Publications

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.

Journal article

Vladescu S, medina S, olver A, Pegg I, Reddyhoff Tet al., 2016,

Lubricant film thickness and friction force measurements in a laser surfacetextured reciprocating line contact simulating the piston ring – linerpairing

, Tribology International, Vol: 98, Pages: 317-329, ISSN: 1879-2464

Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses <50 nanometres to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20 nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing

Journal article

Hills DA, Fleury RMN, Dini D, 2016,

Partial slip incomplete contacts under constant normal load and subject to periodic loading

, International Journal of Mechanical Sciences, Vol: 108-109, Pages: 115-121, ISSN: 0020-7403

All rights reserved. We present a general formulation for the stick slip behaviour of incomplete contact under oscillating loading, but with a constant normal load. An asymptotic description of the contact traction very close to the contact edges is used. The slip zones present in the steady state with cyclically varying bulk tension and shear force (with an arbitrary phase shift) are found. The range of the variation of the state of stress near both of the contact edges and the respective slip zone sizes are defined in terms of the loading parameters, including the phase angle. The quality of the approximations used by the asymptotic approach and the range of applicability of the method is also analysed in detail in this paper.

Journal article

De Laurentis N, Kadiric A, Lugt P, Cann Pet al., 2016,

The influence of bearing grease composition on friction in rolling/sliding concentrated contacts

, Tribology International, Vol: 94, Pages: 624-632, ISSN: 0301-679X

This paper presents new results examining the relationship between bearing grease composition and rolling-sliding friction in lubricated contacts. Friction coefficient and lubricating film thickness of a series of commercially available bearing greases and their bled oils were measured in laboratory tribometers. Test greases were selected to cover a wide spectrum of thickener and base oil types, and base oil viscosities. The trends in measured friction coefficients were analysed in relation to grease composition in an attempt to establish the relative influence of individual grease components on friction. Two distinct operating regions with markedly different friction behaviour are identified for each grease. At relatively high speeds the greases behave approximately as their bled/base oils, while in the low speed region the frictional response is very dependent on their thickener type and properties of the lubricating film. Low viscosity, synthetic base oil seems to offer efficiency advantages in the high speed region regardless of thickener used, while the choice of thickener type is significant under low speed conditions.

Journal article

Khafidh M, Rodriguez NV, Masen MA, Schipper DJet al., 2016,

The dynamic contact area of elastomers at different velocities

, Tribology - Materials, Surfaces and Interfaces, Vol: 10, Pages: 70-73, ISSN: 1751-5831

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Journal article

Wilson R, Dini D, Van Wachem B, 2016,

A numerical study exploring the effect of particle properties on the fluidization of adhesive particles

, AICHE Journal, Vol: 62, Pages: 1467-1477, ISSN: 0001-1541

The effects of varying the elastic modulus, coefficient of restitution, and coefficient of friction of adhesive particles on fluidized bed dynamics have been investigated via numerical simulations. It is found that lower values of the elastic modulus and coefficient of restitution lead to a greater degree of particle clustering, and the formation of smaller bubbles. Coordination numbers are found to initially increase, and then fall, with increasing coefficient of friction, while bubble velocities follow the opposite trend. It is concluded that artificially reducing the elastic modulus of adhesive particles has a significant impact on the fluidization behaviour. The change in dynamics of the fluidized bed due to varying the coefficient of friction is more complex: particle clustering increases up to a point, beyond which clusters become increasingly rigid.

Conference paper

Mastrandrea LN, Giacopini M, Bertocchi E, Strozzi A, Dini Det al., 2016,

A complete 3-D description of the elastic behavior of a piston ring and its influence on the tribological behavior of the piston ring-cylinder liner interface

, Pages: 121-124

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Citations: 5

Conference paper

Kadiric A, Rycerz P, 2016,

Influence of contact conditions on the onset of micropitting in rolling-sliding contacts pertinent to gear applications

, AGMA 2016 - Fall Technical Meeting

Copyright © 2016 American Gear Manufacturers Association. Micropitting is a type of rolling contact fatigue surface damage that occurs in concentrated, rolling-sliding contacts, such as those between gear teeth. In contrast to macropitting, where the damage initiates and progresses on the macro contact level, micropitting damage initiates on the surface asperity level. Despite the fact that it is one of the major modes of gear failure, there are currently no universally accepted design guidelines for prevention of micropitting. This paper attempts to provide further understanding on the tribological conditions that may lead to the onset of micropitting in lubricated, concentrated contacts representative of those occurring between gear teeth. In particular, an attempt is made to establish the effect of slide-roll ratio on the extent of microptting. Experimental results obtained on a triple-disc microptting rig under conditions designed to isolate the effects of slide-roll ratio and specific film thickness are presented. In parallel, the potential effect of sliding on film thickness is studied experimentally using a well-proven optical interferometry technique. The results show that increasing the slide-roll ratio increases the extent of micropitting but that the mechanisms responsible for this are not related to any potential effects of the slide-roll ratio on prevalent film thickness as suggested elsewhere, but rather to the increased number of asperity stress cycles at higher slide-roll ratios.

