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  • Journal article
    Profito FJ, Vladescu S-C, Reddyhoff T, Dini Det al., 2017,

    Experimental validation of a mixed-lubrication regime model for textured piston-ring-liner contacts

    , Materials Performance and Characterization, Vol: 6, Pages: 112-129, ISSN: 2165-3992

    Recent experiments have shown that automotive piston-liner friction may be reduced by up to 50 % if the surface of the liner is laser textured with certain configurations of micro-pockets. It is important to model this behavior to understand and optimize the friction reduction mechanisms that are occurring. However, until now, very few models that predict the lubrication performance of textured surfaces have been successfully validated against experimental data. This is because of the requirement for them to: (1) reproduce experimental configurations with a certain degree of fidelity, (2) conserve mass properly, and (3) account for transient, boundary lubrication conditions. To address this, the current paper presents a comparison between the results from a numerical model, which fulfils these criteria, and an experimental test rig operating under the same conditions. The mathematical modeling is based on the averaged Reynolds’ equation with Patir and Cheng’s flow factors and the p − θ Elrod–Adams mass-conserving cavitation model. Simultaneously to the fluid flow solution, the contact pressures that arise from the asperity interactions are also included into the calculations through the well-known stochastic Greenwood and Tripp model for rough contacts. The experimental data is produced using a reciprocating tribometer, whose contact conditions are closely controlled and accurately mimic those found in an automotive piston–liner conjunction. Data is presented in terms of friction force versus stroke angle, and the similarities and differences between the model and experiment are discussed.

  • Journal article
    Kanazawa Y, Sayles RS, Kadiric A, 2017,

    Film formation and friction in grease lubricated rolling-sliding non-conformal contacts

    , Tribology International, Vol: 109, Pages: 505-518, ISSN: 1879-2464

    This study investigates the film formation and friction in grease lubricated, rolling-sliding, non-conformal contacts over a range of entrainment speeds, surface roughnesses and contact temperatures. The effects of grease composition are assessed by employing custom made, additive free, lithium and diurea thickened greases, whose composition is systematically varied so that the effects of the thickener and the base oil can be isolated. All film thickness and friction measurements were conducted under fully-flooded conditions. It was found that at low speeds all tested greases are able to form thicker films than the corresponding base oils. The thickness and behaviour of these films is determined by the thickener type and is independent of the base oil viscosity and the test temperature. At higher speeds, the film thickness is governed by the base oil properties alone and can be predicted by the EHD theory. At low speeds, films with diurea greases grow with time under constant speed and residual films persist under load after contact motion ceases. The real lambda ratio, based on measured grease film thickness, was shown to correlate well with contact friction. The transition from the thickener dominated behaviour to that dominated by the base oil occurs at a relatively constant film thickness, regardless of the base oil viscosity and test temperatures, rather than at a given entrainment speed. Based on the presented evidence, it is here proposed that the mechanism of formation of grease films at low speeds, is analogous to that reported to operate in EHL contacts lubricated with colloidal dispersions, namely the mechanical entrapment and deposition of thickener fibres, and that, rather than the widely quoted ‘transition speed’, it is the ratio of the thickener fibre size to prevailing film thickness that determines the range of conditions under which the film enhancement due to the action of thickener is present.

  • Patent
    Vladescu S, Reddyhoff T, Olver A, Pegg IGet al., 2017,

    Bearing interface with recesses to reduce friction

    , US20170009886 A1

    A bearing interface of an apparatus, the apparatus having a first element and a second element configured to move relative to each other during operation of the apparatus, the first element comprising a first bearing surface configured to engage at least a portion of a second bearing surface of the second element thereby defining a contact zone between the first bearing surface and the second bearing surface, the first bearing surface having at least one recess indented into the first bearing surface, wherein the dimension of the recess in ...

  • Journal article
    Ciniero A, Le Rouzic J, Baikie I, Reddyhoff Tet al., 2017,

    The origins of triboemission - correlating electron emission with surface damage

    , Wear, Vol: 374-375, Pages: 113-119, ISSN: 1873-2577

    Triboemission – i.e. the emission of photons, electrons and other charged particles that arise from a sliding contact – may play a key role in tribochemical processes, such as lubricant degradation. However, the mechanisms that give rise to this type of emission are not well understood. For the first time, we present spatially resolved measurements of electron emission, obtained as a range materials are worn. These are obtained from scratch tests, carried out under vacuum conditions (10−5 Torr), in which microchannel plates coupled to a phosphor screen are used to image electron emission.The results show that electron emission occurs at specific locations on the worn surface and, depending on the conductivity of the material, these sites remain active and decay with a time constant of up to several seconds. SEM images of the worn surface at these sites reveal that either surface fractures or grain defects are present. This suggests that fractoemission mechanisms are at least partially responsible for triboemission (however, the possible contribution of tribocharging mechanisms are also discussed). Specifically, this provides evidence to support the theory that triboemission results from the imbalance of charge on opposing faces of wear cracks and that this generates an electric field sufficient to accelerate molecular fracture products, which then bombard the surface leading to secondary emission.The strong geometric correlation between damage topography and electron emission distributions shows the potential of using this technique to monitoring wear and crack formation in real time and under high (30x) magnification.

  • Journal article
    Dench J, Morgan N, Wong J, 2016,

    Quantitative viscosity mapping using fluorescence lifetime measurements

    , Tribology Letters, Vol: 65, ISSN: 1573-2711

    Lubricant viscosity is a key driver in both the tribological performance and energy efficiency of a lubricated contact. Elastohydrodynamic (EHD) lubrication produces very high pressures and shear rates, conditions hard to replicate using conventional rheometry. In situ rheological measurements within a typical contact are therefore important to investigate how a fluid behaves under such conditions. Molecular rotors provide such an opportunity to extract the local viscosity of a fluid under EHD lubrication. The validity of such an application is shown by comparing local viscosity measurements obtained using molecular rotors and fluorescence lifetime measurements, in a model EHD lubricant, with reference measurements using conventional rheometry techniques. The appropriateness of standard methods used in tribology for high-pressure rheometry (combining friction and film thickness measurements) has been verified when the flow of EHD lubricant is homogeneous and linear. A simple procedure for calibrating the fluorescence lifetime of molecular rotors at elevated pressure for viscosity measurements is proposed.

