This paper addresses the issue of the determination of the frictional stress distribution from the inversion of the measured surface displacement field for sliding interfaces between a glass lens and a rubber (poly(dimethylsiloxane)) substrate. Experimental results show that high lateral strains are achieved at the periphery of the sliding contacts. As a consequence, an accurate inversion of the displacement field requires that finite strains and non linear response of the rubber substrate are taken into account. For that purpose, a Finite Element (FE) inversion procedure is implemented where the measured displacement field is applied as a boundary condition at the upper surface of a meshed body representing the rubber substrate.Normal pressure is also determined by the same way, if non-diverging values are assumed at the contact edge. This procedure is applied to linearly sliding contacts as well as on twisting contacts.

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This paper reports on the frictional properties of smooth rubber substrates sliding against rigid surfaces covered with various densities of colloidal nano-particles (average diameter 77 nm). Friction experiments were carried out using a transparent Poly(dimethyl siloxane) (PDMS) rubber contacting a silica lens with silica nano-particles sintered onto its surface. Using a previously described methodology (Nguyen et al., J. of Adhesion 87 (2011) 235-250 ), surface shear stress and contactpressure distribution within the contact were determined from a measurement of the displacement field at the surface of the PDMS elastomer. Addition of silica nano-particles results in a strong, pressure-independent enhancement of the frictional shear stress as compared to the smooth lens. The contribution of viscoelastic losses to these increased frictional properties is analyzed in the light of a numerical model that solves the contact problem between the rubber and the rough surface. An order-of-magnitude agreement is obtained between experimental and theoretical results, the latter showing that the calculation of viscoelastic dissipation within the contact is very sensitive to the details of the topography of the rigid asperities.

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A measurement method based on Particle Image Velocimetry (PIV) with refraction of the laser sheet at a window/water interface is proposed for the measurement of the velocity field of a flow, inside a water puddle, due to a tire rolling on. This study focuses on the feasibility and repeatability of this optical measurement method. The characterization of the optical properties of this measurement technique defines its accuracy. The analysis of the overall features of the flow is focused on two main flow zones in front of and around the tire. The flow inside the first zone is characterized by a global velocity of the water displaced in an area located in front of the tire. In the second zone a velocity representative of the flow in the vicinity of the tire shoulder is also defined. Correlations of both characteristic velocities with the car speed and water film height are established. New and the corresponding worn tires were tested in this work.