Evolution of an armour piercing projectile

Projectiles and armour have an antagonistic relationship whereby advances in one against the other spur counter-developments to mitigate or negate changes from the status quo. Projectiles have steadily progressed from inaccurate, low velocity muzzle-loaded lead shot to lead bullets in cartridges, copper jacketed ammunition (allowing better engagement of rifling and higher muzzle velocities) and modern armour piercing ammunition incorporating hardened penetrator cores (e.g. steel, tungsten carbide) and pyrophoric incendiaries such as zirconium to maximise behind armour damage (e.g. NAMMO Raufoss AS NM140 MP). More recently, intermediate 5.7x28mm and 4.6x30mm cartridges have also been developed to allow thusly equipped pistols and submachine guns to pierce modern body armour.

Woven silk clothing was superseded by nylon, aramid and then ultra high molecular weight polyethylene fibres as methods of arresting projectiles and absorbing impact energy. As projectile technology evolved, further measures were needed to protect personnel against armour piercing or high velocity rifle ammunition; these are manifest in the form of hardened armour plates typically made of technical ceramics such as alumina placed in front of a ductile energy-absorbing material. For armoured vehicles, kinetic penetrators, explosively formed penetrators and shaped charges have spurred development in ceramic-laminated armour such as Chobham, explosive reactive armour, holed armour, caged armour and active protection systems such as the IDF Trophy. Requirements for optical clarity have seen the development of glass laminated with tough polycarbonate and polyvinyl butyral, and more recently, aluminium oxynitride, a transparent technical ceramic.

Historically, counter-developments could only be made in response to existing developments in opposing technology. However, as the fundamental purpose of improvements in projectile or armouring technology remain unchanged, with sufficient computational resources it may prove worthwhile to model the dynamic relationship between projectiles and armour rather than varying one design against a fixed benchmark. This will allow us to alter the design philosophy from a retrospective one to a prospective one, and try to anticipate future improvements by designing against opposing optima.

Three armour development epochs will be studied here, consisting of plates made of: 10mm thick aluminium alloy, 10mm thick AISI 4340 steel and a hybrid 17mm Alumina/2.3mm AISI 4340 steel armour. The projectile's configuration will be shown to evolve to counter these stages in armour evolution. Finally, a hybrid armour plate will be evolved against the optimal projectile.

Some caveats:

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Material selection pool:

The material selection pool for the study consisted of 16 different materials. Key materials in projectile and armour design were represented, as well as a range of properties to simulate real-world material variability, rather than specific values. The material selection database can be increased in size to suit the user's requirements.

Material 1: Low alloy steel, AISI 4340 (normalized)
Material 2: Wrought aluminum alloy, 5086, H32
Material 3: Wrought aluminum alloy, 7075, T6
Material 4: Wrought magnesium alloy (AZ80)
Material 5: Alumina (90)
Material 6: Silicon Carbide (HIP)
Material 7: Tungsten Carbide
Material 8: Uranium, depleted, Commercial Purity
Material 9: Lead-1%Antimony,cast
Material 10: Brass: deep-drawing/cartridge brass, CuZn30, soft (wrought) (UNS C26000)
Material 11: Wrought austenitic stainless steel, AISI 304, HT grade D
Material 12: Low alloy steel, AISI 4150 (temperated @ 650C, oil quenched)
Material 13: Wrought aluminium alloy, 5456, H321
Material 14: AerMet 100
Material 15: Carbon steel, AISI 1040 (as-rolled)
Material 16: Carbon steel, AISI 4130 (tempered @ 650 C, H2O quenched)

10mm Al5086-H32 aluminium alloy plate

The co-evolutionary optimisation study will begin with a relatively soft aluminium alloy plate acting as a target. It is worth noting that the geometry of the improved designs are usually such that they encouraged the dynamic formation of a sharpened point in-situ, rather than mushrooming (cylindrical) or flattening (initially pointed) tips.

Evolution of an armour piercing projectile against an aluminium plate
	Baseline steel cylinder design Plate material: Al5086-H32 Impact velocity: 1000ms-1 Exit velocity: 823ms-1 Projectile material: AISI 4340 steel Projectile mass: 0.4087 mass units Composite fitness: 1659 fitness units, VR2M-1 Comments: Steel cylindrical penetrator shear plugs through the softer aluminium plate with significant residual velocity. However, it is heavy. Can savings be made to the projectile mass while not degrading exit kinetic energy too much?
	Best design found (so far) Plate material: Al5086-H32 Impact velocity: 1000ms-1 Exit velocity: 643ms-1 Projectile material: AerMet 100 / Silicon Carbide Projectile mass: 0.1622 mass units Composite fitness: 2548 fitness units, VR2M-1 Comments: 22% drop in exit velocity, 60% reduction in projectile mass from baseline design. Effectively a silicon carbide tip backed by a steel rod, that facilitates shear through the relatively softer aluminium plate. Removing the silicon carbide rear causes a reduction in exit velocity.

