Damage evolution in coal under different loading modes using advanced digital volume correlation based on X-ray computed tomography

To investigate the damage development of coal under different loading conditions, in-situ tests in uniaxial and triaxial compression were carried out. Advanced digital volume correlation based on finite element discretization was utilized to quantify the three-dimensional initial and newborn fractures. With this technique, the low contrast in the coal images was compensated for by mechanical regularization, and the fracture activities were quantified via a damage variable and mesh refinement scheme, including fracture opening/closure displacements, volume fractions of damaged elements, and fractal dimensions. The experiments revealed that the damage growth in coal was substantially affected by randomly distributed initial defects. Prior to a macroscopic failure, the coal samples generally experienced pre-existing fracture closure, newborn fracture initiation, interaction and propagation of the two fracture types. This study aimed to gain in-depth insights into the bulk fracture of coal and provide quantitative evidence for further understanding the damage mechanisms from the microscale to the macroscale.