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In the present work, we carry out molecular dynamics (MD) simulations to investigate mechanism of crack in a single clay sheet. Uniaxial tension is applied to clay sheet in a strain-controlled manner. Crack initiates from the center of clay sheet and extends along the direction perpendicular to the loading direction. Finally crack extends through the whole slab and clay sheet is fractured into two halves. Bond breaking is first observed in silicon-oxygen bond, and later occurs in aluminum-oxygen bond. Numerical results show that crack propagation in single clay sheet is primarily contributed from bond breaking in Al-Ob and Si-Ob bonds. We find that crack length increases almost linearly with tensile strain. Atomic stress is calculated for both tetrahedral layer and octahedral layer considering the layered inhomogeneity of clay crystal structure. Stress intensity factor and energy release rate are computed based on MD simulation and are comparable to experimental results.