Experimental study on fatigue and damage characteristics of coal-rock combined mass subjected to cyclic loading

In order to explore the fatigue and damage characteristics of combined coal and rock mass under cyclic loading, fatigue failure tests under the combined action of high static stress and small-amplitude cyclic loading were carried out based on the RMT-150C test system. The fatigue and damage characteristics of rock-coal-rock combined coal and rock mass specimens under different stress amplitudes, and the variation laws of fatigue strength, hysteresis loop characteristics, axial strain, irreversible deformation with the number of cycles were studied to reveal the fatigue damage law of combined coal and rock mass and analyze the main control factors of fatigue failure. The results show that the fatigue failure of combined coal and rock mass is strictly controlled by the stress-strain under static loading, and the deformation required for fatigue failure should be approximately equal to the deformation of the post-peak part of the uniaxial compression stress-strain curve corresponding to the upper limit stress. The fatigue life of combined coal and rock mass decreases exponentially with the increase of the number of cycles, and the threshold value of the fatigue upper limit stress ratio of combined coal and rock mass is about 0.8, and the stress threshold value is about 22.5 MPa. During the fatigue test, the variation laws of axial strain, axial irreversible strain and damage degree of combined coal and rock mass with the number of cycles are basically similar. Specifically, for specimens that have not failed during the cycle process, there are two stages: initial acceleration and middle and late stage constant speed. For specimens that have failed during the cycle process, there are three stages: initial acceleration, middle stage constant speed and late stage acceleration. The disturbance amplitude has a significant influence on the fatigue mechanical properties of combined coal and rock mass. Increasing the amplitude will significantly increase the growth rate of axial strain, axial irreversible strain and damage degree in the middle stage constant speed, accelerate the damage and deterioration of coal and rock mass, and greatly shorten its fatigue life.