  Cracks in clay are significant in geotechnical and geoenvironmental
engineering (e.g., embankment erosion and stability of landfill cover systems).
This article studies the mechanism of nucleation and growth of cracks in clay
at the nanoscale through full-scale molecular dynamics simulations. The clay
adopted is pyrophyllite, and the force field is CLAYFF. The crack formation in
a pyrophyllite clay layer is evaluated under uniaxial tension and simple shear.
The numerical results show that cracks in the nanoscale pyrophyllite clay layer
are brittle and strain-rate dependent. Small strain rate results in low
ultimate tensile/shear strength. As strain rate increases, clay crack shifts
from a single-crack pattern to a multiple-crack one. The cracking mechanism is
investigated from bond breakage analysis at the atomic scale. It is found that
the first bond breakage occurs in the silicon-surface oxygen bond. As a crack
propagates, the relative percentage of broken silicon-surface oxygen bonds is
the smallest compared to other types of metal-oxygen interactions,
demonstrating that the atomic interaction between silicon and surface oxygen
atoms is the strongest. To understand the propagation of cracks, we also study
the stress intensity factor and energy release rate of pyrophyllite and their
size dependence at the atomic scale.
