Micro-mechanical properties and damage mechanisms of coal under cyclic loading: A nanoindentation experiment and molecular dynamics simulation

ABSTRACT

By studying the key mechanical properties and their damage mechanisms of coal at the nanoscale, a better understanding of the strength and damage of coal in complex stress environments can be obtained. The micro-mechanical properties and damage mechanism of coal under cyclic loading were investigated by nanoindentation experiment and molecular dynamics (MD) simulation of nanoindentation. Molecular dynamics simulations were conducted by building models containing approximately 2.5 million atoms, it was found that the coal substrate showed cyclic hardening under cyclic loading. The hardness and elastic modulus of the coal samples measured by the nanoindentation experiment gradually increased from 0.476 GPa to 0.740 GPa and from 4.70 GPa to 5.997 GPa, respectively. The simulation results display a good agreement with the experimental results. The simulation results show that the plastic damage of coal substrate under cyclic loading is related to the stretching, bending, rotation, and fracture of chemical bonds. At the same time, the formation and recovery of elastic deformation are related to the changes of van der Waals force, Coulomb force, and bond angle. Additionally, the carbon hexatomic ring in the coal macromolecule is critical in resisting the influence of loading.

KEYWORDS:

• Cyclic loading
• coal
• nanoindentation
• molecular dynamics simulation
• dissipated energy

Acknowledgments

The authors are grateful for their support. This work described has not been submitted elsewhere for publication, in whole or in part.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work is financially supported by the National Natural Science Foundation of China [grant number U1704242], Fundamental Research Funds for the Central Universities [grant number 2021YQAQ02] and the Yue Qi Distinguished Scholar Project, China University of Mining & Technology, Beijing.