Study On Physical And Mechanical Properties And Damage Characteristics Of Heat-Treated Granite

The construction of a series of major projects such as deep mining,energy development,and geological disposal of high-level radioactive nuclear waste is an important measure to meet the needs of rapid and sustainable development of the national economy such as energy,resources,and environmental protection.The deep rock mass is mostly in a complex environment of high geotemperature-high geostress coupling.The instability-inducing conditions and damage evolution mechanisms of hard brittle rocks in deep underground projects under the action of high temperature are one of the key scientific problems to be solved urgently in deep engineering.For this reason,this thesis is supported by the National Natural Science Foundation of China "Research on the macro/mesoscopic characterization of mechanical behavior of hard brittle rock and the mechanism of strong unloadingtemperature coupling of surrounding rock damage"(No.51774020).Based on the laboratory experiments and theoretical analysis,the physical and mechanical properties and damage characteristics of heat-treated granite were studied to provide a theoretical basisfor related research and engineering practice.The main research works and results are as follows:(1)Based on X-ray diffraction(XRD)and electron microscope scanning techniques,the effects of high-temperature treatment on mineral composition and microstructure of granite were studied,and the development of thermally-induced cracks in the granite treated by different temperatures(25～800℃)and different cooling methods(high temperature-natural cooling and high temperature-rapid cooling)were quantitatively characterized.The effect of cooling rate on the development of thermally-induced cracks was revealed,and the threshold temperature for thermal damage in granite under different cooling rates was determined.Based on the Brazilian tests,uniaxial compression tests,and conventional triaxial compression tests,the effects of temperature and cooling rate on the physical and mechanical properties of thermally damaged granite such as volume,mass,P-wave velocity,tensile strength,and compressive strength were systematically studied.The results of these studies reveal that the existence of thermally-induced cracks is the main reason for the change in the physical and mechanical properties of thermally damaged granite.(2)Based on acousitic emission(AE)monitoring technique,the damage evolution processes of heat-treated granite discs under Brazilian conditions was studied.The damage parameter DAE based on the cumulative AE is proposed,which can quantitatively characterize the degree of the discs.It is revealed that the existence of thermally-induced cracks is an important reason for the change of AE parameters such as the spatio-temporal evolution,amplitude-frequency characteristics and b value of heat-treated granite.In addition,the stress dependence of b-value was also observed in Brazilian tests.(3)Based on the AE monitoring and digital image correlation(DIC)techniques,the spatio-temporal evolution of AEs and the surface strain evolution of discs during the Brazilian splitting process were studied.It is found that the significant increase of thermally-induced cracks leads to an important change in the crack initiation mode of thermally damaged granite discs in Brazilian tests.(4)Based on the active and passive ultrasonic techniques(active ultrasonic testing and passive AE monitoring),the ultrasonic parameters(ultrasonic velocity,wave velocity anisotropy,ultrasonic amplitude spectrum attenuation)and the AE characteristic parameters(AE temporal characteristics,source locations,b-value,amplitude-frequency characteristics)during uniaxial compression of high temperature-natural cooling granite were studied.It is found that the active ultrasonic waves is more sensitive to the subtle changes of granite before and after heat treatment.The results reveal the control role of thermally-induced cracks and stress-induced cracks on the anisotropic evolution of granite under uniaxial compression.(5)Based on AE monitoring,the physical and mechanical properties and the evolution of AE characteristics of granite treated with high temperature-water cooling were studied.It is found that the compressive strength and AE characteristics of granite are not significantly affected by the high temperature-rapid cooling treatment of no more than 400℃.The effects of temperature and confining pressure on the energy storage capacity,Brittleness and fracture mode of granite are investigated.It is revealed that the confining pressure has a significant limiting effect on the thermally-induced cracks.In addition,it is found that the sudden change of damage parameters based on AE hits is earlier than that based on AE energy before rock failure,which can provide help for the prediction and early warning of rock failure and instability.(6)Based on the generalized plasticity,a double-yield elastic-plastic constitutive model considering the plastic shear mechanism and the volumetric dilatancy mechanism is established.In the proposed constitutive model,two yield functions are quantified based on the modified 3D Hoek-Brown strength criterion,and the temperature-dependent material parameters mq and mv are incorporated into the yield functions,taking the effect of temperature into account.This constitutive model can well describe the mechanical behavior of thermally damaged granite treated at different temperatures,such as confining pressure effect,strain hardening,volume dilatancy,and strain softening.The study can provide help for the constitutive model of thermally damaged rock,and it can also provide a reference for the design,construction,and disaster control of rock engineering facing high temperature.