A story of fibers
I - INTRODUCTION
Following the Second World War, in reaction to the death toll caused by this conflict, thoughts on the combatant led to the concept of "zero death". The use of individual ballistic protections has thus strongly developed, leading to an important research work.
Before the advent of firearms, fighters had to protect themselves against sharp (stabbing or cutting) and/or blunt weapons. The blow of a heavy sword, if it did not cut the flesh because of the thickness of the clothing, could certainly cause injuries inherent to the shock of the weapon. At that time, individual protections took into account the characteristics of this type of weapon. They were generally heavy and rigid. Let's think about the armors and other chainmail ribs, reserved, it is true, to the rich fighters on horseback and under which they wore fleece clothes, ancestors of our anti-trauma materials.
Firearm projectiles and shrapnel are of a completely different nature.
Light, small and fast, they interact, at the moment of their impact on the body, on a very limited surface and therefore penetrate easily. Moreover, the need for greater mobility, which already existed in the time of our valiant knights, led to the search for effective protection, but at the same time light and flexible so as not to hinder the combatant's movements too much.
II - FLEXIBLE BALLISTIC PROTECTIONS
The flexible ballistic protections, intended to stop mainly the projectiles of handguns, are made with fabrics, arranged in superimposed layers, whose fibers present a very strong resistance to traction. They act by stopping the projectile like a net.
The first layers are perforated because their fibers, at the moment of impact, are mainly stressed in shear. Those placed behind the impact move back under the impulse and tend to be arranged in such a way that they are more and more solicited in traction.
At the same time, the projectile generally begins to deform under the mechanical constraints to which it is subjected. This causes a greater number of fibers to come into action, making it easier to stop.
II -1 The used materials
The flexible protections were made with various fibers. Natural at first, then synthetic. We can take the example of silk, already used in medieval Japan, and used in the United States at the end of the 19th century for the manufacture of bulletproof vests. It is said that the Archduke Francois-Ferdinand of Austria wore one when he was assassinated by a bullet in the head (was the assassin aware of this?), which was the beginning of the First World War. This kind of ballistic protection was not used during the first world war because of the high price of silk.
The tensile strength of silk yarns was quickly surpassed by synthetic fibers such as nylon and especially by para aramid and polyethylene fibers.
II-1-1 - Para Aramid fibers: Kevlar®, Twaron® . The fiber battle
Everyone has more or less heard of Kevlar®, which is known to be used in the manufacture of ballistic vest packs. This is the name given by the American company Dupont to the para-aramid fiber it manufactures.
We have heard less about Twaron® which is the same para aramid fiber, but manufactured in Europe by Akzo.
The competition was tough.
• A brief history of Kevlar ®
Kevlar® is the registered name given to a fiber with a very high tensile strength. It is obtained by dissolving an aromatic amide polymerized plastic in a particular solvent. This solution is then injected through a nozzle in order to obtain, once the solvent has evaporated, a very fine wire with a strength/weight ratio approximately five times greater than steel.
The possibility of creating a para-aramid based plastic had been considered as early as 1939. It was synthesized and identified by Dupont in 1960, but para-aramid fibers were not produced until 1965 when the proper solvent was discovered.
When we know that the researcher at the origin of this discovery was called Stephanie Kwolek, we can understand the name given to this fiber.
• The Akzo Company and Twaron®
At the same time, the competition did not remain inactive. This is easy to understand if you take a step back from the restricted field of ballistic protection and realize the importance of the discovery of a material that is five times more resistant than steel for the same weight. The industrial and economic consequences were colossal.
This was the case of the Akzo company, based in Holland, where a team of researchers also discovered a solvent that could be used in the manufacture of para-aramid fibers. A patent on the manufacture of these fibers, called Kevlar® by Dupont and Twaron® by Akzo, was filed by this Dutch company. As one can imagine, in the face of the economic interests presented by these products, legal battles took place and, finally, an agreement was reached in which Akzo was granted the right to manufacture these fibers, but at the same time, Akzo was prohibited from marketing them in the United States until 1990.
Para-aramid fibers were not immediately used in the manufacture of ballistic protection. Their interest was revealed in all fields where the strength/weight ratio of the material was an important parameter. They were widely used, as a replacement for steel, in the structures of automobile tires and in the manufacture of missile components, among others.
In terms of their physical-chemical characteristics, para-aramid fibers lose their mechanical strength as the temperature increases. At very high temperatures (about 600 degrees C), they do not melt, but decompose. They are sensitive to moisture, which also causes them to lose their mechanical strength. This is why ballistic packs for body armor are enclosed in waterproof covers.
The fibers are usually woven together to form what is called a ply, which when superimposed on others, in varying numbers, will create a ballistic pack.
Below is an image of a yarn made of para-aramid fibers.
|Para-aramid Fibers (Kevlar®, Twaron®...)