  • Journal article
    Hajishafiee A, Kadiric A, Ioannides E, Dini Det al., 2016,

    A coupled finite-volume CFD solver for two-dimensional elasto-hydrodynamic lubrication problems with particular application to rolling element bearings

    , Tribology International, Vol: 109, Pages: 258-273, ISSN: 1879-2464

    This paper describes a new computational fluid dynamics methodology for modelling elastohydrodynamic contacts. A finite-volume technique is implemented in the ‘OpenFOAM’ package to solve the Navier-Stokes equations and resolve all gradients in a lubricated rolling-sliding contact. The method fully accounts for fluid-solid interactions and is stable over a wide range of contact conditions, including pressures representative of practical rolling bearing and gear applications. The elastic deformation of the solid, fluid cavitation and compressibility, as well as thermal effects are accounted for. Results are presented for rolling-sliding line contacts of an elastic cylinder on a rigid flat to validate the model predictions, illustrate its capabilities, and identify some example conditions under which the traditional Reynolds-based predictions deviate from the full CFD solution.

  • Conference paper
    Wang P, Zhang J, Spikes HA, Reddyhoff T, Holmes ASet al., 2016,

    Development of hydrodynamic micro-bearings

    , 16th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2016), Publisher: IOP Publishing, ISSN: 1742-6588

    This paper describes the modelling and testing of mm-scale hydrodynamic bearings which are being developed to improve the efficiency of a cm-scale turbine energy harvester, whose efficiency was previously limited by poorly lubricated commercial jewel-bearings. The bearings were fabricated using DRIE and their performance was assessed using a custom built MEMS tribometer. Results demonstrate that acceptably low friction is achieved when low viscosity liquid lubricants are used in combination with an appropriate choice of friction modifier additive. Further reduction in friction is demonstrated when the step height of bearing is adjusted in accordance with hydrodynamic theory. In parallel with the experiments, hydrodynamic lubricant modelling has been carried out to predict and further optimize film thickness and friction performance. Modelling results are presented and validated against experimental friction data.

  • Journal article
    Zhang J, Tan A, Spikes H, 2016,

    Effect of base oil structure on elastohydrodynamic friction

    , Tribology Letters, Vol: 65, ISSN: 1023-8883

    The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.

  • Journal article
    Forte AE, Gentleman SM, Dini D, 2016,

    On the characterisation of the heterogeneous mechanical response of human brain tissue

    , Biomechanics and Modeling in Mechanobiology, Vol: 16, Pages: 907-920, ISSN: 1617-7959

    The mechanical characterization of brain tissue is a complex task scientists have tried to accomplish for over fifty years. The resultsin literatureoften differ by orders of magnitudebecause of the lack of a standard testing protocol. Different testing conditions (including humidity, temperature, strain rate),the methodologyadopted,the variety of the speciesanalysed, are all potential sources of discrepancies in the measurements.In this work we present a rigorous experimental investigation on the mechanical properties of human brain, covering both grey and white matter. The influence of testing conditions isalso shown and thoroughly discussed. The material characterisation performed is finally adopted to provide inputs toa mathematical formulation suitable fornumerical simulations of brain deformation during surgical procedures.

  • Journal article
    van Arkel RJ, Jeffers JRT, 2016,

    In vitro hip testing in the International Society of Biomechanics coordinate system

    , JOURNAL OF BIOMECHANICS, Vol: 49, Pages: 4154-4158, ISSN: 0021-9290
  • Journal article
    Profito FJ, Vladescu S, Reddyhoff, Dini Det al., 2016,

    Transient experimental and modelling studies of laser-textured micro-grooved surfaces with a focus on piston-ring cylinder liner contacts

    , Tribology International, Vol: 113, Pages: 125-136, ISSN: 1879-2464

    This paper presents a comparison between the results from numerical modelling and experiments to shed light on the mechanisms by which surface texture can reduce friction when applied to an automotive cylinder liner. In this configuration, textured features move relative to the piston-liner conjunction and to account for this our approach is to focus on the transient friction response to individual pockets as they pass through, and then leave, the sliding contact. The numerical approach is based on the averaged Reynolds’ equation with the Patir & Cheng’s flow factors and the p-θ Elrod-Adams mass-conserving cavitation model. The contact pressures that arises from the asperity interactions are solved simultaneously to the fluid flow solution using the Greenwood and Tripp method. The experimental data is produced using a pin-on-disc set up, in which laser textured pockets have been applied to the disc specimen. Under certain conditions in the mixed and boundary lubrication regimes, both model and experimental results show i) an increase in friction as the pocket enters the contact, followed by ii) a sharp decrease as the pocket leaves the contact, and then iii) a gradual decay back to the pre-entrainment value. From the evidence obtained for the first time from the proposed combined modelling and experimental investigation conducted under carefully controlled conditions, we suggest that these three stages occur due to the following mechanisms: i) a reduction in fluid pressure due to the increased inlet gap, ii) inlet suction as the cavitated fluid within the pocket draws lubricant into the contact, and iii) film thickness decay as oil is squeezed out of the contact. The interplay of these three mechanisms is shown to control the response of micro-textured surfaces und

  • Journal article
    Rycerz P, Olver A, Kadiric A, 2016,

    Propagation of surface initiated rolling contact fatigue cracks in bearing Steel

    , International Journal of Fatigue, Vol: 97, Pages: 29-38, ISSN: 1879-3452

    Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms. A triple-contact disc machine was used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface is monitored and crack propagation rates extracted. The morphology of the generated cracks is observed by preparing sections of cracked specimens at the end of the test. It was found that crack initiation occurred very early in total life, which was attributed to high asperity stresses due to mixed lubrication regime. Total life to pitting was dominated by crack propagation. Results provide direct evidence of two distinct stages of crack growth in rolling contact fatigue: stage 1, within which cracks grow at a slow and relatively steady rate, consumed most of the total life; and stage 2, reached at a critical crack length, within which the propagation rate rapidly increases. Contact pressure and crack size were shown to be the main parameters controlling the propagation rate. Results show that crack propagation under rolling contact fatigue follows similar trends to those known to occur in classical fatigue. A log-log plot of measured crack growth rates against the product of maximum contact pressure and the square root of crack length, a parameter describing the applied stress intensity, produces a straight line for stage 2 propagation. This provides the first evidence that growth of hereby-identified stage 2 rolling contact fatigue cracks can be described by a Paris-type power law, where the rate of crack growth across the s

