Effect of loading and unloading rates on mechanical properties and energy characteristics of sandstone under true triaxial stress

In order to understand exactly the effect of loading and unloading rates on the mechanical properties and en- ergy characteristics of rock under true triaxial stress conditions,a self-developed true triaxial fluid-solid coupling ex- periment system was applied to investigate the mechanical properties of sandstone under different true triaxial loading- unloading stress paths. During the tests one-side platen was unloaded in the direction of the minimum principal stress to simulate the actual stress evolution of surrounding rock. The results indicate that with the increase of the unloading rates,the maximum principal stress,the maximum principal strain,the minimum principal strain and the volume strain of the sandstone at peak strength are all decreased,the intermediate principal strain is increased,and the volume ex- pansion point is advanced. The failure mode of rock sample gradually changes from shear to tensile,and the tensile cracks are mostly concentrated near the unloading surface. As the loading rate increases, the maximum principal stress,the maximum principal strain,the minimum principal strain and the volume strain increase when the sandstone fails. The volume expansion point gradually lags,and the fracture pattern of rock sample changes from tension-shear to shear,and the non-penetrating crack appears. By introducing a description variable presented as strain-deviatoric stress compliance,the deformation laws of sandstone specimens under the different loading and unloading rates were ana- lyzed. The deviatoric stress sensitivity of the minimum principal strain and the volume strain is positively correlated with the unloading rates. The deviatoric stress sensitivity of the maximum principal strain is positively correlated with the loading rates. Additionally,the energy evolution of rock before peak stress can be divided into obvious different sta- ges,and the energy absorbed before the peak is mostly stored in the form of the elastic strain energy,and the dissipated energy exceeds the elastic strain energy after the peak. The dissipated energy ratio Ud / U tends to increase first,then decrease,and finally increase again with the increase of the maximum principal strain. The rates of Ud / U at peak strength decrease with the increase of unloading rates and increase with the increase of loading rates. When approac- hing the peak stress,the absorbed total energy U,the elastic strain energy Ue ,the dissipated energy Ud and the corre- sponding ratios of strain energy increments to time intervals decrease with the increase of unloading rates and increase with the increase of loading rates.