Damage And Failure Characteristics Of Sandstone Under Cyclic Plus Unloading And Confining Pressure

In the process of cyclic excavation in actual projects,repeated excavation of the rock will cause internal fracture penetration and affect the mechanical properties of the surrounding rock,collapse events and surrounding rock instability and other construction accidents are caused by the negligence of the excavation process of the rock itself and the surrounding rock.So far,there is little research on the damage characteristics of sandstone in cyclic pressure test at home and abroad.In this paper,firstly,the conventional triaxial compression test and triaxial unloading test can be carried out to obtain the peak strength and damage pressure of rock samples,and based on this,the increase cyclic pressure test and equal amplitude cyclic pressure test can be carried out to obtain the damage and acoustic damage of sandstone under cyclic pressure conditions.The paper mainly analyzes three aspects of mechanical characteristics,energy evolution law and acoustic emission,and concludes from the test that:(1)Based on the data from the cyclic addition and removal of siege pressure test,the strength characteristics,stress-strain curve changes and damage mode characteristics of red sandstone under different initial siege pressure and different removal of siege pressure amplitude conditions were analyzed.From the test results,it can be found that: the mechanical parameters such as peak strength,peak strain and elastic modulus of the conventional triaxial compression test gradually increase with the increasing initial enclosure pressure,and the enclosure pressure at the destruction of the triaxial unloading enclosure pressure test also increases continuously;in the cyclic addition and removal enclosure pressure test,the enclosure pressure and the circumferential strain can form a closed "hysteresis loop",and with the cyclic The number of cycles increases,the hysteresis loop continues to migrate to the left,and the hysteresis loop area keeps increasing,and the hysteresis loop gradually tilts to the upper right,indicating that the elastic modulus of the rock sample shows a trend from large to small,in which the change in elastic modulus of the equal-amplitude cyclic unloading enclosure test is greater than that of the increasing cyclic unloading enclosure test,and the damage mode of the cyclic unloading enclosure test is similar to that of the conventional triaxial compression test are shear damage.(2)According to the first law of thermodynamics and energy calculation principle,the energy evolution law of sandstone under cyclic loading and unloading circumferential pressure conditions is investigated,and the research results show that the total energy and elastic energy under increasing cyclic loading and unloading circumferential pressure conditions increase continuously with the increase of the number of cycles,the gap between the proportion of elastic energy and the proportion of dissipated energy becomes smaller and smaller,and the energy conversion rate shows a trend of gradually increasing.In the isoamplitude cyclic pressure test,the elastic energy and total energy show a trend of increasing and then decreasing,and the dissipated energy ratio gradually increases in the later stage of cyclic loading,indicating that the isoamplitude cyclic pressure test produces smaller damage damage inside the rock,and the number of cracks on the surface of the rock sample is smaller.(3)Based on the acoustic emission results of the cyclic compression test,the evolution characteristics of acoustic emission ring counts and amplitudes of rock samples were analyzed.The maximum acoustic emission ringing count is especially obvious when the set lower limit of the enclosing pressure is reached,and the maximum ringing count of the increasing cycle test decreases first and then increases and then decreases with the increase of loading times,while the equal amplitude cycle test shows a trend of decreasing and then increasing.