From Steel to Titanium: The Evolution of Protective Materials

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The traditional helmet materials are increasingly being replaced by new ones, such as titanium. It is a lightweight metal alloy characterized by low density and good specific strength. Due to its susceptibility to adiabatic shear, titanium is typically used in hybrid armour configurations, where a polymer fibre composite backing suppresses unwanted low-energy material failure modes.

Protective helmets must do two things: stop the bullet and prevent severe and deadly backface deformation. Traditional helmets cannot fully deliver these protections, which is why new materials are increasingly replacing the old, namely steel.

Titanium has high strength and deformability at a relatively low weight. It also allows for the best trauma values to the rim while enabling protective surfaces greater than 1,300 cm². Steel in ballistic helmets has less strength at low weight. To achieve the same protective level, it makes the helmet heavier and uncomfortable. One of the first uses of titanium in personal ballistic protection was in a Swiss-made titanium helmet called the PSH-77 in the late 1970s.

Steel helmets were replaced by polymer fiber composite helmets around the world starting from the beginning of the 1980s. (Image: NDR)

Biocompatible Titanium

Titanium was first discovered in 1791, but it was not produced in any quantity until 1937 by Kroll in Luxembourg. The most common titanium alloy, Ti-6Al-4V, was developed in the 1950s. It accounts for over half of the total titanium alloy produced today. It is a very versatile dual-phase titanium alloy, biocompatible and can be used in contact with bone and tissue fluids.

Titanium products are very expensive, yet they have still found a significant number of military applications. The Leopard 2 and M1A2 Abrams main battle tanks (MBTs) and the M2 Bradley infantry fighting vehicle (IFV) use titanium in their ballistic protection. The lightweight towed M777 155-millimetre howitzer and the M134D-H Minigun Gatling-type machine gun utilize titanium components to reduce their weight. These are just a few examples of military applications.

Monolithic or Hybrid

Left: Impact crater formed on a beta-processed titanium plate shows adiabatic shear plugging. This is an undesirable low-energy failure mode.
Right: Alpha-beta processed plate shows higher energy failure modes that absorb more kinetic energy than pure plugging in beta processed plates. Failure modes here are plugging, bulging, and delamination.

Titanium used in personal ballistic protection can be either in a monolithic or hybrid. Monolithic means that titanium is used solely without any backface support, also known as a standalone configuration. Hybrid configuration means there is a supporting material backing, usually a polymer fibre composite. The armour configuration dictates which alloys can be used.

Two dual hardness titanium plate concepts. A dual-phase Ti-6Al-4V can be backed up by commercially pure titanium or a stronger beta-phase plate can be backed up by a Ti-6Al-4V plate. A stronger plate is always the strike face backed up by a more ductile layer.

The titanium shell used in ballistic helmets is usually 2.5-4 millimetres thick. The target thickness to projectile diameter ratio is typically under 0.5 with common handgun calibres. Titanium sheets can bend at these thicknesses, providing an additional kinetic energy absorption mechanism.

When titanium alloy is used in a monolithic configuration in ballistic helmets, lower strength and more ductile alloys are preferable. The bending and stretching (plastic deformation) of a low strength alloy effectively absorbs kinetic energy. The deformation of high-strength alloys generates much more heat and can lead to adiabatic shear of the armour sheet if used in a monolithic configuration. The use of stronger alloys usually requires a polymer fibre composite backing or some other more ductile material to suppress low-energy fracture modes.

High-strength alloys can be made to work effectively against different projectiles in hybrid armour configurations. One must understand the behaviour of the alloy well and choose the appropriate heat treatment. You also need to know your threat projectiles, as this also influences the selection of the right heat treatment. A hybrid armour configuration is always more than the sum of its parts.

There is also a third possibility for using titanium in ballistic protection, which is the dual hardness configuration. This means that an armour plate is made by combining two different alloys with varying mechanical properties. The surface layer is harder and less ductile, while the backing layer is softer and more ductile. The dual-hardness titanium concept was studied extensively in the 1970s in the United States and has been revisited many times since. However, to the best of our knowledge, no practical application has emerged, even though good results were achieved.

Companies like Ulbrichts, a premium helmet manufacturer, are also using aramid in ballistic helmets. Aramid is a synthetic fibre, glued with epoxy or phenolic resin. The benefits are high temperature and tensile strength, but the problem is the missing trauma and edge-per-rim protection. Helmets made of polyethylene enable low surface weight but have weak points at low-temperature resistance and very high trauma values.

Adiabatic Shear

The word “adiabatic” refers to an isolated thermodynamic system. Ballistic events tend to be adiabatic overall, but in titanium, this phenomenon is much more pronounced than in other metals. This is due to titanium’s poor thermal conductivity.

A lot of heat is generated during ballistic penetration, causing thermal softening. On the other side of the coin are physical phenomena that tend to increase a material’s strength, such as strain hardening and strain rate hardening. When thermal softening wins this arm-wrestling match, adiabatic shear can occur.

Titanium has low thermal conductivity, which means it cannot disperse the heat generated by deformation into a larger volume quickly enough during ballistic penetration. A molten phase can be found in the adiabatic shear bands (ASB).

