Understanding vehicle armour: a guide to materials and technologies

With the growing loss rates of armoured fighting vehicles (AFVs) seen on modern battlefields, particularly in Ukraine, it is worth examining the materials and technologies which go into protecting these vehicles, to better understand their limits.

In 2017 a military unit from the UAE was involved in an ambush in Yemen that started with an improvised explosive device (IED) targeted against a single vehicle. The blast impacted the armoured vehicle behind it, a Nimr N35 4×4 or ‘Jais’ as it is now known, which was subsequently engaged with a 6V1 SVD sniper rifle firing 7.62 mm ammunition, as well as 12.7 mm and 14.5 mm KPVT heavy machine guns (HMGs).^[1] The armour on both sides of the vehicle was hit by multiple rounds from these weapons and all of its tyres were punctured. Despite this, the vehicle and its crew self-extracted without casualties. The Jais is an 18.5 tonne vehicle, which is significant, but its ability to stop 14.5 mm rounds is nonetheless impressive. A BMP-2 infantry fighting vehicle (IFV) is lighter at 14.3 tonnes, but its side armour is only sufficient to stop 7.62 mm rounds, or 12.7 mm rounds fired from ranges exceeding 200 m. The Jais likely employs a mix of steel and composite armour, while the BMP relies on steel for most of its hull, along with innovative uses of Aluminium.^[2]

Neither approach is necessarily better than the other, as counter-intuitive as that might seem, vehicle armour is about design balance after all. However, both vehicles reflect different routes to the same effect. The key to understanding how armoured fighting vehicles are protected is to examine the materials used, and the design principles employed. This article will do that through the lens of steel, Aluminium, composite, and glass armour, by exploring the different ways in which these armours are used today.

Steel

Steel armour is the oldest and most mature in fighting vehicles; the first tanks used in the First World War carried armour steel plates. They were made by heating the steel plate alongside Carbon over several weeks, which allowed the Carbon to be absorbed by the face of the plate. This created a very hard face with a malleable core, but the armour tended to splinter when hit.^[3] Armoured plates gave way to rolled homogeneous armour (RHA) in the 1930s in response to the increasing velocity and penetration of projectiles. RHA is created by adding additional metals such as Nickel or Chromium to the Iron and Carbon used to make standard steel. The alloying of steel is what increases hardness, however the rolling process improves its toughness. To produce RHA, steel billets are heated and rolled, which stretches out the grain structures within the steel. The sheets of steel are then heat treated at very high temperatures, which causes the grain structures to align uniformly and face the same direction, the steel is then quenched in water which rapidly reduces its temperature and freezes the grain in the position it was in whilst heated.^[4] At this stage, the armour would be very hard, and it would not be feasible to bend or weld it into an armoured vehicle. As such, the steel is then tempered, which involves heating it up again and allowing it to cool naturally so that it can be bent into shape without losing its hardness and protective properties.

In part due to its ubiquity, RHA has become the industry standard for measuring both armour and projectile penetration performance, both of which are typically measured in terms of millimetres of RHA equivalent (RHAe) – thus providing a universal value for comparison across multiple different material types. RHA was also one of the key materials used in armoured fighting vehicle (AFV) production. The standards used today were broadly reached during World War II.

Production and properties

The performance of armour is measured through a number of criteria such as the Brinell hardness rating (BHN or HB). The BHN refers to a test where a tungsten carbide ball is pressed into the armour until it creates a dent, the diameter of which is then measured. The smaller the diameter of the indentation left by the ball, the harder the armour.^[5] RHA has a Brinell hardness between 250–410 BHN depending on its intended application.

