Publications
456 results found
Ewen J, 2017, Molecular dynamics simulations of lubricants and additives
Kanca Y, Milner P, Dini D, et al., 2017, 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
This research investigated in-vitro tribological performance of the articulation of cartilage-on- polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) blend hydrogels using a custom-designed multi-directional wear rig. The hydrogels were prepared by repeated freezing-thawing cycles at different concentrations and PVA to PVP fractions at a given concentration. PVA/PVP blend hydrogels showed low coefficient of friction (COF) values (between 0.12 ± 0.01 and 0.14 ± 0.02) which were closer to the cartilage-on-cartilage articulation (0.03 ± 0.01) compared to the cartilage-on-stainless steel articulation (0.46 ± 0.06). The COF increased with increasing hydrogel concentration (p = 0.03) and decreasing PVP content at a given concentration (p < 0.05). The cartilage-on-hydrogel tests showed only the surface layers of the cartilage being removed (average volume loss of the condyles was 12.5 ± 4.2mm3). However, the hydrogels were found to be worn/deformed. The hydrogels prepared at a higher concentration showed lower apparent volume loss. A strong correlation (R2 = 0.94) was found between the COF and compressive moduli of the hydrogel groups, resulting from decreasing contact congruency. It was concluded that the hydrogels were promising as hemiarthroplasty materials, but that improved mechanical behaviour was required for clinical use.
Heyes D, Dini D, Smith E, et al., 2017, Nanowire stretching by Non-equilibrium Molecular Dynamics, Physica Status Solidi B: Basic Solid State Physics, Vol: 254, ISSN: 0370-1972
Non-equilibrium Molecular Dynamics (NEMD) simulations of a stretched Lennard-Jones (LJ) model single crystal nanowire with square cross-section are carried out. The microstructural and mechanical properties are examined as a function of strain and strain rate. The instantaneous Poisson's ratio and Young's modulus are shown to be strongly time (strain) dependent from the start of the pulling process. The structural transformation as a result of straining initially involves the (100) layers moving further apart and then slipping at ca. math formula when the shear slip stress along that direction is about 1% of the shear modulus, which is typical of plastic deformation of noble gas solid crystals, and in accordance with Schmid's law.
Shen L, Denner F, Morgan N, et al., 2017, Before the bubble ruptures, Physical Review Fluids, Vol: 2, Pages: 090505-090505, ISSN: 2469-990X
This paper is associated with a video winner of a 2016 APS/DFD Gallery of Fluid Motion Award. The original video is available from the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2016.GFM.V0092
Tan Z, Forte AE, Parisi C, et al., 2017, Composite hydrogel: A new tool for reproducing the mechanicalbehaviour of soft human tissues, WTC 2017
Mackowiak S, Heyes D, Pieprzyk S, et 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
In a recent publication [Maćkowiak et al., J. Chem. Phys. 145, 164704 (2016)] the results of Non-Equilibrium Molecular Dynamics (NEMD) simulations of confined sheared Lennard-Jones molecular films have been presented. The present work builds on that study by focusing on the low wall speed (shear rate) regime. Maps are given of the steady-state structures and corresponding friction coefficients in the region where a transition from static to kinetic friction is observed. The boundary between static and kinetic friction regions is determined as a function of wall speed and applied pressure, which is located for wall speeds up to about 0.8 m s−1. It was found that stick-slip behavior extends to pressures as high as 1000 MPa. The NEMD equations of motion are shown to be consistent with the Prandtl–Tomlinson model in the ‘soft spring’ limit, which leads to a new expression for the friction coefficient. This study provides new details and insights into the nature of anomalous friction behavior in the so-called Plug-Slip part of the nonquilibrium phase diagram regime.
