9 research outputs found
Study of the true contact area of sheared elastomer/glass contacts using an optical method
Nous utilisons une méthode optique récemment développé pour mesurer l'aire de contact réelle à l'interface entre une surface élastomÚre et une plaque de verre. Nous montrons d'abord que l'aire de contact réelle diminue significativement sous cisaillement. Ensuite, nous comparons nos mesures avec deux modÚles de la littérature. Nous utilisons ces résultats pour mieux comprendre le comportement des interfaces multicontacts cisaillées
Continuum contact models for coupled adhesion and friction
We develop two new continuum contact models for coupled adhesion and
friction, and discuss them in the context of existing models proposed in the
literature. Our new models are able to describe sliding friction even under
tensile normal forces, which seems reasonable for certain adhesion mechanisms.
In contrast, existing continuum models for combined adhesion and friction
typically include sliding friction only if local contact stresses are
compressive. Although such models work well for structures with sufficiently
strong local compression, they fail to capture sliding friction for soft and
compliant systems (like adhesive pads), for which the resistance to bending is
low. This can be overcome with our new models. For further motivation, we
additionally present experimental results for the onset of sliding of a smooth
glass plate on a smooth elastomer cap under low normal loads. As shown, the
findings from these experiments agree well with the results from our models. In
this paper we focus on the motivation and derivation of our continuum contact
models, and provide a corresponding literature survey. Their implementation in
a nonlinear finite element framework as well as the algorithmic treatment of
adhesion and friction will be discussed in future work
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Tactile perception of randomly rough surfaces
Most everyday surfaces are randomly rough and self-similar on sufficiently small scales. We investigated the tactile perception of randomly rough surfaces using 3D-printed samples, where the topographic structure and the statistical properties of scale-dependent roughness were varied independently. We found that the tactile perception of similarity between surfaces was dominated by the statistical micro-scale roughness rather than by their topographic resemblance. Participants were able to notice differences in the Hurst roughness exponent of 0.2, or a difference in surface curvature of 0.8 mmâ1 for surfaces with curvatures between 1 and 3 mmâ1. In contrast, visual perception of similarity between color-coded images of the surface height was dominated by their topographic resemblance. We conclude that vibration cues from roughness at the length scale of the finger ridge distance distract the participants from including the topography into the judgement of similarity. The interaction between surface asperities and fingertip skin led to higher friction for higher micro-scale roughness. Individual friction data allowed us to construct a psychometric curve which relates similarity decisions to differences in friction. Participants noticed differences in the friction coefficient as small as 0.035 for samples with friction coefficients between 0.34 and 0.45
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Tactile perception of randomly rough surfaces
Most everyday surfaces are randomly rough and self-similar on sufficiently small scales. We investigated the tactile perception of randomly rough surfaces using 3D-printed samples, where the topographic structure and the statistical properties of scale-dependent roughness were varied independently. We found that the tactile perception of similarity between surfaces was dominated by the statistical micro-scale roughness rather than by their topographic resemblance. Participants were able to notice differences in the Hurst roughness exponent of 0.2, or a difference in surface curvature of 0.8 mmâ1 for surfaces with curvatures between 1 and 3 mmâ1. In contrast, visual perception of similarity between color-coded images of the surface height was dominated by their topographic resemblance. We conclude that vibration cues from roughness at the length scale of the finger ridge distance distract the participants from including the topography into the judgement of similarity. The interaction between surface asperities and fingertip skin led to higher friction for higher micro-scale roughness. Individual friction data allowed us to construct a psychometric curve which relates similarity decisions to differences in friction. Participants noticed differences in the friction coefficient as small as 0.035 for samples with friction coefficients between 0.34 and 0.45
