Detecting impact damage and characterizing pore structure in fractured rocks using nuclear magnetic resonance technology

To investigate the pore structure evolution and fractal characteristics of fractured sandstones under cyclic impact loading, this study applied up to six cyclic impacts to red sandstones with different fracture inclinations (0 degrees, 45 degrees, 90 degrees) using a split Hopkinson pressure bar (SHPB). Nuclear magnetic resonance (NMR) was used to obtain pore structure parameters after each impact. Results showed significant porosity increase, particularly in macropores, with the Rs-90 degrees group increasing by 194.6%, and the increase in total porosity increased with the increase in fracture inclination, in which the Rs-90 degrees, Rs-45 degrees, and Rs-0 degrees groups were improved by 37.71%, 28.06%, and 24.32%, respectively. In addition, the significant increase of MF pores indicated that the pore connectivity was enhanced, and the T2 cutoff value decreased by 42.54% to 82.11% after 6 impacts, and the decreasing degree is positively correlated with the fracture inclination angle. In addition, the fractal dimension D decreases as a power function of the number of impacts, indicating that the pore structure complexity decreases, and the increase of the fracture inclination enhances the decrease of D value after cyclic impacts. Moreover, magnetic resonance imaging (MRI) showed that macroscopic fissures were formed in the 90 degrees group after four impacts, and the rightward shift of the pore gray scale distribution confirmed the macroporous extension. Finally, the correlation between porosity and fractal dimension was modeled for each type of porosity based on pore size gradation and connectivity. The findings provide insights for understanding the progressive damage mechanism of fractured rocks under cyclic impacts.