MD modeling of cracks in clay at the nanoscale

TL;DR

This study uses molecular dynamics simulations to analyze how cracks nucleate and grow in nanoscale pyrophyllite clay, revealing strain-rate effects, atomic bond breakage patterns, and crack propagation mechanisms relevant to geotechnical applications.

Contribution

It provides new insights into nanoscale crack mechanisms in clay using molecular dynamics, including bond breakage analysis and stress intensity factors, which were not previously detailed.

Findings

Cracks are brittle and strain-rate dependent.

Higher strain rates lead to multiple cracks instead of a single crack.

Silicon-surface oxygen bonds are the initial and strongest bonds to break during crack propagation.

Abstract

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.