Most recent Tribology Group publications are Open Access thanks to funding from the EPSRC.

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  • JOURNAL ARTICLE
    Manieri F, Stadler K, Morales-Espejel GE, Kadiric Aet al., 2019,

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

    , International Journal of Fatigue, Vol: 120, Pages: 107-133, ISSN: 0142-1123

    © 2018 The Authors Presence of white etching cracks has been widely associated with early failures of rolling bearings in a number of applications, with wind turbine gearbox bearings being the most frequently cited and practically significant example. Despite the recent research efforts, there is yet no universal agreement on the mechanisms of formation of these cracks and little direct evidence of their significance to bearing reliability. In an attempt to address this, this paper proposes a new theory on the origins and significance of white etching cracks. The paper provides systematic experimental evidence in support of this theory through rolling contact fatigue tests performed with AISI 52100 bearing steel specimens on a triple-disc machine over a wide range of contact conditions. The test results show that white etching cracks can be formed with base oils as well as commercially formulated transmission and engine oils. WECs were generated under slide-roll-ratios ranging from 0.05 to 0.3, under positive and negative sliding, different contact pressures and specific film thicknesses ranging from 0.1 to 0.7. No white etching areas were ever observed without the associate crack being present, and it was also shown that white etching areas themselves can be produced in a pure rubbing contact of bearing steels under both lubricated and unlubricated conditions. These results provide direct evidence that the steel transformations that exhibit themselves as white etching areas are formed through rubbing of the existing crack faces, and that the chemical composition of the lubricant and the magnitude and direction of sliding are not the primary driver of WEC formation, in contrast to literature. Instead, the results presented here show that WECs are formed through the action of a specific stress history in time via the following mechanism: (i) Short-lived high contact stresses, which can be caused by a number of factors, act in the initial stages of the component

  • JOURNAL ARTICLE
    Dzepina B, Balint D, Dini D, 2019,

    A phase field model of pressure-assisted sintering

    , Journal of the European Ceramic Society, Vol: 39, Pages: 173-182, ISSN: 0955-2219

    © 2018 The Authors The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.

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

    Transient aspects of plasma luminescence induced by triboelectrification of polymers

    , TRIBOLOGY INTERNATIONAL, Vol: 130, Pages: 366-377, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Jean-Fulcrand A, Maser MA, Bremner T, Wong JSSet al., 2019,

    Effect of temperature on tribological performance of polyetheretherketone-polybenzimidazole blend

    , TRIBOLOGY INTERNATIONAL, Vol: 129, Pages: 5-15, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Tan Z, Dini D, Rodriguez y Baena F, Forte AEet al., 2018,

    Composite hydrogel: A high fidelity soft tissue mimic for surgery

    , Materials and Design, Vol: 160, Pages: 886-894, ISSN: 0264-1275

    © 2018 The Authors Accurate tissue phantoms are difficult to design due to the complex non-linear viscoelastic properties of real soft tissues. A composite hydrogel, resulting from a mix of poly(vinyl) alcohol and phytagel, is able to reproduce the viscoelastic responses of different soft tissues due to its compositional tunability. The aim of this work is to demonstrate the flexibility of the composite hydrogel in mimicking the interactions between surgical tools and various soft tissues, such as brain, lung and liver. Therefore compressive stiffness, insertion forces and frictional forces were used as matching criteria to determine the hydrogel compositions for each soft tissue. A full map of the behaviour of the synthetic material is provided for these three characteristics and the compositions found to best match the mechanical response of brain, lung and liver are reported. The optimised hydrogel samples are then tested and shown to mimic the behaviour of the three tissues with unprecedented fidelity. The effect of each hydrogel constituent on the compressive stiffness, needle insertion and frictional forces is also detailed in this work to explain their individual contributions and synergistic effects. This study opens important opportunities for the realisation of surgical planning and training devices and tools for in-vitro tissue testing.

  • JOURNAL ARTICLE
    Kontou A, Southby M, Morgan N, Spikes HAet al., 2018,

    Influence of Dispersant and ZDDP on Soot Wear

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Dawczyk J, Ware E, Ardakani M, Russo J, Spikes Het al., 2018,

    Use of FIB to Study ZDDP Tribofilms

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

    Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

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

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

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Dench J, di Mare L, Morgan N, Wong JSSet al., 2018,