Forte AE, galvan S, Dini D, 2017, Models and tissue mimics for brain shift simulations, Biomechanics and Modeling in Mechanobiology, Vol: 17, Pages: 249-261, ISSN: 1617-7940
Capturing the deformation of human brain during neurosurgical operations is an extremely important task to improve the accuracy or surgical procedure and minimize permanent damage in patients. This study focuses on the development of an accurate numerical model for the prediction of brain shift during surgical procedures and employs a tissue mimic recently developed to capture the complexity of the human tissue. The phantom, made of a composite hydrogel, was designed to reproduce the dynamic mechanical behaviour of the brain tissue in a range of strain rates suitable for surgical procedures. The use of a well-controlled, accessible and MRI compatible alternative to real brain tissue allows us to rule out spurious effects due to patient geometry and tissue properties variability, CSF amount uncertainties, and head orientation. The performance of different constitutive descriptions is evaluated using a brain–skull mimic, which enables 3D deformation measurements by means of MRI scans. Our combined experimental and numerical investigation demonstrates the importance of using accurate constitutive laws when approaching the modelling of this complex organic tissue and supports the proposal of a hybrid poro-hyper-viscoelastic material formulation for the simulation of brain shift.
Hu H, Wen J, Jia L, et al., 2017, Significant and stable drag reduction with air rings confined by alternated superhydrophobic and hydrophilic strips, Science Advances, Vol: 3, Pages: 1-10, ISSN: 2375-2548
Superhydrophobic surfaces have the potential to reduce the viscous drag of liquids by significantly decreasing friction at a solid-liquid interface due to the formation of air layers between solid walls and interacting liquids. However, the trapped air usually becomes unstable due to the finite nature of the domain over which it forms. We demonstrate for the first time that a large surface energy barrier can be formed to strongly pin the three-phase contact line of air/water/solid by covering the inner rotor of a Taylor-Couette flow apparatus with alternating superhydrophobic and hydrophilic circumferential strips. This prevents the disruption of the air layer, which forms stable and continuous air rings. The drag reduction measured at the inner rotor could be as much as 77.2%. Moreover, the air layers not only significantly reduce the strength of Taylor vortexes but also influence the number and position of the Taylor vortex pairs. This has strong implications in terms of energy efficiency maximization for marine applications and reduction of drag losses in, for example, fluid transport in pipelines and carriers.
Mueser MH, Dapp WB, Bugnicourt R, et al., 2017, Meeting the Contact-Mechanics Challenge, TRIBOLOGY LETTERS, Vol: 65, ISSN: 1023-8883
This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.
Yu M, Evangelou SA, Dini D, 2017, Model Identification and Control for a Quarter Car Test Rig of Series Active Variable Geometry Suspension, 20th IFAC World Congress, Publisher: Elsevier, Pages: 3376-3381, ISSN: 1474-6670
In this paper, a quarter car test rig is utilized to perform an experimental study of the singlelinkvariant of the Series Active Variable Geometry Suspension (SAVGS). A nonlinear model of the testrig is identified with the use of a theoretical quarter car model and the rig’s experimental frequencyresponse. A linear equivalent modeling method that compensates the geometric nonlinearity is alsoadopted to synthesize an H-infinity control scheme. The controller actively adjusts the single-linkvelocity in the SAVGS to improve the suspension performance. Experiments are performed to evaluatethe SAVGS practical feasibility, the performance improvement, the accuracy of the nonlinear model andthe controller’s robustness.
Bodnarchuk M, Dini D, Heyes D, et al., 2017, Molecular dynamics studies of overbased detergents on a water surface, Langmuir, Vol: 33, Pages: 7263-7270, ISSN: 1520-5827
Molecular dynamics (MD) simulations are reported of model overbased detergent nanoparticles on a model water surface which mimic their behavior on a Langmuir trough or large water droplet in engine oil. The simulations predict that the structure of the nanoparticle on a water surface is different to when it is immersed in a bulk hydrophobic solvent. The surfactant tails are partly directed out of the water, while the carbonate core maximizes its extent of contact with the water. Umbrella sampling calculations of the potential of mean force between two particles showed that they are associated with varying degrees with a maximum binding free energy of ca. 10 kBT for the salicylate stabilized particle, ca. 8 kBT for a sulfurized alkyl phenate stabilized particle, and ca. 5 kBT for a sulfonate stabilized particle. The differences in the strength of attraction depend on the proximity of nearest approach and the energy penalty associated with the disruption of the hydration shell of water molecules around the calcium carbonate core when the two particles approach. This is greatest for the sulfonate particle, which partially loses the surfactant ions to the solution, and least for the salicylate, which forms the weakest water “cage”. The particles are separated by a water hydration layer, even at the point of closest approach.