II-1-2 - Polyethylene fibers compete with para-aramids
High strength fibers are also manufactured from extended chain polyethylene (ECPE). "Spectra®" is the trade name for these fibers manufactured by the company "Allied-Signal". "Dyneema®" is the trade name of the material manufactured by the company DSM Dyneema. The strong point of these fibers is their high mechanical resistance in general, to impacts and punching in particular. Their lightness, their good resistance to mold, chemicals and their hydrophobic character are additional assets.
To make these fibers, long polyethylene molecules are dissolved in a solvent. After heating, the solution is pushed through nozzles. The chemical characteristics of the polyethylene molecule give the fibers a mechanical strength superior to that of para-aramid fibers. With the same weight, they have a mechanical strength ten times greater than that of steel.
Commercially, these fibers appeared around 1985 to replace steel in the manufacture of cables and ropes.
Below is an image of a yarn made of polyethylene fibers.
|FPolyethylene fibers (Spectra®, Dyneema®...)
II-1-3 - Use of fibers
The fibers can be woven (para aramids) for the realization of ballistic protections, but also non woven (polyethylene). In the latter case, they are aligned against each other and then glued with a flexible resin. The result is a single-ply sheet "unidirectional" known as "Spectra Shield". Two sheets of Spectra Shield are layered and glued together so that their fibers intersect at a 90 degree angle. The resulting sheet is coated on both sides with abrasion protection.
Below are two images showing the two methods: weaving and layering.
|Weaving - Kevlar®, Twaron®
||Unidirectional - Spectrashield®
The many qualities recognized in "Spectra" and "Spectra Shield" made this product a great success in the manufacture of ballistic protection (vests and helmets). However, its low resistance to temperature (melting temperature 150 degrees C) and the slight loss of its stopping power from 70 degrees C (temperature that can be reached and measured by ourselves, in summer, by a helmet or a bullet-proof vest left, for operational reasons, in a vehicle exposed to the sun) make para-aramid fibers come back to the taste of the day. We can also note the existence of hybrid solutions (mixtures of para aramid and polyethylene plies) for the manufacture of ballistic packs.
Below, two images showing ballistic fabrics (plies), Kevlar®, Twaron® on the left and Spectra Shield® on the right.
|Kevlar ply®, Twaron®...
||Spectra®p, Dyneema® ply... Note the 90° cross structure
II-1-3 - The "Zylon": a short-lived star
"Zylon" is the name of a para-phenylene-benzobisoxazole (P.B.O.) fiber. It has a higher strength to weight ratio than para-aramid fiber or even polyethylene fiber. It is manufactured by the Japanese company Toyobo. Although this material is known and used for over twenty years in the industrial world.
The idea of using it in the manufacture of bulletproof vests seemed really interesting in recent years. Toyobo set up an office in Europe (Hamburg) and tried to market it. The United States took over the stocks and it was only possible to obtain it in small quantities for testing on our continent.
Problems of perforation of ballistic protections, which cost the life of their wearers, appeared quickly. These accidents seemed to demonstrate the poor performance of the fiber over time and brutally shortened the promising career of this beautiful golden fiber. The Japanese closed their Hamburg office.
– HARD BALLISTIC PROTECTIONS
Flexible protections are mainly designed to stop handgun projectiles.
If you want protection against long weapons (rifles, shotguns), which fire much more powerful projectiles, you have to use rigid protections to reinforce the flexible ones.
This is achieved by adding plates on top of the soft packs, depending on the anatomical areas you wish to protect.
The purpose of a hard protection is to damage the projectile enough that it loses its ability to perforate.
• The used materials
There are various types. The desired level of protection will guide the choice of material.
For ordinary bullets (lead core and brass liner) of the 7.62 Nato or 5.56 mm (.223 Remington) type, plates made from the fibers discussed above are generally used. In this case, the para-aramid or polyethylene plies are not sewn together, but coated with a resin and then stacked. At the same time, the whole is strongly pressed and heated. At the end of the operation a rigid plate is obtained.
The stopping of armour-piercing bullets (steel liner or steel front part of the projectile or more generally brass liner and steel core) requires harder materials. Titanium, a very hard and light metal, has long been used.
Nowadays, ceramic is most often used. Its weight, the main drawback, makes it suitable for heavy protection dedicated to specific operations. Some types of ceramics have a lower density for the same ballistic performance but are very expensive.
- THE BULLETPROOF VEST IN PRACTICE
In this page, we briefly discuss the subject of body armor. A more detailed presentation of bulletproof vests is available by clicking on this link.
When one's mission is to create individual ballistic protection, one is continually forced to make a compromise: maximum protection - maximum ergonomics, two perfectly contradictory notions. Indeed, the designer of ballistic protection will want to protect his wearer more and more while leaving him as much freedom of movement and mobility as possible so that he can accomplish his mission as well as possible.
A piece of equipment that protects all parts of the body from all handgun and rifle projectiles would simply be unacceptable.
