The Micro Heterogeneity Controlling Mechanism Of Tensile Fracture In Crystalline Rocks

Crystalline rocks are composed of crystalline minerals.The mechanical properties of crystalline rocks are controlled by micro-heterogeneity,which includes mineral geometry,mineral type,and mineral orientation.Strong heterogeneity would give rise to tensile stress concentration,leading to engineering geohazards,such as rock burst,large deformation,and exfoliation during the excavation of the major deep projects.These geohazards have put forward higher requirements for the study of the tensile failure,cracking behavior,strength characteristics,and heterogeneous control mechanisms of crystalline rocks.To ensure the safety of construction and operation of major projects,it is of great practical significance to carry out the research on the heterogeneous control mechanism of tensile failure of crystalline rocks from the microscale perspective.This paper focuses on the“three challenges”in the study of micro heterogeneity:uneven mineral geometry,significant mineral transformation,and complex mineral orientation.A comprehensive research framework for the micro-heterogeneity of crystal rocks is proposed.Methods adopted in this research include TIMA micro-mineralogical analysis,MTI micro-tensile system,nano-indentation test,stress field inversion based on digital image correlation,and discrete element numerical simulation based on grain model.Thus,a multi-scale research program for tensile fracture of crystal rocks is established.The systematic study of the effects of micro-heterogeneity on the tensile strength,progressive process,crack initiation and propagation behavior,and mechanism of strength index loss and activation in crystal rocks is carried out.Some innovative conclusions are obtained as follows:（1）Micro heterogeneity affects the tensile strength of crystalline rocks.As the geometric heterogeneity of minerals increases,the average grain size,anisotropism,and discontinuity of mineral grains increase,resulting in a decrease in tensile strength.Changes in mineral type heterogeneity also affect tensile strength,with weak mineral content having a significant impact on tensile strength.For example,the strength of altered granite decreases as clay mineral increases.As for mineral orientation heterogeneity,when the loading angleβ（the angle between orientation of loading and fabric）changes between 0°and 30°,tensile strength increases asβincreases.However,whenβ>30°,it decreases asβincreases;（2）Micro heterogeneity changes the progressive tensile failure of crystalline rocks.The effects of mineral geometric heterogeneity are mainly manifested as follows:with the increase of geometric heterogeneity,the elastic segment becomes shorter,the normalized crack initiation threshold(σ_ci/σ_p)decreases,the normalized crack propagation threshold(σ_cd/σ_p)increases,and the strain localization stage becomes shorter.The effects of mineral characteristic heterogeneity are mainly manifested as follows:with the increase of heterogeneity,the elastic segment slows down,the elastic modulus decreases,σ_ci/σ_pincreases,andσ_cd/σ_p decreases slightly.The effects of mineral directional heterogeneity are mainly manifested as follows:when the orientation angleβchanges between 0°and 30°,σ_ci/σ_p andσ_cd/σ_p both decrease asβincreases.However,whenβ>30°,these increase asβincreases.Atβclose to 30°,the failure of the gneiss tends towards shear failure;（3）Micro heterogeneity plays a crucial role in determining the initiation and propagation of tensile microcracks,which in turn dictates the fracture behavior of crystalline rocks.Different mineral fractureing characteristics are due to differences in crystal structure and anti-fracture mechanical properties.Plasticity is an important factor in inhibiting crack propagation.Biotite and clay minerals have greater ability to withstand plastic deformation.The initiation of micro tensile cracks in crystalline rocks is controlled by three micro heterogeneities:mineral geometry,type,and orientation.These heterogeneities influence the stress redistribution characteristics of the rocks after tensile stress,as well as the density and size of stress concentration zones.The energy field criterion of crack propagation suggests that micro heterogeneity determines the propagation path and interaction mode;（4）Micro heterogeneity dominates the loss and mobilization of tensile strength indicators,which in turn affects macroscopic fracture and strength.The tensile strength T,cohesion c,internal friction angleφ,and stiffness ratioλ_k of contacts within and between grains have the most significant impact on the tensile mechanical properties of crystalline rocks.Micro heterogeneity changes the types of microcracks in the tensile process,macro crack propagation characteristics,and the uneven distribution of stress and strain fields,affecting the loss and mobilization of three strength indicators in macro tensile strength,including microscopic tensile strength,cohesion strength,and frictional strength,thereby affecting the macroscopic fracture process and strength of the entire crystalline rock;（5）Different micro heterogeneities change macro-strength of crystalline rocks through the superposition effect.An empirical model for predicting the tensile strength of crystalline rocks based on micro-scale heterogeneity and mineral fracture toughness is proposed.A cross-scale quantitative relationship is established between the macro-tensile strength of crystalline rock and its micro heterogeneity.The geometric heterogeneity has a non-linear indices relationship with macroscopic strength.The directional heterogeneity has a non-linear logarithmic relationship with macroscopic strength.The directional heterogeneity has a third-order relationship with macroscopic strength.The predictive model is validated using experimental data and numerical simulation results,showing that the proposed empirical model can effectively predict the uniaxial tensile strength of crystalline rocks.Based on the predictive model,the reason for the complex and varied test results is the superposition effect of micro heterogeneities.The macro-strength of a single mineral crystal is significantly higher than that of the rock it composes,while various types of micro heterogeneities together weaken the macro-strength of crystalline rocks.