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Journal article

Ma S, Scaraggi M, Wang D, Wang X, Liang Y, Liu W, Dini D, Zhou Fet al., 2015,

Nanoporous Substrate-Infiltrated Hydrogels: a Bioinspired Regenerable Surface for High Load Bearing and Tunable Friction

, Advanced Functional Materials, Vol: 25, Pages: 7366-7374, ISSN: 1616-301X

Nature has successfully combined soft matter and hydration lubrication to achieve ultralow friction even at relatively high contact pressure (e.g., articular cartilage). Inspired by this, hydrogels are used to mimic natural aqueous lubricating systems. However, hydrogels usually cannot bear high load because of solvation in water environments and are, therefore, not adopted in real applications. Here, a novel composite surface of ordered hydrogel nanofiber arrays confined in anodic aluminum oxide (AAO) nanoporous template based on a soft/hard combination strategy is developed. The synergy between the soft hydrogel fibers, which provide excellent aqueous lubrication, and the hard phase AAO, which gives high load bearing capacity, is shown to be capable of attaining very low coeffcient of friction (<0.01) under heavy load (contact pressures ≈2 MPa). Interestingly, the composite synthetic material is very stable, cannot be peeled off during sliding, and exhibits desirable regenerative (self-healing) properties, which can assure long-term resistance to wear. Moreover, the crosslinked polymethylacrylic acid hydrogels are shown to be able to promptly switch between high friction (>0.3) and superlubrication (≈10−3) when their state is changed from contracted to swollen by means of acidic and basic actuation. The mechanisms governing ultralow and tunable friction are theoretically explained via an in-depth study of the chemomechanical interactions responsible for the behavior of these substrate-infiltrated hydrogels. These findings open a promising route for the design of ultra-slippery and smart surface/interface materials.

Journal article

Leibinger A, Forte AE, Tan Z, Oldfield MJ, Beyrau F, Dini D, Rodriguez Y Baena Fet al., 2015,

Soft tissue phantoms for realistic needle insertion: a comparative study

, Annals of Biomedical Engineering, Vol: 44, Pages: 2442-2452, ISSN: 1573-9686

Phantoms are common substitutes for soft tissues in biomechanical research and are usually tuned to match tissue properties using standard testing protocols at small strains. However, the response due to complex tool-tissue interactions can differ depending on the phantom and no comprehensive comparative study has been published to date, which could aid researchers to select suitable materials. In this work, gelatin, a common phantom in literature, and a composite hydrogel developed at Imperial College, were matched for mechanical stiffness to porcine brain, and the interactions during needle insertions within them were analyzed. Specifically, we examined insertion forces for brain and the phantoms; we also measured displacements and strains within the phantoms via a laser-based image correlation technique in combination with fluorescent beads. It is shown that the insertion forces for gelatin and brain agree closely, but that the composite hydrogel better mimics the viscous nature of soft tissue. Both materials match different characteristics of brain, but neither of them is a perfect substitute. Thus, when selecting a phantom material, both the soft tissue properties and the complex tool-tissue interactions arising during tissue manipulation should be taken into consideration. These conclusions are presented in tabular form to aid future selection.

Conference paper

Mastrandrea LN, Giacopini M, Dini D, Bertocchi Eet al., 2015,

Elastohydrodynamic Analysis of the Conrod Small-End of a High Performance Motorbike Engine via a Mass Conserving Cavitation Algorithm

, ASME International Mechanical Engineering Congress and Exposition (IMECE2015), Publisher: ASME

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

Journal article

Ponjavic A, Dench J, Morgan N, Wong Jet al., 2015,

In situ viscosity measurement of confined liquids

, RSC Advances: an international journal to further the chemical sciences, Vol: 5, Pages: 99585-99593, ISSN: 2046-2069

