Study On Meso-mechanism Of Variability Of Mechanical Properties Of Granite

Rock is a heterogeneous material in nature that composed of various minerals.Meso-mechanical properties of minerals have an important influence on the macroscopic mechanical behavior of rocks.The heterogeneity of mechanical properties of minerals and different development degree of the meso-structure of rock are one of the important reasons for the large variability of macroscopic mechanical properties of rock.The greater heterogeneity of mineral grains and meso-structures,the greater variability of macroscopic mechanical properties of rock.In this thesis,granite was selected as the research object,and the mechanical properties of granite were studied from both macroscopic and mesoscopic aspects.This thesis proposes a set of research methods to study the variability of macroscopic mechanical properties from viewpoint of the mesoscopic level.The main contents and basic understandings of this thesis are as follows:(1)By macroscopic physical and mechanical test,some physical properties and mechanical properties of granite samples were obtained,and the variability of some mechanical parameters was calculated.The results show that the variability of tensile strength and uniaxial compressive strength of the tested granite is large,with a coefficient of variation of 20%.For triaxial compression test,the cracks inside granite gradually close with the increase of confining pressure,resulting in less variability of the results,with a coefficient of variation of about 3%.(2)By CT scanning,the quantitative determination of minerals and the analysis of meso-structure of granite can be realized.In this thesis,the reasonable segmentation threshold was determined by considering the results of X-ray diffraction test to realize the three-valued segmentation of CT images of granite.The results show that the CT images of different minerals have different gray values.By using reasonable segmentation thresholds,the main minerals of granite,i.e.,quartz,feldspar,and mica,can be readily identified and quantitatively determined Based on the 3D reconstruction model,the shape,the particle size,and the spatial distribution of different minerals can be easily obtained.(3)By nanoindentation test,the elastic modulus and hardness parameters of minerals of granite were obtained.This thesis proposes a method to inverse the mechanical strength parameters of minerals using nanoindentation test and numerical simulation.With the help of artificial neural network and numerical simulation,a response surface model of mineral mechanical parameters was established.The cohesion and friction angle of minerals were obtained by inversion of the response surface model using the optimization algorithm.The results show that different minerals have different cohesion and friction angle.The cohesion and friction angle of quartz are the largest among the three minerals,the second is feldspar,the smallest is mica and the results of mica are more discrete.(4)By discrete element method,the numerical simulations of triaxial compression test and Brazilian test of granite were carried out,and the influence of the meso parameters of granite on its macroscopic mechanical properties was analyzed.The results show that the tensile strength of the model depends on the tensile strength of the contact surface and increases with the increase of the tensile strength of the contact surface.The cohesion of the model is affected by the cohesion of the contact surface and the cohesion of the mineral grains,depending on the lowest strength of the two.For friction angle,the variation law is the same as cohesion.Affected by the size effect and the constitutive model,there are great differences between the mineral mechanical parameters obtained by the inversion of the discrete element model and those obtained by the inversion of the nanoindentation test.