  Silicene, a 2D analogue of graphene, has spurred a tremendous research
interest in the scientific community for its unique properties essential for
next generation electronic devices. In this work, for the first time, we
present a molecular dynamics (MD) investigation to determine the fracture
strength and toughness of nanocrystalline silicene (nc silicene) sheet of
varied grain size and pre existing crack length at room temperature. Our
results suggest that the transition from an inverse pseudo Hall Petch to a
pseudo Hall Petch behavior in nc silicene occurs at a critical grain size of
17.32 nm. This phenomenon is also prevalent in nanocrystalline graphene.
However, nc silicene with pre existing cracks exhibits anomalous crack
propagation and fracture toughness behaviour. We have observed two distinct
types of failure mechanisms (crack sensitive and insensitive failure) and
devised the mechanophysical conditions under which they occur. Fracture
toughness calculated from both Griffiths theory and MD simulations indicate
that the former overpredicts the fracture toughness of nc silicene. The most
striking outcome, however, is that despite the presence of a pre existing
crack, the crack sensitivity of nc silicene is found to be dependent on the
grain size and their orientations. This study is the first direct comparison of
atomistic simulations to the continuum theories to predict the anomalous
behaviour in deformation and failure mechanisms of nc silicene.
