A strategy for modeling various interfacial boundary conditions associated with a
higher-order gradient crystal plasticity theory is proposed. The gradient theory
employed is based on the concept of the backstress that is produced in response to
the spatial variation of the geometrically necessary dislocation densities. To set
arbitrary interfacial boundary conditions for the crystallographic slip at the
continuum level, a model with a single scalar quantity that aims to control the
slipping rate at an interface is introduced. This scalar quantity is intended to
represent the resultant effects of microscopic mechanisms such as absorption,
emission, and transmission of the dislocations at an interface or a grain boundary
(GB). As a realistic application of this basic idea, an orientation-dependent GB
model is proposed, which incorporates effects of the degree of misorientation between
the adjacent grains as well as the orientation of the GB plane relative to the
grains. To illustrate capabilities of the proposed model, the bicrystalline
micropillar compression problem is considered. Finite element simulations are
performed for the bicrystalline micropillars including either a large-angle
grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the
compression axis. The numerical results are qualitatively compared with
experimental observations reported in the literature. It is shown that the
proposed GB model has a capability to represent the overall material responses
associated with both LAGB and CTB using the same material parameter
values.