  • Journal article
    Arana C, Evangelou SA, Dini D, 2016,

    Series active variable geometry suspension application to comfort enhancement

    , Control Engineering Practice, Vol: 59, Pages: 111-126, ISSN: 1873-6939

    This paper explores the potential of the Series Active Variable Geometry Suspension (SAVGS) for comfort and road holding enhancement. The SAVGS concept introduces significant nonlinearities associated with the rotation of the mechanical link that connects the chassis to the spring-damper unit. Although conventional linearization procedures implemented in multi-body software packages can deal with this configuration, they produce linear models of reduced applicability. To overcome this limitation, an alternative linearization approach based on energy conservation principles is proposed and successfully applied to one corner of the car, thus enabling the use of linear robust control techniques. An H∞ controller is synthesized for this simplified quarter-car linear model and tuned based on the singular value decomposition of the system's transfer matrix. The proposed control is thoroughly tested with one-corner and full-vehicle nonlinear multi-body models. In the SAVGS setup, the actuator appears in series with the passive spring-damper and therefore it would typically be categorized as a low bandwidth or slow active suspension. However, results presented in this paper for an SAVGS-retrofitted Grand Tourer show that this technology has the potential to also improve the high frequency suspension functions such as comfort and road holding.

  • Journal article
    Leibinger A, Forte AE, Tan Z, Oldfield MJ, Beyrau F, Dini D, Rodriguez y Baena Fet al., 2016,

    Erratum to: Soft Tissue Phantoms for Realistic Needle Insertion: A Comparative Study (Annals of Biomedical Engineering, 10.1007/s10439-015-1523-0)

    , Annals of Biomedical Engineering, Vol: 44, ISSN: 0090-6964

    © 2016 Biomedical Engineering Society. Reference 4 should be changed to: Forte, A. E., S. Galvan, F. Manieri, F. Rodriguez y Baena, and D. Dini. A composite hydrogel for brain tissue phantoms. Mater. Des. 227:238–112, 2016.

  • Journal article
    Ewen J, Echeverri Restrepo S, Morgan N, Dini Det al., 2016,

    Nonequilibrium molecular dynamics simulations of stearic acid adsorbed on iron surfaces with nanoscale roughness

    , Tribology International, Vol: 107, Pages: 264-273, ISSN: 1879-2464

    Nonequilibrium molecular dynamics (NEMD) simulations have been used to examine the structure and friction of stearic acid films adsorbed on iron surfaces with nanoscale roughness. The effect of pressure, stearic acid coverage, and level of surface roughness were investigated. The direct contact of asperities was prevented under all of the conditions simulated due to strong adsorption, which prevented squeeze-out. An increased coverage generally resulted in lower lateral (friction) forces due to reductions in both the friction coefficient and Derjaguin offset. Rougher surfaces led to more liquidlike, disordered films; however, the friction coefficient and Derjaguin offset were only slightly increased. This suggests that stearic acid films are almost as effective on contact surfaces with nanoscale roughness as those which are atomically-smooth.

  • Journal article
    Wang L, Politis DJ, Masen MA, 2016,

    Development of an interactive friction model for the prediction of lubricant breakdown behaviour during sliding wear

    , Tribology International, Vol: 110, Pages: 370-377, ISSN: 1879-2464

    In this paper, a novel interactive friction-lubricant thickness model was developed to predict the evolution of coefficient of friction and the useful life of lubricant film. The developed model was calibrated by experimental results determined from pin-on-disc tests. For these experiments, a grease lubricant was applied on a Tungsten Carbide ball which slides against a disc made from AA6082 Aluminium alloy. In the pin-on-disc tests, the lubricant film thickness decreased with time during single path sliding leading to a rapid increase in the coefficient of friction. The breakdown of lubricant was divided into three stages, namely, the Stage I low and stable coefficient of friction region, Stage II region in which the coefficient of friction sees a rapid rise, and Stage III in which the coefficient of friction reaches a plateau with a value similar to that of dry sliding. In order to characterise the evolution of coefficient of friction throughout these stages, a novel interactive friction model was developed combining the effects of sliding distance, sliding speed, contact pressure and initial lubricant amount on the evolution of the coefficient of friction. This interactive friction model can be applied to situations involving lubricant breakdown in a dynamic environment such as the metal forming industry, where the use of traditional constant coefficient of friction values present limits in predictive accuracy.

  • Journal article
    Shimizu Y, Spikes HA, 2016,

    The tribofilm formation of ZDDP under reciprocating pure sliding conditions

    , Tribology Letters, Vol: 64, ISSN: 1573-2711

    The anti-wear and anti-seizure performance and action mechanisms of zinc dithiophosphate (ZDDP) have been investigated under reciprocating pure sliding conditions to simulate piston ring and cylinder liner assembly, using new techniques. The Mini Traction Machine–Space Layer Imaging is a useful method for monitoring tribofilm formation by ZDDPs. However, tests are generally carried out in mixed sliding–rolling conditions and ZDDP film formation in reciprocating pure sliding conditions is rarely investigated. In this paper, the authors describe an investigation of ZDDP film formation in stationary ball on reciprocating disc pure sliding conditions and compare the results to those obtained in unidirectional pure sliding conditions. In unidirectional pure sliding conditions, the worn area on the ball expands with test time. By contrast, in reciprocating pure sliding conditions, tribofilm forms on the stationary ball and no significant damage occurs. In the initial tribofilm formation under reciprocating pure sliding conditions, solid particulate tribofilm with a high concentration of S forms initially in the contact area and subsequently breaks up. During further rubbing, a Zn- and P-rich tribofilm forms on the comminuted sulphur-rich tribofilm and also the area where the initial tribofilm was removed.

