| With the further development of modern society,human demands on natural resources and living space continue to expand.There are huge energy reserves and development space in the underground space.There are also many problems while going deep into underground,such as the development of geothermal resources,mine gas explosion,and the reconstruction after fire in geotechnical engineering,which are all affected by high temperature to varying degrees.There are great hidden dangers to the structural safety of underground rock engineering,and the thermal damage will lead to drastic changes in the physical and mechanical properties of rock,internal mineral composition,particles and other macroscopic and microscopic changes.Therefore,the influence law of high temperature on all aspects of rock properties has attracted more and more attention of experts and scholars.Yellow sandstone,which is a kind of soft rock in geological rocks,has been paid much attention by researchers to its physical and mechanical properties after being subjected to different high temperatures.The effect of high temperature(25℃ to 800℃)on the macro and micro damage of sandstone and the effect of load on the strength and creep mechanical properties of sandstone were studied by means of laboratory mechanical testing machine and related testing instruments.The rock sample was taken from the gold sandstone of Izmihuang mine,and the standard cylindrical specimen required for the test was prepared by processing.The pretreatment test at different temperatures was carried out in Muffle furnace,and the comprehensive analysis of macro and micro structure and composition changes was carried out by using precision instruments.The focus was on the change of thermal conductivity,the change of sound velocity,and the change of mineral composition.The microscopic characteristics of pores,cracks and crystal structure changes were studied to explore the change mechanism of thermal damage of sandstone.Then,the uniaxial and triaxial compressive strength test,and the uniaxial and triaxial creep test of the sandstone were carried out.The test operating software Test and drawing software Origin were used to collect and analyze the relevant data and explore the mechanical properties of sandstone under the action of loads under different high temperatures.The microscopic influence mechanism of temperature on the variation law of macroscopic physical and mechanical parameters was revealed by studying the stress-strain curve,peak stress,peak strain,tensile strength,poisson’s ratio,elastic modulus,cohesion and internal C.Through the creep test of sandstone,the uniaxial and triaxial creep curves,steady-state creep rate,instantaneous creep,and long-term strength are analyzed,and the influence law of different high temperature treatment on the creep mechanical parameters under the comprehensive and complex stress was explored.The numerical simulation software ABAQUS was used to simulate the sandstone subjected to high temperature.The main research results of this paper are as follows:(1)The sandstone samples need to be heated at high temperature before the test,and the cylindrical specimens were heated to different high temperature levels in Muffle furnace.Acoustic damage detection,scanning electron microscope test,X-ray diffraction test,porosity measurement and other experimental explorations of sandstone subjected to high temperature were carried out,it was found that the change of macroscopic physical and mechanical properties of sandstone subjected to high temperature was caused by mesoscopic damage.With the increase of treatment temperature,the degree of thermal damage was greater,and the development of pores and cracks was accelerated.With the occurrence of chemical reaction,the mineral decomposition,composition change,crystal structure and skeleton were gradually softened and destroyed,further causing the intensification of pores and cracks,which also caused the decrease of sound velocity value with the increase of temperature.The mass loss rate increased with the temperature increasing.As the temperature increased,the yellow color of rock gradually fades,while the red color begins to appear and deepen gradually.When the temperature reached 700℃,the color gradually turned to earthen gray.(2)Uniaxial and triaxial strength tests were carried out on sandstone specimens subjected to different high temperatures.Under uniaxial compression,the stress-strain curves of sandstone specimens subjected to different high temperatures showed regular changes.When the temperature increased,the failure mode changed.The peak strain increases with the increase of temperature.The peak stress,tensile strength,modulus of elasticity and Poisson’s ratio decrease with increasing temperature.Under the condition of triaxial compression,the stress-strain curves of sandstone subjected to different high temperatures also had compaction stage,elastic stage,inelastic deformation stage and failure stage,and the plastic deformation stage and the residual strength stage of sandstone specimen after failure are obvious under triaxial compression.The triaxial peak strength,elastic modulus and shear strength decrease with the increase of temperature,while the peak strain increases with the increase of temperature.Based on the BP neural network model,the uniaxial compressive strength of sandstone in the temperature-sensitive range from 600℃ to 700℃ was predicted.It was found that the error between the predicted value and the experimental value was small.The feasibility of the model method was verified by using the literature data.(3)Uniaxial and triaxial creep tests were carried out on sandstone specimens subjected to different temperatures.It was found that the instantaneous strain of sandstone treated with different temperatures was different at the initial stage of creep.Steady-state creep rate analysis of various loading curves shows that the steady-state creep rate fluctuated with the increase of temperature.Under the action of different confining pressures,the creep curves of sandstone specimens treated at the same temperature varied in different stages.By using the method of isochron curve test,the long-term strength of sandstone after different high temperature treatment was obtained.The analysis shows that the long-term strength changes with the increase of temperature,and the higher the temperature,the closer the long-term strength of sandstone is to its failure strength.(4)The ABAQUS numerical simulation software was used to conduct numerical simulation of the strength test of sandstone after different high temperature treatment,and the gap between the stress-strain and the measured value in the compression failure process of the specimen was explored and simulated.It was found that the numerical simulation results were in high agreement with the test results. |