Knight C, Abdol Azis MH, O'Sullivan C, et al., 2017, Sensitivity analysis of Immersed Boundary Method simulations of fluid flow in dense polydisperse random grain packings, EPD Sciencies, Powders and Grains 2017 – 8th International Conference on Micromechanics on Granular Media, ISSN: 2101-6275
Polydisperse granular materials are ubiquitous in nature and industry. Despite this, knowledge of the momentum coupling between the fluid and solid phases in dense saturated grain packings comes almost exclusively from empirical correlations [2-4, 8] with monosized media. The Immersed Boundary Method (IBM) is a Computational Fluid Dynamics (CFD) modelling technique capable of resolving pore scale fluid flow and fluid-particle interaction forces in polydisperse media at the grain scale. Validation of the IBM in the low Reynolds number, high concentration limit was performed by comparing simulations of flow through ordered arrays of spheres with the boundary integral results of Zick and Homsy [10] . Random grain packings were studied with linearly graded particle size distributions with a range of coefficient of uniformity values (C u = 1.01, 1.50, and 2.00) at a range of concentrations (Φ ∈ [0.396; 0.681]) in order to investigate the influence of polydispersity on drag and permeability. The sensitivity of the IBM results to the choice of radius retraction parameter [1] was investigated and a comparison was made between the predicted forces and the widely used Ergun correlation [3].
Ewen J, Gattinoni C, Zhang J, et al., 2017, On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction, Physical Chemistry Chemical Physics, Vol: 19, Pages: 17883-17894, ISSN: 1463-9084
A detailed understanding of the behaviour of confined fluids is critical to a range of industrial applications, for example to control friction in engineering components. In this study, a combination of tribological experiments and confined nonequilibrium molecular dynamics simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5–2.0 GPa) and strain rates (104–1010 s−1) typical of the elastohydrodynamic lubrication regime, reveals clear relationships between the friction and flow behaviour. Lubricants, which are flexible, broadly linear molecules, give low friction coefficients that increase with strain rate and pressure in both the experiments and the simulations. Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, give high friction coefficients that only weakly depend on strain rate and pressure. The observed differences in friction behaviour can be rationalised through the stronger shear localisation which is observed for the traction fluids in the simulations. Higher pressures lead to more pronounced shear localisation, whilst increased strain rates lead to a widening of the sheared region. The methods utilised in this study have clarified the physical mechanisms of important confined fluid behaviour and show significant potential in both improving the prediction of elastohydrodynamic friction and developing new molecules to control it.
McCarron R, Stewart D, Shipway P, et al., 2017, Sliding wear analysis of cobalt based alloys in nuclear reactor conditions, Wear, Vol: 366-367, Pages: 1489-1501, ISSN: 1873-2577
The study of the wear behaviour of cobalt based alloys in nuclear reactor environmental conditions is the focus of this work. The alloys are used in components within reactors due to their excellent wear and corrosion resistance and their high hardness in the high pressure and temperature water facing environment. In the nuclear reactor core, cobalt is irradiated producing a highly penetrative gamma emitting isotope, cobalt 60 from stable cobalt 59. Wear of the cobalt alloys, producing wear debris, exacerbates this problem as it may be transported and deposited at various locations throughout the primary loop increasing the potential of radiation exposure. Removing this problem will require the removal of cobalt from the system.In order for suitable replacement materials to be identified, a better understanding of the behaviour of these alloys in the prototypical working conditions must be obtained. This work focuses on two cobalt based alloys used in the ball and race components of rolling element bearings in the reactor core, Stellite 20 and Haynes 25, respectively. The sliding wear behaviour of the alloys in an environment designed to replicate reactor conditions is examined using a bespoke pin on disc tribometer. Wear measurement and microstructural and compositional analysis of the samples tested over a range of conditions are presented and discussed.Concurrent to the experimental work is the development of a wear prediction model using a semi analytical method. The model employs Archard’s wear law as the method of predicting wear using data obtained through experimentation. The accuracy of the semi analytical model is limited however it does give a good estimation for maximum wear depth of the test specimens.
