Effect Of Lode Angle In Predicting The Ballistic Resistance Of Weldox 700 E Steel Target

In recent years,many terrorist attacks have attracted extensive academic attentions,and the impact dynamics has become a research focus in explosion mechanics.In the impact of metal targets,geometric nonlinearity,material nonlinearity,high strain rate and high local temperature rise are often involved,and the increase of temperature will affect the flow behavior and fracture failure of materials.The existence of these factors requires accurate characterization of the plastic flow and fracture behavior of the material.Particularly,in recent years,many studies have found that the fracture of many metal materials is related to the stress triaxiality as well as the Lode angle.However,at present,the fracture criterion with only stress triaxial correlation is still widely used in the numerical prediction of the ballistic resistance of metallic target plates.Whether the introduction of Lode angle into the fracture criterion can promote the numerical prediction results of anti-penetration performance still needs to be revealed.In this thesis,the numerical prediction of ballistic resistance of Weldox 700 E steel targets subjected to blunt projectile impact was performed,and the influence of the incorporation of the Lode angle in a fracture criterion on the numerical prediction results of ballistic resistance of Weldox 700 E steel targets was revealed through mechanical test,constitutive model and fracture criterion calibration,ballistic test and numerical simulations.The main work includes:(1)Mechanical properties tests,material model establishment and calibration Mechanical tests include: tensile tests on smooth and notched round bars under quasi-static conditions at room temperature,plane stress tensile tests and plane shear tests;tensile tests at 300℃~ 900℃ temperature range;tensile tests on smooth round rods at room temperature with tension speed from 200 mm/min to 900 mm/min;Split Hopkinson Pressure Bar tests.The corresponding stress-strain curve,yield strength,fracture strain,stress triaxiality and Lode angle at fracture were obtained through the above mechanical properties tests.Based on the experimental results,a constitutive model suitable for the material was established,and the constitutive model and fracture criterion parameters were calibrated.Involved fracture criteria were the modified Johnson-Cook(MJC)fracture criterion and the Modified Mohr-Coulomb(MMC)fracture criterion.(2)Ballistic tests on 4 mm thick Weldox700 E steel targets using projectiles of 5.95 mm diameter and 29.8 mm length in the impact velocity range of 141.2 ~ 275.7 m/s.It was found that the failure mode of the target plate was shear plugging,and the ballistic limit was 190.7m /s.(3)Numerical simulations on the ballistic resistance of Weldox 700 E steel plates.In the simulations,a modified Johnson-Cook constitutive model was adopted with either the MJC fracture criterion or the MMC fracture criterion.Meanwhile,the mesh size dependence of the ballistic resistance prediction was studied.(4)Numerical shooting finite element models with different unit sizes were established,and the above calibrated MJC constitutive model and MJC and MMC fracture criteria were used to conduct numerical simulation analysis of the shooting test.The following conclusions are drawn based on the above mentioned work:(1)The fracture of Weldox 700 E steel is both stress triaxiality and Lode angle dependent.(2)The predicted ballistic limit velocities of the Weldox 700 E steel plates using the MJC fracture criterion and MMC fracture criterion are almost the same,and the fracture mode of shear plugging is reasonably predicted by the two groups of simulations.In other words,the introduction of Lode angle into a fracture criterion affect marginally the ballistic resistance prediction of Weldox 700 E steel plates struck by blunt projectiles.(3)The numerical prediction on the ballistic resistance of Weldox 700 E steel plates struck by blunt projectiles is of high mesh size sensitivity and the simulations with an element size of(17)(15)?m×(17)(15)?m can produce reasonable predictions for both the MJC and MMC fracture criteria.Finally,simulations were conducted on fictitious metals and it was found that for metal targets with low ductility the simulations using a Lode dependent fracture criterion would predict a lower ballistic limit velocity compared with that using Lode independent fracture criterion.However,for targets of high ductility,two series of simulations would give quite close predictions.