A Numerical Simulation Method For Time-dependent Crack Growth In Multi-scale Flawed Rockmass By FLAC

A large number of rock engineering projects show that rock mass instability is a clear tim-dependent process and is closely related to the crack propagation in the rock.So a new simulation method of time-dependent crack growth in rocks is proposed which rely on the National Natural Science Foundation(NO.41172243): rheological properties and aging mechanism of unloading and hardening rock under high residual stress of my tutor.The core idea of this method is that the micro-crack propagation leads to the unit failure,and then the units connected to each other to form a macroscopic fracture surface.Three kinds of microcrack expansion modes are summarized,and the programming of models are realized by FLAC secondary development interface.Crack expansion of complete rock under uniaxial compression,biaxial compression and uniaxial tension loads,sigle-flawed rock mass under uniaxial and biaxial compression and double-flawed rock mass under uniaxial compression are studied.The main work and research conclusions are as follows:(1)According to the law of crack propagation in rock and classical stress intensity factor calculation model,three kinds of microcrack expansion modes(constant crack propagation mode,linear wing crack propagation mode and curved wing crack propagation mode)are summarized.A time-denpendent micro-crack growth model is established combining with the maximum tensile stress criterion,subcritical crack expansion and other theories.Using VS2008 achieve the program of the model,and generate.DLL files.Numerical experiments are achieved by calling the.DLL files in the FLAC configuration.(2)The macro-crack expansion modes in the complete rock of the three types of micro-cracks are quite different.Under the uniaxial compression,the macro-crack in the constant crack propagation mode are wing cracks or secondary cracks which is relied on the distubulation of micro-crack orientation.The macro-crack in the wing crack propagation modes are wing cracks.Under the biaxial compression,the three expansion modes are shown as shear failure,but the actual expansion path is different.Under the uniaxial tensile,the direction of the macro-crack is orthogonal to the direction of the load,but the actual expansion path is different too.The results of the numerical experiments are great agreement with the results of the model tests.These proved that the numerical method is feasible.By comparing the crack propagation in three kinds of micro-crack expansion modes and model tests,we can make conclusions that two kinds of wing crack propagation modes are applied to the specimens under compressive load,and the constant crack propagation mode are applied to the specimens under tensile loads.In addition,the numerical results show that the different distributions of microcracks have different effects on different expansion modes.(3)Based on the numerical modeling of uniaxial and biaxial compression tests,the effect of macroscopic fissure inclination on the bearing capacity of single-flawed rock specimens is revealed.The rock specimens with higher macroscopic flaw angle can bear larger external loads.the concrete manifestation is that the crack initiation and expansion time of specimens with higher macroscopic flaw angle is later than lower one.The paths of macroscopic crcak are different by different micro-crack expansion modes.The macroscopic fractures of the rock specimen in the fixed-direction expansion mode are expanded in the form of "X" shear crack under uniaxial compression,while the two types of wing crack modes are extended in the form of wing crack.The most similar with model tests is the linear wing crack propagation mode.The macroscopic fractures of three extended modes under biaxial compression are the secondary cracks.We make a conclusion that the linear wing crack propagation mode is the most suitable for numerical simulation of macroscopic single-flawed rock specimens.(4)Based on the numerical modeling of four typical rock bridge penetration under uniaxial compression,the macroscopic fissure expansion forms of different rock bridge geometries are analyzed.When the dip angle of the rock bridge is low,the macroscopic fissure surface formed mainly by the shear coalesence.When the dip angle of rock bridge is high,the macroscopic fissure surface formed mainly by the tensile coalesence.Increasing the number of pre-flaw will reduce the carrying capacity of rock specimens,which reflected mainly on the time of crack initiation and rock specimen penetration failure will reduce.