Dynamics of Shear-Transformation Zones in Amorphous Plasticity: Energetic Constraints in a Minimal Theory

We use energetic considerations to deduce the form of a previously uncertain coupling term in the shear-transformation-zone (STZ) theory of plastic deformation in amorphous solids. As in the earlier versions of the STZ theory, the onset of steady deformation at a yield stress appears here as an exchange of dynamic stability between jammed and plastically deforming states. We show how an especially simple ``quasilinear'' version of this theory accounts qualitatively for many features of plasticity such as yielding, strain softening, and strain recovery. We also show that this minimal version of the theory fails to describe certain other phenomena, and argue that these limitations indicate needs for additional internal degrees of freedom beyond those included here.