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

    , Phys Chem Chem Phys

    The viscosity of liquids is a strong function of pressure. While viscosity is relatively easy to measure at low pressure, high-pressure rheology presents significant experimental challenges. As a result, rheological models are often used to extrapolate viscosity from low pressure measurements to higher pressures. Techniques to obtain data over a wide range of pressures and shear rates, as well as understanding the validity and limitations of methods to fill the gaps in the available data, are therefore of crucial practical and theoretical importance. This work examines the viscosity of polyalphaolefin (PAO) by combining average global area averaged measurements at high pressure and local molecular viscosity measurements at moderate pressures. Viscosities spanning five orders of magnitude are examined at pressures up to 720 MPa. High pressure results were obtained with friction measurements where the fluid is sheared between two surfaces in a loaded point contact. The local molecular microviscosity at medium and low pressures was measured by applying a technique based on fluorescence anisotropy, which probes the rotational motion of dye molecules in a nanoscale film under shear. Both sets of measurements are taken in the same configuration, an elastohydrodynamic (EHD) contact. This is the first set of quantitative local viscosity measurements that have been verified against both friction and high pressure rheometry measurements. Commonly used rheological models were compared to experimental results. Our work shows that fluorescence anisotropy and friction measurements can be used to determine the viscosity of liquids over a wide range of conditions from a single experimental setup. The results obtained match results from low- and high-pressure rheometry for PAO. The importance of correcting friction data for pressure non-uniformity, temperature and shear thinning is also highlighted.

  • JOURNAL ARTICLE
    Ma S, Scaraggi M, Yan C, Wang X, Gorb SN, Dini D, Zhou Fet al., 2018,

    Bioinspired 3D Printed Locomotion Devices Based on Anisotropic Friction.

    , Small

    Anisotropic friction plays a key role in natural systems, particularly for realizing the purpose of locomotion and strong attachment for the survival of organisms. Of particular interest, here, is the observation that friction anisotropy is promoted numerous times by nature, for example, by wild wheat awn for its targeted and successful seed anchorage and dispersal. Such feature is, however, not fully exploited in man-made systems, such as microbots, due to technical limitations and lack of full understanding of the mechanisms. To unravel the complex dynamics occurring in the sliding interaction between anisotropic microstructured surfaces, the friction induced by asymmetric plant microstructures is first systematically investigated. Inspired by this, anisotropic polymer microactuators with three-dimensional (3D) printed microrelieves are then prepared. By varying geometric parameters, the capability of microactuators to generate strong friction anisotropy and controllable motion in remotely stretched cylindrical tubes is investigated. Advanced theoretical models are proposed to understand and predict the dynamic behavior of these synthetic systems and to shed light on the parameters and mechanisms governing their behavior. Finally, a microbot prototype is developed and cargo transportation functions are successfully realized. This research provides both in-depth understanding of anisotropic friction in nature and new avenues for developing intelligent actuators and microbots.

  • JOURNAL ARTICLE
    Ebrahimi MT, Dini D, Balint DS, Sutton AP, Ozbayraktar Set al., 2018,

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

    , INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 152, Pages: 12-27, ISSN: 0020-7683
  • JOURNAL ARTICLE
    Porte E, Cann P, Masen M, 2018,

    Fluid load support does not explain tribological performance of PVA hydrogels.

    , J Mech Behav Biomed Mater, Vol: 90, Pages: 284-294

    The application of hydrogels as articular cartilage (AC) repair or replacement materials is limited by poor tribological behaviour, as it does not match that of native AC. In cartilage, the pressurisation of the interstitial fluid is thought to be crucial for the low friction as the load is shared between the solid and liquid phase of the material. This fluid load support theory is also often applied to hydrogels. However, this theory has not been validated as no experimental evidence directly relates the pressurisation of the interstitial fluid to the frictional response of hydrogels. This lack of understanding about the governing tribological mechanisms in hydrogels limits their optimised design. Therefore, this paper aims to provide a direct measure for fluid load support in hydrogels under physiologically relevant sliding conditions. A photoelastic method was developed to simultaneously measure the load on the solid phase of the hydrogel and its friction coefficient and thus directly relate friction and fluid load support. The results showed a clear distinction in frictional behaviour between the different test conditions, but results from photoelastic images and stress-relaxation experiments indicated that fluid load support is an unlikely explanation for the frictional response of the hydrogels. A more appropriate explanation, we hypothesized, is a non-replenished lubricant mechanism. This work has important implications for the tribology of cartilage and hydrogels as it shows that the existing theories do not adequately describe the tribological behaviour of hydrogels. The developed insights can be used to optimise the tribological performance of hydrogels as articular cartilage implants.

