Study on fracturing mechanism of blasting damage in sandstone with different initial porosities based on NMR technology

In order to investigate the effect of porosity on the damage rupture mechanism of sandstone under blasting, the pore distribution characteristics of sandstone before and after blasting and under different impact loads are qualitatively evaluated through the NMR relaxation mechanism and the relaxation signal characteristics of T₂ mapping, the macro and fine damage rupture characteristics of sandstone under blasting are quantitatively characterized, and the fine damage rupture characteristics of sandstone with different porosities are combined with the numerical simulation to explore the macro damage rupture mechanism of sandstone with different porosities in the process of blasting. The results show that: ① after blasting, the sandstone is damaged and ruptured in the macroscopic way. The results show that: after blasting, there is a more significant "convergence" phenomenon between the 1 and 2 peaks in the T2 spectrum, and the amplitude of the signal intensity of the 2 peaks is much larger than the peak signal intensity of the 1 peak; ② the amplitude of the signal intensity of the 1 and 2 peaks of sandstones with large porosities is generally high, and the difference between the internal macroscopic damage of the sandstones and the number of different porosity is even larger, showing a more serious macroscopic rupture,The sandstone shows more serious macroscopic rupture; ③ After blasting sandstone pore number of different apertures are not necessary to increase the degree of sandstone internal damage gradually from the microscopic damage dominated by macroscopic damage dominated; ④ the increase in the charge led to a gradual increase in the average and peak T₂, the average aperture and aperture difference is more pronounced, the pore distribution is gradually non-uniform, sandstone pore connectivity is better, after the explosion of the sandstone damage to the degree of fragmentation more and more intense, the bursting fissure Generation is the result of internal pores in the blasting effect of mutual penetration; ⑤ Sandstone with large porosity in the blasting process of its shock wave propagation and attenuation speed is faster, the shock wave pressure peak is larger, in the pore space and sandstone skeleton propagation alternately more frequent, resulting in more tensile and reflective waves, so that the initial pore throat structure is more likely to reach the load-bearing limit, and promotes the pore space connected to each other through the pore space, resulting in sandstone blasting macroscopic rupture effect is more pronounced.