Another criteria is toughness, which describes how well the metal can bend or deform under stress without breaking. A tougher steel may withstand higher stresses than a very hard steel for a longer period of time. The end use of the armour is a key factor in the type of steel selected. Structural applications, such as the steel that physically makes up the vehicle structure, require a steel that is relatively hard, but also tough and easily welded. RHA is a good candidate in this scenario and is used in the basic construction of the M1 Abrams. However, if an armour is to be fitted onto a structure to provide additional protection, it can be much harder, typically having increased brittleness and reduced weldability. Armour that will be used to resist mine blasts, such as the V-shaped section of a mine resistant ambush protected (MRAP) vehicle is required to be relatively hard – around 400 BHN – which provides a balance of hardness and toughness. Steel with these properties should bend rather than crack under explosive load.^[6] This shows how the armour selected should be optimal for the vehicle requirements, but will likely represent some form of compromise.^[7]

Increases in hardness thereafter result in ultra-high toughness armour (UHTA) with a Brinell hardness of 450 BHN and high hardness armour (HHA) at 512 BHN. There is also a class of steels known as ultra-high hardness armour (UHHA) that can exceed 570 BHN. Previously, these steels were too brittle and would shatter upon impact, but improvements in manufacturing have produced viable UHHAs. UHTA uses fewer alloying elements than HHAs, which means that it is tougher – more resistant to stress and less brittle – and easier to weld than HHA. As a result, UHTA can be better suited than HHA to structural applications within armoured vehicles because it is less prone to cracking. Nevertheless, HHA is occasionally used to provide vehicle structures especially in wheeled armoured vehicles, despite the increased difficulty associated with welding such hard steels together.^[8]

The limits of HHAs led to the development of dual-hardness armour in the 1960s, which combines a UHHA plate together with a softer rear plate. The high hardness plate is typically hardened into the range of 601–712 BHN with the rear plate at around 461–534 BHN, the two are then roll-bonded together.^[9] Roll bonding is a process used to join dissimilar materials; it involves rolling the plates under extreme pressure, often with heat, until plastic deformation (changes to the crystalline structures of the steel) occurs and the plates become bonded.^[10] The production process involves a great deal of complexity, however, it can produce armours that are suitable for use as structural armours as well as providing a good degree of ballistic protection. Furthermore, the combination of very hard armour with softer types is known to increase protection against certain types of projectile over HHA or RHA alone. This design methodology will be revisited later in this article in the discussion of ceramics.

Enhancing protection

When a projectile strikes metal armour, the penetration or damage inflicted on the armour depends on many factors. One of them is the velocity and material of the projectile, another factor is the geometry of the projectile and armour structure, as well as the dimensions of the projectile and target. For example, for an armour to defeat an M993 7.62×51 mm armour piercing projectile, it would ideally be harder and thicker than the tungsten carbide penetrator that the round carries. Tungsten carbide is very dense, at 14.5 g/cm³, compared with just 8 g/cm³ for hardened steel – the penetrator in the M993 weighs just 2.3 g.^[11] Tungsten carbide has a Vickers hardness (VH) rating of 1200-1500, while hardened steel is 600-800 VH. This makes the former one of the most challenging materials fired by a small arms weapon at an armoured vehicle. At 100 m, the M993 can penetrate 18 mm of 300 BHN steel armour set at 0° (vertical). The side armour of a BMP-2 is 13 mm thick, but sloped at 15° from vertical, which increases its line of sight (LoS) thickness to 13.5 mm. The M993 round is therefore theoretically lethal to a BMP-2 at 100 m. However, the BMP-2’s BT-70Sh armour is made from a Nickel-Chromium-Molybdenum alloyed steel with a Brinell hardness of 534, much harder than the armour penetrated by M993 in tests and marketing.^[12]

The sloping of metal armour is an important element in increasing the level of protection it provides. It does this by increasing the LoS thickness. Imagine a vertical sheet of armour that is 100 mm thick; a projectile fired horizontally at this sheet only needs to penetrate 100 mm of material. Imagine the same scenario but the armour plate is sloped backwards at 45° from vertical and the projectile is still travelling horizontally, it must now penetrate 141 mm of material to get through the armour. You can calculate this figure if you know the thickness of the armour and the measurement of its slope. Simply divide the thickness by the Cosine of the angle of the armour. For the scenario used here, this equation would look like this, 100COS(45). Typically, steel armours are combined with other materials to increase the level of protection that they offer. For example, the actual thickness of the glacis plate on an original T-72 is 205 mm, however it is angled at 68° from vertical. Using the equation above, 205COS(68), provides a LoS thickness of 547 mm.

