Hybrid Finite-discrete Element Model For Simulation Fracture In Rocks Under Dynamic Loading

There are two kinds of rock fractures.One kind can benefit the engineering application in terms of demolition,tunneling and mining.Another kind can cause accidents,disaster as fractures in the constructions make them unstable and collapse.Thus it is essential to study the rock fracture processes.Rock is usually considered as continuum.Thus the continuous numerical methods are used to study the rock fracture processes.However,there are lots of joints and existing fractures in rocks.Therefore,the rock is considered as discontinuum.As a consequence,many discontinuous methods are used to simulate the rock fracture processes,which includes deformation of rocks with stress increasing,then fracture and fragmentation.The finite element method,one of continuous method,can simulate the deformation and damage of the rocks while the discrete element method,one of the discontinuous method,can model the fracture and fragmentation processes.However,both methods have limitations to model the entire process of rock fracture and fragmentation.Thus,in order to simulate the entire process,this thesis focuses on study the hybrid finite element method and discrete element method.The main contents are as follows:(1)Establishing the hybrid finite-discrete model.Firstly,the model is considered as discontinuum and the discrete element method is used to simulate the contact detection and interaction of the discrete bodies.Then each discrete body is considered as continuum and the finite element method is used to simulate the strain and stress of the discrete body.In order to simulate the rock fracture process,cohesive element is proposed and inert into the finite elements.Thus the finite element method and discrete element method are combined together to simulate the entire failure process of rock.(2)The rock fracture modes.There are three kinds of fracture modes,mode?,mode ? and mixed mode ?-?.In this thesis,the modelling of fracture is through the deformation and deleting of the cohesive elements,which depends on the rock strength and the energy release rate.The rock strength is used control the start of the rock fracture while the energy release rate is used to indicate that the completion of the fracture.Thus the hybrid finite-discrete model can be used to model the rock fracture processes.(3)Rock fracture processes under static loads,i.e.the calibration of the proposed method.Firstly,the Brazilian disc tensile test and the uniaxial compressive test were conducted.Then the same tests are simulated by the proposed method to calibrate the hybrid model.The fracture pattern,rock strength stress-time curves and stress distribution are compared with the test results to calibrate the proposed method.Finally,the pure mode ?,pure mode ? and mixed mode ?-?fracture are simulated.The modelled fracture toughness are compared with input values.All the tests above indicated that the proposed method can model the entire rock fracture process and the modelled rock strength,fracture toughness and the stress distribution are almost the same as the experimental test.(4)The study of the dynamic constitutive law.The rock fracture under static and dynamic loads are different.On one side,the stress-strain curves under static and dynamic loads are not the same.On another side,the strength and the fracture toughness increases with the loading rates.In this paper,the Hopkinson bar is used to do dynamic rock fracture test.The stress-strain curves of rock under dynamic loads are obtained.The rock dynamic constitutive law was obtained by fitting with the stress-strain curves of rock under dynamic loads.Then the dynamic constitutive law is implemented into the mixed method and the.Then the increase of the dynamic rock fracture toughness with loading rate were analyzed and the criterion for dynamic rock fracture is established and implemented into the hybrid model.Then the hybrid model is used to simulate the dynamic rock fracture process.The model result are almost the same with those in literatures in terms of dynamic characters of rock and the trend of strength increasing.(5)The characters of cohesive element in the proposed method is modified and the gas are allowed to flow into the cohesive element.The interaction of gas and the cohesive element is proposed to follow the rules of gas in the rock fractures.Then the proposed method is used to model the rock fracture process by blast with one borehole and one free surface.The stress propagation process and the reflection are modelled.The rock fracture processes are obtained and the radius of the crack zone almost the same as that obtained by empirical formula.Thus the proposed method is able to simulate the rock fracture processes by blast.After that,the simultaneous and consecutive blast are modeled to demonstrate the interaction of two boreholes.The rock fracture and fragmentation process in sublevel caving by blast is modelled to demonstrate the potential application of the proposed method.