Smith E, Heyes D, Dini D, 2017, Towards the Irving Kirkwood limit of the mechanical stress tensor, Journal of Chemical Physics, Vol: 146, ISSN: 1089-7690
The probability density functions (PDFs) of the local measure of pressure as a function of the sampling volume are computed for a model Lennard-Jones (LJ) fluid using the Method of Planes (MOP) and Volume Averaging (VA) techniques. This builds on the study of Heyes, Dini, and Smith [J. Chem. Phys. 145, 104504 (2016)] which only considered the VA method for larger subvolumes. The focus here is typically on much smaller subvolumes than considered previously, which tend to the Irving-Kirkwood limit where the pressure tensor is defined at a point. The PDFs from the MOP and VA routes are compared for cubic subvolumes, V=ℓ3. Using very high grid-resolution and box-counting analysis, we also show that any measurement of pressure in a molecular system will fail to exactly capture the molecular configuration. This suggests that it is impossible to obtain the pressure in the Irving-Kirkwood limit using the commonly employed grid based averaging techniques. More importantly, below ℓ≈3 in LJ reduced units, the PDFs depart from Gaussian statistics, and for ℓ=1.0, a double peaked PDF is observed in the MOP but not VA pressure distributions. This departure from a Gaussian shape means that the average pressure is not the most representative or common value to arise. In addition to contributing to our understanding of local pressure formulas, this work shows a clear lower limit on the validity of simply taking the average value when coarse graining pressure from molecular (and colloidal) systems.
Gurrutxaga-Lerma B, Shehadeh M, Balint, et al., 2017, The effect of temperature on the elastic precursor decay in shock loaded FCC aluminium and BCC iron, International Journal of Plasticity, Vol: 96, Pages: 135-155, ISSN: 1879-2154
This article offers a comprehensive experimental and theoretical study of the causes of thermal hardening in FCC Al and BCC Fe at high strain rates, with the aim to shed light on important mechanisms governing deformation and failures in materials subjected to shocks and impacts at very high strain rates. Experimental evidence regarding the temperature dependence of the dynamic yield point of FCC Al and BCC Fe shock loaded at 107 s−1 is provided. The dynamic yield point of Al increases with temperature in the range 125K–795K; for the same loading and temperate range, the dynamic yield point of BCC Fe remains largely insensitive. A Multiscale Discrete Dislocation Plasticity (MDDP) model of both Fe and Al is developed, leading to good agreement with experiments. The importance of the Peierls barrier in Fe is highlighted, showing it is largely responsible for the temperature insensitivity in BCC metals. The relevance of the mobility of edge components in determining the plastic response of both FCC Al and BCC Fe at different temperatures is discussed, which leads to developing a mechanistic explanation of the underlying mechanisms leading to the experimental behaviour using Dynamic Discrete Dislocation Plasticity (D3P). It is shown that the main contributing factor to temperature evolution of the dynamic yield point is not the mobility of dislocations, but the temperature variation of the shear modulus, the decrease of which is correlated to the experimental behaviour observed for both FCC Al and BCC Fe.
Bodnarchuk MS, Doncom KEB, Wright DB, et al., 2017, Polyelectrolyte pKa from experiment and molecular dynamics simulation, RSC Advances, Vol: 7, Pages: 20007-20014, ISSN: 2046-2069
The pKa of a polyelectrolyte has been determined experimentally by potentiometric titration and computed using Molecular Dynamics (MD) constant pH (CpH) methodology, which allows the pKa of each titratable site along the polymer backbone to be determined separately, a procedure which is not possible by current experimental techniques. By using experimental results within the CpHMD method, the simulations show that the protonation states of neighbouring residues are anti-correlated so that the charges are well-separated. As found with previous simulation studies on model polyelectrolytes, the end groups are predicted to be the most acidic. CpHMD is shown to result in distinct polymer conformations, brought about by the range of protonation states changes along the polymer; this can now be used in the design of pH-responsive polymers for, amongst other applications, additive formulation and drug delivery devices.
