Uniaxial Reloading Damage Characteristics of Critically Damaged Coal Samples at Different Confining Pressure Unloading Spans

To investigate the damage characteristics of the lower critical damage coal samples under different perimeter pressure unloading spans, a 13-group plus unloading perimeter pressure and then uniaxial loading test scheme was designed. Firstly, the coal samples were subjected to triaxial lowering and raising of the circumferential pressure to determine the critical damage range of the coal samples. Finally, acoustic emission signal, fractal theory, and energy theory were used to study the mechanical characteristics of the coal samples during damage. The results show that the surface cracks of the critically damaged coal samples develop from tension cracks to coexistence of tension and shear cracks to shear cracks as the unloading span of the surrounding pressure of the critically damaged coal samples decreases; through the triaxial unloading-uniaxial reloading experimental scheme, the critical damage range of the coal samples is determined as 60-70% of the triaxial compressive strength during axial pressure loading in the process of surrounding rock stress adjustment. The smaller the unloading span of the critically damaged coal sample, the smaller the peak strength of the sample, the more severe the damage, the weaker the energy storage capacity, the weaker the ability to deform elastically, the easier it is for the specimen to crack and deform plastically, and the larger the weak surface formed in the specimen, resulting in the larger the size of the fragment produced by the damage, the smaller the fractal dimension.