Experimental Study And DEM Simulation On Dynamic Penetration Of Cylindrical Specimen Into Quartz Sand

In order to reveal the penetration mechanism of rapid penetration into granular materials,the dynamic penetration of cylindrical specimens into quartz sand is studied and analyzed from two aspects of experiment and DEM simulation.Twelve cast iron cylindrical specimens with different lengths,diameters and nose shapes were used to impact the sand model at the speed of 3m/s and 6.5m/s,respectively.The high speed camera was used to take pictures at the frame rate of 1000 fps.The image processing and analysis were carried out by Image J,and the penetration velocity-time curves and displacement-time curves were obtained.Then the penetration history curves were analyzed,and the influences of different lengths,diameters and nose shapes of the specimen and the initial penetration velocity on the dynamic penetration depth,velocity and acceleration were obtained.The main work of DEM simulation is as follows: Firstly,the direct shear model was established according to the conducted direct shear tests,and the meso-mechanical parameters of quartz sand were determined by fitting the stress-strain curves obtained from the tests.Then,the penetration test model was established by PFC2 D,and the penetration history curves of different initial penetration speeds,different lengths,diameters and different nose shapes of the specimens were analyzed.To verify the rationality of the model and the accuracy of the test conclusions,we compared the simulation results with the tests.Considering the shape of quartz sand particles,the modeling of irregular particles was carried out,and the results of DEM were compared with those of regular sphere particles.The influence of the sphere and nonshpere models on the penetration time curves and the difference of the penetration results were analyzed.MATLAB was used to fit the kinetic energy and depth curves of the experiment and simulation to analyze the energy change in the penetration process.According to the displacement diagrams of quartz sand and the force chain configurations at different time obtained by DEM,the dynamic response of particles,the interaction between cylinder specimen and particles,the force of particles and changes in the porosity of particles in the process of dynamic penetration were all analyzed on the meso-view.It is found that the penetration depth of dynamic penetration is proportional to the initial penetration velocity,but the penetration time is inversely proportional to the initial penetration velocity when the velocity is of the same order of magnitude.When the mass difference is small,the effect of specimen length on penetration depth is greater than that of specimen mass and diameter.When the diameter and length of the specimen are the same,the influence of the nose shape of the specimen on the penetration depth is greater than that of the mass of the specimen.The velocity variation situations of the long and short specimens are different.The velocities of the long specimens change steadily in the whole penetration process,and the accelerations show no significant difference.The velocities of the short specimens change greatly in the early period and are tend to be stable in the later period.The energy during penetration decreases exponentially with depth.In the process of dynamic penetration,the particles will splash out.There is a certain gap between the cylinder specimen and the surrounding particles,and the V-shaped displacement field of the particles decrease with the distance.The particles are subjected to concentrated force at the beginning,and the force chain configuration is semicircular umbrella-shaped and spreads outward later.After penetration,the particles around the specimen are subjected to less force,which is the tensile force zone,while the particles at the bottom are subjected to greater force,which is the compressive stress zone.