Unloading damage mechanism and pore structure evolution characteristics of different coal and rocks

The final macroscopic fracture of coal rocks is closely related to their internal microscopic structure and micro-defects. The continuous change of the microscopic structure of coal rocks is the main reason for the change of the macroscopic mechanical properties. This change in macroscopic mechanical properties is caused by the unloading damage, which is closely related to the ejection phenomenon of coal rocks. The porosities of the coal rocks before and after tests are measured by the nuclear magnetic resonance (NMR) technique, and the T2 spectrum distribution, porosity, and nuclear magnetic resonance imaging of different coal rocks under the unloading action are obtained. The evolution characteristics of pore structure are analyzed, and the unloading damage mechanisms of different coal rocks are revealed. The damage formula is established based on NMR results, and the damage variables are derived and characterized quantitatively. The results show that the larger the initial damage, the larger the porosity, and the greater the damage under the same unloading confining pressure ratio. This indicates that initial damage plays a leading role in the damage of coal and rock, and ejection is more likely to occur under unloading conditions. With the decrease of confining pressure, the porosity of YTX increases obviously, the connectivity between pores increases, and obvious cracks appear. The research results provide an experimental basis for studying the damage evolution process and micromechanics of coal rock, establishing a bridge between the microdamage mechanism and macro fracture analysis of coal rock.