Study On Damage Mechanism And Rate-Temperature Constitutive Models Of Frozen Soil Under High Strain Rate Loading

The mechanical properties of frozen soil are sensitive to temperature changes and rheology.Therefore,they have attracted substantial attention in the field of cold regions.In the construction process of a few high latitude areas,the frozen soil not only bears quasi-static load but also faces the impact loads due to tunneling,drilling,and explosion.Therefore,studying the strength,deformation,and energy dissipation of frozen soil under different impact states is imperative.The initiation and development of internal defects under impact load is the physical source of failure.Additionally,a high-speed load can raise temperature and cause phase transition,which affects the strength deformation response of frozen soil.Therefore,the frozen soil exhibits a typical thermal and mechanical coupling damage behavior during the impact process.We consider that the mechanical properties of frozen soil at macro scale are closely related to its microstructure.Therefore,a reasonable constitutive model should be built for providing theoretical guidance for engineering design to predict the thermal–mechanical coupling damage response of frozen soil under different loading modes.In this study,the dynamic mechanical characteristics and energy consumption of frozen soil under different impact histories are studied with the help of split Hopkinson pressure bar(SHPB)experiment technology.Furthermore,focusing on the analysis of the damage and deformation mechanisms of frozen soil,the corresponding thermo–mechanical coupled damage models are constructed based on different theoretical frameworks.The major findings and research conclusions include:(1)The mechanical behavior and energy consumption of frozen soil under single impact compression are studied by SHPB.The experimental results show that the compressive strength of frozen soil under different stress states increases linearly with the decrease in temperature and the increase in strain rate.Additionally,when the temperature and stress state are determined,the critical strain and impact toughness increase gradually as a power function with strain rate.The results of energy calculation show that the specific energy absorption value(SEA)of frozen soil increases exponentially with the strain rate and incident energy at the same temperature.When the stress state and loading condition are fixed,an evident negative correlation between SEA and temperature is observed.Additionally,the stress state significantly influences the SEA value of frozen soil due to the difference in energy consumption ratio of defects.(2)Based on the cyclic impact experiment,the strength deterioration,and fatigue deformation characteristics of frozen soil are systematically analyzed.The results show that frozen soil’s strength and deformation modulus show decreases with an increase in cycle time.Impact air pressure and temperature significantly influence the energy consumption of frozen soil,showing that with the increase of impact air pressure and the decrease of temperature,the average dissipated energy shows an increasing trend.In addition,with the accumulation of defects and the attenuation of wave impedance at the later stage of impact,the SEA value exhibits a nonlinear decreasing law with the increase of the number of cycles.In contrast,the cumulative SEA value increases more rapidly than the previous loading,and the two exhibit a certain rate correlation.(3)Within the framework of the statistical damage theory and viscoelastic theory,the damage viscoelastic dynamic constitutive model of frozen soil is reconstructed through the specific relationship between each element and damage in a Maxwell’s body,thereby explaining the physical nature of the damage law of each element due to different mechanical properties.Based on the transient temperature rise effect,the evolution law of stress attenuation,caused by load-induced phase transition,is analyzed,and the rate-temperature sensitive effect and defect evolution law of frozen soil are described.(4)Three defect evolution mechanisms(crack propagation,hole collapse,and phase transformation induced by temperature rise)are introduced under the viscoplastic theory and small deformation framework,with a focus on the multi-step damage behavior caused by defect development under impact load.Additionally,a thermo–mechanical coupled viscoplastic damage model is constructed to describe the dynamic response before the peak.Furthermore,based on the improved cohesive crack model,the mechanical response and toughness evolution law of frozen soil at softening stage are described and predicted,in conjunction with the nonlinear deformation characteristics of brittle materials in the fracture process.(5)Considering the internal microstructure characteristics of the material under the framework of micromechanics,the frozen soil matrix is regarded as a three-phase composite material composed of soil particles,unfrozen water,and ice particles.In contrast,the pore phase and microcrack phase are regarded as internal inclusions.Based on Mori–Tanaka theory,Voigt minimum potential energy principle,and Eshelby equivalent inclusion theory,a mesoscopic multi-scale model of frozen soil are developed.Considering the relationship between defect propagation and macroscopic variables,a damage equation characterizing frozen soil deterioration is obtained based on work(4).Finally,the rate-temperature deformation behavior of frozen soil under different load levels is described.(6)Based on the work(5),considering that the equivalent modulus of frozen soil with pores calculated by Eshelby inclusion theory is similar to that determined by the defect model,the meso thermal–mechanical coupling model is simplified.In view of the evolution characteristics of defects under cyclic loading,the wing crack model,inelastic deformation mechanism,impact temperature rise equation,and hole collapse model are used to reflect the coupling interaction of internal damage.Finally,based on the energy dissipation law,the thermo–mechanical coupling cyclic meso-model of frozen soil under one-dimensional strain is constructed,and the strength deformation characteristics of frozen soil under cyclic disturbance are successfully predicted.