  The plastic events occurring during the process of intergranular fracture in
metals is still not well understood due to the complexity of grain boundary
(GB) structures and their interactions with crack-tip dislocation plasticity.
By considering the local GB structural transformation after dislocation
emission from a GB in the Peierls-type Rice-Beltz model, herein we established
a semi-analytical transition-state-theory-based framework to predict the most
probable Mode-I stress intensity factor (SIF) for dislocation emission from a
cracked GB. Using large-scale molecular dynamics (MD) simulations, we studied
the fracture behaviors of bi-crystalline Fe samples with 12 different symmetric
tilt GBs inside. The MD results demonstrate that the presence of GB could
significantly change the SIF required for the activation of plastic events,
confirming the theoretical predictions that attributes this to the energy
change caused by the transformation of GB structure. Both the atomistic
simulation and the theoretical model consistently indicate that, the critical
dynamic SIF ($K_{I}^{c}(t)$) at which the dynamic SIF $K_{I}(t)$ deviates from
the linearity with respect to the strain ${\epsilon}$, increases with the
increasing loading rate. However, the classical Rice model underestimates the
$K_{I}^{c}(t)$ due to its failure to consider the effects of localized fields.
The present theoretical model provides a mechanism-based framework for the
application of grain boundary engineering in the design and fabrication of
nano-grained metals.
