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 andtriaxial compression were carried out. Advanced digital volume correlation based on finite element discretizationwas utilized to quantify the three-dimensional initial and newborn fractures. With this technique, the lowcontrast in the coal images was compensated for by mechanical regularization, and the fracture activities werequantified 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 damagegrowth 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 andpropagation of the two fracture types. This study aimed to gain in-depth insights into the bulk fracture of coaland provide quantitative evidence for further understanding the damage mechanisms from the microscale to themacroscale