The viscosity of liquids governs crucial physical and engineering phenomena, ranging from diffusion and transport processes of nutrients and chemicals, to the generation of friction and the physics of damping. Engineering fluids frequently experience local conditions that change their bulk rheological properties. While viscosity data can easily be acquired using conventional rheometers, the results are not always applicable to fluids under engineering conditions. This is particularly the case for fluids being sheared at high pressure under severe confinement, which experience very high shear stresses and often show extensive shear thinning. There is a lack of suitable methods for measuring fluid viscosity under such conditions. This work describes a novel in situ viscosity measurement technique to fill this gap. It involves the quantification of the fluorescence lifetime of a fluorescent dye that is sensitive to viscosity. The capability of the developed technique is verified by taking measurements in submicron thick films of two model fluids confined in a ball on flat contact. Viscosity measurements were successfully performed at pressures up to 1.2 GPa and shear rates up to 105 s−1. Spatial heterogeneity in viscosity caused by variations in pressure within the thin fluid film could be observed using the technique. It was also possible to detect differences in the rheological responses of a Newtonian and a non-Newtonian fluid. These first in situ high pressure, high shear viscosity measurements demonstrate the versatility of the proposed technique in providing information on the viscosity in conditions where contemporary techniques are insufficient. More importantly it highlights the complexity of the rheology of engineering fluids and provides a means of verifying existing theories by performing in situ measurements. Information on local viscosity is crucial for understanding the physics of confined fluids and to facilitate improvements in engineering techno

Journal article

van Arkel RJ, Amis AA, Jeffers JRT, 2015,

The envelope of passive motion allowed by the capsular ligaments of the hip

, JOURNAL OF BIOMECHANICS, Vol: 48, Pages: 3803-3809, ISSN: 0021-9290

Journal article

Spikes HA, Costa H, 2015,

Impact of ethanol on the formation of antiwear tribofilms from engine lubricants

, Tribology International, Vol: 93, Pages: 364-376, ISSN: 1879-2464

This paper investigates the impact of contamination of engine lubricants with ethanol fuel on the growth and stability of anti-wear tribofilms from ZDDP-containing lubricants. The MTM-SLIM technique was used to monitor the effects of blending 5 wt% of both anhydrous and hydrated ethanol on tribofilm thickness in a fully-formulated Group I oil and in a solution of ZDDP anti-wear additive dissolved in Group I base oil. Tribofilm thickness was significantly reduced by the addition of ethanol for both oils, and the reduction was more severe for hydrated than for anhydrous ethanol. When a tribofilm was allowed to form during rubbing using an ethanol-free oil, the subsequent addition of hydrated ethanol showed both the destruction of the pre-formed antiwear tribofilm and damage to the rubbed surfaces.

Journal article

Profito FJ, Giacopini M, Zachariadis DC, Dini Det al., 2015,

A general finite volume method for the solution of the reynolds lubrication equation with a mass-conserving cavitation model

, Tribology Letters, Vol: 60, ISSN: 1573-2711

This contribution presents the development of a general discretization scheme for the solution of Reynolds equation with a mass-conserving cavitation model and its application for the numerical simulation of lubricated contacts to be discretized using irregular grids. Such scheme is based on a hybrid-type formulation, here named as element-based finite volume method that combines the flexibility of the FEM to deal with unstructured grids, while preserving the local and global fluid-flow conservation aspect of the FVM throughout the discretized domain. The accuracy and robustness of the method are successfully tested using several benchmark cases proposed in the recent literature. Simulations of fully or partially textured sliding bearings are finally employed to show the advantages of being able to adopt irregular meshes both in terms of flexibility for the discretization of complex surface features and computational speed.

Journal article

Gurrutxaga Lerma B, Balint DANIEL, Dini DANIELE, Sutton APet al., 2015,

Elastodynamic image forces on dislocations

, Proceedings of the Royal Society A: Mathematical, Physical & Engineering Sciences, Vol: 471, ISSN: 1364-5021

The elastodynamic image forces on edge and screw dislocations in the presence of a planar-free surface are derived. The explicit form of the elastodynamic fields of an injected, quiescent screw dislocation are also derived. The resulting image forces are affected by retardation effects: the dislocations experience no image force for a period of time defined by the arrival and reflection at the free surface of the dislocation fields. For the case of injected, stationary dislocations, it is shown that the elastodynamic image force tends asymptotically to the elastotatic prediction. For the case of injected, moving dislocations, it is shown that the elastodynamic image force on both the edge and the screw dislocations is magnified by inertial effects, and becomes increasingly divergent with time; this additional effect, missing in the elastostatic description, is shown to be substantial even for slow moving dislocations. Finally, it is shown that the elastodynamic image force of an edge dislocation moving towards the surface at the Rayleigh wave speed becomes repulsive, rather than attractive; this is suggestive of instabilities at the core of the dislocation, and likely resonances with the free surface.