  • Journal article
    wang P, Reddyhoff T, 2016,

    Wall slip in an EHL contact lubricated with 1-dodecanol

    , Tribology International, Vol: 113, Pages: 197-205, ISSN: 1879-2464

    Astudy was conducted ofthe factors that cause anomalous EHDfilm shapes in 1-dodecanol lubricated contacts –i.e.a thickened central plateau and an exaggerated outlet constriction. This has ledto the hypothesisthat, in the region wherepressuresexceed~0.44 GPa, dodecanol experiences slip, which reduces the average lubricant velocityand causesan elevated film thickness in order to maintain flow continuity. The analysis suggests that slip mustbeoccurringat, or closeto, the faster moving surface, since a discontinuityin the flowmidway through thethickness of the film wouldnot affectthe average speedof the lubricant. Based on this hypothesis, an equation has beenderived to predict the anomalous central film thickness as a function of applied slide-roll ratio and this has beenvalidated by comparison with experimentally measured central film thickness values. Further support of this hypothesis is provided by traction test results showing a discontinuity occurring close tothe onset of slip.

  • Journal article
    Jelita Rydel J, Pagkalis K, Kadiric A, Rivera-Díaz-del-Castillo PEJet al., 2016,

    The correlation between ZDDP tribofilm morphology and the microstructure of steel

    , Tribology International, Vol: 113, Pages: 13-25, ISSN: 0301-679X

    The microstructure of most hard steels used in tribological applications is inhomogeneous at a micro-scale. This results in local variations in chemical composition and mechanical properties. On a similar scale, tribofilms formed by ZDDP and other anti-wear additives are commonly observed to exhibit a patch-like morphology. ZDDP tribofilms formed under controlled contact conditions on four different steel grades were carefully studied with a new AFM technique to analyse the relationship between the steel microstructure and the tribofilm morphology. Tribofilms were found to be thinner on residual carbides than on the martensitic matrix in all grades containing residual carbides. In most cases, the difference in tribofilm thickness is larger than the carbide protrusion.

  • Journal article
    Maćkowiak S, Heyes DM, Dini D, Brańka ACet al., 2016,

    Non-equilibrium phase behavior and friction of confined molecular films under shear: a non-equilibrium molecular dynamics study

    , Journal of Chemical Physics, Vol: 145, ISSN: 1089-7690

    The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potentia

  • Journal article
    Guegan J, Kadiric A, Gabelli A, Spikes Het al., 2016,

    The relationship between friction and film thickness in EHD point contacts in the presence of longitudinal roughness

    , Tribology Letters, Vol: 64, ISSN: 1573-2711

    This study investigates friction and film thickness in elastohydrodynamic contacts of machined, rough surfaces, where roughness is dominated by longitudinal ridges parallel to the rolling/sliding direction. A ball-on-disc tribometer was used to simultaneously measure friction and film thickness in rough contacts as well as with nominally smooth specimens for comparison. The studied rough surfaces were selected so that the influence of the root-mean-square roughness and roughness wavelength can be assessed. Friction and film measurements were taken over a range of slide–roll ratios and speeds and with two lubricating oils with different viscosities, hence covering a wide range of specific film thicknesses. The measurements with the nominally smooth specimens show that friction is strongly influenced by thermal effects at high SRRs and that the transition from mixed/boundary to full EHD lubrication occurs at lambda ratios greater than three. At low speeds, the rough specimens are found to generate higher friction than the smooth ones for all the roughness structures considered, and this is shown to be related to the thinner minimum film thickness. Comparison of friction in rough and smooth contacts shows that the total friction in rough contacts can be divided into two components: one that is equivalent to friction in smooth contacts under the same conditions and is dependent on the slide–roll ratio, and the other that is due to the presence of roughness and is independent of the slide–roll ratio under the conditions tested. Further analysis of the minimum film thickness on tops of roughness ridges indicates that even after the full lift-off, an effect of the roughness on friction persists and is most likely related to the local shear stress in the micro-EHD contacts on the top of roughness ridges. At even higher speeds, the difference in friction between the rough and smooth specimens vanishes.

  • Journal article
    Jonas Verschueren, Gurrutxaga Lerma B, Balint DS, Dini D, Sutton APet al., 2016,

    The injection of a screw dislocation into a crystal: atomistics vs. continuum elastodynamics

    , Journal of the Mechanics and Physics of Solids, Vol: 98, Pages: 366-389, ISSN: 1873-4782

    The injection (creation) process of a straight screw dislocation is compared atomistically with elastodynamic continuum theory. Amethod for injecting quiescent screw dislocations into a crystal of tungsten is simulated using non-equilibrium molecular dynamics.The resulting stress fields are compared to the those of elastodynamic solutions for the injection of a quiescent screw dislocation.A number of differences are found: a plane wave emission is observed to emanate from the whole surface of the cut used to createthe dislocation, affecting the displacement field along the dislocation line (z), and introducing displacement field componentsperpendicular to the line (along x and y). It is argued that, in part, this emission is the result of the finite time required to injectthe dislocation, whereby the atoms in the cut surface must temporarily be displaced to unstable positions in order to produce therequired slip. By modelling this process in the continuum it is shown that the displacements components normal to the dislocationline arise from transient displacements of atoms in the cut surface parallel to x and y. It is shown that once these displacements areincluded in the elastodynamic continuum formulation the plane wave emission in uzis correctly captured. A detailed comparisonbetween the atomistic and continuum models is then offered, showing that the main atomistic features can also be captured in thecontinuum.

  • Journal article
    Woldman M, Van Der Heide E, Tinga T, Masen MAet al., 2016,

    A finite element approach to modeling abrasive wear modes

    , Tribology Transactions, Vol: 60, Pages: 711-718, ISSN: 1040-2004

    Machine components operating in sandy environments will wear because of the abrasive interaction with sand particles. In this work, a method is derived to predict the amount of wear caused by such abrasive action, in order to improve the maintenance concept of the components. A finite element model is used to simulate various tips scratching a smooth surface. The model is verified by comparing the obtained results with a set of experiments performed earlier (M. Woldman, et al., 2013, Wear, 301(1–2), pp 76–81).

