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Chungyuen Hui - One of the best experts on this subject based on the ideXlab platform.
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effect of rate on adhesion and Static Friction of a film terminated fibrillar interface
Langmuir, 2009Co-Authors: Shilpi Vajpayee, Lulin Shen, Anand Jagota, Rong Long, Chungyuen HuiAbstract:A film-terminated fibrillar interface has been shown to result in significant enhancement of adhesion and Static Friction compared to a flat control. This enhancement increases with interfibril spacing. In this, the first of a two-part study, by studying the effect of rate on adhesion and Static Friction, we show that both adhesion and Static Friction enhancement are due to a crack-trapping mechanism. For adhesion, as measured by an indentation experiment, an analytical model is used to relate the applied indenter displacement rate and measured forces to contact line velocity and energy release rate, respectively. The two mechanisms for adhesion enhancement―varying rate and crack-trapping―are found to be coupled multiplicatively.
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strongly enhanced Static Friction using a film terminated fibrillar interface
Soft Matter, 2008Co-Authors: Lulin Shen, Nicholas J Glassmaker, Anand Jagota, Chungyuen HuiAbstract:We examine the behavior under shear of a bio-inspired fibrillar interface that consists of poly(dimethlysiloxane) micro-posts terminated by a thin film. These structures demonstrate significantly enhanced adhesion due to a crack trapping mechanism. We study the response of this structure to shear displacement relative to a spherical indenter placed on its surface under a fixed normal force. The shear force required to initiate sliding between the indenter and the sample, its Static Friction, is strongly enhanced compared to a flat control, and increases with inter-fibril spacing. Examination of the contact region reveals that its area changes with applied shear and that Static Friction is controlled by a mechanical instability. The shear force resisting steady sliding, surprisingly, is independent of fibril spacing and is nearly the same as for the flat unstructured control samples. We interpret dynamic Friction to result from the action of Schallamach-like waves. Our results show that the film-terminated architecture can be used to design an interface with significantly enhanced Static Friction without altering its sliding Frictional resistance.
Izhak Etsion - One of the best experts on this subject based on the ideXlab platform.
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model for the Static Friction coefficient of spherical contact with a soft metal coating
SN Applied Sciences, 2020Co-Authors: Haibo Zhang, Zhou Chen, Izhak EtsionAbstract:An elastic–plastic spherical contact with soft metallic coating under combined normal and tangential loading is studied by finite element analysis. Full-stick contact condition is assumed and sliding inception is related to vanishing tangential stiffness of the contact junction. Previously observed, both experimentally and theoretically, effects of increasing coating thickness on Static Friction coefficient that were published in the literature are thoroughly explained here, to the authors’ best knowledge, for the first time. These effects include initial sharp drop of Friction as soon as a thinnest coating film is applied, followed by a transitional behavior from decrease to increase of Friction when the coating thickness is continuously increased. An intensive parametric study is performed and the effects of substrate and coating material properties on the Static Friction coefficient are revealed and thoroughly explained. An empirical expression for the Static Friction coefficient is derived along with the values for optimum coating thickness that provides the minimum Friction coefficient.
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Model for the Static Friction coefficient in a full stick elastic-plastic coated spherical contact
Friction, 2018Co-Authors: Zhou Chen, Izhak EtsionAbstract:Finite element analysis is used to investigate an elastic-plastic coated spherical contact in full stick contact condition under combined normal and tangential loading. Sliding inception is associated with a loss of tangential stiffness. The effect of coating thickness on the Static Friction coefficient is intensively investigated for the case of hard coatings. For this case, with the increase in coating thickness, the Static Friction coefficient first increases to its maximum value at a certain coating thickness, thereafter decreases, and eventually levels off. The effect of the normal load and material properties on this behavior is discussed. Finally, a model for the Static Friction coefficient as a function of the coating thickness is provided for a wide range of material properties and normal loading.
