  The gradient crystal plasticity framework of Wulfinghoff et al. [53]
incorporating an equivalent plastic strain and grain boundary yielding, is
extended with additional grain boundary hardening. By comparison to averaged
results from many discrete dislocation dynamics (DDD) simulations of an
aluminum type tricrystal under tensile loading, the new hardening parameter in
the continuum model is calibrated. It is shown that although the grain
boundaries (GBs) in the discrete simulations are impenetrable, an infinite GB
yield strength corresponding to microhard GB conditions, is not applicable in
the continuum model. A combination of a finite GB yield strength with an
isotropic bulk Voce hardening relation alone also fails to model the plastic
strain profiles obtained by DDD. Instead, a finite GB yield strength in
combination with GB hardening depending on the equivalent plastic strain at the
GBs is shown to give a better agreement to DDD results. The differences in the
plastic strain profiles obtained in DDD simulations by using different
orientations of the central grain could not be captured, indicating that the
misorientation dependent elastic interaction of dislocations reaching over the
GBs should be included in the continuum model.
