THE BALLISTIC RESPONSE OF CERAMIC TARGETS UNDER VARIOUS CONFIGURATION AND OBLIQUITY

Abstract

The present study is focused on the ballistic evaluation of advanced ceramics (B4C and SIC) against armour piercing incendiary projectiles under normal and oblique incidences as well as under different configurations. A detailed experimental and finite element investigation has been carried out to study the material and structural behaviour of the two identified ceramics in order to explore the suitability of the ceramic-aluminium composite armour under different ballistic impact conditions. The experimental and computational results obtained in the present study with respect to thickness, angle of incidence and target configuration are pertinent with respect to the improvement in the design of ceramic based armour. The material characterisation experiments were initially performed on B4C and SIC hot-pressed ceramic materials. The density, elastic and shear moduli, flexural strength and the uniaxial compression behaviour at quasi-static and high strain rate of the two ceramic materials were studied through Archimedes method, impulse excitation of vibration method, three-point bending test, and universal compression tests performed on UTM and the split-Hopkinson pressure bar apparatus, respectively. The assessment of the ballistic resistance of the ceramic is carried out through the depth-of-penetration (DOP) experiments. The experiments were carried out on ceramic tiles of size, 100 mm × 100 mm, and thicknesses, 5 mm and 10 mm against 7.62 and 12.7 API projectiles. Two different aluminium alloys, 7075-T651 and 6061-T6, were used as the backing layer in the present study with their dimensions, 150 mm × 150 mm × 50 mm and 260 mm × 360 mm × 50 mm, respectively. The experimental setup for DOP experiment had a universal gun that was able to fire various projectiles up to 14.5 mm calibre. Ansys/Autodyn finite element (FE) explicit solver was used in the present study for the modelling, meshing and numerical analysis of high strain rate material tests as well as for the simulations of reference and residual depths of penetration. The original MSHPB used in the experiment had 2000 mm length of the incident and transmission bars. However, in order to simplify the finite element model, the length of incident and transmission bars were reduced to 1200 mm while the other components of MSHPB were modelled as per their actual dimensions. The meshing of all the components of the MSHPB as well as the B4C specimen was carried out using eight node hexahedral elements. The frictional interaction was assigned between the specimen and the inserts. The high strain rate test was numerically simulated for the striker velocity, 13.5 m/s. The 2D and 3D computational models have been developed for simulating the ballistic resistance of ceramics-aluminium targets against 7.62 API and 12.7 API projectiles. The computational model ii for simulating the reference and residual DOP experiments were as such similar except the addition of ceramic target layer in the computational model developed for obtaining the residual DOP. The geometry of target and projectile was not reduced for computational simplification. Plain strain assumption has been adopted in the 2D model. The impact velocity of the projectile in the computational model was assigned based on the respective ballistic condition. The geometry of both 7.62 API and 12.7 API the projectiles have been discretized with the element size of 0.35 mm at X as well as Y direction for all the simulations. The meshing of the aluminium alloy was varied with the ballistic condition. Fixed boundary condition was imposed at the boundaries of the aluminium target. Similarly, the computational model for oblique impact conditions has been established. For 7.62 API projectile, the computational models were developed for 0°, 30º and 45º obliquities and for 12.7 API projectile the models were developed for 0° and 60º obliquities. The Johnson-Cook constitutive model was employed for simulating the material behaviour of projectile as well as for the aluminium alloys while the JH-1 and JH-2 constitutive model was used, respectively, for modelling the behaviour of SIC and B4C ceramics. It has been observed that the B4C ceramic had lesser density, high compressive strength, better elastic properties and negligible strain rate sensitivity (for strain rate below 103 s-1) than that of the SIC ceramic. The uniaxial compressive strength of B4C has been found to have a nominal increase with an increase in the strain rate. It could be concluded from the present study that the geometry of the specimen had a significant influence on the damage mechanism, however, it does not have any influence on the strength characteristics of the B4C. The penetration resistance of 7075-T651 aluminium was superior to that of the 6061-T6 aluminium against both the projectiles. The increase in the performance of 7075-T651 target increased with the increase in the angle of obliquity. The reference DOP was a maximum 73% smaller in 7075-T651 target than the 6061-T6 target against 7.62 API projectile at 60º obliquity. The increase in the obliquity increased the penetration resistance of the target. The aluminium backing material properties affected the B4C and SIC ballistic efficiency factor calculation against both the projectiles. The differential efficiency factor (DEF) has revealed that both the ceramics had poor ballistic resistance against 7.62 API projectile at 30º obliquity as compared to that at 0º obliquity. The DEF of B4C and SIC ceramics at 0º obliquity was 35.66% and 39.69% higher, respectively, than at 30º obliquity. Further, the 12.7 API projectile ricocheted at the aluminium target surface at 60º obliquity and the ballistic resistance was inferior in comparison to that at 0º obliquity. iii The ballistic resistance of B4C ceramic increased with the increase in target thickness and the ballistic resistance of SIC ceramic decreased with the increase in target thickness. The ballistic performance of similar layering of ceramics (B4C-B4C and SIC-SIC) was same to that of the monolithic ceramic target. Both the ceramics exhibited similar ballistic resistance when impacted in a single layer of 10 mm thickness and double layers of 5 mm thickness, each. The layered target of dissimilar material, SIC+B4C, described better ballistic resistance over, B4C+SIC, target against 12.7 API projectile at 0º obliquity. The DEF of 10 mm thick monolithic B4C, double layered, B4C+B4C, SIC+B4C and monolithic 5 mm thick SIC targets was 7.37, 7.20, 7.24 and 7.31, respectively. It shows that the ballistic resistance of these targets are almost same against 12.7 API projectile as compared to the other ceramic target configurations. Further, the ballistic resistance of 10 mm thick monolithic SIC, double layered SIC+SIC and B4C+SIC targets has been found to be close to each other. Among all the layered configurations of ceramic target, SIC+B4C and B4C+B4C target exhibited superior ballistic resistance against 12.7 API projectile. The 2D computational model formulated to simulate the ballistic resistance of B4C and SIC ceramics against 7.62 API projectile was able to reproduce the experimental results. However, for 12.7 API projectile impact, it was not able to reproduce the experimental results accurately. Hence, a 3D computational model was developed, and this model accurately simulated the normal impact experimental results of B4C and SIC ceramics against 7.62 API projectile.