Study on Fatigue Life Prediction and Acoustic Emission Characteristics of Sandstone Based on Mesoscopic Crack Propagation Mechanism

Even when the maximum stress is less than the peak stress under conventional loading, fatigue failure of rock is likely to occur, thereby showing its unique characteristics. The present study summarized the factors affecting rock fatigue life from the perspective of phenomenology and studied the fatigue damage process of rock from the microscopic perspective. However, the meso-mechanical mechanism of fatigue–tension failure of rocks is still not very clear, and there are few studies on rock fatigue life that use meso-crack propagation models. In this paper, a mesoscopic model considering wing crack propagation is introduced to examine the fatigue failure of sandstone. A fatigue life prediction formula of sandstone was deduced via a combination with the Paris formula. This formula can quantitatively characterize the impact of upper limit stress and lower limit stress on the fatigue life of sandstone and explain the reason why upper limit stress has a greater influence on the fatigue process of sandstone. Such a prediction formula is applicable only under the condition of low confining pressures, which mainly cause tensile failure due to mesoscopic wing crack propagation. Acoustic emission signals during fatigue failure were monitored and then analyzed using a clustering method and a moment tensor inversion method. Therefore, the tensile or shear properties of mesoscopic failure could be distinguished according to acoustic emission characteristics in different stages of fatigue crack propagation. The results showed that crack sources causing sandstone fatigue failure are mainly tension-type when confining pressure is less than 10 MPa, which further verifies the proposed prediction model of sandstone fatigue life under low confining pressures.