Dependency of Rubber Friction on Normal Force or Load: Theory and Experiment

Abstract

In rubber friction studies it is often observed that the kinetic friction coefficient µ depends on the nominal contact pressure p. We discuss several possible origins of the pressure dependency of µ: (a) saturation of the contact area (and friction force) due to high nominal squeezing pressure, (b) non-linear viscoelasticity, (c) non-randomness in the surface topography, in particular the influence of the skewness of the surface roughness profile, (d) adhesion, and (e) frictional heating. We show that in most cases the non-linearity in the µ(p) relation is mainly due to process (e) (frictional heating), which softens the rubber, increases the area of contact, and (in most cases) reduces the viscoelastic contribution to the friction. In fact, since the temperature distribution in the rubber at time t depends on on the sliding history (i.e., on the earlier time t ′ < t), the friction coefficient at time t will also depend on the sliding history, i.e. it is, strictly speaking, a time integral operator.The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly affect the area of real contact and the friction force (and the wear-rate). This is the case for rubber sliding on road surfaces at speeds above 1 mm/s. In Ref. [14] we have derived equations which describe the frictional heating for solids with arbitrary thermal properties. In this paper the theory is applied to rubber friction on road surfaces. Numerical results are presented and compared to experimental data. We observe good agreement between the calculated and measured temperature increase.

Effect of simulated tennis steps and slides on tread element friction and wear

“…Friction increases with shape length due to the reduced friction-induced torque experienced. Tread elements with higher second moment of area in the slide direction, produce higher contact areas and lower mean contact pressure which results in a higher friction [33]. As little wear was observed for the step tests (< 1%), differing friction-induced torque explains why the long orientation produced higher friction than the short orientation for the last 15 tests.…”

Enhancing brush tyre model accuracy through friction measurements

“…It should be noted that the commonly observed load-dependency of tyres is captured by the shifting of the master curve. Changes in the contact pressure lead to changes in frictional heating in the rubber, which, in turn, influences the tyre-surface friction coefficient and thus the tyre shear forces [26].…”

A Hybrid Multiscale Approach for Rubber Contact

“…The mechanics of contact taking place at the interface of rough soft solids is attracting a wide scientific interest due to its implications in a number of applications, ranging from biology and biomedical devices to machine elements (Persson, 2006). As an example, passenger cars maneuverability, comfort and energy efficiency in the era of autonomous/assisted driving and novel tires design are strictly related to the ability to adopt advanced physically-based tire friction and wear models during car engineering (Farroni et al, 2017;Fortunato et al, 2017). Moreover, an accurate prediction of contact mechanics, combined with an increasingly realistic physical description of the rubber behavior, plays a key role in term of tire durability, driving safety and sustainability (Lu, 2010), as well as in the prediction of the driving range in electric vehicles (Farfan-Cabrera, 2019).…”