The LoS thickness is not the only outcome of inclining armour that improves its efficacy; the angle imposes asymmetric forces on a projectile as it hits the armour, and unless the projectile is travelling at extreme velocities, it may be deflected into more of the armour. Many MBT programmes have been able to pursue similar designs for their glacis, which is enabled by the rear placement of the engine and a reclined driver’s position. The weight of an MBT is already high because of the size of vehicle that must be protected, and the level of protection required. Introducing sloped armour allows for an effective increase of the frontal protection in a mass-efficient way.

Aluminium

There are two primary types of Aluminium armour; 5083 and 7039, there is also the 5083 is made by rolling Aluminium ingots at temperatures between 350° and 400 °C, the armour is left to cool at room temperature before it is cold-rolled to the desired thickness. The cold-rolling also introduces the desired strength.^[13]

The 5083 alloy was the first developed for armouring purposes. It is an Aluminium‐Magnesium‐Manganese alloy with a BHN of just 75. This gives the armour a mass effectiveness of 0.89 when compared with RHA against a 7.62 mm assault rifle round fired from point blank range. This means that a greater thickness (and mass) of 5083 would be required to stop a 7.62 mm armour piercing round. However, it does offer moderately more effective performance against shell fragments, which lends the alloy to use in howitzers and artillery vehicles.^[14] Furthermore, the added thickness is not necessarily a downside, as it can help minimise failure of the rear of the armoured plate and reduce spalling.

7XXX series alloys offer a Brinell rating up to 150 BHN but are susceptible to stress corrosion cracking, which is where a corrosive – such as water – is introduced into the armour whilst under tensile stress, say driving across difficult terrain. This type of cracking can lead to failure of the material at lower strain rates than the armour would be able to otherwise withstand. One further disadvantage that applies to both types of Aluminium alloys is that they have a lower spalling strength than steel. This means that an explosive impact to the vehicle hull is more likely to result in fragments being generated on the opposite side of the plate, harming the vehicle inhabitants.^[15]

Aluminium has numerous benefits as an armour material, however, with an average BHN of just 75, it must be paired with other harder armours for acceptable survivability. The M2A2 Bradley uses a combination of 5083 and 7039 armour but hardened steel plates were fitted to the side skirts at a distance of 2.5 cm from the hull.^[16] Two plates were used, the first would potentially fracture the hardened penetrator at the core of a bullet and the air gap would force the incendiary element (if present) to activate. A second hardened steel plate was required to shatter the round fully, as the soft Aluminium used to build the structure of the Bradley would not have been capable of achieving the same effect. A similar armour arrangement was used on Soviet BMP-2s deployed to Afghanistan. Although the vehicle’s base armour is hardened steel, the addition of an extra high-hardness steel plate was sufficient to degrade the performance of 12.7 mm and 14.5 mm heavy machine gun rounds, thereby increasing the survivability of the vehicle and crew.^[17] The BMP-2 also employs an Aluminium engine access hatch, which has a significant slope and is fitted with ribs to help deflect bullets. Its thickness is between 10 and 15 mm.^[18]

Aluminium armour has its uses, and many Cold War designs employed it, including the Bradley, Warrior, Scimitar and others. It is still used today and provides the structure of the Turkish Zaha.^[19] The Zaha is built with 5000 series Aluminium, an additional layer of ceramic armour provides it with greater protection from ballistic threats.

