Abstract: An RLW-2000 micro-computer servo-control rock triaxial testing machine was used to carry out triaxial cyclic loading and unloading experiments of saturated and natural shales under different confining pressures. The mechanical properties and hysteresis effects of two hydrated shales were analyzed. The evolution law of the damping ratio was discussed based on the energy principle. The experimental results show that as the cycle number increases, the cumulative residual strain increases gradually. The relative residual strain decreases first and then tends to a stable region, and finally increases rapidly prior to the rock damage. The deformation modulus of the saturated shale is smaller than that of the natural shale during the loading and unloading process. Besides, the loading deformation modulus is smaller than that of the unloading deformation modulus. In the loading and unloading stages, it is observed that the strain always lags the stress. The hysteresis effect of saturated shale is more obvious than that of natural shale. An energy calculation method is proposed considering the hysteresis effect with a smaller error of the calculated energy compared with that of the previous energy calculation. Finally, the calculation formula of the damping ratio is revised based on the energy principle. It is found that the damping ratio of saturated shale is larger than that of natural shale. The change of damping ratio reflects the shale damage mechanism, which can be used as an important indicator to predict the instability of shale.

Key words: shale, mechanical properties, hysteresis effect, energy, damping ratio