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the effect of surface roughness on Static Friction and junction growth of an elastic plastic spherical contact
Journal of Tribology-transactions of The Asme, 2009Co-Authors: Denis Cohen, Y Kligerman, Izhak EtsionAbstract:A model for elastic-plastic spherical contact of rough surfaces under combined normal and tangential loadings, with full stick contact condition, is presented. The model allows evaluation of the effect of surface roughness on the real contact area, Static Friction and junction growth under small normal loads. It is shown that as the normal load approaches a certain threshold value, which depends on the plasticity index, the results of the present rough surface model approach these of previous corresponding models for smooth sphere and a rigid flat. At normal load values below the threshold load, the correlation of the present results and published experimental results is much better in comparison with the results of the smooth surface models.
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a model for contact and Static Friction of nominally flat rough surfaces under full stick contact condition
Journal of Tribology-transactions of The Asme, 2008Co-Authors: Denis Cohen, Y Kligerman, Izhak EtsionAbstract:A model for elastic-plastic nominally flat contacting rough surfaces under combined normal and tangential loading with full stick contact condition is presented. The model incorporates an accurate finite element analysis for contact and sliding inception of a single elastic-plastic asperity in a statistical representation of surface roughness. It includes the effect of junction growth and treats the sliding inception as a failure mechanism, which is characterized by loss of tangential stiffness. A comparison between the present model and a previously published Friction model shows that the latter severely underestimates the maximum Friction force by up to three orders of magnitude. Strong effects of the normal load, nominal contact area, mechanical properties, and surface roughness on the Static Friction coefficient are found, in breach of the classical laws of Friction. Empirical equations for the maximum Friction force, Static Friction coefficient, real contact area due to the normal load alone and at sliding inception as functions of the normal load, material properties, and surface roughness are presented and compared with some limited available experimental results.
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Experimental Study of Adhesive Static Friction in a Spherical Elastic-Plastic Contact
Journal of Tribology-transactions of The Asme, 2008Co-Authors: Andrey Ovcharenko, Gregory Halperin, Izhak EtsionAbstract:The elastic-plastic contact between a deformable sphere and a rigid flat during presliding is studied experimentally. Measurements of Friction force and contact area are done in real time along with an accurate identification of the instant of sliding inception. The Static Friction force and relative tangential displacement are investigated over a wide range of normal preloads for several sphere materials and diameters. Different behavior of the Static Friction is observed in the elastic and in the elastic-plastic regimes of sphere deformation. It is found that at low normal loads, the Static Friction coefficient depends on the normal load in breach of the classical laws of Friction. The presliding displacement is found to be less than 5% of the contact diameter, and the interface mean shear stress at sliding inception is found to be slightly below the shear strength of the sphere material. Good correlation is found between the present experimental results and a recent theoretical model in the elastic-plastic regime of deformation.
Julien Scheibert - One of the best experts on this subject based on the ideXlab platform.
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evolution of real contact area under shear and the value of Static Friction of soft materials
Proceedings of the National Academy of Sciences of the United States of America, 2018Co-Authors: Rahel Sahli, G Pallares, Christophe Ducottet, Al S Akhrass, Marcel Guibert, Julien ScheibertAbstract:The Frictional properties of a rough contact interface are controlled by its area of real contact, the dynamical variations of which underlie our modern understanding of the ubiquitous rate-and-state Friction law. In particular, the real contact area is proportional to the normal load, slowly increases at rest through aging, and drops at slip inception. Here, through direct measurements on various contacts involving elastomers or human fingertips, we show that the real contact area also decreases under shear, with reductions as large as 30%, starting well before macroscopic sliding. All data are captured by a single reduction law enabling excellent predictions of the Static Friction force. In elastomers, the area-reduction rate of individual contacts obeys a scaling law valid from micrometer-sized junctions in rough contacts to millimeter-sized smooth sphere/plane contacts. For the class of soft materials used here, our results should motivate first-order improvements of current contact mechanics models and prompt reinterpretation of the rate-and-state parameters. area of real contact | rough contact | elastomer | Static Friction | rate-and-state Friction R ough solids in dry contact touch only at their highest asper-ities, so that real contact consists of a population of individual microjunctions (Fig. 1B), with a total area A R. A R is usually much