Abstract: In this paper, MTS 815 test system has been used to study the failure behavior of coal-rock combination bodies under different stress level cyclic loading-unloading. Experimental indicates that brittle failure is also the failure mechanism of coal-rock combination body under cyclic loading-unloading, but the failure under cyclic loading-unloading is more fragmentized than that under uniaxial loading. The failure strength under cyclic loading-unloading is higher than that under uniaxial loading; but the axial and circumferential strain in the contrast. The loading curves of coal-rock combination body are usually not coincidence with the unloading curves. In addition, they usually do not form a closed loop. In the first cycle, the residual deformation is large, and the loading-unloading curve forms a horn-shaped curve. The relationships of the residual deformations and the loading-unloading elastic modulus with the advancing of cyclic loading-unloading have been obtained in our tests, respectively. At the reverse position of unloading and loading, when unloading to about 1 MPa, the unloading line is fluctuant; however loading again, loading line is almost straight line in the early. In the all of cycles, some cycle loading lines are nearly coincidence with the unloading curves, some form sharp edges, or some forms cross lines. With the increasing of the loading, the loading curves separate gradually with the unloading curves. With the advancing of loading and unloading, strain phase gradually lags stress phase. The internal factors are the viscosity of rock minerals and the friction between the interface between coal and rock, and the external factors are the irreversible residual deformation under cyclic loading and unloading.

Key words: coal-rock combination, different level loading-unloading, residual deformation, energy dissipation, elastic modulus of loading and unloading