In this interesting paper, the authors address an anomaly which arises when a rigid, square-ended block is pressed against a linear elastic half plane and slid along. The authors note that, within the framework of linear elasticity, the singularity in the contact pressure, and hence shearing traction, produces, adjacent to the edges, regimes in which the implied local relative slip direction dominates the rigid-body sliding velocity, and hence produces a violation of the Coulomb friction law. They seek to resolve the paradox by appealing to a more sophisticated strain definition. All of this is within the context of a quasistatic formulation. The authors recognize, of course, that in any real problem the paradox is unlikely to arise because of (a) the finite strength of the contact giving rise to a yield zone, and (b) the absence of an atomically sharp corner at the contact edge where there is, in all probability, a finite edge radius. Here, we wish to address these issues quantitatively, and so demonstrate that it is unlikely that the paradox described, though interesting, will have any bearing in a real contact.

## Basic Formulation

## Asymptotic Representation

*s*is a coordinate measured from the contact edge. The exponent, $\lambda $, is given by the characteristic equation

## Local Plasticity

We turn now to the question of envelopment of the region of reverse slip by plasticity. As the complete stress field associated with the asymptote is known, through the Muskhelishvili potential, it is straightforward to establish an estimate of the size of the edge plastic zone, simply by seeing where the yield condition is exceeded, in the spirit of the usual fracture mechanics crack-tip plasticity correction.

## Effect of Rounding

*avoid*the paradox occurring we require $R>120nm$. This is such a tiny radius that for all contacts of practical importance the paradox will not exist.

## Concluding Remarks

The region of reverse slip violating the friction law has been quantified and shown to be extremely small. Physical boundaries for its envelopment by plasticity or absence through local rounding have also been explicitly found. It is very probable that plasticity will produce the greater effect.