Study On The Influence Of Prefabricated Cracks On The Mechanical Properties Of Red Sandstone

Due to the long-term experience of complex geological processes,cracks and defects are common in rocks,and most of the rock masses are subjected to dynamic loads in actual projects.This paper takes red sandstone and red sandstone with prefabricated cracks as the research object,which used uniaxial compression testing machine and Split Hopkinson Pressure Bar to carry out the test,and combined with the RFPA numerical test of red sandstone with cracks,to explore the influence of prefabricated cracks on static and dynamic mechanical properties and failure modes of red sandstone.The main research contents and conclusions are as follows:(1)Analysis of static and dynamic mechanical properties and failure modes of intact red sandstone.Use pressure testing machine and SHPB to carry out static uniaxial compression test and dynamic impact loading test on red sandstone.At the same time,determine the relevant physical indicators of red sandstone.The crack strain model is used to analyze the crack propagation of red sandstone under uniaxial static load,and the five stages of the microcrack propagation process of the sample and the corresponding stress threshold values of each stage are obtained.The dynamic test shows that there is a power relationship between the dynamic compressive strength and the strain rate,and the dynamic intensity factor increases as the strain rate increases.The energy consumption density of crushing increases linearly with the increase of strain rate,and there is a critical strain rate that is 43.63s^-1 that makes the energy consumption density of crushing to zero.After collecting the broken samples,it is found that with the increase of strain rate,the average crushing size and fractal dimension after broken linearly increase.(2)Experimental study on static mechanical properties and crack propagation mode of red sandstone specimens under the influence of prefabricated cracks.The prefabricated cracks red sandstone is subjected to uniaxial quasi-static load test and the crack propagation process is recorded.Compared with the complete red sandstone,the static mechanical parameters of the prefabricated cracks red sandstone are all reduced.On the stress-strain curve,there will be a sudden drop of stress before reaching the stress peak,and when the crack angle is 45°,the mechanical parameters are the smallest.During the uniaxial static load process,the first cracks in the red sandstone with prefabricated cracks are tensile cracks,and shear cracks only appear in the 30°and 45°crack samples.The crack initiation point of the 60°and 90°crack specimens is far away from the tip of the prefabricated crack.When the crack angle is less than 60°,all the crack initiation points appear at the tip of the prefabricated crack.(3)Experimental study on dynamic mechanical properties and failure modes of red sandstone affected by prefabricated crack.Applying Split Hopkinson Pressure Bar carry out impact loading test on red sandstone with prefabricated cracks and use high-speed camera to capture crack growth process.It is obtained that the dynamic compressive strength increases with the increase of the strain rate regardless of the crack angle.When the same strain rate is applied,the different prefabricated crack angles will affect the mechanical parameters of red sandstone.In the first stage of crack initiation,the specimens with different angles will generate small shear bands at the tips of the prefabricated cracks,and then cracks where near the shear bands will be generated and propagate.With the increase of the crack angle,the crack initiation time will be prolonged,and the types of secondary cracks in the specimens with different angles will be different,but the final failure mode is centered on the prefabricated crack to form an X-shaped crack fracture zone with a lot of Rock powder.The energy consumption density of fracture of specimens with different crack angle will increase with the increase of strain rate.At lower strain rate levels,the crack angle has basically no effect on the fracture energy density.When the strain rate is increased,the effect of the fracture angle is obvious.Under the impact of different strain rates of red sandstone with prefabricated cracks,the trend of average crushing size with strain rate shows a negative linear relationship,and the fractal dimension increases with the increase of strain rate.(4)Numerical simulation study on dynamic impact of fractured red sandstone.Using RFPA-dynamic calculation software to carry out numerical experiments on red sandstone with prefabricated cracks,it is found that when the peak value of the stress wave is small,there are fewer cracks and fewer acoustic emission events(AE)when the sample is damaged.When the peak value of the stress wave increases,the number of cracks increases when the sample is damaged and the AE number also increases.With the increase of the peak value of the stress wave,when the cracked specimen is destroyed,the cracks gradually increase from a single crack to form an X-shaped shear crack fracture zone.When the peak value of the stress wave is small,the crack angle has little effect on the number of AE events when the sample is broken.When the peak value of the stress wave increases,the crack angle has a significant effect on the number of AE events when the specimen is damaged:With the increase of the crack angle,the cumulative AE number of sample damage first increases and then decreases.In general,the cumulative AE number of sample damage at 45°,60°,and90°fluctuates slightly,but it is basically at the same level.