Ewen J, Gattinoni C, Spikes H, et al., 2017, Nonequilibrium molecular dynamics simulations of organic friction modifiers, 253rd National Meeting of the American-Chemical-Society (ACS) on Advanced Materials, Technologies, Systems, and Processes, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Tan Z, Bernardini A, Konstantinou I, et al., 2017, Diffusion Measurement and Modelling, European Robotics Forum 2017
Ma S, Scaraggi M, Lin P, et al., 2017, Nanohydrogel brushes for switchable underwater adhesion, Journal of Physical Chemistry C, Vol: 121, Pages: 8452-8463, ISSN: 1932-7455
In nature, living systems commonly adopt the switchable friction/adhesion mechanism during locomotion. For example, geckos can move on ceilings, relying on the reversible attachment and detachment of their feet on substrate surfaces. Inspired by this scientists have used different materials to mimic natural dynamic friction/adhesion systems. However, synthetic systems usually cannot work in water environments and are also limited to single-contact interfaces, while nature has provided living systems with complex features to perform energy dissipation and adhere on multiple contact interfaces. Here, for the first time, we report the design, synthesis, and testing of a novel double-sided synthetic construct that relies on nanohydrogel brushes to provide simultaneous friction switching on each side of the membrane that separates the nanohydrogel fibers. This highly tunable response is linked to the swelling and shrinkage of the brushes in basic/acid media. Such a system shows three different friction states, which depend on the combination of pH control of the two membrane sides. Importantly, each side of the membrane can independently provide continuous but stable friction switching from high to ultralow friction coefficients in a wet environment under high load conditions. An in-depth theoretical study is performed to explore the mechanisms governing the hydration state responsible for the observed switching. This novel design opens a promising route for the development of new solutions for intelligent devices, which can adapt to multistimulus-responsive complex environments.
Wilson R, Dini D, van Wachem B, 2017, The influence of surface roughness and adhesion on particle rolling, Powder Technology, Vol: 312, Pages: 321-333, ISSN: 0032-5910
The influence of surface roughness and contact adhesion on the rolling behaviour of dry particles has been investigated. Rough particle surfaces are approximated using an array of spheres, the properties of which are informed by random processes. An analytical model has been derived by considering the torques that a particle experiences. Two mechanisms of rolling resistance are explored – a stationary particle experiencing a tangential force, and a dynamically rolling particle. The analytical model is found to agree well with simulations of the equivalent system using the discrete element model. Adhesive forces are found to increase rolling resistance in all cases. The complex consequences of varying the height variance and length scale of the surface roughness are reproduced accurately by the analytical model.
Profito FJ, Vladescu S-C, Reddyhoff T, et al., 2017, Experimental validation of a mixed-lubrication regime model for textured piston-ring-liner contacts, Materials Performance and Characterization, Vol: 6, Pages: 112-129, ISSN: 2165-3992
Recent experiments have shown that automotive piston-liner friction may be reduced by up to 50 % if the surface of the liner is laser textured with certain configurations of micro-pockets. It is important to model this behavior to understand and optimize the friction reduction mechanisms that are occurring. However, until now, very few models that predict the lubrication performance of textured surfaces have been successfully validated against experimental data. This is because of the requirement for them to: (1) reproduce experimental configurations with a certain degree of fidelity, (2) conserve mass properly, and (3) account for transient, boundary lubrication conditions. To address this, the current paper presents a comparison between the results from a numerical model, which fulfils these criteria, and an experimental test rig operating under the same conditions. The mathematical modeling is based on the averaged Reynolds’ equation with Patir and Cheng’s flow factors and the p − θ Elrod–Adams mass-conserving cavitation model. Simultaneously to the fluid flow solution, the contact pressures that arise from the asperity interactions are also included into the calculations through the well-known stochastic Greenwood and Tripp model for rough contacts. The experimental data is produced using a reciprocating tribometer, whose contact conditions are closely controlled and accurately mimic those found in an automotive piston–liner conjunction. Data is presented in terms of friction force versus stroke angle, and the similarities and differences between the model and experiment are discussed.