  • Journal article
    Laux K, Jean-Fulcrand A, SUE HJ, BREMNER T, Wong JSSet al., 2016,

    The influence of surface properties on sliding contact temperatureand friction for Polyetheretherketone (PEEK)

    , Polymer, Vol: 103, Pages: 397-404, ISSN: 0032-3861

    Polyetheretherketone (PEEK) polymers are increasingly used intribological applications. An important aspect of PEEK tribology is thesurface temperature reached during sliding. However, most knowledge offrictional heating in PEEK is based on post-hoc analysis of debris andwear surfaces. In this study, infrared thermography was used to observethe full field temperature map of PEEK during ball-on-disc sliding.Although the measured temperatures were below any thermal transition, theresults matched closely to those predicted by flash temperature models.Additionally, friction studies were performed with steel and sapphirecounterfaces. It was observed that PEEK debris was readily deposited tosteel but not on sapphire. The friction studies also indicated a greateradhesive friction response for PEEK against steel compared to sapphire.The transfer of PEEK material to the steel surface may elevate thetemperature at the sliding interface. Analysis of films formed on steelsuggests that the transferred PEEK was oriented in the direction ofsliding. The deposition of debris and formation of oriented filmsresembled a high temperature drawing process, which was likely to be dueto localized frictional heating. The results of this study illustrate theimportant role transfer films play in determining both the friction andtemperature response of the PEEK wear interface.

  • Journal article
    Sopher R, Amis A, Davies D, Jeffers Jet al., 2016,

    The influence of muscle pennation angle and cross-sectional area on contact forces in the ankle joint

    , Journal of Strain Analysis for Engineering Design, Vol: 52, Pages: 12-23, ISSN: 0309-3247

    Data about a muscle’s fibre pennation angle and physiological cross-sectional area are used in musculoskeletal modelling to estimate muscle forces, which are used to calculate joint contact forces. For the leg, muscle architecture data are derived from studies that measured pennation angle at the muscle surface, but not deep within it. Musculoskeletal models developed to estimate joint contact loads have usually been based on the mean values of pennation angle and physiological cross-sectional area.Therefore, the first aim of this study was to investigate differences between superficial and deep pennation angles within each muscle acting over the ankle and predict how differences may influence muscle forces calculated in musculoskeletal modelling. The second aim was to investigate how inter-subject variability in physiological cross-sectional area and pennation angle affects calculated ankle contact forces.Eight cadaveric legs were dissected to excise the muscles acting over the ankle. The mean surface and deep pennation angles, fibre length and physiological cross-sectional area were measured. Cluster analysis was applied to group the muscles according to their architectural characteristics. A previously validated OpenSim model was used to estimate ankle muscle forces and contact loads using architecture data from all eight limbs.The mean surface pennation angle for soleus was significantly greater (54%) than the mean deep pennation angle. Cluster analysis revealed three groups of muscles with similar architecture and function: deep plantarflexors and peroneals, superficial plantarflexors and dorsiflexors. Peak ankle contact force was predicted to occur before toe-off, with magnitude greater than five times bodyweight. Inter-specimen variability in contact force was smallest at peak force.These findings will help improve the development of experimental and computational musculoskeletal models by providing data to estimate force based on both surface and deep

  • Journal article
    Tsui S, Tandy J, Myant C, Masen M, Cann PMet al., 2016,

    Friction measurements with yoghurt in a simulated tongue-palate contact

    , Biotribology, Vol: 8, Pages: 1-11

    © 2016 The perception of many food attributes is related to mechanical stimulation and friction experienced in the tongue-palate contact during mastication. Friction in the tongue-palate is determined by the changing film properties (composition, component distribution, thickness) in the conjunction. We suggest this evolution is essentially determined by tongue-palate film loss rather than shear flow entrainment which predominates in conventional bearing lubrication. The paper reports friction measurements in a simulated tongue-palate contact for a range of high and low fat dairy foods. A reciprocating, sliding contact with restricted stroke length ( <  contact width) was used; under these conditions there is negligible shear-entrainment of fluid from outside the contact area. The tongue-palate contact was simulated by a PDMS ball and glass surface. The effect of hydrophobic and hydrophilic surfaces on friction was investigated for different fat contents (0, 4.2, 9.5% wt fat). Friction was measured over 60 s of rubbing. Significant differences were observed in the friction change with time for different fat contents (μ 9.5  <  μ 4.2  <  μ 0 wt%) and for different surface energy conditions (μ hydrophilic  <  μ hydrophobic). Post-test visualisation of the rubbed films showed that low friction coefficient was associated with the formation of a thin oil film on deposited particulate solids.

  • Journal article
    Forte AE, Galvan S, Manieri F, Rodriguez y Baena F, Dini Det al., 2016,

    A composite hydrogel for brain tissue phantoms

    , Materials and Design, Vol: 112, Pages: 227-238, ISSN: 0264-1275

    Synthetic phantoms are valuable tools for training, research and development in traditional and computer aided surgery, but complex organs, such as the brain, are difficult to replicate. Here, we present the development of a new composite hydrogel capable of mimicking the mechanical response of brain tissue under loading. Our results demonstrate how the combination of two different hydrogels, whose synergistic interaction results in a highly tunable blend, produces a hybrid material that closely matches the strongly dynamic and non-linear response of brain tissue. The new synthetic material is inexpensive, simple to prepare, and its constitutive components are both widely available and biocompatible. Our investigation of the properties of this engineered tissue, using both small scale testing and life-sized brain phantoms, shows that it is suitable for reproducing the brain shift phenomenon and brain tissue response to indentation and palpation.

