  The atomic-scale cracking mechanism in clay is vital in discovering the
cracking mechanism of clay at the continuum scale in that clay is a
nanomaterial. In this article, we investigate mechanisms of mode I and mode II
crack propagations in pyrophyllite and Ca-montmorillonite with water adsorption
through reactive molecular dynamics with a bond-order force field. Clay water
adsorption is considered by adding water molecules to the clay surface. During
the equilibration stage, water adsorption could cause bending deformation of
the pre-defined edge crack region. The relatively small orientating angle of
water molecules indicates the formation of hydrogen bonds in the crack
propagation process. The peak number density of adsorbed water decreases with
the increasing strains. The atomistic structure evolution of the crack tip
under loading is analyzed to interpret the nanoscale crack propagation
mechanism. The numerical results show that the crack tip first gets blunted
with a significant increase in the radius of the curvature of the crack tip and
a slight change in crack length. The crack tip blunting is studied by tracking
the crack tip opening distance and O-Si-O angle in the tetrahedral Si-O cell in
mode I and II cracks. We compare bond-breaking behaviors between Al-O and Si-O.
It is found that Si-O bond breaking is primarily responsible for crack
propagation. The critical stress intensity factor and critical energy release
rate are determined from MD simulation results.
