Critical thickness theory for the plastic deformation of small stressed volumes is reviewed, with particular emphasis on its application to the size effect in common micromechanical testing methods, foil flexure, wire torsion, and compression and tension of micropillars and wires. Key predictions which distinguish the theory from others are that plasticity initiates throughout a finite volume when a strain-thickness product or strain-volume integral exceeds a critical value related to the Burgers vector; that misfit or geometrically necessary dislocations will generally be found outside this volume; and that the intrinsic or bulk strength of the material is not changed by the size effect. Experiments designed to test these predictions are briefly described.
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