Shear-Induced Anisotropy in Rough Elastomer Contact

Abstract

True contact between randomly rough solids consists of myriad individual micro-junctions. While their total area controls the adhesive friction force of the interface, other macroscopic features, including viscoelastic friction, wear, stiffness and electric resistance, also strongly depend on the size and shape of individual micro-junctions. Here we show that, in rough elastomer contacts, the shape of micro-junctions significantly varies as a function of the shear force applied to the interface. This process leads to a growth of anisotropy of the overall contact interface, which saturates in macroscopic sliding regime. We show that smooth sphere/plane contacts have the same shearinduced anisotropic behaviour as individual micro-junctions, with a common scaling law over four orders of magnitude in initial area. We discuss the physical origin of the observations in the light of a fracture-based adhesive contact mechanics model, described in the companion article, which captures the smooth sphere/plane measurements. Our results shed light on a generic, overlooked source of anisotropy in rough elastic contacts, not taken into account in current rough contact mechanics models.

Onset of Sliding of Elastomer Multicontacts: Failure of a Model of Independent Asperities to Match Experiments

“…The main outcome is that, although we did our best to incorporate experimentally-based behaviour laws, pa-rameter values and initial conditions into the model, it fails to quantitatively reproduce the measurements of [20,21]. We argue, based on observations at the microjunction scale, that future asperity models should, in order to hope for success, incorporate a description of the tangential elastic interactions between microjunctions.…”

“…[17]), and may still be unsuccessful to quantitatively match friction experiments. We thus urge for the concurrent development of continuum models suitable to reproduce friction experiments like those of [20,21]. Acknowledgements We thank Oleg M. Braun, who initiated the work, but passed away before its completion.…”

“…1b. Those experiments, carried out both for large normal loads [21] and for small (even negative) normal loads [22], have shown that, under increasing shear, the initially circular contact shrinks anisotropically and becomes increasingly ellipse-like. As shown in [21] the shrinking minor axis of the ellipse is parallel to the shear loading direction, while the variations of the major axis (in the direction orthogonal to shear) can be neglected.…”

Shear-induced contact area anisotropy explained by a fracture mechanics model

“…Let us consider the experimental data discussed in the companion Letter [24] and in Sahli et al [15]. The experimental set-up is composed of a cantilever which sustains a glass substrate which is pressed against a PDMS sphere of radius R and then sheared (see Fig.…”

“…A camera was used to track the contact area evolution while a force cell simultaneously measured the tangential force applied. The experimental results reported by Sahli et al [15] and further analyzed in the companion Letter [24] are provided for the following set of normal forces P = [0.27, 0.55, 0.82, 1.10, 1.37, 1.65, 1.92, 2.12] N which span one order of magnitude and for the following sphere radius R = 9.42 mm.…”