Study On Mechanical Properties And Acoustic Emission Characteristics Of Thermal-Damaged Siltstones Under Cyclic Stress

West to east gas transmission,geothermal development and underground compressed air energy storage caverns are important projects invested and developed by the state.Such engineering rock mass is affected by temperature and cyclic stress.Temperature effect will cause thermal damage to the rock mass.Cyclic stress will cause frequent disturbance to the rock mass,thus affecting the mechanical properties of the rock mass.In this paper,the siltstone is taken as the research object,which causes different degrees of thermal damage to the sample through the cyclic heating cooling test.The thermal damage is characterized by wave velocity and elastic modulus.Then carry out cyclic loading and unloading test,and the process of cyclic loading unloading test is monitored in real time through acoustic emission technology,so as to understand the internal damage degree of the sample and explore the siltstone under different thermal damage conditions Mechanical and acoustic emission characteristics under cyclic stress.The main conclusions are as follows:(1)When the peak temperature of heating is 200?,400?and 600?,the degree of thermal damage increases with the increase of the peak temperature.After 20heating cooling cycles,the damage variable D_V is 0.015,0.243 and 0.586 respectively,and the damage variable D_E is 0.016,0.159 and 0.322 respectively.(2)Based on the uniaxial compressive strength (?) of the specimen at room temperature,it is found that the higher the degree of thermal damage,the lower the fatigue threshold.Among them,the fatigue threshold is more than 60% (?) for the sample with the heating peak temperature of 200?,between 40% (?) and 60% (?) for the sample with the heating peak temperature of 400?,and less than 40% (?) for the sample with the heating peak temperature of 600?.(3)When the peak temperature of heating is 200?,400?and 600?,the failure modes of the specimens under uniaxial compression are almost parallel to the axial splitting failure mode,but with the increase of the degree of thermal damage,the failure mode has the sign of transition from splitting failure to compression crushing failure.(4)When the upper limit of cyclic loading stress is lower than the fatigue threshold,the Kaiser effect is obvious,while when it is higher than the fatigue threshold,the Kaiser effect is weakened and the Felicity effect is enhanced.Furthermore,it is found that the accumulation of fatigue damage degree will react on the fatigue threshold value and make the fatigue threshold value decrease continuously,which shows that the Felicity ratio decreases with the increase of cycle times.At the same time,when the Felicity ratio drops to a certain extent(Take the siltstone sample selected in this test as an example,when the Felicity ratio is about 0.5),there is a possibility of fatigue failure.(5)The criteria of judging fatigue damage of rock by acoustic emission technology are verified and put forward:1.Taking ring count as the criterion,the total ring count shows a step-by-step growth during the cycle;2.The three-dimensional positioning points increase with the progression of the cycle.(6)Based on the acoustic emission method and the dissipative energy method,the damage characteristics of the fatigue damaged samples in the process of cyclic loading and unloading are studied.Both methods show that the rock damage mainly occurs in the stress level close to the peak strength segment.At the same time,the acoustic emission method shows that the rock has irreversible damage in the process of loading and unloading,but the loading segment is larger than the unloading segment.