Hajishafiee A, Kadiric A, Ioannides E, et al., 2016, A coupled finite-volume CFD solver for two-dimensional elasto-hydrodynamic lubrication problems with particular application to rolling element bearings, Tribology International, Vol: 109, Pages: 258-273, ISSN: 1879-2464
This paper describes a new computational fluid dynamics methodology for modelling elastohydrodynamic contacts. A finite-volume technique is implemented in the ‘OpenFOAM’ package to solve the Navier-Stokes equations and resolve all gradients in a lubricated rolling-sliding contact. The method fully accounts for fluid-solid interactions and is stable over a wide range of contact conditions, including pressures representative of practical rolling bearing and gear applications. The elastic deformation of the solid, fluid cavitation and compressibility, as well as thermal effects are accounted for. Results are presented for rolling-sliding line contacts of an elastic cylinder on a rigid flat to validate the model predictions, illustrate its capabilities, and identify some example conditions under which the traditional Reynolds-based predictions deviate from the full CFD solution.
Forte AE, Gentleman SM, Dini D, 2016, On the characterisation of the heterogeneous mechanical response of human brain tissue, Biomechanics and Modeling in Mechanobiology, Vol: 16, Pages: 907-920, ISSN: 1617-7959
The mechanical characterization of brain tissue is a complex task scientists have tried to accomplish for over fifty years. The resultsin literatureoften differ by orders of magnitudebecause of the lack of a standard testing protocol. Different testing conditions (including humidity, temperature, strain rate),the methodologyadopted,the variety of the speciesanalysed, are all potential sources of discrepancies in the measurements.In this work we present a rigorous experimental investigation on the mechanical properties of human brain, covering both grey and white matter. The influence of testing conditions isalso shown and thoroughly discussed. The material characterisation performed is finally adopted to provide inputs toa mathematical formulation suitable fornumerical simulations of brain deformation during surgical procedures.
Profito FJ, Vladescu S, Reddyhoff, et al., 2016, Transient experimental and modelling studies of laser-textured micro-grooved surfaces with a focus on piston-ring cylinder liner contacts, Tribology International, Vol: 113, Pages: 125-136, ISSN: 1879-2464
This paper presents a comparison between the results from numerical modelling and experiments to shed light on the mechanisms by which surface texture can reduce friction when applied to an automotive cylinder liner. In this configuration, textured features move relative to the piston-liner conjunction and to account for this our approach is to focus on the transient friction response to individual pockets as they pass through, and then leave, the sliding contact. The numerical approach is based on the averaged Reynolds’ equation with the Patir & Cheng’s flow factors and the p-θ Elrod-Adams mass-conserving cavitation model. The contact pressures that arises from the asperity interactions are solved simultaneously to the fluid flow solution using the Greenwood and Tripp method. The experimental data is produced using a pin-on-disc set up, in which laser textured pockets have been applied to the disc specimen. Under certain conditions in the mixed and boundary lubrication regimes, both model and experimental results show i) an increase in friction as the pocket enters the contact, followed by ii) a sharp decrease as the pocket leaves the contact, and then iii) a gradual decay back to the pre-entrainment value. From the evidence obtained for the first time from the proposed combined modelling and experimental investigation conducted under carefully controlled conditions, we suggest that these three stages occur due to the following mechanisms: i) a reduction in fluid pressure due to the increased inlet gap, ii) inlet suction as the cavitated fluid within the pocket draws lubricant into the contact, and iii) film thickness decay as oil is squeezed out of the contact. The interplay of these three mechanisms is shown to control the response of micro-textured surfaces und
Leibinger A, Forte AE, Tan Z, et al., 2016, Erratum to: Soft Tissue Phantoms for Realistic Needle Insertion: A Comparative Study (Annals of Biomedical Engineering, 10.1007/s10439-015-1523-0), Annals of Biomedical Engineering, Vol: 44, ISSN: 0090-6964
Reference 4 should be changed to: Forte, A. E., S. Galvan, F. Manieri, F. Rodriguez y Baena, and D. Dini. A composite hydrogel for brain tissue phantoms. Mater. Des. 227:238–112, 2016.
