Study On The Damage Characteristics And Triaxial Cyclic Loading And Unloading Constitutive Model Of Sandstone After Freeze-Thaw Cycles

Under the complex environment and climate conditions,the rock mass engineering in the high cold region will inevitably suffer the influence of freeze-thaw action.During freeze-thaw process,the quality of rock mass will be damaged and deteriorated,which is easy to cause all kinds of engineering disasters.The problem of rock mass freeze-thaw damage and deterioration is essentially the fatigue failure caused by the repeated loading and unloading of frost heave force.Based on this,this paper,relying on the projects of National Natural Science Foundation,adopts the methods of literature survey,laboratory test and theoretical analysis to carry out "Research on damage characteristics of sandstone under cyclic freeze-thaw action and triaxial cyclic loading and unloading constitutive model".The main work and achievements are shown as follows:(1)By testing the physical parameters of white and white sandstone,the evolution law of physical parameters of white and white sandstone with the number of freeze-thaw cycles is understood.Among them,the drying mass and p-wave velocity decreased with the increase of freeze-thaw cycles,while the saturation mass,mass change rate,porosity,wave velocity change rate and freeze-thaw coefficient increased with the increase of freeze-thaw cycles.There are obvious stages in the evolution process,and the dividing point of the stages is 20 and 40 freeze-thaw cycles.(2)The mechanical parameters of gray-white sandstone after cyclic freeze-thaw action are tested to understand the evolution law of mechanical parameters of gray-white sandstone with the number of freeze-thaw cycles and the fatigue failure law of gray-white sandstone after cyclic freeze-thaw action.Among them,uniaxial peak strength,elastic modulus and triaxial peak strength all showed a decreasing trend with the increase of the number of freeze-thaw cycles.Moreover,by logarithmic function fitting of the test data,the relationship between mechanical parameters and the number of freeze-thaw cycles could be obtained,and then the relationship was normalized and differentiated.The freezing-thawing cycle deterioration coefficient was introduced to quantitatively compare the normalized and derivative parameters,and the parameter freezing-thawing cycle damage with the largest change value was finally selected to quantitatively describe the freezing-thawing cycle damage.(3)Based on the evolution process of fatigue deformation of cyclic freeze-thawing grey sandstone under triaxial cyclic loading and unloading,the defective components were identified by analyzing each strain stage described by the traditional Nishihara model,and the Able sticky kettle element was introduced to replace the sticky kettle with fixed viscosity coefficient by the fractional calculus theory.The traditional Nishihara model was improved.The fatigue constitutive model of rock under cyclic loading is established.In order to make the model more realistic,the freeze-thaw coefficient and cyclic loading weakening coefficient were introduced to correct the damage effect of rock,and an improved fractional-order nonlinear elastoplastic fatigue model was obtained.