Doctoral thesis viva voce: Danh Toan Nguyen

Local analysis of the friction between a rough surface and an elastomer

Danh Toan Nguyen, doctorant - PhD student Crédits : ESPCI ParisTechThe improvement of the tire performances requires a better understanding of the frictional properties of local multi-contact interface. Simple measurement of quantities that are integrated all over the contact, such as the friction force, allows only a very indirect knowledge of the friction laws and their relationship with surface roughness. In this context, the thesis aims to study the local friction in a contact between a rigid surface and a smooth elastomer. Based on contact imaging techniques, we are able to measure the displacement field induced by friction on the elastomer surface. This displacement is then inverted to obtain the friction law between the local shear stress and the local contact pressure at each point of the contact.

In the first stage of this work, experimental improvements have been carried out to increase the quality of the displacement fields measurement with a better resolution and a better signal/noise ratio. We have then demonstrated the existence of large deformations in the contact region and developed a displacement - stress inversion by finite element method to take into account these deformations.

This approach was first applied to the smooth contacts where we observed a shear stress independent of the local contact pressure and the nominal area of contact, which is not the case of rough interfaces. The experimental local friction law in a rough interface shows a remarkable nonlinearity even at low contact pressures. This law also indicates a tendency of interface saturation at high contact pressure. Different rough surfaces with gaussian and non-gaussian statistics nature allowed us to evaluate the sensitivity of the local friction law to the roughness details and discuss the theoretical models which are based on a purely spectral description of surfaces. The use of surfaces with well controlled roughness has helped us in identifying the contribution of the viscoelastic dissipation to the friction on a rubber surface.