  • Journal article
    Shimizu Y, Spikes HA, 2016,

    The influence of slide–roll ratio on ZDDP tribofilm formation

    , Tribology Letters, Vol: 64, ISSN: 1573-2711

    The anti-wear performance and action mecha-nisms of zinc dithiophosphate (ZDDP) have been investi-gated under various test conditions. The Mini TractionMachine–Space Layer Imaging (MTM–SLIM) is a widelyused and useful method for monitoring tribofilm formationby ZDDPs. However, tests are generally carried out inmixed sliding–rolling conditions, typically between 50 %SRR (slide–roll ratio) and 100 % SRR. In this paper, theauthors describe an investigation of ZDDP film formationat SRRs much higher than 100 % SRR, including puresliding conditions using a novel MTM–SLIM technique. Athigh SRRs, ZDDP tribofilms form without damaging theball surfaces so long as both surfaces move above athreshold speed with respect to the contact, regardless ofwhether the two surfaces move in the same or opposingdirections. In pure sliding conditions, although the wornarea expands with time under pure sliding conditionsshowing that wear takes place, tribofilms are still built upthroughout a test and the ZDDP has a beneficial effect onwear rate. The very early stages of film formation arestudied to show that a tribofilm with a high concentrationof S is formed initially and then replaced with a film havinga high concentration of Zn and P.

  • Conference paper
    Tan Z, Forte AE, Galvan S, Dini D, Rodriguez Y Baena Fet al., 2016,

    Composite Hydrogel: a New Tool for Reproducing the Mechanical Behaviour of Soft Human Tissues

    , Biotribology 2016
  • Conference paper
    Forte AE, Dini D, 2016,

    On the suitability of hydrogels for mimicking the mechano-response of organic soft tissues

    , Biotribology 2016
  • Journal article
    Heyes DM, Dini D, Smith ER, 2016,

    Equilibrium fluctuations of liquid state static properties in a subvolume by molecular dynamics

    , Journal of Chemical Physics, Vol: 145, ISSN: 1089-7690

    System property fluctuations increasingly dominate a physical process as the sampling volume decreases. The purpose of this work is to explore how the fluctuation statistics of various thermodynamic properties depend on the sampling volume, using molecular dynamics (MD) simulations. First an examination of various expressions for calculating the bulk pressure of a bulk liquid is made, which includes a decomposition of the virial expression into two terms, one of which is the Method of Planes (MOP) applied to the faces of the cubic simulation cell. Then an analysis is made of the fluctuations of local density, temperature, pressure, and shear stress as a function of sampling volume (SV). Cubic and spherical shaped SVs were used within a spatially homogeneous LJ liquid at a state point along the melting curve. It is shown that the MD-generated probability distribution functions (PDFs) of all of these properties are to a good approximation Gaussian even for SV containing only a few molecules (∼10), with the variances being inversely proportional to the SV volume, Ω. For small subvolumes the shear stress PDF fits better to a Gaussian than the pressure PDF. A new stochastic sampling technique to implement the volume averaging definition of the pressure tensor is presented, which is employed for cubic, spherical, thin cubic, and spherical shell SV. This method is more efficient for less symmetric SV shapes.

  • Journal article
    Gurrutxaga Lerma B, Balint DS, Dini D, Sutton APet al., 2016,

    A dynamic discrete dislocation plasticity study of elastodynamic shielding of stationary cracks

    , Journal of the Mechanics and Physics of Solids, Vol: 98, Pages: 1-11, ISSN: 0022-5096

    Employing Dynamic Discrete Dislocation Plasticity (D3P), an elastodynamic analysis of theshielding of a stationary crack tip by dislocations is studied. Dislocations are generated via FrankReadsources, and make a negligible contribution to the shielding of the crack tip, whereas dislocationsgenerated at the crack tip via homogeneous nucleation dominate the shielding. Theireffect is found to be highly localised around the crack, leading to a magnification of the shieldingwhen compared to time-independent, elastostatic predictions. The resulting attenuation of KI (t)is computed, and is found to be directly proportional to the applied load and to √t.

  • Conference paper
    Wang A, Zheng Y, Liu J, El Fakir O, Masen M, Wang Let al., 2016,

    Knowledge Based Cloud FE simulation – data-driven material characterization guidelines for the hot stamping of aluminium alloys

    , 10th International Conference and Workshop on Numerical Simulation of 3D Sheet Metal Forming Processes, Numisheet 2016, Publisher: IOP Publishing, Pages: 032042-032042, ISSN: 1742-6588

    The Knowledge Based Cloud FEA (KBC-FEA) simulation technique allows multi-objective FE simulations to be conducted on a cloud-computing environment, which effectively reduces computation time and expands the capability of FE simulation software. In this paper, a novel functional module was developed for the data mining of experimentally verified FE simulation results for metal forming processes obtained from KBC-FE. Through this functional module, the thermo-mechanical characteristics of a metal forming process were deduced, enabling a systematic and data-driven guideline for mechanical property characterization to be developed, which will directly guide the material tests for a metal forming process towards the most efficient and effective scheme. Successful application of this data-driven guideline would reduce the efforts for material characterization, leading to the development of more accurate material models, which in turn enhance the accuracy of FE simulations.

  • Conference paper
    Putignano C, Carbone G, Dini D, 2016,

    A parametrically time-dependent methodology for reciprocating contact mechanics between viscoelastic solids

    , VII European Congress on Computational Methods in Applied Sciences and Engineering, Publisher: National Technical University of Athens (NTUA), Pages: 1856-1863

    We implement an original Boundary Element methodology to study the reciprocating contact mechanics between linear viscoelastic materials. Results are shown for the case of a rigid sphere sinusoidally driven in sliding contact with a viscoelastic half-space. We observe the presence of multi-peaked pressure and displacement distributions; the hysteric friction curve is finally shown for different values of the frequency.

  • Journal article
    Ewen JP, Gattinoni C, Thakkar FM, Morgan N, Spikes HA, Dini Det al., 2016,

    Nonequilibrium Molecular Dynamics Investigation of the Reduction in Friction and Wear by Carbon Nanoparticles Between Iron Surfaces

    , Tribology Letters, Vol: 63, ISSN: 1023-8883
  • Journal article
    Ewen JP, Gattinoni C, Thakkar FM, Morgan N, Spikes HA, Dini Det al., 2016,

    A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants

    , MATERIALS, Vol: 9, ISSN: 1996-1944
  • Journal article
    Zhang J, Spikes HA, 2016,

    On the Mechanism of ZDDP Antiwear Film Formation

    , Tribology Letters, Vol: 63, ISSN: 1573-2711

    Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanismexplains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form.The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.

