INFLUENCE OF TARGET SPAN AND BOUNDARY CONDITIONS ON BALLISTIC RESISTANCE

Abstract

The experiments and finite element simulations have been carried out for studying the behavior of thin 1100-H12 aluminum targets of varying curvature, span diameter and boundary conditions under the normal impact of blunt and ogival shaped projectiles. The perforation phenomenon, failure mechanism, local and global deformation, residual velocities, ballistic limit and energy dissipation in different deformation modes of the target was studied. A pneumatic gun was employed to launch projectiles over the targets in the sub-ordinance velocity range. The incidence velocity of the projectiles was varied in order to evaluate the ballistic limit. The impact and residual velocity of the projectiles was measured with the help of a high speed camera Phantom-V411. The numerical simulation of the problem was carried out using finite element code ABAQUS. An elasto-viscoplastic material model proposed by Johnson and Cook (1983, 1985) was used to predict the flow and fracture behavior of the target material during the numerical simulation. The projectile was prepared of EN24 steel. It was hardened and annealed before being experimented. Thus, the material behavior of projectile was not modeled in view of its very high strength and hardness in comparison to that of the target material. The explicit solution scheme of the finite element code was employed to simulate the perforation phenomenon. The numerical results thus obtained were utilized for calculating the energy dissipation in various plastic deformation mechanisms through a user defined subroutine, Python Script. The influence of boundary condition was studied by varying the region of fixity at the periphery of 1 mm thick aluminum target plate of span diameter 255 mm. The region of fixity was varied as 100%, 75%, 50% and 25%. The effect of continuous and intermittent region of fixity was also investigated by disintegrating the continuously fixed regions of 75% and 50% target periphery into three and two equally intermittent regions respectively. The global deformation and ballistic limit of the target increased with the decrease in region of fixity against both projectile shapes. The increase in global deformation however was more prominent against ogive while the increase in ballistic limit against blunt nosed projectile. Moreover, the global deformation and ballistic limit of the target with intermittent fixity was found to be lower compared to that of the target with the equivalent continuous fixity. The effect of span diameter of the target was studied by varying the target to projectile diameter ratio (D/d) of 1 mm thick aluminum target as 3.6, 5, 7.9, 10 and 15 keeping the diameter of projectile constant, 19 mm. After the validation of numerical results with the corresponding experiments at the above identified D/d ratios, the numerical simulations were further carried out at 20, 25, 30, 35 and 40 D/d ratios. The global plastic deformation as well as ballistic limit of the target has been found to increase significantly with increase in D/d ratio. The energy dissipation in plastic deformation of the target increased initially up to D/d ratio 10 against blunt nosed projectile. However, it decreased subsequently with further increase in D/d ratio. Against ogive nosed projectile however, the D/d ratio could not influence the energy dissipation in plastic deformation. The maximum energy dissipation occurred in circumferential and tangential stretching respectively against blunt and ogive nosed projectile. To study the influence of target curvature the blunt and ogive nosed projectiles were impacted at the crown of hemispherical shell of varying span diameter and thickness. The mechanics of perforation, ballistic resistance, and energy dissipation in local and global deformation of the shell was studied. The thickness of the shell was varied as 0.7, 1 and 1.5 mm whereas the span diameter, 68, 100, 150 and 200 mm. Each shell was hit by ogive as well as blunt nosed projectile. The mechanics of failure of the shells were found quite different compared to those of the plates. The shell failed due to significant dishing followed by formation of petals against ogive nosed projectile. Against blunt nosed projectile however, it has undergone significant global dishing along with tearing at the circumferential clamped region or plugging. The ballistic resistance against blunt nosed projectile has been found to be higher than against ogive nosed projectile. The effect of shell thickness on the ballistic resistance has been found to be more prominent compared to span diameter. The energy dissipation was found maximum in axial stretching and minimum in circumferential stretching, against both projectiles.