Study On Dynamic Mechanical Characteristics Of Frozen Soil In Cold Regions Of Plateau Under Impact Load

The project construction in the plateau area is very important for the security and economic construction of frontier areas.More and more projects are being constructed in the plateau area.For engineering construction in this area,it is undoubtedly necessary to take into account the static load brought by the structure itself and the dynamic impact load brought by some extreme cases(explosion,etc.).As frozen clay is a natural protective material widely existing in this area,it is a protective layer against various blows for all kinds of important engineering construction in this area.So the research on its strength and deformation characteristics is particularly important.It is of great significance to carry out research on the mechanical properties of frozen clay under impact action for engineering construction in this area.In this thesis,the static physical and mechanical tests of frozen clay,the Hopkinson,and a flyer impact test are carried out to reveal the mechanical properties of frozen clay under impact load.Based on the experimental result,the constitutive model and equation of state are constructed.The main research contents and conclusions are as follows:(1)The saturated moisture content of the clay is 23.04%,and the density is 2.09g/cm~3.XRD analysis of frozen clay material shows that there are more clay minerals with the highest content of silicon dioxide.The ultrasonic detector and microcomputer-controlled electro-hydraulic servo triaxial testing machine for rock and frozen soil are used to conduct wave velocity tests,uniaxial compression tests,flexural strength tests,and triaxial compression tests at different temperatures.The results show that the wave velocity of frozen clay increases with the decrease in temperature and gradually tends to be flat.Uniaxial compression strength,flexural strength,and elastic modulus all show a linear increase with the change in temperature.Uniaxial compression and triaxial compression show a"waist drum"type failure,and the flexural test shows a brittle failure from the bottom.Static mechanical parameters are greatly affected by temperature.Overall,the lower the temperature,the higher the strength,and the greater the internal friction angle and cohesion.(2)Dynamic compression tests were carried out on frozen clay under different impact speeds.During the impact test,the frozen clay samples exhibit obvious strain rate effects and temperature effects under the impact load.The dynamic peak strength of the frozen clay samples increases with decreasing temperature and increasing strain rate.The failure characteristics of frozen clay are related to temperature and strain rate.The fractal dimension is used to describe the failure characteristics of frozen clay,which is between 1.5691 and 1.8785.At the same temperature,the fractal dimension exhibits a strain rate effect as the strain rate increases.The larger the strain rate,the greater the fractal dimension and the higher the degree of fragmentation of the sample.Under the same stress condition,the dissipated energy of frozen clay gradually increases with the decrease in temperature and the increase of strain rate,and the relationship between strain rate and dissipated energy is almost linear.At-1℃and-3℃,the increase in strain rate has no significant impact on the energy absorption rate.When the strain rate increases from-5℃to-20℃,the energy absorption rate exhibits a significant strain rate effect.The larger the strain rate,the lower the energy absorption rate.(3)By using the light gas gun system,the flyer impact tests were carried out on the frozen clay at-3℃,-20℃,and normal temperatures.The impact wave velocity,particle velocity after the wave,impact pressure,and volume strain during the impact process are analyzed.The relationship between the particle velocity after the shock wave and the impact wave velocity of the frozen clay at different temperatures was determined.The D-u and P-μimpact adiabatic curves were drawn.When the initial density and water content were the same,the larger the initial strength of the sample,the more difficult it was to be compressed,and the P-μcurve shifted to the left.The energy composition after shock wave and the difference in energy variation between isentropic compression and impact compression are analyzed.(4)The constitutive model and equation of the state of frozen clay were deduced.The damage variable obeying Weibull distribution was introduced into the Z-W-T model,and the temperature term was introduced to construct the constitutive damage model.In addition,based on the test parameter values obtained from the flyer impact test,the state equation of frozen clay is derived,including the P-αmodel,polynomial type Mie-Gruneisen equation of state,Mie-Gruneisen equation of state considering Hugoniot parameters,and three-phase mixture equation of state.The thesis has 71 figures,17 tables,and 140 references.