Study On Mechanical Properties Of Rocks Under Freeze-thaw Cycles And Coupled Static-dynamic Loads

When the rock is exposed to the air for a long time,the internal structure will change after the action of high day-night temperature difference and seasonal replacement cycle,resulting in freeze-thaw damage.In general,the rock itself has the static stress action,in the blasting,mechanical vibration and other operations in the construction of rock at the same time under the impact of the load.This paper takes red sandstone as the research object to explore the dynamic mechanical properties of freezing-thawing damaged rocks under both static and dynamic loads.The Hopkinson bar,which can apply axial pressure,was used for the experiment.Combining with the HJC constitutive model,ANSYS/LS-DYNA software was used for the relevant numerical simulation to study the mechanical properties of red sandstone after freeze-thaw damage under the action of dynamic and static combination.The main research contents and conclusions are as follows:(1)Analysis of static mechanical properties of freeze-thaw damaged samples.Use of freezing and thawing box to freeze and thaw the intact rock,through regular physical test,uniaxial compression test,splitting the experiment measured the physical properties of the rock,and the change of the index such as tensile,compressive strength,found that with the increase of freeze-thaw cycles,peeling phenomenon in a sample,quality gradually reduce,density decreases gradually,tensile,compressive strength decreases gradually.After 10 times freezing-thawing cycle,the stress-strain curve compaction stage increases significantly.Before freezing-thawing,the sample is hard rock,which becomes harder rock after 10 freezing-thawing cycles.During the freezing-thawing process from 10 to 30 times,the sample is all hard rock.(2)RMR quality evaluation of freeze-thaw damaged rock mass.According to the decay function model,the decay constant of red sandstone was calculated to be 0.04335,and the half-life was 16 times according to the results of uniaxial compressive strength test.It can be seen that the samples adopted this time have a weak overall resistance to freeze-thaw weathering.The results of laboratory test and freeze-thaw integrity loss were comprehensively applied to the RMR rock mass quality evaluation system,and the formula of freeze-thaw correction coefficient of rock mass of freeze-thaw damaged red sandstone slope was obtained.Five times before freezing and thawing red sandstone slope rock mass quality levels from ? good rock into ? medium quality of rock mass,5 to 50 times of freeze-thaw interval RMR values gradually reduce and flatten out,but still a category ? medium quality of rock mass.(3)Experiment on dynamic and static combination loading of freeze-thaw rock.The Hopkinson bar with a diameter of 50 mm and the ability to apply axial pressure was used to apply specific axial pressure to rock samples with different damage degrees.The main consideration freeze-thaw damage degree,and the size of the axial compression,and the impact velocity and other three variables influence on dynamic mechanical properties,three common influencing the dynamic mechanical properties of the rock,three factors coupling effect of freeze-thaw damage of rock movement combination loading peak strength,peak strain,the change of the mechanical parameters such as elastic modulus is complex.(4)Numerical simulation analysis of Hopkinson rod experiment.The widely used LS-DYNA software and the HJC constitutive relationship were used to simulate the impact experiment of Hopkinson bar with axial pressure.According to the physical significance of each parameter of the HJC constitutive relation and the corresponding experimental results,the test algorithm was used appropriately to obtain the HJC parameter values of rock samples with different freeze-thaw damage,different static load and different strain rate.LS-DYNA finite element model was used for numerical simulation,and the numerical simulation results were in good agreement with the experimental results.The dynamic strength sensitive parameters of red sandstone are A,B and fc.Based on the results of dynamic and dynamic combination loading experiment and numerical simulation of freeze-thaw rocks,it is found that the sample for the same freeze-thaw injury,the same static load different dynamic load effect,the intensity of rock's dynamic performance strong strain rate effect,strength enhanced with the increase of the dynamic load;Under the same dynamic load and different static load,the dynamic strength of rock sample increases with the increase of static load;when the applied axial pressure exceeds 60%of the static load strength,the dynamic strength of rock sample decreases with the increase of static load.When the loading form of rock is the same,the dynamic strength of rock samples with high degree of damage is lower.