Study On Dynamic Mechanical Properties And Damage Evolution Of Sandstone Under Freezing-Thawing Cycle

With the strategy for large-scale development of western China and“the Belt and Road Initiative” putting into effect,geotechnical engineering in cold areas urgently needs to develop vigorously.The existing researches on rock mechanical properties under freeze-thaw cycles mostly focus on static and dynamic uniaxial aspects,and there is little involvement in dynamic tests under three-dimensional confining pressure.It is inevitable to encounter freeze-thaw cycle damage and dynamic load damage such as blasting and mechanical shock,which are important factors affecting the safety and stability of the actual project.In consequence,research has been carried out in terms of the freeze-thaw cycle damage and dynamic disasters in rock engineering.Particularly,it is significant for us to understand the mechanism and evolution of rock freeze-thaw damage in depth.Taking green sandstone as the object of this study,and the number of freeze-thaw cycles as the control condition,this paper makes a systematic study of the compressive strength,elastic modulus and other mechanical parameters of specimens under static conditions.The dynamic mechanical properties of green sandstone damaged by freeze-thaw cycles under different confining pressures and loading rates were carried out;the energy dissipation characteristics and fragmentation distribution of dynamic compression failure sandstone were analyzed;combined with scanning electron microscopy experiments and computer tomography(CT)technology,the dynamic experimental failure characteristics of different freeze-thaw cycles are analyzed from the microscopic and microscopic perspectives in respectively;finally,a constitutive model based on rock freeze-thaw load damage is established according to the research results.The main conclusions are as follows:(1)The evolution in saturation density,longitudinal wave velocity,porosity,mass loss rate and freeze-thaw coefficient of green sandstone after different freeze-thaw cycles were tested.In addition,a static compression experiment was carried out to compare the stress-strain curves under different freeze-thaw cycles,and obtain the change law of the sandstone’s compressive strength,elastic modulus,Poisson’s ratio,peak strain and other mechanical parameters.The results show that with the increase in the number of freeze-thaw cycles,the water-ice phase change of saturated rock under low temperature will produce corresponding frost heave,which makes the internal pore structure of the sample expand and penetrate,and internal damage continues to accumulate.(2)By using the split Hopkinson pressure bar,the mechanical response of green sandstone under different confining pressures and loading rates after being damaged by freeze-thaw cycles is studied.The results show that under a fixed confining pressure environment,the dynamic compressive strength of green sandstone damaged by different freeze-thaw cycles has obvious rate correlation;the dynamic compressive strength decreases significantly with the increase of the number of freeze-thaw cycles.Under a fixed number of freeze-thaw cycles,the confining pressure effect obviously enhances the dynamic compressive strength of sandstone,but increase of the strength decreases with the increase of confining pressure.(3)The dynamic energy changes of sandstone specimens damaged by different freeze-thaw cycles in the compression failure compaction stage,the linear elastic stage,the yield stage and the failure stage are analyzed.In addition,the distribution of fragmentation of sandstone samples after crushing was studied.The results show that as the number of freeze-thaw cycles increases,the total input strain energy and total dissipation energy of the green sandstone during the entire dynamic compression failure process gradually decrease,and the peak value of the releasable elastic strain energy also gradually decreases.The sensitivity of freeze-thaw cycle damage times is stronger under the condition of lower loading rate,and the sensitivity is relatively weak under the condition of higher loading rate.(4)With the help of scanning electron microscope technology with computed technology(CT),the influence mechanism of freeze-thaw damage on the internal structure of green sandstone specimens from a microscopic and microscopic perspective has been analyzed,and a connection is established between macroscopic failure and local damage,and freeze-thaw-damage to rock Further exploration of the constitutive model reveals the evolution mechanism of its freeze-thaw damage.