Composites, an extra layer of complexity

A composite armour combines two or more distinct materials into layers. The different materials are designed to complement each other and increase protection of the vehicle whilst maximising mass-efficiency. Materials used in creating composite armours might include rubber, glass reinforced fibres, ceramics, and armoured steel. The design considerations in most cases are the order that the layers are arranged in, the thickness of the layers and the materials selected.^[20] The choices are shaped by the severity of threat that the armour must address. Small arms, for example, might be addressed by an armour with fibre reinforced polymer on the face of the armour, which would distribute the force of a projectile and destroy its tip, the next layer could be a ceramic plate designed to break up the projectile and defeat most of its energy, with further layers of rubber reinforced fibres to catch bullet fragments, and hold the ceramic plate together. A fibre reinforced polymer is a type of strong but mass-efficient material that is produced by setting small fibres made of glass or Kevlar in a polymer matrix such as epoxy, which holds all of the fibres together and transfers stress between them.^[21]

The original T-72’s glacis plate consisted of an 80 mm medium-hardness steel plate, followed by 105 mm of glass Textolite known as STB, followed by another 20 mm plate of steel. One source indicates that the steel plates were made from 340 BHN 42 SM steel, and that increasing the hardness did not increase protection as it led to a loss of ductility.^[22] The Textolite filler was reportedly designed by NII Stali to provide increased protection against high explosive anti-tank (HEAT) rounds, making the original T-72 an interesting and well-documented example of increasing vehicle protection through the use of steel armours, composites and by increasing the slope. STB is made by layering glass-fibre mats and bonding them together and there are studies available indicating that when combined with steel, it not only provided mass effective protection against HEAT rounds, but also tungsten kinetic energy penetrators.^[23] Reports from East Germany indicated that the T-72’s glacis plate has a mass effectiveness against HEAT rounds of 2.6, and 2.2 against kinetic energy rounds. This means that its glacis is over twice as effective in terms of weight, than the amount of RHA that would be needed to protect against the same threats.

HEAT warheads have high levels of penetration because they concentrate a lot of energy onto a very small area – a much smaller area than a kinetic energy round. However, as the hardness of the material that it is acting against increases, the size of the crater that the jet creates decreases. This accelerates the erosion of the jet and decreases its penetration potential. It is thought that the glass fibre used in the T-72 exerts a ‘spring-back’ effect against HEAT jets, effectively squeezing the jet and minimising its penetration.^[24]

The late Professor Ogorkiewicz wrote extensively on armour, and in one article published in Composites in 1976 he notes that a monolithic steel armour capable of stopping a 100 mm diameter shaped charge would weigh 3.8 tonnes/m², and would also be 480 mm thick. However, a composite made of Aluminium oxide, Aluminium and high-strength steel could stop the same charge at a thickness of 331 mm and weight of 1.35 tonnes/m².^[25] The armour he references was designed to specifically reduce the penetration of HEAT rounds, however, other composite designs were built to counter kinetic energy penetrators. This approach is able to increase the protection of vehicles within wider design constraints quite typically. Another design patented by the Institute of Saint Louis positioned a steel sandwich of rubber-like material in front of a ceramic plate with an air gap between them. The ceramic had a steel cover plate and was backed by the vehicle itself. In this kind of design, the sandwich would disrupt a HEAT jet before it reached the ceramic, leading to an increase in erosion speed.

Wrap up

It is not clear how the Jais referenced in the opening paragraph to this article was protected. Given the ammunition fired at it, and the weight of the vehicle, it seems likely that a combination of steel and ceramic armour was used. Armouring a modern fighting vehicle is a complex process, primarily because the types of threat have changed significantly. Many of the combat vehicles in use today were designed at a time when the most significant threat infantry could reasonably expect to face – and that their vehicles could be protected against – was a 14.5 mm machine gun, the occasional medium-calibre weapon, and artillery fragmentation. Anti-tank guided missiles (ATGMs) were rare, even unguided shoulder-fired anti-armour weapons were likely to be rationed. This meant that medium armoured fighting vehicles could be relatively light and rely upon hardened steel or Aluminium with some high hardness materials added. In a modern battlespace, however, it is not unreasonable to expect an armoured vehicle to be engaged with 23 mm cannons, as UAE forces in Yemen discovered, as well as ATGMs, IEDs, and drone-delivered munitions.^[26] Shoulder-fired anti-armour weapons are commonplace, and advanced ATGMs are now used by most actors around the world. This places a greater burden on vehicle and armour designers; platforms must now be capable of withstanding munitions fired at the roof.

