Experimental Study On Mechanical Properties And Energy Evolution Of Sandstone Under True Triaxial Loading And Unloading Conditions

The stability of surrounding rock of underground engineering has always been the frontier and hotspot of rock mechanics research.Rock mechanical properties are the basis for the study of surrounding rock stability problems in underground engineering.In this paper,we performed an experimental study on the mechanical properties and energy evolution of sandstone under true triaxial loading and unloading conditions by use of the multi-functional true triaxial fluid-solid coupling test system.The main research results are as follows:?1?The maximum principal stress is loaded in displacement,and unload one-side platen in the direction of the minimum principal stress.During the loading and unloading process,the maximum principal strain and the deviator strain gradually increase,and the intermediate principal strain,the minimum principal strain and the volumetric strain gradually decrease.At the beginning of loading and unloading,the sample is compressed and then expanded.The deformation modulus of the sample first increases and then decreases,the Poisson's ratio remains stable first,then gradually increases,and finally tends to level.When the sample fails,the maximum principal strain,the minimum principal strain and the deviator strain increment increase with the increase of the initial intermediate principal stress coefficient b,while the intermediate principal strain and the volumetric strain increment decrease.Tension rupture is becoming more and more obvious.The peak stress of the sample increases linearly with the increase of the initial minimum principal stress?₃.When the sample fails,the intermediate principal strain,the minimum principal strain and the volumetric strain increment decrease,while the maximum principal strain and the deviator strain increment increase.Shear cracks are more obvious.As the initial shear stress q increases.When the sample fails,the intermediate principal strain and volumetric strain increment decrease,the maximum principal strain,the minimum principal strain and the deviator strain increment decrease first and then increases.The rock mass formed is more broken.?2?When the initial value b increases,the total energy U absorbed by the sample at the peak stress point increases slightly,the energy storage limit U^e_max decreased slightly,the cumulative dissipated energy U^d increases obviously before the peak,and the cracks develop more.The dissipative energy ratio of U^d/U also increases with value b increasing.U,U^e_max and U^d at the peak stress point of the sample show a good linear relationship with the initial?₃.As the initial value q increases,U,U^d and U^d/U decrease first and then increase,and U^e_max increases slightly.?3?As the loading rate increases,the peak stress of rock sample increases obviously,and the corresponding maximum principal strain,deviator strain and intermediate principal strain increment also increased,the minimum principal strain increment decreases,and the volume expansion point gradually lags.The fracture pattern of rock sample changes from tension-shear to shear,and the non-penetrating crack appears.When the unloading rate increases,the peak stress of the sample decreases,the corresponding maximum principal strain,deviator strain and intermediate principal strain increment decreased,the minimum principal strain increment increases,and the volume expansion point advances.The fracture pattern of rock sample changes from shear to tensile.?4?The higher the loading rates,the greater the U,U^e_max,and U^d of the sample at the peak stress,and the corresponding U^d/U also increased.The higher the unloading rates,the U,U^e_max and U^d of the sample at the peak stress gradually decrease,and the corresponding U^d/U also decreases.The strain energy conversion rate u before the peak stress of the sample is linearly related to loading and unloading rates.?5?Both the octahedral shear stress strength criterion and the double shear stress strength criterion have good applicability to the strength data of this paper.