  • Journal article
    Huang L, Guo D, Cann PM, Wan GTY, Wen Set al., 2016,

    Thermal Oxidation Mechanism of Polyalphaolefin Greases with Lithium Soap and Diurea Thickeners: Effects of the Thickener

    , Tribology Transactions, ISSN: 1040-2004

    In this work, lithium and diurea greases formulated by poly-alpha-olefin were aged up to 1,200 h in an oven at 120°C and periodically taken out for testing. Scanning electron microscopy (SEM) and infrared (IR) spectra proved that both physical and chemical degradation occurred during the thermal aging process, such as a decrease in apparent viscosity, thickener destruction, and change in chemical species. Diurea grease showed much better anti-oxidation performance during thermal aging than lithium grease. A dual effect of thermal aging on the grease lubricity was observed and analyzed. Results showed that early oxidation might reduce grease lubricity due to the formation of ketones and aldehydes, and the decrease in grease viscosity and oil bleeding due to thickener destruction would contribute to better replenishment.

  • Conference paper
    Wang A, Zheng Y, Liu J, El Fakir O, Masen M, Wang Let al.,

    Knowledge based cloud FE simulation a multi-objective FEA system for advanced FE simulation of hot stamping process

    , The 10th International Conference and Workshop on Numerical Simulation of 3D Sheet Metal Forming Processes, Numisheet 2016, Publisher: IOP Publishing, ISSN: 1742-6588

    . A knowledge Based Cloud (KBC) FE simulation technique has been developed to enable advanced, efficient and multi-objective finite element analyses (FEA), which operates on a cloud computing environment. The core FE simulation is conducted using a commercial FE code, e.g. PAM-STAMP, and its capability is enhanced through the implementation of advanced functional modules instead of user defined sub-routines. KBC-FE simulation offers great flexibility and variability to engineers as modules could be used collectively or individually. In this paper, case studies were conducted for the hot stamping of tailor welded blanks. Multi-objective FE simulations were performed for ‘Forming Limit’ prediction under non-isothermal and non-linear loading conditions. In addition, ‘Tool-life’ prediction was conducted under multi-cycle loading conditions.

  • Journal article
    Kakavas I, Olver AV, Dini D, 2016,

    Hypoid gear vehicle axle efficiency

    , Tribology International, Vol: 101, Pages: 314-323, ISSN: 0301-679X

    In this paper, a study of a hypoid gear vehicle axle is presented. Using a custom rig, load-independent losses have been accurately measured and the effect of viscosity on spin loss has been quantified. Solution methods for the calculation of component losses are presented and combined into a complete thermally coupled transient model for the estimation of axle efficiency. An analysis of hypoid gear kinematics reveals a simplification, commonly adopted by other researchers, regarding the velocity of the point of contact in hypoid gears, to be in error. As a result, the calculation of lubrication parameters has been improved. Finally, experimental measurements are compared to the generated simulation results for a number of operating scenarios and satisfactory correlation is observed.

  • Journal article
    Hills DA, Dini D, 2016,

    A review of the use of the asymptotic framework for quantification of fretting fatigue

    , Journal of Strain Analysis for Engineering Design, Vol: 51, Pages: 240-246, ISSN: 0309-3247
  • Journal article
    Putignano C, Carbone G, Dini D, 2016,

    Theory of reciprocating contact for viscoelastic solids

    , Physical Review E, Vol: 93, ISSN: 1539-3755

    A theory of reciprocating contacts for linear viscoelastic materials is presented. Results are discussed for the case of a rigid sphere sinusoidally driven in sliding contact with a viscoelastic half-space. Depending on the size of the contact, the frequency and amplitude of the reciprocating motion, and on the relaxation time of the viscoelastic body, we establish that the contact behavior may range from the steady-state viscoelastic solution, in which traction forces always oppose the direction of the sliding rigid punch, to a more elaborate trend, which is due to the strong interaction between different regions of the path covered during the reciprocating motion. Practical implications span a number of applications, ranging from seismic engineering to biotechnology.

  • Journal article
    Ewen JP, Gattinoni C, Morgan N, Spikes HA, Dini Det al., 2016,

    Nonequilibrium molecular dynamics simulations of organic friction modifiers adsorbed on iron oxide surfaces

    , Langmuir, Vol: 32, Pages: 4450-4463, ISSN: 0743-7463

    © 2016 American Chemical Society. For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.

  • Journal article
    Vladescu S-C, Olver AV, Pegg IG, Reddyhoff Tet al., 2016,

    Combined friction and wear reduction in a reciprocating contact through laser surface texturing

    , Wear, Vol: 358-359, Pages: 51-61, ISSN: 0043-1648
  • Journal article
    Klaassen M, Schipper DJ, Masen MA, 2016,

    Influence of the relative humidity and the temperature on the in-vivo friction behaviour of human skin

    , Biotribology, Vol: 6, Pages: 21-28

    © 2016 Elsevier B.V. Both temperature and relative humidity are known to influence the frictional behaviour of human skin. However, literature does not completely cover to what extent both parameters play a role. Measurements were conducted using an in-house built reciprocating tribometer inside an enclosure in which both the humidity and the temperature can be controlled independently. Friction measurements were performed in varying climates ranging from 25 °C and 40% RH to 37 °C and 80% RH at respectively 3 °C and 10% RH intervals. Using the obtained results a ‘friction map’ was created which shows that the coefficient of friction increases by a factor of two when the environment is changed from ‘cold and dry’ to ‘warm and moist’. A statistical analysis shows that the product of the temperature and relative humidity appears to be the driving factor describing the observed frictional behaviour. Results indeed show a more pronounced effect of either parameter at the warmer, moister conditions, in contrast to the colder, drier conditions where a smaller effect on the coefficient of friction is observed. The findings will be of importance, e.g. for bedridden patients who are prone to pressure ulcer development as it indicates the importance of maintaining a healthy microclimate.

