Study On Dynamic Mechanical Properties Of Sandstone With Ice-saturated Flaw Under Impact Load

With the development of mineral resources such as oil and natural gas in the alpine region,the rock mass in the cold region is frequently disturbed by mining disturbance,earthquake,blasting and other loads,and the cracks in the rock mass will be activated and developed,connected and formed a macroscopic penetrating fracture surface,resulting in the instability of the rock mass from time to time,which seriously affects the life safety of the construction personnel.It can be seen that the dynamic characteristics and instability failure characteristics of frozen fractured rock play an important role in the safety of engineering in cold regions.It is an important subject to study the dynamic characteristics and failure characteristics of frozen fractured rock.In this thesis,through the combination of indoor test and numerical simulation,the drop weight impact test of ice-saturated flawed sandstone with different flaw lengths and different flaw dip angles was carried out,and the dynamic response of ice-saturated fractured sandstone was studied.The influence mechanism of negative temperature on the strength and crack propagation of fractured sandstone was revealed,and the influence of flaw length and flaw dip angle on the damage evolution law of ice-saturated flawed sandstone was discussed.The main research results are as follows :Firstly,the drop weight impact tests of ice-saturated single-flawed and ice-saturated parallel-flawed sandstone were carried out,and the effects of flaw length and flaw dip angle on the characteristics of strain time history curves,crack propagation process and failure mode were analyzed.The results show that the variation trend of strain time history curve of ice-saturated flawed sandstone is basically the same,which increases approximately linearly and decreases approximately linearly in loading and unloading stages respectively.With the increase of flaw dip angle,the peak strain of specimens in L8 and L30 groups decreased first and then increased,the peak strain of specimens in L15 group,L20 group,CD group and DD group showed a change of ’ increase-decrease-increase’,while the peak strain of specimens in CC group and DC group showed a change of ’decrease-increase-decrease’.With the increase of flaw length,the peak strain of single-crack specimen decreases first and then increases when the flaw is horizontal,and increases first and then decreases when the flaw is inclined or vertical.The transfixion failure of ice-saturated flawed specimens is mostly dominated by tensile cracks,which tend to split when the flaw is horizontal or vertical,and tend to tensile failure when the flaw is inclined.Secondly,the discrete element particle flow code(PFC2D)was used to simulate the impact test process of ice-saturated flawed sandstone,and the influence of flaw occurrence on the dynamic performance and crack evolution of ice-saturated flawed sandstone was analyzed.The results show that increasing the number of impacts will reduce the slope of the stress-strain curve,but increase the envelope area of the stress-strain curve.The crack initiation position of ice-saturated flawed specimens is at the flaw ice,which is independent of the flaw dip angle and flaw length.Before the stress reaches the peak,the surface of the specimen is dominated by the compression force chain distribution,and after the stress reaches the peak,it is dominated by the tension force chain distribution.With the increase of flaw dip angle,the crack initiation stress and damage stress of specimens in L8 group and L20 group increased first and then decreased.The crack initiation stress and damage stress of specimens in L15 group,L30 group,CC group and DC group decreased first and then increased.With the increase of flaw length,the damage stress of single flaw specimens in horizontal or vertical flaw shows the change law of ’increase-decrease-increase’,and the damage stress in inclined flaw shows the change law of decrease first and increase then.When the ice-saturated single specimen flaw is horizontal or vertical,the maximum displacement is on the upper left and upper right of the specimen.When the flaw is inclined,the maximum displacement is on the left side of the specimen.When the flaw is horizontal or inclined,the minimum displacement is in the stress concentration area of the loading end.When the flaw is horizontal or vertical,the displacement of the left side of the specimen is smaller than that of the right side.Then,The influence mechanism of negative temperature on strength improvement and crack propagation of flawed sandstone is analyzed from macroscopic and mesoscopic perspectives,and the influence law of flaw occurrence on damage evolution is discussed.The results show that the strengthening effect of negative temperature on flawed sandstone is reflected in the filling,support and bonding tensile effect of ice on flaws at the macroscopic level.At the mesoscopic level,it is reflected in the shrinkage of internal mineral particles and the filling and supporting effect of ice on pores.When the flaw dip angle is the same,the single impact damage value of single flaw specimens and DD and CD group specimens increases with the increase of impact times,while the single impact damage variable of DC and CC group specimens decreases with the increase of impact times.When the flaw length is the same,the single impact damage variable of the single flaw specimens when the flaw is level increases first and then decreases with the increase of the impact times.The single impact damage variable increases with the increase of the impact times when the flaw is inclined or vertical.The single impact damage variable of the parallel flaw specimens when the flaw is level or vertical will increase with the increase of the impact times.Finally,the validity of the damage constitutive model of ice-saturated flawed sandstone proposed in this article is verified by numerical simulation results.The results show that the calculated value curve is agree well with the experimental results,which can describe the pre-peak stress-strain relationship of ice-saturated flawed sandstone under impact load,and can be used to analyze the damage evolution characteristics of ice-saturated flawed sandstone in the pre-peak stage.