| Dynamic deformation and failure mechanism of rock are frontier and hot issues in rock mechanics,and also a key scientific problem to be solved in rock dynamic fracture,engineering blasting excavation and engineering safety protection design.Dynamic failure of rock includes two aspects:crack growth mechanism and strain rate effect.Based on theoretical analysis and numerical simulation,this paper focused on two scientific problems:dynamic deformation and failure process and strain rate effect of rock,and simulated and analyzed the fracture and failure characteristics of rock crack initiation,propagation and penetration under dynamic load.The rock dynamic failure criteria,parallel simulation algorithm,selection of meso-parameters,strain rate effect and application of dynamic impact rock breaking were studied.This paper mainly includes the following main contents:(1)The dynamic failure criterion considering dynamic intensity enhancement factor and strain rate effect was introduced into to the dynamic numerical model RFPA(Realistic Failure Process Analysis).The whole process of deformation and failure of a three-dimensional cylinder model under dynamic load was simulated,and the results were compared with theoretical analysis and physical experimental results.The simulation results well reproduced the failure mode and location of dynamic spalling,which proved the reliability of the failure criteria established in this paper.(2)Efficiency of dynamic finite element calculation and efficient solution of large-scale linear equations were realized based on personal PC.Based on the OpenMP library and the parallel algorithm of SSOR-PCG,a reasonable over-relaxation factor was proposed to accelerate the iteration speed of solving equations and realize the parallel and efficient calculation of rock dynamics under three-dimensional conditions on PC.Through detailed simulation research on dynamic failure process of rock samples,the efficient solution performance of parallel calculation was measured.Parallel calculation can significantly reduce the calculation time of finite element solution using a reasonable number of threads and an excess relaxation factor.A calculating step of a 3-D model of 1.55 million elements was finished in 24s on a personal computer with 4 cores and 8 threads.(3)The mesh size in rock dynamic failure simulation is investigated.By comparing and analyzing the grid mesh size effects of homogeneous and heterogeneity models subjected to dynamic loading,the influence of the grid mesh size of heterogeneity models on their dynamic properties,energy characteristics and crack evolution behavior is analyzed,and the correlation between the grid mesh size and the dynamic mechanical deformation and failure of rock in the heterogeneity model is revealed,which provides basis for selecting the grid mesh size of subsequent numerical models.(4)Application of differential evolution algorithm to BP neural network model,a method for determining mesoscopic mechanical parameters in rock dynamic mesoscopic damage model is proposed.Based on the statistical constitutive model of dynamic elastic-brittle damage of rock,the physical meaning of the key mesoscopic characterization parameters in the constitutive model is clarified according to the complete stress-strain curve of rock under uniaxial compression.With the application of BP neural network model,a non-linear mapping relationship between the mesoscopic parameters and rock stress-strain curve characteristics such as peak stress,peak strain,pre-peak nonlinearity and post-peak brittleness is established to construct a mesoscopic mechanical parameters prediction model.In addition to,the connection weights and threshold of nodes of BP neural network model are optimized by using differential evolution algorithm to improve the accuracy of neural network prediction(5)An analytical formula for dynamic bearing capacity of rock reflecting real dynamic failure strength and inertia force is proposed,and the influencing factors of strain rate effect of rock are systematically studied.By establishing a single degree mass-damper components model describing the physical mechanism of macro bearing capacity of rock materials,the model can qualitatively reflect the influence mechanism of rock bearing capacity and.the influence of s strain rate effect,inertia effect,size effect,friction effect and confining pressure on rock bearing capacity can be comprehensively considered.Based on this theory,an analytical formula for rock dynamic bearing capacity,which can comprehensively reflect the true dynamic failure strength and inertia force of rock,is established by using energy conservation and limit equilibrium.The accuracy of the analytical formula is verified by combining rock dynamic physical and mechanical experiments.Moreover,a numerical Split Hopkinson pressure bar(SHPB)is successfully established to investigate the dynamic deformation and failure process of rock.The mechanism of improving the dynamic macroscopic bearing capacity of rock under the coupling action of various factors is systematically studied.The sensitivity of rock stain rate effect is studied.It is found that the heterogeneous is the internal reason for the sensitivity of material rate effect,while the coupling of friction effect and various factors is the external reason.(6)Three-dimensional surface crack growth mechanism of rock under dynamic load is simulated.The initiation and propagation processes wing cracks,anti-wing cracks and shell-like cracks that are reproduced by numerical simulation when specimens subjected to dynamic loading compression.Moreover,the effects of stress wave peak amplitude,wavelength,flaw dip angles,crack excavation thickness heterogeneity of rock were carefully studied.These numerical results of this study demonstrate many phenomena that have already been shown in laboratory experiments;however,many of these fracture phenomena results direct the necessity of additional experiments.Furthermore,the crack initiation and propagation in static loading condition were numerically investigated,and the results are basically in accordance with the experimental results.Finally,starting with the difference of static and dynamic failure process of rock specimens,and the reasons for the differences are explained from the aspects of microcosmic failure mode,mechanical behavior and energy evolution.(7)The mechanical dynamic rock breaking performance in marine channel engineering is simulated and investigated.A numerical model of mechanical rock breaking under impact load was established for dynamic rock breaking of marine navigation channel.The effect of reflection of stress wave at boundary was eliminated by setting absorption boundary and the process of rock micro-breaking under impact load was simulated.The influence of rock impact breaking performance parameters such as bit shape,impact energy,impact speed,impact times,bit center distance and thickness of weak mud layer on rock impact breaking volume,breaking depth and specific energy consumption are simulated and investigated.The rock breaking performance under impact of single and double bits is analyzed by numerical test,and the optimum shape and double bits are given.The optimum center distance provides a reference for effectively improving rock breaking efficiency. |
