Study On Mechanical Behavior And Damage Evolution Characteristics Of Sandstone After High Temperature

The physical-mechanical properties and damage failure characteristics of rocks under high temperature will change significantly.In industries such as coal fire prevention,geothermal resource development,underground coal gasification,and disposal of high-level radioactive waste,the rock properties and damage evolution characteristics under high temperature conditions have become a focus of attention in the engineering field.In this study,sandstone was selected for high temperature treatment(25～1000℃),and the effects of temperature,confining pressure,and stress path on the damage and fracture characteristics of sandstone were investigated from a macroscopic perspective by conventional triaxial compression tests and multi-stage cyclic loading-unloading tests.Additionally,scanning electron microscopy and X-ray diffraction experiments were conducted to analyze the changes in the microstructure and mineral composition of sandstone after high temperature treatment.Furthermore,particle flow code(PFC)was used to verify the conventional triaxial compression experiments carried out in the laboratory,and to analyze the rules of rock crack evolution under the action of thermodynamic coupling.The main research results and conclusions are as follows:(1)The apparent color,mass,volume,and longitudinal wave velocity of sandstone after high temperature treatment were tested and studied.The results show that the physical properties of sandstone change periodically with temperature.At room temperature to 400℃,the mass,volume,and longitudinal wave velocity slightly decrease.From 400℃ to 600℃,the mass and longitudinal wave velocity decrease rapidly,while the volume expands.At 600℃ to 1000℃,the mass and longitudinal wave velocity decrease slowly,and the volume expands rapidly.The color gradually changes to reddish-brown.Among them,the longitudinal wave velocity of sandstone is the most sensitive to temperature,followed by density,and the sensitivity of volume and mass to temperature are almost the same.(2)The effect of temperature on the mineral composition of sandstone microstructure was studied by SEM and X-ray diffraction analysis.The results show that on the microscopic scale,the main changes in sandstone under high temperature conditions are dehydration,decomposition,expansion,melting,and extension of grain boundary cracks and intergranular cracks.(3)The stress-strain curves,strength deformation parameters,failure modes,and damage evolution characteristics were obtained by conducting conventional triaxial compression tests at different temperatures.The results show that when T ≤ 500℃,the peak strength slightly increases,the ratio of damage threshold to peak strength gradually increases,the peak strain tends to stabilize,the elastic modulus and deformation modulus change slowly,and the failure is brittle.When T > 500℃,the peak strength decreases rapidly,the proportion of damage threshold to peak strength decreases rapidly,the peak strain increases rapidly,the elastic modulus and deformation modulus decrease rapidly,and the failure is ductile.Confining pressure limits the thermal effect of sandstone,and temperature accelerates the damage to sandstone while confining pressure slows down the damage to sandstone.(4)Based on the results of conventional triaxial compression tests,multi-stage cyclic loading-unloading tests were carried out to study the effects of temperature and stress limit on stress-strain curves,elastic modulus,residual strain,and energy evolution characteristics.The results show that multi-stage cyclic loading-unloading tests enhances the strength of the samples,especially in the temperature range of 200～800℃.Before the fatigue damage threshold value,cyclic loading-unloading have a strengthening effect on sandstone samples,but after the threshold value,the more cycles,the more severe the degradation of sandstone samples.The damage to the sandstone samples gradually intensifies after 400℃.(5)Sensitivity analysis of mesoscopic parameters was carried out using particle flow code(PFC),and the relationship between macroscopic and mesoscopic parameters was established.Conventional triaxial compression tests were simulated.In addition,crack evolution,displacement field and stress field distribution characteristics of sandstone under thermodynamic coupling were analyzed from the mesoscopic perspective.The results show that the numerical simulation is in good agreement with the laboratory experiment.After compression failure,the force chain of the sample mainly presents axial distribution.With increasing temperature,the number of contact force chains increases firstly and then decreases,reaching a maximum at 500℃,and the number of tensile force chains increases.The damage variable based on crack number evolution corresponds to four stages of crack evolution: no damage stage,initial damage stage,damage development stage,and damage failure stage.When the damage variable based on crack number is 0.5,it is the damage threshold of the model.There are 87 figures,12 tables and 167 references in this thesis.