Publications
458 results found
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.
Forte AE, Galvan S, Manieri F, et al., 2016, A composite hydrogel for brain tissue phantoms, Materials and Design, Vol: 112, Pages: 227-238, ISSN: 0264-1275
Synthetic phantoms are valuable tools for training, research and development in traditional and computer aided surgery, but complex organs, such as the brain, are difficult to replicate. Here, we present the development of a new composite hydrogel capable of mimicking the mechanical response of brain tissue under loading. Our results demonstrate how the combination of two different hydrogels, whose synergistic interaction results in a highly tunable blend, produces a hybrid material that closely matches the strongly dynamic and non-linear response of brain tissue. The new synthetic material is inexpensive, simple to prepare, and its constitutive components are both widely available and biocompatible. Our investigation of the properties of this engineered tissue, using both small scale testing and life-sized brain phantoms, shows that it is suitable for reproducing the brain shift phenomenon and brain tissue response to indentation and palpation.
Forte AE, Dini D, 2016, On the suitability of hydrogels for mimicking the mechano-response of organic soft tissues, Biotribology 2016
Tan Z, Forte AE, Galvan S, et al., 2016, Composite Hydrogel: a New Tool for Reproducing the Mechanical Behaviour of Soft Human Tissues, Biotribology 2016
Heyes DM, Dini D, Smith ER, 2016, Equilibrium fluctuations of liquid state static properties in a subvolume by molecular dynamics, Journal of Chemical Physics, Vol: 145, ISSN: 1089-7690
System property fluctuations increasingly dominate a physical process as the sampling volume decreases. The purpose of this work is to explore how the fluctuation statistics of various thermodynamic properties depend on the sampling volume, using molecular dynamics (MD) simulations. First an examination of various expressions for calculating the bulk pressure of a bulk liquid is made, which includes a decomposition of the virial expression into two terms, one of which is the Method of Planes (MOP) applied to the faces of the cubic simulation cell. Then an analysis is made of the fluctuations of local density, temperature, pressure, and shear stress as a function of sampling volume (SV). Cubic and spherical shaped SVs were used within a spatially homogeneous LJ liquid at a state point along the melting curve. It is shown that the MD-generated probability distribution functions (PDFs) of all of these properties are to a good approximation Gaussian even for SV containing only a few molecules (∼10), with the variances being inversely proportional to the SV volume, Ω. For small subvolumes the shear stress PDF fits better to a Gaussian than the pressure PDF. A new stochastic sampling technique to implement the volume averaging definition of the pressure tensor is presented, which is employed for cubic, spherical, thin cubic, and spherical shell SV. This method is more efficient for less symmetric SV shapes.
Gurrutxaga Lerma B, Balint DS, Dini D, et al., 2016, A dynamic discrete dislocation plasticity study of elastodynamic shielding of stationary cracks, Journal of the Mechanics and Physics of Solids, Vol: 98, Pages: 1-11, ISSN: 0022-5096
Employing Dynamic Discrete Dislocation Plasticity (D3P), an elastodynamic analysis of theshielding of a stationary crack tip by dislocations is studied. Dislocations are generated via FrankReadsources, and make a negligible contribution to the shielding of the crack tip, whereas dislocationsgenerated at the crack tip via homogeneous nucleation dominate the shielding. Theireffect is found to be highly localised around the crack, leading to a magnification of the shieldingwhen compared to time-independent, elastostatic predictions. The resulting attenuation of KI (t)is computed, and is found to be directly proportional to the applied load and to √t.
Putignano C, Carbone G, Dini D, 2016, A parametrically time-dependent methodology for reciprocating contact mechanics between viscoelastic solids, VII European Congress on Computational Methods in Applied Sciences and Engineering, Publisher: National Technical University of Athens (NTUA), Pages: 1856-1863
We implement an original Boundary Element methodology to study the reciprocating contact mechanics between linear viscoelastic materials. Results are shown for the case of a rigid sphere sinusoidally driven in sliding contact with a viscoelastic half-space. We observe the presence of multi-peaked pressure and displacement distributions; the hysteric friction curve is finally shown for different values of the frequency.