IV-1 - The protection versus ergonomics trade-off or a difficult choice
When one wishes to create a ballistic protection, one is quickly confronted with two obvious facts. On the one hand, it will not be possible to make a useful bulletproof vest capable of stopping all the handgun and rifle projectiles in the world. On the other hand, it will be impossible to protect all areas of the body. Therefore, two very important choices have to be made: against which projectiles should the wearer be protected and which body regions should be preserved?
IV-2 - The choice of the ballistic level
The type of missions of the wearer of the protection already allows a discrimination between handguns and long guns. Will it be worn continuously, will the vest have to be discreet, will it be used punctually in the face of well-characterized aggressions? These questions are absolutely decisive.
The final choice will result from a statistical study of the weapons and ammunition that the wearer is most likely to encounter.
If you are not a retailer but an experimenter, you will come to the abrupt but realistic conclusion that a bullet-proof vest does not stop anything except the projectiles for which it was designed and those of a lower ballistic level than the latter, a characteristic that is however difficult to define without testing.
IV-3 - The choice of anatomical regions to protect
The fact that an integral ballistic protection would not be wearable, with the materials we currently have, leads de facto to the need to make a choice of the anatomical regions to be imperatively protected.
These are the most vascularized areas and are where the vital organs are located, which, if hit by a projectile, will result in severe bleeding. These are the thorax and the abdomen.
When one chooses to protect these areas of the body, one is aware that the neck, the face, the hatred, the top of the inner thighs (Scarpa's area or femoral trigone) are areas at risk given the large blood vessels that are found there. But this is the compromise that we have to accept. And it is obvious that at equal ballistic level, a "discreet" protective vest will protect less than a visible vest (with pelvic protection) because of the difference in the surface area of protection.
–THE BACK TRAUMA OR THE "BACK EFFECT
This concept has become very important with the use of flexible body armor. During the impact of the projectile, the vest, due to its flexibility, deforms backwards where the bullet hits it. This deformation is essential for the ballistic fabric fibers to be solicited in traction mode. A dynamic cone of depression is formed, which high-speed imaging shows to be more proportional to the amount of momentum of the projectile than to its kinetic energy. However, we can speak in terms of kinetic energy and consider that it is not entirely absorbed by the ballistic protection, but transmitted behind the vest, i.e. to the wearer. This phenomenon can be the cause of internal trauma identical to what can be observed experimentally with blunt shocks (no perforation).
In order to minimize this effect, a material is placed behind the ballistic protection (between the latter and the wearer's body) in charge of diffusing and absorbing all or part of this transmitted energy.
This "rear effect" is being studied in order to characterize it. Various avenues are being followed to understand its causes: contusions due to direct impact, overpressures due to a mechanical effect observed in certain pulmonary regions and at a distance from the impact in the case of thoracic injury, pressure wave propagation phenomena with possible interference phenomena. Some of these causes have been experimentally proven, others have yet to be definitively demonstrated.
- BALLISTIC HELMETS
To finish this quick presentation on ballistic protections, we will review the subject of ballistic helmets.
Contrary to common belief, even among users, bulletproof helmets are very new.
Some will raise their eyebrows and say: "ballistic helmets have been around for a long time". Yes, but... They were shrapnel-proof helmets designed to stop standard shrapnel (Stanag 2920) weighing 1.101 g at a speed of 600 m/s.
There is nothing abnormal about this, since these helmets were intended for the army and current statistics show that 96% of battlefield injuries were caused by shrapnel and 4% by bullets.
Sentence : before you put on any ballistic protection, check what it can stop. It's usually written on it. If it doesn't say so... beware.
Since then, really bulletproof helmets have been designed, especially for handguns, because the problem of weight is particularly present for this accessory.
The problem of the "back effect" is just as important because, as everyone knows, the skull is difficult to deform.
– BALLISTIC STANDARDS
The manufacture and testing of these materials follow specific standards that may vary from country to country, as the threats are sometimes different. They do not take into account all projectiles, but only the most common ones. For a synoptic view of the different standards, please consult the table proposed by DSM Dyneema.
The standards most known by specialists are those established by the American organization "National Institute of Justice". Europe has difficulties to achieve a homogeneity however desirable.
The major purchasers (Ministry of Defense, Ministry of the Interior, etc.) define specifications or special technical clauses imposing specific ballistic levels established according to the needs of the missions.
VIII - CONCLUSION
The field of ballistic protection is vast and complex. It is illusory to want to circumscribe it in a few pages. We hope, however, to have aroused your curiosity.
The compromise between the level of protection and the ergonomics of a bulletproof vest demystifies ballistic protection. It often appears to the layman as a universal shield capable of protecting its wearer against any projectile; its wearer was, in the mind of the uninitiated, invincible. This is unfortunately not the case. An analogy can be made with the world of the automobile: the safety belt regularly saves lives, provided that it is used within the limits for which it was designed.
The same is true of ballistic protection, it has its own limits. This is the most important message to convey.
Research in the field of ballistic protection continues. It promises more effective and lighter protections. Ammunition, on the other hand, is also evolving. As the battle between armor and sword is far from over, techniques for protecting humans still have a long way to go.