Publications
456 results found
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.
Dini D, Bodnarchuk MS, Heyes DM, et 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.
Bodnarchuk MS, Heyes DM, Breakspear A, et 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.
Gurrutxaga-Lerma B, Balint DS, Dini D, et 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.
Medina S, Fowell MT, Vladescu S-C, et 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
Forte AE, D'Amico F, Charalambides MN, et 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
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- Citations: 62
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.
Smith ER, Heyes DM, Dini D, et al., 2015, A localized momentum constraint for non-equilibrium molecular dynamics simulations, JOURNAL OF CHEMICAL PHYSICS, Vol: 142, ISSN: 0021-9606
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- Citations: 5
Heyes DM, Dini D, BraĆka 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.
Rathbone D, Dini D, Marigo M, et al., 2015, An accurate force–displacement law for the modelling of elastic–plastic contacts in discrete element simulations, Powder Technology, Vol: 282, Pages: 2-9, ISSN: 1873-328X
This paper presents an accurate model for the normal force–displacement relationship between elastic–plastic spheres in contact for use in discrete element method (DEM) simulations. The model has been developed by analysing the normal force–displacement relationship between an elastic–perfectly plastic sphere and a rigid surface using the finite element method (FEM). Empirical relationships are found that relate the parameters of the new model to material properties. This allows the model to be used in the DEM for direct simulation of well characterised elastic–plastic materials without fitting parameters to experimental results. This gives the model an advantage over models in the literature for which fitting to experimental results is required. The implementation of the model into an existing DEM code is discussed and validated against the results from FEM simulations. The new model shows a good match to the FEM results and the DEM implementation correctly distinguishes between the loading, unloading and re-loading phases of contact between two spheres.
Arana C, Evangelou SA, Dini D, 2015, Series Active Variable Geometry Suspension for Road Vehicles, IEEE-ASME TRANSACTIONS ON MECHATRONICS, Vol: 20, Pages: 361-372, ISSN: 1083-4435
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- Citations: 34
Flicek RC, Hills DA, Barber JR, et al., 2015, Determination of the shakedown limit for large, discrete frictional systems, EUROPEAN JOURNAL OF MECHANICS A-SOLIDS, Vol: 49, Pages: 242-250, ISSN: 0997-7538
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- Citations: 15
Dini D, Giacopini M, Giuseppe MA, et al., 2015, The influence of textured surfaces on the tribological behaviour of hip replacements employng a mass conserving complementarity algorithm, Pages: 4-6
Mastrandrea LN, Giacopini M, Dini D, et al., 2015, Elastohydrodynamic analysis of the conrod small-end of a high performance motorbike engine via a mass conserving cavitation algorithm
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.
Profito FJ, Dini D, Zachariadis DC, 2015, A general finite volume method solution for the reynolds lubrication equation with Mass-Conserving cavitation model, Pages: 303-305
Tavasci A, Arizzi F, Dini D, et al., 2014, Heat flux evaluation in high temperature ring-on-ring contacts, Wear, Vol: 330-331, Pages: 320-326, ISSN: 1873-2577
A comprehensive methodology to investigate heat flux in a ring-on-ring tribometer is presented. Thermal fluxes under high contact pressures and temperature differences were evaluated through an experimental campaign and by a numerical procedure of inverse analysis applied to surface temperature measurements. An approximation of a two-dimensional time-dependent analytical solution for the temperature distribution was first developed and subsequently adapted to mimic the specific testing configuration characteristics; the problem was finally simplified to enable further inverse analysis. Experiments were performed using an innovative high temperature ring-on-ring tribometer. The evaluated contact heat-transfer rates were reported as a function of normal load and temperature difference between the discs under steady-state conditions; the results reported here show that, in the present test configuration, the temperature difference has stronger influence than the applied load in terms of heat transfer induced by contact.
Parkes M, Myant C, Dini D, et al., 2014, Tribology-optimised silk protein hydrogels for articular cartilage repair, Tribology International, Pages: ---, ISSN: 0301-679X
Gurrutxaga-Lerma B, Balint DS, Dini D, et al., 2014, Dynamic Discrete Dislocation Plasticity, Advances in Applied Mechanics, Vol: 47, ISSN: 0065-2156
This chapter concerns with dynamic discrete dislocation plasticity (D3P), a two- dimensional method of discrete dislocation dynamics aimed at the study of plastic relaxation processes in crystalline materials subjected to weak shock loading. Traditionally, the study of plasticity under weak shock loading and high strain rate has been based on direct experimental measurement of the macroscopic response of the material. Using these data, well-known macroscopic constitutive laws and equations of state have been formulated. However, direct simulation of dislocations as the dynamic agents of plastic relaxation in those circumstances remains a challenge. In discrete dislocation dynamics (DDD) methods, in particular the two-dimensional discrete dislocation plasticity (DDP), the dislocations are modeled as discrete discontinuities in an elastic continuum. However, current DDP and DDD methods are unable to adequately simulate plastic relaxation because they treat dislocation motion quasi- statically, thus neglecting the time-dependent nature of the elastic elds and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. This chapter reproduces the ndings by Gurrutxaga-Lerma, Balint, Dini, Eakins, and Sutton (2013), who proved that under shock loading, this assumption leads to models that invariably break causality, introducing numerous artifacts that invalidate quasi- static simulation techniques. This chapter posits that these limitations can only be overcome with a fully time-dependent formulation of the elastic elds of dislocations. In this chapter, following the works of Markensco and Clifton (1981) and Gurrutxaga- Lerma et al. (2013), a truly dynamic formulation for the creation, annihilation, and nonuniform motion of straight edge dislocations is derived. These solutions extend the DDP framework to a fully elastodynamic formulation that has been called dynamic discrete dislocation plasticity (D3P). This chapter describes the s
