RESPONSE OF METALLIC PLATES TO SMALL ARMS PROJECTILE IMPACT

Abstract

The present study is based on the experimental and finite element investigations of the response of metallic plates subjected to small arms projectile impact. The mild steel, Armox 500T steel and 7075-T651 aluminium alloy plates of various thicknesses were impacted by Armor Piercing Incendiary (API) projectiles at increasing angles of incidence until the occurrence of projectile ricochet. The Armox 500T steel targets of thicknesses 6, 8 and 10 mm were impacted by 7.62 API projectiles and those of thicknesses 5, 10, 15 and 20 mm impacted by 12.7 API projectiles. The 7075-T651 aluminium targets of thicknesses 20, 32, 40 and 50 mm were impacted by 12.7 API projectiles. The 7.62 API projectiles were fired through sniper rifle and 12.7 API projectiles through air defence gun. The incidence velocity of both of these projectiles was close to 820 m/s. The incidence and residual projectile velocities were measured with the help of infrared optical measurement device. The high speed video camera was employed for recording the residual projectile velocity as well as for studying the mechanics of penetration and perforation. The experimental results thus obtained were simulated by carrying out the three-dimensional finite element analysis on ABAQUS/Explicit finite element code. The ballistic performance of mild steel targets of thicknesses 4.7, 6, 10, 12, 16, 20 and 25 mm was also studied against 7.62 API projectiles by performing the finite element simulations and the results thus obtained were validated through the experiments performed by Gupta and Madhu (1992, 1997). The numerical simulations enabled the determination of ballistic limit for all the target materials at normal impact. The characterization of the target material was carried out under varying stress state, strain rate and temperature. The possible anisotropy of a material was studied by extracting vi the flat specimens in three different orientations i.e., 0°, 45° and 90° from the middle of the thickness of the plate. The mild steel and Armox 500T steel plates were found to be isotropic while 7075-T651 aluminium alloy plates have been found to possess high degree of anisotropy. The tension tests were carried out on smooth cylindrical specimens at a constant strain-rate, 6 x 10-4 s-1. The gauge diameter of the cylindrical specimen was 6.25 and the gauge length 25 mm. The diameter reduction of the specimen was measured up to fracture and the true stress-strain relationship was obtained. The influence of stress triaxiality was studied by performing quasi-static tests under tension on notched cylindrical specimens with initial notch radius varying from 0.4 to 10 mm. The results thus obtained revealed a decrease in ductility and an increase in strength of the material with increase in stress triaxiality. The low, medium and high strain rate tension tests were performed on smooth cylindrical specimens of diameter 3 mm and gauge length 10 mm. The strain rate in the range 1 x 10-4 s-1 - 1500 s-1 was obtained on Universal Testing Machine and Split Hopkinson pressure bar apparatus. In general, the strength of the material has been found to increase and the ductility decreased with increase in strain rate. The thermal sensitivity of the material was studied by performing quasi-static tension tests at varying temperature on cylindrical specimens of diameter 6.25 mm and gauge length 25 mm. A portable furnace with 220 mm height and 60 mm internal diameter was employed for controlling the temperature of the specimens during testing. The tests were carried out in the temperature range of 27 °C to 900 °C. The flow stress of all the materials has been found vii to increase initially with increase in temperature from 100 °C to 300 °C due to blue brittle effect, and subsequently it decreased with further increase in temperature. The material of 7.62 API and 12.7 API projectiles was also characterized at varying stress triaxility, strain rate and temperature. The material coupons were extracted from the hardened steel core of the bullet. The material tests thus performed enabled the calibration of the Johnson-Cook (JC) elasto-viscoplastic constitutive model for the target as well as the projectile material. All the parameters of Johnson-Cook flow and fracture model were calibrated through curve fitting method. The calibrated material parameters of the JC model were validated by simulating the high strain rate material tests conducted on Split Hopkinson Pressure Bar (SHPB). The ABAQUS/Explicit finite element code was employed for carrying out the axi-symmetric simulations for the validation of the material model. The experimental and numerical results with respect to failure mechanism, residual projectile velocity, maximum angle of perforation and critical angle of ricochet have been compared. A close correlation between the experimental findings and the predicted results has been found. In general, the ballistic resistance has been found to increase with increase in angle of obliquity. Moreover, the critical angle of projectile ricochet and maximum angle of perforation was found to decrease with increase in target thickness. The ballistic limit of 10 mm thick Armox 500T steel has been found to be 100% higher than the equivalent mild steel target against 7.62 API projectile. The ballistic limit of 20 mm thick Armox 500T steel has been found to be 55% higher than the equivalent 7075-T651 aluminium target against 12.7 API projectile. viii The ballistic performance of mild steel and Armox 500T steel as a function of target areal density has been compared against 7.62 API projectile. It has been found that 78 kg/m2 Armox 500T steel offered the ballistic limit equivalent to that of the 196 kg/m2 mild steel. Therefore, with respect to thickness as well as areal density the ballistic limit of Armox steel is 2.5 time higher than mild steel against 7.62 API threat. The ballistic performance of Armox 500T and 7075-T651 aluminium materials has also been compared against 12.7 API projectiles as a function of areal density. The 108 kg/m2 of 7075-T651 aluminium offered the ballistic limit equivalent to that of 157 kg/m2 of Armox 500T steel. Therefore, if the performance of Armox steel and aluminium is compared with respect to thickness, the aluminium target should be two times thicker than Armox steel target to stop the 12.7 API projectile. However, if the areal density of the two materials is compared, Armox steel would be two times heavier than the equivalent aluminium target to stop the 12.7 API projectile. The study thus presents a detailed investigation of the material behaviour and ballistic performance of mild steel, Armox 500T steel and 7075-T651 aluminium alloys and leads to some important conclusions pertaining to the mechanics of projectile and target interaction.