Mechanical Properties And Crack Propagation Of Rocks Containing Three-Dimensional Flaws Under Dynamic Loading

Rocks naturally contain many three-dimensional flaws,and when subjected to different dynamic loads,three-dimensional crack propagation can lead to rock damage.In order to study the effect of three-dimensional flaws on the mechanical properties and crack propagation of rocks under dynamic loading,dynamic loading tests were conducted on rocks containing surface flaws at different angles and depths,and the effects of dynamic strain rate and combined dynamic and static loading were investigated in this paper.Based on the dynamic test results,the damage of rocks containing three-dimensional surface and internal flaws under dynamic loading is simulated using elastic-plastic state-based peridynamics,and the main research contents and conclusions are as follows.(1)Dynamic loading tests were conducted on sandstone specimens containing a three-dimensional surface flaw with different angles and depths using a modified split Hopkinson pressure bar(SHPB)device.A high-speed camera with digital image correlation method was used to record the crack initiation,propagation and failure process of specimens and to monitor the surface strain field.The test results show that both the angle and depth of the surface flaw have an important influence on the dynamic mechanical properties and energy dissipation of the specimens.The specimen is least weakened at a surface flaw angle of 90°,with the angle having a greater effect on dynamic strength and energy absorption at greater depths.Analysis of the strain fields by DIC shows that the crack initiation is due to a combination of tensile and shear stresses.Both the angle and depth of the surface flaw have a significant effect on the failure patterns.Specimens show shear failure at surface flaw angles less than or equal to 60° and tensile failure at surface flaw angles of 75° and 90°.When the angle of surface flaw is less than or equal to 60°,there are more shear cracks at greater depths.(2)Dynamic compression tests were conducted on sandstones containing surface flaws of different angles and depths at three impact air pressures.Correlation analysis of the dynamic mechanical results was carried out using Pearson correlation coefficient.Crack initiation,propagation and failure characteristics were monitored using a high-speed camera combined with digital image correlation methods.The results show that strain rate plays an important role in the dynamic strength,peak strain,elastic modulus,energy evolution,crack propagation and failure modes of surface-flawed rocks.The strain rate has different effects on the dynamic strength of specimens containing surface flaws at different angles and depths.The dynamic strength of the surface-flawed specimen is more sensitive to the strain rate when the surface flaw is deeper.The peak strain and elastic modulus of specimens containing surface flaws increase with increasing strain rate.The absorbed energy of the surface-flawed specimen increases as the strain rate increases,but the energy absorption rate decreases.As the strain rate increases,the strain concentration zone appears earlier and cracks propagate more rapidly.At higher strain rates,there are more shear cracks and a shift in failure mode towards shear failure.(3)Different dynamic and static combination loading tests were conducted on sandstone with surface flaws,and the results showed that the effect of axial pressure on the mechanical properties of specimens with surface flaws of different angles and depths was different,and it was related to the angle and depth of the surface flaws.The dynamic strength of the specimens with surface flaws generally decreases with the increase of the axial pressure,and the dynamic strength of the combined dynamic and static strength generally increases with the increase of the axial pressure and is greater than the dynamic strength.The absorbed energy of specimens with surface flaws decreases with the increase of axial pressure,which is consistent with the change of strength.The crack initiation and propagation processes of the specimens indicate that the axial pressure inhibits the propagation of the wing cracks.The failure mode of specimens with surface flaws without axial pressure is mixed tensile-shear damage or tensile damage,and the failure mode is X-type shear damage or mixed tensile-shear damage under combined dynamic and static loading,and the axial pressure changes the failure mode to shear damage.(4)Based on the results of previous experiments,an elastic-plastic ordinary state-based peridynamic model that considers the compressive and tensile strength ratio is used to simulate the failure process of rocks containing three-dimensional surface flaws at different angles and depths under dynamic loading.The effect of the surface flaw angle,depth and double surface flaws on the dynamic strength and failure of the specimen is investigated.The simulation results show that the proposed peridynamic model can well simulate the propagation of three-dimensional cracks in rocks and the failure process of specimens.The dynamic strength of the specimen increases as the angle of the surface flaw increases.The weakening effect of surface flaw depth on the strength of the specimen is more pronounced at angles less than 60° and limited at angles greater than 60°.Peridynamic simulations can be used to obtain strength variation regularity that are consistent with the experimental results.The crack generated on the surface of the specimen penetrates to a certain depth inside the specimen,which is related to the depth of the surface flaw.Shell-like cracks generated inside the specimen will join with cracks generated on the surface to form complex three-dimensional cracks.Surface flaws have a greater effect on the damage of the rock when rotated around the X and Z axes,and a smaller effect when rotated around the Y axis.The depth of the double surface flaws has a greater effect on the strength of the rock.(5)The mechanical properties and crack propagation patterns of rocks with internal coin-type flaws at different angles and positions under dynamic loading are simulated using elastic-plastic state-based peridynamics.The results show that the peridynamics can well simulate the propagation of the internal cracks in the rock and obtain the variation law of the dynamic strength of the rock.The mechanical properties and crack propagation of rocks containing coin-type flaws are related to the position and angle of the flaws.When the coin-type flaw rotates around the X and Z axes,the strength of the specimen increases with the increase of the crack angle,and the strength of the specimen is lower when the crack is closer to the surface of the specimen.When the flaw rotates around the Y-axis,the strength of the specimen does not change significantly with the angle.When the flaw is not in the center of the specimen,the crack inside the specimen is more likely to extend to the close specimen surface.A strength estimation method considering the equivalent cross-sectional moment of inertia is proposed,which can effectively estimate the strength of rocks containing three-dimensional flaws and also accurately present the variation of rock strength with flaw shape.