Conference paper

Forte AE, Galvan S, Dini D, 2015,

Biomechanics of the brain: experimental investigation, numerical simulation, design of advanced phantoms for robotic surgery

, ITC 2015

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Journal article

Woldman M, Tinga T, Van Der Heide E, Masen MAet al., 2015,

Abrasive wear based predictive maintenance for systems operating in sandy conditions

, Wear, Vol: 338-339, Pages: 316-324, ISSN: 0043-1648

Machines operating in sandy environments are damaged by the abrasive action of sand particles that enter the machine and become entrapped between components and contacting surfaces. In the case of the military services the combination of a sandy environment and the wide range of tasks to be fulfilled results in extreme and uncertain operating conditions. All of this hinders the ability to establish efficient maintenance strategies prior to deployment and increases the risk of mechanical failure. To prevent such problems, it would be desirable to perform maintenance based on the prevailing condition of both the components and the environment. By monitoring the loading situation as well as the characteristics of the sand particles, the wear of components is quantified, allowing maintenance to be performed when necessary. The development and implementation of such a predictive maintenance concept requires knowledge of the operational and environmental conditions and how they relate to the principal wear mechanisms. Based on previously established relationships between the abrasive particles and the resulting abrasive wear, the current work focuses on the implementation of these results into a predictive maintenance concept for vehicles that operate in a sandy environment. For this, the local parameters that govern the wear mechanism, such as the normal forces and sliding distances need to be linked to machine usage parameters including the type of terrain and the driving distance. The proposed concept is demonstrated using a case study on the sprockets of a military vehicle, where the sprockets wear progressively during use of the vehicle due to the abrasive action of sand. The predictive maintenance concept is shown to support the determination of maintenance intervals under a range of usage profiles and sand varieties.

Journal article

Spikes HA, 2015,

Friction modifier additives

, Tribology Letters, Vol: 60, ISSN: 1573-2711

The need for energy efficiency is leading to the growing use of additives that reduce friction in thin film boundary and mixed lubrication conditions. Several classes of such friction modifier additive exist, the main ones being organic friction modifiers, functionalised polymers, soluble organo-molybdenum additives and dispersed nanoparticles. All work in different ways. This paper reviews these four main types of lubricant friction modifier additive and outlines their history, research and the mechanisms by which they are currently believed to function. Aspects of their behaviour that are still not yet fully understood are highlighted.

Journal article

Masen M, Myant C, 2015,

Second International Conference on Biotribology (ICoBT)

, Tribology International, Vol: 89, Pages: 1-1, ISSN: 1879-2464

Journal article

Aqil A, Sheikh HQ, Masjedi M, Jeffers J, Cobb Jet al., 2015,

Birmingham Mid-Head Resection Periprosthetic Fracture.

, Clin Orthop Surg, Vol: 7, Pages: 402-405

Total hip arthroplasty in the young leads to difficult choices in implant selection. Until recently bone conserving options were not available for younger patients with deficient femoral head bone stock. The novel Birmingham Mid-Head Resection (BMHR) device offers the option of bone conserving arthroplasty in spite of deficient femoral head bone stock. Femoral neck fracture is a known complication of standard resurfacing arthroplasty and is the most common reason for revision. It is unknown whether this remains to be the case for the BMHR neck preserving implants. We report a case of a 57-year-old male, who sustained a periprosthetic fracture following surgery with a BMHR arthroplasty. This paper illustrates the first reported case of a BMHR periprosthetic fracture. The fracture pattern is spiral in nature and reaches to the subtrochanteric area. This fracture pattern is different from published cadaveric studies, and clinicians using this implant should be aware of this as revision is likely to require a distally fitting, rather than a metaphyseal fitting stem. We have illustrated the surgical technique to manage this rare complication.

Journal article

Gurrutxaga Lerma BENAT, Balint DS, Dini D, Sutton APet al., 2015,

The mechanisms governing the activation of dislocation sources in aluminum at different strain rates

, Journal of the Mechanics and Physics of Solids, Vol: 84, Pages: 273-292, ISSN: 1873-4782

This article examines the time to activate Frank–Read sources in response to macroscopic strain rates ranging from 101 s−1 to 1010 s−1 in aluminium under athermal conditions. We develop analytical models of the bowing of a pinned dislocation segment as well as numerical simulations of three dimensional dislocation dynamics. We find that the strain rate has a direct influence on both the activation time and the source strength of Frank–Read sources at strain rates up to 106 s−1, and the source strength increases in almost direct proportion to the strain rate. This contributes to the increase in the yield stress of materials at these strain rates. Above 106 s−1, the speed of the bowing segments reaches values that exceed the domain of validity of the linear viscous drag law, and the drag law is modified to account for inertial effects on the motion of the dislocation. As a result the activation times of Frank–Read sources reach a finite limit at strain rates greater than 108 s−1, suggesting that Frank–Read sources are unable to operate before homogeneous nucleation relaxes elastic stresses at the higher strain rates of shock loading. Elastodynamic calculations are carried out to compare the contributions of Frank–Read sources and homogeneous nucleation of dislocations to plastic relaxation. We find that at strain rates of 5×107 s−1 homogeneous nucleation becomes the dominant generation mechanism.