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Arana C, Evangelou SA, Dini D, 2016, Series active variable geometry suspension application to comfort enhancement, Control Engineering Practice, Vol: 59, Pages: 111-126, ISSN: 1873-6939
This paper explores the potential of the Series Active Variable Geometry Suspension (SAVGS) for comfort and road holding enhancement. The SAVGS concept introduces significant nonlinearities associated with the rotation of the mechanical link that connects the chassis to the spring-damper unit. Although conventional linearization procedures implemented in multi-body software packages can deal with this configuration, they produce linear models of reduced applicability. To overcome this limitation, an alternative linearization approach based on energy conservation principles is proposed and successfully applied to one corner of the car, thus enabling the use of linear robust control techniques. An H∞ controller is synthesized for this simplified quarter-car linear model and tuned based on the singular value decomposition of the system's transfer matrix. The proposed control is thoroughly tested with one-corner and full-vehicle nonlinear multi-body models. In the SAVGS setup, the actuator appears in series with the passive spring-damper and therefore it would typically be categorized as a low bandwidth or slow active suspension. However, results presented in this paper for an SAVGS-retrofitted Grand Tourer show that this technology has the potential to also improve the high frequency suspension functions such as comfort and road holding.
Ewen J, Echeverri Restrepo S, Morgan N, et al., 2016, Nonequilibrium molecular dynamics simulations of stearic acid adsorbed on iron surfaces with nanoscale roughness, Tribology International, Vol: 107, Pages: 264-273, ISSN: 1879-2464
Nonequilibrium molecular dynamics (NEMD) simulations have been used to examine the structure and friction of stearic acid films adsorbed on iron surfaces with nanoscale roughness. The effect of pressure, stearic acid coverage, and level of surface roughness were investigated. The direct contact of asperities was prevented under all of the conditions simulated due to strong adsorption, which prevented squeeze-out. An increased coverage generally resulted in lower lateral (friction) forces due to reductions in both the friction coefficient and Derjaguin offset. Rougher surfaces led to more liquidlike, disordered films; however, the friction coefficient and Derjaguin offset were only slightly increased. This suggests that stearic acid films are almost as effective on contact surfaces with nanoscale roughness as those which are atomically-smooth.
Maćkowiak S, Heyes DM, Dini D, et al., 2016, Non-equilibrium phase behavior and friction of confined molecular films under shear: a non-equilibrium molecular dynamics study, Journal of Chemical Physics, Vol: 145, ISSN: 1089-7690
The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potentia
Jonas Verschueren, Gurrutxaga Lerma B, Balint DS, et al., 2016, The injection of a screw dislocation into a crystal: atomistics vs. continuum elastodynamics, Journal of the Mechanics and Physics of Solids, Vol: 98, Pages: 366-389, ISSN: 1873-4782
The injection (creation) process of a straight screw dislocation is compared atomistically with elastodynamic continuum theory. Amethod for injecting quiescent screw dislocations into a crystal of tungsten is simulated using non-equilibrium molecular dynamics.The resulting stress fields are compared to the those of elastodynamic solutions for the injection of a quiescent screw dislocation.A number of differences are found: a plane wave emission is observed to emanate from the whole surface of the cut used to createthe dislocation, affecting the displacement field along the dislocation line (z), and introducing displacement field componentsperpendicular to the line (along x and y). It is argued that, in part, this emission is the result of the finite time required to injectthe dislocation, whereby the atoms in the cut surface must temporarily be displaced to unstable positions in order to produce therequired slip. By modelling this process in the continuum it is shown that the displacements components normal to the dislocationline arise from transient displacements of atoms in the cut surface parallel to x and y. It is shown that once these displacements areincluded in the elastodynamic continuum formulation the plane wave emission in uzis correctly captured. A detailed comparisonbetween the atomistic and continuum models is then offered, showing that the main atomistic features can also be captured in thecontinuum.
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