Mechanisms of strain rate-dependent response of naturally fractured coal

The mechanical characteristics of naturally fractured coal under strain rate-dependent loadings can affect engineering activities such as coal seam gas production, gas drainage and CO₂ sequestration. While the macro-(core) scale strain-rate dependent response of fractured coal has been investigated previously, the micro-scale mechanisms driving this core-scale behaviour particularly under recoverable (elastic) bulk deformation remains unexplored. In this study, we conduct a series of systematic multi-scale experiments to shed light on the mechanisms controlling the strain rate dependency of coal.

Core-scale triaxial tests are initially performed on coal specimens under isotropic and deviatoric loading conditions at different strain rates to identify their strain rate dependency. The results indicate a clear strain rate dependency in dry specimens only under isotropic loading, where the bulk modulus increases with increasing strain rates. Notably, unloading of the specimens shows a considerable strain rate-dependent energy dissipation without any permanent deformation in these isotropic loading tests.

To explore the identified micro-scale processes causing the energy dissipation and strain rate-dependency, a series of micro-scale mechanical tests are conducted on a coal joint specimen, coupled with microscopy and digital image correlation (DIC) analysis. The results from triaxial and micro-scale tests indicate that the asperity damage within pre-existing fractures during their closure is the primary driver of strain rate dependency at core-scale, without inducing any permanent deformation in bulk specimens.

To gain further insights into the relationship between asperity damage and energy dissipation under varying strain rates, a series of normal stress loading tests are conducted on identical synthetic joint specimens. These tests confirm a strong correlation between asperity damage and energy dissipation in the identical specimens, demonstrating that slower strain rates lead to greater asperity damage, higher energy dissipation, and reduced stiffness. These findings substantially enhance our understanding of asperity damage-driven strain rate dependency of fractured rock through the evolution of energy dissipation.