Effect of cyclic load on mechanical properties and failure mechanisms of different rank coals

Underground coal is often subjected to periodic loads caused by mining stress. To determine the effect of cyclic load on mechanical properties of coal of different ranks and failure mechanisms, nanoindentation experiments and molecular dynamics (MD) simulations were used to study different rank coals. The results indicated that cyclic hardening occurred in all the coal matrices under cyclic loading. With the increase in coal rank, the change of the elastic modulus and the hardness, the energy dissipation rate, and the damage layer decreased, the cyclic hardening weakened, and the resistance of the six-membered ring structure increased. The plastic failure of the coal matrix was mainly connected with the bending and stretching of the bond, the dihedral angle twisting, and the out-of-plane bending vibration. The elastic deformation was correlated with the variation of the Coulomb force and the van der Waals force, the twisting and recuperation of the dihedral angle, and the out-of-plane bending vibration. With increasing coal rank, the effect of the elastic deformation of the Coulomb force weakened. When the coal rank exceeded a certain range, the influence of the plastic failure of the out-of-plane bending vibration was transformed into the elastic deformation, and the effect of the elastic deformation of the van der Waals force was transformed into the plastic failure.