Investigation on crack recovery effect of coal-rock combined body under the influence of unloading

The test machine MTS 815 was applied to carry out cyclic loading-unloading experiments for coal-rock com- bined body. The axial crack recovery effect was analyzed and the axial crack closure stress (strain) and recovery stress (strain) were obtained. The results show that the axial crack closure stress (strain) and axial crack closure recovery stress (strain) increase with increasing cyclic times. Due to the increase of stress level,new cracks are generated,and the axial crack closure strain difference,axial crack recovery strain difference and axial crack stress difference increase with increasing cyclic times. Based on natural strain and engineering strain,the axial crack recovery model is proposed and verified by experimental data. The good agreement between theoretical data and test data shows the model is rea- sonable. Also,the proposed model can describe the axial crack recovery effect during unloading process well.