Length scale effect on mechanical behavior due to strain gradient plasticity

The characteristic material length scale l as introduced in the phenomenolo gical theory of strain gradient plasticity by Fleck and Hutchinson [1] is c rucial for the application of the theory. Three dislocation models are prop osed in this paper for the derivation of the material length scales as ther e is a rational connection between dislocation theory and phenomenological theory. The length scale is determined as a function of material fundamenta l parameters and is independent of how the 'overall effective strain' is me asured, or how the dislocations interact. The length scale is generally abo ut 1.5 mum or less, agreeing with the observation that the size effect appe ars approximately at 1 mum and sub-micrometer levels for indentation deform ations. The dislocation interaction is also discussed in the framework of s train gradient plasticity. (C) 2001 Elsevier Science B.V. All rights reserv ed.