Journal article

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

Friction induced vibration in windscreen wiper contacts

, Journal of Vibration and Acoustics: Transactions of the ASME, Vol: 137, Pages: 1-7, ISSN: 1048-9002

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

Conference paper

Cheng C, Evangelou SA, Arana C, Dini Det al., 2015,

Active Variable Geometry Suspension robust control for improved vehicle ride comfort and road holding

, American Control Conference (ACC), 2015, Publisher: IEEE, Pages: 3440-3446, ISSN: 0743-1619

This paper investigates the design of robust ℋ∞ control for road vehicle Series Active Variable Geometry Suspension (SAVGS). The objective is to improve ride comfort and road holding, while guaranteeing operation inside existing physical constraints. The study utilizes a nonlinear quarter car model that represents accurately the vertical dynamics and geometry of one quarter of a high performance car with a double wishbone suspension. The control objective is to reduce the body vertical acceleration, tire deflection and suspension travel under the impact of road perturbations. Therefore, the selection of the weighting functions for a linear ℋ∞ control, designed for the linearized quarter car, is based on these objectives. The proposed controller is then applied to the nonlinear quarter car model and investigated by nonlinear simulation for a range of road disturbance inputs. The results show that the designed controller when applied on the SAVGS is effective in improving the vehicle ride comfort and road holding.

Journal article

Arana C, Evangelou SA, Dini D, 2015,

Series Active Variable Geometry Suspension application to chassis attitude control

, IEEE/ASME Transactions on Mechatronics, Vol: PP, ISSN: 1083-4435

This paper explores the application of the recently introduced Series Active Variable Geometry Suspension (SAVGS) to the control of chassis attitude motions and the directional response of cars. A co-design methodology, involving a component dimensioning framework and a multi-objective control scheme, is developed to maximize the SAVGS control capabilities while respecting vehicle and actuator design constraints. The dimensioning framework comprises: a steady-state mathematical model based on the principle of virtual work; a parameter sensitivity analysis that sheds light on the dependencies that exist between the properties of the passive suspension, the SAVGS and the chassis; and an algorithm to size the main SAVGS components for any given vehicle and steady-state performance objectives. The general multi-objective control scheme is presented for general application, and the particular case of combined chassis attitude control and overturning couple distribution control is developed in detail. The proposed scheme is subsequently applied to a high performance sports car and a fully laden SUV and tested under a wide range of operating conditions through the simulation of standard open-loop maneuvers. Results demonstrate the SAVGS potential to favorably regulate the attitude motions and directional response in both vehicle classes.

Journal article

Guegan J, Kadiric A, Spikes HA, 2015,

A Study of the Lubrication of EHL Point Contact in the Presence of Longitudinal Roughness

, Tribology Letters, Vol: 59, ISSN: 1023-8883

This work investigates the effect on elastohydrodynamic lubrication of roughness ridges oriented along the rolling–sliding direction, such as may be present on rolling bearing raceways. The roughness of the three specimens tested is characterised by the RMS of surface heights and a dominant wavelength. Optical interferometry and a ball-on-disc set-up were employed to map the oil film thickness. The paper first describes a novel procedure to carry out optical interferometry measurements on rough surfaces. Film thickness maps from the central part of the contact were obtained for a range of speeds in pure rolling and rolling–sliding conditions. The evolution of the film distribution with increasing speed along with the in-contact RMS and the real area of contact was calculated. The film maps show that the lift-off speed increases when roughness is introduced compared with smooth surfaces, while the average film thickness remains very close to the smooth case. The general horseshoe film shape that becomes visible at higher speeds is discussed. Using an inverse solution approach based on measured in-contact roughness, the pressure distribution is estimated in a rough, lubricated contact and its evolution with speed is explained. The findings provide important insights into the transition from boundary, through mixed, to full EHL lubrication for longitudinal roughness.

Journal article

Spikes H, Tysoe W, 2015,

On the Commonality Between Theoretical Models for Fluid and Solid Friction, Wear and Tribochemistry