  • JOURNAL ARTICLE
    Verschueren J, Gurrutxaga-Lerma B, Balint DS, Sutton AP, Dini Det al., 2018,

    Instabilities of High Speed Dislocations

    , PHYSICAL REVIEW LETTERS, Vol: 121, ISSN: 0031-9007
  • JOURNAL ARTICLE
    Yu M, Arana C, Evangelou SA, Dini D, Cleaver GDet al., 2018,

    Parallel Active Link Suspension: A Quarter-Car Experimental Study

    , IEEE-ASME TRANSACTIONS ON MECHATRONICS, Vol: 23, Pages: 2066-2077, ISSN: 1083-4435
  • JOURNAL ARTICLE
    Hartinger M, Reddyhoff T, 2018,

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

    , TRIBOLOGY INTERNATIONAL, Vol: 126, Pages: 144-152, ISSN: 0301-679X
  • JOURNAL ARTICLE
    Gattinoni C, Ewen JP, Dini D, 2018,

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

    , JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 20817-20826, ISSN: 1932-7447
  • JOURNAL ARTICLE
    Campen S, Smith B, Wong J, 2018,

    Deposition of Asphaltene from Destabilized Dispersions in Heptane-Toluene

    , ENERGY & FUELS, Vol: 32, Pages: 9159-9171, ISSN: 0887-0624
  • JOURNAL ARTICLE
    Reddyhoff T, Underwood RJ, Sayles RS, Spikes HAet al., 2018,

    Temperature measurement of debris particles in EHL contacts

    , SURFACE TOPOGRAPHY-METROLOGY AND PROPERTIES, Vol: 6, ISSN: 2051-672X
  • JOURNAL ARTICLE
    Marx N, Fernández L, Barceló F, Spikes Het al., 2018,

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

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

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

  • JOURNAL ARTICLE
    Marx N, Fernandez L, Barcelo F, Spikes Het al., 2018,

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

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Vakis AI, Yastrebov VA, Scheibert J, Nicola L, Dini D, Minfray C, Almqvist A, Paggi M, Lee S, Limbert G, Molinari JF, Anciaux G, Aghababaei R, Restrepo SE, Papangelo A, Cammarata A, Nicolini P, Putignano C, Carbone G, Stupkiewicz S, Lengiewicz J, Costagliola G, Bosia F, Guarino R, Pugno NM, Mueser MH, Ciavarella Met al., 2018,

    Modeling and simulation in tribology across scales: An overview

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

    Capillary waves with surface viscosity

    , JOURNAL OF FLUID MECHANICS, Vol: 847, Pages: 644-663, ISSN: 0022-1120
  • JOURNAL ARTICLE
    Stevenson H, Parkes M, Austin L, Jaggard M, Akhbari P, Vaghela U, Williams HRT, Gupte C, Cann Pet al., 2018,

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

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

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

  • JOURNAL ARTICLE
    Kanca Y, Milner P, Dini D, Amis AAet al., 2018,

    Tribological evaluation of biomedical polycarbonate urethanes against articular cartilage

    , JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS, Vol: 82, Pages: 394-402, ISSN: 1751-6161
  • JOURNAL ARTICLE
    Heyes DM, Dini D, Smith ER, 2018,

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

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

    Slip of Alkanes Confined between Surfactant Monolayers Adsorbed on Solid Surfaces

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

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

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

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

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

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

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

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

  • JOURNAL ARTICLE
    Lu J, Reddyhoff T, Dini D, 2018,

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

    , TRIBOLOGY LETTERS, Vol: 66, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Spikes H, 2018,

    Stress-augmented thermal activation: Tribology feels the force

    , FRICTION, Vol: 6, Pages: 1-31, ISSN: 2223-7690
  • JOURNAL ARTICLE
    Menga N, Carbone G, Dini D, 2018,

    Do uniform tangential interfacial stresses enhance adhesion?

    , Journal of the Mechanics and Physics of Solids, Vol: 112, Pages: 145-156, ISSN: 0022-5096

    © 2017 Elsevier Ltd We present theoretical arguments, based on linear elasticity and thermodynamics, to show that interfacial tangential stresses in sliding adhesive soft contacts may lead to a significant increase of the effective energy of adhesion. A sizable expansion of the contact area is predicted in conditions corresponding to such scenario. These results are easily explained and are valid under the assumptions that: (i) sliding at the interface does not lead to any loss of adhesive interaction and (ii) spatial fluctuations of frictional stresses can be considered negligible. Our results are seemingly supported by existing experiments, and show that frictional stresses may lead to an increase of the effective energy of adhesion depending on which conditions are established at the interface of contacting bodies in the presence of adhesive forces.

