Study On Mechanical Response Characteristics Of Drop Hammer Impact Of Frozen Single-fractured Sandstone

In the mining process of mineral resources in plateau alpine region,the instability phenomenon of engineering rock mass affected by internal and external factors such as joint cracks,low temperature environment and dynamic load disturbance occurs frequently.In order to further explore the influence mechanism of main control factors such as fracture occurrence,freezing temperature and impact energy on rock instability and failure characteristics,this paper studies the drop hammer impact mechanical response characteristics of frozen single-fractured sandstone,aiming to provide theoretical instruction for the implementation of related engineering activities.The main contents and results are as follows:First,the drop hammer impact test of single-fractured sandstone under different fracture dip angles,different drop hammer heights and different freezing temperatures was carried out.The influence of different factors on the failure characteristics of single-fractured sandstone was discussed by controlling the test variables.The orthogonal experiment with 3 factors and 5 levels was designed and the factors were analyzed.The results show that with the increase of fracture dip angle,the ultimate strain and its corresponding time ratio of sandstone change in the form of“M”.With the increase of drop hammer height and the decrease of freezing temperature,the ultimate strain increases and its corresponding time ratio decreases.At the same time,the influence of fracture dip angle and freezing temperature on the ultimate strain is remarkable,and the influence of freezing temperature on the time ratio corresponding to ultimate strain is remarkable.The final failure mode of sandstone is tensile failure,which is more affected by the fracture dip angle,and the cracks are mainly tensile cracks.At the same time,after sandstone failure,the macroscopic cracks are mostly parallel to the direction of impact force,that is,the failure angle is generally 0°～10°,and is most affected by the fracture dip angle.Secondly,the numerical analysis of impact failure of frozen single-fractured sandstone under impact load was carried out by PFC^2D software.The results show that the deformation and failure process of numerical simulation of sandstone samples under different conditions has experienced four stages:elastic deformation stage,crack initiation and stable propagation stage,crack unstable propagation stage and post-peak stage.The peak stress decreases with the increase of fracture dip angle,increases with the increase of drop height and the decrease of freezing temperature.The sandstone specimen mainly produces tensile cracks,and most of them are compressive stress near the macro failure surface.The displacement at the failure surface of the sample is large,in addition,the particle displacement of the sample increases with the drop height and freezing temperature.The crack initiation location,propagation process and coalescence mode of sandstone specimens under different conditions are different,but the crack propagation process below the prefabricated crack is often slow and the number of cracks is small.Subsequently,the parameters of the damage evolution constitutive model are calculated based on the Logistic function,and the strain-dependent damage evolution equation of frozen single-fractured sandstone is established.Then,the damage evolution constitutive model of frozen single-fractured sandstone under impact load is constructed.The obtained model can accurately describe the stress-strain curve characteristics of frozen single-fractured sandstone.Finally,NMR technology was used to measure the pore parameters of sandstone under freezing state and after thawing,and the strength strengthening mechanism of frozen single-fractured sandstone was discussed.The results show that the change rate of porosity of frozen sandstone continues to decrease with the decrease of freezing temperature.After freezing and thawing,the change rate of sandstone porosity continues to increase with the decrease of freezing temperature,and the frost heaving damage variable basically shows a linear growth trend.The strength enhancement after freezing mainly depends on the effect of pore ice.At the same time,the frost heaving damage and the supercooled water after the impact will inhibit the strength growth of frozen sandstone.