreement was found to be highly dependent on contact geometry and applied conditions.

  • Journal article
    Bodnarchuk MS, Heyes DM, Dini D, Chahine S, Edwards Set al., 2014,

    Role of Deprotonation Free Energies in pK(a) Prediction and Molecule Ranking

    , JOURNAL OF CHEMICAL THEORY AND COMPUTATION, Vol: 10, Pages: 2537-2545, ISSN: 1549-9618
  • Journal article
    Myant C, Cann P, 2014,

    On the matter of synovial fluid lubrication: Implications for Metal-on-Metal hip tribology

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 34, Pages: 338-348, ISSN: 1751-6161

    Artificial articular joints present an interesting, and difficult, tribological problem. These bearing contacts undergo complex transient loading and multi axes kinematic cycles, over extremely long periods of time (>10 years). Despite extensive research, wear of the bearing surfaces, particularly metal–metal hips, remains a major problem. Comparatively little is known about the prevailing lubrication mechanism in artificial joints which is a serious gap in our knowledge as this determines film formation and hence wear. In this paper we review the accepted lubrication models for artificial hips and present a new concept to explain film formation with synovial fluid. This model, recently proposed by the authors, suggests that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins. The implications of this new mechanism for the tribological performance of new implant designs and the effect of patient synovial fluid properties are discussed.

  • Journal article
    Zhang J, Yamaguchi E, Spikes H, 2014,

    The Antagonism between Succinimide Dispersants and a Secondary Zinc Dialkyl Dithiophosphate

    , TRIBOLOGY & LUBRICATION TECHNOLOGY, Vol: 70, Pages: 60-+, ISSN: 1545-858X
  • Journal article
    Myant CW, Cann P, 2014,

    The effect of transient conditions on synovial fluid protein aggregation lubrication

    , Journal of the Mechanical Behavior of Biomedical Materials, Vol: 34, Pages: 349-357, ISSN: 1751-6161

    Little is known about the prevailing lubrication mechanisms in artificial articular joints and the way in which these mechanisms determine implant performance. The authors propose that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins within the contact inlet region. A direct relationship between contact film thickness and size of the protein aggregation within the inlet region has been observed.In this paper the latest experimental observations of the protein aggregation mechanism are presented for conditions which more closely mimic joint kinematics and loading. Lubricant films were measured for a series of bovine calf serum solutions for CoCrMo femoral component sliding against a glass disc. An optical interferometric apparatus was employed to study the effects of transient motion on lubricant film formation. Central film thickness was measured as a function of time for a series of transient entrainment conditions; start-up motion, steady-state and non-steady-state uni-directional sliding, and bi-directional sliding. The size of the inlet aggregations was found to be dependent upon the type of transient condition. Thick protective protein films were observed to build up within the main contact region for all uni-directional tests. In contrast the inlet aggregation was not observed for bi-directional tests. Contact film thickness and wear was found to be directly proportional to the presence of the inlet protein phase. The inlet phase and contact films were found to be fragile when disrupted by surface scratches or subjected to reversal of the sliding direction.

  • Journal article
    Fowell M, Ioannides S, Kadiric A, 2014,

    An Experimental Investigation into the Onset of Smearing Damage in Nonconformal Contacts with Application to Roller Bearings

    , Tribology Transactions, Vol: 57, Pages: 472-488, ISSN: 1547-397X

    The onset of smearing damage was studied under controlled conditions in a custom test rig that simulates the passage of a rolling element through loaded and unloaded zones of a rolling bearing. The setup includes a spherical roller that is intermittently loaded between two bearing raceways driven at a prescribed speed. The roller is free to accelerate during the loading phase. Contact load, roller speed and acceleration, and electrical contact resistance are recorded during the test. Contact shear stress, friction coefficient, frictional power intensity, and elastohydrodynamic film thickness are calculated from the recorded kinematics data. Results suggest that the first onset of smearing occurs early in the loading phase where the roller is near stationary and the frictional power intensity is high. The raceway speed at the onset of damage decreases with increasing load and increasing lubricant supply temperature. The maximum frictional power intensity is found to be relatively constant at all contact conditions that led to smearing. An existing thermomechanical contact model is used to estimate the contact temperature distribution under smearing conditions and the potential for elastohydrodynamic lubrication (EHL) film thickness reduction due to forward heat conduction.

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