Mechanically Equivalent Elastic-plastic Deformations and the Problem of Plastic Spin *

The problem of plastic spin is phrased in terms of a notion of mechanical equivalence among local intermediate configurations of an elastic/plastic crystalline solid. This idea is used to show that, without further qualification, the plastic spin may be suppressed at the constitutive level. However, the spin is closely tied to an underlying undistorted crystal lattice which, once specified, eliminates the freedom afforded by mechanical equivalence. As a practical matter a constitutive specification of plastic spin is therefore required. Suppression of plastic spin thus emerges as merely one such specification among many. Restrictions on these are derived in the case of rate-independent response.