Fractal and pore structure analysis of structural anisotropic coal under different impact loads

Pore structure is the key factor that affects the adsorption/desorption of coal. In order to study the change characteristics of pore structure in structural anisotropic coal under different impact loads, the low- temperature liquid nitrogen adsorption tests were carried out after the split Hopkinson bar (SHPB) impact, then the pore structure variation and fractal dimension were calculated and analyzed. The results show that before and after impact, the adsorption isotherms change from type II (raw coal) to type III (impacted coal). With impact loads increasing, adsorption and desorption isotherms change from hysteresis loops to overlap which are determined by open pore structure and dead end pore structure. Meanwhile, mesoporous and micropores in coal samples are much more obviously damaged by larger impact load, and the adsorption capacity becomes smaller, especially for coal samples in parallel to the bedding. Pore volume mainly distribute in mesoporous and macropores, and SSA mainly distribute in mesoporous and micropores. On the whole, the pore volume and SSA decrease with the increase of impact load. Not only pore volume but also SSA in perpendicular to the bedding are larger than that in parallel to the bedding under same impact load. Fractal dimension D2 decreases with impact load increasing, which causes the adsorption capacity linearly reduces.