Whilst this was always a possibility when facing the Soviet Union with its cluster munitions and masses of artillery, it is now almost a certainty. The proliferation of drones enables even moderately ambitious insurgents to hold a vehicle’s roof armour at risk. Vehicles must be protected from this kind of threat, as well as considering the risk posed by ATGMs and everything else mentioned above. Intelligent use of composites with steel as a structural base has provided survivability until now, but it is likely that future designs will rely more heavily on active and reactive armour types that provide mass-effective protection with combinations of composites as a backing. The lethality-protection cycle will continue with more and more effort dedicated to reducing the impacts of various weapons, and counter-efforts to side-step the capabilities of the modern state of the art in protection. Fundamentally, the material properties discussed here will always be important, there is little else that guarantees protection to the same extent as hardened steel, and that is why it is useful to understand the materials and technologies of vehicle armour.

Sam Cranny-Evans

^[1] https://www.youtube.com/watch?v=_gT3IndWXYs

^[2] https://thesovietarmourblog.blogspot.com/2016/05/bmp-2.html#prot

^[3] http://norfolktankmuseum.co.uk/types-of-armour/

^[4] https://www.youtube.com/watch?v=Q-YtRtBC7MA

^[5] https://foundrax.co.uk/how-to-calculate-brinell-hardness/

^[6] https://masteel.co.uk/armour-plate-steel/; https://apps.dtic.mil/sti/pdfs/ADA601455.pdf

^[7] https://apps.dtic.mil/sti/pdfs/AD1027340.pdf

^[8] https://apps.dtic.mil/sti/pdfs/AD1027340.pdf

^[9] https://apps.dtic.mil/sti/pdfs/AD0759506.pdf; https://apps.dtic.mil/sti/pdfs/AD1027340.pdf

^[10] https://www.mdpi.com/2075-4701/11/9/1344; https://plastometrex.com/blogs/plastic-deformation-of-metals

^[11] https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2015/smallarms/17379_Erninge.pdf

^[12] https://thesovietarmourblog.blogspot.com/2016/05/bmp-2.html#prot

^[13] Paul J. Hazell, Armour Materials, Theory and Design. Taylor and Francis, 2015, p. 194

^[14] Unknown author, Advances in Armor Materials. Janes International Defence Review, 1991

^[15] Paul J. Hazell, Armour Materials, Theory and Design. Taylor and Francis, 2015, p. 195

^[16] http://afvdb.50megs.com/usa/m2bradley.html

^[17] http://btvt.info/5library/vbtt_1991_afgan.htm

^[18] https://thesovietarmourblog.blogspot.com/2016/05/bmp-2.html#prot

^[19] https://euro-sd.com/2024/03/articles/36849/beachheads-and-amphibious-armour/

^[20] https://apps.dtic.mil/sti/pdfs/ADA481733.pdf

^[21] https://www.researchgate.net/publication/309205526_Armour_Materials_Theory_and_Design

^[22] https://thesovietarmourblog.blogspot.com/2017/12/t-72-part-2.html#8010520; https://vdocuments.site/catalogue-of-plates-new1.html?page=25

^[23] http://btvt.info/5library/vbtt_1987_08_stb.htm

^[24] R.M. Ogorkiewicz, “Combat Vehicle Armour Progress.” International Defense Review, June 1995.

^[25] R.M. Ogorkiewicz, “Composite Armour,” Composites, 1976.

^[26] https://www.amazon.co.uk/25-Days-Aden-Arabian-Forces-ebook/dp/B0B94QSF65