Quantitative in situ CT analysis of fatigue damage evolution in coal pillars under progressive and cyclic loading

This is the authors' abstract. We don't add key points for this paper.

Underground energy storage in coal mines requires rigorous assessment of the long-term structural stability of residual coal pillars. During operation, the coal pillars experience complex stress changes due to mining disturbances and cyclic loading. However, the quantitative link between macroscopic fatigue behavior and internal fracture evolution in mining-disturbed coal remains unclear. To simulate mining-induced loading conditions, coal pillar specimens were examined under progressive and cyclic uniaxial loading using in situ X-ray computed tomography (CT) to visualize and quantify damage evolution. The results show that damage evolution varies with loading conditions. Under progressive loading, a three-stage response occurs, with slow growth below 60% of peak stress and rapid growth near failure. Under cyclic loading, deformation is stable at 70% of uniaxial compressive strength but increases rapidly and leads to failure at 80% as fractures connect. Tomography-based indicators, such as crack porosity and fractal dimension, are sensitive to early-stage damage and the formation of connected shear networks. These results provide a basis for stability assessment and early warning of coal pillars in underground storage.