All in all, titanium’s low thermal conductivity, low strain hardening, and strain rate hardening, combined with high-strength alloys, make it sensitive to adiabatic shear. Blunt and hard projectiles also tend to encourage this phenomenon due to the way they load an armour plate. Titanium alloys have poor shear strength, which blunt projectiles exploit to their advantage.

This is what happens when a conventional composite helmet is penetrated by a rifle bullet. It leaves quite a mess. (Image: NDR)

Ballistic Titanium Alloys

Information is scarce about the modern titanium alloys used in current Western titanium helmets. However, quite a lot of information is available about the alloys used by the United States in their army titanium helmet program in the 1960s. Information about Russian alloys can also be found. Therefore, we will focus on the alloys used by these two countries.

The United States manufactured five hundred ballistic titanium helmets in the late 1960s using the Greer process. This was a feasibility study to replace the M1 steel helmet, and good results were achieved. It was found that, from all the candidates, the Ti-5Al-2.5Sn alloy was the most suitable for ballistic helmets at that time. It was commercially available and possessed the required formability. Other studied alloys included Ti-6Al-4V and commercially pure grades 50A and 75A.

It is quite odd that public US military documents from 1968 state that titanium is the only metal that offers better protection than the manganese steel used in the M1 steel helmet. That was not true even back then. Ballistic steel alloys for helmets were not developed after World War II in the United States. The manganese steel found in the M1 helmet was inferior in protection to the alloys used in the German World War II steel helmet, Stahlhelm.

Russians Have Halved Costs

Ulbrichts titanium helmets provide good multihit performance. Here we can see excellent ductility of titanium alloys used in ballistic protection. (Image: NDR)

The Russians have used the OT4-1 pseudo-alpha alloy in monolithic titanium armour configurations. This is a very common aerospace-grade titanium alloy and can be found in many other military applications, such as aircraft engine firewalls. OT4-1’s fracture mode is ductile rather than brittle (high energy vs. low energy mode).

The ductility of the OT4-1 titanium alloy is due to its classification as a low-strength alloy. It can be categorized as a medium-strength alloy, albeit at the very beginning of the scale. Low-strength alloys do not generate as much heat as high-strength ones and exhibit better ductility.

In hybrid armour solutions, the Russians have used high-strength VT-6 alloy backed by aramid composite (TSVM-J). VT-6 is the same alloy as Western Grade 5 (Ti-6Al-4V). The aramid backing supports the titanium sheet, which might otherwise exhibit brittle behaviour. Choosing the right heat treatment is also especially important.

Modern Russian titanium helmets use the economically alloyed VST-2 alloy. This high-strength dual-phase alloy was first demonstrated in 2015 by VSMPO-AVISMA Corporation. There are also plans to use ceramic coatings on the surface of this alloy to defeat armour-piercing projectiles.

The Russians have managed to halve the price compared to previous-generation titanium armour products. They claim that VST-2 demonstrates better ballistic properties than steel. This brings us to an interesting question: which is better in ballistic helmet applications, steel or titanium?

One would have to perform objective ballistic testing on both materials at equal areal weights against different projectiles. That is the only way to find out, no matter how much scientific literature you study. One thing is certain: with steel, you get smaller backface deformation.

Ulbrichts manufactures two types of titanium helmets: the Optio and the Zenturio helmets. In the image, a VPAM 6-rated Zenturio helmet. Optio helmets can also be upgraded to VPAM 6 level by a Fortis add-on composite plate.(Image: Ulbrichts)

Heat Build-up and Weight Are Crucial

Ergonomics play a vital role in ballistic protection. No one wants to wear ballistic protection gear that is uncomfortable. The weight of a helmet dictates how long it can be worn without causing neck injury. The heavier a helmet, the more important it is to have the right balance, that is, the centre of gravity. Heavy visors without a counterweight exert pressure on your forehead and incapacitate you surprisingly quickly. First, your vision starts to blur, and then you begin to get a terrible headache.

For purely military operations, lightweight and highly ergonomic ballistic helmets are required. However, for short-duration storming operations, no substitute for heavy ballistic helmets provide maximum protection.

The most important aspect is the fatigue caused by excessive heat build-up under the helmet. This can lead to incapacitation very quickly, especially in hot climates. Heat build-up is a more significant incapacitating factor than the weight of a helmet.

Future Alloys

Ultra-fine grain (UFG) and nano grain (NG) alloys offer new possibilities. Refining the grain size of a material makes it stronger and harder according to the Hall-Petch effect. Ultra-fine and nano-grain alloys also exhibit very good ductility.

It is not an exaggeration to say that it is possible to fracture the hard steel core of an armor-piercing bullet with advanced titanium alloys. This was even possible in the 1970s when the United States developed dual-hardness titanium armour. It required a minimum of 57 Rockwell C scale hardness on the strike face to fracture the hard steel core of an armour-piercing bullet. The strike-face hardness of a ballistic plate should approach the hardness of an armour-piercing bullet’s core to break it.

Titanium alloys that end up in ballistic protection products must be commercially available. It means nothing if you can produce a very advanced alloy in laboratory conditions in small amounts. It must be available commercially and in large quantities.

The battle between steel and titanium continues in the ballistic helmet industry. Titanium has the lead. Some of this might be due to its ‘wonder metal’ aura, but steel is making a comeback. The fight is on! ​​

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