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Effects of stretching on the contact and frictional properties of rubber.

We are investigating the effects surface stretching on the frictional properties of rubber within smooth contact interfaces with glass.

We provide new experimental evidences of a dependence of frictional stress on surface stretch within macroscopic sliding contacts between a smooth silicone rubber and rigid probes. From optical contact imaging methods, we determine both surface displacement and stress fields with a space resolution of about 10 $\mu$m. Using this approach, we show that the local frictional stress is proportional to the local in-plane surface stretch independently on contact pressure, sliding velocity and contact geometry. Additional experiments using pre-stressed silicone strips also show that the increase in frictional stress with stretch ratio is isotropic, i.e. it is insensitive to the orientation of the sliding direction with respect to the stretch direction. Potential explanations for this phenomenon are discussed within the framework of the Schallamach’s model of rubber friction.


Dependence of the local frictional stress on the local stretch ratio for glass/PDMS contacts. The different curves correspond to varied contact conditions (normal force, sliding velocity) and to different contact geometries (spheres with different radii of curvature, triangular flat punch).

 

This overlooked stretch-dependence of rubber friction pertains to many different situations. As mentioned in the introduction, frictional contact with statistically rough surfaces are clearly an example where frictional stresses achieved locally at micro-contact scale will be affected by surface strains induced at the macroscopic contact scale. Interestingly, such surface strains are a consequence of the finite size of the contact. It therefore turns out that the stretch-dependence of friction introduces a coupling between the macroscopic and microscopic contact length scales which is clearly not accounted for in existing rough contact mechanics theories dealing with extended - infinite - contact interfaces.

 

Related investigations are dealing with the normal contact of uniaxially stretched rubbers with spherical probes. A contact model for neo-Hokean substrates has been developped which allows to describe the elliptical contact shape of the contacts in both adhesive and non-adhesive situations.

References

A.Chateauminois, D.T. Nguyen and C. Frétigny
Effects of local stretch on the frictional stress of rubber
Soft Matter->http://pubs.rsc.org/en/content/articlelanding/2017/sm/c7sm01092c#!divAbstract]
 

Contact on a stretched rubber
C. Frétigny and A. Chateauminois
Physical Review E to appear (2017)
Arxiv