Study On Rock Failure Mechanism Based On Influence Of Hydrostatic Stress On Lode Angle Effect

The strength criterion of rock plays an important role in the prevention and control of underground engineering and geological disasters.The strength of the rock varies with its composition,stress state and other factors.In this thesis,we use homogeneous sandstone as the research object,and comprehensively use experimental research,theoretical analysis and numerical simulation to analyze and study various effects of rock,and to reveal the essential mechanism of these various effects during rock failure process.The self-developed multi-functional true triaxial fluid-solid coupling system is used to study the intermediate principal stress effect,hydrostatic stress effect and Lode angle effect of homogeneous sandstone.According to the experimental results,the mechanism of various effects of rock is proposed.Based on these mechanism,a rock composite failure model is proposed.This model can be explaine the mechanics of influence of hydrostatic stress on the Lode effect.According to the characteristics of the rock composite damage model,a rock non-associated flow is proposed.Based on the influence of hydrostatic stress on Lode angle effect,a rock strength criterion is proposed.At the same time,this composite failure model is experimentally and numerically verified by the acoustic emission phenomenon generated during the rock failure process and the discrete element simulation method.The research results of this thesis are as follows:(1)As a result of the combined action of hydrostatic stress and Lode angle effect,the intermediate principal stress effect has a significant influence on the failure process of rock.The ratio of the deviatoric stress to the volume stress at the peak stress of rock decreases with the increase of the intermediate principal stress.As the intermediate principal stress increases,the dominant shear stress along the fracture surface increases,and the internal fracture surface becomes smooth and clear.The dominant shear stress and the plasticity of the sandstone may contribute to this phenomenon.In the zone without dominant shear stress,a large number of discrete cracks are generated inside the rock.These cracks consume some energy during rock loading process,which increase the rock brittleness.The dominant shear stress induced the increase of rock brittleness with the increase of the intermediate principal stress.(2)The hydrostatic stress affacte the characteristic of rock strain significantly.Under a relatively high hydrostatic stress conditions,the rock exhibits rheological properties before the peak strength,and the volume strain of rock is continuously compressed as the maximum principal strain increases.With the increase of hydrostatic stress,the sandstone Lode angle effect is gradually reduced,the plastic deformation stage of rock increases significantly,and the expansion effect of rock failure gradually disappears.The strength envelope of the rock on the π plane gradually changes to a circle.It can be seen that the hydrostatic stress has a significant effect on the Lode angle effect.(3)Based on the experimental results,this thesis proposes a new strength criterion.This criterion can be applied to investigate the hydrostatic stress effect of rock,the Lode angle effect and the influence of hydrostatic stress on the Lode angle effect.In order to verify the strength criterion,the previous true triaxial experimental data are used.This criterion has a good fitting result for these experimental data.The fitting results also show that the Lode angle effect of different rock materials varies with the rock materials and stress states.(4)Based on the rock properties of particle and friction,a composite failure model which reveals the mechanism of influence of hydrostatic stress on Lode angle effects and its flow rule are proposed.The model explains that the dilatancy effect on the sliding surface of the rock leads to the Lode effect.At the same time,the hydrostatic stress state directly affects the proportion of collapse of the cement skeleton and rock particle damage inside the rock.The collapse and damage proportion can affect the friction coefficient and friction angle,which changes the dilatancy effect of the rock.Thus this will affect the Lode angle effect of rock.A non-associative flow rule is proposed based on the composite failure model.The flow law contains three plastic potential faces and introduces a parameter β that reflects the dilatancy effect during rock loading.During the rock loading process,the interior rock can be divided into a steady state and an unstable state according to the stress state of the fracture surface.For unstable micro-elementes,the strain energy may release during loading,which is the reason why the Drucker postulation is not suite for rock.(5)Acoustic emission data and discrete element numerical(DEM)simulation are used to verify the composite failure model.Based on the molecular dynamics simulation result,the acoustic emission signal characteristics of the tensile crack were obtained.This feature can be used to distinguish the type of fracture generated by rock.The results of acoustic emission experiments show that there is obvious intermittency in the process of rock failure.It can be seen that the failure process of rock is a combined process of internal damage and stress transfer after stress concentration,which further validates the rock composite failure model proposed in this thesis.The composite failure model was verified by whether the internal particles of the rock are breakable with the discrete element method.By comparing the simulation results of the particle model and the particle cluster model(cluster is breakable),the simulation results of the particle cluster model is found to be closer to the experimental results.Under different hydrostatic stress conditions,the fracture of the model particle clusters has certain differences,and this difference leads to the hydrostatic stress effect of the rock.At the same time,the deflection of the bias strain path on the π plane during the reconstruction of failure model also changes,thus changing the Lode effect of rock.