Study On Deterioration Mechanism Of Tensile Performance Of Sandstone Under Freeze-thaw Cycles

The tensile strength of freeze-thaw rock is a key mechanical index for the stability evaluation of geotechnical engineering structures in cold regions.Taking sandstone as the research object,this paper studies the deterioration mechanism of tensile properties of sandstone under freeze-thaw cycles,which has theoretical and practical significance for the stability evaluation of geotechnical engineering in cold regions.The Brazilian splitting acoustic emission real-time test of two kinds of sandstone with different freeze-thaw cycles was completed.With the increase of freeze-thaw cycles,the tensile strength of the two sizes of sandstone decreases;The failure mode of fine-grained sandstone is central splitting,and that of coarse-grained sandstone is non central splitting;Under the action of freeze-thaw cycles,the Brazilian fracturing damage of sandstone has the characteristics of continuous evolution,and the damage development rate has a sudden change at the critical moment.When the freeze-thaw cycle is 5 times,the damage velocity increases,and the AE energy and event frequency increase abruptly.With the increase of freeze-thaw cycles,the failure of sandstone changes from brittle failure to ductile failure.High energy acoustic emission events often occur in the rapid expansion period of damage,which can be used as an important reference for damage.conduction and aggravation.The ringing count and cumulative energy increase to a higher order simultaneously,which is the precursor information of the failure time domain of freeze-thaw sandstone.With the increase of freeze-thaw cycles,b value first decreases and then increases,which can reflect the failure mode of sandstone,from internal large fracture failure to continuous small fracture failure.The microscopic damage observation experiments of two kinds of sandstone under different freeze-thaw cycles are completed,and the pseudo color enhancement technology of CT image of freeze-thaw rock is realized.The pseudo color enhancement technology improves the visual resolution of CT image.The micro structure and damage information in the pseudo color enhanced image indicate that the damage propagation of sandstone under freeze-thaw cycle starts from the initial damage.After the crack formation and propagation stage,the freeze-thaw damage increases gradually,and the effective bearing area of rock decreases.The numerical simulation of Brazilian splitting failure process of two kinds of grain size sandstone under different freeze-thaw cycles and loading rates and three kinds of grain size sandstone under zero freeze-thaw cycles was completed.With the increase of freeze-thaw cycles,the tensile strength decreased;The tensile strength decreases with the increase of particle size;With the increase of loading rate,the tensile strength increases.After 20 freeze-thaw cycles,the effect of loading rate is no longer obvious,and the tensile strength changes little with the increase of loading rate.The initial position of acoustic emission event is the same as that of rock crack propagation.When the acoustic emission event begins to appear,there is no obvious crack,and the crack propagation direction is the same as that of acoustic emission,and it is basically in the same path.