A reformulation of strain gradient plasticity

A class of phenomenological strain gradient plasticity theories is formulat ed to accommodate more than one material length parameter. The objective is a generalization of the classical J(2) flow theory of plasticity to accoun t for strain gradient effects that emerge in deformation phenomena at the m icron scale. A special case involves a single length parameter and is of si milar form to that proposed by Aifantis and co-workers. Distinct computatio nal advantages are associated with this class of theories that make them at tractive for applications requiring the generation of numerical solutions. The higher-order nature of the theories is emphasized, involving both highe r-order stresses and additional boundary conditions. Competing members in t he class of theories will be examined in light of experimental data on wire torsion, sheet bending, indentation and other micron scale plasticity phen omena. The data strongly suggest that at least two distinct material length parameters must be introduced in any phenomenological gradient plasticity theory, one parameter characterizing problems for which stretch gradients a re dominant and the other relevant to problems when rotation gradients (or shearing gradients) are controlling. Flow and deformation theory versions o f the theory arc highlighted that can accommodate multiple length parameter s. Examination of several basic problems reveals that the new formulations predict quantitatively similar plastic behavior to the theory proposed earl ier by the present authors. The new formulations improve on the earlier the ory in the manner in which elastic and plastic strains are decomposed and i n the representation of behavior in the elastic range. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.