On the basis of a fitness metric of residual velocity squared normalised by the initial projectile mass, the optimisation algorithm has identified an improved projectile design that was 54% better than the baseline steel cylinder after a few hundred iterations. The optimisation algorithm modified the sectional profile and material of the projectiles, whilst accounting for real-world material variability. The subsequent design epoch will see the monolithic 10mm Al5086-H32 "upgraded" to a monolithic 10cm plate of AISI4340 steel, at the expense of weight.

10mm AISI4340 steel plate

In response to the penetration of the previous Al5086 aluminium alloy plate, the material was changed to AISI4340 steel. The material type change was performed whilst keeping the thickness fixed, rather than at constant areal density, so that the improvement over the previous aluminium plate in terms of ballistic resistance would be unambiguous.

Evolution of an armour piercing projectile against steel plate
	Epoch 1 Optimal design Plate material: AISI4340 steel Impact velocity: 1000ms-1 Exit velocity: 36ms-1 Projectile material: AerMet 100 / Silicon Carbide Projectile mass: 0.1622 mass units Composite fitness: 8 fitness units, VR2M-1 Comments: The best design found against an Al5086-H32 plate failed dismally against the upgraded AISI 4340 steel armour. The co-evolutionary optimiser will have to evolve the projectile from scratch to defeat the upgraded armour.
	Projectile #1 Plate material: AISI4340 steel Impact velocity: 1000ms-1 Exit velocity: 733ms-1 Projectile material: Brass core/body Projectile mass: 0.9981 mass units Composite fitness: 538 fitness units, VR2M-1 Comments: The optimiser has clearly been working on improving the profile. An elongated but heavy projectile that penetrates the target with a significant residual velocity. It's heavy mass penalises its fitness, but represents a marked improvement in terms of geometry and mass over the earlier design shown.
	Projectile #2 Plate material: AISI4340 steel Impact velocity: 1000ms-1 Exit velocity: 585ms-1 Projectile material: Tungsten carbide/Brass Projectile mass: 0.4804 mass units Composite fitness: 713 fitness units, VR2M-1 Comments: The projectile is evolving a harder, denser tip of tungsten carbide whilst maintaining a small cross section to keep mass down. Dense but ductile brass is located at the rear of the projectile to drive the tungsten carbide tip through the steel plate.
	Projectile #3 Plate material: AISI4340 steel Impact velocity: 1000ms-1 Exit velocity: 556ms-1 Projectile material: Tungsten carbide/Lead Projectile mass: 0.4217 mass units Composite fitness: 734 fitness units, VR2M-1 Comments: The optimal shape of the projectile is becoming increasingly compact. Sectional density has increased through the use of lead behind the tungsten carbide tip to facilitate piercing the target plate.

Early on in the evolutionary process, the projectiles had ballistically inefficient profiles and materials. As the designs evolved, the projectiles took on more elongated or compact profiles. Improved materials such as tungsten carbide were introduced to maximise penetration. Next, we will look at ceramic/metal armour systems and evolve projectiles against those.

17mm Alumina (90% purity) / 2.3mm AISI 4340 steel armour

Modern composite armour tends to be made out of a hard but brittle ceramic front, backed with a ductile material. The purpose of the ceramic strike plate is to blunt the incoming projectile and spread out the impact energy over a wider amount of backing material. The ratio of the thickness of Al₂O₃ to steel is calculated using a closed form formula* whilst keeping the areal density the same as the previous 10mm thick steel plate.

