Macro And Micro Analysis Of Rock Fracture Behavior And Crack Evolution Based On Fine Discrete Element Simulation

It is of great scientific significance and engineering application value to fully consider all aspects of rock mass properties,such as inherent microcracks and tensile properties when the fracture mechanism and crack propagation law of rock mass are analysed numerically.In this paper,based on laboratory tests,a fine discrete element numerical model that can fully reflect the mechanical characteristics of granite is established.Using this model,the fracture properties of granite and the crack propagation mechanism are analyzed and discussed from macro and micro perspectives.The main contents and achievements are as follows:(1)A series of laboratory tests were carried out on the granite specimens to analyze the failure characteristics,and to obtain as many macro parameters as possible to characterize the mechanical properties.Semi-circular bend tests on small-sized grooved specimens were carried out,and the apparent fracture toughness was calculated from the test results.By analyzing the evolution of the global strain field,the length of the fracture process zones was calculated from the strain concentration.(2)Based on the principle of flat joint model,combined with the results of laboratory tests,a discrete element method for finely simulating the properties of granite is proposed.Utilizing the established flat joint model,several numerical tests were conducted.Some macro mechanical characteristics of rocks are explained and analyzed in depth.The results show that not only the macroscopic mechanical parameters are consistent with the parameters obtained from the laboratory tests,but also the microcrack closure characteristics,the elastic stage characteristics,the triaxial shear characteristics,the elastic bimodularity and the crack evolutions under different stress paths are reproduced.The corresponding relationship between macroscopic parameters and microscopic damage are also revealed.With the help of numerical tests,the quantitative analysis of the contact properties between the grains in the rock,which affects the Hoek-Brown parameter,reveals the meso physical meaning of the parameter.For the semi-circular bend test,the formation mechanism of the fracture process zone is explained from the perspective of particle displacement field and energy diffusion.A method for estimating the length of the fracture process zone based on the distribution characteristics of microcracks in the numerical test is proposed.The fracture toughness considering the fracture process zone is calculated.The exponential law of rock fracture toughness changing with the specimen size is summarized,and the real fracture toughness that is not affected by the specimen size is obtained.(3)The established model was used to simulate the compression of prefabricated flawed rock,and the process from the initiation to the formation of macrocracks was analyzed from the perspectives of strain field,stress-strain curve,strength loss,microcrack distribution,stress field,and energy release.For prefabricated singleflawed granite specimen,the macro and mesoscopic effects of flaw inclination on crack propagation are mainly analyzed.It is found that the initiation and propagation of cracks are jointly affected by the geometry of prefabricated flaw and the inherent microcracks.Which one is the primary depends on the flaw inclination.With the increase of the prefabricated flaw inclination angle,the influence of prefabricated flaw on the initiation and propagation of microcracks is gradually weakened,which is getting weaker than that of the inherent microcracks in the specimen.As a result,the stress-strain curves gradually approach those of the intact specimen.The peak strength,the microcrack initiation stress and so on all increase accordingly.The longer the prefabricated flaw length,the greater the influence of the inclination angle on the strength.For prefabricated double-flawed granite specimens,the macro and micro mechanisms of crack initiation and coalescence when the bridge angle changes are mainly discussed.The microcrack initiation is dominated by wing mirocracks perpendicular to the direction of the prefabricated flaw,but the coalescence paths of the prefabricated flaws are mainly directed to the principal stress direction.Through the in-depth analysis of the stress field distribution characteristics of the double falwed specimen,the intrinsic driving force for the initiation and coalescence of microcracks near the prefabricated flaw and the influence of the bridge angle are revealed.(4)Based on the fact that the contact interface can be partially broken in the failure of the flat joint contact and the inherent microcracks exist in the specimen,the preconditions for the opening or closing of the flow channel in the hydraulic coupling algorithm are reconsidered,so that the coupling action between the Flat joint model particles and the fluid are realized.By analyzing the fracturing results without axial stress,the superiority of the proposed hydraulic coupling method is verified from three aspects: fracturing path,fracturing hydraulic pressure and tensile-shear mechanism.Numerical tests were carried out using the proposed coupling algorithm.The fracture propagation law of fracturing was analyzed from different perspectives.The internal driving mechanism of fracturing is revealed by the evolution of displacement field and force chain.The fracturing process is described by the evolution of strain field and microcrack distribution.The crack morphology of fracturing is discussed in terms of the initiation and the ultimate distribution of microcracks.Finally,according to the numerical test results,a prediction of fracturing pressure is proposed,which is compared with the calculated results of the fracture theory.