Study On Dynamic Mechanical Properties Of Red Sandstone Under Freezing And Thawing Cycles

China’s transportation engineering construction in the northeast and central and western regions has been strengthened,and mining resources extraction has flourished.As the water in the microfractures of the rock in the cold areas of the northeast and the plateau areas of the central and western regions is subject to phase changes due to freeze-thaw action,the rock is subject to freeze-thaw damage and deterioration of its physical and mechanical properties,which can easily cause safety hazards to the tunnel envelope and building foundations.In addition,during the tunneling and resource extraction process,the rock is also subjected to impact loading from blasting.Therefore,we study the deterioration of dynamic mechanical properties,energy dissipation law and fractal characteristics of rocks under the action of freeze-thaw cycles,and establish a dynamic ontological model of rocks considering freeze-thaw damage,and finally improve the test results through numerical simulation,in order to provide reference for engineering construction and engineering disaster prevention in cold regions.In this paper,the SHPB test system was used to conduct impact tests on red sandstone in Jiulongpo,Gansu Province,to investigate the effects of strain rate and freeze-thaw damage on the dynamic mechanical properties,energy dissipation patterns and fractal characteristics of red sandstone,respectively.The results show that the dynamic compressive strength,peak strain,dissipation energy,dissipation energy per unit volume and fractal dimension of fragmentation are all positively correlated with strain rate under the same number of freeze-thaw cycles,and have obvious rate effects,while the variation of elastic modulus with strain rate is not obvious.The strain rate sensitivity of dynamic compressive strength and dissipation energy per unit volume decreases with increasing freeze-thaw damage,while the strain rate sensitivity of peak strain increases with increasing freeze-thaw damage.At the same impact pressure,dynamic compressive strength,modulus of elasticity and dissipation energy per unit volume are negatively correlated with the number of freeze-thaw cycles,while peak strain and fractal dimension of fragmentation are positively correlated with the number of freeze-thaw cycles.The sensitivity of dynamic compressive strength,modulus of elasticity,peak strain and dissipation energy per unit volume to freeze-thaw damage increases with increasing impact pressure.Based on the combined element model theory and statistical damage theory,a dynamic visco-elastic-plastic model for freeze-thaw red sandstone was constructed,and the reasonableness and applicability of the model were verified,and the influence of the model parameters on the stress-strain curve was analysed.It is found that F₀reflects the average strength of the rock and affects the yielding stage of the stress-strain curve;E₁and E₂affect the pre-peak stage of the stress-strain curve and have the same effect pattern,with E₁having a more obvious effect at low strain rates and E₂at high strain rates;η₁andη₂have a certain enhancement on the curve as a whole,but the effect is small.Under the coupling effect of strain rate and freeze-thaw damage,the parameter m decreases with increasing strain rate（number of freeze-thaws）,F₀is positively correlated with strain rate and negatively correlated with number of freeze-thaws,both of which become less sensitive to strain rate as the number of freeze-thaws increases;η₁andη₂decrease with increasing strain rate（number of freeze-thaws）.The HJC model,which can well describe the dynamic mechanical behaviour of rocks,was chosen to combine the parameters of the concrete HJC intrinsic model with the experimental data to determine the parameters of the HJC model for red sandstone at different numbers of freeze-thaw cycles,and the impact compression test was simulated using ANSYS/LS-DYNA finite element software.It is found that the simulated curves are in good agreement with the test curves and the simulated damage states are similar to the actual damage states.The HJC model parameters determined in this paper can well characterise the dynamic mechanical behaviour and damage processes of red sandstone under different numbers of freeze-thaw cycles,which verifies the reasonableness of the model parameter determination method.