  • JOURNAL ARTICLE
    Guo Y, di Mare L, Li RKY, Wong JSSet al., 2018,

    Cargo Release from Polymeric Vesicles under Shear

    , POLYMERS, Vol: 10, ISSN: 2073-4360
  • JOURNAL ARTICLE
    Masen M, Cann PME, 2018,

    Friction measurements with molten chocolate

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

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

  • JOURNAL ARTICLE
    Forte AE, Galvan S, Dini D, 2018,

    Models and tissue mimics for brain shift simulations

    , BIOMECHANICS AND MODELING IN MECHANOBIOLOGY, Vol: 17, Pages: 249-261, ISSN: 1617-7959
  • JOURNAL ARTICLE
    Kanca Y, Milner P, Dini D, Amis AAet al., 2018,

    Tribological properties of PVA/PVP blend hydrogels against articular cartilage

    , JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS, Vol: 78, Pages: 36-45, ISSN: 1751-6161
  • CONFERENCE PAPER
    Ferretti A, Giacopini M, Mastrandrea L, Dini Det al., 2018,

    Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

    , WCX World Congress Experience

    © 2018 SAE International. All Rights Reserved. Bearings represent one of the main responsible of friction losses in internal combustion engines and their lubrication performance has a crucial influence on the operating condition of the engine. In particular, the conrod small-end bearing is one of the most critical engine parts from a tribological point of view since limited contact surfaces have to sustain high inertial and combustion forces. In this contribution an analysis is performed of the tribological behaviour of the lubricated contact between the piston pin and the conrod small-end of a high performance motorbike engine. An algorithm is employed based on a complementarity formulation of the cavitation problem. A comparison between two different approaches to simulate the asperity contact problem is performed, the former based on the standard Greenwood-Tripp theory and the latter based on a complementarity formulation of the asperity contact problem. A model validation is performed by comparing the results with those obtained adopting the commercial software AVL Excite Power Unit. Similar results are obtained from both the approaches, if a proper calibration of the model input data is performed. However, a remarkable sensitivity is highlighted of the results obtained using the Greenwood/Tripp model to the adjustment parameters. The realistic (engineering) difficulty in defining and identifying the roughness data and their purely statistical nature returns results that may be afflicted by a dose of uncertainty. Considering that results of such simulations usually offer guidelines for a correct design of the coupling, further investigations are suggested to identify a relationship between simply available roughness data and model input, starting from a direct experimental measurements of real roughness profiles.

  • JOURNAL ARTICLE
    Milner PE, Parkes M, Puetzer JL, Chapman R, Stevens MM, Cann P, Jeffers JRTet al., 2018,

    A low friction, biphasic and boundary lubricating hydrogel for cartilage replacement

    , ACTA BIOMATERIALIA, Vol: 65, Pages: 102-111, ISSN: 1742-7061
  • JOURNAL ARTICLE
    Yang S, Wong JSS, Zhou F, 2018,

    Ionic Liquid Additives for Mixed and Elastohydrodynamic Lubrication

    , TRIBOLOGY TRANSACTIONS, Vol: 61, Pages: 816-826, ISSN: 1040-2004
  • JOURNAL ARTICLE
    Vladescu SC, Putignano C, Marx N, Keppens T, Reddyhoff T, Dini Det al., 2018,

    The percolation of liquid through a compliant seal-an experimental and theoretical study

    , Journal of Fluids Engineering, Transactions of the ASME, Vol: 141, ISSN: 0098-2202

    Copyright © 2019 by ASME New apparatus is described to simulate a compliant seal interface, allowing the percolation of liquid to be viewed by a fluorescence microscope. A model, based on the boundary element (BE) methodology, is used to provide a theoretical explanation of the observed behavior. The impact of contact pressure, roughness, and surface energy on percolation rates are characterized. For hydrophilic surfaces, percolation will always occur provided a sufficient number of roughness length scales are considered. However, for hydrophobic surfaces, the inlet pressure must overcome the capillary pressure exerted at the minimum channel section before flow can occur.

  • JOURNAL ARTICLE
    Yu M, Arana C, Evangelou SA, Dini Det al., 2017,

    Quarter-Car Experimental Study for Series Active Variable Geometry Suspension

    , IEEE Transactions on Control Systems Technology, ISSN: 1063-6536

    CCBY In this paper, the recently introduced series active variable geometry suspension (SAVGS) for road vehicles is experimentally studied. A realistic quarter-car test rig equipped with double-wishbone suspension is designed and built to mimic an actual grand tourer real axle, with a single-link variant of the SAVGS and a road excitation mechanism implemented. A linear equivalent modeling method is adopted to synthesize an H-infinity control scheme for the SAVGS, with the geometric nonlinearity compensated. Simulations with a theoretical nonlinear quarter-car indicate the SAVGS potential to enhance suspension performance, in terms of ride comfort and road holding. Practical features in the test rig are further considered and included in the nonlinear model to compensate the difference between the theoretical and testing behaviors. Experiments with a sinusoidal road, a smoothed bump and hole, and a random road are performed to evaluate the SAVGS practical feasibility and performance improvement, the accuracy of the model, and the robustness of the control schemes. Compared with the conventional passive suspension, ride comfort improvements of up to 41&#x0025; without any deterioration of the suspension deflection are demonstrated, while the SAVGS actuator power is kept very low, at levels below 500 W.