smaller than the apparent contact area, A A , that one would expect for smooth surfaces. Since the seminal work of Bowden and Tabor (1), it is recognized that the Frictional properties of such multicontact interfaces are actually controlled by A R rather than by A A. In particular, direct measurements of A R on transparent interfaces have been developed (2, 3) and repeatedly found proportional to the Friction force, both for multicontacts (4–10) and for single contacts between smooth bodies (1, 11, 12), with the proportionality constant being the contact's Frictional shear strength, σ. A R is a dynamic quantity with three major causes for variations. First, A R is roughly proportional to the normal load applied to multicontacts (5, 6, 10). This result, which provides an explanation for Amontons–Coulomb's law of Friction (Friction forces are proportional to the normal force), has been reproduced by many models of weakly adhesive rough contacts under purely normal load (1, 4, 13–16). In the case of independent elastic microjunctions, although each of them grows nonlinearly with normal load, proportionality arises statistically due to random-ness in the surface asperities' heights (13). Second, in Static conditions , A R slowly increases, typically logarithmically, with the time spent in contact (5, 17). This phenomenon, so-called geometric aging (18), is interpreted as plastic (5, 19, 20) or viscoelas-tic (21) creep at the microjunctions, depending on the materials in contact, and is different from contact strengthening with time at constant contact area (18, 22), so-called structural aging. Third, at the onset of sliding of the interface, the population of already aged microjunctions gradually slips and is replaced by new, smaller microjunctions. Slip inception is thus accompanied by a drop of A R (5, 17), by up to a few tens of percent. This effect is often considered to be the origin of the difference between the peak (Static) and steady sliding (kinematic) Friction forces (18). Accounting for these three dependencies together has been a major success in the science of Friction because it provides a consistent picture of the physical mechanisms underlying the ubiquitous state-and-rate Friction law (5, 18, 20–31), which is obeyed by multicontacts in a variety of materials, from polymer glasses to rocks, through rubber and paper. However, a series of experimental observations reported here and there in the literature over recent decades suggest that the picture may not be fully comprehensive yet. These observations, made on smooth contacts, have repeatedly indicated that the area of apparent contact, A A , depends on the value of the tangential load, Q, applied to the interface. For instance, smooth metallic sphere/plane contacts typically grow as Q increases (1, 2), due to plastic deformations in the vicinity of the contact (1, 32). Conversely, A A decreases when smooth elastomer-based sphere/plane contacts as well as fingertip contacts are increasingly sheared (9, 33–38), presumably due to viscoelastic and/or adhesion effects (33, 36, 38–40). It is therefore tempting to hypothesize that not only smooth but also rough interfaces have a dependence of their contact area on the tangential load, Q. Such a dependence would directly affect the resistance to sliding of a rough contact, the way we use current contact and Friction models to predict the Static Friction force, and the physical meaning of the parameters of the rate-and-state Friction law. To test this hypothesis, we carried out experiments to monitor, in multicontacts involving elastomers or human fingertips, the evolution of A R when Q is increased from 0 to macroscopic sliding. Significance We investigate the origin of Static Friction, the threshold force at which a Frictional interface starts to slide. For rough contacts involving rubber or human skin, we show that the real contact area, to which Static Friction is proportional, significantly decreases under increasing shear, well before the onset of sliding. For those soft materials, our results will impact how we use and interpret current contact mechanics and Friction models.
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evolution of real contact area under shear and the value of Static Friction of soft materials
Proceedings of the National Academy of Sciences of the United States of America, 2018Co-Authors: Riad Sahli, G Pallares, Christophe Ducottet, Al S Akhrass, I Ben E Ali, Matthieu Guibert, Julien ScheibertAbstract:The Frictional properties of a rough contact interface are controlled by its area of real contact, the dynamical variations of which underlie our modern understanding of the ubiquitous rate-and-state Friction law. In particular, the real contact area is proportional to the normal load, slowly increases at rest through aging, and drops at slip inception. Here, through direct measurements on various contacts involving elastomers or human fingertips, we show that the real contact area also decreases under shear, with reductions as large as 30%, starting well before macroscopic sliding. All data are captured by a single reduction law enabling excellent predictions of the Static Friction force. In elastomers, the area-reduction rate of individual contacts obeys a scaling law valid from micrometer-sized junctions in rough contacts to millimeter-sized smooth sphere/plane contacts. For the class of soft materials used here, our results should motivate first-order improvements of current contact mechanics models and prompt reinterpretation of the rate-and-state parameters.