, Tribology Letters, Vol: 59, ISSN: 1573-2711

Tribology is concerned with the influence ofmechanically applied forces on interfacial phenomena thataccompany and control sliding. A wide range of modelshas been developed to describe these phenomena, whichinclude frictional dissipation, wear and tribochemical reactions.This paper shows that these apparently disparatemodels are based on the same fundamental concept that anexternally applied force accelerates the rate of thermaltransition of atoms or molecules across energy barrierspresent in solid and liquid materials, thereby promotingflow, slip or bond cleavage. Such ‘‘stress-assisted’’ effectsand the associated thermal activation concepts were developedindependently and in different forms by Prandtl (ZAngew Math Mech 8:85, 1928) and Eyring (J Chem Phys4(4):283–291, 1936). These two works have underpinnedsubsequent theories of dry friction, boundary lubrication,EHD rheology, tribochemistry and nanoscale wear modelling.This paper first reviews the historical developmentof the concepts, focussing in particular on the models ofPrandtl and Eyring and how they have subsequently beenused and adapted by others. The two approaches are thencompared and contrasted, noting that although superficiallysimilar, they contain quite different assumptions and constraints.First, the Prandtl model assumes that the force isexerted through a compliant spring, while constant forcesliding is assumed by Eyring. Second, different approximationsare made in the two models to describe the change in energy barrier with external force. Prandtl exploresthe asymptotic behaviour of the energy barrier as theapplied force become sufficiently high to reduce it to zero,while Eyring assumes that the energy barrier is reduced byan amount equal to the external work carried out on thesystem. The theoretical underpinnings of these differencesare discussed along with the implications of compliantcoupling and constant force sliding on the velocity andtemperature dependence of t

Journal article

Gurrutxaga-Lerma B, Balint DS, Dini D, Eakins DE, Sutton APet al., 2015,

The Role of Homogeneous Nucleation in Planar Dynamic Discrete Dislocation Plasticity

, Journal of Applied Mechanics-Transactions of the ASME, Vol: 82, ISSN: 1528-9036

Homogeneous nucleation of dislocations is the dominant dislocation generation mechanismat strain rates above 108 s1; at those rates, homogeneous nucleation dominates theplastic relaxation of shock waves in the same way that Frank–Read sources control theonset of plastic flow at low strain rates. This article describes the implementation ofhomogeneous nucleation in dynamic discrete dislocation plasticity (D3P), a planarmethod of discrete dislocation dynamics (DDD) that offers a complete elastodynamictreatment of plasticity. The implemented methodology is put to the test by studying fourmaterials—Al, Fe, Ni, and Mo—that are shock loaded with the same intensity and astrain rate of 1010 s1. It is found that, even for comparable dislocation densities, the latticeshear strength is fundamental in determining the amount of plastic relaxation a materialdisplays when shock loaded. [DO

Journal article

Dini D, Bodnarchuk MS, Heyes DM, Breakspear A, Chahine S, Edwards Set al., 2015,

Response of calcium carbonate nanoparticles in hydrophobic solvent to pressure, temperature, and water

, Journal of Physical Chemistry C, Vol: 119, Pages: 16879-16888, ISSN: 1932-7455

Molecular Dynamics (MD) simulations of surfactant-stabilized calcium carbonate, CaCO3, nanoparticles in hydrophobic solvent have been carried out to characterize their response to changes in temperature (T) and pressure (P), and also their interaction with trace water and water droplets. The response to increasing temperature and pressure is sensitive to the type of model surfactant, with the sulfonate-stabilized particle, which is the most spherical, showing a weak temperature-pressure dependence, while the sulfurized alkyl phenol (SAP) and salicylate-stabilized particles distort into a more spherical shape with increasing temperature and pressure. The atom-atom radial distribution functions of the core ions reveal consolidation of the calcium carbonate structure with increasing temperature and pressure. The simulations show that the nanoparticles adsorb onto the surface of water droplets through a water bridge transitional mechanism, in agreement with evidence from experimental studies. In the case of the sulfonate surfactant particle, only, a number of surfactant molecules detached from the calcium carbonate core and transferred to the surface of the water droplet. Consequently this type of particle had the greatest interaction with and affinity for water which may explain its rapid neutralization characteristics observed in experiments. The detachment free energy of the sulfonate obtained by potential of mean force (PMF) calculations was the largest of the three, which is consistent with the core being more embedded in the water and less well stabilised on returning to the hydrophobic medium. The salicylate nanoparticle had about half the detachment free energy, which could give rise to a more dynamic equilibrium of attached-to-detached states for this class of nanoparticle.

Journal article

Putignano C, Carbone G, Dini D, 2015,

Mechanics of rough contacts in elastic and viscoelastic thin layers

, International Journal of Solids and Structures, Vol: 69-70, Pages: 507-517, ISSN: 1879-2146

