PSI - Issue 14

Aisha Ahmed et al. / Procedia Structural Integrity 14 (2019) 507–513 Aisha Ahmed/ Structural Integrity Procedia 00 (2018) 000–000

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protection levels require resistance against high velocity (above 800 m/s) armour piercing projectiles which is possible with hard ceramic materials. Ballistic ceramics such as aluminium oxide, silicon carbide, boron carbide, etc., are very hard materials with high compressive strength along with low density, are better alternatives to rolled hard armour (RHA) steels. But the issue of amorphization or disintegration of ceramics at very high velocities needs to address to make it multi-hit capable. For better performance of armour, ‘hardness’ is the desirable material property which helps in reflecting the impacting energy and deforming the projectile. But there are other properties as well which are required for the improved ballistic resistance as for thinner ceramic tile most common failure mode is the radial which arise due to tensile stresses at the rear part of the tile. Therefore, an extensive research has been going on the materials and methods of confinement of the ceramic to hold the comminuted particles together to maintain the integrity of the ceramic plate even after multiple impacts Rahbek and Johnsen (2015).

Nomenclature t

Time duration

E

Young’s modulus of bar material Cross-sectional area of bar Cross-sectional area of striker Elastic wave velocity in the bars

A b A s C o L s

Specien length Arbitrary strain Transmitted strain

ε i ε t

ε r Reflected strain � s Strain rate ԑ s Averae strain σ(t) Average stress

Shockey, Simons, and Curran (2010) Shockey et al. had studied the failure mechanism of the ceramic block (25mm thick) at moderate impact velocities and found out that failure begins with the formation of radial crack at the rear surface, followed by cone crack and the inelastic deformation beneath the point of impact. But all the cracks mentioned above did not caused the penetration of the projectile, as the fragments were not moved out of impacting zone. Thus, the ceramic could only be penetrated if the fragments pushed out of their place and/or micro-cracks are formed underneath the point of impact. Hence, the penetration can be prevented by making the movement of fragmented particles and formation of micro-cracks as difficult as possible. Therefore, to hold the comminuted parts together the confinement of ceramic is focused upon. Lundberg et al. (2000); Westerling et al. (2001) have done an in-depth study on the penetration of long rods into the ceramic plate. The study was focussed upon how the penetration is resisted and the moment when the penetration is being started. While the projectile is being impacted on the ceramic target, when the penetrator does not have enough velocity to penetrate the ceramics, the materials in front of the rod (projectile) is forced to flow outward and the rear part of the rod continues to move forward into the ceramic, this moment is called the Interface defeat of the penetrator. Thus, the time period between the first contact of the penetrator to the target to the point when actual penetration begins is called the dwell time . There are numerous studies on the metallic confinement of ceramic such as tempered steel by Chi et al. (2015), Aluminium alloy by Savio et al. (2011), Titanium by Sarva et al. (2007), etc., that showed improvement in energy absorption but at the cost of increase in areal density. Reddy et al. (2008) has studied that effective of polymeric matrix confinement (UHMWPE and Kevlar fabric) can doubles the energy absorption with 13 per cent increase in areal density. According to a number of studies (Chi et al. (2015); Savio et al. (2011)), there is a positive effect of pre-stress conditions such as radial, axial and hydrostatic, on resisting the ballistic impact. It was observed that by restraining the impact-face (impacting with a velocity of 900m/s) of ceramic tiles with a membrane of suitable tensile strength, the ballistic efficiency can be improved by nearly 25% for a mere 2.5% increase in areal density Sarva et al. (2007).