Lulin Shen - One of the best experts on this subject based on the ideXlab platform.
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effect of rate on adhesion and Static Friction of a film terminated fibrillar interface
Langmuir, 2009Co-Authors: Shilpi Vajpayee, Lulin Shen, Anand Jagota, Rong Long, Chungyuen HuiAbstract:A film-terminated fibrillar interface has been shown to result in significant enhancement of adhesion and Static Friction compared to a flat control. This enhancement increases with interfibril spacing. In this, the first of a two-part study, by studying the effect of rate on adhesion and Static Friction, we show that both adhesion and Static Friction enhancement are due to a crack-trapping mechanism. For adhesion, as measured by an indentation experiment, an analytical model is used to relate the applied indenter displacement rate and measured forces to contact line velocity and energy release rate, respectively. The two mechanisms for adhesion enhancement―varying rate and crack-trapping―are found to be coupled multiplicatively.
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strongly enhanced Static Friction using a film terminated fibrillar interface
Soft Matter, 2008Co-Authors: Lulin Shen, Nicholas J Glassmaker, Anand Jagota, Chungyuen HuiAbstract:We examine the behavior under shear of a bio-inspired fibrillar interface that consists of poly(dimethlysiloxane) micro-posts terminated by a thin film. These structures demonstrate significantly enhanced adhesion due to a crack trapping mechanism. We study the response of this structure to shear displacement relative to a spherical indenter placed on its surface under a fixed normal force. The shear force required to initiate sliding between the indenter and the sample, its Static Friction, is strongly enhanced compared to a flat control, and increases with inter-fibril spacing. Examination of the contact region reveals that its area changes with applied shear and that Static Friction is controlled by a mechanical instability. The shear force resisting steady sliding, surprisingly, is independent of fibril spacing and is nearly the same as for the flat unstructured control samples. We interpret dynamic Friction to result from the action of Schallamach-like waves. Our results show that the film-terminated architecture can be used to design an interface with significantly enhanced Static Friction without altering its sliding Frictional resistance.
Anand Jagota - One of the best experts on this subject based on the ideXlab platform.
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effect of rate on adhesion and Static Friction of a film terminated fibrillar interface
Langmuir, 2009Co-Authors: Shilpi Vajpayee, Lulin Shen, Anand Jagota, Rong Long, Chungyuen HuiAbstract:A film-terminated fibrillar interface has been shown to result in significant enhancement of adhesion and Static Friction compared to a flat control. This enhancement increases with interfibril spacing. In this, the first of a two-part study, by studying the effect of rate on adhesion and Static Friction, we show that both adhesion and Static Friction enhancement are due to a crack-trapping mechanism. For adhesion, as measured by an indentation experiment, an analytical model is used to relate the applied indenter displacement rate and measured forces to contact line velocity and energy release rate, respectively. The two mechanisms for adhesion enhancement―varying rate and crack-trapping―are found to be coupled multiplicatively.
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strongly enhanced Static Friction using a film terminated fibrillar interface
Soft Matter, 2008Co-Authors: Lulin Shen, Nicholas J Glassmaker, Anand Jagota, Chungyuen HuiAbstract:We examine the behavior under shear of a bio-inspired fibrillar interface that consists of poly(dimethlysiloxane) micro-posts terminated by a thin film. These structures demonstrate significantly enhanced adhesion due to a crack trapping mechanism. We study the response of this structure to shear displacement relative to a spherical indenter placed on its surface under a fixed normal force. The shear force required to initiate sliding between the indenter and the sample, its Static Friction, is strongly enhanced compared to a flat control, and increases with inter-fibril spacing. Examination of the contact region reveals that its area changes with applied shear and that Static Friction is controlled by a mechanical instability. The shear force resisting steady sliding, surprisingly, is independent of fibril spacing and is nearly the same as for the flat unstructured control samples. We interpret dynamic Friction to result from the action of Schallamach-like waves. Our results show that the film-terminated architecture can be used to design an interface with significantly enhanced Static Friction without altering its sliding Frictional resistance.
