Mechanical properties and failure mechanism of mudstone subjected to cyclic loads with variable loading limit

Elucidating the damage evolution characteristics of roof mudstone under cyclic loading with different loading amplitudes is of great significance for the prevention and control of roof disasters and the safe mining of coal resources. Therefore, multi-stage cyclic loading experiments of mudstone with variable amplitude were carried out. First, the deformation characteristics of mudstone were analyzed. Second, the relationship between acoustic emission parameters and multi-stage cyclic loading was elucidated. Finally, the damage mechanism of mudstone under multi-stage cyclic loading is discussed by analyzing the variation trend of cumulative dissipated energy and cumulative acoustic emission count, supplemented by three-dimensional scanning results. The results show that with the increase in cyclic loading and unloading times, the elastic modulus and cumulative irreversible strain of mudstone increase, and the increase in both of them is relatively large when the amplitude is low. The acoustic emission signal fluctuates greatly in the compression and damage stages and shows a steady increase during the elastic stage. In addition, by analyzing the variation in the combination of activity frequency and rise time to amplitude, it is observed that the mudstone specimen damage follows a shear-dominated tensile–shear combination failure mode, and the higher the amplitude, the greater the proportion of tensile damage. In addition, computed tomography scanning reveals from the microscopic point of view that with the increase in stress amplitude, the degree of mudstone fragmentation is weakened, the fragmentation speed is accelerated, obviously.