  • JOURNAL ARTICLE
    Putignano C, Dini D, 2017,

    Soft Matter Lubrication: Does Solid Viscoelasticity Matter?

    , ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 42287-42295, ISSN: 1944-8244
  • JOURNAL ARTICLE
    Parkes M, Sayer K, Goldhofer M, Cann P, Walter WL, Jeffers Jet al., 2017,

    Zirconia phase transformation in retrieved, wear simulated, and artificially aged ceramic femoral heads

    , JOURNAL OF ORTHOPAEDIC RESEARCH, Vol: 35, Pages: 2781-2789, ISSN: 0736-0266
  • JOURNAL ARTICLE
    Mueser MH, Dapp WB, Bugnicourt R, Sainsot P, Lesaffre N, Lubrecht TA, Persson BNJ, Harris K, Bennett A, Schulze K, Rohde S, Ifju P, Sawyer WG, Angelini T, Esfahani HA, Kadkhodaei M, Akbarzadeh S, Wu J-J, Vorlaufer G, Vernes A, Solhjoo S, Vakis AI, Jackson RL, Xu Y, Streator J, Rostami A, Dini D, Medina S, Carbone G, Bottiglione F, Afferrante L, Monti J, Pastewka L, Robbins MO, Greenwood JAet al., 2017,

    Meeting the Contact-Mechanics Challenge

    , TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
  • JOURNAL ARTICLE
    De Laurentis N, Cann P, Lugt PM, Kadiric Aet al., 2017,

    The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

    , TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
  • JOURNAL ARTICLE
    Wang A, Liu J, Gao H, Wang L-L, Masen Met al., 2017,

    Hot stamping of AA6082 tailor welded blanks: Experiments and knowledgebased cloud - finite element (KBC-FE) simulation

    , JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, Vol: 250, Pages: 228-238, ISSN: 0924-0136
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    Mackowiak S, Heyes DM, Pieprzyk S, Dini D, Branka ACet al., 2017,

    Non-Equilibrium Phase Behavior of Confined Molecular Films at Low Shear Rates

    , PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Vol: 254, ISSN: 0370-1972
  • JOURNAL ARTICLE
    Heyes DM, Dini D, Smith ER, Branka ACet al., 2017,

    Nanowire Stretching by Non-Equilibrium Molecular Dynamics

    , PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, Vol: 254, ISSN: 0370-1972
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    Tan Z, Parisi C, Di Silvio L, Dini D, Forte AEet al., 2017,

    Cryogenic 3D printing of super soft hydrogels

    , Scientific Reports, Vol: 7, ISSN: 2045-2322

    Conventional 3D bioprinting allows fabrication of 3D scaffolds for biomedical applications. In this contribution we present a cryogenic 3D printing method able to produce stable 3D structures by utilising the liquid to solid phase change of a composite hydrogel (CH) ink. This is achieved by rapidly cooling the ink solution below its freezing point using solid carbon dioxide (CO2) in an isopropanol bath. The setup was able to successfully create 3D complex geometrical structures, with an average compressive stiffness of O(1) kPa (0.49 ± 0.04 kPa stress at 30% compressive strain) and therefore mimics the mechanical properties of the softest tissues found in the human body (e.g. brain and lung). The method was further validated by showing that the 3D printed material was well matched to the cast-moulded equivalent in terms of mechanical properties and microstructure. A preliminary biological evaluation on the 3D printed material, coated with collagen type I, poly-L-lysine and gelatine, was performed by seeding human dermal fibroblasts. Cells showed good attachment and viability on the collagen-coated 3D printed CH. This greatly widens the range of applications for the cryogenically 3D printed CH structures, from soft tissue phantoms for surgical training and simulations to mechanobiology and tissue engineering.

  • JOURNAL ARTICLE
    Shen L, Denner F, Morgan N, van Wachem B, Dini Det al., 2017,

    Marangoni effect on small-amplitude capillary waves in viscous fluids

    , PHYSICAL REVIEW E, Vol: 96, ISSN: 2470-0045

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