  • Journal article
    Marx N, Guegan J, Spikes HA, 2016,

    Elastohydrodynamic film thickness of soft EHL contacts using optical interferometry

    , Tribology International, Vol: 99, Pages: 267-277, ISSN: 1879-2464

    Robust, chromium, semi-reflective coatings have been applied to transparent polymethylmethacrylate and polyurethane discs and this has enabled conventional, normal incidence optical interferometry to be used to measure lubricant film thickness in soft EHL conditions for the first time. High quality interferograms comparable to those obtained from coated glass discs are obtained. Measured film thickness has been compared with existing soft EHL film thickness equations obtained using computer modelling and revised central and minimum film thickness equations have been proposed. These film thickness measurements and measurement technique have applicability to our understanding of the performance and design of lubricated gears and bearings manufactured from polymeric materials.

  • Journal article
    Vladescu S-C, Medina S, Olver AV, Pegg IG, Reddyhoff Tet al., 2016,

    The Transient Friction Response of a Laser-Textured, Reciprocating Contact to the Entrainment of Individual Pockets

    , Tribology Letters, Vol: 62, ISSN: 1573-2711

    To shed light on the mechanisms with which surface texture improves the tribological performance of piston–liner contacts, we have measured the transient friction response as individual pockets pass through a reciprocating sliding contact. Tests were performed at different sliding speeds and results compared to those from a non-textured, reference specimen under different lubrication regimes. At low speed when the contact is in the boundary regime, friction force falls abruptly as each pocket leaves the contact zone, before gradually returning to an approximately steady-state value. This suggests that each pocket acts to temporarily increase the film thickness, which then decays to its non-textured value as oil is squeezed out. At higher speeds, friction is seen to reduce in a stepwise fashion, since the period between pockets being entrained is less than the time taken for the film to decay. In addition, friction results obtained when the contact is operating in the middle of the mixed regime point to a temporary film thickness collapse as the pocket enters the contact, and this agrees with recent modelling predictions. At higher speeds, the compound effect of successive pockets is to shift the contact to the right on Stribeck curve. These results imply that each pocket gives rise to an increase in film thickness that is both short-lived and small in magnitude (we estimate a few tens of nm). However, the resulting effect on friction can be significant (up to 82 % in this study) for two reasons: (1) provided the pocket frequency is sufficiently high, each successive pocket entrainment builds the film up without there being time for it to reduce back to its steady-state value; (2) when the contact is in the mixed regime, the Stribeck curve is at its steepest and friction is therefore most sensitive to film thickness changes. This has important practical implications in that pocket spacing on piston liners should be varied as a function of reciprocating sliding

  • Journal article
    Xu Y, Balint DB, Dini DD, 2016,

    A method of coupling discrete dislocation plasticity to the crystal plasticity finite element method

    , Modelling and Simulation in Materials Science and Engineering, Vol: 24, ISSN: 1361-651X

    A method of concurrent coupling of planar discrete dislocation plasticity (DDP) and a crystal plasticityfinite element (CPFE) method was devised for simulating plastic deformation in large polycrystals withdiscrete dislocation resolution in a single grain or cluster of grains for computational efficiency;computation time using the coupling method can be reduced by an order of magnitude compared toDDP. The method is based on an iterative scheme initiated by a sub-model calculation, which ensuresdisplacement and traction compatibility at all nodes at the interface between the DDP and CPFEdomains. The proposed coupling approach is demonstrated using two plane strain problems: (i)uniaxial tension of a bi-crystal film and (ii) indentation of a thin film on a substrate. The latter was alsoused to demonstrate that the rigid substrate assumption used in earlier discrete dislocation plasticitystudies is inadequate for indentation depths that are large compared to the film thickness, i.e. theeffect of the plastic substrate modelled using CPFE becomes important. The coupling method can beused to study a wider range of indentation depths than previously possible using DDP alone, withoutsacrificing the indentation size effect regime captured by DDP. The method is general and can beapplied to any problem where finer resolution of dislocation mediated plasticity is required to studythe mechanical response of polycrystalline materials, e.g. to capture size effects locally within a largerelastic/plastic boundary value problem.

  • Journal article
    Putignano C, Reddyhoff T, Dini D, 2016,

    The influence of temperature on viscoelastic friction properties

    , Tribology International, Vol: 100, Pages: 338-343, ISSN: 0301-679X

    Viscoelastic friction strongly depends on temperature, which determines the material stiffness and, therefore, given a constant load, the volume that is deformed and dissipates energy. We compare the results obtained by a numerical approach introduced by Carbone and Putignano (2013) [1] with measurements that separate viscoelastic losses from Coulomb contribution. This is done for a range of temperatures. We show that viscoelastic friction curves for different temperatures can be arranged into a single master curve using a frequency shift coefficient, which can be found from the characterization of the viscoelastic material response. This shows that it is possible to accurately (a) use dynamic material analysis data to extrapolate viscoelastic friction measurements to values outside the tested range, and (b) use a tribometer to obtain fundamental viscoelastic material properties.

  • Journal article
    Galmiche B, Ponjavic A, Wong J, 2016,

    Flow measurements of a polyphenyl ether oil in an elastohydrodynamic contact

    , Journal of Physics: Condensed Matter, Vol: 28, ISSN: 0953-8984

    A novel methodology, based on the use of phosphorescence imaging, is applied to determine the local through-thickness velocity profile of lubricant in an elastohydrodynamic (EHD) contact. The technique has spatial and temporal resolutions of 40 µm and 340 µs respectively and thus allows lubricant rheology to be investigated at conditions close to service conditions. The capability of the newly-developed method is verified by examining the flow of 5P4E polyphenyl ether, a lubricant base fluid used in very high temperature applications and well-known for its high viscosity-pressure coefficient. Experimental results highlight the effect of the contact pressure on the velocity profile of this fluid in lubricated contacts. At low pressures, the velocity profile of 5P4E is close to linear, characteristic of Couette flow. As the local pressure increases, its velocity profile progressively deviates from a Couette profile and shear banding is evident at high pressure.

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