Microstructure And Time Dependent Fracture Of Hard Brittle Rock Based On Distinct Element Method

Hard brittle rock commonly found in deep underground engineering presents strong time-dependent behavior of deformation and fracture under high stress condition.The time-dependent fracture behavior of rock will cause the failure of underground engineering in a period of time after excavation and unloading and its later operation stage,and even hysteresis rockburst phenomenon under high in-situ stress condition.Different from metal materials,the time-dependent fracture behavior of hard brittle rock is not caused by dislocation plug in crystal,but is closely related to its mineral composition,micro-damage and microstructure characteristics that exhibit specific brittle behavior,so it is also called brittle creep.Mastering the microstructure of rock is the basis of reproducing its macroscopic mechanical characteristics in numerical simulation,which is also the focus and hotspot of international scholars in the field of computational rock mechanics.Based on the particle distinct element method,this paper studies the influence of mineral composition,initial microcrack and mineral grain structure characteristics on the macroscopic mechanical properties of hard brittle rock,as well as the time-dependent behavior of fracture.The main results are as follows:(1)Aiming at the interface contact problem of mineral components in hard brittle rocks,the elastic theoretical relations between different mineral grains are derived,and the heterogeneity of numerical models of multi-mineral components in rocks is studied.The effects of grain size heterogeneity on macroscopic physical and mechanical properties of rock are studied by establishing isodiametric grain model and anisometric grain model.Based on the established models of various mineral composition,ternary contours of the main mineral composition are used to analyze the influence of mineral composition heterogeneity on the macro physical and mechanical properties of rocks.The above results indicate that the numerical model of mineral composition can reflect geometric heterogeneity and material heterogeneity in rocks to a certain extent.(2)Aiming at the phenomenon of crack closure in the initial loading section of hard brittle rock,an initial microcrack characterization model is established and the calculation formula of crack opening gap is improved.The numerical results show that the numerical sample based on the initial microcrack model can well fit the crack closure segment and the transverse expansion behavior of the sample,and the poisson’s ratio obtained by the simulation is very close to the physical test value.A series of numerical simulation experiments are carried out based on the multi-mineral composition numerical sample with the initial microcrack.The results show that the microstructure of rock has a significant effect on the progressive damage process.(3)A grain texture model(GTM)is proposed for anisotropy induced by grain aspect ratio and grain orientation in hard brittle rocks.On this basis,the effects of grain aspect ratio and grain dip angle on the macroscopic mechanical properties of rock are studied.By analyzing the proportion of mineral possession and contact type,it shows that the increase of grain aspect ratio increases the proportion of grain boundary contact,while the change of dip angle has little influence on the proportion of mineral possession and contact type but has significant influence on the number of grain boundary tensile cracks and the ratio of uniaxial compression strength and tensile strength.Therefore,grain anisotropic texture has a significant control effect on the macroscopic mechanical behavior of rock.(4)Based on the stress corrosion theory,the concept of "bonding surface" is introduced,and by defining the damage of bonding surface,the equivalent strength degradation model and the corresponding strength iteration theoretical relation are derived.The equivalent strength degradation model and flat joint contact model are combined to simulate creep of homogeneous material model.The creep time curve of the equivalent strength degradation model shows a strong stable creep characteristic,and the shear crack and tensile crack increase in equal proportion during the creep process.The relation between the creep time and stress ratio is close to that obtained by the parallel bond stress corrosion model.(5)A viscous joint contact model is proposed based on the initial microcrack model and rate process theory,and corresponding model is developed by VC++.The viscous joint contact model is applied to the creep simulation of Ld B granite.The creep time curve has the characteristics of creep three stages,and it is found that tensile cracks mainly occur in creep process.In addition,the creep time under different stress ratios is more consistent with the measured data of physical tests than the simulated results of the equivalent strength degradation model.(6)A GTM model with initial microcracks is established,which considered mineral composition,initial micro-cracks and crystal structure,and uniaxial compression and direct tensile tests are carried out.Compared with Ld B granite physical test data,the GTM model with initial microcracks can fully describe the macroscopic mechanical properties of hard brittle rock(the ratio of uniaxial compression strength and tensile strength,characteristic stresses,elastic bimodularity,etc.).By adding the equivalent strength degradation model and viscous joint model to the GTM model with initial micro-cracks,creep simulation tests are carried out.It is found that the creep curves have obvious characteristics of three stages,and the failure mode is mainly transgranular failure.The simulated creep results are close to the physical test data,and the fitting curves show two different characteristics,which indicates that two creep mechanisms(intragrain creep and grain boundary creep)play a dominant role at various stress levels respectively.