Forte AE, Dini D, 2014, Modelling and Characterisation of Soft Tissue Deformation and Indentation with Particular Application to Articular Cartilage and Brain, STLE Frontiers 2014
Gattinoni C, Mackowiak S, Heyes DM, et al., 2014, Boundary-controlled barostats for slab geometries in molecular dynamics simulations, PHYSICAL REVIEW E, Vol: 90, ISSN: 1539-3755
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- Citations: 17
Strozzi A, Giacopini M, Bertocchi E, et al., 2014, Formulation of the tangential velocity slip problem in terms of variational inequalities, Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol: 228, Pages: 1122-1135, ISSN: 2041-305X
Borges PDN, Forte AE, Vincent TL, et al., 2014, Rapid, automated imaging of mouse articular cartilage by microCT for early detection of osteoarthritis and finite element modelling of joint mechanics, Osteoarthritis and Cartilage, Vol: 22, Pages: 1419-1428, ISSN: 1063-4584
ObjectiveMouse articular cartilage (AC) is mostly assessed by histopathology and its mechanics is poorly characterised. In this study: (1) we developed non-destructive imaging for quantitative assessment of AC morphology and (2) evaluated the mechanical implications of AC structural changes.MethodsKnee joints obtained from naïve mice and from mice with osteoarthritis (OA) induced by destabilization of medial meniscus (DMM) for 4 and 12 weeks, were imaged by phosphotungstic acid (PTA) contrast enhanced micro-computed tomography (PTA-CT) and scored by conventional histopathology. Our software (Matlab) automatically segmented tibial AC, drew two regions centred on each tibial condyle and evaluated the volumes included. A finite element (FE) model of the whole mouse joint was implemented to evaluate AC mechanics.ResultsOur method achieved rapid, automated analysis of mouse AC (structural parameters in <10 h from knee dissection) and was able to localise AC loss in the central region of the medial tibial condyle. AC thickness decreased by 15% at 4 weeks and 25% at 12 weeks post DMM surgery, whereas histopathology scores were significantly increased only at 12 weeks. FE simulations estimated that AC thinning at early-stages in the DMM model (4 weeks) increases contact pressures (+39%) and Tresca stresses (+43%) in AC.ConclusionPTA-CT imaging is a fast and simple method to assess OA in murine models. Once applied more extensively to confirm its robustness, our approach will be useful for rapidly phenotyping genetically modified mice used for OA research and to improve the current understanding of mouse cartilage mechanics.
Putignano C, Le Rouzic J, Reddyhoff T, et al., 2014, A theoretical and experimental study of viscoelastic rolling contacts incorporating thermal effects, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART J-JOURNAL OF ENGINEERING TRIBOLOGY, Vol: 228, Pages: 1112-1121, ISSN: 1350-6501
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- Citations: 16
Forte AE, galvan S, manieri F, et al., 2014, A Novel Composite Phantom for Brain Tissue, EMBC 2014
Bodnarchuk MS, Dini D, Heyes DM, et al., 2014, Self-assembly of calcium carbonate nanoparticles in water and hydrophobic solvents, Journal of Physical Chemistry C, Vol: 118, Pages: 21092-21103, ISSN: 1932-7455
The self-assembly of Ca2+ and CO32– ions into nanoparticles in water and hydrophobic solvents is investigated using molecular dynamics (MD) computer simulation. A new three-stage particle assembly procedure is used which relaxes the structure of the nanoparticle toward a lower energy state. In hydrophobic solvent the bare particle is essentially spherical whereas in water it is ellipsoidally shaped. With surfactant stabilizer the nanoparticles typically exhibit nonspherical cores in model hydrophobic solvents. Binary surfactant systems exhibit synergistic effects where for example a salicylate-sulfonate combination forms a cage which promotes a compact core. Synergistic effects on the shape of the particle were also observed in a hydrophobic solvent for surfactant-stabilized systems with trace water as a third component. The simulations show that rather than being a rigid structure the carbonate core shape and stabilizing shell coverage are sensitive to solvent, surfactant, and small polar molecules which act as cosurfactants.
Flicek RC, Hills DA, Dini D, 2014, Refinements in the characterisation of mode-mixity and small scale yielding at sharp notch roots, ENGINEERING FRACTURE MECHANICS, Vol: 126, Pages: 73-86, ISSN: 0013-7944
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- Citations: 10
Cattilino M, Secoli R, Galvan S, et al., 2014, Development of a Dynamic Soft Tissue Phantom for Cooperative Control Testing in Robotic Surgery, Hamlyn Symposium
Rasin I, Pekar Z, Sadowsky O, et al., 2014, Real-Time Modelling of Intra-operative Brain Shift Based on Video Tracking, The Hamlyn Symposium on Medical Robotics 2014
Medina S, Dini D, 2014, A numerical model for the deterministic analysis of adhesive rough contacts down to the nano-scale, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 51, Pages: 2620-2632, ISSN: 0020-7683
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- Citations: 56
Bodnarchuk MS, Heyes DM, Dini D, et al., 2014, Role of Deprotonation Free Energies in p<i>K</i><sub>a</sub> Prediction and Molecule Ranking, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, Vol: 10, Pages: 2537-2545, ISSN: 1549-9618
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- Citations: 12
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