Experimental Study on Fatigue Damage of Sandstone Based on NMR and Acoustic Emission

In order to evaluate the long-term stability and safety of engineering rock mass scientifically, cyclic loading and unloading test was carried out on saturated sandstone. Acoustic emission technology and nuclear magnetic resonance analysis technology were used to explore the macroscopic and microscopic deformation and fatigue damage evolution law. The conclusions are as follows:under cyclic loading, the distribution law of sandstone's acoustic emission energy was consistent with the development law of axial cumulative residual deformation and the variation law of the stress-strain curve hysteresis loop. They all reflected the rock' gradual deterioration to damage under repeated stress. During rock fatigue damage, the power law characteristics of acoustic emission energy had scale invariance in time series, and the medium-term power index could effectively represent the energy distribution of the whole process. According to the result of T₂ spectrum, the ratio of the fine fracture to the larger fracture volume was defined as K₂. During the cycle, K₂ was always increasing. When K₂ dropped sharply, the sandstone showed symptoms of destruction; the AE and NMR characteristics of the specimen could reflect the cracks in the internal pores of the rock, and the results are consistent, which indicates rock damage is the overall fatigue damage caused by local deformation accumulation.