Analysis Of The Effect Mechanism Of Lode Angle In Predicting Ballistic Resistance Of Typical Metal Targets

In recent years,due to the challenges in the numerical simulation of flat-headed projectile impacting metal targets,some scholars have tried to introduce the Lode angle into the fracture criterion to improve the accuracy of numerical simulation.At present,limited research has found that the introduction of Lode angle can improve the accuracy of numerical simulations for aluminum alloy targets,however,it cannot improve the accuracy of numerical simulations of high-strength steel targets.(1)Aiming at the problem that the role of Lode angle in the numerical simulation of high-strength steel target ballistic limit velocity(BLV)is too little studied,in this thesis,ballistic experiments of a 5.95 mm diameter rigid flat-headed projectile impacting 4 mm thick40 Cr Ni Mo A and Weldox 900 E high-strength steel targets were conducted using a onestage light gas gun in the velocity range of 114.7 ～ 414.66 m/s.In order to obtain the material model parameters,mechanical tests such as quasi-static tensile,shear tensile,plane strain tensile,dynamic compression and high temperature tensile tests were carried out on 40 Cr Ni Mo A at room temperature;A modified form of the JohnsonCook(MJC)constitutive model is used to describe the stress flow behavior of the material,a modified form of the Lode angle-independent MJC fracture criterion and the Lode angle-dependent Mohr-Coulomb(MMC)fracture criterion to describe the fracture trajectory of material,the model parameters are calibrated by a combination of numerical simulation and experiment;The finite element calculation model of the impacting experiment was established,the numerical simulation results of Lodedependent and Lode-independent fracture criteria are compared,the result shows:(1)The fracture trajectory of 40 Cr Ni Mo A obviously depends on the Lode angle.(2)The failure modes of the two high-strength steels are shear plugging,the ballistic limits of40 Cr Ni Mo A and Weldox 900 E high-strength steel are 167.16 m/s and 172.93 m/s,respectively.(3)The ballistic limit predicted by the two fracture criteria are both very similar to the experiment,the shear plugging failure mode of the target is reasonably predicted.This indicates that the Lode angle has no effect in predicting ballistic limits of two high-strength steel targets against the impact of flat-headed projectiles.(2)In view of the problem that introducing the Lode angle into the fracture criterion shows two completely different effects in the numerical simulation of aluminum alloy and high-strength steel resisting the impact of flat-headed projectiles,in this paper,based on two failure mechanisms of adiabatic shear and ductile fracture,the ballistic limit of7075-T651 and 2024-T351 aluminum alloys,Weldox 700 E,Weldox 900 E,40 Cr Ni Mo A high-strength steel and 316 L stainless steel targets under the impact of flat-headed projectiles was simulated,where adiabatic shear is simulated by stress collapse.The result shows:(1)The adiabatic shear instability strains of 40 Cr Ni Mo A,Weldox 900 E and Weldox 700 E high strength steels are lower than the ductile fracture strains,the shear plugging failure of the target is mainly caused by adiabatic shear instability,therefore,even if the Lode angle is introduced into the fracture criterion,it does not affect the numerical simulation results.(2)For 7075-T651 aluminum alloy,2024-T351 aluminum alloy and 316 L stainless steel,the ductile fracture strain is lower than the adiabatic shear instability strain,the shear plugging failure of the target plate is mainly caused by ductile fracture,therefore,the introduction of the Lode angle into the fracture criterion can improve the numerical simulation accuracy.Therefore,whether the Lode angle can affect the numerical simulation results of the ballistic resistance behavior of the metal target is related to the failure mechanism of the target.