The design of mosaic armour: The influence of tile size on ballistic performance
Introduction
The ballistic performance of ceramic materials used in armour applications is well known and has been extensively studied since the 1960s [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. A ceramic-faced armour design usually consists of the hard disrupting face of the ceramic and some kind of absorbing element behind. The purpose of the ceramic is to induce fragmentation in the projectile or induce erosion thereby redirecting and dispersing the kinetic energy. The absorber on the other hand, acts to transfer the kinetic energy of the projectile to a lower form of energy – such as heat, through inelastic deformation (for example). Ceramics are inherently brittle materials and consequently have fracture toughness (KIc) values in the 1–5 MPa m½ range as opposed to the 5–170 MPa m½ range for metals [11]. Consequently, when a projectile impacts and penetrates the ceramic face, brittle failure ensues leading to extensive fragmentation of the tile. If the fragments are not retained in place then the multi-hit capability of the armour is compromised.
Many modern-day armours are regularly subjected to automatic weapons fire where multiple bullets are fired towards a single location. Accordingly for multi-hit protection, it is necessary to retain as much ceramic material intact as possible after each subsequent hit. One of the ways that this can be achieved is by reducing the tile size such that if one tile has been destroyed protecting against a single projectile, the exposed area to subsequent strikes is minimized. Reducing the tile size inevitably leads to an increase in the number of interfaces between tiles for a given area. Bless and Jurick [12] have conducted a probability-based analysis of such mosaics to determine how multi-hit protection varies with tile size. They concluded that the impact of interfaces is likely for most armour system designs of interest. de Rosset [13] has also studied such patterned armours to examine the probability of defeating automatic weapons fire and similarly shown the vulnerability of joins between individual cells. However, for these types of analyses there is a requirement to know how the ballistic performance is affected by the proximity of the impact to the tile edge. Without this knowledge, only crude assumptions can be made.
There is a little published work in the open literature on the effect of tile size on the ballistic performance of ceramic-faced armours. Researchers have, however, studied the effect of applying radial confinement to ceramic targets on their behaviour under dynamic loading conditions. The effect of the radial confinement on the behaviour of a ceramic tile has been studied by Sherman [14] who impacted a confined ceramic tile by a 0.30″ armour piercing projectile. He showed that the addition of a steel confinement frame reduces the damage to the tile significantly whereas using other supporting materials of lower acoustic impedance leads to greater ceramic tile damage. Others have shown that the effect of adding steel radial confinement to ceramics subjected to high velocity long rod penetration also results in the resistance to penetration increasing [15], [16].
The size of the tile is also important for ballistic testing of the ceramic. Good reviews of the various techniques are provided by James [17] and Normandia and Gooch [18]. There are clear advantages in using small tiles, not only in the cost of the ceramic but also the cost of the backing materials. Therefore it is advantageous to the design engineer to know the smallest tile size that will provide the most accurate data on the material’s ballistic resistance.
In most cost-effective mosaic armour designs, the sides of the tiles are unlikely to be ground flat and therefore there will be little or no intimate contact between each tile. Therefore to evaluate the worst case scenario it should be assumed that each tile is performing independently of its neighbour. In this work we have evaluated the effect of the proximity of a central impact point to a free surface on the ceramic armour’s ballistic performance. Both the type of ceramic and the size of the tile were varied. This work is part of a wider study on the resistance of ceramic-faced armour to penetration by tungsten carbide-cored projectiles.
Section snippets
Experimental setup
The depth-of-penetration technique as described by Rozenberg and Yeshurun [19] was used to measure the ballistic performance of the ceramic tiles (see Fig. 1). In this work, polycarbonate was chosen as the backing material instead of more commonly used materials such as RHA or aluminium. The use of polycarbonate, which is less resistant to ballistic penetration, has the advantage that any small differences in the ballistic performance of the tile will result in relatively large differences in
Numerical model
To elucidate the mechanisms of penetration and the effect of the tile edges on the penetrating projectile for the sSiC case we have conducted a series of computations. All computations were carried out using 2D axial symmetry using a Lagrangian mesh in the explicit non-linear transient dynamic numerical code – AUTODYN-2D. This software is explained in detail elsewhere [21] and a useful overview of these types of codes is provided by Anderson [22]. However in brief, this code solves the
Experimental
In Fig. 5 the recorded depth-of-penetration data for each of the ceramic-faced armour targets tested is presented. Each reported data point refers to the average of the number of shots per tile and the error bars represent the spread in the data. Note that for the LPS SiC the depth-of-penetration is significantly higher than the sSiC and consequently represents a lower ballistic performance. It has been previously noted by Ray et al. [44] that the ballistic efficiency of liquid-phase-sintered
Conclusions
Experiments have been carried out on two differently made silicon carbides to evaluate the effect of border proximity on each material’s ballistic performance. Further, a series of computations have been carried out using a commercial hydrocode to elucidate the penetration mechanism in two different areal sizes of tile.
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The measured depth-of-penetration after completely penetrating 7.5-mm thick sSiC was dependent on the tile size. For this ceramic, the effect of the border was insignificant at a
Acknowledgements
Some of this work was carried out during Capt Moutinho’s study for a Forensic Engineering and Science M.Sc. at Cranfield University. We would like to thank the Brazilian Army for funding his studies. We would also like to acknowledge Mr. David Miller, Mr. Gary Cooper and Mr. Adrian Mustey for their technical assistance. Finally, our thanks to Morgan AM&T for supplying the samples and funding other materials used in this project.
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