Contact mechanics between rough solids usually relies on the half-space approximation, which assumes that the contact area dimension is much smaller than the thickness of the layers of materials that characterize the surfaces of the contacting bodies. However, such simplifying assumption is often inadequate when industrially relevant applications are considered, in particular those of biomechanical interest. Indeed, a large variety of systems, including not only classical engineering applications such as gear boxes, shafts, tyres, etc., but also biological tissues such as human skin, is characterized by superficial coatings; very often the mechanical properties of these coatings are very different from those of the bulk region of the bodies in contact. The aim of this paper is to shed light on the role played by the thickness of the layer of material used as a coating, with specific focus on the contact between a rigid rough surface and a thin deformable layer bonded to a rigid substrate. Starting from a recently developed boundary element formulation (Carbone and Putignano, 2013), we derive a methodology which accounts for finite thickness by a corrective coefficient modulating the classical Greens function, and extends our analyses to periodic domains. This enables to avoid border effects and provides an innovative tool to tackle viscoelastic contacts with realistic roughness. This is exploited to perform a thorough investigation of the mechanisms responsible for frictional losses in layered systems characterized by different materials, thickness and loading conditions. Results show that decreasing the layer thickness corresponds to an increase in the contact stiffness. Furthermore, in the case of viscoelastic layer, particular attention has to be paid to the changes in the viscoelastic dissipation due to the finite thickness of the surface layer.

Journal article

Leong JY, Zhang J, Sinha SK, Holmes A, Spikes H, Reddyhoff Tet al., 2015,

Confining Liquids on Silicon Surfaces to Lubricate MEMS

, Tribology Letters, Vol: 59, ISSN: 1573-2711

Liquid lubrication may provide a solution tothe problem of high friction and wear in micro-electromechanicalsystems. Although the effectiveness of thisapproach has been demonstrated in laboratory-based frictiontests, practical constraints prevent it from being appliedin commercial devices. The main problem is how toposition the lubricant on a silicon surface in order to limitspreading and evaporation. This paper describes twotechniques to address this issue. First, low concentrationsof additives are used to promote autophobic behaviour.Tests’ results show that certain concentrations of bothmultiply alkylated cyclopentane and amine additives areeffective in halting the spread of hexadecane on silicon,and, in the latter case, cause the hexadecane drop to subsequentlyretract. The second approach involves applying amicro-contact printing technique previously used on goldsurfaces. Here, silicon surfaces are coated with octadecyltrichlorosilanemono-layers that are then selectively removed,using oxygen plasma, to leave regions ofcontrasting surface energy. Results from spin tests showthat surfaces treated in this way can anchor 1 ll drops ofhexadecane and water when forces of up to 22 and 230 lN,respectively, are applied.

Journal article

Bodnarchuk MS, Heyes DM, Breakspear A, Chahine S, Dini Det al., 2015,

A molecular dynamics study of CaCO3 nanoparticles in a hydrophobic solvent with a stearate co-surfactant.

, Physical Chemistry Chemical Physics, Vol: 17, Pages: 13575-13581, ISSN: 1463-9084

Stearates containing overbased detergent nanoparticles (NPs) are used as acid neutralising additives in automotive and marine engine oils. Molecular dynamics (MD) simulations of the self-assembly of calcium carbonate, calcium stearate as a co-surfactant and stabilising surfactants of such NPs in a model explicit molecular hydrophobic solvent have been carried out using a methodology described first by Bodnarchuk et al. [J. Phys. Chem. C, 2014, 118, 21092]. The cores and particles as a whole become more elongated with stearate, and the surfactant molecules are more spaced out in this geometry than in their stearate-free counterparts. The rod dimensions are found to be largely independent of the surfactant type for a given amount of CaCO3. The corresponding particles without stearate were more spherical, the precise shape depending to a greater extent on the chemical architecture of the surfactant molecule. The rod-shaped stearate containing nanoparticles penetrated a model water droplet to a greater depth than the corresponding near-spherical particle, which is possibly facilitated by the dissociation of nanoparticle surfactant molecules onto the surface of the water in this process. These simulations are the first to corroborate the nanoparticle-water penetration mechanism proposed previously by experimental groups investigating the NP acid neutralisation characteristics.

Journal article

Parkes M, Myant C, Cann PM, Wong JSSet al., 2015,

Synovial fluid lubrication: The effect of protein interactions on adsorbed and lubricating films

, Biotribology, Vol: 1-2, Pages: 51-60, ISSN: 2352-5738

© 2015 Elsevier Ltd. All rights reserved. Synovial fluid lubrication is dependent on protective protein films that form between joint surfaces. Under static conditions surface film formation occurs through adsorption, while under dynamic conditions protein aggregation under shear and load becomes the dominant mechanism. This work examines how the protein content of six model synovial fluids affects film formation under static and rolling conditions and if the changes in properties can be correlated. With an increase in the statically adsorbed mass and the rate of adsorption the film thickness under rolling increased. These increases did not correlate with the total protein content of the fluid, but were dependent on the type of protein. An increase in pH reduced the adsorbed mass, rate of adsorption and film thickness, but was of secondary importance to the type of protein. The rolling film thickness was also correlated with the viscoelastic properties of the films formed under static conditions. In this case thinner rolling films corresponded to the more hydrated, viscoelastic adsorbed films. The strong correlations found between the properties of the adsorbed films and those formed under rolling indicate that the same protein-protein and protein-surface interactions may govern both mechanisms of film formation despite the differences in the film structures.