Ewen J, Gattinoni C, Thakkar F, et al., 2016, Nonequilibrium Molecular Dynamics Investigation of the Reduction in Friction and Wear by Carbon Nanoparticles Between Iron Surfaces, Tribology Letters, Vol: 63, ISSN: 1573-2711
For the successful development and application of novel lubricant additives, a full understanding of their tribological behaviour at the nanoscale is required, but this can be difficult to obtain experimentally. In this study, nonequilibrium molecular dynamics simulations are used to examine the friction and wear reduction mechanisms of promising carbon nanoparticle friction modifier additives. Specifically, the friction and wear behaviour of carbon nanodiamonds (CNDs) and carbon nano-onions (CNOs) confined between α-iron slabs is probed at a range of coverages, pressures, and sliding velocities. At high coverage and low pressure, the nanoparticles do not indent into the α-iron slabs during sliding, leading to zero wear and a low friction coefficient. At low coverage and high pressure, the nanoparticles indent into, and plough through the slabs during sliding, leading to atomic-scale wear and a much higher friction coefficient. This contribution to the friction coefficient is well predicted by an expression developed for macroscopic indentation by Bowden and Tabor. Even at the highest pressures and lowest coverages simulated, both nanoparticles were able to maintain separation of the opposing slabs and reduce friction by approximately 75 % compared to when no nanoparticle was present, which agrees well with experimental observations. CNO nanoparticles yielded a lower indentation (wear) depth and lower friction coefficients at equal coverage and pressure with respect to CND, making them more attractive friction modifier additives. Potential changes in behaviour on harder and softer surfaces are also discussed, together with the implications that these results have in terms of the application of the studied nanoparticles as lubricants additives.
Ewen JP, Gattinoni C, Thakkar FM, et al., 2016, A Comparison of Classical Force-Fields for Molecular Dynamics Simulations of Lubricants, MATERIALS, Vol: 9, ISSN: 1996-1944
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- Citations: 75
Vannuccini E, Paccagnini E, Cantele F, et al., 2016, Two classes of short intraflagellar transport train with different 3D structures are present in <i>Chlamydomonas</i> flagella, JOURNAL OF CELL SCIENCE, Vol: 129, Pages: 2064-2074, ISSN: 0021-9533
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- Citations: 33
Ewen JP, Gattinoni C, Morgan N, et al., 2016, Nonequilibrium molecular dynamics simulations of organic friction modifiers adsorbed on iron oxide surfaces, Langmuir: the ACS journal of surfaces and colloids, Vol: 32, Pages: 4450-4463, ISSN: 0743-7463
For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.
Kakavas I, Olver AV, Dini D, 2016, Hypoid gear vehicle axle efficiency, Tribology International, Vol: 101, Pages: 314-323, ISSN: 0301-679X
In this paper, a study of a hypoid gear vehicle axle is presented. Using a custom rig, load-independent losses have been accurately measured and the effect of viscosity on spin loss has been quantified. Solution methods for the calculation of component losses are presented and combined into a complete thermally coupled transient model for the estimation of axle efficiency. An analysis of hypoid gear kinematics reveals a simplification, commonly adopted by other researchers, regarding the velocity of the point of contact in hypoid gears, to be in error. As a result, the calculation of lubrication parameters has been improved. Finally, experimental measurements are compared to the generated simulation results for a number of operating scenarios and satisfactory correlation is observed.
Hills DA, Dini D, 2016, A review of the use of the asymptotic framework for quantification of fretting fatigue, Journal of Strain Analysis for Engineering Design, Vol: 51, Pages: 240-246, ISSN: 0309-3247
Putignano C, Carbone G, Dini D, 2016, Theory of reciprocating contact for viscoelastic solids, Physical Review E, Vol: 93, ISSN: 1539-3755
A theory of reciprocating contacts for linear viscoelastic materials is presented. Results are discussed for the case of a rigid sphere sinusoidally driven in sliding contact with a viscoelastic half-space. Depending on the size of the contact, the frequency and amplitude of the reciprocating motion, and on the relaxation time of the viscoelastic body, we establish that the contact behavior may range from the steady-state viscoelastic solution, in which traction forces always oppose the direction of the sliding rigid punch, to a more elaborate trend, which is due to the strong interaction between different regions of the path covered during the reciprocating motion. Practical implications span a number of applications, ranging from seismic engineering to biotechnology.
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.
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.
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.
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.
Mastrandrea LN, Giacopini M, Bertocchi E, et 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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Ma S, Scaraggi M, Wang D, et 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.
Leibinger A, Forte AE, Tan Z, et 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.
Forte AE, 2015, BRAIN TISSUE BIOMECHANICS: NEW TISSUE PHANTOMS, MECHANICAL CHARACTERISATION AND MODELLING STRATEGIES FOR ENHANCED SURGICAL PROCEDURES
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, 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.
Profito FJ, Giacopini M, Zachariadis DC, et 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.
Gurrutxaga Lerma B, Balint DANIEL, Dini DANIELE, et 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.
Forte AE, Galvan S, Dini D, 2015, Biomechanics of the brain: experimental investigation, numerical simulation, design of advanced phantoms for robotic surgery, ITC 2015
Gurrutxaga Lerma BENAT, Balint DS, Dini D, et 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.
Reddyhoff T, Dobre O, Le Rouzic J, et 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.
Cheng C, Evangelou SA, Arana C, et 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.
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