Influence of Rock Strength on the Mechanical Characteristics and Energy Evolution Law of Gypsum–Rock Combination Specimen under Cyclic Loading–Unloading Condition

Abstract

The stability of coal–rock composite structures is essential for the safety of coal mine production. To explore the failure characteristics and energy evolution law of coal–rock composite structures, uniaxial cyclic loading–unloading tests of gypsum–rock combination specimens are conducted. Test results show that the peak stress and elastic modulus of combination specimens increase linearly with increasing the rock strength, as well as the failure intensity. When the rock strength increases, the energy accumulation rate of a combination specimen decreases, but the total accumulated energy increases before the prepeak stress. The energy evolution processes of rock component and gypsum component are similar to the combination specimen. During the loading process, the energy stored in the rock is low due to the small deformation, but the energy stored in the gypsum is high due to the large deformation, which is almost equal to the combination specimen. Finally, the energy-driven failure mechanism of the coal–rock composite structure is revealed as, in the failure process of the composite structure, the coal will fail first and release elastic strain energy, accompanied with the release of the elastic strain energy of the rock. The energy released by the rock will compensate the coal, which can intensify the coal failure.