Study On Freeze-thaw Damage Degradation Mechanism Of Sandstone And Its Mechanical Response Characteristics Under Impact Load

Three quarters of China’s land area are cold regions.The rock engineering in the cold region is facing serious freeze-thaw（FT）weathering problems,and the rock mass in the engineering geological environment involved in blasting excavation,mechanical mining,earthquake,train vibration,rock burst and so on is vulnerable to serious dynamic disturbance and complex tectonic stress.However,the damage and deterioration mechanism of rock mass under the coupling of complex dynamic stress and FT cycle is unclear,and the mechanical properties are unknown.Based on this,the sandstone in cold region-Tongchuan area is taken as the research object in this paper.Nuclear magnetic resonance（NMR）test,scanning electron microscope（SEM）test,split Hopkinson pressure bar（SHPB）test and other technical methods are applied,indoor experimental research and theoretical analysis are comprehensively used,the deterioration mechanism of FT damage is proved,and the mechanical response mechanism of rock under complex impact load is revealed.The main research results of this paper include:（1）The pore structure evolution process and microstructure damage degree of FT sandstone are analyzed in depth.Through a large number of microscopic tests on FT sandstone specimens,it is found that the clay mineral content of sandstone decreases after freezing and thawing,and tends to be stable after FT20 cycles.The increased pores in FT sandstone are mainly micropores,and the macropores have fractal characteristics.The uniformity of pore distribution is closely related to the initiation rate and expansion rate of extremely fine pores.The growth rates of porosity,permeability and porosity of sandstone specimens after freezing and thawing reached the maximum at FT80 cycles.The SEM image of FT sandstone has multifractal characteristics.After FT20 cycles,the multifractal phenomenon is more and more obvious,and the heterogeneity becomes larger.（2）The damage degradation mechanism and chaotic characteristics of porous rock in FT process are revealed.Based on the law of water migration in the FT process and the theory of water/ice phase transition,the FT damage degradation mechanism of rock is proposed,which is mainly based on ice segregation and ice growth,supplemented by water/ice phase transition volume expansion,and accompanied by hydration.Numerical analysis and elastic-plastic mechanics theory are used to analyze the stress of porous rock pore edge.It is found that macropores are more likely to cause rock failure,and the weights of micropores,mesopores and macropores in this study are calculated.A macroscopic FT damage evolution model considering saturation,phase transition critical pore size and pore weight is established,and the rationality of the model is verified.Based on the theory of dynamic fracture mechanics,the theoretical model of crack propagation velocity in FT rock is established,and the stability of the fixed point of the model and the chaotic characteristics are analyzed.（3）The effects of FT and loading rate on the dynamic splitting tensile properties of rock are analyzed.The FT sandstone specimen shows a failure mode dominated by tensile failure and supplemented by shear failure after splitting,and the specimen after dynamic splitting failure has a local fracture zone.The weaker the sandstone,the larger the area of the fracture zone.The effect of loading rate delays the half-life of sandstone,and the strengthening effect of loading rate of weak rock is more obvious.The absorption energy of FT sandstone is positively correlated with the loading rate and dynamic splitting tensile strength,and negatively correlated with the FT cycles,and its downward trend satisfies the exponential function relationship.（4）The effects of FT,strain rate and confining pressure on the dynamic compressive properties of sandstone are clarified.The fracture form of FT sandstone specimen after compression is mainly splitting failure.The sandstone specimen after dynamic uniaxial impact compression has obvious fractal characteristics,but the mass-frequency method for calculating its fractal dimension shows certain limitations due to the excessive weight of large fragments.The weaker the FT sandstone is,the more obvious the rate correlation is.An empirical model of dynamic compressive strength considering FT damage and impact velocity is proposed,and the rationality of the model is verified.The absorption energy and absorption rate in the process of dynamic uniaxial compressive failure are negatively correlated with the FT cycles and positively correlated with the strain rate,which satisfies the convex-power function relationship.Through mathematical analysis,considering the interaction between FT,strain rate and confining pressure factors,the order of influence is:strain rate>confining pressure>FT.The final result of the coupling of the three factors is the improvement of the dynamic compressive strength of sandstone.（5）The dynamic damage constitutive model of FT rock is constructed.Based on the existing strength-strain rate empirical model and unified strength theory,a new nonlinear dynamic strength criterion of rock is proposed,and the rationality of the criterion is verified.On this basis,a dynamic damage constitutive model of FT sandstone considering the double damage of FT and impact loading is established.The fitting degree between the model and the test data is greater than 0.97.Based on the Sobol-QMC method,the global sensitivity analysis of many input parameters in the constitutive model is carried out.It is found that the three factors₍（8）,_mes^′ and(8 in this study always have little influence,while,and_min^′ have great influence.