An extended crystal plasticity theory that accounts for the length-scale effects in plastic strain gradient fields is presented. First, foundations and kinematics of crystal plasticity theory is reviewed. Then, experimental evidences for the size-effects in small-sized bent single crystals are presented. Total amounts of apparent strain hardening, which were experimentally observed, are decomposed into isotropic and kinematic hardening components. Physically-based models are formulated to describe the size-dependent isotropic and kinematic hardening behaviors, utilizing possible micromechanical information with respect to dislocations and their motions. Roles of the geometrically necessary dislocations (GNDs) in strain hardening behavior are studied in detail. Furthermore, some aspects of numerical computations of the extended size-dependent crystal plasticity theory are presented. The developed theory involves extra boundary conditions for crystallographic slips and/or the GND densities. Effects of these extra boundary conditions are demonstrated through numerical simulations for some basic boundary value problems. Finally, a phenomenological strain gradient plasticity theory is revisited, based on the knowledge from the present size-dependent crystal plasticity theory.
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