Mechanical properties and damage evolution of coal under cyclic loading conditions

The uniaxial and triaxial strength tests on raw coal samples under cyclic load conditions were conducted u- sing acoustic emission technology to systematically study and compare the damage evolution characteristics under sin- gle and triaxial cyclic loading conditions. During the whole process of cyclic loading,the rock internal damage process is divided into three stages,namely the initial damage stage,the micro-crack initiation and evolution stably stage,and the destruction stage. The peak strain,unloading stiffness and damage variable in the process of cyclic loading were quantitatively analyzed. The uniaxial cyclic test shows the most significant damage in the first stage and the triaxial cy- clic test also shows significant damage in the first stage,while the third stage is not significant. The upper limit of the cyclic stress shows a stronger acoustic emission phenomenon, indicating the most intense internal damage occurs around the cyclic stress upper limit. Incremental step uniaxial cyclic loading tests show the damage variable under sta- ble damage stage and the mean stress is in an exponential relationship. The damage variable value slightly elevates when approaching destruction under the triaxial cyclic loading condition,which shows a great diversity under uniaxial cyclic loading.