Journal article

Gurrutxaga-Lerma B, Balint DS, Dini D, Eakins DE, Sutton APet al., 2015,

Attenuation of the dynamic yield point of shocked aluminum using elastodynamic simulations of dislocation dynamics

, Physical Review Letters, Vol: 114, Pages: 1-5, ISSN: 0031-9007

When a metal is subjected to extremely rapid compression, a shock wave is launched that generates dislocations as it propagates. The shock wave evolves into a characteristic two-wave structure, with an elastic wave preceding a plastic front. It has been known for more than six decades that the amplitude of the elastic wave decays the farther it travels into the metal: this is known as “the decay of the elastic precursor.” The amplitude of the elastic precursor is a dynamic yield point because it marks the transition from elastic to plastic behavior. In this Letter we provide a full explanation of this attenuation using the first method of dislocation dynamics to treat the time dependence of the elastic fields of dislocations explicitly. We show that the decay of the elastic precursor is a result of the interference of the elastic shock wave with elastic waves emanating from dislocations nucleated in the shock front. Our simulations reproduce quantitatively recent experiments on the decay of the elastic precursor in aluminum and its dependence on strain rate.

Journal article

Ku ISY, Chong WWF, Reddyhoff T, Rahnejat Het al., 2015,

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

, Meccanica, Vol: 50, Pages: 1915-1922, ISSN: 1572-9648

The paper presents measurements of friction of any ultra-thin film entrained into the contact of a pair of very smooth specimen subjected to entrainment in a converging micro-wedge of a special-purpose micro-tribometer. An ultra-thin film is expected to form at the boundary solids through adsorption of boundary active molecules. Fluids with linear and branched molecules are used in the investigation. It is found that the frictional characteristics of these films can be adequately described through use of Eyring thermal activation energy and a potential energy barrier to sustain conjunctional sliding motion. The combined experimental measurement and the simple activation energy approach shows that the thin molecular adsorbed films act like hydro Langmuir–Blodgett layers, the formation and frictional characteristics of which are affected by the competing mechanisms of adsorption, forced molecular re-ordering and discrete-fashion drainage through the contact by the solvation effect. This process is a complex function of the contact sliding velocity as well as a defined Eyring activation density (packing density of the molecules within the conjunction). It is shown that the contribution of solvation to friction is in the form of energy expended to eject layers of lubricant out of the contact, which unlike the case of micro-scale hydrodynamic films, is not a function of the sliding velocity.

Journal article

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

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

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

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

Journal article

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

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

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

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

Journal article

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

Transient effects in lubricated textured bearings

, Proceedings of the Institution of Mechanical Engineers Part J - Journal of Engineering Tribology, Vol: 229, Pages: 523-537, ISSN: 1350-6501

Journal article

Forte AE, D'Amico F, Charalambides MN, Dini D, Williams JGet al., 2015,

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

, FOOD HYDROCOLLOIDS, Vol: 46, Pages: 180-190, ISSN: 0268-005X

Journal article

Ma G, Wang L, Gao H, Zhang J, Reddyhoff Tet al., 2015,

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

, Applied Surface Science, Vol: 345, Pages: 109-115, ISSN: 0169-4332

Journal article

Spikes H, Zhang J, 2015,

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

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

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

Journal article

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

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

, SCIENCE CHINA-TECHNOLOGICAL SCIENCES, Vol: 58, Pages: 551-558, ISSN: 1674-7321

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Citations: 11

Journal article

van Kuilenburg J, Masen MA, van der Heide E, 2015,

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

, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART J-JOURNAL OF ENGINEERING TRIBOLOGY, Vol: 229, Pages: 243-258, ISSN: 1350-6501

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Citations: 56

Journal article

Flicek RC, Hills DA, Dini D, 2015,

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

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

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

Journal article

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

A localized momentum constraint for non-equilibrium molecular dynamics simulations

, JOURNAL OF CHEMICAL PHYSICS, Vol: 142, ISSN: 0021-9606

Journal article

Ingram M, Hamer C, Spikes H, 2015,

A new scuffing test using contra-rotation

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

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

Journal article

Heyes DM, Dini D, Brańka AC, 2015,

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

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

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

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A complete 3-D description of the elastic behavior of a piston ring and its influence on the tribological behavior of the piston ring-cylinder liner interface

Influence of contact conditions on the onset of micropitting in rolling-sliding contacts pertinent to gear applications