*Ben-Dor, G., Dubinsky, A., Elperin, T. (2004) "Optimization of two-component composite armor against ballistic impact." Composite Structures, Vol. 69, Issue 1, June 2005. http://dx.doi.org/10.1016/j.compstruct.2004.05.014)

Evolution of an armour piercing projectile against an ceramic/steel plate
	Previous optimal projectile vs. current hybrid armour Plate material: 17mm Alumina / 2.3mm AISI 4340 steel Impact velocity: 1000ms-1 Exit velocity: 571ms-1 Projectile material: Tungsten carbide penetrator / lead jacket Projectile mass: 0.4217 mass units Composite fitness: 774 fitness units, VR2M-1 Comments: The optimal projectile against a 10mm AISI 4340 steel plate is now used against a ceramic/AISI 4340 plate. It performs well, although its exit velocity is somewhat lacking.
	Current optimal projectile vs. current hybrid armour Plate material: 17mm Alumina / 2.3mm AISI 4340 steel Impact velocity: 1000ms-1 Exit velocity: 634ms-1 Projectile material: AISI 304 steel penetrator / Al5086-H32 body Projectile mass: 0.4907 mass units Composite fitness: 819 fitness units, VR2M-1 Comments: The projectile appears to have a sacrificial tip which creates a guided channel for the hard tungsten carbide core to pierce the armour.
	Current optimal projectile vs. previous armour (thick steel plate) Plate material: 10mm AISI 4340 steel Impact velocity: 1000ms-1 Exit velocity: 447ms-1 Projectile material: AISI 304 steel penetrator / Al5086-H32 body Projectile mass: 0.4907 mass units Composite fitness: 407 fitness units, VR2M-1 Comments: The projectile does not perform as well against a thick steel plate than it does against an alumina/steel hybrid.

The best projectile found whilst evolving against a hybrid ceramic/metal laminate was elongated, with a relatively soft sacrificial tip and harder steel core. It performed adequately against a monolithic, thick steel plate of equivalent areal density to the alumina/steel armour. The previous optimal projectile against a monolithic, thick steel plate also performed adequately against the alumina/steel armour. Thus in the context of two possible armour configurations, either projectile is robust; although it appears that the projectile optimised against a monolithic, thick steel plate is more so.

Geometrically optimised, hybrid laminated armour plate evolving against the current optimal projectile

With the evolution of the armour piercing projectile frozen, we now examine how a hybrid armour assembly may adapt to counter the optimal projectile. The following section shows two possible good designs against the previously identified optimal tungsten carbide core / lead jacketed projectile striking the assembly at 1km/s.

Evolution of an armour plate against a projectile
	Optimal projectile vs. optimised hybrid armour Plate material: AISI 4340 steel with periodic voids, 2x AISI 4130 steel with periodic voids Impact velocity: 1000ms-1 Exit velocity: Arrested Projectile material: Tungsten carbide penetrator / lead jacket Armour plate mass: 9.598 mass units Composite fitness: 1944 fitness units Comments: A lightweight steel sandwich structure was developed to resist penetration by the optimal projectile.
	Optimal projectile vs. optimised hybrid armour Plate material: AISI 4130 steel / Aermet 100 steel / AISI 304 steel Impact velocity: 1000ms-1 Exit velocity: Arrested Projectile material: Tungsten carbide penetrator / lead jacket Armour plate mass: 8.423 mass units Composite fitness: 2642 fitness units Comments: A lighter sandwich structure designed to arrest the projectile.

Optimising the geometric and material parameters of a triple layered hybrid armour resulted in the optimiser converging upon steel plates (of different compositions and tempers) with voids and a "defence-in-depth" strategy. Although the optimiser could set the void periodicity to zero (hence monolithic armour plates with no voids), thicker armour plates with periodic cavities to reduce weight were the preferred option. Thick steel plates with cavities appeared to be the strategy of choice over other possible options such as monolithic ceramic/metal armours. Energy absorption was facilitated by deformation and collapse of the voids. Should the optimiser be faced with a broad spectrum threat environment, such as impacts off the central axis, it is likely that the periodicity of the voids would increase to prevent any regions where a lucky hit could pierce the armour.

Future work possibilities:

Avenues for future research include:

1. Expand the scale of the work from a simple computational pilot study to a more complex one involving both experimental calibration and validation.
2. Increase the size of the material selection pool to include more interesting materials such as fibre-reinforced composites and foams.
3. Introduce topological and geometric feature optimisation into the target plates, such as sandwich panels.
4. Improve the modelling fidelity (part of the funds for the consulting contract will go towards computational hardware purchases).
5. Introduce robust design of projectiles to ensure they work against a variety of armour configurations and impact velocities/obliquity. This is very computationally expensive.
6. Introduce robust design of armour to make them resistant to a broad spectrum of possible impact threats.
7. Examine other fitness metrics such as projectile aerodynamics, blunt trauma and behind-armour hydrodynamic shock.

If you are interested in funding further research, please contact me for details.