Study On Damage And Fracture Characteristics Of Rock Mass With Irregular Joints Based On 3D Printing

The joints in rock mass have been a problem for the safe construction of rock mass engineering such as mining because of the various morphology,different penetration degrees,and complex occurrence elements.From the whole to the local and from the macro to the micro,it has important theoretical and practical significance for revealing the fracture mechanism of the complex jointed rock mass by multiple methods to separate and extract a single joint and a small number of joint combinations from the complex joint network and carry out targeted and systematic research on the mechanics and fracture characteristics of relevant rock mass,which constitutes the foundation and necessary link for the study of instability and failure of the complex jointed rock mass.This paper combines the laboratory test,theoretical analysis,and numerical simulation.Four kinds of irregular joint models,rough single joints,rough discontinuous joints,rough cross joints,and undulating joints,were made through 3D printing technology.The rock-like samples of standard size with relevant joints were poured with cement mortar.Through uniaxial compression test,digital image correlation technology(DIC),CT scanning technology,Yade discrete element numerical simulation,statistics,fracture mechanics,damage mechanics,and fractal geometry methods have been adopted to deeply study the strength characteristics,deformation behavior,and fracture evolution mechanism of rock samples with irregular joints.The main research contents and innovative conclusions are as follows:(1)The fracture mode and stability analysis method of rock mass based on full-field damage and full-field stress nephogram of rock mass surface were proposed.Based on DIC technology and damage mechanics method and by combining DIC strain field with damage evolution constitutive equation,the characterization method of rock mass surface damage field and stress field under load was explored,and the visualization of damage field and stress field on the surface of rough discontinuous jointed rock mass was realized.Through the local damage value of damage field on the rock mass surface,the damage degree of the area can be judged intuitively,which provides a quantitative criterion for whether the rock mass is broken or not.Through the stress field on the rock mass surface,the stress concentration area can be effectively identified and the development trend of rock mass surface stress can be obtained,which provides a reference basis for the determination of rock mass support level,the selection of support methods and support parameters,and the accurate early warning of rock mass instability and failure.(2)The solution method of stress intensity factors(SIF)at the joint tip based on DIC technology was proposed,which provides a quantitative judgment basis for the identification of fracture types.Based on the principle of DIC technology and fracture mechanics,the DIC displacement field at the tip of rough cross joints is brought into Williams polynomial by combining laboratory test and theoretical analysis.The solution method of type Ⅰ and type Ⅱ SIF(KⅠ and KⅡ)at the tips of joints was studied.By referring to the distribution form of crack at the tip of joint and the size of KⅠ and KⅡ,the types of tension and shear crack initiation were explored.It provides a quantitative index for the discrimination of fracture type,and reveals the fracture evolution mechanism of rough cross jointed rock mass from a quantitative point of view.(3)Based on Yade discrete element,the numerical models of rough discontinuous jointed rock mass and rough cross jointed rock mass were established.Six types of coalescence of rough discontinuous joints,eight meso fracture mechanisms and the meso fracture evolution mechanism causing the final failure of rough discontinuous jointed rock mass and rough cross jointed rock mass were obtained.Based on the stress-strain characteristics and fracture mode of laboratory tests,the numerical models of rough discontinuous jointed rock mass and rough cross jointed rock mass were established by Yade discrete element numerical simulation method,and the effects of joint dip angle,rock bridge dip angle and joint roughness on the strength characteristics,deformation behavior and fracture evolution characteristics of rough discontinuous jointed rock mass under uniaxial compression were investigated.The laboratory test results of rough cross jointed rock mass were verified by discrete element simulation.Six types of coalescence of rough discontinuous joints were obtained.Through the analysis of the velocity and displacement of particles at the crack position,eight kinds of meso fracture mechanisms were obtained.It is revealed that the macro evolution of the main fracture from the near tips to the far tips of joints corresponds to the meso evolution from oblique tensile(OT)fracture to tensile shear(ST)fracture.The results provide a reference for solving the fracture problems of jointed rock mass in engineering.(4)Based on DIC technology,a set of damage constitutive model of jointed rock mass with SIF as the medium was established.The solution of damage constitutive characteristics is closely related to the actual data and has higher reliability.Based on DIC technology and fracture mechanics method,the SIF at the tips of undulating joints were solved and analyzed.Taking SIF as the bridge,the damage mechanics method was adopted.Based on the theory that the SIF in the initial damage equation of joint under compression and shear is directly proportional to the stress of sample and the secondary damage of load meets Weibull distribution model,the damage evolution equation of jointed rock mass under load was determined through DIC strain field.Finally,the damage evolution equation including two parameters,joint dip angle a and loading strain ε,was established according to Lemaitre strain equivalence principle.Based on practice,the damage constitutive characteristics of jointed rock mass were explored by combining theory and experiment,which provides a new idea with higher reliability for revealing the damage and fracture evolution process of rock mass.