Research On Several Problems Of The Ballistics Material Response Under Impact

At present,concrete is used as most of the assessment target material for the earth penetration weapon(EPW)already equipped.While the existing major military projects are mostly built in rock to provide protection capabilities.Generally,the difference between rock and concrete(density and strength)is large,especially the resistance formed by dense hard rock is much higher than that of concrete.During the process of rock penetration,the projectile is easy to break,and even cannot invade the rock effectively.This means that the EPW successful in the concrete target assessment probably have poor ability to damage the high-strength rock targets.In addition,investigations on the rock penetration are not so rich and deep as those on concrete penetration.And the experimental data of rock penetration already published is far less than that of concrete data.Taken into account this situation,projectiles with the same shape but different shell thickness are designed and then the experiments for penetrating concrete targets and high-strength rock targets at about 1000m/s are carried out.The experimental results of concrete penetration indicate the projectiles invade effectively and only a small amount of abrasion marks without significant deformation can be found on the projectile surface.While those of rock penetration indicate that all projectiles are completely fractured and fail to effectively penetrate the rock targetand only the surface of rock target is comminuted.For projectile fracture,both factors of the material strength failure and the projectile structure damage are discussed to theoretically analyze the projectile fragmentation with the fracture morphology,the latter of which in the form of the dynamic plastic buckling failure gives an satisfactory explanation for the fracture pattern of petal shaped fragments with three-hinges structures employed.In addition,the numerical model is established by Autodyn based on experimental results.By taking the failure model of Mott stochastic distribution into account for the projectiles,the numerical simulations are carried out with different discrete forms to reveal the projectile fracture mechanism and show high credibility.Furthermore,the influences of the filling and different impact conditions including velocities,angles of attack(AOA)and incidence angles on the projectile fracture pattern are discussed.At the same time,the projectile structure during concrete penetration is verified and the concrete penetration process is reproduced.The experimental results and the adopted numerical method with high credibility can be referred to further study on the projectile fracture and the structure stability during high-velocity penetration.Commercial softwares(such as Ls-dyna,Autoduyn)deal with temperature rise using an adiabatic model in impact dynamics simulations.In fact,impact dynamics problems,especially adiabatic shearing,usually involve large localized plastic deformations.Heat is highly concentrated at local locations(causing temperature rise).Increased temperature can affect material properties and inevitably cause heat diffusion.That,heat conduction,will change the temperature field.In addition,when the solid temperature changes,once it is constrained,it will form "thermal stress"or "temperature-varying stress" which limits the generation of thermal deformation corresponding to the temperature change.Obviously,the formation of thermal stress will also affect the stress state of the material,which means that the instantaneous temperature field and stress field changes will affect the overall performance of the material,and then have an impact on the entire impact process.Then based on the study of Finite Element Method(FEM)and Smoothed Particle Hydrodynamics(SPH)method,the thermal-mechanical coupling with thermal conduction and thermal stress is introduced into these methods,and then the numerical methods including thermal-mechanical coupling are established.The numerical calculations of some typical problems are conducted and the influences of thermal-mechanical coupling on calculations are analyzed.And the simulation results indicate that the heat conduction has a certain effect on impact shear simulation.The thermal conduction influences the temperature field and the stress field obviously.And the change of the thermo stress has little effects on the temperature field but great effects on the pressure field.And the thermo-mechanical coupling eventually has a certain effect on the final impact results.It is foreseeable that thermo-mechanical coupling is even more important in physics where the temperature is higher,the deformation is more intense,and the pressure is higher.Therefore,it is necessary to consider thermal coupling in the numerical calculation of impact shear,and it will make the numerical calculation result more accurately reflect the physical facts.The work of this paper can provide reference for the research of thermo-mechanical coupling in the process of impact dynamics.