| Understanding the dynamic mechanical characteristics and fracture behavior of rock under impact load is crucial to improving the rock-breaking efficiency during engineering blasting and reducing the damage of blasting stress wave to retaining rock mass.It is well known that a rock is an aggregate composed of one or more mineral grains,and its mesoscopic structural geometric characteristics and the mechanical properties of the mineral grains themselves have a significant impact on the macroscopic mechanical behavior of the rock.Therefore,investigating the deformation and failure process of rock from the scale of mineral grains will help to deeply understand the mechanism of the strain rate effect of rock strength,and provide a theoretical basis for solving rock engineering problems.However,it is difficult to precisely control the mesostructure of rocks in laboratory experiments,which will bring difficulties to quantitatively study the influence of mesostructure.With the rapid development of computer technology,numerical simulation has become an alternative tool for studying rock mechanics.A numerical simulation method will be able to accurately reproduce the rock mesostructure,in addition to allowing for quantifiable control of it.As a consequence,this paper will mainly rely on numerical simulations to systematically study the dynamic mechanical characteristics and fracture behavior of brittle rocks at the macro and micro scales by constructing a multi-scale grain-based discrete element model.The main research work of this paper is as follows:(1)Establish a two-scale grain-based model(GBM)with statistical characteristics and a contact model suitable for mineral grains.In order to achieve quantitative characterization of grain structure,the study uses Neper software,so by dynamically adjusting the centroid of the Voronoi block,a granite numerical sample whose mesostructure satisfies a specific statistical distribution is generated.After that,mineral grains are further subdivided based on multi-scale Voronoi tessellation in order to generate a breakable grain-based model.Finally,the C++ program is used to establish the topological description conversion channel from the multi-grain model to the discrete element program UDEC.In order to enable the GBM to truly reflect the nonlinear fracture between mineral grains,based on the cohesive zone model framework,the contact interface model of UDEC is improved,and a bilinear contact constitutive equation that can consider single tension,tension shear and compression shear is constructed.(2)Establish a calibration procedure for the meso-parameters of the GBM.At present,the main reason for limiting the application of GBM is the determination of meso-parameters.Taking a granite containing 4 minerals(quartz,k-feldspar,plagioclase,and biotite)as an example,the GBM requires input of more than 80 meso-parameters.Since some parameters have no physical meaning,they cannot be obtained through laboratory tests.However,using the traditional trial and error method to calibrate parameters not only faces the problem of non-uniqueness of GBM parameters,but also consumes a lot of time.Therefore,this study proposes a set of recommended GBM meso-parameter calibration procedures,which greatly improves the applicable scope of GBM.Plackett-Burman experimental design was first used to evaluate the sensitivity of the meso-mechanical parameters in the model to the macro-response,and the parameters with significant effects on the macro-response values were screened out.Then,by using response surface methodology,the interaction between meso-parameters and macro-response was studied and the nonlinear relationship between significant meso-parameters and macro-response values was established.Finally,the optimization of the meso-parameters of the GBM is completed by the particle swarm optimization algorithm,and a set of meso-parameters that can reflect the macro-mechanical properties of the rock is given.(3)Carry out dynamic compression and splitting tensile simulations to quantify the influence of mesostructure and mineral composition on the strain rate correlation of rock dynamic mechanical properties and failure process.The results show that the UDEC-GBM can reproduce the strain rate effect of granite strength well.In comparison with mineral composition,mesostructure has a more obvious impact on the dynamic compressive or tensile strength of rock.Under the same strain rate,the rate effect of dynamic compressive or tensile strength of "poor quality materials" is more significant.That is,the dynamic strength increase factor DIF of the heterogeneous sample is higher than that of the homogeneous sample;the dynamic strength increase factor DIF of the coarse grain size sample is higher than that of the medium and fine grain size sample;and the dynamic strength increase factor DIF of the low roundness sample is greater than that of the high roundness sample.Although the mechanical parameters of mineral grains and contact interfaces in the simulation are strain rate insensitive,the compressive or tensile strength of all models has strain rate effects.This indicates that the meso-structure of rock has an effect on the rate effect of strength.Further statistics on the types of microcracks during the shock compression failure process were carried out,and it was found that the strain rate dependence was related to the choice of crystal breakage,which was affected by the rock mesostructure(grain size distribution,average particle size and grain circularity).The transition from intergranular fracture to transgranular fracture on the mesoscale is considered to be the root cause of the strain rate effect mechanism.(4)The LS-DYNA / UDEC-GBM coupling method was used to simulate the failure process of rock blasting,and to investigate the effect of rock mesostructure on rock fracture during blasting.According to the results,only the average grain size of the rock significantly influences blasting fracture morphology under blast loading.The distribution of microcracks becomes more dispersed with larger grain sizes,and its likelihood of connecting to form radial main cracks decreases.While grain size distribution and mineral composition appear to have no effect on the blasting fracture mode,they do have a significant effect on the number or type of microcracks.(5)A synthetic rock mass model is constructed using discrete fracture network(DFN)and UDEC-GBM,and the effect of in-situ stress and fracture network on blasting behavior of deep rock mass is studied.The results show that with the increase of burial depth,it is more difficult for blast cracks to penetrate between blastholes.Rock masses with different fracture network properties have significant differences in blasting behavior.Finally,according to the simulation results,the guidelines for optimizing the blasting parameters of the deep fractured rock mass are given. |
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