Effect of coating thickness on the friction properties of rubber-sphere on rubber-coated-plane contacts
Nous prĂ©sentons une Ă©tude expĂ©rimentale du frottement d'un contact entre une sphĂšre caoutchouteuse et une lame de verre revĂȘtue d'un film de caoutchouc d'Ă©paisseur variable. Nous montrons que le dĂ©pĂŽt joue un rĂŽle clĂ© dans les propriĂ©tĂ©s de rupture de l'interface de contact : des variations d'un facteur trois du coefficient de frottement statique sont observĂ©es lorsque l'Ă©paisseur est modifiĂ©e. Certaines interprĂ©tations sont discutĂ©es pour expliquer le phĂ©nomĂšne impliquant notamment le rĂŽle de la viscoĂ©lasticitĂ© du caoutchouc
Mise en glissement des interfaces multicontacts élastomÚres : étude expérimentale par visualisation in situ
The onset of sliding of a contact interface is a phenomenon the space-time dynamics of which are still poorly understood. In this thesis, we have developed and implemented an original experimental device allowing us to visualize in situ the local phenomena involved during the onset of sliding of rough elastomer interfaces, with a good temporal resolution. We have shown a strong reduction of the real contact area within a sheared contact interface, well before the beginning of macroscopic sliding. This reduction affects the value of the static friction force of the interface. We have shown that the parameter that quantifies the amplitude of the reduction obeys a well-defined scaling law ranging from millimetric mono-contacts to the micrometric junctions involved in rough interfaces. We have then shown that the shear strength of an interface is not a constant for a couple of materials in contact. Indeed, by systematically changing the thickness of an elastic coating on one of the bodies in contact, we could vary the value of the shear strength by a factor three. This effect is interpreted semi-quantitatively via a model incorporating dissipation both at the interface and in the bulk of the materials. We have finally shown that the space-time dynamics of the onset of sliding is influenced by the torque applied to the interface by the friction force, when the latter is not exerted in the plane of the interface. In particular, via a digital image correlation-based measurement, we performed the first quantitative comparison with a recent model describing this torque effect.La mise en glissement dâune interface de contact est un phĂ©nomĂšne dont la dynamique spatiotemporelle est encore mal comprise. Dans cette thĂšse, nous avons dĂ©veloppĂ© et mis en oeuvre un dispositif expĂ©rimental original permettant de visualiser in situ les phĂ©nomĂšnes locaux en jeu lors de la mise en glissement dâinterfaces rugueuses Ă©lastomĂšres, avec une bonne rĂ©solution temporelle. Nous avons mis en Ă©vidence une forte rĂ©duction de lâaire de contact rĂ©elle au sein dâune interface de contact sous cisaillement, et ce bien avant le dĂ©but du glissement macroscopique. Cette rĂ©duction influence la valeur de la force de frottement statique de lâinterface. Nous avons montrĂ© que le paramĂštre qui quantifie lâamplitude de la rĂ©duction vĂ©rifie une loi dâĂ©chelle valable largement, allant des monocontacts millimĂ©triques jusquâaux jonctions micromĂ©triques impliquĂ©es dans les interfaces rugueuses. Nous avons ensuite montrĂ© que la contrainte de cisaillement critique de mise en glissement dâune interface nâest pas une constante pour un couple de matĂ©riaux en contact. En effet, en changeant systĂ©matiquement lâĂ©paisseur dâun revĂȘtement Ă©lastique sur lâun des corps en contact, on peut varier cette contrainte dâun facteur trois. Cet effet est interprĂ©tĂ© semi-quantitativement via un modĂšle couplant dissipation Ă lâinterface et dans le volume des matĂ©riaux. Nous avons enfin montrĂ© que la dynamique spatio-temporelle de mise en glissement est influencĂ©e par le couple appliquĂ© Ă lâinterface par la force de frottement, lorsque celle-ci nâest pas exercĂ©e dans le plan de lâinterface. En particulier, via une mesure du champ de dĂ©placement par corrĂ©lation dâimages, nous avons rĂ©alisĂ© la premiĂšre comparaison quantitative avec un modĂšle rĂ©cent dĂ©crivant cet effet de couple